P. A. Naik

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.

Pulsed laser deposition of metal films and nanoparticles in vacuum using subnanosecond laser pulses

A study of silver, chromium, stainless-steel, and indium thin films prepared by subnanosecond laser deposition in vacuum is reported. We compare the laser ablation in vacuum at the weak- and tight-focusing conditions of a Ti:sapphire laser beam and analyze the nanoparticles synthesized in the latter case using absorption spectroscopy, x-ray fluorescence, atomic force microscopy, and scanning electron microscopy. Our results show that the nanoparticle formation can be accomplished using long laser pulses under tight-focusing conditions.

Dependence of soft x-ray conversion on atomic composition in laser produced plasma of gold-copper mix-Z targets

A comparative experimental study of soft x-ray emission from laser-irradiated Au–Cu mix-Z targets of different atomic compositions has been performed. Plasma was produced from planar targets using second-harmonic laser pulses from an Nd:glass laser at a focused intensity of ∼1013 W cm−2. Radiation intensity in the spectral region ∼15–150 A and integrated x-ray yield for mix-Z target were observed to be higher than those for individual elements. The maximum conversion occurred for an atomic composition of Au 0.43–Cu 0.57. These observations are consistent with the variation of Rosseland mean opacity with atomic composition calculated using a screened hydrogenic average atom model.

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.

Nano-ripple formation on different band-gap semiconductor surfaces using femtosecond pulses

Nano-ripple formation from ultrashort laser pulse irradiation of semiconductors of different band gaps has been studied using a Ti-sapphire laser with 8 mJ energy, 45 fs pulse duration and 800 nm wavelength (1.5eV) at a fluence in the range of ∼100 mJ/cm2 −1J/cm2. The effects of the number of laser shots, angle of incidence, laser polarization, fluence, incident laser wavelength, bandgap, and ambient medium on the ripple period, have been studied. Depending upon the experimental parameters nano-ripple sizes varied in the range of λ-λ/9. The studies clearly show that narrower nano-ripples are formed from wide bandgap semiconductors. In addition, the width of the nano-ripples decreases with the laser wavelength and fluence. The observed results are explained considering the transient metallic nature of the semiconductor surface on irradiation with intense femtosecond pulse which excites surface plasmon leading to the nano-ripple formation. The critical role of the surface plasma electron density in deciding ripple period is identified which helps in generation of narrow sub-wavelength nano-ripples.Nano-ripple formation from ultrashort laser pulse irradiation of semiconductors of different band gaps has been studied using a Ti-sapphire laser with 8 mJ energy, 45 fs pulse duration and 800 nm wavelength (1.5eV) at a fluence in the range of ∼100 mJ/cm2 −1J/cm2. The effects of the number of laser shots, angle of incidence, laser polarization, fluence, incident laser wavelength, bandgap, and ambient medium on the ripple period, have been studied. Depending upon the experimental parameters nano-ripple sizes varied in the range of λ-λ/9. The studies clearly show that narrower nano-ripples are formed from wide bandgap semiconductors. In addition, the width of the nano-ripples decreases with the laser wavelength and fluence. The observed results are explained considering the transient metallic nature of the semiconductor surface on irradiation with intense femtosecond pulse which excites surface plasmon leading to the nano-ripple formation. The critical role of the surface plasma electron density in deciding...

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.

Formation of metal nanoparticles of various sizes in plasma plumes produced by Ti:sapphire laser pulses

In this paper, an experimental study on generation of nanoparticle various sizes using Ti:sapphire laser pulses, is reported. Nanoparticle formation in plasma plumes of metals like silver and copper, expanding in vacuum, has been studied using stretched pulses of 300 ps duration [subnanoseconds (sub-ns)] from a Ti:sapphire laser. It has been compared with the nanoparticle formation (of the same materials) when compressed pulses of 45 fs duration were used under similar focusing conditions. Nanoparticle formation is observed at intensities as high as 2×1016 W/cm2. The structural analysis of the nanoparticle deposition on a silicon substrate showed that, using 45 fs pulses, smaller nanoparticles of average size ∼20 nm were generated, whereas on using the sub-ns pulses, larger particles were produced. Also, the visible light transmission and reflection from the nanoparticle film of Ag on glass substrate showed surface plasmon resonance (SPR). The SPR curves of the films of nanoparticles deposited by femtosec...

Simultaneous measurement of pulse-front tilt and pulse duration of a femtosecond laser beam

Simultaneous single-shot measurements of pulse-front tilt and pulse duration of ultrashort laser pulses are performed using a single-shot autocorrelator. It uses non-collinear second-harmonic generation by overlapping an ultrashort laser pulse with its spatially inverted replica in a KDP crystal. The pulse-front tilt is determined from the rotation angle of the axis of the elliptical shaped autocorrelation trace. Performance of the system is demonstrated from measurements of the pulse duration and known pulse-front tilt introduced in 250 fs (FWHM) transform-limited laser pulses from a CW mode-locked Nd:glass laser.

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.