Optically-Induced Modulation and Programming of Diamond Photonic Cavities

Abstract: Single-crystal diamond combines exceptional optical, thermal, and mechanical properties while hosting optically addressable and spin-coherent colour centres, making it a promising platform for integrated quantum and nonlinear photonics. However, practical post-fabrication mechanisms for dynamically controlling and tuning monolithic diamond photonic circuits remain limited. Here, we demonstrate all-optical modulation and long-lived tuning of suspended diamond Fabry--Perot nanobeam cavities containing nitrogen-vacancy centres. Under 532~nm illumination, the infrared (1000 to 1100~nm) cavity response is governed by two competing contributions: a well-understood thermo-optic red-shift, and a novel charge-mediated blue-shift that we attribute to the free-carrier plasma dispersion effect. With 10~mW of 20~kHz-modulated green light, we achieve an infrared modulation depth of approximately 45\% of the available reflection contrast. Green illumination also produces quasi-permanent photo-refractive resonance blue-shifts, with a maximum observed tuning of 3.15~nm (0.87~THz), without measurable degradation of the cavity linewidth or contrast. Achieving the same cavity shift in a lithium-niobate Pockels modulator would require 200 to 500~V across a (5~μ\mathrm{m}) electrode gap. These results establish a monolithic diamond nanophotonic platform combining agile and reversible modulation with long-lived optical reconfiguration, two key ingredients in photonic technologies.