Development of efficient optical interfaces for quantum photonics with color centers

Abstract

Point defects in solid-state materials are one of the fascinating quantum resources since they serve single photon emission as well as defect spin states, which are widely used for various quantum applications. The well-defined defect states embedded in the wide bandgap of host materials enable us to utilize these defects even at ambient conditions. Besides, these defect spin states provide spin-dependent fluorescence with relatively longer coherent properties than other solid-state quantum emitters, thus, these spin states can be coherently controlled optically. Therefore, point defects in a crystal have been attracted as a promising candidate for practical quantum photonic devices by exploiting their quantum optical and spin properties to photonics. Since the single photon emissions as well as the optically addressable defect spins are characterized optically, it is important to build proper optical characterization systems for efficiently interfacing them to desired photonic platforms. However, the host materials generally have a high refractive index, thus, lacking light extraction efficiency, and the diffraction limit of the optical systems limits their optical resolution in defect characterization. Further, the widespread far-field propagation originating from the omnidirectional emission from the color center hinders the coupling efficiency between their emission and desired photonic platforms. Such limited brightness, resolution, and low coupling efficiency from color centers to the optical setup are great challenges for feasible and practical quantum photonic devices. To overcome these challenges, we tried two different approaches for improving the optical interface between defects in the solid-state system and photonic platforms. First, we positioned a dielectric, off-the-shelf microlens, which is a microsphere, on the surface of bulk diamond hosting negatively charged nitrogen-vacancy (NV-) color centers, followed by the implementation of confocal scanning microscopy. We used a bulk diamond hosting single NV- centers due to their optically stable, and long coherent spin properties and a barium titanate glass microsphere as micro-optics, which has a high refractive index, thus, it can reduce the index contrast between the bulk diamond and the environment, so the light extraction efficiency is increased. Further, the microsphere lens on the bulk diamond can move an image plane from the sample surface to a magnified virtual image under the diamond surface, which results in the improvement of spatial resolution and the signal contrast between the defect emissions and background fluorescence. Such great improvements in optical interface stem from the optical phenomena from the microsphere lens, which are composed of a photonic nanojet effect for a highly localized excitation beam and near-field collection from the microsphere acting as a near-field probe. With microsphere-assisted confocal microscopy, the spatial resolution is improved up to ~𝜆𝜆/5, which value is beyond the diffraction limit of the conventionally used confocal microscopy. Also, the signal-to-noise ratio between the defect emissions and the background fluorescence is increased 4 times greater in microsphere-assisted confocal microscopy. From these results, we also investigated the capabilities of individually addressing each single defect by using microsphere-assisted microscopy, where these single defects are unresolvable multi-defects in a conventional optical setup. From these results, the microsphere-assisted confocal microscopy is expected to perform an improved widefield fluorescence quantum imaging as well as investigate a photon-mediated quantum interaction among the single defects such as superradiant emission. The other approach that we tried is engineering the photonic environment by fabricating the photonic cavity on the host material where the defects are embedded. We used a hexagonal boron nitride (hBN) as a host material which has a defect kind of negatively charged boron vacancy (VB -). As the hBN is a representative of a Van der Waals 2-dimensional material, it has notable physical characteristics in easy to integrate with a desired photonic platform and to respond to the environment sensitively. In addition, the matured fabrication techniques on hBN flakes are well-established, therefore, it is easy to engineer the optical properties by fabricating photonic structures on the hBN flakes themselves. We optimize a new photonic cavity design, which is hole-circular Bragg grating (hole-CBG), where the radially repeated ring patterns are replaced by a nanoholes array by exploiting a finite difference time domain (FDTD) simulation. The hole-CBG narrows the far-field emission angle and cavity-induced Purcell enhancement resulting in the strong vertical emission which can couple efficiently even in low-NA optics such as an optical fiber. From the simulation of our optimized cavity design, it is anticipated that the cavity-enhanced emission shows a Purcell factor of ~50 and the photon collection is above 60% in single mode fiber (NA=0.12) with the hole-CBG cavity, which value is an order greater than in the case of bulk hBN flakes. Further from the simulation, we fabricated the optimized hole-CBG on the hBN flakes and integrated them into the optical fiber core. We expect that our hBN photonic cavity integrated optical fiber performs as a fiber optic sensor for magnetometer and thermometer. To investigate such capabilities, we performed a photoluminescence collection and an optically detected magnetic resonance through the cavity-integrated optical fiber channel and we successfully detected the cavity-enhanced PL emission from the VB - defects and ODMR spectrum via the optical fiber channel. Our approach can be applied not only for the fiber optics quantum sensor but also for the distributed quantum photonics such as quantum memory and quantum node via quantum channel. From this thesis, we developed optical interfaces for efficient coupling between the defect qubits in solid-state materials and photonic platforms with two distinct approaches. Our approaches can be applied from the improved wide-field quantum imaging, and fundamental studies for quantum interactions among the defects to the compact, alignment-free, fiber optic quantum sensing platform. Further, it is expected that our research will pave the way for practical quantum photonic devices based on point defects in solid-state systems, which will become more feasible beyond the experimental test in a laboratory frame, reaching toward the upcoming quantum era.