A broad and tunable 250- to 430-nm source for microscopy and lifetime measurements by frequency doubling of a 78-MHz-picosecond white-light laser

Applied Physics B, 2014, DOI 10.1007/s00340-014-5774-5, Volume 116, Issue 4 , pp 875-882 published on 01.09.2014
Applied Physics B
Broadly tunable picosecond pulses in the UV for nonlinear microscopy and lifetime measurements are not yet readily available. Complex synchronously pumped optical parametric oscillators with subsequent frequency doubling are typically used. We show that direct second harmonic generation of a visible picosecond supercontinuum source at 78 MHz renders pulses easily tunable from 250 to 430 nm. We find that an unexpectedly large numerical aperture and the use of thick crystals increase the efficiency of the frequency doubling process dramatically. The observed spectral width and efficiency are nearly two orders of magnitude larger than predicted by conventional theory. With broadband achromatic doubling, a 130 nm wide spectrum is achieved. Pulse durations of 17–35 ps are found in the UV and an average power between 1 and 70 μW. This qualifies the setup for most UV-based microscopic investigations. As first application, the fluorescence lifetime of two differing conformations of 2-(2′-hydroxyphenyl) benzothiazole is measured.
TU München