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Optics Express

Suppression of fluorescence phonon sideband from nitrogen vacancy centers in diamond nanocrystals by substrate effect

Hong-Quan Zhao, Masazumi Fujiwara, Shigeki Takeuchi

Published Jun 26, 2012• 33 citations
Publisher

Abstract

Substrates effect is observed on the suppression of the phonon sideband from nitrogen vacancy (NV) centers in 50nm diamond nanocrystals at cryogenic temperatures. As a quantitative parameter of the population of phonon sidebands, the Debye-Waller factor is estimated from fluorescence spectra on glass, silicon, and silica-on-silicon substrates. Fluorescence spectra of negatively charged NV centers in nanodiamonds on silica-on-silicon substrates have average and maximum Debye-Waller factors of 12.7% (which is about six times greater than that of samples on glass substrates) and 19.3%, respectively. This effect is expected to be very important for future applications of NV centers in quantum information science and nanosensing.

DOI
10.1364/oe.20.015628

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Substrates effect is observed on the suppression of the phonon sideband from nitrogen vacancy (NV) centers in 50nm diamond nanocrystals at cryogenic temperatures. As a quantitative parameter of the population of phonon sidebands, the Debye-Waller factor is estimated from fluorescence spectra on glass, silicon, and silica-on-silicon substrates. Fluorescence spectra of negatively charged NV centers in nanodiamonds on silica-on-silicon substrates have average and maximum Debye-Waller factors of 12.7% (which is about six times greater than that of samples on glass substrates) and 19.3%, respectively. This effect is expected to be very important for future applications of NV centers in quantum information science and nanosensing.
We report the substrate effects on the zero-phonon transitions and suppression of phonon side bands in the NV center spectrum. Fluorescence spectra of NV centers in cryogenic temperatures were measured by depositing diamond nanocrystals on different substrates including glass slides, undoped Si, and silica (1~2μm) on undoped Si (SiO2/Si). We found that SiO2/Si substrate was an effective substrate to suppress the phonon side band from spectra of NV- centers. Temperature dependence of NV- zero-phonon line Debye-Waller factor on Si and SiO2/Si were measured, from 2.5K to 230K, Debye-Waller factor decreased linearly on both of the two substrates.
With in-built advantages (high quantum efficiency and room temperature photostability1) for deployment in quantum technologies as a bright on-demand source of single photons, the nitrogen vacancy (NV) center is the most widely studied optical defect in diamond. Despite significant success in controlling its spontaneous emission2, the fundamental understanding of its photo-physics in various environments and host material remains incomplete. Studying NV photoemission from nanodiamonds on a glass substrate, we recently pointed out a disparity between the measured and calculated decay rates (assuming near unity quantum efficiency)3. This indicates the presence of some strong nonradiative influences from factors most likely intrinsic to nanodiamond itself. To obtain a clearer picture of the NV emission, here we remove the substrate contributions to the decay rates by embedding our nanodiamonds inside silica aerogel, a substrate-free environment of effective index n ~ 1.05. Nanodiamond doped aerogel samples were fabricated using the “two-step” process4. Time-resolved fluorescence measurement on ~20 centers for both coverslip and aerogel configurations, showed an increase in the mean lifetime (~37%) and narrowing of the distribution width (~40%) in the aerogel environment, which we associate with the absence of a air/cover-glass interface near the radiating dipoles3. Finite difference time domain (FDTD) calculations showed the strong influence of the irregular nanodiamond geometry on the remaining distribution width. Finally a comparison between measurements and calculations provides an estimate of the quantum efficiency of the nanodiamond NV emitters as ~0.7. This value is apparently consistent with recent reports concerning the oscillation of the NV center between negative and neutral charge states5.
Photoluminescence excitation spectroscopy of nitrogen vacancy (NV) centers in type Ib diamond nanocrystals shows that in spite of spectral diffusion and spin-nonconserving transitions, zero-phonon linewidth as small as 16MHz16\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}16MHz, which is nearly lifetime limited, can be observed at low temperature. Relatively small effects of spectral diffusion have also been observed in a small fraction of NV centers.
We discuss charge state ratio-dependent spectral and temporal emission properties of nitrogen-vacancy (NV) centers in bulk diamonds. An understanding of the charge state ratio in artificially grown diamonds is important to design them for NV-based magnetometry. We show that the NV charge state ratio strongly correlates with the intensity of the zero-phonon line (ZPL) of NV−, which is supported by an emission dip at 2.87 GHz in spin-selective optical transitions. The Debye–Waller factor of NV centers depends on the charge state ratio, and it increases for NV− with an increase in the charge state ratio. We observe that the energy separation between the second phonon sideband (PSB) and ZPL does not scale multiplicatively with that of the energy separation between first PSB and ZPL. This deviation becomes more pronounced with an increase in charge state ratio, suggesting anharmonicity of vibrational energy levels, rather than the usually reported harmonicity. Further, we discuss the inherent relation between decay rates and charge states as a function of emission wavelengths. The excited state lifetime of NV0 remains constant, whereas the NV− lifetime decreases with an increase in the charge state ratio, with an overall decrease of 37%. Our results show an emission spectra-based approach to estimate the charge state ratio in diamond crystals, which is helpful for quantum-enhanced sensing and magnetometry.
ABSTRACT The negatively charged nitrogen‐vacancy (NV) center in diamond is a leading solid‐state quantum emitter, offering spin‐photon interfaces over a wide temperature range with applications from electromagnetic sensing to bioimaging. While NV centers in bulk diamond are well understood, embedding them in nanodiamond (ND) introduces complexities from size, NV location, and NV polarizations. NVs in ND show altered fluorescence properties, including longer lifetimes, lower quantum efficiency, and higher sensitivity to dielectric surroundings, which arise from radiative suppression, surface‐induced non‐radiative decay, and escape inefficiency at the diamond‐background interface. Prior models typically addressed isolated aspects, such as dielectric contrast or surface quenching, without integrating full quantum‐optical NV behavior with classical electrodynamics. We present a hybrid framework coupling rigorous electromagnetic simulations with a quantum‐optical NV model including phonon sideband dynamics. NV emission is found to depend strongly on ND size, NV position, and surrounding refractive index. Our results explain observations such as shallow NVs in water‐coated ND appearing brighter than deeper ones in air. This integrated model provides a unified framework for realistic NV in ND emission scenarios and informs the design of efficient NV‐based sensors and quantum devices, advancing understanding of quantum emitter photophysics in nanoscale crystals.
Recently, the nitrogen vacancy (NV) center has emerged as a deterministic single-photon source at room temperature. However, the presence of phonon sideband emission limits the use of NV centers in various quantum optical applications. Here we report a novel way to suppress the phonon sideband emission of NV centers aided with the enhancement of zero phonon line intensity using photonic crystals at room temperature. We study the photonic stop gap mediated charge state conversion in NV centers. The enhancement of phonon sideband emission from the same structures by implementing a different measurement geometry is also discussed. The studies open new avenues in studying the NV center-based quantum nanophotonics and biophotonics.