Publications

2002
C Gmachl, EE Narimanov, F Capasso, JN Baillargeon, and AY Cho. 2002. “Kolmogorov-Arnold-Moser transition and laser action on scar modes in semiconductor diode lasers with deformed resonators.” OPTICS LETTERS, 27, 10, Pp. 824-826.Abstract
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.
M Troccoli, C Gmachl, F Capasso, DL Sivco, and AY Cho. 2002. “Mid-infrared (lambda approximate to 7.4 mu m) quantum cascade laser amplifier for high power single-mode emission and improved beam quality.” APPLIED PHYSICS LETTERS, 80, 22, Pp. 4103-4105.Abstract
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.
C Gmachl, A Soibel, R Colombelli, DL Sivco, F Capasso, and AY Cho. 2002. “Minimal group refractive index dispersion and gain evolution in ultra-broad-band quantum cascade lasers.” IEEE PHOTONICS TECHNOLOGY LETTERS, 14, 12, Pp. 1671-1673.Abstract
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.
V Spagnolo, G Scamarcio, M Troccoli, F Capasso, C Gmachl, AM Sergent, AL Hutchinson, DL Sivco, and AY Cho. 2002. “Nonequilibrium optical phonon generation by steady-state electron transport in quantum-cascade lasers.” APPLIED PHYSICS LETTERS, 80, 23, Pp. 4303-4305.Abstract
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.
F Capasso, C Gmachl, DL Sivco, and AY Cho. 2002. “Quantum cascade lasers.” PHYSICS TODAY, 55, 5, Pp. 34-40.
F Capasso, R Paiella, R Martini, R Colombelli, C Gmachl, TL Myers, MS Taubman, RM Williams, CG Bethea, K Unterrainer, HY Hwang, DL Sivco, AY Cho, AM Sergent, HC Liu, and EA Whittaker. 2002. “Quantum cascade lasers: Ultrahigh-Speed operation, optical wireless communication, narrow linewidth, and far-infrared emission.” IEEE JOURNAL OF QUANTUM ELECTRONICS, 38, 6, Pp. 511-532.Abstract
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.
K Unterrainer, R Colombelli, C Gmachl, F Capasso, HY Hwang, AM Sergent, DL Sivco, and AY Cho. 2002. “Quantum cascade lasers with double metal-semiconductor waveguide resonators.” APPLIED PHYSICS LETTERS, 80, 17, Pp. 3060-3062.Abstract
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.
C Gmachl, HM Ng, R Paiella, R Martini, HY Hwang, DL Sivco, F Capasso, AY Cho, SV Frolov, SNG Chu, and HC Liu. 2002. “Recent results in quantum cascade lasers and intersubband transitions in GaN/AlGaN multiple quantum wells.” PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, 13, 2-4, Pp. 823-828.Abstract
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.
AA Belyanin, F Capasso, VV Kocharovsky, VV Kocharovsky, DS Pestov, and MO Scully. 2002. “Resonance parametric generation of IR radiation on intersubband transitions in quantum-well heterostructures.” IZVESTIYA AKADEMII NAUK SERIYA FIZICHESKAYA, 66, 2, Pp. 247-249.
G Gagliardi, F Tamassia, P De Natale, C Gmachl, F Capasso, DL Sivco, JN Baillargeon, AL Hutchinson, and AY Cho. 2002. “Sensitive detection of methane and nitrous oxide isotopomers using a continuous wave quantum cascade laser.” EUROPEAN PHYSICAL JOURNAL D, 19, 3, Pp. 327-331.Abstract
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.
A Straub, C Gmachl, DL Sivco, AM Sergent, F Capasso, and AY Cho. 2002. “Simultaneously at two wavelengths (5.0 and 7.5 mu m) singlemode and tunable quantum cascade distributed feedback lasers.” ELECTRONICS LETTERS, 38, 12, Pp. 565-567.Abstract
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.
C Gmachl, A Straub, R Colombelli, F Capasso, DL Sivco, AM Sergent, and AY Cho. 2002. “Single-mode, tunable distributed-feedback and multiple-wavelength quantum cascade lasers.” IEEE JOURNAL OF QUANTUM ELECTRONICS, 38, 6, Pp. 569-581.Abstract
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.
R Colombelli, A Straub, F Capasso, C Gmachl, MI Blakey, AM Sergent, SNG Chu, KW West, and LN Pfeiffer. 2002. “Terahertz electroluminescence from superlattice quantum cascade structures.” JOURNAL OF APPLIED PHYSICS, 91, 6, Pp. 3526-3529.Abstract
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.
AA Kosterev, FK Tittel, R Kohler, C Gmachl, F Capasso, DL Sivco, AY Cho, S Wehe, and MG Allen. 2002. “Thermoelectrically cooled quantum-cascade-laser-based sensor for the continuous monitoring of ambient atmospheric carbon monoxide.” APPLIED OPTICS, 41, 6, Pp. 1169-1173.Abstract
We report the first application of a thermoelectrically cooled, distributed-feedback quantum-cascade laser for continuous spectroscopic monitoring of CO in ambient air at a wavelength of 4.6 mum. A noise-equivalent detection limit of 12 parts per billion was demonstrated experimentally with a 102-cm optical pathlength and a 2.5-min data acquisition time at a 10-kHz pulsed-laser repetition rate. This sensitivity corresponds to a standard error in fractional absorbance of 3 x 10(-5). (C) 2002 Optical Society of America.
A Straub, TS Mosely, C Gmachl, R Colombelli, M Troccoli, F Capasso, DL Sivco, and AY Cho. 2002. “Threshold reduction in quantum cascade lasers with partially undoped, dual-wavelength interdigitated cascades.” APPLIED PHYSICS LETTERS, 80, 16, Pp. 2845-2847.Abstract
A dual-wavelength quantum cascade (QC) laser with an interdigitated cascade is presented. Aside from providing two-wavelength operation at 8.0 and 9.5mum wavelength, this laser design was used to test the role of extrinsic carriers in the injectors. An interdigitated cascade was grown with undoped injectors bridging 9.5 and 8.0 mum active regions, but doped injectors bridging 8.0 and 9.5 mum active regions. Clear laser action on both wavelengths demonstrates that doping of all injector regions is not a firm requirement for QC lasers. Comparison with a conventionally doped interdigitated cascade QC laser shows a threshold reduction by a factor of approximately 2 for the laser based on the active regions preceded by the undoped injector. This can be understood from the absence or strong reduction of impurity scattering related to the dopant ions. (C) 2002 American Institute of Physics.
AA Kosterev, RF Curl, FK Tittel, R Kohler, C Gmachl, F Capasso, DL Sivco, and AY Cho. 2002. “Transportable automated ammonia sensor based on a pulsed thermoelectrically cooled quantum-cascade distributed feedback laser.” APPLIED OPTICS, 41, 3, Pp. 573-578.Abstract
A compact ammonia sensor based on a 10-mum single-frequency, thermoelectrically cooled, pulsed quantum-cascade laser with an embedded distributed feedback structure has been developed. To measure NH3 concentrations, we scanned the laser over two absorption lines of its fundamental nu(2) band. A sensitivity of better than 0.3 parts per million was achieved with just a 1-m optical path length. The sensor is computer controlled and automated to monitor NH3 concentrations continuously for extended periods of time and to store data in the computer memory. (C) 2002 Optical Society of America.
C Gmachl, DL Sivco, R Colombelli, F Capasso, and AY Cho. 2002. “Ultra-broadband semiconductor laser.” NATURE, 415, 6874, Pp. 883-887.Abstract
The fundamental mechanism behind laser action leads in general only to narrowband, single-wavelength emission. Several approaches for achieving spectrally broadband laser action have been put forward, such as enhancing the optical feedback in the wings of the gain spectrum(1,2), multi-peaked gain spectra(3,4), and the most favoured technique at present, ultrashort pulse excitation(5,6). Each of these approaches has drawbacks, such as a complex external laser cavity configuration, a non-flat optical gain envelope function, or an inability to operate in continuous mode, respectively. Here we present a monolithic, mid-infrared `super-continuum' semiconductor laser that has none of these drawbacks. We adopt a quantum cascade(7,8) configuration, where a number of dissimilar intersubband optical transitions are made to cooperate in order to provide broadband optical gain from 5 to 8 mum wavelength. Laser action with a Fabry-Perot spectrum covering all wavelengths from 6 to 8 mum simultaneously is demonstrated with this approach. Lasers that emit light over such an extremely wide wavelength range are of interest for applications as varied as terabit optical data communications(9) or ultra-precision metrology(10) and spectroscopy(11).
WH Weber, JT Remillard, RE Chase, JF Richert, F Capasso, C Gmachl, AL Hutchinson, DL Sivco, JN Baillargeon, and AY Cho. 2002. “Using a wavelength-modulated quantum cascade laser to measure NO concentrations in the parts-per-billion range for vehicle emissions certification.” APPLIED SPECTROSCOPY, 56, 6, Pp. 706-714.Abstract
Measurements of NO concentrations at sub-ppm levels in vehicle exhaust are needed for emissions certification of future ultra-low emission vehicles. We demonstrate a wavelength-modulation, laser-based, NO detection system suitable for this purpose. A quantum cascade distributed feedback laser (QC-DFB) operating continuous wave (cw) at similar to100 K is frequency modulated at f = 10 kHz and locked to the center of a transition at similar to1921 cm(-1) in the fundamental band of NO. The demodulated signal at 2f of the beam passing through the sample cell directly measures the NO concentration. The cell is a multipass Herriott-type with a 100-m path length. Doppler broadening, pressure broadening, and unresolved A doubling combine to yield a pressure for optimum sensitivity of 100 torr and a modulation amplitude of similar to600 MHz. A flowing gas system is used to avoid problems with adsorption and desorption of NO from the cell walls. The reduced pressure eliminates interference from other gas species. Detection of NO concentrations in the few parts-per-billion (ppb) range is demonstrated in diluted exhaust-gas bag samples collected in the vehicle certification process.
Claire Gmachl, Deborah L. Sivco, Alex Soibel, Raffaele Colombelli, Federico Capasso, and Alfred Y. Cho. 2002. “Laser Optics: Ultrabroadband QuantumCascade Lasers.” Opt. Photon. News 13 (12), Pp. 23–23. Publisher's Version
2001
AA Kosterev, RF Curl, FK Tittel, C Gmachl, F Capasso, DL Sivco, JN Baillargeon, AL Hutchinson, and AY Cho. 2001. “Absorption spectroscopy with quantum cascade lasers.” LASER PHYSICS, 11, 1, Pp. 39-49.Abstract
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.

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