S. J. Bingham

Origin of the red luminescence in Mg-doped GaN

Optically detected magnetic resonance and positron annihilation spectroscopy experiments have been employed to study magnesium-doped GaN layers grown by metal-organic vapor phase epitaxy. As the Mg doping level is changed, the combined experiments reveal a strong correlation between the vacancy concentrations and the intensity of the red photoluminescence band at 1.8eV. The analysis provides strong evidence that the emission is due to recombination in which electrons both from effective-mass donors and from deeper donors recombine with deep centers, the deep centers being vacancy-related defects.

Optical detection of transition metal ion electron paramagnetic resonance by coherent Raman spectroscopy

Coherent Raman scattering in combination with optical heterodyne detection provides an attractive new technique for the measurement of electron paramagnetic resonance (EPR). The technique is applicable to both electronic ground and excited states. In contrast to conventional ODMR techniques, it monitors the precessing magnetisation and produces phase-sensitive spectra. We have demonstrated the feasibility of the experiment using the ‘Al to 4E(4T1) transition of Cr 3+ in Al 2 O 3 (ruby).

The design and sensitivity of microwave frequency optical heterodyne receivers

Recent advances in high speed photodetector and microwave receiver technology make microwave frequency optical heterodyning an attractive approach for the detection of a number of coherent Raman and Brillouin scattering experiments. We have therefore analyzed the sensitivity of microwave frequency optical heterodyne receivers. Experimental tests on a visible wavelength receiver operating at 13.5 GHz confirm the expectation of shot noise limited sensitivity. The relative merits of microwave frequency optical heterodyne detection and the alternative Fabry–Perot interferometry approach are discussed.

Magnetic circular dichroism anisotropy from coherent Raman detected electron paramagnetic resonance spectroscopy: Application to spin-1/2 transition metal ion centers in proteins

Measurement of magnetic circular dichroism (MCD) anisotropy has contributed greatly to the understanding of the electronic structure of transition metal ion centers in both biological and nonbiological materials. Compared to previous methods, optically detected electron paramagnetic resonance experiments can measure MCD anisotropy with dramatically improved orientational resolution. In this paper the relevant theory for systems with an isolated Kramers doublet ground level is derived and its application illustrated using a transition metal ion center in a protein: low spin ferric haem.

Optically detected electron paramagnetic resonance by microwave modulated magnetic circular dichroism

Electron paramagnetic resonance (EPR) can be detected optically, with a laser beam propagating perpendicular to the static magnetic field. As in conventional EPR, excitation uses a resonant microwave field. The detection process can be interpreted as coherent Raman scattering or as a modulation of the laser beam by the circular dichroism of the sample oscillating at the microwave frequency. The latter model suggests that the signal should show the same dependence on the optical wavelength as the MCD signal. We check this for two different samples [cytochrome c-551, a metalloprotein, and ruby (Cr3+:Al2O3)]. In both cases, the observed wavelength dependence is almost identical to that of the MCD signal. A quantitative estimate of the amplitude of the optically detected EPR signal from the MCD also shows good agreement with the experimental results.Electron paramagnetic resonance (EPR) can be detected optically, with a laser beam propagating perpendicular to the static magnetic field. As in conventional EPR, excitation uses a resonant microwave field. The detection process can be interpreted as coherent Raman scattering or as a modulation of the laser beam by the circular dichroism of the sample oscillating at the microwave frequency. The latter model suggests that the signal should show the same dependence on the optical wavelength as the MCD signal. We check this for two different samples [cytochrome c-551, a metalloprotein, and ruby (Cr3+:Al2O3)]. In both cases, the observed wavelength dependence is almost identical to that of the MCD signal. A quantitative estimate of the amplitude of the optically detected EPR signal from the MCD also shows good agreement with the experimental results.

Magnetic circular dichroism anisotropy of the CuA centre of nitrous oxide reductase from coherent Raman detected electron spin resonance spectroscopy

Coherent Raman detected electron spin resonance spectroscopy is a technique that bridges the established fields of magnetic resonance and magneto-optics. By exploiting the orientational selectivity of the microwave resonance condition it becomes possible to measure the relative orientations of the magnetic and optical anisotropies of paramagnetic chromophores, and thereby to test models of their electronic structure. This paper reports the application of this method to the CuA centre from Paracoccus pantotrophus nitrous oxide reductase, an unusual mixed valence copper, Cu(I)/Cu(II), dimer centre also found in some heme-copper terminal oxidases. Data from the principal visible bands (at 476, 514 and 750 nm) shows that their magnetic circular dichroism is almost entirely aligned with the g-value z-axis. This is consistent with previous models of the electronic structure in which the optical transitions are polarized within the copper-thiolate plane of the centre, and the g-value z-axis is orientated normal to this plane.

Breaking the Stokes–anti-Stokes symmetry in Raman heterodyne detection of magnetic-resonance transitions

Coherent Raman scattering can generate Stokes and anti-Stokes fields of comparable intensities. When the Raman shift is due to a magnetic resonance transition (usually in the MHz to GHz range), the Raman fields are generally detected by optical heterodyne detection, using the excitation laser as the local oscillator. In this case, the two sidebands generate beat signals at the same frequency and are therefore indistinguishable. Separation of the two contributions becomes possible, however, by superheterodyne detection with a frequency-shifted optical local oscillator. We compare the two scattering processes, and show how the symmetry between them can be broken in

Coherent Raman detected electron spin resonance spectroscopy of metalloproteins: linking electron spin resonance and magnetic circular dichroism

{\mathrm{Pr}}^{3+}{:\mathrm{Y}\mathrm{A}\mathrm{l}\mathrm{O}}_{3}.

Ultra-high resolution studies of the strain dependence of electron g-values in ZnSe

The simultaneous excitation of paramagnetic molecules with optical (laser) and microwave radiation in the presence of a magnetic field can cause an amplitude, or phase, modulation of the transmitted light at the microwave frequency. The detection of this modulation indicates the presence of coupled optical and ESR transitions. The phenomenon can be viewed as a coherent Raman effect or, in most cases, as a microwave frequency modulation of the magnetic circular dichroism by the precessing magnetization. By allowing the optical and magnetic properties of a transition metal ion centre to be correlated, it becomes possible to deconvolute the overlapping optical or ESR spectra of multiple centres in a protein or of multiple chemical forms of a particular centre. The same correlation capability also allows the relative orientation of the magnetic and optical anisotropies of each species to be measured, even when the species cannot be obtained in a crystalline form. Such measurements provide constraints on electronic structure calculations. The capabilities of the method are illustrated by data from the dimeric mixed-valence Cu(A) centre of nitrous oxide reductase (N(2)OR) from Paracoccus pantotrophus.