The detection and measurement of gas concentrations using the characteristic optical absorption of the gas species is important for both understanding and monitoring a variety of phenomena from industrial processes to environmental change. This study reviews the field, covering several individual gas detection techniques including non-dispersive infrared, spectrophotometry, tunable diode laser spectroscopy and photoacoustic spectroscopy. We present the basis for each technique, recent developments in methods and performance limitations. The technology available to support this field, in terms of key components such as light sources and gas cells, has advanced rapidly in recent years and we discuss these new developments. Finally, we present a performance comparison of different techniques, taking data reported over the preceding decade, and draw conclusions from this benchmarking.

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Optical gas sensing: a review

A broadband, compact, all-electrically driven mid-infrared frequency comb based on a quantum cascade laser widens the scope of application of combs in this frequency range beyond that of sources which depend on a chain of optical components. Optical frequency combs are light sources that produce a comb-like spectrum, with sharp equidistant frequency modes, and have many uses in metrology and spectroscopy applications. The mid-infrared regime is particularly important for molecular fingerprinting, but so far the comb sources in this wavelength regime are bulky and rely on a chain of optical components. For wide practical applications, an electrically injected, compact scheme is desired. Andreas Hugi et al. now demonstrate a mid-infrared frequency comb generator based on a semiconductor device, a continuous-wave quantum cascade laser. Optical frequency combs1 act as rulers in the frequency domain and have opened new avenues in many fields such as fundamental time metrology, spectroscopy and frequency synthesis. In particular, spectroscopy by means of optical frequency combs has surpassed the precision and speed of Fourier spectrometers. Such a spectroscopy technique is especially relevant for the mid-infrared range, where the fundamental rotational–vibrational bands of most light molecules are found2. Most mid-infrared comb sources are based on down-conversion of near-infrared, mode-locked, ultrafast lasers using nonlinear crystals3. Their use in frequency comb spectroscopy applications has resulted in an unequalled combination of spectral coverage, resolution and sensitivity4,5,6,7. Another means of comb generation is pumping an ultrahigh-quality factor microresonator with a continuous-wave laser8,9,10. However, these combs depend on a chain of optical components, which limits their use. Therefore, to widen the spectroscopic applications of such mid-infrared combs, a more direct and compact generation scheme, using electrical injection, is preferable. Here we present a compact, broadband, semiconductor frequency comb generator that operates in the mid-infrared. We demonstrate that the modes of a continuous-wave, free-running, broadband quantum cascade laser11 are phase-locked. Combining mode proliferation based on four-wave mixing with gain provided by the quantum cascade laser leads to a phase relation similar to that of a frequency-modulated laser. The comb centre carrier wavelength is 7 micrometres. We identify a narrow drive current range with intermode beat linewidths narrower than 10 hertz. We find comb bandwidths of 4.4 per cent with an intermode stability of less than or equal to 200 hertz. The intermode beat can be varied over a frequency range of 65 kilohertz by radio-frequency injection. The large gain bandwidth and independent control over the carrier frequency offset and the mode spacing open the way to broadband, compact, all-solid-state mid-infrared spectrometers.

/pdf/mid-infrared-frequency-comb-based-on-a-quantum-cascade-laser-1a0a174joi.pdf

Mid-infrared frequency comb based on a quantum cascade laser

Quantum cascade (`QC') lasers are reviewed. These are semiconductor injection lasers based on intersubband transitions in a multiple-quantum-well (QW) heterostructure, designed by means of band-structure engineering and grown by molecular beam epitaxy. The intersubband nature of the optical transition has several key advantages. First, the emission wavelength is primarily a function of the QW thickness. This characteristic allows choosing well-understood and reliable semiconductors for the generation of light in a wavelength range unrelated to the material's energy bandgap. Second, a cascade process in which multiple - often several tens of - photons are generated per electron becomes feasible, as the electron remains inside the conduction band throughout its traversal of the active region. This cascading process is behind the intrinsic high-power capabilities of the lasers. Finally, intersubband transitions are characterized through an ultrafast carrier dynamics and the absence of the linewidth enhancement factor, with both features being expected to have significant impact on laser performance. The first experimental demonstration by Faist et al in 1994 described a QC-laser emitting at 4.3 µm wavelength at cryogenic temperatures only. Since then, the lasers' performance has greatly improved, including operation spanning the mid- to far-infrared wavelength range from 3.5 to 24 µm, peak power levels in the Watt range and above-room-temperature (RT) pulsed operation for wavelengths from 4.5 to 16 µm. Three distinct designs of the active region, the so-called `vertical' and `diagonal' transition as well as the `superlattice' active regions, respectively, have emerged, and are used either with conventional dielectric or surface-plasmon waveguides. Fabricated as distributed feedback lasers they provide continuously tunable single-mode emission in the mid-infrared wavelength range. This feature together with the high optical peak power and RT operation makes QC-lasers a prime choice for narrow-band light sources in mid-infrared trace gas sensing applications. Finally, a manifestation of the high-speed capabilities can be seen in actively and passively mode-locked QC-lasers, where pulses as short as a few picoseconds with a repetition rate around 10 GHz have been measured.

https://iopscience.iop.org/article/10.1088/0034-4885/64/11/204/pdf

Recent progress in quantum cascade lasers and applications

Thermoelectric (TE) materials have the capability of converting heat into electricity, which can improve fuel efficiency, as well as providing robust alternative energy supply in multiple applications by collecting wasted heat, and therefore, assisting in finding new energy solutions. In order to construct high performance TE devices, superior TE materials have to be targeted via various strategies. The development of high performance TE devices can broaden the market of TE application and eventually boost the enthusiasm of TE material research. This review focuses on major novel strategies to achieve high-performance TE materials and their applications. Manipulating the carrier concentration and band structures of materials are effective in optimizing the electrical transport properties, while nanostructure engineering and defect engineering can greatly reduce the thermal conductivity approaching the amorphous limit. Currently, TE devices are utilized to generate power in remote missions, solar-thermal systems, implantable or/wearable devices, the automotive industry, and many other fields; they are also serving as temperature sensors and controllers or even gas sensors. The future tendency is to synergistically optimize and integrate all the effective factors to further improve the TE performance, so that highly efficient TE materials and devices can be more beneficial to daily lives.

https://espace.library.uq.edu.au/view/UQ:722903/UQ722903_OA.pdf

High Performance Thermoelectric Materials: Progress and Their Applications

Simulations Complement Experimental Studies Jessica Nasica-Labouze,† Phuong H. Nguyen,† Fabio Sterpone,† Olivia Berthoumieu,‡ Nicolae-Viorel Buchete, Sebastien Cote, Alfonso De Simone, Andrew J. Doig, Peter Faller,‡ Angel Garcia, Alessandro Laio, Mai Suan Li, Simone Melchionna, Normand Mousseau, Yuguang Mu, Anant Paravastu, Samuela Pasquali,† David J. Rosenman, Birgit Strodel, Bogdan Tarus,† John H. Viles, Tong Zhang,†,▲ Chunyu Wang, and Philippe Derreumaux*,†,□ †Laboratoire de Biochimie Theorique, Institut de Biologie Physico-Chimique (IBPC), UPR9080 CNRS, Universite Paris Diderot, Sorbonne Paris Cite, 13 rue Pierre et Marie Curie, 75005 Paris, France ‡LCC (Laboratoire de Chimie de Coordination), CNRS, Universite de Toulouse, Universite Paul Sabatier (UPS), Institut National Polytechnique de Toulouse (INPT), 205 route de Narbonne, BP 44099, Toulouse F-31077 Cedex 4, France School of Physics & Complex and Adaptive Systems Laboratory, University College Dublin, Belfield, Dublin 4, Ireland Deṕartement de Physique and Groupe de recherche sur les proteines membranaires (GEPROM), Universite de Montreal, C.P. 6128, succursale Centre-ville, Montreal, Quebec H3C 3T5, Canada Department of Life Sciences, Imperial College London, London SW7 2AZ, United Kingdom Manchester Institute of Biotechnology, University of Manchester, 131 Princess Street, Manchester M1 7DN, United Kingdom Department of Physics, Applied Physics, & Astronomy, and Department of Biology, Rensselaer Polytechnic Institute, Troy, New York 12180, United States The International School for Advanced Studies (SISSA), Via Bonomea 265, 34136 Trieste, Italy Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, 02-668 Warsaw, Poland Institute for Computational Science and Technology, SBI Building, Quang Trung Software City, Tan Chanh Hiep Ward, District 12, Ho Chi Minh City, Vietnam Instituto Processi Chimico-Fisici, CNR-IPCF, Consiglio Nazionale delle Ricerche, 00185 Roma, Italy School of Biological Sciences, Nanyang Technological University, 60 Nanyang Drive, 637551 Singapore Department of Chemical and Biomedical Engineering, Florida A&M University-Florida State University (FAMU-FSU) College of Engineering, 2525 Pottsdamer Street, Tallahassee, Florida 32310, United States National High Magnetic Field Laboratory, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310, United States Institute of Complex Systems: Structural Biochemistry (ICS-6), Forschungszentrum Julich GmbH, 52425 Julich, Germany School of Biological and Chemical Sciences, Queen Mary University of London, London E1 4NS, United Kingdom Institut Universitaire de France, 75005 Paris, France

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Amyloid β Protein and Alzheimer’s Disease: When Computer Simulations Complement Experimental Studies

Mid-infrared photonics in silicon needs low-loss integrated waveguides. While monocrystalline germanium waveguides on silicon have been proposed, experimental realization has not been reported. Here we demonstrate a germanium strip waveguide on a silicon substrate. It is designed for single mode transmission of light in transverse magnetic (TM) polarization generated from quantum cascade lasers at a wavelength of 5.8 μm. The propagation losses were measured with the Fabry–Perot resonance method. The lowest achieved propagation loss is 2.5  dB/cm, while the bending loss is measured to be 0.12 dB for a 90° bend with a radius of 115 μm.

/pdf/low-loss-germanium-strip-waveguides-on-silicon-for-the-mid-489n34xs1s.pdf

Low-loss germanium strip waveguides on silicon for the mid-infrared

Controlling Bloch surface waves in a multilayer dielectric yields a new platform for creating two-dimensional photonic circuitry. Libo Yu and co-workers from the Swiss Federal Institute of Technology in Lausanne used an ultrathin (λ/15) polymer coating to modify the local effective refractive index of a dielectric multilayer stack that guides Bloch surface waves. Careful design of the shape of a polymer layer makes it possible to deflect, diffract and focus the waves within a planar geometry. The researchers used this approach to construct a flat lens capable of operating at a wavelength of 1.5 µm, and used a multiheterodyne scanning near-field optical microscope to observe the near-field behavior of the platform. They say that many other forms of photonic component should be possible.

/pdf/manipulating-bloch-surface-waves-in-2d-a-platform-concept-4u7nem4j2i.pdf

Manipulating Bloch surface waves in 2D: a platform concept-based flat lens

We demonstrate room-temperature, single-mode, continuous-wave operation of a λ≃5.4μm quantum-cascade laser up to the temperature of 30°C. Processing is done using standard lithography in a ridge waveguide mounted junction-up. The active region is based on a bound-to-continuum transition. The high performances were achieved with a low active region doping and a thick electroplated gold deposition, resulting in a characteristic temperature of T0=155K in continuous-wave with a threshold current density of jth=2.05kA∕cm2 at 300K.

Room-temperature, continuous-wave, single-mode quantum-cascade lasers at λ≃5.4μm

A germanium (Ge) strip waveguide on a silicon (Si) substrate is integrated with a microfluidic chip to detect cocaine in tetrachloroethylene (PCE) solutions. In the evanescent field of the waveguide, cocaine absorbs the light near 5.8 μm, which is emitted from a quantum cascade laser. This device is ideal for (bio-)chemical sensing applications.

/pdf/cocaine-detection-by-a-mid-infrared-waveguide-integrated-516yow2xb7.pdf

Cocaine detection by a mid-infrared waveguide integrated with a microfluidic chip

A measurement of the linewidth enhancement factor α of a distributed feedback quantum cascade laser is presented. The measurement is based on a heterodyning experiment, in which one of the lasers is modulated at radio frequency. A value of α=0.02±0.20 is obtained for a modulation frequency of 500MHz. As the frequency is decreased, α increases and is consistent with a thermal chirp effect.

Lubos Hvozdara

Papers

Low-loss germanium strip waveguides on silicon for the mid-infrared

Manipulating Bloch surface waves in 2D: a platform concept-based flat lens

Room-temperature, continuous-wave, single-mode quantum-cascade lasers at λ≃5.4μm

Cocaine detection by a mid-infrared waveguide integrated with a microfluidic chip

Direct measurement of the linewidth enhancement factor by optical heterodyning of an amplitude-modulated quantum cascade laser