"Apparatus for measuring optical activity and/or optical anisotropy"

DESCRIPTION

Technical field

This disclosure relates to the field of measurement on optical activity and/or optical anisotropy.

Background

As it is known, optical activity is the ability of a chemical substance to rotate the direction of polarization of light to the right or left, independent of sample orientation. Optical activity occurs in chiral materials, those lacking mirror symmetry.

The interaction of polarized light with a chiral sample can be formally expressed by means of the differential optical activity susceptibility Δχ(ω)=χι_Η(ω)-χπΗ(ω), where the subscripts LH and RH stand for left-handed and right-handed circularly polarized light, respectively. Δχ is a complex function of the frequency ω, whose real and imaginary parts describe, respectively, the circular birefringence (CB, often also called optical rotatory dispersion ORD) and the circular dichroism (CD) . The former is intimately dependent on the difference of the refractive indexes of the chiral material ex- perienced by LH and RH circularly polarized light (ηυ-ι(ω)-ηρΐΗ(ω)) , while the latter depends on the difference of the extinction coefficients for LH and RH circularly polarized light (ku-i(u))-kRH(u))) . Linearly polarized light can be regarded as a superposition of LH and RH circularly polarized light. As a result, the differential absorption (CD) of a chiral sample turns linearly polarized light into ellip- tically polarized light, while the difference in refractive index (CB) causes the rotation of the plane of polarization. Both correspond to the generation of a polarization component perpendicular to the incom ing polarization direction, which is called chiral signal. CB is associated with the component of the chiral signal that is in phase with the incident light, while CD gives rise to a 90° phase-shifted component.

Spectropolarimeters are known ; these apparatuses typically use a lamp as an incoherent light source and are based on the following principle of measurement: a monochromator selects a particular wavelength of the incoming light, which then passes through a linear polarizer. A photoelas- tic modulator (PE ) induces a polarization modulation (usually at tens of kHz frequency) , between LH and RH light. A detector, typically a photom ultiplier tube (PMT) or an InGaAs photodiode, measures the intensity of light transmitted by the sample. For an isotropic sample of chiral mole- cules, this intensity is slightly different for LH and RH polarization states (typically by 1 part in 1 0 ² -

1 0 ⁵ ). To extract this small difference signal, a lock-in amplifier (referenced to the PEM frequency) is used. This procedure is then repeated for all wavelengths of interest in order to retrieve the CD spectrum . Spectropolarimeters have the disadvantages of being very expensive, often bulky (double prism monochromators require long optical path length) and measuring only CD (typically, fur- ther accessories must be inserted into the beam path in order to measure also CB).

Document US 3,737,235 discloses a polarization interferometer with beam polarizing compensator wherein no polarizer coupled with the sample and the detector, so as to provide a second polychromatic radiation having a linear polarization, is described or suggested. The absence of the said polarizer coupled with the sample and the detector limits the measurable quantities to linear dichro- ism and circular dichroism , whereas, as explained later, the presence of such a polarizer in our setup allows one to measure not only linear dichroism and circular dichroism, but also linear bire- fringence and circular birefringence.

Document US2012/0268740 describes an apparatus for vacuum ultraviolet circular dichroism spectroscopy consisting of a broadband source followed by the sample under test, a compensator (which can be continuously rotated during measurement) , an analyzer and a spectrometer- detector. The compensator imparts a circular polarization component to the beam . The spectrometer detector comprises both dispersive/diffractive elements and a multi-element array detector, like a photodiode array or a charge coupled device. The broadband light from the optical system is spatially separated by the dispersive/diffractive element, such that light of different wavelength illuminates the detector array at different locations.

It is noticed that the apparatus described by US2012/0268740 requires a multi-element detector and measures a signal as a function of the rotation angle of a compensator, for several angles, and so it performs slow measurements. Moreover, the apparatus described by US2012/0268740 is specifically designed for vacuum ultraviolet experiments (between 10 and 200nm) for integration into a beam line at a synchrotron radiation facility. It is also observed that according to US2012/0268740 the generated signals have very low intensity and it is difficult to extract them from the instrumental noise. The principle of operation of this apparatus is technically and conceptually different from our ap-paratus: in fact, the apparatus described by US2012/0268740 requires a multi-element detector, whereas our apparatus requires a single-element detector, and it is based on the rotation of a compensator, whereas our apparatus is based on the linear translation of a compensator.

In the infrared spectral range (2-10 micrometers) CD and CB apparatuses are often based on a time-domain measurement rather than a frequency dispersion device like a monochromator: light from the (incoherent) source, like a glow bar, is split into two replica of variable time delay by a Mi- chelson interferometer. Spectra are obtained by scanning the time delay and Fourier-transform ing the measured interferogram . The additional elements that allow for the detection of chiral signals

(CD and CB) are the same as in the UV-visible range: a linear polarizer is followed by a fast polarization modulator, which changes the state of polarization from LH to RH circular polarization at high frequency. Lock-in demodulation then separates the chiral from the achiral signal.

It has also been demonstrated with infrared light that (vibrational) CD can be measured without a polarization modulator, if the two arms of the Michelson interferometer produce replica that have mutually perpendicular polarization (Prasad L. Polavarapu, Z. Deng, and G.-C. Chen, "Polarization- Division Interferometry: Time-Resolved Infrared Vibrational Dichroism Spectroscopy", Appl. Spectr. Volume 49, Number 2, pages 229-236 (1 991 ) and others). Alternatingly LH and RH states of polarization are then automatically created when the interferometer delay is varied.

If the sample is chiral and absorbs LH and RH polarized light differently, the intensity at the detector will be modulated accordingly and an interferogram can be recorded. Without sample, of with an achiral sample, on the other hand, the two perpendicular polarized replica created by the interfer ^¬ ometer do not interfere and no interferogram is seen.

The following documents also deal with optical activity measurement or observation techniques: - N . Ragunathan, N. S. Lee, T. B. Freedman, L. A. Nafie, C. Tripp, and H. Buijs, "Measurement of

Vibrational Circular Dichroism Using a Polarizing Michelson Interferometer", Applied Spectroscopy 44, 5-7 (1990) ; - Lombardi, R. A.; Nafie, L. A., Observation and calculation of vibrational circular birefringence: A new form of vibrational optical activity. Chirality 2009, 21 (1 E), E277-E286;

- Helbing, J. ; Bonmarin, M ., Vibrational Circular Dichroism signal enhancement using self- heterodyning with elliptically polarized laser pulses. J. Chem . Phys. 2009, 131 1 74507;

- Rhee, Y.-G . June, J.-S. Lee, K.-K. Lee, J.-H. Ha, Z. H. Kim , S.-J. Jeon and . Cho, "Femtosecond characterization of vibrational optical activity of chiral molecules", Nature 458, 310-313 (2009) ;

- Hanju Rhee, Intae Eom , Sung-Hyun Ahn and inhaeng Cho, "Coherent electric field characterization of molecular chirality in the time domain", Chem . Soc. Rev. 41 , 4457-4466 (2012) ; - Hiramatsu et al. , "Observation of Raman Optical Activity by Heterodyne-Detected Polarization-

Resolved Coherent Anti-Stokes Raman Scattering" Phys. Rev. Lett. 1 09, 083901 (2012) ;

- Hiramatsu et al., "Raman optical activity by coherent anti-Stokes Raman scattering spectral inter- ferometry", Opt. Expr. 21 , 13515 (2013) ;

- Hiramatsu et al., "Raman optical activity spectroscopy by visible excited coherent anti-Stokes Raman scattering", Opt. Lett. 40, 41 70 (201 5) .

Summary

The Applicant notices that the known optical activity measurement techniques show lim itations due to the complexity and costs of the apparatuses employed and/or in their performances, such as, the measurement time, the inherent stability, the detectable properties and the light wavelength range.

The Applicant addressed the technical problem of providing an apparatus alternative to the known ones and, according to a first aspect, the present invention relates to an optical apparatus as de ^¬ fined by the appended independent claims 1 and 9. Particular embodiments of the optical apparatus are defined by the dependent claims 2-8 and 1 0 - 1 5. A method for measuring optical activity and optical anisotropy is defined by the appended claim 1 6.

Brief description of the drawings

Further characteristics and advantages will be more apparent from the following description of an embodiment and of its alternatives given as a way of an example with reference to the enclosed drawings in which:

Figure 1 schematically shows an example of an optical apparatus configured to analyse optical activity and optical anisotropy of a sample;

Figure 2 schematically shows a first optical apparatus which is a first particular embodiment of the apparatus of Figure 1 , employable to obtain Circular Dichroism spectra;

Figure 3 schematically shows a second optical apparatus which is a second particular embodiment of the apparatus of Figure 1 , employable to obtain Circular Dichroism and Circular Birefringence spectra;

Figure 4a shows the Circular Birefringence on spectra experimentally obtained using an apparatus analogous to the one of Figure 3;

Figure 4b shows the Circular Dichroism spectra experimentally obtained using an apparatus analo- gous to the one of Figure 3;

Figure 5 schematically shows a third optical apparatus which is a third particular embodiment of the apparatus of Figure 1 , employable to measure Stimulated Raman Optical Activity; Figure 6 shows examples of time profiles of radiation pulses propagating into said third optical apparatus.

Detailed description

Optical apparatus 100

Figure 1 shows schematically an optical apparatus 1 00 which can be employed to analyse optical activity and optical anisotropy of a sample 1 .

Particularly, the optical apparatus 100 can be employed to measure the following optical properties: Circular Dichroism (CD) and Circular Birefringence (CB, sometimes also called Optical Rotatory Dispersion, ORD), both connected to the optical activity. Optical activity is present in chiral materi- als/samples lacking mirror symmetry. The optical apparatus 1 00 can be employed to measure also the Stim ulated Raman Optical Activity (Stimulated ROA) . Moreover, optical apparatus 1 00 can be employed to measure Linear Birefringence (LB) and Linear Dichroism (LD) which are connected to optical anisotropy.

The sample 1 to be tested with the optical apparatus 1 00 can be an isotropic sample, e.g. ensem- bles of randomly oriented molecules such as chromophores in solution. In anisotropic samples, e.g. molecular ensembles in which the molecules are to some extent aligned, such as in samples with "texture", LD and LB signals are usually m uch larger than CD and CB. However, optical activity spectra are by definition independent of the orientation of the sample, while LB and LD completely vanish by proper orientation of the sample 1 . Hence, the optical apparatus 1 00 can be used at will to measure LB and LD or, by proper rotation of the sample or the optical bench, CD and CB spectra.

The optical apparatus 1 00 comprises an electromagnetic radiation source module 2 and an optical path 20 including an adjustable optical birefringent module 3 and a support 6 for the sample 1 under test. The first optical path 20 is coupled to a detector 4 connected with a processing and control module 5. As it will be also clarified subsequently, the order of the components/elements of the optical apparatus 100 along the optical path 20 can be different from the one shown in the drawings. With reference to the optical structure of the optical apparatus 100, the following axes/directions can be defined :

• a propagation direction z,

· a first direction x, which is orthogonal to the propagation direction z, and

• a second direction y, which is orthogonal to the first direction x and the propagation direction z.

The first direction x and the second direction y define a plane which is orthogonal to the propagation direction z.

The electromagnetic radiation source module 2 (hereinafter, the source module 2) is configured to generate a polychromatic radiation having a linear polarization. The source module 2 includes a radiation generation device 7 which can be an incoherent source (e.g. a lamp) or a coherent source (e.g. a laser) .

The source module 2 is structured to generate a polychromatic (i.e. m ulti-wavelength) radiation having a broad bandwidth , particularly, comprising light in the UV (1 80-400nm), visible (400-750 nm) and infrared (750ηιη-20μιη) range. The bandwidth of the radiation produced by source module 2 can be chosen depending on the supposed property of the sample 1 under test. According to a specific embodiment, if the radiation generation device 7 provides an unpolarized or not purely polarized radiation, the source module 2 also comprises a polarization control module 8 which allows selecting a controllable polarized radiation having a high purity. As an example, it could be linear polarization with pre-established direction, selected by a polarizer with very high ex- tinction ratio (typically >10 ⁵ ). As another example, it could be a circular/elliptical polarization with selectable ellipticity and orientation of the axes.

The source module 2 is configured to produce an incident radiation RIN having, as an example, a linear polarization Pe lying in the plane x-y and forming a non-null angle ^■ & with the first direction x and a non-null angle with the second direction y.

The adjustable optical birefringent module 3 is an optical device configured to produce first and second linearly polarized radiations reciprocally delayed by an adjustable time delay. In accordance with the example of Figure 1 , the adjustable optical birefringent module 3 is structured to receive the incident radiation RIN and produce a transmitted radiation RTR which is formed by a first transmitted radiation component Rxi propagating along the propagation direction z (having a linear polarization parallel to the first direction x) and a second transm itted radiation component R _y i prop ^¬ agating along the propagation direction z (having a linear polarization parallel to the second direction y) . The first transmitted radiation component R _x i and the second transmitted radiation component R _y i are replicas (particularly, attenuated replica) of the incident radiation RIN entering the optical birefringent module 3. Particularly, the second transmitted radiation component R _y i is delayed, by an adjustable time delay x, with respect to the first transmitted radiation component Rxi . The above defined delay x can have a negative or a positive value.

It is observed that by varying the time delay x between the first and the second transm itted radiation components R*i and R _y i also the resultant polarization state of the transm itted radiation RTR which exits the adjustable optical birefringent module 3 is varied .

In other words, by suitably adjusting the time delay x introduced by the optical birefringent module 3 it is possible to obtain a transm itted radiation RTR also assuming a right-handed or a left-handed circular/elliptical polarization.

The adjustable optical birefringent module 3 includes at least one movable birefringent component. Particular embodiments of the adjustable optical birefringent module 3 will be described later. The sample 1 can be any type of material (suitably housed) employed, as an example, in biological and pharmaceutical areas for which there is the need to investigate its optical activity. As an example, CD and CB measurements in the UV spectral region can be performed in order to retrieve crucial information on the structural conformation of macromolecules, molecules, DNA/RNA and proteins, to study and characterize, e.g. their structure, stability (under heat, stress, denaturation), fold- ing, kinetics and interactions. As another example, CD and CB measurements can be performed from the near-infrared to the long-wavelength infrared region, especially in the 3-10 μιη region, in resonance with the so-called fingerprint vibrational region of molecules (often called Vibrational Circular Dichroism, VCD) , to provide three-dimensional structural information, not only on small molecules, but also on large and complex biopolymers such as proteins, polypeptides and nucleic acids/DNA.

It is observed that the sample 1 under test when reached by the transmitted radiation RTR (i.e. the first and second transmitted radiation components Rxi and R _y i) can provide an output radiation Rou having a polarization state depending on the optical activity of the sample itself. It is observed that if the sample 1 shows an optical anisotropy the output radiation Rou has a polarization state depending on the optical anisotropy of the sample.

Detector 4 is configured to convert a polychromatic radiation resulting from the optical path 20 into an electrical signal. Particularly, the detector 4 is an optical-to-electrical converter to be coupled to the sample 1 and configured to convert at least a portion of the output radiation Rou into a detected electrical signal S(T) (e.g. an electrical current Ι(τ), or an electrical voltage V(x)), proportional to the output radiation Rou. The detected electrical signal S(v) represents an interference electrical signal (that is also called interferogram) , having a behavior depending on said optical activity and/or said optical anisotropy of the sample 1 , that oscillates as a function of the said adjustable time delay τ between the first radiation component R _x i and the second radiation component R _y i imparted by the adjustable optical birefringent module 3. According to an example, the detector 4 is a photodiode or a photom ultiplier with suitable bandwidth. In accordance with further examples, the detector 4 can be a bolometer or a MCT (Mercury Cadmium Telluride) detector or other suitable I R-detector. Particularly, the detector 4 is a single-element device having a single continuous detection surface which is illuminated by the polychromatic radiation. Moreover, it is observed that the detector 4 has, particularly, a single output for the detected electrical signal S( ).

The processing and control module (5) is configured to process the electrical signal S(T) and provide an electromagnetic spectrum which depends on the optical activity and/or the optical anisotro- py of the sample 1 . Particularly, the processing and control module 5 is configured to perform a

Fourier Transform of the electrical signal S( ) (the interferogram) with respect to the time delay τ to obtain an electromagnetic spectrum . Preferably, the processing and control module 5 is also configured to vary said adjustable time delay τ within a delay range which is a multiple of the duration of the optical cycle of the central wavelength of the polychromatic incident radiation and which de- termines the resolution of the retrieved electromagnetic spectrum .

According to the example shown in Figure 1 , the processing and control module 5 is provided with an analog-digital converter 14 and a computing device 15 (CU) . The computing device 15 (as an example, a computer apparatus or an oscilloscope) can include a module configured to compute a Fourier Transform of an input digital signal.

Particularly, the processing and control module 5 can cooperate with an actuator 9 acting on the adjustable optical birefringent module 3 to vary the time delay τ. The actuator 9 may include a motor, which allows the adjusting of the movable component of the optical birefringent module 3.

Operation of the optical apparatus 00

In operation , the processing and control module 5 acts on the actuator 9 to vary, preferably, in con- tinuous manner the adjustable time delay τ introduced by the optical birefringent module 3 within a delay range, defined by a minimum and a maximum value. The processing and control module 5 generates control signal Sc to control the actuator 9.

In operation, the optical birefringent module 3 varies the adjustable time delay τ within a given delay range, preferably in continuous manner. The processing and control module 5 either generates a control signal Sc to control the actuator 9 and/or it receives a suitable signal that uniquely identifies the actuator position at every measurement, and hence allows determining the delay τ with suitable precision.

During the variation of the delay τ the electromagnetic radiation source module 2 generates the in ^¬ cident radiation RIN which propagates along the optical path 20 comprising the adjustable optical bi- refringent module 3 and the sample 1 . The adjustable optical birefringent module 3 provides a transmitted radiation RTR reaching the sample 1 which provides an output radiation Rou. The output radiation Rou is detected by the detector 4 which converts it into the electrical signal S(T). The electrical signal S(T) is processed by the processing and control module 5.

In greater detail, the highly polarized incident radiation RIN, generated by the electromagnetic radiation source module 2, reaches the adjustable optical birefringent module 3 which produces the transmitted radiation RTR having the two delayed replicas Rxi and R _y i of the incident radiation RIN.

Particularly, the adjustable optical birefringent module 3 is designed to introduce a delay τ up to, as an example, hundreds or thousands optical cycles, where the optical cycle is evaluated with reference to the central carrier wavelength of the incident radiation RIN.

Preferably, the adjustable optical birefringent module 3 is so as to allow controlling with very high precision (on the order of attoseconds) the relative delay τ between the two transmitted radiation components Rxi and R _y i . In greater detail, the adjustable optical birefringent module 3 introduces the said delay τ by means of a linear translation of preferably one birefringent optical element. The relative strength of these two replicas Rxi and R _y i can be tuned by changing the direction of the polarization of the incident radiation RIN (angle θ in Figure 1 ) with respect to the optical axes of the birefringent materials inside the adjustable optical birefringent module 3.

The first transmitted radiation component Rxi and the second transmitted radiation component R _y i reach the sample 1 : if such sample does not show optical activity (i.e. it is not a chiral sample) and it is not anisotropic (i.e. it does not show LD nor LB) the polarization states of the two perpendicular incom ing electrical fields Rxi and R _y i are not rotated during their propagation through the sample 1 . As a consequence, the output radiation Rou shows only the following two components:

a first output radiation component RX _X 2 propagating along the propagation direction z (having polarization parallel to the first direction x) which has been induced by the first transmitted radiation Rxi after propagation in the sample 1 ;

a second output radiation component RY _y 2 propagating along the propagation direction z (having polarization parallel to the second direction y) which has been induced by the second transmitted radiation R _y i after propagation in the sample 1 .

Therefore, if the sample does not show optical activity and it is not anisotropic, these two output radiations RXx2 and RY _y 2 do not interfere at the detector 4 because they are perpendicularly polarized, so that the provided electrical signal S( ^T ) is constant (i.e. it does not vary for the various de- lays ^T ).

On the contrary, if the sample 1 shows an optical activity (i.e. it is a chiral sample) and/or it is anisotropic (i .e. it shows LD and/or LB) such sample produces the output radiation Rou which shows not only the aforementioned RXx∑and RY _y 2, but also two more components:

a third output radiation component RX _y 2 propagating along the propagation direction z (having po- larization parallel to the second direction y) which is due to the interaction of the chiral (and/or anisotropic) sample 1 with the incident first transm itted radiation R _x i ; a fourth output radiation component RYx2 propagating along the propagation direction z (having polarization parallel to the first direction x) which is due to the interaction of the chiral and/or anisotropic sample 1 with the incident second transmitted radiation R _y i .

The output radiation Rou reaches the input port of the detector 4 on which the radiation compo- nents having the same polarization direction interfere. Particularly, the third output radiation component RXy2 having polarization parallel to the second direction y interferes with the second output radiation component RY _y 2, and the fourth output radiation component RY _X 2 having polarization parallel to the first direction x interferes with the first output radiation component RX _X 2. So, for a chiral or anisotropic sample, detector 4 generates the electrical signal Sf ¹ ) that is not constant and rep- resents the interferogram (oscillating as a function of the delay ^τ ) generated by the interference between the radiation components.

Particularly, the processing and control module 5 performs the complex (comprising both the real and imaginary parts) Fourier Transform with respect to the delay ^τ of the electrical signal Sft), so obtaining a complex spectrum as a function of the radiation wavelength. Depending on the meas- urement conditions, this spectrum carries information on the optical activity of a chiral sample (CD,

CB) and on LB and/or LD of an anisotropic sample.

With reference to the optical activity and the type of detectable phenomena, it is observed that interaction of polarized radiation with a chiral sample can be formally expressed by means of the differential optical activity susceptibility:

Δχ(ω)=χι_Η(ω)-χΒΗ(ω)

where the subscripts LH and RH stand for left-handed and right-handed circularly polarized radiation, respectively.

Δχ is a complex function of the frequency ω, whose real and imaginary parts describe, respectively, the circular birefringence (CB) and the circular dichroism (CD) .

The circular birefringence (CB) is intimately dependent on the difference of the refractive indexes of the material experienced by LH and RH circularly polarized radiation (n_H((o)-nRH(u))), while the circular dichroism (CD) depends on the difference of the extinction coefficients for LH and RH circularly polarized light (ku-i(u>)-kRH((d)) .

Linearly polarized radiation can be regarded as a superposition of LH and RH circularly polarized radiation. As a result, the differential absorption (CD) of a chiral sample turns linearly polarized radiation into elliptically polarized radiation , while the difference in refractive index (CB) causes the rotation of the plane of polarization. Both correspond to the generation of a polarization component perpendicular to the incoming polarization direction, which is called chiral signal. CB is associated with the component of the chiral signal that is in phase with the incident radiation, while CD gives rise to a 90° phase-shifted component.

Particular embodiments of the optical apparatus 100 will be described in the following. Identical or analogous elements, components or devices are indicated in the figures of the present description with the same reference numbers or symbols.

First embodiment: first optical apparatus 110

Figure 2 schematically shows a first optical apparatus 1 1 0, which is a first embodiment of the optical apparatus 100 and can be used to measure a CD (Circular Dichroism) signal and/or LD signals. In accordance with a particular example of the first optical apparatus 1 10, the polarization control module 8 is a first linear polarizer employed to obtain the linearly polarized incident radiation RIN. According to this first embodiment, the adjustable optical birefringent module 3 comprises a first adjustable wedge pair 10 and an optical element 1 1 . The first adjustable wedge pair 1 0 comprises a first optical wedge 12 and a second optical wedge 13. Both first 12 and second 13 optical wedges are made of a birefringent material and , as an example, show a fast polarization direction (also called fast axis) parallel to the first direction x and a slow polarization direction (also called slow axis) parallel to the second direction y. As it is clear from Figure 2, the first optical wedge 12 and the second optical wedge 13 are optical prisms.

At least one of the two optical wedges 12 and 13 is movable along the second direction y by means of the actuator 9. As an example, the first optical wedge 12 is movable and the second optical wedge 13 is fixed.

The time delay τ between the two replicas (i.e. the first transmitted radiation component Rxi and the second transmitted radiation component R _y i) is adjusted by changing the position (i.e. the insertion) of the first optical wedge 12 along the second direction y.

The second (fixed) wedge 13 is used to m inimize the spatial separation of the different wavelengths (prism effect due to dispersion, i.e. variable index of refraction with wavelength) and of the two replicas R _x i , R _y i (due to different refraction because of birefringence, i.e. different index of refraction for the two polarizations) .

The optical element 1 1 is a birefringent plate having the optical axis perpendicular both to the propagation direction z and to the optical axis of the two optical wedges. The optical element 1 1 causes a delay between the replicas (i.e. the components Rxi , R _y i), which is of opposite sign with respect to that induced by the wedges pair 10, thus permitting both positive and negative delays ^τ between the two radiation components having perpendicular polarizations. The optical element 1 1 exchanges the fast and slow polarization directions with respect to the first optical wedge pair 1 0, reversing the relative delay ^τ between the components Rxi and R _y i . In the previously mentioned example, the slow axis of the optical element 1 1 must be oriented along the first direction x and its fast axis along the second direction y.

It is observed that using the first optical apparatus 1 1 0 of figure 2 the CD and/or LD signals can be measured while the CB and/or LB signals cannot be detected. The two radiations having polarizations parallel to the first direction x and the second direction y at the output of sample 1 do not interfere at the detector if the interaction of the light with the sample does not cause a rotation of the polarization state. Linear and circular birefringence can thus not be measured. On the other hand, the presence of (linear or circular) dichroism causes a projection of part of the two incoming or- thogonal polarizations onto a common polarization plane, so that CD and LD can be measured.

The optical intensity and therefore the electrical signal S(T) provided by the detector 4 shows the following expression :

S (T ) = I ₀ e ^~KL {l + 2 sinfl cosfl [CD * sin(coT)]} (i ) where lo is proportional to the light intensity after the first polarizer 8, ^K is the extinction coefficient (i.e. the imaginary part of the complex refractive index), L is the length of sample 1 , θ is the angle of the first polarizer 8 with respect to the first direction x, ^ω is the carrier frequency and ^τ is the above defined (variable) delay.

Particularly, the analog-to-digital converter 1 4 (Figure 1 ) of the processing and control module 5 converts the electrical signal S(T) into a digital signal which is processed by the computing device 1 5 to provide a Fourier Transform with respect to the variable delay τ. The Fourier Transform pro- vided by the processing and control module 5, after a proper calibration of the intensity and wavelength axes, represents the CD and/or LD spectrum of the sample 1 . To suppress the contribution of LD and measure a pure CD spectrum of a chiral sample 1 it is possible to properly orient the sample 1 or the optical bench.

In the case the polarization control module 8 produces a linear polarization, the chiral signals are extracted by computing the sine Fourier transform of (1 ) (in the following indicated with sFT) with respect to ^τ . In the case the polarization control module 8 produces an elliptical polarization, the equation (1 ) is not valid anymore: if the mayor axes of the polarization ellipse are x and y, the CD signal will be multiplied by a cosine term , so that the chiral signal will be extracted by computing a cosine Fourier (in the following indicated with cFT) with respect to ¹ . In the general case, for other input polarization states, a mixture of cosine and sine Fourier transforms m ust be employed.

It is observed that the optical apparatus 100 and the first embodiment of figure 2 (first optical apparatus 1 10) make possible extending time-domain Fourier-Transform chiral spectroscopy to the visible and UV spectral range since the interferometer is based on the adjustable optical birefringent module 3 in which the two replicas of the radiation are not separated in space (that would cause in- creasing instabilities at shorter wavelengths, thus destroying the interference pattern) but rather in polarization.

In this way, the two replicas always travel via the same optics and follow the same optical path 20, so that the interferometer can be called "common mode". This inherently guarantees excellent stability and reproducibility of the imparted delay τ even for shorter wavelengths such as the visible and the UV. This consideration is also valid for the following second optical apparatus 120.

Second embodiment: second optical apparatus 120

Figure 3 schematically shows an example of a second optical apparatus 120, which is a second embodiment of the optical apparatus 1 00. The second optical apparatus 120 is suitable to simultaneously provide CD and CB spectra and/or LB and LD spectra.

The second optical apparatus 1 20 can be analogous to the first optical apparatus 1 10 but it also comprises a second polarizer 16 which is placed , as an example, between the sample 1 and the detector 4.

The second polarizer 16 is oriented in such a way to select one of the two orthogonal polarizations of the output radiation Rou providing a selected output radiation Roup. This means that, as clearly and unambiguously disclosed in figure 3 and figure 5 and in the description of the present invention, the polarizer 1 6 preferably is directly coupled with the sample 1 , which is in the support 6, and the detector 4 as to provide a second polychromatic radiation (R _ou p) having linear polarization. The second polarizer 1 6 can be oriented with its optical axis parallel to one of the optical axes of the adjustable optical birefringent module 3, i.e. to the first direction x or second direction y (angle P = 0 or P ⁼ in Figure 3). In this case, this condition can be fulfilled by rotating the second polarizer 1 6 so as to minimize the amplitude of the interference pattern in the S(T) (i.e. the interfer- ogram) when the sample 1 is removed or replaced by an achiral sample such as pure solvent. In the second embodiment, the optical intensity and therefore the electrical signal S(T) at the detec ^¬ tor 4 for y orientation of the second polarizer 16 (P ⁼ 90°) _anC j f ₀ r x orientation of the second polarizer 16 ( β ⁼ 0°) are, respectively:

S _y (r) = I _Q e ^~KL {sin ^z § + sin ϋ cos fi [CB * εοε(ωτ) + CD * είη(ωτ)]} ^a)

S _X ( r) = I ₀ e ^~KL {cos ² d + sin fi cos O [—CB * εοε(ωτ + CD * sin (ωτ)]} (2b) where /o, ^K , L, -θ, ^ω , ^τ have already been defined. These expressions (2a-2b) are valid in the weak signal lim it, in which the terms with quadratic dependence in CB and CD can be neglected, as is usually the case in practice. A sine or cosine Fourier transform of (2) with respect to ^Έ gives the CD or CB spectrum , respectively. This is valid in the case the polarization control module 8 produces a linear polarization at the entrance of the adjustable optical birefringent module 3; in case it produces an elliptical polarization with major axes along x and y, the roles of sFT and cFT are inverted. For any other input polarization state an appropriate phase shift must be included.

Calibration

The (absolute) magnitude of the CD and CB signals can be retrieved by suitable calibration via a second measurement where the second polarizer 1 6 is tilted by a well-defined angle Bcai (different from 0° and 90°) with respect to the vertical direction (as indicated in Fig. 3). In this case, the two replicas generated by the adjustable optical birefringent module 3 interfere at the detector 4 and the much weaker chiral and anisotropy signals can be neglected, giving : i ₀ £? ^~Ki f (1 + cos2P _cal cos 2ϋ ^' )/2 + sin2fi _cal sin ϋ cos ϋ [COS (WT)]) (3) As an example, the orientations of both first polarizer 8 and second polarizer 1 6 can be oriented parallel at θ= Pcai =45° to balance the intensities of the two replicas generated by the adjustable optical birefringent module 3 and to maximize the light throughput at the detector 4. In this way, the amplitude of the ^ωϊ ( ^ωτ ) interference term in equation (3) (proportional to sin2 sin ϋ cos \ _s maximized and the calibration procedure is facilitated (stronger signals are retrieved) .

By normalizing the Fourier transforms of signals (2a, 2b) with respect to the Fourier Transform of signal (3) , the ^{e KL} term cancels and the dependence on the angle θ vanishes and we obtain:

CB = είπ2β _∞ι cFT[I _h (T)]/cFT[I _cal (T^ _cal )] _(4a)

CD = Είη2β _εα1 sFr[/ _h ,(r)]/cFr[l _Cfli ( _¾ ft _al )] _(4b)

The extraction of these spectra is the ultimate goal of the second optical apparatus 120.

In the aforementioned calibration example with θ= Pcai =45°, however, as the orientation of the first polarizer 8 is varied with respect to the measurement of the chiral signals, the dependence on the angle θ does not vanish anymore when normalizing equations (2a) and (2b) with equation (3). This m ust be taken into account to extract the (absolute) magnitude of the CD and CB signals.

In the case of an anisotropic sample, equations (2a), (2b) , (4a) and (4b) still hold if the sample or the optical bench are properly oriented in space. If this is not the case, equations (4a) and (4b) will yield the LD and LB spectra, respectively, instead of CB and CD.

Experimental results

The Applicant has realized (employing discrete optical components) the second optical apparatus 120 and has tested it with the two enantiomers of a chiral sample (Nickel Tartrate) in the near- infrared spectral region. Figure 4a shows the CB spectra experimentally obtained : the spectrum for the RH enantiomer is, as expected, the mirror image of the one obtained for the LH enantiomer. Figure 4b shows the CD spectra experimentally obtained using the same chiral sample of Figure 4a: the spectrum for the RH enantiomer is, as expected, the mirror image of the one obtained for the LH enantiomer.

Phasing of the interferoqrams

It is observed that the signal recorded using the second polarizer 1 6 in a tilted (angle β different from 0° and 90°) orientation (equation 3) can also be used for calibrating the optical delay ^τ of the interferometer, precisely determ ining the zero-delay position. This calibration relies on the fact that in this case the signal must be an even function (symmetric in τ)_ so that a proper algorithm can flatten its Fourier Transform phase around zero, i.e. to minim ize the imaginary part of the Fourier Transform . This can be achieved, as an example, finding the peak of the self-convolution of the in- terferogram .

Signal amplification

It is noticed that interferograms in equations (1 ) and (2a, 2b) lie over a background offset. To improve the signal-to-noise ratio in the procedure of chiral signal extraction, maximization of the modulation depth is needed. The "modulation depth" is the ratio between the amplitude of the fringe pattern in the interferogram and its offset.

For the first embodiment (first optical apparatus 1 1 0, Figure 2) equation (1 ) shows that the offset is 1 and the amplitude of the fringe pattern is proportional to sin d cos d _{^ so} that the modulation depth for the CD spectrum is ^s ∞ $ cos tf ^■ CD The maximum fringe visibility is achieved when x- and y-components have equal intensity (angle $ = 45° j _n case of linear input polarization) and the corresponding modulation depth is CD/2. This means that, for typical CD signals, the modulation depth is limited to 1 0 ² -l 0 ⁵ .

For the second optical apparatus 120 (Figure 3), equation (2a) (second polarizer 16 oriented along the second direction y, β=90°) the offset is ^ίη ² ϋ _anc j the amplitude of the fringe pattern is proportional to s ^" ™ ^cos so that the modulation depth for the CD and CB spectra is proportional to:

(5)

From equation (5) it can be inferred that, in principle, to facilitate the extraction of CD and CB spectra it is possible to increase the modulation depth at will by reducing the angle θ, because ^cot & di ^¬ verges to infinity. In the practical case, the optimal choice of the angle θ of the first polarizer 8 will be of a few degrees or a fraction of a degree, depending on the amount of radiation available and the sensitivity of the detector 4. Approaching θ=0°, in fact, the second polarizer 1 6 has perpendicular polarization with respect to the first polarizer 8 and (in case of perfect extinction ratio of the polarizer) the amplitude of the fringe pattern goes to zero. However, an amplification of the modulation depth by 1 -2 orders of magnitude can be easily obtained for UV/visible light sources, up to a modulation depth of 1 -1 0%, thus increasing the signal-to-noise ratio considerably.

For angles approaching 0° or 90° the total radiation intensity at the detector 4 will become very small. The interferogram due to (unpolarized or partially polarized) leakage radiation will eventually become comparable or larger than the signal. Even with perfect polarizers, the condition leading to equations 1 -4 that the chiral signal is m uch smaller than the total intensity, will no longer hold any- more. A sim ilar reasoning also applies to equation (2b) (second polarizer 1 6 oriented along the second direction y, β=0°) , for which the optimal orientation of the first polarizer 8 is close to the second direction y (θ approaching 90°) .

It is noticed that, in contrast to embodiment 1 10, a strong asymmetry between the x and y components of the incident radiation is now desired. In this case, the first transmitted radiation component Rxi of the transmitted radiation RTR (having polarization parallel to the first direction x) is more intense than the other component R _y i and generates a chiral signal (i.e. the third output radiation component RX _y ∑) having a polarization along the second direction y (Fig 3) . Only this third output radiation component RX _y 2 and the local oscillator RY _y 2 are transmitted by the second polarizer 1 6, while RXx2 and RYx2 are blocked.

The third output radiation component RX _y 2 interferes with the second output radiation component

RY _y 2, which acts as a local oscillator for self-heterodyne amplification (sometimes also called ho- modyne amplification) as a function of delay τ. For the purposes of the present description, with the term "self-heterodyne amplification" it is meant the (non-linear) mixing of the weak chiral electric field emitted by the sample 1 with the electric field of the local oscillator: their interference at the de- tector 4 causes the appearance of a m ixing term (double product of the two fields) that bears in ^¬ formation on the optical activity of the sample and is proportional to the local oscillator (that acts as an amplifier with tunable gain). It is noticed that the self-heterodyne amplification analogously occurs with the m ixing of the LB signal and/or the LD signal and the local oscillator.

Modulation

It is observed that conventional CD spectropolarimeters achieve their high sensitivity thanks to fast polarization modulation at m ulti-kHz frequencies. This removes low-frequency fluctuations of the light source, thus improving the signal-to-noise ratio.

It is noticed that the above described optical apparatus 100 (together with the first and second embodiments 1 1 0 and 120) performs a high-frequency detection because the information on the CD/CB/LD/LB spectra is contained in the high-frequency oscillating pattern of the interferogram . In other words, the information is distributed in the interferograms as variations in the amplitude, period and sign of the fringes. The spacing between a maximum and the adjacent m inimum of the interferogram corresponds to a path-length difference, generated by the interferometer of the optical apparatus 1 00, of approximately half the central wavelength of the spectrum generated by the elec- tromagnetic radiation source module 2.

Over this distance the sign of the local oscillator signal inverts with respect to the chiral signal. This corresponds to a change in the insertion of the moving wedge of the adjustable optical birefringent module 3 by a much larger distance, by a factor that is called gear ratio. The gear ratio depends only on the apex angle of the first wedge 12 and the birefringence (in particular the difference Δη ₉ in the group index of refraction experienced by the ordinary and extraordinary waves) of the optical glass employed, and is typically -50 for small apex angles (a few degrees) and standard birefrin- gent materials (with birefringence Δη ₉ «0.ΐ ) . In this case, a motor speed of 10 mm/second at 200- nm central wavelength corresponds to a 1 -kHz modulation (i.e. 1000 fringes per second) . This enables the optical apparatus 100 (together with the first and second embodiments 1 1 0 and 120) to remove low-frequency fluctuations of the light source, thus improving the signal-to-noise ratio and reaching sensitivities as high as those achieved by spectropolarimeters, or even higher.

Preferably, the actuator 9 has a motor which allows adjusting the movable component of the optical birefringent module 3 with a speed higher than 1 0 m m/second.

As a first alternative, the optical apparatus 100 can also be operated in combination with a polarization modulator (not shown) , placed, as an example, between the first polarizer 8 and the adjustable optical birefringent module 3. For example, the first polarizer 8 can then be oriented at θ=0 and the polarization modulator can create LH and RH elliptically polarized radiation, thus inverting the relative sign of local oscillator and chiral signal. Since the radiation entering the adjustable optical birefringent module 3 needs to have only very small horizontal polarization component (local oscillator) , the requirements (material, driving voltage) for this modulator are much more relaxed than for one that has to produce a full λ/4 modulation, as required in conventional CD spectrometers.

As a second alternative, modulation of the signal sign can also be induced purely mechanically by periodically changing the orientation of the first polarizer 8 between -9 to 9.

It is observed that the position of the sample 1 shown in the figures 1 , 2 and 3 is only an example. Particularly, the sample 1 can be also placed between the electromagnetic radiation source module 2 and the adjustable optical birefringent module 3. Moreover, the components of the adjustable op- tical birefringent module 3 can be placed according to a different order from the one shown in the drawings. As an example, the sample 1 can be placed between two of the components of the adjustable optical birefringent module 3.

Third embodiment: third optical apparatus 130

Figure 5 shows a third optical apparatus 130 which represents a third embodiment of the optical apparatus 1 00. The third optical apparatus 130 can be used for measuring Raman Optical Activity

(ROA) spectra and is based on Stimulated ROA (called SROA in the following) and comprises, inter alia, an optical pump path 30 including a pulsed pump radiation source module 17 and an op ^¬ tional third polarizer 27.

The pulsed pump radiation source module 1 7 can be a coherent source, such as a pulsed laser producing a pump radiation Rp which is a pulsed radiation with spectrum narrower or equal to the width of the Raman features to be measured.

The third polarizer 27 is oriented to selected a linear polarization parallel , according to the example shown Figure 5, to the first direction x. The third polarizer 27 is configured to transmit on the sample 1 a pump transm itted radiation Rp _x , having linear polarization along the first direction x.

In accordance with the example of figure 5, the optical pump path 30, along which the pulsed pump radiation source module 1 7 and the third polarizer 27 are aligned, defines a further propagation direction r which is not collinear with the propagation direction z. In accordance with another embod- iment, the propagation direction z and the further propagation direction r can be collinear.

With reference to the radiation generation device 7, it can be analogous to the pulsed pump radiation source module 1 7 and is configured to produce an incident radiation F½ having pulsed form , but possibly with a broader spectral bandwidth, which ultimately defines the accessible spectral re- gion to observe the SROA signals.

The adjustable optical birefringent module 3 shown in Figure 5 comprises, as an example, a second adjustable wedge pair 21 and a first fixed wedge pair 22 optically coupled to the optical element 1 1 . The second adjustable wedge pair 21 and the first fixed wedge pair 22 are structurally analogous to the first adjustable wedge pair 1 0 of Figure 2.

In accordance with Figure 5, the second adjustable wedge pair 21 comprises a third optical wedge

23 and a fourth optical wedge 24. The third optical wedge 23 shows the optical axis parallel to the second direction y and the fourth optical wedge 24 shows the optical axis parallel to the propagation direction z. According to the embodiment shown , the third optical wedge 23 is fixed with respect to the fourth optical wedge 24 and the whole second adjustable wedge pair 21 is movable by a corresponding actuator 9 along the second direction y.

The first fixed wedge pair 22 comprises a fifth optical wedge 25 showing an optical axis parallel to the propagation direction z and a sixth optical wedge 26 showing an optical axis parallel to the second direction y.

The transm itted radiation RTR exiting the adjustable optical birefringent module 3 and reaching the sample 1 plays the role of a Stokes beam in a standard stim ulated Raman scattering measurement. Its frequency detuning with respect to the pump pulse determ ines the frequencies of the vibrational modes accessible in the SROA measurement.

The sample 1 is placed so as to be reached by the transmitted radiation RTR and the pump transmitted radiation Rp _x . Their interaction with the sample 1 produces the output radiation Rou. The second polarizer 1 6 shown in figure 5 allows one to select the desired output radiation Roup, which shows polarization directed parallel to the second direction y, in accordance with the example of figure 5.

It is noticed that the ROA allows characterizing chiral molecules. This phenomenon is based on inelastic Raman scattering of radiation which monitors the vibrational response of a molecule. Spontaneous ROA spectra can be measured as the difference between the Raman spectra of the sample under illum ination with RH and LH circularly polarized incident and/or scattered light. ROA signals are typically extremely small, of the order of 1 0 ³ in comparison with spontaneous Raman scattering, i.e. the achiral background.

Spontaneous ROA can be used as an analytical tool in stereochemistry and biochem istry. Thanks to its high sensitivity to three-dimensional molecular structure, ROA spectroscopy can also be used to investigate the conformation of biological molecules such as proteins and DNAs in aqueous solutions where vibrational CD is limited by strong solvent absorption.

As regard to the angle θ of the first polarizer 8, considerations analogous to the ones made with reference to the second optical apparatus 1 20 (Figure 3) also apply to the embodiment of Figure 5. In operation, in accordance with the example of figure 5 (first polarizer 8 oriented along an angle d close to 0°) the pulsed incident radiation RIN (i.e. the Stokes beam) reaches the adjustable optical birefringent module 3 which imposes a relative delay between the major x-component and its minor y-replica.

The x component of the transm itted radiation RTR interacts on the sample 1 with the pump transm itted radiation Rpx to generate a chiral SROA signal polarized along the second direction y, while the y-component of the pulsed incident radiation RIN will serve as local oscillator signal to amplify the chiral SROA signal.

In other words, the third optical apparatus 130 is based on a coherent interaction of two pulsed radiations sim ultaneously interacting with the sample 1 , thus amplifying the resulting signal by orders of magnitude with respect to the spontaneous ROA apparatuses.

The second polarizer 16 allows one to remove the achiral major component included into the out- put radiation Rou exiting the sample 1 . The transmitted chiral signal and the local oscillator signal included into the selected output radiation Roup interfere on the detector 4, producing the electrical signal S'(T) defining an interferogram as a function of the delay ^T introduced by the adjustable optical birefringent module 3.

The processing and control module 5 performs a Fourier Transform of this interferogram with re- spect to the delay ^T yielding a spectrum , i.e. the SROA signal as a function of optical frequency.

It is noticed that the fourth optical wedge 24 and the fifth optical wedge 25, that are additional components with respect to the embodiments of Figure 2 and 3, allow scanning the delay ^τ without changing the temporal-overlap between the major component of the Stokes pulse and the pump pulse.

Figure 6 shows the temporal profiles of the pump beam pulse Pp (polarized along the first direction x) , the first replica Psi of the Stokes beam (polarized along the first direction x) and the second replica Ps2 (polarized along the second direction y) . By moving the second adjustable wedge pair 21 , the time delay ^τ can be varied (preferably in a continuous manner) between a maximum and a minim um value.

It is noticed that also the Stokes replica polarized along the y (i.e. the second replica Ps2) direction overlaps in time with the pump beam (pulse Pp). This overlap induces an achiral signal (oriented along the y direction) which is thus transmitted by the second polarizer 1 6. Nonetheless, this achiral signal, if measured by a lock-in amplifier, does not give rise to another interferogram because it is not heterodyned by a local oscillator signal.

In other words, the retrieved signal does not vary over the period of the carrier frequency (a few femtoseconds) , but it rather presents a slow variation of its profile over the course of the pulse envelope following its overlap with the pump pulse (over the picosecond time scale) . For these reasons, this achiral signal constitutes a (large but slowly varying) offset in the retrieved interferogram , so that the Fourier Transform of the achiral signal is centred at low frequencies, out of the spectral region of interest given by the Fourier Transform of the aforementioned chiral signal and thus it does not perturb the SROA signal.

Similarly to the second optical apparatus 120 of figure 3, the modulation depth of the interferogram obtainable with the third optical apparatus 130 of figure 5 is proportional to cot( ) and the signal-to- noise ratio can be optim ized by adjusting the angle d.

Rotating the second polarizer 1 6 by an angle β and recording an interferogram under otherwise identical conditions enables to properly phase the interferograms (i.e. finding the position of the ze- ro path-length difference) and to measure the achiral SRS signal for absolute amplitude calibration. The above method is compatible with high-frequency modulation of the pump-pulse intensity at MHz frequency combined with lock-in detection, which is known to significantly enhance the sensitivity of achiral SRS spectroscopy because the Fourier Transform operator is linear (i.e. the Fourier Transform of the difference of two signals is the difference of the two Fourier Transforms, where the difference is computed by the lock-in). In this case, a high-frequency modulator (not shown) can be placed at the input or at the output of the third polarizer 27 and a lock-in amplifier (not shown) can be placed between detector 4 and the processing and control module 5.

Fast delay scanning and/or modulation of the polarization angle θ (either optically or mechanically) induces a modulation of the chiral SRS sign and further enhances signal-to-noise ratio.

With reference to third optical apparatus 130, it is noticed that the sequence order of the optical components of the adjustable optical birefringent module 3 can be modified with respect to the one shown in Figure 5. Moreover, in accordance with another embodiment of the third optical apparatus 130, the adjustable optical birefringent module 3 can be provided with a single optical wedge pair, analogous to the optical wedge pair 1 0 (Figure 3) , which can be placed between the sample 1 and the second polarizer 16.

Further objects of the present invention are the followings embodiments:

1 . Optical apparatus (1 00) comprising:

- a radiation source module (2) configured to generate a first polychromatic radiation (RIN) ;

- an optical path (20) optically coupled to the radiation source module (2) and comprising:

an adjustable optical birefringent module (3) configured to produce first and second radiations (Rxi , R _y i ) reciprocally delayed by an adjustable time delay and having reciprocally orthogonal linear polarizations,

a support (6) structured to support a sample (1 ) adapted to provide, in response to an input radia- tion, an output radiation (Rou) having a linearly polarized component (RX _y 2 or alternatively RY _X 2) depending on optical activity and/or optical anisotropy of the sample (1 )) ;

- a detector (4) configured to convert a second polychromatic radiation (Rou) resulting from the optical path (20) into an interference electrical signal (S(z)) representative of an interference of said linearly polarized component (RX _y 2 or alternatively RYx2) with a further linearly polarized component (θ) of the second polychromatic radiation (Rou);

- a processing and control module (5) configured to process the interference electrical signal (S(-z)) and provide an electromagnetic spectrum depending on said optical activity and/or said optical anisotropy of the sample (1 ) .

2. The apparatus (100; 1 1 0; 120; 130) according to the embodiment 1 , wherein the processing and control module (5) is configured to:

perform a Fourier Transform of the interference electrical signal (S(T)) to obtain said electromagnetic spectrum ;

vary and/or monitor said adjustable time delay within a delay range which is a function of a duration of an optical cycle of a central wavelength of the polychromatic radiation.

3. The apparatus (100) according to the embodiment 1 , wherein the adjustable optical birefringent module (3) comprises:

an adjustable wedge pair (1 0; 21 ) including a first optical wedge (12; 23) and a second optical wedge (13; 26) configured to produce said first and second radiations (R _x i , R _y i) from the first polychromatic radiation (RIN) reciprocally delayed by the adjustable time delay;

an optical birefringent element (1 1 ) optically coupled with the adjustable wedge pair (10) to modify the sign of said time delay; and

wherein the apparatus further comprises an actuator module (9) connected to the processor and control module (5) and mechanically coupled with at least one between said first optical wedge (12 ; 23) and second optical wedge (13; 26) to modify said time delay.

4. The apparatus (100) according to the embodiment 1 , wherein the detector 4 is configured to receive said output radiation (Rou) comprising said linearly polarized component (RX _y 2 or alterna- tively RY _X 2) corresponding to a chiral signal and/or a signal connected to optical anisotropy of the sample (1 ) and an additional linearly polarized component (RY _y 2 or alternatively RX _X 2) having the function of local oscillator signal for self-heterodyne amplification ; the linearly polarized component (RXy2 or alternatively RYx2) and the additional linearly polarized component (RY _y 2 or alternatively RXx2) having parallel polarizations.

5. The apparatus (120; 130) according to the embodiment 4, further comprising a polarizer (16) to be coupled with the sample (1 ) and the detector (4) so as to provide a second polychromatic radiation (Roup) having a linear polarization.

6. The apparatus (1 10) according to the embodiment 1 , wherein the apparatus is structured to provide an electromagnetic spectrum representing at least one of the following measures: a measure of CD (Circular Dichroism) associated with the sample (1 ) , a measure of VCD (Vibrational Circular

Dichroism) associated with the sample (1 ) , a measure of LD (Linear Dichroism) associated with the sample (1 ) if this is anisotropic.

7. The apparatus (120) according to the embodiments 4 and 6, wherein the apparatus is further structured to provide at least one of the following spectra: a first electromagnetic spectrum repre- senting a measure of CB (Circular Birefringence, also called Optical Rotatory Dispersion) associated with the sample (1 ) , a second electromagnetic spectrum representing a measure of LB (Linear Birefringence) associated with the sample (1 ) , a third electromagnetic spectrum representing a measure of LD (Linear Dichroism) associated with the sample (1 ) .

8. The apparatus (1 30) according to the embodiment 4, further comprising:

an additional optical path (30) including an additional radiation source apparatus (1 7, 27) configured to generate a linearly polarized pulsed pump radiation (RPX) to be transm itted to the sample (1 ) ; wherein said optical path and the additional optical path are structured to cause Stim ulated Raman Optical Activity in said sample (1 ) and said interference electrical signal (S(-r)) representing the Stimulated Raman Optical Activity.

9. The apparatus (130) according to the embodiments 3 and 8, wherein said adjustable optical birefringent module (3) further comprises a fixed optical wedge pair (22) optically coupled with said adjustable optical wedge pair (21 ) ; wherein both first optical wedge (12; 26) and a second optical wedge (13; 24) are movable by said actuator module (9) .

1 0. Method for measuring optical activity or optical anisotropy, comprising :

- generating a first polychromatic radiation (RIN) ;

- transmitting the first polychromatic radiation (RIN) to an optical path (20) including an adjustable optical birefringent module (3) ; - producing, by said optical birefringent module (3) , first and second radiations (Rxi , R _y i) reciprocally delayed by an adjustable time delay and having reciprocally orthogonal linear polarizations,

- providing the optical path (20) with a sample (1 ) generating, in response to an input radiation, an output radiation (Rou) having a linearly polarized component (RX _y ∑ or alternatively RYx2) depending on optical activity and/or optical anisotropy of the sample (1 )) ;

- converting a second polychromatic radiation (R _ou ) resulting from the optical path (20) into an interference electrical signal (S( z)) representative of an interference of said linearly polarized compo ^¬ nent (RXy2 or alternatively RYx2) with a further linearly polarized component (RY _y 2 or alternatively RXx2) of the second polychromatic radiation (Rou) ;

- processing the interference electrical signal (S( z)) and providing an electromagnetic spectrum depending on said optical activity and/or said optical anisotropy of the sample (1 ).

Further objects of the present invention are the followings embodiments:

I. Optical apparatus (100) comprising:

- a radiation source module (2) configured to generate a first polychromatic radiation (RIN) ;

- an optical path (20) optically coupled to the radiation source module (2) and comprising :

an adjustable optical birefringent module (3) configured to produce first and second radiations (R _x i , Ryi ) reciprocally delayed by an adjustable time delay and having reciprocally orthogonal linear polarizations,

a support (6) structured to support a sample (1 ) adapted to provide, in response to an input radia- tion, an output radiation (Rou) having a linearly polarized component (RXy2 or alternatively RY _X 2) depending on optical activity and/or optical anisotropy of the sample (1 )) ;

- a detector (4) configured to convert a second polychromatic radiation (Rou) resulting from the optical path (20) into an interference electrical signal (S(z)) representative of an interference of said linearly polarized component (RX _y 2 or alternatively RY _X 2) with a further linearly polarized compo- nent (RYy2 or alternatively RX _X 2) of the second polychromatic radiation (Rou) ;

- a processing and control module (5) configured to process the interference electrical signal (S(z)) and provide an electromagnetic spectrum depending on said optical activity and/or said optical anisotropy of the sample (1 ) ;

wherein the detector 4 is configured to receive said output radiation (Rou) comprising said linearly polarized component (RX _y 2 or alternatively RYx2) corresponding to a chiral signal and/or a signal connected to optical anisotropy of the sample (1 ) and an additional linearly polarized component (RYy2) having the function of local oscillator signal for self-heterodyne amplification ; the linearly po ^¬ larized component (RXy2 or alternatively RYx2) and the additional linearly polarized component (RY _y 2 or alternatively RX _X ∑) having parallel polarizations;

said apparatus further comprising a polarizer (1 6) to be coupled with the sample (1 ) and the detector (4) so as to provide a second polychromatic radiation (Roup) having a linear polarization;

II. The apparatus (100; 1 10; 120; 130) according to the embodiment I, wherein the processing and control module (5) is configured to:

perform a Fourier Transform of the interference electrical signal (S(x)) to obtain said electromag- netic spectrum ;

vary and/or monitor said adjustable time delay within a delay range which is a function of a duration of an optical cycle of a central wavelength of the polychromatic radiation. III . The apparatus (1 00) according to the embodiment I, wherein the adjustable optical birefringent module (3) comprises:

an adjustable wedge pair (1 0; 21 ) including a first optical wedge (12; 23) and a second optical wedge (13; 26) configured to produce said first and second radiations (R _x i , R _y i ) from the first poly- chromatic radiation (RIN) reciprocally delayed by the adjustable time delay;

an optical birefringent element (1 1 ) optically coupled with the adjustable wedge pair (10) to modify the sign of said time delay; and

wherein the apparatus further comprises an actuator module (9) connected to the processor and control module (5) and mechanically coupled with at least one between said first optical wedge (12 ; 23) and second optical wedge (13; 26) to modify said time delay.

IV. The apparatus (100) according to the embodiment I, wherein the polarizer (16) is placed between the sample (1 ) and the detector (4).

V. The apparatus (1 1 0) according to the embodiment I , wherein the apparatus is structured to provide an electromagnetic spectrum representing at least one of the following measures: a measure of CD (Circular Dichroism) associated with the sample (1 ) , a measure of VCD (Vibrational Circular

Dichroism) associated with the sample (1 ) , a measure of LD (Linear Dichroism) associated with the sample (1 ) if this is anisotropic.

VI. The apparatus (120) according to the embodiments I and V, wherein the apparatus is further structured to provide at least one of the following spectra: a first electromagnetic spectrum repre- senting a measure of CB (Circular Birefringence, also called Optical Rotatory Dispersion) associated with the sample (1 ) , a second electromagnetic spectrum representing a measure of LB (Linear Birefringence) associated with the sample (1 ) , a third electromagnetic spectrum representing a measure of LD (Linear Dichroism) associated with the sample (1 ) .

VII . The apparatus (130) according to the embodiment I, further comprising:

an additional optical path (30) including an additional radiation source apparatus (1 7, 27) configured to generate a linearly polarized pulsed pump radiation (RPX) to be transm itted to the sample (1 ) ; wherein said optical path and the additional optical path are structured to cause Stim ulated Raman Optical Activity in said sample (1 ) and said interference electrical signal (S(-r)) representing the Stimulated Raman Optical Activity.

VII I. The apparatus (130) according to the embodiments III and VII , wherein said adjustable optical birefringent module (3) further comprises a fixed optical wedge pair (22) optically coupled with said adjustable optical wedge pair (21 ) ; wherein both first optical wedge (12; 26) and a second optical wedge (13; 24) are movable by said actuator module (9).

IX. Optical apparatus (1 00) comprising :

- a radiation source module (2) configured to generate a first polychromatic radiation (RIN) ;

- an optical path (20) optically coupled to the radiation source module (2) and comprising :

an adjustable optical birefringent module (3) configured to produce first and second radiations (Rxi , R _y i ) reciprocally delayed by an adjustable time delay and having reciprocally orthogonal linear polarizations,

a support (6) structured to support a sample (1 ) adapted to provide, in response to an input radiation, an output radiation (Rou) having a linearly polarized component (RX _Y 2 or alternatively RY _X 2) depending on optical activity and/or optical anisotropy of the sample (1 )) ; - a detector (4) configured to convert a second polychromatic radiation (Rou) resulting from the optical path (20) into an interference electrical signal (S(z)) representative of an interference of said linearly polarized component (RX _Y 2 or alternatively RY _X 2) with a further linearly polarized component (RY/2 or alternatively RX _X ∑) of the second polychromatic radiation (Rou);

- a processing and control module (5) configured to process the interference electrical signal (S(-z)) and provide an electromagnetic spectrum depending on said optical activity and/or said optical ani- sotropy of the sample (1 ) ;

wherein the adjustable optical birefringent module (3) comprises:

an adjustable wedge pair (1 0; 21 ) including a first optical wedge (12; 23) and a second optical wedge (13; 26) configured to produce said first and second radiations (R _x i , R _y i) from the first polychromatic radiation (RIN) reciprocally delayed by the adjustable time delay;

an optical birefringent element (1 1 ) optically coupled with the adjustable wedge pair (10) to modify the sign of said time delay; and

wherein the apparatus further comprises an actuator module (9) connected to the processor and control module (5) and mechanically coupled with at least one between said first optical wedge (12 ;

23) and second optical wedge (13; 26) to modify said time delay;

said apparatus further comprising a polarizer (1 6) to be coupled with the sample (1 ) and the detector (4) so as to provide a second polychromatic radiation (R _ou p) having a linear polarization.

X. The apparatus (1 00; 1 10; 1 20; 130) according to the embodiment IX, wherein the processing and control module (5) is configured to:

perform a Fourier Transform of the interference electrical signal (S(T)) to obtain said electromag ^¬ netic spectrum ;

vary and/or monitor said adjustable time delay within a delay range which is a function of a duration of an optical cycle of a central wavelength of the polychromatic radiation;

XI. The apparatus (100) according to the embodiment IX, wherein the polarizer (1 6) is placed between the sample (1 ) and the detector (4) ;

XI I. The apparatus (1 10) according to the embodiment IX, wherein the apparatus is structured to provide an electromagnetic spectrum representing at least one of the following measures: a measure of CD (Circular Dichroism) associated with the sample (1 ) , a measure of VCD (Vibrational Cir- cular Dichroism) associated with the sample (1 ), a measure of LD (Linear Dichroism) associated with the sample (1 ) if this is anisotropic;

XI II. The apparatus (120) according to the embodiments IX and XI I, wherein the apparatus is further structured to provide at least one of the following spectra: a first electromagnetic spectrum representing a measure of CB (Circular Birefringence, also called Optical Rotatory Dispersion) associ- ated with the sample (1 ), a second electromagnetic spectrum representing a measure of LB (Linear

Birefringence) associated with the sample (1 ) , a third electromagnetic spectrum representing a measure of LD (Linear Dichroism) associated with the sample (1 ) .

XIV. The apparatus (130) according to the embodiment IX, further comprising:

an additional optical path (30) including an additional radiation source apparatus (1 7, 27) config- ured to generate a linearly polarized pulsed pump radiation (Rpx) to be transm itted to the sample

(1 ) ; wherein said optical path and the additional optical path are structured to cause Stim ulated Raman Optical Activity in said sample (1 ) and said interference electrical signal (S(-r)) representing the Stimulated Raman Optical Activity.

XV. The apparatus (130) according to the embodiments IX and XIV, wherein said adjustable optical birefringent module (3) further comprises a fixed optical wedge pair (22) optically coupled with said adjustable optical wedge pair (21 ) ; wherein both first optical wedge (12; 26) and a second optical wedge (13; 24) are movable by said actuator module (9).

Further objects of the present invention are the followings embodiments:

A. Optical apparatus (100) comprising :

- a radiation source module (2) configured to generate a first polychromatic radiation (RIN) ;

- an optical path (20) optically coupled to the radiation source module (2) and comprising :

an adjustable optical birefringent module (3) configured to produce first and second radiations

(Rx1 , Ry1 ) reciprocally delayed by an adjustable time delay and having reciprocally orthogonal linear polarizations,

a support (6) structured to support a sample (1 ) adapted to provide, in response to an input radiation, an output radiation (ROU) having a linearly polarized component (RXy2 or alternatively RY _X 2) depending on optical activity and/or optical anisotropy of the sample (1 )) ;

- a detector (4) configured to convert a second polychromatic radiation (ROU) resulting from the optical path (20) into an interference electrical signal (S( z)) representative of an interference of said linearly polarized component (RX _y 2 or alternatively RY _X 2) with a further linearly polarized component (RX/2 or alternatively RYx2) of the second polychromatic radiation (ROU) ;

- a processing and control module (5) configured to process the interference electrical signal (S(z)) and provide an electromagnetic spectrum depending on said optical activity and/or said optical anisotropy of the sample (1 ) ;

said apparatus further comprising a polarizer (1 6) to be coupled with the sample (1 ) and the detector (4) so as to provide a second polychromatic radiation (Roup) having a linear polarization.

B. The apparatus (100; 1 10; 120; 130) according to the embodiment A, wherein the processing and control module (5) is configured to:

perform a Fourier Transform of the interference electrical signal (S(i)) to obtain said electromagnetic spectrum ;

vary and/or monitor said adjustable time delay within a delay range which is a function of a duration of an optical cycle of a central wavelength of the polychromatic radiation;

C. The apparatus (100) according to the embodiment A, wherein the adjustable optical birefringent module (3) comprises:

an adjustable wedge pair (1 0; 21 ) including a first optical wedge (12; 23) and a second optical wedge (13; 26) configured to produce said first and second radiations (Rx1 , Ry1 ) from the first pol- ychromatic radiation (RIN) reciprocally delayed by the adjustable time delay;

an optical birefringent element (1 1 ) optically coupled with the adjustable wedge pair (10) to modify the sign of said time delay; and

wherein the apparatus further comprises an actuator module (9) connected to the processor and control module (5) and mechanically coupled with at least one between said first optical wedge (12 ; 23) and second optical wedge (13; 26) to modify said time delay;

D. The apparatus (1 00) according to the embodiment A, wherein the detector 4 is configured to receive said output radiation (ROU) comprising said linearly polarized component (RX _y 2 or alterna- tively RYx2) corresponding to a chiral signal and/or a signal connected to optical anisotropy of the sample (1 ) and an additional linearly polarized component (RY 2 or alternatively RXx2) having the function of local oscillator signal for self-heterodyne amplification ; the linearly polarized component (RXy2 or alternatively RY _X 2) and the additional linearly polarized component (RY _V 2 or alternatively RXx2) having parallel polarizations;

E. The apparatus (120; 130) according to the embodiment A, wherein the polarizer (16) is placed between the sample (1 ) and the detector (4) ;

F. The apparatus (1 10) according to the embodiment A, wherein the apparatus is structured to provide an electromagnetic spectrum representing at least one of the following measures: a measure of CD (Circular Dichroism) associated with the sample (1 ) , a measure of VCD (Vibrational Circular

Dichroism) associated with the sample (1 ) , a measure of LD (Linear Dichroism) associated with the sample (1 ) if this is anisotropic.

G. The apparatus (120) according to the embodiments A, D and F, wherein the apparatus is further structured to provide at least one of the following spectra: a first electromagnetic spectrum repre- senting a measure of CB (Circular Birefringence, also called Optical Rotatory Dispersion) associated with the sample (1 ) , a second electromagnetic spectrum representing a measure of LB (Linear Birefringence) associated with the sample (1 ) , a third electromagnetic spectrum representing a measure of LD (Linear Dichroism) associated with the sample (1 ) .

H. The apparatus (130) according to the embodiments A and D, further comprising :

an additional optical path (30) including an additional radiation source apparatus (1 7, 27) configured to generate a linearly polarized pulsed pump radiation (RPX) to be transmitted to the sample (1 ) ; wherein said optical path and the additional optical path are structured to cause Stim ulated Raman Optical Activity in said sample (1 ) and said interference electrical signal (S(-r)) representing the Stimulated Raman Optical Activity.

I. The apparatus (130) according to the embodiments C and H, wherein said adjustable optical bire- fringent module (3) further comprises a fixed optical wedge pair (22) optically coupled with said adjustable optical wedge pair (21 ) ; wherein both first optical wedge (12; 26) and a second optical wedge (13; 24) are movable by said actuator module (9) ..

Further comments

The described optical apparatus 1 00 (and the corresponding embodiments) shows several advantages. As an example, it does not require a PE (photoelastic modulator) or a lock-in amplifier, thus it can be much less expensive than the known ones. In addition, it does not require a mono- chromator, thus it is much more compact.

With reference to the second optical apparatus 120 of figure 3, it is observed that it can automati- cally measure CD and CB signals, simultaneously.

Moreover, it is observed that commercial spectropolarimeters measure the spectra wavelength by wavelength, so that the measurement time increases with increasing spectral coverage required. On the other hand, the described optical apparatus 1 00 (and the corresponding embodiments) illuminates the sample with all the light colours sim ultaneously, so that it measures the entire spec- trum at once (m ultiplex advantage) .

It is also noticed that spectral resolution can be varied just by varying the scan range of the adjustable optical birefringent module 3. The optical apparatus 100 is based on the heterodyned temporal interferometry technique, which combines heterodyne amplification of the chiral signal with a local oscillator signal and Fourier Transform detection of a temporal interferogram using a single- channel detector (i.e. a single-element device having only one continuous detection surface) . The optical apparatus 1 00 allows extending the measurements to the UV and visible region thanks to the use of a common-path interferometer (while apparatuses employing polarization-division Mi- chelson interferometer require active stabilization) . Also in the infrared region, the optical apparatus 1 00 can afford much higher sensitivities due to the intrinsic higher stability of its interferometer path. The above teachings make it possible and practical to perform in the time-domain CD and CB (and/or LD and LB) measurements without an expensive polarization modulator in a wide spectral range from the far UV to the far I R. It is noticed that the optical apparatus 1 00 and, particular, the embodiments of figure 2 and 3 can be employed to provide electromagnetic spectra representing a measure of VCD (Vibrational Circular Dichroism) , in the relevant spectral range (e.g. I R) .

In addition, as above described, the optical apparatus 1 00 perm its to measure the LB and LD signals.

Particularly, the high intrinsic stability and precision of the described devices allows extending this measurement principle to a new form of chiral Raman spectroscopy. With reference to the third optical apparatus 130, it is noticed that Stimulated Raman signals can be fast acquired, so that SROA spectra could require acquisition times of only a few minutes, compared to hours required with current technology.