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ULTRAFAST NANOSCIENCE GROUP
SCHOOL OF ELECTRICAL AND COMPUTER ENGINEERING
GEORGIA INSTITUTE OF TECHNOLOGY
PROF. DAVID S. CITRIN
David S. Citrin is an Associate Professor in the School of Electrical
and Computer Engineering at Georgia Tech where his interests are in
ultrahigh-speed devices, nanoscale engineering, multifunctional
materials, nonlinear optics, and terahertz technology. He earned the
MS (1987) and PhD (1991) degrees in Physics from the University
of Illinois at Urbana-Champaign. Following the PhD, he was a postdoctoral research fellow at the Max Planck Institute for Solid State
Research, Stuttgart, Germany (1992-1993), and then Center Fellow
at the Center for Ultrafast Optical Science at the University of
Michigan (1993-1995). He was an Assistant Professor of Physics and of Materials
Science at Washington State University (1995-2001). During that time, he was awarded a
Presidential Early Career Award for Scientists and Engineers (PECASE) and an award
under the Young Investigator Program (YIP) of the US Office of Naval Research. He is
the author of over 100 refereed scientific publications. Citrin is currently serving as an
associate editor of the IEEE Journal of Quantum Electronics and has chaired the
workshops Radiative Processes and Dephasing in Semiconductors (Ann Arbor, Michigan
1994; Coeur d’Alene, Idaho 1998) and co-chaired Fundamental Optical Processes in
Semiconductors (Estes Park. Colorado 2004). Citrin currently serves as the Chair of the
Topical Interest Group on Optics and Photonics in the School of Electrical and Computer
Engineering at Georgia Tech and is also a member of the Microsystems and of the
Electromagnetic Topical Interest Groups. He was recently named a recipient of a
Friedrich Wilhelm Bessel Research Awards by the Alexander von Humboldt Foundation
of Germany.
RESEARCH ACTIVITIES
The Ultrafast Nanoscience Group works on the theory and simulation of novel materials,
structures, and devices for applications in photonics and electronics. The majority of the
work deals with enabling technologies at the intersection of the ultrafast and the
ultrasmall where quantum effects may dominate and provide for functionality not
available with conventional approaches.
Terahertz Technology
Lying between high-frequency electronics
(radio frequency and microwaves) and infrared
optics is the terahertz portion of the
electromagnetic spectrum. Broadly speaking,
from about 500 GHz through 10 THz there is a
shortage of tools to deploy compact, highpower,
turn-key
terahertz
systems—
specifically for time-domain applications. Nonetheless, numerous potential applications
exist, including carrier dynamics in high-speed devices (lower left frame of the figure),
imaging within packages (lower right frame), as well as many applications in molecular
and solid-state spectroscopy. Emerging areas of interest include security screening and
standoff detection.
Prof. Citrin has been instrumental in elucidating
terahertz phenomena in semiconductor materials and
nanostructures for over 10 years. The Ultrafast
Nanoscience Group is active in several areas that may
enable the future penetration of terahertz technology in
many areas. In collaboration with Prof. S. E. Ralph at
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the Georgia Institute of Technology and Dr. D.
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50 kV, 10
Denison of the Georgia Tech Research Institute, Prof.
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Citrin’s PhD student DaeSin Kim is carrying out
simulations of carriers (electrons and holes) in
photoconductive terahertz sources excited by ultrafast
optical pulses. The simulations rely on a homedeveloped self-consistent Monte Carlo-PoissonMaxwell solver. An example of computed terahertz
pulses following ultrafast optical excitation of a GaAs
photoconductor is shown to the left. In this example, a comparison is made between flat
electrodes and structured electrodes, as shown in the inset. Structured electrodes lead to
larger local electric field in the photoconductor producing larger carrier accelerations, and
hence stronger terahertz pulses.
Another area of interest within the Group’s
terahertz efforts involves the use of terahertz
photonic crystals for sensing applications. The
figure on the right shows simulations of terahertz
propagation based on the finite-difference timedomain method through a coupled resonator
optical waveguide (CROW) in a photonic crystal.
The calculations were carried out by PhD student
Hamza Kurt. The presence of DNA within the
black holes (circles in figure) alter the characteristics of the waveguide transmission
(transmission peak within the photonic-crystal bandgap in frames at right). Simulations
indicate that sensitivity to picoliter volumes may be possible. A number of avenues are
being pursued, including integration with microfluidic systems for realtime, highthroughput industrial applications and laboratory-on-a-chip concepts for biomedical
applications.
Dynamics and Nonlinearities in Long-Wavelength Quantum-Cascade Lasers
Quantum-cascade lasers are proving attractive for high-power mid-infrared applications,
and more recently are making inroads into the far-infrared portion of the spectrum.
Devices with output between 2 and 3 THz have been reported. These long-wavelength
lasers rely on a number of physical mechanisms that are of little importance or absent in
conventional near-infrared or visible semiconductor lasers. In particular, the internal laser
field in long-wavelength quantum-cascade lasers can be comparable to the bias field. In
addition, because of the low gain, the mechanism for passive mode locking is not well
understood. Two PhD students in the Group, Jing Bai and Shih-Hsuan Hong, are working
on theoretical issues concerning intracavity nonlinearities in long-wavelength quantumcascade lasers and modelocking, including the role of noise. A laser-cavity simulator
coupled with a detailed physical model for the gain medium is being developed to
address these issues.
Photonic Nanostructures
The unprecedented ability to fabricate, in a controlled fashion, nanoscale structures in
semiconductor, metals, and other materials will allow us to access physical phenomena
that were hitherto not available to us. For example, together with former PhD student and
post-doc Alex Maslov, Citrin has predicted that nanoscale rings of compound
semiconductors can exhibit a strong magnetooptic effect, even though the constituent
material does not show a significant intrinsic magnetooptic effect. Other work in the
group has examined the dynamics of electrons in semiconductor superlattice nanorings in
magnetic field, and has identified a new type of Bloch oscillations, the results being
published in Physical Review Letters.
While work continues on the semiconductor nanostructures, a more recent area of interest
to the group is nanoplasmonics. Specifically, the group is studying waveguides and
related structures based on chains or arrays of noncontacting noble-metal (e.g., Ag, Au)
nanoparticles deposited on dielectric substrates. Such nanoparticles exhibit strong
surface-plasmon resonances at optical frequencies. A surface plasmon in one nanoparticle
emits light, which can then
coherently
excite
surface
plasmons in other nanoparticles
a chain arrangement, to form a waveguide, as shown in the figure. Note that the
transverse dimension of the nanoplasmonic waveguide thus formed is the nanoparticle
diameter, ~10-100 nm, which may be much less than the optical wavelength. Ordinary
dielectric waveguides, however, cannot have transverse dimensions of much less than the
optical wavelength. Thus, nanoplasmonic waveguides are of potential interest for
nanoscale optical interconnects, as well as for example for biophotonic applications in
order to deliver optical energy selectively to subcellular structures.
The implementation of such structures, however, is currently limited by both nonradiative
and radiative attenuation. The former is associated with intrinsic nonradiative damping of
single-nanoparticle surface plasmons. This source of damping can be controlled
potentially by judicious choice of nanoparticle materials and geometries. The latter is
related to electromagnetic energy scattered into directions other than the longitudinal
direction down the chain axis. The Group’s focus is on the development of quasi-analytic
models to describe electromagnetic propagation in such structures that can serve as useful
design tools as well as to understand basic physical issues associated with propagation
and radiation. One aim is to design structures to minimize radiative losses. In addition,
the effects of chain disorder, bends, junctions, and terminations are being studied. Other
structures such as nanowires are also being explored.
Polymer Integrated Optics
The ability to literally print photonic integrated circuits on flexible substrates will enable
inexpensive displays as well as all optical systems.
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Together with Prof. B. Kipellen and the post-doc
Sungwon Kim at the Georgia Institute of Technology,
the Group is designing, fabricating, and characterizing
passive and active polymer photonic components, such
as waveguides and ring resonators. In close cooperation
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with chemists, these materials are selectively doped with
molecules exhibiting strong optical nonlinearities, and soon, with gain.
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Multifunctional Epitaxially Grown Oxides
The unprecedented ability to grow epitaxially layers of ferroelectric, ferromagnetic,
semiconducting, and superconducting materials (though, not yet all in the same sample),
promises to enable nanoscale multifunctional materials (e.g., piezo-electric/magnetooptic,
superconductor/semiconductor, etc.) that have hitherto been unavailable. The Group is
involved in a major multiuniversity research effort headed by Prof. W. A. Doolittle at the
Georgia Institute of Technology to grow novel epitaxially structures multifunctional
oxide materials and to exploit them by demonstrating new classes of devices. Thomas
Backes, a PhD student in Citrin’s Ultrafast Nanoscience Group is designing acoustically
controlled nonlinear optical materials based on multifunctional oxide epilayers.
Interdigitated transducers will be used to control optical nonlinearities in space and time.
Chaotic Communications
The ability to encode high data-rate optical information securely remains a challenge.
PhD student Alexandre Locquet is working on schemes to modulate semiconductor lasers
in a chaotic fashion using external feedback. The recipient, having precise knowledge of
a small number of system parameters can accurately reproduce the chaotic modulation,
and hence decode the signal. An evesdropper with an inaccurate knowledge of the system
parameters is unable to recover the signal. The work in the group involves physical
modeling of the laser system as well as time-series analysis of chaotic signals.
Intermediate Solar Cells
Together with Prof. C. Honsberg of the University of Delaware, PhD student Michael
Levy is designing and modeling intermediate solar cells with predicted high efficiency
based on multilevel semiconductor quantum dots. The scheme is expected to circumvent
limitations associated with lost excess energy above the bandgap due to photocarriers
excited by high-energy solar photons. The modeling involves multiband effective mass
treatment of the quantum-dot electronic states, carrier relaxation, optical processes, and
transport.
Terahertz-Modulated Photonics
PhD student DongKwon Kim is studying theoretical issues involving the modulation of
semiconductor-based photonic and optoelectronic devices by freespace terahertz fields.
Due to the high speed of the modulation, coherent nonlinear mixing between optical and
terahertz signals occurs, and can be understood in terms of hyper-Raman processes in the
terahertz photons. Schemes to optimize and exploit this ultrahigh-speed modulation are
under exploration.
Other Activities
In addition, the Group in interested in physical modeling of electrons in quantum-dot
based quantum logic gates, the nonlinear terahertz dynamics of wide quantum wells, and
electromagnetic propagation in metamaterials. Citrin is active in teaching; recent courses
include undergraduate electromagnetics, graduate quantum mechanics, and graduate
device physics. He is also involved in teaching at Georgia Tech Lorraine in Metz, France,
where he will spend the 2005-2006 academic year.
Contact Information
Address: Prof. David S. Citrin, School of Electrical and Computer Engineering,
Georgia Institute of Technology, Atlanta, Georgia 30332-0250
Tel: +1 404 385-1579, Fax: +1 404 894-4700
Email: [email protected]