🔬 mesoscale physics

Optical Switching of $χ^{(2)}$ in Diamond Photonics

This paper demonstrates the first optical switching of diamond's effective second-order susceptibility ( $\chi^{(2)}$ ) by utilizing a nanoscale cavity to modulate nitrogen-vacancy center charge states via photoionization, thereby enabling controllable second-harmonic generation.

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

Published 2026-07-07

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CC BY 4.0

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

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a diamond not just as a sparkling gem, but as a tiny, high-tech factory for light. For a long time, scientists knew diamonds were great at handling light, but they had a major limitation: they were too "symmetrical" to perform a specific, useful trick called Second-Harmonic Generation (SHG).

Think of SHG like a magic trick where you take two low-energy light particles (photons) and smash them together to create one high-energy particle with double the energy. It's like taking two slow-moving cars and merging them into one super-fast race car. Normally, diamonds are too perfectly balanced to let this happen.

The Problem: The Diamond is Too Perfect

The researchers explain that because a diamond's internal structure is perfectly symmetrical (like a perfectly balanced seesaw), it cancels out the ability to perform this "two-to-one" light trick. To make it work, you have to break that symmetry.

The Solution: The "Defect" Switch

In this study, the team used a tiny, microscopic disk made of diamond. Inside this diamond, there are natural "flaws" or defects called Nitrogen-Vacancy (NV) centers. You can think of these defects as little switches or batteries trapped inside the diamond crystal.

Usually, these switches are in a "negative" state (charged like a battery). The researchers discovered that these negative switches are actually the secret sauce that allows the diamond to perform the SHG magic trick. When the switches are negative, the diamond can turn two light particles into one.

The Experiment: Flipping the Switch with Light

The team set up an experiment with two lasers:

The IR Laser (Infrared): This is the "worker" laser. It shines into the diamond disk to try and perform the SHG trick.
The Green Laser: This is the "control" switch.

Here is what happened:

When the Green Laser was OFF: The NV switches stayed in their "negative" state. The diamond successfully performed the magic trick, and the researchers saw a strong signal of the new, high-energy light.
When the Green Laser was ON: The green light hit the diamond and "photoionized" the defects. This is a fancy way of saying the green light stripped an electron away from the defect, changing it from a "negative" state to a "neutral" state.
The Result: As soon as the switches flipped to neutral, the magic trick stopped. The high-energy light signal vanished (or was "quenched").

The Analogy: The Orchestra and the Conductor

Imagine the diamond is an orchestra.

The SHG process is the music being played.
The NV defects are the conductors holding the batons.
When the conductors are in the "negative" state, they are actively waving their batons, and the orchestra plays loud, beautiful music (the SHG signal).
When the Green Laser shines on them, it forces the conductors to drop their batons and sit down (changing to the "neutral" state).
Without the conductors waving, the music stops immediately.

By simply turning the green laser on and off, the researchers could start and stop the diamond's ability to generate this special light. They did this repeatedly over 40 hours, proving they could toggle the diamond's properties like a light switch.

Why This Matters (According to the Paper)

The paper claims this is the first time anyone has shown that you can control a diamond's ability to do this specific light trick just by changing the electrical charge of the tiny defects inside it.

They also mapped out exactly which colors of light cause this change. They found that only certain colors (energies) are strong enough to flip the switch. If the light is too weak (like deep red light), nothing happens. If it's strong enough (like green or blue light), it flips the switch and stops the magic trick.

In summary: The researchers proved that by using a green laser to change the electrical charge of tiny flaws inside a diamond, they can instantly turn the diamond's ability to generate new light on and off. This turns the diamond into a controllable optical switch, a feat that was previously thought impossible for this material.

Technical Summary: Optical Switching of $\chi^{(2)}$ in Diamond Photonics

Problem Statement
Diamond possesses exceptional physical properties for quantum and nonlinear photonic technologies, yet its centrosymmetric crystal structure results in a vanishing second-order nonlinearity ( $\chi^{(2)} = 0$ ). This symmetry constraint limits diamond's utility in nonlinear optics to third-order processes. While spatial symmetry breaking (e.g., via surfaces or defects) can induce an effective second-order susceptibility ( $\chi^{(2)}_{\text{eff}} \neq 0$ ), the mechanisms governing this in diamond, particularly the role of native defects like nitrogen-vacancy (NV) centres, remain to be fully elucidated. Specifically, it is unclear if the electronic configuration of these defects can be optically manipulated to dynamically control $\chi^{(2)}_{\text{eff}}$ .

Methodology
The authors fabricated a diamond microdisk cavity hosting uniform distributions of NV centres ( $\sim 10^{13} - 10^{14} \text{ cm}^{-3}$ ) and substitutional nitrogen ( $N_s$ , $\sim 10^{17} \text{ cm}^{-3}$ ). The device supports whispering-gallery modes (WGMs) with quality factors $Q = 10^3 - 10^5$ , evanescently coupled to a fibre-taper waveguide.

The experimental setup employed two laser sources:

Infrared (IR) Pump: A tunable continuous-wave (CW) laser ( $\lambda_{\text{IR}} \approx 1546$ nm, $P_{\text{IR}} = 77$ mW) coupled into the microdisk to drive second-harmonic generation (SHG). The SHG process is doubly resonant, enhanced by modes at both $\lambda_{\text{IR}}$ and $\lambda_{\text{IR}}/2$ .
Visible Control: A green CW laser ( $\lambda_{\text{vis}} = 532$ nm) focused on the microdisk edge to excite NV centres and induce charge-state transitions.

The researchers monitored the SHG signal intensity and NV photoluminescence (PL) while toggling the green laser. To map the underlying mechanisms, they varied the green laser power and, in a separate experiment, substituted the green laser with a pulsed supercontinuum laser to scan excitation wavelengths from 480 nm to 800 nm.

Key Results

Observation of SHG: The team demonstrated SHG in a diamond nanophotonic cavity using low-power CW IR light, confirming the presence of a non-zero $\chi^{(2)}_{\text{eff}}$ .
Optical Switching of $\chi^{(2)}$ : Illuminating the microdisk with the green laser resulted in a deterministic quenching of the SHG signal by approximately 80%. This effect was reversible; toggling the green laser on and off allowed for the switching of the device's $\chi^{(2)}_{\text{eff}}$ over a 40-hour period without degradation.
Correlation with Charge State: The degree of SHG quenching ( $R_{\text{SHG}}$ ) showed a strong correlation with the suppression of NV photoluminescence ( $R_{\text{PL}}$ ). This indicates that the change in $\chi^{(2)}_{\text{eff}}$ is directly linked to the photoionization of NV centres from the negatively charged state ( $NV^-$ ) to the neutral state ( $NV^0$ ).
Wavelength-Dependent Mechanisms: By scanning the visible excitation wavelength, the authors identified distinct regimes based on photoionization thresholds:
- Region I ( $\lambda < 564$ nm): Strong quenching occurs due to the photoionization of both $N_s$ and both NV charge states, significantly altering the local charge environment.
- Region II & III (564 nm $< \lambda <$ 637 nm): Quenching persists but is reduced, corresponding to regimes where only specific charge-state transitions (e.g., $NV^- \to NV^0$ ) are energetically allowed without ionizing $N_s$ .
- Region IV ( $\lambda > 637$ nm): No quenching is observed, as photoionization is energetically forbidden, leaving the charge environment unchanged.
Mechanism Identification: The quenching is attributed to the reduction of the local electric field ( $E_{\text{DC}}$ ) generated by charged defects. The conversion of $NV^-$ to $NV^0$ (and associated changes in $N_s$ charge states) reduces the effective dipole moments or charge separation responsible for the field-induced second-order nonlinearity ( $\chi^{(2)}_{\text{EFISH}} \approx 3\chi^{(3)}E_{\text{DC}}$ ).

Significance and Claims
The paper claims the first demonstration of second-order nonlinear integrated photonics in diamond and the first observation of optical switching of $\chi^{(2)}_{\text{eff}}$ in a nanophotonic device. The authors establish that the magnitude of diamond's effective second-order susceptibility is not a static material property but is dynamically dependent on the electronic configuration of crystal defects.

The work suggests that defect engineering offers a pathway to enable and control second-order nonlinear processes in centrosymmetric materials. By monitoring SHG intensity, the device can also serve as a probe for solid-state charge-state environments. The authors conclude that this capability opens the door for future applications in frequency conversion, modulators, and optical switching via $\chi^{(2)}$ engineering within the diamond photonics toolbox. The paper notes that while the mechanism is identified as charge-state dependent, further time-domain measurements are required to definitively isolate the dominant physical contribution (microscopic $\chi^{(2)}$ of the defect vs. macroscopic field effects).