The method of photonic band structure is used to calculate the frequencies of light that propagate in lattice models of the cornea and sclera of the mammalian eye, providing an explanation for transparency in the cornea that first properly accounts for multiple scattering of light. Each eye tissue is modeled as an ordered array of collagen rods, and photonic band structure methods are used to solve Maxwell's equations exactly for these models, a procedure that automatically effectively includes all orders of multiple scattering. These calculations show that the dispersion relation for the cornea is linear in the visible range, implying that the cornea is transparent. We show that the transmissivity is approximately 97% by using an effective medium approximation derived from the photonic band structure results and applicable in the visible region. In contrast, the dispersion relation for the model in the sclera is not linear in the visible region, and there are band gaps in this region that could play an important role in the transmission of light in the sclera.

In this paper we present the results of calculations of the lattice thermal conductivity of layered complex metal oxide Na x CoO 2 within the Green-Kubo theory. Using Na x CoO 2 we identify the two competing mechanisms responsible for the favorable scaling properties of the Green-Kubo method for calculating the lattice thermal conductivity. The artificial correlations of the heat flux fluctuations due to the finite size of the supercells are partially cancelled by the missing long wavelength acoustic phonon modes. We compute the lattice thermoelectric properties of bulk Na x CoO 2 with varying stoichiometry, structural defects, and temperature. We also calculate the thermal conductivity of Na x CoO 2 in the nanosheet geometry. While the dependence of thermal conductivity on Na fractions x in the middle range (0.5 < x < 0.8) is relatively weak, introducing Co vacancies results in significant lattice thermal conductivity reduction. The material exhibits strong anisotropy of lattice thermal conductivity due to a layered crystal structure and relatively weak bonding between layers. This structure leads to the possibility of manufacturing relatively large nanosheets of Na x CoO 2. However, the weak inter-layer binding also results in the insensitivity of thermal conductivity to the nanosheet thickness.

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Planar interdigitated metal-semiconductor-metal (MSM) photodetectors have long been used in optoelectronic integrated receivers due to their ease of fabrication and their compatibility with FET and HEMT technology. Additionally, their low-capacitance structure leads to widebandwidth and high-sensitivity performance. Unfortunately, Schottky barrier heights on undoped InGaAs are rather low (0.2-0.3 eV), which lead to excessive dark currents when Schottky contacts are formed directly on this material. A thin InAlAs barrier-enhancement epi-layer interposed between the bulk active region and the surface contacts raises the effective Schottky barrier height and leads to significantly lower dark currents [I]. The effect of this intermediate layer on high-speed carrier transport, however, is of concem, and potential carrier pile-up at heterojunction interfaces may lead to degraded bandwidth performance [2,3]. In this work, we investigate the dc and transient response of quantum-well InGaAs MSM photodetectors that incorporate a thin barrier-enhancement region between the two-dimensional quantum channel and the three-dimensional metal contact. To obtain a truly one-dimensional and uniformly strong electricfield throughout the transport direction in the quantum channel, resulting in higher speed performance for this transit-time-limited device, it is desirable to contact this quantum channel laterally, and not by surface contacts separated by a barrier-enhancement epi-layer. It is proposed that this can be accomplished by forming a local barrier using ion-induced quantumwell intermixing in a thin region adjacent to the recessed trench electrodes. This thin bamerenhancement region is modeled by a quantum mechanical boundary condition in the numerical carrier transport simulations. Effects of this thin boundary on MSM photodetector dark and transient current responses are presented.The Figure 1 inset depicts the cross-sectional view of a single cell between the fingers of the trench electrodes in a quantum-well InGaAs MSM photodetector. The buried electrodes may contact multiple quantum wells or wires in a stack or array. The thickness and length of the quantum channel in this study are d = IO nm and L = 1 .O pm, respectively. The darkly shaded regions between the metal contacts and the quantum well indicate the areas of quantum well intermixing. Of the various techniques to induce this intermixing, ion implantation is the most precise in its control of layer disordering by varying the implantation ion species, dose and energy. A representative energy band diagram is shown in Figure 1 for an applied DC bias voltage of VA = IV. Emission of electrons and holes through and over the thin barrier regions near the contacts is calculated from quantum-mechanical transmission coefficients. New current density boundary conditions for electrons and holes have been formulated and self-consistently incorporated into a numerical thermionicihydrodynamic carrier transport simulator.The performance of the barrier-enhanced quantum device was c...