Ultrafast Science and Terahertz Photonics Group
Steve Andrews THz charge oscillations in quantum wells Metamaterial waveguide
Dr Steve Andrews (group leader), Dr Ali Muir (postdoc), Mrs Siti Norbaieah Mohamad Hashim (PhD student), Mr Jonathan Archer (PhD student)
Previous members: Dr Lin Sun (Academic Visitor from Chinese Defence University in Changsha), Dr Mia Zangui (PhD student), Dr Yi Pan (PhD student), Dr Mukul Misra (postdoc), Dr Chris Williams (PhD student), Dr Wei Ding (PhD student, joint supervised with Stefan Maier), Dr Ali Hussain (PhD student + 3 month postdoc), Dr Adam Armitage (postdoc), Dr Peter Huggard (postdoc), Dr Julian Cluff (PhD student), Dr Graeme Moore (PhD student), Chris Shaw (PhD student)
Recent or active research projects
· THz surface and volume guiding by microstructured materials; near field probing of metamaterials and waveguides (initially funded by Air Force Office of Scientific Research and Royal Society)
· High sensitivity THz circular dichroism spectroscopy and its applications (Leverhulme Trust)
· THz bio and chemical sensing and imaging (Malaysian Government)
· Development of high power THz gas-filled waveguide sources (EPSRC)
· Optical/THz Pump-THz probe studies of condensed matter
Our main specialisation at present is in the applications of time domain THz spectroscopy and the development of related ‘T-ray’ technology. Previous THz work can be divided up into fundamental materials physics and THz technology, as listed below.
1. Ultrafast studies of electron and phonon dynamics in materials
‘Mechanism of THz emission from coupled quantum wells’ P G Huggard, C J Shaw, S R Andrews, J A Cluff and R Grey Phys. Rev. Letts. 84, 1023-1026 (2000)
‘Ultrafast Optical Excitation of Coherent Two-Dimensional Plasmons’ A Armitage, S R Andrews, J A Cluff, E H Linfield and D A Ritchie Phys Rev B 69, 125309 (2004)
‘Coherent control of cyclotron emission from a semiconductor using sub-picosecond electric field transients’ P G Huggard, J A Cluff, C J Shaw, S R Andrews, E HLinfield and D A Ritchie Appl. Phys. Lett. 71, 2647 (1997)
‘Magnetic field suppression of THz charge oscillations in a double quantum well’ S R Andrews, P G Huggard, C J Shaw, J A Cluff, O E Raichev and R Grey Phys. Rev B57, R9443 (1998)
'Magnetic field dependence of THz emission from an optically excited GaAs p-i-n diode', S R Andrews, A Armitage, P G Huggard, C J Shaw and G P Moore, Phys Rev. B66, 085307 (2002)
'Absence of phase sensitive noise in time resolved reflectivity measurements of coherent phonons', A. Hussain and S. R. Andrews, Phys. Rev. B 81, 224304 (2010)
2. Development of ultrafast THz techniques, near field measurements
'Polarization dependent efficiency of photoconducting THz transmitters and receivers' P G Huggard, J A Cluff, C J Shaw, S R Andrews Appl. Phys. Lett. 72(17) (1998)
'Optimization of photoconducting receivers for THz spectroscopy' S R Andrews, A Armitage, P G Huggard and A Hussain, Phys. Med. Biol. 47, 3705-3710 (2002)
‘Dynamic range of ultrabroadband terahertz detection using GaAs photoconductors’ A. Hussain and S. R. Andrews, Appl. Phys. Lett. 88, 143514 (2006)
'Internal excitation and superfocusing of surface plasmon polaritons on a silver-coated optical fiber tip', W. Ding, S. R. Andrews and S. A. Maier, Phys. Rev. A, 75, 63822 (2007)
‘Waveguide artefacts in THz near field imaging’, M. Misra, S. R. Andrews and S. A. Maier, Appl. Phys. Lett. 100, 191109 (2012)
‘Characterization of a hollow-core fibre-coupled near field terahertz probe’, M. Misra, C. R. Williams, Y. Pan, S. A. Maier and S. R. Andrews, J. Appl. Phys. 113, 193104 (2013)
3. THz waveguiding and metamaterials
‘Terahertz Surface Plasmon-Polariton Propagation and Focusing on Periodically Corrugated Metal Wires’ Physical Review Letters 97, 176805-1-3 (2006)
'Terahertz pulse propagation using plasmon-polariton-like surface modes on structured conductive surfaces', S. A. Maier and S. R. Andrews, Appl. Phys. Lett. , 88, 251120-1 to 3 (2006)
‘Dual band THz waveguiding on a planar metal surface patterned with annular grooves’, C. R. Williams, M. Misra, S. R. Andrews, S. Carretero Palacios, L. Martin-Moreno, F. J. Garcia Vidal and S. A. Maier, Appl. Phys. Lett. 96, 11101 (2010)
' Terahertz surface plasmon polaritons on a helically grooved wire A.I. Fernandez-Dominguez, C. R. Williams, F. J. Garcia-Vidal, L. Martin-Moreno, S. R. Andrews and S. A. Maier, Appl.Phys. Lett. , 93,141109 (2008)
'Highly confined guiding of terahertz surface plasmon polaritons on structured metal surfaces', C. R. Williams, S. R. Andrews , S. A. Maier and A I Fernandez-Dominguez and L Martin-Moreno and F. J. Garcia-Vidal, Nature Photonics 2, 175-179 (2008)
‘Microstructured Terahertz Waveguides (review)’ S. R. Andrews, J. Phys. D: Appl. Phys. 47, 374004 (2014)
‘Terahertz waveguiding between parallel dielectric films’, Y. Pan and S. R. Andrews, Opt. Express 23, 274 (2015)
4. High energy pulsed THz sources and their applications (new project)
This project is now our main focus – after a very long build and learning period we are starting to generate new results – watch this space,
4. Fiber nanophotonics
- in collaboration with Dr Wei Ding in the Key Laboratory of Optical Physics
'Modal coupling in fiber tapers decorated with metallic surface gratings', W. Ding, S. R. Andrews, T. A. Birks and S. A. Maier, Optics Lett., 31, 2556-8 (2006)
'Internal excitation and superfocusing of surface plasmon polaritons on a silver-coated optical fiber tip', W. Ding, S. R. Andrews and S. A. Maier, Phys. Rev. A, 75, 63822 10pp (2007)
'Surface corrugation Bragg gratings on optical fiber tapers created via plasma etch postprocessing', W. Ding, S. R. Andrews and S. A. Maier, Optics Lett., 32, 2499-2501(2007)
'Modal coupling in surface-corrugated long-period-grating fiber tapers', W. Ding, S. R. Andrews and S. A. Maier, Optics Lett., 33, 717-719 (2008)
‘Demonstration of broad photonic crystal stop band in a freely-suspended microfiber perforated by an array of rectangular holes, Y. Yu, W. Ding, L. Gan, Z-Y Li, Q. Luo and S. R. Andrews, Opt. Express 22, 2528-2535 (2014)
5. Historical research interests
Inelastic light and diffuse X-ray scattering studies of phase transitions, optical spectroscopy of semiconductor nanostructures, optoelectronic devices, semiconductor nanofabrication.
Background on the THz research area
What are T-rays? The terahertz part of the electromagnetic spectrum (1 THz = 1012 Hz) lies in the far infrared and is usually taken to mean the region between about 0.1 and 10 Hz. Radiation in this region is sometimes referred to as T-rays. It lies between the domains of electronics (which deals with real currents) and optics (displacement currents). THz science and technology thus blends concepts from both extremes.
The electromagnetic spectrum
Until recently, the far infrared was relatively unused for practical purposes. This is because the available sources were limited to weak black body radiators and difficult to use and narrow band optically pumped gas lasers. Detection was also hampered by insensitive and slow room temperature bolometers or sensitive but inconvenient helium cooled bolometers. In the mid 1980s there was a revolution in experimental access to this part of the spectrum pioneered by researchers at Bell Labs and IBM in the USA who applied ultrashort (~100 fs) pulsed lasers to the generation and highly sensitive, femtosecond resolved detection of pulsed THz radiation. Commercial availability of even shorter pulse lasers (10 fs) now allows extension of the same technology to the mid infrared and beyond (20-100 THz).
What use are T-rays? Currently, T-rays are most widely applied to the detailed characterization of materials, particularly solid state materials like semiconductors, superconductors and other correlated electron materials and polymers but also liquids and gases. The ability to time resolve the real and imaginary parts of the dielectric response of materials to optical and other stimuli makes time domain THz spectroscopy particularly useful in fundamental scientific studies of semiconductors and correlated electron materials. Pulsed techniques are also being widely explored for possible real world applications such as security screening, imaging of skin cancer and industrial quality control although in many cases cw approaches, using quantum cascade THz lasers for example, are likely to be more effective. Use outside the research lab is presently limited but has great potential.
How are T-rays generated and detected? In the 1890s work by von Bezold and Hertz culminated in the first demonstration of the generation, transmission and detection of radio waves. Hertz’s apparatus used a spark gap, akin to the ignition plugs in car, to create an air plasma in a region of high electric field. Charges in an electric field accelerate and emit radiation with a spread of frequencies – in this case around 1 MHz. The frequency is mainly determined by the speed with which the plasma is created and the design of the antenna surrounding the spark gap. Ultrafast lasers can similarly create plasmas in a region of high electric field within a semiconductor on femtosecond time scales. This idea lead in the mid 1980s to the first generation of sub-picosecond pulses of THz radiation with bandwidths of several THz. This was soon followed by all optical generation exploiting the spectral breadth of ultrashort pulses and difference frequency generation in nonlinear optical crystals. A relatively new, pulsed THz generation-detection technique involves two colour laser ionization and four wave mixing in gases. Currently this requires very high energy pump pulses but it has the advantages of being relatively independent of laser wavelength and less limited by material absorption.
Using the most common, few nJ pulse energy femtoscond lasers, the radiation is quite weak (nW-10 µW average power) but extremely sensitive, time resolved and coherent detection systems were developed at the same time. Coherent detection means that the electric field is measured, as in TV and radio, rather than intensity as is usually the case in optics. This is especially important in allowing very sensitive detection and in providing both amplitude and phase information. The first coherent detection technique to be developed involved photoconducting dipole antennas. As in Hertz’s experiments, a second plasma is involved. In Hertz’ case, the radio waves themselves created another spark across a small gap between the arms of a receiving antenna and a transient current in the antenna, proportional to the instantaneous electric field, that can be measured. In the THz case, a beam splitter takes part of the same laser pulse that excites the T-rays to create a conducting region in an otherwise insulating semiconductor ‘bridge’ between the arms of a microfabricated planar dipole antenna. If the plasma is excited so as to coincide with the ‘gating’ laser pulse then a current flows in an external transimpedance amplifier (‘current meter’) attached to the antenna arms. In this way the electric field can be sampled as a function of time delay between THz and gating pulses. This delay is typically varied at a few Hz or less so that the no fast electronics is required to time reconstruct the signal as a function of time on femtosecond time scales. Perhaps surprisingly, this detection technique can be used up to about 30 THz (although the dynamic range isn’t large at the higher frequencies). There is also an electro-optical technique for detection which was developed a little later and has some advantages at higher frequency and a gas phase coherent scheme based on 4-wave mixing. We use both of these techniques in our high field THz work.
There are a growing number of continuous wave (or long pulse) THz sources based on optical parametric oscillators, photomixers and cryogenically cooled quantum cascade lasers. It is likely that these more compact, although still expensive, devices will play an important part in real world applications. The cascade laser is particularly interesting because it offers relatively high power together with a coherent detection capability and great efforts are being made to overcome the current restrictions on operating temperature and tunability.
Our Terahertz systems
· System 1, driven by 10 nJ optical pulses from 70 fs or 10 fs, 80 MHz rep. rate oscillators (shared pump) can be configured to allow any one of the following 3 types of measurement (shared laser system so that only one at a time):
(a) Polarisation sensitive THz emission and transmission between 0.1 and 3 THz with the possibility of low resolution imaging over 20x20 mm (based on photoconducting transmitters and receivers made in house).
3 THz bandwidth spectrometer for low temperature and polarisation resolved spectroscopy and imaging and typical signal. Plastic boxes purged with dry air cover parts of the apparatus where there are THz beams because atmospheric water absorption is a significant nuisance in THz spectroscopy.
Transmitters and receivers THz image of rose leaf
(b) THz emission and transmission between 0.1 and 40 THz (with a gap between 8 and 10 THz) at temperatures down to 5 K using ultrabroadband and ultra low noise photoconducting and nonlinear/electro-optic transmitters and receivers respectively. This apparatus can also be used for time domain reflectivity and reflective electro-optic sampling.
(a) Multi-THz spectroscopy and optical pump-optical probe set up
(b) Signal from 30 µm thick GaSe source detected with ultrabroadband photoconductive receiver
(c) A flexible geometry 3 THz bandwidth photonic crystal fibre (PCF) coupled THz spectrometer for room temperature scattering and near field imaging measurements using freely positionable photonic crystal fibre coupled sources and detectors. The near field probe is currently based on a photoconducting antenna behind an aperture in a metal screen and has a best resolution of 20µm. It is mounted on a motorised xyz stage. This system is mainly used for studies of THz metamaterials and waveguides.
Fibre coupled apparatus Near field THz probe
Map showing electric field above surface of a metamaterial Silica PCF for 80 fs pulse delivery
System 2 is uses a 1-10 kHz repetition rate, 5-0.5 mJ pulse energy, 35 fs laser system (Newport Spectra-Physics Spitfire Ace) and associated THz spectrometer which is based on a laser excited plasma source and ultraboadband gas phase detection. It is being used to test high THz pulse energy source concepts and has been adapted (although not yet used) to study materials in extreme THz fields. The system incorporates a variable pressure chamber for studying plasma generation of THz in gas filled waveguides. A 7 T superconducting magnet with variable temperature insert and THz and optical windows previously used with system 1a is integrated with this system, together with a 2 kHz OPA.
Cell used for studying gas filled waveguides Front end of 2 kHz rep rate, mJ laser system
· Access to in-house fabrication facilities: cleanrooms for optical and e-beam lithography, metal and dielectric deposition, wet and dry etching, metrology, fibre and capillary drawing, machine shop. Dedicated remote access workstations for computer modelling.
Last updated January 2015