Frequency-Tunable DUV Bi-Chromatic Pulse Pairs Directly Generated from NIR Ultrashort Pulses

Our experimental investigations reveal that plasma interactions can segment intense, ultrashort pulses of intermediate soliton order into bi-chromatic tunable frequency pairs. These pairs, referred to as bi-chromatic pulse pairs (BCPP), can be adjusted in frequency through dispersion control. The process begins with the propagation of intense ultrashort pulses, having a central wavelength of around 800 nm, through a 1-meter-long gas-filled hollow circular waveguide with a radius of 70 μm. These pulses, with durations in the range of 14 ± 3 fs and energy levels around 140 ± 30 μJ, possess intensities sufficient to ionize the gas within the first few millimetres of the waveguide. However, the soliton self-compression and resonant dispersive wave generation, as estimated by the fission length, are expected to occur only after several tens of centimetres. The ionization process creates a plasma environment that facilitates soliton-plasma interactions. We generated two individual adjustable frequency components through controlled soliton dynamics and plasma-induced fission mechanisms under anomalous dispersion conditions. The observed wavelength tunability range of the DUV peak was from 225 nm to 270 nm for shorter wavelengths, while for longer wavelengths, it was observed to range approximately from 250 nm to 390 nm. The first ultraviolet pulse corresponds to a conventional resonant dispersive wave (RDW) [1], [2], while the second arises from a plasma-induced RDW [3], triggered by the blue-shifted plasma-induced frequency components. Adjusting the gas pressure in the waveguide allowed precise tuning of the central wavelengths of both bi-chromatic pulse pairs, leveraging the dispersion-dependent phase-matching requirements for RDWs. This behaviour was observed across various nonlinear gaseous media, including argon, nitrogen, and ambient air. As pressure increases, pulses with shorter frequencies exhibit an increase in spectral power density. This spectral power density of BCPPs is influenced by the dispersion and nonlinear characteristics of the gas medium. Experimental spectra (Fig. 1 a) illustrating these results were obtained in an argon-filled hollow waveguide and show bi-chromatic spectral power densities peaked in the UV and DUV. Fig. 1 b. illustrates that by adjusting the waveguide's dispersion through changes in gas pressure, both bi-chromatic pulse pairs shift their central wavelengths to lower frequencies.