We report the compression of intense, carrier-envelope phase stable mid-IR pulses down to few-cycle duration using an optical filament. A filament in xenon gas is formed by using self-phase stabilized 330 J 55 fs pulses at 2 m produced via difference-frequency generation in a Ti:sapphire-pumped optical parametric amplifier. The ultrabroadband 2 m carrier-wavelength output is self-compressed below 3 optical cycles and has a 270 J pulse energy. The self-locked phase offset of the 2 m difference-frequency field is preserved after filamentation. This is to our knowledge the first experimental realization of pulse compression in optical filaments at mid-IR wavelengths ͑Ͼ0.8 m͒. © 2007 Optical Society of America OCIS codes: 190.5530, 320.5520. Progress in strong-field physics has been accelerated by the development of lasers operating near the 0.8 m wavelength that feature high peak power, few-cycle duration, and reliable control over the carrier-envelope phase 1 (CEP). Furthermore, the fundamental scaling laws 2,3 governing the intense laseratom interaction suggest that the advancement of longer-wavelength mid-IR laser sources capable of similar optical quality will have a major impact in strong-field physics. The most compelling examples include the generation of shorter attosecond x-ray bursts and the rescattering of electrons at kilovolt energies. [3][4][5] A recently demonstrated 80 J, 2 m prototype system 6 based on optical parametric chirped-pulse amplification via difference-frequency generation defines a standard for future development of longwavelength drivers. However, the optical parametric chirped-pulse amplification architecture is faced with important technical challenges, 7 such as the need for specific pump laser design and unwanted generation of parasitic fluorescence underlying the primary pulse for high parametric gain configurations. 6 Currently, femtosecond optical parametric amplifiers (OPAs) pumped by multimillijoule Ti:sapphire chirped-pulse amplification systems can deliver multicycle pulses in the mid-IR with sufficient peak power to investigate the efficacy of the nonlinear pulse compression techniques developed at shorter wavelengths. In particular, optical filaments formed in a noble gas by intense 0.8 m pulses have demonstrated pulse compression down to the few-cycle regime with excellent beam stability and spatial mode quality. 8This Letter demonstrates, for the first time to our knowledge, the self-compression in an optical filament of high-peak-power mid-IR pulses derived by difference-frequency generation in a Ti:sapphire pumped OPA. This efficient scheme produces fluorescence-free, sub-3 optical cycle pulses near the 2 m wavelength with 270 J energy at a 1 kHz repetition rate. The intense 2 m field carries a constant CEP offset, thus making it an attractive longwavelength driver for benchmark strong-field experiments.A schematic of the experimental setup is shown in Fig. 1. High-peak-power multicycle mid-IR pulses are produced in a slightly modified traveling-wave OPA (TOPAS, L...

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We report on the generation and measurement of a >10 8 contrast ratio between main pulse and amplified spontaneous emission (ASE) from a relativistic kHz chirped-pulse amplified laser. We have enhanced the ASE contrast ratio as much as >400 times by employing a pulse cleaner composed of a µJ preamplifier and a saturable absorber. A third-order cross-correlator with a dynamic range of >10 9 and a scanning range of up to 4 ns has been developed for the contrast measurement. Detailed analysis of the cross-correlation trace shows that the random noise of spectral phase generates 20-ps pedestal structure starting from 10 −6 level of the main pulse.

Isolated attosecond pulses and electron buncht-s can be efficiently yeiif rated in the interaction of intense lasers with pliismn in the confined voliiine of the }? regime. Scaling with intensitj' is found m improve pulse hre\ ity and focusabilit>-greatly vvhile the efficiency of the attoseeond pulse generation continues to remain high. Practical consideration of the tools needed to generate such pulses indicates that sueh interactions are surprisingly accessible. We mention some introductory experiments whereby we may verify the theoretical predictions of this new class of attosecond pulses. This techni(.|ue may enable us to reach the Schwinger intensity 10^''Wcm~ .