Abstract

Recently a number of new femtosecond sources have been developed at near ultraviolet (190 nm < λ < 310 nm) wavelengths,^1,2 opening up new applications in ultrafast molecular spectroscopy and high energy density physics.³ These sources include intracavity frequency-doubled dye lasers¹ and high power excimer amplifiers.² Nevertheless, pulsewidth measurement in the UV remains problematic compared to routine autocorrelation measurements in the visible and near-infrared based on phase-matched second harmonic generation (SHG) in transparent crystals. The lack of frequency doubling crystals in the UV necessitates alternative, usually less convenient and more expensive, methods. Here we present a simple, low-cost method of obtaining background-free second-order autocorrelation measurements of femtosecond pulses at least as far as 220 nm in the UV, and at pulse energy at least as low as the nanojoule regime, by using two-photon absorption (TPA) in diamond. Because undoped diamond has a wide band gap (5.45 eV), no linear optical absorption occurs at wavelengths λ >220 nm, the range of most currently available UV femtosecond sources. Our measurements with 310-nm, 10-50-nJ pulses of duration 100 is <t_P < 2 ps from a frequency doubled, amplified CPM source show that they induce TPA in diamond strong enough for autocorrelation measurements within path lengths of 0.25 mm small enough that temporal broadening from group velocity dispersion is negligible and small enough that the diamond samples needed are available commercially at inexpensive prices. Because phase matching is not required, beam alignment with respect to crystalline axes is not critical. Consequently, this method provides simple, accurate, low cost autocorrelation measurements for nearly all currently available UV femtosecond sources.

© 1991 Optical Society of America