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Transcript
Template for Abstract Submission – A Concrete
Example of the Connection between Basic Physics
and the Detection of Explosives and CBRN Agents
using THz Radiation
M.F. Pereira1
1
Materials and Engineering Research Institute, Sheffield Hallam University, S1 1WB, Sheffield, United Kingdom IEEE
Abstract- Interconduction band gain without global inversion is
obtained by engineering conduction band dispersion relations
with dilute N resonant centres. This may lead to high temperature
operation of THz Quantum Cascade Lasers, which are candidate
sources for Detection of Explosives and CBRN, using Terahertz
radiation. The abstract with this format should have a maximum
length of two pages.
I.
INTRODUCTION
The main focus of this workshop is on novel materials, concepts and
device designs for generating and detecting THz (0.3 THz to 10 THz)
and Mid Infrared (10 THz to 100 THz) radiation for the detection of
explosives and CBRN agents. CBRN is an initialism for chemical,
biological, radiological and nuclear. CBRN agents, explosives and
illegal drugs can be detected by their characteristic absorption spectra
at THz frequencies with high selectivity and resolution in applications
fields as industrial quality inspection control, customs inspection and
security screening. Moreover, MIR and THz radiation has no
endangering effects on human beings and enables higher contrast for
“soft matter” than x-rays. In comparison to standard optical
technologies for wavelengths up to about 2μm, sources and detectors
for MIR and THz have not yet reached this level of maturity and there
is still a large gap for features like wavelength tunability, spectral
purity, high power and room temperature operation, which all are
necessary for commercial applications.
generation of THz radiation; Millimetre wave and microwave
generation and detection – bridging the gap to THz; Quantum
Cascade Lasers; Quantum Cascade Detectors; Intersubband Transport
and Optics; THz and MIR Metamaterials; Intersubband Polaritons
and Antipolaritons.
Intersubband (ISB) devices like the quantum cascade laser
QCL [1], can unleash the potential for many applications in the far
infrared, notably in the THz range [2], notably detection of explosives
and CBRN, but THz QCLs still require cryocooling. The main
difficulty at high temperatures is achieving population inversion and
ISB lasing without inversion (LWI) may be a solution for this
microscopic bottleneck [3-5].
Modeling of the optical and THz response of semiconductors can be
achieved using Nonequilibrium Many Body Simulations, proven to
deliver realistic results for both interband and intersubband transitions
[6-14]. Here we exploit extreme nonparabolicity in the conduction
bands to engineer LWI. This can be achieved, e.g., in dilute nitride
quantum wells. Indeed, we have recently demonstrated the possibility
of interconduction band gain without global inversion by engineering
the conduction band effective masses so that the upper lasing subband
has an effective mass considerably smaller than the lower lasing
subband that could not be obtained in conventional III-V materials.
Next we summarize the main steps of the technique used to address
the ISB LWI case [7].
II.
Papers are requested on ether direct THz detection of CBRN agents
and explosives, or on basic or applied science with a clear introduction
on how the investigation can be useful for CBRN and explosives
detection.
Furthermore, detection using THz radiation implies that people are
exposed to it, thus progress in understanding the interaction of THz
radiation with bio matter is also of relevance.
The underlying topics include, but are not restricted to: Detection of
explosives and CBRNs; Interaction of MIR or THz radiation with
living biological material; Applications of MIR and THz to medical
diagnosis MIR and THz imaging; Integration of THz and MIR with
optical fibers; Our of the box methods for MIR and THz generation
and detection; Lasers without inversion; Nonlinear MIR and THz
optics; Difference frequency and other nonlinear methods of
NUMERICAL RESULTS AND DISCUSSION
The strong interaction between the N resonant states and the
conduction band edge means that the conventional eight-band
k·p method cannot be applied directly to GaAsN and related
heterostructures. One must include the interaction between the
N resonant states and the conduction band edge to describe the
variation of the zone-center conduction band edge energy with
N. In this paper we use a ten-band k·p Hamiltonian to describe
the average interaction of the N, NN-pairs and other N-cluster
states with the host material.
The absorption α(ω), or equivalently the gain spectra g(ω)
are evaluated within the context of a NGF approach. The
susceptibility is directly related to the carriers Green’s
functions G, which satisfies a Dyson equation.
The first step of the numerical scheme is the solution of the
ten-band k·p Hamiltonian which includes the dilute nitrogen
levels responsible for the extra nonparabolicity that gives rise
to strong differences in effective subband masses. The Green’s
functions and self-energies are expanded using eigenstates and
eigenvalues of this Hamiltonian. The model system actually
investigated in this paper is globally out of equilibrium but the
electrons are assumed to be thermalized within each subband.
For a feasibility study we control the total number of electrons
in each subband, which can in practice be achieved, e.g., by
optical pumping, selective doping or a combination of both
methods. Thus, the full NGF scheme is simplified and reduces
to the self-consistent evaluation of chemical potentials and
self-energy matrix elements which lead to subband energy
renormalization, dephasing constants, and occupation
functions. Finally, absorption and gain are given by the
solution of the integro-differential equation for the optical
susceptibility obtained from the carriers Green’s function in
linear response. The numerical matrix inversion technique used
to solve this equation is similar to the interband methods used
under different approximations for both absorption and gain for
III-V and II-VI materials [6-14]. Figure 1 illustrates the ISB
LWI effect for a 10 nm Ga0.98 N0.02As/Al0.3Ga0.7As QW.
In summary, this paper shows that it is possible to engineer
conduction subbands with substantial differences in effective
masses which make gain without global inversion feasible in
dilute nitride heterostructures. In the full presentation we shall
extend the results of Ref. [7] for different structures in an effort
to push the gain towards the THz domain, where the devices
can be used for detection of explosives and CBRN agents.
REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
Figure 1. Gain without global population inversion for a 10 nm
Ga0.98 N0.02As/Al0.3Ga0.7As QW. The density for the top e2+
subband is the same in all curves. nup=1.0 ×1011 cm−2. From top
to bottom, the equal density of subbands e1− and e3- increase by
nlow=1.0, 1.1, 1.2, 1.3, 1.4, 1.5×10 11. The insets show
occupations for nup = nlow and the corresponding bandstructure
for the e1- (dashed), e3- (dot-dashed) and e2+ (dotted) subbands
[9]. The curves with smaller mass have larger occupation.
[13]
[14]
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inversion through dilute nitride band engineering,” Appl.
Phys. Lett. 98, 061101 (2011).
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approach, Phys Rev B75, 195301 (2007).
R. Nelander, A. Wacker, M.F. Pereira Jr., D.G. Revin,
M.R.Soulby, L.R. Wilson, J.W. Cockburn, A.B. Krysa, J.S.
Roberts, and R.J. Airey, Fingerprints of spatial charge
transfer in quantum cascade lasers, Journal of Applied Physics
102, 113104 (2007).
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of intersubband devices combining a nonequilibrium many
body theory with transmission spectroscopy experiments, J.
Mater Sci: Mater Electron 18, 689 (2007).
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the
Optical
Properties
of
Coulomb-Correlated
Semiconductors, Physica Status Solidi (b) 206, 477 (1998).
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Lasing in II-VI Compounds, Phys. Stat. Sol. B202, 751
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many-body effects, Appl. Phys. Lett. 64, 279 (1994).
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