Sub-second quantum cascade laser based infrared spectroscopic ellipsometry

Ebner, Alexander; Zimmerleiter, Robert; Cobet, Christoph; Hingerl, Kurt; Brandstetter, Markus; Kilgus, Jakob

doi:10.1364/ol.44.003426

Cited by 23 publications

(21 citation statements)

References 15 publications

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“…Spectroscopic ellipsometry, however, can be a comparatively expensive and slow method due to the use of thermal radiation sources requiring long integration times. Ebner et al., 37 recently demonstrated improved acquisition times and detection limits using an IR broadband quantum cascade laser (QCL) with phase-modulated polarization to acquire broadband (1204–900 cm −1 ), high-resolution (1 cm −1 ) ellipsometry spectra in less than 1 s. By integrating multiple QCLs, the possibility for rapid, high-resolution SE over a broader spectral range in the IR (i.e., 2700–800 cm −1 ) may be feasible.…”

Section: Resultsmentioning

confidence: 98%

Improved Infrared Optical Constants from Pressed Pellets: II. Ellipsometric n and k Values for Ammonium Sulfate with Variability Analysis

et al. 2020

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Infrared reflectance analysis is facilitated via the comparison of spectra recorded in situ to a databank of actual or synthetic infrared reflectance spectra. It has recently been shown that reference spectra corresponding to the many different morphological forms of the same chemical can be generated synthetically using the imaginary, k, and real, n, components of the complex refractive index, [Formula: see text] = n + i k. One method to obtain the n and k vectors is infrared ellipsometry, which measures the changes in amplitude, tan Ψ, and phase, Δ, of polarized light reflected from the sample both as a function of wavenumber and angle of incidence. The method requires specularly reflected light, so best results are usually obtained with polished planar samples of large surface area. Due to the difficulties of obtaining such samples, however, we investigate the possibility of pressing powders of neat materials and obtaining the corresponding optical constants from the pellets. In this paper, variability in the sample pellet and preparation method is investigated, as is variability in the fitting procedure for the derived optical constants. The n/k vectors are derived from the measured ellipsometric parameters, tan ψ and Δ, as they are fit by an oscillator model which yield n([Formula: see text]) and k([Formula: see text]) vectors as a function of wavenumber, [Formula: see text]. Construction of the oscillator model is not automatic and depends on significant input from the analyst as well as the sample’s physical characteristics. For pellet pressing, the experimental variability was found to be minimized for size-selected powdered samples as gauged by the minimal variance in ψ and Δ for three different pellets; similarly, the analytical precision for multiple measurements of the same pellet was also quite good, suggesting that a pressed pellet is a viable sample preparation method. Experimental variabilities were comparatively small; the greatest variability came in the analytic fitting procedure with differences in the k-peak values up to 10% for only the sharpest bands arising from four different fits to the same data set. The final ellipsometric n/k data are compared to literature values obtained from crystalline ammonium sulfate ((NH4)2SO4) samples as well as single-angle reflectance measurements that also used pressed pellets. Comparison with the previous literature values shows generally good agreement, although larger k-values are observed for the independent sets of data derived from pressed pellets. These data are suggested as an improved set of optical constants for (NH4)2SO4.

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Section: Resultsmentioning

confidence: 98%

Improved Infrared Optical Constants from Pressed Pellets: II. Ellipsometric n and k Values for Ammonium Sulfate with Variability Analysis

et al. 2020

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“…Therefore, Ebner et al proposed the quantum cascade laser open-path system (QCLOPS) with spectroscopic ellipsometry. Because of the combination of the fast tunability of the QCL and the polarization of the phase modulation, a broadband between 900 and 1024 cm −1 with a high resolution of 1 cm −1 of elliptically polarized spectra can be obtained in less than 1 s [21]. Yin used a continuous wave room temperature, a high-power QCL, and external diffraction grating cavity geometry to develop a sensitive photoelectron sensor system to detect sulfur dioxide (SO 2 ) in the pseudo-units of parts per billion [22].…”

Section: Introductionmentioning

confidence: 99%

Air Pollution: Sensitive Detection of PM2.5 and PM10 Concentration Using Hyperspectral Imaging

et al. 2021

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This paper proposes a method to detect air pollution by applying a hyperspectral imaging algorithm for visible light, near infrared, and far infrared. By assigning hyperspectral information to images from monocular, near infrared, and thermal imaging, principal component analysis is performed on hyperspectral images taken at different times to obtain the solar radiation intensity. The Beer–Lambert law and multivariate regression analysis are used to calculate the PM2.5 and PM10 concentrations during the period, which are compared with the corresponding PM2.5 and PM10 concentrations from the Taiwan Environmental Protection Agency to evaluate the accuracy of this method. This study reveals that the accuracy in the visible light band is higher than the near-infrared and far-infrared bands, and it is also the most convenient band for data acquisition. Therefore, in the future, mobile phone cameras will be able to analyze the PM2.5 and PM10 concentrations at any given time using this algorithm by capturing images to increase the convenience and immediacy of detection.

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“…Such instrumentation naturally faces limitations imposed by the low brightness of thermal sources. During the last two decades new measurement technologies have been developed to overcome these limitations, facilitated by novel laser technology, such as quantum cascade lasers [ 1 , 2 ] and supercontinuum laser sources [ 3 , 4 ]. However, these laser sources are rather expensive, which limits their range of application and triggers the development of alternative speed- and sensitivity-enhanced measurement approaches.…”

Section: Introductionmentioning

confidence: 99%

Spectral-Coding-Based Compressive Single-Pixel NIR Spectroscopy in the Sub-Millisecond Regime

Gattinger

Zorin

Rankl

et al. 2021

Sensors

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In this contribution, we present a high-speed, multiplex, grating spectrometer based on a spectral coding approach that is founded on principles of compressive sensing. The spectrometer employs a single-pixel InGaAs detector to measure the signals encoded by an amplitude spatial light modulator (digital micromirror device, DMD). This approach leads to a speed advantage and multiplex sensitivity advantage atypical for standard dispersive systems. Exploiting the 18.2 kHz pattern rate of the DMD, we demonstrated 4.2 ms acquisition times for full spectra with a bandwidth of 450 nm (5250 cm−1–4300 cm−1; 1.9 µm–2.33 µm). Due to the programmability of the DMD, spectral regions of interest can be chosen freely, thus reducing acquisition times further, down to the sub-millisecond regime. The adjustable resolving power of the system accessed by means of computer simulations is discussed, quantified for different measurement modes, and verified by comparison with a state-of-the-art Fourier-transform infrared spectrometer. We show measurements of characteristic polymer absorption bands in different operation regimes of the spectrometer. The theoretical multiplex advantage of 8 was experimentally verified by comparison of the noise behavior of the spectral coding approach and a standard line-scan approach.

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Sub-second quantum cascade laser based infrared spectroscopic ellipsometry

Cited by 23 publications

References 15 publications

Improved Infrared Optical Constants from Pressed Pellets: II. Ellipsometric n and k Values for Ammonium Sulfate with Variability Analysis

Improved Infrared Optical Constants from Pressed Pellets: II. Ellipsometric n and k Values for Ammonium Sulfate with Variability Analysis

Air Pollution: Sensitive Detection of PM2.5 and PM10 Concentration Using Hyperspectral Imaging

Spectral-Coding-Based Compressive Single-Pixel NIR Spectroscopy in the Sub-Millisecond Regime

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