Speaker
Description
Next generation wireless communication technologies will rely on techniques able to rapidly change the properties of an optical filter in the far-infrared region. Here we demonstrate ultrafast modulation of a metasurface’s transmission spectrum containing a resonance around the optical frequency of 1 terahertz (THz). The metasurface consists of an array of sub-wavelength gold crosses deposited on a silicon substrate. A femtosecond optical pulse in the visible region is used to inject free carriers in the semiconductor to modify the metasurface properties, while the transmission spectrum is monitored with a broadband time-resolved THz spectroscopy system. As we increase the density of optically injected carriers, we gradually damp the filter resonance, broaden its linewidth and blueshift its frequency. At a carrier density of $2\times10^{17}$ $cm^{-3}$, we completely bleach the resonance, and the full transmission spectrum becomes flat with an overall transmission coefficient approaching T = 0.25. We also investigate the effect of injected carriers in a dynamical context: when the excitation pulse modifies the properties of the metasurface less than 1 ps after the arrival of the THz probe pulse. Interestingly, this scheme prevents spectral components from remaining trapped inside the metasurface and yields a transmission spectrum free of any resonances. In the case of a notch filter, we show that this innovative technique increases the transmission at the resonance by more than 2 orders of magnitude while the off-resonance part of the spectrum decreases by less than 30%. Our experimental results are in good agreement with numerical simulations based on a finite-difference time-domain (FDTD) method, allowing us to reproduce the transmission spectrum and visualize the electric field distribution within the metasurface. Our tunable frequency selection technique has a great potential for applications in adaptive communication devices, THz pulse shaping, and the sub-cycle modulation of nanomaterials with ultrafast carrier dynamics.
