V. Arora

Single-harmonic enhancement by controlling the chirp of the driving laser pulse during high-order harmonic generation from GaAs plasma

We report a study of the enhancement of the intensity of the 27th-harmonic radiation produced during the high-order harmonic generation from the GaAs plasma by controlling the chirp of the fundamental Ti:sapphire laser radiation. The influence of Ga and As ions on the enhancement of the 27th-harmonic radiation is also studied.

Dependence of high order harmonics intensity on laser focal spot position in preformed plasma plumes

The dependence of the high-order harmonic intensity on the laser focal spot position in laser produced plasma plumes is experimentally studied. High order harmonics up to the 59th order (λ∼13.5 nm) were generated by focusing 48 fs laser pulses from a Ti:sapphire laser system in silver plasma plume produced using 300 ps uncompressed laser radiation as the prepulse. The intensity of harmonics nearly vanished when the best focus was located in the plume center, whereas it peaked on either side with unequal intensity. The focal spot position corresponding to the peak harmonic intensity moved away from the plume center for higher order harmonics. The results are explained in terms of the variation of phase mismatch between the driving laser beam and harmonics radiation produced, relativistic drift of electrons, and defocusing effect due to radial ionization gradient in the plasma for different focal spot positions.

X-ray enhancement in a nanohole target irradiated by intense ultrashort laser pulses

In this paper, we present a comparative study of the laser energy absorption, soft x-ray emission (in the water window region: 2.3–4.4 nm) and hard x-ray emission (in the 2–20 keV range) from planar aluminum and nanohole alumina of 40 nm average diameter, when irradiated by Ti:sapphire laser pulses. The laser pulse duration was varied from 45 to 500 fs, and the focused intensity on the target ranged from ∼3 × 1016 W/cm2 to 3×1017 W/cm2. The x-ray yield enhancement from the nanoholes shows an increased coupling of the laser energy to the target. The effect of laser pulse duration on the x-ray emission was also studied, where a resonance like phenomenon was observed. The laser energy absorption measurements in the nanoholes showed a marginal enhancement in absorption as compared to planar Al. The integrated keV x-ray yield, from nanohole alumina and planar Al, at an intensity of 3 × 1017 W/cm2, was 25 and 3.5 μJ, respectively. The results can be explained by considering the hydrodynamic expansion of the las...

Conversion efficiency and spectral broadening of the K-α line emitted from planar titanium targets irradiated with ultra-short laser pulses of high intensity

A study of the conversion efficiency and line shape of the K-α x-ray line radiation from a planar titanium target irradiated by an ultra-short laser pulse is performed. The conversion efficiency and spectral broadening are studied as a function of laser intensity (5 × 1016–1018 W cm−2), laser pulse duration (45 fs–800 fs), and laser fluence (2 × 103–4.2 × 104 J cm−2). The K-α1 line (4510 eV) is observed to be broadened (up to ∼9 eV), predominantly towards the higher energy side and strongly depends on the laser fluence rather than on laser intensity. The reason for the spectral broadening is attributed to K-α emission in warm dense plasma. The role of hot electrons and direct laser heating on spectral broadening is outlined. In addition to this, our observations indicates that the presence of pre-plasma strongly contribute to the observed broadening through the inner-shell transitions in multiply charged titanium ions in the pre-plasma. The appropriate laser irradiation parameters to achieve high conversi...

Study of fast electron jet produced from interaction of intense laser beam with solid target at oblique incidence

Fast electrons generated along target normal direction from the interaction of intense ultrashort Ti:sapphire laser pulses (λ0 = 800 nm) with planar copper target at 45° incidence angle have been experimentally studied under different interaction conditions. Angular spread and energy spectrum of the fast electrons was measured for both p- and s-polarized laser irradiation at intensities in the range 4 × 1016 – 4 × 1017 W/cm2 (for a fixed pulse duration of 45 fs) and for pulse duration in the range 45 fs–1.2 ps (for a fixed laser fluence of 1.8 × 104 J/cm2). The fast electrons were observed consistently along the target normal direction over the entire range of interaction conditions in the form of a collimated jet, within a half cone angle of 20°. The fast electrons have continuous energy spectrum with effective temperature 290 keV and 160 keV, respectively, for p- and s-polarized 45 fs laser pulse irradiation at intensity 4 × 1017 W/cm2. Scaling laws for temperature of fast electrons with laser intensity...

Effect of gold on keV x-ray emission yield from laser produced plasma of gold-copper mix-Z targets

The effect of gold on keV x-ray emission from gold-copper (Au–Cu) mix-Z plasma has been experimentally studied. The intensity of the copper L-shell line radiation (λ∼7.8–10.9A) as well as the integrated keV x-ray yield were observed to decrease sharply with increasing atomic fraction of gold in the mix-Z target. The decrease was observed to be by a factor of ∼2.1 for 0.12 atomic fraction of gold in the Au–Cu mix-Z target with respect to the pure copper target. The results can be explained from physical consideration of the high value of free-bound opacity of gold in the spectral region of the L-shell emission of copper ions and downconversion of the absorbed keV line radiation.

Study of strain propagation in laser irradiated silicon crystal by time-resolved diffraction of K-α x-ray probe of different photon energies

An experimental study on the time resolved x-ray diffraction from laser shocked silicon crystal, carried out using a 10 TW Ti:sapphire laser system, is presented. The characteristic Kα x-ray line radiation generated by 45 fs laser produced plasmas of two different target materials (iron and copper) is used as the probe, whereas the stretched pulse of sub-nanosecond duration (pump), derived from the same laser, is used to compress the sample. The use of x-ray probe of different photon energies yields information about the strain over a greater crystal depth. The dynamics of the strain propagation is inferred by monitoring the evolution of rocking curve width of the shocked sample at different time delays between the pump and the probe pulse. The shock velocity deduced from these measurements is ∼106 cm/s, consistent with the sound velocity in bulk silicon. The maximum elastic compression observed is 0.4%, indicating a pressure of 0.8 GPa.

Angular distribution and dose measurements of hard x-ray emission from intense laser-plasma interaction

An experimental study on hard x-ray Bremsstrahlung radiation due to fast electron generation from a planar solid copper target irradiated by 150 mJ, 45 fs Ti:sapphire laser pulses at I∼1018 W∕cm2 is reported. Angular distribution of the hard x-ray dose rate (hν≥40 keV) is observed to be strongly forward peaked in the direction of target normal with a measured peak value of 40 μSv∕h, at a distance of 500 mm from the target. Two sources of this radiation, one at the laser irradiated target and the other at the chamber wall facing the target, are inferred from the measurements of angular distribution and distance dependence of the x-ray dose rate.

Generation of higher harmonics in laser plasma formed on the surface of a silver target

The results of investigation of the higher harmonics (from the 21st to the 61st order) of radiation of a femtosecond laser (793 nm, 48 fs) that are generated in silver plasma are presented. The changes in the shape and spectra of harmonics for different chirps of the laser radiation are studied. It is found that the wavelengths of harmonics generated in the region of the plateaulike distribution of the converted radiation intensity can be tuned. It is shown that the conditions of phase matching between harmonics and laser radiation are different for different modulations of the femtosecond pulse spectrum. The tuning range of the spectrum of the converted radiation in the region of lower harmonics, which lie in the beginning region of the plateau, is shown to amount to 0.8 nm.

Control of the intensity of a single harmonic generated in laser plasma by varying the chirp of intense femtosecond pulses

A significant increase, as well as a decrease, in the intensity of single higher harmonics of femtosecond radiation of a Ti:sapphire laser that are generated in a weakly excited laser plasma on the surface of GaAs, Cr, InSb, In, and stainless steel is demonstrated. The intensities of some higher harmonics in different regions of their plateaulike distribution varied from a 23-fold excess over the intensity of the neighboring harmonics to the complete absence in the extreme ultraviolet spectrum. The specific features of a 200-fold increase in the intensity of the 13th harmonic generated in indium plasma are analyzed. These variations in the intensity distribution of harmonics in the plateau region are obtained by varying the chirp of laser pulses.