We report single-pass difference-frequency generation of mid-infrared femtosecond pulses tunable in the 3.2-4.8 microm range from a two-branch mode-locked erbium-doped fiber source. Average power levels of up to 1.1 mW at a repetition rate of 82 MHz are obtained in the mid infrared. This is achieved via nonlinear mixing of 170 mW, 65 fs pump pulses at a fixed wavelength of 1.58 microm, with 11.5 mW, 40 fs pulses tunable in the near-infrared range between 1.05 and 1.18 microm. These values indicate that the tunable near-infrared input component is downconverted with a quantum efficiency that exceeds 30%.

Observation of soft x rays from a tungsten plasma produced by intense subpicosecond laser pulses is reported. X radiation of several keV photon energy was observed with just a few millijoules of laser energy. Using two consecutive pulses for the generation of the plasma the x-ray emission is maximal for a delay of about 30 to 50 ps between the laser pulses.

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