arXiv subjects

Sanjay Kumar Mishra

Publications and source records attributed to Sanjay Kumar Mishra.

5 recordsLinked to original sources

arXiv2025

Optimization of airgap in a monocentric lens assembly and metasurface based anti-reflecting coating in the long-wave IR regime

Owing to minimal aberration, larger field-of-view, and high resolution, monocentric lenses are preferred over other imagers in hemispherical image surface scenarios, particularly in the long-wave infrared (LWIR) region. Herein, we study a monocentric lens assembly consisting of a ball lens and two hemispherical shell lenses of given radii of curvature and lens materials to enhance the transmission of LWIR ranging from 8μm to 12μm wavelength. We optimize the air gaps between the lenses of the assembly for a wider range of incidence angles, assuming antireflecting coating (ARC) on all surfaces of the lenses. Additionally, the effect of temperature variation on the transmittance is also studied to attain an optimum air gap between the lenses. With the variation in incidence angle and temperature, modulations in focal length and spot size are reported for different wavelengths. Further, we propose a metasurface design to replace conventional multilayer ARCs to enhance the transmittance through the lens assembly over the given wavelength range for a wide field-of-view.

physics.optics↗

arXiv2024

Evolutions of ring Airy vortex beam and ring Pearcey vortex beam in turbulent atmosphere and a comparative analysis of their channel efficiencies and OAM spectra

An optical vortex beam propagating through turbulent atmosphere encounters distortions in the wavefront that results in modal scattering. Abruptly autofocussing (AAF) beams with orbital angular momentum have gained significant attention due to their non-diffracting and self-healing nature. These warrants understanding of the behaviour of these beams through turbulent atmosphere absolutely necessary. With this intuition, in the present work we investigate the behaviour of two AAF beams namely ring Airy vortex beam (RAVB) and ring Pearcey vortex beam (RPVB) through the turbulent atmosphere in two cases-multiplexed and non-multiplexed. We propagate multiplexed as well as non-multiplexed RAVB and RPVB in different levels of turbulent atmosphere. In non-multiplexed case, channel efficiency declines for both the beams with increase in modes numbers. In multiplexed case, increasing the gap between the mode sets results in decrease in channel efficiency. We also report that in weak atmospheric turbulence RAVB outperform RPVB in terms of channel efficiency. We use optical transformation sorting (log-polar) method to demultiplex the optical beams at the output. Furthermore, we investigate and compare the OAM spectra of both beams in different levels of atmospheric turbulence and at different propagation distances. The comparison reveals that the spectra of RPVB are more dispersive as compared to that of RAVB.

physics.optics↗

arXiv2023

Ring Pearcey vortex beam dynamics through atmospheric turbulence

The subject area of free space optical communication with optical beam carrying orbital angular momentum has attracted a great deal of research attention since last two decades. Efforts to understand, model and execute communication links through turbulent atmosphere with OAM beams have gained particular importance. In this regard, different types of shape preserving beams, which can withstand turbulences of varying strengths, have been proposed and studied. In this paper, we present a numerical investigation of the propagation characteristics of ring Pearcey vortex beam through turbulent atmosphere. The study details on both moderate as well as strong atmospheric turbulences. Modified von Karman model has been relied on to model random phase screen. In moderate turbulence, the ring PVB preserved its singularity. In strong turbulence, the ring PVB preserved its singularity for short propagation distances but lost its singularity at longer propagation distances. We found that upon increasing the value of topological charge, aperture averaged scintillation index increases. We calculated the aperture averaged SI for different truncation factors and noticed that the ring PVB with a truncation factor 0.1 performed better in stronger turbulence. In moderate turbulence, the aperture averaged SI performed better for shorter propagation distances and relatively larger truncation factors. Further, we calculated the aperture averaged SI for spatially chirped ring PVB, and it has been found that aperture averaged SI improved largely for negatively chirped ring PVB. Furthermore, on comparing the aperture averaged SI of ring PVB and ring Airy vortex beam, it has been noticed that in strong turbulence ring PVB exhibited better aperture averaged SI. Additionally, we have calculated the beam wander for ring PVB and ring AVB and found that ring PVB demonstrates better beam wander.

physics.optics↗

arXiv2017

Anomalous Lattice Behavior of Vanadium Pentaoxide (V2O5): X- Ray Diffraction, Inelastic Neutron Scattering and ab-initio Lattice Dynamics

We present the structural and dynamical studies of layered vanadium pentaoxide (V2O5). The temperature dependent X-ray diffraction measurements reveal highly anisotropic and anomalous thermal expansion from 12 K to 853 K. The results do not show any evidence of structural phase transition or decomposition of α-V2O5, contrary to the previous transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) experiments. The inelastic neutron scattering measurements performed up to 673 K corroborate the result of our X-ray diffraction measurements. The analysis of the experimental data is carried out using ab-initio lattice dynamics calculation. The important role of van der-Waals dispersion and Hubbard interactions on the structure and dynamics is revealed through the ab-initio calculations. The calculated anisotropic thermal expansion behavior agrees well with temperature dependent X- ray diffraction. The mechanism of anisotropic thermal expansion and anisotropic linear compressibility is discussed in terms of calculated anisotropy in Grüneisen parameters and elastic coefficients. The calculated Gibbs free energy in various phases of V2O5 is used to understand the high pressure and temperature phase diagram of the compound. Softening of elastic constant (C66) with pressure suggests a possibility of shear mechanism for α to \b{eta} phase transformation under pressure.

cond-mat.mtrl-sci↗

arXiv2008

Structural Basis for the Anomalously Low Spontaneous Polarisation Values of the Polar Phase of Sr1-xCaxTiO3 (x=0.02, 0.04): Evidence for a Ferrielectric Ordering

Full pattern Le-Bail refinement using x-ray powder diffraction profiles of Sr1-xCaxTiO3 for x=0.02, 0.04 in the temperature range 12 to 300 K reveals anomalies in the unit cell parameters at 170, 225 K due to an antiferrodistortive (cubic to tetragonal I4/mcm) phase transition and at ~32, ~34 K due to a transition to a polar phase (tetragonal I4/mcm to orthorhombic Ic2m), respectively. The lower transition temperatures obtained by us are in excellent agreement with those reported on the basis of the dielectric studies by Bednorz and Muller, [10] who attributed these to ferroelectric transition. Rietveld analysis of the diffraction profiles of the polar phase reveals off-centre displacements of both Sr2+/Ca2+ and Ti4+ ions in the X-Y plane along <110> pseudocubic directions, in agreement with the experimentally reported direction of easy polarization by Bednorz and Muller, but the resulting dipole moments are shown to be ferrielectrically coupled in the neighbouring (001) planes along the [001] direction leading to anomalously low values of the spontaneous polarization at 12K.

cond-mat.mtrl-sci↗