Yia-Chung Chang

Wide-angle polarization independent infrared broadband absorbers based on metallic multi-sized disk arrays

Two-dimensional metallic broadband absorbers on a SiO(2)/Ag/Si substrate were experimentally studied. The absorptivity of such structure can be increased by tailoring the ratio of disk size to the unit cell area. The metallic disk exhibits a localized surface plasmon polariton (LSPP) mode for both TE and TM polarizations. A broadband thermal emitter can be realized because the LSPP mode is independent of the periodicities. By manipulating the ratios and disk sizes, a high-performance, wide-angle, polarization-independent dual band absorber was experimentally achieved. The results demonstrated a substantial flexibility in absorber designs for applications in thermal photovoltaics, sensors, and camouflage.

Temperature-dependent absorption measurements of excitons in GaN epilayers

Optical absorption measurements were performed on a series of thin GaN epilayers. Sharp spectral features were observed due to the 1s A and B exciton transitions. Using polarization dependent absorption, the C exciton transition was identified. A broad absorption feature was observed at ∼3.6 eV, which is attributed to indirect exciton-phonon absorption. The excitonic structure was found to persist well above room temperature. A fit to the Varshni formula yielded a temperature dependence of E(T)=E(T=0)−11.8×10−4T2(1414+T) eV for the A and B excitons. The exciton absorption linewidth was studied as a function of temperature, indicating that GaN exhibits very large exciton-phonon coupling.

A resonant spin lifetime transistor

We present a device concept for a spintronic transistor based on the spin relaxation properties a two-dimensional electron gas (2DEG). The device design is very similar to that of the Datta and Das spin transistor. However, our proposed device works in the diffusive regime rather than in the ballistic regime. This eases lithographical and processing requirements. The switching action is achieved through the biasing of a gate contact, which controls the lifetime of spins injected into the 2DEG from a ferromagnetic emitter, thus allowing the traveling spins to be either aligned with a ferromagnetic collector or randomizing them before collection. The device configuration can easily be turned into a memory and a readout head for magnetically stored information.

Broadband and omnidirectional antireflection employing disordered GaN nanopillars

Disordered GaN nanopillars of three different heights: 300, 550, and 720 nm are fabricated, and demonstrate broad angular and spectral antireflective characteristics, up to an incident angle of 60? and for the wavelength range of lambda=300-1800 nm. An algorithm based on a rigorous coupled-wave analysis (RCWA) method is developed to investigate the correlations between the reflective characteristics and the structural properties of the nanopillars. The broadband and omnidirectional antireflection arises mainly from the refractive-index gradient provided by nanopillars. Calculations show excellent agreement with the measured reflectivities for both s- and p- polarizations.

Valence‐band mixing effects on the gain and the refractive index change of quantum‐well lasers

The effects of valence‐band mixing on the gain and on the refractive index change of the quantum‐well laser and the effect of an applied electric field perpendicular to the quantum wells for gain switching are studied theoretically. Our calculations are based on the multiband effective‐mass theory (k⋅p method) and the density‐matrix formalism with the intraband relaxation taken into account. First, we calculate the nonparabolic valence‐band structure by the finite difference method after making a unitary transformation of the Luttinger‐Kohn Hamiltonian. The calculated gain for our model shows remarkable differences in both spectral shape and peak amplitude as compared with those for the conventional model of the parabolic valence band. The peak gain is reduced considerably and the gain spectrum is more symmetric in our model compared with that for the conventional model. The refractive index change shows a negative increment in the active region for both the TE and TM polarizations resulting in the antigu...

Modification of optical properties of GaAs‐Ga1−xAlxAs superlattices due to band mixing

A theoretical calculation of the optical properties of GaAs‐Ga1−xAlxAs superlattices is presented. The calculation includes the detailed atomic nature of the superlattice electronic states in a realistic tight‐binding model. It is found that the mixture of the bulk heavy hole and light hole states in the superlattice wave function substantially affects the optical properties.

Thermoelectric and thermal rectification properties of quantum dot junctions

The electrical conductance, thermal conductance, thermal power, and figure of merit

Monte Carlo simulation of impact ionization in GaAs including quantum effects

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Submonolayer quantum dot infrared photodetector

of semiconductor quantum dots (QDs) embedded into an insulator matrix connected with metallic electrodes are theoretically investigated in the Coulomb blockade regime. The multilevel Anderson model is used to simulate the multiple QDs junction system. The charge and heat currents in the sequential tunneling process are calculated by the Keldysh Greens function technique. In the linear-response regime the

Reflection and emission properties of an infrared emitter.

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