Transformation of strong picosecond pulses in radiation with an extended quasirotational spectrum during self-focusing in high-pressure hydrogen

Abstract

Radiation with a spectrum representing a discrete analog of the extended spectrum observed in the generation of a supercontinuum in gases is generated in the self-focusing of 30-ps pulses with a wavelength of 1.06 μ]m in hydrogen at pressures up to 120 atm. The spectrum contains lines with similar intensities, an average frequency spacing approximately equal to the rotational transition frequency in hydrogen (587 cm−1), and a smooth spatial profile. The lines consist of several vibrational-rotational components. As the pressure is increased, the spectral lines are transformed so that at a pressure above 60 atm each line in the spectrum contains one or two components formed as a result of the smaller number of cascade (rotational and vibrational) processes. Self-focusing is manifested in the occurrence of a radiating channel up to 12 cm in length. The formation of a channel of this length is associated mainly with the variation of the refractive index in vibrational excitation of the hydrogen molecules by electrons heated in the pump field.