arXiv subjects

Bhupesh Kumar

Publications and source records attributed to Bhupesh Kumar.

16 recordsLinked to original sources

arXiv2026

Topology of Plasma Wakefields Driven by Two Color Laguerre Gaussian Laser Pulses

Plasma wakefield excitation driven by two color Laguerre Gaussian laser pulses carrying orbital angular momentum is investigated analytically and through quasi-cylindrical particle in cell simulations. Using a perturbative framework together with the quasistatic approximation, the influence of the transverse laser mode structure on the longitudinal and transverse wakefields in an underdense plasma is examined in the weakly relativistic regime. The results show that drivers with finite azimuthal index produce reduced and less regular on-axis longitudinal wakefields compared to conventional Gaussian drivers. However, radial longitudinal field distributions reveal that this reduction originates from a redistribution of the wakefield energy toward finite radii rather than a simple loss of wake excitation. Orbital angular momentum carrying modes generate hollow and ring shaped wake structures accompanied by strongly modified transverse electric fields and broader plasma density perturbations. Mixed Gaussian Laguerre Gaussian configurations exhibit intermediate behavior, combining weak on-axis acceleration with pronounced off axis wake excitation. The study demonstrates that structured two-color laser drivers fundamentally modify the topology of plasma wakefields and provide an additional mechanism for controlling transverse plasma dynamics, off-axis acceleration, and angular momentum mediated wakefield structures in plasma based accelerator schemes.

physics.plasm-ph↗

arXiv2026

Chirp-controlled plasma wake excitation by an exponential laser pulse in underdense plasma

The excitation of plasma wakefields driven by chirped laser pulses is investigated using a reduced relativistic fluid Poisson model supported by fully relativistic particle in cell (PIC) simulations. The study considers exponential, linear, quadratic, and unchirped phase-modulated laser drivers propagating in an underdense plasma. Numerical solutions of the governing equations demonstrate that exponential chirping produces enhanced wakefield amplitudes compared to polynomial and unchirped cases due to nonlinear phase variation across the pulse envelope. The analytical predictions are validated using quasi cylindrical PIC simulations performed under identical plasma and laser parameters. The simulations reveal strong chirp dependent wakefield modification, with positively chirped pulses generating peak accelerating fields exceeding 58 GV per m, accompanied by pronounced density compression and enhanced electron momentum gain. These results demonstrate that exponential chirping provides an effective mechanism for controlling wakefield strength and improving plasma based particle acceleration.

physics.plasm-ph↗

arXiv2026

Wakefield amplification via coherent Resonant excitation with two copropagating laser pulses in homogeneous plasma

In the present study, wakefield amplification via coherent resonant excitation using two co propagating laser pulses in a homogeneous plasma is investigated. The proposed scheme is based on linearly polarized leading seed pulse followed by a trailing pulse with identical or controlled parameters, enabling phase synchronized energy transfer to the plasma wave. By systematically varying the temporal pulse widths and inter pulse separation, conditions for resonant enhancement of the wakefield are established. Analytical modelling, supported by particle in cell simulations, reveals that maximum amplification occurs when the pulse separation approaches a quarter of the plasma wavelength, ensuring constructive interference of the plasma oscillations driven by successive pulses. Under optimal conditions, the coherent resonant excitation leads to a significant enhancement of the wakefield amplitude, reaching up to three times of that produced by a single laser pulse. The results demonstrate that precise control of pulse spacing and duration enables efficient energy coupling into plasma waves, providing a robust pathway for enhanced wakefield generation in laser plasma interaction regimes.

physics.plasm-ph↗

arXiv2026

Azimuthally polarized terahertz radiation generation using radially polarized laser pulse in magnetized plasma

An analytical formulation of a radially polarized laser pulse propagating in a homogeneous, magnetized plasma is presented using Lorentz force, continuity and Maxwells equations. Perturbation technique and quasi-static approximation (QSA) have been used to study the generated fields in nonlinear regime. The generated slow, oscillating, transverse electric and magnetic fields having equal amplitude, constitute a radiation field having frequency in the terahertz (THz) range. Particle-in-cell (PIC) simulation code FBPIC is used to validate analytical findings. Simulation studies also show that the generated THz radiation field propagates beyond the plasma boundary, indicating coherent electromagnetic radiation emission. Furthermore, the field amplitude scales nonlinearly with plasma density and increases linearly with external magnetic field strength, highlighting the role of these parameters in controlling radiation amplitude.

physics.plasm-ph↗

arXiv2026

Enhanced wakefield generation in homogeneous plasma via two co-propagating laser pulses

This investigation deals with enhanced plasma wakefield amplitude generated using two co-propagating laser pulses in homogeneous plasma. The configuration consists of a seed pulse followed by a trailing pulse, both linearly polarized and sharing identical laser parameters. The enhancement in wakefield amplitude corresponding to fixed spatial separation is optimized for various pulse widths and intensities of the seed and trailing lasers. Analytical modelling and particle-in-cell simulations reveal that the maximum amplification in wakefield amplitude is obtained when spatial separation equals the plasma wavelength (\lambda_p). The spatial intervals between laser pulses critically influence the wakefield amplification. These findings confirm that the two co-propagating lasers scheme provides a promising route toward stronger plasma wakefield excitation, potentially important for various applications.

physics.plasm-ph↗

arXiv2025

Performance limit of on-chip speckle spectrometers

Disorder-driven, integrated speckle spectrometers offer exceptional spectral resolution within a compact design. They benefit from enhanced optical path lengths due to multiple light scattering events, however, often at the cost of low optical throughput. Here, we investigate the relationship between these two figures of merit by systematically varying the scattering strength of random-uniform disorder distributions. Furthermore, we also investigate the temperature stability of such spectrometers. Our study shows that the device resolution can be tuned from 2 nm to 20 pm, while the operating temperature ranges from 1 to more than 6 degrees and throughput can be varied by more than a factor of 10, paving the way for application-tailored design of microscale high-resolution spectrometers.

physics.optics↗

arXiv2024

Wakefield generation and electron acceleration via propagation of radially polarized laser pulses in homogeneous plasma

The paper presents a study of wakefield generation and electron injection via propagation of radially polarized laser pulses in homogeneous pre-ionized plasma. The analytical study is based on Lorentz force and continuity equations. Perturbation technique and quasi-static approximation are used for evaluating the generated longitudinal wakefields. Trapping and acceleration of electrons are examined by injecting a test electron in the generated wakefields. The results are compared with those obtained via linearly polarized laser pulses. The validation of analytical results is performed using the Fourier-Bessel particle-in-cell (FBPIC) simulation code. It is seen that there is a significant enhancement in amplitude of the longitudinal wakefield generated and electron energy gain via radially polarized laser pulses as compared to linearly polarized laser pulse case.

physics.plasm-ph↗

arXiv2024

Second harmonic generation by radially polarized laser beam propagating in homogeneous plasma

This study presents an investigation of second harmonic generation (SHG) resulting from interaction of radially polarized laser beam propagating in homogeneous, unmagnetized plasma. Lorentz force and continuity equations have been used to derive the radial and axial current density components. Further, using these densities in the wave equation leads to analytical expressions for the SHG field amplitudes. These amplitudes exhibit periodic oscillations along the propagation axis, characterized by detuning length dependent on plasma density and laser parameters. Radial and axial contributions to SHG are found to be highly enhanced near the beam axis due to the Gaussian beam profile of the laser. The analytical findings are validated using Fourier Bessel Particle-In-Cell (FBPIC) simulations. Notably, unlike linearly or circularly polarized beams which require either inhomogeneous or magnetized plasma, radially polarized beams facilitate efficient harmonic generation in homogeneous unmagnetized plasmas.

physics.plasm-ph↗

arXiv2024

Twisted terahertz radiation generation using Laguerre-Gaussian laser pulse propagating in axially magnetized plasma

We present analytical and simulation study of twisted terahertz (THz) radiation generation via propagation of a circularly polarized Laguerre Gaussian (LG) laser pulse in homogeneous plasma embedded in an axial magnetic field. Analytical formulation is based on perturbation technique and quasistatic approximation. Longitudinal and transverse wakefields generated via laser plasma interactions are evaluated using Lorentz force and Maxwells equations in the mildly nonlinear regime. It is observed that two linearly polarized twisted terahertz (THz) radiation beams are generated in mutually perpendicular planes. Superposition of the two beams result in a single linearly polarized twisted THz radiation beam with modified amplitude and polarization direction. Three dimensional (3D) particle in cell (PIC) simulations are performed for this configuration using FBPIC code. Graphical comparison of amplitude of the resultant THz beam obtained via analytical and simulation studies is presented.

physics.plasm-ph↗

arXiv2023

High-throughput speckle spectrometers based on multifractal scattering media

We present compact integrated speckle spectrometers based on monofractal and multifractal scattering media in a silicon-on-insulator platform. Through both numerical and experimental studies we demonstrate enhanced optical throughput, and hence signal-to-noise ratio, for a number of random structures with tailored multifractal geometries without affecting the spectral decay of the speckle correlation functions. Moreover, we show that the developed multifractal media outperform traditional scattering spectrometers based on uniform random distributions of scattering centers. Our findings establish the potential of low-density random media with multifractal correlations for integrated on-chip applications beyond what is possible with uncorrelated random disorder.

physics.optics↗

arXiv2023

Broadband Continuous Spectral Control of a Single Wavelength Polymer-Based Solid-State Random Laser

We demonstrate temperature-controlled spectral tunability of a partially-pumped single-wavelength random laser in a solid-state random laser based on DCM (4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran) doped PMMA (polymethyl methacrylate) dye. By carefully shaping the spatial profile of the pump, we first achieve low-threshold, single-mode random lasing with excellent side lobes rejection. Notably, we show how temperature-induced changes in the refractive index of the PMMA-DCM layer result in a blue-shift of this single lasing mode. Continuous tunability of the lasing wavelength is demonstrated over an 8nm-wide bandwidth.

physics.optics↗

arXiv2022

2D tunable all-solid-state random laser in the visible

A two-dimensional (2D) solid-state random laser emitting in the visible is demonstrated, in which optical feedback is provided by a controlled disordered arrangement of air-holes in a dye-doped polymer film. We find an optimal scatterer density for which threshold is minimum and scattering is the strongest. We show that the laser emission can be red-shifted by either decreasing scatterer density or increasing pump area. We show that spatial coherence is easily controlled by varying pump area. Such a 2D random laser provides with a compact on-chip tunable laser source and a unique platform to explore non-Hermitian photonics in the visible

physics.optics↗

arXiv2022

Impact of Non-Hermiticity and Nonlinear Interactions on Disordered-Induced Localized Modes

If disorder-induced Anderson localized states have been observed experimentally in optics, their study remains challenging leaving a number of open questions unsolved. Among them, the impact on Anderson localization of non-Hermiticity, optical gain and loss, and more generally, nonlinearities has been the subject of numerous theoretical debates, without yet any conclusive experimental demonstration. Indeed, in systems where localized modes have reasonable spatial extension to be observed and investigated, their mutual interaction and coupling to the sample boundaries make it extremely difficult to isolate them spectrally and investigate them alone. Recently, we successfully exhibited localized lasing modes individually in an active disordered medium, using pump-shaping optimization technique. However, a one-to-one identification of the lasing modes with the eigenmodes of the passive system was not possible, as the impact of non-Hermiticity and nonlinear gain on these localized states was unknown. Here, we apply the pump-shaping method to fully control the non-Hermiticity of an active scattering medium. Direct imaging of the light distribution within the random laser allows us to demonstrate unequivocally that the localized lasing modes are indeed the modes of the passive system. This opens the way to investigate the robustness of localized states in the presence of nonlinear gain and nonlinear modal interactions. We show that, surprisingly, gain saturation and mode competition for gain does not affect the spatial distribution of the modes.

physics.optics↗

arXiv2021

Localized modes revealed in Random Lasers

In sufficiently strong scattering media, light transport is suppressed and modes are exponentially localized. Anderson-like localized states have long been recognized as potential candidate for high-Q optical modes for low-threshold, cost effective random lasers. Operating in this regime remains however a challenge since Anderson localization is difficult to achieve in optics and nonlinear mode interaction compromise its observation. Here, we exhibit individually each lasing mode of a low-dimension solid-state random laser by applying a non-uniform optical gain. By undoing gain competition and cross-saturation, we demonstrate that all lasing modes are spatially localized. We find that selective excitation reduces significantly the lasing threshold while lasing efficiency is greatly improved. We show further how their spatial location is critical to boost laser power-efficiency. By efficiently suppressing spatial hole burning effect, we can turn on the optimally-outcoupled random lasing modes. Our demonstration opens the road to the exploration of linear and nonlinear mode interactions in the presence of gain, as well as disorder-engineering for laser applications.

physics.optics↗

arXiv2020

Selective Pumping of Localized States in a Disordered Active Medium

Light scattering and localization in strongly scattering disordered systems is governed by the nature of the underlying eigenmodes, specially their spatial extension within the system. One of the main challenges in studying experimentally Anderson like localized states resides in the difficulty to excite these states independently and observe them individually anywhere in the sample. The modes mutual interaction and their coupling to the sample boundaries makes it extremely challenging to isolate them spectrally and image them alone. In the presence of gain, random lasing occurs above threshold and lasing modes are excited. It was shown recently that shaping the intensity profile of the optical pump is a very effective way to control and force random lasing in singlemode regime at any desired emission wavelength. Active random media therefore offer a unique platform to select modes individually. By using pump shaping technique in a strongly scattering random laser, we successfully select localized lasing modes individually. Direct imaging of the light distribution within the random laser confirms the confined nature of the modes and enables us to elucidate long standing questions on the role of non-Hermiticity in active scattering media and the influence of nonlinearities on Anderson wave localization.

physics.optics↗

arXiv2014

Fatigueless response of spider draglines in cyclic torsion facilitated by reversible molecular deformation

We demonstrate that spider draglines exhibit a fatigueless response in extreme cyclic torsion up to its breaking limit. The well defined Raman bands at $1095$ and $1245 cm^{-1}$ shifted linearly towards lower wavenumbers versus increasing twist in both clockwise and counter-clockwise directions. Under thousands of continuous loading cycles of twist strain approaching its breaking limit, all the Raman bands were preserved and the characteristic Raman peak shifts were found to be reversible. Besides, nanoscale surface profile of the worked silk appeared as good as the pristine silk. This unique fatigueless twist response of draglines, facilitated by reversible deformation of protein molecules, could find applications in durable miniatured devices.

cond-mat.mtrl-sci↗