Department of Physics

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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 For the case of ultrashort pulses, where the paraxial model is no longer valid, we adopt a subcycle vector beam description of the field.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.
Strong-field ionization and Coulomb explosion of argon clusters by few-cycle laser pulses
(APS, 2010-08) Holkundkar, Amol R.
Energy distributions are measured for ions emitted upon Coulomb explosion of Arn clusters (n=400−900) upon irradiation by intense three-cycle pulses (10 fs) of 800-nm laser light of peak intensity 5×1014Wcm−2. With few-cycle pulses, there is insufficient time for the cluster to undergo expansion; this results in overall dynamics that are significantly different from those in the many-cycle regime. The peak ion energies are much lower than those obtained when 100-fs pulses of the same intensity are used; they are almost independent of the size of the cluster (over the range 400−900 atoms). Ion yields are measured to be larger in the direction that is perpendicular to the laser-polarization vector than along it. Model molecular dynamics calculations are used to qualitatively rationalize this unexpected anisotropy in terms of shielding by a spatially asymmetric electronic-charge cloud within the cluster.
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
Molecular dynamic simulation for laser–cluster interaction
(AIP, 2011-05) Holkundkar, Amol R.
A three dimensional relativistic molecular dynamic model for studying the laser interaction with atomic clusters is presented. The model is used to simulate the interaction dynamics of deuterium, argon, and xenon clusters when irradiated by the short and high intensity laser pulses. The interaction of 82 Å argon cluster by 100 fs, 806 nm laser pulse with the peak intensity of 8 1015 W/cm2 is studied and compared with the experimental results. The maximum ion energy in this case is found to be about 200 keV. Ion energies along and perpendicular to laser polarization direction is calculated and asymmetry along laser polarization direction is detected which is further explained on the basis of charge flipping model. The effect of cluster density on the energetics of the laser–cluster interaction is also being studied, which provides a qualitative understanding of the presence of optimum cluster size for maximum ion energies.
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.
Ultra-intense laser pulse characterization using ponderomotive electron scattering
(IOP, 2019-12) Holkundkar, Amol R.
We present a new analytical solution for the equation of motion of relativistic electrons in the focus of a high-intensity laser pulse. We approximate the electron's transverse dynamics in the averaged field of a long laser pulse focused to a Gaussian transverse profile. The resultant ponderomotive scattering is found to feature an upper boundary of the electrons' scattering angles, depending on the laser parameters and the electrons' initial state of motion. In particular, we demonstrate the angles into which the electrons are scattered by the laser scale as a simple relation of their initial energy to the laser's amplitude. We find two regimes to be distinguished in which either the laser's focusing or peak power are the main drivers of ponderomotive scattering. Based on this result, we demonstrate how the intensity of a laser pulse can be determined from a ring-shaped pattern in the spatial distribution of a high-energy electron beam scattered from the laser. We confirm our analysis by means of detailed relativistic test particle simulations of the electrons' averaged ponderomotive dynamics in the full electromagnetic fields of the focused laser pulse.
Wakefield generation in magnetized plasmas
(APS, 2011-09) Holkundkar, Amol R.
We consider wakefield generation in plasmas by electromagnetic pulses propagating perpendicular to a strong magnetic field, in the regime where the electron cyclotron frequency is equal to or larger than the plasma frequency. Particle-in-cell simulations reveal that for moderate magnetic field strengths previous results are reproduced, and the wakefield wave number spectrum has a clear peak at the inverse skin depth. However, when the cyclotron frequency is significantly larger than the plasma frequency, the wakefield spectrum becomes broadband, and simultaneously the loss rate of the driving pulse is much enhanced. A set of equations for the scalar and vector potentials reproducing these results are derived, using only the assumption of a weakly nonlinear interaction.
Thomson scattering in high-intensity chirped laser pulses
(AIP, 2015-10) Holkundkar, Amol R.
We consider the Thomson scattering of an electron in an ultra-intense laser pulse. It is well known that at high laser intensities, the frequency and brilliance of the emitted radiation will be greatly reduced due to the electron losing energy before it reaches the peak field. In this work, we investigate the use of a small frequency chirp in the laser pulse in order to mitigate this effect of radiation reaction. It is found that the introduction of a negative chirp means the electron enters a high frequency region of the field while it still has a large proportion of its original energy. This results in a significant enhancement of the frequency and intensity of the emitted radiation as compared to the case without chirping.
Transition from wakefield generation to soliton formation
(APS, 2018-04) Holkundkar, Amol R.
It is well known that when a short laser pulse propagates in an underdense plasma, it induces longitudinal plasma oscillations at the plasma frequency after the pulse, typically referred to as the wakefield. However, for plasma densities approaching the critical density, wakefield generation is suppressed, and instead the EM-pulse (electromagnetic pulse) undergoes nonlinear self-modulation. In this article we have studied the transition from the wakefield generation to formation of quasi-solitons as the plasma density is increased. For this purpose we have applied a one-dimensional relativistic cold fluid model, which has also been compared with particle-in-cell simulations. A key result is that the energy loss of the EM-pulse due to wakefield generation has its maximum for a plasma density of the order 10% of the critical density, but that wakefield generation is sharply suppressed when the density is increased further.
Role of radial nonuniformities in the interaction of an intense laser with atomic clusters
(AIP, 2008-01) Holkundkar, Amol R.
A model for the interaction of an intense laser with atomic clusters is presented. The model takes into account the spatial nonuniformities of the cluster as it evolves in time. The cluster is treated as a stratified sphere having an arbitrary number of layers. Electric and magnetic fields are obtained by solving the vector Helmholtz equation coupled with one-dimensional Lagrangian hydrodynamics. Results are compared with the uniform density nanoplasma model. Enhancement in the amount of energy absorbed is seen over the uniform density model. In some cases the absorbed energy increases by as much as a factor of 40.
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.
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.
Proton acceleration by circularly polarized traveling electromagnetic wave
(APS, 2012-09) Holkundkar, Amol R.
The acceleration of charged particles, producing collimated monoenergetic beams, over short distances holds the promise to offer new tools in medicine and diagnostics. Here, we consider a possible mechanism for accelerating protons to high energies by using a phase modulated circularly polarized electromagnetic wave propagating along a constant magnetic field. It is observed that a plane wave with dimensionless amplitude of 0.1 is capable to accelerate a 1 keV proton to 386 MeV under optimum conditions. Finally, we discuss possible limitations of the acceleration scheme.
Regulating the higher harmonic cutoffs via sinc pulse
(IOP, 2020) Holkundkar, Amol R.; Bandyopadhyay, Jayendra N
We theoretically investigate the generation of higher harmonics and the construction of a single attosecond pulse (ASP) by means of two oppositely polarized sinc-shaped driver pulses. In comparison to a few-cycle Gaussian pulse of the same energy, here we observe a significant broadening in the bandwidth of the XUV/soft x-ray supercontinuum spectrum in the synthesized pulse. Furthermore, we observe that the harmonic cutoff and its corresponding intensity follow a well-defined scaling with the delay parameter between the two pulses. In principle, this delay can easily be tuned on an optical bench. The typical nature of the synthesized pulse ensures the generation of a single ASP instead of a pulse train. In this case, we obtain a single ASP with a duration of ∼27 attoseconds in the XUV/soft x-ray regime of the electromagnetic spectrum. Depending on the delay parameter we observe an enhancement in some satellite harmonics. The proposed setup promises a highly tunable source of energetic photons, wherein the energy of the photons can easily be controlled from the XUV to the soft x-ray regime by simply changing the delay between two oppositely polarized sinc-pulses.
Polarization control of attosecond pulses using bi-chromatic elliptically polarized laser
(IOP, 2021) Holkundkar, Amol R.; Bandyopadhyay, Jayendra N
We study the high harmonic generation (HHG) using elliptically polarized two-color driving fields. The HHG via bi-chromatic counter-rotating laser fields is a promising source of circularly polarized ultrashort XUV radiation at the attosecond time scale. The ellipticity or the polarization of the attosecond pulses (APs) can be tweaked by modifying the emitted harmonics' ellipticity, which can be controlled by varying the driver fields. A simple setup is used to control the polarization of the driving fields, which eventually changes the ellipticity of the APs. A well-defined scaling for the ellipticity of the AP as a function of the rotation angle of the quarter-wave plate is also deduced by solving the time-dependent Schrödinger equation in two dimensions. The scaling can further be explored to obtain the APs of the desired degree of polarization, ranging from linear to elliptical to circular polarization.
Particle-in-cell simulations of electron spin effects in plasmas
(CUP, 2013-02) Holkundkar, Amol R.
We present a particle-in-cell code accounting for the magnetic dipole force and for the magnetization currents associated with the electron spin. The electrons are divided into spin-up and spin-down populations relative to the magnetic field, where the magnetic dipole force acts in opposite directions for the two species. To validate the code, we study wakefield generation by an electromagnetic pulse propagating parallel to an external magnetic field. The properties of the generated wakefield are shown to be in good agreement with previous theoretical results. Generalizations of the code to account for other quantum effects are discussed.
Laser induced neutron production by explosion of the deuterium clusters
(AIP, 2014-01) Holkundkar, Amol R.
The high energy deuterium ions serve as compact source of neutrons when fused with either deuterium or tritium atoms. In view of this, the explosion of the deuterium clusters under the influence of the laser pulse with intensity ranging from 1015 to 1019 W/cm2 is being studied along with the effect of the cluster radius and inter-cluster distance. The objective of this article is to study the efficiency of the deuterium cluster as a compact source of neutrons under various laser and cluster parameters. It is being observed that the cluster density (number of clusters per unit volume) is quite important to gain high neutron yield.
Parallel implementation of three-dimensional molecular dynamic simulation for laser-cluster interaction
(AIP, 2013-11) Holkundkar, Amol R.
The objective of this article is to report the parallel implementation of the 3D molecular dynamic simulation code for laser-cluster interactions. The benchmarking of the code has been done by comparing the simulation results with some of the experiments reported in the literature. Scaling laws for the computational time is established by varying the number of processor cores and number of macroparticles used. The capabilities of the code are highlighted by implementing various diagnostic tools. To study the dynamics of the laser-cluster interactions, the executable version of the code is available from the author.
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.
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 (