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We present a high-resolution electron tomography study of buried ultra-thin layers and their interfaces from the active region of a (Ga,In)As/(Al,In)As quantum cascade laser (QCL) test structure. Using a high-angle annular dark-field scanning transmission electron microscopy image series, a three-dimensional (3D) reconstruction of the complex layer structure is obtained. From this 3D information, we determine quantitative values for the chemical width and, simultaneously and independently, the root mean square roughness (rms) and lateral correlation length of the individual interfaces of a cascade using topographic height maps. The interfacial widths are comparably small for all interfaces within a cascade and the layer thicknesses show only a small standard deviation of less than one monolayer. The rms roughness is systematically lower at the direct (Ga,In)As-on-(Al,In)As interface compared to its inverse. In addition, using the one-dimensional height–height correlation function, different lateral correlation lengths along the two perpendicular ⟨ 011 ⟩ in-plane directions are detected indicating a distinct anisotropy of the interface morphology. Our accurate and comprehensive results on the QCL test structure will serve as feedback to evaluate the growth process and help to assess the performance of corresponding future laser devices in detail.

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.

A general eigen equation has been deduced that can handle ideal and non-ideal quarter-wavelength cases of Bragg-reflection waveguide structure for nonlinear interaction with matching layers on each side of the active region. In particular, this equation allows for solving the cases when the Bragg reflectors are non-ideal quarter-wavelength thick. With this equation, we propose a Bragg-reflection waveguide structure based on the A l x G a 1 − x A s / G a A s material system with high figure-of-merit | ξ d e f f | total nonlinearity and checked the influence if the thickness of the quarter-wavelength Bragg reflectors is off by 10%.

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.

We perform 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. Using a MIR EC-QCL with a tuning range from 7.5 μm to 10 μm, detection of a large variety of explosives, e.g. TNT, PETN and RDX and precursor materials such as Ammonium Nitrate could be demonstrated. In a real world scenario stand-off detection over distances of up to 20 m could be successfully performed. This includes measurements in a post blast scenario demonstrating the potential of the technique for forensic investigations.

In the recent past infrared laser backscattering spectroscopy using Quantum Cascade Lasers (QCL) emitting in the molecular fingerprint region between 7.5 μm and 10 μm proved a highly promising approach for stand-off detection of dangerous substances. In this work we present an active illumination hyperspectral image sensor, utilizing QCLs as spectral selective illumination sources. A high performance Mercury Cadmium Telluride (MCT) imager is used for collection of the diffusely backscattered light. Well known target detection algorithms like the Adaptive Matched Subspace Detector and the Adaptive Coherent Estimator are used to detect pixel vectors in the recorded hyperspectral image that contain traces of explosive substances like PETN, RDX or TNT. In addition we present an extension of the backscattering spectroscopy technique towards real-time detection using a MOEMS EC-QCL.

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.

The versatility of solid-source molecular beam epitaxy for the growth of InP/InGaAs heterojunction bipolar transistors (HBTs) is provided by its excellent control of doping and composition grading profiles in combination with its efficiency for carbon doping. Various designs using doping grading or composition grading in the base are investigated to provide a built-in quasi-electric field that enhances electron transport. All graded-base devices exhibit higher current gains (beta), as compared to uniform-base structures, but the beta improvements are found to be nonproportional to the generated built-in drift fields. The best performances are obtained with a 9% linear composition grading profile. As compared to conventionally grown uniform-base structures, the linearly graded-base HBTs show higher current gains (up to 42%), which is of particular importance particularly in analog and mixed-signal applications.

The near-field intensity distributions of quantum-cascade lasers emitting at λ = 7.7 μm were investigated with a Gaussian telescope, comprising standard optical components together with a microbolometer focal plane array. Although the technique is operated at the diffraction limit, the quality of the near-field patterns obtained allows unambiguous determination of the lateral-mode structure of the laser even for individual laser pulses. The high quality of the recorded near-field intensity distribution allows us to calculate analytically the corresponding far-field intensity distributions, leading to an excellent match with measured far-field patterns. The present experimental technique allows fast and efficient characterization of the beam propagation properties of mid-infrared semiconductor lasers.

We investigated the beam-steering phenomenon of quantum-cascade lasers emitting at ∼7.8 µm wavelength, operated in pulsed mode at heat sink temperatures around room-temperature. The lasers have a dimension of 12 µm × 1500 µm. The observed beam-steering phenomenon has been explained by considering spatial hole burning in the population inversion of conduction electrons along the lateral direction. Rate equations have been set up to describe the lateral hole burning effect. The simultaneous oscillation of at least two lateral modes, which are a consequence of the lateral hole burning and at the same time possible cause of beam steering, has been verified by high-resolution spectroscopy. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

Molecular beam epitaxial (MBE) growth and lasing operation of quaternary GaInAsSb/AlGaAsSb-based optically-pumped vertical-external-cavity surface-emitting lasers (VECSEL) emitting at a wavelength of 2.0 μm are reported. MBE growth of such structures is particularly challenging as it requires, apart from the large total thickness of the epitaxial layer stack of 10–12 μm a change of group-III fluxes for the growth of AlAsSb and low Al-content AlGaAsSb. Two different growth schemes are compared. The first one is the conventional growth procedure in which the wafer remains in the growth chamber and the growth is interrupted at the interfaces before and after the active region to adjust the group-III cell temperatures for different flux settings. At the interfaces where the growth had to be interrupted, secondary-ion mass-spectrometry revealed the unintentional incorporation of In at a concentration equivalent to 1–2 monolayers. Here we introduce a new growth procedure—the sequential growth scheme where each section of the VECSEL is grown separately, and after the growth of a section, the sample is taken out of the growth chamber and stored in the buffer chamber while the group-III cell temperatures are adjusted. After the new group-III fluxes have been stabilized, the wafer is transferred back into the growth chamber and the next section of the laser is grown. This way the unintentional incorporation of In at interfaces between different sections of the VECSEL structure can be avoided. A comparison of nominally identical VECSEL structures grown within the same growth campaign using the two different growth procedures reveals an increase of the maximum output power of nearly 100% for a 2.0 μm emitting VECSEL structure grown with the sequential growth scheme accompanied by an improvement of the optical-to-optical power conversion efficiency from 14.4% to 21.5%.

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 .

In this work we present a hyperspectral image sensor based on MIR-laser backscattering spectroscopy for contactless detection of explosive substance traces. The spectroscopy system comprises a tunable Quantum Cascade Laser (QCL) with a tuning range of 7.5 μm to 9.5 μm as an illumination source and a high performance MCT camera for collecting the backscattered light. The resulting measurement data forms a hyperspectral image, where each pixel vector contains the backscattering spectrum of a specific location in the scene. The hyperspectral image data is analyzed for traces of target substances using a state of the art target detection algorithm (the Adaptive Matched Subspace Detector) together with an appropriate background extraction method. The technique is eye-safe and allows imaging detection of a large variety of explosive substances including PETN, RDX, TNT and Ammonium Nitrate. For short stand-off detection distances (

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.

Short wavelength (lambda < 5 mu m) quantum cascade lasers are of current interest as they operate in the technologically important atmospheric 3-5 mu m transparency window. For this wavelength range the GaInAs/AlAsSb-on-InP material system offers the advantage of a large conduction band offset of about 1.6 eV over the well-established lattice-matched GaInAs/AlInAs-on-InP material combination with a band offset of only 0.5 eV. In this paper, we report on molecular beam epitaxial growth and subsequent structural and compositional analysis of GaInAs/AlAsSb quantum wells as well as quantum cascade laser structures. Special emphasis has been laid on establishing a growth procedure which allows the growth of these structures without growth interruption at the GaInAs/AlAsSb interfaces. The epitaxial layer sequences were analyzed by high-resolution X-ray diffraction, secondary ion mass spectrometry, and transmission electron microscopy. Finally, mesa waveguide GaInAs/AlAsSb QC lasers were fabricated emitting around 4.5 mu m, which could be operated in pulsed mode up to 400 K.

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).

Molecular beam epitaxial growth and operation of quaternary GaInAsSb/AlGaAsSb-based optically pumped vertical-external-cavity surface-emitting lasers (VECSEL) emitting at wavelengths 2.X mu m are reported here. The epitaxial layer sequence of a VECSEL consists of three different sections: a distributed Bragg reflector (DBR), an active region, and a window layer terminated by a capping layer. The Al content of the AlGaAsSb layers Varies between 100% in the DBR and 30% in the barriers separating the compressively Strained GaInAsSb quantum wells (QW) in the active region. While the active region was grown without growth interruption, growth had to be interrupted at the interfaces between the above-mentioned sections to adjust the group-HI cell temperatures for different flux settings. At the growth interruption interfaces secondary-ion mass-spectrometry revealed the unintentional incorporation of In at a concentration equivalent to 1-2 monolayers. Inspire of the formation of InAsSb-like interfacial layers between the DBR and the active region on the one hand, and the active region and the window layer on the other, high-performance VECSELs emitting in the 2.0-2.3 mu m wavelength range have been grown. A maximum output power of 3.4 W at 2.25 mu m and -10 degrees C heat-sink temperature has been achieved with a maximum optical-to-optical power conversion efficiency of 24%.

In this work we demonstrate imaging standoff detection of solid traces of explosives using infrared laser backscattering spectroscopy. Our system relies on active laser illumination in the 7 μm-10 μm spectral range at fully eye-safe power levels. This spectral region comprises many characteristic absorption features of common explosives, and the atmospheric transmission is sufficiently high for stand-off detection. The key component of our system is an external cavity quantum cascade laser with a tuning range of 300 cm -1 that enables us to scan the illumination wavelength over several of the characteristic spectral features of a large number of different explosives using a single source. We employ advanced hyperspectral image analysis to obtain fully automated detection and identification of the target substances even on substrates that interfere with the fingerprint spectrum of the explosive to be detected due to their own wavelength-dependent scattering contributions to the measured backscattering spectrum. Only the pure target spectra of the explosives have to be provided to the detection routine that nevertheless accomplishes reliable background suppression without any a-priory-information about the substrate.

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.

We report on investigations of far-field intensity patterns and angle-resolved lasing spectra emitted fromquantum cascade (QC) lasers at wavelengths around Lamda = 7.7 µm in pulsed operation. Measurements of the angle-dependent intensity noise in the far-field reveal that small driving currents lead to an incoherent superposition of multiple lateral cavity modes. Elevated currents result in a phase-coupled superposition of the light fields arising from the excited lateral modes. This superposition leads to beam steering and strong pulse-to-pulse intensity fluctuations. We present for both the incoherent and the phase-coupled superposition, a model which allows us to explain the observed far-field intensity patterns and their noise properties. The angle-resolved lasing spectra of the investigated QC lasers show distinct sets of longitudinal modes with distinctively different dependences of the lasing spectrum on the far-field angle. They can be assigned to the incoherent and the phase-coupled superposition of the lateral modes.

We demonstrate contactless detection of solid residues of explosives using mid-infrared laser spectroscopy. Our detection scheme relies on active laser illumination, synchronized with the collection of the backscattered radiation by an infrared camera. The key component of the system is an external cavity quantum cascade laser with a tuning range of 300 cm -1 centered at 1220 cm -1 . Residues of TNT (trinitrotoluene), PETN (pentaerythritol tetranitrate) and RDX (cyclotrimethylenetrinitramine) could be identified and discriminated against non-hazardous materials by scanning the illumination wavelength over several of the charact eristic absorption features of the explosives. 1. INTRODUCTION Different techniques for the detection of explosives have been proposed and demonstrated during the last years. They may be grouped into two different classes, characterized by the scenario in which they can be applied. So-called cooperative situations pr ovide the possibility of sampling cl ose to the object to be inve stigated, e.g. by accumulation of air, preparation of swipe samples, and setting up well-defined measurement conditions. Under these conditions, very high sensitivity can be achieved with st ate-of-the-art techniques such as ion-mobility spectrometry and related methods [1]- [4]. However, there also exists a strong demand for techniques th at allow contactless identification of hazardous substances without any sample preparation, ideally from safe distances. In case of improvised explosive devices, these distances can reach several tens to hundreds of meters e.g. for car bombs. But also detection distances of a few meters are highly useful, if the detection system allows identification of unknown substances without making physical contact, thus minimizing the risk of ignition. An example for the latter situation may be the identification of chemicals in an illicit laboratory. While no established technique is able to fulfill these demands yet, there have been numerous reports on possible approaches. For the detection of larger amounts of hidden explosive material (> 100 g) microwave based techniques show potential for short distances (

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 demonstrate imaging standoff detection of traces of explosives using mid-infrared laser spectroscopy. An infrared camera for collection of the diffusely backscattered laser light is used. Hyperspectral image analysis yields low false alarm rates.

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.

In this paper, we report the design, fabrication, and characterization of an InP/InGaAs heterojunction bipolar transistor (HBT) employing a lattice-mismatched In(0.53)Ga(0.47)As/GaAs strained-layer-superlattice (SLS) base structure. The performance of the SLS-base device is also compared with that of an In(0.53)Ga(0.47)As uniform-base structure. The digitally graded-base devices exhibit a slightly lower gain at low bias voltage and a higher current gain at high currents. While the offset voltage remains comparable in the two structures, the graded-base InP/InGaAs HBTs, with built-in fields of - 66 kV/cm, have typically a maximum dc current gain of 18% larger than that of transistors with uniform base.

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

In this contribution we present the results of an imaging stand-off detection system based on a mid-IR external-cavity quantum cascade laser (EC-QCL) with a broad tunable range of 200 cm-1. Traces of TNT (trinitrotoluene) and PETN (pentaerythritol tetranitrate) as well as various non-hazardous substances such as flour or skin cream on different substrate-materials were investigated by illuminating them with the EC-QC laser and collecting the diffusely backscattered light. By tuning the EC-QCL across the significant absorption spectra we were able to detect the explosives

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.

We present experimental results on a Quantum cascade laser (QC laser) embedded in an external cavity. These results were obtained with a broadly tunable laser exceeding 80 cm-1 covering a characteristic absorption band of trinitrotoluene (TNT). By combining the laser source with a high performance IR imager a stand-off detection setup based on multi- spectral MIR backscattering spectroscopy has been realized. With this technique TNT surface-contaminations of as low as 10 μg/cm2 could be detected on surfaces such as an aluminum-sheet and standard car paint. The contrast of the detection technique depends on the reflectance of the surface. A surface leading to mirror-like reflectance of the IR laser radiation leads to absorbance-like signatures of the TNT contamination, while surfaces showing high absorbance of the laser light may induce a contrast-reversal in the resulting image of the TNT coverage. This effect can be explained by a theoretical model for thin film coated substrates taking into account differences in the reflectance. Limitations and further work needed to explore the full potential of the IR backscattering technique are also discussed.

We report on the concept, realization and performance data of infrared semiconductor laser modules serving as compact and robust laser sources for a Directed Infrared Countermeasures (DIRCM) system. While the 2-2.5 µm atmospheric transmission window is covered by a GaSb-based optically pumped semiconductor disk laser (OPSDL), delivering a continuous-wave (cw) or temporally modulated output of t 1 W with a high beam quality (M 2 < 3), an external cavity (EC) quantum cascade (QC) laser module is used to cover the 4.5-5 µm spectral range. The EC-QC laser concept allows efficient spectral beam combining of the output of several QC laser located side-by-side on the same semiconductor chip, while preserving the high-quality output beam of a single emit ter. Both the OPSDL and the EC-QC laser have been inte-grated into rugged laser modules, comprising also all necessary power supply and control electronics, ready for use in field trials. Keywords: Infrared semiconductor lasers, infrared laser module s, directed infrared counter measures (DIRCM), quantum cascade laser, QC laser, optically pumped semiconductor disk laser (OPSDL), vertical-external-cavity surface-emitting laser (VECSEL), spectral beam combining

In this contribution, we demonstrate that spectral beam combining in an external cavity (EC), a technique which has been applied previously to shorter wavelength diode laser bars [1], is also applicable to mid-infrared QC lasers. Within this concept, the output of multiple emitters from a 4.6 μm emitting QC laser chip is combined in a single, collinear beam. The average power of an EC-QC laser module realized that way surpasses the output of a corresponding single emitter by more than a factor of 4. Furthermore, the EC-concept allows a certain degree of wavelength tuning during operation. The EC, consisting of a collimating lens, a grating and a partially reflecting outcoupling mirror, forces each laser to emit at a unique wavelength defined by its offset relative to the main optical axis. The EC approach further ensures the collinear directional and spatial overlap of the individual QC laser output beams forming a single combined output beam.

We report on the temporal and spectral evolution of the lasing spectrum of an external-cavity quantum-cascade (QC) laser operated in pulsed mode. Time-resolved lasing spectra, recorded on nanosecond time scale, revealed that after turn-on of the 100 ns current pulse multiple-mode lasing starts at the gain maximum of the QC laser controlled by the cavity formed by the chip facets. Depending on the spectral tuning after a time delay of 15–35 ns, the QC laser couples to the external cavity as seen from a change in lasing wavelength to that defined by the external grating as wavelength selective element.

We report on the development and characteristics of infrared semiconductor lasers as compact and robust light sources for Directed Infrared Countermeasures (DIRCM). The short-wavelength side of the 2-5 μm wavelength band of interest can be covered by GaSb-based optically pumped semiconductor disk lasers (OPSDLs), delivering a continuous-wave (cw) or temporally modulated multiple-Watt output with a high beam quality (M2