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To support stand-off detection, spectral libraries are being developed that include the optical constants, n(ν) and k(ν). Variable angle spectroscopic ellipsometry and single-angle reflectance are used to derive these data, enabling the reference spectra to be modeled for different parameters.

This paper updates recent work to populate spectral databases for (near-) infrared sensing of solids in different modalities including passive and active sensing of minerals, ores and other mining chemicals and minerals.

This paper presents the methods for developing synthetic spectral libraries using the real n(λ) and imaginary k(λ) parts of the complex refractive index for sensors, including hyperspectral sensors, and presents results for solid materials.

We present an overview of solids reflectance spectra that report quantitative reflectance spectra between 1.3 and 16 µm of both natural and anthropogenic materials. The spectra can be used for myriad applications.

We report on recent work on spectral databases that can be used for (near-) infrared sensing of solids and liquids relevant to both passive/active sensing of materials related to mining, oil and gas exploration.

A method for deriving the optical constants ( n / k ) of organic powdered materials using pressed pellets in the mid-infrared spectral range is introduced that combines variable angle spectroscopic ellipsometry and transmission spectroscopy. The approach is applied to anhydrous lactose, in which three different forms of pellets were pressed and measured: a pure lactose pellet and a mixed lactose/potassium bromide (KBr) pellet with a large analyte percentage were used for ellipsometric measurements, and a KBr transmission pellet with only a small analyte percentage was used for transmission measurements. The transmittance data provide an initial set of oscillators and improve the spectral fitting of weak absorption features ( k < 0.01 ). Ellipsometric data for the pure and mixed pellets are then fit simultaneously to derive the final n / k values for lactose from 6000 − 400 c m − 1 . An alternative method just using the ellipsometric data from the mixed pellet and the transmittance data is also presented and shows good agreement with the multi-sample analysis, providing a simpler method for powders that do not press easily into pure pellets. Finally, the derived optical constants were used to model the reflectance data, demonstrating a good match with the measured reflectance spectra if non-idealities are included.

Broadband high-speed absorption spectroscopy using swept-wavelength external cavity quantum cascade lasers (ECQCLs) is applied to measure multiple pyrolysis and combustion gases in biomass burning experiments. Two broadly-tunable swept-ECQCL systems were used, with the first tuned over a range of 2089-2262 cm−1 (4.42–4.79 µm) to measure spectra of CO2, H2O, and CO. The second was tuned over a range of 920-1150 cm−1 (8.70–10.9 µm) to measure spectra of ammonia (NH3), ethene (C2H4), and methanol (MeOH). Absorption spectra were measured continuously at a 100 Hz rate throughout the burn process, including inhomogeneous flame regions, and analyzed to determine time-resolved gas concentrations and temperature. The results provide in-situ, dynamic information regarding gas-phase species as they are generated, close to the biomass fuel source.

We describe and characterize an external cavity quantum cascade laser designed for detection of multiple airborne chemicals, and used with a compact astigmatic Herriott cell for sensing of acetone and hydrogen peroxide.