III-Nitride Intersubband Devices
Intersubband (ISB) transitions in semiconductor quantum structures – i.e. electronic transitions between quantized states within a same energy band – offer several attractive features for device applications, including great design flexibility, ultrafast relaxation lifetimes, and giant optical nonlinearities.  Compared to the more widely studied As-based materials, GaN/AlGaN quantum wells can provide much larger conduction-band offsets and hence accommodate ISB transitions at record short wavelengths, into the near-infrared transmission window of optical fibers.  Combined with the aforementioned features, this property opens up new opportunities in the area of all-optical switching for future ultrafast fiber-optic communications.  Additionally, the large LO-phonon energies of III-nitride semiconductors are advantageous for the development of ISB terahertz emitters, as the forbidden reststrahlen band of these materials lies well outside the 1-10 THz range.  Such devices are expected to find a wide range of imaging and sensing applications, e.g. for security screening, manufacturing quality control, and medical diagnostics.  Theoretical and experimental efforts in both areas are ongoing, with the quantum-well samples provided by the MBE group of Ted Moustakas at BU.  These projects are funded by NSF and ARL. 

Surface-Plasmon Enhanced LEDs
Surface plasmon polaritons (SPPs) are bound electromagnetic waves guided at metal/dielectric interfaces, whose fields are strongly localized near the interface and whose density of modes is highly enhanced near their resonance frequency.  Due to these features, if a metal film is deposited in close proximity of an LED active layer, the device internal quantum efficiency can be strongly increased through the emission of SPPs.  These guided modes can then be converted into radiative waves with a grating, or even by the roughness of the metal surfaces.  We are currently pursuing this approach to increase the photo- and ultimately the electro-luminescence efficiency of visible and ultraviolet solid-state sources.  To this purpose, we are studying ways to tune by design the SPP resonance to closely match the LED emission frequency over a broad spectral range, for example using hybridized SPP modes in strongly coupled metallo-dielectric heterostructures.  This project is funded by DOE.

Silicon-Based Quantum Cascade Lasers
The demonstration of silicon-based injection lasers, which is complicated by the indirect bandgap of silicon, has long been the subject of extensive research efforts, as it would enable the monolithic integration of electronic and photonic devices on an unprecedented scale.  The use of intersubband transitions in SiGe quantum wells is a promising approach, as these transitions involve states within a same energy band so that the nature of the bandgap is irrelevant.  Additionally, due to their nonpolar nature and resulting lack of reststrahlen absorption, SiGe heterostructures are also very attractive for emission in the terahertz spectral region.  On the other hand, the development of SiGe intersubband gain media (based on the quantum-cascade-laser scheme) has been hindered by design issues specific to these heterostructures, including generally small band offsets and heavy effective masses, as well as by material issues related to strain.  Ways to circumvent these obstacles are being studied theoretically, with experimental efforts upcoming. 

Ultraviolet Electroabsorption Modulators
The bound-state energy levels and envelope functions of a semiconductor quantum well (and the resulting excitonic and interband absorption spectra) can be modified in a highly controlled fashion through the application of an electric field along the growth direction – a phenomenon known as the quantum-confined Stark effect.  This effect already forms the basis of an important class of optical modulators used in fiber-optics communications employing As-based heterostructures.  We are currently developing similar devices using wide-bandgap GaN/AlGaN quantum wells, for operation at ultraviolet wavelengths.  A number of physical properties characteristic of these materials, including their large intrinsic electric fields and their highly anisotropic optical response on non-polar crystal planes, will be exploited to optimize the device performance.  These ultraviolet optical modulators are expected to find important applications in areas such as non-line-of-sight free-space optical communications, sensing and spectroscopy, and Q-switched pulsed lasers.  This project is a collaboration with the MBE group of Ted Moustakas at BU, and is funded by NSF.