Observation of light driven band structure via multi-band high harmonic spectroscopy

Intense light-matter interactions have revolutionized our ability to probe and manipulate quantum systems at sub-femtosecond time scales, opening routes to all-optical control of electronic currents in solids at petahertz rates. Such control typically requires electric field amplitudes $\sim V/\AA$, when the voltage drop across a lattice site becomes comparable to the characteristic band gap energies. In this regime, intense light-matter interaction induces significant modifications of electronic and optical properties, dramatically modifying the crystal band structure. Yet, identifying and characterizing such modifications remains an outstanding problem. As the oscillating electric field changes within the driving field's cycle, does the band-structure follow, and how can it be defined? Here we address this fundamental question, proposing all-optical spectroscopy to probe laser-induced closing of the band-gap between adjacent conduction bands. Our work reveals the link between nonlinear light matter interactions in strongly driven crystals and the sub-cycle modifications in their effective band structure

Utilization of stimulated Raman excitation and coherent anti-stokes Raman scattering in studies of bond- and mode-selective chemistry

The state resolved photodissociation of rovibrational states of the fundamental symmetric stretch of water, H2O (1,0,0), and of the O-H or O-D stretch vibrations of HOD was studied using laser induced fluorescence and Doppler polarization spectroscopy. The control that the initially selected state exerts over the product state distribution, vector correlations and bond selectivity was demonstrated. The specific energy deposition via the lowest level of vibrational excitation leads to the enhancement of bond breaking and to structured rotational state distributions of the OH fragment which depends on the prepared rotational state of the parent. The correlations are close to the maximum attainable values expected for an idealized orientation where the transition dipole moment of the parent is parallel to the fragment angular momentum and perpendicular to its velocity. The photodissociation of HOD in which the O-H stretch is initially excited, leads to enhanced bond breaking and selective production of OD + H, while when the O-D is excited no enhancement is obtained. These results show that experiments which prepare the parent molecule in a particular state before a second laser dissociates it, provide a powerful means for bond- and mode- slective chemistry and molecular dynamics studies

MODE-SPECIFICITY IN PHOTODISSOCIATION OF FUNDAMENTAL STRETCHINGS OF HOD

Stimulated Raman excitation and coherent anti-Stokes Raman scattering prepare and detect, respectively, HOD molecules with one quantum of vibrational excitation in the O-H or O-D stretch vibrations. An ArF excimer laser at 193 nm promotes vibrationally excited molecules to the first electronic surface A(1B1) where they dissociate to produce OH + D or OD + H fragments. The photodissociation products OD and OH are detected via laser induced fluorescence. The photodissociation cross section of HOD (0,0,l) vibrationally excited molecules with one quantum of vibration in the O-H stretch is greatly enhanced over that for HOD (0,0,0). The dissociation of this vibrationally excited molecule is also very selective, the OD + H yield being 2.5±0.5 times greater than the OH + D yield. In the photodissociation of the O-D stretch, excited HOD (l,0,0), no enhancement of the yield of the fragments is obtained. Also, these results demonstrate that bond cleavage does not necessarily occur on the weakened bond, and they agree with theoretical calculations indicating that the yield of OD and OH fragments depends on the Franck-Condon overlap of the ground state vibrational wavefunction with the continuum wavefunction on the repulsive surface of the upper state. Our results show that even the very lowest possible level of vibrational excitation can be "leveraged" to effect selective bond breaking

Robust enhancement of high harmonic generation via attosecond control of ionization

We demonstrate up to two orders of magnitude enhancements in high harmonic generation efficiency via sub-cycle control and scaling of the ionization rate in a two colour laser field

Robust enhancement of high harmonic generation via attosecond control of ionization

We demonstrate up to two orders of magnitude enhancements in high harmonic generation efficiency via sub-cycle control and scaling of the ionization rate in a two colour laser field