We demonstrate a novel pulse-compression technique that uses the self-confinement of two-dimensional spatial solitons propagating in bulk nonlinear media to increase the spectral bandwidth followed by a grating pair for recompression. Output pulses of 19-fs duration with 0.6-,J energies are routinely obtained at a repetition rate of 8.6 kHz. Unlike other high-energy compression methods, soliton compression offers both high repetition rates and a potentially unlimited wavelength range. Femtosecond pulse compression techniques that employ self-phase modulation in an optical fiber to generate spectral bandwidth have developed to the point where it is now possible to generate optical pulses as short as 6 fs.1 However, fiber damage thresholds and parasitic higher-order nonlinear processes typically limit the amount of energy that can effectively be compressed to less than 10 nJ. Applications such as mode-selective excitation of coherent phonons by means of impulsive stimulated Raman scattering 2 and strong-field physics'-' require new methods of compression that produce shortduration optical pulses while maintaining high energies. In recent years progress has been made in extending the energy range of compressed pulses. Efforts by Rolland and Corkum, who used self-phase modulation in bulk materials, have succeeded in generating 100-,J, 24-fs pulses. In this Letter we report on a new method of pulse compression, which produces 19-fs, 0.6-,uJ optical pulses at a repetition rate of 8.6 kHz. Our method relies on the self-trapping and stable propagation of two-dimensional bright spatial optical solitons in bulk nonlinear media. In close analogy with temporal solitons, in which the balancing of group-velocity dispersion and self-phase modulation lead to dispersion-free propagation, 9 the balancing of diffraction by the spatial nonlinear index profile results in diffraction-free propagation.' 0 Although selftrapping of beams in three dimensions is unstable and leads to catastrophic self-focusing, recent experiments have demonstrated the stable propagation of two-dimensional spatial solitons in CS 2 liquid"' and in guided-wave geometries.12l' 4 The self-trapped propagation of the spatial soliton itself maintains the high intensity necessary for large phase modulation, which generates the necessary bandwidth for pulse compression. Unlike other high-energy compression methods, soliton compression offers both high repetition rates and a potentially unlimited wavelength range. The basic experimental apparatus for generating and compressing spatial solitons is as follows. Pulses of 75-fs duration and 0.1-nJ energies from a balanced colliding-pulse mode-locked ring dye laser operating at 620 nm were amplified to 30 /.tJ at a repetition rate of 8.6 kHz in a two-stage optical amplifier pumped by a 20-W copper-vapor laser. To achieve these pulse energies, we used a dye cell in the second stage.' 5 Following recompression to 75 fs with a two-prism sequence in a double-pass geometry, the pulses were spatially filtered to improve beam quality and ensure the formation of clean spatial solitons. The energy throughput of the prism sequence-spatial filter was 11 /%J. We chose an 8-mm-thick piece of bulk fused silica as the nonlinear medium, which has a positive nonlinear index (n 2 = 2.7 x 10-16 cm 2 /W), as required for bright spatial solitons as well as minimal linear and twophoton absorption. Pulses were focused on the front face of the glass in an elliptical profile by a cylindrical-spherical lens combination. We used beam diameters of w = 900 gum (l/e peak intensity) in the long dimension (which we denote x; see the graph o
