Researchers at the University of Michigan (Ann Arbor, MI) and the Ecole Polytechnique (Palaiseau, France) have focused millijoule-level, sub-ten-femtosecond pulses into a spatial region on the order of a wavelength cubed to achieve a focal-point intensity of 7x1017 W/cm2, approaching the level of relativistic interaction. To achieve this, the researchers constructed a system based on a high-numerical-aperture, f/1 off-axis paraboloid to focus ultrashort pulses and on adaptive optics to correct wavefront distortion. Seed pulses were generated by a modelocked Ti:sapphire laser pumped by a Nd:YVO4 laser. An all-reflective grating stretcher expanded the pulses to 40 ps. A diode-pumped Nd:YLF laser was used to pump the system amplifier, yielding 1-mJ pulses that were then compressed to 21 fs with 700-µJ energy.
An output spectrum of 700-900 nm was obtained by focusing 420-mJ pulses into a hollow, 85-cm-long, 320-mm-core-diameter fiber at 9-10 Torr of argon gas pressure. The spectrum was compressed using chirped mirrors, producing 8-fs FWHM pulses of 0.17-mJ energy. Using a wedge, 0.5% of the beam was reflected and then expanded using a telescope to the 50-mm aperture of a deformable mirror that directed the beam to the paraboloid. A thin BBO crystal provided second-harmonic generation for small spot size. Contact Gerard Mourou at [email protected].