Andrea Burzo

Multiphoton ionization of molecular hydrogen by single-cycle pulses

Multiphoton ionization, first observed by N.B. Delone in 1965, has been a subject of intense studies ever since. In this paper we consider multiphoton ionization of molecules in the limit of subopticalcycle pulse duration. Moreover, we study the regime where the molecules are first prepared in a coherent vibrational superposition state, and then are subjected to sub-cycle laser pulses synchronized with respect to the phase of the coherent molecular motion. The present approach is based on the Keldysh formalism, which assumes that the final free electron’s state is much more sensitive to the pulse than the bound initial wavefunction [1]. We find that the ionization rate depends not only on the sub-cycle shape of the laser pulses, but also on the time delay between the arrival of pulses and molecular motion.

Absolute phase measurement for broadband collinear Raman generation

We use collinear Raman generation in cooled hydrogen and obtain a wide comb of equidistant frequencies spanning over an octave of bandwidth. We adjust the frequencies to be commensurate and observe an f-to-2f interference.

Interplay of molecular modulation technique and stimulated raman scattering for generation of ultra-broadband radiation

Simultaneous rotational and vibrational Raman generation was observed in cooled deuterium gas. Strong vibrational generation leads unexpectedly to efficient rotational generation, while under different conditions the vibrational generation may suppress a self-starting stimulated rotational generation.

Femtosecond-scale response of semiconductors to laser pulses

We report simulations of the response of InSb, GaAs, and Si to 70-femtosecond laser pulses of various intensities. In agreement with the experiments of Mazur and coworkers, and other groups, there is a nonthermal phase transition for each of these semiconductors above a threshold intensity. Our simulations employ tight-binding electron-ion dynamics (TED), a technique which is briefly described in the text. In the experimental pump-probe observations, the dielectric function (epsilon) ((omega) ) and the second-order susceptibility (chi) (2) can be measured. These same quantities can be calculated during a TED simulation, and there is good agreement in the behavior with respect to both time and frequency. The simulations provide much additional microscopic information which is experimentally inaccessible: for example, the time-dependence of the atomic pair-correlation function, electronic energy bands, occupancies of excited states, kinetic energy of the atoms, and excursions of atoms from their initial positions.