Ultrafast Science and Terahertz
Photonics Group
Steve in the Annapurnas THz charge oscillations in quantum
wells Metametrial
waveguide
Group members
Dr Steve Andrews
(group leader), Dr Ali Muir (posdoc), Mr Yi Pan (PhD
student), Miss Mia Zangui (PhD student), Mrs Siti Norbaieah Mohamad Hashim (PhD student), Mr
Jonathan Archer (PhD student)
Previous
members: Mukul
Misra (postdoc), Chris Williams (PhD student), Wei
Ding (PhD student, joint supervised with Stefan Maier), Ali Hussain
(PhD student + 3 month postdoc), Adam Armitage
(postdoc), Peter Huggard (postdoc), Julian Cluff (PhD student), Graeme Moore (PhD student), Chris Shaw
(PhD student)
Current research Interests
Our
main specialisation at present is in the applications of time domain THz
spectroscopy and the development of related ‘T-ray’ technology, including:
1. Ultrafast studies of electron and phonon dynamics in materials
Examples
of work in this area:
‘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
Examples of work in this area:
'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)
'Ultrabroadband polarization analysis of terahertz pulses', A. Hussain and S. R. Andrews, Optics Express, 16, 7251-7257 (2008)‘Waveguide artefacts in THz near field imaging’, M. Misra, S. R. Andrews and S. A. Maier, Appl. Phys. Lett. 100, 191109 (2012)
'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)
3. THz waveguiding
and metamaterials
Examples of work in this area:
‘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) 4. High energy pulsed THz sources and their applications (new 2012 project)
Historical research interests
Inelastic light and diffuse X-ray scattering studies of phase transitions, optical spectroscopy of semiconductor nanostructures, optoelectronic devices, semiconductor nanofabrication.
Current 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,
Yi)
·
High
sensitivity THz circular dichroism spectroscopy and
its applications (Leverhulme Trust, Mia)
·
THz
bio and chemical sensing and imaging (Malaysian Government, Siti)
·
Development
of high power THz gas-filled waveguide sources; physics of strongly THz
driven materials (EPSRC, Jonathan
and Ali)
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. 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-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. There is also an electro-optical
technique for detection which was developed a little later and has advantages
at higher frequency.
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 80 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 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 Typical
signals
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.
