Coupling of Sub-Terawatt Laser Into Hollow Core Waveguide for High-Harmonic Generation Above 200 eV

Abstract

Hollow core waveguides are widely used in extreme field research to guide laser pulses. The laser peak intensity is usually several orders higher than the damage threshold of waveguide cladding material in these applications. The nonlinear drift of the spatial focus of such high power laser with the power or intensity fluctuation can easily destroy the waveguide in an avalanche manner. In this letter, we experimentally investigate the nonlinear drift of focal position and size of a sub-terawatt laser beam with respect to pulse energy and pulse duration. A theoretical model is derived and qualitatively explains the experimental observations. A reliable procedure to couple 3 mJ laser pulses at 27 fs into a hollow core waveguide with 150 $\mu \text{m}$ bore diameter is developed. When the waveguide is filled with Helium, the high harmonics up to 212 eV are generated which confirms that the intense laser pulses with peak intensity of $1.8\times 10 ^{\mathbf {15}}$ W/cm $^{\mathbf {2}}$ are successfully coupled into the waveguide.