Hybrid CO$\mathrm{_2}$-Ti:sapphire laser with tunable pulse duration for mid-infrared-pump terahertz-probe spectroscopy

Abstract: Ultrafast optical excitation with intense mid-infrared and terahertz pulses has emerged as a new tool to control materials dynamically. As most experiments are performed with femtosecond pulse excitation, typical lifetimes of most light-induced phenomena in solids are of only few picoseconds. Yet, many scientific applications require longer drive pulses and lifetimes. Here, we describe a mid-infrared pump - terahertz-probe setup based on a CO$\mathrm{_2}$ laser seeded with 10.6 $\mathrm{\mu}$m wavelength pulses from an optical parametric amplifier, itself pumped by a Ti:Al$\mathrm{_2}$O$\mathrm{_3}$ laser. The output of the seeded CO$\mathrm{_2}$ laser produces high power pulses of nanosecond duration, which are synchronized to the femtosecond laser. Hence, these pulses can be tuned in pulse duration by slicing their front and back edges with semiconductor-plasma mirrors irradiated by replicas of the femtosecond seed laser pulses. Variable pulse lengths from 5 ps to 1.3 ns are achieved, and used in mid-infrared pump, terahertz-probe experiments with probe pulses generated and electro-optically sampled by the femtosecond laser.