Subhasish Dutta Gupta

Atomic absorbers for controlling pulse propagation in resonators.

We consider pulse propagation through a Fabry-Perot cavity with silver mirrors that contain macroscopic samples of resonant absorbers. We show that the pulse velocity can be tuned from subluminal to superluminal in a strongly coupled atom-cavity system. We delineate the effects of the interplay of cavity and absorbers. We demonstrate the saturation effects of pulse advancement with increasing mirror thickness and atomic damping.

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 interference of the transverse electric and transverse magnetic scattering modes enhances both the magnitudes and the spatial extent of the transverse SAM and the transverse momentum components.

Manifestations of geometric phase and enhanced spin Hall shifts in an optical trap

The spin orbit interaction (SOI) of light has been the focus of recent research due to the fundamental consequences and potential applications in diverse systems, ranging from inhomogeneous anisotropic media to engineered plasmonics and metamaterial strutures. Here, we demonstrate perhaps one of the simplest means to study SOI and the spin Hall shift (SHS) using a standard Gaussian TEM00 beam in an optical trap. Our system exploits the versatility and interference generated in a stratified medium to control and manipulate SOI and transfer the resulting angular momentum to optically trapped microparticles. We show that even such a simple setup can lead to an order of magnitude enhancement in the SHS compared to the subwavelength shifts typically obtained. Importantly, this leads to the generation of doughnut-like mode structures from a fundamental Gaussian beam, as well as controlled rotation of mesoscopic particles using a linearly polarized Gaussian beam that lacks intrinsic angular momentum. The local optical torque leading to rotation of the particles is a direct measure of the local spin angular momentum (SAM) density of the field. Our measurement is the first experimental demonstration of using a probe particle to measure the SAM density for nonparaxial fields.

Coherent perfect absorption mediated enhancement of transverse spin in a gap plasmon guide

We consider a symmetric gap plasmon guide (a folded Kretschmann configuration) supporting both symmetric and antisymmetric coupled surface plasmons. We calculate the transverse spin under illumination from both the sides like in coherent perfect absorption (CPA), whereby all the incident light can be absorbed to excite one of the modes of the structure. Significant enhancement in the transverse spin is shown to be possible when the CPA dip and the mode excitation are at the same frequency. The enhancement results from CPA-mediated total transfer of the incident light to either of the coupled modes and the associated large local fields. The effect is shown to be robust against small deviations from the symmetric structure. The transverse spin is localized in the structure since in the ambient dielectric there are only incident plane waves lacking any structure.

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.

Enhancing transverse spin and transverse spin momentum in micro- and nano-optical systems

We show that scattering of plane waves leads to helicity-independent transverse spin angular momentum (SAM) and helicity-dependent transverse Poynting vector components. The in uence of plasmon resonance and avoided crossing for a sphere on these quantities is studied.

Coherent Perfect Absorption Mediated Enhancement and Optical Bistability in Phase Conjugation

We study phase conjugation in a nonlinear composite slab when the counter propagating pump waves are completely absorbed by means of coherent perfect absorption. Under the undepleted pump approximation the coupling constant and the phase conjugated reflectivity are shown to undergo a substantial increase and multivalued response. The effect can be used for efficient switching of the phase conjugated reflectivity in photonic circuits.

Number-phase uncertainty and quantum dynamics of bosons and fermions interacting with a finite range and large scattering length in a double-well potential

We define the standard quantum limit (SQL) for phase and number fluctuations, and describe two-mode squeezing for number and phase variables. When phase is treated as a unitary quantum-mechanical operator, number and phase operators satisfy an uncertainty relation. As a result, the usual definition of number squeezing parameter becomes modified. Two-mode number squeezing occurs when the number fluctuation goes below the SQL at the cost of enhanced phase fluctuation. As an application of number-phase uncertainty, we consider bosons or fermions trapped in a quasi-one dimensional double-well (DW) potential interacting via a 3D finite-range two-body interaction potential with large scattering length

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

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Interesting manifestations of spin orbit interaction and spin Hall shift of light in an optical trap

. Under tight-binding or two-mode approximation, we describe in detail the effects of the range of interaction on the quantum dynamics and number-phase uncertainty in the strongly interacting or unitarity regime