Professor Andrew Turberfield

Abstract:

The term 'photonics' describes a technology whereby data transmission and processing occurs largely or entirely by means of photons. Photonic crystals are microstructured materials in which the dielectric constant is periodically modulated on a length scale comparable to the desired wavelength of operation. Multiple interference between waves scattered from each unit cell of the structure may open a 'photonic bandgap'--a range of frequencies, analogous to the electronic bandgap of a semiconductor, within which no propagating electromagnetic modes exist. Numerous device principles that exploit this property have been identified. Considerable progress has now been made in constructing two-dimensional structures using conventional lithography, but the fabrication of three-dimensional photonic crystal structures for the visible spectrum remains a considerable challenge. Here we describe a technique--three-dimensional holographic lithography--that is well suited to the production of three-dimensional structures with sub-micrometre periodicity. With this technique we have made microperiodic polymeric structures, and we have used these as templates to create complementary structures with higher refractive-index contrast.

Abstract:

We report measurements of the spin relaxation time (Tln) for nuclei in the potential well confining a high-mobility two-dimensional electron system at a single GaAs-GaAlAs heterojunction. At low temperatures nuclear spin relaxation is dominated by electron-nuclear spin scattering: we find that Tln displays sharp maxima at incompressible states throughout the v = 2/3 hierarchy of the fractional quantum Hall effect. This behaviour is consistent with the existence of low-energy spin excitations only where the electron system is compressible. Our measurements also provide evidence for a gap in the spin excitation spectrum at ν = 1/2 .

Abstract:

The fabrication of three-dimensional photonic crystal structures with sub-micron periodicity was performed by holographic lithography. The photonic crystals of titanium dioxide and polymeric materials were characterized by scanning electron microscopy and optical diffraction measurements. The interference pattern generated at the intersection of four beams from a neodymium laser was employed for the exposure of the photoresist.

Abstract:

We report a remarkably sensitive optical technique for detecting nuclear magnetic resonance from a single ultra-high mobility two-dimensional electron system at a GaAs heterojunction in the fractional quantum Hall regime. Resonant inter-band optical excitation of the 2DES provides a very high degree of dynamic nuclear polarization, three times greater than that previously achieved; this is detected by using inelastic light scattering to measure the Overhauser shift in the energy of the electron spin wave. Our optical detection scheme is sensitive only to the polarization of nuclei in the illuminated volume at the heterojunction. This factor, together with the highly resonant excitation, increases the sensitivity of the technique and has allowed us to measure Knight-shifted magnetic resonance bands of As and Ga nuclei within a single 2DES at filling Factor v = 1/3 at temperatures <100 mK. These measurements can be made over a wide range of temperatures and filling factors and have great potential for the study of collective spin excitations in the fractional quantum Hall regime. (C) 1998 Elsevier Science B.V. All rights reserved.