Harold Y. Hwang

Terahertz-field-induced insulator-to-metal transition in vanadium dioxide metamaterial

Electron–electron interactions can render an otherwise conducting material insulating, with the insulator–metal phase transition in correlated-electron materials being the canonical macroscopic manifestation of the competition between charge-carrier itinerancy and localization. The transition can arise from underlying microscopic interactions among the charge, lattice, orbital and spin degrees of freedom, the complexity of which leads to multiple phase-transition pathways. For example, in many transition metal oxides, the insulator–metal transition has been achieved with external stimuli, including temperature, light, electric field, mechanical strain or magnetic field. Vanadium dioxide is particularly intriguing because both the lattice and on-site Coulomb repulsion contribute to the insulator-to-metal transition at 340 K (ref. 8). Thus, although the precise microscopic origin of the phase transition remains elusive, vanadium dioxide serves as a testbed for correlated-electron phase-transition dynamics. Here we report the observation of an insulator–metal transition in vanadium dioxide induced by a terahertz electric field. This is achieved using metamaterial-enhanced picosecond, high-field terahertz pulses to reduce the Coulomb-induced potential barrier for carrier transport. A nonlinear metamaterial response is observed through the phase transition, demonstrating that high-field terahertz pulses provide alternative pathways to induce collective electronic and structural rearrangements. The metamaterial resonators play a dual role, providing sub-wavelength field enhancement that locally drives the nonlinear response, and global sensitivity to the local changes, thereby enabling macroscopic observation of the dynamics. This methodology provides a powerful platform to investigate low-energy dynamics in condensed matter and, further, demonstrates that integration of metamaterials with complex matter is a viable pathway to realize functional nonlinear electromagnetic composites.

Impact ionization in InSb probed by terahertz pump—terahertz probe spectroscopy

Indium antimonide InSb is a model system for the study of hot-electron dynamics due to its low band gap of 170 meV at room temperature 1 and the fact that it has the highest electron mobility and saturation velocity among all known semiconductors. The wealth of nonequilibrium transport phenomena that have been observed in this material 2–4 is of special interest due to the large nonparabolicity of the conduction band, 5 which results in negative differential mobility

Observation of nonequilibrium carrier distribution in Ge, Si, and GaAs by terahertz pump-terahertz probe measurements

We compare the observed strong saturation of the free-carrier absorption in

Terahertz Kerr effect

n

THz-pump/THz-probe spectroscopy of semiconductors at high field strengths [Invited]

-type semiconductors at 300 K in the terahertz (THz) frequency range when single-cycle pulses with intensities up to

Nonlinear terahertz metamaterials via field-enhanced carrier dynamics in GaAs.

150\text{ }\text{MW}/{\text{cm}}^{2}

A review of non-linear terahertz spectroscopy with ultrashort tabletop-laser pulses

are used. In the case of germanium, a small increase in the absorption occurs at intermediate THz pulse energies. The recovery of the free-carrier absorption was monitored by time-resolved THz pump\char21{}THz probe measurements. At short probe delay times, the frequency response of germanium cannot be fitted by the Drude model. We attribute these unique phenomena of Ge to dynamical overpopulation of the high mobility

Nonlinear THz conductivity dynamics in P-type CVD-grown graphene.

\ensuremath{\Gamma}

Fiber laser pumped high average power single-cycle terahertz pulse source

conduction-band valley.

Tunable multi-cycle THz generation in organic crystal HMQ-TMS

We have observed optical birefringence in liquids induced by single-cycle terahertz pulses with field strengths exceeding 100 kV/cm. The induced change in polarization is proportional to the square of the terahertz electric field. The time-dependent terahertz Kerr signal is composed of a fast electronic response that follows the individual cycles of the electric field and a slow exponential response associated with molecular orientation.