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arXiv:2412.06792 · Physical Review Letters

Optical Switching of χ ( 2 ) in Diamond Photonics

Sigurd Flågan, Joe Itoi, Prasoon Kumar Shandilya, Vinaya K. Kavatamane, Matthew Mitchell, David P. Lake, Paul E. Barclay

Published Oct 28, 2025• 3 citations

Abstract

Diamond's unique physical properties make it a versatile material for a wide range of nonlinear and quantum photonic technologies. However, unlocking diamond's full potential as a nonlinear photonic material with nonzero second-order susceptibility, χ^{(2)}≠0, requires symmetry breaking. In this Letter, we use a nanoscale cavity to demonstrate second-harmonic generation (SHG) in diamond, and demonstrate, for the first time, that the magnitude of diamond's effective χ^{(2)} strongly depends on the electronic configuration of defects in the diamond crystal, such as nitrogen-vacancy centers. The modification of χ^{(2)} arises from photoionization from the negative to neutral charge state, and is manifested by quenching of SHG upon green illumination. Toggling the green illumination allows for optical switching of the device's χ^{(2)}. Optical control of χ^{(2)} by defect engineering opens the door for second-order nonlinear processes in diamond.

DOI: 10.1103/yx3z-cr5v

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Diamond's unique physical properties make it a versatile material for a wide range of nonlinear and quantum photonic technologies. However, unlocking diamond's full potential as a nonlinear photonic material with nonzero second-order susceptibility, χ^{(2)}≠0, requires symmetry breaking. In this Letter, we use a nanoscale cavity to demonstrate second-harmonic generation (SHG) in diamond, and demonstrate, for the first time, that the magnitude of diamond's effective χ^{(2)} strongly depends on the electronic configuration of defects in the diamond crystal, such as nitrogen-vacancy centers. The modification of χ^{(2)} arises from photoionization from the negative to neutral charge state, and is manifested by quenching of SHG upon green illumination. Toggling the green illumination allows for optical switching of the device's χ^{(2)}. Optical control of χ^{(2)} by defect engineering opens the door for second-order nonlinear processes in diamond.

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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.

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.

The photodynamics of diamond nitrogen-vacancy (NV) centers limits their performance in many quantum technologies. Quenching of photoluminescence, which degrades NV readout, is commonly ascribed to a dark state that is not fully understood. Using a nanoscale cavity to generate intense infrared fields that quench NV emission nonlinearly with field intensity, we show that the dark state is accessed by two-photon pumping into the

^4\!A_2

quartet state of the neutrally charged NV (NV

^0

). We constrain this state's energy relative to the NV

^0

ground-state (

^2\!E

) to

{<}0.58

\,eV and the recombination energy threshold to the NV

^-

ground state (

^3\!A_2

) to

\leq2.33\,\text{eV}

. Furthermore, we estimate the intrinsic lifetime of

^4\!A_2

state to be

1.78-6.06\,μ\text{s}

and show that accessing this state allows sensing of local infrared fields. This new understanding will allow predictions of the limits of NV technologies reliant upon intense fields, including levitated systems, spin--optomechanical devices, and absorption--based magnetometers.