Femtosecond laser fabrication and optical characterization of low-loss diamond waveguides

In addition to its excellent mechanical and thermal properties, diamond also possesses favorable linear and nonlinear optical properties, including a wide spectral transparency window from the ultraviolet to infrared, a high refractive index, and a reasonably high nonlinear refractive index, among others. In recent years, diamond has also been demonstrated as a promising platform for quantum information and sensing applications due to the presence of spin-active nitrogen vacancy centers (NV) with long coherence times at room temperature. These NV centers generate spin-dependent fluorescence when excited at 532 nm and enable the measurement of numerous physical quantities such as strain, magnetic field, and temperature with high sensitivity. In such quantum sensing applications, optical waveguides play a crucial role, since they can be used for addressing and spatially linking NV centers or for guiding fluorescence between different locations inside the diamond crystal. In this thesis, fabrication and optical characterization of femtosecond (fs) laser written waveguides with varying geometries and parameters in a single crystal CVD grown diamond has been demonstrated. Depressed circular cladding, half-ring, and double-line waveguides were fabricated. Design parameters such as core size and number of written tracks were varied to minimize propagation losses. Characterization of the waveguides was performed at 633 nm, which is close to the peak fluorescence wavelength of the nitrogen NV centers in diamond. Important experimental results revealing the dependence of the propagation loss and refractive index contrast on the design parameters were obtained. The maximum refractive index contrast was estimated as 22.7x10-5 for the fabricated waveguides. The measured propagation loss values of 2.05 dB/cm and 1.20 dB/cm, obtained with circular depressed cladding and half-ring waveguides, respectively, are, to the best of our knowledge, the lowest propagation loss values reported so far among fs laser written diamond waveguides. It is forseen that the photonic devices based on the diamond waveguides examined in this thesis can find applications in quantum communication.