Hybrid fs Dye/Excimer Lasers for Pump-Probe and High-Intensity Experiments

For many experiments in physics, chemistry, and biology one needs a femtosecond pump-probe spectrometer with a strong pump pulse, preferably in the ultraviolet, and a jitter-free synchronized probe pulse, tunable over a wavelength range from the near ultraviolet to the near infrared. Such a spectrometer is usually very complex consisting of several lasers, synchronization and other electronics and electro-optical instrumentation. By contrast, we have designed a relatively simple, reliable spectrometer using only a single laser for the generation of the pump and the probe pulses absolutely jitter-free and without any electronic or electro-optic instrumentation. The spectrometer is schematically shown in Fig. 1. It contains a commercial excimer laser with two discharge channels operated by one thyratron for stable synchronization of the two discharges. One channel is operated as a XeCl oscillator whose output pumps a series of dye lasers giving finally an output pulse of .5 ps duration at 497 nm with more than 100 μJ pulse energy. Part of the output pulse is frequency-doubled and sent into the second discharge channel which is filled with KrF mixture. After a triple pass in an off-axis arrangement the output energy is 20 mJ in less than .5 ps. Since the output pulse has a strong positive chirp it can be compressed to less than 100 fs. The rest of the green pulse passes a two-grating compressor and is focussed into a 5 mm thick piece of quartz glass to produce a continuum. The collimated white light is sent through a dye amplifier cell pumped by the rest of the XeCl laser pulse energy through a delaying quartz fiber. After a second pass through the dye amplifier cell, the pulse is compressed in a pulse compressor consisting of two LiF prisms and a retroreflector to give a probe pulse output of less than 30 fs duration and 1 μJ energy. An aperture allows fine tuning the spectral position and width to give transform-limited pulses in a well collimated probe beam. With 7 different dyes one can cover the spectral region from 420 to 650 nm. In a similar scheme we have also used other excimers in order to reach different ultraviolet wavelengths.