This PDF file contains the front matter associated with SPIE Proceedings Volume 8631, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

Wide band gap dilute magnetic semiconductors have recently been of interest due to theoretical predictions of room temperature ferromagnetism in these materials. In this work Ga_1-xGdxN thin films were grown by Metalorganic Chemical Vapor Deposition. These films were found to be ferromagnetic at room temperature and electrically conducting. However, only GaN:Gd layers and devices grown with a TMHD₃Gd precursor that contained oxygen showed strong ferromagnetism, while materials grown with an oxygen-free Cp₃Gd precursor did not show ferromagnetic behavior. This experimental observation was consistent with first-principles calculations based on density functional theory calculations that we completed that showed the ferromagnetism was mediated by interstitial oxygen. The results confirmed the first successful realization of Ga_1-xGd_xN-based spin-polarized LED with 14.6% degree of polarization at 5000 Gauss is obtained.

Modern Nano electronics involves the use of heterojunctions in forming energy steps based on band-edge alignments in effecting quantum confinements. When the electron meanfree- path exceeds couple of periods, man-made quantum states appeared, mimicking natural solids with sharpness determined by the degree of coherence dictated by a relatively long meanfree- path. When a single quantum well is involved, the structure is represented by resonant tunneling. This process can further be extended to 3D (3-dimension), known as QD, for quantum dot, however, thus far only few systems have been found possible, mostly involving InAs, or InN. However, the real problem lies in I/O, making contact to a single quantum dot, seems to be impractical on account of difficulties in making contacts in Nano scale regime. The issue with impedance matching, is the most important aspect for efficient devices, whether as detectors, or as generator in frequencies between THz to visible light. As size shrinks to Nano-regime, even the wavelength of IR is too large for effective coupling to the quantum dots without some sort of coupling such as the use of Fabry-Perrot mirrors, which is in fact unsuited for quantum dots, unless these dots are arranged in an array mimicking a solid with translational symmetry, which in fact defeating the purpose of going to quantum dots, except when the distribution of these quantum dots are arranged either representable by some distribution functions suitable for arriving at a meaningful average, or periodically mimicking a solid, such as the man-made superlattice, SL, originally proposed by Esaki and Tsu. [1, 2]. Interestingly Esaki and Tsu were asked to remove the reference on doping in the barrier region for increased mobility by the reviewer for the IBM’s own J. of Research and Development. We did protest to the Editor-in- Chief of the Journal to no avail! Because of this experience, it did occur to me of requiring something beyond the regular reviewing process in technical journals. Some ten years ago, I proposed to M. Henini the need to have a journal with two outlets for publications, one ‘regular’, and another as ‘special’: rejected by reviewer, but accepted by the editorial staff. For some reason, we did not get enough support then.

We demonstrate room temperature terahertz (THz) quantum cascade laser (QCL) sources with a broad spectral coverage based on intracavity difference-frequency generation. Dual mid-infrared (mid-IR) active cores based on the single-phonon resonance scheme are designed with a THz nonlinearity specially optimized for the high operating fields that correspond to the highest mid-infrared output powers. Integrated dual-period distributed feedback (DFB) gratings with different grating periods are used to purify and tune the mid-IR and THz spectra. Two different phase matching schemes are used for THz generation. The first is the collinear modal phase matching scheme, wherein the wafer is grown on a n+ InP substrate. Room temperature single mode operation THz emission with frequency tuning range from 3.3 to 4.6 THz and THz power up to 65 mW at 4.0 THz are realized. The mid-IR to THz power conversion efficiency is 23 uW/W2. The second is the Čerenkov phase-matching scheme, wherein the wafer is grown on a semi-insulating InP substrate, and device’s facet is polished into 20-30 degrees for THz extraction. Room temperature single mode emissions from 1.0 to 4.6 THz with a side-mode suppression ratio and output power up to 40 dB and 32 µW are obtained, respectively. The mid-IR to THz power conversion efficiency is 50 uW/W2.

Quantum cascade lasers (QCLs) are powerful testing grounds for the fundamental physical parameters determined by their quantum nature. The associated wealth of unique physical properties makes the QCL a subject of extensive research, especially across the far-infrared, where the electron-phonon relaxation dynamics, the gain mechanisms and the related intrinsic QCL features need to be extensively investigated. Here, we report a complete overview of the frequency-noise power spectral density of a THz QCL, giving an experimental and analytical evaluation of its intrinsic linewidth and a full investigation of the physics beyond it, offering a new perspective of use of ultra-narrow THz QCL sources as a metrological grade tool for a widespread range of photonic applications.

We present experimental demonstrations using a broadly tunable external cavity quantum cascade laser (ECQCL) to perform Reflection-Absorption InfraRed Spectroscopy (RAIRS) of thin layers and residues on surfaces. The ECQCL compliance voltage was used to measure fluctuations in the ECQCL output power and improve the performance of the RAIRS measurements. Absorption spectra from self-assembled monolayers of a fluorinated alkane thiol and a thiol carboxylic acid were measured and compared with FTIR measurements. RAIRS spectra of the explosive compounds PETN, RDX, and tetryl deposited on gold substrates were also measured. Rapid measurement times and low noise were demonstrated, with <1E-3 absorbance noise for a 10 second measurement time.

A field deployable Compound Specific Isotope Analyzer (CSIA) coupled with capillary chromatogrpahy based on Quantum Cascade (QC) lasers and Hollow Waveguide (HWG) with precision and chemical resolution matching mature Mass Spectroscopy has been achieved in our laboratory. The system could realize 0.3 per mil accuracy for 12C/13C for a Gas Chromatography (GC) peak lasting as short as 5 seconds with carbon molar concentration in the GC peak less than 0.5%. Spectroscopic advantages of HWG when working with QC lasers, i.e. single mode transmission, noiseless measurement and small sample volume, are compared with traditional free space and multipass spectroscopy methods.

We are building prototype chip-scale low-power integrated-optic gas-phase chemical sensors based on mid-infrared (3-5μm) Tunable Diode Laser Absorption Spectroscopy (TDLAS). TDLAS is able to sense many gas phase chemicals with high sensitivity and selectivity. Novel gas sensing elements using low-loss resonant photonic crystal cavities or waveguides will permit compact integration of a laser source, sampling elements, and detector in configurations suitable for inexpensive mass production. Recently developed Interband Cascade Lasers (ICLs) that operate at room temperature with low power consumption are expected to serve as monochromatic sources to probe the mid-IR molecular spectral transitions. Practical challenges to fabricating these sensors include: a) selecting and designing the high-Q microresonator sensing element appropriate for the selected analyte; b) coupling laser light into and out of the sensing element; and c) device thermal management, especially stabilizing laser temperature with the precision needed for sensitive spectroscopic detection. This paper describes solutions to these challenges.

We first discuss and review a novel technique to obtain efficient surface-emission in terahertz quantum cascade lasers (THz-QCLs). We report high-power, single-mode surface-emission by using graded photonic heterostructures (GPH) as resonators. Peak output powers of ~100 mW and differential efficiency of 230mW/A are achieved at frequency ~ 3.4THz, the maximum operation temperature being 120K in pulsed mode. Combining with the active region based on the bound-to-continuum design, we also achieved continuous-wave (CW) operation of GPH THz QC lasers, with the maximum output power of 3.8 mW at 2.7THz. We then discuss the prospects to develop phase-locked arrays of these devices, in order to take advantage of their good wall-plug efficiency (<0.3%) and single-lobed directional beam pattern. As an initial result, a very robust approach to phase-lock second-order distributed feedback (DFB) THz QC laser arrays (pairs in this case) is demonstrated.

We studied mid-infrared sensors based on the wavelength shift of Surface Plasmon Polariton resonances upon solid substance deposition on subwavelength hole arrays in a thin metal film (metal meshes). We present an experimental and numerical investigation of the mid-infrared transmission of metal meshes with and without a dielectric substrate, and we develop an analytical model which describes the Fano interference between the Bethe continuum and the SPP resonances. Fitting the differential transmission signal, measured before and after deposition of a target solid film, we demonstrate sensitivity down to few molecular monolayers, at least one order of magnitude better than mid-infrared vibrational spectroscopy. Sensor calibration was performed on thin polymer films and an example of real application is then provided by measuring the optical density of phospholipid membrane complexes with thickness in the range 2-10 nm.

Quantum Cascade Lasers (QCLs), operating in continuous wave (cw) at room temperature (rt) in 3-3.5μm spectral range, which overlaps the spectral fingerprint region of many hydrocarbons, is essential in spectroscopic trace gas detection, environment monitoring, and pollution control. A 3μm QCL, operating in cw at rt is demonstrated. This initial result makes it possible, for the most popular material system (AlInAs/GaInAs on InP) used in QCLs in mid-infrared and long-infrared, to cover the entire spectral range of mid-infrared atmospheric window (3-5μm). In_0.79Ga_0.21As/In_0.11Al_0.89As strain balanced superlattice, which has a large conduction band offset, was grown. The strain was balanced with composite barriers (In0.11Al0.89As /In0.4Al0.6As) in the injector region, to eliminate the need of extremely high compressively strained GaInAs, whose pseudomorphic growth is very difficult.

This paper describes our development efforts at Northwestern University regarding dual-section sampled grating distributed feedback (SGDFB) QCLs. These devices are the same size, but have much wider electrical tuning, than a traditional DFB laser. In this paper, I will show how we have dramatically extended the monolithic tuning range of high power quantum cascade lasers with high side mode suppression. This includes individual laser element tuning of up to 50 cm^-1 and 24 dB average side mode suppression. These lasers are capable of room temperature continuous operation with high power (<100 mW) output. Additionally, we have demonstrated a broad spectral coverage of over 350 cm^-1 on a single chip, which is equivalent to 87.5% of the gain bandwidth. The eventual goal is to realize an extended array of such laser modules in order to continuously cover a similar or broader spectral range, similar to an external cavity device without any external components.

Infrared focal plane arrays (FPAs) covering broad mid- and long-IR spectral ranges are the central parts of the spectroscopic and imaging instruments in several Earth and planetary science missions. To be implemented in the space instrument these FPAs need to be large-format, uniform, reproducible, low-cost, low 1/f noise, and radiation hard. Quantum Well Infrared Photodetectors (QWIPs), which possess all needed characteristics, have a great potential for implementation in the space instruments. However a standard QWIP has only a relatively narrow spectral coverage. A multi-color QWIP, which is compromised of two or more detector stacks, can to be used to cover the broad spectral range of interest. We will discuss our recent work on development of multi-color QWIP for Hyperspectral Thermal Emission Spectrometer instruments. We developed QWIP compromising of two stacks centered at 9 and 10.5 μm, and featuring 9 grating regions optimized to maximize the responsivity in the individual subbands across the 7.5-12 μm spectral range. The demonstrated 1024x1024 QWIP FPA exhibited excellent performance with operability exceeding 99% and noise equivalent differential temperature of less than 15 mK across the entire 7.5-12 μm spectral range.

Modulation transfer function (MTF) is the ability of an imaging system to faithfully image a given object. The MTF of an imaging system quantifies the ability of the system to resolve or transfer spatial frequencies. In this presentation we will discuss the detail MTF measurements of 1024x1024 pixels multi-band quantum well infrared photodetector and 320x256 pixels long-wavelength InAs/GaSb superlattice infrared focal plane arrays.

It is not clear whether the tunneling current in QWIPs depends just on the energy corresponding to motion perpendicular to the plane of the quantum well or on the total energy. In order to get a quantitative assessment of the contribution of energy corresponding to motion in the plane of the quantum well to the dark current we use the following approach. We calculate the dark current in GaAs/Al_xGa_1-x s quantum well infrared detectors for both tunneling dependent only on Ez, and tunneling dependent on the total energy, and compare the results to experimental data. Comparison of theoretical results with experimental data at 40K shows that motion in the plane of the quantum well plays a significant role in determining the tunneling dark current. Corrections are made to Levine's original formula. Variation of the dark current with barrier width and doping density is systematically studied. It is shown that increasing the barrier width and/or decreasing the doping density in the well do not always reduce the dark current.

Recent advances in the development of compact sensors based on mid-infrared continuous wave (CW), thermoelectrically cooled (TEC) and room temperature operated quantum cascade lasers (QCLs) for the detection, quantification and monitoring of trace gas species and their applications in environmental and industrial process analysis will be reported. These sensors employ a 2f wavelength modulation (WM) technique based on quartz enhanced photoacoustic spectroscopy (QEPAS) that achieves detection sensitivity at the ppb and sub ppb concentration levels. The merits of QEPAS include an ultra-compact, rugged sensing module, with wide dynamic range and immunity to environmental acoustic noise. QCLs are convenient QEPAS excitation sources that permit the targeting of strong fundamental rotational-vibrational transitions which are one to two orders of magnitude more intense in the mid-infrared than overtone transitions in the near infrared spectral region.

Two areas of current activity in the Earth sciences are the development of ground-based sensor networks and sensor payloads for unmanned aircraft. This paper reviews a few of our sensor development efforts, highlighting how design elements meet specific sensor measurement needs.

Chemically reactive short-lived species play a crucial role in tropospheric processes affecting regional air quality and global climate change. Contrary to long-lived species (such as greenhouse gases), fast, accurate and precise monitoring changes in concentration of atmospheric short-lived species represents a real challenge due to their short life time (~1 s for OH radical) and very low concentration in the atmosphere (down to 10⁶ molecules/cm3, corresponding to 0.1 pptv at standard temperature and pressure). We report on our recent progress in instrumentation developments for spectroscopic sensing of trace reactive species. Modern photonic sources such as quantum cascade laser (QCL), distributed feedback (DFB) diode laser, light emitting diode (LED), difference-frequency generation (DFG) parametric source are implemented in conjunction with highsensitivity spectroscopic measurement techniques for : (1) nitrous acid (HONO) monitoring by QCL-based long optical pathlength absorption spectroscopy and LED-based IBBCEAS (incoherent broadband cavity-enhanced absorption spectroscopy); (2) DFB laser-based hydroxyl free radical (OH) detection using WM-OA-ICOS (wavelength modulation off-axis integrated cavity output spectroscopy) and FRS (Faraday rotation spectroscopy), respectively; (3) nitrate radical (NO3) and nitrogen dioxide (NO2) simultaneous measurements with IBBCEAS approach. Applications in field observation and in smog chamber study will be presented.

We will report here on the design and realization of optoacoustic sensors based on an external cavity QCL laser source emitting at 10,54 μm, fiber-coupled with a QEPAS spectrophone module. SF₆ has been selected as the target gas. Single mode laser delivery through the prongs of the quartz tuning fork has been realized using a hollow waveguide fiber with internal core size of 300 μm. The achieved sensitivity of the system was 50 part per trillion in 1 s corresponding to a record for QEPAS normalized noise-equivalent absorption of 2,7•10^-10 W•cm^-1•Hz-^1/2.

Infrared scattering scanning near-field optical microscopy (s-SNOM) is an apertureless superfocusing technique that uses the antenna properties of a conducting atomic force microscope (AFM) tip to achieve infrared spatial resolution below the diffraction limit. The instrument can be used either in imaging mode, where a fixed wavelength light source is tuned to a molecular resonance and the AFM raster scans an image, or in spectroscopy mode where the AFM is held stationary over a feature of interest and the light frequency is varied to obtain a spectrum. In either case, a strong, stable, coherent infrared source is required. Here we demonstrate the integration of a broadly tunable external cavity quantum cascade laser (ECQCL) into an s-SNOM and use it to obtain infrared spectra of microcrystals of chemicals adsorbed onto gold substrates. Residues of the explosive compound tetryl was deposited onto gold substrates. s-SNOM experiments were performed in the 1260-1400cm^-1 tuning range of the ECQCL, corresponding to the N0₂ symmetric stretch vibrational fingerprint region. Vibrational infrared spectra were collected on individual chemical domains with a collection area of ~500 nm² and compared to ensemble averaged far-field reflection-absorption infrared spectroscopy (RAIRS) results.

Gallium Nitride (GaN) is a unique material system that has been heavily exploited for photonic devices thanks to ultraviolet-to-terahertz spectral tunability. However, without a cost effective approach, GaN technology is limited to laboratory demonstrations and niche applications. In this investigation, integration of GaN on Silicon (100) substrates is attempted to enable widespread application of GaN based optoelectronics. Controlled local epitaxy of wurtzite phase GaN on on-axis Si(100) substrates is demonstrated via metal organic chemical vapor deposition (MOCVD). CMOScompatible fabrication scheme is used to realize [SiO₂-Si{111}-Si{100}] groove structures on conventional 200-mm Si(100) substrates. MOCVD growth (surface treatment, nucleation, initiation) conditions are studied to achieve controlled GaN epitaxy on such grooved Si(100) substrates. Scanning electron microscopy and transmission electron microscopy techniques are used to determine uniformity and defectivity of the GaN. Our results show that aforementioned groove structures along with optimized MOCVD growth conditions can be used to achieve controlled local epitaxy of wurtzite phase GaN on on-axis Si(100) substrates.

We present a scheme for the realization of high performances, large tuning range, fully integrated and possibly low cost mid infrared laser source based on quantum cascade lasers and silicon based integrated optics. It is composed of a laser array and a laser combiner. We show that our metal grating approach gives many advantages for the fabrication yield of those laser arrays. We show the results of such a fabrication at 1350 cm^-1 with 60 cm^-1 tuning range. The silicon is a low cost option for the size consuming combiner. In the development of the SiGe platform, we present the loss measurement set up and we show losses below 1dB/cm at 4.5μm.

Several molecules of interest have their absorption signature in the mid-infrared. Spectroscopy is commonly used for the detection of these molecules, especially in the short-wave infrared (SWIR) region due to the low water absorption. Conventional spectroscopic systems consist of a broadband source, detector and dispersive components, making them bulky and difficult to handle. Such systems cannot be used in applications where small footprint and low power consumption is critical, such as portable gas sensors and implantable blood glucose monitors. Silicon-On-Insulator (SOI) offers a compact, low-cost photonic integrated circuit platform realized using CMOS fabrication technology. On the other hand, the GaSb material system allows the realization of high performance SWIR lasers and detectors. Integration of GaSb active components on SOI could therefore result in a compact and low power consumption integrated spectroscopic system. In this paper, we report the study on thin-film GaSb Fabry-Perot lasers integrated on a carrier substrate. The integration is achieved by using an adhesive polymer (DVS-BCB) as the bonding agent. The lasers operate at room temperature at 2.02μm. We obtain a minimum threshold current of 48.9mA in the continuous wave regime and 27.7mA in pulsed regime. This yields a threshold current density of 680A/cm² and 385A/cm², respectively. The thermal behaviour of the device is also studied. The lasers operate up to 35 °C, due to a 323 K/W thermal resistance

This paper addresses the interaction between light wave technologies and semiconductors devices at the nanoscale. Research works aiming at the development of emerging 1D and 2D nano materials such as nanodots, nanowires, nanotubes and nanoribbons open the way to overcome the performances bottleneck of conventional microwave photoconductive switches. Such new materials offer new opportunities for the confinement of light/matter interaction and exhibit interesting energy band diagram in an optical wavelength spectrum covering visible to NIR. Strong material interests stays for the generation of very high local density of carriers in contrast with a high dark resistivity, in association with a high carrier mobility. These challenges can be reached today thanks to nanotechnology processes with a high compatibility constraint with submicrometer light coupling solutions and microwave devices and circuits technologies. Modeling and design tools dedicated to photoconductive effect description at nanometer scale, for its implementation in passive and active components must be set up in order to exalt this effect for microwave signal processing functionalities such as switching, generation, amplification and emission over a large frequency bandwidth. This paper will report on latest demonstrations of high performance photoconductive switches for high frequency applications at 0.8μm and 1.5μm based on LT-GaAs, GaAs nanowires and GaInAsSb semiconductor materials.

In recent years, transistor technology has scaled down to sub-20 nm channel length with many performance-boosting techniques at the material and device levels in order to meet the increasing demand for higher performance electronics. The nanowire (NW) device architecture has proven itself as a viable candidate for the sub-20 nm generation transistors. Compared to Si NWs, the Ge/Si core/shell NW alternative can supply larger on-current due to the increased confined hole mobility and ohmic behavior at the Ni-alloyed drain/source contacts. It is thus important to understand transport mechanisms in this core/shell structure, and develop pathway to realize ultra-short channel core/shell NW field effect transistors (FETs). In this paper, we report the growth of Ge/Si concentric NWs with precise control of Si shell thickness. Performance of FETs fabricated from core/shell NWs exhibited a clear dependence on NWs’ diameters, with steeper sub-threshold slopes for smaller NWs. An 18 nm diameter Ge/Si heterostructure FET exhibited sub-threshold swing of 102 mV/decade, with a maximum transconductance of 3.4 μS at VDS =-100 mV. Finally, transmission electron microscopy was utilized to monitor and control the solid state reaction between Ni contacts and Ge/Si NWs, resulting in ultrascaled channel lengths, as short as 5 nm.

We reported the controlled growth and optical properties of porous nanocolumnar structures by the oblique angle deposition (OAD) method using an electron-beam evaporation system. Distributed Bragg reflectors, graded refractive index films, and antireflective coatings were studied to apply to various devices including vertical cavity surface emitting lasers, light-emitting diodes, solar cells, and sensors. The characteristics of the fabricated devices were measured. These results suggest that the tailoring of optical properties by a simple OAD technique is very promising for optical and optoelectronic device applications.

We designed and characterized Silicon Single-Photon Avalanche Diodes (SPADs) fabricated in a high-voltage 0.35 μm CMOS technology, achieving state-of-the-art low Dark Counting Rate (DCR), very large diameter, and extended Photon Detection Efficiency (PDE) in the Near Ultraviolet. So far, different groups fabricated CMOS SPADs in scaled technologies, but with many drawbacks in active area dimensions (just a few micrometers), excess bias (just few Volts), DCR (many hundreds of counts per second, cps, for small 10 μm devices) and PDE (just few tens % in the visible range). The novel CMOS SPAD structures with 50 μm, 100 μm, 200 μm and 500 μm diameters can be operated at room temperature and show DCR of 100 cps, 2 kcps, 20 kcps and 100 kcps, respectively, even when operated at 6 V excess bias. Thanks to the excellent performances, these large CMOS SPADs are exploitable in monolithic SPAD-based arrays with on-chip CMOS electronics, e.g. for time-resolved spectrometers with no need of microlenses (thanks to high fillfactor). Instead the smaller CMOS SPADs, e.g. the 10 μm devices with just 3 cps at room temperature and 6 V excess bias, are the viable candidates for dense 2D CMOS SPAD imagers and 3D Time-of-Flight ranging chips.

We designed, fabricated and tested a new planar InGaAs/InP Single-Photon Avalanche Diode (SPAD). By optimizing design and fabrication processes, we obtained low afterpulsing and very good timing jitter, with very fast tail. The detector has a separate absorption, charge and multiplication structure, with double p-type Zn diffusion into n-type InP for defining the p-n high-field avalanching junction. The SPAD can be operated at temperatures achievable with thermoelectric coolers mounted in compact packages (like TO-8). When operated in gated mode with 5 V excess bias, the 25 μm active area diameter InGaAs/InP SPAD reaches good performance at 225 K: i) photon detection efficiency of 40% at 1 μm and 25% at 1.55 μm; ii) dark count rate below 100 kcps (counts per second); iii) low afterpulsing allowing to set a hold-off time as short as 1 μs, corresponding to 1 Mcps; iv) timing jitter less than 90 ps (full width at half maximum) and time constant of decaying tail of just 30 ps. Overall this new planar InGaAs/InP SPAD can be exploited in many near-infrared (up to 1.7 μm) applications where low light, wide dynamic range waveforms have to be acquired, e.g. in Time-Correlated Single-Photon Counting (TCSPC) measurements or Time-of-Flight LIDAR applications for eye-safe 3D ranging.

In this paper we present an array of 48 Single Photon Avalanche Diodes (SPADs) specifically designed for multispot Single Molecule Analysis. The detectors have been arranged in a 12x4 square geometry with a pitch-to-diameter ratio of ten in order to minimize the collection of the light from non-conjugated excitation spots. In order to explore the tradeoffs between the detectors’ performance and the optical coupling with the experimental setup, SPADs with an active diameter of 25 and of 50μm have been manufactured. The use of a custom technology, specifically designed for the fabrication of the detectors, allowed us to combine a high photon detection efficiency (peak close to 50% at a wavelength of 550nm) with a low dark count rate compatible with true single molecule detection. In order to allow easy integration into the optical setup for parallel single-molecule analysis, the SPAD array has been incorporated in a compact module containing all the electronics needed for a proper operation of the detectors.

We present a low-power Time-to-Digital Converter (TDC) chip, fabricated in a standard cost-effective 0.35 μm CMOS technology, which provides 160 ns dynamic range, 10 ps timing resolution and Differential Non-Linearity better than 0.01 LSB rms. This chip is the core of a compact TDC module equipped with an USB 2.0 interface for user-friendly control and data-acquisition. The TDC module is suitable for a wide variety of applications such as Fluorescence Lifetime Imaging (FLIM), time-resolved spectroscopy, Diffuse Optical Spectroscopy (DOS), Optical Time-Domain Reflectometry (OTDR), quantum optics, etc. In particular, we show the application of our TDC module to fluorescence lifetime measurements.

We present a photon-counting module based on InGaAs/InP SPAD (Single-Photon Avalanche Diode) for detecting single photons up to 1.7 μm. The module exploits a novel architecture for generating and calibrating the gate width, along with other functions (such as module supervision, counting and processing of detected photons, etc.). The gate width, i.e. the time interval when the SPAD is ON, is user-programmable in the range from 500 ps to 1.5 μs, by means of two different delay generation methods implemented with an FPGA (Field-Programmable Gate Array). In order to compensate chip-to-chip delay variation, an auto-calibration circuit picks out a combination of delays in order to match at best the selected gate width. The InGaAs/InP module accepts asynchronous and aperiodic signals and introduces very low timing jitter. Moreover the photon counting module provides other new features like a microprocessor for system supervision, a touch-screen for local user interface, and an Ethernet link for smart remote control. Thanks to the fullyprogrammable and configurable architecture, the overall instrument provides high system flexibility and can easily match all requirements set by many different applications requiring single photon-level sensitivity in the near infrared with very low photon timing jitter.

Ga-free InAs/InAsSb type-II superlattice (T2SL) nBn photodetectors with very low dark current are fabricated and characterized. The typical device without antireflection coating and surface passivation has a cut-off wavelength of 13.2 micrometers, quantum efficiency (QE) of 2.5% and a background limited operating temperature of 70 K. Our analysis shows that the anticipated highest operating temperature of a 10.6 micrometer cut-off Ga-free T2SL nBn device can be 108 K, with a potential to reach 135 K if 20% QE or lower noise is achieved.

Spatial noise and the loss of photogenerated current due material non-uniformities limit the performance of long wavelength infrared (LWIR) HgCdTe detector arrays. Reducing the electrical activity of defects is equivalent to lowering their density, thereby allowing detection and discrimination over longer ranges. Infrared focal plane arrays (IRFPAs) in other spectral bands will also benefit from detectivity and uniformity improvements. Larger signal-to-noise ratios permit either improved accuracy of detection/discrimination when an IRFPA is employed under current operating conditions, or provide similar performance with the IRFPA operating under less stringent conditions such as higher system temperature, increased system jitter or damaged read out integrated circuit (ROIC) wells. The bulk passivation of semiconductors with hydrogen continues to be investigated for its potential to become a tool for the fabrication of high performance devices. Inductively coupled plasmas have been shown to improve the quality and uniformity of semiconductor materials and devices. The retention of the benefits following various aging conditions is discussed here.

Active and passive imaging in a single camera based on the combination of short-wavelength and mid-wavelength infrared detection is highly needed in a number of tracking and reconnaissance missions. Due to its versatility in band-gap engineering, Type-II InAs/GaSb/AlSb superlattice has emerged as a candidate highly suitable for this multi-spectral detection. In this paper, we report the demonstration of high performance bias-selectable dual-band short-/mid-wavelength infrared photodetectors based on InAs/GaSb/AlSb type-II superlattice with designed cut-off wavelengths of 2 μm and 4 μm. Taking advantages of the high performance short-wavelength and mid-wavelength single color photodetectors, back-to-back p-i-n-n-i-p photodiode structures were grown on GaSb substrate by molecular beam epitaxy. At 150 K, the short-wave channel exhibited a quantum efficiency of 55%, a dark current density of 1.0x10^-9 A/cm² at -50 mV bias voltage, providing an associated shot noise detectivity of 3.0x1013 Jones. The mid-wavelength channel exhibited a quantum efficiency of 33% and a dark current density of 2.6x10^-5 A/cm² at 300 mV bias voltage, resulting in a detectivity of 4.0x10¹¹ Jones. The operations of the two absorber channels are selectable by changing the polarity of applied bias voltage.

The noise behavior of InAs/GaSb superlattice photodiodes for high-performance thermal imaging in the mid- and longwavelength infrared atmospheric windows at 3-5 μm and 8-12 μm is complex and up to now not very well understood. In order to characterize these devices we have developed a noise measurement setup with a noise current resolution in the femtoampère range. First, we show that, when sidewall leakage is absent, InAs/GaSb superlattice photodiodes with a low dark current very close to the generation-recombination limited dark current level of the bulk behave according to the well-known shot noise expression. Next, we investigate a set of 18 large-area diodes with a bandgap in the midwavelength infrared regime, which show an increased dark current depending linearly on the applied reverse bias. For these diodes the common shot noise model generally fails to describe the noise experimentally observed in the white part of the noise spectrum. Instead, we find that McIntyre’s excess noise model for electron-initiated avalanche multiplication processes fits our data remarkably well for the entire set of diodes, which covers about three orders of magnitude in dark current and a wide range of reverse bias voltage. Thus, to explain the mechanism leading to the increased reverse dark current and observed excess noise we tentatively suggest that primary electrons originating from Shockley-Read-Hall states within the space charge region might initiate avalanche multiplication processes within high electric field domains localized around sites of macroscopic crystallographic defects.

In this paper we describe the bulk crystal growth and characterization of low defect mono-crystalline InSb and GaSb substrates suitable for use in the epitaxial deposition of infrared detector structures. Results will be presented on the production of single crystal InSb and GaSb ingots grown by both standard and modified forms of the Czochralski (Cz) technique. Material quality has been assessed by a new method of fully automated defect recognition microscopy (DRM) that enables crystallographic defect structures (etch pits) to be mapped and presented in real time. X-Ray Diffraction (XRD) assessments have been used to derive information on the spatial uniformity of bulk quality and this shows that very high quality crystals have been grown. Consideration has also been given to the requirements for manufacture of ≥4" diameter ingots that will be necessary to support the fabrication of very large area, Sb-based detector structures. The scaling challenges associated with InSb and GaSb production will also be discussed.

We discuss two distinct approaches to realizing distributed-feedback (DFB) interband cascade lasers (ICLs) for emission in the mid-IR. In the top-grating approach, the first-order gratings are produced by patterning high-index germanium layers on top of narrow ridges with relatively thin top claddings. One 7-μm-wide device emitting at λ = 3.8 μm generated over 27 mW of cw single-mode output at 40°C, with a side-mode-suppression ratio <30 dB, while at 80°C it still emitted <1 mW. At 20°C, a second device lased in a single spectral mode with <100 mW of drive power. The tuning range was 21.5 nm with temperature and 10 nm with current. The corrugated-sidewall approach relies on a fourth-order grating defined by optical lithography and etched into the sidewalls of the laser ridge. For a 13-μm-wide ICL ridge emitting at λ = 3.6 μm, the maximum power at T = 25°C was 55 mW, and at 40°C the device still produced 11 mW. We compare the physical requirements and performance characteristics for the two DFB classes and conclude that top-grating DFBs generally exhibit greater stability and reproducibility, although the efficiency is reduced by extra loss induced by modal overlap with the top metallization.

Various semiconductor laser concepts have been researched during recent years to cover the mid-infrared (MIR) spectral range with robust, compact and reliable coherent sources operating at ambient temperatures. Among them, interband cascade lasers [1] (ICLs), which unite interband transitions with the concept of cascading active regions, have recently shown great promise as a low power consumption devices. They can emit in the tens of mW output power range between 3 and 6 μm. In this work, we describe improvements of our quantum-engineered ICLs leading towards lower threshold current densities and higher operation temperatures. This includes the active region design as well as the doping levels in different parts of the structure. Broad area lasers with unprecedented threshold current densities as low as 97 A/cm² at room temperature are presented.

In this work we report on the characterization of InAsNSb dilute nitride alloys and mutli-quantum well structures. InAsN epilayers with room-temperature photoluminescence emission have been successfully grown by MBE on InAs and GaAs substrates. By careful attention to growth conditions, device quality material can be obtained for N contents up to ~3% with band gap reduction which follows the band anti-crossing model. Mid-infrared light-emitting diodes containing ten period InAsNSb/InAs multi-quantum wells within the active region were fabricated. These devices exhibited electroluminescence up to room temperature consistent with e-hh₁ and e-lh₁ transitions within type I quantum wells in good agreement with calculations. Comparison of the temperature dependence of the EL with that of type II InAsSb/InAs reveals more intense emission at low temperature and an improved temperature quenching up to T~200 K where thermally activated carrier leakage becomes important and further increase in the QW band offsets is needed. This material system shows promise for use in mid-infrared diode lasers and other optoelectronic devices.

An external cavity lasing system, which operates at 2.4 μm and involves an InGaAsSb Fabry-Pérot gain chip, was developed and characterized. The system was set up in a Littrow configuration to achieve a tuning range of about 100 nm. Continuous, mode-hope free tuning was observed with variation of grating angle. The threshold current of the lasing chip varied from 86 mA down to 70 mA for lasing in Fabry-Pérot (FP) and external cavity (ECL) modes, respectively. A dependence of the threshold current on lasing wavelength was observed. The tuning range was found to depend on the semiconductor gain profile and also on the intensity of the optical feedback through the collimation-dispersion system. Drifting of the wavelength, as observed in intra cavity semiconductor lasers, was not observed for drive currents of up to 350 mA in regions close to the gain peak. Single mode ECL lasing could be sustained in this range at a significantly weaker optical feedback. Traces of FP lasing were, however, observed at currents in the range of 100-200 mA for wavelengths in the shoulder regions of the gain profile. In these cases, an increase of drive current resulted in mode hopping to multi-mode FP lasing. Our results confirm the competing nature of FP lasing with that of ECL mode of lasing. The onset of multi-mode FP lasing, and eventual hopping to these modes, can be correlated with the gain profile, drive current, and optical feedback.

This paper reviews the recent breakthroughs in the field of highly miniaturized optical parametric oscillators for midinfrared spectroscopy.

The growth, fabrication, and properties of GaN/AlN/sapphire with periodically poled surface polarity for second harmonic generation are investigated. The periodic inversion of the surface polarity is achieved by the growth of a thin AlN buffer layer and subsequent partial removal by using either wet etching with potassium hydroxide (KOH) or reactive-ion etching (RIE). GaN growth on these substrates by MOCVD leads to Gapolar GaN on the AlN buffer and N-polar GaN on the bare sapphire. Using atomic force microscopy and scanning electron microscopy, it is demonstrated that a sufficient combination of H₂ and NH₃ surface treatment before the growth of the GaN layers removes surface defects introduced by RIE etching. Thus, films with comparable quality and properties independent of the etching technique could be grown. However, in contrast to RIE etching, the interfaces between the Ga-polar and N-polar GaN is rough if KOH etching is applied. Thus, it is concluded that MOCVD in combination with RIE etched AlN/sapphire substrates can be a versatile process to fabricate GaN with periodically poled surface polarity as desired for UV light generation via frequency doubling.

GaInAs/AlInAs/InP quantum cascade lasers (QCLs) have established themselves as reliable and versatile semiconductor laser sources in the mid-infrared wavelength region. Due to the presence of unique molecular absorption lines, in combination of water-free atmospheric transmission windows, this spectral range is of particular importance for sensing, medical, material processing and homeland security applications. Being compact electrically pumped and able to operate at room-temperature, QCLs are ideal choice for wavelengths between 3.5 - 12 microns. However, wavelengths above and below are more challenging to obtain. In our work, we use intracavity nonlinear frequency mixing in mid-infrared QCLs to extend the spectral coverage for GaInAs/AlInAs/InP devices. We demonstrate that passive nonlinear structures, consisting of coupled quantum wells can be grown on top of the mid-IR QCL active region. Such nonlinear structures can be designed to possess a resonant nonlinear response for the pump frequency. Such concept, in combination with quasi-phase-matching technique can be used for efficient short-wavelength lasing by second-harmonic generations. We demonstrated room-temperature lasing down to 2.6 micrometer. For long-wavelengths, particularly THz frequencies, a novel waveguide concept was introduced. Here, we used a leaky THz waveguide concept, for a difference-frequency generation device. Phase matching was achieved by Cherenkov phase-matching scheme. This concept led to ultra-broadband THz emission at room-temperature (1.2-4.5 THz) with pulsed output powers as high as 14 μW.

Two-color, high-power, tunable lasers are highly beneficial for the generation of new wavelengths through various nonlinear methods. Vertical external cavity surface emitting lasers (VECSEL) are particularly flexible for intracavity wavelength control. We report a novel VECSEL cavity capable of generation high-power, tunable, two-color generation. The laser employs overlapping VECSEL cavities in a two-chip T-shape cavity configuration which uses a polarizing beam splitter/combiner to fold one cavity and thus allows for two-color orthogonally polarized high power outputs. The tunable collinear, orthogonally polarized two-color is ideal for type II nonlinear conversion. A continuous wave (CW) output power in excess of 13 W for the two-color emissions was demonstrated to be the sum of the output from each of the overlapped cavities. In a high Q cavity, birefringent filters were used to facilitate tunability, and wavelength separation was varied from 35 nm to 52 nm. In a modified T-Cavity configuration, high power intracavity type-II sum frequency generation resulted in tunable blue-green emission with more than 750 mW output. By selecting identical or different chips in the cavity, the wavelength separation and tuning can cover a wide range from zero to several hundreds of nm opening doors for broad mid- to far-infrared applications.

Electrical and optical characterizations of highly-doped InAsSb layers lattice matched to GaSb substrates show the possibility to control their plasma frequency in the mid-infrared range. Reflectance experiments performed on InAsSb sub-wavelength arrays evidence localized surface plasmon resonances which can be modeled by finite difference time domain method. By adjusting the refractive index of the surrounding material and the geometry of the periodic arrays it is possible to control the frequency of the plasmonic resonances. Our results show that GaSb-based materials can be the building block of all-semiconductor mid-infrared plasmonic devices.

We study experimentally and theoretically band-pass filters based on guided-mode resonances in free-standing metal-dielectric structures with subwavelength gratings. A variety of filters are obtained: polarizing filters with lD gratings, and unpolarized or selective polarization filters with 2D gratings, which are shown to behave as crossed-lD structures. In either case, a high transmission (up to ≈ 79 %) is demonstrated, which represents an eight-fold enhancement compared to the geometrical transmission of the grating. We also show that the angular sensitivity strongly depends on the rotation axis of the sample. This behavior is explained with a detailed description of the guided-mode transmission mechanism.

Here, an in-depth study of a novel electro-optic phenomenon suggested earlier in optical waveguides, referred to as the conducting interfaces, is carried out. It is shown that it would be possible to achieve a very strong optical modulation effect in slab waveguides fabricated on the standard III-V InAlGaAs compound platform. The electro-optic effect is obtained by controlling the density and population of the electron and hole states in the well layer. The additional phase shift contributed to the reflection phase of the guided electromagnetic wave constitutes an ultrastrong source of optical modulation and phase control. It is estimated that a Mach-Zhender configuration based on this optical modulator, will have a relatively compact size of around 50 microns in length, compared to the other conventional technologies.

Lattice-matched GaP-based nanostructures grown on silicon substrates is a highly rewarded route for coherent integration of photonics and high-efficiency photovoltaic devices onto silicon substrates. We report on the structural and optical properties of selected MBE-grown nanostructures on both GaP substrates and GaP/Si pseudo-substrates. As a first stumbling block, the GaP/Si interface growth has been optimised thanks to a complementary set of thorough structural analyses. Photoluminescence and time-resolved photoluminescence studies of self-assembled (In,Ga)As quantum dots grown on GaP substrate demonstrate a proximity of two different types of optical transitions interpreted as a competition between conduction band states in X and Γ valleys. Structural properties and optical studies of GaAsP(N)/GaP(N) quantum wells coherently grown on GaP substrates and GaP/Si pseudo substrates are reported. Our results are found to be suitable for light emission applications in the datacom segment. Then, possible routes are drawn for larger wavelengths applications, in order to address the chip-to-chip and within-a-chip optical interconnects and the optical telecom segments. Finally, results on GaAsPN/GaP heterostructures and diodes, suitable for PV applications are reported.

In this paper, two kinds of colloidal quantum dots, PbS and HgTe, are explored for SWIR photodetectors application. The colloidal dots are prepared by hot injection chemical synthesis, with organic ligands around the dots keeping them stable in solution. For the purpose of achieving efficient carrier transport between the dots in a film, these long organic ligands are replaced by shorter, inorganic ligands. We report uniform, ultra-smooth colloidal QD films without cracks realized by dip-coating and corresponding ligand exchange on a silicon substrate. Metal-free inorganic ligands, such as OH^- and S^2-, are investigated to facilitate the charge carrier transport in the film. Both PbS and HgTe-based quantum dot photoconductors were fabricated on interdigitated gold electrodes. For PbS-based detectors a responsivity of 200A/W is measured at 1.5μm, due to the large internal photoconductive gain. A 2.2μm cut-off wavelength for PbS photodetectors and 2.8μm for HgTe quantum dot photodetectors are obtained.

This paper summarizes recent progress primarily achieved in authors' laboratory on synthesizing hexagonal boron nitride (hBN) epilayers by metal organic chemical vapor deposition (MCVD) and studies of their structural and optoelectronic properties. The structural and optical properties of hBN epilayers have been characterized by x-ray diffraction (XRD) and photoluminescence (PL) studies and compared to the better understood wurtzite AIN epilayers with a comparable energy bandgap. These MOCVD grown hBN epilayers exhibit highly efficient band-edge PL emission lines centered at around 5.5 eVat room temperature. The band-edge emission of hBN is two orders of magnitude higher than that of high quality AlN epilayers. Polarization-resolved PL spectroscopy revealed that hEN epilayers are predominantly a surface emission material, in which the band-edge emission with electric field perpendicular to the c-axis (E_emi⊥c) is about 1.7 times stronger than the component along the c-axis (E_emillc). This is in contrast to AIN, in which the band­ edge emission is known to be polarized along the c-axis, (E_emillc). Based on the graphene optical absorption concept, the estimated band-edge absorption coefficient of hBN is about 7x10⁵ cm^-1, which is more than 3 times higher than the value for AlN (~2x10⁵ cm^-1 . The hBN epilayer based photodetectors exhibit a sharp cut-off wavelength around 230 nm, which coincides with the band-edge PL emission peak and virtually no responses in the long wavelengths. The dielectric strength of hBN epilayers exceeds that of AlN and is greater than 4.5 MV/cm based on the measured result for an hBN epilayer released from the host sapphire substrate.

We used the two-wire 3ω method to measure the in-plane and out-of-plane thermal conductivity of thin films and analyzed the error for all fitting parameters. We find the heater half-width, the insulating layer thickness and the out-of-plane thermal conductivity of the insulating layer the most sensitive parameters in an accurate fitting. The data of a 2.5 μm GaAs thin film suggests that the phonon mean free path in the film is limited to the film thickness, far shorter than that in the bulk material at low temperatures.

We propose a novel set of boundary conditions, based on the continuity of a generalized velocity and on the continuity of the probability current at the interface of heterojunctions, which is well suited to construct the solution of the tunneling problem when spin-orbit interaction is taken into account. We illustrate this procedure in a model case: tunneling of conduction electrons through a [110]-oriented GaAs barrier. In that case, the new boundary conditions reduce to two set of equations: the first one expresses the discontinuity of the envelope function at the interface while the other one is close to the standard condition on the derivative of the envelope function.

We report on the design, fabrication and optical investigation of AlGaAs microcavities for THz Difference Frequency Generation (DFG) between Whispering Gallery Modes (WGMs), where the pump and DFG wavelengths (λ ≈ 1.3 μm and λ ≈ 75-150 μm, respectively) lie on opposite sides of the Restrahlen band. For the pump modes, we demonstrate CW lasing of quantum-dot layers under electrical injection at room temperature. We control the number of lasing WGMs via vertical notches on the pillars sidewalls, providing a selection mechanism for funneling the power only to the modes contributing to DFG. In parallel with the optimization of the pump lasers and in order to validate design and material parameters before the DFG experiments, we have performed linear measurements on two sets of passive samples. For the telecom range, the micropillars have been integrated with waveguides for distributed coupling and characterized via transmission measurements. In the THz range we have measured reflectivity spectra on 2D arrays of identical cylinders. In both cases, we demonstrate a good agreement between experimental results and simulations. On a more speculative note, we numerically show that etching a hole along the pillar axis can facilitate phase matching, while single-lobe farfield pattern can be obtained for the THz mode by micro-structuring the metallic ground plane around the microcavity. Finally, we suggest a real-time fine-tuning mechanism for the forthcoming active devices.

We report on the experimental measurement of active region lattice (T_L) and electronic temperatures (T_e) in terahertz quantum cascade devices based on the phonon-photon-phonon scheme, by means of microprobe band-to-band photoluminescence spectroscopy. Three mesa devices, differing for doping region and number of quantum wells composing the active region, have been investigated. With device on, under band alignment for lasing condition, we measured a difference (T_e - T_L) ~ 40 K much smaller than the typical value (T_e - T_L ~ 100 K) reported for resonantphonon THz QCLs.

Semiconductor nanowires (NWs) represent an ideal building block for implementing rectifying diodes or plasma­ wave detectors that could operate well into the THz, thanks to the typical attofarad-order capacitance. Despite the strong effort in developing these nanostructures for a new generation of complementary metal-oxide semi­ conductors (CMOS), memory and photonic devices, their potential as radiation sensors into the Terahertz is just starting to be explored. We report on the development of NW-based field effect transistors operating as high sensitivity THz detectors in the 0.3 - 2.8 THz range. By feeding the radiation field of either an electronic THz source or a quantum cascade laser (QCL) at the gate-source electrodes by means of a wide band dipole antenna, we measured a photovoltage signal corresponding to responsivity values up to 100 V IW, with impressive noise equivalent power levels < 6 x 10^-11W/Hz at room temperature and a > 300kHz modulation bandwidth. The potential scalability to even higher frequencies and the technological feasibility of realizing multi-pixel arrays coupled with QCL sources make the proposed technology highly competitive for a future generation of THz detection systems.

Analytical instruments based on InfraRed Absorption Spectroscopy (IRAS) and Gas Chromatography (GC) are today available only as bench-top instrumentation for forensic labs and bulk analysis. Within the 'DIRAC' project funded by the European Commission, we are developing an advanced portable sensor, that combines miniaturized GC as its key chemical separation tool, and IRAS in a Hollow Fiber (HF) as its key analytical tool, to detect and recognize illicit drugs and key precursors, as bulk and as traces. The HF-IRAS module essentially consists of a broadly tunable External Cavity (EC) Quantum Cascade Laser (QCL), thermo-electrically cooled MCT detectors, and an infrared hollow fiber at controlled temperature. The hollow fiber works as a miniaturized gas cell, that can be connected to the output of the GC column with minimal dead volumes. Indeed, the module has been coupled to GC columns of different internal diameter and stationary phase, and with a Vapour Phase Pre-concentrator (VPC) that selectively traps target chemicals from the air. The presentation will report the results of tests made with amphetamines and precursors, as pure substances, mixtures, and solutions. It will show that the sensor is capable of analyzing all the chemicals of interest, with limits of detection ranging from a few nanograms to about 100-200 ng. Furthermore, it is suitable to deal with vapours directly trapped from the headspace of a vessel, and with salts treated in a basic solution. When coupled to FAST GC columns, the module can analyze multi-components mixes in less than 5 minutes.

The development of a continuous wave (CW), thermoelectrically cooled (TEC), distributed feedback (DFB) laser diode based spectroscopic trace-gas sensor for ultra sensitive and selective ethane (C₂H₆) concentration measurements is reported. The sensor platform used tunable laser diode absorption spectroscopy (TDLAS) and wavelength modulation spectroscopy (WMS) as the detection technique. TDLAS was performed with an ultra-compact 57.6 m effective optical path length innovative spherical multipass cell capable of 459 passes between two mirrors separated by 12.5 cm. For an interference free C₂H₆ absorption line located at 2976.8 cm_-1 a 1σ minimum detection limit of 130 pptv with a 1 second lock-in amplifier time constant was achieved.

We report standoff detection of explosives using quantum cascade laser (QCL) and photoacoustic technique. In our experiment, a QCL with emission wavelength near 7.35 μm was used and operated at pulsed mode. The output light was focused on Trinitrotoluene (TNT) sample in its powder form. Photoacoustic signals were generated and detected by an ultra-sensitive low-noise microphone with one inch diameter. A detection distance up to 8 inches was obtained using the microphone alone. With the increasing detection distance the measured photoacoustic signal not only decayed in amplitude but also delayed in phase, which clearly verified the source location. To further increase the detection distance, a parabolic sound reflector was used for effective sound collection. With the help of the sound reflector, standoff photoacoustic detection of TNT with distance of 8 feet was demonstrated.

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 demonstrate a common-path optical interferometer based on a quantum-cascade laser (QCL), in which the QCL acts both as source and detector of the infrared radiation. The collinear arms of the interferometer are terminated by a plastic surface (acting as the beam splitter) and by a metallic one (acting as the mirror). The different reflectivity of the surfaces allows for high contrast feedback-interferometry fringes exhibited on the laser-emitted power and revealed by voltage compliance measurement at the QCL terminals. The displacement of each surface can be identified and measured with sub wavelength resolution. The experimental results are in excellent agreement with the numerical simulations based on the Lang-Kobayashi model for multiple cavities. Applications to microfluidics and resonant chemical detection can be envisaged.

In this contribution, we discuss progresses on ultra-sensitive chemical sensing in the mid-infrared (MIR; 3-20 μm) spectral regime by combining microfabricated GaAs/Al_0.2Ga_0.8As waveguides and sensing structures with quantum cascade lasers (QCL). Modern epitaxial grown methods including molecular beam epitaxy (MBE) and metal–organic vapor-phase epitaxy (MOVPE) were applied facilitating epitaxial growth of on-chip MIR GaAs/Al_0.2Ga_0.8As (6 μm core / 6 μm cladding) semiconductor slab waveguides, which were then structured with reactive ion etching (RIE) and/or focused ion beam milling (FIB) for establishing a variety of substrate-integrated GaAs/Al_0.2Ga_0.8As waveguide geometries. A distributed feedback (DFB) QCL lasing at a wavelength of 10.3 μm was combined with planar waveguide slabs and strip waveguides, respectively. Exemplary detection of acetic anhydride on strip waveguides (50 μm waveguide width) result in a limit of detection (LOD) of 0.05 pL, which is among the most sensitive direct evanescent field absorption measurements with substrate-integrated waveguides using MIR sensing systems reported to date. The first mid-infrared Mach-Zehnder interferometers (MIR-MZI) was recently design, fabricated, and functionally verified using a broadly tunable quantum cascade laser (tQCL) providing access to a spectral window of 5.78-6.35 μm. Finally, the development of first MIR ring resonators via microfabrication is shown providing an outlook toward next-generation miniaturized MIR sensor devices based on substrate-integrated semiconductor waveguides.

While conventional semiconductor lasers employ electrical injection for carrier excitation, optically pumped semiconductor lasers (OPSLs) have demonstrated high output powers and high brightness in the mid-infrared. An important consideration for optically pumped lasers is efficient absorption of the pump beam, which can be achieved through increasing the number of periods in the active region, by placing the active region in a cavity with an optical thickness of twice the pump wavelength between distributed Bragg reflectors (Optical Pumping Injection Cavity), or by periodically inserting the active quantum wells into an InGaAsSb waveguide designed to absorb the pump radiation (Integrated Absorber). A tunable optical pumping technique is utilized by which threshold intensities are minimized and efficiencies are maximized. The near-IR idler output of a Nd:YAG-pumped optical parametric oscillator (10 Hz, ~4 ns) is the tunable optical pumping source in this work. Results are presented for an OPSL with a type-II W active region embedded in an integrated absorber to enhance the absorption of the optical pump beam. Emission wavelengths range from 4.64 μm at 78 K to 4.82 μm at 190 K for optical pump wavelengths ranging from 1930-1950 nm. The effect of wavelength tuning is demonstrated and compared to single wavelength pumping (1940 nm) at a higher duty cycle (20- 30%). Comparisons are also made to other OPSLs, including a discussion of the characteristic temperature and high temperature performance of these devices.

There are reviewed the optical properties of two kind of active regions of mid infrared laser devices both grown on GaSb substrates: GaInAsSb/AlGaInAsSb type I QWs for laser diodes and InAs/GaInAsSb type II QWs for interband cascade lasers. There are presented their crucial optical properties and the related current challenges with respect to the device performances. This covers such issues as spectral tenability of the emission via the structure parameters, the band gap discontinuities, carrier loss mechanisms and oscillator strengths. For that, spectroscopic techniques have been used (photoluminescence and its temperature dependence, and photoreflectance) and combined with the energy level calculations based on effective mass approximation and kp theory. Eventually, the potential for further material optimization and prospects for the improved device performances are also discussed.

In this communication, we studied the influence of the SL period on the electrical performances of MWIR pin photodiodes, fabricated by MBE on p-type GaSb substrate. These SL structures are made of symmetric or asymmetric SL period designs and exhibited cut-off wavelength around 5μm at 77K. Experimental measurements carried out on several SL pin photodiodes show the superiority, in terms of dark current density, of the asymmetric SL structure composed of 7 InAs monolayers (MLs) and 4 GaSb MLs. As a result, the 7/4 SL diode exhibits dark current density values as low as 40nA/cm² and R₀A product greater than 1.7x10⁶ Ohm.cm² at 77K, one decade larger than the value obtained with equivalent symmetric 10/10 SL diode. This result obtained demonstrates the strong influence of the SL period design on the performances, and then on temperature operation, of MWIR SL photodiodes.

In this work, an analytical study of the temperature dependence of current gain and ideality factor (η) has been performed for the heterojunction bipolar light emitting transistor (HBLET). In order to utilize the radiative recombination, the structure of HBT embedded two quantum wells in the base region which can improve the radiation efficiency. Compare with the convention HBT, the temperature dependence of current gain increases 42.5% with increasing temperature from 350K followed by a decrease towards 300K. Variation of gain with temperature is different from that the characteristic of HBT adding another advantage in favor of the HBLET. The η_B of these devices are similar, revealing that the space-charge recombination dominates the overall base current. The high output power of HBLET is 962 μW at 88 mA. These results reveal that the HBLET which combine electrical and optical characteristic device.

Enhanced transmissions at infra-red wavelengths are measured through hole arrays made in gold-covered silicon nitride free-standing membranes. The membranes are made by a standard photolithography batch process. They are cheap to fabricate, reproducible and robust. The optical transmission of the membranes are investigated with varying hole size (down to 1μm), period, and thickness. The membranes show enhanced optical transmission. The spectra show good agreement with a very simple mode matching model which can be used for design. Calculations are also shown giving absorption enhancements of 5.7 normalized to the same material on a silicon membrane. Finite difference time domain calculations are also presented to show the spatial distribution of the enhanced field. Field enhancements of 3.3 are calculated. The field enhancements are concentrated in the hole which makes the membranes ideally suited for a microfluidic setup. Hence, this paper shows that through enhanced transmission cheap, disposable membranes in a simplified transmission can be used for measurements for molecular absorption.