Dynamics and Nonlinearities in Long-Wavelength Quantum-Cascade Lasers
Multifunctional Epitaxially Grown Oxides
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, high-power, 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 the Georgia Institute of Technology and Dr. D. Denison of the Georgia Tech Research Institute, Prof. 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 home-developed self-consistent Monte Carlo-Poisson-Maxwell solver. An example of computed terahertz pulses following ultrafast optical excitation of a GaAs photoconductor is shown above. 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 above figure shows simulations of terahertz propagation based on the finite-difference time-domain 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, high-throughput 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 quantum-cascade 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. 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 with chemists, these materials are selectively doped with molecules exhibiting strong optical nonlinearities, and soon, with gain.
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 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.
Page Last Updated: 4 February 2005