All in One‐Chip, Electrolyte‐Gated Graphene Amplitude Modulator, Saturable Absorber Mirror and Metrological Frequency‐Tuner in the 2–5 THz Range

Abstract Layered 2D materials display unique optical and electrical properties that can enable manipulation, propagation, and detection of electromagnetic waves over a broad spectral range, with a high level of control, offering the potential to activate different functionalities, by optical or electrical means, in a single chip. Here, a compact optoelectronic device behaving as an amplitude modulator, saturable absorber mirror (SA mirror), and frequency‐tuner is conceived at terahertz (THz) frequencies. It comprises a gate‐tunable single layer graphene (SLG), embedded in a quarter‐wave cavity, operating in the 1–5 THz range. The use of electrolyte ionic liquid gate ensures 40% optical amplitude modulation depth. Z‐scan self‐mixing interferometry reveals 60% reflectivity modulation, with ≈4.5 W cm⁻² saturation intensity. By integrating the modulator/SA mirror with a heterogeneous THz quantum cascade laser frequency comb, in an external cavity configuration, fine‐tuning of the intermode beatnote frequency is also demonstrated. This opens intriguing perspectives for short pulse generation, phase‐locking, frequency tuning/chirping, phase modulation, and metrological referencing, inter alia.