Our research is aimed at the development of novel optoelectronic devices based on artificially-structured materials systems, whose properties can be tailored by design to meet specific applications in a way that is not afforded by simply using bulk materials. One important example is that of semiconductor quantum structures, in which nanoscale layers (or wires or dots) of different semiconductor materials are assembled to create an energy landscape in which electrons behave in a markedly quantum-mechanical fashion. By controlling the dimensions and geometry of these structures, one can tune their most basic electronic and optical properties to enable entirely new device concepts – an approach known as bandgap engineering. Photonic nanostructures involving materials with different optical properties (both metals and dielectrics) can also be designed in a similar manner, and used to control the flow of light and its interaction with the underlying matter in novel and useful ways.
Using this general approach, we are investigating several device concepts to address a wide range of different applications. Specific activities involve both theoretical and experimental efforts, including design and simulations (often based on the proverbial particle-in-a-box problem of quantum mechanics, or on the FDTD method for electromagnetic modeling), device micro- and nano-fabrication, and electrical and optical characterization.
The THz spectral region, which lies at the intersection between the traditional domains of microwave electronics and photonics, has so far remained relatively underutilized, mainly due to the lack of a practical device technology (most notably sources and detectors). At the same time, THz devices could provide unique imaging and sensing capabilities (e.g., for security screening, manufacturing quality control, and medical diagnostics), related to the ability of THz light to penetrate through common packaging materials with little attenuation, and to the presence of distinctive THz absorption resonances in many molecules of interest. Coherent THz radiation can now be generated using AlGaAs intersubband quantum cascade lasers, whose operation however is fundamentally limited to cryogenic temperatures and to an incomplete coverage of the THz spectrum. Work in our lab is focused on the development of THz light sources and photodetectors based on new materials systems that can potentially address this technology gap. Specific systems under study include AlGaN and SiGe quantum cascade structures, whose large optical-phonon energies and nonpolar nature, respectively, can in principle allow overcoming the intrinsic limitations of AlGaAs devices. Novel THz radiation mechanisms based on the unique electronic and mechanical properties of graphene are also being explored.
LEFT: Far-infrared photodetection with AlGaN double-step quantum wells. Inset: conduction-band diagram of the device active material. MIDDLE: Design of a THz quantum cascade laser based on electronic intersubband transitions in the L valleys of Ge/SiGe quantum wells. RIGHT: Far-infrared absorption spectra of Si/SiGe quantum wells grown on a strain-relaxed nanomembrane (red trace) and on a rigid Si substrate (blue trace).
Plasmonic excitations are collective oscillations of the electron gas on metallic surfaces or nanostructures, which can produce electromagnetic field distributions featuring sub-wavelength “hot spots” of highly enhanced local field intensity. As a result, metallic nanoparticles can be used as optical antennas to concentrate incident light into suitably positioned absorbers. By reciprocity, the same nanostructures can also dramatically enhance the spontaneous emission rate of radiating dipoles located within their hot spots. Under these conditions, the far-field properties of the emitted light can also be engineered based on the nature of the plasmonic excitations involved in the radiation process, for example to enable beaming along geometrically tunable directions or the formation of more complex wavefront patterns. Ongoing work in our lab is focused on developing a fundamental understanding of these phenomena, and exploring its potential for applications in a wide range of disciplines, including optoelectronics, solid-state lighting, sensing, and photovoltaics.
LEFT: Schematic illustration of plasmon-enhanced light emission in the presence of a periodic array of metallic nanoparticles (NPs). MIDDLE: Plasmon-enhanced near-green photoluminescence (PL) from InGaN quantum wells coated with a square-periodic array of cylindrical Ag NPs. RIGHT: Plasmonic off-axis unidirectional beaming of quantum-well photoluminescence, based on a periodic array of triangular Ag NPs on an ultrathin Ag film deposited on the light-emitting sample. Bottom: SEM image of the sample top surface (the scale bar is 500 nm).
The development of practical light sources based on group-IV semiconductors, which provide the leading materials platform of microelectronics, represents one of the major outstanding challenges of optoelectronics research. This goal, which is complicated by the indirect energy bandgap of silicon, germanium, and related alloys, has far reaching technological implications, as it would enable the monolithic integration of electronic and photonic functionalities on an unprecedented scale. Our group is pursuing two separate approaches to address this challenge. One approach is based on the use of intersubband transitions in SiGe quantum cascade structures, i.e., transitions between quantum confined states derived from the same energy band, so that the nature of the bandgap becomes irrelevant. The second approach involves the use of Ge nanomembranes (ie, single-crystal sheets with nanoscale thicknesses that are either completely released from, or partially suspended over, their native substrates) in the presence of externally applied mechanical stress By virtue of their extreme aspect ratios, these nanomembranes can sustain extremely large tensile strain levels, which in turn can transform Ge into a direct-bandgap semiconductor capable of providing optical gain under realistic pumping conditions.
LEFT: Schematic band structure of Ge, showing the transition from indirect bandgap at zero strain to direct bandgap at 2% biaxial tensile strain. MIDDLE: Schematic illustration of the sample mount used to demonstrate direct-bandgap Ge in mechanically stretched nanomembranes (NMs), and optical micrograph of an experimental sample. RIGHT: Strain-enhanced photoluminescence (PL) from a mechanically-stressed Ge NM, coated with a square periodic array of amorphous-Ge pillars. The dashed lines indicate the appearance of photonic-crystal cavity modes.