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AIP Advances

Multi-color laser excitation of diamond nitrogen vacancy centers embedded in nanophotonic structures

Axel Hochstetter, Elke Neu

Published Jun 1, 2021• 4 citations
Publisher

Abstract

Negatively charged nitrogen vacancy centers (NV−) in diamond serve as highly sensitive, optically readable sensors for magnetic fields. Improved sensing approaches rely on NV− centers embedded in diamond nanopillar waveguides, which enable scanning probe imaging and use multi-color laser schemes for efficient spin readout. In this work, we investigate the free-beam coupling of the most relevant laser wavelengths to diamond nanopillars with different geometries. We focus on cylindrical pillars, conical pillars, and conical pillars with an added parabolic dome. We study the effects of the pillar geometry, NV− position, laser wavelength, position of laser focus, and excitation geometry (excitation from the top facet or from the substrate side). We find a pronounced impact of the laser wavelength that should be considered in multi-color excitation of NV−. Within the pillars, exciting laser fields can be enhanced up to a factor of 11.12 compared to bulk. When focusing the laser to the interface between the substrate and the nanopillar, even up to 29.78-fold enhancement is possible. Our results are in accordance with the experimental findings for green laser excitation of NV− in different pillar geometries.

DOI
10.1063/5.0050390

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Negatively charged nitrogen vacancy centers (NV−) in diamond serve as highly sensitive, optically readable sensors for magnetic fields. Improved sensing approaches rely on NV− centers embedded in diamond nanopillar waveguides, which enable scanning probe imaging and use multi-color laser schemes for efficient spin readout. In this work, we investigate the free-beam coupling of the most relevant laser wavelengths to diamond nanopillars with different geometries. We focus on cylindrical pillars, conical pillars, and conical pillars with an added parabolic dome. We study the effects of the pillar geometry, NV− position, laser wavelength, position of laser focus, and excitation geometry (excitation from the top facet or from the substrate side). We find a pronounced impact of the laser wavelength that should be considered in multi-color excitation of NV−. Within the pillars, exciting laser fields can be enhanced up to a factor of 11.12 compared to bulk. When focusing the laser to the interface between the substrate and the nanopillar, even up to 29.78-fold enhancement is possible. Our results are in accordance with the experimental findings for green laser excitation of NV− in different pillar geometries.
Nitrogen-vacancy (NV) color centers in diamond are excellent quantum sensors possessing high sensitivity and nano-scale spatial resolution. Their integration in photonic structures is often desired, since it leads to an increased photon emission and also allows the realization of solid-state quantum technology architectures. Here, we report the fabrication of diamond nano-pillars with diameters up to 1000 nm by electron beam lithography and inductively coupled plasma reactive ion etching in nitrogen-rich diamonds (type Ib) with [100] and [111] crystal orientations. The NV centers were created by keV-He ion bombardment and subsequent annealing, and we estimate an average number of NVs per pillar to be 4300 ± 300 and 520 ± 120 for the [100] and [111] samples, respectively. Lifetime measurements of the NVs’ excited state showed two time constants with average values of τ1 ≈ 2 ns and τ2 ≈ 8 ns, which are shorter as compared to a single color center in a bulk crystal (τ ≈ 10 ns). This is probably due to a coupling between the NVs as well as due to interaction with bombardment-induced defects and substitutional nitrogen (P1 centers). Optically detected magnetic resonance measurements revealed a contrast of about 5% and average coherence and relaxation times of T2 [100] = 420 ± 40 ns, T2 [111] = 560 ± 50 ns, and T1 [100] = 162 ± 11 μs, T1 [111] = 174 ± 24 μs. These pillars could find an application for scanning probe magnetic field imaging.
Diamond-embedded optical fibers with negatively charged nitrogen-vacancy (NV) centers have great potential for remote and distributed magnetic field sensing. However, the magnetic field sensitivity of the currently proposed fiber configuration is limited. This work uses a numerical approach to investigate the effect of structured diamonds on enhancing the excitation and collection efficiency in an optical fiber through a hybrid modelling approach. The results revealed that a 690-times enhancement is achievable with a pillar-shaped diamond embedded in a fiber.
The long-lived electronic spin of the nitrogen-vacancy (NV) center in diamonds is a promising quantum sensor for detecting nanoscopic magnetic and electric fields in various environments. However, the poor signal-to-noise ratio (SNR) of prevalent optical spin-readout techniques presents a critical challenge in improving measurement sensitivity. Here, we address this limitation by coupling individual NVs to optimized diamond nanopillars, thereby enhancing the collection efficiency of fluorescence. Guided by near-field optical simulations, we predict improved performance for tall (≥5 μm) pillars with tapered sidewalls. This is subsequently verified by fabricating and characterizing a representative set of structures using a newly developed nanofabrication process. We observe increased SNR for optimized devices, owing to improved emission collimation and directionality. Promisingly, these devices are compatible with low-numerical-aperture collection optics and a reduced tip radius, reducing experimental overhead and facilitating improved spatial resolution for scanning applications.
Nitrogen vacancy (NV) centers in diamond have been identified over the past few years as promising systems for a variety of applications, ranging from quantum information science to magnetic sensing. This relies on the unique optical and spin properties of the negatively charged NV. Many of these applications require shallow NV centers, i.e., NVs that are close (a few nm) to the diamond surface. In recent years there has been increasing interest in understanding the spin and charge dynamics of NV centers under various illumination conditions, specifically under infrared (IR) excitation, which has been demonstrated to have significant impact on the NV centers' emission and charge state. Nevertheless, a full understanding of all experimental data is still lacking, with further complications arising from potential differences between the photodynamics of bulk and shallow NVs. Here we suggest a generalized quantitative model for NV center spin- and charge-state dynamics under both green and IR excitation. We experimentally extract the relevant transition rates, providing a comprehensive model which reconciles all existing experimental results in the literature, except for highly nonlinear regimes. Moreover, we identify key differences between the photodynamics of bulk and shallow NVs, and use them to significantly enhance the initialization fidelity of shallow NVs to the useful negatively charged state.
Nitrogen-vacancy (NV) centers in diamond are versatile room-temperature quantum sensors whose sensitivity is often limited by photon collection. Shaping the diamond into nanopillar antennas increases photon collection, and packing these pillars into a dense array promises to combine this collection gain with the parallel throughput of the widefield quantum diamond microscope. However, optical interactions among the pillars in the array can limit its performance. The study combines finite-difference time-domain (FDTD) simulation with two experimental modalities, a home-built confocal microscope and a widefield microscope, on a (111) diamond parabolic nanopillar array of four geometries spanning two tip diameters (0.4 and 0.8 \textmu m) and edge-to-edge gaps of 0.5-1.6 \textmu m. The pillars enhance NV collection broadband, with band-integrated enhancements of 10.4×10.4\times10.4× and 6.0×6.0\times6.0× for the two tips, consistent with simulation. The geometry-mediated transport is quantified through the photons transported from the pillars to the gap and to neighboring pillars by the gap-to-pillar ratio (G/PG/PG/P) and the neighbor fraction (χ\chiχ): G/PG/PG/P falls with increasing gap across all three methods (widefield 50.4%→31.4%50.4\%\rightarrow31.4\%50.4%→31.4%, confocal 26.2%→12.1%26.2\%\rightarrow12.1\%26.2%→12.1%, simulated χ\chiχ 7.7%→2.6%7.7\%\rightarrow2.6\%7.7%→2.6%), establishing the coupling as a robust physical feature governed by the base-to-base gap rather than the pitch. These optical interactions can influence the performance of the sensor where χ\chiχ biases the accuracy while G/PG/PG/P blurs the spatial resolution, and both grow as the pitch shrinks, capping the density-for-sensitivity gain and identifying the base-to-base gap and the pillar taper as the geometric parameters a purpose-designed sensing array must control. Whether the transported light is spin-active NV−^-− emission, and therefore biases ODMR readout rather than merely adding background, remains to be confirmed, and is identified as the central next step toward array-based widefield sensing.
The nitrogen vacancy (NV) center in diamond is the most widely studied color center in diamond for its wide range of applications in both physics and biology. The negatively charged state ${\mathrm{NV}}^{\ensuremath{-}}$ is often favored for its better spin-optical properties, whereas the presence of the neutral charge state NV0{\mathrm{NV}}^{0}NV0 is often minimized or neglected by using a single optimized excitation wavelength. In many cases, the use of additional laser wavelengths has an important impact on the NV optical properties and can lead to a dramatically quenched fluorescence. Through measurements of the photophysics of the NV center we show in this work that this quenching mechanism is mediated by charge state interconversion providing additional channels for nonradiative decay. Our results show that this mechanism is also sensitive to the spin state of the ${\mathrm{NV}}^{\ensuremath{-}}$. A better understanding of this physical mechanism will help mitigate the spin and charge state depolarization effects that are detrimental for quantum technology applications.