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 | Dr Jason M SmithLecturer in Materials |
[ Quicklinks: Research Summary Current Research Projects Recent Publications D.Phil. Projects Available ]
Summary of Research Interests
Leader of the Photonic Nanomaterials Group (PNG) in which research is focused on optical and electronic properties of solid state nanostructures for applications such as optoelectronics devices, quantum information processing and photovoltaics. At present the Group’s research involves two such systems - semiconductor nanocrystals and nitrogen-vacancy defects in diamond which we investigate using luminescence-based optical techniques. More information about our research can be found in the project descriptions below, or on the PNG website.
Current Research Projects
Solar cells based on semiconductor nanocrystals and nanocrystal/polymer composites
A. Stavrinadis, S. Fairclough, A.A.R. Watt, H.E. Assender, and J.M. Smith
The attraction of semiconductor nanocrystals for photovoltaic devices is that they are strong absorbers of light and the ability to tune their bandgap provides a means to control the optical absorption spectrum of the device. Moreover they are cheap to make and can be incorporated into mechanically flexible devices, either on their own or in a composite with a polymer. Our work focuses primarily on synthesis of II-VI nanocrystals such as PbS that absorb light in the near-infrared (lambda < 1600 nm), on the surface chemistry that provides efficient charge extraction, and on device fabrication.
Excitons in semiconductor nano-heterostructures
E. Tyrrell, S. Fairclough, and J.M. Smith
Semiconductor nanocrystals are becoming increasingly important in a wide range of applications in fields as diverse as medicine, renewable energy, and telecommunications. An important feature of many applications is that nanocrystals absorb and emit light at wavelengths that is tuneable with the size of the nanocrystal as a result of quantum confinement. In nanocrystals made from a single constituent material choosing the size also determines other important optical properties, such as the luminescence lifetime and linewidth. However it is also possible to grow heterostructures of different materials, to explore different quantum confinement geometries which modify the behaviour in both quantitative and qualitative fashion. In this project we are calculating the properties of quantum confined excitons in a number of different heterostructures to explore the range of properties that can be achieved.
Fluorescence stability in semiconductor nanocrystals
P.-H. Sher and J.M. Smith
It is well known that if the surface of a nanocrystal is not well passivated, electrons and/or holes can escape into surface trap states, and cause random spectral drift and blinking in the luminescence intensity. This behaviour could limit the use of nanocrystals in some application areas, and could potentially offer opportunities for exploitation in others (such as sensing). In this project we study the fundamental carrier dynamics that lead to the instabilities in the luminescence from nanocrystals. Our characterisation techniques focus on photoluminescence spectroscopy and time-resolved photoluminescence of single nanocrystals. We analyse the experimental results by using Kinetic Monte Carlo techniques to simulate how the quantum state of the system changes with time.
Measurement based quantum entanglement of solid state spin qubits
F. Grazioso, P. Dolan, Dr. J.M. Smith, Dr. B.W. Lovett, and Dr. S.C. Benjamin
Recent advances in the theory of quantum information processing have shown that high fidelity quantum entanglement can be generated between remote systems simply by performing measurements on them in a certain way. From a materials perspective this is a huge advantage over traditional QIP schemes, which require controlled interaction between two or more systems - usually on nanometre length scales - to generate the necessary entanglement. Here we are developing the capability to perform a basic quantum measurement from which a measurement-based entanglement apparatus will be built. The ‘qubit’ that we are measuring is the spin state of a negatively charged Nitrogen Vacancy (NV-) defect in diamond; a system that has been demonstrated to have excellent coherence properties and on which some exquisite single qubit manipulation experiments have already been performed. Our focus is currently on characterising the optical properties of single NV- defects in ultra-pure diamond produced by Element Six Ltd, and on controlling the coupling of light between the defect and the measurement apparatus. The project involves collaboration with Element Six Ltd, the University of Melbourne, the University of Bristol, and the Institute of Photonics at the University of Strathclyde. Funding is provided by the EPSRC and MoD through the QIP IRC, and by Hewlett Packard Laboratories in Bristol.
4 public active projects
Research Publications
S. C. Benjamin, B. W. Lovett, and J. M. Smith, Prospects for measurement based quantum computing using solid state spins, (Laser & Photonics Reviews, in press)
P. K. Santra, R. Viswanatha, S. Daniels, N. L. Pickett, J. M Smith, P. O'Brien and D. D. Sarma, Investigation of the internal heterostructure of highly luminescent quantum dot - quantum well nanocrystals, J. Am. Chem. Soc. 131, 470 (2009).
B. A. Fairchild, P. Olivero, A. D. Greentree, F. Waldermann, R. A. Taylor, J. M. Smith, S. Rubanov, S. Huntington, B. Gibson, D. N. Jamieson, and S. Prawer, Fabrication of ultra thin membranes from single crystal diamond for photonic and NEMS applications, Advanced Materials 20, 4793 (2008).
A. Stavrinadis, R. Beal, J. M. Smith, H. E. Assender, and A. A. R. Watt, Direct formation of PbS nanorods in a conjugated polymer, Advanced Materials 20, 3105 (2008).
S. H. Kim, P. H. Sher, Y. B. Hahn, and J. M. Smith, Luminescence from single CdSe nanocrystals embedded in ZnO thin films using atomic layer deposition, Nanotechnology 19, 365202 (2008).
P. A. Dalgarno, J. M .Smith, J. MacFarlane, B. D. Gerardot, K. Karrai, A. Badolato, P. M. Petroff, and R. J. Warburton, Coulomb interactions in single charged self-assembled quantum dots: Radiative lifetime and recombination energy, Phys. Rev. B 77, 245311 (2008).
P. A. Dalgarno, M. Ediger, B. D. Gerardot, J. M. Smith, S. Seidl, M. Kroner, K. Karrai, P. M. Petroff, A. O. Govorov, and R. J. Warburton, Optically induced hybridization of a quantum dot state with a filled continuum, Phys. Rev. Lett. 100, 176801 (2008).
P. H. Sher, J. M. Smith, P. A. Dalgarno, R. J. Warburton, X. Chen, P. J. Dobson, S. M. Daniels, N. L. Pickett, and P. O'Brien, Power law carrier dynamics in semiconductor nanocrystals at nanosecond time scales, Appl. Phys. Lett. 92, 101111 (2008).
S. Seidl, M. Kroner, P. A. Dalgarno, A. Hogele, J. M. Smith, M. Ediger, B. D. Gerardot, J. M. Garcia, P. M. Petroff, K. Karrai, and R. J. Warburton, Absorption and photoluminescence spectroscopy on a single self-assembled charge-tunable quantum dot, Phys. Rev. B 72 195339 (2005).
P. A. Dalgarno, J. M. Smith, B. D. Gerardot, A. O. Govorov, K. Karrai, P. M. Petroff, and R. J. Warburton, Dark exciton decay dynamics of a semiconductor quantum dot, Phys. Stat. Solid. A-Applications and Materials Science 202 p.2591-2597 (2005).
A. J. Moore, J. Smith, and N. J. Lawson, Volume three-dimensional flow measurements using wavelength multiplexing, Optics Lett. 30, p.2569 (2005).
J. M. Smith, P. A. Dalgarno, R. J. Warburton, A. O. Govorov, K. Karrai, B. D. Gerardot and P. M. Petroff, Voltage control of the spin dynamics of an exciton in a semiconductor quantum dot, Phys. Rev. Lett. 94, 197402 (2005).
M. Ediger, P. A. Dalgarno, J. M. Smith, B. D. Gerardot, R. J. Warburton, K. Karrai and P. M. Petroff, Controlled generation of neutral, negatively-charged and positively-charged excitons in the same single quantum dot, Appl. Phys. Lett. 86, 2119209 (2005).
B. Urbaszek, E. J. McGhee, J. M. Smith, R. J. Warburton, K. Karrai, B. D. Gerardot, J. M. Garcia and P. M. Petroff, Charged excitons in individual quantum dots: effects of vertical electric fields and optical pump power, Physica E 17, 35 (2003).
J. M. Smith, P. A. Dalgarno, B. Urbaszek, E. J. McGhee, G. S. Buller, G. J. Nott and R. J. Warburton, Carrier storage and capture dynamics in quantum-dot heterostructures, Appl. Phys. Lett. 82, 3761 (2003).
M. A. Malik, P. O'Brien, S. Norager and J. Smith, Gallium arsenide nanoparticles: synthesis and characterisation, J. Mater. Chem. 13, 2591 (2003).
J. M. Smith, G. S. Buller, D. Marshall, A. Miller and C. C. Button, Microsecond carrier lifetimes in InGaAsP quantum wells emitting at lambda=1500 nm, Appl. Phys. Lett. 80, 1870 (2002).
J. M. Smith, P. A. Hiskett, I. Gontijo, L. Purves and G. S. Buller, A picosecond time-resolved photoluminescence microscope with detection at wavelengths greater than 1500 nm, Rev. Sci. Instrum. 72, 2325 (2001).
J. M. Smith, P. A. Hiskett and G. S. Buller, Picosecond time-resolved photoluminescence at detection wavelengths greater than 1500 nm, Opt. Lett. 26, 731 (2001).
P. A. Hiskett, J. M. Smith, G. S. Buller and P. D. Townsend, Low-noise single-photon detection at wavelength 1550 nm.Electron. Lett. 37, 1081 (2001).
S. Pellegrini, G. S. Buller, J. M. Smith, A. M. Wallace and S. Cova, Laser-based distance measurement using picosecond resolution time-correlated single-photon counting, Meas. Sci. Technol. 11, 712 (2000).
P. A. Hiskett, G. S. Buller, A. Y. Loudon, J. M. Smith, I. Gontijo, A. C. Walker, P. D. Townsend and M. J. Robertson, Performance and design of InGaAs/InP photodiodes for single- photon counting at 1550 nm, Appl. Optics 39, 6818 (2000).
Projects Available
Optical resonators in ultrapure diamond
J.M. Smith
Diamond is optically transparent throughout the infrared, visible and into the ultraviolet region of the spectrum, and its refractive index is relatively insensitive to temperature, making it highly attractive for producing stable optical resonator structures. Such structures could have a number of uses, from classical optical switches, to microcavities for housing quantum bits (qubits) for quantum computing. The difficulty is that processing diamond into microstructures is highly challenging as its chemical inertness makes it resistant to most etches. New plasma etching techniques developed by our collaborators at the University of Strathclyde Institute of Photonics look extremely promising in this regard, and the project will be to design and characterize resonators and waveguides based on thin films of diamond. The project will also involve interaction with groups at Bristol, with whom we are seeking to fund the project with an EPSRC Programme grant.
Also see homepages:Jason Smith
Quantum photonics using colour centres in diamond
J.M. Smith
The optical transition in the nitrogen-vacancy defect in diamond can be used to read out the spin state of the defect’s electrons, and therefore is an important ingredient for constructing quantum information devices based on these defects. However there are several fundamental questions that need to be answered and techniques developed before reproducible devices can be realized. How does the shelving state act to spin polarize the defect? What is the mechanism behind photo-ionization? To what extent do local field fluctuations limit the ability to control, and read out the spin state optically? This project will seek to find answers to some of these questions using laser excitation and fluorescence techniques. Experiments will be performed using the latest ultra-pure diamond material from Element Six Ltd.
Also see homepages:Jason Smith
Manipulating spin qubits in diamond
J.M. Smith / J.J.L. Morton
Electron spins on colour centres in diamond show excellent prospects for use in quantum information technologies, offering long coherence times (tens of milliseconds) and the ability to initialize, manipulate, and read out the spin state – all prerequisites for quantum information processing. The aims of this project will be to develop advanced spin control techniques for single colour centres using pulsed microwave signals. It will be co-supervised by Dr Jason Smith, head of the photonic nanomaterials group, and Dr John Morton, a Royal Society URF working on electron spin resonance. Focus will be on adapting techniques from ‘bulk’ ESR spectroscopy (eg ‘Bang Bang’ pinning), to use on single electrons and nuclei in the solid state.
Also see homepages:John Morton Jason Smith
Engineering excitons in semiconductor nanocrystals
J M Smith / A A R Watt
The ability to grow heterostructured semiconductor nanocrystals using wet chemical techniques opens up a plethora of new possibilities for engineering their optical and electrical properties. For instance (i) in type II heterostructures, the electron and hole that form the ‘exciton’ are separated spatially, so that the recombination lifetime is increased; and (ii) alloyed structures have recently been grown in which luminescence blinking is absent – a discovery that may hold the key to developing nanocrystal-based LEDs, sensors, and even quantum optical devices. The aim of this project will be to investigate the excitonic behaviour of heterostructured and alloyed nanocrystals. In particular, low temperature spectroscopy of single nanocrystals will be used to gain information free from inhomogeneous and thermal broadening. Experimental results will be modeled using 8-band effective mass calculations that are currently being developed. Applicants interested in the synthesis of alloyed and heterostructured nanocrystals will also be considered.
Also see homepages:Jason Smith Andrew Watt
Semiconductor nanocrystal sensors
J.M. Smith
Semiconductor nanocrystals grown by wet chemical methods are rapidly becoming a key functional material that are finding applications in fields as diverse as life sciences, solar power generation, and telecommunications. It is well known that the chemistry of the nanocrystals’ surface profoundly affects their electronic properties, and the fluorescence from single nanocrystals displays a number of interesting dynamic phenomena such as random blinking and spectral drift due to the Quantum Confined Stark Effect which can potentially be harnessed for use as sensors of the local environment. The aim of this project will be to develop techniques for using changes in the fluorescence properties of nanocrystals to sense changes in the surroundings. It will build on recent work to develop a fluidics device for monitoring single nanocrystal fluorescence under changing local conditions.
Also see homepages:Jason Smith
Enhancing the efficiency of thin film solar cells using optical confinement
J M Smith / H E Assender / A A R Watt
Thin film solar cells offer an inexpensive means to generate ‘clean’ energy, but current efficiencies are limited to around five percent, about three times lower than commercial polycrystalline silicon cells. One of the main reasons behind the low efficiency is that a tension exists between the desire to absorb as much as possible of the incident light, in which case the optical path length should be thick (at least several hundred nanometres), and the desire to extract the photogenerated charge carriers efficiently from the cell, in which case the exciton transport path length should be short (no more than a few tens of nanometers). Most attempts to solve this problem involve using a thick cell, and focusing the advanced aspects of cell design on building in some means for ensuring a short transport path length. Here we take the opposite viewpoint; that the optical path can be elongated for a given cell geometry by the use of wave guiding and cavitation, thereby reducing the burden placed on the transport related features of the device. The project will involve the design, fabrication, and testing of devices that explore this theme. It would best suit a student with at least a basic knowledge of optics, who is confident in the laboratory, and who can work both independently and within a team.
Also see homepages:Hazel Assender Jason Smith Andrew Watt
Also see a full listing of New projects available within the Department of Materials.
