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Reliable standoff detection of traces of explosives is still a challenging task. Imaging MIR backscattering spectroscopy has been shown to be a promising technique for non-contact detection of traces of explosives on various surfaces. This technique, which is eye-safe, relies on active imaging with MIR laser illumination at various wavelengths. Recording the backscattered light with a MIR camera at each illumination wavelength, the MIR backscattering spectrum can be extracted from the three-dimensional data set recorded for each point within the laser illuminated area. Applying appropriate image analysis algorithms to this hyper-spectral data set, chemically sensitive and selective images of the surface of almost any object can be generated. This way, residues of explosives can be clearly identified on the basis of characteristic finger print backscattering spectra and separated from the corresponding spectra of the underlying material. To achieve a high selectivity, a large spectral coverage is a key requirement. Using a MIR EC-QCL with a tuning range from 7.5 μm to 9.5 μm, different explosives such as TNT, PETN and RDX residing on different background materials, such as painted metal sheets, cloth and polyamide, could be clearly detected and identified. For short stand-off detection distances (

The (AlGaIn)(AsSb) material system has been shown to be ideally suited to realize VECSELs for the 2-3 μm wavelength range. In this report we will present results on increasing the output power of the SDL chips with special emphasis on the 2.8 μm emission wavelength by means of low quantum defect pumping. Further on we have investigated concepts for a VECSEL-pumped Q-switched Ho:YAG laser in order to convert the high cw-power of the VECSEL into pulses with a high peak power. Up to 3.3 mJ of pulse energy were achieved with a compact setup (corresponding to a peak power of 30 kW at 110 ns pulse length) combined with stable pulsing behavior.

In this contribution, we report on real-time mid-IR spectroscopy enabled by rapidly tunable External Cavity Quantum Cascade Lasers (EC-QCLs). High speed spectral scanning in a Littrow-type resonator is realized by employing a resonantly driven micro-opto-electro-mechanical-systems (MOEMS) grating as wavelength selective element. Oscillating at a frequency of 1 kHz with mechanical amplitudes of up to 10°, the MOEMS grating is able to cover the whole spectral range provided even by broad-gain QCL chips in just 500 μs. In addition to the high spectral scanning frequency, the MOEMS approach also allows for a miniaturized and rugged design of the EC-QCL. An evaluation of this laser source with regard to spectral reproducibility of consecutive scans, pulse intensity noise, and spectral resolution will be given. Furthermore, we present spectroscopic measurements in backscattering as well as in transmission geometry, demonstrating the real-time capability in different scenarios.

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.

Using the (AlGaIn)(AsSb) material system, VECSELs covering the 2 – 3 μm wavelength range can be realized. The best laser performance of GaSb-based VECSELs was achieved so far at emission wavelengths around 2.0 μm with a slope efficiency of more than 30 %, a low threshold pump power density of 1.1 kW/cm2 at 20°C heatsink temperature and concomitant a high output power exceeding 7 W in CW operation (depending on the mounting technology). These parameters were degrading significantly for longer wavelength devices emitting around 2.5 μm and 2.8 μm. But for applications like the generation of MWIR light (3-8 μm) by pumping ZGP-OPOs, high-power VECSELs around 2.5 μm are required to suppress absorption losses, while for medical laser treatment, high-power operation near the water absorption peak at around 2.9 μm is desirable. We will present results of our ongoing research strand for further optimization of the semiconductor heterostructure design of ≥ 2.5 μm emitting GaSb-based VECSELs. By using a low quantum deficit design (i.e. optical pumping at around 1.5 μm) in combination with highly strained QWs (compressive strain 2.1 %) we were able to realize a 2.5 μm emitting VECSEL with a slope efficiency above 30 %, corresponding to an external quantum efficiency exceeding 50 %, and a low threshold pump power density of 0.8 kW/cm2. These values are as good as those for the best performing 2.0 μm VECSELs. With a frontside SiC heatspreader and operated in a standard linear cavity, over 7 W of CW output power were achieved for this 2.5 μm emitting VECSEL structure when operated at 20°C. Furthermore, we will compare laser structures with different emission wavelengths and discuss the role of the QW strain, band-offset and active region composition on laser performance.

In phacoemulsification-based cataract surgery, a corneal incision is made and is then closed by hydration of the wound lips, or by suturing. We developed a system for sealing such an incision by soldering with a semiconductor disk laser (λ=1.9μm), under close temperature control. The goal was to obtain stronger and more watertight adhesion. The system was tested on incisions in the corneas of 15 eyes of pigs, in-vivo. Optical Coherent Tomography (OCT) and histopathologic examination showed little thermal damage and good apposition. The measured average burst pressure was 1000±30mmHg. In the future, this method wound may replace suturing of corneal wounds, including in traumatic corneal laceration and corneal transplantation.

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.

We present spectroscopic measurements performed with an EC-QCL combining a broadly tunable quantum cascade laser chip with a tuning range of more than 300 cm-1 and a resonantly driven MOEMS scanner with an integrated diffraction grating for wavelength selection in Littrow configuration. The grating geometry was optimized to provide high diffraction efficiency over the wide tuning range of the QCL, thus assuring high power density and high spectral resolution in the MIR range. The MOEMS scanner has a resonance frequency of 1 kHz, hence allowing for two full wavelength scans, one up and the other downwards, within 1 ms. 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 gaseous samples of carbon monoxide. For the latter one, a large portion of the characteristic CO absorption band containing several absorption lines in the range of 2070 cm-1 to 2280 cm-1 can be monitored in real-time.

Long-term stability and reliability of AlGaN/GaN high electron mobility transistors (HEMT) can be validated by various stress tests which allow studying the physical mechanisms responsible for degradation. As the electroluminescence (EL) intensity is related to the kinetic energy and density of the channel electrons accelerated in the electric field, both local current and electric field changes can result in an increase or decrease of the EL intensity. The electric field distribution in source drain direction peaks at the drain side edge of the gate foot and the field plates. It is strongly dependent on the gate design and the passivation /semiconductor interface trap density. Although the spatial resolution of the electroluminescence microscopy (ELM) set-up is limited to a few microns, the technique allows to monitor submicron changes in the device geometry and hence to identify elevated electric field peaks being responsible for the acceleration of the degradation process. ELM also benefits from the fact that submicron imperfections at the Schottky interface of the gate electrode result in strong local current variations. It has been used to selectively identify suitable positions for further failure analysis of focused ion beam prepared cross sections by scanning and transmission electron microscopy. Process induced imperfections as voids at the Schottky interface after stress have been localized.

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 (

In this paper recent advances in broadband-tuneable mid-infrared (MIR) external-cavity quantum cascade lasers (EC-QCL) technology are reported as well as their use in spectroscopic process analysis and imaging stand-off detection of hazardous substances, such as explosive and related precursors. First results are presented on rapid scan EC-QCL, employing a custom-made MOEMS scanning grating in Littrow-configuration as wavelength-selective optical feedback element. This way, a scanning rate of 1 kHz was achieved, which corresponds to 2000 full wavelength scans per second. Furthermore, exemplary case studies of EC-QCL based MIR spectroscopy will be presented. These include timeresolved analysis of catalytic reactions in chemical process control, as well as imaging backscattering spectroscopy for the detection of residues of explosives and related precursors in a relevant environment.

In situ process information in the chemical, pharmaceutical or food industry as well as emission monitoring, sensitive trace detection and biological sensing applications would increasingly rely on MIR-spectroscopic anal­ysis in the 3 μm - 12 μm wavelength range. However, cost effective, portable, low power consuming and fast spectrometers with a wide tuning range are not available so far. To provide these MIR-spectrometer properties, the combination of quantum cascade lasers with a MOEMS scanning grating as wavelength selective element in the external cavity is addressed to provide a very compact and fast tunable laser source for spectroscopic analysis.

This paper presents recent advances of 2-μm GaSb-based vertical external cavity surface emitting laser (VECSEL) with special emphasis on quantum deficit reduction and miniaturization. Operating the VECSEL in a 5-cm long cavity, we could demonstrate an increase in maximum cw output power from 4.2 W to 7.2 W at room temperature when barrier pumping a 2.0-μm emitting VECSEL at a pump wavelength of 1.5 μm instead of 980 nm. Furthermore, miniaturized VECSELs were realized by depositing a high-reflectivity (~97 %) coating on top of a 375-μm thick SiC heat spreader, which acts as output coupler of the micro cavity (μC) formed. This planar cavity is rendered stable by thermal lensing induced by the absorption of pump light. At the same time, thermal lensing influences the beam quality. We will report a detailed study of the influence of the thermal lens on the stability and beam diameter of the μC-VECSEL by using two different VECSEL structures optimized for 980 nm and 1.5 μm barrier pumping, respectively. Using different pump photon energies results in different amounts of heat generated at a given pump photon flux, and thus thermal lenses with different focal lengths. Using the low-quantum deficit pumping scheme we could achieve a factor-7 increase in output power in TEM 00 emission from the μC-VECSEL compared to the 980 nm pumped device, as well as a maximum output power of 2.2 W. This 2-μm μC-VECSEL exhibits 110-nm tunable single-frequency emission at a 7-MHz linewidth at an output power of up to 90 mW. The linewidth of the μC-VECSEL is comparable to that of VCSELs, which typically emit output powers in the milli-Watt range.

We demonstrate pulse periods from 0:13 to 10 ns of GaN{based ridge waveguide laser diodes with monolithically integrated absorbers in the regimes of relaxation oscillations and self{Q{switching as function of gain current and absorber voltage. We introduce a simple model for the self{Q{switching regime, describing the pulse period in terms of current injection and spontaneous emission (including Auger recombination), only. At reverse voltages larger than 35V the modal absorption exceeds 500 cm-1, which cannot be explained solely by transitions of bound states in the quantum wells. Calculations based on wavefunction overlap and quantum con ned Stark e ect (QCSE) predict a decrease of absorption at such large bias. In contrast, we show experimental ndings, proving that the absorption further increases. Due to the strong tilt of the band pro le in this regime, we take into account the Franz{Keldysh e ect in the barriers and the waveguide and discuss its possible in uence on the absorption, leading to an increased absorption at large reverse bias.

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.

Dielectric elastomer actuators (DEAs) can be optimized by modifying the dielectric or mechanical properties of the electroactive polymer. In this work both properties were improved simultaneously by a simple process, the one-step film formation for polyurethane and silicone films. The silicone network contains polydimethylsiloxane (PDMS) chains, as well as cross-linker and grafted molecular dipoles in varying amounts. The process leads to films, which are homogenous down to the molecular level and show higher permittivities as well as reduced stiffnesses. The dipole modification of a new silicone leads to 40 times higher sensitivities, compared to the unmodified films. This new material reaches the sensitivity of the widely used acrylate elatomer VHB4905. A similar silicone modification was obtained by using smart fillers consisting of organic dipoles and additional groups realizing a high compatibility to the silicon network. Polyurethanes are alternative elastomers for DEAs which are compared with the silicones in important properties. Polyurethanes have an intrinsically high dielectric constant (above 7), which is based on the polar nature of the polyurethane fragments. Polyurethanes can be made in roll-to-roll process giving constant mechanical and electrical properties on a high level.

Electroactive polymers are soft capacitors made of thin elastic and electrically insulating films coated with compliant electrodes offering a large amount of deformation. They can either be used as actuators by applying an electric charge or they can be used as energy converters based on the electrostatic principle. These unique properties enable the industrial development of highly efficient and environmentally sustainable energy converters, which opens up the possibility to further exploit large renewable and inexhaustible energy sources like wind and water that are widely unused otherwise. Compared to other electroactive polymer materials, polyurethanes, whose formulations have been systematically modified and optimized for energy harvesting applications, have certain advantages over silicones and acrylates. The inherently higher dipole content results in a significantly increased permittivity and the dielectric breakdown strength is higher, too, whereby the overall specific energy, a measure for the energy gain, is better by at least factor ten, i.e. more than ten times the energy can be gained out of the same amount of material. In order to reduce conduction losses on the electrode during charging and discharging, a highly conductive bidirectional stretchable electrode has been developed. Other important material parameters like stiffness and bulk resistivity have been optimized to fit the requirements. To realize high power energy harvesting systems, substantial amounts of electroactive polymer material are necessary as well as a smart mechanical and electrical design of the generator. In here we report on different measures to evaluate and improve electroactive polymer materials for energy harvesting by e.g. reducing the defect occurrence and improving the electrode behavior. © (2013) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

We develop a monolithic picosecond laser pulse generator, based on the classical design of a group-III-nitride Fabry-Perot laser diode with electrically separated ridge sections. We use two different multi-section design variants, with the absorber section placed either in the center or at the end of the ridge. Profiting from the very low lateral conductivity in the p-type GaN top contact layer, we implement the multi-section concept just by etching off small sections of the top metalization on the ridge. The physical mechanism underlying short pulse generation within such system, operating in the 400 - 435nm wavelength range, strongly depends both on the reverse bias applied to the absorber and the forward current in the gain section. Varying the applied reverse bias affects both the absorption and the carrier lifetime in the absorber section through changes in the QW internal field. In consequence we can distinguish between different modes of operation. For moderately long carrier lifetimes the absorber stabilizes relaxation oscillations in the GHz frequency range and self-pulsation occurs, of relatively long duration. With increasing reverse bias, and thus decreasing carrier lifetime, we observe a transition to self-Q-switching. Finally, at large enough negative bias, the carrier life time in the absorber is so short that the laser diode operates in a passive self-mode-locking regime with a repetition rate of 87 GHz and pulse duration of 2 ps for a cavity length of 540 μm.

In this paper electro-optic cavity dumping of a 2 μm semiconductor disk laser is reported. Using this approach, pulsed mode operation with 3 ns pulse length, 30 W of peak power and pulse repetition frequencies between 87 kHz and 1 MHz has been achieved. For cavity dumping, a birefringent polarizer prism was inserted into the V-shaped external cavity formed by high-reflectivity mirrors, in order to establish a linear polarization of the intra-cavity field. The polarization of the intra-cavity field could be rotated by 90° when applying a voltage to a Pockels cell placed inside the cavity close to the end mirror. Photons with rotated polarization undergo then total internal reflection inside the polarizing prism and are coupled out of the cavity sideways. Furthermore, a model based on standard laser rate equations has been developed, which excellently reproduces the measured pulse shape and temporal evolution of the intracavity power. Based on this model, we have studied the pulse length and shape as well as peak power and pulse energy as a function of cavity length in order to explore routes for further increasing peak output power and pulse energy.

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.

Picosecond (ps) lasers provide a universal tool for material processing. Due to the short pulse length material is removed by a process called "cold ablation", with minimal thermal damage to neighbouring regions. As a result, better defined structures with smother and cleaner side walls can be fabricated than with nanosecond (ns) laser pulses. This offers new possibilities for laser processing in semiconductor technology for both semiconductor materials as well as contact and bond metallizations. The fabrication of optoelectronic devices such as light-emitting diodes (LEDs) typically involves photolithography steps, requiring specific lithography masks be fabricated which, in particular for prototyping, is expensive and time consuming. Therefore it would be attractive for a range of applications to replace these steps by direct writing techniques such as laser processing, which will speed up e.g. the development and prototyping of new devices. We report on fully laser processed planar GaN-based LEDs fabricated without any photolithography steps. On the bare semiconductor wafer, isolation trenches and mesa structures are formed directly by ultraviolet ps laser pulses. For the direct deposition of patterned ohmic contact metallizations, the ps laser fabrication and subsequent use of high resolution shadow masks is presented, which exhibit a significantly reduced sidewall roughness compared to masks produced by ns laser pulses. Due to the higher precision of the laser defined masks it becomes possible to deposit multiple layers, through the use of alignment marks, similar to multiple mask level photolithography. Finally, the ps laser processed LEDs are electrically and optically characterized and their characteristic compared with that of conventionally fabricated mesa LEDs.

The (AlGaIn)(AsSb) semiconductor materials system has been shown to be ideally suited to realize optically pumped Vertical External Cavity Surface Emitting Lasers (VECSELs) for the 2-3 μm wavelength range. For a 2 μm emitting single-chip VECSEL with a ternary GaInSb quantum well active region, employing a linear planar-concave resonator geometry, a maximum continuous wave (cw) output power of 4.2 W at 20°C heatsink temperature (6 W at 0°C) has been achieved. Standard fiber-coupled 980 nm diode lasers have been used for optical barrier pumping. Employing a W-shaped resonator with two optically pumped gain chips acting as planar folding mirrors, a maximum cw output power of 5.5 W has been achieved at a heatsink temperature of 20°C, increasing to 10.4 W when reducing the heatsink temperature to -10°C. The resonator versatility of a VECSEL also allows the insertion of additional optical elements into the optical cavity for wavelength selection and linewidth control. Employing a V-shaped folded cavity with the gain chip acting as the planar end mirror, single-mode operation with a linewidth

We develop (Al, In)GaN ridge waveguide laser diodes in the violet-blue spectral region. Varying the indium content of the InGaN quantum wells, we tailor the emission wavelength of our devices for specific applications in the range from 390 nm to 425 nm. Using different commercially available free standing GaN substrates, we adjust the epitaxial design and optimize the processing sequence for the different kinds of substrates. In particular we focus on the ridge formation. We compare different fabrication methods to obtain devices with ridge widths around ≤2 μm. So far, we have achieved threshold currents around 60 mA and slope efficiencies exceeding 1 W/A.

In this paper we report on the development of narrow-linewidth vertical-external-cavity surface-emitting laser (VECSEL) at a wavelength of >2 μm. Starting from a laboratory setup, we designed a highly stable VECSEL module machined from a solid block of aluminum. For linewidth precise measurements, heterodyne beatnote measurements were employed. For this firstgeneration module a linewidth of 9 kHz was achieved when actively stabilizing the laser wavelength, whereas without stabilization the linewidth amounted to 45 kHz at an output power of 100 mW, both data referring to a 100-μs sampling time. To further increase the output power, a second-generation module was fabricated, for which the on-chip mode diameter was increased. This allowed operation at a larger pump-spot diameter and still maintaining TEM00 operation, while increasing the maximum pump power and hence the output power. This module yielded an output power above 1 W in single-mode operation at a linewidth of 60 kHz (100 μs sampling time) without active wavelength stabilization. Modehop-free single-mode operation could be maintained for more than 18 hours. This new multiple-Watt, narrow-linewidth VECSEL module is apt for plane-to-ground communications without the necessity of amplifiers.

Improving the crystal quality of AlGaN epitaxial layers is essential for the realization of efficient III-nitride-based light emitting diodes (LEDs) with emission wavelengths below 365 nm. Here, we report on two different approaches to improve the material quality of AlGaN buffer layers for such UV-LEDs, which are known to be effective for the MOVPE growth of GaN layers. Firstly, we grew AlGaN on thin GaN nucleation islands which exhibit a threedimensional facetted structure (3D GaN nucleation). Lateral overgrowth of these islands results in a lateral bending of dislocation lines at the growing facets. Secondly, in-situ deposited SiNx interlayers have been used as nano-masks reducing the dislocation density above the SiNx layers. Both approaches result in reduced asymmetric HRXRD ω-scan peak widths, indicating a reduced edge-type dislocation density. They can be applied to the growth of AlGaN layers with an Al concentration of at least 20%, thus suitable for LEDs emitting around 350 nm. On-wafer electroluminescence measurements at 20 mA show an increase in output power by a factor of 7 and 25 for LED structures grown on 3D GaN nucleation and SiNx interlayer, respectively, compared to structures grown on a purely 2D grown low Al-content AlGaN nucleation layer. Mesa-LEDs fabricated from the LED layer sequences grown on buffers with SiNx interlayer exhibit a low forward voltage of 3.8 V at 20 mA and a maximum continuous wave (cw) output power of 12.2 mW at 300 mA.

The recent break-through in semiconductor laser-module technology in the infrared region between 2 μm and 10 μm opens up new windows of opportunity for active sensing, imaging, and modulated power projection. Optically pumped semiconductor disk lasers and quantum cascade lasers cover the infrared spectral range continuously, either with tuneable small bandwidths for active spectral sensing or with broad bandwidth, high power (Watt level), and good diffraction properties (M2

High-power diode lasers in the mid-infrared wavelength range between 1.8μ;m and 2.3μm have emerged new possibilities for applications like processing and accelerated drying of materials, medical surgery, infrared countermeasures or for pumping of solid-state and semiconductor disc lasers. We will present results on MBE grown (AlGaIn)(AsSb) quantum-well diode laser single emitters with emitter widths between 90μm and 200μm. In addition laser bars with 20% or 30% fill factor have been processed. More than 30% maximum wall-plug efficiency in cw operation for single emitters and laser bars has been reached. Even at 2200nm more than 15W have been demonstrated with a 30% fill factor bar. Due to an increasing interest in pulsed operation modes for these mid-infrared lasers, we have investigated single emitters and laser bars at 1940nm for different pulse times and duty cycles. More than 9W have been measured at 30A with 500ns pulse time and 1% duty cycle without COMD for a single emitter. Most applications mentioned before need fiber coupled output power, therefore fiber coupled modules based on single emitters or laser bars have been developed. Single-emitter based modules show 600mW out of a 200μm core fiber with NA=0.22 at different wavelengths between 1870nm and 1940nm. At 2200nm an output power of 450mW ex fiber impressively demonstrates the potential of GaSb based diode lasers well beyond wavelengths of 2μm. Combining several laser bars, 20W out of a 600μm core fiber have been established at 1870nm. Finally for a 7 bar stack at 1870nm we have demonstrated more than 85W at 50A in qcw mode.

Reflective (thick) and semi-transparent (thin) Ni/Ag/Ni contacts were prepared on GaInN-based light-emitting diodes (LEDs) and p-GaN/p-Al0.15Ga0.85N layer sequences. A light output power enhancement of 41% and forward voltage reduction of 0.59 V were obtained compared to a Ni/Au contact for LEDs emitting at 400 nm with thick p-GaN contact layers. The specific contact resistance of the Ni/Ag/Ni contacts on p-GaN/p-Al0.15Ga0.85N with varying p-GaN thickness (5-20 nm) were determined by transmission line method and compared to Ni/Au contacts. Low resistive ohmic contacts were obtained for a p-GaN thickness of less than 10 nm. The p-GaN layer can be completely omitted for the reflective Ni/Ag/Ni contact. In addition, reflection and transmission of the Ni/Ag/Ni metallization schemes were investigated in the ultra-violet spectral range. Thick Ni/Ag/Ni and thin Ni/Ag/Ni covered by Al are promising to serve as reflective contacts for ultra-violet LEDs. The former for wavelength around 350 nm and the latter for wavelengths below 350 nm.

In recent years, optically pumped semiconductor disk lasers (OPSDLs) have attracted increasing interest due to their capability of delivering simultaneously high output power and excellent beam quality. Here we report on group-III-Sbbased OPSDLs allowing to cover the wavelength range around and above 2 μm. First the current state-of-the-art and recent progress for OPSDLs emitting in the 2.0-to-2.3 μm spectral range is presented, which includes power scaling through the use of multiple gain elements and as well as spectral tuning and line width narrowing, exploiting in both cases the versatility of the external cavity concept. Then, results on III-Sb-based OPSDLs emitting at 2.8 μm with a cw output power of up to 0.12 W and a peak output power in pulsed mode of >0.5 W, both data referring to roomtemperature operation, are presented. In both cases, the active region of the OPSDL chip consists of compressively strained GaInAsSb quantum well (QW) layers embedded between AlGaAsSb barrier and pump-light-absorbing layers. The emission wavelength is controlled by adjusting the composition of the quaternary QW material. The active region is grown on top of an epitaxial GaSb/AlAsSb Bragg mirror. For efficient heat extraction, SiC intra-cavity heat spreaders were bonded to the surface of the cleaved laser chips. An N-shaped resonator with one OPSDL chip acting as an end mirror and the second OPSDL chip as a folding mirror was used for power scaling, while a V-shaped resonator configuration with a birefringent tuner inserted into the collimated beam path of the resonator was employed for wavelength tuning. Optical pumping was achieved by standard fiber-coupled diode laser modules emitting at 980 nm.

High-power diode lasers in the mid-infrared wavelength range between 1.8μm and 2.3μm have emerged new possibilities either for direct military applications or as efficient pump sources for laser sources in the 2-4μm wavelength range for infrared countermeasures. GaSb based diode lasers are naturally predestinated for this wavelength range and offer clear advantages in comparison to InP based diode lasers in terms of output power and wall-plug efficiency. We will present results on different MBE grown (AlGaIn)(AsSb) quantum-well diode laser single emitters and linear laser arrays, the latter consisting of 19 emitters on a 1cm long bar, emitting at different wavelengths between 1.8μm and 2.3μm. Single emitters have resonator lengths between 1.0 and 1.5mm and stripe widths between 90μm and 200μm. Laser bars with 20% and 30% fill factors have been processed. For single emitters the electro-optical behaviour, beam quality and wavelength tunability have been investigated in detail. For diode laser bars mounted either on actively or passively cooled heat sinks by Indium or AuSn solder, more than 20W at 1.9μm in continuous-wave mode have been achieved at a heat sink temperature of 20°C resulting in maximum wall-plug efficiencies of 30%. Even at 2.2μm more than 16W have been measured, impressively demonstrating the potential of GaSb based diode lasers well beyond wavelengths of 2μm. Application driven fiber coupled single emitter based modules with 600mW as well as fiber coupled bar based modules with 20W have been realized.

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.

Many applications exist for high performance lasers in the short-wave, mid-infrared spectral regime between 1.9 and 2.5 P m – from long-range communications systems through to remote atmospheric gas sensing and pollution monitoring. However, a simple, efficient laser source offering the desired performance characteristics and flexibility has not been available. In the last few years considerable progress has been made in the development of optically-pumped (AlGaIn)(AsSb) quantum well semiconductor disk lasers emitting in the 2.XP m mid-infrared spectral region - continuous-wave and pulsed-pumped output power levels now exceed 6W and 16W respectively. Furthermore, single-frequency operation with linewidths

Near-UV LEDs emitting at around 400 nm can be used e.g. as pump light source in tri-phosphor RGB white luminescence-conversion LEDs with high color rendering. 1 Although non-thermal roll-over decreases towards shorter emission wavelengths in GaInN-based LEDs, this effect still limits the efficiency of 400 nm emitting LEDs at current densities above 50 A/cm 2 . One way to overcome non-thermal roll-over is to combine a GaInN wide-well active region with the growth on low dislocation density (DD) substrates. Single-well LEDs with GaInN layer widths between 3 nm and 18 nm were grown (a) directly on sapphire substrates with a resulting DD of 10 9 cm -2 , (b) on low DD GaN templates on sapphire (DD of 10 8 cm -2 ), and (c) on freestanding GaN substrates (FS-GaN, DD of 4×107 cm -2 ). At low current densities (pulsed mode operation) the LEDs with a 3 nm GaInN QW active region showed the highest efficiency, irrespective of the substrate. However, the electroluminescence (EL) efficiency peaks at around 50 A/cm 2 and shows a clear non-thermal roll-over towards higher current densities. The efficiency of LEDs with well widths >3 nm grown on sapphire decreases with increasing well width over the whole range of current densities (≤300 A/cm 2 ). However, when grown on low DD GaN templates or FS-GaN, the efficiency of the LEDs with 11 and 18 nm wide GaInN layers surpasses that of the conventional LEDs (well widths ≤6 nm) for current densities ≥250 A/cm 2 , yielding the highest EL efficiency of all LED-structures.

The realization of a compact green-emitting solid state laser source for applications like laser TV or head-up displays is still a challenging task. One way to generate green light with a solid state laser source is nonlinear frequency upconversion (frequency-doubling) of e.g. 1064 nm to 532 nm. In order to achieve good conversion efficiencies tunable laser sources with output powers of several watts, narrow bandwidth and good beam quality are required. We have realized tapered laser diodes based on the GaInAs/AlGaAs material system emitting at a central wavelength of 1064 nm. These devices have an AR-coating on the front facet as well as on the ridge facet. Therefore, these laser diodes can be frequency stabilized in an external cavity setup consisting either of a grating in Littrow mounting placed on the rear side or by an integrated Fiber Bragg grating. The latter configuration allows a compact low footprint integration of the laser diodes into compact laser modules. The optical output power of these devices frequency stabilized at 1064 nm exceeds 4 W with beam qualities suitable for frequency doubling (M 2 < 2) and a tuning range from 1030 nm to 1070 nm. For laser diodes with a HR coating on the ridge facet even higher output powers of more than 8 W are achieved. The ridge and tapered section of the tapered diode amplifiers are contacted separately in order to enable the modulation of the light source by the variation of the ridge current. The rapid temporal modulation achieved this way is a prerequisite for the use of such lasers in flying spot display applications.

A novel active region concept for GaSb-b ased optically pumped mid-infrared ve rtical external cavity surface emitting lasers (VECSELs, also referred to as optically pumped semico nductor disk lasers - OPSDLs) is presented. The concept is based on Ga x In 1-x As y Sb 1-y type-I quantum wells (QWs) embedded between AlAs 0.08 Sb 0.92 barrier layers designed for optical in-well pumping where the pump absorption at pump wavelengths between 1 µm and 2 µm takes place exclusively in the active QWs. This concept provides several advantages such as a high modal gain, the suppression of thermal leakage currents, and an improved thermal conductivity of the active region compared to a conventional GaInAsSb/AlGaAsSb active region design. Using the novel design approach an in-well pumped VECSEL emitting at 2.24 µm has been realized, yielding at a heat sink temperature of 20°C in continuous-wave operation a power slope efficiency of more than 32% and an ab sorption of the 1.96 µm pump light of more than 50% without pump recycling, These data constitute a significant improvement in device perfor mance compared to previously reported data on in-well pumped GaSb-based VECSELs. Keywords: Vertical-external-cavity surface-emitting laser (VECSEL), optically pumped semiconductor disk laser (OPSDL), infrared, (AlGaIn)(AsSb)

There is a high demand for compact low-cost ozone (O 3 ) sensors. It has been shown that indium oxide (In 2 O 3 ) thin films grown by metal-organic vapor-phase epitaxy (MOVPE) act as an O3 sensitive material, which can be activated at roomtemperature by ultraviolet (UV) light. In the present work we integrated the In 2 O 3 sensing thin film and an (AlGaIn)N based near-UV LED back-to-back on a single sensor chip. The integrated In 2 O 3 film-LED sensor was exposed to varying ozone concentrations ranging from 38 ppb to 726 ppb and found to be sensitive to even the lowest O 3 concentration.

We report on recent advances in the performance of GaSb-based optically pumped semiconductor disk lasers (OPSDLs), emitting in the 2.0 - 2.3 μm wavelength range. Both barrier pumped OPSDL (using 980 nm laser diodes as pump source) and in-well pumped OPSDL (using 1.96 μm pump radiation) have been fabricated and characterized. Using alternative SiC or diamond intracavity heatspreader, multiple-watt CW-output powers have been achieved (e.g. >3W at 2.3 μm and >5W at 2.0 μm), with power efficiencies in the range of 18 % - 25 %. For an optimised resonator setup, the beam profile is close to the diffraction limit with M2 values around 1.2; and even for the highest power levels, M2 is in the range of 2-5.© (2008) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Phonons are considered as a most important origin of scattering and dissipation for electronic coherence in nanostructures. The generation of coherent acoustic phonons with femtosecond laser pulses opens the possibility to control phonon dynamics in amplitude and phase. We demonstrate a new experimental technique based on two synchronized femtosecond lasers with GHz repetition rate to study the dynamics of coherently generated acoustic phonons in semiconductor heterostructures with high sensitivity. High-speed synchronous optical sampling (ASOPS) enables to scan a time-delay of 1 ns with 100 fs time resolution with a frequency in the kHz range without a moving part in the set-up. We investigate the dynamics of coherent zone-folded acoustic phonons in semiconductor superlattices (GaAs/AlAs and GaSb/InAs) and of coherent vibration of metallic nanostructures of non-spherical shape using ASOPS.

GaSb based diode laser both as single emitters and as arrays, emitting between 1.9 and 2.2 μm, have a huge potential especially for materials processing, medical applications and as optical pump sources for solid state laser systems emitting in the 2-4 μm wavelength range. Determined by the absorption characteristics of thermoplastic materials at wavelengths around 2 μm, the light emitted by the diode laser will be absorbed by the material itself and can thus be used for marking and welding without the addition of e.g. colour pigments. We will present results on different (AlGaIn)(AsSb) quantum-well diode laser single emitters and linear laser arrays, the latter consisting of 20 emitters on a 1 cm long bar, emitting at different wavelengths between 1.9 and 2.2 μm. To improve on the typically poor fast axis beam divergence of diode lasers emitting at these wavelengths, we abandoned the broadened waveguide concept and changed over to a new waveguide design which features a rather narrow waveguide core. This results in a remarkable reduction in fast axis beam divergence to 43° FWHM for the new waveguide design. Electro-optical and thermal behaviour and the wavelength tunability by current and temperature have been carefully investigated in detail. For single emitters cw output powers of 2 W have been demonstrated. For diode laser arrays mounted on actively cooled heat sinks, more than 20 W in continuous-wave mode have been achieved at a heat sink temperature of 20 °C resulting in wall-plug efficiencies of more than 26%.

The spectroscopic detection of complex molecules, such as explosives, requires a much broader spectral tuning range of the employed laser compared to the well-established tunable diode laser spectroscopy of light molecules. In this paper we demonstrate a new type of spectrally-tunable laser modules, which enables an increase of the tuning range by at least one order of magnitude in comparison to the established current-temperature tuning techniques. This was achieved by increasing the operating temperature range through the use of a temperature-variable diamond submount. The module comprises a quantum cascade (QC) laser mounted on the diamond submount with an integrated heater element and temperature sensor, enabling a controlled temperature change of the laser between 77 K and 400 K at a rate of temperature change of up to 2500 K/s. The favorable temperature characteristics allow us to easily tune the laser wavelength on- and off-resonance with the characteristic absorption bands of the material to be detected. With the QC laser module we demonstrate the detection of surface contaminations of around 10 μg/cm2 TNT performing imaging backscattering spectroscopy.

The internal quantum efficiency as a function of the internal electric field was studied in InGaN/GaN based quantumwell heterostructures. Most striking, we find the IQE to be independent of the electron hole overlap for a standard green-emitting single quantum-well LED structure. In standard c-plane grown InGaN quantum wells, internal piezo-fields are responsible for a reduced overlap of electron and hole wavefunction. Minimization of these fields, for example by growth on non-polar m- and a-planes, is generally considered a key to improve the performance of nitride-based light emitting devices. In our experiment, we manipulate the overlap by applying different bias voltages to the standard c-plane grown sample, thus superimposing a voltage induced band-bending to the internal fields. In contrast to the IQE measurement, the dependence of carrier lifetime and wavelength shift on bias voltage could be explained solely by the internal piezo-fields according to the quantum confined Stark effect. Measurements were performed using temperature and bias dependent resonant photoluminescence, measuring luminescence and photocurrent simultaneously. Furthermore, the doping profile in the immediate vicinity of the QWs was found to be a key parameter that strongly influences the IQE measurement. A doping induced intrinsic hole reservoir inside the QWs is suggested to enhance the radiative exciton recombination rate and thus to improve saturation of photoluminescence efficiency.

We report on the experimental study of the electronic and thermal properties in state of art Sb-based quantum-cascade lasers (QCLs) operating in the range 4.3-4.9 mm. This information has been obtained by investigating the band-to-band photoluminescence signals, detected by means of an InGaAs-array detector. This technique allowed to probe the spatial distribution of conduction electrons as a function of the applied voltage and to correlate the quantum design of devices with their thermal performance. We demonstrate that electron transport in these structures may be insufficient, thus affecting the tunneling of electrons and the electronic recycling and cascading scheme. Finally, we present the first measurement of the electronic and lattice temperatures and of the electron-lattice coupling in Sb-based QCLs based on a quaternary-alloy. We 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 Ga 0.47 In 0.53 As/Al 0.62 Ga 0.38 As 1-x Sb x structures operating at 4.9 mm, in the lattice temperature range 60 K - 90 K. The corresponding electron-lattice coupling a= 9.5 Kcm 2 /kA) reflects the efficient electronic cooling via optical phonon emission. The experimental normalized thermal resistance R L * = 3.9 Kxcm/W demonstrates the beneficial use of quaternary thicker barriers in terms of device thermal management.

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.

We report on the characteristics of (AlGaIn)(AsSb)-based optically pumped vertical-external-cavity surface-emitting lasers (VECSELs) emitting at wavelengths around 2.35 μm. For barrier-pumped VECSELs mounted substrate-side down without substrate thinning, typical room temperature cw output powers of 2 mW were achieved, limited by premature thermal rollover due to strong device overheating. The thermal impedance of the VECSEL semiconductor chip could be considerably reduced by bonding an intra-cavity polycrystalline CVD diamond heat spreader to the top surface of the chip. This way, at -18°C a maximum cw output power of 0.6 W and a slope efficiency of 10% were obtained for a multiple transverse mode output beam limited by the available pump power rather than by thermal rollover. Optimising the resonator for TEM00 mode operation (M2≈1.1), an output power exceeding 0.4 W was achieved. To reduce the large quantum deficit of more than 50% inherent to barrier-pumped (1.06 μm pump wavelength) GaSb-based VECSELs which emit at wavelengths above 2 μm, we realized a first in-well pumped (AlGaIn)(AsSb) VECSEL where the pump light is absorbed directly in the quantum wells, with the amount of absorbed light enhanced by a higher order microcavity resonance. Using a pump wavelength of 1.94 μm, the quantum deficit is reduced to only 18% and an output power of 5mW, limited by the available pump power, and a slope efficiency of 10% were achieved. Further optimisation of the pump optics is expected to result in a significant increase in device performance.

GaSb based diode laser arrays emitting at wavelengths around 2 μm have a significant potential for a variety of applications including material processing, such as welding of transparent plastic materials, and optical pumping of mid-infrared solid state lasers. Even though high output power broad area single emitters and laser arrays have already been demonstrated, they all suffer from a large fast axis beam divergence of typically 67° FWHM due to the broadened waveguide design employed. Here we will present results on (AlGaIn) (AsSb) quantum-well diode laser single emitters and linear arrays consisting of 19 emitters on a 1 cm long bar emitting at around 1.9 μm. To improve on the poor fast axis beam divergence we abandoned the broadened waveguide concept and changed over to a novel waveguide design which features a rather narrow waveguide core. This results in a remarkable reduction in fast axis beam divergence to 44° FWHM for the new waveguide design. For single emitters a cw output power of more than 1.9 W have been observed. 16.9 W in continuous-wave mode at a heat sink temperature of 20 °C have been achieved for arrays. The maximum wall-plug efficiency amounts to 26% both for the single emitters and the laser arrays. These efficiencies are among the highest values reported so far for GaSb based diode lasers, and allow us to use passively cooled and thus less expensive heat sinking technologies.

We present results on low beam divergence, low threshold current GaSb-based quantum-well diode lasers emittingin the 1.9 - 2.4 µ m wavelength range. By carefully designing the active quantum-well region, low threshold currentdensities in the range of 148 to 190A/cm 2 could be achieved in the entire wavelength range. A novel structurefor the epitaxial waveguide was designed and realized experimentally, leading to a reduced beam divergence inthe fast axis of 44 full width at half maximum (FWHM), compared to 67 FWHM of a conventional broadenedwaveguide design. This improvement was achieved without any sacri“ce in the laser performance, i.e. the novellaser structure showed the same threshold current I th and dierential quantum eciency d as the standard one.Ridge-waveguide lasers employing the new waveguide design and emitting at 2.3 µ mwereoperatedinanexternalcavity con“guration. Due to the improved coupling eciency of the laser beam into the collimating optic, a widetuning range of 130nm could be achieved, limited only by the gain bandwidth of the active material.Keywords: GaSb, infrared diode laser, waveguide, divergence, fast axis, external cavity, tuning range

Type-I diode lasers based on the (AlGaIn)(AsSb) material family are ideally suited to cover the 2-3 μm wavelength range. In this paper recent progress in terms of output power, beam quality and wavelength tunability is reported, achieved for broad-area and tapered single emitters as well as for linear broad-area laser arrays. Special attention has been paid to the reduction of the fast axis far-field beam divergence, employing improved vertical waveguide laser designs. Furthermore, tapered diode lasers have been developed in order to increase the slow axis beam quality at high output powers. An improved beam quality is of particular importance as many applications, including coupling the laser output into an optical fiber or into an external resonator, require diode lasers with a low beam divergence and a high brightness rather than sheer output power.

The influence of the Mg doping profile on the electroluminescence efficiency of GaInN light emitting diodes (LED) has been investigated. The Mg doping profile is influenced by segregation as well as by diffusion during the growth. The diffusion of the Mg dopants into the active region can be controlled by the growth temperature of the Mg doped layers. An increase in Mg concentration close to the active region results in an improved hole injection and thus in a higher electroluminescence efficiency of the GaInN quantum wells. However an excessive spread of the Mg doping atoms towards the GaInN quantum well active region leads to nonradiative recombination and thus a lower output power of the LEDs. An LED test structure containing multiple quantum wells which differ in In content and emission wavelength was used to probe the spatial distribution of the radiative recombination of electrons and holes in the active region and to clarify the influence of Mg dopants in the active region on nonradiative recombination.