THE REFRACTIVE INDEX AND ASSOCIATED OPTICAL METHOD

TECHNICAL FIELD

The goal of the current invention is a polarimetric microscope working in reflection for the measurement of the refractive index of materials, including both isotropic and anisotropic materials, in liquid and solid phases, and in the latter case, measuring both planar and non- planar surfaces.

The optical method associated to the invention, which is also purpose of the present invention, allows measuring the refractive index of materials that are integrated into devices and optical systems, and thus, which cannot be measured in transmission (as lenses integrated into cameras). For the accurate and precise measure of the refraction index, the microscope includes confocal measurements.

BACKGROUND OF THE INVENTION

The use of polarimetric microscopes capable of measuring the Mueller matrix of a sample is known. Most of these microscopes work in transmission, but few of them are also capable of working with samples in reflection. Among the latter, most of them are able to perform the image of the sample, and then, the polarimetric content is studied.

In addition, there are also known microscopes using optical systems able to imaging the Fourier plane, i.e., not directly visualizing the object, but the plane wave angular spectrum. This collection of different plane wave directions includes in a single image all the information of a spot in the sample, instead of that of its surface. One previous patent exploiting this configuration is that of M. P. Davidson, “Interferometric back focal plane scatterometry with Koehler illumination (US7061623B2) (2006)", where a microscope in reflection, using an epi-illumination scheme, uses data of the back-focal plane of the microscope objective to image the sample. Another patent is that of W. D. Mieher et al. “Apparatus and methods for detecting overlay errors using scatterometry (US7280212B2) (2007)”, in which a microscope in reflection is set. This microscope uses an epi-illumination configuration, works with monochromatic light and also includes means to control the polarization state of light incident on the sample and an additional means to analyze the state of polarization of light scattered by the sample. Likewise, in the patent fS¾I¾ and Jit “Polarization state tomography micro-imaging device and method thereof (CN 103134756A) (2013)”, the authors propose a microscope which is mounted in reflection, using an epi-illumination configuration, that comprises a polarization generator, a microscope objective, and three beamsplitters to separate the reflected beam in four beams to measure-in parallel four Stokes vectors. Finally, in the patent Liang-Chia Chen et al. , “Method and system for three-dimensional polarization-based confocal microscopy (US8416491B2) (2009)", linearly polarized structured light formed by an optical grating is projected on the object underlying profile measurement. The purpose of the instrument is not the measurement of the refractive index (and absorption coefficient) of the sample as the microscope disclosed in the present invention but the measure of the topography of the sample (which can be non-planar). Moreover, the use of polarized light is not intended to be used to measure polarization sensitive properties, such reflectivity or Stokes parameters, but to create different illumination patterns to be projected on the sample.

We want to note that the last three patents, although using polarization, are not intended to determine a full Mueller matrix, but a partial polarimetric measurement resulting in the polarized reflectance of the sample according to known directions respect to the plane of incidence and scattering.

Some other works are able to measure the angle resolved Mueller matrix of the sample. It can be found a manuscript of P. Smith, “Characterizing dielectric tensors of anisotropic materials from a single measurement. PhD thesis, The University of Arizona (2013)”, where a mechanical arm is used to measure different angular ranges. By using a low numerical aperture microscope objective, a small range of angles is taken. This small angular range can be taken starting from an initial angle, which is set through mechanical movements.

Other manuscripts, as those published by C. Fallet “Angle resolved Mueller polarimetry, applications to periodic structures. PhD thesis, Ecole Polytechnique X (2011)” or A. de Martino and B. Drevillon “Metrological characterization of microelectronic circuits (EP1828712B1) (2004)” provide instrumentation able to measure the angle-resolved Mueller matrix by using a high numerical aperture microscope objective, in both cases, using the systems for the characterization of microstructures. Nowadays, there is no method capable of measuring the refractive index of samples with non-planar surfaces. In turn, none of the manuscripts previously stated allow performing confocal measurements of the angular-resolved Mueller matrix of samples.

DESCRIPTION OF THE INVENTION

One first focal point of the current invention describes a polarimetric microscope for the measure of the refractive index of multiple samples (including non-planar surfaces). The microscope, which is adapted to perform confocal measures, images the angular-resolved Mueller matrix of the studied sample (the wave plane angular spectrum of the sample, i.e., the Fourier plane), this allowing to determine, according to an optical model developed by the inventors, parameters of interest of studied materials, as refractive index in isotropic materials, ordinary and extraordinary refractive indices in uniaxial anisotropic media, optical axis orientation, etc.

On the one hand, the polarimetric microscope working in reflection of the present invention is used for the first time for measuring the refractive index of optical elements already integrated into devices (thus, they can not be measured with metrological systems working in transmission). Importantly, none of the above stated instrumentation working in reflection is applied to this goal. What is more, most of the optical elements of interest that are integrated in devices, presents non-planar surfaces (with certain curvature or roughness), which can be perfectly measured with our proposed microscopic based system and its associated optical method.

The polarimetric microscope working in reflection of the present invention is adapted to measure confocal images of the Fourier plane, allowing studying the plane wave angular spectrum of light reflected by the sample without non-desired contributions of other planes, and thus, to obtain the refractive index of the sample with high accuracy.

The confocal polarimetric microscope working in reflection for the measure of the refractive index comprises:

• a positioning system of a sample to be analysed;

• an imaging system configured to focus a polarized light beam into a spot over the surface of the studied sample which comprises a high numerical aperture microscope objective; • an illumination arm that comprises a laser source, a collimator, and a polarization state generator; and

• a detection arm that comprises a polarization state analyser, a camera and a confocal system; wherein the confocal system comprises:

• a pinhole;

• an additional microscope objective; and

• an imaging lens; being the pinhole, the additional microscope objective and the imaging lens placed between the camera and the polarization state analyser in the detection arm.

The developed confocal configuration allows performing the metrology of a very specific plane, in particular, the focal plane of the high numerical aperture objective, where the surface of the sample to be studied is placed. Being able to measure the plane wave spectrum of light reflected by the sample allows measuring the angular-resolved Mueller matrix without non-desired contributions of other planes. From this Mueller matrix image, and by means of the optical method in the following described, the optical characteristics of the sample are fully determined.

The proposed optical instrumentation includes a high-precision sample positioning. This is achieved because it comprises spatial positioners, preferably comprising transversal and axial platforms, and/or goniometers and/or piezoelectrics configured to move the set-up elements and the sample in the order of nanometers. In addition, the set-up includes a high-precision optical system for the spatial determination of the high numerical aperture objective focal plane, which comprises a light source, cylindrical and spherical lenses, chromatic filters and a four-quadrant photodetector. This system minimizes mechanical vibrations and allows keeping still the sample surface in the focal plane of the high numerical aperture microscope objective, being crucial for the proper confocal system performance.

The invention is also refereed to an optical method for the measure of the refractive index of samples by using the polarimetric microscope above described, where the method comprises: a stage for the positioning of the studied sample; • a stage for the focussing of a polarized light beam in a spot over the analysed sample;

• an illumination stage in which different polarization states are generated;

• a detection stage in which the polarization states modified by the sample are analysed; and

• a stage to conduct confocal measurements.

In this way, only the focal plane of the high numerical aperture microscope objective, where the sample surface is placed, is measured. This allows eliminating non-desired contributions coming from different spatial planes of the sample, leading to an accurate measure of the angle-resolved Mueller matrix.

Optionally, the optical method also comprises:

• a stage for the polarimetric image recording of the Fourier plane of the high numerical aperture microscope objective.

Optionally, the optical method also comprises:

• a stage for the numerical processing of intensity images obtained in the previous stage, for the determination of the angle-resolved Mueller matrix corresponding to a high range of angles incident to the sample.

Optionally, the optical method also comprises:

• a stage for the filtering of the experimentally measured Mueller matrices through polarimetric methods.

Optionally, the optical method also comprises:

• a stage for the determination of the characteristic optical parameters of the sample, among which are the refractive index of isotropic samples, ordinary and extraordinary refractive indices of uniaxial anisotropic materials, and the optical axis orientation, through comparison between experimental data and a theoretical model developed by the inventors.

Optionally, the optical method also comprises:

• a stage of high-precision metrology for the calibration of the focal plane of the high numerical aperture microscope objective. BRIEF DESCRIPTION OF THE DRAWINGS

The previous and other advantages and features will be more fully understood from the following detailed description of embodiments, according to a preferred example of practical realization thereof, with reference to the attached drawings, which must be considered in an illustrative and non-limiting manner, in which:

Figure 1.- Shows an sketch of the polarimetric microscope objective working in reflection of the present invention, for the measurement of the refractive index of liquid, solid, isotropic, anisotropic, planar or non-planar materials, where the arrows indicate the direction of light propagation.

DETAILED DESCRIPTION OF THE INVENTION

In de following, and in a detailed manner, the conoscopic polarimetric microscope working in reflection for the measurement of the refractive index of samples and the associated method of the present invention, are described.

The conoscopic polarimetric microscope working in reflection for the measurement of refraction indices comprises:

• a positioning sample holder (SH) to place the sample (S) to be analysed in the focal plane of a high numerical aperture objective (HNAO);

• an imaging system configured to focus a polarized light beam into an spot over the sample (S) surface, which comprises the high numerical aperture objective (HNAO);

• an illumination arm that comprises a laser source (LASER), a collimator (CO), and a polarization state generator (PSG); and

• a detection arm that comprises a polarization state analyzer (PSA), a camera (CA) and a confocal system; wherein the confocal system comprises:

• a pinhole (PH);

• an additional objective (O), this being a microscope objective preferably;

• an imaging convergent lens (L); being the pinhole (PH), the additional objective (O) and the imaging convergent lens (L) placed between the polarization state analyser (PSA) and the camera (CA) in the detection arm.

The polarization state generator (PSG) in the illumination arm comprises two parallel aligned liquid crystal displays (LCD) and a linear polarizer (LP).

The polarization state analyzer (PSA) in the detection arm comprises two parallel aligned liquid crystal displays (LCD) and a linear polarizer (LP).

In addition, the polarimetric microscope comprises as well a non-polarizing beam splitter (NP-BS) to steer the incident beam coming from the laser source (LASER) to the sample and the reflected light to the detection arm.

An experimental prototype of the polarimetric microscope has been implemented. Together with the optical instrumentation, an optical method has been also developed, in order to measure with high accuracy the refractive index of optical elements presenting different surfaces, preferably non-planar surfaces, either in the case of isotropic and anisotropic materials.

This method is very useful to perform “in situ” measures (e.g., when transmission measurements can not be applied, as it is the case of elements already integrated into optical systems of assembled devices, as lenses into cameras).

Importantly, the polarization microscope comprises a high numerical aperture objective (HNAO) that focus light to a spot over the surface of the sample (S) to be analysed. This allows illuminating the sample with a high range of incident angles, which permits to instantaneously obtain redundant information of the sample (S) to be analysed. In fact, the reflected set of incident angles are of great interest because the incident polarization is transformed in different ways depending on the incident angle (the light-matter interactions depends on this parameter). In this way, with a single image recorded by using the polarizing microscope of the current invention, an instantaneous high data redundancy is obtained. What is more, by recording a set of images related to different incident polarizations, the corresponding angle-resolved images allow fitting the sample (S) physical parameters of interest (as can be the refractive index of an isotropic material, the ordinary and extraordinary refractive indices of uniaxial anisotropic materials or the optical axis orientation) to a model developed by the authors (which is based on the Mueller-Stokes formalism and the calculation of the Fresnel coefficients in reflection). One major advantage of the developed polarization microscope is that it does not require mechanical movements, and so, it is not affected by systematic errors related to mechanical arms.