Towards a setup for narrowband terahertz generation through difference-frequency mixing of chirped ultrashort pulses

Abstract: Terahertz radiation is a useful tool for inducing lower-energy excitations in matter, where it can be used to cause structural changes through direct interaction with the crystal lattice. Sub-picosecond optical pulses can be used to generate intense broadband terahertz pulses through optical rectification in nonlinear crystals, reaching peak electric fields of several MV/m and conversion efficiencies as high as a few percent. For certain applications it may be desirable to only generate terahertz frequencies within a narrow band. One such way, using broadband optical pulses, is through chirped-pulse beating, where two linearly chirped pulse copies are delayed relative each other in time, causing them to beat. This results in an output optical pulse with an intensity envelope modulated at a single beat frequency. When incident on a nonlinear crystal lacking inversion symmetry, such a modulated pulse generates difference-frequencies only within a narrow band, centered around the beat frequency of the intensity modulation. This scheme for narrowband terahertz generation was implemented in the project, and an electro-optic sampling setup was built to characterize the temporal profile of the terahertz pulses. In the project, terahertz radiation was generated using the organic crystal DSTMS pumped with 1500 nm light from an optical parametric amplifier. Pulse characterization was done using the electro-optic crystals ZnTe and GaP with a frequency-doubled 750 nm probe pulse. Chirp and delay was implemented using a transmission grating chirp filter and a Michelson interferometer. Few-cycle terahertz pulses were successfully generated and sampled with electro-optic sampling. The pulse stretching and the delay between pulse copies were implemented, but narrowband terahertz pulses were not sampled with the electro-optic sampling within the time frame of the Master thesis.