We predict and study the effect of parametric self-induced excitation of a molecule moving above the dielectric or conducting medium with periodic grating. In this case the radiation reaction force modulates the molecular transition frequency which results in a parametric instability of dipole oscillations even from the level of quantum or thermal fluctuations. The present mechanism of instability of electrically neutral molecules is different from that of the well-known Smith-Purcell and transition radiation in which a moving charge and its oscillating image create an oscillating dipoles We show that parametrically excited molecular bunches can produce an easily detectable coherent radiation flux of up to a microwatt.

We report the first application of pulsed, near-room-temperature quantum cascade laser technology to the continuous detection of biogenic CO production rates above viable cultures of vascular smooth muscle cells. A computer-controlled sequence of measurements over a 9-h period was obtained, resulting in a minimum detectable CO production of 20 ppb in a 1-m optical path above a standard cell-culture flask. Data-processing procedures for real-time monitoring of both biogenic and ambient atmospheric CO concentrations are described.

Quantum cascade lasers operating at lambda > 20 mum wavelength are reported. Pulsed operation as obtained up to 140 K with a peak power of 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, but we also report on a 21.5 mum wavelength laser based on a double-sided interface-plasmon waveguide. This latter, contrary to the single-sided surface plasmon, is a viable waveguiding solution in the THz range, i.e. at wavelengths between 60 and 100 mum. Finally, intersubband electroluminescence is reported in a quantum cascade structure based on asymmetric superlattice active regions and designed for emission in the THz range at lambda approximate to 80 mum. (C) 2002 Published by Elsevier Science B.V.

The intrinsic frequency fluctuations of two single-mode quantum cascade (QC) distributed-feedback lasers operating continuously at a wavelength of 8.5 mum are reported. A Doppler-limited rovibrational resonance of nitrous oxide is used to transform the frequency noise into measurable intensity fluctuations. The QC lasers, along with recently improved current controllers, exhibit a free-running frequency stability of 150 kHz over a 15-ms time interval. (C) 2002 Optical Society of America.

Experimental results for an optical free-space high-speed link using direct modulated mid-infrared (lambda = 8.1 mum) quantum cascade lasers are presented, A total of 800 digitally encoded multimedia channels were transmitted. The reliability of the system against weather influence (fog) was experimentally compared to that of a near-infrared (lambda=0.85 mum) link.

We report a heterodyne beat with a linewidth of 5.6 +/- 0.6 Hz between two cavity-stabilized quantum-cascade lasers operating at 8.5 mum. We also present a technique for measuring this beat that avoids the need for extreme isolation of the optical cavities from the environment, that of employing a third servo loop with low bandwidth to force one cavity to track the slow drifts and low-frequency fluctuations of the other. Although it is not fully independent, this technique greatly facilitates heterodyne beat measurements for evaluating the performance of cavity-locked lasers above the bandwidth of the third loop. (C) 2002 Optical Society of America.

We demonstrate the generation of cw tunable far-infrared radiation by mixing a quantum cascade laser and a CO2 laser in a W-Ni metal-insulator-metal diode. The first known spectroscopic application to the recording of an H Br-79 transition near 4.47 THz is reported. (C) 2002 Optical Society of America.

Measurements of the optical spectra of semiconductor injection lasers with deformed cylinder resonators show strong indications of the classical Kolmogorov-Arnold-Moser transition from integrability to chaos for devices with small deformation. At larger deformation, evidence for laser action on scar modes is obtained. The diode lasers operate with TE polarization, resulting in laser action on (partially) chaotic whispering-gallery modes for all deformations. (C) 2002 Optical Society of America.

A quantum cascade laser amplifier has been developed. It was used to obtain high power single-mode emission at lambdaapproximate to7.4 mum from a quantum cascade distributed feedback laser, together with enhanced beam quality. Laser and amplifier are directly coupled in a master oscillator power amplifier configuration. Peak optical powers of 0.5 W at 80 K have been obtained. Ninety percent of the total power is thereby emitted within a divergence of 20degrees in the lateral direction. The device showed single mode operation with a side mode suppression ratio of 30 dB in the temperature range from 10 to 280 K. This allowed tuning of the emission wavelength in the range from 7.36 to 7.46 mum. The estimated peak amplifier gain is 6.4 and 4.9 dB at 80 and 300 K, respectively, and the cavity losses are 12.5 and 22 cm-1 at the corresponding temperatures. (C) 2002 American Institute of Physics.

The group refractive index dispersion in ultra-broad-band quantum cascade (QC) lasers has been determined using Fabry-Perot spectra obtained by operating the lasers in continuous wave mode below threshold. In the wavelength range of 5-8 mum, the global change of the group refractive index is as small as +8.2 X 10(-3) mum(-1). Using the method of Hakki and Paoli, the subthreshold gain of the lasers has furthermore been measured as a function of wavelength and current. At the wavelength of best performance, 7.4 mum, a modal gain coefficient of 16 cm(.)kA(-1) at threshold and a waveguide loss of 18 cm(-1) have been estimated. The gain evolution confirms an earlier assumption that cross-absorption restricted laser action to above 6 mum wavelength.

Observation of the nonequilibrium optical phonons population associated with electron transport in quantum-cascade lasers is reported. The phonon occupation number was measured in the range 75-280 K by using a combination of microprobe photoluminescence and Stokes/anti-Stokes Raman spectroscopy. The excess phonon population is observed to decrease as the lattice temperature increases. From the nonequilibrium phonon population, we extracted interface phonon lifetimes of 5 ps at 75 K and 2 ps at 280 K. (C) 2002 American Institute of Physics.

Following an introduction to the history of the invention of the quantum cascade (QC) laser and of the band-structure engineering advances that have led to laser action over most of the mid-infrared (IR) and part of the far-IR spectrum, the paper provides a comprehensive review of recent developments that will likely enable important advances in areas such as optical communications, ultrahigh resolution spectroscopy and applications to ultrahigh sensitivity gas-sensing systems. We discuss the experimental observation of the remarkably different frequency response of QC lasers compared to diode lasers, i.e., the absence of relaxation oscillations, their high-speed digital modulation, and results on mid-IR optical wireless communication links, which demonstrate the possibility of reliably transmitting complex multimedia data streams. Ultrashort pulse generation by gain switching and active and passive modelocking is subsequently discussed. Recent data on the linewidth of free-running QC lasers (similar to150 kHz) and their frequency stabilization down to 10 kHz are presented. Experiments on the relative frequency stability (similar to5 Hz) of two QC lasers locked to optical cavities are discussed. Finally, developments in metallic waveguides with surface plasmon modes, which have enabled extension of the operating wavelength to the far IR are reported.

Quantum cascade (QC) lasers with double metal-semiconductor waveguide resonators are reported for operating wavelengths of 19, 21, and 24 mum. The waveguides are based on surface-plasmon modes confined at the metal-semiconductor interfaces on both sides of the active region/injector stack and are not restricted by a cutoff wavelength for the TM polarized intersubband radiation. The double metal-semiconductor resonator devices are fabricated using an epilayer transfer process. Optical confinement factors close to 1 are obtained, with low waveguide losses. The performance of the devices is compared with that of QC lasers based on single-sided surface-plasmon waveguides. The concept of QC laser with double metal-semiconductor waveguide is applicable to a much wider wavelength range. (C) 2002 American Institute of Physics.

Quantum cascade (QC) lasers have been fabricated with Ge0.25Se0.75 chalcogenide lateral waveguide claddings. Aside from a strongly (up to similar to50%) reduced waveguide loss this device lay-out also displays a significantly reduced stray capacitance and improved high-speed modulation properties. This has been exploited for the first use of QC lasers in optical wireless communications as well as for gain-switched and actively mode-locked QC-lasers. Optical devices based on intersubband (IS) transitions face a rising interest also in other wavelength ranges due to their anticipated ultrafast electron dynamics. We present initial measurements of IS-transitions in the fiber-optics wavelength range in GaN/AlGaN samples grown by molecular beam epitaxy. IS-absorption at wavelengths of 1.44, 1.41, and 1.52 mum are measured for 11, 12, and 13 Angstrom wide GaN quantum wells, respectively. We also measured the IS electron scattering time by conventional pump-probe technique. Using 1.55 mum as pump- and 1.70 mum as probe-wavelength, we obtain an electron scattering time of 370 fs. (C) 2002 Elsevier Science B.V. All rights reserved.

A continuous wave quantum cascade laser (QCL), operating near 8.1 mum, was used for wavelength modulation spectroscopy of methane (CH4) and nitrous oxide (N2O) stable isotopes. Several rotational transitions of (N2O)-N-14-O-16, (NNO)-N-15-N-14-O-16, (N2O)-N-14-O-18, (N2O)-N-14-O-17, (CH4)-C-13 and (CH4)-C-12 fundamental bands were detected. The noise-equivalent absorbance was measured to be less than 10(-5) in a 1-Hz bandwidth. A characterization of the laser source was also performed. The use of a QCL spectrometer for high-precision isotope ratio measurements is discussed.

Quantum cascade distributed feedback (QC-DFB) lasers based on a heterogeneous-cascade two-wavelength active waveguide core and a multisectioned cavity featuring two different Bragg gratings are demonstrated. Optimised lasers display singlemode emission at lambda similar to 5.0 and 7.5 mum simultaneously and a tunability on both modes equal to single-wavelength QC-DFB lasers.

Single-mode and tunable quantum cascade distributed feedback (QC-DFB) lasers in the wavelength range from 4.5 to 16.5 mum are reviewed. In the case of QC lasers with dielectric waveguides, DFB lasers are fabricated either with a top-grating approach, which is simpler to manufacture, or a buried grating with epitaxial regrowth, which generally has a higher single-mode yield as a result of a larger coupling factor. Long-wavelength QC-DFB lasers based on surface plasmon waveguides use bi-metal gratings for Bragg reflection. Single-mode emission with a side-mode suppression ratio of 30 dB and a tunability (depending on wavelength) of 0.3-1.0 nm/K heat sink temperature or of 2040 nm/A CW current are customarily achieved. These features together with the potential for high optical power, room-temperature operation, and narrow intrinsic linewidth make QC-DFB lasers prime choices as narrow-band light sources in mid-infrared trace gas sensors. As a result of their unipolar nature and the possibility to serially stack, or ``cascade,'' many active regions, QC lasers also have an intrinsic potential for multiple-wavelength operation in a wide variety of device concepts. Multiple different optical transitions in single active regions stacked in a homogeneous cascade as well as multiple single-wavelength active regions cascaded in various schemes of heterogeneous cascades have been demonstrated. Based upon multiple-wavelength QC lasers, multiple single-mode QC-DFB lasers have been fabricated using sectioned laser cavities with multiple gratings. Adjusting the length of each Bragg-grating section as well the mode-overlap factor by tailoring the heterogeneity of the cascade has lead to a doubly single-mode QC-DFB laser with simultaneous single-mode emission around 5.0 and 7.5 mum and a tunability at each wavelength as expected from equivalent single-wavelength single-mode lasers. Finally, the concept of multiple-wavelength emission was extended to ultrabroad-band emission, with a QC laser that exhibited gain from 5 to 8 mum and simultaneous laser action from 6 to 8 Am.

Intersubband electroluminescence is reported in a quantum-cascade structure based on asymmetric superlattice active regions and designed for emission in the THz range (lambdaapproximate to80 mum). Comparison with a structure based on a ``vertical transition'' in a single quantum well shows an increased full width at half maximum (2.8 vs 0.9 meV) of the emission line. In both cases the dependence of the optical power on the injected current is linear or sublinear and remains in the pW range. (C) 2002 American Institute of Physics.