A tunable quantum-cascade (QC) laser has been flown on NASA's ER-2 high-altitude aircraft to produce the first atmospheric gas measurements with this newly invented device, an important milestone in the QC laser's future planetary, industrial, and commercial applications. Using a cryogenically cooled QC laser during a series of 20 aircraft flights beginning in September 1999 and extending through March 2000, we took measurements of methane (CH4) and nitrous oxide (N2O) gas up to similar to 20 km in the stratosphere over North America, Scandinavia, and Russia. The QC laser operating near an 8-mum wavelength was produced by the groups of Capasso and Cho of Bell Laboratories, Lucent Technologies, where QC lasers were invented in 1994. Compared with its companion lead salt diode lasers that were also flown on these flights, the single-made QC laser cooled to 82 K and produced higher output power (10 mW), narrower laser linewidth (17 MHz), increased measurement precision (a factor of 3), and better spectral stability (similar to0.1 cm(-1) K). The sensitivity of the QC laser channel was estimated to correspond to a minimum-detectable mixing ratio for methane of approximately 2 parts per billion by volume. (C) 2001 Optical Society of America OCIS codes: 010.0010, 120.0120, 140.0140, 300.0300.

Quantum cascade ('QC') lasers are reviewed. These are semiconductor injection lasers based on intersubband transitions in a multiple-quantum-well (QW) heterostructure, designed by means of band-structure engineering and grown by molecular beam epitaxy. The intersubband nature of the optical transition has several key advantages. First, the emission wavelength is primarily a function of the QW thickness. This characteristic allows choosing well-understood and reliable semiconductors for the generation of light in a wavelength range unrelated to the material's energy bandgap. Second, a cascade process in which multiple-often several tens of-photons are generated per electron becomes feasible, as the electron remains inside the conduction band throughout its traversal of the active region. This cascading process is behind the intrinsic high-power capabilities of the lasers. Finally, intersubband transitions are characterized through an ultrafast carrier dynamics and the absence of the linewidth enhancement factor, with both features being expected to have significant impact on laser performance. The first experimental demonstration by Faist et al in 1994 described a QC-laser emitting at 4.3 mum wavelength at cryogenic temperatures only. Since then, the lasers' performance has greatly improved, including operation spanning the mid- to far-infrared wavelength range from 3.5 to 24 mum, peak power levels in the Watt range and above-room-temperature (RT) pulsed operation for wavelengths from 4.5 to 16 mum. Three distinct designs of the active region, the so-called `vertical' and `diagonal' transition as well as the `superlattice' active regions, respectively, have emerged, and are used either with conventional dielectric or surface-plasmon waveguides. Fabricated as distributed feedback lasers they provide continuously tunable single-mode emission in the mid-infrared wavelength range. This feature together with the high optical peak power and RT operation makes QC-lasers a prime choice for narrow-band light sources in mid-infrared trace gas sensing applications. Finally, a manifestation of the high-speed capabilities can be seen in actively and passively mode-locked QC-lasers, where pulses as short as a few picoseconds with a repetition rate around 10 GHz have been measured.

A new scheme for the generation of coherent radiation on the intersubband transition without population inversion between subbands is presented. The scheme is based on the resonant nonlinear mixing of the optical laser fields on the two interband transitions that are intracavity generated in the same active region. The two-wavelength lasing on the interband transitions can be achieved at a substantially lower threshold current than gain on the intersubband transition. This may ensure cw room-temperature operation. Due to the parametric, inversionless nature of generation, the lasers proposed are especially promising for long-wave length operation above 20 mum.

Two configurations of a continuous wave quantum cascade distributed feedback laser-based gas sensor for the detection of NO at a parts per billion (ppb) concentration level, typical of biomedical applications, have been investigated. The laser was operated at liquid nitrogen temperature near lambda = 5.2 mum. In the first configuration, a 100 m optical path length multi-pass cell was Employed to enhance the NO absorption. Tn the second configuration, a technique based on cavity-enhanced spectroscopy (CES) was utilized, with an effective path length of 670 m. Both sensors enabled simultaneous analysis of NO and CO2 concentrations in exhaled air. The minimum detectable NO concentration was found to be 3 ppb with a multi-pass cell and 16 ppb when using CES. The two techniques are compared, and potential future developments are discussed.

The local temperature of quantum-cascade lasers operating in continuous wave mode is reported. This information is extracted from the thermal shift of the band-to-band photoluminescence peaks in the AlInAs and InP cladding layers of quantum-cascade laser facets using a high-resolution microprobe setup. Interpolation by means of a two-dimensional heat diffusion model allows to obtain the temperature profile and the thermal conductivity in the waveguide core. Comparison between substrate and epilayer-side mounted lasers shows the superior thermal dissipation capability of the latter, and explains their better performance with respect to threshold current and maximum operating temperature. (C) 2001 American Institute of Physics.

Surface-plasmon quantum cascade laser.The two metals grating select a singlemode by periodically modulatingthe skin depth.

Cavity ringdown spectra of ammonia at 10 parts in 10(9) by volume (ppbv) and higher concentrations were recorded by use of a 16-mW continuous-wave quantum-casacde distributed-feedback laser at 8.5 mu m whose wavelength was continuously temperature tuned over 15 nm. A sensitivity (noise-equivalent absorbance) of 3.4 x 10(-9) cm(-1) Hz(-1/2) was achieved for ammonia in nitrogen at standard temperature and pressure, which corresponds to a detection limit of 0.25 ppbv. (C) 2000 Optical Society of America.

A long wavelength (1 similar or equal to 11 mu m) quantum cascade laser based on inter-miniband transitions in semiconductor superlattices is reported. The device operates continuous wave up to a temperature of 85K, with a maximum output power of 75mW at 25K, both record values for unipolar lasers of comparable wavelength.

A variable duty cycle quasi-cw frequency scanning technique was applied to reduce thermal effects resulting from the high heat dissipation of type I quantum-cascade lasers. This technique was combined with a 100-m path-length multipass cell and a zero-air background-subtraction technique to enhance detection sensitivity to a parts-in-10(9) (ppb) concentration level for spectroscopic trace-gas detection of CH4, N2O, H2O, and C2H5OH in ambient air at 7.9 mu m. A new technique for analysis of dense high resolution absorption spectra was applied to detection of ethanol in ambient air, yielding a 125-ppb detection limit. (C) 2000 Optical Society of America OCIS codes: 140.5960, 280.3420, 300.6320, 010.1280.

The electron population in the excited miniband of quantum cascade structures with intrinsic superlattice active regions is extracted from the fine structure analysis of spontaneous interminiband electroluminescence spectra. At current densities typical of laser thresholds, the electrons injected into the excited miniband of a (GaInAs)(6 nm)/(AlInAs)(1.8 nm) superlattice are described by a nonequilibrium thermal distribution characterized by temperatures T-e> 200 K, much higher than the lattice temperature T-L=15 K. (C) 2000 American Institute of Physics. [S0003-6951(00)04534-4].

Semiconductor lasers and detectors based on intersubband electron transitions are used to generate and measure high-speed pulses of mid-infrared radiation. In particular, we use a commercial comb generator to gain-switch a state-of-the-art 8-mu m quantum cascade laser mounted in a high-speed package, The output pulses of this device are then detected with a small-area quantum-well infrared photodetector, also packaged for high-speed operation. Pulse widths shorter than 90 ps are directly measured with this system. Accounting for the finite response time of the detection electronics, a deconvolved duration of approximately 45 ps is extrapolated.

Quantum cascade (QC) lasers are a fundamentally new semiconductor laser source designed by methods of `bandstructure engineering' and realized by molecular beam epitaxy (MBE). One of their most intriguing features is the cascading scheme, which results in the lasers' intrinsic potential for high optical output power. QC-lasers with varying numbers, from one to 75, of cascaded active regions and injectors have been studied. Pulsed peak output power levers of greater than or equal to 500 mW at room temperature and greater than or equal to 1 W at 200 K have been obtained for a 2.25 mm long and approximate to 12 mu m wide Fabry-Perot laser-stripe with 75 cascades. In continuous wave operation, 200 mW have been measured from one facet at 80 K and still 60 mW at 110 K, both from lasers with 30 stages. These lasers have an InP top cladding layer grown by MBE using solid source phosphorous. Widely tunable single-mode QC-distributed feedback (DFB) lasers have been fabricated in the wavelength range around 8.5 mu m. A side-mode suppression ratio of 30 dB and a 140 nm single-mode tuning range (thermal tuning between 10 and 320 K for lasers operated in pulsed mode) have been obtained. QC-DFB lasers driven in cw-mode display a tunability of approximate to 70 nm as a result of thermal tuning between 20 and 120 K. (C) 2000 Elsevier Science S.A. All rights reserved.

High temperature pulsed operation of quantum cascade lasers is reported. At 425K and 8.4 mu m wavelength a peak output power of 17mW was measured for a laser incorporating 75 stages of alternated active regions and injectors.

An optimized design of quantum cascade lasers with electric field free undoped superlattice active regions is presented. In these structures the superlattice is engineered so that: 1) the first two extended states of the upper miniband are separated by an optical phonon to avoid phonon bottleneck effects and concentrate the injected electron density in the lower state and 2) the oscillator strength of the laser transition is maximized. The injectors' doping profile is also optimized by concentrating the doping in a single quantum well to reduce the electron density in the active material. These design changes result in major improvements of the pulse/continuous-wave performance such as a weak temperature dependence of threshold (T-0 = 167 K), high peak powers (100-200 mW at 300 K) and higher CW operating temperatures for devices emitting around at lambda similar to 8.5 mu m.

Quantum-cascade distributed-feedback lasers with high-power, continuous-wave (cw), tunable, single-mode emission are reported. The emission wavelengths are near 5.2 and 7.95 mu m. The lasers are operated at liquid-nitrogen temperature and above. A maximum output power of >100 mW is obtained per facet at 80 K for both wavelengths, which is the result of careful positioning of the peak gain with respect to the Bragg wavelength. Continuous tuning with either heat-sink temperature or cw current is demonstrated. The tuning coefficients are 0.35 nm/K (5.2 mu m) and 0.51 nm/K (7.95 mu m) for thermal tuning and vary from 20 to 40 nm/A for tuning with current. The lasers are being used in high-resolution and high-sensitivity gas-sensing applications. (C) 2000 Optical Society of America OCIS codes: 140.3070, 140.3600, 140.5960.

A technique is reported which allows the observation of intersubband spontaneous emission in unipolar quantum-cascade lasers above threshold. The technique consists of cleaving the laser stripe in the direction perpendicular to its facets. This does not negatively affect the operation of the lasers thanks to their unipolar nature. To show the potential of the method, we apply it to superlattice quantum-cascade (QC) lasers with various active region designs. We directly observe the saturation of the luminescence intensity at the laser transition, and a bottleneck effect for transitions separated from the lasing one by less than one optical phonon. This technique should help in the optimization of QC lasers. (C) 2000 American Institute of Physics. [S0003-6951(00)04949-4].

We demonstrate active mode locking of a high-speed 8 mu m quantum cascade laser in a monolithic configuration, at a repetition rate of 11.6 GHz. Evidence of mode locking is obtained from the measured optical spectra and corresponding interferograms, as well as from the power spectra of the photocurrent detected with a fast quantum-well infrared photodetector. An estimate for the pulse width of approximately 5 ps is inferred from the experimental results. Mode-locked operation is observed up to a maximum temperature of over 120 K. (C) 2000 American Institute of Physics. [S0003-6951(00)01628-4].

Recent advances and new directions in quantum cascade (QC) lasers are discussed in this paper. Invented in 1994 following many years of research on band-structure engineered semiconductors and devices grown by molecular beam epitaxy, this fundamentally new laser has rapidly advanced to a leading position among midinfrared semiconductor lasers in terms of wavelength agility as well as power and temperature performance. Because of the cascaded structure, QC lasers have a slope efficiency proportional to the number of stages. Devices with 100 stages having a record peak power of 0.6 W at room temperature are reported here. QC lasers in the AlInAs-GaInAs lattice matched to InP material system can now be designed to emit in the whole midinfrared range from 4 to 20 mum by appropriately choosing the thickness of the quantum wells in the active region. Using strained AlInAs-GaInAs, wavelengths as short as 3.4 mum have been produced, New results on QC lasers emitting at 19 mum, the longest ever realized in a III-V semiconductor laser, are reported. These devices use innovative plasmon waveguides to greatly enhance the mode confinement factor, thereby reducing the thickness of the epitaxial material. By use of a distributed feedback (DFB) geometry, QC lasers show single-mode emission with a 30-dB side-mode suppression ratio. Broad continuous single-mode tuning by either temperature or current has been demonstrated in these DFB QC lasers at wavelengths in two atmospheric windows (3-5 and 8-13 mum), with continuous-wave linewidths <1 MHz when freerunning and 10 KHz with suitable locking to the side of a molecular transition. These devices have been used in a number of chemical sensing and spectroscopic applications, demonstrating the capability of detecting parts per billion in volume of several trace gases. Sophisticated band-structure engineering has allowed the design and demonstration of bidirectional lasers, These devices emit different wavelengths for opposite bias polarities. The last section of the paper deals,vith the high-speed operation of QC lasers, Gain switching with pulse widths similar to 50 ps and active modelocking with a few picosecond-long pulses have been demonstrated. Finally a new type of passive modelocking has been demonstrated in QC lasers, which relies on the giant and ultrafast optical Kerr effect of intersubband transitions.

We have demonstrated quantitative chemical vapor detection with a multimode quantum cascade (QC) laser. Experiments incorporated pseudorandom code (PRC) modulation of the laser intensity to permit sensitive absorption measurements of isopropanol vapor at 8.0 pm. The demonstration shows the practicality of one technical approach for implementing low-peak-power QC lasers in the transmitter portion of a differential absorption lidar (DIAL) system. With a 31-chip, 300-ns/chip PRC sequence, the measured isopropanol detection limit was 12 parts in 10(6) by volume times meters (similar to 3 X 10(-3) absorption) for a simple backscatter-absorption measurement configuration. (C) 2000 Optical Society of America OCIS codes: 280.1910, 140.3070.