High-repetition rate XUV laser for trARPES

Until recently, there have been two branches in the implementation of trARPES, one based on high repetition rate laser systems providing high counting statistics but limited probe photon energies, which limits the accessible range in the Brillouin zone, and another based on probing with extreme ultraviolet (XUV) light generated by high harmonic generation employing high power laser systems of limited repetition rate.

In order to bridge this technology gap, we developed a 0.5 MHz XUV laser source based on high-harmonic generation (HHG) driven with the output of a femtosecond parametric laser amplifier. Its output is frequency-doubled to yield 10 µJ pulses centered at 400 nm wavelength. These pulse are focused into a jet of Ar atoms where frequency up-conversion occurs through HHG. The 7th harmonic of the 400 nm (photon energy: 3.1 eV) pulses is filtered by a combination of XUV mirror and transmission filter and focused on the sample to trigger photoemission. The XUV laser pulses (photon energy: ~21.7 eV) have a pulse duration of approximately 20 fs and a bandwidth of 100 meV.

Scheme of the trARPES experiment.

High-power OPCPA laser system

Our approach of generating XUV pulses at 0.5 MHz repetition rate requires high-average power laser system providing sufficient pulse energy for high-harmonic generation. We developed an optical parametric chirped-pulse amplifier (OPCPA) pumped with a hybrid fiber-slab laser system as front end, see Figure 1. This laser provides spectrally tunable, both in terms of central wavelength and bandwidth, femtosecond laser pulses with average output power exceeding 15 W (pulse energy: >30 µJ). The OPCPA output has a long-term power stability of 0.3% and and a spectral stability of 0.2%.

We employ two schemes for frequency up-conversion:

Deep-ultraviolet (DUV) pulses in the photon energy range 6.0-6.4 eV are generated by frequency-quadrupling in cascaded frequency-mixing in nonlinear crystals. The spectral bandwidth of these pulses support sub-30 fs pulse durations.

Extreme-ultraviolet (XUV) pulses of approximately 20 eV photon energy (wavelength: 60 nm) are generated by high harmonic generation in a jet of noble gas. This process is driven by focusing either the fundamental OPCPA output or its second harmonic to intensities in the range 10¹³-10¹⁴ W/cm².

Further reading:

• M. Puppin, Y. Deng, O. Prochnow, J. Ahrens, Th. Binhammer, U. Morgner, M. Krenz, M. Wolf, and R. Ernstorfer:
  500 kHz OPCPA delivering tunable sub-20 fs pulses with 15 W average power based on an all-ytterbium laser.
  Opt. Exp. 23 1491-1497 (2015) [doi: 10.1364/OE.23.001491].