arXiv subjects

Nirmalya Ghosh

Publications and source records attributed to Nirmalya Ghosh.

At least 55 records · Page 3Linked to original sources

arXiv2020

Direct observation of the effects of spin dependent momentum of light in optical tweezers

We demonstrate that tight focusing of a circularly polarized Gaussian beam in optical tweezers leads to spin-momentum locking - with the transverse spin angular momentum density being independent of helicity, while the transverse momentum (Poynting vector) becomes helicity dependent. Our theoretical calculations, numerical simulations, and experiments reveal that the presence of a stratified medium in the path of the trapping beam significantly enhances the magnitude of transverse momentum in the radial direction with respect to the beam axis, and likewise, also leads to high off-axial intensity. This overlap allows us to experimentally observe the circular motion of a birefringent particle, trapped off-axis, in response to an input circularly polarized fundamental Gaussian beam carrying no intrinsic orbital angular momentum. The circular motion is dependent on the helicity of the input beam, so that we can identify it to be the signature of the elusive Belinfante spin in propagating light beams obtained in our optical tweezers setup. Our work can be extended to higher-order beams carrying intrinsic orbital angular momentum leading to simple routes of achieving complex particle manipulation using optical tweezers.

physics.optics↗

arXiv2019

Spin selective scattering modes in random anisotropy optical medium

Geometrical and dynamical phase have competing effects as far a scattering of light form inhomogeneous anisotropic optical medium is concerned. If fine-tuned appropriately, these effects can completely cancel each other for a chosen spin component while having an additive effect on the orthogonal components. Here, we show a manifestation of extraordinary spin selective modes in Fourier spectrum of Gaussian beam transmitted through anisotropic disordered medium. We realize the concept using a twisted nematic liquid crystal-based spatial light modulator (SLM) with random gray level distribution for incident Gaussian beam.

physics.optics↗

arXiv2019

Single-shot measurement of space varying polarization state of light through interferometric quantification of geometric phase

Light beam carrying spatially varying state of polarization generates space varying Pancharatnam-Berry geometric phase while propagating through homogeneous anisotropic medium. We show that determination of such space varying geometric phase provides a unique way to quantify the space varying polarization state of light using a single-shot interferometric measurement. We demonstrate this concept in a Mach-Zehnder interferometric arrangement using a linearly polarized reference light beam, where full information on the spatially varying polarization state is successfully recovered by quantifying the space varying geometric phase and the contrast of interference. The proposed method enables single-shot measurement of any space varying polarization state of light from the measured interference pattern with a polarized reference beam. This approach shows considerable potential for instantaneous mapping of complex space varying polarization of light in diverse applications, such as astronomy, biomedical imaging, nanophotonics, etc., where high precision and near real-time measurement of spatial polarization patterns are desirable.

physics.optics↗

arXiv2019

Adversarially Trained Convolutional Neural Networks for Semantic Segmentation of Ischaemic Stroke Lesion using Multisequence Magnetic Resonance Imaging

Ischaemic stroke is a medical condition caused by occlusion of blood supply to the brain tissue thus forming a lesion. A lesion is zoned into a core associated with irreversible necrosis typically located at the center of the lesion, while reversible hypoxic changes in the outer regions of the lesion are termed as the penumbra. Early estimation of core and penumbra in ischaemic stroke is crucial for timely intervention with thrombolytic therapy to reverse the damage and restore normalcy. Multisequence magnetic resonance imaging (MRI) is commonly employed for clinical diagnosis. However, a sequence singly has not been found to be sufficiently able to differentiate between core and penumbra, while a combination of sequences is required to determine the extent of the damage. The challenge, however, is that with an increase in the number of sequences, it cognitively taxes the clinician to discover symptomatic biomarkers in these images. In this paper, we present a data-driven fully automated method for estimation of core and penumbra in ischaemic lesions using diffusion-weighted imaging (DWI) and perfusion-weighted imaging (PWI) sequence maps of MRI. The method employs recent developments in convolutional neural networks (CNN) for semantic segmentation in medical images. In the absence of availability of a large amount of labeled data, the CNN is trained using an adversarial approach employing cross-entropy as a segmentation loss along with losses aggregated from three discriminators of which two employ relativistic visual Turing test. This method is experimentally validated on the ISLES-2015 dataset through three-fold cross-validation to obtain with an average Dice score of 0.82 and 0.73 for segmentation of penumbra and core respectively.

eess.IV↗

arXiv2019

The 2019 July 2 Total Solar Eclipse: Prediction of the Coronal Magnetic Field Structure and Polarization Characteristics

On 2019 July 2 a total solar eclipse -- visible across some parts of the Southern Pacific Ocean, Chile and Argentina -- will enable observations of the Sun's large-scale coronal structure. The structure of the Sun's corona and their emission characteristics are determined by underlying magnetic fields which also govern coronal heating and solar eruptive events. However, coronal magnetic field measurements remain an outstanding challenge. Computational models of coronal magnetic fields serve an important purpose in this context. Earlier work has demonstrated that the large-scale coronal field is governed by slow surface flux evolution and memory build-up which allows for prediction of the coronal structure on solar rotational timescales. Utilizing this idea and based upon a 51 day forward run of a data-driven solar surface flux transport model and a Potential Field Source Surface model, we predict the Sun's coronal structure for the 2019 July 2 solar eclipse. We also forward model the polarization characteristics of the coronal emission from the predicted magnetic fields. We predict two large-scale streamer structures and their locations on the east and west limbs of the Sun and discuss the possibility of development of a pseudo-streamer based on an analysis of field line topology. This study is relevant for coronal magnetometry initiatives from ground-based facilities such as the Daniel K. Inouye Solar Telescope and Coronal Multichannel Polarimeter, and upcoming space-based instruments such as the Solar Ultraviolet Imaging Telescope and the Variable Emission Line Coronagraph onboard ISRO's Aditya-L1 space mission.

astro-ph.SR↗

arXiv2019

Spectral singularity enhances transverse spin

We study the transverse spin and the Belinfante spin momentum in a gain-loss balanced waveguide, known to be one of the first and most studied examples of $\mathcal{PT}$-symmetric systems. Such a guide supports the spectral singularities leading to infinite scattering amplitudes for both reflection and transmission. We show that near the spectral singularity there can be dramatic enhancement of the transverse spin and the transverse spin momentum. Note that these exotic spin and spin momentum have recently been observed experimentally despite having tiny magnitudes, making it worthy to explore ways and means to enhance these fundamentally important elusive quantities.

physics.optics↗

arXiv2019

Controlling Fano resonance using the Geometrical Phase of light in spatially tailored waveguided plasmonic crystals

Fano resonance exhibiting an asymmetric spectral line shape is a universal phenomenon observed in diverse physical systems. Here we experimentally establish a direct link between the spectral asymmetry parameter and a physically realizable phase factor of interference between a continuum and a discrete mode that leads to Fano resonance. Using a specially designed metamaterial, namely waveguided plasmonic crystal with a spatially varying orientation axis of plasmonic grating, we demonstrate control on the spectral asymmetry of the Fano resonance through changes in the geometric phase of polarized light. In this scenario, the changes in the geometric phase for input left, and right circular polarized light arises due to varying orientation angle of the grating axis. The systematic changes in the geometric phase and the resulting q-parameter of Fano resonance is interpreted by an appropriate theoretical model connecting the two physical entities. The demonstrated control over the spectral line shape of Fano resonance achieved by tailoring geometric phase may open up novel routes for polarization-based photonic applications.

physics.optics↗

arXiv2019

Unraveling linguistic patterns in dog behaviour

Apparently random events in nature often reveal hidden patterns when analysed using diverse and robust statistical tools. Power-law distributions, for example, project diverse natural phenomenon, ranging from earthquakes1 to heartbeat dynamics2 onto a common platform of statistical self-similarity. A large range of human languages are known to follow a specific regime of power-law distributions, the Zipf-Mandelbrot law, in addition to showing properties like the Pareto principle and Shannon entropy3,4. Animal behaviour in specific contexts have been shown to follow power-law distributions5,6. However, the entire behavioural repertoire of a species has never been analysed for the existence of underlying patterns. Here we show that the frequency-rank data of randomly sighted behaviours at the population level of free-ranging dogs follow a scale-invariant power-law behaviour. While the data does not display Zipfian trends, it obeys the Pareto principle and Shannon entropy rules akin to languages. Interestingly, the data also exhibits robust self-similarity patterns at different scales which we extract using multifractal detrended fluctuation analysis7. The observed multifractal trends suggest that the probability of consecutive occurrence of behaviours of adjacent ranks is much higher than behaviours widely separated in rank. Since we observe such robust trends in random data sets, we hypothesize that the general behavioural repertoire of a species is shaped by a syntax similar to languages. This opens up the prospect of future multifractal and other statistical investigations on true time series of behavioural data to probe the existence of possible long and short-range correlations, and thereby develop predictive models of behaviour.

physics.soc-ph↗

arXiv2019

Weak value amplification using spectral interference of Fano resonance

The extraordinary concept of weak value amplification and a number of other seemingly counter-intuitive wave phenomena are manifestations of fine interference effects. Taking example of one such intriguing wave interference phenomenon, namely, optical Fano resonance, we show that weak value amplification naturally arises in such scenario due to near destructive spectral interference between a continuum mode and a narrow resonance mode which mimics the corresponding near destructive interference between the eigenstates of the measuring observable as a consequence of nearly mutually orthogonal pre and post selections of the system states. In order to elucidate this, we first experimentally demonstrate a weak measurement concept that uses near destructive interference of two paths of an interferometer having slightly rotated linear polarization states of light. By generating nearly destructive interference between the two paths with small amplitude and phase offsets of the waves, we observe both real and imaginary (respectively) weak value amplification of the small polarization rotation leading to large rotation of the polarization vector orientation and dramatic changes in the circular polarization descriptor 4th Stokes vector element of light, respectively. We go on to demonstrate that the giant enhancement of small Faraday rotation in the vicinity of the Fano spectral dip in waveguided magneto-plasmonic crystal is a manifestation of such natural interferometric weak value amplification.

physics.optics↗

arXiv2018

Transverse spin and transverse momentum in structured optical fields

It has been recently recognized that in addition to the conventional longitudinal angular momentum, structured (inhomogeneous) optical fields exhibit helicity-independent transverse spin angular momentum (SAM) and an unusual spin (circular polarization)- dependent transverse momentum, the so-called Belinfante's spin momentum. Such highly nontrivial structure of the momentum and the spin densities in the structured optical fields (e.g., evanescent fields) has led to a number of fundamentally interesting and intricate phenomena, e.g., the quantum spin Hall effect of light and the optical spin-momentum locking in surface optical modes similar to that observed for electrons in topological insulators. In this review, we introduce the basic concepts and look into the genesis of transverse SAM and transverse spin momentum in structured light. We then discuss few illustrative examples of micro and nano optical systems where these illusive entities can be observed. The studied systems include planar and spherical micro and nano structures. We also investigate the ways and means of enhancing the elusive extraordinary spin. In particular, we show that dispersion management leading to avoided crossing along with perfect absorption mediated by recently discovered coherent perfect absorption can positively influence the resonant enhancement of the transverse spin and spin momentum. The role of mode mixing and interference of neighboring TE and TM scattering modes of diverse micro and nano optical systems are illustrated with the selected examples. The results demonstrate possibilities for the enhancement of not only the magnitudes but also the spatial extent of transverse SAM and the transverse momentum components, which opens up interesting avenues for experimental detection of these illusive fundamental entities and may enhance the ensuing spin-based photonic applications.

physics.optics↗

arXiv2018

Experimental probe of weak value amplification and geometric phase through the complex zeroes of the response function

The extraordinary concept of weak value amplification has attracted considerable attention for addressing foundational questions in quantum mechanics and for metrological applications in high precision measurement of small physical parameters. Here, we experimentally demonstrate a fundamental relationship between the weak value of an observable and complex zero of the response function of a system by employing weak value amplification of spin Hall shift of a Gaussian light beam. Using this relationship, we show that arbitrarily large weak value amplification far beyond the conventional weak measurement limit can be experimentally obtained from the position of the minima of the pointer intensity profile corresponding to the real part of the complex zero of the response function. The imaginary part of the complex zero, on the other hand, is related to the spatial gradient of geometric phase of light, which in this particular scenario evolves due to the weak interaction and the pre and post selections of the polarization states. These universal relationships between the weak values and the complex zeros of the response function may provide new insights on weak measurements in a wide class of physical systems. Extremely large weak value amplification and extraction of system parameters outside the region of validity of conventional weak measurements may open up a new paradigm of weak measurement enhancing its metrological applications.

physics.optics↗

arXiv2018

Fully Convolutional Model for Variable Bit Length and Lossy High Density Compression of Mammograms

Early works on medical image compression date to the 1980's with the impetus on deployment of teleradiology systems for high-resolution digital X-ray detectors. Commercially deployed systems during the period could compress 4,096 x 4,096 sized images at 12 bpp to 2 bpp using lossless arithmetic coding, and over the years JPEG and JPEG2000 were imbibed reaching upto 0.1 bpp. Inspired by the reprise of deep learning based compression for natural images over the last two years, we propose a fully convolutional autoencoder for diagnostically relevant feature preserving lossy compression. This is followed by leveraging arithmetic coding for encapsulating high redundancy of features for further high-density code packing leading to variable bit length. We demonstrate performance on two different publicly available digital mammography datasets using peak signal-to-noise ratio (pSNR), structural similarity (SSIM) index and domain adaptability tests between datasets. At high density compression factors of >300x (~0.04 bpp), our approach rivals JPEG and JPEG2000 as evaluated through a Radiologist's visual Turing test.

cs.CV↗

arXiv2018

Mueller matrix spectroscopy of Fano resonance in Plasmonic Oligomers

Fano resonance in plasmonic oligomers originating from the interference of a spectrally broad superradiant mode and a discrete subradiant mode is under intensive recent investigations due to numerous potential applications. In this regard, development of experimental means to understand and control the complex Fano interference process and to modulate the resulting asymmetric Fano spectral line shape is highly sought after. Here we present a polarization Mueller matrix measurement and inverse analysis approach for quantitative understanding and interpretation of the complex interference process that lead to Fano resonance in symmetry broken plasmonic oligomers. The spectral Mueller matrices of the plasmonic oligomers were recorded using a custom designed dark-field Mueller matrix spectroscopy system. These were subsequently analyzed using differential Mueller matrix decomposition technique to yield the quantitative sample polarimetry characteristics, namely, polarization diattenuation (d) and linear retardance (δ) parameters. The unique signature of the interference of the superradiant dipolar plasmon mode and the subradiant quadrupolar mode of the symmetry broken plasmonic oligomers manifested as rapid spectral variation of the diattenuation and the linear retardance parameters across the Fano spectral dip. The polarization information contained in the Mueller matrix was further utilized to desirably control the Fano spectral line shape. The experimental Mueller matrix analysis was complemented with finite element based numerical simulations, which enabled quantitative understanding of the interference of the superradiant and the subradiant plasmon modes and its link with the polarization diattenuation and retardance parameters.

physics.optics↗

arXiv2018

Transverse spin in scattering of focused radially and azimuthally polarized vector beams

We study the effect of focusing of the radially and azimuthally polarized vector beams on the spin angular momentum (SAM) density and Poynting vector of scattered waves from a Mie particle. Remarkably, the study reveals that the SAM density of the scattered field is solely transverse in nature for radially and azimuthally polarized incident vector beams; however the Poynting vector shows usual longitudinal character. We also demonstrate that the transverse SAM density can further be tuned with wavelength and focusing of the incident beam by exploiting the interference of different scattering modes. These results may stimulate further experimental technique of detecting the transverse spin and Belinfante's spin-momentum densities.

physics.optics↗

arXiv2017

Consequences of nonreciprocity in reflection from a truncated spatial Kramers-Kronig medium

We study the truncated Kramers-Kronig (KK) medium to reveal the effects of nonreciprocity in reflection on the scattering and the wave guiding properties. We focus on the simplest possible spatial profile mimicking the dielectric response. We show that a finite slab of KK medium can support only one sided null scattering under bidirectional identical illumination. However, for a phase lag between the two incident beams it is possible to have null scattering on the other side. This is shown to be a consequence of the nonreciprocity in reflection due to the inherent asymmetry of the KK medium. We further look at the possibility of surface modes for unidirectional illumination. For specific range of parameters, we show the existence of TM polarized surface plasmons only for illumination from one side. The origin of such modes is traced to the inhomogeneous metal like layer with negative real part of the permittivity in the KK medium.

physics.optics↗

arXiv2017

Effects of mode mixing and avoided crossings on the transverse spin in a metal-dielectic-metal sphere

We study transverse spin in a sub-wavelength metal-dielectric-metal (MDM) sphere when the MDM sphere exhibits avoided crossing due to hybridization of the surface plasmon with the Mie localized plasmon. We show that the change in the absorptive and dipersive character near the crossing can have significant effect on the transverse spin. An enhancement in the transverse spin is shown to be possible associated with the transparency (suppression of extinction) of the MDM sphere. The effect is attributed to the highly structured field emerging as a consequence of competition of the electric and magnetic modes.

physics.optics↗

arXiv2017

Submicron scale tissue multifractal anisotropy in polarized light scattering

A number of disordered systems exhibit local anisotropy in the fractal or multifractal correlation and in the resulting scaling behavior, which contain wealth of information on the system. Here, we demonstrate that the spatial dielectric fluctuations in a random medium like biological tissue exhibit such multifractal anisotropy, leaving its unique signature in the wavelength variation of the light scattering Mueller matrix and manifesting as an intriguing spectral diattenuation effect. We have thus developed an inverse analysis method for the quantification of the multifractal anisotropy from the scattering Mueller matrix. The method is based on processing the relevant Mueller matrix elements in Fourier domain using Born approximation followed by multifractal analysis. Application of this technique on tissues of human cervix ex vivo demonstrate the potential of the multifractal anisotropy parameters as novel biomarkers for screening subtle micro-structural changes associated with precancers. Sensing structural anisotropy in the sub-micron length scale via the multifractal anisotropy parameters may prove valuable for non-invasive characterization of a wide class of complex materials and disordered scattering media.

physics.optics↗

arXiv2016

Transverse spin and transverse momentum in scattering of plane waves

We study the near field to the far field evolution of spin angular momentum (SAM) density and the Poynting vector of the scattered waves from spherical scatterers. The results show that at the near field, the SAM density and the Poynting vector are dominated by their transverse components. While the former (transverse SAM) is independent of the helicity of the incident circular polarization state, the latter (transverse Poynting vector) depends upon the polarization state. It is further demonstrated that the magnitudes and the spatial extent of the transverse SAM and the transverse momentum components can be controllably enhanced by exploiting the interference of the transverse electric and transverse magnetic scattering modes.

physics.optics↗