Nano-optical studies of silicon-vacancy color centers in thin polycrystalline diamond membranes

Quantum information and computing advancements are contingent on the efficacy of spin-photon interfaces, wherein atomic transitions are coupled to the resonant mode of an optical cavity, thereby enhancing the photon emission rate and augmenting the overall efficiency of quantum networks. However, the substantial enhancement of the emission rate (Purcell effect) is only achievable when a quantum emitter interacts with a resonant optical antenna of specific dimensions. Amidst the various semiconductor quantum emitters, silicon-vacancy (SiV) color centers in diamond hold great promise for scalable operations, owing to their limited inhomogeneous broadening and room temperature stability. Nevertheless, their restricted out-coupling efficiency due to the high refractive index of bulk diamonds presents a significant challenge. Addressing this limitation involves the manipulation of the local density of states ( LDOS ) by altering the photonic environment through the integration of dielectric microstructures or plasmonic nanostructures.
To achieve resilient heterostructure devices, integrated quantum photonics, hybrid quantum systems, and other intricate functional materials, it is imperative that the single-color centers be embedded in thin, free-standing membranes with thicknesses ranging from a few micrometers to hundreds of nanometers. While the utilization of high-quality single-crystalline diamond ( SCD) seems intuitive, the technological intricacies involved in fabricating single-crystal diamond membranes present challenges, as they cannot be cultivated on a substrate other than diamond, and their physical and chemical properties lag behind advanced materials such as silicon technology.
Initially, a comprehensive examination of the optical properties of SiV centers in polycrystalline diamond ( PCD) membranes of varying thickness was conducted. A combined study of microphotoluminescence measurements and electron microscopy on PCD membranes housing SiV color centers underscored the pivotal role of diamond grain size in the effective isolation of individual SiV centers, mitigating background interference originating from grain boundaries (GBs). Additional depth-resolved elemental analysis using time-of-flight secondary ion mass spectrometer (TOF-SIMS) conducted on both un-implanted and implanted PCD membranes indicated that PCD membranes with a thickness in the micrometer (μm) range hold potential for quantum optical applications, provided that the nano-crystalline regions are adequately removed, either through back thinning or the substantial enhancement of spatial resolution in optical excitation through near-field or antenna-enhanced microscopy.
Subsequently, finite-difference time-domain ( FDTD) calculations were employed to explore the design considerations for nanoantennas. The research suggested that a gold nano-cone with an aspect ratio of ≈ 1 yields a substantial enhancement in the Purcell factor (F) exceeding four orders of magnitude, accompanied by an antenna efficiency of 80% under efficient near-field coupling with a quantum emitter. The fabrication process involved the creation of ultra-sharp gold nano-cones with dimensions of approximately 100 nm, boasting an aspect ratio of ≈ 1 through gold sputtering and focused ion beam (FIB) milling procedures on commercially available scanning near-field probes (atomic force microscope ( AFM) probes), thus enabling precise near-field manipulation of quantum emitters. The optical studies on nano-cones showed that the localized surface plasmon resonance ( LSPR) linewidth is relatively narrow (around 30 – 50 nm) and spans the required near-infra-red ( NIR) spectral region, enabling them to implement near-field coupling with NIR quantum emitters. Since the LSPR shift is proportional to the refractive index modification of the embedding medium, the optical response of the nano-cone LSPR was fine-tuned by adjusting the aspect ratio of the nanostructure, ensuring a spectral matching with the quantum emitter.
To unravel this novel observation for the hybrid quantum system, we performed the nearfield coupling with nanometer precision by utilizing AFM integrated confocal microscopy. The controlled near-field interaction of SiV centers with gold nano-cones fabricated on commercial AFM probes ensured a Purcell factor of F ≈ 57. Discrepancies between theoretical predictions and experimental results primarily stem from two factors: spectral mismatch and random dipole orientations of SiV color centers on PCD membranes. The future objective focuses on precisely tailoring the dimensions of the nano-cones and the creation of SiV color centers within high-quality PCD membranes, thereby achieving perfect spectral matching and ideal dipole orientation. This, in turn, will effectively enhance the near-field coupling.