Ultrafast THz light-matter interactions

Prof. David G. Cooke
Department of Physics, McGill University

Abstract

Phase-locked, few-cycle pulses of THz frequency light are powerful tools for both probing and driving ultrafast dynamics of low energy (meV scale) excitations in condensed matter. As an example of using THz pulses as a time-resolved probe, I discuss recent multi-THz spectroscopy experiments on the widely researched hybrid organometallic halide perovskites. These solution processable materials have been successfully applied to a variety of optoelectronic devices, most notably high efficiency photovoltaics achieving up to 22% power conversion efficiency in the lab (comparable to silicon). Their long carrier lifetimes and relative insensitivity of their electronic transport properties to the presence of impurities have been puzzling when considering their similarities to other direct band gap semiconductors like GaAs. This led to a proposal that charge carriers exist as large polarons, protected against scattering by their correlation to polar lattice vibrations. We show ultrafast THz measurements provide direct evidence for the existence of polarons in these materials, resolving the quantum dynamics of their formation.

In addition, strong field THz pulses can now be used to control the motion of charged particles on sub-cycle time scales. Along these lines, I will discuss our recent work on sub-cycle THz field emission of femtosecond electron wave packets from metal nanotips. The nanotip provides a local field enhancement of the intense THz fields to an astounding 10’s of GV/m near the tip apex. We show that through field-assisted tunneling directly from the metal’s Fermi level, impressive electron bunch charges up to 106/shot are emitted on a sub-cycle time scale. These electrons are subsequently accelerated quasi-statically within the half-cycle of the THz pulse to keV energies. I discuss implications for this new ultrafast electron source for both time-resolved electron scattering experiments and as a test bed for high field physics. 

Finally, I discuss a new platform for dynamic THz photonics based on a light-addressable silicon-filled waveguide. The platform allows virtually any metal-dielectric photonic structure to be created within the 2D waveguide, both statically and dynamically on time scales that are faster than the pulse transit time and even the THz carrier wave. I will discuss several functionalities enabled by this platform, including arbitrary pulse shaping and the trapping light.