Papers by Ravindra Kumar G
Applied Physics B, Jul 1, 2003
We study forward stimulated Raman emission from weakly fluorescent dye 4'diethylamino-N-methyl-4-... more We study forward stimulated Raman emission from weakly fluorescent dye 4'diethylamino-N-methyl-4-stilbazolium tosylate (DEST) in 1,2,dichloroethane solution excited by a 28 ps, 532 nm Nd: YAG laser. Neat 1, 2, dichloroethane emits the first Stokes line at 631 nm with a spectral width of 1.6 nm corresponding to a Raman shift of 2956 cm -1 . We observe reduction of spectral width with the addition of DEST in 1, 2, dichloroethane solution. The single pass conversion efficiency for forward Raman emission is as high as 20% in a 1 cm path length sample. The pulse duration of forward stimulated Raman emission measured by a third order autocorrelation technique is 10 ps in neat 1, 2, dichloroethane, whereas it is ~3 ps for 4 x 10 -5 mol/l of DEST solution.
Optics Letters, Jul 21, 2021
We demonstrate the highest efficiency (∼80%) second harmonic generation (SHG) of Joule level, 27 ... more We demonstrate the highest efficiency (∼80%) second harmonic generation (SHG) of Joule level, 27 femtosecond, high contrast pulses in a type-I lithium triborate (LBO) crystal. In comparison, potassium dihydrogen phosphate (KDP) gives a maximum efficiency of 26%. LBO thus offers high intensity (>10 19 W/cm 2 ), ultra-high contrast femtosecond pulses, which have great potential for high energy density science particularly with nanostructured targets as well as technological applications.
Physical Review Letters, Feb 3, 2010
We present direct measurements of the absolute energy distribution of relativistic electrons gene... more We present direct measurements of the absolute energy distribution of relativistic electrons generated in intense, femtosecond laser interaction with a solid. Cherenkov emission radiated by these electrons in a novel prism target is spectrally dispersed to obtain yield and energy distribution of electrons simultaneously. A crucial advance is the observation of high density electron current as predicted by particle simulations and its transport as it happens inside the target. In addition, the strong sheath potential present at the rear side of the target is inferred from a comparison of the electron spectra derived from Cherenkov light observation with that from a magnet spectrometer.
arXiv (Cornell University), Sep 12, 2019
We demonstrate pump-probe techniques, namely the Doppler spectrometry and the reflectometry in de... more We demonstrate pump-probe techniques, namely the Doppler spectrometry and the reflectometry in detail, which directly capture the time-resolved ultrafast evolution of high intensity femtosecond laser-driven hot, dense plasma. These techniques are capable of capturing ultrafast plasma dynamics on time scales of sub 100 femtosecond. We have shown the dynamics of high intensity femtosecond laser-driven shock like disturbance into the plasma at densities more than 10 22 cm -3 . This can help understanding the physics related to shock ignition, supernova explosion and many other astrophysical scenarios and also can have implications in medicine and chemistry. Furthermore, we have investigated the ultrafast acoustic phenomena due to hydrodynamics inside an expanding hot, dense plasma in its transient phase by the correlated measurements of Doppler spectrometry and reflectometry.
arXiv (Cornell University), Jan 3, 2001
The hard x-ray emission in the energy range of 30 -300 keV from copper plasmas produced by 100 fs... more The hard x-ray emission in the energy range of 30 -300 keV from copper plasmas produced by 100 fs, 806 nm laser pulses at intensities in the range of 10 15 -10 16 W cm -2 is investigated. We demonstrate that surface roughness of the targets overrides the role of polarization state in the coupling of light to the plasma. We further show that surface roughness has a significant role in enhancing the x-ray emission in the above mentioned energy range.
Bulletin of the American Physical Society, Nov 12, 2013
HIMAN K. KAW, Institute for Plasma Research, Gandhinagar, India -We present experiments on the sp... more HIMAN K. KAW, Institute for Plasma Research, Gandhinagar, India -We present experiments on the spatio-temporal dynamics of megagauss magnetic fields created in solid-density plasmas by intense femtosecond laser pulses. Our results display distinctive signatures of the Kolmogorov -5/3 scaling in the k-spectra of the turbulent magnetic fields. About 50 picoseconds after the incidence of the main interaction laser pulse, the spectrum displays two distinct turbulent regimes, characterized by a -5/3 scaling (for smaller values of k) and a -7/3 scaling (for larger values of k), separated by a spectral "kink." Similar spectral "knees" have been observed previously in Alfven ion cyclotron waves in the turbulent magnetosheath of the earth as well as in turbulent processes in the solar photosphere.
arXiv (Cornell University), Jan 19, 2004
We report experimental evidence of electrostatic inhibition of fast electrons, generated in a hig... more We report experimental evidence of electrostatic inhibition of fast electrons, generated in a highly resistive material upon irradiation with an intense ultra-short (10 16 W/cm 2 , 100 f msec) laser pulse. The experiment involves measurement of temporal evolution of self-generated magnetic pulses using pump-probe polarimetry. A comparison is made between the temporal behaviour of magnetic pulses generated with Aluminum and Glass targets. It is found that in contrast to Aluminium, selfgenerated magnetic pulse decays much faster in glass. This is attributed to the absence of return shielding currents in glass, which results in build up of electrostatic field, which in turn inhibits the movement of fast electrons. Fitting of experimental measurements using a one dimensional model, yields estimate of conductivity of Aluminium and glass, and penetration depth of hot electrons in these materials.
arXiv (Cornell University), Jan 8, 2004
We make a comparative study of hard x-ray emission from 15 µm methanol microdroplets and a plain ... more We make a comparative study of hard x-ray emission from 15 µm methanol microdroplets and a plain slab target of similar atomic composition at similar laser intensities. The hard X-ray yield from droplet plasmas is ≃ 35 times more than that obtained from solid plasmas. A prepulse that is about 10ns and about 5% of the main pulse is essential for hard x-ray generation from the droplets. A hot electron temperature of 36 keV is measured from the droplets at 8×10 14 W cm -2 ; three times higher intensity is needed to obtain similar hot electron temperature from solid plasmas with similar composition. We use 1D PIC simulation to obtain qualitative correlation to the experimental observations.
arXiv (Cornell University), Oct 27, 2021
Using spectral interferometry technique, we measured subpicosecond time-resolved pre-plasma scale... more Using spectral interferometry technique, we measured subpicosecond time-resolved pre-plasma scale lengths and early expansion (< 12 ps) of the plasma produced by a high intensity (2×10 18 W/cm 2 ) pulse with ultra-high contrast (10 -9 ). We measured pre-plasma scale lengths in the range of 3-15 nm. This measurement plays a crucial role in understanding the mechanism of laser coupling its energy to hot electrons and hence important for laser-driven ion acceleration and fast ignition approach to fusion.
Bulletin of the American Physical Society, Oct 30, 2012
In view of the crucial role played by the relativistic mega-ampere electron currents in mediating... more In view of the crucial role played by the relativistic mega-ampere electron currents in mediating the energy transfer to the core of the DT fuel in fast ignition (FI), it is imperative to obtain a direct unambiguous measure of the fast electron transport, laden with the well-known filamentary instabilities. Here, we report the first direct pump-probe Cotton-Mouton polarimetric measurements of the spatial and the temporal evolution of the megagauss magnetic fields, which bear the signature of fast electron transport, generated at the rearside of a petawatt-laser-irradiated solid target in an intensity regime (3 ps pulses at 10 20 -10 21 W/cm 2 ) of direct relevance to FI. The experiment was performed at the Vulcan Petawatt laser facility. The temporal and the spatial evolution of the megagauss magnetic fields will be presented, emphasizing on the filamented beam transport to the rearside of 50 µm thick Al targets, as well as a central hollow in the rearside magnetic field profile in 50 µm thick plastic targets, previously predicted in simulations.
We report the observation of magnetic domain formation and reorientation in a Fe 2 O 3 magnetic t... more We report the observation of magnetic domain formation and reorientation in a Fe 2 O 3 magnetic tape by intense laser induced high magnetic field. Earlier reports have shown that high intensity lasers (femtosecond, terawatt) can create relativistic electron jets in a solid which can generate Mega Gauss magnetic field pulses. Irradiation of such laser pulse on pre-magnetized iron/iron-oxide films creates large domain formation. The magnetic domains are imaged through Magneto-optical imaging technique based on Faraday Effect. We see that the intense laser induced domain formation depends on the laser power and that a threshold laser power is required for nucleating the large domain. With micro-magnetic simulations involving LLG equations we study the magnetization dynamics of the magnetic films under high magnetic pulse. Comparison of the results with the simulations indicates the formation of the observed domain patterns due to the transient effect of the azimuthal magnetic field pulse. We believe our work holds the potential to explore different routes towards efficient ways of encoding magnetic information which may be useful for magnetic memory devices.
arXiv: Plasma Physics, 2017
For finite systems boundaries can introduce remarkable novel features. A well known example is th... more For finite systems boundaries can introduce remarkable novel features. A well known example is the Casimir effect [1, 2] that is observed in quantum electrodynamic systems. In classical systems too novel effects associated with finite boundaries have been observed, for example the surface plasmon mode [3] that appears when the plasma has a finite extension. In this work a novel instability associated with the finite transverse size of a beam owing through a plasma system has been shown to exist. This instability leads to distinct characteristic features of the associated magnetic field that gets generated. For example, in contrast to the well known unstable Weibel mode of a beam plasma system which generates magnetic field at the skin depth scale, this instability generates magnetic field at the scales length of the transverse beam dimension [4]. The existence of this new instability is demonstrated by analytical arguments and by simulations conducted with the help of a variety of P...
AIP Advances, 2021
Table-top, femtosecond lasers provide the highest light intensities capable of extreme excitation... more Table-top, femtosecond lasers provide the highest light intensities capable of extreme excitation of matter. A key challenge, however, is the efficient coupling of light to matter, a goal addressed by target structuring and laser pulse-shaping. Nanostructured surfaces enhance coupling but require “high contrast” (e.g., for modern ultrahigh intensity lasers, the peak to picosecond pedestal intensity ratio >1012) pulses to preserve target integrity. Here, we demonstrate a foam target that can efficiently absorb a common, low contrast 105 (in picosecond) laser at an intensity of 5 × 1018 W/cm2, giving ∼20 times enhanced relativistic hot electron flux. In addition, such foam target induced “micro-optic” function is analogous to the miniature plasma-parabolic mirror. The simplicity of the target—basically a structure with voids having a diameter of the order of a light wavelength—and the efficacy of these micro-sized voids under low contrast illumination can boost the scope of high in...
Journal of Physics: Conference Series, 2016
We report a study on the dynamics of a near-solid density plasma using an ultraviolet (266 nm) fe... more We report a study on the dynamics of a near-solid density plasma using an ultraviolet (266 nm) femtosecond probe laser pulse, which can penetrate to densities of ∼ 10 22 cm −3 , nearly an order of magnitude higher than the critical density of the 800 nm, femtosecond pump laser. Time-resolved probe-reflectivity from the plasma shows a rapid decay (picosecondtimescale) while the time-resolved reflected probe spectra show red shifts at early temporal delays and blue shifts at longer delays. This spectral behaviour of the reflected probe can be explained by a laser-driven shock moving inward and a subsequent hydrodynamic free expansion in the outward direction.
Study of Coherent and Incoherent Plasma Emissions: Role of Plasma Wave Excitation and Breaking
Here we experimentally map the dynamics of electron plasma waves in laser solid interaction. We d... more Here we experimentally map the dynamics of electron plasma waves in laser solid interaction. We do time resolved measurements of second harmonic and hard X-ray generation from interaction of intense ($10^{16} W cm^{-2}$, 100 fs, 800nm) laser with a pre-plasma generated on a solid surface. The parameter space explored in this time resolved study includes variation of scale length, laser
Short laser pulse induced generation of hot electrons and their anomalous stopping in overdense plasmas
Nuclear Fusion, 2005
Evidence of anomalous resistivity for hot electron propagation through a dense fusion core in fast ignition experiments
New Journal of Physics, 2009
Scientific Reports, Oct 7, 2022
Bright, energetic, and directional electron bunches are generated through efficient energy transf... more Bright, energetic, and directional electron bunches are generated through efficient energy transfer of relativistic intense (~ 10 19 W/cm 2), 30 femtosecond, 800 nm high contrast laser pulses to grating targets (500 lines/mm and 1000 lines/mm), under surface plasmon resonance (SPR) conditions. Bi-directional relativistic electron bunches (at 40° and 150°) are observed exiting from the 500 lines/ mm grating target at the SPR conditions. The surface plasmon excited grating target enhances the electron flux and temperature by factor of 6.0 and 3.6, respectively, compared to that of the plane substrate. Particle-in-Cell simulations indicate that fast electrons are emitted in different directions at different stages of the laser interaction, which are related to the resultant surface magnetic field evolution. This study suggests that the SPR mechanism can be used to generate multiple, bright, ultrafast relativistic electron bunches for a variety of applications. High energy particle bunches are a major tool in modern science and technologies 1-4. In recent years ultra-short, high intensity laser pulses have emerged as compact drivers for high energy particle beams from various targets in different phases of matter 1-4. It has however become obvious that further progress in laser driven particle beams requires smart manipulations of laser pulses and/or target properties. One nice example, is the invocation of the enhanced surface electric field created when such lasers impinge on a solid target that has modulated surface structures 5-24. The structures can either be randomly oriented as in nanoparticle 5 or nanotube 6,7 coatings, or somewhat defined like aligned structures like nanorods 8 , nanotubes 9 , microspheres 10,11 or well-defined modulations as in grating structures 11-26. Although all these structures 5-30 contribute to the enhancement of the laser absorption because of increase of interaction area and effects like the lightning rod 31 , grating structures 11-30 were found to generate hotter and more copious flux of electrons via resonant excitation of surface plasmon, the so-called surface plasmon resonance (SPR). The extrapolation of such behaviour to higher intensities is however still to be established because of potential damage to the grating structures by the rising edges and precursors of the ultra-intense laser pulses, which has so far been limited to moderate intensities 14-16. Even after observation of higher energy of accelerated protons 17 and efficient conversion of laser energy to electrons 32-34 (using two-dimensional particle-in-cell simulations) on a grating target at relativistic intensity, it is still unclear whether SPR or local field concentration (similar to nanosize structure) governs the absorption mechanism. The Brunel absorption (vacuum heating) 35 , a well-known mechanism for absorption at high intensity laser pulses, has an optimum incident angle, and is governed by the plasma scale length at target front 36. Such angular dependence also complicates the observation of SPR. Interestingly, periodically modulated/grating targets are found to be efficient for X-rays 12 , electron emission from target surface and front side 15,16,27 , and proton acceleration 17. Also, these targets are explored intensively to generate higher harmonic 18-21,28 , and steady magnetic fields 26. A recent summary of the subject can be found in Ref. 22 .
Plasma Physics and Controlled Fusion, 2021
Structure and transportation of electron vortices in near-critical density plasmas driven by ultr... more Structure and transportation of electron vortices in near-critical density plasmas driven by ultrashort intense laser pulses have been studied by multi-dimensional particle-in-cell simulations. Dimensional features of electron vortices are revealed. In two-dimensional geometry, two electron vortices and a quasi-static magnetic dipole are closely coupled. In three-dimensional geometry, a moving electron vortex ring associated with a closed magnetic ring moves in near-critical density plasmas. Such structure can transport some energy to the region where the laser pulse cannot reach. It is found that the motion of plasma ions makes the vortex magnetic energy dissipate quickly. These studies provide possible connection of electron vortices in nature with laser plasma experiments.
Bulletin of the American Physical Society, 2015
Submitted for the DPP15 Meeting of The American Physical Society Controlling the dynamics of a fe... more Submitted for the DPP15 Meeting of The American Physical Society Controlling the dynamics of a femtosecond laser-driven shock in hot dense plasma AMITAVA ADAK, PRASHANT KUMAR SINGH, GOURAB CHATTERJEE, AMIT D. LAD, P. BRIJESH, G. RAVINDRA KUMAR, Tata Inst of Fundamental Res-We present the dependence of the dynamics of a plasma super-critical layer on the laser intensity contrast in the regime of intense femtosecond laser-solid interaction. Time-resolved pump-probe diagnostics reveal the interplay of inward shock strength and laser contrast of a femtosecond laser at an intensity of 10 18 W cm −2. The measurements show that the pulse with 2 orders of magnitude higher intensity contrast than that with a usual lower contrast one (∼ 10 −5) launches the shock-like disturbance (into the target) having 10 times more speed. This observation is further supplemented by employing an external prepulse (for manipulating the preplasma scale length) which helps to control the inward motion of the critical surface. This opens up the possibility of controlling the inward moving shock disturbance and leads to medical, science and engineering applications.
Uploads
Papers by Ravindra Kumar G