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The output spectrum of both gas and semiconductor lasers usually contains more than one frequency. Multimode operation in gas versus semiconductor lasers arises from different physics. In gas lasers, slow equilibration of the electron populations at different energies makes each frequency an independent single-mode laser. The slow electron diffusion in semiconductor lasers, combined with the spatially varying optical intensity patterns of the modes, makes each region of space an independent single-mode laser. We develop a rate equation model for the photon number in each mode which captures all these effects. Plotting the photon number versus pumping rate for the competing modes, in both subthreshold and above threshold operation, illustrates the changes in the laser output spectrum due to either slow equilibration or slow diffusion of electrons.

In a point contact NS junction, perfect Andreev reflection occurs over a range of voltages equal to the superconducting energy gap, producing an excess current of $I_{exc} = (4/3)(2e\Delta/h)$. If the superconductor has a finite width, rather than the infinite width of the point contact, one cannot neglect superfluid flow inside the superconducting contact. The energy range available for perfect Andreev reflections then becomes larger than the superconducting gap, since superfluid flow alters the dispersion relation inside the finite width superconductor. We find a maximum excess current of approximately $(7/3)(2e\Delta/h)$ when the width of the superconductor is approximately 7/3 times the width of the normal metal.

Control of the Josephson current by varying a gate current has recently been demonstrated in both 4-terminal and 3-terminal junctions. We show that, when the the gates are weakly coupled to the Josephson junction, the Josephson current versus gate current (or versus gate voltage) relation is the same for both the 4- and 3- terminal geometries. At low temperature, the supercurrent switches abruptly as a function of the gate voltage, but only slowly as a function of the gate current., Comment: 5 pages, 5 figures

We calculate the electrical currents through a superconductor - insulator - superconductor junction which is also weakly coupled to a normal metal side probe. The voltage $V$ applied to the normal metal terminal controls the occupation of Andreev energy levels $E_n$, and therefore controls the Josephson current flowing through these levels. Whenever the probe voltage crosses an Andreev level, the Josephson current changes abruptly by an amount equal to the current flowing through the Andreev level. The differential conductance along the normal metal terminal permits spectroscopy of the Andreev levels. In a short junction $(L \ll \xi_0)$, the critical current switches abruptly from the Ambegaokar-Baratoff value to zero when the probe voltage is approximately equal to the superconducting energy gap ($|eV| \simeq \Delta$). The magnitude of the Josephson current switching in a long junction $(L \gg \xi_0)$, and the range of probe voltages over which the Josephson current differs from its equilibrium value, are much smaller than for three-terminal ballistic superconductor - normal metal - superconductor junctions., Comment: 17 pages, 19 figures (RevTeX)