We introduce the concept of a photovoltaic band gap Epvg for amorphous solar cells. This is the minimum photon energy thermodynamically required for the generation, of two free carriers in an operating solar cell. For hydrogenated amorphous silicon the photovoltaic band gap is 1.57 eV at 1-sun illumination.

Summary form only given.Optical single sideband modulation has received great interest as a way to reduce the dispersion penalty of analog fiber-optic systems. We propose and demonstrate a tandem single sideband fiber-optic link, which uses a dual-electrode Mach-Zehnder modulator to place different information in each of the two sidebands, thus doubling the bandwidth efficiency.

We demonstrate a technique that coherently frequency shifts a direct sequence encoded optical channel by mixing it with an identically encoded RF local oscillator in a dual-electrode Mach Zehnder modulator (DE-MZM). The spectral separation would reduce the effects of cumulative shot noise and speckle, and full code orthogonality is possible since the bipolar nature of the optical field is recognized by the frequency shifting process.

Three independent methods were used to evaluate transpiration of a boreal forest: the branch bag, sap flow and eddy covariance methods. The branch bag method encloses several thousand needles and gives a continuous record of branch transpiration. The sap flow method provides a continuous record of sap velocity and an estimate of tree transpiration. The eddy covariance method typically measures evaporation rates between a forest and the atmosphere. We deployed an extra eddy covariance system below the forest to estimate canopy transpiration by difference. The three systems detected small water vapor fluxes despite a plentiful supply of energy to drive evaporation. We also observed that transpiration rates were low even when the soil was well supplied with water. Low rates of transpiration were attributed to the canopy's low leaf area index and the marked reduction in stomatal conductance as vapor pressure deficits increased. Water vapor fluxes, derived from the sap flow method, lagged behind those derived by the branch bag method by 1 to 2 h. The sap flow method also suffered from sampling errors caused by the non-uniformity of flow across the sapwood and the spatial variability of sapwood cross section throughout the forest. Despite technical difficulties associated with hourly measurements, daily totals of transpiration agreed well with values derived from micrometeorological systems.

Using a dual-electrode Mach-Zehnder modulator, a 'tandem' single sideband modulator has been constructed that doubles the spectral efficiency of a system by enabling the transmission of different data streams in the upper and lower sidebands of the same optical carrier.

We have achieved long-term stability in producing high-power single-axial-mode CO2 laser pulses with a hybrid oscillator (TEA laser + low-pressure section) by using a novel digital feedback system that electronically adjusts cavity length.

We present a systematic study of the infrared multiphoton excitation and dissociation of several polyatomic molecules. The molecules range in size from SO2, with three vibrational modes, to C3F7I, with 27. A gradual transition occurs from intensity dependent excitation, characteristic of a sparse density of vibrational states in small molecules, to a fluence dominated absorption characteristic of a quasicontinuum of vibrational states in large, heavy molecules. Molecules with ten or more atoms showed no intensity dependence under pulsed CO2 laser excitation at a fixed energy fluence. For a molecule to have a high probability of dissociation under realizable conditions of laser intensity and fluence, quasicontinuum region excitation must dominate. All molecules appeared to have statistically coupled vibrational modes at high levels of vibrational energy.

Ultrathin metallic films have an interesting electromagnetic behavior as the frequency of the incident field is varied over several orders of magnitude, because of the dramatic dispersion exhibited by the metal permittivity. We study a finite multilayer of periodically placed planar conducting films for frequencies varying from the dc limit to the far ultraviolet. We provide the optimized reflectivity and transmittivity of the system for the various frequency regimes involved. Further, we produce the dispersion diagrams of the corresponding photonic bandgap structures, which clearly show the transition of the system from a metallic (low frequencies) to a dielectric (optical frequencies) behavior. In addition, simple design formulas for maximum reflectivity of finite film number N are presented in terms of film thickness and film spacing in each of the representative frequency ranges.

Summary form only given. Two-photon exposure can be advantageous for the improving of spatial resolution of an image. Two-photon exposure uses the square of the pulse intensity profile, resulting in decrease of the spot size. More important, it has been shown recently, that multi-frequency beams can interfere with each other to produce a doubling of spatial resolution (although at 2:1 pedestal) if the media exposure is due to two-photon. We performed our experiments using Kodak commercial film.

A high-impedance electromagnetic surface is a new type of metallic structure exhibiting high surface impedance and the suppression of propagating surface currents at a particular frequency band. We experimentally characterize such a high-impedance surface designed near 2.4 GHz. We describe an antenna built on such a surface, integrated into a printed circuit board that was designed for the form factor of a portable handset. Measurement shows high radiation efficiency near 2.4 GHz.

Nonmethane hydrocarbons are ubiquitous trace atmospheric constituents yet they control the oxidation capacity of the atmosphere. Both anthropogenic and biogenic processes contribute to the release of hydrocarbons to the atmosphere. In this manuscript, the state of the science concerning biosynthesis, transport, and chemical transformation of hydrocarbons emitted by the terrestrial biosphere is reviewed. In particular, the focus is on isoprene, monoterpenes, and oxygenated hydrocarbons. The generated science during the last 10 years is reviewed to explain and quantify hydrocarbon emissions from vegetation and to discern impacts of biogenic hydrocarbons on local and regional atmospheric chemistry. Furthermore, the physiological and environmental processes controlling biosynthesis and production of hydrocarbon compounds are reported on. Many advances have been made on measurement and modeling approaches developed to quantify hydrocarbon emissions from leaves and forest ecosystems. A synthesis of the atmospheric chemistry of biogenic hydrocarbons and their role in the formation of oxidants and aerosols is presented. The integration of biogenic hydrocarbon kinetics and atmospheric physics into mathematical modeling systems is examined to assess the contribution of biogenic hydrocarbons to the formation of oxidants and aerosols, thereby allowing us to study their impacts on the earth's climate system and to develop strategies to reduce oxidant precursors in affected regions.