Publications


DNA scaffolds and electron crystallography: a method for protein structure determination

BIOPHYS J 86 (2004) 80A-80A

J Malo, JC Mitchell, C Venien-Bryan, JR Harris, LN Johnson, DJ Sherratt, AJ Turberfield


Self-assembly of chiral DNA nanotubes.

J Am Chem Soc 126 (2004) 16342-16343

JC Mitchell, JR Harris, J Malo, J Bath, AJ Turberfield

A system of DNA "tiles" that is designed to assemble to form two-dimensional arrays is observed to form narrow ribbons several micrometers in length. The uniform width of the ribbons and lack of frayed edges lead us to propose that they are arrays that have curled and closed on themselves to form tubes. This proposal is confirmed by the observation of tubes with helical order.


DNA fuel for free-running nanomachines

Physical Review Letters 90 (2003) article 118102 4pp-

AJ Turberfield, J. C. Mitchell, B. Yurke, A. P. Mills


DNA as an engineering material

PHYSICS WORLD 16 (2003) 43-46

A Turberfield


Diamond-like photonic crystals by holographic lithography

Technical Digest - Summaries of Papers Presented at the Quantum Electronics and Laser Science Conference, QELS 2001 (2001) 13-

AJ Turberfield

© 2001 Optical Soc. Of America. Summary form only given. Holographic lithography is a general and flexible technique for the fabrication of three-dimensional photonic crystals. Three-dimensional microstructure is generated when a four-beam laser interference pattern is used to expose a thick layer of photoresist. Highly exposed regions in the resulting three-dimensional intensity grating are rendered insoluble; unexposed areas are then dissolved away to produce a three-dimensional photonic crystal formed of cross-linked polymer with air-filled voids. The polymeric structure may be used as a template for the production of photonic crystals with higher refractive index contrast. This technique is particularly well adapted to the production of structures with the sub-micron periodicity required for applications in the visible optical spectrum.


Photonic crystals made by holographic lithography

MRS Bulletin 26 (2001) 632-636

AJ 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.


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.


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.


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.


Investigation of inter-valley scattering and hot phonon dynamics in GaAs quantum wells using femtosecond luminescence intensity correlation

Superlattices and Microstructures 6 (1989) 199-202

AM de Paula, RA Taylor, CWW Bradley, AJ Turberfield, JF Ryan

Photoluminescence intensity correlation measurements of GaAs quantum wells using 120 fs laser pulses show relatively slow relaxation times ≤ 10 ps at high energy close to the L valley conduction band minimum. This value is consistent with recent measurements of the L → Γ scattering time. However, theoretical estimates show that nonequilibrium phonon effects can also give rise to slow relaxation on this timescale.


Time-resolved photoluminescence from hot two-dimensional carriers in GaAsGaAlAs MQWS

Surface Science 170 (1986) 511-519

JF Ryan, RA Taylor, AJ Turberfield, JM Worlock

Picosecond time-resolved measurements of luminescence from hot carriers confined in GaAsGaAlAs multiple quantum wells show that energy loss rates are substantially slower than those predicted for 2D carriers. We review our recent experiments and present results for photoexcitation of (1) GaAs layers only, (2) both GaAs and GaAlAs layers. We compare the energy loss rates in samples with different well widths. Finally, we present measurements of hot 2D carrier relaxation in the presence of high magnetic fields; at low fields the energy loss rate is reduced, but for B > 9 T we observe a rapid increase. © 1986.


HOT ELECTRON RELAXATION AND TRAPPING IN MODULATION-DOPED GaAs/GaA1As MULTIPLE QUANTUM WELL HETEROSTRUCTURES.

(1985) 567-570

JF Ryan, RA Taylor, AJ Turberfield, A Maciel, JM Worlock, AC Gossard, W Wiegmann

There is much current interest in determining the energy relaxation processes of hot carriers confined in quantum well heterostructures. We present measurements of time-resolved photoluminescence from hot carriers and from carrier recombination at shallow traps. The experimental technique is basically the same as described previously except that the laser (h upsilon //L equals 1. 76 eV) excites carriers only in the GaAs layers.


Picosecond photoluminescence measurements of Landau level lifetimes and time dependent Landau level linebroadening in modulation-doped GaAs-GaAlAs multiple quantum wells

Physica B+C 134 (1985) 318-322

JF Ryan, RA Taylor, AJ Turberfield, JM Worlock

We report the first picosecond time-resolved photoluminescence measurementsof hot-carrier relaxation in a modulation-doped GaAs-GaAlAs MQW in the presence of strong magnetic fields. We have measured the lifetimes of carriers in excited Landau levels and have determined the time-dependent carrier temperature. We find that the cooling rate is slower in applied field tthan at B=0; also there is a significant increase in the cooling rate for B {greater-than or approximate} 10T. We report also the observation of highly time-dependent linewidths of the Landau levels. © 1985.


MEASUREMENTS OF HOT CARRIER RELAXATION AND RECOMBINATION IN GaAs QUANTUM WELLS BY PICOSECOND OPTICAL PROBING.

IEE Colloquium (Digest) (1984)

JF Ryan, RA Taylor, AJ Turberfield

In this paper the authors show how picosecond time-resolved photoluminescence can be used to probe the energy relaxation of hot carriers in semiconductor structures. In addition, by measuring the variation of luminescence lifetime with carrier density one can obtain information about radiative and nonradiative decay processes. These processes are important in all photonic devices, but especially so in structures where interfaces may have a particularly strong influence.


Time-resolved photoluminescence of two-dimensional hot carriers in GaAs-AlGaAs heterostructures

Physical Review Letters 53 (1984) 1841-1844

JF Ryan, RA Taylor, AJ Turberfield, A Maciel, JM Worlock, AC Gossard, W Wiegmann

We have studied the picosecond time dependence of luminescence from a two-dimensional electron system following absorption of an ultrashort light pulse. From our measurements we determine the temporal evolution of the carrier temperature, finding that the cooling of hot carriers is suppressed by a factor 60 below that predicted on a three-dimensional nondegenerate-electron model. Additionally, we determine the electron-hole radiative life-time and invoke a hole trap to explain shortened luminescence lifetimes at low carrier densities. © 1984 The American Physical Society.


Modelling DNA origami self-assembly at the domain level

Journal of Chemical Physics American Institute of Physics (AIP) (0)

JN Bath, AJ Turberfield, TO Ouldridge, KE Dunn, F Dannenberg, M Kwiatkowska

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