H. Singhal

Strong enhancement and extinction of single harmonic intensity in the mid- and end-plateau regions of the high harmonics generated in weakly excited laser plasmas.

Strong intensity enhancement or extinction of some single harmonics is observed in high-harmonic generation from 48 fs Ti:sapphire laser pulses propagating through preformed low-excited laser-produced plasmas of various materials (GaAs, Cr, InSb, stainless steel). The intensities of some of the harmonics in the mid- and end-plateau regions vary from ~23-fold enhancement to near disappearance compared with those of the neighboring ones. It is also shown that the observed intensity enhancement (or extinction) can be varied by controlling the chirp of the driving laser radiation.

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.

Study of high-order harmonic generation from nanoparticles

An experimental study on high-order harmonic generation from the interaction of 45 fs Ti:sapphire laser pulses with preformed plasma plumes of metal nanoparticles was carried out. Highly efficient harmonic generation in the range of 9th order to 19th order was observed for Ag nanoparticles. The stability of harmonic generation was enhanced by utilizing special target fabrication techniques and through optimizing the conditions of plasma plume formation. Broadband harmonic generation was observed through the optimization of femtosecond laser intensity and through the use of spectrally broadened laser pulses. The harmonic generation was compared for various target materials (nano and bulk) and for Ag nanoparticle targets prepared from different fabrication techniques. Efficient generation of even- and odd-order harmonics was observed through the use of two-colour pulses. The observations can be explained qualitatively from symmetry breaking of high-order harmonic generation through the introduction of second harmonic pulses. The spectral broadening and shift of harmonic radiation can be understood from the self-modulation of the laser and harmonic radiation in the plasma.

Influence of C60 morphology on high-order harmonic generation enhancement in fullerene-containing plasma

The morphologies of the fullerene targets and the ablated fullerenes to determine the optimal conditions of excitation of the C60-containing targets have been analyzed. The optimization of fullerene-containing plasma conditions allowed the enhanced harmonic generation in these plasmas using laser radiation of different wavelengths, pulse durations, and phase modulation. A comparison between the harmonic generation in single-atom/ion-containing plasmas (using bulk carbon, silver, and indium targets) and fullerene-rich plasma plumes showed better conversion efficiency for the latter medium. The influence of phase modulation of the fundamental radiation in fullerene plasmas on the spectral properties of harmonics has been studied.

Carbon aerogel plumes as an efficient medium for higher harmonic generation in the 40–90 nm range

We have performed a study on the preparation, characterization, and high-order harmonic generation (HHG) of a femtosecond laser in carbon aerogel plasma plumes. A higher conversion of the femtosecond laser radiation into HHG in the range of 40–90 nm is observed, as compared to that in plasma plumes of silver and C60 targets, where the highest conversion efficiency has been previously reported. Next, the application of a two-color pump scheme allowed further enhancement of the harmonic yield compared to a single-color pump scheme in the carbon aerogel, for both odd and even harmonics. The conversion efficiency from the carbon aerogel plasma was estimated at not less than 5×10−5.

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.

Enhanced coherent extreme ultraviolet emission through high order harmonic generation from plasma plumes containing nanoparticles

A new method to calculate the high order harmonic generation from the interaction of ultra-short laser pulses with plasma plumes containing solid nanoparticles has been presented. It is shown that enhancement for a particular harmonic order is almost independent of the nanoparticle size, in agreement with the observed experimental results which are not explainable by the existing theories applicable to high order harmonic generation from gases or gas clusters.

Variation of harmonic spectra in laser-produced plasmas at variable phase of femtosecond laser pulses of different bandwidth

The intensity enhancement of harmonics and change in harmonic profiles have been observed in a few plasma plumes (La, In, Mn) under variable phase modulation of Ti:sapphire laser pulses. It is seen that variation of relative harmonic intensities in the plateau region could be achieved by chirp variation of narrowband (~10 nm bandwidth) radiation. The effect of the self-phase modulation (SPM) of broadband (~20 nm bandwidth) laser radiation on the harmonic emission from nanoparticles, C60, and Ag plasma while passing through a glass slab is also studied. The observation of broadband harmonic emission and redshift in harmonic wavelengths for an unchirped laser pulse are explained in terms of SPM. The observation of blueshift of harmonic radiation with both positively and negatively chirped pulses was attributed to the different effects of SPM on the chirped pulses.

Study of the spatial coherence of high order harmonic radiation generated from pre-formed plasma plumes

A study of the spatial coherence of the high order harmonic radiation generated by the interaction of 45 fs Ti:sapphire laser beam with carbon (graphite) plasma plume has been carried out using Youngs double slit interferometry. It is observed that the spatial coherence varies with harmonic order, laser focal spot size in plasma plume, and peaks at an optimal spot size. It is also observed that the spatial coherence is higher when the laser pulse is focused before the plasma plume than when focused after the plume, and it decreases with increase in the harmonic order. The optimum laser parameters and the focusing conditions to achieve good spatial coherence with high harmonic conversion have been identified, which is desirable for practical applications of the harmonic radiation.

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...