Color-Center-Compatible Freestanding Diamond Directional Couplers for Quantum Photonics

Freestanding all-diamond color-center photonics is a promising platform for optical integration of spin-based quantum defects. Within this geometry, we realize a key building block for quantum-network interconnects: a directional coupler that acts as an on-chip beam splitter. We design and simulate directional couplers with triangular cross sections using eigenmode and finite-difference time-domain simulations and target near-50:50 splitting at visible wavelengths. We fabricate the devices directly from bulk single-crystal diamond by angled oxygen reactive-ion-beam etching followed by a dry postrelease hard-mask removal process. Room-temperature measurements at λ0 ≈ 637 nm yield a mean coupling ratio of Cmeas = 46(16) %. Finally, we integrate tin-vacancy (SnV–) centers into the nanophotonic structures and observe near-lifetime-limited optical line widths and coherent optical Rabi oscillations without postfabrication annealing, identifying the platform as a viable route toward integrated diamond quantum photonics.