Strong-Field Excitation of Liquid and Solid Xe Using Intense Femtosecond Pulses

Abstract

Strong-field excitation of condensed xenon, both solid and liquid, are investigated using focused 60 fs pulses at 400 and 800 nm. Both wavelengths lead to efficient generation of excitons, which are monitored through their VUV emissions. The self-trapped Xe2* excitons are observed in both solid and liquid Xe; and although weaker, emission from the free Xe* excitons is also detected in the crystalline samples. The photoionization mechanism, which ultimately leads to the creation of excitons through electron−hole recombination, is investigated through power dependence measurements of fluorescence and transmission. At 400 nm, the ionization proceeds through the multiphoton mechanism, while at 800 nm, field-induced tunneling ionization prevails. It is observed that the ionization process is self-limited, preempting the possibility of dielectric breakdown. The failure of an electron avalanche process to develop is understood to arise from the small scattering cross-section of electrons in condensed Xe whereby the field-driven electron energy distribution localizes at the deep scattering minimum, near 0.7 eV. Additionally, the achievable field intensities are limited by beam divergence, due to the negative refraction in the generated electron plasma. This is established in the liquid-phase samples by transmission measurements through a limiting aperture. In the solid state, damage triggered by defects limit the achievable irradiation intensities. It is estimated that exciton densities of ∼1018 cm-3 are reached (at pump intensities of 1012 W/cm2). Although amplification of stimulated emission can be expected at the achieved number densities, no evidence of gain is found in the on-axis spectral profile of the excitonic emission.