Research Highlight and Overview
Our research activity is mainly in the area of experimental optics and spectroscopy with special emphasis on nano-optics (investigation of light-matter interactions at nano or mesoscopic scale) and biophotonics (biomedical optics, spectroscopy and imaging) .A brief account of the research activity of the group (during 2010-2014) is provided below.
A. Plasmonics and Nano Optics:1. Quantitative polarimetry in nano plasmonics:
'Plasmonics' dealing with localized/ surface plasmon resonances in metal nanoparticles (nanostructures) / planar metal-dielectric interfaces is a rapidly developing field and is under recent intensive investigations owing to fundamental interests and numerous potential applications. Various optical effects of such plasmonic nanostructures have been studied, ranging from extinction, absorption, scattering properties to non-linear properties and so forth. Despite this, there has only been limited literature available dealing with polarization characteristics of such plasmonic systems. Since the polarization properties of light play an important role in the interaction of light with such plasmonic systems, knowledge on the polarimetry parameters and their spectral characteristics are crucial for both fundamental understanding of the interactions and for optimizing experimental parameters for many practical applications. We have thus carried out pioneering studies on quantitative polarimetry of plasmon resonant metal nanoparticles / nano-structures. Our studies have revealed intriguing spectral diattenuation d (differential attenuation of orthogonal polarization states) and retardance d (phase shift between orthogonal polarization states) effects in non-spherical (rods and spheroids) metal nanoparticles. These were attributed to the inherent differences in amplitudes and phases respectively of the two competing orthogonal (transverse along the short axis and longitudinal along the long axis) dipolar plasmon polarizabilities of the metal nano rods (or spheroids) . These studies also indicated that such enhanced diattenuation and retardance effects (having distinct spectral characteristics) of the plasmonic (non-spherical) nanoparticles can be exploited to develop polarization-controlled novel schemes for contrast enhancement in nanoparticle-based imaging and for optimizing/enhancing sensitivity of plasmonic sensors2. Spin-Orbit interaction of light in plasmonic nanostructures:
Figure1: Spin orbit interaction (SOI) of light in plasmonic nano-structures. Left panel: Variations of the polarimetry parameters, diattenuation d (left axis, black lines) and retardance d (right axis, red lines) with wavelength ? for silver nanorods having equivalent sphere radius r = 20 nm and for two aspect ratio values e = 0.65 (corresponding d and d: solid lines) and 0.95 ( d and d: dashed lines). The three different regimes of pure SOI effects are marked as Region 1 (red), 2 (blue) and 3 (black) respectively. Right panel: The butterfly-like azimuthal polarization patterns (in the scattering Mueller matrix elements) from plasmonic nanorods are characteristic signature of SOI and subsequent generation of topological phase. These results demonstrated that various interesting regimes of SOI can be realized by a clever manipulation of diattenuation d and retardance d polarimetry parameters, by selecting suitable wavelength, size, shape and orientation of nanoparticles (adopted from J. Soni et al, Optics Letters, 38, 1748 -1750 (2013)).
Our current research is directed towards basic studies on Spin Orbit Interaction (SOI) of light in micro and nano systems. The SOI phenomenon signifies inter-conversion between the spin angular momentum (SAM, circular polarization) and orbital angular momentum (OAM, helical phase) of light, which is of considerable current interest due to both fundamental interests and potential nano-optical applications [4, 9, 10]. We have recently developed a generalized theoretical treatment based on Jones and Mueller matrix algebra for analysis, interpretation and quantification of SOI via experimentally measurable polarimetry characteristics. Importantly, we have demonstrated that SOI can be significantly enhanced/optimized in plasmonic metal nanoparticles/nanostructures (e.g., nanorods and nanospheres) by exploiting the interference of neighbouring modes (orthogonal electric dipolar modes in rods or electric dipolar and quadrupolar modes in spheres) . In fact, it was shown that various interesting regimes of SOI can be realized by a clever manipulation of diattenuation d and retardance d polarimetry parameters, by selecting suitable wavelength, size, shape and orientation of nanoparticles. Illustrative example of realizing three pure cases of SOI in plasmonic nano-rod is shown in Figure 1. Characteristic angular polarization pattern as signatures of SOI and subsequent generation of topological phase is also shown in the right panel .3. Giant Goos-Hanchen shift in scattering from plasmonic structures: role of the interfering localized plasmon modes
The longitudinal and the transverse beam shifts, namely, the Goos-Hanchen (GH) and the Imbert-Federov (IF) shifts (a variant of this is also known as Spin-Hall shift) are usually observed at planar interfaces. We have recently shown that both the longitudinal GH shift and the transverse SH shift may also arise due to scattering of plane waves from micro / nano systems . We have studied both the GH and the SH shifts in plasmonic metal nanoparticles/ nanostructures and dielectric micro-particles employing a unified framework that utilizes the transverse components of the Poynting vector of the scattered wave.
Figure2: Giant Goos-Hanchen (GH) shift in plasmon resonant Ag nanosphere: role of the interfering plasmon modes. Left panel: GH shift DGH for Ag nanosphere (a=50 nm), at two wavelengths, l=380 nm (red solid line), 430 nm (black dotted line). The shifts are shown for incident X-polarized light (|H> state) in the scattering plane f = 0. The inset shows the corresponding shifts for incident Y-polarized light (|V> state). Right panel: The wavelength dependence of the strength of the interference of the dipolar a1 and the quadrupolar a2 plasmon modes ( ) (blue dotted line, left axis) and the corresponding phase difference (in radian) between them (red solid line, right axis). The |H> polarized light exhibits a giant shift at 380 nm, where the strength of interference between the two plasmon modes (dipolar and quadrupolar) is maximal. (adopted from J. Soni et al, Optics Letters, 39, 4100-4103 (2014) ).
The results demonstrated that interference of neighboring resonance modes in plasmonic nanostructures (e.g., electric dipolar and quadrupolar modes in metal spheres) leads to giant enhancement of GH shift in scattering from such systems (shown in Figure 2). Note that these shifts are usually tiny (in sub-wavelength domain) and are extremely sensitive to the local dielectric environment. Giant enhancement of these shifts in plasmonic structures may lead to development of novel plasmonic sensors, nano displacement probes based on these schemes, and such efforts are currently underway in our laboratory.4. A novel dark field Mueller matrix spectroscopic microscopy system for studying coupled plasmons and plasmonic Fano resonances:
We have developed a novel dark field Mueller matrix spectroscopic microscopy system for conducting basic studies on plasmonic nanoparticles and nanostructures. A schematic of this experimental system is shown below (Figure 3). The system consists of three components; (a) conventional dark field microscopic imaging system (b) simultaneous single particle spectroscopy set-up and (c) combined spectral Mueller matrix (a 4×4 matrix representing the transfer function of any optical system in its interaction with polarized light containing complete information about all the polarimetry characteristics of the system) polarimetry measurement method . Briefly, the system is built around an inverted microscope operating in dark-field imaging mode. Part I containing the lasers (diode lasers with wavelength l = 405 nm, 473 nm shown here; other laser sources can also be coupled based on the requirement of the excitation wavelength) and the Xe-lamp as excitation sources and the polarization state generator (PSG) unit. Part II is the dark field microscopic arrangement coupled with the excitation light source. Part III comprises of the polarization state analyzer (PSA) unit and the spectroscopy (spectrograph / CCD) and imaging (liquid crystal tunable filter LCTF and EMCCD camera) devices.
Figure3: The developed dark field Mueller matrix spectroscopic microscopy system. (a) A schematic of the system built around an inverted microscope operating in dark-field imaging mode. PSG: Polarization state generator unit comprising of a fixed linear polarizer and two liquid crystal variable retarders (LVRs); PSA: Polarization state analyzer unit comprising of the same polarizing components but arranged in reverse order. The sample-scattered light is collected by an objective and passed through the PSA unit and a liquid crystal tunable filter (for wavelength selection) and the multispectral images is acquired using an EMCCD camera. The system is capable of performing near simultaneous spectroscopy, imaging and full 4×4 Mueller matrix polarimetry from single isolated nanostructure. (b) Image of a single gold nanorods in the dark field microscope. The corresponding SEM image is shown on top right corner. (c) The wavelength variation of Mueller matrix-derived diattenuation d and retardance d polarimetry parameters from a single isolated gold nanorod. (adopted from J. Soni et al, Optics Express, 21, 15475 – 15489 (2013)  and “Quantitative spectral Mueller polarimetry of single isolated nanostructure” (manuscript under preparation)).
Collimated output of a Xe-lamp excitation source (and lasers operating at different wavelengths) are coupled through the bottom port of the microscope. The sample-scattered light is collected by an objective and passed through a LCTF (for wavelength selection, resolution ~ 7.5 nm) and the multispectral images (simultaneous spectroscopy and imaging) is acquired using an EMCCD camera. The spectra of the scattered light are also simultaneously recorded at higher spectral resolution (~ 1.5 nm) using another spectrograph / CCD system. The PSG unit consist of a horizontally oriented fixed linear polarizer and a two liquid crystal variable retarders (LVRs). Analogously, the PSA unit consists of the same system arranged in the opposite order with analyzer oriented vertically. The strategy for recording spectral Mueller matrices of sample scattered light is based on recording sixteen set of spectra for four different combinations of the optimized retardation (birefringence) levels of the LVRs in the PSG and the PSA unit (generating four optimized elliptical polarization states and analyzing them via four optimized analyzer basis states). The system has been Eigenvalue calibrated to correct for the wavelength response of the PSG and the PSA unit ensuring high accuracy of Mueller matrix measurements in the entire wavelength range of 400-800 nm . Full electronic control of the retardance levels of the two LVRs and the unique combination of the LVRs, LCTF and the EMCCD (and spectrograph / CCD system) allows one to record spectral Mueller matrices (l = 400 – 800 nm) and images from single isolated nanostructures, without involving any movable optics. Motivated by our initial promising results on spectroscopic polarimetry of non-spherical (rods and spheroids) plasmonic nanoparticles, we are currently expanding our investigations further on different complex plasmonic structures (nano particle arrays, clusters etc.). Specifically, the developed system is being used to conduct studies on some of the intriguing aspects of nano-plasmonics, like coupled plasmons, plasmonic fano resonances, spin orbit interaction of light in complex metallic, metal-dielectric nano structures. Finally, information obtained from such basic studies would be explored for developing ultra-high sensitive plasmonic sensors and for optical nano-probing applications. The system may also be explored in other diversified interdisciplinary research, including, spectroscopic characterization of nano-materials, ultra-sensitive bio-sensing, single molecule detection, biomedical imaging etc.
5. Weak measurements and other novel methods for amplifying the sub-wavelength optical effects:
The weak measurement of the beam shifts is an optical analogue of the well known quantum mechanical weak measurements. When a system is subjected to a weak perturbation, a hugely amplified signature of that can be found through cleverly manipulating the initial state and measurement process. We have recently initiated such novel weak measurement schemes in context to amplification of the tiny beam shifts, namely, the Goos-Hänchen (GH), the transverse Imbert-Federov (IF) and the Spin Hall (SH) shifts mediated by a number of processes involving light-matter interactions. In the optical weak measurement schemes, the optical beam shifts (GH, IF and SH) act as weak measuring effects and the state of polarization serves as pre and post selection mechanisms. Indeed, we have recently demonstrated simultaneous weak value amplification of both the angular GH and the IF shifts in partial reflection of fundamental Gaussian beam at planar dielectric interfaces . We have employed pre- and post selection schemes with appropriate linear polarization basis states for simultaneous weak measurements and amplification of both these shifts (shown below in Figure 4a). The experimentally observed enhancement of the beam shifts and their dependence on the angle of incidence were analyzed / interpreted via quantum mechanical treatment of weak measurements. Our current studies are also directed towards extending the weak measurement schemes beyond the polarization of light by implementing weak measurement on Orbital Angular Momentum (OAM) state of light. We are also expanding our investigations towards enhancing the sub-wavelength optical effects in plasmonic and other micro/nano systems by employing appropriate weak measurement schemes. This may open-up a whole new range of possibilities of studying light matter interactions at the sub-wavelength scale and explore potential nano optical applications.
Figure4(a): Simultaneous weak value amplification of the angular GH and the IF shifts in partial reflection. Diagonal shift in beam’s centroid between the two post selected states +e (left panel) and -e (right panel) away from the orthogonal state for the input state ?|yinñ=[1,1]?^T(45o linearly polarization state) is apparent. The observed transverse shift in beam’s centroid for input states (b)?|yinñ=[1,0]?^T(input p- polarization state) and (c)?|yinñ=[0,1]?^T (input s- polarization state). (adopted from S. Goswami et al, Optics Letters (2014) (in press) . Simultaneous weak value amplification of the angular GH and the IF shifts in partial reflection. Diagonal shift in beam’s centroid between the two post selected states +e (left panel) and -e (right panel) away from the orthogonal state for the input state ?|yinñ=[1,1]?^T(45o linearly polarization state) is apparent. The observed transverse shift in beam’s centroid for input states (b)?|yinñ=[1,0]?^T(input p- polarization state) and (c)?|yinñ=[0,1]?^T (input s- polarization state). (adopted from S. Goswami et al, Optics Letters (2014) (in press) .]).
Figure4(b): Giant Spin Hall (SH) effect of light in an exotic optical system, an inhomogeneous anisotropic medium (retarder) having complex spatially varying birefringent structure. Top panel: The inhomogeneous retarder is realized by modulating the individual pixels of a twisted nematic liquid crystal spatial light modulator through user defined birefringence map. Giant SH shift is manifested as a large shift in the centroid of the transmitted beam (intensity distribution) in the first element of the measured Stokes vector (I). Bottom panel: The opposite direction of transversal energy flow (determined by the transverse component of the Poynting vector S^) for input left (LCP) and right circular polarization (RCP) states was identified as the origin of the observed giant SH shift in the inhomogeneous anisotropic medium. (adopted from A. Bag et al, Proc. of SPIE, Vol. 9126, 91262E, Nanophotonics V (2014) and “Giant Spin Hall effect of light in an exotic optical system” (manuscript under preparation)).
We have extended our studies on SOI and the resulting beam shifts to other interesting optical systems – for tightly focused fundamental Gaussian beam propagating through stratified medium and for propagation of light through inhomogeneous anisotropic medium consisting of twisted nematic liquid crystal layers. Each of these systems showed interesting manifestation of SOI effects [4, 10]. SOI phenomenon is associated with two distinct effects – (a) evolution of geometric (topological) phase due to the effect of trajectory of light on polarization and (b) spin hall effect of light (SHEL) arising due to the effect of polarization on the trajectory itself. We have observed giant enhancement of SH shift even for normal incidence in an exotic optical system, an inhomogeneous anisotropic medium having complex spatially varying birefringent structure. In this experiment, the individual pixels of a twisted nematic liquid crystal spatial light modulator (TNSLM) were electronically addressed to form complex spatially varying birefringence effect. The polarization dependent spatial variation of the transmitted light beam through such inhomogeneous anisotropic medium was recorded using an Eigenvalue calibrated Stokes-Mueller imaging system. Giant SH shift was manifested as distinctly different spatial distribution of the recorded output Stokes vector elements for two orthogonal (left and right) input circular polarization states. Rigorous three dimensional analysis of polarization evolution revealed that the generation of input circular polarization-dependent large magnitude of transverse energy flow originating from SOI in the inhomogeneous birefringent medium lead to the observation of such a large spin dependent deflection of the trajectory of light (shown in Figure 4b).
B. Biophotonics:1. Tissue multifractality in Born approximation of light scattering: A novel approach for pre-cancer detection:
Multifractal, a special class of complex self-affine processes, are under recent intensive investigations because of their fundamental nature and potential applications in diverse physical systems. Such complex self-affine processes are also ubiquitous in nature, and are observed for example in physiological time series of heartbeat, turbulence, Sun’s magnetic field dynamics, stock market fluctuations and so forth. We have recently demonstrated that the spatial distribution of refractive index (RI) of biological tissues exhibit multifractality, indicative of its morphological and ultra-structural tissue content . The multifractal parameters, namely, the generalized Hurst exponent and the width of the singularity spectrum, of tissue RI fluctuations were successfully quantified by employing multi-resolution analysis on differential interference contrast (DIC) images of human cervical tissue. In order to explore possibility of in-situ extraction/quantification of multifractality in the spatial distribution of refractive index of tissues, we have also subsequently developed a novel light scattering based inverse method. The method is based on Fourier domain pre-processing of the light scattering signal (either wavelength or angular variation of scattering) via the Born approximation, followed by the Multifractal Detrended Fluctuation Analysis . The approach was experimentally validated in synthetic multifractal scattering phantoms, and tested on ex-vivo tissues. The derived multifractal properties appeared sensitive in detecting cervical precancerous alterations through an increase of multifractality with pathology progression, demonstrating the potential of the developed methodology for novel precancer biomarker identification and tissue diagnostic tool  (shown in Figure 5). The novel ability to delineate the multifractal optical properties from light scattering signals may also prove useful for characterizing a wide variety of complex scattering media of non-biological origin.
Figure5: Tissue multifractality in Born approximation of light scattering: A novel approach for pre-cancer detection. Top panel: Evidence of multifractality in the spatial variation of tissue refractive index (RI). (a) DIC image of typical Grade I precancerous human cervix. (b) Fourier power spectrum of the generated 1D RI fluctuation series (shown in natural logarithm scale). Fitting at two selected spatial frequency n-ranges (lower (green) and higher (red), respectively) and overall fitting (black) yield different values for slope (manifestation of multifractality) (c) The light scattering signal [P(n)=I (n)?×k?^(-4)] recorded from the same tissue also yields multiple spectral slope exponents (fitting at two selected n ranges and overall fitting on the entire n range are shown). Bottom panel: Results of the inverse multifractal analysis performed on the light scattering spectra recorded from the grade-1 precancer human cervical tissue. (d) The variation of the derived index fluctuations function log Fq(s) vs logs for different moments q (=-4 to +4 shown here). Multifractality in index fluctuation is evident from significant variations in the slopes with varying q. Comparison of the variation of derived (e) generalized Hurst exponent h (q) and (f) singularity spectrum f(a) for grade I and grade III precancerous tissues. Higher grades of precancers are associated with increased anti-correlations of index fluctuations (reduction in h(q=2) ) and stronger multifractality (increase in the value for Da). These results provide support that normal and precancerous tissues with different pathology grades can be discriminated using the derived multifractal parameters. (adopted from N. Das et al, Scientific Reports (Nature), 4, 6129 (2014)  and N. Das et al, Optics Letters, 38, 211-213 (2013) ).
2. A comprehensive turbid polarimetry platform for biophotonics and other interdisciplinary research:
Figure6: A comprehensive turbid polarimetry platform for probing biological and other complex systems. Schematic of the turbid polarimetry platform. (a) Experimental system for the measurement of spectral (wavelength: 400 – 800 nm) Mueller matrices; (b) Polarization-sensitive forward Monte Carlo simulations for forward modelling of simultaneous polarization effects in the presence of turbidity, and experimental turbid phantoms to mimic simultaneous polarization and scattering effects; (c) Different variants of the Mueller matrix decomposition approaches to inverse calculate the constituent polarization contributions in complex turbid media. Illustrative example of Mueller matrix decomposition from complex experimental turbid phantom is shown; (d) Illustrative example of tissue polarimetry inverse analysis. The Mueller matrix decomposition -derived linear retardance dlog-M (in degrees), net depolarization Dlog-M (scaled ´ 100), the orientation angle of retardance qlog-M (in degrees) and the uncertainty of linear retardance Ddlog-M (in degrees) images from transmission Mueller matrix measurements in 1-mm-thick sections from rat myocardial tissue. Such quantitative tissue polarimetry measurements / analysis was successfully explored for monitoring stem cell therapy-based regenerative treatment of myocardial infarction. (adopted from S. Kumar et al, Journal of Biomedical Optics, 17, 105006 (2012)  and J. Soni et al, Optics Express, 21, 15475 – 15489 (2013) ).
Polarization properties of light scattered from any medium contain rich morphological and functional information which can be exploited for non-destructive evaluation of a variety of complex materials. In principle, all the sample polarization properties are encoded in the various elements of the 4×4 Mueller matrix recorded from it. However, in various naturally occurring complex turbid (random) media, many optical polarization effects occur simultaneously and contribute in a complex interrelated way to the Mueller matrix elements. Hence, these represent several ‘lumped’ effects, masking potentially interesting ones and hindering unique data interpretation. Moreover, numerous complexities due to multiple scattering also present formidable challenges, both in terms of accurate measurement and in terms of unique interpretation of the polarization parameters. Thus, despite the wealth of interesting parameters that can be probed with polarized light, quantitative polarimetry in complex random medium is severely compromised due to the aforementioned problems. The challenges in ‘turbid medium polarimetry’ are therefore to minimize or compensate for multiple scattering, and to decouple and quantify the individual contributions of simultaneously occurring polarization effects. In order to address some of these challenges, we have recently built-up a comprehensive polarimetry platform consisting of (a) high sensitive experimental polarimetry system , (b) polarization-sensitive forward Monte Carlo model for simulating simultaneous polarization effects in the presence of multiple scattering (in complex turbid medium) [13, 16, 21, 24], (c) polarimetry inverse analysis models for extraction, quantification and unique interpretation of the individual, intrinsic polarimetry characteristics of complex systems [13, 16, 21, 24]. A schematic of the ‘turbid polarimetry platform’ is shown in Figure 6. The developed experimental system is capable of recording full 4×4 Mueller matrix over the wavelength range of l = 400 – 800 nm. The experimental Mueller matrix system is complemented with novel polarimetry inverse analysis models. These inverse analysis models are mainly based on various forms of decompositions (polar decomposition, differential matrix decomposition etc. ) of Mueller matrices into the basis matrices of contributing polarization effects. The three fundamental contributing polarization effects are retardance (represented by a matrix MR), diattenuation (MD) and depolarization (MD). Once decomposed, the constituent matrices can further be analyzed to derive quantitative individual polarization medium properties, diattenuation d (linear and circular), retardance (linear d and circular y) and depolarization coefficient D. The developed Mueller matrix polarimetry platform was successfully used to test / validate the applicability of various such inverse analysis models. After successful evaluation, the comprehensive polarimetry platform was explored in biophotonics and in other interdisciplinary research. In biophotonics, this was successfully explored for monitoring stem cell therapy-based regenerative treatment of myocardial infarction in rat via the derived polarimetry parameters [13, 22]. Elastic scattering spectroscopic Mueller matrix measurements in combination with Born approximation – based fractal modelling of light scattering, was successfully employed for the characterization of fractal growth of bacterial colony under different environments . Similarly, the Mueller matrix-derived polarimetry parameters have been explored for quantitative monitoring of deswelling kinetics of polymer hydrogels .
3. Fluorescence Mueller matrix: A novel spectroscopic tool:
Conventionally, the Mueller matrix is defined for optical interactions like reflection/refraction, transmission and elastic scattering. We have recently made its important extension to measurement, analysis and interpretation of the complex polarimetry effects associated with the fluorescence emission process. Specifically, we have developed a novel experimental system for recording full 4×4 fluorescence spectroscopic Mueller matrix from any complex system [6,7]. This is complemented with inverse analysis strategy to extract/quantify all the contributing fluorescence polarimetry effects, namely, depolarization, fluorescence (linear and circular) dichroism and fluorescence (linear and circular) polarizance . These Mueller matrix-derived fluorescence polarimetry parameters have initially been explored for the characterization of complex systems such as biological tissues  and inhomogeneous, low quantum yield polymeric nanoparticle systems . Initial exploration has shown considerable promise of the fluorescence linear diattenuation (differential excitation of fluorescence by orthogonal linear polarization states) and fluorescence linear polarizance (preferential emission of fluorescence with orthogonal linear polarizations) parameters as novel biomarkers for pre-cancer detection . This is illustrated in Figure 7 below.
Figure7: Fluorescence Mueller matrix: A novel spectroscopic tool. (a) The 4 ×4 Fluorescence spectroscopic Mueller matrix (lem = 475 – 800 nm) recorded from P3HT polymeric nanoparticles with 405 nm excitation. Significant intensities of the off-diagonal Mueller matrix elements indicate presence of anisotropic ground state absorption and anisotropic excited state emission properties. These were subsequently quantified using Mueller matrix inverse analysis via fluorescence diattenuation and fluorescence Polarizance parameters. (b) The fluorescence linear diattenuation and fluorescence linear polarizance parameters as novel diagnostic metrics for pre-cancer detection. Considerable magnitudes of fluorescence linear diattenuation and polarizance in tissues were attributed to anisotropic absorption and emission from organized and cross-linked collagen molecular structure. Destruction of the collagen cross-links and the resulting randomization of the fibrous collagen structures with the progression of precancers lead to significant reduction in these parameters. (adopted from J. Soni et al, Applied Physics Letters, 104, 131902 (2014)  and S. Chandel et al, Optics Letters, 39, 243-246 (2014) ).