Single-stage GaSe OPCPA delivering high-energy few-cycle pulses at 11 µm wavelength - INVITED

. The generation of sub-five optical cycle pulses centered at 11.2 µm wavelength with 50 µJ energy at a 1 kHz repetition rate is reported. A GaSe optical parametric chirped pulse amplifier (OPCPA) is driven by the residual 2.0 µm pump and 5 µm idler of a high-energy midwave-IR OPCPA. The latter serves as driver for hard X-ray generation and this makes the achieved fs longwave-IR pulses available for X-ray pump-probe experiments.


Introduction
Ultrafast X-ray diffraction is a proven technique for studying femtosecond structural dynamics at atomic length scales.Using the compact laser-driven hard X-ray sources established in recent years, time-resolved X-ray absorption and diffraction studies became available on a laboratory scale [1].The use of high-energy few-cycle pulses with a long optical period allow for accelerating electrons from metal targets to very high kinetic energies resulting in high X-ray photon flux.This scaling concept was impressively demonstrated recently using pulses at a 5-µm driver wavelength with 3 mJ energy and 85 fs duration at a repetition rate of 1 kHz [2].In order to perform time-resolved measurements, a pump-probe line was implemented, whereby pulses at different wavelengths are required as pump for the X-ray pumpprobe experiments.The setup of our midwave-IR (MWIR) optical parametric chirped pulse amplifier (OPCPA) used as driver a hard X-ray source is shown in Fig. 1.So far, a part of the 5 µm driver pulses has been used as a pump for the X-ray pump-probe experiments.The availability of pump pulses in the longwave-IR (LWIR) with sufficient energy would enable the intended investigation of low-frequency excitations.
In order to generate wavelengths in the spectral range beyond 10 µm, nonlinear frequency conversion methods are used.These are difference frequency generation (DFG) or optical parametric amplification (OPA).A challenge, however, is the limited availability of nonlinear crystals that exhibit a high damage threshold and are transparent to the pump and idler wavelengths.As a result, the development of laser systems with microjoule energy and femtosecond duration has stagnated in the last two decades [3][4][5].Only very recently, pulse energies around 10 µJ at 12 µm with sub-200 fs duration were achieved via DFG in gallium selenide [6,7].
Here we report on the implementation of fs pump pulses beyond 10 µm in our system by adding a single-stage GaSe OPCPA that delivers few-cycle idler pulses with 50 µJ energy.

Experimental setup and results
The architecture of our MWIR OPCPA system, which operates at a repetition rate of 1 kHz, is described in [8].The three-color front-end is based on a femtosecond Er:fiber laser, which provides the seeds for the pump and signal at 2.05 µm and at 3.4 µm wavelength, respectively.A Ho:YLF chirped pulse amplifier is used as pump, which emits up to 50 mJ energy in 6 ps long pulses at 2.05 µm.The parametric amplification takes place in four stages, in each of which a high nonlinear ZnGeP 2 crystal is implemented (Fig. 1).The parametric amplifier exhibits high pump-to-idler conversion efficiency of >10% resulting in 3.4 mJ idler pulses centered at 4.9 µm wavelength.The recompressed idler pulses have a duration of 85 fs corresponding to only sub-five optical cycles.
The system is expanded to provide ancillary emissions beyond 10 µm by adding a LWIR OPCPA (Fig. 1).The latter is also pumped at 2.05 µm using the residual pump energy after the fourth stage of the MWIR-OPCPA.For creating the signal pulses for the LWIR OPCPA, second-harmonic generation (SHG) of the 4.9 µm MWIR OPCPA emission is required.For this purpose a small part (about 10%) of the idler pulse energy is applied.GaSe is used as the nonlinear crystal for generating the SH of 4.9 µm.The LWIR OPCPA signal pulses produced in this way exhibit a bandwidth of 100 nm (FWHM) centered at 2.4 µm with energy of 40 µJ.Prior to parametric amplification these pulses are stretched to a duration of 1.5 ps in bulk sapphire.
For the generation of the LWIR pulses, a single parametric amplifier stage is implemented, which contains a 2 mm thick GaSe crystal.The latter was positioned for Type-II phase matching because it supports a broader bandwidth than Type-I.To prevent damage of GaSe, the maximum pump intensity is limited to 30 GW/cm 2 .Taking into account the 7-mm diameter of the GaSe crystal a maximum pump energy of 4 mJ can be applied.Idler pulses with 70 µJ energy are generated in the collinear amplification stage which corresponds to a notable pump-to-idler conversion efficiency of 1.7%.The idler output spectrum extends from 10.2 to 12.9 µm at 1/e 2 -level with the center at 11.2 µm (Fig. 2a) corresponding to a Fourier limit of 140 fs.Recompression is performed in anti-reflection-coated ZnSe windows which is accompanied with 28% loss.From the SH frequency-resolved optical gating (FROG) measurement of the 11-µm pulses, a duration of 180 fs was derived (Fig. 2b) which corresponds to less than five optical cycles.Taking into account the compressed pulse energy of 50 µJ, it translates into a remarkable peak power of 0.3 GW.

Conclusion
The generation of high-energy pulses at a center wavelength of 11.2 µm with sub-five optical cycles was demonstrated.These parameters were achieved in a single-stage GaSe OPCPA at 1 kHz repetition rate.With the unprecedented pulse energy of 50 µJ, a peak power of 300 MW is realized, which represents a record value.Comparable results beyond 10 µm have only been reported for a three-stage LWIR OPCPA so far [9].An additional pump wavelength for planned fs X-ray pumpprobe experiments was thus successfully implemented.