Generation of 100-fs light pulses down to 170 nm by sum-frequency mixing in lithium tetraborate

Lithium tetraborate Li/sub 2/B/sub 4/O/sub 7/ (LB4) has proved to be very promising for the generation of the fourth and fifth harmonics of Nd:YAG lasers. The birefringence of LB4 is higher than that of LBO. LB4 is negative uniaxial and belongs to the 4 mm point group which means that its effective nonlinearity is maximized under the conditions of 90/spl deg/ phase-matching. The clear transmittance spans from 170 nm to 3300 nm. We demonstrate sum-frequency generation of tunable VUV femtosecond pulses with LB4 from 185 nm down to 170 nm (7.3 eV), the lower limit being set by the available fourth harmonic tunability. A comparison with an LBO reference sample reveals that LB4 has a superior performance in the VUV spectral region below 180 nm.