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International Conference on Luminescence - 1984

Phase Coherence in the 2E state of Cr3+ ions in MgO detected via ODMR

B. Henderson

Published Jan 1, 1984• 0 citations
Publisher

Abstract

The coupling between the Zeeman sublevels of an atom by a resonant microwave magnetic field also induces a phase coherence between the states. The effect is equivalent to the precession at the Zeeman frequency of a transverse magnetic moment about the static magnetic, and is manifest as a modulation of the circularly polarized emission at the Zeeman frequency. We have investigated such phase coherence in the 2E excited state of Cr3+ ions occupying sites of both cubic and tetragonal symmetry in magnesium oxide, MgO. The experiments were carried out at 1.6K, in the X-band frequency range (ν ~ 9.5 GHz) with the magnetic field parallel to the crystal [111] direction. In this orientation the optically detected magnetic resonance spectra from Cr3+ ions in cubic and tetragonal symmetry sites are well-separated in magnetic field. In addition all three resonances from the mutually orthogonal tetragonal species occur at the same magnetic field. The purpose of this geometry is to eliminate cross-relaxation between cubic and tetragonal sites.

DOI
10.1364/icol.1984.tug4

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The coupling between the Zeeman sublevels of an atom by a resonant microwave magnetic field also induces a phase coherence between the states. The effect is equivalent to the precession at the Zeeman frequency of a transverse magnetic moment about the static magnetic, and is manifest as a modulation of the circularly polarized emission at the Zeeman frequency. We have investigated such phase coherence in the 2E excited state of Cr3+ ions occupying sites of both cubic and tetragonal symmetry in magnesium oxide, MgO. The experiments were carried out at 1.6K, in the X-band frequency range (ν ~ 9.5 GHz) with the magnetic field parallel to the crystal [111] direction. In this orientation the optically detected magnetic resonance spectra from Cr3+ ions in cubic and tetragonal symmetry sites are well-separated in magnetic field. In addition all three resonances from the mutually orthogonal tetragonal species occur at the same magnetic field. The purpose of this geometry is to eliminate cross-relaxation between cubic and tetragonal sites.
Optically induced Zeeman coherences between various pairs of sublevels in the ground state 4 A 2 of a sample ruby (0.05 wt.%) were observed at room temperature by using impact excitation with circularly polarized resonant light pulses from a mode-locked ruby laser. The precessing magnetization associated with Zeeman coherence was detected magnetically in UHF, VHF and X-band microwave regions. The observed signals can be regarded as various types of ESR free induction decay signals and the decay times were of the order of nanosecond. Sustained precession signals were also observed. Characteristics of the signals were explained by numerical solutions of the Liouville equation.
The optical energy transfer between Cr 3+ ions from cubic centres to tetragonal centres is measured. Transfer is observed only when the donor is very close to the acceptor and the measured transfer times are relatively short ( approximately 100 mu s). The tetragonal acceptors are Cr 3+ -ion pairs which form different triads with the donor. It is deduced that transfer to rhombic sites is also rapid only from very close cubic centres. The shortness of the transfer times explains why a spin memory is observed during the energy transfer from cubic to rhombic centres. The localisation of optical excitation in triads is due to the presence of charge-compensating Mg 2+ vacancies and the inefficiency of the dipole-dipole interaction.
A longitudinal radio-frequency (rf) magnetic field resonating at the Larmor frequency along the $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{z}$ direction in the presence of a static transverse magnetic field s investigated. Optical-radio-frequency double resonance in 85Rb^{85}\mathrm{Rb}85Rb characterizes the strength of an external magnetic field through the precession frequency of 85Rb^{85}\mathrm{Rb}85Rb atoms. The measurements are carried out in a paraffin-coated 85Rb^{85}\mathrm{Rb}85Rb cell with a spin alignment configuration. An intriguing discovery of magnetic resonance peaks is observed at subharmonics of the Larmor frequency, accompanied by the emergence of nonlinear effects in the vicinity of the resonance. Notably, when the Larmor frequency reaches zero, the optical rotation signals induced by the rf magnetic field demonstrate a superposition of high-order harmonics of the rf frequency. Based on the simplified density matrix formalism and perturbation treatment, analytical expressions for the optical rotation are consistent with experimental outcomes. These findings hold the potential of extending our understanding on unveiling the dynamics of population and coherence among Zeeman sublevels, thereby advancing our knowledge in the field of alignment magnetometry.
An ion is located in a crystal at a site that is not a centre of inversion, with a Zeeman field directed at right angles to the crystal axis c. Periodic intensity variations in the optical spectrum that occur when the crystal is rotated about c are due to magnetic dipole transitions between levels of different type and to electric dipole transitions between levels having admixed opposite-parity parts of different type. Use of an antiunitary operator that commutes with the total Hamiltonian demonstrates that interference between electric and magnetic dipole radiation (or interference between any opposite-parity multipole radiation) does not occur for transitions appearing in the axial spectrum, but does occur for those appearing in the pi or sigma transverse spectrum. Depending on the site symmetry, two or four of the Zeeman components of a doublet-doublet transition have an additional periodicity as a result of this interference. Eigenstates are constructed that show the relationship between the M values of a Zeeman level, the crystal quantum number that labels it, and its type, and are used to calculate the angular dependence of certain doublet-doublet dipole transitions for ions at sites of point symmetries D 2d and C 3h . With explicit expressions for dipole matrix elements, it is discovered that two Zeeman components having the same angular dependence when interference is neglected exhibit dissimilar angular behaviour when the interference cross-term is included.
Paramagnetic ions in solid state crystals form the basis for many advanced technologies like lasers, masers, frequency standards, and quantum-enhanced sensors. One of the most-studied examples is the Cr3+ ion in sapphire (Al2O3), also known as ruby, which has been intensely studied in the 1950s and 1960s. However, despite decades of research on ruby, some of its fundamental optical and spin properties have not been characterized at ultra low-temperatures, yet. In this article, we present optical measurements on a ruby crystal in a dilution refrigerator at ultra-low temperatures down to 20 mK. Analysing the relative populations of its 4A2 ground state spin levels, we extract a lattice temperature of 143(7) mK under continuous laser excitation. We perform spin lattice relaxation T1 measurements in excellent agreement with the direct, one-phonon model. Furthermore, we perform optically detected magnetic resonance measurements showing magnetically driven transitions between the ground state spin levels for various magnetic fields. Our measurements characterize some of ruby's low temperature spin properties, and lay the foundations for more advanced spin control experiments.