Research activity

As a research group we are involved in training the next generation of experts in photonics and we have 20 PhD students working on photonics related projects. We also extensively publish our work in leading photonics journals and present results at major international conferences.

PhD projects available

Project summary

Supervisor Stephen Sweeney
Research Group Photonics
Type Experimental, Theoretical
Collaborations  This project will involve collaborations with other universities and companies based in Europe and North America
Techniques used

The project will largely involve simulating the properties of semiconductors computationally using both bespoke and commercial software. The outputs will be:

  • Models of the semiconductor bandstructure
  • Determination of the temperature dependence of key properties, e.g. band gap, effective mass, band offsets
  • Prediction of device properties based upon these materials
Student will require Potential students should have a strong interest in modelling the physics of semiconductors whilst keeping in mind device applications
Objectives The aims of this project are to take a radical look at alloy physics and the extent to which mixed semiconductors encompassing the II-VIs, III-Vs and II-IVs may be used to produce semiconductors with good photonic and electronic properties that are also temperature stable. The project will principally be theoretical, but as the project progresses the aim will be to produce some real material and devices.

Project description

One of principal problems with all modern electronic and optoelectronic/photonic devices is the fact that the semiconductors on which they are based change their properties as the ambient temperature varies. For example, as an LED or laser heats-up, its band gap changes meaning that the emission wavelength changes. This is particularly problematic if one wishes to make a stable high efficiency light source, or for example, sending information down an optical fibre at a particular wavelength. In electronics, properties such as the effective mass are linked to the band gap and changes in mass change the transport of electrons through the material (i.e. change its resistivity - important in electronics). This means that temperature stabilisation electronic are often required to maintain a constant temperature. Such systems typically demand 10x more energy than the devices themselves. There are therefore considerable energy savings to be made by producing temperature insensitive semiconductors, with consequent positive environmental and economic benefits.

Project summary

Supervisor Stephen Sweeney
Research Group Photonics
Type Experimental, Theoretical
Collaborations Arizona State University, USA
Techniques used Experimental:
  • Temperature and pressure depdendence of steady-state optical and electronic properties.
  • Set-up of a simple OCT interferometer.

Theoretical:

  • Active region design and optimisation.
  • Modelling of carrier recombination in lasers, amplified spontaneous emission and non-equilibrium conditions in semiconductor lasers and LEDs.
Student will require A strong interest in photonics and its application in medicine and diagnostics.
Objectives The aim of this project is to investigate and develop high brightness optical sources for applications in Optical Coherence Tomography.

Project description

The project will consider approaches based on quantum well and quantum dot based devices (semiconductor lasers and superluminescent LEDs) using a combination of experimental and theoretical approaches. Please see the optical coherence tomography pages on Wikipedia for more information.

Project summary

Supervisor Jeremy Allam (Supervisor) and Dr Richard Curry (Co-supervisor)
Research Group Photonics
Type Experimental
Collaborations University of Southampton
Additional support This studentship is in conjunction with the EPSRC project "Nanotube Nonlinear Waveguides for Next Generation Electrophotonics"
Techniques used Nonlinear optical methods using femtosecond titanium-sapphire laser system, including:
  • Z-scan measurements
  • Degenerate four-wave mixing
  • Upconversion of propagating femtosecond pulses

Preparation of carbon nanotube samples, including:

  • Growth of tubes
  • Size selection
  • Embedding in polymer waveguides, cavities, etc.
Student will require
  • Good hands-on skills with complex experiments.
  • Some knowledge of lasers and optics, and/or electronic and optical properties of solid-state materials.
  • Physical insight to guide the experiments performed.
  • Willingness to talk and collaborate with theorists, and industrial collaborators.
Objectives
  • Accurately quantify second- and third-order nonlinear coefficients in nanotube-doped polymers and glasses, and correlate with nanotube properties (diameters, chirality, etc)
  • Investigate nonlinear propagation of ultrashort laser pulses in nanotube waveguides.
  • Demonstrate prototypical waveguide devices for ultrafast optical switching, sum-frequency generation, etc.

Project description

The discovery that carbon atoms can form molecules with spherical and tubular geometry has provided researchers with a new class of materials with unique properties called carbon nanotubes (CNTs). In particular these CNTs possess remarkable mechanical and electrical properties that have been used to produce ultra-strong fibres and electron sources for displays amongst many other applications. Recently, studies on the optical properties of these materials have shown that they have much to offer in this area as well. These materials exhibit unique absorption and emission properties that can be controlled through changing the diameter of the carbon tubes. They also have significant potential through the use of their so called non-linear optical properties. These properties are generally observed when high light intensities are present and can be used to control and adapt this light. For example it is possible to rapidly switch the light on and off, change the wavelength (colour) of the light and even form a continuum of light from a short pulse using non-linear properties. The aim of this proposal is to develop a new generation of electrophotonic materials by embedding these carbon nanotubes in polymer and chalcogenide glass hosts. Within these CNT-doped hosts waveguides will be formed and their linear and non-linear optical properties studied. We will exploit them to realise highly functional planar lightwave devices.

Project summary

Supervisor Stephen Sweeney
Research Group Photonics
Type Experimental
Collaborations
  • University of Marburg, Germany
  • Stanford University, USA
  • Arizona State University, USA
  • University of Sheffield, UK
Objectives The aim of this project is to investigate new semiconductor materials for applications in photonics and electronics. The core theme of this work is understand and exploit the properties of III-V alloys containing group V elements such as Nitrogen and Bismuth. Whilst these are quite dissimilar atoms, they can both have remarkable effects on III-V semiconductors giving rise to the possibility of efficient lasers and solar cells, optical computing, spintronics and THz applications.

Project description

In this project we will investigate III-V semiconductors both in terms of their fundamental physical properties and their application in real devices such as lasers and solar cells. Experimental activities will focus on developing novel spectroscopic techniques to determine the nature of the band structure of these materials; such techniques include photocurrent spectroscopy, photo-, and electro-luminescence and modulation techniques, eg. modulation of the structure through external fields etc. Part of this study will also utilise the unique high pressure and low temperature facilities at Surrey which offer a useful way of manipulating, for example, the band structure and physical properties of the semiconductors.

The second aspect of this project will be to characterise fully-functional devices based on these materials, eg. lasers, light-emitting diodes or solar cells, as they become available. The knowledge gained from the spectroscopic studies with be used to help develop optimum devices where we will be interested in maximising parameters such as efficiency or producing temperature insensitive operation.

The project will be backed up with strong theory both at Surrey and other leading international groups. Furthermore, the experimental work will be highly collaborative, involving academic and commercial groups in the USA, Europe and Japan.

Project summary

Supervisor Jeremy Allam (Supervisor) and Stephen Sweeney (Co-supervisor)
Research Group Photonics
Type Experimental
Techniques used
  • Ultrafast optical methods using femtosecond titanium-sapphire laser systems
  • Optical Parametric Oscillator (OPO) and Optical Parametric Amplifier (OPA) light sources
  • Mixed temporal-spectral methods (e.g. "FROG" - frequency-resolved optical gating)
  • Design of new photonic structures with optimised dynamics
Student will require
  • Good hands-on skills with complex experiments
  • Some knowledge of lasers, optics and semiconductor physics
  • Physical insight to guide the experiments performed
  • Willingness to talk and collaborate with theorists
Objectives
  • To apply spectral-temporal techniques to the study of femtosecond pulses interacting with a range of photonic devices and materials
  • To understand physical processes limiting the speed of real-world optical devices
  • To investigate new physical effects in high-intensity, coherent, short-duration pulses propagating in semiconductor optical devices

Project description

Femtosecond optical pulses have become an essential tool for the study of photonic materials and devices:

  1. The short duration (<50fs where 1fs =10-15s) allows us to study dynamics of the fundamental processes which limit the switching rate of present photonic devices to the GHz range
  2. The high instantaneous light intensity allows us to study extreme nonlinear processes, from material modification (e.g. laser writing of optical gratings or waveguides) to fundamental studies of relativistic electron dynamics
  3. The inherent spectral width allows us to explore the full spectral-temporal response of materials; for example, measuring the chromatic dispersion of light propagating in artificial photonic crystals
  4. The coherent optical pulses can be shaped in the spectral domain to allow "coherent control" of quantum systems

A number of projects are available based on spectral-temporal measurements of femtosecond pulses interacting with different photonic materials and devices. The latter include semiconductor lasers, nonlinear waveguides, and photonic crystals.

Project summary

Supervisor Ben Murdin
Research Group Photonics
Type Experimental
Collaborations
  • University College London
  • Radboud University Nijmegen
  • Rice University Houston
Techniques used • Numerical image processing techniques encoded with a language such as fortran or matlab
• There is the possibility to perform experiments in megagauss magnetic fields with low temperature terahertz (THz) spectroscopy systems
Student will require A good degree in a physics or engineering related subject
Objectives • To develop a new method of image processing related to the maximum entropy method that includes correlations
• To use high magnetic fields to produce spectra of defects in semiconductors up to 30T, at facilities in the Netherlands and Rice University Houston
• To make significant enhancement in the field dependent spectra, to detect the features within the noise, by making use of the correlations between adjacent spectra
• To use the processed spectra to gain insight into the behavior of atoms and molecules in fields up to 100,000T on the surface of white dwarf stars
• To generalize the technique to other areas such as geological seismological sequences, time series image snapshots etc.
• To commercialise the technique

Project description

Image processing is a technology that is ubiquitous. Many of the disruptive advances were made by physicists such as astronomers trying to improve the data produced with telescopes, by fighting with diffraction problems and noise. A very important advance was made by the description of the problem with Bayesian probability and the application of the idea of information entropy. The concept is that the probability of arriving at the measured data is easy to calculate if you know the true image and the noise distribution, and Bayes gives a way to calculate the reverse, i.e. the probability of the true image given the data. An important requirement in the calculation is knowledge of the intrinisic probability of any possible true image in the absence of any data, and this is where the entropy comes in. Intrinsically the most likely images are the ones with the maximum entropy, i.e. the smoothest ones - the ones with the minimum information content.  The image with the maximum entropy that is consistent with the data and the expected noise distribution is the most likely one. The useful thing about finding the maximum entropy image, over alternatives such image processing by fitting peaks etc, is that it has the least bias. The only assumption is the type of noise.

There are a number of variations of the maximum entropy technique, but there are few that take account of situations with knowledge that the features have known shape. A good example of this is the case of images of the magnetic field dependence of absorption spectra. In such images the useful information is in the form of absorption lines that move smoothly from one spectrum to the other. In the same way, a stack of movie stills taken in a time series containing a moving object also has information that moves smoothly from frame to frame in sequence.

In this project we will develop a new method of maximum entropy processing that uses the knowledge that the frames or spectra have correlations. The application we will use as a testbed is the high magnetic field spectroscopy of silicon impurities [see our article in Nature Communications]. Silicon and the technology developed with it has revolutionised industry, entertainment and communication, and there is always interest in new ways to remember and process information. The challenge is to find methods of encoding information in the smallest possible volume and manipulating it in the most complex ways with the lowest energy cost and the highest speed. Our group leads a project called COMPASSS (Coherent Optical and Microwave Physics for Atomic Scale Spintronics in Silicon, www.compasss.net), a £multi-million activity aiming to develop methods for encoding information in a single electron, orbiting a single impurity atom in a silicon crystal.

Project summary

Supervisor Steven Clowes
Research Group Photonics
Type Experimental
Collaborations
  • Lancaster University
  • Tsinghua University, Beijing
Techniques used
  •  Spin polarised photocurrent measurements using optical orientation of electron spins using circularly polarised light
  • High Magnetic Fields (7 Tesla) and low temperatures (>1.8 K) using superconducting magnet with optical cryostat
  • Clean-room processing techniques such as electron beam lithography, focused ion beam (FIB) lithography, oxide deposition, reactive ion etching and optical lithography
Student will require  A good degree in physics or engineering related subject
Objectives
  • Perform the first ever direct observation of spin polarised transport in a two dimensional topological insulator
  • Fabrication of spintronic devices as demonstrators of spin transport in the topologically protected edge states and the ability of photonic control of these spin polarised currents

Project description

topological insulator (TI) is a material that behaves as an insulator in its bulk while conductive surface states exhibit unusual behaviour. These unique properties of TIs may result in new spintronic or magnetoelectric devices and lead to a new architecture for topological quantum bits. The surface states have Dirac-like band structure and are similar to graphene. Whereas, graphene has two Dirac cones and each is spin-degenerate, TI’s have only one cone that is spin-polarized. Generally TI’s arise at the interface between an inverted gap material and the vacuum (or a normal gap material). The fact that the surface states belong to a Dirac cone means they all have the same group velocity, and the fact that the cone is polarized means that the spin direction is determined by the direction of travel. Two dimensional (2D) TIs are heterostructure materials with a quantum well band structure that is insulating, with conducting 1D edge states, as shown in Fig 1. Therefore, they are ideal spin filters when charge current flows, while in equilibrium they carry dissipationless spin currents (so called helical states where spin up and down travel with equal speed in opposite directions). Although charge current is not perfectly dissipationless, it is topologically protected – i.e. momentum scattering is only allowed in conjunction with a spin flip. In the case of the quantum Hall effect edge states are formed by very high magnetic fields where both spins propagate along the edge in the same direction. Conversely, 2D TI’s require no field and opposite spins counter-propagate along the same edge due to time reversal symmetry; indeed a magnetic field destroys the effect.

In this project you will use circularly polarised light to optically orientate spin polarised photocurrents in both III-V and V-VI semiconductor materials. Using this technique we will be able to directly observe spin dependent transport in the topologically protected edge states. We have already successfully applied this to observed spin dependent electron transport in highly spin-orbit coupled InSb mesoscopic devices [Physical Review B 85 (2012) 045431Applied Physics Letters 101 (2012) 152407].

Project summary

Supervisor  
Research Group Photonics
Type Experimental
Collaborations Dr Malgosia Kaczmarek, University of Southampton
Techniques used Organic Synthesis, Schlenk Techniques, Column Chromatography, Uv-vis Spectroscopy, all standard organic characterization techniques
Student will require The successful applicant will hold a first degree in chemistry or chemical engineering. The candidate should be able to demonstrate a working knowledge of chemical synthesis either through peer reviewed journal articles or experience performing novel experimental synthetic research. A recommendation letter should be provided directly by the applicant’s recommender, upon request. An interest in photonics and materials chemistry is desirable. Applicants should be able to write quality technical reports in English.
Objectives The project aims to establish the working criteria for optimizing the solution-phase non-linear optical molecular properties of intramolecular charge-transfer chromophores (ICT) that double as organic metal ligands as a function of a bound metal, its geometry and its binding position along the ICT backbone. From the results, the design of ordered functional materials will then be developed.

Project description

In general, it is now becoming increasingly recognized that there is a greater advantage in mixing inorganic and organic components to achieve desired materials properties than to operate within the confines of either the organic or inorganic material subclass. This Ph. D. project will address the effectiveness of the use of non-covalent coordination chemistry as a facile means to enhancing the molecular properties of pre-existing organic non-linear optical structural motifs. It will explore the structural, electronic and spectroscopic consequences of metal coordination to organic intramolecular charge-transfer (ICT) ligand systems. This work will explore these fundamental questions by organic and inorganic synthesis, computational chemistry and spectroscopy. Once the fundamentals are established, efforts will be expanded towards dynamic light-up organopolymers, metallomesogens and metallochromophores. Non-linear optical effects in ordered media will be investigated through a partnership with the University of Southampton’s Quantum, Light and Matter Group under the direction of Dr. Malgosia Kaczmarek. The candidate will be expected to spend some time being trained in the use of the facilities at Southampton towards the end of their work at the University of Surrey.

Further information

  1. Reutenauer, P.; Kivala, M.; Jarowski, P. D.; Boudon, C.; Gisselbrecht, J. –P.; Gross, M.; Diederich, F. "New Strong Organic Super-Acceptors by Cycloaddition of TCNE and TCNQ to Donor-Substituted Cyanoalkynes." Chem. Commun., 2007, 46, 4898-4900. (Citations: 16)
  2. Jarowski, P. D.; Wu, Y.-L.; Schweizer, W. B.; Diederich, F. "1,2,3-Triazoles as Conjugative p-Linkers in Push-Pull Chromophores: Importance of Substituent Positioning on Intramolecular Charge-Transfer." Org. Lett., 2008, 10, 3347-3350. (Citations: 12)
  3. Gottschalk, T.; Jarowski, P. D.; Diederich, F. "Reversable Controllable Guest Binding in Precisely Defind Cavities: Selectivity, Induced Fit, and Switching in Novel Resorcin[4]arene-Based Container Molecules." Tetrahedron, 2008, 64, 8307-8317. (Citations: 7)
  4. Kivala, M.; Boudon, C.; Gisselbrecht, J. –P.; Enko, B.; Seiler, P.; Müller, I. B.; Langer, N.; Jarowski, P. D.; Gescheidt, G.; Diederich, F. “Organic Super-Acceptors with Efficient Intramolecular Charge-Transfer Interactions by [2+2] Cycloadditions of TCNE, TCNQ, and F4-TCNQ to Donor-Substituted Cyanoalkynes.” Chem. Eur. J., 2009, 15, 4111-4123. (Citations: 14)
  5. Jarowski, P. D.; Wu, Y.-L.; Gisslebrecht, J.-P.; Schweizer, W. B.; Diederich, F. "New Donor-acceptor Chromophores by Formal [2+2] Cycloaddition of Donor-substituted Alkynes to Dicyanovinyl Derivatives." Org. Biomol. Chem., 2009, 7, 1312-1322. (Citations 9)
  6. Wu, Y.-L.; Jarowski, P. D.; Schweizer, W. B.; Diederich, F. "Mechanistic Investigation of the Formal [2+2] Cycloaddition-Cycloreversion Reaction between 4-(N,N-Dimethylamino)phenylacetylene and Arylated 1,1-Dicyanovinyl Derivatives to Form Intramolecular Charge-Transfer Chromophores." Chem. Eur. J. 2009, 16, 202-211. (Citations 3)
  7. Wu, Y.-L.; Bureš, F.; Jarowski, P. D.; Schweizer, W. B.; Boudon, C.; Gisselbrecht, J.-P.; Diederich, F. "Proaromaticity: Organic Charge-Transfer Chromophores with Small HOMO-LUMO Gaps." Chem. Eur. J., 2010, 16, 9592-9605.
  8. Breiten, B.; Wu, Y. -L.; Jarowski, P. D.; Gisselbrecht, J. –P.; Corinne, B.; Griessar, M.; Onitsch, C.; Gescheidt, G.; Schweizer, W. B.; Langer, N.; Lennartze, C.; Diederich, F. "Donor-substituted Octacyano[4]dendralenes: a New Class of Cyano-rich Nonplanar Organic Acceptors." Chem. Sci., 2010, ASAP.

Project summary

Supervisor  
Research Group Photonics
Type Experimental
Student will require  
The project seeks a chemists or materials chemist. The student will benefit from gained experience in experimental and preparative lab work along with computer simulations. The student should have experience in organic synthesis.
Objectives This project aims to establish the link between the degree of molecular non-planarity and the emission enhancement or quenching in aggregated states.

Project description

Aggregation Induced Emission (AIE)1 is a newly identified photophysical phenomenon whereby certain propeller-shaped (non-planar) organic molecules emit absorbed light efficiently only when aggregated in poor solvents. When well solvated (mono-dispersed) the molecules tend to release excitation energy as heat (vibrations and rotations) rather than light. Thus, the phenomenon is likely caused by the nature of the supramolecular structure in the aggregated state, which may be restricting specific energy-dissipating rotational modes at the molecular level.

This behaviour is in strong contrast to the more typical Aggregation Caused Quenching (ACQ) observed for planar organic molecules. In general, emission quenching, such as ACQ, is an important problem in materials science. The unwanted quenching arises from the strong interaction between molecules in the condensed state. It seems that molecular non-planarity, as found in AIE systems, reduces these interactions, while, at the same time, the rigidity imposed by the medium likely turns-off thermal energy dissipation. Most importantly, however, molecular non-planarity also deleteriously affects chromophoric properties such as absorptivity (uptake of photonic energy) and HOMO-LUMO gap (energy of the emitted light). Both are key factors in the design of fluorescent, semiconductor and photovoltaic devices.

This project aims to establish the link between the degree of molecular non-planarity and the emission enhancement or quenching in aggregated states. The goal would be to identify geometric and electronic features that optimize fluorescence and minimize intermolecular interactions in the aggregates: How close to planarity can the systems become before quenching begins to dominate? By a convergent and straightforward synthesis, a target set of molecules will be made and studied for their AIE behaviour. The series represents a systematic alteration of certain geometric parameters of a prototypical AIE system. The planarity of these systems can be seamlessly tuned and connected to emissivity. This work will be supported by computational methods and may lead to transient IR measurements to probe molecular rotations following photo-absorption.

Further information

Hong, Y.; Lama, J. W. Y.; Tang, B. Z., “Aggregation-induced emission: phenomenon, mechanism and applications.” Chem. Commun., 2009, 4332.

Project summary

Supervisor Ben Murdin
Research Group Photonics
Type  
Collaborations
  • The project is in collaboration with Universities of Heriot-Watt (Edinburgh), Linz (A), Cambridge, Neuchatel (CH)
  • Industrial collaborators are Thales (Paris), Teraview (Cambridge).
Techniques used We shall investigate the transition rates between states in quantum wells using femtosecond light pulses, both in the mid-infrared from the new "ultrafast" laser facility here at Surrey or with the far-infrared FELIX facility in Holland
Student will require A good degree in a physics or engineering related subject
Objectives
  • To study the transition probability of holes in valence band quantum wells made from silicons-germanium.
  • To learn how to control these rates for application to lasers.

Project description

Quantum Cascades are a type of semiconductor structure that can be used to produce a new type of laser. In this structure the electrons skip down a staircase staying all the while within one band, unlike in conventional "interband" lasers where electrons jump across the bandgap and recombine with holes to give out the photons. The staircase is designed using the simplest quantum mechanics - the particle-in-a-box problem - and the most demanding crystal growth technology. When one type of semiconductor is grown on top of another, the electron usually prefers to sit in one of them. This means a potential well can be made and if the layers are thin enough a "quantum well" is formed where there are allowed states with forbidden energies between. If wells are stacked very close to each other quantum-mechanical tunneling can occur from one to the next. Putting on an electric field can make the ground state of one well line up with the excited state of the next, producing the stair-case. Quantum Cascade Lasers were first produced ten years ago using the semiconductor InP, and recently GaAs, which is much cheaper, but still several thousand Euro a piece! The ultimate would be to produce a QCL from silicon, which could make it affordable for many applications such as environmental pollution monitoring etc.

We are already collaborating with a large EU consortium to produce Quantum Cascade Lasers from silicon. These devices use holes in the valence band rather than electrons, but the principle is just the same. The main difference is the variety of the set of allowed states because the valence band is actually three different bands mixed together. The aim of this project is to observe the transitions of holes between excited and ground states and measure the transition time. By looking at the change in time with well width, alloy composition etc we hope to learn how to control it, and hence help to produce working lasers.

Project summary

Supervisor Jeremy Allam (Supervisor) and Dr Gabriela Slavcheva (Co-supervisor)
Research Group Photonics
Type Experimental
Collaborations Prof P Roussignol (Ecole Normale Superieure, Paris, France)
Techniques used
  • Propagation of femtosecond laser pulses in semiconductor waveguides
  • Nonlinear optical transmission of semiconductor microcavities in the SIT regime
  • Spatial probing of SIT electron density gratings
Student will require
  • Good hands-on skills with complex experiments
  • Some knowledge of lasers and optics, and/or electronic and optical properties of semiconductors
  • Physical insight to guide the experiments performed
  • Willingness to talk and collaborate with theorists
Objectives
  • To study the interaction of coherent ultrashort light pulses with quantum wells / quantum dots embedded in semiconductor microcavities
  • To establish experimentally the conditions for self-induced transparency (SIT) and soliton formation
  • To investigate possible device applications of SIT solitons

Project description

Self-induced transparency (SIT), namely, solitary propagation of an optical pulse in near-resonant media, is one of the most striking and important effects in nonlinear optics. It is a fundamental example of coherent nonlinear light-matter interaction in a discrete-level system, initially discovered in atomic vapours. If the pulse duration is much shorter than the typical material coherence lifetimes, and the pulse area is a multiple of 2π, then reemission of the absorbed radiation in phase with the driving optical field leads to SIT. The nearly lossless ultrashort pulse propagation and the preservation of the pulse shape is extremely attractive for a wide range of applications, such as optical storage and processing of information, optical communication and pulse generation and compression techniques.

Quantum dots (atomic-like nanostructures in which electrons are localised in all 3 dimensions)are the most promising candidates for observation of SIT due to their long coherence times, and some evidence for SIT has been found in pulse propagation measurements performed on semiconductor quantum dots at low temperature. Theoretical calculations [1] suggest that SIT is enhanced in optical cavities such as semiconductor microcavities, leading to spatio-temporal solitons or "light bullets".

In this project, the student will look for evidence of SIT in semiconductor waveguides and microcavities, both bul ikand quantum dot. The power threshold for SIT will be determined and compared to theoretical predictions. Techniques for imaging the spatial solitons will be developed, and the prospects of cavity SIT-solitons for novel optoelectronic devices will be assessed.

Further information

G. Slavcheva, J. M. Arnold and R. W. Ziolkowski, "Ultrafast pulse lossless propagation through a degenerate three-level medium in nonlinear optical waveguides and semiconductor microcavities, Journal of Selected Topics in Quantum Electronics 9, 929 (2003)

Project summary

Supervisor Stephen Sweeney (Supervisor) and Alf Adams (Co-supervisor)
Research Group Photonics
Type Experimental
Collaborations University of Marburg, Germany
Techniques used High pressure and low temperature measurements will be used to:
  • Determine the band structure and band offsets of GaAsPN/GaP using photocurrent and electroluminesce measurements on LEDs and lasers
  • Investigate the dominant carrier recombination processes in this material and the extent to which they limit device efficiency and maximum operating temperature
  • Determine the modulation properties of GaAsPN lasers
Student will require  
Objectives The aim of this project is to investigate a new optoelectonic materials system that is directly compatible with silicon electronics.

Project description

The eventual aim of the project is to produce lasers on silicon which are compatible with standard CMOS electronics technology and which will enable fast optical buses within computers. This would revolutionise computing by reducing power consumption and increasing data transmission rates.

In this project you will investigate GaAsPN-based lasers which may be grown directly on silicon. The project will focus on the carrier recombination processes which limit the efficiency of the lasers and their maximum operating temperature.

Project summary

Supervisor Ben Murdin
Research Group Photonics
Type Experimental
Collaborations
  • University College London
  • Technical University of Vienna
Techniques used • Femtosecond pulsed laser spectroscopy
• Clean-room processing techniques such as electron beam lithography, oxide deposition and optical lithography
• Ion beam implantation
Student will require A good degree in a physics or engineering related subject
Objectives • To develop a high frequency single cycle terahertz (THz) light pulse generator based on the femtosecond plasma ionization technique
• To use the system to perform coherent non-linear optics experiments on hydrogen-like impurities in semiconductors
• To produce single electron transistors (SETs) with implanted impurities
• To use the THz pulses to observe non-linear excitations and dynamics of the single atom SET

Project description

Silicon and the technology developed with it has revolutionised industry, entertainment and communication, and there is always interest in new ways to remember and process information. The challenge is to find methods of encoding information in the smallest possible volume and manipulating it in the most complex ways with the lowest energy cost and the highest speed. Our group leads a project called COMPASSS (Coherent Optical and Microwave Physics for Atomic Scale Spintronics in Silicon), a £multi-million activity aiming to develop methods for encoding information in a single electron, orbiting a single impurity atom in a silicon crystal.

A donor in silicon is a single impurity with one (or more) extra valence electrons that is not used in the bonding. For example, phosphorus impurities in silicon crystals (Si:P) look like silicon atoms with an extra proton in the nucleus and an extra electron – very much like a hydrogen atom [see our article in Nature Communications]. The energies of the allowed impurity orbits follow the same pattern (the Rydberg series) except that the transitions are in the terahertz (THz) range of the spectrum, and the radius of the orbit is many nanometres rather than sub-angstrom. We aim to extend the analogy between the atom in vacuo and the impurity in a crystal. For example we already demonstrated that electrons can be put into orbital superpositions that live for long times, and the superposition can be manipulated as required for a quantum computation [see our article in Nature].

In this project we aim to put together two well established but state-of-the-art technologies that have not been used in conjunction before, namely single electron transistors (SETs) and terahertz (THz) transient pulsed spectroscopy. SETs are devices in which only one electron is allowed into the conduction channel at a time. They are based on extremely small islands of semiconductor, for which the electrical capacitance is tiny. When one electron is on the island the capacitor charges to such a high voltage that no other electrons can enter until the first one leaves. The voltages that are needed to push the electrons through are very sensitive to the local environment, so they can be used to sense the state of nearby impurity atoms. We aim to implant individual impurities near to the SETs and sense their state changes when exposed to light.

The light pulses needed to controllably change the state of the impurity are in the far-infrared, or THz region of the spectrum, as mentioned above. We have been using two types of THz light source for our previous experiments: weak, broad-band light from a black-body lamp; and intense laser pulsed from a very large, expensive accelerator based source in Holland. In this project we will build a table-top source based on our near-infrared femtosecond laser. When the pulses from this laser are focused strongly in air, the air becomes ionized, and emits a flash of electromagnetic radiation. With appropriate surrounding optics the radiation can contain what is essentially a single cycle pulse of THz. We aim to optimize the emission for the excitation and control of a variety of impurity atoms, and increase the frequency to 10THz where the most important impurities in siicon have their hydrogen-like orbital transitions.

Project summary

Supervisor Steven Clowes
Research Group Photonics
Type Experimental
Techniques used
  • Spin polarised photocurrent measurements using optical orientation of electron spin using circularly polarised light
  • High Magnetic Fields (7 Tesla) and low temperatures (>1.8 K) using superconducting magnet with optical cryostat
  • Clean-room processing techniques such as electron beam lithography, focused ion beam (FIB) lithography, oxide deposition, reactive ion etching and optical lithography 
Student will require A good degree in physics or engineering related subject
Objectives
  • To develop new innovative techniques for the fabrication of electrical surface gates on InSb quantum well structures
  • Fabrication of spintronic devices for the detection of electron spins using the spin refraction phenomenon
  • Spin dependent electron transport measurements in mesoscopic devices using electrical detection of optically orientated spin polarised photocurrents

Project description

These surface gated structures of materials with high spin-orbit coupling would enable spatial spin control in a ballistic transport device. Work at Surrey has demonstrated spin sensitive focusing of photocurrents using small applied magnetic fields [Physical Review B 85 (2012) 045431]. However, applied electric fields offer much greater control of the spins by directly influencing the strength of the spin-orbit interaction (SOI) in quantum wells via the Rashba interaction. Nanoscale patterning of surface gates would allow unprecedented control of a spatially varying SOI and enable spin separation via theoretically predicted spin refraction[Physical Review Letters 92 (2004) 086602]. The devices will be studied at using the 7 T optical access magnet facility at Surrey which can achieve base temperatures down to 1.8 K

InSb is an attractive material for applications in quantum information processing (QIP) and spintronics due to its large g-factor, light effective mass and strong spin-orbit coupling. However unlike GaAs, it possesses properties that make it a challenging material to process. To date there has been only one reported InSb single electron transistor, fabricated by the QIP group at QinetiQ Ltd [New Journal of Physics 9 (2007) 261]. In this particular device, Schottky barriers were formed at the InAlSb surface to produce the surface gated structure. However, the yield from this method is poor and the fabrication is complex requiring delicate air-bridged contacts. An alternative approach is to adopt the route of dielectric layers as the gate insulator.

Thisproject will develop surface gated InSb QW devices using novel direct write techniques made possible by the world leading focussed ion beam (FIB) expertise at the Surrey. FIB enables direct writing of device structures using ion or electron (e-) beam assisted deposition, where a precursor gas (such as TEOS used for silicon oxide deposition) is dissociated by the energetic gallium ions and deposited with tens of nanometre resolution. The advantage of this approach is that gating can be done post the photolithographic processing stage, such as the etched spintronic devices currently produced and measured at Surrey [Applied Physics Letters 101 (2012) 152407]. A prototype of the techniques has been achieved using e-beam assisted deposition of gate oxide and ion-beam deposition of the gate (see Fig 1). This fabrication route requires optimisation by studying the effects localised heating and induced damage so that the electronic properties of semiconductor structures can be maintained. A more radical approach which would completely eliminate the possibility of beam damage, as well as enabling much greater flexibility and resolution, would be to use dielectric membranes. In this method, a free standing membrane can be patterned by FIB deposition (not in proximity to the InSb material) and then be positioned using micromanipulation probes on a prepared InSb device. Using thin membranes will reduce proximity effects that limit resolution; also it will make possible very complex device architectures as it allows the ability to deposit both oxide layer and gate electron in-situ without the need for realignment. The unique combination of having both state-of-the-art FIB lithography and InSb device processing capabilities at surrey make this ambitious processing strategy a very real and attractive prospect for spintronics and QIP activities.

Project summary

Supervisor Ben Murdin
Research Group Photonics
Type Experimental
Collaborations
  • University College London
  • Radboud University Nijmegen
Techniques used In this project you will set up the apparatus necessary for detection of DX electrons, and a microwave pulse generator for putting the donors into spin superpositions. You will use the system to investigate how to efficiently and reproducibly spin superpositions can be manipulated and then read out at the end of a quantum computation. You will investigate the effect of intense THz pulses which produce orbital state excitations on the DX readout. It is hoped that a full understanding of the DX readout process will enable it to become a workhorse tool for quantum information processing.
Student will require A good degree in a physics or engineering related subject
Objectives • To probe the spin and orbital state of impurity atoms in a silicon crystal using the “donor-bound-exciton” effect described below
• To use this probe to demonstrate coherent control of impurity atoms, i.e. the creation and destruction of superposition states
• To extend the probe towards the single atom level using single electron transistors

Project description

Silicon and the technology developed with it has revolutionised industry, entertainment and communication, and there is always interest in new ways to remember and process information. The challenge is to find methods of encoding information in the smallest possible volume and manipulating it in the most complex ways with the lowest energy cost and the highest speed. Our group leads a project called COMPASSS (Coherent Optical and Microwave Physics for Atomic Scale Spintronics in Silicon, www.compasss.net), a £multi-million activity aiming to develop methods for encoding information in a single electron, orbiting a single impurity atom in a silicon crystal.

A donor in silicon is a single impurity with one (or more) extra valence electrons that is not used in the bonding. For example, phosphorus impurities in silicon crystals (Si:P) look like silicon atoms with an extra proton in the nucleus and an extra electron – very much like a hydrogen atom [see our article in Nature Communications]. The energies of the allowed impurity orbits follow the same pattern (the Rydberg series) except that the transitions are in the terahertz (THz) range of the spectrum, and the radius of the orbit is many nanometres rather than sub-angstrom. We aim to extend the analogy between the atom in vacuo and the impurity in a crystal. For example we already demonstrated that electrons can be put into orbital superpositions that live for long times, and the superposition can be manipulated as required for a quantum computation [see our article in Nature]. In separate projects we shall develop the technology for manipulating that information with terahertz speed by its magnetic connection with adjacent impurity electrons.

In this project we aim to develop the technology for read-out of the state of the atom at the end of the computation. Naturally, reading out the state of a single atom by optical means is impossible because the size is many orders of magnitude smaller than the wavelength, so the route must be to make electrical contacts. However, light beams can play an important role by exciting the atom into a state that is more easily sensed by the read-out electronics. This is where the donor-bound exciton comes in.

An exciton is another hydrogen-like object, where an electron and a hole orbit around each other. In fact it is more like positronium because the electron and hole are in some sense anti-particles – they can be created and destroyed in pairs by a photon. Excitons feel attracted to donors in the same way that hydrogen atoms bind together to form molecules, and one of the aims of this project will be to use the donor-bound exciton (DX) as a model system to probe the energy states of hydrogen molecules. The prediction of the energy levels in H2 is one of the classic “hard” problems in quantum physics because it is a 4-body problem, and it gets even more difficult when it is subjected to a magnetic field, and almost impossible when the field is very high like on the surface of a white dwarf star. In these conditions controlled experiments are difficult – the temperatures and fields are too high to reproduce on Earth. The DX can help here because the effects of magnetic fields are amplified by the silicon, but so far no one has really tried to study it.

The reason why the DX helps with state readout of the D part of the object is because the total energy of the DX has very well defined allowed values that are different for the different states of the donor. Illuminating the donor with photons will only produce a DX if they have the energy that corresponds to the state of the D. The DX can then be read out by electronic contacts because when the X self-annihilates the energy goes into kicking out the electron from the D, which can then be picked up as a current. Measuring the current as the photon energy is changed at the end of the calculation will give us the number of donors in each different state.  

For background please see the COMPASSS project website.

Project summary

Supervisor Ben Murdin
Research Group Photonics
Type Experimental
Techniques used
  • You will use the state-of-the-art ultra-short pulsed laser system in the ATI to shock a semiconductor, that will react by producing (harmless) T-rays
  • You will then develop a new type of spectrometer that can convert the polarisation of the T-rays so that it becomes circular, and hence sensitive to cirality (handedness) in molecules
Student will require A good degree in a physics or engineering related subject
Objectives
  • To devolop a system capable of recognising the difference between chiral (left- and right-handed) molecules, using T-rays (the THz frequency or far-infrared region of the spectrum)
  • To use the system to observe known simple enantiomers and then to research the utility for larger molecules of pharmaceutical interest

Project description

Light is normally thought of as a plane-polarised transverse wave, but in the quantum picture it is made up of equal numbers of photons with spin +1 and -1. The photons can be separated with some simple optical tricks, making a beam whose electric field makes a helical path around the axis of propagation either to the right or left, called circular polarisation. The beam therefore has a handedness, and is sensitive to handedness in the subtances it passes through.

The study of optical activity is of interest for the study of low symmetry crystals, molecules and other systems. There are various different types of optical activity. Linear birefringence is the phenomenon of a difference in the refractive index of a material measured for two orthogonal linear polarisations, and circular dichroism (or birefringence) refers to differences absorption coefficient (or refractive index) for opposite circular polarisations. Cicular dichroism is currently of interest to biochemists for its application in identification and isolation of pharmaceutical enantiomers, where it is important to isolate the left from the right handed kinds and study the effects of each seperately. The classic example is thalidomide, which in one form is a very potent and side-effect-free morning-sickness drug, but in the other form causes infant mutations. Other examples include limonen, which in one form smells of lemons and in the other smells of oranges. The reason for the different chemistry is that the natural molecules in your body are normally purely one enantiomer so its reaction to synthetic compounds is very chirally specific.

Spectroscopic techniques for analysis of optical activity rely on the use of birefringent crystals that convert plane polarised light into circular polarised light. I propose that you should build a new type of spectrometer that enables optical activity spectroscopy without birefringent crystals, and is therefore applicable to the T-ray region of the electromagnetic spectrum where suitable birefringent crystals are almost impossible to find, but where important information on chemical identification can be found.

Funding for Photonics research

Current and recent research projects are listed below:

  • COMPASSS (EPSRC, 2010-2015, £2.6m)
  • SILAMPS (EU-ERC, 2008-2013, £1.8m)
  • Efficient photonic devices for near- and mid-infrared applications (EPSRC, 2010-2015, £1m)
  • Non-magnetic semiconductor spintronics (EPSRC, 2007-2012, £600k)
  • Bismide and nitride components for high temperature operation (BIANCHO) (EU, 2010-2013, £280k)
  • Space based solar power (EADS, 2009-2011, £280k)
  • III-V bismide materials for IR and mid IR semiconductors (EPSRC, 2009-2012, £250k)
  • Extended temperature optoelectronics 2 (ETOE2) (TSB, 2008-2011, £200k)
  • Metrology for solid-state lighting (EU-EURAMET, 2010-2013, £190k)
  • Optical orientation of spins in semiconductors using the FELIX and FELBE free-electron laser facilities (EPSRC, 2007-2011, £180k)
  • Exploring short wavelength limits for high performance quantum cascade lasers (EPSRC, 2010-2013, £170k) 

Publications in the Photonics Group

For full text items please see the Photonics Group work on the Surrey Research Insight pages.

For a full list of publications see the Advanced Technology Institute section of the Surrey Research Insight database, or individual's publications can be found under the publications tabs on the relevant staff profile.

Find us

Address
Photonics and Quantum Sciences Group
Advanced Technology Institute
University of Surrey
Guildford
Surrey
GU2 7XH