B. P. Nelson

We have used infrared transmission and nuclear-reaction analysis to determine the ir absorption strength of the Si-H wagging and stretching modes in hydrogenated amorphous silicon (a-Si:H). The films were deposited by plasma-assisted chemical vapor deposition and reactive magnetron sputtering. We show that the widely used ir-data-analysis method of Brodsky, Cardona, and Cuomo can lead to significant errors in determining the absorption coefficients, particularly for films less than \ensuremath{\sim}1 \ensuremath{\mu}m thick. To eliminate these errors we explicitly take into account the effects of optical interference to analyze our data. We show that the hydrogen content can be determined from the stretching modes at \ensuremath{\omega}=2000 and 2100 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ as well as the wagging mode at \ensuremath{\omega}=640 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. By assigning different oscillator strengths to the 2000- and 2100-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ modes, we show that the absorption strength of the stretching modes does not depend on the details of sample preparation, contrary to hypotheses previously invoked to explain experimental data. We obtain ${\mathit{A}}_{640}$=(2.1\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}${10}^{19}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$, ${\mathit{A}}_{2000}$=(9.0\ifmmode\pm\else\textpm\fi{}1.0)\ifmmode\times\else\texttimes\fi{}${10}^{19}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$, and ${\mathit{A}}_{2100}$=(2.2\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}${10}^{20}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ for the proportionality constants between the hydrogen concentration and the integrated absorbance of the wagging and stretching modes. The value of ${\mathit{A}}_{640}$ is \ensuremath{\sim}30% larger than the generally used value. We show that previously published data for both the wagging and stretching modes are consistent with the proportionality factors determined in the present study.

Device-quality hydrogenated amorphous silicon containing as little as 1/10 the bonded H observed in device-quality glow discharge films have been deposited by thermal decomposition of silane on a heated filament. These low H content films show an Urbach edge width of 50 mV and a spin density of ∼1/100 as large as that of glow discharge films containing comparable amounts of H. High substrate temperatures, deposition in a high flux of atomic H, and lack of energetic particle bombardment are suggested as reasons for this behavior.

We describe the two-wavelength operation of the nonlinear fiber loop mirror. In this mode of operation a high-power signal at one wavelength switches a low-power signal at another wavelength. This device is investigated both theoretically and experimentally. The experimental results show that the nonlinear loop mirror performs as an optical modulator that consists of all-fiber components.

The all fibre nonlinear optical loop mirror (NOLM) is used in the two wavelength version to show good contrast stable switching of selected pulses from a stream of short pulses. The polarisation properties of the device enable operation as a genuine four port demultiplexer.

We demonstrate that solitons are not essential for the operation of the soliton laser. The external cavity employed contains an optical fiber with negative group-delay dispersion and therefore does not support bright solitons. Thus the improved mode locking cannot be attributed to the injection of an N = 2 soliton.