Terahertz emission mediated by ultrafast time-varying metasurfaces

Systems with ultrafast time-varying dielectric properties represent an emerging physical framework. We demonstrate here the observation of subcycle dynamics interacting directly with an electromagnetic source comprised of morphologically constrained photoexcited carriers in a surface nanostructure. A transition to a metallic metasurface state occurs on time scales faster than the terahertz-field period, inducing large nonlinear ultrafast phase shifts in the terahertz emission and exposing an interesting physical setting.

Figure 1

(a) The full experimental setup for THz generation and ultrafast metallization of black silicon substrates. BS: beam splitter, GP: generation pump, MP: metallization pump, $B$-Si: black silicon, ZnTe: zinc telluride, HWP: half-wave plate, WP: Wollaston prism. (b) Schematic illustration of the three major regimes in the transient measurements as the $B$-Si transitions from a semiconducting to a metallic state. (c) The corresponding shift in the real part of the time-dependent refractive index induced by the MP. The inset shows an SEM image of the $B$-Si surface (scale bar $5\mu \mathrm{m}$).

Figure 2

(a) The full transient THz dynamics spanning tens of picoseconds before and after photoexcitation. (b) The $B$-Si THz wave forms taken at the pump-probe delay times depicted by the lines marked in (a). With $\tau =-10\mathrm{ps}$, $\tau =0\mathrm{ps}$, and $\tau =10\mathrm{ps}$ corresponding to the standard unexcited $B$-Si, the transient metallization region, and the metallic state respectively. (c) The THz spectra obtained via the fast Fourier transform of the temporal wave forms shown in (b).

Figure 3

(a) The fully simulated THz time-dependent dynamics, where all wave forms have been individually normalized to highlight the phase shift induced by the material metallization. (b) The temporal THz wave forms corresponding to the pump-probe delay times marked in (a), with $\tau =-7.5\mathrm{ps}$ and $\tau =7.5\mathrm{ps}$ being the semiconducting and metallic states, respectively. (c) The experimental dynamics shown in Fig. 2 with the same normalization to demonstrate the behavior similar to that shown in (a). (d) The experimental wave forms taken at the same values of the delay $\tau$ as in (b).