Experimental Condensed Matter Physics

Solid State Subgroup

Phonon-mediated X-ray detectors

(Professor J K Wigmore and Dr A G Kozorezov)
Increasingly astronomers are turning for improved sensitivity and resolution of their instruments, both earth- and satellite-based, to photon detectors which operate at very low temperatures, as low as a few mK (thousandths of a degree). The basic principle is that the energy of an incoming photon creates a pulse of phonons (lattice vibrations) which is then converted into electrical charge either by a bolometer or a superconducting tunnel junction (STJ). The STJ is especially promising for the X-ray region of the spectrum, which is at the focus of many current astrophysical problems. An understanding of the behaviour of the high frequency phonons which mediate the detection process is crucial to achieving high performance.
In a collaborative project with the astrophysics research group of the European Space Agency at ESTEC in the Netherlands, we are using high resolution phonon pulse techniques to study the interaction of phonons with the superconducting electrons in STJs. Via phonon pulses we can simulate the conditions produced by photons over the energy range from below 1 keV to above 1 MeV. Such experiments lead not only to more sensitive detectors but also to better understanding of superconductivity at very low temperatures. Theoretical modelling of the detection processes, also carried out at Lancaster, involves many different topics in superconductivity and phonon physics, such as elastic scattering, frequency down-conversion and transmission at disordered interfaces of THz phonons, and non-equilibrium quasiparticles, localised traps and hot spots in superconductors. In parallel work we are studying also the physics of bolometers, both superconducting and semiconducting types.

Semiconductor Optoelectronics

(Drs A Krier, A Iraqi and Z Labadi)
The physical properties of a wide range of inorganic and organic semiconductors as well as polymers are being investigated. Materials which exhibit luminescence or photoconductivity in the mid-infrared are of special interest together with the various optoelectronic devices which can be made from them. We have specialist expertise in the following areas: LEDs and diode lasers for the mid-infrared; liquid phase epitaxial growth of III-V semiconductors; luminescence spectroscopy; optical and amperometric gas sensors; polymer/organic LEDs for flat panel display applications; infrared detectors.
One of the main areas of activity is the liquid phase epitaxial (LPE) growth of narrow gap semiconductors and the fabrication of III-V light emitting diodes and lasers. These novel light sources operate in the mid-infrared region (2-5 µm) of the spectrum which contains the fundamental absorption bands of gases such as methane, carbon dioxide and sulphur dioxide, so they can be used in pollution monitoring instruments. At Lancaster we have made sources which correspond to key wavelengths for making gas sensors, and we have a number of industrial collaborators. Compound semi-conductor materials (InGaAs and InAsSb) are grown in-house by LPE and investigated using low temperature luminescence techniques in order to understand the basic physics of light generation in heterojunctions. Recently we have produced a 2.6 µm diode laser and a 2.5-3.5 µm photodiode detector for monitoring boiler flue emissions. At the moment, research is underway to fabricate an efficient infrared photodiode for flame/fire detection. We are also developing a unique rapid-slider LPE technique for the growth of ultrathin semiconductor layers and have grown the world's first InAs quantum wells using this approach. Low-dimensional structures containing quantum dots are of much current interest, and infrared optoelectronic devices based on them offer considerable performance advantages which we hope to exploit in practical devices.
Exciting advances have recently been made in organic electroluminescence (EL) devices for large area flat-panel display applications. In our laboratory we have observed visible light emission from block co-polymer LEDs based on polyphenylene-vinylene (PPV) as well as from smaller molecule organics such as tris (8-hydroxyquinolino)aluminium (A1Q). In aiming towards practical EL display devices, we need to improve the efficiency, lifetime and stability and to extend the emission wavelengths into the pale blue and deep red regions using various fluorescent dyes and different electrode materials. The work is interdisciplinary with many industrial and commercial applications. At the moment, we are investigating a variety of multilayer organic device structures. Voltage tunable emission colour and white EL devices are especially interesting. We are also investigating the metal substituted phthalocyanines and related compounds. When prepared in thin film form, either by thermal evaporation or by the Langmuir Blodgett technique, the phthalocyanines behave as sensitive gas sensors for the detection of NO₂ NH₃ and Cl₂ down to ppb levels.

Further Details

Mid-Infrared Network  

Surface Physics and Micro-thermal Analysis

(Drs I J Saunders, R Jones and N S Lawson )
In a joint project with the Biological Sciences department, our recently patented-technique known as photothermal micro-spectroscopy (PTMS) is being applied to cancer diagnostics, in particular to breast and prostate cancers. Our objective is to develop a technology sensitive enough to identify cancerous or pre-malignant cells even when such cells make up only a very small proportion of the overall cell numbers; a very important limitation in conventional cancer diagnosis. Such a development would reduce the number of false negatives that arise using conventional methodologies and could result in better prognosis. In collaboration with Loughborough University and two American companies, we have developed the revolutionary technique of micro-thermal analysis for the microscopic study of thermal phenomena in solids. We also use atomic */?>force spectroscopy*/?> to measure forces between surfaces as a function of separation, and have devised a unique */?>nanoindentation tester*/?>. These techniques are important in the study of coatings and the surface-mechanical behaviour of powder materials.

Plastic-Optical-Fibre-Consortium

Until very recently, femtosecond laser processing was applied to glass and semiconductors for waveguides and surface/3-D profiling but not to polymers. Now, new initial studies of femtosecond laser processing of undoped polymethylmethacrylate (PMMA) indicate for the first time that refractive index changes in un-doped PMMA are significant; ( n=10-2 - 10-3)[1]; and can be written at precise depths into bulk material. This facilitates writing 3-D waveguide and diffractive structures in undoped perspex; Bragg/Long Period gratings in ordinary undoped polymer-optical-fibres (POFs), and refractive index profiles into couplers and tapers. Polymer Microstructured Optical Fibers and photonic crystal fibre based on polymer optical fibre (PC-POF) (www.redfernpolymer.com) is easier to make than glass PC fiber as only one polymer is involved and no dopants are required. As with glass photonic crystal fibre, PC-POF has periodic air holes running along its entire length, giving it exotic properties; confining the light to a central core via a modified form of total internal reflection, rather than the refractive index step of a standard fibre. These advances, coupled with the recent availability of new high-intensity semiconductor light sources in the visible, blue, and near-UV (300-650 nm) opens up the exciting opportunity to develop new polymeric waveguide and active polymeric optical devices for chemical/environmental/medical sensors where measurands such as blood, algae, transition metal complexes and pollutants interact with visible/UV light. Current silica/III-IV semiconductor optoelectronic systems generally do not cover the visible/UV spectrum and are thus unsuited. Furthermore, incorporation of state-of-the-art polymer optical amplifiers [2] and 90% efficient, enhanced, light-source to fibre coupling [3] with photovoltaic technology [4] will realise the transfer of optical, interference-free power to remote electronics in hazardous (explosive/high-voltage/radio frequency) environments; completely revolutionising sensor applications in this domain.

Key References (see here for full listing)

1. P.J.Scully, D.Jones & D.A.Jaroszynski, "Writing Refractive Index Gratings in Perspex and Polymer Optical Fibres using Femto-second Laser Irradiation"; submitted to Photon02, Applied Optics and Optoelectronics Conference, Cardiff, Sept 2002.

2. J R Lawrence, G A Turnbull & I D W Samuel, "Broadband optical amplifier based on a conjugated polymer", Appl. Phys. Lett., 80, 3036-3038 (2002).

3. M.J.Lazarus, V.Ellarby & D.Campbell, "Enhanced coupling of light emitters to plastic optical fibre using bulb-lens attenna systems". Microwave & Opt. Tech. Letts. April 5, 2002.

4. K.W.J.Barnham,"Photovoltaics for the 21st Century II", Electrochem. Soc.Proc., 2001-10, 30, (2001).

5. Zang. J,. Du. X., Yuan. W. D., Deakin. A., Spencer. J. W., Jones. G R., Gibson, J. R., (Hall. W. B., McGrail. A., A., Tonge. H.,) 2001."Tracking trends in the Chemical Composition of Systems using Chromatic Mapping" Proc. IEE Seminar on "Intelligent and Self Validating Instruments" (Sensors and Actuators) (Dec 2001 London).

6. Z. Xiong, G.D.Peng, B.Wu and P.L.Chu. "Highly tunable Bragg gratings in single mode polymer optical fibres", IEEE Photonics Tech.Letts. 11, (3), 352-354, (1999).

CONTACTS AND FURTHER INFORMATION