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Physics Procedia

Effects of magnetic field on the low temperature emission of nitrogen vacancy centres in diamond

Roger Mc Murtrie, Lachlan J. Rogers, Neil B. Manson

Published Feb 1, 2010• 2 citations
Publisher

Abstract

Under optical illumination in the blue or green, negatively charged nitrogen vacancy centres in diamond emit in the red. The intensity of this emission varies slightly depending on spin state occupation. Optical transitions occur predominantly without change of spin projection. However, excited ms=±1 states can decay non-radiatively to the ground ms=0 state via intermediary singlet states. With continuous excitation, this effect transfers most of the population to the ms=0 state resulting in decay becoming almost entirely radiative so that optical emission is stronger than when all spin states are occupied equally. However, under optical illumination the ms=0 polarization is reduced when an applied magnetic field induces avoided crossings between energy levels. The spin states are mixed and some population is diverted into the ms=±1 states with consequent reduction in optical emission. The change of emission can be calculated from a rate equation model involving the spin states of the ground and excited levels plus one singlet level [5]. The spin states of the excited levels are also affected by strain and in this work we calculate the variation in optical emission with changing magnetic field for various fixed values of strain.

DOI
10.1016/j.phpro.2010.01.225

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Under optical illumination in the blue or green, negatively charged nitrogen vacancy centres in diamond emit in the red. The intensity of this emission varies slightly depending on spin state occupation. Optical transitions occur predominantly without change of spin projection. However, excited ms=±1 states can decay non-radiatively to the ground ms=0 state via intermediary singlet states. With continuous excitation, this effect transfers most of the population to the ms=0 state resulting in decay becoming almost entirely radiative so that optical emission is stronger than when all spin states are occupied equally. However, under optical illumination the ms=0 polarization is reduced when an applied magnetic field induces avoided crossings between energy levels. The spin states are mixed and some population is diverted into the ms=±1 states with consequent reduction in optical emission. The change of emission can be calculated from a rate equation model involving the spin states of the ground and excited levels plus one singlet level [5]. The spin states of the excited levels are also affected by strain and in this work we calculate the variation in optical emission with changing magnetic field for various fixed values of strain.
We investigate the magnetic field dependent photophysics of individual nitrogen-vacancy (NV) color centers in diamond under cryogenic conditions. At distinct magnetic fields, we observe significant reductions in the NV photoluminescence rate, which indicate a marked decrease in the optical readout efficiency of the NV's ground state spin. We assign these dips to excited state level anticrossings, which occur at magnetic fields that strongly depend on the effective, local strain environment of the NV center. Our results offer new insights into the structure of the NVs' excited states and a new tool for their effective characterization. Using this tool, we observe strong indications for strain-dependent variations of the NV's orbital g factor, obtain new insights into NV charge state dynamics, and draw important conclusions regarding the applicability of NV centers for low-temperature quantum sensing.
The emission intensity of diamond samples containing negatively charged nitrogen-vacancy centres are measured as a function of magnetic field along the ⟨111⟩ direction for various temperatures. At low temperatures the responses are sample and stress dependent and can be modelled in terms of the previous understanding of the 3 E excited state fine structure which is strain dependent. At room temperature the responses are largely sample and stress independent, and modelling involves invoking a strain independent excited state with a single zero field spin-level splitting of 1.42 GHz. The change in behaviour is attributed to a temperature dependent averaging process over the components of the excited state orbital doublet. It decouples orbit and spin and at high temperature the spin levels become independent of any orbit splitting. One significant implication of this averaging is that it simplifies the development of room temperature applications.
We investigate spin-dependent decay and intersystem crossing (ISC) in the optical cycle of single negatively charged nitrogen-vacancy (NV) centres in diamond. We use spin control and pulsed optical excitation to extract both the spin-resolved lifetimes of the excited states and the degree of optically induced spin polarization. By optically exciting the centre with a series of picosecond pulses, we determine the spin-flip probabilities per optical cycle, as well as the spin-dependent probability for ISC. This information, together with the independently measured decay rate of singlet population, provides a full description of spin dynamics in the optical cycle of NV centres. The temperature dependence of the singlet population decay rate provides information about the number of singlet states involved in the optical cycle.
The negative Nitrogen-Vacancy (NV) colour centre in diamond is an atomic-level impurity that is remarkably well suited for quantum information processing and quantum metrology applications. Underlying these properties is the fact that optical illumination causes the NV centre to preferentially populate one of its three spin levels. The mechanism that gives rise to this spin polarisation has not been well understood, despite intense research interest in the NV centre and its applications. This thesis arrives at an account of the spin polarisation mechanism, although a detailed theoretical treatment remains outstanding. The energy level structure of the NV centre is investigated using techniques of solid state spectroscopy. These include absorption and photoluminescence measurements made on diamonds at cryogenic and room temperatures in the presence of uniaxial stress, magnetic fields, and microwaves. Pulsed light experiments allow the dynamic processes within the NV centre to be measured. It is shown that at room temperature the orbital doublet excited state behaves like an orbital singlet, and this description is elegantly confirmed by single-site observations. This picture of the excited state is inconsistent with cryogenic measurements, and its behaviour is examined across a range of temperatures in order to resolve this tension. It is shown that electron-vibration interaction accounts for the quenching of orbital properties in the excited state with temperature. A new optical transition within the NV centre is reported at a wavelength of 1042 nm. It is established that this transition occurs between the intermediate singlet levels which are thought to provide the decay pathway that produces spin polarisation. The results are consistent with this understanding, and the order and properties of these singlet levels are established. Electron-vibration interaction is observed to be important within the singlet system. It is shown that electron-vibration interaction is an important detail in the NV optical pumping cycle. Taking it into consideration leads to the first plausible physical description of the spin polarisation mechanism. -- provided by Candidate.
With the creation of nitrogen (NV) in 1b diamond it is common to find that the absorption and emission is predominantly of negatively charged NV centres. This occurs because electrons tunnel from the substitutional nitrogen atoms to NV to form NV − –N + pairs. There can be a small percentage of neutral charge NV 0 centres and a linear increase of this percentage can be obtained with optical intensity. Subsequent to excitation it is found that the line width of the NV − zero-phonon has been altered. The alteration arises from a change of the distribution of N + ions and a modification of the average electric field at the NV − sites. The consequence is a change to the Stark shifts and splittings giving the change of the zero-phonon line (ZPL) width. Exciting the NV − centres enhances the density of close N + ions and there is a broadening of the ZPL. Alternatively exciting and ionizing N 0 in the lattice results in more distant distribution of N + ions and a narrowing of the ZPL. The competition between NV − and N 0 excitation results in a significant dependence on excitation wavelength and there is also a dependence on the concentration of the NV − and N 0 in the samples. The present investigation involves extensive use of low temperature optical spectroscopy to monitor changes to the absorption and emission spectra particularly the widths of the ZPL. The studies lead to a good understanding of the properties of the NV − –N + pairs in diamond. There is a critical dependence on pair separation. When the NV − –N + pair separation is large the properties are as for single sites and a high degree of optically induced spin polarization is attainable. When the separation decreases the emission is reduced, the lifetime shortened and the spin polarization downgraded. With separations of <12 A 0 there is even no emission. The deterioration occurs as a consequence of electron tunneling in the excited state from NV – to N + and an optical cycle that involves NV 0 . The number of pairs with the smaller separations and poorer properties will increase with the number of nitrogen impurities and it follows that the degree of spin polarization that can be achieved for an ensemble of NV − in 1b diamond will be determined and limited by the concentration of single substitutional nitrogen. The information will be invaluable for obtaining optimal conditions when ensembles of NV − are required. As well as extensive measurements of the NV − optical ZPL observations of Stark effects associated with the infrared line at 1042 nm and the optically detected magnetic resonance at 2.87 GHz are also reported.
The nitrogen-vacancy (NV) centre in diamond is a unique optical defect that is used in many applications today and methods to enhance its fluorescence brightness are highly sought after. We observed experimentally an enhancement of the NV quantum yield by up to 7% in bulk diamond caused by an external magnetic field relative to the field-free case. This observation is rationalised phenomenologically in terms of a magnetic field dependence of the NV excited state triplet-to-singlet transition rate. The theoretical model is in good qualitative agreement with the experimental results at low excitation intensities. Our results significantly contribute to our fundamental understanding of the photophysical properties of the NV defect in diamond.