The terahertz (THz) frequency range lies in the far-infrared region of the electromagnetic spectrum. Picosecond THz pulses are well suited for both probing and exciting low-energy (meV) dynamics in a sample. Such experiments require intense, quasi-single-cycle THz pulses. A well-established way to generate them is optical rectification, a nonlinear optical process in which a femtosecond infrared laser pulse produces intense THz radiation. We use two different approaches to study material properties with THz radiation.
The first approach is THz time-domain emission microscopy (THz-TDEM), which detects the THz radiation emitted by the sample to provide detailed insights into its material properties. By tightly focusing the optical excitation beam and raster-scanning the sample perpendicular to the propagation direction, spatially resolved characterization is achieved based on local THz emission spectra.
The second approach explores sample dynamics on the picosecond timescale in a transmission electron microscope (TEM). Here the sample is first excited by a THz pulse and subsequently probed by ultrashort electron pulses. By varying the temporal delay between the two pulses, the spatial resolution of the TEM can be combined with temporal resolution of sample dynamics such as plasmons, phonons, or collective spin excitations.
