Toward Terawatt-Peak-Power Single-Cycle Infrared Fields

1.55-μm pulses with up to 12.5-mJ energy are generated from a four-stage KTPOPCPA and recompressed to 79 fs. By focusing these pulses into a noble-gas cell, we demonstrate single 4-mJ supercontinuum filaments supporting 8-fs pulses. ©2009 Optical Society of America OCIS codes: (320.7110) Ultrafast nonlinear optics; (190.4970) Parametric oscillators and amplifiers. The current quest for generating multimillijoule few-cycle IR pulses has been fuelled by intriguing applications in attosecond physics and high-field science [1] and, in particular, by the scientists’ dream to realize bright coherent Xray sources with up to keV photon energies [2,3]. Optical Parametric Amplification (OPA) and Optical Parametric Chirped-Pulse Amplification (OPCPA) have emerged as promising candidates for this task because of their unique optical properties including ultrabroadband gain and absence of thermal load problems. Carrier-envelope phase (CEP)-stable few-cycle IR seed pulses obtained from difference-frequency generation (DFG) can be parametrically amplified to close to the millijoule level [4]. The standard technology for the generation of few-cycle driver pulses based on Ti:sapphire amplifier systems is spectral broadening of millijoule-level femtosecond pulses in noble gases. Recently, Hauri et al. demonstrated that filamentation of longer ~55-fs OPA pulses at 2 μm in a xenon cell allows the generation of 17-fs, 0.27-mJ pulses at 2 μm [5]. This result motivated us to pursue a hybrid OPCPA-filamentation approach to realize a TW-peak-power single-cycle IR source. In general, in a more narrowband parametric amplifier, uniform gain saturation across the pulse spectrum can more easily be achieved, thus avoiding back-conversion into the pump. The resulting parametrically amplified spectra, however, exhibit steep slopes leading to a poor quality of the compressed pulses in the time domain. Subsequent spectral broadening in a gas filament provides low-intensity broad spectral wings essential for few-cycle pulse formation. At present, the energy throughput of gas-phase broadening schemes, such as the hollow-core fiber compressor [6] and filamentation [5], is limited to 4-5 mJ at 800 nm due to ionization losses. In filamentation of intense femtosecond pulses, a self-guiding channel (“filament”) is formed due to the dynamical balance between selffocusing by the nonlinear Kerr effect and defocusing by the plasma created by gas ionization. During the nonlinear pulse propagation, the spectrum is significantly broadened by the Kerr nonlinearity and plasma-induced blueshifting. Since no waveguiding structure is required in filamentation and therefore coupling losses are eliminated, the filamentation technique permits higher energy throughput than the hollow-core fiber compressor. As the critical power of self-focusing scales as λ, we expect to surpass the current energy limitation (4-5 mJ at 800 nm [7]) with a multimillijoule femtosecond pulse generated in an IR OPCPA. Recently, we have reported on multimillijoule optically synchronized and passively CEP-stable OPCPA at 1.5 μm [8]. Our 4-stage IR OPCPA system (see Fig. 1) is based on a diode-pumped solid-state (DPSS) Yb-MOPAdriven CEP-stable front-end [9] and two successive power-amplification stages pumped by a flash-lamp-pumped Nd:YAG amplifier (Ekspla Ltd.). Following the pioneering work of Miller and coworkers [10], we employ (nearly) collinear type-II phase matching in KTP in the amplification stages 2-4 reaching pulse energies up to 12.5 mJ at a 20-Hz repetition rate before a grating compressor. Here, we describe in detail our recent improvements on the power-amplification OPCPA stages 3 and 4 that allowed upscaling of the pulse energy to >10 mJ, and we present first SHG-FROG characterization data of the 20Hz output from the 4-stage OPCPA system. Finally, we discuss what we believe to be the first generation of a 600nm-wide infrared supercontinuum from a multimillijoule filamentation in noble gases starting from ~1.6-μm OPCPA pulses. a366_1.pdf NFA3.pdf © 2009 OSA/IPNRA/NLO/SL 2009 . f