An experimental study has been made of photovoltaic effects which occur at semiconductor-electrolyte interfaces. Single crystal specimens of CdS and several other compounds were used. It was found that in a number of cases the photovoltaic effect results from a chemical reaction of the electrode materials. In such cases the observations may be explained by a simple mechanism which relates the sign of the photo-emf to the conductivity type of the semiconductor and to the chemical reaction which is occurring. The reaction may be predicted using readily available thermodynamic data. A different process occurs when the electrode material is GaAs. It acts as an inert electrode which exchanges electrons with an oxidation-reduction couple in the solution.

Following an introduction to the history of the invention of the quantum cascade (QC) laser and of the band-structure engineering advances that have led to laser action over most of the mid-infrared (IR) and part of the far-IR spectrum, the paper provides a comprehensive review of recent developments that will likely enable important advances in areas such as optical communications, ultrahigh resolution spectroscopy and applications to ultrahigh sensitivity gas-sensing systems. We discuss the experimental observation of the remarkably different frequency response of QC lasers compared to diode lasers, i.e., the absence of relaxation oscillations, their high-speed digital modulation, and results on mid-IR optical wireless communication links, which demonstrate the possibility of reliably transmitting complex multimedia data streams. Ultrashort pulse generation by gain switching and active and passive modelocking is subsequently discussed. Recent data on the linewidth of free-running QC lasers ( 150 kHz) and their frequency stabilization down to 10 kHz are presented. Experiments on the relative frequency stability ( 5 Hz) of two QC lasers locked to optical cavities are discussed. Finally, developments in metallic waveguides with surface plasmon modes, which have enabled extension of the operating wavelength to the far IR are reported. I N THIS paper, we concentrate on reviewing recent developments in quantum cascade (QC) laser research in the areas of high-speed modulation, optical wireless, ultrashort pulse and Manuscript A. Michael Sergent has been with Bell Laboratories, Murray Hill, NJ, since July 1960. He has been in the semiconductor research area since the latter part of 1967, working on the luminescence properties of CdS and ZnSe materials systems and performing C-V, C-T, and deep-level transient spectroscopy measurements on GaAs. Since the early 1990s, he has been involved in semiconductor laser research, working on the electroabsorption modulated laser and most recently with the quantum-cascade laser. Most of his work in this endeavor revolves around the cleaving, mounting, and packaging of the devices.

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We have investigated an energy-storage cycle in which CO2 is electrochemically reduced to formic acid, HCOOH. The product can be used in several ways. By means of a catalyst, it can be converted to hydrgen for use as a fuel or raw material. We have obtained data on the efficiency of the process and analyzed the energetics.

To investigate how surface oxygen participates in the reaction of important aromatic oxygenates, the surface chemistry of benzyl alcohol (PhCH2OH) and benzaldehyde (PhCHO) has been studied on oxygen-precovered Pd(111) (O/Pd(111)) using temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). On both Pd(111) and O/Pd(111), TPD using isotopically labeled benzyl alcohol and low-temperature HREEL spectra show that the oxidation of benzyl alcohol proceeds through a benzyl alkoxide (PhCH2O-) intermediate to adsorbed benzaldehyde so that the sequence of bond scission is O-H followed by C(α)-H. In the presence of surface O, some benzaldehyde desorbs from the surface below 300 K, consistent with the presence of a weakly adsorbed η(1) aldehyde state that is bound to the surface through its oxygen lone pair. Benzaldehyde also reacts with surface oxygen to produce benzoate (PhCOO-). Shifts in the OCO stretching frequency suggest that the benzoate orientation changes as the surface becomes less crowded, consistent with a strong interaction between the phenyl group and the surface. Adsorbed benzaldehyde and benzoate undergo decomposition to CO and CO2, respectively, as well as benzene. Deoxygenation of benzyl alcohol to toluene occurs at high coverages of benzyl alcohol when the relative surface O coverage is low. Experiments conducted on (18)O/Pd(111) reveal exchange occurring between surface O and the benzaldehyde and benzoate intermediates. This exchange has not been reported for other alcohols, suggesting that aromatic binding effects strongly influence alcohol oxidation on Pd.