Browsing by Author "Holkundkar, Amol R."

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Bi-chromatic driver enabled control of high harmonic generation in atomic targets
(IOP, 2023-04) Holkundkar, Amol R.
We investigated the high-order harmonic generation from interacting time-delayed, linearly polarized bi-chromatic laser pulses with the atomic target. The frequency ratio of secondary to primary fields ($\omega_2/\omega_1$), along with the carrier-envelope-phase (CEP) and respective time-delay between the two pulses, are very instrumental in controlling the ionization of the electron and so the quantum dynamics in the continuum. We observed an optimum CEP (φ) and time delay ($t_\mathrm{d}$) for a given bi-chromatic frequency ratio, giving maximum cutoff energy. We studied the effect of CEP and time delay for a fixed frequency ratio and observed that the harmonic cutoff could be controlled in an experimentally feasible way. Moreover, the harmonic yield in the energy range 140 eV–240 eV was also calculated, and it was observed that the harmonic yield scales as $\propto -\phi^{5/6}$. The attosecond pulses for different CEP were also calculated by superposing the harmonics in the same energy range, and ${\sim}200$ pulses were observed with peak intensity decreasing with CEP, following the variation of respective harmonic yield with CEP
Characterization of ultra-intense laser in radiation damping regime using ponderomotive scattering
(IOP, 2022-02) Holkundkar, Amol R.
We present a novel approach to analyzing phase-space distributions of electrons ponderomotively scattered off an ultra-intense laser pulse and comment on the implications for the thus conceivable in-situ laser-characterization schemes. To this end, we present fully relativistic test particle simulations of electrons scattered from an ultra-intense, counter-propagating laser pulse. The simulations unveil non-trivial scalings of the scattered electron distribution with the laser intensity, pulse duration, beam waist, and energy of the electron bunch. We quantify the found scalings by means of an analytical expression for the scattering angle of an electron bunch ponderomotively scattered from a counter-propagating, ultra-intense laser pulse, also accounting for radiation reaction (RR) through the Landau–Lifshitz (LL) model. For various laser and bunch parameters, the derived formula is in excellent quantitative agreement with the simulations. We also demonstrate how, in the radiation-dominated regime, a simple re-scaling of our model's input parameter yields quantitative agreement with numerical simulations based on the LL model.
Chirp assisted ion acceleration via relativistic self-induced transparency
(AIP, 2018-10) Holkundkar, Amol R.
We study the effect of the chirped laser pulse on the transmission and associated ion acceleration by the sub-wavelength target. In the chirped laser pulses, the pulse frequency has a temporal variation about its fundamental frequency, which manifests to the temporal dependence of the critical density (nc). In this work, we used a chirp model which is beyond the linear approximation. For negatively (positively) chirped pulses, the high (low) frequency component of the pulse interacts with the target initially followed by the low (high) frequency component. The threshold plasma density for the transmission of the pulse is found to be higher for the negatively chirped laser pulses as compared to the unchirped or positively chirped pulses. The enhanced transmission of the negatively chirped pulses for higher densities (6nc) results in very efficient heating of the target electrons, creating a very stable and persistent longitudinal electrostatic field behind the target. The void of the electrons results in expansion of the target ions in either direction, resulting in the broad energy spectrum. We have introduced a very thin, low density (
Complete characterization of ultra-intense laser pulses in radiation damping regime
(ARXIV, 2020-10) Holkundkar, Amol R.
We report the first closed, analytical expression for the scattering angle of an electron bunch ponderomotively scattered from a counter-propagating, ultra-intense laser pulse, also accounting for radiation reaction (RR). The found formulation depends nontrivially on the laser intensity, pulse duration, beam waist, and energy of the electron bunch. For various laser and bunch parameters the proposed formula is in excellent quantitative agreement with full, relativistic test particle simulations in a realistic electromagnetic field configuration of a focused laser pulse. We also demonstrate how in the radiation dominated regime a simple rescaling of our model's input parameters yields excellent quantitative agreement with numerical simulations based on the Landau-Lifshitz model. Finally, we discuss how the model can be applied for an in-situ characterization of current and future ultra-high power laser systems, but also to experimentally probe fundamental properties of RR during ultra-intense laser electron interaction.
Control over the secondary collision of electron in high-order harmonic generation
(IOP, 2024-06) Holkundkar, Amol R.
We investigated the high-order harmonic generation by interacting linearly polarized laser pulses with the atomic target. The temporal evolution of harmonic emission and the underlying mechanisms of rescattering electrons are thoroughly investigated through a combination of quantum analysis and classical trajectory simulations. The manipulation of the carrier-envelope phase (CEP) provides a promising avenue for controlling electron recollisions, revealing a systematic linear relationship between ionization and recombination times across varying CEP values. Moreover, examining phase properties in emitted harmonics during secondary collisions presents intriguing modulations, offering a potential experimental approach to verify the presence of secondary recollisions.
Controlling resonant enhancement in higher-order harmonic generation
(ARXIV, 2021) Holkundkar, Amol R.; Bandyopadhyay, Jayendra N.
We present a method to tune the resonantly enhanced harmonic emission from engineered potentials, which would be experimentally feasible in the purview of the recent advances in atomic and condensed matter physics. The recombination of the electron from the potential dependent excited state to the ground state causes the emission of photons with a specific energy. The energy of the emitted photons can be controlled by appropriately tweaking the potential parameters. The resonant enhancement in high-harmonic generation enables the emission of very intense extreme ultra-violet or soft x-ray radiations. The scaling law of the resonant harmonic emission with the model parameter of the potential is also obtained by numerically solving the time-dependent Schrödinger equation in two dimensions.
Determining the duration of an ultra-intense laser pulse directly in its focus
(Springer, 2019-12) Holkundkar, Amol R.
Ultra-intense lasers facilitate studies of matter and particle dynamics at unprecedented electromagnetic field strengths. In order to quantify these studies, precise knowledge of the laser’s spatiotemporal shape is required. Due to material damage, however, conventional metrology devices are inapplicable at highest intensities, limiting laser metrology there to indirect schemes at attenuated intensities. Direct metrology, capable of benchmarking these methods, thus far only provides static properties of short-pulsed lasers with no scheme suggested to extract dynamical laser properties. Most notably, this leaves an ultra-intense laser pulse’s duration in its focus unknown at full intensity. Here we demonstrate how the electromagnetic radiation pattern emitted by an electron bunch with a temporal energy chirp colliding with the laser pulse depends on the laser’s pulse duration. This could eventually facilitate to determine the pulse’s temporal duration directly in its focus at full intensity, in an example case to an accuracy of order 10% for fs-pulses, indicating the possibility of an order-of magnitude estimation of this previously inaccessible parameter.
Dispersion of the laser pulse through propagation in underdense plasmas
(ARXIV, 2018-11) Holkundkar, Amol R.
The propagation of the laser pulses in the underdense plasma is a very crucial aspect of laser-plasma interaction process. In this work, we explored the two regimes of laser propagation in plasma, one with a0<1 and other with a0≳10. For a0<1 case, we used a cold relativistic fluid model, wherein apart from immobile ions no further approximations are made. The effect of the laser pulse amplitude, pulse duration, and plasma density is studied using the fluid model and compared with the expected scaling laws and also with the PIC simulations. The agreement between the fluid model and the PIC simulations are found to be excellent. Furthermore, for a0≳10 case, we used the PIC simulations alone. The delicate interplay between the conversion from the electromagnetic field energy to the longitudinal electrostatic fields results in the dispersion and so the red-shift of the pump laser pulse. We also studied the interaction of the dispersed pulse (after the propagation in underdense plasma) with the sub-wavelength two-layer composite target. The ions from the thin, low-density second layer are found to be efficiently accelerated to ∼70 MeV, which is not found to be the case without dispersion.
Effect of initial plasma density on laser induced ion acceleration
(AIP, 2008-12) Holkundkar, Amol R.
The effect of initial plasma density on the energetics of the laser accelerated ions is studied using one dimensional particle in cell simulations. It is observed that the initial plasma density plays an important role in the generation of high energy particles. In the case of a spatially constant initial density, there exists an optimum value for the maximum ion acceleration. Similarly for the case of a density ramp, an optimum value of ramp length exists for the maximum ion acceleration. At a laser intensity of ⁠, a maximum energy of about 1 GeV is seen with an optimum initial density ramp
Effect of laser pulse time profile on its absorption by argon clusters
(CUP, 2011-07) Holkundkar, Amol R.
The interaction of medium sized Argon clusters (30 Å) with high-intensity femtosecond laser pulses (806 nm, 8 × 1016 W/cm2) of durations ranging from 10 fs to 120 fs have been studied using a three-dimensional relativistic time dependent molecular dynamic approach. The dynamics of cluster expansion is explained in terms of temporal evolution of electron population in the cluster and snapshots of particle positions at various times. The effects of inter-cluster distance on ionization dynamics are presented. It is observed that the collisional ionization increases with decreasing inter-cluster distance. The effect of pulse duration on laser energy absorption is also studied. For a laser pulse of gaussian time profile, there exists an optimum pulse duration for maximum absorption. No such optimum exists for a nearly flat top (super-gaussian) laser pulse. Results indicate the existence of resonance absorption inside the cluster. It is also observed that the high energy component of ion emission from cluster is anisotropic, showing a preferential direction of emission along laser polarization while the low energy ions emerge almost isotropically.
Effect of Radial Density Profile on Resonance Absorption in Laser-Cluster Interaction
(Wiley, 2009-07) Holkundkar, Amol R.
The spatial non-uniformities in the hydrodynamic parameters of an expanding plasma in laser-cluster interaction plays an important role in determining the region of resonance absorption. It is shown that, in the case of uniform density, the surface resonance at three times the critical density is responsible for laser absorption. However, it is volume resonance at critical density responsible for the laser absorption, if the non-uniform nature of plasma density inside the hydrodynamically expanding cluster is taken into account (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Effects of cluster size and spatial laser intensity distribution on fusion neutron generation by laser driven Deuterium clusters
(ARXIV, 2016-02) Holkundkar, Amol R.
three dimensional molecular dynamic code is used to study the generation of fusion neutrons from Coulomb explosion of Deuterium clusters driven by intense near infra-red (NIR) laser (λ=800nm) of femtosecond pulse duration (τ=50fs) under beam-target interaction scheme. We have considered various clusters of average sizes (⟨R0⟩=80,140,200Å) which are irradiated by a laser of peak spatial-temporal intensity of 1×1018W/cm2. The effects of cluster size and spatial laser intensity distribution on ion energies due to the Coulomb explosion of the cluster are included by convolution of single cluster single intensity ion energy distribution function (IEDF) over a range of cluster sizes and laser intensities. The final convoluted IEDF gets broadened on both lower and higher energy sides due to this procedure. Furthermore, the neutron yield which takes into account the convoluted IEDF, also gets modified by a factor of ∼2 compared to the case when convolution effects are ignored.
Efficient ion acceleration by relativistic self-induced transparency in subwavelength targets
(Springer, 2016-11) Holkundkar, Amol R.
We studied the effect of target thickness on relativistic self-induced transparency (RSIT) and observed that, for subwavelength targets, the corresponding threshold target density (beyond which the target is opaque to an incident laser pulse of given intensity) increases. The accelerating longitudinal electrostatic field created by RSIT from the subwavelength target is then used to accelerate the ions from a thin, low density layer behind the main target to ~100 MeV using a 6 cycle flat-top (with rise and fall of one cycle each) circularly polarized laser with peak dimensionless amplitude of 20. A suitable scaling law for optimum laser and target conditions is also deduced. We observed that, as far as energy spectrum is concerned, an extra low density layer is more advantageous than relying on target ions alone.
Energetic electron bunch generation by laser interaction with xenon clusters
(AIP, 2018-10) Holkundkar, Amol R.
We study the interaction of intense, sub-cycle, and few-cycle laser pulses with xenon clusters for the generation of mono-energetic electron bunches. For this purpose, we used three dimensional, relativistic, molecular dynamics simulations. In this work, we used two mutually perpendicularly polarized (MPP) pulses separated by a finite temporal phase delay. The first pulse is responsible for the generation of electrons by field ionization of atomic clusters. However, the second pulse tends to accelerate the electrons (created by the first pulse) as a bunch. The effect of phase delay, pulse duration, and peak laser intensity on the generation of energetic electron bunches is studied. Under optimum conditions, the electrons are found to be accelerated to energies as high as 2.5 MeV. The feasibility of further acceleration of these electron bunches utilizing laser wakefield acceleration is also explored in this work by treating the accelerated electron bunch by MPP pulses as an initial condition to the nonlinear one-dimensional laser wakefield equations. The rough estimate of the final accelerated electron energies after laser wakefield acceleration has also been made
Engineering harmonic emission through spatial modulation in a Kitaev chain
(APS, 2025-09) Bandyopadhyay, Jayendra N.; Holkundkar, Amol R.
We investigate high-harmonic generation (HHG) in a dimerized Kitaev chain. The dimerization in the model is introduced through a site-dependent modulating potential, determined by a parameter 𝜆∈[−1:1] . This parameter also determines the strength of the hopping amplitudes and tunes the system's topology. Depending upon the parameter 𝜆 , the HHG emission spectrum can be classified into three segments. The first segment exhibits two plateau structures, with the dominant one resulting from transitions to the chiral partner state, consistent with quasiparticle behavior in the topological superconducting phase. The second segment displays multiple plateaus, where intermediate states enable various transition pathways to higher conduction bands. Finally, the third segment presents broader plateaus, indicative of active interband transitions. In the 𝜆≤0 regime, we observe the midgap states (MGSs) hybridize with the bulk, suppressing the earlier observed harmonic enhancements. This highlights the key role of the intermediate states, particularly when MGSs are isolated. These results demonstrate that harmonic emission profiles can be selectively controlled through the modulating parameter 𝜆 , offering new prospects for tailoring HHG in topological systems.
Focusing effects in laser-electron Thomson scattering
(APS, 2016-09) Holkundkar, Amol R.
We study the effects of laser pulse focusing on the spectral properties of Thomson scattered radiation. Modeling the laser as a paraxial beam we find that, in all but the most extreme cases of focusing, the temporal envelope has a much bigger effect on the spectrum than the focusing itself. For the case of ultrashort pulses, where the paraxial model is no longer valid, we adopt a subcycle vector beam description of the field. It is found that the emission harmonics are blue shifted and broaden out in frequency space as the pulse becomes shorter. Additionally the carrier envelope phase becomes important, resulting in an angular asymmetry in the spectrum. We then use the same model to study the effects of focusing beyond the limit where the paraxial expansion is valid. It is found that fields focussed to subwavelength spot sizes produce spectra that are qualitatively similar to those from subcycle pulses due to the shortening of the pulse with focusing. Finally, we study high-intensity fields and find that, in general, the focusing makes negligible difference to the spectra in the regime of radiation reaction.
Generation of ultraintense proton beams by multi-ps circularly polarized laser pulses for fast ignition-related applications
(AIP, 2011-05) Holkundkar, Amol R.
A scheme of generation of ultraintense proton beams relevant for proton fast ignition (PFI) which employs multi-ps, circularly polarized laser pulse irradiating a thick (≥ 10 μm) H-rich target is proposed and examined using one-dimensional particle-in cell-simulations. It is shown that a 5-ps laser pulse of intensity ∼ (2–5) × 1020W/cm2 irradiating the target of the areal proton density ∼ 2 × 1020cm−2 can produce – with a high energetic efficiency – a proton beam (plasma block) of parameters (intensity, energy fluence, pulse duration, proton energy spectrum) close to those required for PFI. At a fixed total laser energy, the proton beam parameters can be controlled and fitted to the PFI requirements by changing the laser intensity (energy fluence) and/or the target thickness as well as by using a shaped (curved) target inserted into a guiding cone.
Harmonic emission as a probe to coherent transitions in the topological superconductors
(2025-07) Bandyopadhyay, Jayendra N.; Holkundkar, Amol R.
We investigate the dynamical behavior of a topological superconducting system, demonstrating that its static configuration undergoes a transition driven by an intrinsic supercurrent. By analyzing the band population, we confirm the quasiparticle nature of the system both in the presence and absence of an external laser field. Under laser driving, we observe an enhancement in static emission forming a plateau-like structure, accompanied by multiple coherent transitions in the population. These transitions exhibit Rabi-like oscillations, attributed to the presence of Majorana bound states (MBS), further reinforcing the quasiparticle character of the model. Our results highlight the efficacy of laser driving as a probe of the system's topological and dynamical stability.
High-order harmonic generation by sub-cycle laser pulses and associated scaling laws
(Elsevier, 2023-02) Holkundkar, Amol R.; Bandyopadhyay, Jayendra N.
We studied the high-harmonic generation by the interaction of sub-cycle laser pulses with the He atom. The sub-cycle pulses are modeled using the complex source vector beam model, which is an exact solution to Maxwell's equations and accurately models the sub-cycle field profiles. We observed that the harmonic cutoff could be extended with the variation of the sub-cycle pulse duration, mainly because of the inherent blueshift associated with shorter pulses. The scaling laws for the harmonic yield and harmonic cutoff energy with pulse duration and fundamental driver wavelengths are also deduced. Furthermore, a detailed wavelet analysis of harmonic generation by sub-cycle pulses is carried out. The appropriate filtering and superposition of the harmonics gave rise to a single attosecond pulse of duration ∼100 as.
Higher harmonics and attosecond pulse generation by laser induced Thomson scattering in atomic clusters
(APS, 2019-08) Holkundkar, Amol R.
The generation of higher harmonics of intense lasers and associated attosecond pulses is a field of contemporary interest which promises a variety of applications ranging from the fundamental to applied sciences. In this work, we have probed the interaction of the intense (≳1019 W/cm2) 248 nm laser with Deuterium clusters using classical molecular dynamics simulation. The Thomson scattered radiation emitted by the electrons is considered by using standard Liénard-Wiechert potentials. We have studied the angular distribution of the radiation emitted by electrons and observed that the ponderomotive force exerted by these highly intense laser pulses leaves a very distinct signature of the radiated energy along a particular direction, which in principle has its own diagnostic potential to directly measure the intensities of incident laser pulses. Furthermore, the interaction of lasers with intensities ∼1019–1021 W/cm2 with atomic clusters results in the attosecond burst of energy in form of electromagnetic radiations, which fall under the XUV to soft x-rays regime of electromagnetic spectrum. The parameters of the atomic clusters, e.g., size (number of atoms), atomic species, etc. can be easily controlled experimentally and these in turn, change the number of electrons participating in the interaction process and hence, the properties of Thomson scattered radiation can be tuned accordingly.