Novel pulsed and cw quantum cascade distributed feedback (QC-DFB) lasers operating near lambda = 8 mum were used for detection and quantification of trace gases in ambient air by means of sensitive absorption spectroscopy. N2O, (CH4)-C-12, (CH4)-C-13, and different isotopic species of H2O were detected. Also, a highly selective detection of ethanol vapor in air with a sensitivity of 125 parts per billion by volume (ppb) was demonstrated.

Distributed-feedback quantum-cascade (QC) lasers are expected to form the heart of the next-generation mid-IR laser absorption spectrometers, especially as they are applied to measurements of trace gases in a variety of environments. The incorporation of room-temperature-operable, single-mode QC lasers should result in highly compact and rugged sensors for real-world applications. We report preliminary results on the performance of a laser absorption spectrometer that uses a QC laser operating at room temperature in a quasi-cw mode in conjunction with balanced ratiometric detection. We have demonstrated sensitivities for N2O [10 parts in 10(6) volume-mixing ratio for a 1-m path (ppmv-m)] and NO [520 parts in 10(9) volume-mixing ratio for a 1-m path (ppbv-m)] at 5.4 mum System improvements are described that are expected to result in a 2 orders of magnitude increase in sensitivity. (C) 2001 Optical Society of America OCIS codes: 010.1120, 120.1740, 140.3070, 300.1030, 300.6260, 140.5960.

A spectroscopic gas sensor for nitric oxide (NO) detection based on a cavity ringdown technique was designed and evaluated. A cw quantum-cascade distributed-feedback laser operating at 5.2 mum was used as a tunable single-frequency light source. Both laser-frequency tuning and abrupt interruptions of the laser radiation were performed through manipulation of the laser current. A single ringdown event sensitivity to absorption of 2.2 x 10(-8) cm(-1) was achieved. Measurements of parts per billion (ppb) NO concentrations in N-2 with a 0.7-ppb standard error for a data collection time of 8 s have been performed. Future improvements are discussed that would allow quantification of NO in human breath. (C) 2001 Optical Society of America.

Continuous wave laser action has been achieved in a superlattice quantum cascade device operating on surface plasmon waveguide modes. The emission wavelength lambda - 19 mum is by far the longest ever reported for continuous wave III-V semiconductor lasers. The output power at cryogenic temperature is of the order of the mW.

Quantum-cascade lasers operating above 20 mum (at lambda similar to 21.5 mum and lambda similar to 24 mum) wavelength are reported. Pulsed operation was obtained up to 140 K and with a peak power of a few milliwatts at cryogenic temperatures. Laser action originates from interminiband transitions in ``chirped'' superlattice active regions. The waveguides are based on surface-plasmon modes confined at a metal-semiconductor interface. The wavelengths were chosen in order to avoid major phonon absorption bands, which are particularly strong at energies just above the reststrahlen band. We also report on a 21.5-mum-wavelength laser based on a two-sided interface-plasmon waveguide. (C) 2001 American Institute of Physics.

High duty cycle operation of quantum cascade superlattice lasers with graded superlattice active regions is investigated with the goal of achieving high average optical power. The optical output power increases with pulse width and decreases with heat sink temperature. This behavior is explained on the basis of the laser core temperature oscillations during the pulsed, high duty cycle operation. Between 175 and 325 K heat sink temperature, optimum duty cycles vary from 10% to 1% and average power levels vary from 50 to 1 mW for various lasers used in this study. (C) 2001 American Institute of Physics.

The improved optical power performance of superlattice quantum cascade lasers with a novel injector design allow ing the tunnelling of electrons into high energy states of the excited miniband is reported. At 8.4 mum and temperatures less than or equal to 120K, peak powers > 2W per facet are measured, corresponding to a record power of 88mW/stage. A slope efficiency of 160mW/A over a current range six times larger than the laser threshold is observed.

Quantum cascade (QC) lasers, based on intersubband transitions in semiconductor quantum wells, are characterized by ultrafast (picosecond) carrier lifetimes. An important consequence of this unique property is the expected absence of relaxation oscillations in the transient response of these devices. Here, we discuss and experimentally verify this prediction by measuring the modulation response of several 8-mum-QC lasers, properly processed and packaged for high-speed operation, up to 10 GHz. (C) 2001 American Institute of Physics.

High-speed mid-infrared quantum cascade lasers with direct modulation bandwidth of approximately 7GHz have been developed. Error-free digital data transmission at 2.5Gbit/s is demonstrated with devices emitting at 8 mum and operating at temperatures up to 85 K.

The high-speed direct modulation response of mid-infrared quantum cascade (QC) lasers is investigated up ro a frequency of 2GHz, showing high-frequency data transmission capabilities. The application uf QC lasers to optical wireless communications is discussed and demonstrated in a free-space television link over a distance of 70m.

A scheme for infrared generation without population inversion between subbands in quantum-well and quantum-dot lasers is presented. The scheme is based on the resonant nonlinear mixing of the optical laser fields on the two interband transitions that are generated in the same active region and that serve as the coherent drive for the infrared field. This mechanism for frequency down-conversion does not rely upon any ad hoc assumptions of long-lived coherences in the semiconductor active medium, and it should work efficiently at room temperature with injection current pumping. For optimized waveguide and cavity parameters, the intrinsic efficiency of the down-conversion process can reach the limiting quantum value corresponding to one infrared photon per one optical photon. Due to the parametric nature of infrared generation, the proposed inversionless scheme is especially promising for long-wavelength (far-infrared) operation.

An ``injectorless'' quantum-cascade (QC) laser is presented. The requirement of using injector regions to transport electrons from the lower laser level and other low-lying energy levels of one active region to the upper laser level of the next electron-downstream active region was eliminated by using an appropriately designed double-quantum-well ``chirped'' superlattice active region. The major advantage of the ``injectorless'' QC laser is the close packing of the active regions and the concomitant large optical confinement factor. Using a cascade of 75 consecutive active regions, designed for emission at lambda = 11.5 mum, a pulsed peak output power of 270 mW is achieved at 7 K and approximately 10 mW at the maximum operating temperature of 195 K. (C) 2001 American Institute of Physics.

We propose and study resonant ladder schemes of inversionless lasing on a fast-decaying transition when the population inversion is not possible or Very difficult to achieve. The driving field on a neighboring, slowly decaying transition is not imposed externally, but is self generated in the same active medium. Inversionless lasing in such schemes avoids the problem of driving-field absorption and leads to considerable reduction in the required pumping rate. Applications of such schemes include inversionless ultraviolet and soft x-ray lasing in gas lasers and inversionless generation of coherent mid/far-infrared emission on intersubband (interlevel) transitions in multiple quantum-well or quantum-dot laser diodes.

We report line width measurements of a quantum cascade distributed feedback laser by a heterodyne experiment. At currents slightly above threshold and a laser output power higher than I mW, the full width at half maximum of the beat signal was narrower than 0.5 MHz, which gives us an upper limit of the laser line width. As reference laser we used a carbon monoxide laser. Both lasers were operating unstabilized in continuous wave mode emitting light at about 5.2 mum. (C) 2001 Elsevier Science B.V. All rights reserved.

The Casimir force between uncharged metallic surfaces originates from quantum-mechanical zero-point fluctuations of the electromagnetic field. We demonstrate that this quantum electrodynamical effect has a profound influence on the oscillatory behavior of microstructures when surfaces are in close proximity (less than or equal to 100 nm). Frequency shifts, hysteretic behavior, and bistability caused by the Casimir force are observed in the frequency response of a periodically driven micromachined torsional oscillator.

There is presently a lack of efficient, compact, solid-state sources for the spectral range 1-10 THz, also known as the ``terahertz gap''. In fact Gunn diodes fail at such high frequencies, while, from the other side, conventional semiconductor lasers are limited to the mid-infrared. Intersubband or interminiband transitions, which constitute the basis of the very successful quantum cascade (QC) lasers, possess the potential for the efficient generation of far-infrared light, although many important physical questions have to be addressed in the case of THz transition frequencies. Furthermore, the problem of confining long wavelength radiation inside waveguides with thickness compatible with existing growing techniques, minimizing at the same time the absorption losses, poses an interesting technological challenge. Recent significant progresses in this direction are presented here. Surface plasmon modes at the interface between a metal and the semiconductor are exploited in the design of a high performance lambda similar to 17 mum (17.6 THz) superlattice QC laser. Thanks to adoption of this novel waveguide the total epitaxial thickness of the structure is reduced by a factor of 2 with respect to a conventional waveguide with semiconductor claddings. The emission is made single mode with the adoption of a dual-metal Bragg grating which modulates the skin depth of the surface plasmon. The same approach is used in the realization of a lambda similar to 19 mum (15.8 THz) QC laser, which represents the longest wavelength III-V semiconductor laser to date. (C) 2001 Elsevier Science B.V. All rights reserved.

The recent expansion of quantum cascade lasers to the far-infared wavelength range and short-pulsed operating further broadens the lasers' scope in trace-gas sensing applications.

A quantum cascade (QC) laser with a heterogeneous cascade containing two substacks previously optimized to emit at 5.2 mum and 8.0 mum wavelengths, respectively, is presented. The low-temperature performance of the two-wavelength laser is comparable to the respective homogeneous stack lasers, indicating no penalty from the heterogeneity of the cascade. Each substack is apportioned the optimum fraction of the applied bias. This demonstrates the general applicability of this scheme. In addition, an etch-stop layer inserted between the two substacks allowed fabrication of a ``tap'' into the cascade. The latter was used to selectively manipulate the laser threshold of one substack, turning the 8.0 mum laser on and off while the adjacent 5.2 mum QC laser was operating undisturbed. (C) 2001 American Institute of Physics.

A quantum-cascade laser using a double-quantum-well graded superlattice as the active region is presented. Each SL period consists of two strongly coupled quantum wells resulting in the splitting of the lowest miniband into two minibands, These two minibands can be designed to be pat and to contain delocalized, spatially symmetric wavefunctions under an applied electric field which in turn leads to a high optical dipole for the interminiband transition. In addition, the new design allows independent control of the energy levels of the lowest two minibands, their width and the splitting separating them, enhancing design flexibility. Using a cascade design of 55 pairs of alternated active regions and injectors, pulsed laser action is achieved at lambda = 11.6 mum, Peak output powers reach 120 mW at 7 K and approximately 12 mW at the maximum operating temperature of 195 K.

Quantum cascade lasers have been fabricated with chalcogenide lateral waveguide claddings. Several-micrometer-thick Ge0.25Se0.76 glass has been deposited by pulsed laser ablation onto the sidewalls of narrow, deep etched laser ridges. Due to the intrinsically low mid-infrared attenuation of the chalcogenide material, the waveguide loss of the lasers is significantly reduced, by up to similar to 50%, depending on the ridge width, when compared to conventional ridge waveguides, This resulted in an improved overall laser performance, such as a reduction of the threshold current density, an increase in slope efficiency, and an improved temperature performance.