Sub-ten-femtosecond pulses approach relativistic intensity

Researchers at the University of Michigan (Ann Arbor, MI) and the Ecole Polytechnique (Palaiseau, France) have focused ...

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 mourou@umich.edu.

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