Time resolution limits for two-color pump/probe spectroscopy

Abstract

The characteristics of various cw mode-locked dye lasers were thoroughly explored, and the characteristics of a pump/probe apparatus employing two such lasers pumped by a single mode-locked pump laser. First, it was observed that the rf source used to drive the acousto-optic mode-locking element in the pump laser, while not affecting the average power or obtainable pulse width, had a profound effect on the noise characteristics of the pump laser, which are in turn passed along to the dye lasers. For a driver based on a tunable frequency synthesizer, pulse jitter on the order of 300 ps was observed, which represents ≊2.5% of the total interpulse time interval. This was deemed unacceptably high, and use of this mode-locker driver was discontinued. For a driver with a fixed frequency reference source (an oscillating quartz crystal), pulse jitter on the order of 5 ps was measured, a great improvement over the other driver. Next we studied various dye laser configurations pumped by this pump laser. We found that for time resolution of several picoseconds, a pair of synchronously pumped dye lasers employing birefringent filter tuning elements was an excellent source of independently tunable pulse trains, and that these pulse trains were well synchronized with each other despite the absolute timing jitter in the pump pulse train. To obtain better temporal resolution with independently tunable lasers was more problematic, however. A hybridly mode-locked laser (employing a saturable absorber within a synchronously pumped dye laser) can produce shorter pulses (≤100 fs has been reported), but the lack of temporal synchronization between two of these lasers limits the resolution of a two wavelength experiment to several picoseconds. An attempt was made to reduce this relative timing jitter by crossing two independently tunable hybrid lasers through a common intracavity saturable absorber, thereby temporally locking the pulse trains together. This mechanism failed to work well on time scales longer than a millisecond.