Projects

Development of Coherent Raman detected Electron Spin Resonance Spectroscopy

Dr Daniel Wolverson, Prof John Davies, Prof Andrew Thomson FRS

When a paramagnetic material is excited with resonant microwave radiation a coherent precession of the electronic spins is generated about the polarising magnetic field axis. In conventional Electron Spin Resonance (ESR) spectroscopy the associated precession in magnetisation is measured. However it is also often possible to measure the microwave induced precession of the Magnetic Circular Dichroism (MCD). Experimentally, a microwave frequency intensity modulation of a circularly polarised optical beam is observed when it propagates along an axis perpendicular to the polarising magnetic field. The optical modulation sidebands can also be viewed as resulting from coherent Raman mixing of the optical and microwave radiation fields. This picture of the experiment is especially useful in the very rare circumstance that optical linewidths are smaller than the microwave frequency and a simple magneto-optical modulation treatment is not valid. Coherent Raman detected ESR (CR-ESR) does not require any special magnetic or optical relaxation characteristics, which makes it very widely applicable. In semiconductor materials it frequently offers orders of magnitude better sensitivity than conventional microwave detected ESR measurements. Samples with very small volumes, such as single epitaxial layers, can be studied. In biomolecular spectroscopy the primary motivation for using CR-ESR is the extra information that results from the enhanced selectivity of double resonance. This allows the optical and magnetic properties of a particular paramagnetic species in the sample to be correlated. Selective study of each centre in a multi-centre protein or of unstable minority species in a catalytic pathway is particularly useful. It is also possible to measure the relative orientations of the magnetic and optical anisotropies, which is very powerful in the assignment of electronic transitions and hence in the identification of excited levels. Current efforts in this area are focused on improving the theory of magneto-optical spectroscopies.

Selected publications

• Bingham, S.J., B. Borger, J. Gutschank, D. Suter, and A.J. Thomson, Probing the electronic structure of transition metal ion centres in proteins by coherent Raman-detected electron paramagnetic resonance spectroscopy. Journal of Biological Inorganic Chemistry, 2000. 5(1): p. 30-35. Article
• Bingham, S.J., J. Gutschank, B. Borger, D. Suter, and A.J. Thomson, Magnetic circular dichroism anisotropy from coherent Raman detected electron paramagnetic resonance spectroscopy: Application to spin-1/2 transition metal ion centers in proteins. Journal of Chemical Physics, 2000. Article
• Bingham, S.J., J.J. Davies, and D. Wolverson, High-resolution optical detection of electron spin resonance in epitaxial semiconductor layers by coherent Raman spectroscopy. Physical Review B, 2002. 65(15): art. no.-155301. Article
• Bingham, S.J., T. Rasmussen, J. Farrar, D. Wolverson, and A.J. Thomson, Magnetic circular dichroism anisotropy of the Cu_A centre of nitrous oxide reductase from coherent Raman detected electron spin resonance spectroscopy. Molecular Physics, 2007. 105(15-16): p.2169. Article

Optically detected magnetic resonance of semiconductors

Dr Daniel Wolverson, Prof John Davies, Dr Gazi Aliev, Dr Lowenna Smith

In addition to application of the coherent Raman detected ESR methods described above, I have been involved in the use of conventional luminescence detected Optically Detected Magnetic Resonance (ODMR) methods to the study of semiconductor materials. Many of the materials investigated have been II-VI materials of relevance to opto-electronic and spin-tronic devices.

Selected publications

• Aliev, G.N., S.J. Bingham, D. Wolverson, J.J. Davies, H. Makino, H.J. Ko, and T. Yao, Optically detected magnetic resonance of epitaxial nitrogen- doped ZnO. Physical Review B, 2004. 70(11): art. no.-115206. Article
• Aliev, G.N., S. Zeng, J. J. Davies, D. Wolverson, S. J. Bingham, P. J. Parbrook and T. Wang, Nature of acceptor states in magnesium-doped gallium nitride, Physical Review B, 2005. 71: art. no.-195204. Article
• Smith, L.C., S.J. Bingham, J.J. Davies, and D. Wolverson, Electron paramagnetic resonance of manganese ions in CdTe detected by coherent Raman spectroscopy. Applied Physics Letters, 2005. 87(20): art. no. 202101. Article
• Zeng, S., G.N. Aliev, D. Wolverson, J.J. Davies, S.J. Bingham, D.A. Abdulmalik, P.G. Coleman, T. Wang, and P.J. Parbrook, Origin of the red luminescence in Mg-doped GaN. Applied Physics Letters, 2006. 89(2): art. no. 022107. Article

Photoconductivity of solar cells probed by microwave reflectivity measurements

Dr Halina Dunn, Prof Laurie Peter, Eric Maluta, Prof Alison Walker

Microwave reflectivity provides a contactless method of measuring the electrical conductivity of functioning solar cell devices. I am assisting colleagues in their work on Dye Sensitised Solar Cells.

Recent projects

Other recent projects have included work to develop an Electron Spin Resonance Imaging instrument and the application of Electron Spin Resonance to the study of drug delivery materials. Unfortunately these projects have been shelved following problems with our laboratory space, including a serious asbestos contamination incident.