Abstract
Recent years have seen dramatic developments in the technology of intense pulsed light sources in the THz frequency range. Since many dipole-active excitations in solids and molecules also lie in this range, there is now a tremendous potential to use these light sources to study linear and nonlinear dynamics in such systems. While several experimental investigations of THz-driven dynamics in solid-state systems have demonstrated a variety of interesting linear and nonlinear phenomena, comparatively few efforts have been made to drive analogous dynamics in molecular systems. In the present Perspective article, we discuss the similarities and differences between THz-driven dynamics in solid-state and molecular systems on both conceptual and practical levels. We also discuss the experimental parameters needed for these types of experiments and thereby provide design criteria for a further development of this new research branch. Finally, we present a few recent examples to illustrate the rich physics that may be learned from nonlinear THz excitations of phonons in solids as well as inter-molecular vibrations in liquid and gas-phase systems.
Highlights
Ultrafast pump-probe experiments, which were developed in parallel with short pulse lasers starting from the early 1960s paved the way for the field of femtochemistry, i.e., the study of real-time dynamics of chemical reactions in molecular systems, for which Zewail was awarded with the Nobel prize in 1999.1 Ultrafast pump-probe experiments have made strong contributions to other areas of the physical sciences, in particular, the study of strongly correlated solid state materials
Given the huge transitions dipoles to low-lying electronic states in solids, which make it relatively easy to reach the interaction energies just discussed with readily available THz pulses with field strengths in the order of 10–100 kV/cm, it can be understood why by far the most nonlinear THz experiments have been performed on such transitions, e.g., in semiconductors,55–58,60,68,106–108 superconductors,109–112 or graphene
The electric field of THz pulses couple most directly to the charges of the nuclei, thereby opening a window to a fundamentally new approach to control the structure of solid state materials and molecular systems
Summary
Ultrafast pump-probe experiments, which were developed in parallel with short pulse lasers (initially Q-switched and later mode-locked) starting from the early 1960s paved the way for the field of femtochemistry, i.e., the study of real-time dynamics of chemical reactions in molecular systems, for which Zewail was awarded with the Nobel prize in 1999.1 Ultrafast pump-probe experiments have made strong contributions to other areas of the physical sciences, in particular, the study of strongly correlated solid state materials. It has not yet been realized experimentally, THz excitation could in principle induce a phase transition by coherently driving a set of structural coordinates over an energy barrier; the displacive first order structural transition in BaTiO3 could be a good candidate for this.52 Another possible method is to use strong midinfrared excitation of high frequency vibrational modes to induce an effective renormalization of low frequency structural coordinates via anharmonic coupling, leading to a structural symmetry change.. The ultimate nonlinear THz experiment is 2D-THz spectroscopy, which has the potential to observe the nonlinear response function completely, typically to all orders.54–60 In such an experiment, both pump and probe processes act in the THz regime, and the experiment measures the nonlinear component of the system response to the applied THz fields.
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