Tunable, Grating‐Gated, Graphene‐On‐Polyimide Terahertz Modulators

An electrically switchable graphene terahertz (THz) modulator with a tunable‐by‐design optical bandwidth is presented and it is exploited to compensate the cavity dispersion of a quantum cascade laser (QCL). Electrostatic gating is achieved by a metal grating used as a gate electrode, with an HfO2/AlOx gate dielectric on top. This is patterned on a polyimide layer, which acts as a quarter wave resonance cavity, coupled with an Au reflector underneath. The authors achieve 90% modulation depth of the intensity, combined with a 20 kHz electrical bandwidth in the 1.9–2.7 THz range. The modulator is then integrated with a multimode THz QCL. By adjusting the modulator operational bandwidth, the authors demonstrate that the graphene modulator can partially compensate the QCL cavity dispersion, resulting in an integrated laser behaving as a stable frequency comb over 35% of the operational range, with 98 equidistant optical modes and a spectral coverage ~1.2 THz. This paves the way for applications in the terahertz, such as tunable transformation‐optics devices, active photonic components, adaptive and quantum optics, and metrological tools for spectroscopy at THz frequencies. An electrically tunable graphene modulator for terahertz (THz) applications is presented. The modulator design, comprising a grating gated graphene on a polyimide quarter wave waveguide, shows a high degree of tenability and optimum modulation efficiency, and acts as an efficient dispersion compensator for quantum cascade THz frequency combs. The device concept can be extended to alternative photonic applications, paving the way to novel applications in quantum optics.