arXiv subjects

Subhasish Dutta Gupta

Publications and source records attributed to Subhasish Dutta Gupta.

At least 19 recordsLinked to original sources

arXiv2026

Probing the Broken Spatial Symmetry of a Stratified Medium with Structured Light

We study theoretically near-symmetric resonant stratified media to show how a tiny broken spatial symmetry can effectively be probed by structured light with or without orbital angular momentum. This is achieved by examining both the in-plane and out of plane Goos-Hanchen and Imbert Fedorov shifts, respectively, in the reflected light, magnified by resonant enhancement and when needed with weak value amplification. We show that non-reciprocity in reflection for illumination from opposite ends can result in different shifts, even to the extent of shifts with opposite signs for tiny imbalance resulting from the broken symmetry. Our methodology is applicable to any general stratified medium with homogeneous and isotropic constituent layers being responsive to any entity that may cause such symmetry breaking - including temperature, refractive index, displacement etc. in the near symmetric case. This may have possible use for sensing applications.

physics.optics↗

arXiv2026

Tunable Optical Torque by Asymmetry-Induced Spin-Hall Effect in Tightly Focused Spinless Gaussian Beams

A linearly polarized Gaussian beam, carrying zero net spin angular momentum, is conventionally not expected to exert optical torque or induce rotational motion in birefringent microparticles. When such a beam is tightly focused, the constituent left- and right-circular polarization components separate spatially due to spin-orbit interaction, commonly known as the spin Hall effect of light. However, this separation is at wavelength scales and is also axially symmetric, resulting in zero net spin angular momentum, and concomitantly no optical torque near the focal plane. Here, we demonstrate that this limitation can be overcome using several commonly encountered asymmetric illumination modalities that break the axial symmetry of the focusing system, thereby disrupting the symmetric separation of the spin components for the same linearly polarized Gaussian beam. As a consequence, trapped microparticles experience a tunable optical torque and exhibit rotational motion with distinct rotational frequencies at the same input power. The particles also undergo controlled reversal of the rotation direction simply by rotating the incident plane of polarization using a half-wave plate. Despite their apparent diversity, all these methods share the same physical origin rooted in asymmetric illumination. These results establish an experimentally accessible and minimal strategy for realizing controllable optical rotation devices exploiting spin-orbit optomechanics without requiring intrinsic angular momentum in the light.

physics.optics↗

arXiv2025

Surface Plasmon Mediated Giant Goos-Hanchen and Imbert-Fedorov Shifts on a Corrugated Metal Surface

Enhanced beam shifts mediated by surface plasmon resonance (SPR) at metal-dielectric interfaces have been widely investigated. However, research on the associated Imbert-Fedorov or spin Hall shifts, driven by the spin-orbit interaction of structured light in structured interfaces, has been comparatively scarce and limited. We explore the reflection characteristics of generic polarized, non-paraxial light beams from a corrugated silver (Ag) interface, since surface corrugation can naturally couple the incident radiation modes to the surface excitations. In the vicinity of SPR, we report a significant enhancement in the beam shifts, attributed to the rapid variation of the specular reflection coefficient near its minima, resulting in amplified weak values. By carefully selecting the incident and projected polarization states of the beam, we achieve a pronounced spatial spin Hall effect. We also investigate vortex-induced beam shifts within this resonant regime, revealing distinctive signatures of the angular momentum of the beam. Furthermore, a comprehensive analysis is also presented for the conical diffraction geometry, wherein polarization conversions between p and s states are fully incorporated. Our work establishes the interplay of the spin-orbit interaction of light and the weak measurement approach as an important methodology in amplifying SPR effects, which may have important connotations in applications involving light at nanoscales.

physics.optics↗

arXiv2025

Spin-Split Dispersion of Leaky Surface plasmons in Inversion- Symmetric System

Spin-dependent dispersion and Rashba effect are manifestations of universal spin orbit interaction associated with the breaking of the spatial inversion symmetry in condensed matter and in optical systems. In sharp contrast to this, we report a spin-split dispersion effect of leaky surface plasmons in an inversion-symmetric one dimensional plasmonic grating system. In our system, the signature of spin-momentum locking and the resulting spin-polarization dependent splitting of dispersion of the surface plasmons are observed through the leakage radiation detected in a Fourier (momentum) domain optical arrangement. The setup enables single-shot recording of the full polarization-resolved dispersion (frequency vs transverse momentum (k)) of the leaky surface plasmons. Momentum domain polarization analysis identified a transverse momentum (k) dependent linear birefringence-linear dichroism effect (referred to as the geometric LB-LD effect) responsible for the observed spin-split dispersion. This unconventional SOI effect is reminiscent of the recently reported LB-LD effect resulting in giant chirality in centrosymmetric crystal, albeit with geometric origin. It is demonstrated that the interplay of the geometrical polarization transformation in focused polarized light and subsequent interaction of the structured field polarization with the plasmonic grating leads to the evolution of strong geometrical phase gradient or spin(circular polarization)-dependent transverse momentum of light resulting in spin-split dispersion. Our study offers a new paradigm of spin-based dispersion engineering and spin-enabled nano-optical functionalities in simple symmetric metasurfaces using geometric LB-LD effect.

physics.optics↗

arXiv2025

Can a vector beam be critically coupled leading to perfect absorption?

Critical coupling has emerged as a prominent area of research in recent years. However, most theoretical models are based on scalar theories (and occasionally coupled mode theories), which inadequately account for the polarization states of the incident light. To bridge this gap, we revisit the concept of critical coupling in planar multilayer structures using a full vectorial theory, where conventional plane wave illumination is replaced by well-defined vector beams with and without orbital angular momentum (OAM). Our investigation explores the possibility of complete absorption of monochromatic beams without and with intrinsic OAM (such as Gaussian and Laguerre-Gaussian (LG)), incident on the multilayer structure at normal or oblique incidence. A two-component metal-dielectric composite film is chosen as the absorbing layer in the system. Our results demonstrate a significant reduction in the intensities of the reflected and transmitted beams at normal incidence, with reduced efficiency for oblique incidence due to the lack of spatial overlap of multiply reflected components. Interestingly, we also observe super-scattering from the same structures when conditions for constructive interference of the various reflected components are satisfied. This work highlights the need to incorporate the vector nature of beams by retaining the complete polarization information of off-axis spatial harmonics in future studies.

physics.optics↗

arXiv2025

Quasi-coherent perfect absorption of counter-propagating vector beams of finite spatial extent through an absorptive slab

Coherent perfect absorption (CPA) has been a topic of considerable contemporary research interest. Most of the theoretical treatment of CPA with beams, to the best of our knowledge, relies on a scalar (in some cases coupled mode) theories with inadequate input about the polarization states of the incoming light. In view of the lack of a full vectorial theory even for the original CPA configuration by Wan et al \cite{timereversedlasing_science}, we revisit the same when the incident plane waves are replaced by well defined vector beams with or without OAM. We study the absorption characteristics of two counter-propagating monochromatic structured beams, e.g., Gaussian and Laguerre-Gaussian (LG) beams with and without orbital angular momentum, respectively, incident normally on a composite slab from both sides by fulfilling the CPA condition exclusively for the central plane wave component. We show that though perfect absorption is not achievable, there can be a substantial reduction of the scattered light. We also consider the limitations of CPA for oblique incidence and discuss the difficulties. We believe that our study will motivate and necessitate the study of recent advancements with input vector beams, retaining the full polarization information of the off-axis spatial harmonics.

physics.optics↗

arXiv2024

A comprehensive study of the Spin-Hall effect of tightly focused linearly polarized light through a stratified medium in optical tweezers

The optical Spin-Hall effect originates from the interaction between the spin angular momentum (SAM) and extrinsic orbital angular momentum (OAM) of light, leading to mutual interrelations between the polarization and trajectory of light in case of non-paraxial fields. Here, we extensively study the SHE and the resultant Spin-Hall shifts (SHS) in optical tweezers (OT) by varying the numerical aperture of objective lenses, and the refractive index (RI) stratification of the trapping medium. Indeed, we obtain much larger values of the SHS for particular combinations of NA and stratification compared to the sub-wavelength orders typically reported. We also observe that the longitudinal component of the spin angular momentum (SAM) density - which is responsible for the spin of birefringent particles in optical tweezers - changes more-or-less monotonically with the lens numerical aperture, except around values of the latter where the angle subtended by the focused light equals the critical angle for a particular RI interface. Our results may find applications in designing experiments for tuning the SHS and SAM induced due to SOI to generate exotic optomechanics of trapped particles in optical tweezers.

physics.optics↗

arXiv2024

Micromotors Driven by Spin-Orbit Interaction of Light: Mimicking Planetary Motion at the Microscale

We introduce a new class of optical micromotors driven by the spin-orbit interaction of light and spin-driven fluid flows leading to simultaneous rotation and revolution of the micromotors. The micromotors are essentially birefringent liquid crystal particles (LC) that can efficiently convert the angular momentum of light into high-frequency rotational motion. By tightly focusing circularly polarized Gaussian beams through a high numerical aperture objective into a refractive index stratified medium, we create a spherically aberrated intensity profile where the spinning motion of a micromotor optically trapped at the centre of the profile induces fluid flows that causes orbiting motion of the off-axially trapped surrounding particles (secondary micromotors). In addition, the interaction between the helicity of light and the anisotropic properties of the LC medium leads to the breaking of the input helicity and drives the conversion of right to left-circular polarization and vice-versa. This spin-to-spin conversion, causes the orbiting secondary micromotors to spin in certain cases as well so that the entire system of spinning primary micromotor and revolving and spinning secondary micromotors is reminiscent of planetary motion at mesoscopic scales. Our findings, supported by both theoretical modeling and experimental validation, advance the understanding of light-matter interactions at the microscale.

physics.optics↗

arXiv2024

Spatially resolved spin angular momentum mediated by spin-orbit interaction in tightly focused spinless vector beams in optical tweezers

We demonstrate an effective and optimal strategy for generating spatially resolved longitudinal spin angular momentum (LSAM) in optical tweezers by tightly focusing first-order azimuthally radially polarized (ARP) vector beams with zero intrinsic angular momentum into a refractive index (RI) stratified medium. The stratified medium gives rise to a spherically aberrated intensity profile near the focal region of the optical tweezers, with off-axis intensity lobes in the radial direction possessing opposite LSAM (helicities corresponding to $σ= +1$ and -1) compared to the beam centre. We trap mesoscopic birefringent particles in an off-axis intensity lobe as well as at the beam center by modifying the trapping plane, and observe particles spinning in opposite directions depending on their location. The direction of rotation depends on particle size with large particles spinning either clockwise (CW) or anticlockwise (ACW) depending on the direction of spirality of the polarization of the ARP vector beam after tight focusing, while smaller particles spin in both directions depending on their spatial location. Numerical simulations support our experimental observations. Our results introduce new avenues in spin-orbit optomechanics to facilitate novel yet straightforward avenues for exotic and complex particle manipulation in optical tweezers.

physics.optics↗

arXiv2024

A rectangular loop interferometer for scalar optical computations and controlled generation of higher-order vector vortex modes using spin-orbit interaction of light

We have developed a rectangular loop interferometer (RLI) that confines light in a rectangular path and facilitates various interesting applications. Such a device can yield the sum of numerous geometric series converging to different values between zero and one by the use of simple intra-cavity beam splitters - both polarization-independent and dependent. Losses - principally due to alignment issues of the beam in the RLI - limit the average accuracy of the series sum value to be between 90 - 98\% with the computation speed determined by the bandwidth of the detectors. In addition, with a circularly polarized input Gaussian beam, and a combination of half-wave plate and q-plate inserted into the interferometer path, the device can generate a vortex beam that carries orbital angular momentum (OAM) of all orders of topological charge. The OAM is generated due to the spin-orbit interaction of light, and the topological charge increases with each successive pass of the beam inside the interferometer. However, experimentally, only the third order of OAM could be measured since projecting out individual orders entailed a slight misalignment of the interferometer, which caused higher orders to go out of resonance. Furthermore, with input linear polarization, the device can generate a vector beam bearing a superposition of polarization states resembling the multipole expansion of a charge distribution. Even here, experimentally, we were able to quantify the polarization distribution up to the third order using a Stokes vector analysis of the vector beam, with the size of the polarization singularity region increasing as the polarization states evolve inside the interferometer. Our work demonstrates the ubiquitous nature of loop interferometers in modifying the scalar and vector properties of light to generate simple mathematical results and other complex but useful applications.

physics.optics↗

arXiv2024

Reflectionless propagation of beams through a stratified medium

Reflectionless potentials following the prescription of Kay and Moses allow for total transmission of incoming waves of any kinetic energy. The optical analogue of such potentials occur as dielectric stratified media that can offer null reflectivity and near total transmission over a large range of incidence angles and wavelengths. In a previous work (S. Dutta Gupta and G. S. Agarwal, Opt. Express 15, 9614-9624, 2007), this was demonstrated for linearly polarized plane waves. We extend the earlier work valid for plane waves to structured beams to show near-total transmission of beams across the reflectionless dielectric profile. The analysis is based on the angular spectrum decomposition treating the beam as a collection of plane waves. Gaussian and Laguerre-Gaussian beams are shown to be transmitted through the film with <1% reflection in most scenarios. We also discuss the superlative performance of our proposed profile in preserving the beam shape during transmission comparing these results to a conventional lambda/2 antireflection coating.

physics.optics↗

arXiv2024

Doppler-assisted quantum resonances through swappable excitation pathways in Potassium vapor

We report the observation of two additional sub-natural line width quantum interference in the $D_2$ manifold of $^{39}K$ vapor, in addition to the usual single Electromagnetically induced transparency peak. The other two features appear exclusively because $^{39}K$ ground hyperfine splitting is smaller than the Doppler broadened absorption profile. This allows probe and control beams to swap their transition pathways. The control beam detuning captures the nature of the coherence, therefore an unusual phenomenon of conversion from perfect transparency to enhanced absorption is observed and explained by utilizing adiabatic elimination of the excited state in the Master equation. Controlling such dark and bright resonances leads to new applications in quantum technologies viz. frequency offset laser stabilization and long-lived quantum memory.

physics.atom-ph↗

arXiv2024

Inhomogeneous spin momentum induced orbital motion of birefringent particles in tight focusing of vector beams in optical tweezers

Spin orbit interaction (SOI) due to tight focusing of light in optical tweezers has led to exciting and exotic avenues towards inducing rotation in microscopic particles. However, instances where the back action of the particles influences and modifies SOI effects so as to induce rotational motion are rarely known. Here, we tightly focus a vector beam having radial/azimuthal polarization carrying no intrinsic angular momentum, into a refractive index stratified medium, and observe orbital rotation of birefringent particles around the beam propagation axis. In order to validate our experimental findings, we perform numerical simulations of the underlying equations. Our simulations reveal that the interaction of light with a birefringent particle gives rise to inhomogeneous spin currents near the focus, resulting in a finite spin momentum. This spin momentum combines with the canonical momentum to finally generate an origin-dependent orbital angular momentum which is manifested in the rotation of the birefringent particles around the beam axis. Our study describes a unique modulation of the SOI of light due to interaction with anisotropic particles that can be used to identify new avenues for exotic and complex particle manipulation in optical tweezers.

physics.optics↗

arXiv2024

Observation of Larmor-like precession of a single birefringent particle due to spin-dependent forces in tilted optical tweezers

We observe clear precessional motion of highly birefringent liquid crystal (LC) particles trapped in a spherically aberrated optical trap which is built around a tilted refractive index stratified medium. For input circularly polarized light, the breaking of azimuthal symmetry induced by the tilt leads to an asymmetric intensity distribution in the radial direction near the trap focal plane, which - in combination with the spin-orbit conversion effects for input circularly polarized light - results in non-uniform canonical and spin momentum densities in those regions. In addition, while the canonical momentum remains always oriented towards the axial direction, the spin momentum reverses direction along spatial loops in the radial direction. As a consequence, the total momentum precesses around the canonical momentum vector along elliptical spatial loops - akin to a Larmor-like precession of magnetic moment (total momentum in our case) around a magnetic field (canonical momentum). We probe this precession experimentally using the single trapped LC particles - with the direction of precession determined by the helicity of the input light and the precession frequency varying linearly with the laser power. Our experimental results are validated by numerical simulations of the system where we employ the Debye-Wolf theory for tight focusing in the presence of a tilted stratified media.

physics.optics↗

arXiv2023

Enhanced beam shifts mediated by Bound States in Continuum

The interaction of light beams with resonant structures has led to the development of various optical platforms for sensing, particle manipulation, and strong light-matter interaction. In the current study, we investigate the manifestations of the bound states in continuum (BIC) on the in plane and out of plane shifts (referred to as Goos-Hanchen (GH) and Imbert-Fedorov (IF) shifts, respectively) of a finite beam with specific polarization incident at an arbitrary angle. Based on the angular spectrum decomposition, we develop a generic formalism for understanding the interaction of the finite beam with an arbitrary stratified medium with isotropic and homogeneous components. it is applied to the case of a Gaussian beam with p and circularly polarized light incident on a symmetric structure containing two polar dielectric layers separated by a spacer layer. For p-polarized plane wave incidence one of the coupled Berreman modes of the structure was recently shown to evolve to the bound state with infinite localization and diverging quality factor coexisting with the other mode with large radiation leakage (Remesh et al. Optics Communications, 498:127223, 2021). A small deviation from the ideal BIC resonance still offers resonances with very high quality factors and these are exploited in this study to report giant GH shifts. A notable enhancement in the IF shift for circularly polarized light is also shown. Moreover, the reflected beam is shown to undergo distortion leading to a satellite spot. The origin of such a splitting of the reflected beam is traced to a destructive interference due to the left and right halves of the corresponding spectra.

physics.optics↗

arXiv2023

Probing inhomogeneous and dual asymmetric angular momentum exploiting spin-orbit interaction in tightly focused vector beams in optical tweezers

The spin-orbit interaction (SOI) of light generated by tight focusing in optical tweezers has been regularly employed in generating angular momentum - both spin and orbital - in trapped mesoscopic particles. Specifically, the transverse spin angular momentum (TSAM), which arises due to the longitudinal component of the electromagnetic field generated by tight focusing, is of special interest, both in terms of fundamental studies and associated applications. We provide an effective and optimal strategy for generating TSAM in optical tweezers by tightly focusing radially and azimuthally polarized first-order Laguerre Gaussian beams with no intrinsic angular momentum, into a refractive index stratified medium. Our choice of such input fields ensures that the longitudinal spin angular momentum (LSAM) arising from the electric (magnetic) field for the radial (azimuthal) component is zero, which leads to the separate and exclusive effects of the electric and magnetic TSAM in the case of input radially and azimuthally polarized beams on single birefringent particles. We also observe the emergence of origin-dependent intrinsic orbital angular momentum causing the rotation of birefringent particles around the beam axis for both input beam types, which opens up new and simple avenues for exotic and complex particle manipulation in optical tweezers.

physics.optics↗

arXiv2022

Zero-threshold correlated-photon laser with a single trapped atom in a bimodal cavity

We demonstrate theoretically the feasibility of correlated entangled photon-pair generation with vanishing threshold in a bimodal cavity setup that uses a single V-type three level atom pumped by dual incoherent sources and driven by two coherent fields. The photon-pair is shown to be entangled only for low levels of the incoherent pumps and owes its origin solely to the coherent drives. Our results show that the dual incoherent pumping with no coherent drive can lead to amplification of the cavity fields with strong inter-mode antibunching but no entanglement. Though only coherent drives with no incoherent pumping can produce entangled photon-pairs, the entangled cavity fields can not be amplified beyond a certain limit using only coherent drives. However, the use of even small incoherent pumping in the presence of the coherent drives can amplify the generated entangled photon-pairs significantly. We analyse our results in terms of an interplay between coherent and incoherent processes involving cavity-dressed states. Both the inter- and intra-mode HBT functions exhibit temporal oscillations in the strong-coupling cavity QED regime. Our theoretical scheme for the generation of nonclassical and entangled photon pairs may find interesting applications in quantum metrology and quantum information science.

quant-ph↗

arXiv2022

Diffraction of an Off-axis Vector-Beam by a Tilted Aperture

Manifestations of orbital angular momentum induced effects in the diffraction of a radially polarized vector beam by an off-axis tilted aperture are studied both experimentally and theoretically. Experiments were carried out to extract the degree of circular polarization, which was shown to be proportional to the on-axis component of the spin angular momentum density. We report a clear separation of the regions of dominance of the right and left circular polarizations associated with positive and negative topological charges respectively, which is reminiscent of the standard vortex-induced transverse shift, albeit in the diffraction scenario. The experimental results are supported by model simulations and the agreement is quite satisfactory. The results are useful to appreciate the orbit-orbit related effects due to unavoidable misalignment problems (especially for vortex beams).

physics.optics↗