Two-Dimensional Photonic Crystal Cavities in Bulk Single-Crystal Diamond

Color centers in diamond are leading candidates for quantum information processing. Recent demonstrations of entanglement between separated spins of the nitrogen-vacancy (NV) color center constitute a major milestone in generating and distributing quantum information with solid-state quantum bits. However, the generation of entanglement in local quantum nodes containing NV centers is an inefficient process due to the largely incoherent NV optical transitions, as the zero-phonon-line (ZPL) constitutes only 4% of the NV’s spontaneous emission. This fraction can be modified if the NV center is placed in a photonic cavity, which modifies the electromagnetic environment, and thus, the NV’s emission properties via the Purcell effect. Photonic crystal (PhC) slab nanocavities offer high-quality factors (Q) and small mode volumes (V), which considerably increase the fraction of emission into the ZPL. The electric field profile is shown in such a nanocavity, where the lattice constant is a = 214 nm and the thickness of the slab is H = a.

The fabrication of such structures, however, typically requires laborious reactive-ion etching (RIE) thinning of a bulk diamond down to a thickness of H. This need arises because high-quality single-crystal diamond thin films are not available and the
chemically inert nature of diamond precludes wet undercutting techniques. In this work, we fabricate PhC nanocavities in diamond directly from bulk diamond. Electron beam lithography and reactive ion etching (RIE) first defines the PhC structures, after which alumina deposited using atomic layer deposition conformally coats and protects the diamond sidewalls. Then, anisotropic oxygen plasma undercuts the diamond slabs and, finally, hydrofluoric acid removes the hard mask and alumina to reveal suspended diamond structures. We find high Q resonances near the NV ZPL wavelength of 637 nm, as shown in the photoluminescence spectra. The fabrication details and cavity measurements are in the last reference.

In conclusion, we report the first fabrication of photonic crystal slab nanocavities in bulk diamond. Immediate steps include the coherent coupling of a single NV center to the nanocavity, which will serve as a node in a quantum repeater and for solid-state cavity quantum electrodynamics investigations. This 2-D platform considerably expands the toolkit for classical and quantum nanophotonics in diamond.