Photonics Technology Development To Accelerate The Application Of Artificial Diamond

Advances in the production of synthetic diamonds have made new photonics technologies possible, but many challenges remain for these new technologies in serving quantum applications.
Over the past decade or so, driven by a number of key technology trends and market demand, many commercial, emerging photonics technologies that take advantage of the special physical properties of diamond have seen significant advances. Innovations in the synthesis of optical quality diamond by chemical vapor deposition (CVD), diamond color center engineering, and technologies for the fabrication of diamond optical components and photonic structures have made these advances possible.
Photonics applications based on diamond's excellent intrinsic properties
High purity diamond exhibits transparency in the frequency range from ultraviolet to terahertz and beyond. It has the highest room temperature thermal conductivity of any bulk material (>5 times that of copper), while having a low thermo-optical coefficient. These properties make diamond optics ideal for high-power industrial laser applications, including machining, welding and additive manufacturing, where it is applicable to many different parts of the electromagnetic spectrum.
In addition, diamond is the hardest known substance on earth, and it is extremely hard and rugged, making it also ideal for defense and security applications that require rugged optical and infrared components and the ability to function in very challenging environments.
Optical quality CVD diamond is available in single crystal and polycrystalline forms. The advantage of polycrystalline diamond is that it can be used for large size large area devices up to 135 mm in diameter. For example, it can be used as a window for high-power 10.6 μm CO2 lasers for extreme ultraviolet (EUV) lithography systems for the most advanced semiconductor device fabrication nodes.
This technology, which is driven by keeping pace with Moore's Law, relies heavily on synthesizing and processing diamond windows to stringent optical quality standards, as no other optical material can operate under the extreme laser conditions required.
Scattering losses in polycrystalline CVD diamond at wavelengths shorter than about 1.5 μm mean that most applications in that range are addressed using single crystal diamond. Due to the size limitations of currently available diamond substrates, single-crystal diamond elements are typically around 5-10 mm in length, and although some manufacturers are developing large-area single-crystal diamonds on non-diamond substrates, this material cannot be used for all optical applications due to its relatively high internal strain.
Despite the size limitations, some single-crystal CVD diamond photonics techniques have been developed, such as diamond Raman lasers based on Element Six's unique low-light-absorbing, low-birefringence crystals.
These nonlinear lasers exploit the phenomenon of excited Raman scattering to convert the pump beam to a Stokes-shifted output beam, thus expanding the range of available laser sources for new applications covering the UV to IR, including: material welding, 3D printing, directed energy, LIDAR, remote sensing, and laser guided stars (LGS).
Diamond has one of the highest Raman gain coefficients, which, combined with its excellent thermal conductivity, makes it an ideal gain medium for demonstrating power scaling and brightness enhancement, including in the "human eye safe" spectral region of 1.4-1.8 μm. In this range, the choice of available laser sources has previously been limited.
Expanding Diamond's Applications through Color Core Engineering
While diamond has an excellent set of intrinsic optical properties, it also has hundreds of different optically active defects (color centers). Some of these are important for technical 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 that has been the subject of intensive research due to the ability to easily manipulate its quantum state by the application of light and RF fields at room temperature.
Depending on the final application process, one can create NV color centers in two ways. One is by controlling the doping of nitrogen during the CVD growth process so that the nitrogen atoms are distributed throughout the material at the desired concentration. On the other hand, precise spatial control of the individual color centers is required, using nitrogen injection. The lattice vacancies are then created by high-energy electron irradiation, and the crystal is annealed at high temperatures to mobilize the vacancies to bond with the nitrogen atoms in the crystal, resulting in NV color centers. A similar approach can be used to form other customized color centers such as silicon vacancies (SiV) or germanium vacancies (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 the chemical inertness of diamond and the lack of widespread market availability, considerable effort and funding is still required to develop the nanofabrication techniques required for such structures; however, in recent years, researchers have made great progress in this area, including the fabrication of complex nanostructures in the form of waveguides, columns, cavities, and disks, using a variety of photolithography techniques, and employing plasma and reactive ion beams for etching.
Future challenges for achieving 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 techniques.
However, a number of challenges remain before diamond applications in quantum photonics can be successfully implemented as scalable chips for applications such as quantum information processing. These include: improvement of color-centered engineering and robustness of quantum bits; fabrication of wafers; and hybrid integration with other photonic materials and components. Despite these challenges, current research directed toward these areas is very active and substantial progress is expected in the coming years.