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Journal of Applied Physics

Multiphonon effects on the optical emission spectra of the nitrogen-vacancy center in diamond at different temperatures

Wen-Long Ma, Shu‐Shen Li

Published Mar 15, 2012• 1 citation
Publisher

Abstract

We theoretically investigate the multiphonon effects on the optical emission spectra of the negatively charged nitrogen-vacancy (NV−) center in diamond and find that the fluorescence emission spectra described by the single frequency model agree quite well with the experimental data. From a comparison between our theory and the experimental results, we also find a reduction of the anti-Stokes shift of the emission spectra at high temperature. This is contradictory to the linear hole-phonon interaction model. We resolve this contradiction by adding the quadratic hole-phonon interaction to our model. Our theoretical analyses show that this may reduce the lattice relaxation energy. As a consequence, fewer phonons are emitted in the optical transition, therefore explaining the reduction of the anti-Stokes shift at high temperature.

DOI
10.1063/1.3696968

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We theoretically investigate the multiphonon effects on the optical emission spectra of the negatively charged nitrogen-vacancy (NV−) center in diamond and find that the fluorescence emission spectra described by the single frequency model agree quite well with the experimental data. From a comparison between our theory and the experimental results, we also find a reduction of the anti-Stokes shift of the emission spectra at high temperature. This is contradictory to the linear hole-phonon interaction model. We resolve this contradiction by adding the quadratic hole-phonon interaction to our model. Our theoretical analyses show that this may reduce the lattice relaxation energy. As a consequence, fewer phonons are emitted in the optical transition, therefore explaining the reduction of the anti-Stokes shift at high temperature.
We study the spectroscopy of an ensemble of negatively charged nitrogen-vacancy centers in diamond at high temperatures between room temperature and 700K under high vacuum conditions. Spin resonances are studied using optical detection of magnetic resonance (ODMR), and optical spectroscopy is employed to study radiative transitions. Upon increasing the temperature, the intensity of radiative decay in visible and infrared decreased. In addition, the ODMR resonance frequencies were decreased, and the phonon line emission shifted to higher wavelengths. Density functional theory calculation of the zero-field splitting parameter (D) revealed that thermal expansion is not enough to explain the shift in the ODMR frequencies. Fitting the measured intensity of photo-luminescence with the theoretical predictions of the Mott–Seitz model yields the value of 0.22eV for the energy barrier associated with nonradiative decay.
Anti-Stokes optical cooling in diamond nitrogen-vacancy (NV) centers is investigated using photoluminescence (PL) spectroscopy, time-resolved measurements, and rate-equation modeling. Under excitation below the zero-phonon line, anti-Stokes emission from the negatively charged NV- state appears, but optical excitation also induces charge-state conversion between NV- and neutral NV0. Excitation-dependent PL dynamics reveal a reduction of the NV- population under strong excitation. A minimal rate-equation model constrained by nanosecond and millisecond PL measurements predicts that photoionization suppresses the NV--mediated cooling contribution and leads to self-limiting cooling behavior. The calculations further show that net cooling requires exceptionally high quantum efficiency and does not increase monotonically with excitation density. These results highlight photoinduced charge-state conversion as a key photophysical bottleneck for defect-based anti-Stokes optical cooling.
The nitrogen-vacancy (NV) centre in diamond is a leading platform for room-temperature quantum sensing. Improvements in sensitivity require precise control of the NV charge state. Transitions from the neutral NV0^00 charge state to the negative NV−^-− charge state can occur during excitation with photon energies below the ZPL transition of NV0^00. These sub-resonant charge transitions limit modern initialisation protocols and have not been studied in full detail. In this paper we show that sub-resonant charge cycling arises from phonon-assisted anti-Stokes excitation. We further uncover the phonon states which contribute most strongly to the anti-Stokes transition via the development of novel quantitative models. The models indicate that low energy acoustic phonons strongly contribute to the transition close to the ZPL. At longer wavelengths a 43\,meV mode additionally impacts the charge cycling dynamics.
Magnetic resonance and fluorescence spectra of nitrogen-vacancy (NV) color centers ensemble in high purity diamond sample were measured, with temperature ranging from 5.6 K to 295 K. Both microwave and optical transition energies have similar nonlinear temperature dependent changes, which might mainly originate from the local thermal expansion. As the frequency shifts will reduce the fidelity of resonant quantum control, the present results demonstrate the necessity of taking temperature fluctuation into consideration. For temperature below 100 K, the transition energies show tendencies to be constant, which indicate higher stability and performance in applications with NV centers.
Anti-Stokes optical cooling in diamond nitrogen-vacancy (NV) centers is experimentally and numerically investigated. Photoluminescence-excitation spectroscopy reveals pronounced phonon-assisted anti-Stokes emission under excitation below the zero-phonon line (ZPL). However, the below-ZPL excitation drives photoinduced charge-state conversion between negatively-charged NV- and neutral NV0, thereby suppressing the NV- mediated cooling channel. Time-resolved photoluminescence (PL) measurements reveal an increase in the effective PL lifetime with excitation density, reflecting an increasing NV0 contribution. By fitting nanosecond and millisecond PL dynamics with a minimal rate-equation model, we extract effective optical pumping and charge-conversion rates, which enables us to quantitatively simulate the cooling performance. The simulations predict a self-limiting behavior of anti-Stokes cooling and clarify the excitation conditions under which net cooling can be sustained within this effective model. The estimated cooling power per NV center is comparable, on a microscopic basis, to values discussed for semiconductor quantum dots and rare-earth optical coolers. These results identify charge-state conversion as a key bottleneck for defect-based optical refrigeration.
Abstract The neutral nitrogen‐vacancy (NV0) defects in diamond are photostable color centers, suitable for a wide range of applications in science and engineering. However, the photophysical properties of the centers have not yet been fully characterized. This work measured the stimulated emission cross sections of NV0 in a single‐crystal diamond by two‐photon excitation of its matrix at 344 nm. From the measured photoluminescence spectrum and the fluorescence lifetime of 20 ± 1 ns, we determined a peak stimulated emission cross section of 1.43 ± 0.07 × 10−17 cm2 at 650 nm for the NV0 centers. In addition, we have also examined the thermal shifts of the zero‐phonon line of NV0 centers in nanoscale diamonds (~100 nm in diameter) over the temperature range of 30–120°C. A temperature measurement sensitivity of 0.2 K·Hz−1/2 was achieved, which is about two‐fold better than that of NV−, despite that the fluorescence intensity of NV0 is about six‐fold lower than that of NV− in the same nanoparticles. The result is attributed to the smaller electron–phonon coupling strength of the neutral center, compared with its negatively charged counterpart.