BACKGROUND

[0001] Propagating optical radiation— which may be referred to as "light"— can include optical frequencies from at least mid-infrared (IR) frequencies to ultra-violet (UV) frequencies. The light can be polarized, e.g. , linearly polarized (along one of two orthogonal axes that may be referred to as "s" and "p") or circularly polarized (either right- hand or left-hand circularly polarized). Circularly polarized light (CPL) has a polarization vector that rotates in a right-hand or left-hand direction as the light propagates. CPL is a specific case of elliptically polarized light (EPL), which for non-degenerate cases (i.e., not linear polarization, which is a one-dimensional polarization) can be referred to as two- dimensionally (2D) EPL, or chiral light carrying spin angular momentum.

[0002] EPL can be used in many applications, including optical communications, quantum computation, spin-optoelectronic devices, optical trapping, analysis of chiral metamaterials and biological molecules (e.g. , to determine structures of biomolecules, e.g., proteins), analysis of biopharmaceutical products (which may be referred to as "biologies"), coupling to or from electron spin in semiconductors, and applications involving use or control of photonic spin.

[0003] However, detecting the polarization of EPL, i.e. , determining whether the polarization is right-handed or left-handed, and separating right-handed from left-handed EPL (e.g. , when both polarizations are collinear) generally requires equipment for the following: (i) conversion from circular polarization to linear polarization (using a quarter- wave plate), and (ii) filtering (i.e. , separation of the converted light based on its polarization) using a linear filter. For some applications, this equipment may be too bulky, too expensive, or too difficult to integrate accurately. Some polarization-sensitive devices may be less bulky, but may have insufficient extinction ratios for some applications, or may be sensitive only to linear polarizations. Polarization-sensitive devices that are sensitive only to linear polarization lose an important degree of freedom (i.e., spin), which may be important in some applications. There is a need for improved polarization- sensitive detectors and filters for 2D EPL.

[0004] It is desired to address or ameliorate one or more disadvantages or limitations associated with the prior art, or to at least provide a useful alternative.

SUMMARY

[0005] In accordance with the present invention, there is provided an apparatus including:

a conductive body that supports propagating plasmons; and

at least one structure that generates the propagating plasmons when illuminated by two-dimensionally (2D) elliptically polarized light (EPL),

wherein the structure is formed to give opposite initial phases to the plasmons corresponding to opposite elliptical polarizations of the EPL.

[0006] The present invention also provides a method including the steps of:

forming at least one structure in or on a conductive body to generate propagating plasmons when illuminated by two-dimensionally (2D) elliptically polarized light (EPL), wherein the structure is formed to give opposite initial phases to the plasmons corresponding to opposite elliptical polarizations of the EPL.

[0007] The present invention also provides a method including the steps of:

generating propagating plasmons from two-dimensionally (2D) elliptically polarized light (EPL); and

controlling initial phases of the plasmons using the elliptical polarization of the

EPL.

[0008] The present invention also provides a photodetector including the apparatus above.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Preferred embodiments of the present invention are hereinafter described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

[0010] Figure 1 is a side diagrammatic view of an apparatus for optical polarization filtering;

[0011] Figure 2 is a top diagrammatic view of the apparatus;

[0012] Figure 3 is a scanning electron microscope (SEM) image of an embodiment of the apparatus;

[0013] Figure 4 is an enlarged portion of Figure 3;

[0014] Figure 5 is a diagram of an array of the apparatus;

[0015] Figure 6 is a diagram of plasmonic elements in the array;

[0016] Figure 7 is a diagram of an optical system including an optical detector with the apparatus;

[0017] Figure 8 is a graph of simulated normalized transmission spectra of left- handed and right-handed circularly polarized light (CPL) through the apparatus;

[0018] Figure 9 is a graph of measured normalized transmission spectra of left- handed and right-handed EPL through the apparatus;

[0019] Figure 10 is a graph of measured transmission spectra of left-handed and right-handed EPL for a wavelength of 785 nanometres (nm);

[0020] Figure 11 is a polar graph of polarization measurements from a photodetector including the apparatus configured for 785-nm light;

[0021] Figure 12 is a diagram of two columns the plasmonic elements;

[0022] Figures 13A and 13B are graphs of phases of plasmons generated by the plasmonic elements in Figure 12, for orthogonal circular polarizations; and [0023] Figures 14A and 14B are graphs of the total phase of the plasmons at an aperture of the apparatus (for the orthogonal polarizations), and the phase difference of the polarizations.

DETAILED DESCRIPTION Apparatus

[0024] Described herein is an apparatus 100 for optical polarization filtering and optical polarization sensing. The apparatus 100 is sensitive to "chiral" polarization of light, i.e., two-dimensional elliptical polarization, including the particular case of circular polarization. Light with a chiral polarization is referred to as two-dimensionally (2D) elliptically polarized light (EPL) 104, i.e., EPL not including linear polarization cases. In many applications, the EPL is circularly polarized light (CPL).

[0025] As shown in Figure 1, the apparatus 100 includes a conductive body 106 that supports propagating plasmons 114, and is opaque to the EPL 104 (i.e., the body 106 is optically thick for the EPL 104). The body 106 includes a conductive surface 102 that is illuminated by the EPL 104. The body 106 may be a thin film or a layer supported by a substrate. The body 106 may include a conductive material that provides conductivity for the conductive surface 102. The conductive surface 102 and the body 106 may include gold, silver and/or aluminium, e.g., a 300-nm thick metal film.

[0026] As shown in Figure 1, the apparatus 100 includes at least one polarization- sensitive plasmonic structure 112 that generates or provides the propagating plasmons 114 when illuminated by the EPL 104. Each structure 112 may be referred to as a "metasurface" because it is a patterned surface. The structure 112 is in or on the conductive surface 102. The structure 112 generates surface plasmons 114 from the EPL 104. The plasmons 114 propagate along the surface 102. The plasmons may be referred to as charge-density waves on the surface 102.

[0027] The structure 112 (which may be referred to as a "plasmonic structure") is formed to give opposite initial phases to the plasmons 114 corresponding to opposite elliptical polarizations of the EPL 104. Thus, when illuminated by the EPL 104, the initial phases (from 0 to 360 degrees) of surface plasmons 114 are controlled by the elliptical polarization (from 0 to 360 degrees) of the EPL 104. The structures 112 may be referred to as having "planar chirality" because the plasmons 114 are generated with opposite initial phases (e.g. , zero, and pi or 180 degrees) for respective opposite polarizations of the EPL 104. The initial phases of these generated surface plasmons 114 are controlled by the polarization state of the incident EPL 104 and the structure 112. The structure 112 is formed to give opposite initial phases to the surface plasmons 114 corresponding to the opposite polarizations of the EPL 104. For example, if the phase of surface plasmons 114 generated by right-handed EPL 104 is zero, then the phase of surface plasmons 114 generated by left-handed EPL 104 is pi. Thus, the surface plasmons 114 carry different initial phases based on the polarization of the EPL 104.

[0028] The structure 112 includes a plurality of plasmonic elements (which may be referred to as "plasmonic features", "sub-wavelength features", or "nanostructures") that are formed in or on the body 106 to convert the EPL 104 to the plasmons 114, and to give opposite initial phases to the plasmons 114 corresponding to opposite elliptical polarizations of the EPL 104. The plasmonic elements act as scatterers of the EPL 104. The plasmonic elements may include cavities 118 (which may be troughs or slots or holes) in the body 106 that are open at the surface 102 and otherwise closed (and thus do not form further apertures for transmissions of the EPL 104 through the surface 102 or through the apparatus 100). Accordingly, the cavities may be referred to as "non-penetrating cavities" because they do not penetrate the thickness of the body 106, and thus do not transmit the EPL 104. The non-penetrating cavities differ in this way from the aperture 108, which does penetrate the body 106 and allows the transmission of the portion of the EPL 104. The plasmonic elements may include protrusions (which may be referred to as "antennas") on or from the surface 102. The form of the structure 112 includes a pattern comprising a mutual arrangement of the plasmonic elements, and respective shapes of the plasmonic elements. The plasmonic elements have dimensions less than the wavelength of the EPL 104, which may be at least 2 or 5 or 10 times less. The plasmonic elements include at least one group of different plasmonic elements with a mutual spacing less than the wavelength of the EPL 104, which may be at least 2 or 5 or 10 times less. The plasmonic elements include a plurality of the groups arranged in an array. [0029] As shown in Figure 1, the apparatus 100 includes at least one sub-wavelength aperture 108 (which may be referred to as a "slit") through the body 106. The body 106 may be one layer of a plurality of layers, including one or more transparent layers above the body 106 that transmit the EPL 104 (where "above" refers to the light-receiving side of the body 106), and one or more transparent layers below the body 106 that transmit the light transmitted through the aperture 108. The at least one structure 112 may include a plurality of structures 112 on a respective plurality of sides of the aperture 108. Each portion of the structure 112 is formed or configured to generate the plasmons 114 from a first selected optical polarization of the EPL 104 with a first plasmon phase, and to generate the plasmons 114 from a second optical polarization (that is opposite to the first polarization) of the EPL 104 with a second plasmon phase that is opposite to the first plasmon phase.

[0030] The sub-wavelength aperture 108 has cross-sectional dimensions that are less than the free-space wavelength of the EPL 104 such that, if the sub-wavelength aperture 108 were to receive the EPL 104 in the absence of the structure 112, there would be little transmission of the light. However, the aperture 108 and the structure 112 are formed such that, when both the aperture 108 and the structure 112 are illuminated by the EPL 104, transmission of one elliptical polarization through the aperture 108 is selectively enhanced relative to transmission of the opposite elliptical polarization through the aperture 108. Thus the apparatus 100 selectively transmits light, specifically a portion 110 of the EPL 104, to the far side of the surface 102 and the body 106 based on the elliptical or circular handedness of the EPL 104. The aperture 108 extends through the body 106, and the transmitted portion 110 (which may be referred to as the "transmitted light") is also transmitted through the body 106. The aperture 108 and the structure 112, formed together, may be referred to as a "meta-aperture".

[0031] The structure 112 is formed to direct at least a portion of the surface plasmons 114 towards the aperture 108. The structure 112 may also direct the plasmons 114 in all directions on the surface 102, including a plurality of directions 116A, 116B, including away from the aperture 108, regardless of the polarization of the EPL 104, as shown in Figure 1. [0032] The plasmons 114 (having with an initial phase controlled by the handedness of the EPL 104) propagate from the structure 1 12 to the aperture 108, and accumulate phase due to the propagation. The aperture 108 and the structure 112 are formed relative to each other such that one handed portion of the EPL 104 undergoes constructive interference at the aperture 108, and the other handed portion of the EPL 104 destructive. The form of the aperture 108 and the structure 112 includes their mutual arrangement and position. The distance between the structure 112 and the aperture 108 is selected such that constructive interference occurs between the incident EPL 104 of one selected polarization and the surface plasmons 114 when they reach the aperture 108 from all active portions of the structure 112 around the aperture 108. Due to the selected propagation distance between the structure 1 12 and the aperture 108, destructive interference occurs for the opposite, non-selected polarization state of EPL 104. Therefore, a difference in transmission of the transmitted portion 110 tells the polarization state of EPL 104.

[0033] The transmission enhancement caused by the structure 112 is similar to extraordinary optical transmission (EOT), which relates to enhanced transmission of linearly polarized light through a linear aperture when a set of periodic grooves is etched near the linear aperture at both the entry and exit surfaces, and the linear polarization is perpendicular to the aperture.

[0034] As shown in Figure 2, the apparatus 100 includes a surface spacing Δ (which may also be referred to as "d") between the aperture 108 and each structure 112. The surface spacing Δ is selected such that the surface plasmons 114 from the structures 112 interfere constructively at the aperture 108 with the EPL 104 to enhance transmission of the transmitted portion of the EPL 104 through the aperture 108. The surface spacing Δ provides a phase delay to the plasmons 114. The surface spacing Δ is selected based on the free-space wavelength of the EPL 104 such that the phase of the surface plasmons 114 at the aperture 108 corresponds to the phase of the EPL 104 at the aperture 108 to enhance transmission of a selected elliptical polarization. The total phase of surface plasmons 114 at the aperture is φ _ίο ι=π/2+φ (λο)+βΑ+φ and thus is controlled by the coupling phase (<p _c ), the propagating phase (βΑ) and the binary phase induced by the structure 112 or metasurface (^ _ms ). The coupling phase <p _c describes the phase of the surface plasmons 114 when it is first converted from the EPL 104 at the structure 112. The coupling phase <p _c is controlled by material parameters of the surface 102, the design or arrangement of the structure 112, the depth and width of the plasmonic elements, and any fabrication imperfections. The metasurface phase (p _m$ is either 0 or pi (180 degrees). The distance or spacing Δ is chosen to make the total phase <p _tot equal to 0 or pi (0 for high transmission, pi for low transmission). However, as <p _c can be affected by many parameters during fabrication (including some that are difficult to control in experiments), a practical way to select a value for Δ is to use trial fabrications and experimental testing, which may include scanning a series of values of Δ. The phase delay may be controlled by selecting: the surface spacing Δ; and/or the material properties of the surface 102 along which the surface plasmons 114 propagate. Selecting the material properties may include selecting a surface material of the conductive surface 102 (which may include selecting a material for the body 106), and selecting dielectric properties adjacent to (and above in Figure 1, i.e., in a direction towards the EPL 104 from the body 106) the conductive surface 103. The apparatus 100 may include a dielectric with dielectric properties, and the surface plasmons 114 may propagate along an interface defined by the dielectric and the conductive surface 102. The dielectric may be a transparent material and/or a low-loss material that is substantially non-conductive and substantially transmissive of the EPL 104. The dielectric may be air, water, a vacuum and/or a solution.

[0035] The at least one structure 112 may include structures 112 around the aperture 108, and/or on opposite sides of the aperture 108, as shown in Figures 1 and 2. Each of the one or more structures 112 has no particular size limit in a direction along the surface 102 parallel to the length or long side of the aperture 108 (which can be a slit); however, in the other direction along the surface 102 (i.e., perpendicular to the length of the aperture 108, or, equivalently, parallel to a direction from the structure 112 to the aperture 108), the size of the structure 112 is limited by a propagation length of surface plasmon 114 along the surface 102: as the array of the structure 112 gets bigger, the surface plasmon 114 launched from a distant plasmonic element will not be able to reach the aperture 108 due to the propagation loss. For visible light wavelengths, the size limit in the direction towards the aperture 108 can be tens of microns. [0036] The apparatus 100 may include a thin film providing the body 106 and the conductive surface 102. The structure 112 and the aperture 108 may be formed in or on the thin film, as shown in Figure 1. The thin film may be gold.

[0037] As shown in Figures 3 - 6, the at least one structure 112 may include at least one array 122 with a plurality of repeating groups 124 of the plasmonic elements 120. Each group 124 may include a pair of the plasmonic elements 120.

[0038] As shown in Figure 6, each group 124 includes two of the sub-wavelength "plasmonic" elements 126 A, 126B. The plasmonic elements 126 A, 126B are mutually spaced such that the two centres are spaced by a distance D in a direction of propagation of the surface plasmons 114. The distance D is selected based on a frequency of the EPL 104, and is selected to be half a wavelength of surface plasmons 114. The two plasmonic elements 126 A, 126B are arranged to receive respectively the two perpendicular polarization components in the EPL 104. The two orthogonal plasmonic elements form the mutually cooperative group 124. The two plasmonic elements 126A, 126B in each group 124 are aligned mutually perpendicularly, and arranged along, or parallel to, the surface 102. The two plasmonic elements 126A, 126B may lie flat relative to the conductive surface 102, and may be formed as troughs or slots on or in the surface 102 with generally constant depths along their lengths. This design of the metasurface enables it to "sense" both of the orthogonal electric fields of the two-dimensional polarization. From a symmetry view point, the metasurface loses its mirror symmetry with respect to the direction between the aperture 108 and the structure 112, i.e., the direction of travel of the plasmons 114. That is, by placing a first structure 112 that is a mirror image of a second structure 112 at opposite sides of the aperture 108, the apparatus 100 forms the meta- aperture with planar chirality, rendering it possible to distinguish the polarization helicity of the chiral light.

[0039] Each plasmonic element 126 A, 126B has a length L, a width W and a height H (which may be referred to as a "depth" or a "thickness") that are all less than the surface plasmon wavelength. The length L, the width W and the height H represent three orthogonal distances or dimensions that define the shape and size of each plasmonic element. The width W may be substantially less than the length L. The L may have a maximum of one third of the surface plasmon wavelength for the L to remain substantially less than the surface plasmon wavelength. The W may have a maximum of one quarter of the surface plasmon wavelength for the W to remain substantially less than the surface plasmon wavelength. The W may have a minimum of 10 nm for sufficient coupling with the EPL and the surface plasmons. The height H may be equal to the length L. For optical frequencies in the EPL, the L may be 200 nanometres (nm), and the W may be 40 to 60 nm, particularly 50 nm. For a body 106 in the form of a gold film of thickness 300 nm, the plasmonic elements 126 can be fabricated with a focused ion-beam milling system, the central aperture can have a width of 200 nm and a depth of 300 nm (i.e., through the body 106), and the dimensions of each element 126 can be 200 nm in length (L), by 50 nm in width (W), by 200 nm in height (H), separated by 340 nm in both the lateral and vertical directions (i.e., X = Y = 340nm, for a wavelength of 680 nm).

[0040] Each plasmonic element is oriented in the plane of the surface 102, which may be rotationally, such that the initial phases of the plasmons 114 are generated as selected. The plasmonic elements 126 A, 126B may be oriented at 45 degrees to the directions 116A, 116B. One plasmonic element in each group 124 is susceptible to a particular linear polarization state, while the other plasmonic element in each group 124 is susceptible to an orthogonal polarization state, and the orientations of the elements in each group 124 are orthogonal to each other.

[0041] As shown in Figure 5, adjacent plasmonic elements 126 of the same orientation may be spaced by a distance X parallel to (or along) the surface plasmon propagation direction 116A, 116B. The distance X is selected based on the surface plasmon wavelength, and is equal to the surface plasmon wavelength so that each group 124 generates surface plasmons 114 with the same phase in the direction towards the aperture 108. This spacing X enhances the emission efficiency of the surface plasmons.

[0042] Adjacent plasmonic elements 126 of the same orientation may be spaced by a distance Y perpendicular to (or transverse) the surface plasmon propagation direction 116A, 116B. The Y may be less than the surface plasmon wavelength. The values for Y can be 300 to 350 nm for visible wavelengths in the EPL 104.

[0043] The aperture 108 may have at least one dimension less than the surface plasmon wavelength. The aperture 108 may include a slit with a slit width less than the surface plasmon wavelength. For optical EPL 104 with a free-space wavelength of 633 nm or 800 nm, the slit width may be 200 nm to 300 nm.

[0044] The apparatus 100 may include a plurality of the apertures 108, each arranged to receive in-phase surface plasmons from at least one of the structures 112. The structures 112 may surround each such aperture 108. The apertures 108 may be round (or circular) and the structures 112 may form respective annuli around the round apertures 108.

[0045] The apparatus 100 may be configured for operation with EPL 104 including frequencies from mid-infrared (IR) to ultraviolet (UV).

[0046] The apparatus 100 may be relatively simple to manufacture, may add little complexity or bulk to optical systems, and may be simply integrated with existing optical components and systems. Existing polarization- sensitive detectors for EPL may require multiple spaced devices, e.g. , a quarter-wave plate (to convert circular to linear polarization), a linear polarizer, and a photodetector. In contrast, the apparatus 100 may be applied directly across a photodetector (PD) to form a EPL detector or demodulator or filter. The photodetector may be a photodiode, or a photosensitive array.

[0047] As shown in Figure 7, a photodetector (PD) 702 including the apparatus 100 may operate as an elliptical polarization detector that generates a signal for a voltmeter V. The PD 702 can detect polarization modulation on a light beam 704, where the beam 704 is modulated by a linear polarizer LP and a quarter-wave plate QW.

Method

[0048] The apparatus 100 provides a method including the steps of:

[0049] illuminating at least one aperture 180 by 2D elliptically polarized light

(EPL) 104; [0050] generating propagating plasmons from the 2D EPL;

[0051] controlling initial phases of the plasmons using the at least one polarization- sensitive structure 112 based on the elliptical polarization of the EPL; and

[0052] enhancing transmission of one elliptical polarization of the EPL 104 through the aperture 108 over transmission of the other elliptical polarization based on the phases of the plasmons.

[0053] The method is for optical polarization filtering or sensing.

[0054] The method includes a step of transmitting a portion of the EPL 104 through the aperture 108 in the conductive surface 102.

[0055] The apparatus 100 may be manufactured by a method including steps of:

[0056] forming the at least one structure 112 in or on the conductive body 106 to generate the propagating plasmons when illuminated by the 2D EPL such that the structure is formed to give opposite initial phases to the plasmons corresponding to the opposite elliptical polarizations of the EPL;

[0057] forming the at least one aperture 108 for illumination by the EPL 104; and

[0058] forming the structure 112 and the aperture 108 relative to each other to enhance transmission of one elliptical polarization of the EPL 104 through the aperture 108 over transmission of the other elliptical polarization.

[0059] The apparatus 100 may be manufactured by focussed ion beam milling, photolithography, electron-beam lithography, nanoimprinting, or soft lithography.

[0060] The method includes a step of arranging a conductive surface for illumination by elliptically polarized light (EPL).

[0061] The step of forming the aperture 108 includes forming the aperture 108 in the conductive surface 102 for transmitting the portion of the EPL 104. [0062] The step of forming the at least one polarization- sensitive structure 112 includes forming the at least one structure 112 in or on the conductive surface for generating the surface plasmons 114, from the EPL 104, with the different initial phases based on the polarizations of the EPL 104.

[0063] The photodetector (PD) may be manufactured by coating a light-receiving side or surface of the PD with the apparatus.

Meta-Aperture Design

[0064] As shown in Figure 12, the metasurface may include a plurality of columns (or "rows") forming an array: the array may include a single pair of columns of the plasmonic elements (or equivalently, a single column of the groups of plasmonic elements, each element in each group having a different orientation). When at least a portion of the EPL 104 illuminates the metasurface an normal direction (i.e. , from a direction normal to the surface 102), as the plasmonic elements have dimension much smaller than the incident wavelength, each plasmonic element acts as an in-plane dipolar plasmon source with dipole polarization perpendicular to its major axis. The superposition of the plasmon wavelets from the entire column gives rise to a plasmon plane wave propagating perpendicular to and away from the column. The total z-component electric field of the plasmons from the metasurface after the interference between the plasmon plane waves emitted from the two columns (in the right half-space where the central aperture locates) is determined using Equation (1):

that is, the total electric field is based on the amplitude of the incident field Ζ¾, the light- to- plasmon coupling efficiency η, the coupling phase <p _c , the wave-vector of the plasmons β, the spin state of the incident light a _s (+1 for a left-handed CPL and -1 for a right-handed CPL respectively), the distance between plasmonic elements in the y direction (along the surface, and perpendicular to the direction to the aperture) Ad (which is equivalent to "Y" in Figures 4 and 5), and the distance between plasmonic elements in the x direction (along the surface, and parallel to the direction to the aperture) t (which is equivalent to "D" in Figure 6, and equivalent to half of "X" in Figures 4 and 5). The z-component of the plasmon field is the dominant component for determining the near-field properties of the plasmons. The terms within the bracket in Equation (1) indicate the differential response of the metasurface to the incident polarization helicity, and their consolidated phase is denoted as ^ _ms , representing the phase component of the plasmons induced by the metasurface, where:

[0065] For each helicity, the two columns (shown in Figure 12) yield conjugated phase terms, thus the consolidated phase is determined by the sign of an oscillation function with pitch 2π in terms of βΐ, which leads to a plasmonic binary phase repeating at "0" and "π", as shown in Figure 13 A. The helicity of the photons (σ ₈ ) appearing in the oscillation function causes a half-cycle shift between the binary phases from the right- and left-handed CPLs. Such a half-cycle shift gives rise to multiple wave-vector regions with π phase difference, shown as regions " 1 ", "2" and "3" between the unbroken curve (representing the LHCP) and the broken curve (representing the RHCP) in Figure 13 A. The binary phases can be converted into real space by using a surface plasmon dispersion relation: where n _eff denotes the effective index of SPs. A shown in Figure 13B, a notable out-of-phase region (labelled " 1 " in Figure 13B) exists, between the curves, with a large spectral spread from visible to near-infrared region.

[0066] The aperture 108 is set at the surface spacing Δ (or "d") from the plasmonic elements, thus the total accumulated phase of the plasmons when travelling to the aperture 108 can be determined based on the total accumulated phase,

that is: the total accumulated phase is based on the coupling phase (<p _c ), the propagating phase {fid) and the binary phase induced by the metasurface (^ _ms ). As shown in Figure 14A, the total phase (obtained numerically using a simulation) of the plasmons is opposite for the two orthogonal polarizations, LHCP and RHCP. As shown in Figure 14B, the phase difference is about 180 degrees, or pi. The surface plasmons couple back to free space radiation at the aperture 108, and interfere with the incident photons in the EPL 104 right above the aperture 108. The constructive or destructive interference leads - Ir respectively to an enhancement or depression of the optical transmission through the aperture 108. As a result, the peak transmission (in-phase with incident photons) for one handedness will results in a valley transmission (out-of-phase) for the other handedness. The total accumulated phase is modulated with the propagating phase that monotonically increases when decreasing the wavelength, as shown in Figures 8 and 9. Increasing the number of column pairs can improve emission efficiency of the plasmons from the metasurface, and hence the depth of the enhancement/depression of the optical transmission.

Experiments

[0067] As shown in Figure 8, in a numerical simulation of transmission through an example of the apparatus 100, when the transmission coefficient is normalized to one through a slit-only structure, there are clear transmission differences between transmission of the left-handed CPL (LCP) and the right-handed CPL (RCP) at the centre wavelength, illustrating modulation by the structures 112. As shown in Figure 9, measured experimental results from an experimental example of the apparatus 100 showed similar results to the simulation of Figure 8. The experimental structure used for Figures 8 and 9 was similar to that shown in Figure 3, in which the whole image is about 10 micrometer (um) by 8 um: the meta-aperture was fabricated with a focused ion-beam lithographic system, the central aperture had a width of 200nm, and the dimensions of each element were 200 nm by 50 nm by 200 nm deep, and the elements of each orientation were separated by 340 nm in both the lateral and vertical directions.

[0068] As shown in Figure 10, an experimental apparatus 100 designed for a free- space wavelength of 785 nm can provide differential transmission of the polarizations at the selected wavelength. In the experiment, the polarization of incident photons was adjusted by rotating a quarter waveplate behind a linear polarizer. The peak and valley of the measured curve corresponded to the left- and right-handed CPLs, hence validating the effectiveness of the photodiode for discerning the photon spins, as shown in Figure 11. The experimental structure was similar to that shown in Figure 3, in which the whole image is about 10 um by 8 um, and in which the distance Δ (also referred to in some cases as "d") from the structure 112 to the aperture 108 was about 2.1 times the surface plasmon wavelength (which may be referred to as the pitch, "P").

[0069] At least some numerical simulations, neglecting fabrication and experimental imperfections, indicate that the meta-aperture provides a much higher (at least 10 times) extinction ratio (the ratio of high transmission over low transmission) than using a plasmonic structure configured to direct plasmons in different directions from incident light of respective different polarizations.

Applications

[0070] The apparatus 100 may be described as an electro-optic plasmonic modulator.

[0071] To form a spin- sensitive photodiode, one or more of the meta-apertures may be coated onto a window or a front (light-receiving) surface of a compact, and potentially low-cost, photodiode. When the EPL 104 illuminates the photodiode, its spin information is be converted directly to a voltage signal (high or low level, depending on the handedness of the photons and the meta-aperture), making it ready be integrated with other electronic devices. The performance of the spin photodiode (in terms of extinction ratio) may be improved further by optimizing the structural parameters of the meta-aperture. Numerical simulation indicates that the extinction ratio can be on the order of 102-103. In contrast to a known CPL analyzer using a three-dimensional metamaterial, the meta-aperture described herein may have a simpler fabrication process, smaller devices, and may extend to working wavelengths from mid-infrared to visible/near- infrared regions. A far- field detection arrangement of the optical transmission may have advantages over near-field approaches that work typically for only a single wavelength and require a near-field probe (e.g. , near- field scanning microscopes, or fluorescent molecules) to read out the spin information, which complicates the processes and impairs the performance.

[0072] In an optical communication system, the apparatus 100 may enable polarization- sensitive sensing and switching, and thus an additional form of modulation for optical communication systems. The apparatus 100 may be used in the optical communication systems to filter EPL at the PD 702, and this may enable usage of an additional modulation scheme to approximately double the bandwidth for the optical communications systems. The apparatus 100 may be integrated to the PD 702 by applying the apparatus 100 as a thin film on a light-receiving face of the PD 702. Circular polarization may be maintained better than linear polarization in optical fibres, including those used in communication systems, and thus may be used more widely than linear polarization in optical communications.

[0073] Detecting or measuring absorption of EPL, including measuring differential absorption of left and right CPL (which may be referred to as measuring "circular dichroism", or CD, of a material) is useful in the study of optically active chiral molecules, including many biological molecules. CD occurs when a molecule contains one or more chiral chromophores (light- absorbing group). CD can be used compare two macromolecules, or the same molecule under different conditions, and determine if they have a similar structure. CD can be used to ascertain if a newly purified protein is correctly folded, to determine if a mutant protein has folded correctly in comparison to the wild-type, or to analyse biopharmaceutical products to confirm that they are in a correctly folded active conformation. Existing CD spectroscopy systems measure the two opposite circular polarizations separately, whereas the apparatus and method described herein may enable the both polarizations to be measured simultaneously. Accordingly, the apparatus 100 may enable a more efficient CD spectroscopy process.

INTERPRETATION

[0074] The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

[0075] Many modifications will be apparent to those skilled in the art without departing from the scope of the present invention as hereinbefore described with reference to the accompanying drawings.