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Photonics North (PN)

Photorefractive Resonance Tuning and Frequency Conversion in a Diamond Nanocavity

Joe Itoi, Elham Zohari, Waleed El-Sayed, Nicholas J. Sorensen, Sigurd Flågan, Paul E. Barclay

Published May 20, 2025• 0 citations
Publisher

Abstract

Weak optical nonlinear interactions can be greatly enhanced by tight confinement and recirculation of light in an optical resonator. In this work, we demonstrated cavity-enhanced strong green third-harmonic generation from a single-crystal diamond photonic crystal nanocavity. Furthermore, we observe a novel photorefractive resonance blue-shift of the cavity, arising from charge excitation by intense IR pump photons and generated third-harmonic photons. The resulting photorefractive blue-shift greatly exceeds the cold cavity linewidth.

DOI
10.1109/pn66844.2025.11097146

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Weak optical nonlinear interactions can be greatly enhanced by tight confinement and recirculation of light in an optical resonator. In this work, we demonstrated cavity-enhanced strong green third-harmonic generation from a single-crystal diamond photonic crystal nanocavity. Furthermore, we observe a novel photorefractive resonance blue-shift of the cavity, arising from charge excitation by intense IR pump photons and generated third-harmonic photons. The resulting photorefractive blue-shift greatly exceeds the cold cavity linewidth.
Single-crystal diamond nanocavities have tremendous potential for use in quantum and nonlinear optical technologies. The ability to precisely control their resonant frequencies is essential for many applications, and in situ tuning is particularly desirable. In this work, we demonstrate deterministic resonance tuning of a diamond nanocavity. We observed a photorefractive effect in concert with the generation of third-harmonic light within the device. This effect blue-shifted the cavity resonance frequency by 20.2 (2) GHz, exceeding the cavity linewidth. The shift corresponded to a fractional change in refractive index of −1.05(1)×10 −4 , and its relaxation occurred over several tens of hours. Although photorefraction is a second-order nonlinear effect and has previously not been observed in diamond owing to its vanishing χ (2) , the observed behavior is consistent with the generation of non-zero χ (2) by electric fields from charged crystal defects. In addition to allowing tuning of diamond cavities for resonant nonlinear and quantum photonics applications, this observation could enable the realization of diamond frequency converters and electro-optical modulators that rely on second-order nonlinearity.
Resonant recirculation of light in optical microcavities can greatly enhance light-matter interactions. In this work, we demonstrate cavity-enhanced second-harmonic and third-harmonic generation from a single-crystal diamond microdisk. The presence of crystal defects – here nitrogen-vacancy centers – breaks the inversion symmetry of the diamond crystal, allowing for observation of the otherwise forbidden second-harmonic generation. We further show that the photon emission follows a clear dependency with cavity detuning of both the pump and emission modes, a clear indication of a doubly-resonant configuration.
Diamond constitutes a promising material platform for a myriad of photonic applications. In this work, we demonstrate several novel nonlinear optical phenomena in fibre-taper coupled diamond micro and nanocavities. Central to these observations are ensembles of nitrogen-vacancy (NV) centres native to the diamond devices. We first show photoinduced modifications of second-harmonic generation (SHG) from a microdisk, and demonstrate, for the first time, that the strength of the SHG strongly depends on the charge state of the NV centres. Illuminating the microdisk with a green laser quenches the SHG signal. Removing the green laser recovers the original SHG signal, allowing for deterministic optical switching of the device’s χ(2). We next show strong photorefractive blue-shifting of the resonance frequency in a diamond photonic crystal cavity. The total frequency shift exceeds the intrinsic linewidth, corresponding to a fractional change in refractive index of ∼ −10−4.
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 μm5~μ\mathrm{m}5 μm 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.