High-Q optical nanocavities in bulk single-crystal diamond (1408.5973v1)

Abstract: Single-crystal diamond, with its unique optical, mechanical and thermal properties, has emerged as a promising material with applications in classical and quantum optics. However, the lack of heteroepitaxial growth and scalable fabrication techniques remain major limiting factors preventing more wide-spread development and application of diamond photonics. In this work, we overcome this difficulty by adapting angled-etching techniques, previously developed for realization of diamond nanomechanical resonators, to fabricate racetrack resonators and photonic crystal cavities in bulk single-crystal diamond. Our devices feature large optical quality factors, in excess of 10^5, and operate over a wide wavelength range, spanning visible and telecom. These newly developed high-Q diamond optical nanocavities open the door for a wealth of applications, ranging from nonlinear optics and chemical sensing, to quantum information processing and cavity optomechanics.

• The paper introduces an innovative angled-etching approach to fabricate free-standing diamond nanocavities with optical Q-factors exceeding 10^5.
• Methodological advances include racetrack resonators and photonic crystal cavities, achieving Q-factors of 151,000 for TE and 113,000 for TM modes in the telecom range.
• These high-performance devices enable scalable integration of diamond in nanophotonic circuits, with promising applications in quantum information and nonlinear optics.

High-Q Optical Nanocavities in Bulk Single-Crystal Diamond

The paper presents an advanced approach to fabricating high-quality factor (Q) optical nanocavities using single-crystal diamond, which stands out due to its unique optical, mechanical, and thermal properties. This research overcomes a critical limitation in diamond photonics—namely, the absence of heteroepitaxial growth and scalable fabrication techniques. Leveraging angled-etching, a method traditionally used for realizing diamond nanomechanical resonators, the authors have successfully developed racetrack resonators and photonic crystal cavities with optical Q-factors surpassing $10^5$, operable across a broad wavelength range, including visible and telecom spectrums.

Key Methodological Advances

The core of this work lies in the deployment of an innovative angled-etching approach, performed in a standard ICP-RIE setup, to construct free-standing, triangular cross-section optical structures within diamond. This involves anisotropic oxygen plasma etching at an oblique angle to ensure the creation of high-quality optical resonators supported by novel structural designs, including tapered vertical support structures. These structural innovations play a pivotal role in minimizing scattering losses that often plague conventional free-standing optical structures.

Significant Results

Utilizing their novel methodology, the researchers achieved notable Q-factors of $Q_{L,TE} \sim 151,000$ and $Q_{L,TM} \sim 113,000$ for racetrack resonators in the telecom range. Intrinsic Q-factors were measured as $Q_{i,TE} \sim 302,000$ and $Q_{i,TM} \sim 226,000$. These resonators exhibited a light transmission loss ($\alpha$) of about $1.5 \, \text{dB/cm}$, comparable to those obtained through membrane thinning but notably superior to polycrystalline approaches. Furthermore, the reported diamond waveguides were found to sustain high-Q operation over visible wavelengths, evidencing impressive flexibility and scalability in device functionality.

For photonic crystal nanobeam cavities, the research yielded loaded Q-factors such as $Q_{TE} \sim 183,000$ for TE-like modes. This performance is akin to the best practices in silicon-based photonic crystal technologies, underscoring the competitive standing of single-crystal diamond platforms in high-Q, compact footprint photonic devices.

Implications and Future Directions

The successful realization of high-Q optical nanocavities in bulk diamond, with such scalably fabricated devices, signifies a substantial advancement for the integration of diamond into practical nanophotonic circuits. From a theoretical standpoint, the developed structures could significantly enhance the light-matter interactions critical for quantum information processing through mechanisms like the Purcell effect. High-Q factors coupled with the low loss inherent in these devices point to practical applications in nonlinear optics, including the generation of on-chip frequency combs and Raman lasers.

Looking ahead, this technology paves the way for embedding diamond-based photonic systems in broader quantum networks and exploring hybrid systems integrating color centers like NV$^-$. Beyond quantum applications, the robustness and thermal properties of diamond suggest its suitability for high-power and high-temperature photonic applications. Furthermore, the methodology demonstrated could be extrapolated to other crystalline materials where thin-film growth is not feasible, thereby broadening the scope of high-performance integrated photonics.