arXiv subjects

Sauvik Roy

Publications and source records attributed to Sauvik Roy.

11 recordsLinked to original sources

arXiv2026

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

We study 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-H\"anchen and Imbert Fedorov shifts, respectively, in the reflected light, magnified by resonant enhancement and 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. We believe that our results can lead to new type of extra-sensitive sensors for any agent (eg. refractive index, displacement, etc.) that can break the symmetry.

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

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↗

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

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

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 $\sigma = +1$ 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

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↗

arXiv2022

Manipulating the transverse spin angular momentum and Belinfante momentum of spin-polarized light by a tilted stratified medium in optical tweezers

In the recent past, optical tweezers incorporating a stratified medium have been exploited to generate complex translational and rotational dynamics in mesoscopic particles due to the coupling between the spin and orbital angular momentum of the light, generated as a consequence of the tight focusing of light by a high numerical aperture objective lens into the stratified medium. Here, we consider an optical tweezers system with a tilted stratified medium (direction of stratification at an angle with the axis of the incident beam), and show that for input circularly polarized Gaussian beams, the resulting spin-orbit interaction deeply influences the generation of transverse spin angular momentum (TSAM) and Belinfante momentum of light, and allows additional control on their magnitude. Importantly, the TSAM generated in our system consists of both the orthogonal components, which is in sharp contrast to the case of evanescent waves and surface plasmons, where only one of the TSAM components are generated. The broken symmetry due to the tilt ensures that, depending upon the helicity of the input beam, the magnitude of the mutually orthogonal components of the TSAM depend entirely on the tilt angle. This may prove to be an effective handle in exotic spin-controlled manipulation of particles in experiments.

physics.optics↗

arXiv2020

Realizing negative index of refraction in an ensemble of ground-state polar molecules with lasers

We propose a coherent optical method for creating negative refractive index (NRI) for a gaseous ensemble of ground-state polar molecules possessing both permanent electric and magnetic moments. Exploiting the pure rotational transition between the two lowest rotational levels of the ground vibrational state one can generate two dressed states of mixed parity using a microwave laser. These dressed states are then used as the two lower states of a {\Lambda}-type three-level scheme using two infrared lasers to couple them to another ro-vibrational level in the ground-state manifold. One of the infrared lasers is used as a weak probe and the other as a control field with a fixed detuning. We take into account local-field effects on the dielectric response of the medium at the probe frequency in terms of Clausius-Mossoti relation. We extract magnetic response out of purely dielectric response and calculate the magnetic permeability of the medium in terms of dielectric susceptibility. Our results show that there is a small frequency window near the two-photon resonance where both electric permittivity and magnetic permeability are negative with vanishingly small absorption. The figure of merit for the medium is shown to exceed unity. We interpret our results in terms of the proximity of EIT and quantum interference. We discuss the possible realization of our method using cold polar molecules that are recently experimentally produced.

physics.optics↗