Harsh Purwar

Quantitative fluorescence and elastic scattering tissue polarimetry using an Eigenvalue calibrated spectroscopic Mueller matrix system

A novel spectroscopic Mueller matrix system has been developed and explored for both fluorescence and elastic scattering polarimetric measurements from biological tissues. The 4 × 4 Mueller matrix measurement strategy is based on sixteen spectrally resolved (λ = 400 - 800 nm) measurements performed by sequentially generating and analyzing four elliptical polarization states. Eigenvalue calibration of the system ensured high accuracy of Mueller matrix measurement over a broad wavelength range, either for forward or backscattering geometry. The system was explored for quantitative fluorescence and elastic scattering spectroscopic polarimetric studies on normal and precancerous tissue sections from human uterine cervix. The fluorescence spectroscopic Mueller matrices yielded an interesting diattenuation parameter, exhibiting differences between normal and precancerous tissues.

Comparative study of differential matrix and extended polar decomposition formalisms for polarimetric characterization of complex tissue-like turbid media

Abstract. 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. After successful evaluation, the potential advantage of the differential matrix decomposition over the polar decomposition formalism was explored for monitoring of myocardial tissue regeneration following stem cell therapy.

Differing self-similarity in light scattering spectra: a potential tool for pre-cancer detection

The fluctuations in the elastic light scattering spectra of normal and dysplastic human cervical tissues analyzed through wavelet transform based techniques reveal clear signatures of self-similar behavior in the spectral fluctuations. The values of the scaling exponent observed for these tissues indicate the differences in the self-similarity for dysplastic tissues and their normal counterparts. The strong dependence of the elastic light scattering on the size distribution of the scatterers manifests in the angular variation of the scaling exponent. Interestingly, the spectral fluctuations in both these tissues showed multi-fractality (non-stationarity in fluctuations), the degree of multi-fractality being marginally higher in the case of dysplastic tissues. These findings using the multi-resolution analysis capability of the discrete wavelet transform can contribute to the recent surge in the exploration for non-invasive optical tools for pre-cancer detection.

Elementary and collective excitations as probes for order parameter symmetry in Fe-based superconductors

We show that the superconducting quasiparticle energy of Fe-based superconductors would be different for a sign-changing order parameter than that for a non–sign-changing order parameter. This particularly happens in the low-doping region when the electron and hole Fermi surfaces nest and superconductivity coexists with either magnetic or orbital or both density waves. This difference in the nature of superconducting quasiparticles is also detectable from collective mode excitations. We further show that while the magnetic spin density wave order competes with superconductivity, the orbital density wave grows when the superconductivity order parameter changes sign accross the electron-hole Fermi surface. However, when the superconducting order parameter does not change sign both the density waves compete with superconductivity. Irrespective of the nature of the superconducting order parameter the spin and orbital density waves are concurrent indicating simultaneous magnetic and structural transition. The significance of the various results is discussed comparing the various experimental observations.

Probing the fractal pattern and organization of Bacillus thuringiensis bacteria colonies growing under different conditions using quantitative spectral light scattering polarimetry

Abstract. Development of methods for quantification of cellular association and patterns in growing bacterial colony is of considerable current interest, not only to help understand multicellular behavior of a bacterial species but also to facilitate detection and identification of a bacterial species in a given space and under a given set of condition(s). We have explored quantitative spectral light scattering polarimetry for probing the morphological and structural changes taking place during colony formations of growing Bacillus thuringiensis bacteria under different conditions (in normal nutrient agar representing favorable growth environment, in the presence of 1% glucose as an additional nutrient, and 3 mM sodium arsenate as toxic material). The method is based on the measurement of spectral 3×3 Mueller matrices (which involves linear polarization measurements alone) and its subsequent analysis via polar decomposition to extract the intrinsic polarization parameters. Moreover, the fractal micro-optical parameter, namely, the Hurst exponent H, is determined via fractal-Born approximation-based inverse analysis of the polarization-preserving component of the light scattering spectra. Interesting differences are noted in the derived values for the H parameter and the intrinsic polarization parameters (linear diattenuation d, linear retardance δ, and linear depolarization Δ coefficients) of the growing bacterial colonies under different conditions. The bacterial colony growing in presence of 1% glucose exhibit the strongest fractality (lowest value of H), whereas that growing in presence of 3 mM sodium arsenate showed the weakest fractality. Moreover, the values for δ and d parameters are found to be considerably higher for the colony growing in presence of glucose, indicating more structured growth pattern. These findings are corroborated further with optical microscopic studies conducted on the same samples.

Development and eigenvalue calibration of an automated spectral Mueller matrix system for biomedical polarimetry

We present a novel spectral Mueller matrix measurement system for both elastic and inelastic scattering (fluorescence) polarimetric measurements. The system comprises of a Xenon lamp as excitation source, a polarization state generator (PSG) and a polarization state analyzer (PSA) unit to generate and analyze polarization states required for 4 x 4 sample Mueller matrix measurements, coupled to a spectrometer for spectrally resolved (λ ~ 400 - 800 nm) signal detection. The PSG unit comprises of a fixed linear polarizer (polarization axis oriented at horizontal position) followed by a rotatable broadband quarter wave plate. The sample-scattered light is collected and collimated using an assembly of lenses, then passes through the PSA unit, and is finally recorded using the spectrometer. The PSA unit essentially consists of a similar arrangement as that of the PSG, but positioned in reverse order, and with the axis of the linear polarizer oriented at vertical position. A sequence of sixteen measurements are performed by changing the orientation of the fast axis of the quarter wave plates of the PSG unit (for generating the four required elliptical polarization states) and that of the PSA unit (for analyzing the corresponding polarization states). The orientation angles (35°, 70°, 105° and 140°) were chosen based on optimization of the PSG and PSA matrices to yield most stable system Mueller matrices. The performance of the polarimeter was calibrated using Eigenvalue calibration method which also yielded the actual values of the system PSG and PSA matrices at each wavelength. The system has been automated and is capable of Mueller matrix measurement with high accuracy over the entire spectral range 400 - 800 nm (elemental error < 0.01). For recording the elastic scattering Mueller matrix of sample, the PSG and PSA matrices for each wavelength are used, while for fluorescence Mueller matrix measurements, the PSG for the excitation wavelength (chosen to be 405 nm) and PSA for varying emission wavelengths (450 - 800 nm) are used. The developed spectral Mueller matrix system has been initially used to record both elastic scattering and fluorescence Mueller matrices from normal and cancerous cervical tissues.

Specific Heat and Characteristic Ratio of Some Fe-Based Superconductors: A Model Study

Within two band model of Fe-based superconductors we calculate specific heat and characteristic ratio. Within combined inter-intra band pairing for sign changing order parameters we show that the ΔC/Tc is proportional Tc2 in contrast to ΔC/Tc nearly proportional to Tc for other classes of superconductors. This agrees very well with experimental observations. We show within the model calculation that Fe-based superconductors have characteristic ratios for the small and large gap as around 4 and 12 respectively consistent with experimental observations of 2.5±1.5 and 7±2.At the request of all authors of the paper, and with the agreement of the Proceedings Editor, an updated version of this article was published on 3 September 2014. The original article supplied to AIP Publishing was not the final version and contained an error in Figure 4. The error has been corrected in the updated and re-published article.

Orbital density wave, spin density wave and superconductivity in Fe-based materials

Using the two-band model, we study the interplay among orbital density wave, spin density wave and superconductivity in Fe based materials. The susceptibilities corresponding to orbital modulation and inter orbital spin density show enhanced peak at around the nesting wave vector indicating the possibility of respective density waves. It is shown that both the orbital and spin density waves are concurrent and irrespective of their zero temperature magnitudes, they have the same transition temperature. Superconductivity is studied in three scenarios – intra, inter and a mixed inter-intra orbital pairing scenarios. Inter orbital pairing is found to be dominant whereas mixed inter-intra orbital pairing scenario provides higher Tc. The S±-wave superconductivity is found to be most stable within the model.