Down-scaling grating couplers and waveguides in single-crystal diamond for VIS-UV operation

We report on the design, fabrication and experimental demonstration of single-crystal diamond (SCD) waveguide devices with integrated grating couplers optimized for 850 nm, 635 nm, and 405 nm wavelengths, respectively. The devices are fabricated on SCD thin films using electron-beam lithography and reactive-ion etching technologies. To reduce the wafer wedge typically present in commercial diamond plates, we introduce a novel tilted-etching technique in the preparation of the thin films. We obtain 60% reduction of the wafer wedge, namely from 300 to 120 nm mm(-1), enabling us to properly fabricate the devices designed for the short to ultra-short wavelengths considered here. Using light in- and out-coupling with 50 mu m core tapered fibers, we measure total (input plus output) grating coupling losses of 20.6 dB and 22.7 dB, and waveguide losses of 11 dB mm(-1)and 20.5 dB mm(-1)for the 850 nm and 635 nm wavelength devices, respectively. The 405 nm wavelength devices, tested with a lensed 9 mu m core input fiber and with the same tapered output fiber as employed for the other devices, also demonstrate light guidance, and feature total grating coupling and waveguide losses on the order of 33.1 dB and 46.7 dB mm(-1), respectively. These results showcase the possibility of down-scaling grating-coupled SCD devices for VIS-UV operation, and pave the way for exploiting diamond's properties on photonic chips at extremely short wavelengths.