Conventional crime scene investigation is slow as traces are collected at the scene and are only subsequently analyzed in the lab. Rapid and contactless detection and examination of various trace materials at the scene without alteration is highly desirable as it avoids degradation of traces, significantly speeds up the investigations, and reduces the time to capture the perpetrator. Furthermore, contactless identification is crucial in guaranteeing the health and safety of crime scene investigators for chemical or explosive threats. Here we present a quantum-cascade laser-based backscattering infrared sensor that allows instantaneous detection and identification of a wide range of forensic relevant samples, from explosives to drugs, and precursors to bodily fluids. The system is based on external-cavity tunable quantum cascade lasers (EC-QCL) using resonant MOEMS (micro-opto-electromechanical system scanners) diffraction gratings, offering kilohertz spectral scan speeds. Two such MOEMS EC-QCL modules are multiplexed in the system and spectrally combined into a single laser beam. The (scalable) approach almost doubles the spectral coverage and increases the selectivity without sacrificing spectral scan speed. We report here on the system design and show first results.
Fastly detecting hazardous, non-volatile chemical substances on paved roads and streets is a topic of utmost military importance in an area denial scenario. Since the 1980s, inherently slow manual sampling has been avoided on armored vehicles using a small silicone wheel that continuously accumulates surface contaminations. After a given sampling period, collected (and potentially hazardous) contaminants on the wheel are thermally desorbed and analyzed by mass spectrometry. This approach led to further technological advancements, including implementing a double-wheel sampling system for automated, uninterrupted operation. Suspicious areas are examined at low driving speeds (approximating a fast-walking speed) with comparatively low spatial resolution, as the silicone wheels can only be rolled comparatively slowly to ensure continuous surface contact. Incremental improvements may further optimize the double-wheel sampling system. In that context, we are currently investigating laser desorption technology to achieve a more targeted heat treatment of the complete silicone wheel and increase spatial resolution and sensitivity. In addition, we also contribute to the development of advanced ion mobility spectrometers, which are both fast scanning and highly sensitive, as a viable alternative to cumbersome mass spectrometers. As a radically different approach, we report here on a measurement system using back-scattering IR-spectroscopy to optically interrogate samples at a standoff distance and process the information without delay. The used IR light source consists of three coupled broadband quantum cascade laser modules, each with an integrated micro-opto-electro-mechanical grating scanner (MOEMS EC-QCL). The elaborate coupling of three such pulsed laser modules provides an ultra-broadband spectral scan within the IR-fingerprint area (covered by those three MOEMS EC-QCLs) at a repetition rate of almost one kilohertz, thus resulting in measurement times of as short as one millisecond per (ultra-broadband) spectrum. We found that even minor contaminations of hazardous substances are identified using this setup. Furthermore, preliminary laboratory tests revealed a successful detection after the measurements on a fast-moving contaminated object. The experiments were performed at different observation angles with a considerable focal depth. The proof of concept shows that this novel QCL-based chemical detection approach is fast enough and promising to continuously monitor the ground with sufficient geometric resolution at cruise speeds on uneven and textured surfaces.
Traditional crime scene investigation is slow as traces are collected at the scene and only subsequently analyzed in the lab. Rapid contactless detection and examination of various trace materials at the crime scene without any alteration avoids degradation of traces, significantly speeds up investigation and reduces the time to capture the originator before he can commit further offences. Furthermore, contactless identification is crucial in guaranteeing the health and safety of crime scene investigators for chemical, biological and explosives treats. Here we report on the development of a quantum-cascade laser based infrared sensors, that allows instantaneous detection and identification of a wide range of forensic relevant samples, from explosives to drugs, their precursors, but also biological traces such as blood. The system is based on MOEMS-EC-QCLs that allow kilohertz spectral scan speed. Two such sources are combined in the sensor to extend the spectral coverage to increase the selectivity without sacrificing scan speed. We report on the system design and show first results on drugs and explosive identification.
Step-taper active-region (STA) quantum cascade lasers (QCLs) allow for both carrier-leakage suppression and ultrafast, miniband-like carrier extraction. In turn, that has led to very high internal-efficiency ni values: ~ 77 % and 80-86 % from ~ 5.0 μm- and 8-9 μm-emitting QCLs, respectively. Based on extracted parameters that characterize the interfaceroughness (IFR) scattering, a study has been performed of the effects of elastic scattering, both IFR and alloy-disorder (AD) scattering, on 5.0 μm-emitting STA-QCLs. We find that the laser-transition efficiency ntr is enhanced by ~15 % (i.e., from 83 % to ~ 95 %) due to the much stronger effect of elastic scattering on the lower-laser-level lifetime than on the effective upper-laser-level lifetime. In turn, the injection efficiency: ninj = ni /ntr , reaches ~ 81 %; that is, the highest injection-efficiency value obtained to date from QCLs. Furthermore, we find that the projected upper limit for the pulsed wall-plug efficiency can reach values as high as 44.4 % for 4.6 μm-emitting devices; thus, raising the possibility of CW operation of 4.5-5.0 μm-emitting QCLs with wallplug-efficiency values as high as 40 %.
Quantum cascade lasers (QCLs) that employ metamorphic buffer layers as substrates of variable lattice constant have been designed for emission in the 3.0- to 3.5-μm wavelength range. Theoretical analysis of the active-region (AR) energy band structure, while using an 8-band k•p model, reveals that one can achieve both effective carrier-leakage suppression as well as fast carrier extraction in QCL structures of relatively low strain. Significantly lower indium-content quantum wells (QWs) can be employed for the AR compared to QWs employed for conventional short-wavelength QCL structures grown on InP, which, in turn, is expected to eliminate carrier leakage to indirect-gap valleys (X, L). An analysis of thermo-optical characteristics for the complete device design indicates that high-Al-content AlInAs cladding layers are more effective for both optical confinement and thermal dissipation than InGaP cladding layers. An electroluminescence-spectrum full-width half-maximum linewidth of 54.6 meV is estimated from interface roughness scattering and, by considering both inelastic and elastic scattering, the threshold-current density for 3.39-μm-emitting, 3-mm-long back-facet-coated QCLs is projected to be 1.40 kA/cm2.
The quantum-cascade laser (QCL) emitting in the mid-infrared region of 4 to 8 m has been refined to the point that its internal quantum efficiency is approaching fundamental limits. QCLs designed for power typically contain 30-40 cascades, are less than two wavelengths in width, and laser ridge lengths are typically between 3 and 6 mm. Even with state-of-the-art efficiency and thermal management, room temperature operation of such lasers is fundamentally limited to several watts. This paper describes a path to power scaling that is not fundamentally limited. Power requires volume and thermal conductance. We propose that this combination is best achieved using fewer than 15 cascades combined with broad areas. We demonstrate the first room temperature continuous-wave emission of broad-area QCLs and discuss how this scaling concept can deliver MIR emission of 10's of watts at room temperature with beam quality required for high brilliance.
We have recently described a method to analyze the leakage current (Jleak) in quantum-cascade lasers (QCLs) for carriers scattering into higher minibands due to LO-phonon absorption. In his presentation we analyze Jleak due to elastic scattering. We illustrate how at low temperature, when inelastic scattering is negligible, this current becomes significant for devices operating at high electron temperatures. Measuring Jleak above threshold we are able to investigate the effect of electron temperature on the differential quantum efficiency. This procedure is supported by a self-consistent calculation of the rate equations based on a phenomenological scattering-rate model. We apply our approach and measure Jleak above threshold as a function of electron temperature for a QCL emitting near 5.4 μm operated at a low duty cycle and a heat sink temperature of 80 K. This current is then modeled using a thermally activated, electron temperature-driven, scattering model based on intersubband interface roughness scattering. As a result, a reduction in the upper laser state population by ∼35% is estimated due to the effect of increased electron temperature. A decrease of the quantum efficiency of ∼80% is estimated for an electron temperature of 400 K.
Active region designs of QCLs containing composite barriers based on AlAs allow short wavelength emission, improved injection efficiency, and high values of T0 and T1. On the other hand, AlAs introduces challenges, not only in strain compensation and growth, but also in effects on thermal management, thermal stability, and scattering. Leakage current, allowing electrons to bypass transitions between upper and lower laser levels occur due to scattering of electrons into higher-lying states via phonons and interface roughness scattering. This interface roughness scattering is exacerbated by large values of ΔEc and by the rms roughness itself, both of which are pronounced at the AlAs/InGaAs interface. The resulting leakage current noticeably reduces the slope efficiency, leading to more heating to achieve a given emission power. Efficient thermal management requires a buried heterostructure design; the re-growth of InP:Fe, however, needs to be carried out at temperatures consistent with maintaining the highly strained AlAs/InGaAs interfaces. This paper describes the physics of intersubband electron scattering due to strained interfaces and some partially optimized structures with Jth = 1.7 kA/cm2 at 300 K, slope efficiency η = 1.4 W/A, T0 = 175 K, and T1 = 550 K. Re-growth of InP:Fe using gas-source MBE at substrate temperatures below 550°C results in packaged lasers with 7 μm width having high thermal conductance.
We present progress on bandpass infrared interference filters with very narrow passbands to be used for sensitive trace gas and volatile compound imaging and detection and are suitable for mode selection and tuning in singlemode External Cavity Quantum Cascade Lasers. The process parameters for fabrication of such filters with central wavelengths in the 3-12 μm range are described. One representative fillter has a passband width of 6 nm or 0.14% with peak transmission of 62% and a central wavelength of 4.4μm. Theoretically, it can be tuned through about 4% by tilting with respect to the incident beam and offers orders of magnitude larger angular dispersion than diffraction gratings. We compare filters with single-cavity and coupled-cavity Fabry-Perot designs. The filters pass the tests for adhesion and abrasion as stated in MIL-C-48497.
The quantum-cascade lasers (QCL), first demonstrated in 1994, has since been developed into a mature laser emitting within nearly the entire spectrum from 2.6 to 250 μm, particular within the mid-infrared part of the spectrum from 3 to 12 μm for applications in gas sensing for security, environmental and medical uses, as well as for defense-related IR countermeasures. The QCL heterostructure is generally based on the InGaAs/InAlAs system lattice-matched to InP or on its strain-compensated extension to maximize the conduction band discontinuity between well and barrier material. A refinement is the use of mixed-height barriers to engineer the interface scattering of the different levels involved in the lasing process. This design strategy appears to be universally applicable, across the entire range of QCL emission wavelengths. By using low barriers where the upper laser state has its maximum probability and high barriers where the lower laser state has its maximal probability in strain-compensated designs for short wavelength emission, the lifetime of the upper laser state can be increased, while decreasing the lifetime of the lower laser state. First realizations of this design result in Jth = 1.7kA/cm2 at 300 K, slope efficiency η = 1.4 W/A, T0 = 175 K, and T1 = 550 K. Further increases in efficiency can be achieved through designs in which parasitic states near the upper laser level are separated from it, either energetically or oscillator strength. These states may be associated with other k values, or with higher-lying subbands.
The leakage current in two quantum-cascade (QCL) structures is measured and analyzed. The structures illustrate a new design feature, exploiting the interface roughness scattering at the well/barrier interfaces to intentionally shorten the lifetime of the lower laser state while increasing that of the upper laser state. By using low barriers where the upper laser state has its maximum probability and high barriers where the lower laser state has its maximal probability in strain-compensated designs for short wavelength emission, the lifetime of the upper laser state can be increased, while decreasing the lifetime of the lower laser state. First realizations of this design result in Jth = 1.7 kA/cm2 at 300 K, slope efficiency η = 1.4 W/A, T0 = 175 K, and T1 = 550 K for lasers emitting at 3.9 μm. A further analysis allows the extraction of the leakage current into higher minibands from the temperature dependence of the threshold current density and to reconstruct the energies of the higher-lying states from this current. The modeling includes the thermal population of LO phonons that drive the leakage.
The laser threshold and lateral mode confinement of blue (440 nm) InGaN multiple quantum well (MQW) laser diodes
have been investigated. Ridge-waveguide (RW) laser diodes with different ridge etch depth ranging from 25 nm above
the active region (deep-ridge waveguide) to 200 nm above the active region (shallow-ridge waveguide) have been
fabricated. The comparison of devices with the same resonator length shows that the threshold current densities are
significantly lower for deep-ridge waveguide laser diodes. The difference in lasing threshold becomes more eminent for
narrow ridges, which are required for single mode operation. For shallow-ridge devices the threshold current density
increases by more than a factor of three when the ridge width is decreased from 20μm to 1.5μm. For the deep-ridge
waveguide devices instead, the lasing threshold is almost independent of the ridge waveguide width.
The effect has been analyzed by 2D self-consistent electro-optical simulations. For deep-ridge devices, the simulated
thresholds and far-field patterns are in good agreement with the simulations. For shallow-ridge devices, however,
questionable theoretical assumptions are needed. Two possible causes are discussed: extremely large current spreading
and strong index anti-guiding.
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