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Journal of Physics Condensed Matter

The 'H2' optical transition in diamond: the effects of uniaxial stress perturbations, temperature and isotopic substitution

Simon C. Lawson, G. Davies, Alan T. Collins, Alison Mainwood

Published Mar 30, 1992• 60 citations
Publisher

Abstract

Uniaxial stress perturbations establish the 1.257 eV 'H2' zero-phonon line as an electric-dipole allowed transition at the centre of a C 2v point group. Taken in conjunction with a simple one-electron model, the magnitudes of the perturbations support recent suggestions that the H2 centre is a negative charge state of the well-known nitrogen-vacancy-nitrogen 'H3' centre. The H2 zero-phonon line is 0.9 meV higher in energy in 13 C diamond than in 12 C diamond. The shift is consistent, within large uncertainties, with the shift expected from the temperature dependence of the line. A local vibrational mode involving predominantly carbon atoms is identified.

DOI
10.1088/0953-8984/4/13/008

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•Nov 1, 2018•0 citations
Uniaxial stress perturbations establish the 1.257 eV 'H2' zero-phonon line as an electric-dipole allowed transition at the centre of a C 2v point group. Taken in conjunction with a simple one-electron model, the magnitudes of the perturbations support recent suggestions that the H2 centre is a negative charge state of the well-known nitrogen-vacancy-nitrogen 'H3' centre. The H2 zero-phonon line is 0.9 meV higher in energy in 13 C diamond than in 12 C diamond. The shift is consistent, within large uncertainties, with the shift expected from the temperature dependence of the line. A local vibrational mode involving predominantly carbon atoms is identified.
The N3 centre in diamond consists of three nitrogen atoms and one vacancy. We show that the N3 zero-phonon line, at 2.985 eV in natural diamond, is shifted by in diamond doped with , with no detectable change in the vibronic bandshape. In diamond, the energies of all the phonons seen in the vibronic band appear to be reduced from diamond in the ratio , and the zero-phonon is shifted by . The carbon isotope shift can be understood in terms of contributions from the lattice expansion, of from the different vibrational frequencies in the ground and excited electronic states, and from the Jahn - Teller effect in the excited state.
The neutrally charged silicon-vacancy defect (SiV 0 ) is a colour centre in diamond with spin S = 1, a zero-phonon line (ZPL) at 946 nm and long spin coherence, which makes it a promising candidate for quantum network applications. For the proper performance of such colour centres, all of them must have identical optical characteristics. However, in practice, there are factors that influence each individual centre. One of these factors is non-uniform isotope composition for both carbon atoms in diamond lattice and silicon atoms of dopant. In this work, we studied the isotopic shifts of SiV 0 centres for CVD-grown epitaxial layers of isotopically enriched 12 C and 13 C diamonds, as well as for diamond with natural isotope composition but doped only with one isotope of Si ( 28 Si, 29 Si and 30 Si). The detected shift was 1.60 meV for 12 C/ 13 C couple and 0.33 meV for 28 Si/ 29 Si and 29 Si/ 30 Si couples, which are close to the previously obtained values of the isotopic shift for the negatively charged silicon vacancy (SiV − ), which indicates a similar model of interaction with the environment for these two charge states of the SiV colour centres. This article is part of the Theo Murphy meeting issue 'Diamond for quantum applications'.
An emission band in the infrared (IR) is shown to be associated with a transition within the negative nitrogen-vacancy centre in diamond. The band has a zero-phonon line at 1046 nm, and uniaxial stress and magnetic field measurements indicate that the emission is associated with a transition between 1 E and 1 A 1 singlet levels. Inter-system crossing to these singlets causes the spin polarization that makes the NV - centre attractive for quantum information processing, and the IR emission band provides a new avenue for using the centre in such applications.
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 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.
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 NV0 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 N0 in the lattice results in more distant distribution of N+ ions and a narrowing of the ZPL. The competition between NV− and N0 excitation results in a significant dependence on excitation wavelength and there is also a dependence on the concentration of the NV− and N0 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 A0 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 NV0. 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.