SPECTRAL BLUE SHIFTING OF A FEMTOSECOND LASER-PULSE PROPAGATING THROUGH A HIGH-PRESSURE GAS

Plasma-induced spectral blue shifting in rare gases has been investiga ted with a subpicosecond KrF excimer laser focused to a peak intensity in the region of 10(14) W/cm2 (adiabaticity parameter in the range 8 < gamma < 10). The quiver energy of a free electron under these condit ions is sufficiently small to ensure that ionization occurs solely by optical-field-induced processes. Blue shifts as large as 2 nm have bee n observed, and the blue-shifted spectrum shows an interferencelike os cillatory structure. Experimental results are compared with numerical simulations to show that the blue-shifted spectra are the result of pl asma-induced self-phase modulation and can be modeled qualitatively by assuming tunneling ionization and plane-wave pulse propagation. The s tructure in the spectrum is closely related to that observed in earlie r experiments on self-phase modulation in quite different systems.