Publications by Andrew Turberfield


Fabrication of photonic crystals for the visible spectrum by holographic lithography

Nature 404 (2000) 53-56

M Campbell, DN Sharp, MT Harrison, RG Denning, AJ Turberfield

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.


Low-energy electronic spin excitations between filling factors ν = 1 and 1/2 studied by optically detected nuclear magnetic resonance

Physica E: Low-Dimensional Systems and Nanostructures 6 (2000) 56-59

RL Brockbank, HDM Davies, JF Ryan, MA Thomson, AJ Turberfield

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 .


A DNA-fuelled molecular machine made of DNA.

Nature 406 (2000) 605-608

B Yurke, AJ Turberfield, AP Mills, FC Simmel, JL Neumann

Molecular recognition between complementary strands of DNA allows construction on a nanometre length scale. For example, DNA tags may be used to organize the assembly of colloidal particles, and DNA templates can direct the growth of semiconductor nanocrystals and metal wires. As a structural material in its own right, DNA can be used to make ordered static arrays of tiles, linked rings and polyhedra. The construction of active devices is also possible--for example, a nanomechanical switch, whose conformation is changed by inducing a transition in the chirality of the DNA double helix. Melting of chemically modified DNA has been induced by optical absorption, and conformational changes caused by the binding of oligonucleotides or other small groups have been shown to change the enzymatic activity of ribozymes. Here we report the construction of a DNA machine in which the DNA is used not only as a structural material, but also as 'fuel'. The machine, made from three strands of DNA, has the form of a pair of tweezers. It may be closed and opened by addition of auxiliary strands of 'fuel' DNA; each cycle produces a duplex DNA waste product.


Photonic crystals for the visible spectrum by holographic lithography

Conference on Lasers and Electro-Optics Europe - Technical Digest (2000) 68-

DN Sharp, AJ Turberfield, M Campbell, RG Denning

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.


Variable sample temperature scanning superconducting quantum interference device microscope

APPLIED PHYSICS LETTERS 74 (1999) 4011-4013

JR Kirtley, CC Tsuei, KA Moler, VG Kogan, JR Clem, AJ Turberfield


Skyrmion-hole excitations at v=1 studied by photoluminescence spectroscopy

PHYSICA B 251 (1998) 544-548

HDM Davies, JC Harris, RL Brockbank, JF Ryan, AJ Turberfield, MY Simmons, DA Ritchie

We present measurements of photoluminescence from the incompressible two-dimensional electron system at filling factor v = 1. By varying the doping density, sample structure and angle between magnetic field and confinement plane we investigate the influence of electron-electron and electron-valence hole interactions on the photoluminescence spectrum. We find that a strong electron-valence hole interaction suppresses a characteristic discontinuity in the photoluminescence energy. In all samples studied the electron-valence hole interaction is strong enough to prevent observation of a theoretically predicted tilt-induced shrinkage of skyrmion-charged spin texture excitations. (C) 1998 Elsevier Science B.V. All rights reserved.


Optically detected nuclear magnetic resonance from a single heterojunction in the fractional quantum Hall regime

PHYSICA B 256 (1998) 104-112

HDM Davies, RL Brockbank, JF Ryan, AJ Turberfield

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.


Role of spin excitations in the fractional quantum Hall effect at nu=1/3

PHYSICA B 251 (1998) 44-48

JC Harris, HDM Davies, JF Ryan, AJ Turberfield

We report resonant inelastic light scattering measurements of collective electronic excitations at filling factor v = 1/3 in a high mobility 2DES at temperatures between 50 mK and 3 K. In a narrow range of temperatures near 1 K the intensity of light scattering from the charge density mode, and the exchange enhancement of the energy of a collective spin excitation, fall rapidly to zero. We deduce that the electron correlation characteristic of the spin-polarized electron liquid at v = 1/3 collapses at 1 K: this collapse is triggered by thermal fluctuations of the electron spin. (C) 1998 Elsevier Science B.V. All rights reserved.


Spin and charge density excitations and the collapse of the fractional quantum hall state at ν = 1/3

Physical Review Letters 78 (1997) 4095-4098

HDM Davies, CJ Harris, JF Ryan, AJ Turberfield

Inelastic light scattering from 2D electrons below 100 mK reveals collective charge and spin density excitations of the incompressible electron liquid at Landau level filling factor ν = 1/3 and provides experimental evidence for a magnetoroton minimum in the charge density dispersion curve. The temperature scale for the collapse of electron correlation in the spin-polarized electron liquid is determined by thermal excitation of long-wavelength spin waves. © 1997 The American Physical Society.


ELECTRON-DIFFRACTION FROM A 2D ELECTRON WIGNER CRYSTAL

EUROPHYSICS LETTERS 29 (1995) 333-338

IN HARRIS, HDM DAVIES, JF RYAN, AJ TURBERFIELD, CT FOXON, JJ HARRIS


PICOSECOND PHOTOLUMINESCENCE INTENSITY CORRELATION-MEASUREMENTS OF HOT CARRIERS IN GAAS/ALXGA1-XAS QUANTUM-WELLS

JOURNAL OF LUMINESCENCE 59 (1994) 303-313

AM DEPAULA, JF RYAN, HJW EAKIN, M TATHAM, RA TAYLOR, AJ TURBERFIELD


QUASI-PARTICLE RECOMBINATION AND SPATIAL ORDERING OF 2D ELECTRONS IN THE EXTREME QUANTUM LIMIT

SURF SCI 305 (1994) 61-66

IN HARRIS, HDM DAVIES, RA FORD, JF RYAN, AJ TURBERFIELD, CT FOXON, JJ HARRIS


Picosecond photoluminescence intensity correlation measurements of hot carriers in GaAs/Al<inf>x</inf>Ga<inf>1-x</inf>As quantum wells

Journal of Luminescence 59 (1994) 303-313

AM de Paula, JF Ryan, HJW Eakin, M Tatham, RA Taylor, AJ Turberfield

We have measured the energy relaxation of hot carriers in doped and undoped GaAs/AlxGa1-xAs quantum wells by detecting time-resolved hot luminescence using a two-pulse intensity correlation technique. The results are compared with time-dependent intensity correlation functions calculated using a model of energy relaxation by optical phonon interactions which includes non-equilibrium phonon effects. The excellent agreement between calculated and experimental correlation functions shows the great potential of this method in measuring ultrafast relaxation processes. © 1994.


PHOTOLUMINESCENCE SPECTROSCOPY OF INCOMPRESSIBLE ELECTRON LIQUID STATES IN GAAS

NATO ADV SCI INST SE 248 (1993) 45-56

JF RYAN, AJ TURBERFIELD, RA FORD, IN HARRIS, CT FOXON, JJ HARRIS


Optical investigation of tunneling in AlAs/GaAs/AlAs double-barrier diodes.

Phys Rev B Condens Matter 47 (1993) 15705-15716

SR Andrews, AJ Turberfield, BA Miller


Incompressible electron liquid states studied by optical spectroscopy.

Phys Rev B Condens Matter 47 (1993) 4794-4797

AJ Turberfield, RA Ford, IN Harris, JF Ryan, CT Foxon, JJ Harris


OPTICAL SPECTROSCOPY OF CORRELATED PHASES OF DEGENERATE 2-DIMENSIONAL ELECTRONS

SURF SCI 263 (1992) 1-8

AJ TURBERFIELD, SR HAYNES, PA WRIGHT, RA FORD, RG CLARK, JF RYAN, JJ HARRIS, CT FOXON


CORRELATED STATES OF DEGENERATE 2D ELECTRONS STUDIED BY OPTICAL SPECTROSCOPY

PHYS SCRIPTA T45 (1992) 164-167

AJ TURBERFIELD, RA FORD, IN HARRIS, JF RYAN, CT FOXON, JJ HARRIS


LUMINESCENCE FROM DEGENERATE 2-DIMENSIONAL ELECTRONS AT AN ULTRAHIGH MOBILITY HETEROJUNCTION

SURF SCI 263 (1992) 614-617

SR HAYNES, RA FORD, AJ TURBERFIELD, PA WRIGHT, RG CLARK, JF RYAN, JJ HARRIS, CT FOXON


Optical spectroscopy of GaAs in the extreme quantum limit: integer and fractional quantum Hall effect, and onset of the electron solid

Physica B: Physics of Condensed Matter 169 (1991) 336-354

RG Clark, RA Ford, SR Haynes, JF Ryan, AJ Turberfield, PA Wright, FIB Williams, G Deville, DC Glattli, JR Mallett, M van der Burgt, PMW Oswald, F Herlach, CT Foxon, JJ Harris

Our recent optical detection of the integer and fractional quantum Hall effects in GaAs, by intrinsic band-gap photoluminescence at dilution refrigerator temperatures, is reviewed. This work has been extended to the extreme quantum limit where a photoluminescence peak develops close to Landau level filling factor v = 1 5 which correlates both with the onset of threshold behaviour in current-voltage characteristics of the two-dimensional electron system and a resonant radio-frequency absorption; the latter are quantitatively accounted for by a model of crystalline electronic structure broken up into domains. Preliminary mK transport experiments in intense, pulsed magnetic fields are also described, which establish a basis to access the electron solid phase transition in a hitherto unattainable region of the (B, T) plane. © 1991.

Pages