Autocorrelation measurements of femtosecond ultraviolet pulses using two-photon conductivity in wide-gap dielectrics

Summary form only given. We report on the application of planar photoconductive switches on wide-gap dielectrics to measurements of the intensity autocorrelation of femtosecond pulses at 270 nm. In our scheme, metal layers were deposited on the wafers of fused silica and magnesium fluoride (bandgaps of approximately 8 eV and 11 eV, respectively). These large bandgaps allow for the observation of two-photon conductivity for pulses with wavelengths below 300 nm. The third harmonic of the output of a Ti:sapphire regenerative laser system was injected into a Michelson-type autocorrelator, and the output was focused by a mirror onto the structure. The interferometric single-scan and accumulated (averaged) autocorrelation traces are presented for the SiO/sub 2/ substrate. The averaged trace fitted by a Gaussian function corresponds to pulses of 380-fs duration. The photocurrent became saturated for incident pulse energies above 400-600 pJ for the SiO/sub 2/ wafer. We have also made measurements using MgF/sub 2/ substrates and observed nonlinear response in the photoconductivity, but the signal and saturation current were found to be lower, which resulted in noisier autocorrelation traces with reduced contrast ratio. The simplicity of the device and its sensitivity are promising for accurate ultrashort pulse duration measurements in the UV and should allow for pulse characterization at wavelengths below 200 nm.