The terahertz spectral range (lambda = 30-300 mu m) has long been devoid of compact, electrically pumped, room-temperature semiconductor sources(1-4). Despite recent progress with terahertz quantum cascade lasers(2-4), existing devices still require cryogenic cooling. An alternative way to produce terahertz radiation is frequency down-conversion in a nonlinear optical crystal using infrared or visible pump lasers(5-7). This approach offers broad spectral tunability and does work at room temperature; however, it requires powerful laser pumps and a more complicated optical set-up, resulting in bulky and unwieldy sources. Here we demonstrate a monolithically integrated device designed to combine the advantages of electrically pumped semiconductor lasers and nonlinear optical sources. Our device is a dual-wavelength quantum cascade laser(8) with the active region engineered to possess giant second-order nonlinear susceptibility associated with intersubband transitions in coupled quantum wells. The laser operates at lambda(1) = 7.6 mu m and lambda(2) = 8.7 mu m, and produces terahertz output at lambda = 60 mu m through intracavity difference-frequency generation.

A time-resolved mid-infrared upconversion technique based on sum-frequency generation was applied to measure pulse propagation in lambda similar to 5.0 mu m quantum cascade lasers operated in continuous wave at 30 K. The wavelength-dependent propagation delay of femtosecond mid-infrared pulses was measured to determine the total group-velocity dispersion. The material and waveguide dispersion were calculated and their contributions to the total group-velocity dispersion were found to be relatively small and constant. The small-signal gain dispersion was estimated from a measurement of the electroluminescence spectrum without a laser cavity, and was found to be the largest component of the total GVD. A negative group-velocity dispersion of beta(2) ( = d(2)beta/d omega(2)) approximately -4.6 x 10(-6) ps(2)/mu m was observed at the peak emission wavelength, and good agreement was found for the measured and calculated pulse-broadening. (c) 2007 Optical Society of America.

We present a systematic study of the current - voltage characteristics and electroluminescence of gallium nitride ( GaN) nanowire on silicon ( Si) substrate heterostructures where both semiconductors are n- type. A novel feature of this device is that by reversing the polarity of the applied voltage the luminescence can be selectively obtained from either the nanowire or the substrate. For one polarity of the applied voltage, ultraviolet ( and visible) light is generated in the GaN nanowire, while for the opposite polarity infrared light is emitted from the Si substrate. We propose a model, which explains the key features of the data, based on electron tunnelling from the valence band of one semiconductor into the conduction band of the other semiconductor. For example, for one polarity of the applied voltage, given a sufficient potential energy difference between the two semiconductors, electrons can tunnel from the valence band of GaN into the Si conduction band. This process results in the creation of holes in GaN, which can recombine with conduction band electrons generating GaN band-to-band luminescence. A similar process applies under the opposite polarity for Si light emission. This device structure affords an additional experimental handle to the study of electroluminescence in single nanowires and, furthermore, could be used as a novel approach to two- colour light-emitting devices.

We demonstrate a compact, single-mode quantum cascade laser source continuously tunable between 8.7 and 9.4 mu m. The source consists of an array of single-mode distributed feedback quantum cascade lasers with closely spaced emission wavelengths fabricated monolithically on a single chip and driven by a microelectronic controller. Our source is suitable for a variety of chemical sensing applications. Here, we use it to perform absorption spectroscopy of fluids.