arXiv subjects

Akhilesh Kumar Mishra

Publications and source records attributed to Akhilesh Kumar Mishra.

14 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↗

arXiv2025

Where Should I Study? Biased Language Models Decide! Evaluating Fairness in LMs for Academic Recommendations

Large Language Models (LLMs) are increasingly used as daily recommendation systems for tasks like education planning, yet their recommendations risk perpetuating societal biases. This paper empirically examines geographic, demographic, and economic biases in university and program suggestions from three open-source LLMs: LLaMA-3.1-8B, Gemma-7B, and Mistral-7B. Using 360 simulated user profiles varying by gender, nationality, and economic status, we analyze over 25,000 recommendations. Results show strong biases: institutions in the Global North are disproportionately favored, recommendations often reinforce gender stereotypes, and institutional repetition is prevalent. While LLaMA-3.1 achieves the highest diversity, recommending 481 unique universities across 58 countries, systemic disparities persist. To quantify these issues, we propose a novel, multi-dimensional evaluation framework that goes beyond accuracy by measuring demographic and geographic representation. Our findings highlight the urgent need for bias consideration in educational LMs to ensure equitable global access to higher education.

cs.CL↗

arXiv2025

Statistics of Non-Rayleigh Speckles Generated from Nonlinear Media

We analytically derive an expression for a speckle field's intensity probability density function (PDF) in a nonlinear medium. The analytically driven results are in good agreement with the numerical outcomes. In a focusing nonlinear medium, the local intensity of the speckle is enhanced as manifested through the longer tail of the PDF. In contrast, the local intensity of speckle is reduced in the presence of a defocusing nonlinearity, and the tail of the probability density function also reduces. This change in local intensity of the speckles arises due to the cubic Kerr nonlinearity, which eventually modifies the second-order statistics. Hence, the intensity correlation is altered as per the nature of the associated nonlinearity while the field correlation remains invariant of both types of the nonlinear conditions.

physics.optics↗

arXiv2025

Space-time non-separable dynamics of ultrashort vortex pulse with power exponential spectrum

Ultrashort optical pulses with orbital angular momentum (OAM) owing to their applications in classical as well as quantum domains attract a lot of research attention. The evolution of their spatio-temporal dynamics is of particular interest. In the present work, the spatio-temporal evolution of ultrashort Laguerre Gauss (LG) vortex with power exponential spectrum, also called power exponential (PE) pulse is numerically explored, in dispersive and nonlinear media. The evolution of space-time non-separable ultrashort vortex pulse is specifically addressed. The study reveals the asymmetric spatio-temporal evolution and splitting in the spatio-temporal plane. Compression at the trailing edge of the pulse is observed with increase in the strength of the nonlinearity. The study further reveals that the temporal, spectral, and chirp evolution patterns of the pulse vary uniquely across different spatial regions and these variations depend upon the strength of nonlinearity and propagation distance. In weak nonlinear regime, a redshift in the spectrum is observed as one moves radially outward from the bright caustic of the ultrashort LG vortex, while a blueshifted spectrum emerges as one approaches the phase singularity. Interestingly, as the pulse furthers in the medium, nonlinearity induced spectral shift is witnessed and with stronger nonlinearity, a complex spectral evolution is observed.

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↗

arXiv2024

A Comparative Study of Coherent and Incoherent Drives in Four-Level Quantum Dot-Based Spaser

In this article, we theoretically investigate a spaser (surface plasmon amplification by stimulated emission of radiation), which consists of a spherical silver nanoparticle surrounded by four-level gain medium of quantum dots (QDs). The spaser system is pumped coherently and incoherently with the same excitation rate, and the characteristics of coherent localized surface plasmon (LSP) mode, thus produced, are compared for the two pumping scenarios. We provide a detailed analytical expression for the steady state and show that the incoherent pump is more suitable for the continuous spaser mode. The reason is better understood by studying the temporal evolution of number of LSP (N_n ), where the oscillation of LSP starts early for incoherent drive and relaxes to steady state with a large value of N_n. At a large pump rate, spaser curve shows saturation. In addition, we have found that the resonance peak of spaser field is independent of coherent as well as incoherent pumping, while the peak amplitude of field depends on the pump rate.

cond-mat.mes-hall↗

arXiv2024

Eigenfrequency splitting with EPs order tunability in a coupled triple cavity system

Degeneracies of non-Hermitian Hamiltonian i.e., exceptional points (EPs) of parity-time (PT)-symmetric systems have received considerable research attention due to their various possible applications in optical devices. At EPs, at least two eigenvalues as well as their eigenvector coalesce. Recently, the effect of the eigenfrequency splitting on transfer function near EP was studied for an optical system consisting of two micro ring resonators, which led to complex splitting in PT-symmetric and anti-PT-symmetric sensors. In present work, we propose a simple system of three coupled ring resonators to show real splitting in both PT-symmetric and anti-PT-symmetric parameter domains by exploiting higher-order EPs. We indirectly couple two rings with equal amount of gain and loss via an intermediate neutral ring. This system is then tested for refractive index (RI) sensing by modulating the cladding index and we numerically show a huge enhancement in sensitivity as compared to those reported in previous studies of micro ring resonators. Importantly, the enhancement is found to be of the order of 10^8. Further, we have found that the order of EP can be tuned by perturbating the cladding. The outcomes of this study may set up a wide range of applications in non-Hermitian triplet cavity systems.

physics.optics↗

arXiv2024

Reflection and Transmission of Airy Pulse from Controllable Periodic Temporal Boundary

We numerically investigate the interaction between two Airy pulses propagating at different wavelengths. The periodically varying peak intensity of the soliton that emerges from stronger Airy pulse (pump pulse) leads to the formation of periodic temporal boundary. The relatively weaker Airy pulse (probe pulse) on interaction with this boundary gets partially reflected as well as transmitted. As a result, the probe pulse spectrum splits into two parts -- the reflected pulse spectrum undergoes red shift while transmitted pulse exhibits blue shift. The probe pulse witneses maximum reflection when point of interaction lies on the intensity maxima of the emergent soliton from pump Airy pulse. On the other hand, maximum transmission occurs when probe Airy pulse interacts at the intensity minima of the soliton. The reflection and transmission processes can be manipulated by tuning the time delay between pump and probe Airy pulses. In case of sufficiently intense pump pulse, the temporal boundary mimics the artificial optical event horizon, and the weak probe Airy pulse is completely reflected. This phenomenon is equivalent to the temporal version of total internal reflection. The time delay and truncation parameters play a vital role in the interaction. The results of the study hold potential applications in optical manipulation and temporal waveguiding.

physics.optics↗

arXiv2023

Effect of Electron-Phonon Interactions on Three-Level QD-based Spaser: Linear and Quadratic Potentials

In this article, a spaser (surface plasmon amplification by stimulated emission of radiation) system consisting of a metal nanoparticle surrounded by a large number of quantum dots (QDs) is studied. Usually, the effect of electron-phonon interaction is neglected in the spaser related literature. But some gain media, attributed by the large Raman scattering cross-section, exhibit stronger electron-phonon interaction. No such study has been performed for a QD-based spaser. Hence, it warrants investigation of the same in spaser system. In the present work, we investigate the effects of electron-phonon interaction on a three-level QD-based spaser. We consider two types of interaction potentials, linear and quadratic, and analyze their effects individually. First, we focus on the linear electron-phonon interaction that perturbs the electrons present in the excited state. This yields a periodic steady-state number of localized surface plasmon (LSP). The accompanying analytic solution reveals that the population inversion of the gain medium depends on the linear potential strength (Frohlich constant) but does not affect the threshold of spaser considerably for the given numerical parameters. In addition to the LSP, phonons may be generated during this process, the temporal dynamics of which are also detailed. Initially, the number of phonons exhibit decaying periodic oscillations, whose amplitude depends on the strength of the electron-phonon interaction. Under continuous pumping, at later times, the number of phonons reaches a steady-state value, which may find application in realization of continuous phonon nanolasers. Further, the effect of the quadratic potential is studied phenomenologically by increasing the excited-state decay rate. This results in a large number of LSP and an intense spaser spectrum.

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↗

arXiv2021

Soliton Shedding from Airy Pulses in a Highly Dispersive and Nonlinear Medium

We present a numerical investigation of the propagation dynamics of a truncated Airy pulse in a highly dispersive and nonlinear medium by employing the split-step Fourier transform method and look, in particular, into the effects of fourth order dispersion (FOD) and cubic-quintic-septic nonlinearity on pulse evolution. Presence of FOD cancels the Airy pulse self-acceleration along with eclipsing the oscillatory tail during propagation in the linear regime. Further, we observe soliton shedding at low input pulse power in the presence of cubic and quintic nonlinearity and negative FOD. The emergent soliton exhibits temporal shift and the direction and the extent of the shift depend upon the strengths of cubic and quintic nonlinearities. In the presence of anomalous group velocity dispersion (GVD) with negative FOD, soliton shedding is observed at relatively high input pulse power. The strengths of GVD and nonlinearity play a vital role in the temporal shifting of emergent soliton. Furthermore, we have explored the effects of septic nonlinearity on soliton shedding in different scenarios of nonlinearity and dispersion.

physics.optics↗

arXiv2021

Dynamics of chirped Airy pulse in a dispersive medium with higher-order nonlinearity

Chirp can control the dynamics of the Airy pulse, making it an essential factor in pulse manipulation. Finite energy chirped Airy pulse (FECAP) has potential applications in underwater optical communication and imaging. Hence, it's critical to study the propagation of FECAP. We present a numerical investigation of the propagation dynamics of a FECAP in a dispersive, and highly nonlinear medium. The nonlinearity under study includes self-phase modulation (SPM), self-steepening (SS), as well as intra-pulse Raman scattering (IRS) terms. We have observed soliton shedding and the chirp parameter is demonstrated to have a considerable impact on the pulse dynamics. In particular, the emergent soliton does not propagate in a straight path instead, depending on the sign of the chirp parameter, it delays or advances in the time. Furthermore, it has been established that the chirp can be employed as an alternate control parameter for the spectral manipulation. The results of our study may have implications in supercontinuum generation and for producing tunable sources.

physics.optics↗

arXiv2021

Ultrafast nonlinear pulse propagation dynamics in metal-dielectric periodic photonic architectures

One-dimensional (1D) metal-dielectric (MD) periodic structures take advantage of large refractive index contrast between metal and dielectrics to invoke extremely high nonlinear ultrafast responses of metal. These structures are also special due to their extremely high laser damage threshold. The Bragg like 1D MD structure (Ag/SiO2)4 enables strong optical field confinement with much enhanced nonlinear features as compared to simple metal or single (Ag/SiO2)1 structure. In the present work, the ultrafast nonlinear optical responses of the above structures are investigated via femtosecond broadband optical pump-probe technique. The enhanced nonlinear optical absorption is of reverse saturation of absorption (RSA) nature, resulted due to free-carrier absorption (FCA) and excited-state absorption (ESA) processes. The spectral nonlinearities are closely related to the pump-induced modification of the metal's dielectric functions, which are qualitatively visualized by transfer matrix and two-temperature models. The ultrafast temporal evolution of nonlinear absorption clearly demonstrated enhanced optical nonlinearity, disentangled by the electron-electron and electron-phonon dynamic interactions at picosecond time scales. A phenomenological pulse propagation model is employed that incorporates the experimentally obtained nonlinear absorption coefficients and different nonlinear effects exhibited by the system. Nonlinearity plays a crucial role in controlling the ultrafast pulse propagation and could open a new window for many nonlinear device applications. The findings of these new optical materials could possibly pave the way for promising applications in ultrafast photonics.

physics.optics↗

arXiv2016

Coherent control in room-temperature quantum dot semiconductor optical amplifiers using shaped pulses

We demonstrate the ability to control quantum coherent Rabi-oscillations in a room-temperature quantum dot semiconductor optical amplifier (SOA) by shaping the light pulses that trigger them. The experiments described here show that when the excitation is resonant with the short wavelength slope of the SOA gain spectrum, a linear frequency chirp affects its ability to trigger Rabi-oscillations within the SOA: A negative chirp inhibits Rabi-oscillations whereas a positive chirp can enhance them, relative to the interaction of a transform limited pulse. The experiments are confirmed by a numerical calculation that models the propagation of the experimentally shaped pulses through the SOA.

physics.optics↗