Long-wave Infrared Research Articles

Long-wave bilayer graphene/HgCdTe based GBp type-II superlattice unipolar barrier infrared detector

Type-II superlattice (T2SL) material is used to design highly efficient next-generation infrared (IR) detectors. The flexibility of the material allows the integration of unipolar barrier IR detector architecture to suppress generation-recombination currents by blocking the majority charge carriers and improve device performance. This paper presents a self-powered T2SL unipolar barrier detector that operates in the long-wave IR regime. It comprises a contact layer made of p+-bilayer graphene (G) and a barrier layer made of p–-Hg1–xCdx=0.41Te (B) placed over a p+-Hg1–xCdx=0.2217Te absorber layer (p) and is known as the GBp detector. The optoelectronic characterization of the unipolar GBp detector is carried out using the Silvaco Atlas TCAD software. Under self-powered mode operation, the proposed GBp detector exhibits an improvement in photocurrent density of about 108 times. The dark current density of 0.4nA/cm2, the photocurrent responsivity of 7.82 A/W, the quantum efficiency of 91.44%, and the detectivity of 4.48 × 1014 cmHZ1/2/W are found at the liquid nitrogen temperature with a bias of –0.5 V. A dichroic ratio of 249 and a response time of less than 1 ps are likewise exhibited by the GBp detector. The substantial electric field at the contact/barrier heterointerface and the high barrier height in the conduction band, which result in an increased photodetection response, are attributed to this improved performance.

Utilizing Alignment Loss to Advance Eye Center Detection and Face Recognition in the LWIR Band

Geometric normalization is an integral part of most of the face recognition (FR) systems. To geometrically normalize a face, it is essential to detect the eye centers, since one way to align the face images is to make the line joining the eye centers horizontal. This paper proposes a novel approach to detect eye centers in the challenging Long-Wave Infrared (LWIR) spectrum (8-14 μm). While using thermal band images for face recognition is a feasible approach in low-light and nighttime conditions, where visible face images cannot be used, there are not many thermal or dual band (visible and thermal) face datasets available to train and test new eye center detection models. This work takes advantage of the available deep learning based eye center detection algorithms in the visible band to detect the eye centers in thermal face images through image synthesis. While we empirically evaluate different image synthesis models, we determine that StarGAN2 yields the highest eye center detection accuracy, when compared to the other state-of-the-art models. We incorporate alignment loss that captures the normalized error between the detected and actual eye centers as an additional loss term during training (using the generated images during training, ground truth annotations, and an eye center detection model), so that the model learns to align the images to minimize this error. During test phase, visible images are generated from the thermal images using the trained model. Then, the available landmark detection algorithms in the visible band, namely, MT-CNN and HR-Net are used to detect the eye centers. Next, these eye centers are used to geometrically normalize the source thermal face images before performing same-spectral (thermal-to-thermal) face recognition. The proposed method improved the eye center detection accuracy by 60% over the baseline model, and by 14% over training only the StarGAN2 model without the alignment loss. The proposed approach also reports the highest improvement in the face recognition accuracy by 36% and 3% over the baseline and original StarGAN2 models, respectively, when using deep learning based face recognition models, namely, Facenet, ArcFace, and VGG-Face. We also perform experiments by augmenting the train and test datasets with images rotated in-plane to further demonstrate the efficiency of the proposed approach. When CycleGAN (another unpaired image translation network) is used to generate images before incorporating the alignment loss, it failed to preserve the alignment at the slightest, therefore the eye center detection accuracy was extremely low. With the alignment loss, the accuracy increased by 20%, 50%, and 80% when the normalized error (e) ≤ 0.05, 0.10 and 0.25 respectively.

Long-wave-infrared pulse production at 11 µm via difference-frequency generation driven by femtosecond mid-infrared all-fluoride fiber laser.

We present an ultrafast long-wave infrared (LWIR) source driven by a mid-infrared fluoride fiber laser. It is based on a mode-locked Er:ZBLAN fiber oscillator and a nonlinear amplifier operating at 48 MHz. The amplified soliton pulses at ∼2.9 µm are shifted to ∼4 µm via the soliton self-frequency shifting process in an InF3 fiber. LWIR pulses with an average power of 1.25-mW centered at 11 µm with a spectral bandwidth of ∼1.3 µm are produced through difference-frequency generation (DFG) of the amplified soliton and its frequency-shifted replica in a ZnGeP2 crystal. Soliton-effect fluoride fiber sources operating in the mid-infrared for driving DFG conversion to LWIR enable higher pulse energies than with near-infrared sources, while maintaining relative simplicity and compactness, relevant for spectroscopy and other applications in LWIR.

Long‐Term Variation of the Quasi‐Five‐Day Wave in the Top Layer of Venus Clouds

AbstractPlanetary waves play an essential role in the momentum budget of Venus's atmosphere superrotation. Using the Longwave Infrared Camera (LIR) observations of the Akatsuki and their cloud‐tracking wind fields, we study the characteristics of planetary waves and their long‐term variations. The dominant planetary wave is a wavenumber 1 wave with a period of 5–6 days (Quasi‐Five‐Day wave or QFDW) and is symmetrically distributed in the midlatitudes of both hemispheres. The wave phase speed is close to the background zonal wind, indicating the possible existence of a critical level near the LIR emission altitude of 65 km. The QFDW tends to appear at ∼8:00 a.m. local time, suggesting a modulation by the thermal tide. The QFDW amplitude is correlated with the vertical shear of the background zonal wind modulated by the thermal tide. This indicates some nonlinear interaction between the thermal tide and the QFDW.

Hyperspectral core scanner: An effective mineral mapping tool for apatite in the Upper Zone, northern limb, Bushveld Complex

The technological advances in efficient, rapid, and non-destructive hyperspectral core logging systems for systematic mineral mapping have led to the discovery and exploitation of new mineral deposits Hyperspectral imaging in the long-wave infrared range has been recently used successfully to identify various phosphate-bearing minerals (monazite, xenotime, and britholite), with limited work on apatite associated with mafic-ultramafic layered intrusions. In this study we investigate the effectiveness of a hyperspectral imaging (HSI) system with long-wave infrared (LWIR) bandwidthsto identify apatite in the Upper Zone of the Bushveld Complex. The accuracy of the HSI results was validated by mineralogical and geochemical data. The two apatite-enriched zones detected by HSI suggesting widespread development of apatite throughout the uppermost 600 m of the Upper Zone. The lower apatite-enriched zone is approximately 40 m thick, while the upper apatite-enriched zone is about 23 m thick, consistent with previous thickness determinations by traditional logging and analytical methods. Spectral mixing observed in the response of apatite is ascribed either to the common association of apatite and olivine in these rocks, or to differences between the spatial resolution of the hyperspectral image and the size of apatite grains. The VNIR-SWIR wavelength region did not show prominent spectral features of apatite. Nonetheless, HSI in the LWIR range is effective in mapping apatite and should therefore be considered as an exploration tool. This research advances our knowledge of the reflectance spectroscopy of REE-bearing minerals, which makes it easier to detect, identify, and quantify REE-bearing silicate minerals by HSI.

Qualitative Comparison of Lock-in Thermography (LIT) and Pulse Phase Thermography (PPT) in Mid-Wave and Long-Wave Infrared for the Inspection of Paintings

When studying paintings with active infrared thermography (IRT), minimizing the temperature fluctuations and thermal shock during a measurement becomes important. Under these conditions, it might be beneficial to use lock-in thermography instead of the conventionally used pulse thermography (PT). This study compared the observations made with lock-in thermography (LIT) and pulse phase thermography (PPT) with halogen light excitation. Three distinctly different paintings were examined. The LIT measurements caused smaller temperature fluctuations and, overall, the phase images appeared to have a higher contrast and less noise. However, in the PPT phase images, the upper paint layer was less visible, an aspect which is of particular interest when trying to observe subsurface defects or the structure of the support. The influence of the spectral range of the cameras on the results was also investigated. All measurements were taken with a mid-wave infrared (MWIR) and long wave infrared (LWIR) camera. The results show that there is a significant number of direct reflection artifacts, caused by the use of the halogen light sources when using the MWIR camera. Adding a long-pass filter to the MWIR camera eliminated most of these artifacts. All results are presented in a side-by-side comparison.

A layer-2 solution for inspecting large-scale photovoltaic arrays through aerial LWIR multiview photogrammetry and deep learning: A hybrid data-centric and model-centric approach

Defective components within solar photovoltaic (PV) arrays overheat, resulting in particular temperature patterns under the long-wave thermal infrared (LWIR) spectrum. The detection and on-field localization of these patterns is of paramount aid to the operations and maintenance of PV installations. In this context, we develop a two-layer end-to-end inspection solution for the detection, quantification and on-field localization of overheated regions on PV arrays from LWIR UAV imagery. Layer 1 generates a georeferenced orthomosaic of the inspected site via a Structure from Motion-MultiView Stereo (SfM-MVS) photogrammetric acquisition/post-processing workflow. Layer 2 is a tile-based deep semantic segmentation stage that extracts and quantifies the affected regions from the generated orthomosaic. We collect aerial images from 103 PV sites, comprising approximately 342000 modules. After a SfM-MVS workflow, we produce and annotate 7910 orthorectified unique affected tiles, posteriorly augmented to prepare the state-of-the-art dataset in terms of size and representativeness. Through a training/cross-validation and test process, we investigate the implementation of 9 models in the segmentation process: FCN, U-Net, FPN, DeepLab, LinkNet, DANet, CFNet, ACFNet and TransU-Net, each of which experimented with 2 backbones: ResNet50 and DenseNet121. The models feature efficient encoder-to-decoder feature map transfers, pyramidal feature recognition, spatial and channel attention, feature co-occurrence, class center as well as vision transformers. The best performance is achieved by FPN-DenseNet121, with a mean mIoU of 93.44% and an F1-score of 96.39% on our test set. The two-layer solution takes the best of the data-centric and model-centric paradigms, alongside addressing the limitations of conventional inspection procedures. It is put into a concrete application framework, where it provides a pixel-based and a tile-based quantification of the affected regions within a PV plant. The results are promising, and the selected model can be deployed efficiently for extensive aerial monitoring of large-scale PV plants.

Femtosecond long-wave-infrared generation in hydrogen-filled hollow-core fiber

Femtosecond long-wave-infrared (LWIR) pulses have found applications in several fields, but their generation is limited to CO2 lasers and solid-state frequency converters. Waveguide-based Raman red shifting provides another promising solution to efficiently generate LWIR pulses. Here, we numerically study LWIR pulse generation in a hydrogen-filled hollow-core fiber. Several excitation schemes are considered, involving one or two pulses at either the same or different wavelengths. The analysis reveals that a waveguide structure enables tailoring of the Raman gain, which is required to produce pulses at LWIR wavelengths. With ∼5-mJ and 50-fs input pulses, clean 400-µJ and 88-fs pulses at 12 µm are theoretically generated with 41% total quantum efficiency. The simulations also provide insight into the nonlinear dynamics of the Raman gain, where the concept of a phonon amplifier underlies the optimal performance that can be achieved. Only the two-pulse scheme with a two-color source creates a good phonon amplifier for efficient LWIR generation.

Improving transmittance of long-wave infrared guided-mode resonant filters

The long-wave infrared (LWIR) spectral region spanning from 8 to 12 μm is useful for many scientific and industrial applications. Many of these applications require use of either a bandpass or a bandstop filter that can be realized by the guided-mode resonance (GMR) effect with subwavelength periodic features in layered dielectric materials transparent in the LWIR. The GMR filters operating in the LWIR region are fabricated by depositing an amorphous germanium (Ge) film to form a zero-contrast (ZC) waveguide-grating (WGG) on a polished zinc selenide (ZnSe) substrate. In general, the backside of a ZnSe substrate with refractive index 2.41 is uncoated causing a 17% Fresnel-reflection loss in the light transmitted through the filter due to a large impedance mismatch at the ZnSe/air interface. Because we use such filters in the LWIR laser experiments for notch filtering, to improve the filter transmittance we used ZnSe substrates coated on one-side with broadband antireflection coating (ARC) covering the 7 to 12 μm spectral range to fabricate GMRFs with one-dimensional (1D) Ge ZC WGG. We employed high-spatial resolution e-beam lithography and reactive-ion etching nanofabrication techniques to achieve high-performance large-area (12 × 12 mm2) 1D notch filters with subwavelength periods. We characterized polarization dependent spectral performance of the prototype filters with both coherent and incoherent incident light using a tunable quantum cascade laser system that spans the 7 to 12 μm region, and a Fourier transform infrared spectrometer with collimated incident beam to achieve close to 15% improvement in the peak transmittance as well as significant reduction in coherent noise compared to our earlier results with GMRFs without ARC. Here, we present the filter design simulation and measurement results.

Classification of Micromobility Vehicles in Thermal-Infrared Images Based on Combined Image and Contour Features Using Neuromorphic Processing

Trends of environmental awareness, combined with a focus on personal fitness and health, motivate many people to switch from cars and public transport to micromobility solutions, namely bicycles, electric bicycles, cargo bikes, or scooters. To accommodate urban planning for these changes, cities and communities need to know how many micromobility vehicles are on the road. In a previous work, we proposed a concept for a compact, mobile, and energy-efficient system to classify and count micromobility vehicles utilizing uncooled long-wave infrared (LWIR) image sensors and a neuromorphic co-processor. In this work, we elaborate on this concept by focusing on the feature extraction process with the goal to increase the classification accuracy. We demonstrate that even with a reduced feature list compared with our early concept, we manage to increase the detection precision to more than 90%. This is achieved by reducing the images of 160 × 120 pixels to only 12 × 18 pixels and combining them with contour moments to a feature vector of only 247 bytes.

Optimization of multispectral chalcogenide glass for large-size fabrication.

Chalcogenide glass has become one of the essential IR lens materials in passively athermalized long-wave IR devices. However, that there is no multispectral chalcogenide glass capable of large-size fabrication raises challenges to the development and popularization of multispectral imaging systems combining visible, near-IR, and mid-IR. In this work, we developed a novel chalcogenide glass capable of a record-big (Ø120 mm) fabrication through the compositional optimization of GeS2-Ga2S3-CsCl glass with introduction of Sb2S3. Its transmission window is characterized as ranging from 0.51 to 11.2 µm, which means it could be employed as a multispectral lens transmitting visible and IR signals in a co-aperture IR optical system. In addition, a method of three-stage thermal analysis is proposed to evaluate the glass-forming ability of chalcogenide glass through simulating the melt-quenching process of chalcogenide melt in a vacuum-sealed silica ampoule. This work not only shows an innovative multispectral chalcogenide glass with promising applications but also introduces a simple and convenient technique for screening chalcogenide glass with ultrahigh glass-forming ability capable of large-size fabrication.

Multiband longwave infrared reflectance removal using blackbody channel prior

Drawing from techniques used to dehaze visible, three-channel RGB images, we propose an approach to separate emissive and reflected radiance in images at short distances with a microbolometer-based longwave infrared (LWIR) camera system. The best case for optimal contrast and with the most descriptive information about the scene in an LWIR image would be where no external sources are radiating toward the scene. We introduce the concept of a blackbody channel prior (BCP) to multiband LWIR imaging to describe pixels that represent objects that behave most similarly to perfect blackbodies with an emissivity near unity. Most LWIR images of outdoor scenes are degraded largely in part by reflected sky path radiance. We can estimate scene radiance with a minimized reflective component producing images with enhanced contrast. Experiments on a number of multiband images are present to demonstrate this spectral-based BCP technique and show its potential for preserving scene information while achieving contrast enhancement.

Characterisation of coal using hyperspectral core scanning systems

Commercially available hyperspectral drill core scanning systems in the Visible and Near-Infrared (VNIR), Short-Wave Infrared (SWIR) and Long-Wave Infrared (LWIR) regions, along with a handheld spectrometer for the Middle-Wave Infrared (MWIR) region, were used to characterise a rank suite of coal samples (0.45% to 1.76% Rr) with different preparations (slabbed surface, outer core and compressed powder pellets). The samples used in this study came from Australian basins of Permian and Jurassic ages. The spectral characteristics of the coals measured using these scanning systems were compared to those obtained using well-established spectroscopy techniques that operate in the same region of the electromagnetic spectrum, such as Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectroscopy. The observed spectral features of the coals changed with increasing coalification. The features occurring at 1910 nm in the SWIR region and 2940 nm in the MWIR region tended to disappear as rank increased; these features arise from OH bonds related to structural water and hence correlate with the moisture content. The CH stretching fundamentals at 3280 nm correspond to the aromatic fraction of the coal, while the spectral region between 3380 nm and 3500 nm corresponds to the CH2 and CH3 stretching fundamentals occurring in the aliphatic fraction of the coal. The ratio of the area of the aromatic absorption feature to that of the aliphatic absorption features, herein called Rank Spectral Index, displayed a strong linear correlation with coal rank parameters, such as vitrinite reflectance. Other spectral features in the SWIR and MWIR regions also correlated with coal rank parameters. In addition to rank, trends in the magnitudes of the spectral features were influenced by maceral composition, particularly in low rank samples with abundant liptinite or inertinite group macerals. The variable spectral features related to composition were mapped onto the slabbed core surface, discriminating bright and dull bands and fusain lenses enabling the classification of coal lithotypes, regardless of rank. Among the less banded lithotypes, it was established that high liptinite content coals can be distinguished by their aliphatic spectral features in core samples. Overall, this study has shown that hyperspectral drill core scanners can be used to characterise coal and non-destructively classify rank and lithotype.

Zero‐Bias Long‐Wave Infrared Nanoantenna‐Mediated Graphene Photodetector for Polarimetric and Spectroscopic Sensing

AbstractGraphene has attracted great interest for integrated photonic platforms in the long‐wave infrared (LWIR) for spectroscopic and polarimetric sensing due to the capability of on‐chip integration, fast response, and broadband operation. However, graphene suffers from low photoresponsivity and thus poor sensing performance due to weak absorption. Polarization detection using graphene is hindered by its small in‐plane anisotropy. Here, nanoantenna‐mediated graphene photodetectors (NMGPDs) are proposed to enhance responsivity by tailoring the nanoantenna near‐field distribution to generate a strong photoresponse. The devices demonstrate a high responsivity of 6.3 V W−1 under zero bias at room temperature with low noise‐equivalent power of 1.6 nW Hz−1/2. Furthermore, polarization detection is enabled by artificial near‐field anisotropy enabled by double L‐shaped nanoantennas. The proposed LWIR NMGPDs achieve subtle polarization‐angle detection down to 0.05° thanks to the unusual negative polarization ratio of −1. To demonstrate the advances of LWIR NMGPDs for molecule detection, acetone is chosen as an analyte for spectroscopic sensing. The devices show a low limit of detection of 115 ppm, and fast dynamic gas sensing response of 6 s for real‐time monitoring. These results reveal the potential of the device as a multi‐functional on‐chip miniaturized optoelectronic platform for polarimetric and spectroscopic sensing toward real‐time environmental monitoring and biomedical screening.

High-Speed 9.6-μm Long-Wave Infrared Free-Space Transmission With a Directly-Modulated QCL and a Fully-Passive QCD

Free-space optics (FSO) in the mid-infrared (mid-IR) contains rich spectral resources for future ultrahigh-speed wireless communications yet is currently under-exploited. Two atmospheric transmission windows at the mid-IR, namely, the mid-wave IR (MWIR, 3–5 μm) and the long-wave IR (LWIR, 8–12 μm), show great potential in supporting free-space communications for both terrestrial and space application scenarios. Particularly, the LWIR signal with a longer wavelength has high intrinsic robustness against aerosols’ scattering and turbulence-induced scintillation and beam broadening effects, which are the main concerns hindering the wide deployment of practical FSO systems. In this context, high-bandwidth semiconductor-based mid-IR FSO transceivers will be desirable to meet the requirements of low energy consumption and small footprints for large-volume development and deployment. Quantum cascade devices, including quantum cascade lasers (QCLs) and quantum cascade detectors (QCDs), appear promising candidates to fulfill this role. In this work, we report a high-speed LWIR FSO transmission demonstration with a 9.6-μm directly-modulated (DM)-QCL and a fully passive QCD without any active cooling or bias voltage. Up to 8 Gb/s, 10 Gb/s, and 11 Gb/s signal transmissions are achieved when operating the DM-QCL at 10 °C, 5 °C, and 0 °C, respectively. These results indicate a significant step towards an envisioned fully-connected mid-IR FSO solution empowered by the quantum cascade semiconductor devices.

Modified Two-Point Correction Method for Wide-Spectrum LWIR Detection System.

Non-uniformity commonly exists in the infrared focal plane, which behaves as the fixed-pattern noise (FPN) and seriously affects the image quality of long-wave infrared (LWIR) detection systems. The two-point correction (TPC) method is commonly used to reduce image FPN in engineering. However, when a wide-spectrum LWIR detection system calibrated with a black body is used to detect weak and small targets in the sky, FPN still appears in the image, affecting its uniformity. The effects of atmospheric transmittance characteristics of long-range paths on the non-uniformity of wide-spectrum long-wave infrared systems have not been studied. This paper proposes a modified TPC model based on spectral subdivision that introduces atmospheric transmittance. Additionally, the effects of atmospheric transmittance characteristics on the long-wave infrared non-uniform correction coefficient are analyzed. The experimental results for a black body scene and sky scene using a weak and small target detection system with a long-wave Sofradir FPA demonstrate that the wide-spectrum LWIR detection system fully considers atmospheric transmittance when performing calibration based on the TPC method, which can reduce the non-uniformity of the image.

Fabrication life prediction model for HDPE/Cr2O3 composites toward roof passive cooling

Passive cooling materials have been widely investigated to reduce building energy consumption. However, references about life prediction of the materials are insufficient. In this work, a novel life prediction model based on the passive cooling composites was established. The inorganic pigment chromium oxide (Cr2O3) was introduced into the high density polyethylene (HDPE) matrix to prepare HDPE/Cr2O3 composites with high long wave infrared (LWIR) emissivity and near infrared (NIR) reflectivity. The composites showed excellent cooling performance and color stability. The addition of ultraviolet absorber (UVA) and hindered amine light stabilizer (HALS) further improved the UV resistance of HDPE/Cr2O3 composites. HALS performs better and meets the needs of long-life outdoor use. Furthermore, a new failure point selection method is proposed and a model is established to predict the actual service life of HDPE/Cr2O3 composites, which provides theoretical guidance for the outdoor long-life use of the polymer composites. Based on the model, the fabricated HDPE/Cr2O3/HALS system can be used for more than 15 years. The long-term cooling composites can be used for building roof and wall to effectively reduce the indoor temperature and realize the sustainable use of energy. Besides, if used in military engineering, it has excellent cooling function and military camouflage effect. In addition, the composites can also be widely used in outdoor decoration, artificial lawn, floating materials and other fields.

Hybrid emitters with raspberry-like hollow SiO2 spheres for passive daytime radiative cooling

Passive daytime radiative cooling (PDRC) has been vigorously investigated in recent years because it does not require electrical energy, reducing greenhouse gas emissions for a more sustainable society. Representative approaches to improve radiative cooling performance are the design of metamaterials with a long-wave infrared (LWIR) emissivity and porous polymer structures with a high-solar reflectivity. Furthermore, coating materials for a simple process should be researched to improve the applicability of radiative cooling. In this study, we investigate a daytime radiative cooling coating using raspberry-like hollow SiO2 spheres (RHSs) to achieve effective radiative cooling performance and a simple coating process. The raspberry-like morphologies of the synthesized hollow SiO2 spheres contribute to multiple scattering of sunlight, resulting in enhanced performance of the radiative cooling coating. Moreover, the results of an outdoor experiment and energy-saving demonstration suggest that the RHS coating is suitable for effective heat management of the buildings. The RHS coating achieves a cooling temperature of 9.7 °C under solar irradiance of 800 W m−2. The inner temperature of a house coated with PDRC is 6.6 °C lower than that of a bare house under direct sunlight.

Comparative Long-Wave Infrared Laser-Induced Breakdown Spectroscopy Employing 1-D and 2-D Focal Plane Array Detectors.

Long-wave infrared (LWIR) emissions of laser-induced plasma on solid potassium chloride and acetaminophen tablet surfaces were studied using both a one-dimensional (1-D) linear array detection system and, for the first time, a two-dimensional (2-D) focal plane array (FPA) detection system. Both atomic and molecular infrared emitters in the vicinity of the plasma were identified by analyzing the detected spectral signatures in the infrared region. Time- and space-resolved long-wave infrared emissions were also studied to assess the temporal and spatial behaviors of atomic and molecular emitters in the plasma. These pioneer temporal and spatial investigations of infrared emissions from laser-induced plasma would be valuable to the modeling of plasma evolutions and the advances of the novel LWIR laser-induced breakdown spectroscopy (LIBS). When integrated both temporally (≥200 µs) and spatially using a 2-D FPA detector, the observed intensities and signal-to-noise-ratio (SNR) of single-shot LWIR LIBS signature emissions from intact molecules were considerably enhanced (e.g., with enhancement factors up to 16 and 3.76, respectively, for a 6.62 µm band of acetaminophen molecules) and, in general, comparable to those from the atomic emitters. Pairing LWIR LIBS with conventional ultraviolet-visible-near infrared (UV/Vis/NIR) LIBS, a simultaneous UV/Vis/NIR + LWIR LIBS detection system promises unprecedented capability of in situ, real-time, and stand-off investigation of both atomic and molecular target compositions to detect and characterize a range of chemistries.

Spectral Characterization of Parent Soils From Globally Important Dust Aerosol Entrainment Regions

AbstractWe investigated the variation in the mineralogical composition of surface soil samples from a wide range of atmospheric dust aerosol generating regions. These soils likely have mineral compositions similar to dust aerosols. We measured the visible, shortwave infrared (VSWIR) and long wave infrared (LWIR) reflectance spectra, as well as LWIR transmission spectra and carried out linear spectral mixture modeling on the transmission data. The spectroscopically identified minerals were compared with the mineralogy previously obtained using X‐ray diffraction (XRD) and optical microscopy (OM). The results showed that VSWIR is very sensitive to iron‐bearing minerals and better identifies their presence and type compared to XRD and OM. Clay minerals and carbonates have multiple distinct absorptions features in SWIR but are often overlapping, limiting our ability to uniquely assign spectral features. LWIR reflectance easily distinguishes carbonates, even in trace amounts that were not identified with other techniques. LWIR reflectance spectra for silicates were more complicated to interpret. In some samples, we were able to identify Si‐O features diagnostic of common silicates; however, in most cases these features were absent, and an unusual new peak (∼7.4–7.9 μm) was observed. LWIR transmission is characterized by strong absorption that arises from overlapping features from multiple silicate minerals, but carbonates are readily identifiable. Spectroscopy is complementary to XRD and can help identify additional minerals in these soil samples. Our results can help improve global soil atlases and can be used to support interpretation and validation of data acquired by current and future remote sensing instruments.