Underwater Optical Wireless Communication (UOWC) is not a new idea, but it has recently attracted renewed interest since seawater presents a reduced absorption window for blue-green light. Due to its higher bandwidth, underwater optical wireless communications can support higher data rates at low latency levels compared to acoustic and RF counterparts. The paper is aimed at those who want to undertake studies on UOWC. It offers an overview on the current technologies and those potentially available soon. Particular attention has been given to offering a recent bibliography, especially on the use of single-photon receivers.

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We have studied the adsorption of hydrogen at room temperature and liquid-nitrogen temperature on a Ge(100) single crystal. As in the Si case, two temperature-dependent phases are observed: The monohydride and dihydride phases which show a (2×1) and (1×1) low-energy electron diffraction (LEED) pattern, respectively. The monohydride-phase saturation is observed at about 500 L (1 L≡${10}^{\mathrm{-}6}$ Torr?c) of hydrogen exposure at room temperature; after that value the 105-meV loss, typical of the dihydride-phase presence, appears but the full (1×1) LEED pattern is obtained only after about 4000-L exposure. The width of the specularly reflected elastic peak grows with increasing hydrogen exposure up to a maximum value at 500 L, indicating that the monohydride phase on Ge(100) has a lower surface resistivity with respect to clean or dihydride-covered Ge. Hydrogen exposure at 100 K shows a new peak at 195 meV which is not completely due to water contamination; the origin of this peak is not clear at present.

In this paper we describe a superposition model for Bessel-Gauss beams, in which higher orders are included. An analogous model leads to a different class of beams, namely the modified Bessel-Gauss beams. Then a generalized set of beams, containing the previous beams as particular cases, is introduced. The behaviour of these beams upon propagation is investigated, both analytically and numerically.