Ultrafast modulation of the spectral filtering properties of a THz metasurface.

We demonstrate ultrafast tuning of a plasmonic spectral filter at terahertz (THz) frequencies. The device is made of periodically spaced gold crosses deposited on the surface of an undoped silicon wafer in which transient free carriers can be optically injected with a femtosecond resonant pulse. We demonstrate the concept by measuring the transmission spectrum of a notch filter using time-domain THz spectroscopy. Proper synchronization of the THz probe and visible excitation pulses leads to an enhanced transmission at the resonance by more than two orders of magnitude. Finite-difference time-domain simulations, which are in agreement with the experimental results, show that the underlying mechanisms responsible for the resonance blueshift and linewidth broadening can be attributed to the photoinduced change in dielectric properties of the substrate. This is supported by the numerically simulated field distribution and reflection/transmission coefficients. The device can be used in future pulse shaping and ultrafast switching experiments.