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Photonics Technology Development Accelerates Synthetic Diamond Applications

September 05,2023

Advances in synthetic diamond production have enabled new photonics technologies, but these new technologies still present many challenges in serving quantum applications.

Over the past decade or so, a number of commercial and emerging photonics technologies that exploit the unique physical properties of diamond have ushered in significant progress, driven by a series of key technology trends and market demands. These advances have been made possible by innovations in the synthesis of optical-quality diamond by chemical vapor deposition (CVD), diamond color center engineering, and techniques for fabricating diamond optical components and photonic structures.

Photonics Applications Based On The Excellent Intrinsic Properties Of Diamond


High-purity diamond is transparent in the frequency range from ultraviolet to terahertz and even higher. It has the highest room temperature thermal conductivity (>5 times that of copper) of any bulk material, while having a low thermo-optic coefficient. These properties make diamond optics ideal for high-power industrial laser applications, including machining, welding and additive manufacturing, where it is suitable for many different parts of the electromagnetic spectrum.

Additionally, diamond, the hardest known substance on Earth, is extremely hard and strong, making it ideal for defense and security applications that require robust optical and infrared components capable of functioning in extremely challenging environments. Optical quality CVD diamond is available in single and polycrystalline forms. The advantage of polycrystalline diamond is that it can be used for large-scale large-area devices up to 135 mm in diameter. For example, it can serve as a window for high-power 10.6 μm CO2 lasers used in extreme ultraviolet (EUV) lithography systems for state-of-the-art semiconductor device fabrication nodes. The technology, driven to keep pace with Moore's Law, relies heavily on synthesizing, processing diamond windows to stringent optical quality standards, as no other optical material can operate under the extreme laser conditions required.


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For industrial lasers, large-area polycrystalline CVD diamond optics are more robust and reliable than the ZnSe optics they typically replace
Image credit: Element Six


Scattering losses in polycrystalline CVD diamond at wavelengths shorter than about 1.5 μm mean that most applications in this range are addressed using single crystal diamond. Due to size limitations of currently available diamond substrates, single crystal diamond elements are typically around 5-10 mm in length, although some producers are developing large-area single crystal diamond on non-diamond substrates due to their relatively high internal strain, this material cannot be used in all optical applications. Despite the size constraints, several single-crystal CVD diamond photonics technologies have been developed, such as diamond Raman lasers based on Element Six's unique low-absorption, low-birefringence crystals. These nonlinear lasers exploit the phenomenon of stimulated Raman scattering to convert the pump beam into a Stokes-shifted output beam, expanding the range of available laser sources for new applications spanning the UV to the IR, including: materials welding, 3D printing, directed energy, lidar, remote sensing and laser guide stars (LGS).

Diamond has one of the highest Raman gain coefficients, combined with its excellent thermal conductivity, making it an ideal gain medium to demonstrate power scaling and brightness enhancement, including in the "eye-safe" spectral region of 1.4-1.8 μm. In this range, the choice of available laser sources has traditionally been limited.

Expanding Diamond Applications Through Color Center Engineering

While diamond has an excellent set of inherent optical properties, it also has hundreds of different optically active defects (color centers). Some of these are important for technological applications that exploit the quantum state of light and the electron spin properties of color centers, including quantum communications, quantum computing and a range of sensing applications.

Of particular note is the nitrogen-vacancy (NV) color center—a luminescent point defect in diamond whose quantum state can be easily manipulated at room temperature by the application of light and radiofrequency fields. has been the subject of intensive research. Depending on the final application process, one can create NV centers in two ways. One is by controlling nitrogen incorporation during CVD growth so that nitrogen atoms are distributed throughout the material at a desired concentration. On the other hand, precise spatial control of individual color centers is required, using nitrogen implantation. Then, lattice vacancies are generated by high-energy electron irradiation, and the crystal is annealed at high temperature to mobilize the vacancies to combine with nitrogen atoms in the crystal, thereby forming NV color centers. A similar approach can be used to form other custom-made color centers, such as silicon-vacancy (SiV) or germanium-vacancy (GeV) centers.
For quantum information processing, arrays of color centers are needed—both to control their quantum properties and to efficiently couple individual centers together through photonic cavities. Due to diamond's chemical inertness and lack of broad market availability, considerable effort and money is still required to develop the nanofabrication techniques required for such structures; however, researchers have made great strides in this area in recent years, This includes the fabrication of complex nanostructures in the form of waveguides, pillars, cavities, and disks, using various photolithographic techniques, and etching with plasma and reactive ion beams.

Facing Future Challenges For Diamond Quantum Photonics

In recent years, researchers have made significant progress in producing diamonds with high intrinsic optical quality and high-quality color centers, and have enabled many new and existing advanced photonics technologies.

However, diamond applications in the field of quantum photonics still have some challenges before the successful realization of scalable chips for applications such as quantum information processing. These include: improvements in color center engineering and qubit robustness; fabricating wafers; and hybrid integration with other photonic materials and components. Despite these challenges, current research in these areas is very active and substantial progress is expected in the coming years.