Tunable, grating-gated, graphene-on-polyimide terahertz modulators

TL;DR

This paper introduces a tunable graphene-based terahertz modulator with high modulation depth and bandwidth, used to stabilize a quantum cascade laser's frequency comb, advancing applications in THz photonics and spectroscopy.

Contribution

The work presents a novel electrically switchable graphene THz modulator with a tunable bandwidth integrated with a QCL to stabilize its frequency comb.

Findings

Achieved 90% modulation depth at 20 kHz bandwidth in 1.9-2.7 THz range.

Demonstrated partial compensation of QCL cavity dispersion.

Enabled stable frequency comb with 98 modes over 1.2 THz spectral coverage.

Abstract

We present an electrically switchable graphene terahertz (THz) modulator with a tunable-by-design optical bandwidth and we exploit it 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. We get 90% modulation depth of the intensity, combined with a 20 kHz electrical bandwidth in the 1.9 _ 2.7 THz range. We then integrate our modulator with a multimode THz QCL. By adjusting the modulator operational bandwidth, we demonstrate that the graphene modulator can partially compensates the QCL cavity dispersion, resulting in an integrated laser behaving as a stable frequency comb over 35% of the laser operational range, with 98 equidistant optical modes and with a spectral coverage of ~ 1.2 THz. This has significant potential for frontier applications in the terahertz, as tunable transformation-optics devices, active photonic components, adaptive and quantum optics, and as a metrological tool for spectroscopy at THz frequencies.