Broadband Light Extraction from Shallow NV Centers Using Low-Q Silicon Antennas

Negatively charged nitrogen-vacancy (NV) centers are fluorescent lattice defects whose energy levels are highly sensitive to external conditions, making them good candidates for sensing applications. The NV zero-phonon transition line is at a wavelength of 637 nm, with most room-temperature fluorescence occurring in the vibrational sideband from 630 to 800 nm [1]. However, diamond’s high refractive index (~2.4) results in total internal reflection for NV fluorescence at angles exceeding the critical angle of approximately 25°, and even at normal incidence the Fresnel reflectance at the interface is high. Therefore, efficiently extracting fluorescence from NV centers in diamond is a challenge [2]. Previous methods for enhancing photon collection from bulk diamond involve etching the diamond substrate [2-3]. However, this method can cause surface damage and potentially compromise NV properties by increasing surface roughness or altering chemical termination [4].<br/>We have developed a silicon-based antenna that is placed directly on a flat diamond surface and enhances the output of NV centers across their entire room-temperature emission spectrum. This design is effective for shallow NV centers, and for this work we targeted an emitter depth of 10 nm. The antenna, consisting of a crystalline silicon (Si) pillar with a diameter of 500 nm and a height of 220 nm, outcouples a substantial portion of light from the NV centers. The Si pillar is compatible with the standard silicon device layer used in silicon photonics [5]. We used crystalline Si because, due to its indirect bandgap, it exhibits sufficiently low loss at wavelengths longer than 500 nm, whereas polycrystalline and amorphous silicon have higher absorption coefficients at these wavelengths [6].<br/>We fabricated the antenna by transferring a single-crystal silicon membrane from a silicon-on-insulator (SOI) wafer onto the diamond surface via an epitaxial lift-off technique. The structure was defined using electron-beam lithography and silicon etching, leaving the diamond surface unetched. Our process is as follows: (1) Undercut the SOI piece using hydrogen fluoride (HF) and buffered oxide etchant (BOE) solution; (2) Transfer the Si membrane onto the diamond substrate; (3) Deposit a SiO2 hard mask using plasma-enhanced chemical vapor deposition (PECVD); (4) Spin ZEP520A resist onto the substrate; (5) E-beam exposure and development; (6) Etch the SiO2 layer using CHF3 and O2 to create the features in the hard mask; (7) Etch the Si layer using HBr/O2 to form Si pillars; (8) Remove the resist residue with O2 plasma.<br/>The optical performance of the fabricated antenna was evaluated using a home-built confocal microscope, with excitation from a 515 nm green laser. Utilizing an objective lens with a numerical aperture of 0.95 to both excite and collect light, we achieved a 3.6-fold increase in the saturated single-photon count rate and observed a decrease in lifetime to about 7 ns, compared to 18 ns for similar shallow NV centers in bare diamond. For at least one NV and silicon pillar, we observed a second-order correlation g^(2)(0) =0.31, confirming that this fluorescence originated from a single NV center. Our method can be adapted to enhance the emission into free space of various color centers in diamond and in other wide-gap materials.<br/>We acknowledge support from the NSF (1839174-CHE). Work performed at the Center for Nanoscale Materials, a U.S. Department of Energy Office of Science User Facility, was supported by the U.S. DOE, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.<br/><b>References</b><br/>[1] R. Schirhagl, et al., Annu. Rev. Phys. Chem. 65, 83–105 (2014).<br/>[2] T. Y. Huang, et al., Nat. Commun. 10, 1-10 (2019).<br/>[3] N. H. Wan, et al., Nano Lett. 18, 2787-2793 (2018).<br/>[4] B. Ofori-Okai, et al., Phys. Rev. B 86, 1-10 (2012).<br/>[5] D. X. Xu, et al., IEEE J. Sel. Topics Quantum Electron. 20, 1-10 (2014).<br/>[6] D. Sell, et al., ACS Photonics 3, 1919–1925 (2016).