Frequency conversion of Ti:sapphire-based femtosecond laser systems to the 200-nm spectral region using nonlinear optical crystals

We review the linear and nonlinear optical properties of crystals transparent near and below 200 nm and suitable for up conversion of femtosecond Ti:sapphire laser sources, β-BaB₂O₄ , the crystal with the largest birefringence of all presently available materials, is investigated experimentally as a quadrupler by mixing the fundamental and the third harmonic both using a 1-kHz repetition rate Ti:sapphire regenerative amplifier and a 82-MHz mode-locked Ti:sapphire laser. Milliwatt average powers near 200 nm are achieved in both cases. The sub-200-fs pulses at the fourth harmonic are almost bandwidth limited. Sum-frequency generation as a method for upconversion of femtosecond pulses is experimentally studied by mixing the fourth harmonic generated down to 189 nm by the regenerative amplifier with a parametrically generated femtosecond pulse in the infrared. Pulse energies at the microjoule level are produced with LiB ₃O₅ above 180 nm. Li₂B₄O₇ shows superior performance in the 170-180-nm range, and the shortest wavelength achieved with KB₅ O₈·4H₂O is 166 nm