Optimization of high-order harmonic brightness in the space and time domains

Brightness of high-order harmonics is optimized in the space domain by profile flattening and self-guiding of intense femtosecond laser pulse and in the time domain by controlling the laser chirp. The profile flattening and self-guiding of the laser pulse propagating through a long gas jet effectively increased the phase-matched harmonic generation volume, thereby obtaining strong harmonics with low beam divergence, and the laser chirp control allowed the generation of spectrally sharp harmonics. At optimized conditions, the 61st harmonic, obtained at 134 Å from Ne, had a brightness of about 1×10¹⁵ W∕cm²∕srad with a beam divergence of 0.5 mrad and a spectral bandwidth of 0.7 Å.