The availability of intense light fields at terahertz (THz) frequencies has opened new opportunities for nonlinear spectroscopy of molecules and materials. Some advances have involved excitation of low-frequency resonances including those of lattice vibrational modes, molecular rotations, and collective and localized spins. Others have involved THz excitation of electronic responses that ordinarily would be accessed with light in the visible spectrum or nearby regions. In these cases, the THz pulse is acting much like a fast but quasi-DC classical field, liberating and accelerating carriers or inducing charge transfer to produce responses that may be inaccessible to true DC fields due to unwanted effects such as dielectric breakdown. THz fields have been used to generate high densities of carriers in insulating materials. The fields can liberate valence-band or locally trapped electrons through various ionization channels, and can accelerate carriers to multi-eV energies at which they liberate additional electrons through impact ionization. The newly generated carriers can be accelerated in turn, leading to a cascade of ionization events that may result in high carrier densities produced within the duration of a field half-cycle on the order of 1 picosecond. In some cases including vanadium dioxide (VO2) and single-layer molybdenum ditelluride (MoTe2), the result is an electronic phase transition accompanied by a structural phase transition that may be observed through femtosecond x-ray diffraction measurements. In arrays of semiconductor quantum dots (QDs), THz fields have induced electroluminescence (EL) which, unlike conventional EL that occurs through field emission from electrodes in contact with the luminescent material, arises from direct QD-to-QD charge transfer. THz field-assisted photoluminescence (PL) also has been observed in which near-IR photons that are as much as ½ eV below the QD bandgap are able to induce emission when they are time-coincident with THz fields that drive extensive Stark shifts and electronic state mixing. THz field control over isolated QD emission also has been demonstrated. The effects observed thus far indicate rich prospects for THz field control over molecular and material electronic states and dynamics.

Terahertz fields induce dramatic electronic responses

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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