Self-compression of millijoule pulses from a 1.5 μm OPCPA

We demonstrate a four-stage optical parametric chirped-pulse amplification (OPCPA) system delivering carrierenvelope phase (CEP)-stable ∼1.5-μm pulses with energies up to 12.5 mJ before recompression. The system is based on a fusion of femtosecond DPSS Yb technology and a picosecond 100-mJ Nd:YAG pump laser. Pulses with 62-nm bandwidth are recompressed to a 74.4-fs duration close to the transform limit. To show the way toward a terawatt-peak-power single-cycle IR source, we demonstate self-compression of 2.2-mJ pulses down to 19.8 fs duration in a single filament in argon with a 1.5-mJ output energy and 66% energy throughput. Recently, OPCPA has attracted a lot of attention as a promising path toward intensity scaling of few-cycle laser pulses. Intense CEP-stable fewcycle laser pulses have numerous intriguing applications in attosecond science and strong-field physics [1]. In particular, few-cycle high-power IR sources featuring high ponderomotive energy Up ∝ λ2I [2, 3, 4] also open the door to experimental studies of the λ-scaling laws of strongfield physics [5], and high-harmonic generation driven by intense few-cycle IR pulses [4, 6] represents a promising route toward bright coherent X-ray sources with several keV photon energies. Parametric amplification of CEP-stable twocycle IR seed pulses obtained from differencefrequency generation (DFG) to the energy level close to 1 mJ has been demonstrated [2, 3]. However, the inherently low DFG seed energy causes a sizeable superfluorescence background [2] that prevents further energy upscaling. By narrowing the bandwidth of an optical parametric amplifier (OPA), one can optimize the spectral brightness of the seed at the expense of the seed energy, achieve a more uniform saturation across the pulse spectrum, and minimize energy backconversion into the pump. In saturation, however, the parametrically amplified spectra exhibit steep slopes that lead to a poor fidelity of the compressed pulses in the time domain. Recently, Hauri et al. [7] demonstrated that filamentation of ∼55-fs OPA pulses at 2μm in a xenon cell allows the generation of selfcompressed spectrally broadened 17-fs 0.27-mJ pulses. The limited pulse energy available in that experiment implied the use of xenon as a noble gas with the highest nonlinearity. Detailed numerical investigations of self-compression of 2μm laser filaments in gases with a moderate ionization potential (Ip < 20 eV) by Bergé [8] predicted a number of highly attractive features of femtosecond filamentation at longer wavelength including higher filament energies, broader supercontinua with a flatter spectral phase, and the feasibility to reach single-cycle pulse durations as compared to 2-3 cycle durations in the visible. Fig. 1. Scheme of the OPCPA power-amplification stages 3 and 4: AOPDF, acousto-optic programmable dispersive filter; RA, regenerative amplifier; PA, double-pass post amplifier; A, aperture; W1/W2, input/output windows; BP, beam profiler. These fascinating numerical findings motivated us to explore the filamentation aproach with the pulses from a multimillijoule IR OPCPA. The experimental scheme of our fourstage IR OPCPA is depicted in Fig. 1. The frontend of the OPCPA is based on a femtosecond Yb:KGW DPSS MOPA (Pharos, Light Conversion, Ltd.) and two stages of CEP-stable parametric preamplifiers [9]. Adding two OPCPA booster stages 3 and 4 allows us to reach pulse energies above 10 mJ. Amplification stages 2-4 employ type-II KTP crystals (θ = 45.5◦, φ = 0◦) which exhibit a relatively broad bandwidth E-mail: [email protected] E-mail: [email protected] Oliver Muecke 54