J. Jha

Energy pooling in multiple ionization and Coulomb explosion of clusters by nanosecond-long, megawatt laser pulses

We report the results of experiments that establish the possibility of bringing about multiple ionization and Coulomb explosion of molecular clusters with nanosecond laser pulses at intensities as small as 10(9) W cm(-2). We demonstrate several new facets of the laser-cluster interaction in the low-intensity, long-pulse domain: (i) The choice of laser wavelength for a given cluster species is very crucial. (ii) Excited electronic states play a very important role in the ionization dynamics. (iii) When field ionization is insignificant and ponderomotive energies are very small, it is energy pooling rather than inverse bremsstrahlung that determines how clusters absorb energy from the optical field.

Enhancement of x-ray yields from heteronuclear cluster plasmas irradiated by intense laser light

We report a new method to enhance the x-ray emission from nano-cluster plasmas formed upon irradiation by intense femtosecond-duration laser pulses. Our experiments demonstrate that when Ar clusters are doped with H2 Ot he time-integrated yield of Ar K x-ray emission is enhanced by approximately 12fold in comparison to that obtained from pure Ar clusters under otherwise identical experimental conditions. A significant alteration in the timedependent electron density is achieved by the presence of an H2O dopant, and this could be the possible reason for the enhancement that is observed. (Some figures in this article are in colour only in the electronic version)

Hotter electron generation in doped clusters

We present electron energy measurements from nano-cluster plasmas that are formed when molecule-doped rare-gas clusters are irradiated by intense, 100 fs laser pulses of intensity ~1015 W cm−2. In pure Ar clusters the high temperature component (energy ~1400 eV) is less than 1% of the low temperature component (energy ~130 eV), while for water-doped Ar clusters the high temperature component is as high as 7% of the low temperature component. Numerical estimates based on collisional ionization and inverse bremsstrahlung absorption indicate that the easily ionizable dopant molecules enhance the propensity for ionization ignition by significantly altering the temporal profile of the inner-ionized electron density within the cluster.

Engineering clusters for table-top acceleration of ions

Upon irradiation by ultrashort, intense laser light, inert gas clusters show an uncanny ability to absorb nearly all the incident optical energy, and to subsequently disburse this energy by producing very energetic ions and electrons. Practical realization of a table-top accelerator requires such a laser-cluster “ion source” to demonstrate sufficiency in terms of brightness, ion yield, and charge state. We show that by “engineering” the constituents of the cluster, using low ionization energy dopants, it is possible to significantly enhance the high-energy ion yield and ion charge states.

Evolution of dopant-induced helium nanoplasmas

Two-component nanoplasmas generated by strong-field ionization of doped helium nanodroplets are studied in a pump–probe experiment using few-cycle laser pulses in combination with molecular dynamics simulations. High yields of helium ions and a pronounced resonance structure in the pump–probe transients which is droplet size dependent reveal the evolution of the dopant-induced helium nanoplasma with an active role for He shells in the ensuing dynamics. The pump–probe dynamics is interpreted in terms of strong inner ionization by the pump pulse and resonant heating by the probe pulse which controls the final charge states detected via the frustration of electron–ion recombination.

Collisionless phenomena in heteronuclear clusters

Argon K-shell x-ray yields and electron fluxes are measured for water doped Ar clusters as a function of laser pulse width. The high energy electron and x-ray yields are significantly enhanced in doped clusters. We show that doping of the clusters can be used to enhance collisionless absorption.

Ignition of Doped Helium Nanodroplets in Intense Few-Cycle Laser Pulses

The ultra-fast dynamics of He nanodroplets (103–105 atoms) in intense (1…7 ×1014 W/cm2), few-cycle (∼10 fs), infrared (∼790 nm) laser pulses has been investigated as a function of the number of dopant rare-gas atoms, the laser intensity and the rare-gas species. We find the “ignition” behaviour predicted by theory for 20 fs resulting in the complete ionisation and disintegration of the droplet, otherwise entirely transparent, initiated by just a few, less than 5 dopant atoms.