Research Papers Featuring Block Engineering's Products

The following papers feature Block Engineering's QCL-based products.

Surface Analysis and Hyperspectral Imaging with Quantum Cascade Lasers

Block Engineering's pulsed external-cavity Quantum Cascade Lasers (QCLs) combine the optical brightness of lasers with very broad wavelength tuning to provide a breakthrough capability for spectroscopy. Over the past decade QCLs have been refined, become accepted as a spectroscopy tool, and applied to heretofore challenging sampling scenarios such as transmission through liquids or scattering off of low-reflectivity surfaces. More recent miniaturization of hardware and electronics are now enabling QCLs to become embedded in portable and hand-held instruments. This presentation will briefly review the current state of the art of QCL technology and describe systems recently developed for portable medical diagnostics and hyperspectral imaging. (doi: 10.1117/12.2655234)

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Laser-based Long-wave-infrared Hyperspectral Imaging System for the Standoff Detection of Trace Surface Chemicals

A trace chemical detector is described that combines external-cavity quantum cascade lasers and a mercury cadmium telluride camera to capture hyperspectral images of the diffuse reflectance from a target surface in the long-wave infrared. The system is able to generate individual hypercubes in <0.1 s. When raster scanning the laser beam over the target surface, areal coverage rates of >60 cm2∕s have been achieved. Results are presented for standoff distances ranging from 0.1 to 25 m. Hyperspectral images generated by the system are analyzed for spectral features that indicate the presence of trace surface contaminants. This approach has been found to be highly capable of detecting trace chemical residues on a wide variety of surfaces, and we present a collection of detection results to demonstrate the capabilities of this technology. Examples include the detection of 10 μg of saccharin powder on a wide range of substrates, 0.2 µg of an explosive residue on a computer keyboard, residual pharmaceuticals within a plastic baggie, and a contaminated fingerprint on cell phone case.

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Mid-Infrared Reflection Signatures for Trace Chemicals on Surfaces

Results are presented for the detection of trace explosive residues on real-world surfaces using active mid-infrared (MIR) hyperspectral imaging. The target surface is illuminated using miniature, rapidly tunable, external-cavity quantum cascade lasers (EC-QCLs) and the reflected light is imaged using a HgCdTe camera with a spatial resolution of 70 um. Hypercubes with 128x128 pixels are captured with more than 256 wavelengths that span 7.7 - 11.8 um. The samples consisted of PETN residues which were applied to keyboard keys at various levels of chemical loading. We estimate a limit of detection of less than 6 ng per pixel for the as-deposited chemical. The explosive residue remains detectable by HSI even after wiping the surface several times using isopropyl alcohol. Simple signature models for solid particles (i.e., Mie scattering) and thin-films account for the many of the spectral features observed in the chemical signatures.

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High-speed and large-area scanning of surfaces for trace chemicals using wavelength-tunable quantum cascade lasers

A standoff chemical detection system is being developed to detect and identify a wide range of trace chemicals on a variety of natural and artificial surfaces. The system is based on active mid-infrared (MIR) hyperspectral imaging in which the target surface is illuminated using miniature, rapidly tunable, external-cavity quantum cascade lasers (ECQCLs). These lasers are tuned across the wavelength range of 7.7 – 11.8 um while a HgCdTe camera captures images of the reflected light. Hypercubes with 128x128 pixels and more than 130 wavelengths are captured within 0.1 s. By operating the camera in sub-window mode, hypercubes with 16x96 pixels and 138 frames are captured in only 14 ms. To the best of our knowledge, these represent the world's fastest acquisition of active MIR hypercubes. Raster-scanning of the laser beam is used to scan large regions. In this talk, we will present results for detecting traces of solid chemicals (with loadings on the order of 100 ug) on natural outdoor surfaces such as roofing shingles, concrete, sand, and asphalt at a standoff distance of 5 m. The measured spectra are found to correlate very well with those of reference measurements made of pure chemicals after accounting for the substrate reflectance.

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Mid-Infrared Hyperspectral Simulator for Laser-Based Detection of Trace Chemicals on Surfaces

Laser-based, mid-infrared (MIR) hyperspectral imaging (HSI) has the potential to detect a wide range of trace chemicals on a variety of surfaces under standoff conditions. The major challenge of MIR reflection spectroscopy is that the reflection signatures for surface chemicals can be complex and exhibit significant spectral variability. This paper describes a MIR Hyperspectral Simulator that is being developed to model the reflectance signatures from surfaces including the effects of speckle and other sources of spectral variability. Simulated hypercubes will be compared with experiments.

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High-speed mid-infrared hyperspectral imaging using quantum cascade lasers

We report on a standoff chemical detection system using widely tunable external-cavity quantum cascade lasers (ECQCLs) to illuminate target surfaces in the mid infrared (λ = 7.4 – 10.5 μm). Hyperspectral images (hypercubes) are acquired by synchronously operating the EC-QCLs with a LN2-cooled HgCdTe camera. The use of rapidly tunable lasers and a high-frame-rate camera enables the capture of hypercubes with 128 x 128 pixels and >100 wavelengths in <0.1 s. Furthermore, raster scanning of the laser illumination allowed imaging of a 100-cm2 area at 5-m standoff. Raw hypercubes are post-processed to generate a hypercube that represents the surface reflectance relative to that of a diffuse reflectance standard. Results will be shown for liquids (e.g., silicone oil) and solid particles (e.g., caffeine, acetaminophen) on a variety of surfaces (e.g., aluminum, plastic, glass). Signature spectra are obtained for particulate loadings of RDX on glass of <1 μg/cm2.

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Detection of chemical clouds using widely tunable quantum cascade lasers

Widely tunable quantum cascade lasers (QCLs) spanning the long-wave infrared (LWIR) atmospheric transmission window and an HgCdTe detector were incorporated into a transceiver having a 50-mm-diameter transmit/receive aperture. The transceiver was used in combination with a 50-mm-diameter hollow retro-reflector for the open-path detection of chemical clouds. Two rapidly tunable external-cavity QCLs spanned the wavelength range of 7.5 to 12.8 um. Open-path transmission measurements were made over round-trip path-lengths of up to 562 meters. Freon-132a and other gases were sprayed into the beam path and the concentration-length (CL) product was measured as a function of time. The system exhibited a noise-equivalent concentration (NEC) of 3 ppb for Freon-132a given a round-trip path of 310 meters. Algorithms based on correlation methods were used to both identify the gases and determine their CL-products as a function of time.

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Standoff detection of chemical and biological threats using miniature widely tunable QCLs

Standoff detection and identification of chemical threats has been the "holy grail" of detection instruments. The advantages of such capability are well understood, since it allows detection of the chemical threats without contact, eliminating possible operator and equipment contamination and the need for subsequent decontamination of both. In the case of explosives detection, standoff detection might enable detection of the threat at safe distances outside the blast zone. A natural extension of this capability would be to also detect and identify biological threats in a standoff mode and there are ongoing efforts to demonstrate such capability.

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Active and passive infrared spectroscopy for the detection of environmental threats

Block MEMS/Engineering develops mid-infrared spectroscopy systems based on both Fourier transform infrared (FTIR) spectrometers and quantum cascade lasers (QCLs). Our recently developed miniaturized external-cavity QCLs are widely tunable over a spectral range of >250 cm-1 and tuning can be accomplished at rates of >25 cm-1 per millisecond. This enables high-speed mid-infrared spectroscopy of gases and surface contaminants for a variety of military and commercial applications. This paper provides an overview of our FTIR and QCL systems and their defense-related applications.

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Highly sensitive and specific noninvasive in-vivo alcohol detection using wavelength-modulated differential photothermal radiometry

This paper reports the application of wavelength modulated differential photothermal radiometry (WM-DPTR) to blood alcohol (ethanol) concentration (BAC) measurements in the mid-infrared range to prevent impaired driving. In-vivo alcohol consumption measurements performed in the BAC range of interest (0-80 mg/dl) with an optimal wavelength pair demonstrated the alcohol detection capability of WM-DPTR with high resolution (~5 mg/dl) and a low detection limit (~10 mg/dl). Oral glucose tolerance tests using both glucose and alcohol sensitive wavelength pairs in the normal-to-hyperglycemia range (~80-320 mg/dl) proved the blood glucose screening ability and ethanol detection specificity of WM-DPTR. The immunity of WM-DPTR to temperature and glucose variation makes the differential signals alcohol sensitive and specific, yielding precise and accurate noninvasive alcohol measurements in the interstitial fluid.

Click to read the paper (OSA site)

Detection of Highly Energetic Materials on Non-Reflective Substrates Using Quantum Cascade Laser Spectroscopy

A quantum cascade laser spectrometer was used to obtain the reflection spectra of highly energetic materials (HEMs) deposited on nonideal, low-reflectivity substrates, such as travel-bag fabric (polyester), cardboard, and wood. Various deposition methods were used to prepare the standards and samples in the study. The HEMs used were the nitroaromatic explosive 2,4,6-trinitrotoluene (TNT), the aliphatic nitrate ester pentaerythritol tetranitrate (PETN), and the aliphatic nitramine 1,3,5-trinitroperhydro-1,3,5-triazine (RDX). The results demonstrate that the infrared vibrational method described in this study is well suited for the rapid screening analysis of HEMs on low-reflectivity substrates when a supervised model has been previously constructed or when a reference spectrum of the clean substrate can be acquired to be subtracted from the HEM-substrate spectrum.

Click to read the paper (Sage Journals site)

Differential Excitation Spectroscopy for detection of chemical threats: DMMP and thiodiglycol

Differential Excitation Spectroscopy (DES) is a new pump-probe detection technique (patent-pending) which characterizes molecules based on a multi-dimensional parameterization of the rovibrational excited state structure, pump and probe interrogation frequencies, as well as the lifetimes of the excited states. Dimethyl methylphosphonate (DMMP) is used as a simulant for G series nerve agents and thiodiglycol as a simulant for sulfur mustard (HD). Optimal detection parameters were determined and mixtures of the two materials were used to demonstrate the immunity of the DES technique to interference from other materials, even those whose IR spectra show significant overlap.

Click to read the paper (SPIE site)

Discrete frequency infrared imaging using quantum cascade lasers for biological tissue analysis

Infrared (IR) spectroscopic imaging is an emerging modality for biological tissue analysis that has traditionally employed an interferometer for spectral discrimination. Recent technology developments have made discrete frequency sources, both lasers and filters, practical for imaging. The use of quantum cascade lasers in particular, presents new opportunities as well as challenges. Here we describe results from a novel point scanning confocal IR microscope and demonstrate the performance imaging several important spectral bands of lung tissue. Results show the possibility of discrete frequency (DF) absorbance measurements with RMS noise levels down to 0.34 mAU in 0.25 ms.

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High contrast GeTe4 waveguides for mid-infrared biomedical sensing applications

Realization of single-mode waveguides is essential for ultra-sensitive biosensing in the mid-infrared molecular "fingerprint" region for biomedical lab-on-chip applications. High contrast (Δn ≈ 1) germanium telluride (GeTe4) single mode rib waveguides were fabricated on zinc selenide (ZnSe) substrates for evanescent field based sensing to detect analytes at low concentration. Amorphous GeTe4 thin films were deposited by RF-sputtering and were found to transmit over the spectral range from 2µm - 20µm. Photolithography followed by reactive ion etching was carried out to etch the film, forming rib waveguide structures with minimum surface roughness and vertical sidewalls. It was found that films deposited at room temperature have average roughness of about 5nm. Optical constants were determined by IR-VASE ellipsometry.

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Linewidth-narrowing phenomena with intersubband cavity polaritons

Absorption spectra of strongly coupled intersubband cavity polaritons have been measured, using a tunable midinfrared quantum cascade laser, with high angular and spectral resolution. Pronounced linewidth narrowing of the polaritons around the anticrossing was found, with polariton linewidths narrower (4.2 meV) than both the bare intersubband transition linewidth and empty cavity linewidth (6.2 and 6 meV, respectively), at room temperature. This is due to variations in the degree of spatial averaging of the in-plane quantum-well disorder as the polariton's extended coherence length is increased by the photonic coupling over the value corresponding to the bare intersubband transition coherence length.

Click to read the paper (Researchgate site)

Multi-modal, ultrasensitive detection of trace explosives using MEMS devices with quantum cascade lasers

Multi-modal chemical sensors based on microelectromechanical systems (MEMS) have been developed with an electrical readout. Opto-calorimetric infrared (IR) spectroscopy, capable of obtaining molecular signatures of extremely small quantities of adsorbed explosive molecules, has been realized with a microthermometer/microheater device using a widely tunable quantum cascade laser. A microthermometer/microheater device responds to the heat generated by nonradiative decay process when the adsorbed explosive molecules are resonantly excited with IR light. Monitoring the variation in microthermometer signal as a function of illuminating IR wavelength corresponds to the conventional IR absorption spectrum of the adsorbed molecules. Moreover, the mass of the adsorbed molecules is determined by measuring the resonance frequency shift of the cantilever shape microthermometer for the quantitative opto-calorimetric IR spectroscopy. In addition, micro-differential thermal analysis, which can be used to differentiate exothermic or endothermic reaction of heated molecules, has been performed with the same device to provide additional orthogonal signal for trace explosive detection and sensor surface regeneration. In summary, we have designed, fabricated and tested microcantilever shape devices integrated with a microthermometer/microheater which can provide electrical responses used to acquire both opto-calorimetric IR spectra and microcalorimetric thermal responses. We have demonstrated the successful detection, differentiation, and quantification of trace amounts of explosive molecules and their mixtures (cyclotrimethylene trinitramine (RDX) and pentaerythritol tetranitrate (PETN)) using three orthogonal sensing signals which improve chemical selectivity.

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Nanoscale chemical imaging by photoinduced force microscopy

Correlating spatial chemical information with the morphology of closely packed nanostructures remains a challenge for the scientific community. For example, supramolecular self-assembly, which provides a powerful and low-costway to create nanoscale patterns and engineered nanostructures, is not easily interrogated in real space via existing nondestructive techniques based on optics or electrons. A novel scanning probe technique called infrared photoinduced force microscopy (IR PiFM) directly measures the photoinduced polarizability of the sample in the near field by detecting the time-integrated force between the tip and the sample. By imaging at multiple IR wavelengths corresponding to absorption peaks of different chemical species, PiFM has demonstrated the ability to spatially map nm-scale patterns of the individual chemical components of two different types of self-assembled block copolymer films. With chemical-specific nanometerscale imaging, PiFM provides a powerful new analytical method for deepening our understanding of nanomaterials.

Click to read the paper (Science Advances site)

Sensitive and selective detection of adsorbed explosive molecules using opto-calorimetric infrared spectroscopy and micro-differential thermal analysis

Multi-modal explosive sensors based on microelectromechanical systems (MEMS) have been developed. Opto-calorimetric infrared (IR) spectroscopy, capable of obtaining molecular signatures of extremely small quantities of adsorbed explosive molecules, has been realized with the use of micro-heater/thermometer devices and a widely tunable quantum cascade laser. These devices respond to the heat generated by non-radiative decay process when the adsorbed explosive molecules are resonantly excited with IR light. Monitoring the variation in microthermometer signal as a function of illuminating IR wavelength corresponds to the conventional IR absorption spectrum of the adsorbed molecules. Moreover, the mass of the adsorbed molecules is determined by measuring the resonance frequency shift of the device for the quantitative analysis. In addition, micro-differential thermal analysis, which can be used to differentiate exothermic or endothermic reaction of heated molecules, has been performed with the same devices to provide additional orthogonal signal for trace explosive detection and sensor surface regeneration. We have demonstrated successful detection, differentiation, and quantification of trace amounts of explosive molecules (cyclotrimethylene trinitramine (RDX) and pentaerythritol tetranitrate (PETN)) and their mixtures using three orthogonal sensing signals.

Click to read the paper (ScienceDirect site)

Study of the Exhaled Acetone in Type 1 Diabetes Using Quantum Cascade Laser Spectroscopy

The acetone concentration exhaled in the breath of three type 1 diabetes patients (two minors and one adult) and one healthy volunteer is studied using a quantum cascade laser-based spectroscopic system. Using the acetone signature between 1150 and 1250 cm-1 and a multiline fitting method, the concentration variations on the order of parts per billion by volume were measured. Blood glucose and ketone concentrations in blood measurements were performed simultaneously to study their relation with acetone in exhaled breath. We focus on personalized studies to better understand the role of acetone in diabetes. For each volunteer, we performed a series of measurements over a period of time, including overnight fastings of 11 ± 1 h and during ketosis-hyperglycemia events for the minors. Our results highlight the importance of performing personalized studies because the response of the minors to the presence of ketosis was consistent but unique for each individual. Also, our results emphasize the need for performing more studies with T1D minors, because the acetone concentration in the breath of the minors differs, with respect to those reported in the literature, which are based on adults.

Click to read the paper (ACS site)

Multicomponent Gas Analysis using Broadband Quantum Cascade Laser Spectroscopy

We present a broadband quantum cascade laser-based spectroscopic system covering the region between 850 and 1250 cm-1. Its robust multipass cavity ensures a constant interaction length over the entire spectral region. The device enables the detection and identification of numerous molecules present in a complex gas mixture without any pre-treatment in two minutes. We demonstrate that we can detect sub-ppmv concentration of acetone in presence of 2% of water at the same wavenumber region.

Click to read the paper (Optics InfoBase site)

Optical Re-Injection in Cavity-Enhanced Absorption Spectroscopy

Cavity ringdown spectroscopy and integrated cavity output spectroscopy are often used for the ultrasensitive detection of trace gases. This article describes a method of re-injecting light to deliver more light to the photodetector and thus increase the signal-to-noise ratio of the absorption measurement. The re-injected CRDS system was used to measure the spectrum of several volatile organic compounds, demonstrating the improved ability to resolve weakly absorbing spectroscopic features.

Click to read the paper (AIP site)

Fast Infrared Chemical Imaging with a Quantum Cascade Laser

In this paper, a rapidly tunable QCL was coupled with a high performance microscope equipped with a cooled focal plane array (FPA) detector. Multiple QCL units were multiplexed together to provide spectral coverage across the fingerprint region (776.9 to 1904.4 cm-1) in a DF-IR microscope capable of broad spectral coverage, wide-field detection, and diffraction limited spectral imaging. The spectral and spatial fidelity of this system was shown to be at least as good as the best FT-IR imaging systems.

Click to read the paper (ACS site)

High-Temperature Iso-Butene Absorption Diagnostic for Shock Tube Kinetics using a Pulsed Quantum Cascade Laser near 11.3 µm

A high-bandwidth absorption sensing technique for iso-butene (iC4H8) was developed to measure transient species concentration behind reflected shock waves for combustion kinetics studies. Direct measurements of iC4H8 were enabled by monitoring absorption in the infrared near 11.3 µm using a novel pulsed external-cavity quantum cascade laser (ECQCL). First-ever shock tube measurements of iC4H8 yields from iso-octane pyrolysis were also produced, with a detection limit of ~100 ppm.

Click to read the paper (ScienceDirect site)

Detection and Discrimination of Microorganisms on Various Substrates with Quantum Cascade Laser Spectroscopy

In this contribution, infrared spectroscopy detection based on QCL was used to obtain the mid-infrared (MIR) spectral signatures of Bacillus thuringiensis, Escherichia coli, and Staphylococcus epidermidis. These bacteria were used as microorganisms that simulate biothreats (biosimulants) very truthfully. The experiments were conducted in reflection mode with biosimulants deposited on various substrates including cardboard, glass, travel bags, wood, and stainless steel.

Click to read the paper (SPIE site)

Broadly Tunable Quantum cascade Laser in Cantilever-Enhanced Photoacoustic Infrared Spectroscopy of Solids

An external cavity quantum cascade laser (ECQCL) was applied in the photoacoustic detection of solid samples. The high spectral power density of the EC-QCL was combined with an extremely sensitive optical cantilever microphone of the photoacoustic detector to achieve an ultimate sensitivity. The EC-QCL photoacoustic setup yielded roughly a decade better signal-to-noise ratios than the FTIR setup with the same measurement time.

Click to read the paper (SpringerLink site)