Research Interests 

The dynamics of photoexcited electrons and holes can be measured on the timescale of ~100fs by using ultrafast optical spectroscopy. The carriers interact strongly with each other to form a hot plasma, which then cools by interacting with the lattice. In my experiments I make real-time measurements of fundamental optical properties such as absorption and luminescence using pulses generated by modelocked lasers. These measurements are important from a fundamental point of view, since they provide basic information about electron-electron and electron-phonon interactions, and they are of great practical significance since the speed of these processes limits the speed ultimately obtainable in electronic and opto-electronic devices. 

My main interests have been in making resonant measurements on wide-bandgap semiconductors and low-dimensional systems such as quantum wells, wires and dots. Topics currently being investigated include coherent electron-hole dynamics, excitonic dynamics in wide band gap materials such as GaN and InGaN, and gain mechanisms in bulk and quantum well lasers. Please have a look at my publication list for references to relevant recent work. 

I am now involved in a LINK grant with The Department of Materials in Oxford, Hitachi Cambridge and the Microelectronic Research Centre in Cambridge. The grant is entitled "Nanoelectronics at the Quantum Edge", I am looking at InGaN and GaN quantum dots, studying dephasing, carrier lifetime and inter-dot interactions. For more details see the project website at www.nanotech.org.

Work on Quantum Dots

Under the LINK grant entitled "Nanoelectronics at the Quantum Edge" I have been investigating the optical properties of single quantum dots using micro-photoluminescence CW and time-resolved spectroscopy. The text below comes from the project web page.

Overview & Relation to Quantum Computation

Quantum dots are small nanometre sized inclusions of a lower bandgap material in a solid matrix of higher bandgap. These materials, when excited optically, show evidence of the "quantum confinement" of electrons and holes in the dot material. There is currently much activity in the field of quantum dots as there are many potential applications in optoelectronic devices and other novel applications. One such application is quantum computation.

In this field, quantum dots are a useful playground for investigation, as it is possible to grow arrays of quantum dots in vertical stacks with well defined separations. This is important, as one of the most important aspects of any quantum logic device will be the ability to control the interactions between separate qubits. The variation in spacing between dots will change their interaction strength. However, in order to build a quantum computer there are many practical difficulties to overcome.

Our Work

Although much previous work has been done in the InAs family of quantum dots, we are interested in the III-Nitrides. There are reasons for believing that they may be more promising in the long term for the development of quantum logic circuits:

  • The exciton binding energy is high (25meV) so that excitons are stable up to room temperature.
  • Quantum dots in the nitride system have strong piezo-electric fields such that the dots levels can be shifted to both higher and lower energies under the influence of an applied electric field.
  • The LO phonon energy is also high (92meV), which suggests that the quantum dots may be more decoupled from the thermal environment than in other systems.

    If this is shown to be the case, it may manifest itself in longer coherence lifetimes in such dots. The coherence lifetime is a parameter of great importance, as successful quantum manipulations demand that the wavefunctions are coherent over the time period of operation and error correction of the system.

    Experimental Methods

    Currently we are interested in the characterization of Nitride quantum dots, in order to measure certain parameters. The experimental tools we have available include:
     

  • A time-correlated single photon counting system.
  • A micro-photoluminescence measurement system.
  • A macro-photoluminescence measurement system.
  • A coherent control system.
  • A high resolution spectrometer for single dot spectroscopy.





    Fig. 1: A schematic of our sample and a method to isolate single dots for optical measurements. An Aluminium mask is deposited, then windows as small as 100nm x 100nm are etched away, leaving some holes that contain a single dot.

    Fig.2 This is a photograph of the micro-PL system that we have set up in order to look at the time- and spectrally-resolved emission from a single quantum dot.
     


    Fig 3. Decay of a single InGaN quantum dot at 4.2K

    People

    Our work is based in the Clarendon Laboratory, at the Department of Physics in the University of Oxford. We are also collaborate with colleagues in the Department of Materials in Oxford, as well as with Cambridge University Microelectronics Group and Hitachi in a LINK project aimed at the development of a solid state quantum computer. The people working directly in the field of Nitride dots are:
     

  • Dr Robert Taylor - has a great deal of experience in optical measurements of GaN and related materials. Past research has included work on ultrafast carrier relaxation, gain mechanisms, plasma effects and the Mott transition in both bulk and quantum well materials.
     
  • Dr James Rice is a postdoctoral researcher, with experience of ultrafast measurements on zero-dimensional materials such as fullerenes and dots.
     
  • Jonathan Smith is a 3rd year DPhil student. He has been working on time-resolved investigations in bulk, quantum wells and recently GaN quantum dot materials.
     
  • Rachel Oliver is a 3rd year DPhil student in the Department of Materials working on the development on S-K growth of InGaN quantum dots.
     
  • James Robinson is a 1st year DPhil student investigating GaN quantum dots.
     
  • Anas Jarjour is a visiting student from ENS Paris, investigating GaN quantum dots.

    • Work on  bulk GaN

    Some years ago I began a project at the CLRC in the Rutherford Appleton Laboratory near Didcot in Oxfordshire. There I used their amplified femtosecond Ti:sapphire laser system to excite epilayers of GaN. This was achieved by frequency shifting the 800nm Ti:sapphire beam in an optical parametric amplifier to wavelengths resonant with, and shorter than the bandgap of GaN. This meant that both hot-carrier dynamics and cold exciton dynamics could be studied. The technique used was time-resolved reflectance or absorption spectroscopy. Here the sample was excited using an ultrafast laser pulse (250fs) and probed at various time delays after excitation by a white light continuum generated by focusing some of the amplified laser light into a transparent non-linear medium such as water of sapphire. This continuum had a duration of 800fs when generated in water, and 250fs when generated in sapphire. A detailed description of the experiments and results can be found in my recent publications, some of which are available as .PDF files at this Web site.

    Recently the Department of Physics here in Oxford purchased a Ti:sapphire amplifier system with a dedicated optical parametic amplifier (OPA) from Spectra Physics. This consists of a MaiTai oscillator and an Evolution pump laser boxed in a system known as a Hurricane. This is capable of producing 125fs pulses at a repetition frequency of 1kHz with pulse energies of up to 0.9mJ. The output from the amplifier can be shifted to cover most of the near-UV, visible and near-IR using the OPA. We are currently using the system in conjunction with a femtosecond Kerr gate to investigate time-resolved emission dynamics from InGaN MQWs with sub-picosecond time resolution.

    I have included some photographs of the experimental system, both at RAL and in the Clarendon (and some dedicated researchers!) below.

    Above are pictured a Spectra Physics Hurricane Ti:sapphire amplifier pumping an optical parametric amplifier (OPA). The OPA output at 1.36mm is frequency-doubled twice to produce 125fs long pulses at a repetition frequency of 1kHz with energies of ~1mJ at 340nm.

    This photograph shows part of the inside of the Hurricane. Here we see the gratings used to stretch the pulses before amplification, and to recompress them afterwards.

    The inside of the OPA is pictured above. Some of the 800nm light from the Ti:sapphire amplifier is used to create a white-light continuum. This is used to seed the OPA, where the pulses are split up spectrally such that the sum of the frequencies of the two parametric beams equals the frequency of the input beam. The amplified output is then passed through two doubling crystals to shift the wavelength into the UV.

    Femtosecond white-light continuum pulses generated by focussing some of the amplified beam into a disc of sapphire are passed through a cell containing the dye Coumarin 102. As can be seen on the screen, the dispersed continuum light has a missing region in the blue, where it has been absorbed by the dye.

     


    This photograph shows an optical parametric amplifier (OPA). The fundamental 800nm light from the Ti:sapphire laser is first frequency doubled, then focused into the non-linear crystal seen above. The 400nm photons are split in two to form a red beam (seen above) and an infrared beam (known as signal and idler). The red light at ~640nm is mixed with residual amplified 800nm light to produce a pump beam at ~355nm. The pump wavelength can be tuned by rotating the non-linear OPA crystal.

    In the top section of this photograph a white light continuum centred at 400nm is shown. The photograph was taken by dispersing the white light continuum generated by focusing the laser beam into a cell of  D2O (shown in lower portion of photograph) using a diffraction grating.

    Here we see the experimental system. The GaN sample sits in a continuous flow liquid helium cryostat, capable of maintaining the sample at temperatures between 4K and room temperature. Also shown in the picture are Stefan Hess - my graduate student (now has his D.Phil. and is working for Corning Glass in France), Francois Walraet - a project student from Paris, and of course, me!


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