High-order harmonic generation (HHG) in carbon-nanotube (CNT)-containing plasma plumes has been demonstrated. Various targets were ablated to produce the plasma plumes containing nanotubes for the HHG in these media. Harmonics up to the 29th order were generated. Odd and even harmonics were generated using a two-color pump. The integrity of CNTs within the plasma plume, indicating nanotubes as the source of high-order harmonics, was confirmed by structural studies of plasma debris.

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.

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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 Young's 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.

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Soft x-ray spectra of the toroidal grating monochromator (TGM) at the reflectivity beamline of Indus-1 synchrotron source are analyzed for higher harmonic contribution. A diffraction grating of central line spacing 1200 l/mm is used to disperse the monochromatic beam received from TGM to quantify the harmonic contents in the 50-360 Å wavelength range. In order to calculate the harmonic contamination, conventionally the intensity of higher order peak is divided by first order peak intensity of the desired wavelength. This approach is found to give wrong estimate as first order peak itself is overlapped by higher order peaks. In the present study, a modified approach has been proposed to calculate harmonic contamination where the intensity contributions of overlapping orders have been removed from the first order diffraction peak of the desired wavelength. It is found that the order contamination in the TGM spectra is less than 15% in the wavelength range of 90-180 Å. The total harmonic contribution increases from 6%-60% in the wavelength range of 150-260 Å. The critical wavelength of Indus-1 is 61 Å hence the harmonic contamination below 90 Å is significantly low. The results obtained with modified approach match well with those obtained by quantitative analysis of multilayer reflectivity data. The obtained higher harmonics data are used to fit the transmission of aluminum edge filter in the 120-360 Å wavelength range.

Multicolor radiography is used for the characterization of atoms, ions, and molecules of carbon plasma plumes formed by focusing a 200 ps Ti:sapphire laser pulse on a solid graphite target. The radiography of the plume was carried out using a high order harmonic generation based radiation source consisting of the 11th (λ ≈ 72.7 nm) to 21st (λ ≈ 38 nm) odd harmonic orders of the Ti:sapphire laser pulse. The density profile of CI, CII, and carbon dimer molecule (C2) is estimated from the 2D-transmission profile of the harmonics, recorded after passing through the carbon plume. The peak densities of CI, CII, and C2 at a 50 ns delay are estimated to be 8 × 1024 m−3, 4 × 1024 m−3, and 3.5 × 1023 m−3 at distances of 150 μm, 170 μm, and 120 μm away from the target surface, respectively. The expansion speed of the plasma plume front is estimated to be 2 × 104 m/s and the speed of the C2 molecule to be 4 × 103 m/s at a laser intensity of 1011 W cm−2. The present study demonstrates ultrafast multicolor radiography as a simple and versatile tool for a simultaneous estimation of the density profile of neutral atoms, ions, and molecular species of the plasma plume.

The role of nanosecond duration pre-pulse pedestal (Amplified Spontaneous Emission (ASE) pre-pulse) in the propagation of 45 fs, 4 TW Ti:Sapphire laser pulse through a helium gas jet target has been investigated. We observed that the pre-pulse pedestal of about 1 ns duration and intensity 3 × 1012 W/cm2 creates pre-formed plasma with optical guiding channel like structure in the gas-jet at density around 3 × 1019 cm−3. Guiding of the 45 fs laser pulse (IL = 3 × 1018 W/cm2) in the pre-formed plasma channel, over a distance much longer than the Rayleigh length was also observed. The guiding of the laser pulse resulted in the generation of high energy electron beam by laser wake-field acceleration of self-injected electrons. The accelerated electron beam was quasi-monoenergetic with peak energy up to 50 MeV, low divergence in the range of 3–6 mrad, and bunch charge up to 100 pC.