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Applied Physics Letters

Microsphere-enhanced fluorescence collection for nitrogen vacancy centers in diamond

Shu-Hao Wu, Ignas Lekavicius, Hailin Wang

Published Oct 21, 2024• 2 citations
Publisher

Abstract

We report experimental studies of microsphere-enhanced fluorescence collection of nitrogen vacancy (NV) centers using silica microspheres with diameters ranging between 15 and 50 μm and employing 20× and 40× objectives with numerical aperture of 0.42 and 0.64, respectively. Photoluminescence-excitation saturation counts as high as 95 kHz have been observed. These studies show that due to the effective collimation of fluorescence by the microsphere, objectives with relatively low numerical aperture (NA) can be used without sacrificing collection enhancement, in agreement with a theoretical model based on Mie scattering. The large enhancement of fluorescence collection with relatively low NA objectives, which feature extralong working distance and are relatively inexpensive, can potentially enable wider use of NV-based quantum sensing in real world applications.

DOI
10.1063/5.0230091

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•May 12, 2021•0 citations
We report experimental studies of microsphere-enhanced fluorescence collection of nitrogen vacancy (NV) centers using silica microspheres with diameters ranging between 15 and 50 μm and employing 20× and 40× objectives with numerical aperture of 0.42 and 0.64, respectively. Photoluminescence-excitation saturation counts as high as 95 kHz have been observed. These studies show that due to the effective collimation of fluorescence by the microsphere, objectives with relatively low numerical aperture (NA) can be used without sacrificing collection enhancement, in agreement with a theoretical model based on Mie scattering. The large enhancement of fluorescence collection with relatively low NA objectives, which feature extralong working distance and are relatively inexpensive, can potentially enable wider use of NV-based quantum sensing in real world applications.
Diamond nitrogen-vacancy (NV) center ensemble magnetometer is a novel solid quantum magnetometer with high sensitivity and high spatial resolution, which can be used at normal temperature and pressure. However, the low fluorescence collection efficiency of the magnetometer in confocal optical system hinders further improvement in sensitivity. To solve the problem, based on silica microspheres, we propose a method to enhance the excitation and collection efficiency of NV center ensemble in massive diamond. Experimental results show, compared with the NV diamond magnetometer without microspheres, the photon collection efficiency of NV diamond magnetometer coated with a layer of microspheres is increased by 110%. Thus, the sensitivity of the NV center magnetometer is increased by 4 times to 5.3μT/√Hz.
Using an optical fiber to both excite the nitrogen-vacancy (NV) center in diamond and collect its fluorescence is essential to build NV-based endoscope-type sensor. Such endoscope-type sensor can reach inaccessible fields for traditional NV-based sensors built by bulky optical components and extend the application areas. Since single NV's fluorescence is weak and can easily be buried in fluorescence from optical fiber core's oxide defects excited by the green laser, fixing a micrometer size diamond containing high-density NVs rather than a nanodiamond containing single NV or several NVs on the apex of an optical fiber to build an endoscope-type sensor is more implementable. Unfortunately, due to small numerical aperture (NA), most of the optical fibers have a low fluorescence collection efficiency, which limits the sensitivity and spatial resolution of the NV-based endoscope-type sensor. Here, using a tapered optical fiber (TOF) tip, we significantly improve the efficiency of the laser excitation and fluorescence collection of the NV ensembles in diamond. This could potentially enhance the sensitivity and spatial resolution of the NV-based endoscope-type sensor. Numerical calculations show that the TOF tip delivers a high NA and has a high NV excitation and fluorescence collection efficiency. Experiments demonstrate that such TOF tip can obtain up to over 7-fold excitation efficiency and over 15-fold fluorescence collection efficiency of that from a flat-ended fiber (non-TOF) tip.
We present both theoretical and experimental results on fluorescence of single defect centers in diamond nanocrystals embedded in a planar dielectric microcavity. From a theoretical point of view, we show that the overall fluorescence collection efficiency using moderate numerical aperture microscope objective can be enhanced by using a low quality factor microcavity. This could be used in particular for low temperature applications where the numerical aperture of collection microscope objectives is limited due to the experimental constraints. We experimentally investigate the control of the fluorescence spectrum of the emitted light from a single center. We show the simultaneous narrowing of the room temperature broadband emission spectrum and the increase of the fluorescence spectral density.
Diamond particles with color centers have found numerous applications in quantum optics, bioimaging and nanosensing. Here, we report on an experimental study of the optical Mie resonances of subwavelength diamond particles and their effect on the fluorescence of nitrogen-vacancy centers. We employ single-particle scattering spectroscopy to reveal the fundamental optical resonances of individual diamond particles and measure the fluorescence saturation curves using continuous-wave excitation to evidence the difference in the spontaneous emission rate of the color centers in different particles. A simple analytical model has been used to predict the dependence of the saturation intensity on the particle size for a free-standing spherical diamond. Experimentally, we have shown that the magnetic dipole and quadrupole modes are excited in the fluorescence spectral range for particles of approximately 250-350 nm in size and that the saturation intensity for such particles is by an order of magnitude higher than for a smaller one, which is in agreement with our calculations. The results demonstrate that the efficiency of diamond-based sensors and single-photon sources can be improved by simply choosing particles of a proper size.
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We propose and optimize a vertically-loaded diamond microdisk resonator (VLDMoRt) coupled to a nitrogen-vacancy (NV) center in diamond for efficient collection of fluorescence emission into low numerical aperture (NA\text{NA}NA) free-space modes. The VLDMoRt achieves a Purcell enhancement of 172 with 39\% of the emitted light collected within a NA\text{NA}NA of 0.6, leading to a total external spin-photon collection efficiency of 0.33. As the design is compatible with established nanofabrication techniques and couples to low-NA\text{NA}NA modes accessible by cryogenic free-space optical systems, it is a promising avenue for increasing the efficiency and scalability of quantum devices based on diamond quantum emitters.