UV-induced terahertz wave modulation in free-standing ZnO nanowire films

We present terahertz (THz) wave modulation by using free-standing ZnO nanowire (NW) network films. The ZnO NW films were virtually transparent against THz waves without UV illumination. Conversely, the THz waves were attenuated under very low-intensity UV illumination, making the ZnO NW film a promising platform for low-loss, low-power and all-optical THz modulators. The complex dielectric constants measurements reveal that the UV laser induces an enhancement in the ac conductivity while leaving the real part of the dielectric constant unchanged. The relatively slow time response implies that the UV-induced modulation is closely linked to the surface trap states. The THz attenuations showed clear saturation behavior with respect to the UV intensity, from which we extracted the ZnO NW surface trap density.