New Record Set for Low-Loss Diamond Waveguides Using Femtosecond Lasers

## The Future of Photonics and Quantum Sensing is Diamond Diamond is more than just a gemstone; its exceptional physical properties make it a superstar material for advanced technology. With a wide spectral transparency window (from UV to infrared), a high refractive index, and a high nonlinear refractive index, diamond is a superior platform for linear and nonlinear optics. Furthermore, the presence of spin-active nitrogen-vacancy (NV) centers allows diamond to be used in quantum information and sensing. These NV centers, whose spin-dependent fluorescence can be excited with a laser, are highly sensitive to physical quantities like magnetic fields, strain, and temperature, making them ideal for next-generation sensors. For these applications to become a reality, we need a reliable way to guide light and connect these NV centers within the diamond crystal. This is the crucial role of optical waveguides. ## The Challenge: Crafting Low-Loss Waveguides in Diamond Femtosecond (fs) laser writing is a powerful technique for fabricating three-dimensional waveguide structures inside transparent materials. It works by focusing high-intensity, ultra-short laser pulses inside the material, locally modifying its refractive index to create a path for light. However, in diamond, this process has a significant challenge: graphitization. The intense laser energy can convert the diamond's carbon atoms into graphite, which absorbs light and increases the waveguide's propagation loss. High loss means a weaker signal and less efficient devices. Prior to this research, the lowest reported propagation loss for an fs-laser-written diamond waveguide was 4.20 dB/cm, a figure that limited the potential for high-efficiency quantum devices. ## The Breakthrough: Record-Low Loss in Appsilon CVD Diamond In a recent study, researchers utilized a high-quality, single-crystal CVD-grown diamond from Appsilon to systematically tackle this challenge. By experimenting with different waveguide geometries and laser-writing parameters, the team successfully fabricated waveguides with unprecedentedly low propagation losses. Three different designs were fabricated and tested: * **Depressed Circular Cladding:** A core guiding region is surrounded by a series of laser-written tracks with a lower refractive index. * **Half-Ring:** A variation of the cladding design, forming a semi-circular barrier. * **Double-Line:** Two parallel laser-written lines confine light between them. The waveguides were characterized at a wavelength of 633 nm, chosen specifically for its proximity to the peak fluorescence wavelength of the critical NV centers. ## The Results: A New Industry Benchmark The study yielded groundbreaking results, setting a new standard for the industry. The optimized waveguides showed significantly lower propagation losses than any previously reported: * **1.20 dB/cm** for the half-ring waveguide. * **2.05 dB/cm** for the circular depressed cladding waveguide. These values, particularly the 1.20 dB/cm result, represent a greater than 3.5x improvement over the previous record. The research also revealed a key insight: propagation loss decreases as the waveguide's core size increases and as the number of laser-written tracks decreases. This provides a clear roadmap for future optimization. These ultra-low-loss waveguides are a pivotal achievement. Lower loss directly translates to higher signal collection efficiency, paving the way for more sensitive quantum sensors and more robust quantum computing platforms built with diamond.