100 % Amplitude modulation of an external cavity terahertz QCL using an optoelectronic chopper based on metamaterials and graphene

Abstract

The continuous development of terahertz (THz) sources has opened up many potential applications in spectroscopy, imaging and communications. One popular THz source is the quantum cascade laser (QCL), which has many desirable properties including compactness and high output power with a narrow emission frequency. For such a source to be successfully integrated into a THz communication system, it is necessary to have control over the amplitude, frequency and phase. For wireless communication purposes, amplitude modulators must have a reasonable modulation depth and be capable of fast modulation speeds to take full advantage of the greater bandwidth opened up by using a THz carrier wave. To this purpose, we have developed optoelectronic split ring resonator (SRR) and graphene amplitude modulators which have been combined with a THz QCL thus realising an external cavity set-up which uses the SRR/graphene devices to efficiently modulate the light feedback into the laser cavity. The SRR/graphene device is lithographically designed to have maximum reflectivity at the QCL emission frequency (2.9 THz) and the graphene acts as a variable dampener, capable of electrically modulating the reflectivity. Similar SRR/graphene device architectures have been used previously for amplitude modulation by varying the reflection from a standard CW QCL output, achieving modulation speeds >100 MHz with a modulation depth limited to around 20 % [1].