Continuous wave operation of quantum cascade lasers is reported up to a temperature of 312 kelvin. The devices were fabricated as buried heterostructure lasers with high-reflection coatings on both laser facets, resulting in continuous wave operation with optical output power ranging from 17 milliwatts at 292 kelvin to 3 milliwatts at 312 kelvin, at an emission wavelength of 9.1 micrometers. The results demonstrate the potential of quantum cascade lasers as continuous wave mid-infrared light sources for high-resolution spectroscopy, chemical sensing applications, and free-space optical communication systems.The mid-infrared portion of the spectrum, covering approximately the wavelength range from 3 to 12 m, is sometimes referred to as "underdeveloped" because of its lack of convenient coherent optical sources. Especially when compared to the visible or near-infrared spectral range, where interband semiconductor lasers are now produced very economically with continuous wave (CW) output power of tens of milliwatts, this assertion holds true. In the mid-infrared, a new class of semiconductor lasers-intersubband quantum cascade (QC) lasers (1)-has become a promising alternative to interband diode lasers (2, 3) in the past 7 years. In these devices, photon emission is obtained by electrons making optical transitions between confined energy lev-

Self-imaging properties of generalized N × N multimode interference couplers are derived. Positions, amplitudes, and phases of the self-images are directly related to the lengths and widths of the coupler by solving the eigenmode superposition equation analytically for any arbitrary length. Devices of length (M/N) 3L(c), where M is the multiple occurrence of the N self-images, are analyzed in detail. The general formalism is applied to practical N × N couplers used in integrated optics, and simple phase relations are obtained.

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We report on imaging of three-dimensional precessional orbits of the magnetization vector in a magnetic field by means of a time-resolved vectorial Kerr experiment that measures all three components of the magnetization vector with picosecond resolution. Images of the precessional mode taken with submicrometer spatial resolution reveal that the dynamical excitation in this time regime roughly mirrors the symmetry of the underlying equilibrium spin configuration and that its propagation has a non-wavelike character. These results should form the basis for realistic models of the magnetization dynamics in a largely unexplored but technologically increasingly relevant time scale.

We have investigated the oxidation kinetics of TiN thin films in dry O2 in view of a possible application of TiN as material for gate electrodes and interconnections in large-scale integrated circuits. We found that in the temperature range of 500 to 650 °C the oxidation is thermally activated with an activation energy of 2.05±0.05 eV. Thereby the diffusion of oxygen through the oxide is the rate-limiting process. Analysis with x rays indicates that dry oxidation transforms TiN to the rutile form of TiO2. Films of TiO2 formed in such a manner are found to be semi-insulating with resistivities in the order of 106 Ω cm and at higher applied electric fields the injection of space-charge–limited currents is observed.