Photonic crystals (PC) can fundamentally alter the emission behavior of light sources by suitably modifying the
electromagnetic environment around them. Strong modulation of the photonic density of states especially by full three-dimensional
(3D) bandgap PCs, enables one to completely suppress emission in undesired wavelengths and directions
while enhancing desired emission. This property of 3DPC to control spontaneous emission, opens up new regimes of
light-matter interaction in particular, energy efficient and high brightness visible lighting. Therefore a 3DPC composed
entirely of gallinum nitride (GaN), a key material used in visible light emitting diodes can dramatically impact solid state
lighting. The following work demonstrates an all GaN logpile 3DPC with bandgap in the visible fabricated by a template
directed epitaxial growth.
The novel properties of semiconductor nanowires, along with their potential for device applications in areas including
nanoscale lasers and thermoelectrics, have led to a resurgence of interest in their growth and characterization over the
past decade. However, the further development and optimization of nanowire-based devices will depend critically on an
understanding of carrier relaxation in these nanostructures. For example, the operation of GaN-based photonic devices is
often influenced by the presence of a large defect state concentration. Ultrafast optical spectroscopy can address this
problem by measuring carrier transfer into and out of these states, which will be important in optimizing device
In this work, we use ultrafast wavelength-tunable optical spectroscopy to temporally resolve carrier dynamics in
semiconductor nanowires. Wavelength-tunable optical pump-probe measurements enable us to independently measure
electron and hole dynamics in Ge nanowires, revealing that the lifetime of both electrons and holes decreases with
decreasing nanowire diameter. Similar measurements on CdSe nanostructures reveal that the surface-to-volume ratio
strongly influences carrier relaxation. Finally, ultrafast optical experiments on GaN nanowires probe carrier dynamics in
the defect states that influence device operation. These experiments provide fundamental insight into carrier relaxation in
these nanosystems and reveal information critical to optimizing their performance for applications.
An AlGaN Light-emitting diode (LED) emitting with a peak wavelength at 291 nm and a radiant power of 0.5 mW @ 100 mA was fabricated on a sapphire substrate. A compact gated fluorescence detection system was built using this LED as the excitation light source. We demonstrate that it provides sufficient power using Terbium enhanced fluorescence to detect subnanomolar concentrations of dipicolinic acid (DPA, 2, 6-pyridinedicarboxylic acid), a substance uniquely present in bacterial spores such as that from B. anthracis, providing a basis for convenient early warning detectors. We also describe initial results from a novel approach for biological aerosol detection using long lived fluorescence from a Europium tagged dye that binds to proteins.
Deep ultraviolet light emitting diodes (LEDs) with emission wavelengths shorter than 300 nm have been grown by metalorganic vapor phase epitaxy. A bottom emitting LED design is used which requires a high-Al content AlxGa1-xN (x = 0.5 - 0.8 ) buffer layer which has sufficient conductivity and is transparent to the quantum well emission wavelength. LEDs were flip chip mounted to a silicon submount which provides for good thermal performance as well as improved light extraction. For large area 1 mm x 1 mm LEDs emitting at 297 nm, an output power as high as 2.25 mW under direct current operation has been demonstrated at 500 mA with a forward voltage of 12.5 volts. For shorter wavelength LEDs emitting at 276 nm, an output power as high as 1.3 mW has been demonstrated under direct current operation at 300 mA with a forward voltage of 9.2 volts. Recent improvements in heterostructure design have resulted in quantum well emission at 276 nm with a peak intensity that is 330 times stronger than the largest sub-bandgap peak. LEDs with emission wavelengths as short as 237 nm have also been demonstrated.
In this paper, we overview the critical materials challenges in the development of AlGaN-based deep ultraviolet light emitting diodes (LEDs) and present our recent advances in the performance of LEDs in the 275-290 nm range. Our primary device design involves a flip-chip, bottom emitting, transparent AlGaN (Al = 47-60%) buffer layer structure with interdigitated contacts. To date, under direct current operation, we have demonstrated greater than 1 mW of output power at 290 nm with 1 mm x 1 mm LEDs, and greater than 0.5 mW output power from LEDs emitting at wavelengths as short at 276 nm. Electroluminescence spectra demonstrate both a main peak from quantum well emission as well as sub-bandgap emission originating from radiative recombination involving deep level states. The heterostructure designs that we have employed have greatly suppressed this deep level emission, resulting in deep level peak intensities that are 40-125X lower than the primary quantum well emission for different LED designs and applied current densities.
Several thousand glass optical fibers fused together is routinely used as fiber image guides for medical and other image remoting applications. Fiber image guides also offer possibility for flexible optical interconnect links with potentially thousands of bi-directional parallel channels with data rates as high as 10 Gbps per channel, leading to more than Tera bits per second aggregate data transfer rates. A fair number of fiber image guide based link demonstrations using vertical cavity surface emitting lasers have been reported. However, little is known about designable parameters and optimization paradigms for applications to massively parallel optical interconnects. This paper discusses critical optical parameters that characterize a massively parallel link. Experimental characterizations were carried out to explore some of the fundamental interactions between single-mode 850 nm VCSELs and fiber image guides having different numerical apertures, 0.25, 0.55 and 1.00. Preliminary optical simulation results are given. Finally, potential directions for further experimental and analytical explorations, and for applicability into designable link systems are suggested.
Femtosecond pump-probe (P-P) and four-wave mixing (FWM) experiments were performed simultaneously at 11 K on gallium nitride epilayers to study the initial temporal line-shapes of the exciton. A-B exciton beats were found in both P-P and FWM experiments near the exciton resonance. However, the differential reflection spectra showed a much slower rise time that persisted at longer negative time delay than the FWM signal or differential transition spectra at the exciton resonance. A numerical solution of a six band semiconductor Bloch equation model including all Hartree Fock nonlinearities shows that this slow rise results from excitation induced dephasing, that is, the strong density dependence of the dephasing time which changes with the laser excitation energy.
Femtosecond pump-probe measurements were performed in GaN epilayers to study carrier dynamics in the band edge region. Excitonic absorption was found to begin saturating at a pump fluence of 20 (mu) J/cm2 which corresponds to an estimated carrier density of 1 X 1018 cm-3. At zero delay between pump and probe, induced absorption is observed below the unpumped band gap due to ultrafast bandgap renormalization. After 375 fs, a large induced transparency is observed just below the excitonic resonance which is due to a transient electron-hole plasma. After 1 ps, the absorption has partially recovered to a level associated with excitonic phase-space filling. The absorption then recovers with a characteristic time of approximately 20 ps, a value which increases with increasing excitation density.
Femtosecond four-wave-mixing (FWM) is used to study the coherent dynamics of excitons in thin epilayers of GaN grown by metalorganic chemical vapor deposition on sapphire substrates. Temperature dependent FWM is used to accurately measure the exciton homogeneous linewidth and it is shown that the exciton-LO phonon interaction is larger in GaN than in other III-V materials. Furthermore, the excitonic resonances in our samples are shown to be very nearly homogeneously broadened even at low temperature. We have observed strong beating behavior in the FWM signal corresponding to the energy separation of the A and B free exciton transitions. The beats were studied as a function of relative position across the B exciton linewidth in order to determine that the beating is due to a coherent exchange of population between the A and B excitons and not to, so called, polarization interference. The quantum beats were further studied as a function of polarization geometry and a phase shift of 180 degrees was observed when changing from collinear to cross-linear polarization geometries. The FWM signal was calculated in the ultrashort pulse limit in order to theoretically model the observed phase change.
A variety of spectroscopic techniques has been used to study the optical properties of epitaxial GaN based materials grown by metalorganic chemical vapor deposition and molecular beam epitaxy. The emphasis was on the issues vital to device applications such as stimulated emission and laser action, as well as carrier relaxation dynamics. Sharp exciton structures were observed by optical absorption measurements above 300 K, providing direct evidence of the formation of excitons in GaN at temperatures higher than room temperature. Using a picosecond streak camera, the time decay of free and bound exciton emissions was studied. By optical pumping, stimulated emission and lasing were investigated over a wide temperature range up to 420 K. In addition, the optical nonlinearity of GaN was studied using wave mixing techniques.