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Design and growth characteristics of broadly tunable hetero-cascading quantum cascade lasers are discussed. We show the influence of electric field variation, and interface roughness on the gain spectra. Interface roughness is shown to play an important role for these broadband lasers. External cavity configuration has been used to extract narrow-band emission, thus rendering wide tuning range of the device. We achieve a single-chip tuning range of 400 cm−1 (from 1030 cm−1 to 1430 cm−1, namely 7.0–9.7 µm in wavelength) by using tri-stack active regions.

In this paper, we give an overview of quantum cascade detector technology for the near- and mid-infrared wavelength range. Thanks to their photovoltaic operating principle, the most advanced quantum cascade detectors offer great opportunities in terms of high detection speed, reliable room temperature operation, and excellent Johnson noise limited detectivity. Besides some important features dealing with their fabrication and their general characteristics, we will also briefly present some possibilities for performance improvement. Elementary theoretical considerations adopted from photoconductive detectors confirm that optimization of such devices always involves various trade-offs.

There is an increasing demand for wavelength agile laser sources covering the mid-infrared (MIR, 3.5–12 µm) wavelength range, among others in active imaging. The MIR range comprises a particularly interesting part of the electromagnetic spectrum for active hyperspectral imaging applications, due to the fact that the characteristic ‘fingerprint’ absorption spectra of many chemical compounds lie in that range. Conventional semiconductor diode laser technology runs out of steam at such long wavelengths. For many applications, MIR coherent light sources based on solid state lasers in combination with optical parametric oscillators are too complex and thus bulky and expensive. In contrast, quantum cascade lasers (QCLs) constitute a class of very compact and robust semiconductor-based lasers, which are able to cover the mentioned wavelength range using the same semiconductor material system. In this tutorial, a brief review will be given on the state-of-the-art of QCL technology. Special emphasis will be addressed on QCL variants with well-defined spectral properties and spectral tunability. As an example for the use of wavelength agile QCL for active hyperspectral imaging, stand-off detection of explosives based on imaging backscattering laser spectroscopy will be discussed.

Performance modeling of the characteristics of mid-infrared quantum cascade lasers (MIR QCL) is an essential element in formulating consistent component requirements and specifications, in preparing guidelines for the design and manufacture of the QCL structures, and in assessing different modes of operation of the laser device. We use principles of system physics to analyze the electro-optical characteristics of high power MIR QCL, including thermal backfilling of the lower laser level, hot electron effects, and Stark detuning during lasing. The analysis is based on analytical modeling to give simple mathematical expressions which are easily incorporated in system-level simulations of defense applications such as directed infrared countermeasures (DIRCM). The paper delineates the system physics of the electro-optical energy conversion in QCL and the related modeling. The application of the performance model to a DIRCM QCL is explained by an example.

We employ active hyperspectral imaging using tunable mid-infrared (MIR) quantum cascade lasers for contactless identification of solid and liquid contaminations on surfaces. By collecting the backscattered laser radiation with a camera, a hyperspectral data cube, containing the spatially resolved spectral information of the scene is obtained. Data is analyzed using appropriate algorithms to find the target substances even on substrates with a priori unknown spectra. Eye-save standoff detection of residues of explosives and precursors over extended distances is demonstrated and the main purpose of our system. However, it can be applied to any substance with characteristic reflectance / absorbance spectrum. As an example, we present first results of monitoring food quality by distinguishing fresh and mold contaminated peanuts by their MIR backscattering spectrum.

In recent years, the technology of the quantum cascade laser (QCL) has drastically improved, as proven by room temperature continuous wave (CW) operation, high optical power, low dissipation devices, and accessible spectral range [1]. As a result, QCLs have drawn a great deal of interest for their various applications in trace-gas sensing, defense countermeasures, spectroscopy and free-space communications [2]. The performance of QCLs is primarily dependent on the design of the active region, usually gown by MBE (molecular beam epitaxy). Nevertheless, the laser performance are also influenced by the thermal properties and optical losses introduced by the fabrication technology, in particular in continuous-wave operation. In that respect, the buried heterostructure (BH) approach, which is a standard and reliable telecommunications laser manufacturing technique, offers unique possibilities to achieve effective thermal dissipation and lateral single modes. In standard BH designs, the process starts by the definition and etching of the ridge mesa using a dielectric mask. The latter (SiO2 or SiN) is also used as the masking material for the selective regrowth by metal-organic vapor phase epitaxy (MOVPE) of a thick insulating InP (Fe) layer over the entire laser structures. Unfortunately, the selective regrowth of InP:Fe is not a trivial process. Indeed, it is extremely difficult to achieve an ideal planarized regrowth, i.e, completely burying the laser ridge, while maintaining a planar surface. In addition to growth conditions, the characteristics of the regrown InP layer strongly depend on the sidewall profile of the ridges, as well as on the physical parameters of the dielectric mask (thickness, width…) [3]. The complexity of the etching and regrowth process further increases for thick layer structures, such as for long wave QCLs, whose ridges can be >10 µm deep. In effect, deep ridge etching generates big undercuts in the structures and consequently, highly non uniform active region widths. Moreover, the thick selective regrowth step, needed for planarization, tends to favor the appearance of crystal growth artifacts (crystallites, voids, “rabbit ears”…) in the vicinity of the ridges, which deteriorate the performance of the regrown structures. In some cases, additional photolithographic and back-etching steps might even be required to improve the planarity of the structure. Another method that allows overcoming the aforementioned limitations is to use a modified buried heterostructure concept, with two MOVPE regrowth steps [4]. In this case, the waveguide pattering and etching is constituted only by the active layers ( In this perspective, we report an optimized BH-QCL approach obtained by adding an InP layer and a sacrificial InGaAs cap layer on top of the MBE grown active region. As shown in the attached figures (FIB/SEM cross-section profiles), the lateral thickness of the insulating and planar regrowth is increased and reduces potential leakage paths. Using this approach, BH QCL regrowths were successfully carried out with reproducible and good epitaxial growth quality (smooth and uniform regrown layers, planar surfaces, no “rabbit-ears” and defect free interfaces) around the etched ridges, permitting the fabrication of BH-QCLs with good beam quality and hundreds of mW of optical power at room temperature. The growth and optimization of the BH process, as well as the fabrication and testing of the laser devices will be presented and compared to conventional double trench devices. References [1] Faist, J., et al. "Quantum cascade lasers," University Press, 2013. [2] Razeghi, M., et al. "Quantum cascade lasers: from tool to product," Optics express 23.7 (2015): 8462-8475. [3] Cheng, L., et al., "Analysis of InP regrowth on deep-etched mesas and structural characterization for buried-heterostructure quantum cascade lasers." J. of electronic materials 41.3 (2012): 506-513. [4] Süess, M. J., et al. "Advanced Fabrication of Single-Mode and Multi-Wavelength MIR-QCLs." Photonics. Vol. 3. No. 2. Multidisciplinary Digital Publishing Institute, 2016. Figure 1

We present the time-resolved spectral analysis of the settling process in a pulsed external-cavity quantum-cascade laser (EC-QC laser) emitting in the wavelength range around 7.4 μm. Due to mode competition between the two cavities formed by the laser-chip on one hand and the external-cavity on the other hand, a time-dependent spectral emission on a ns time-scale can be observed. Depending on the spectral characteristic of the external cavity with respect to the chip gain curve time-delays of the external cavity emission of 15–35 ns within the 100 ns drive pulse can be observed.

This paper gives an overview on the design, fabrication, and characterization of quantum cascade detectors. They are tailorable infrared photodetectors based on intersubband transitions in semiconductor quantum wells that do not require an external bias voltage due to their asymmetric conduction band profile. They thus profit from favorable noise behavior, reduced thermal load, and simpler readout circuits. This was demonstrated at wavelengths from the near infrared at 2 μm to THz radiation at 87 μm using different semiconductor material systems.

Molecular beam epitaxial growth of quaternary barrier GaInAs/AlGaAsSb quantum cascade lasers grown lattice-matched on n -InP substrates is reported. By adjusting the As/Sb flux ratio as well as the group-III growth rate and the Al/Ga ratio for the AlGaAsSb layers, a growth technique has been optimized employing a continuous growth mode (i.e. no growth interruption) for the GaInAs/AlGaAsSb layer sequence of GaInAs/AlGaAsSb on InP quantum cascade structures. We found that the As/Sb incorporation ratio is determined by an interplay of arrival rate and chemical bonding of the involved species. The lasers operate in pulsed mode up to temperatures higher than 400 K, with a characteristic temperature of T 0 = 169 K .

The influence of injector doping concentration Ninj on the performance of InP-based quantum-cascade (QC) lasers is investigated for devices emitting around 9.2-mum wavelength and injector doping concentrations between 1times1017 cm-3 and 3times1017 cm-3. The threshold current density, the dynamic range, the maximum emitted output power as well as the maximum operating temperature are found to increase with increasing N inj. All in all, there exists no optimal value of Ninj per se. In fact, the injector doping concentration has to be adjusted individually depending whether emphasis is placed on obtaining low threshold current densities or on high-power operation

High-temperature (up to at least 400 K) pulsed-mode operation of quantum cascade (QC) lasers based on GaInAs/Al(Ga)AsSb grown lattice-matched on InP substrates are presented in this article. The emission wavelength of the QC lasers is in the range from 4 µm to 5 µm at room temperature. For a typical device with the size of 18 µm × 2.8 mm, based on a 25-stage GaInAs/AlAsSb active region, mounted substrate-side down with as-cleaved facets, a maximum peak power per facet of 750 mW has been achieved at room temperature. The maximum pulsed operation temperature for the GaInAs/AlGaAsSb-based QC lasers is estimated to be 450 K. For a typical device with the size of 14 µm × 1.5 mm mounted substrate-side down with as-cleaved facets, a maximum peak power per facet of 190 mW has been achieved at 400 K for the GaInAs/AlGaAsSb based QC lasers. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Die vorliegende Arbeit befasst sich mit dem Entwurf, der Herstellung sowie der Charakterisierung von einmodigen DFB-Quantenkaskadenlasern für Anwendungen in der Spurengasanalytik. Die Laser eignen sich zur Detektion der fundamentalen Rotations-Schwingungs-Übergänge von Stickstoffmonoxid im Wellenlängenbereich um 5,35 μm (1870 cm−1) und von Kohlenmonoxid um 4,67 μm (2140 cm−1) mittels hochauflösender Absorptionsspektroskopie. Im Pulsbetrieb ist die Seitenmodenunterdrückung >27dB und die maximale Betriebstemperatur > 400K (127 °C). Typische Linienbreiten liegen bei etwa 100MHz.

Conventional strain-compensated quantum cascade (QC) lasers are fabricated with both tensile strained AlInAs and compressively strained GaInAs layers. We show that strain compensation can also be achieved using tensile strained AlInAs barrier layers and lattice-matched GaInAs with thin InAs layers inserted. In the present paper, we use Al 0.6 In 0.4 As as barriers. The tensile strain caused by these barriers is compensated by thin InAs layers inserted into the lattice-matched GaInAs wells. The lasers, mounted epilayer up with as-cleaved facets, operate in pulsed mode up to 400 K (127 °C) at a wavelength of λ∼5.3um. For a laser with a ridge size of 18 μm x 3 mm, the maximum output peak power per facet obtained is 1 W at room temperature (27 °C).

Broadband tunable external cavity quantum cascade lasers (EC-QCL) have emerged as attractive light sources for midinfrared (MIR) “finger print” molecular spectroscopy for detection and identification of chemical compounds. Here we report on the use of EC-QCL for the spectroscopic detection of hazardous substances, using stand-off detection of explosives and sensing of hazardous substances in water as two prototypical examples. Our standoff-system allows the contactless identification of solid residues of various common explosives over distances of several meters. Furthermore, results on an EC-QCL-based setup for MIR absorption spectroscopy on liquids are presented, featuring a by a factor of ten larger single-pass optical path length of 100 μm as compared to conventional Fourier transform infrared spectroscopy instrumentations.

External-cavity quantum cascade lasers (EC-QCL) are now established as versatile wavelength-tunable light sources for analytical spectroscopy in the mid-infrared (MIR) spectral range. We report on the realization of rapid broadband spectral tuning with kHz scan rates by combining a QCL chip with a broad gain spectrum and a resonantly driven micro-opto-electro-mechanical (MOEMS) scanner with an integrated diffraction grating in Littrow configuration. The capability for real-time spectroscopic sensing based on MOEMS EC-QCLs is demonstrated by transmission measurements performed on polystyrene reference absorber sheets, as well as on hazardous substances, such as explosives. Furthermore, different applications for the EC-QCL technology in spectroscopic sensing are presented. These include the fields of process analysis with on- or even inline capability and imaging backscattering spectroscopy for contactless identification of solid and liquid contaminations on surfaces. Recent progress in trace detection of explosives and related precursors in relevant environments as well as advances in food quality monitoring by discriminating fresh and mold contaminated peanuts based on their MIR backscattering spectrum is shown.

We present standoff detection of various explosives by backscattering spectroscopy, using a sensing system based on mid-IR external-cavity quantum cascade lasers (EC-QCL) with a broad tunable range of about 300 cm-1. Traces of TNT (trinitrotoluene), PETN (pentaerythritol tetranitrate) and RDX (cyclotrimethylenetrinitramine) as well as different nonhazardous substances were investigated by illuminating them with the EC-QC laser and collecting the diffusely backscattered light. Tuning the EC-QCL across the characteristic absorption spectra enables us to detect and identify the explosives against a background of non-hazardous materials.

We present a comprehensive analysis of the wall-plug efficiency of mid-infrared (λ ∼ 4–10 μm) quantum cascade (QC) lasers, by taking into account the transition efficiency of the active region, the light out-coupling efficiency from the emission facet, the voltage efficiency across the superlattice, and the current efficiency including the leakage and thermal backfilling current densities at threshold. All relevant parameters including their interdependencies have been taken into account and analyzed in order to optimize the wall-plug efficiency of practical QC lasers. Expressions for optimizing the extraction energy Δinj and the mirror loss αm are deduced, which are then used to maximize the wall-plug efficiency of QC lasers. Finally, predictions for the wall-plug efficiency of realistic QC lasers are made both at room temperature and at low temperature. Good agreement is found with corresponding experimental data from the literature.

We report the improvement of quantum cascade lasers emitting at λ∼5 μm by introducing AlAs blocking barriers together with strain-compensating InAs layers into the active regions. The blocking barriers are designed to selectively prevent electrons in the initial laser state from tunneling out of the active region, while maintaining the high tunneling probabilities of the electrons in the final laser states. Adopting blocking barriers, the maximum peak power per facet at 77 K (300 K) is increased from 285 (30 mW) to 900 mW (240 mW), and the maximum operation temperature in pulsed mode has been improved from 320 to 350 K with respect to a reference sample without blocking barriers.

An overview of quantum cascade detector technology for the near- and mid-infrared wavelength range will be given. Thanks to photovoltaic instead of photoconductive operation, quantum cascade detectors offer great opportunities in terms of detection speed, room temperature operation, and detectivity. Besides some crucial issues dealing with fabrication and general characteristics, some possibilities for performance improvement will also be briefly presented. In a theory section, some basic considerations adopted from photoconductive detectors confirm the necessity of various trade-offs for the optimization of such devices. Nevertheless, we will show several possible measures to push the key performance figures of these detectors closer to their physical and technological limits.

A midinfrared quantum cascade detector with a spectrally broad (ΔE/E = 27.3%) response is designed, fabricated, and tested. This detector consists of 26 differently designed active region stages in order to cover a wavelength region from 4.7 to 7.4 μm. The device could be operated above room temperature and showed peak responsivities of 13 mA/W at 10 K and 1.25 mA/W at room temperature. A background limited detectivity of 1.55×1010 Jones was seen up to a temperature TBLIP of 110 K.

Spectral beam combining of linear arrays of quantum cascade (QC) lasers emitting at 4.6 µm is demonstrated as a viable approach towards power-upscaling. By using an external cavity setup equipped with a diffraction grating and a partially transmitting output mirror providing wavelength selective feedback to each emitter, up to 8 individual QC lasers are combined with an optical coupling efficiency of 60% for an array of 6 emitters. The resulting beam quality (M2

Results on the detection of traces of trinitrotoluene (TNT) on different substrate-materials like Aluminum and standard car paint are presented. We investigated different samples with a movable imaging standoff detection system at angles of incidence far away from specular reflection. The samples were illuminated with a tunable mid-infrared external-cavity quantum cascade laser. For collection of the diffusely backscattered light a highperformance infrared imager was used. Trace concentrations of TNT corresponding to fingerprints on realworld- substrates were detected, while false alarms of cross-contaminations were successfully suppressed.

We present a comprehensive analysis of an external-cavity (EC) quantum cascade (QC) laser system, which is based on a coupled-mode system with the length of the EC set equal to that of the laser chip and accommodating the large difference in physical cavity length by introducing an effective speed of light for light propagating in the EC. By analyzing the rate equations and discussing the cross sections and losses of the EC QC lasers, numerical results as well as suggestions for a further optimization of the spectral tuning range of the EC system are presented. Besides, the delayed onset of the EC modes with respect to that of the internal Fabry–Perot modes in pulsed operation has been simulated using the rate equations, yielding a good agreement with the experimental data.

The authors report on short-wavelength In0.53Ga0.47As/AlAs0.56Sb0.44 quantum cascade detectors (QCDs). At room temperature, one device detects at 505 meV (2.46 mu m) with a responsivity of 2.57 mA/W, while a second QCD is sensitive at 580 meV (2.14 mu m) with a responsivity of 0.32 mA/W.

At present GaInAs/AlInAs based quantum cascade (QC) lasers represent the state-of-the-art with respect to the short-wavelength ( 400K for devices emitting at 4.6 μm and an impressive maximum peak output power of 8W at 77K (corresponding to a total power efficiency of 23 %) for a QC laser emitting at 3.7 μm. Furthermore, current limitations of the GaInAs/AlAsSb QC laser concept and challenges for future research are discussed.

The authors extracted the thermal resistance (R-L=9.6 K/W) and the electrical power dependence of the electronic temperature (R-e=12.5 K/W) of Ga0.47In0.53As/Al0.62Ga0.38As1-xSbx quantum-cascade lasers (QCLs) operating at 4.9 mu m, in the lattice temperature range of 60-90 K. The low electron-lattice coupling constant alpha=10.4 K cm(2)/kA can be related to the beneficial effect of the high conduction band offset, peculiar to the GaInAs/AlGaAsSb material system, on the electron leakage. The authors found an active region cross-plane thermal conductivity value k(perpendicular to)=1.8 +/- 0.1 W/(K m), which is approximately three times larger than that measured in QCLs with GaInAs/AlInAs heterostructures. (c) 2007 American Institute of Physics.

We present a study of the spectral characteristics of Fabry-Perot quantum cascade lasers in pulsed mode operation applying a time resolution of 3 ns in combination with a high spectral resolution of 0.02 cm-1. With this technique the laser spectra were investigated applying pulse lengths ranging between 100 ns and 20 μs and duty cycles between 0.01% and 10%. Depending on the current density and operation temperature, the spectra exhibit complex line patterns, which indicate mode competition caused by gain saturation effects.

Short-wavelength (lambda < 4 µm) GaInAs-AlAsSb quantum cascade (QC) lasers have been demonstrated using a "bound-to-continuum" design for the purpose of reducing the electric injection power density. As a result, we have reduced the low-temperature electric injection power density of the lasers by 40%, compared to that of GaInAs-AlAsSb QC lasers emitting at the same wavelength but adopting a triple-quantum-well design. The lasers in the present report can operate up to room temperature (300 K) in pulsed mode, emitting at short-wavelength lambda about 3.7 - 3.9 µm.

This paper presents the operation and fabrication of GaInAs/Al(Ga)AsSb quantum cascade lasers. This article also focuses on the development of processing technology to enhance the heat removal from the active region of the lasers by depositing a thick gold layer on both sides of the laser ridge. Due to the improved heat removal devices emitting at around 4.9 /spl mu/m, the quantum cascade laser can be operated at high duty cycle (/spl ges/ 50%) up to a heat-sink temperature of 130 K.

We report on the demonstration of continuous-wave (CW) operation of GaInAs-AlGaAsSb quantum cascade (QC) lasers. By placing a 2.5-/spl mu/m-thick gold layer on both sides of the laser ridge to extract heat from the active region in the lateral direction, together with mounting the device epilayer down, we have achieved CW operation of GaInAs-AlGaAsSb QC lasers composed of 25 stages of active/injection regions. The maximum CW operating temperature of the lasers is 94 K, and the emission wavelength is around /spl lambda//spl sim/4.65 /spl mu/m. For a device with the size of 10/spl times/2000 /spl mu/m/sup 2/, the CW optical output power per facet is 13 mW at 42 K and 4 mW at 94 K. The CW threshold current density is 1.99 kA/cm/sup 2/ at 42 K, and 2.08 kA/cm/sup 2/ at 94 K, respectively.

We present pulsed operation of index-coupled distributed feedback quantum cascade lasers based on the GaInAs/AlInAs/InP materials system emitting at a wavelength around 5.4 μm. The emission is single mode in the entire investigated temperature range between 240K and 350K with a side mode suppression ratio larger than 27 dB. These devices are employed in a fast gas detection experiment for the quantitative detection of nitric oxide. With the present measurement system minimum noise equivalent concentrations between 16.7 ppbv and 23.3 ppbv are obtained, corresponding to minimum detectable optical densities between 4.7•10-5 and 6.5•10-5.

Summary form only given. We report on a new design of GaInAs-AlInAs quantum cascade lasers (QCLs) improving the concept of Bragg reflectors by incorporating "blocking barriers" into the active regions. These barriers significantly enhance the confinement of electrons in the initial state of the laser transition by selectively blocking direct tunneling through the exit barrier (reducing the tunneling probability by a factor of 2). The high tunneling probability of electrons in the final state into the injector's miniband is maintained by reducing the total width of the exit barrier appropriately.

We point out the difference between the two mostly used methods for calculating the effective refractive index of mid-infrared quantum cascade lasers, namely by solving the Maxwell's wave equation and by analyzing the frequency spacing of the longitudinal modes of the Fabry-Perot cavity. The effective refractive indices obtained by these methods are shown to be different, as one refers to the phase effective refractive index while the other refers to the group effective refractive index, respectively. Dispersion relationships for these two effective refractive indices are deduced for mid-infrared quantum cascade lasers, which show an increase in group refractive index and a decrease in phase refractive index with wavelength. Experiments are conducted to obtain the group effective refractive indices of quantum cascade lasers emitting at various wavelengths, and good agreement has been achieved between the experimental data and theory.

Employing a “slightly diagonal” active region design for the quantum cascade lasers compared to a reference sample based on the conventional vertical transition design [R. Kohler et al., Appl. Phys. Lett. 76, 1092 (2000)], we have improved the maximum operation temperature, room-temperature maximum peak power per facet, and room-temperature slope efficiency from 320 K, 200 mW, and 570 mW/A to higher than 360 K, 3.2 W, and 2200 mW/A, respectively, for the device size of 16 μm×3 mm with as-cleaved facets operated in pulsed mode.

The authors present a technique for broadband study of the optical waveguides used for midinfrared quantum cascade lasers. This method allows insight into the waveguide properties and helps in improving the waveguide design. Using this method the waveguide losses in the whole range of waveguide transparency of about 1.8–9μm have been found for a λ∼4.4μm In0.53Ga0.47As∕AlAs0.56Sb0.44∕InP quantum cascade laser. The polarization dependence of waveguide losses in a quantum cascade laser is discussed.

Short-wavelength (λ∼4μm) single-mode distributed-feedback GaInAs∕AlAsSb quantum-cascade lasers operating in pulsed mode up to room temperature (300K) have been demonstrated by etching an index-coupled first-order distributed-feedback grating into the upper GaInAs separate confinement layer. The temperature-dependent wavelength shift of the distributed-feedback lasers is −0.14cm−1∕K (0.238nm∕K). For devices with a size of 18μm×2.9mm mounted epilayer-up with as-cleaved facets, a maximum peak power per facet of 840mW has been achieved at 77K and 4mW at 300K. The characteristic temperature T0 of the threshold current density is 105K.

We discuss the possible limitations on realizing short-wavelength GaInAs∕AlAsSb quantum-cascade lasers and consequently demonstrate room-temperature (Tmax=310K) short-wavelength (λ∼3.7–3.9μm) GaInAs∕AlAsSb quantum-cascade lasers based on triple-well vertical-transition active regions. For a device with the size of 14μm×3.0mm with as-cleaved facets, the maximum peak powers per facet are around 31mW at 300K and 17mW at 310K. The characteristic temperature of the laser is 170K in the temperature range between 220 and 310K.

Above room-temperature (T⩾400K) operation of GaInAs∕AlAsSb-based quantum-cascade lasers has been demonstrated. The lasers are based on vertical-transition active regions and consist of 25 periods of Ga0.47In0.53As∕AlAs0.56Sb0.44 active∕injection regions grown lattice-matched on InP substrates by molecular-beam epitaxy. They emit at a wavelength of λ∼4.5μm. For a device with the size of 18μm×2.8mm mounted substrate-side down with as-cleaved facets, a maximum peak power per facet of 750mW has been achieved at 300K and remains as high as 30mW at 400K. The characteristic temperature T0 of the threshold current density is 171K in the temperature range between 280K and 400K.

Room-temperature (300 K) intersubband electroluminescence has been obtained from a quantum cascade structure adopting triple-well vertical-transition active regions, based on Ga0.47In0.53As/AlAs0.56Sb0.44 heterostructures grown lattice-matched on InP substrate by molecular beam epitaxy. The emission peak wavelength varies from 4.3 µm at 77 K to 4.5 µm at 300 K.

A report is presented on the continuous-wave operation of λ≃5 µm GaInAs/AlInAs/InP-based quantum cascade lasers up to a heatsink temperature of 222K (−51°C). The devices are mounted epi-side down with high-reflection coated facets.

Based on broadly tunable external cavity quantum lasers (EC-QCL) an imaging system has been developed for stand-off detection of traces of explosives. A similar EC-QCL serves for detection of hazardous chemicals in drinking water.

Quantum cascade lasers emitting at λ ∼ 5µm based on different ac- tive region designs are investigated. Using lattice-matched GaInAs/AlInAs on InP substrates the maximum peak optical power as well as the maximum pulsed-mode operating temperature is enhanced by incorporating AlAs blocking barriers together with strain-compensating InAs layers into the active regions. Further improvement is achieved by employing strain-compensated GaInAs/AlInAs quantum wells for which maximum pulsed-mode operating temperatures in excess of 350K are ob- served. High-reflectivity coated devices mounted substrate-side down show a maxi- mum continuous-wave operating temperature of 194 K. Also the normalized relative intensity noise is investigated. Finally, a comparison trace-gas sensing experiment employing one of the present quantum cascade lasers and a lead-chalcogenide laser is presented. Detecting the P(25) absorption line of CO, higher stability is obtained using a quantum cascade laser.

External-Cavity-Quantum Cascade Lasers are attractive sources for mid-infrared spectroscopy. We demonstrate the possibilities offered by EC-QCLs by two examples: Standoff detection of explosives and spectroscopic detection of contaminations in water