Terahertz (THz) radiation is ideal for probing many different materials and processes. Photons in the THz regime have energies on the order of an meV, which is an important energy scale for many electronic processes. In this paper we will describe the use of optical rectification of 50 fs IR pulses to generate THz pulses. Using this method, spectrally broad THz pulses with durations on the order of ps can be produced. This feature allows us to obtain time and frequency resolved information about the transmission of THz radiation during transient processes. A 50 fs IR pulse is used to optically excite a material and the relaxation as a function of time can be observed with the THz probe.
We are developing a transient THz spectroscopy to study non-equilibrium processes in thin film superconducting YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-δ</sub> (YBCO). We use ultrafast optical pulses to excite the sample, breaking a fraction of the Cooper pairs responsible for the film's superconductivity. This process produces highly energetic quasiparticles which thermalize and recombine on a picosecond timescale as the superconducting state recovers. Transient THz spectroscopy allows us to follow the evolution of this process with the required resolution, while simultaneously providing valuable spectroscopic information.