The rise of optical biopsy as an alternative to classical biopsy is dictated by ongoing technological progress: any type of measurements has to be fast, precise, non-invasive and implemented in-vivo. The use of polarized light for optical biopsy has a long history. As Mueller-Stokes formalism provides the most complete description of polarized light interaction with any type of sample (even depolarizing one) we explored the capabilities of in-house multi-wavelength Mueller imaging polarimeter for the detection of pre-malignancy and malignancy. Our studies were performed with both scattering phantom tissues (in transmission configuration) and specimens of human colon and uterine cervix (in backscattering configuration).
For the interpretation of measurement results we decomposed Mueller matrix of a sample into product of elementary Mueller matrices of homogeneous diattenuator, retarder, and depolarizer. This phenomenological approach does not require the exact solution of Maxwell equations and provides the “effective” values of polarimetric properties of sample.
Exploring differential Mueller matrix formalism for fluctuating medium we showed that depolarization in homogeneous turbid medium varied parabolically with the pathlength of transmitted light, while the standard deviation of elementary polarization properties of medium depends linearly on the concentration of scatterers.
Neither scattering phantoms nor human tissue possessed any measurable diattenuation in backscattering configuration. The polarimetric images of tissue depolarization power, scalar birefringence and orientation of optical axis were compared with the analysis of histological slides. The spectral dependence of depolarization power and scalar birefringence values ascertained the potential of imaging Mueller polarimetry to discriminate healthy and diseased tissue zones.
The keystone to realize a monolithic integrated source on silicon with germanium is to optimize tensile strain and n-doping. In order to realize an integrated compact source, we demonstrate highly strained n-doped germanium microdisks obtained by two approaches using initially compressed silicon nitride (SiN) deposition. In the first approach, the microdisks are fabricated from relaxed Ge. In a second approach, we use tensile-strained Ge grown on a mismatched buffer layer, thus increasing the global strain in the Ge volume and lowering its gradient. A photoluminescence red-shift up to 450 nm is observed, corresponding to more than 1% biaxial strain.
Development of methodologies for quantification/unique interpretation of the intrinsic polarimetry characteristics of biological tissues are important for various applications involving tissue characterization/diagnosis. A detailed comparative evaluation of the polar decomposition and the differential matrix decomposition of Mueller matrices for extraction/quantification of the intrinsic polarimetry characteristics (with special emphasis on linear retardance δ , optical rotation Ψ and depolarization Δ parameters was performed, because these are the most prominent tissue polarimetry effects) from complex tissue-like turbid media exhibiting simultaneous scattering and polarization effects. The results suggest that for media exhibiting simultaneous linear retardance and optical rotation polarization events, the use of retarder polar decomposition with its associated analysis which assumes sequential occurrence of these effects, results in systematic underestimation of δ and overestimation of Ψ parameters. Analytical relationships between the polarization parameters (δ , Ψ ) extracted from both the retarder polar decomposition and the differential matrix decomposition for either simultaneous or sequential occurrence of the linear retardance and optical rotation effects were derived. The self-consistency of both decompositions is validated on experimental Mueller matrices recorded from tissue-simulating phantoms (whose polarization properties are controlled, known a-priori, and exhibited simultaneously) of increasing biological complexity. Additional theoretical validation tests were performed on Monte Carlo-generated Mueller matrices from analogous turbid media exhibiting simultaneous depolarization (Δ ), linear retardance (δ ) and optical rotation (Ψ ) effects.
Light depolarization occurs whenever different polarization responses add up incoherently, as it may be the case with inhomogeneous samples. The most convenient technique to characterize such samples is Mueller matrix polarimetry, as it is the only one providing all the relevant information in presence of depolarization. We studied the case of small grating boxes surrounded by bare silicon, in conditions where both the gratings and the substrate were illuminated by the Mueller polarimeter beam. The grating optical response is modeled by using rigorous coupled-waves analysis, and added incoherently to that of the substrate by merely summing the corresponding Mueller matrices. The line width and the depth of the grating as well as the percentage of substrate in irradiated spot area were obtained by fitting the experimental data taken with controlled displacement of the light spot in the boundary region between grating and substrate. Accurate grating parameters could be obtained with the fraction of the spot area within the grating box was larger than 30%. Moreover, these parameters remained relatively constant when this fraction was further decreased to 5%.
Modeling of optical properties of nanogratings (sub-wavelength gratings) is of scientific and technological interests
for (i) application of nanogratings as new artificial effective materials with unusual optical properties and
(ii) application in non-destructive optical testing of nanogratings using optical spectroscopic ellipsometry and
polarimetry. This paper deals with anisotropic lamellar nanogratings described by Effective Medium Approximation
(EMA). Analytical formulae for effective medium optical parameters of nanogratings from arbitrary
anisotropic materials are derived using approximation of zero-order diffraction mode. The method is based on
Rigorous Coupled Wave Analysis (RCWA) combined with proper Fourier factorization method. Good agreement
between EMA and the rigorous model is observed, where slight differences are explained by the influence
of higher Fourier harmonics in the nanograting. Analytical spectral formulae for ordinary and extraordinary
effective optical functions are derived for nanogratings consisting of material described by Sellmeier, damped
harmonic oscillator, and Drude formulae. Spectral origin for birefringence of dielectric nanogratings and linear
dichroism for absorbing ones is discussed.
A new null ellipsometer has been recently proposed that uses photoelastic modulator (PEM). The phase modulation adds a good signal-to-noise ratio, high sensitivity, and linearity near null positions to the traditional high-precision nulling system. The ellipsometric angles Delta and psi are obtained by azimuth measurement of the analyzer and the polarizer--PEM system, for which the first and second harmonics of modulator frequency cross the zeros. In this paper we discuss influence of component imperfection on precision of null measurement. Particular interest is devoted to azimuth angle error of compensator and modulator. Effect of residual birefringence of PEM is discussed. We show that the null system is insensitive to ellipsometer misadjustment and component imperfections and modulator calibration is not needed.
An analytic computer-oriented model for the Er / Yb co-doped silicate fiber amplifier is presented. The model is based on iterative solving the rate and propagation equations in their recursive forms at uniform discrete points along the fiber. It is capable of handling double-clad multimode-pumped fiber configurations, as well as conventional single-mode-pumped ones (with the signal featuring single-mode propagation in both cases). Arbitrary number of signals and pumps including wavelength-multiplexed ones from any direction can be treated. Amplification-deteriorating physical effects characteristic of the Er / Yb co-doped system, such as Yb amplified spontaneous emission, Er direct pumping, Er ions up-conversion and pairs formation, pump excited-state absorption and Er - Yb back-transfer, have been accounted for, in addition to the direct Yb - Er energy transfer. The relative importance of these effects in the amplification process has been modeled and discussed on the basis of experimentally determined fiber physical parameters.
A new optical instrument allowing photoellipsometric measurements is presented. Photoellipsometry (PE) is a modulation spectroscopy technique which uses ellipsometry in presence of a chopped external light excitation. PE measurements are obtained using a double modulation system, combining spectroscopic phase-modulated ellipsometry (SPME) with a laser pump beam. The experimental system described here takes advantage of the high frequency polarization of SPME (approximately equals 50 kHz). As a consequence the frequency of the pump beam can be varied up to 5 kHz. The field-induced changes in the real and imaginary parts of the bulk dielectric function can be directly measured and analyzed in terms of the pump beam power or the probe beam photon energy. Demonstration of this method is made with measurements, recorded in the band-gap E0 region (approximately equals 1.4 eV), on n-type GaAs sample. In particular, Franz-Keldish oscillations are observed with a very good sensitivity. More generally, PE measurements are compared with a theoretical model. From this preliminary study, it can be concluded that PE appears as a promising technique for semiconductor characterization.
A new infrared phase modulated ellipsometer (IRPME) is presented here. The polarization phase modulation technique takes advantage of the high frequency modulation (37 kHz) provided by a ZnSe photoelastic modulator. In order to increase the signal to noise ratio, the conventional globar source was superseded by a cascade arc, which emitted intensity corresponds to that of a blackbody at a temperature > 10,000 K. Ellipsometric measurements can be recorded from 700 up to 4000 cm-1 combining photovoltaic InSb and MCT detectors. A monochromator is used to record spectra, with a 2 - 5 cm-1 spectral resolution, depending on the wavelength domain. The signal acquisition and data processing is based on the use of a numerical electronic system. The improvements of both optical and electronic parts of the ellipsometer result in increased performances by more than one order of magnitude. The precision on (Psi) and (Delta) is now approximately equals 0.01 deg., achieving a submonolayer sensitivity.