A broad variety of fundamental excitations in solids and molecules occur on THz ( = 10-12/s) time and energy scales. Among them are e.g. molecular and lattice vibrations (phonons), collective spin waves (magnons), Cooper pairs in high-temperature superconductors and internal excitations of bound electron-hole pairs (excitons). To directly access these quasi-particles in their natural ground or excited states, we generate resonant electromagnetic fields in the form of ultrashort laser pulses in the THz and MIR frequency range (0.1 THz – 40 THz), corresponding to 0.4 – 160 meV photon energy. The oscillation periods of these THz pulses (25 fs – 10 ps) also cover typical time scales of charge carrier scattering, electron-phonon equilibration or structural transitions, and can be therefore used as non-resonant, contact free bias fields. In the Department of Physical Chemistry, we employ the following three approaches to transiently observe or specifically control these ultrafast phenomena via phase-stable THz electric fields:
1. THz probing
Time-domain probing of THz fields in amplitude and phase (electro-optic sampling) provides direct access to the transient evolution of material properties such as complex valued dielectric function, refractive index and frequency dependent conductivity.
2. THz emission
Resolving the weak THz emission of accelerated charges provides a contact free and ultrafast Ampere-meter to measure currents during carrier transport, during trapping processes or during spin-charge conversion.
3. Strong field THz source
Our tabletop strong-field THz sources with field strengths exceeding 1 MV/cm at 1 THz and 50 MV/cm above 20 THz allow for specific creation and control resonant of low-energy excitations. Extending these excitation schemes into the nonlinear coupling regime, enables us to even control non-IR-active (e.g. dipole forbidden) modes. Moreover, we are specifically interested in exploiting tailored THz electric fields to drive structural dynamics along desired pathways.