Abstract

We report on a novel coherent soft X-ray source based on high harmonic generation of few-cycle laser pulses provided by a high repetition rate and average power Yb-based fiber laser system. We use a fiber chirped-pulse amplification system incorporating coherent combination of two fiber amplifiers [1] delivering ~210 fs, 1 mJ pulses which are subsequently shortened in two post-compression stages. The first stage consists of a 1 m long hollow-fiber (250 µm diameter) filled with 2.5 bar of argon and a subsequent chirped mirror compressor (total GVD: −1400 fs²). It delivers ~30 fs, 550 µJ pulses which are then coupled to a second compression stage consisting of a 0.5 m long hollow-fiber (250 µm diameter) filled with 8 bar of neon. The pulses are recompressed to 7.8 fs by a pair of ultra-broadband chirped mirrors (total GVD −200 fs²) and 5.3 mm fused silica. The compressed pulse energy of 353 µJ is achieved at up to 150 kHz repetition rate corresponding to an average output power of 53 W [2]. This is the highest average power of any few-cycle laser system so far. Phase-matched and, therefore, efficient HHG into the soft X-ray spectral region has been enabled by the short pulses and by providing the target gas through a 150µm diameter nozzle with adequately high density (backing pressure up to 15 bar for neon and 45 bar for helium).When optimizing the generation conditions for the lower soft X-ray energies in neon, we obtain 3.1·10⁹ photons/s in a 1% bandwidth at 120 eV which represents the highest photon flux generated in this spectral region so far. In helium, when increasing the laser intensity to ~2·10¹⁵ W/cm², we were able to generate high harmonics beyond the C absorption edge (283 eV) with 10⁶ photons/s for harmonics in the water window. The carbon absorption edge is clearly visible when comparing a measurement without (green line in Fig.1 a) and with (blue line in Fig. 1a) an additional 0.5 µm Parylene filter. Moreover, this measurement allows determining fine structure details of the carbon absorption edge with only 20 seconds of integration time demonstrating the unique capabilities of this table-top light source. In a next step, the soft X-ray source will be utilized for table-top few-nm resolution coherent diffractive imaging [3] with potential of state-selective imaging of organic materials.

© 2015 IEEE