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High-power continuous-wave quantum cascade lasers
Abstract
High-power continuous-wave (CW) laser action is reported for a
GaInAs-AlInAs quantum cascade structure operating in the mid-infrared
(λ≃5 μm). Gain optimization and reduced heating effects
have been achieved by employing a modulation-doped funnel injector with
a three-well vertical-transition active region and by adopting InP as
the waveguide cladding material to improve thermal dissipation and
lateral conductance. A CW optical power as high as 0.7 W per facet has
been obtained at 20 K with a slope efficiency of 582 mW/A, which
corresponds to a value of the differential quantum efficiency
η<sub>d</sub>=4.78 much larger than unity, proving that each
electron injected above threshold contributes to the optical field a
number of photons equal to the number of periods in the structure. The
lasers have been operated CW up to 110 K and more than 200 mW per facet
have still been measured at liquid nitrogen temperature. The high
overall performance of the lasers is also attested by the large
“wall plug” efficiency, which, for the best device, has been
computed to be more than 8.5% at 20 K. The spectral analysis has shown
finally that the emission is single-mode for some devices up to more
than 300 mW at low temperature
... At present, intensive researches to develop the semiconductor lasers of infrared range are carried out [1][2][3][4][5][6][7][8]. They have a broad variety of uses, including trace gas analysis for pollution monitoring, environmental sensing, medical diagnostics, and military applications. ...
... They have a broad variety of uses, including trace gas analysis for pollution monitoring, environmental sensing, medical diagnostics, and military applications. One approach is based on development of the superlattice quantum cascade (QC) structures with optical transitions between conduction minibands due to miniband transport in the active and in the injector regions [1][2][3][4][5][6][7][8]. In present work, calculations of the energy spectra and injection coefficient at optical-phonon scattering are made. ...
... The modern superlattice QC lasers have the complex structure with the several periodic stages consisting of the number of potential wells and barriers [2][3][4][5][6][7] At the analysis of electronic transport [9,10] in the quantum cascade heterostructures the rate equations it is necessary to solve [7]. In addition, injection coefficients have significant influence on the recombination intensity of the charge carriers [11]. ...
Numerical self-consistent calculations of the energy levels and wave functions for superlattice quantum cascade structure are performed. Theoretical model for the calculations of the injection coefficients at optical-phonon scattering are suggested
... Current QC devices have demonstrated room-temperature CW operation at 4.3µm and 4.8µm 3,4 , but with very low wallplug-efficiency values (< 2.5%) due to inherently high voltages (10-11V). Furthermore, the devices have extremely temperature-sensitive characteristics 3,4 at and near RT, due to thermal runaway triggered by the backfilling effect 5,6 , which raises serious issues of device reliability. In fact, no device reliability has been demonstrated to date from any type of QC lasers. ...
... IS-QC lasers have fundamentally poor radiative efficiencies since the nonradiative, LO-phonon-assisted relaxation time for electrons in the upper laser states is about 1.8 ps, 5 whereas the radiative relaxation time is 4.2 ns. That is, nonradiative processes are about 2300 times faster than radiative processes. ...
... In QW structures electron relaxation between subbands occurs 2,5 in about 1-2 ps, primarily via LO-phonon absorption or emission. 5,7 Making quantum boxes (QBs) causes discrete states in the subbands, 6,17 which in turn causes the LO-phononassisted electron relaxation time to increase 7,9 by a factor β [ Ref 6]. Experimental results [10][11][12][13][14] from unipolar QBs (such that electron-hole scattering 10 does not circumvent the phonon bottleneck) and photocurrent-response/dark-current measurements from QB IR detectors 11,15 indicate electron-relaxation times of the order of 100 ps, in good agreement with theory. ...
IntroductionIntersubband Quantum Box LasersPreliminary Experimental ResultsConclusions
AcknowledgmentsReferences
... Since their realization, MIR QCLs have been continuously improved achieving significant performances in many of their properties like frequency range [33], maximum operating temperature [34] [35] ( fig. 1.2) and output power [36]. ...
... Finally, regarding the different kinds of EO crystals, the ZnTe [36] and GaP [84] are the mostly commonly used as the group velocity of the near-infrared optical pulse is roughly equal to the phase velocity of the THz pulse in these materials. This phase-matching condition allows to maximize the Pockels effect and permits to treat the varying THz waves as a static electric field. ...
THz QCLs are nowadays considered a promising platform for the generation of intense and ultrashort THz pulses. Owing to their fast gain recovery time, passive modelocking of THz QCLs has so far proved to be difficult. On the contrary, active modelocking with a microwave modulation has been successfully applied. The pulse duration, however, has been arduous to reduce despite years of research. In 2017, THz pulses as short as 4ps have been generated by our group with the application of an integrated structure (a GTI) aiming to reduce the chromatic dispersion. The research in this thesis starts from this point.In particular, I present dispersion engineering in THz QCLs in order to obtain very short pulses even from relatively narrow-band devices. This is achieved using proven active modulation methods that can tune the QCL emission from high to low dispersion regimes. I also show that THz QCLs can present a strong amplitude modulation of their emission profile and that they can spontaneously emit pulses as a result of a self-locking mechanism, contrary to the expected frequency modulated response. As a consequence, this indicates that the fast gain recovery time is not a limiting factor for the generation of pulses. I also show this passive self-locking scheme for passive pulse generation in the framework of the first demonstrations of harmonic modelocking of THz QCLs. Finally, a new phenomenon is presented where the modes of a free running THz QCL can beat together to generate free space microwave emission.
... The other fundamental feature of QCLs is the multistage cascade scheme, where electrons are recycled from period to period, contributing each time to the gain and the photon emission. Thus each electron injected above threshold generates N p laser photons, where N p is the number of stages, leading to a differential efficiency and therefore an optical power proportional to N p (Faist et al. 1998b). The first demonstration of a QCL was achieved in 1994 at AT&T (now Lucent Technologies) Bell Laboratories by Faist et al. (1994) using an Al 0.48 In 0.52 As/Ga 0.47 In 0.53 As/InP heterostructure. ...
... In the following years the Bell Laboratories group made a strong effort to improve the performance and illustrate the potential of these devices as a revolutionary light source for molecular spectroscopy (Puldus et al. 1999; Sharpe et al. 1998). Many important milestones for semiconductor lasers with emission wavelength in the mid-IR (3–15 µm), such as room-temperature operation and high continuous-wave (CW) output power (ca. 1 W) at cryogenic temperatures, were demonstrated for the first time using QCLs (Faist et al. 1996Faist et al. , 1998b Sirtori et al. 1997 ). However, for five years after its demonstration, all results were accomplished using a single semiconductor system: Al 0.48 In 0.52 As/Ga 0.47 In 0.53 As/InP. ...
Quantum engineering of the electronic energy levels and tailoring of the wave functions in GaAs/AlxGa1–xAs heterostructures allows one to obtain the correct matrix elements and scattering rates which enable laser action betwee subbands. This article reviews the state–of–the–art of GaAs–based quantum cascade lasers. These new light sources operate with peak power in excess of 1 W at 77 K, in the 8–13 μm wavelength region, greatly extending the wavelength range of GaA optoelectronic technology. Waveguides are based on an Al–free design with an appropriate doping profile, which allows optica confinement, low losses and optimal heat dissipation. Finally, new active–region designs aimed at improving the laser temperatur dependence are discussed. Recent results on these devices confirm that the ratio between the conduction–band discontinuit and the photon energy (ΔEc/Elaser) is the doms inant parameter controlling their thermal characteristic. The maximum operating temperature of these device is 280 K for lasers with emission wavelength at ca. 11 μm.
... Since the invention of quantum cascade lasers (QCL) [1] , a unipolar type of semiconductor midinfrared laser source based on intersubband transitions , their performance has been greatly improved2345 . Pulse lasing above room temperature and CW lasing near TE cooling temperature has been demonstrated. ...
... This phenomenon has been observed andFig. 6. Measured lasing spectra of a QCL at different driving current with driving pulse duration of 100 ns and pulse frequency of 20 kHz, the heat sink temperature is 110 K. investigated in the CW driving of the QCL [2]. The measured results shown in Figs. 5 and 6 means that, in the pulse driving of the QCL with quite short pulse duration, the lasing mode character transition also exists. ...
... Deuxièmement, l'émission en II.2. SOURCES LASER cascade des photons en fait des sources relativement puissantes (de 1 mW à environ 1 W) [71]. Enfin, tout comme les diodes laser, les QCL sont continûment accordables en longueur d'onde par ajustement de leur point de fonctionnement (réglage du courant d'injection et de la température) et peuvent être rendus monomodes (structure DFB ou DBR, cavité externe). ...
Infrared heterodyne sensing is a technique which has been developed primarily to improve the detectivity of infrared detectors, particularly in the 8−12 μm window. This technique has long been closely associated with the use of gas lasers. The fields of application were mainly astrophysical and atmospheric studies. Due to the complexity of implementation and the size of this type of instrument, few other applications could have been envisaged. Recent progress in the field of semiconductor lasers (Quantum Cascade Laser - QCL - cover a large part of the infrared spectrum) enable to consider new developments and new applications for infrared heterodyne sensing, for example for the remote detection and identification of atmospheric molecules, such as pollutants. The main advantages of heterodyne sensing concern spectral and directional selectivity of the instrument. It is applicable in civil sector to atmospheric molecules such as ozone and carbon dioxide, and for the military one to detect hazardous species. A heterodyne receiver has been developed with a QCL emitting at around10 μm and a temperature stabilized black body. To this end, several systems were considered: a system based on lens, another one based on off-axis parabolic mirrors and a last one based on mid-infrared optical fibers. Meanwhile, a heliostat has also been developed in order to do atmospheric measurements.
Keywords: Heterodyne sensing; Laser spectrometry; Mid infrared; Quantum cascade lasers; Heliostat
... Compound semiconductors III-As (where atoms III are Ga, In, Al) and their ternary alloys, due to direct band structure and high carrier mobility, are prospective materials for high electron mobility transistors (HEMT) [1] as well as for a large number of optoelectronic devices [2,3]. In the last decade, following the general trend to reduce the devise sizes down to the nanometer scale, III-As nanowires (NW)s started to be employed for device engineering [4][5][6]. ...
Origin of the Fermi level pinning at oxidized (110) surfaces of III-As nanowires (GaAs, InAs, InGaAs, AlGaAs) is studied. Using scanning gradient Kelvin probe microscopy, we find that positions of the Fermi level pinning on oxidized cleavage surfaces of ternary alloys Al$_{x}$Ga$_{1-x}$As (0$\le$x$\le$0.45) and Ga$_{x}$In$_{1-x}$As (0$\le$x$\le$1) are situated at the same distance of 4.8$\pm$0.1 eV with respect to vacuum level. The finding implies an unified mechanism of the Fermi level pinning for such surfaces. Then, we study photooxidation of Al$_{x}$Ga$_{1-x}$As and Ga$_{x}$In$_{1-x}$As, nanowires by Raman scattering and photoluminescence spectroscopy. The experiments discover the growth of excess arsenic layer on the crystal nanowire surfaces which is accompanied by simultaneous strong decrease of the band-edge photoluminescence intensity. The surface excess arsenic in crystalline or amorphous forms is concluded to be responsible for the Fermi level pinning at oxidized (110) surfaces of III-As nanowires.
... Engineered bound states in quantum wells are the heart of quantum cascade lasers (QCLs), and proper understanding of these quantum states-both individually and collectively-has enabled rapid evolution in the performance of QCLs. Since the first demonstration in 1994 [1], QCLs with remarkable performance have been realized, including: continuous wave operation at room temperature [2]; high electrical-to-optical conversion efficiency [3][4][5][6][7][8]; and, nonlinear generation of mid-infrared [9] and THz radiation [10]. These advances, and many others, are partly the result of improved understanding of intersubband physics and clever quantum engineering of the conduction band profile. ...
We present a three-level model based on a density matrix to examine the influence of coherence and dephasing on the gain spectrum of mid-infrared quantum cascade lasers. The model is used to examine a quantum cascade active region with multiple optical transitions. We show how coherence can explain the origin of additional peaks in the gain spectrum. We also analyze the spectra calculated using the three-level model with a rate equation formalism to demonstrate the importance of considering interface roughness and limitations of the rate equation formalism. Specifically, we present how interface roughness influences the broadening and oscillator strength that are recovered using a rate equation analysis. The results of this work are important when considering the design of active regions with multiple optical transitions and could lead to devices with improved performance.
... A standard technique to determine the gain is the Hakki-Paoli technique [118]. Here, optical gain is extracted from the fringe contrast of the Fabry-Perot modes of the cavity below threshold using a Fourier analysis of the subthreshold spectra [119,116]. Detailed theoretical studies on gain were carried out by Wacker and Lee [120]. They used a fully self-consistent quantum mechanical approach based on the theory of nonequilibrium Green's functions [121,13]. ...
In dieser Arbeit untersuchen wir die ultraschnelle Dynamik von Ladungsträgern und kohärenten Intersubbandpolarisationen in quasi-zweidimensionalen Halbleiternanostrukturen und Halbleiterbauelementen. Insbesondere werden n-Typ modulationsdotierte multiple Quantentöpfe und Quantenkaskadenlaserstrukturen basierend auf dem Materialsystem GaAs/AlGaAs mit der Methode der ultraschnellen Spektroskopie im mittleren Infrarot (3-20 mu) studiert. Ein neuartiger experimenteller Aufbau ist entwickelt worden, der zum ersten Mal das phasen- und amplitudenkontrollierte Formen von ultraschnellen Feldtransienten im mittelinfraroten Spektralbereich erlaubt. Wir untersuchen die Möglichkeit der kohärenten Kontrolle von Intersubbandübergängen. Amplituden- und phasenkonntrollierte Feldtransienten im mittleren Infrarot, die mit unserer neuen Laserquelle erzeugt werden, induzieren resonante Intersubbandanregungen in n-Typ modulationsdotierten GaAs/AlGaAs Quantentöpfen. Die transmittierten elektrischen Feldtransienten werden mit Hilfe des ultraschnellen elektro-optischen Abtastverfahrens gemessen. Unter Anwendung zweier phasengekoppelter Mittinfarotpulse variabler relativer Phase zeigen wir erstmalig die kohärente Kontrolle an linearen Intersubbandpolarisationen mit Dephasierungszeiten unterhalb einer Pikosekunde. Eine Sättigung von mehr als 0.2 wird bei einer Mittinfrarotpulsenergie von nur 1 pJ erreicht. Es wird erstmalig ein direktes, zeitaufgelöstes Experiment an elektrisch betriebenen Quantenkaskadenstrukturen vorgestellt. Diese Untersuchung ermöglicht den Einblick in die Dynamik des Elektronentransports, der mit stationären Methoden nicht meßbar ist. Der ultraschnelle Quantentransport der Elektronen vom Injektor durch die Injektionsbarriere in das obere Lasersubband wird in Femtosekunden-Mittinfrarot-Anreg-Abtast-Experimenten untersucht. Auf diese Weise beobachten wir die ultraschnelle Sättigung und die nachfolgende Wiederherstellung des elektrisch induzierten Gains. Wir beobachten ausgeprägte Gainoszillationen bei angelegtem Vorwärtsstrom und an spektralen Positionen am Gainmaximum. Dies ist ein direkter Beweis für eine kohärente Wellenpaketspropagation vom Injektor in das obere Lasersubband mittels resonantem Tunneln trotz der hohen Ladungsträgerdichte in Quantenkaskadenlasern. Nach der Sättigung ist der elektrisch induzierte Gain bei niedrigen Gitter- und Ladungsträgertemperaturen innerhalb einer Pikosekunde vollständig wiederhergestellt.
... With more gain stages in the QC laser, both carriers in the upper and lower subbands decrease, while the photon number remains almost constant. Additionally, the threshold current of the free running laser is substantially reduced while the external differential efficiency (dS/dI) is enhanced, as reported in [Faist98]. When compared to the free running laser, the carrier number of the upper subband (N 3 ) in the locked laser decreases while that of the lower subband (N 2 ) slightly increases. ...
High performance semiconductor lasers are strongly demanded in the rapidly increasing optical communication networks. Low dimensional nanostructure lasers are expected to be substitutes of their quantum well (Qwell) counterparts in the next-generation of energy-saving and high-bandwidth telecommunication optical links. Many efforts have been devoted during the past years to achieve nanostructure lasers with broad modulation bandwidth, low frequency chirp, and reduced linewidth enhancement factor. Particularly, 1.55-μm InP-based quantum dash (Qdash)/dot (Qdot) lasers are preferable for long-haul transmissions in contrast to the 1.3-μm laser sources. In this dissertation, we investigate the dynamic characteristics of InPbased nanostructure semiconductor lasers operating under direct current modulation, including the amplitude (AM) and frequency (FM) modulation responses, the linewidth enhancement factor (also known as α-factor), as well as large-signal modulation responses. Using a semi-analytical analysis of the rate equation model, it is found that the modulation bandwidth of the quantum dot laser is strongly limited by the finite carrier capture and relaxation rates. In order to study the α- factor and chirp properties of the quantum dot laser, we develop an improved rate equation model, which takes into account the contribution of carrier populations in off-resonant states to the refractive index change. It is demonstrated that the α-factor of quantum dot lasers is strongly dependent on the pump current as well as the modulation frequency, in comparison to the case of Qwell lasers. The α-factor remains constant at low modulation frequencies (<0.1 GHz) and higher than the value derived at high modulation frequencies (beyond several GHz) from the FM/AM technique. These unique features are mostly attributed to the carrier populations in off-resonant states. Further simulations show that the α-factor can be reduced by enlarging the energy separation between the resonant ground state (GS) and off-resonant states. Lasing from the excited state (ES) can be a promising alternative to enhance the laser’s dynamic performance. The laser exhibits a broader modulation response and the α-factor can be reduced by as much as 40%. The optical injection technique is attractive to improve the laser’s dynamical performance, including bandwidth enhancement and chirp reduction. These are demonstrated both theoretically and experimentally. The phase-amplitude coupling property is altered as well in comparison with the free-running laser and the optical gain depends on the injection strength and the frequency detuning. This work proposes a new method derived from the Hakki-Paoli method, enabling to measure the α-factor of semiconductor lasers under optical injection both below and above threshold. In addition, it is demonstrated theoretically that the α-factor in nanostructure lasers exhibits a threshold discontinuity, which is mainly attributed to the unclamped carrier populations in the off-resonant states. It is a fundamental limitation, preventing the reduction of the α-factor towards zero. Quantum cascade (QC) lasers rely on intersubband electronic transitions in multi-quantum well heterostructures. QC lasers show flat broadband AM response (tens of GHz) without resonance, which constitutes promising features for free-space communications. Surprisingly, calculations show that the QC laser exhibits an ultrabroad FM bandwidth on the order of tens of THz, about three orders of magnitude larger than the AM bandwidth. Optically injection-locked QC lasers also exhibit specific characteristics by comparison to interband semiconductor lasers. Both positive and negative frequency detunings enhance the modulation bandwidth.
We present the results of experiments on the fabrication and study of the properties of quantum-cascade lasers of the spectral range of 7–8 μm in the geometry of a waveguide with a thin top cladding based on indium phosphide. The heterostructure is synthesized by molecular beam epitaxy on an InP substrate with an active region based on a heteropair of In0.53Ga0.47As/Al0.48In0.52As solid alloys. Laser emission at a wavelength of 7.8 μm at a temperature of 300 K with a threshold current density of ~6 kA/cm² was achieved. The values of characteristic temperatures T0 and T1 for the studied quantum-cascade lasers are of the order of 150 and 450 K, respectively. The results obtained confirm that the design of the waveguide with a thin top cladding for devices for detecting liquids, the fabrication of microfluidic devices, and photonic circuits on silicon holds promise.
Aufgrund der hohen Sensitivität bei der Absorptionsmessung von Gasen im Spektral- bereich des mittleren Infrarot steigt die Nachfrage nach monolithischen, kompakten und energieeffizienten Laserquellen in Wellenlängenfenster zwischen 3 und 6 μm ste- tig. In diesem Bereich liegen zahlreiche Absorptionsbanden von Gasen, welche sowohl in der Industrie als auch in der Medizintechnik von Relevanz sind. Mittels herkömm- licher Diodenlaser konnte dieser Bereich bisher nur unzureichend abgedeckt werden, während Quantenkaskadenlaser infolge ihrer hohen Schwellenleistungen vor allem für portable Anwendungen nur bedingt geeignet sind. Interbandkaskadenlaser kom- binieren die Vorteile des Interbandübergangs von konventionellen Diodenlasern mit der Möglichkeit zur Kaskadierung der Quantenkaskadenlaser und können einen sehr breiten Spektralbereich abdecken. Das übergeordnete Ziel der Arbeit war die Optimierung von molekularstrahlepitak- tisch hergestellten Interbandkaskadenlasern auf GaSb - Basis im Spektralbereich des mittleren Infrarot für den Einsatz in der Gassensorik. Dies impliziert die Ermögli- chung von Dauerstrichbetrieb bei Raumtemperatur, das Erreichen möglichst geringer Schwellenleistungen sowie die Entwicklung eines flexiblen Konzepts zur Selektion von nur einer longitudinalen Mode. Da die Qualität der gewachsenen Schichten die Grundvoraussetzung für die Herstel- lung von performanten Bauteilen darstellt, wurde diese im Rahmen verschiedener Wachstumsserien eingehend untersucht. Nachdem das Flussverhältnis zwischen den Gruppe -V Elementen Sb und As ermittelt werden konnte, bei dem die InAs/AlSb - Übergitter der Mantelschichten verspannungskompensiert hergestellt werden können, wurde die optimale Substrattemperatur beim Wachstum dieser zu 450 ◦C bestimmt. Anhand von PL - sowie HRXRD- Messungen an Testproben konnte auch die opti- male Substrattemperatur beim Wachstum der charakteristischen W- Quantenfilme zu 450 ◦C festgelegt werden. Als weiterer kritischer Parameter konnte der As - Fluss beim Wachstum der darin enthaltenen InAs - Schichten identifiziert werden. Die bes- ten Ergebnisse wurden dabei mit einem As - Fluss von (1.2 ± 0.2) × 10−6 torr erzielt. Darüber hinaus konnte in Kooperation mit der Technischen Universität Breslau eine sehr hohe guteWachstumshomogenität auf den verwendeten 2′′ großen GaSb -Wafern nachgewiesen werden. Im Anschluss an die Optimierung des Wachstums verschiedener funktioneller Be- standteile wurden basierend auf einem in der Literatur veröffentlichten Laserschicht- aufbau diverse Variationen mit dem Ziel der Optimierung der Laserkenndaten unter- sucht. Zum Vergleich wurden 2.0 mm lange und 150 μm breite, durch die aktive Zone geätzte Breitstreifenlaser herangezogen. Eine erhebliche Verbesserung der Kenndaten konnte durch die Anwendung des Kon- zepts des Ladungsträgerausgleichs in der aktiven Zone erreicht werden. Bei einer Si - Dotierkonzentration von 5.0 × 1018 cm−3 in den inneren vier InAs - Filmen des Elektroneninjektors konnte die niedrigste Schwellenleistungsdichte von 491W/cm2 erreicht werden, was einer Verbesserung von 59% gegenüber des Referenzlasers ent- spricht. Mithilfe längenabhängiger Messungen konnte gezeigt werden, dass der Grund für die Verbesserung in der deutlichen Reduzierung der internen Verluste auf nur 11.3 cm−1 liegt. Weiterhin wurde die Abhängigkeit der Laserkenngrößen von der Anzahl der verwendeten Kaskaden in den Grenzen von 1 bis 12 untersucht. Wie das Konzept der Kaskadierung von Quantenfilmen erwarten ließ, wurde eine mo- notone Steigerung des Anstiegs der Strom - Lichtleistungskennlinie sowie eine Pro- portionalität zwischen der Einsatzspannung und der Kaskadenzahl nachgewiesen. Für ICLs mit einer gegebenen Wellenleiterkonfiguration und einer Wellenlänge um 3.6 μm wurde bei einer Temperatur von 20 ◦C mit 326W/cm2 die niedrigste Schwel- lenleistungsdichte bei einem ICL mit vier Kaskaden erreicht. Des Weiteren konnte für einen ICL mit 10 Kaskaden und einer Schwellenstromdichte von unter 100A/cm2 ein Bestwert für Halbleiterlaser in diesem Wellenlängenbereich aufgestellt werden. Eine weitere Reduktion der Schwellenleistungsdichte um 24% konnte anhand von Lasern mit fünf Kaskaden durch die Reduktion der Te - Dotierung von 3 × 1017 cm−3 auf 4 × 1016 cm−3 im inneren Teil der SCLs erreicht werden. Auch hier wurde mit- tels längenabhängiger Messungen eine deutliche Reduktion der internen Verluste nachgewiesen. In einer weiteren Untersuchung wurde der Einfluss der SCL - Dicke auf die spektralen sowie elektro - optischen Eigenschaften untersucht. Darüber hin- aus konnten ICLs realisiert werden, deren Mantelschichten nicht aus kurzperiodigen InAs/AlSb - Übergittern sondern aus quaternärem Al0.85Ga0.15As0.07Sb0.93 bestehen. Für einen derartig hergestellten ICL konnte eine Schwellenstromdichte von 220A/cm2 bei einer Wellenlänge von 3.4 μm gezeigt werden. Mithilfe der durch die verschiedenen Optimierungen gewonnenen Erkenntnisse so- wie Entwurfskriterien aus der Literatur wurden im Rahmen diverser internationaler Kooperationsprojekte ICLs bei verschiedenen Wellenlängen zwischen 2.8 und 5.7 μm hergestellt. Der Vergleich der Kenndaten zeigt einen eindeutigen Trend zu einer stei- genden Schwellenstromdichte mit steigender Wellenlänge. Die charakteristische Tem- peratur der untersuchten Breitstreifenlaser nimmt von circa 65K bei lambda=3.0 μm mit steigender Wellenlänge auf ein Minimum von 35K im Wellenlängenbereich um 4.5 μm ab und steigt mit weiter steigender Wellenlänge wieder auf 45K an. Ein möglicher Grund für dieses Verhalten konnte mithilfe von Simulationen in der Anordnung der Valenzbänder im W-Quantenfilm gefunden werden. Zur Untersuchung der Tauglichkeit der epitaktisch hergestellten Schichten für den in der Anwendung hilfreichen Dauerstrichbetrieb oberhalb von Raumtemperatur wur- den Laser in Stegwellenleitergeometrie mit einer aufgalvanisierten Goldschicht zur verbesserten Wärmeabfuhr hergestellt. Nach dem Aufbau der Laser auf Wärmesen- ken wurde der Einfluss der Kavitätslänge sowie der Stegbreite auf diverse Kennda- ten untersucht. Des Weiteren wurden eine Gleichung verifiziert, welche es erlaubt die maximal erreichbare Betriebstemperatur im Dauerstrichbetrieb aus der auf die Schwellenleistung bezogenen charakteristischen Temperatur sowie dem thermischen Widerstand des Bauteils zu berechnen. Mithilfe von optimierten Bauteilen konn- ten Betriebstemperaturen von mehr als 90 ◦C und Ausgangsleistungen von mehr als 100mW bei einer Betriebstemperatur von 20 ◦C erreicht werden. Im Hinblick auf die Anwendung der Laser in der Absorptionsspektroskopie wurde ab- schließend ein DFB-Konzept, welches zuvor bereits in konventionellen Diodenlasern zur Anwendung kam, erfolgreich auf das ICL - Material übertragen. Dabei kommt ein periodisches Metallgitter zum Einsatz, welches seitlich der geätzten Stege aufge- bracht wird und aufgrund von Verlustkopplung eine longitudinale Mode bevorzugt. Durch den Einsatz von unterschiedlichen Gitterperioden konnten monomodige ICLs basierend auf dem selben Epitaxiematerial in einem spektralen Bereich von mehr als 100nm hergestellt werden. Ein 2.4mm langer DFB- Laser konnte einen Abstimmbe- reich von mehr als 10nm bei Verschiebungsraten von 0.310nm/K und 0.065nm/mA abdecken. Der DFB- ICL zeigte im Dauerstrichbetrieb in einem Temperaturbereich zwischen 10 und 35 ◦C monomodigen Betrieb mit einer Ausgangsleistung von mehre- ren mW. Basierend auf dem in dieser Arbeit gewachsenem Material und dem DFB- Konzept konnte im Rahmen verschiedener Entwicklungsprojekte bereits erfolgreich Absorptionsspektroskopie in einem breiten Spektralbereich des mittleren Infrarot be- trieben werden.
The fabrication and study of the characteristics of a lattice-matched quantum cascade laser structure on an indium-phosphide substrate, designed for a wavelength of ~4.8 μm corresponding to one of the atmospheric windows are described. The heterostructure grown by molecular-beam epitaxy consisted of thirty cascades. Lasing was experimentally observed at temperatures up to 200 K at a wavelength coinciding with the calculated one, which confirms the high heterointerface quality and high precision of the layer thicknesses and active-region doping levels.
This chapter describes lasers and other sources of coherent light that operate in a wide wavelength range. First, the general principles for the generation of coherent continuous-wave and pulsed radiation are treated including the interaction of radiation with matter, the properties of optical resonators and their modes as well as such processes as Q-switching and mode-locking. The general introduction is followed by sections on numerous types of lasers, the emphasis being on today’s most important sources of coherent light, in particular on solid-state lasers and several types of gas lasers. An important part of the chapter is devoted to the generation of radiation coherent coherent radiation coherent radiation by nonlinear processes with optical parametric oscillators, difference- and sum-frequency generation, and high-order harmonics. Radiation in the extended ultraviolet (EUV) and x-ray ranges can be generated by free electron lasers (FEL) and advanced x-ray sources. Ultrahigh light intensities up to 10²¹ W/cm² open the door to studies of relativistic laser-matter interaction and laser particle acceleration. The chapter closes with a section on laser stabilization.
By optimizing the molecule beam epitaxy growth condition, the quality of quantum cascade (QC) material has greatly been improved. The spectrum of double x-ray diffraction indicates that the interface between the constituent layers is very smooth, the lattice mismatch between the epilayer and the substrate is less than 0.1%, and the periodicity fluctuation of the active region is not more than 4.2%. The QC laser with the emission wavelength of about 5.1 mum is operated at the threshold of 0.73 kA/cm(2) at liquid nitrogen temperature with the repetition rate of 10kHz and at a duty cycle of 1%. Meanwhile, the performance of the laser can be improved with suitable post process techniques such as the metallic ohmic contact technology.
Intersubband quantum-box (IQB) lasers, that is, devices consisting of 2D arrays of single-stage intersubband QB emitters, are proposed as an alternative to the 30-40-stage (conventional) quantum-cascade (QC) devices as sources for efficient room-temperature (RT) emission in the mid- and far-IR (3-5 μm and 8-12 μm) wavelength ranges. The devices rely on the much larger electron relaxation times in unipolar QBs than in quantum-well structures (i.e., the phonon bottleneck effect) to achieve low RT threshold-current densities (<0.5 kA/cm2) and high RT CW wallplug efficiencies of 20-25%. Preliminary experimental results include the following: (i) The realization of the first mid-IR (λ = 4.7 μm) single-stage emitters operating at room temperature. These involve GaAs-based deep-well devices that, due to tight carrier confinement to the active QWs, do not need Bragg mirror/transmitter regions; (ii) etching and regrowth at the nanoscale level by employing in situ gas etching and regrowth in an MOCVD crystal-growth system; (iii) a pattern employing a high-resolution e-beam resist of 30-nm-diameter dots on 80-nm centers, and the transfer of the pattern into SiO2, thus forming a mask for the fabrication of IQBs via in situ etching and regrowth.
This chapter discusses quantum cascade lasers. Most solid-state and gas lasers rely on narrow optical transitions connecting discrete energy levels in which population inversion is achieved by optical or electrical pumping. In contrast, semiconductor diode lasers, including quantum-well lasers, rely on transitions between energy bands in which conduction electrons and valence-band holes, injected into the active layer through a forward-biased p-n junction, radiatively recombine across the bandgap. A unipolar intersubband laser or quantum cascade laser differs in many fundamental ways from diode lasers. It relies only on one type of carrier, making electronic transitions among conduction-band states (subbands) arising from size quantization in a semiconductor heterostructure. In contrast to interband transitions, the gain linewidth depends indirectly on temperature through collision processes. In quantum cascade lasers, the gain spectrum has basically the same shape of the absorption spectrum unlike interband diode lasers.
Inhalt der Arbeit sind Untersuchungen zu mit der Molekularstrahlepitaxie (MBE) realisierten Materialkonzepten für ultra-schnelle Anwendungen in der Photonik. Nominell undotierte und Be dotierte GaInAs/AlInAs Vielfach-Quantenfilm Strukturen (MQW) wurden auf semi-isolierenden InP Substraten bei Wachstumstemperaturen bis zu 100°C mittels MBE (LT-MBE) abgeschieden. Untersucht wurden die kristallinen, elektrischen und optischen Eigenschaften dieser Schichtstrukturen im unbehandelten und ausgeheilten Zustand. Die elektrischen und optischen Eigenschaften der LT-MQWs sind auf Zustände nahe der Leitungsbandkante von GaInAs zurückzuführen. Die Dynamik der Ladungsträgerrelaxation wurde durch Anrege- und Abtastexperimente bestimmt. Messungen der differentiellen Transmission mit zusätzlicher Dauerstrichanregung, sowie Messungen mit zwei kurz aufeinander folgenden Anregepulsen, belegen das Potential von Be dotierten unbehandelten (ausgeheilten) LT GaInAs/AlInAs MQW Strukturen für die Verwendung in optischen Schaltern mit Schaltfrequenzen in der Größenordnung von 1 Tbit/s (250 Gbit/s). Die spannungsinduzierten Änderung der Interband-Transmission von Quantenkaskadenlaser (QCL) im gepulsten Betrieb wurde anhand von 8 Band k*p Berechnungen analysiert. Die Auswirkungen unterschiedlicher Ladungsträgerverteilungen und Probenerwärmung sind gegenüber dem dominierenden Effekt des elektrischen Feldes auf die Interband Transmission zu vernachlässigen. Der Einfluss von MBE Wachstumsparameter auf die Grenzflächenqualität von AlAsSb/GaInAs Heterostrukturen wurde anhand von Hall Messungen, temperatur- und intensitätsabhängigen PL Messungen, spektralen Messungen der Interband- und Intersubbandabsorption bestimmt. Bandstruktur-Näherungsrechnungen ermöglichten, den Einfluss von In Segregation und Sb Diffusion auf die Intersubbandabsorption zu analysieren. Intersubband Übergänge bei Wellenlängen von ca. 1800 nm (1550 nm) wurden in MQW (gekoppelten QW) Strukturen realisiert.
The interband cascade lasers (IC) represent a new class of mid-IR light sources, which take advantage of the broken-gap alignment in type-II quantum wells to reuse electrons for sequential photon emissions from serially connected active regions. Here, we describe recent progress in InAs/GaInSb type-II IC lasers at emission wavelengths of 3.8-4 μm; these semiconductor lasers have exhibited significantly higher differential quantum efficiencies and peak powers than previously reported. Also, these lasers were able to operate at temperatures up to 217 K, which is higher than the previous record (182 K) for an IC laser at this wavelength. We observed from several devices at temperatures above 80 K a slope efficiency of approximately 800 mW/A per facet, corresponding to a differential external quantum efficiency of approximately 500%. A peak optical output power exceeding 4 W/facet and peak power efficiency of approximately 7% were observed from a device at 80 K. Also, we report the first cw operation of IC lasers.
We report on results of electronic features of parabolic quantum well (PQW) of diluted magnetic semiconductor (DMS) under crossed fields. The electric field is applied parallel to the growth direction and the magnetic field is transversal to this one. For specific values of the magnetic and electric fields the effective potential associated to the conduction band resembles a double quantum well (DQW). The barrier of the heterostructure is made of DMS, consequently the interaction between the localized spins with external magnetic field is considered. We show that in DQW configuration the wave function of this system is strongly dependent on the spin polarization. In this particular situation, the energy levels with different spin polarization can cross as the external field varies, this is interesting for tuning electromagnetic transitions in opto-electronic devices.
A density-matrix based theory of transport and lasing in quantum-cascade lasers reveals that large disparity between lasing linewidth and tunneling broadening changes the design guidelines to favor strong coupling between injector and upper laser level.
form only given. We address practical issues involving quantum cascade laser (QCL) fabrication, introduce a new fabrication technique which permits a reduction in the waveguide width which is simple to implement, and report on quantum cascade devices lasing at /spl lambda/=5 /spl mu/m fabricated using this new scheme. The QC wafer, based on the design demonstrated by Faist et al. (1996), was grown in-house. Production of QC lasers with reduced threshold currents are envisaged through a reduction in the laser waveguide core width.
A theoretical investigation of the optical gain of electrically pumped intersubband lasers is presented. Using a prototype four-level near-infrared (1.55 μm) triple quantum well structure, self-consistent numerical simulations of the rate equations, energy density equations, optical gain and spontaneous emisson spectra of triple quantum well structures have been implemented at temperatures of 100 K and 300 K. It is predicted that the threshold currents of such lasers are significantly smaller than those for mid-infrared quantum cascade lasers.
Optical waveguide structures of InP-based quantum cascade lasers (QCLs) are theoretically analyzed and designed using the finite element method. The optical confinement factor (Γ) and absorption coefficient (α) of the waveguide structures for λ ∼ 4.6, 6, 8.8, and 9.5 µm QCLs with different numbers of active/injector stages (Ns) are calculated. As Ns is decreased at longer wavelength, Γ is gradually decreased and α is increased. For double-channel (DC) ridge waveguide structures of QCLs operating at λ ∼ 9.5 µm, a relatively high optical loss is caused by the lossy insulation layer. To improve the performance, the buried heterostructure (BH) by InP regrowth is used for waveguide design in terms of Ns and waveguide width. For a narrow BH width of ∼10 µm, a low α value of 3.92 cm−1 is obtained compared to 12.32 cm−1 in the DC ridge waveguide structure. With the calculated optical properties of waveguide structures, the device characteristics of QCLs operating at λ ∼ 4.6 and 9.5 µm are theoretically investigated in comparison with the experimental results.
Mid-infrared quantum cascade lasers are semiconductor injection lasers whose active core implements a multiple-quantum-well structure. Relying on a designed staircase of intersubband transitions allows free choice of emission wavelength and, in contrast with diode lasers, a low transparency point that is similar to a classical, atomic four-level laser system. In recent years, this design flexibility has expanded the achievable wavelength range of quantum cascade lasers to similar to 3-25 mu m and the terahertz regime, and provided exemplary improvements in overall performance. Quantum cascade lasers are rapidly becoming practical mid-infrared sources for a variety of applications such as trace-chemical sensing, health monitoring and infrared countermeasures. In this Review we focus on the two major areas of recent improvement: power and power efficiency, and spectral performance.
InAs/AlSb intersubband quantum cascade lasers based on bound-to-continuum transitions are fabricated and operation at 10 μm is demonstrated. A spatially indirect intersubband transition together with a double plasmon waveguide structure is employed. Threshold current density is 4.9 kA/cm2 at 4 K. Temperature dependence of the threshold current density is also presented.
Quantum cascade (QC) lasers emitting at λ ≈ 8 μm with a power performance equal to short-wavelength (λ ≈ 5 μm) QC lasers are reported. The device improvement is mainly achieved by a design of the injector/relaxation region, which at laser threshold allows resonant carrier injection between the ground state of the preceding and the upper laser level of the subsequent active region. In pulsed operation a peak output power of 1.3 W per facet has been measured at 100 K. At room temperature a record peak power of 325 mW and a record slope efficiency of 180 mW/A have been measured. In continuous-wave operation the maximum power at 30 K was 510 mW per facet and still 200 mW per facet at 80 K. The high values of the output power and slope efficiency demonstrate the validity of the cascading scheme, in which electrons above threshold generate one photon per each active region they successively traverse.
Mid-infrared (λ = 3.25 μm) broadened-waveguide diode lasers with active regions consisting of 5 type-II “W” quantum wells operated in continuous-wave (cw) mode up to 195 K. At 78 K, the threshold current density was 63 A/cm2, and up to 140 mW of cw output power was generated. A second structure with ten quantum wells operated up to 310 K in pulsed mode.
X-ray diffraction, as an effective probe and simple method, is used to ascertain the precise control of the epilayer thickness and composition. Intersubband absorption from the whole structure of the QC laser is used to monitor the wavelength of the QC laser and the material quality. Path for growth of high-quality InP-based InGaAs/InAlAs quantum cascade laser material is realized. The absorption between two quantized energy levels is achieved at ∼4.7 μm. Room temperature laser action is achieved at λ≈5.1–5.2 μm. For some devices, if the peak output power is kept at 2 mW, quasi-continuous wave operation at room temperature can persist for more than 1 h.
In this paper, we report results on the material quality of InGaAs/AlInAs lattice-matched to InP and the performance of a Fabry-Perot QC lasers grown by gas-source molecular beam epitaxy using a one-growth step procedure. The surface defect density of epi-layer with 10/cm2 over a 2′′ diameter wafer are achieved. The Fabry-Perot QC lasers uncoated exhibit operation in quasi-continuous-wave at room temperature with a low threshold current density of 1.75 KA/cm2 and 7.95 μm. The emitting wavelength is in good agreement with the predicted emission wavelength by design.
Operation of a unipolar injection quantum cascade laser grown in a GaAs/AlGaAs material is reported. In pulsed mode an optical peak power exceeding 100 mW at 77 K and at an emission wavelength of 9.3 mu m is obtained. The optimum threshold current density is 6.3 kA/cm(2). A strong influence of external white light illumination on device operation is observed. This behaviour can be explained by the presence of DX centers in the AlGaAs cladding layers.
We report on electroluminescence and photoluminescence studies of arsenic rich InAs1-xSbx heterostructure LED's for the MIR region. Single-quantum- well LED's have demonstrated 300 K of approximately 24 (mu) W and approximately 50 (mu) W and approximately 8 micrometers , respectively, with corresponding internal quantum efficiencies of 0.8% and 1.6%. We also demonstrate 4.2 micrometers , 300 K emission from strained-layer superlattice (SLS) LED's with AlSb electron confining barriers with output powers > 0.1 mW. In reverse bias, these SLS devices exhibit negative luminescence efficiencies of approximately 14% at 310 K.
We review our recent results in development of high-precision laser spectroscopic instrumentation using midinfrared quantum cascade lasers (QCLs). Some of these instruments have been directed at measurements of atmospheric trace gases where a fractional precision of 10-3 or better of ambient concentration may be required. Such high precision is needed in measurements of fluxes of stable atmospheric gases and measurements of isotopic ratios. Instruments that are based on thermoelectrically cooled midinfrared QCLs and thermoelectrically cooled detectors have been demonstrated that meet the requirements of high-precision atmospheric measurements, without the need for cryogens. We also describe the design of and results from a new dual QCL instrument with a 200-m path-length absorption cell. This instrument has demonstrated 1-s noise of 32 ppt for formaldehyde (HCHO) and 9 ppt for carbonyl sulfide (OCS).
Intersubband quantum-box (IQB) lasers, that is, devices consisting of 2D arrays of single-stage intersubband QB emitters, are proposed, as an alternative to 30-stage quantum-cascade (QC) devices, as sources for efficient room-temperature (RT) emission in the mid- and far-IR (3–5 and 8–12 μm) wavelength ranges. Preliminary results include: (1) the realization of the first mid-IR (λ = 4.7 μm) single-stage emitters operating at RT; (2) etch-and-regrowth at the nanoscale level by employing in situ gas etching and MOCVD regrowth; (3) the formation of 30 nm-diameter SiO2 posts on 80 nm centers, thus forming the mask for the fabrication of IQBs via in situ etch and regrowth.
Citation
F. Rana, S. G. Patterson, and R. J. Ram, "Integrated Photonics Research Noise in Semiconductor Cascade Lasers,"
in Integrated Photonics Research,
OSA Technical Digest Series
(Optical Society of America, 1999), paper RMH6.
http://www.opticsinfobase.org/abstract.cfm?URI=IPR-1999-RMH6
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 levels of ≥500 mW at room temperature and ≥1 W at 200 K have been obtained for a 2.25 mm long and≈12 μm 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 μm. 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 ≈70 nm as a result of thermal tuning between 20 and 120 K.
The need for higher sensitive detection technology for trace gas samples, either in the laboratory setup or in the atmospheric remote sensing has been a goal for several decades. The development of the tunable diode laser has propelled the progress of trace detection technology, and modulation technology enables the improvement of the detection sensitivity. As a result, the detection of 10 or 10 absorbance is possible for some applications, and modulation technology has been applied to the ultraviolet as well as mid-infra red wavelength range. In this review, recent progress of tunable diode lasers and diode laser-based modulation technologies are presented. Wavelength modulation, frequency modulation, and two-tone frequency modulation techniques are mainly described along with the actual application of the techniques. In addition, the state-of-the-art of diode laser development which can be adopted for the trace detection is presented.
Double X-ray diffraction has been used to investigate InGaAs/InAlAs quantum cascade (QC) laser grown on InP substrate by molecule beam epitaxy, by means of which, excellent lattice matching, the interface smoothness, the uniformity of the thickness and the composition of the epilayer are disclosed. What is more, these results are in good agreement with designed value. The largest lattice mismatch is within 0.18% and the intersubband absorption wavelength between two quantized energy levels is achieved at about λ=5.1μm at room temperature. At 77K, the threshold density of the QC laser is less than 2.6kA/cm2 when the repetition rate is 5kHz and the duty cycle is 1%.
The optimization of Bragg-confined double quantum wells with respect to resonant second harmonic generation and electro-optic coefficients is considered. The method of optimization relies on solving a system of nonlinear equations which involve the geometric parameters of the structure and bound state energies. Advantage is taken of the above-the-barrier bound states in Bragg-confined structures, in order to extend the range of allowable incident photon energies. Numerical results are given for Al0.48In0.52As/Ga0.47In0.53As based structure and the incident photon energy homega=240 meV.
Mid-infrared intersubband light-emitting diodes based on InAs/GaSb/AlSb type-II cascade structure have been investigated. The observed emission energy is in good agreement with calculation based on the multi-band k·p theory. In contrast to interband cascade structures, dominant polarization of the emitted light is perpendicular to the quantum well layers. Structure dependence of intersubband electroluminescence is also presented.
The development of mid-infrared interband diode lasers has been hindered by factors such as Auger recombination and intervalence band absorption, which become increasingly important at longer wavelengths. A number of structures have been proposed in which the effects of these processes are reduced. The maximum gain per unit volumetric current density can be used as a figure of merit for comparing different active region materials. Using this figure of merit, we compare a series of structures with band gaps near 0.3 eV (i.e., wavelengths near 4 microns). The figure of merit is obtained from gain spectra calculated using superlattice K(DOT)p theory and a combination of calculated and measured recombination rates. We show that devices based on active regions incorporating type-I InAsSb/AlInAsSb or InAsSb/InAsP quantum wells should have room temperature threshold currents 7 - 13 times smaller than those of devices based on bulk InAs. However, devices using type-II superlattice active regions should have room temperature threshold currents that are a factor of 3 - 4 times smaller than those of the type-I quantum wells. The figure of merit can also be used to determine the optimal thickness of the active region as a function of waveguide loss and optical mode width.
A method is described for the optimized design of quantum-well structures, with respect to maximizing the second-order susceptibilities relevant for second harmonic generation. The possibility is explored of obtaining resonantly enhanced nonlinear optical susceptibilities in quantum wells with two bound states and a continuum resonance state as the dominant third state. The method relies on applying the isospectral (energy structure preserving) transformations to an initial Hamiltonian in order to generate a parameter-controlled family of Hamiltonians. By changing the values of control parameters one changes the potential shape and thus the values of matrix elements relevant to susceptibility to be maximized. The method was used for the design of AlxGa1-xAs -based QWs. The results indicate the possibility of employing continuum states in resonant second harmonic generation at higher photon energies, ℏomega= 200-300 meV.
Demonstration of short-range multispectral remote sensing, using 3 to
4-micrometers mid- infrared Sb semiconductor lasers based on
code-division multiplexing (CDM) architecture, is described. The system
is built on a principle similar to intensity- modulated/direct-detection
optical-CDMA for communications, but adapted for sensing with
synchronous, orthogonal codes to distinguish different wavelength
channels with zero interchannel correlation. The concept is scalable for
any number of channels, and experiments with a two-wavelength system are
conducted. The CDM-signal processing yielded a white-Gaussian-like
system noise that is found to be near the theoretical level limited by
the detector fundamental intrinsic noise. With sub-mW transmitter
average power, the system was able to detect an open-air acetylene gas
leak of 10-2 STP ft3/hr from 10-m away with
time-varying, random, noncooperative backscatters. A similar experiment
detected and positively distinguished hydrocarbon oil contaminants on
water from bio-organic oils and detergents. Projection for more advanced
systems suggests a multi-kilometer-range capability for watt-level
transmitters, and hundreds of wavelength channels can also be
accommodated for active hyperspectral remote sensing application.
A rate equation analysis on the modulation response of an optical injection-locked quantum cascade laser is outlined. It is found that the bifurcation diagram exhibits both bistable and unstable locked regions. In addition, the stable locked regime widens as the linewidth enhancement factor increases. It is also shown that both positive and negative optical detunings as well as strong injection strength enhance the 3 dB modulation bandwidth by as much as 30 GHz. Finally, the peak in the modulation response is significantly influenced by the optical frequency detuning.
Possible ways to overcome difficulties encountered in the creation of cascade lasers with collisionless
tunneling of electrons are discussed. A structure of an IR laser using a single three-barrier structure and
capable of working at a frequency of 4.5 THz is proposed
The realization of the first noncascaded intersubband injection lasers based on a single optical transition is reported. The unipolar lasers are based on an active region consisting of three InGaAs quantum wells closely coupled by thin AlInAs barriers. The lasers emit at λ ≈ 7.7 μm wavelength and operate in pulsed mode up to 110 K. Peak power levels of 20 mW at 10 K and 4 mW at 110 K are obtained. The low-temperature threshold current density is 25.6 kA cm−2 in good agreement with calculations. Several advantages arise from this novel type of intersubband laser. First, only few layers are necessary to build the active region, simplifying sample preparation. Second, low operating voltages can be achieved, which is essential for many applications. Finally, the noncascaded intersubband laser allows studying fundamental properties of quantum cascade lasers without possible artifacts introduced by the sequential stacking of many active regions. © 1998 American Institute of Physics.
A new class of quantum cascade lasers is presented. They are based on interminiband transitions in chirped superlattices (SL), where the applied electric field is compensated by the quasielectric field resulting from a gradually varying SL period length and average composition. In this way “flat” minibands can be obtained without the need for dopants. At room temperature record high peak (0.5 W) and average (14 mW) powers are obtained for a laser of 7.6 μm wavelength, with the lowest threshold current densities (5 kA/cm2) reported so far for quantum cascade lasers. The maximum temperature for continuous wave operation is an unprecedented 160 K. © 1998 American Institute of Physics.
We report on a simple technique for the generation of short pulses of midinfrared (5 and 8 μm) radiation, based on gain-switched quantum cascade lasers. In particular, an integrated step-recovery diode source (comb generator) is used to drive the lasers, properly packaged for high-speed operation. Using a fast HgCdTe detector, we measure optical pulses with duration as short as 200 ps, broadened by the detector response time, and peak power of a few tens of mW. The maximum operating temperature of these gain-switched sources is approximately 120 K. © 1999 American Institute of Physics.
Lateral current spreading in shallow ridge processed unipolar semiconductor lasers is described using a two-dimensional flow model. In these devices, contrary to bipolar diode lasers, the density of carriers can be considered constant also in the active region. Therefore electron diffusion is a negligible effect and the spatial distribution of the current can be obtained by solving a two-dimensional differential equation for the electric potential. Our calculations prove that the major contribution to the current spreading takes place right before electrons enter the active region and is caused by the discontinuity of the conductivity at the cladding–active region interface. © 2001 American Institute of Physics.
A semiconductor injection laser that differs in a fundamental way from diode lasers has been demonstrated. It is built out
of quantum semiconductor structures that were grown by molecular beam epitaxy and designed by band structure engineering.
Electrons streaming down a potential staircase sequentially emit photons at the steps. The steps consist of coupled quantum
wells in which population inversion between discrete conduction band excited states is achieved by control of tunneling. A
strong narrowing of the emission spectrum, above threshold, provides direct evidence of laser action at a wavelength of 4.2
micrometers with peak powers in excess of 8 milliwatts in pulsed operation. In quantum cascade lasers, the wavelength, entirely
determined by quantum confinement, can be tailored from the mid-infrared to the submillimeter wave region in the same heterostructure
material.
Gain spectra for GaAs double-heterostructure junction lasers have been obtained with high resolution. This is accomplished by using an automated data aquisition system to analyze the Fabry-Perot resonance modulation in the spontaneous emission spectra. For active regions doped with Ge at a level of 4×10<sup>17</sup> cm<sup>-3</sup>, the gain in the TE polarization at a fixed wavelength increases linearly with current, below lasing threshold. However, the peak gain (at a variable wavelength) increases slightly faster than linearly with current. The photon energy at which gain is a maximum increases logarithmically with current. Gain in the TM polarization depicts the same general behavior as that for the TE case, except that it is slightly less than the TE gain. It is concluded that for this particular doping the spectral gain characteristics are intermediate between those for undoped and heavily doped active regions. Above the threshold for lasing in the TE mode the TE gain spectra are well saturated, with new fine details revealed in the saturated spectra. On the other hand, gain in the nonlasing TM polarization is not well saturated above threshold, with marked differences in gain between high and low photon energies relative to the TE lasing energy.
Growth of lattice matched GaInAsP on (100) InP was achieved using all solid source molecular beam epitaxy (MBE). Two valved cracking cells, one for phosphorus and the other for arsenic, were employed to supply the column V fluxes. The ability to obtain lattice matched conditions to InP was found to be highly reproducible and readily achievable using two valved cracking cells. X‐ray diffraction and photoluminescence measurements showed run‐to‐run variations in the arsenic/phosphorus mole fractions of less than 1%. Lattice matched Ga 0.30 In 0.70 As 0.68 P 0.32 layers displayed 300 K luminescence full width at half maximums as low as 40.6 meV at λ∼1.43 μm. The results suggest all solid source MBE offers a viable alternative to existing heterojunction growth technologies.
The authors propose two novel sources emitting in the mid-IR:
type-II and type-I interband cascade lasers, and perform detailed gain
calculations. High radiative efficiencies are expected, since the phonon
processes which dominate relaxation in the intersubband quantum cascade
laser are effectively eliminated
The extension of maser techniques to the infrared and optical region is considered. It is shown that by using a resonant cavity of centimeter dimensions, having many resonant modes, maser oscillation at these wavelengths can be achieved by pumping with reasonable amounts of incoherent light. For wavelengths much shorter than those of the ultraviolet region, maser-type amplification appears to be quite impractical. Although use of a multimode cavity is suggested, a single mode may be selected by making only the end walls highly reflecting, and defining a suitably small angular aperture. Then extremely monochromatic and coherent light is produced. The design principles are illustrated by reference to a system using potassium vapor.
Continuous wave operation of a quantum cascade laser at lambda=4.6 mu m is reported above liquid nitrogen temperature. Optical powers of 15 mW at 50 K and 2 mW at 85 K are reported. The single mode spectrum is temperature tunable over 1.8 cm(-1). A linewidth of 115 Mhz is measured with a Fabry-Perot on those free running devices. Gain measurements show evidence for ultra low linewidth enhancement factor alpha<0.1. These devices also operated in pulse mode with 20 mW peak power at 200 K. (C) 1996 Academic Press Limited
AlInAs/GaInAs quantum cascade (QC) lasers operating at λ=11.2 μm wavelength are reported. In pulsed operation the peak power is in excess of 50 mW at 110 K heat sink temperature and the devices have been operated at temperatures as high as 200 K. Continuous wave single‐mode operation with powers ≂7 mW at 10 K has been achieved. This work, combined with our previous reports on QC lasers, demonstrates that these new light sources can be tailored, by suitable quantum design, over a wide wavelength range (4–11 μm) using the same heterostructure material. © 1996 American Institute of Physics.
IV‐VI multiple‐quantum‐well lasers with seven wells made from molecular‐beam‐epitaxy grown PbSe/PbSrSe have been operated in pulsed mode up to 282 K at a wavelength of λ=4.2 μm. This is the highest midinfrared quantum well laser operation temperature observed to date. © 1995 American Institute of Physics.
Strained quantum‐well lasers emitting at 4.5 μm have been fabricated. The laser structure, grown on a GaSb substrate by molecular beam epitaxy, consists of compressively strained InAsSb active layers and tensile‐strained InAlAs barrier layers, surrounded by AlAsSb cladding layers. Under electrical injection, the laser exhibited pulsed operation up to 85 K, with threshold current density of 350 A/cm2 at 50 K. Under optical pumping, the laser operated pulsed up to 144 K, with peak power at 95 K of 0.54 W. © 1995 American Institute of Physics.
The high power operation of mid‐infrared quantum cascade lasers at temperatures up to T=320 K is reported. Gain at high temperature is optimized by a design combining low doping, a funnel injector, and a three‐well vertical transition active region. A molecular beam epitaxy grown InP top cladding layer is also used to optimize heat dissipation. A peak pulsed optical power of 200 mW and an average power of 6 mW are obtained at 300 K and at a wavelength λ=5.2 μm. The devices also operate in continuous wave up to 140 K. © 1996 American Institute of Physics.
We report a high power mid-infrared interband cascade laser operating at temperatures up to 170 K. The threshold current densities of this laser are considerably lower than the previously reported values in cascade lasers. The structure was grown by molecular beam epitaxy on a GaSb substrate and comprises 23 periods of active regions separated by digitally graded multilayer injection regions. A peak optical output power of ∼ 0.5 W/facet and a slope of 211 mW/A per facet, corresponding to a differential external quantum efficiency of 131%, are observed at 80 K and at a wavelength of ∼ 3.9 μm. © 1997 American Institute of Physics.
The design and temperature dependence of the performance characteristics of a quantum cascade intersubband laser operating pulsed in the midinfrared (λ≂4.3 μm) are reported. The threshold current density varies exponentially with temperature [exp(T/T 0 )] from ≊6.0 kA/cm<sup>2</sup> at 50 K to ≊9.3 kA/cm<sup>2</sup> up to the maximum operating temperature (125 K) with a T 0 ∼112 K. This weak temperature dependence, compared to interband lasers operating at similar wavelengths, is due to the intersubband nature of the laser transition, to the physics of optical phonons scattering, and to the negligible intersubband Auger transition rates. The measured peak optical power varies from 32 mW at 10 K to 18 mW at 80 K for a 1.2‐mm cavity length. The measured slope efficiency is 52 mW/A at 80 K which corresponds to an estimated differential quantum efficiency of ≂3.4×10<sup>-2</sup> per facet per stage. © 1994 American Institute of Physics.
A new midinfrared (λ∼4.5 μm) intersubband quantum cascade laser based on a vertical transition is reported. A superlattice graded gap region was incorporated in the design to provide strong electron confinement in the upper state using a Bragg reflector. Pulsed operation at 100 K is reported with a threshold current density of Jth=3 kA/cm2 and a measured slope efficiency of 300 mW/A.
The high power operation of mid-infrared quantum cascade lasers at temperatures up to T=320 K is reported. Gain at high temperature is optimized by a design combining low doping, a funnel injector, and a three-well vertical transition active region. A molecular beam epitaxy grown InP top cladding layer is also used to optimize heat dissipation. A peak pulsed optical power of 200 mW and an average power of 6 mW are obtained at 300 K and at a wavelength lambda=5.2 mu m. The devices also operate in continuous wave up to 140 K. (C) 1996 American Institute of Physics.
The quantum cascade laser is a new semiconductor laser that is based on one type of carrier making transitions between energy levels created by quantum confinement. This work demonstrates that the design based on a vertical transition can be optimized to provide continuous wave operation. The InGaAs/AlInAs heterostructures consist of 25 stages, each comprising a coupled-well active region and a graded-gap superlattice injection/relaxation region. The samples are processed into mesa etched ridge waveguides by wet chemical etching and SiO2 insulation. The devices are also investigated in pulsed operation in which there is negligible heating.
The fundamental limits of the operation of quantum cascade intersubband lasers are investigated. Band nonparabolicities combined with the nonthermal electron distribution in the active region make laser action possible even in the absence of global k-space population inversion between subbands, i.e., when the lifetime of the lower subband exceeds that of the upper one. A laser based on local k-space population inversion with single-quantum-well active regions is demonstrated and its performance compared to that of quantum cascade lasers with double-quantum-well active regions.
An AlInAs/GaInAs quantum cascade (QC) laser operating at 8.4 μm wavelength was presented. The outer portions of the core consist of GaInAs layers to improve the optical confinement by increasing the average refractive index of the core. The low doped (100) InP substrate was the lower waveguide cladding layer. The doped AlInGaAs compositionally graded layers served to smooth the conduction band at the interfaces between the claddings and the other layers so as to not hinder electron transport. Meanwhile, the top n++ GaInAs layer played a critical role in suppressing the coupling between the fundamental mode of the waveguide and the high-loss plasmon mode propagating along the metal contact-semiconductor interface. The peak optical power versus drive current for different heat-sink temperatures were obtained for a 1.3 mm long device.
AIInAs/GaInAs quantum cascade (QC) lasers operating at lambda=11.2 mu m wavelength are reported. In pulsed operation the peak power is in excess of 50 mW at 110 K heat sink temperature and the devices have been operated at temperatures as high as 200 K. Continuous were single-mode operation with powers similar or equal to 7 mW at 10 K has been achieved. This work, combined with our previous reports on QC lasers, demonstrates that these new light sources can be tailored, by suitable quantum design, over a wide wavelength range (4-11 mu m) using the same heterostructure material. (C) 1996 American Institute of Physics.
Continuous wave operation of a quantum cascade laser at lambda=4.6 mu m is reported above liquid nitrogen temperature. Optical powers of 15 mW at 50 K and 2 mW at 85 K are reported. The single mode spectrum is temperature tunable over 1.8 cm(-1). A linewidth of 115 Mhz is measured with a Fabry-Perot on those free running devices. Gain measurements show evidence for ultra low linewidth enhancement factor alpha<0.1. These devices also operated in pulse mode with 20 mW peak power at 200 K. (C) 1996 Academic Press Limited
Pulsed single mode operation of distributed feedback quantum cascade lasers is reported above room temperature at both 5.3 and 8 mu m wavelengths, Peak optical powers up to 60 mW at 300 K are obtained with a tuning range of similar to 60 nm from 100 to similar to 320 K. The linewidth is limited by thermal drift during the pulse with a typical value of 0.3 cm(-1) for a 10 ns long pulse at 300 K. (C) 1997 American Institute of Physics.
The room-temperature pulsed operation of a semiconductor laser emitting at 8.5 /spl mu/m is reported. This device is an optimized vertical transition quantum cascade (QC) laser. At 300 K the peak output power from a single facet is 15 mW, and the current density at threshold is /spl sim/8 kA/cm/sup 2/. The temperature dependence of the threshold current density is described by a high T/sub 0/ (107 K) in the 200-320 K temperature range.
The operation of quantum cascade lasers at a wavelength
(λ≃9.3 μm) well within the 8-13-μm atmospheric window
is reported. A detailed study of intersubband luminescence in a vertical
transition structure shows linewidths as narrow as ~10 meV at cryogenic
temperatures, increasing to 20 meV at room temperature. Pulsed operation
is demonstrated up to 220 K with a peak power ≈10 mW and ≈35 mW at
140 K. The temperature dependence of the threshold current density (J
<sub>th</sub>) is described by a high T<sub>0</sub> (128 K), J<sub>th
</sub> is also systematically studied as a function of cavity length to
determine the peak gain and waveguide losses. Continuous-wave,
single-mode operation is demonstrated up to 30 K with powers ≈2 mW