We demonstrate that high-field terahertz (THz) pulses trigger transient insulator-to-metal transition in a nanoantenna patterned vanadium dioxide thin film. THz transmission of vanadium dioxide instantaneously decreases in the presence of strong THz fields. The transient THz absorption indicates that strong THz fields induce electronic insulator-to-metal transition without causing a structural transformation. The transient phase transition is activated on the subcycle time scale during which the THz pulse drives the electron distribution of vanadium dioxide far from equilibrium and disturb the electron correlation. The strong THz fields lower the activation energy in the insulating phase. The THz-triggered insulator-to-metal transition gives rise to hysteresis loop narrowing, while lowering the transition temperature both for heating and cooling sequences. THz nanoantennas enhance the field-induced phase transition by intensifying the field strength and improve the detection sensitivity via antenna resonance. The experimental results demonstrate a potential that plasmonic nanostructures incorporating vanadium dioxide can be the basis for ultrafast, energy-efficient electronic and photonic devices.

We show that the transmission of a terahertz (THz) pulse through single-layer graphene is strongly nonlinear. As the peak electric field of the THz pulse exceeds 50 kV/cm, the graphene becomes increasingly transparent to the THz radiation. When field strength reaches 800 kV/cm, the increased transparency corresponds to a two-fold decrease in the time-average sheet conductivity of the graphene (time averaged over the duration of the pulse). Time-resolved measurements reveal that the leading portion of the pulse creates transparency for the trailing portion, with a 10-fold suppression in sheet conductivity at the tail of the strongest THz pulse. Comparing the THz-induced transparency phenomena in different sample geometries shows that substrate-free graphene is the best geometry for maximizing the nonlinear transparency effect.

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Following femtosecond (fs) laser pulse irradiation, the formation of a new type of low-spatial-frequency laser-induced periodic surface structure (LSFL) patterns, namely, omnidirectional LSFLs (OD-LSFLs) with the periodic ordering of their orientations, are investigated on Ni in this Letter. Using a liquid crystal polymer patterned depolarizer, we periodically rotate the polarization of fs laser pulses across the laser spot and create OD-LSFLs by raster scanning fs laser pulses. We also show that the period of the OD-LSFL orientation rotation can be controlled with the defocused distance, and OD-LSFLs can significantly expand the viewing angle of the structural colors in the azimuthal direction without noticeable color degradations.