Dynamics of the Stratification Process in Drying Colloidal Dispersions Studied by Terahertz Time-Domain Spectroscopy

Mechelen, J. L. M. van

doi:10.1021/la503322v

Cited by 13 publications

(4 citation statements)

References 26 publications

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“…Such expressions have been used successfully in a range of works discussing refractive index extraction from single-layer samples. − Indeed, such an analysis is accurate for many cases where we can write out the transfer function, and Peretti et al have provided an open source package to implement this approach for several sample geometries . Such approaches are limited to predefined sample geometries, however, and as we add more layers to the sample itself or to other features of the setup (e.g., a cuvette containing a liquid sample), the transfer function can become complicated. Each of these additional layers will require its own set of Fresnel coefficients and may require Fabry–Perot terms as well.…”

Section: Resultsmentioning

confidence: 99%

“…have provided an open source package to implement this approach for several sample geometries. 21 Such approaches are limited to predefined sample geometries, however, and as we add more layers to the sample itself 22 or to other features of the setup (e.g., a cuvette containing a liquid sample 23 ), the transfer function can become complicated. Each of these additional layers will require its own set of Fresnel coefficients and may require Fabry−Perot terms as well.…”

Section: ■ Experimental Sectionmentioning

confidence: 99%

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Nelly: A User-Friendly and Open-Source Implementation of Tree-Based Complex Refractive Index Analysis for Terahertz Spectroscopy

et al. 2021

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Terahertz (THz) spectroscopy is a powerful tool for unambiguously extracting complex-valued material properties (e.g., refractive index, conductivity, etc.) from a wide range of samples, with applications ranging from materials science to biology. However, extracting complex refractive indices from THz time-domain spectroscopy data can prove challenging, especially for multilayer samples. These challenges arise from the large number of transmission-reflection paths the THz pulse can take through the sample layers, leading to unwieldy strings of Fresnel coefficients. This issue has often been addressed using various approximations. However, these approximations are only applicable to specific classes of samples and can give erroneous results when misapplied. An alternative to this approach is to programmatically model all possible paths through the sample. The many paths through the sample layers can be modeled as a tree that branches at every point where the paths diverge, i.e., whenever the pulse can either be transmitted or reflected. This tree can then be used to generate expressions relating the unknown refractive index to the observed time domain data. Here, we provide a freely available open-source package implementing this method as both a MATLAB library and a corresponding graphical user interface, which can also be run without a MATLAB license (https://github.com/YaleTHz/nelly). We have tested this method for a range of samples and compared the results to commonly used approximations to demonstrate its accuracy and wide applicability. Our method consistently gives better agreement than common approximations.

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Section: Resultsmentioning

confidence: 99%

Section: ■ Experimental Sectionmentioning

confidence: 99%

Nelly: A User-Friendly and Open-Source Implementation of Tree-Based Complex Refractive Index Analysis for Terahertz Spectroscopy

et al. 2021

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“…Terahertz technology is pushing forward strongly into industrial applications. This is due to its extraordinary features: Contact-free determination of multi-layer thicknesses of coatings [1][2][3] as well as sensing for thickness variations or defects in polymers, foams, and other non-conductive materials [4]. The measurement is contact-free, non-destructive, and based on harmless, non-ionizing radiation.…”

Section: Introductionmentioning

confidence: 99%

Ultra-fast, High-Bandwidth Coherent cw THz Spectrometer for Non-destructive Testing

Liebermeister

Nellen

Kohlhaas

et al. 2019

J Infrared Milli Terahz Waves

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Continuous wave THz (cw THz) systems define the state-of-the-art in terms of spectral resolution in THz spectroscopy. Hitherto, acquisition of broadband spectra in a cw THz system was always connected with slow operation. Therefore, high update rate applications like inline process monitoring and non-destructive testing are served by time domain spectroscopy (TDS) systems. However, no fundamental restriction prevents cw THz technology from achieving faster update rates and be competitive in this field. In this paper, we present a fully fibercoupled cw THz spectrometer. Its sweep speed is two orders of magnitude higher compared to commercial state-of-the-art systems and reaches a record performance of 24 spectra per second with a bandwidth of more than 2 THz. In the single-shot mode, the same system reaches a peak dynamic range of 67 dB and exceeds a value of 100 dB with averaging of 7 min, which is among the highest values ever reported. The frequency steps can be as low as 40 MHz. Due to the fully homodyne detection, each spectrum contains full amplitude and phase information. This demonstration of THz-spectroscopy at video-rate is an essential step towards applying cw THz systems in non-destructive, in line testing.

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“…In scientific applications, intensive THz pulses can control the electronic, ionic, and spin degrees of freedom in matter 3 . In NDT, broadband terahertz spectroscopy has been successfully explored for non-contact thickness measurements of multilayer dielectric coatings [4][5][6][7] and sensing of defects in polymers, foams, and other non-conductive materials [8][9][10] . Today, most of the aforementioned applications make use of THz TDS systems, which convert the enormous bandwidth of an optical femtosecond pulse into the THz domain, using either an ultrafast photoconductive switch or a nonlinear crystal [11][12][13][14] .…”

mentioning

confidence: 99%

Optoelectronic frequency-modulated continuous-wave terahertz spectroscopy with 4 THz bandwidth

et al. 2021

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Broadband terahertz spectroscopy enables many promising applications in science and industry alike. However, the complexity of existing terahertz systems has as yet prevented the breakthrough of this technology. In particular, established terahertz time-domain spectroscopy (TDS) schemes rely on complex femtosecond lasers and optical delay lines. Here, we present a method for optoelectronic, frequency-modulated continuous-wave (FMCW) terahertz sensing, which is a powerful tool for broadband spectroscopy and industrial non-destructive testing. In our method, a frequency-swept optical beat signal generates the terahertz field, which is then coherently detected by photomixing, employing a time-delayed copy of the same beat signal. Consequently, the receiver current is inherently phase-modulated without additional modulator. Owing to this technique, our broadband terahertz spectrometer performs (200 Hz measurement rate, or 4 THz bandwidth and 117 dB peak dynamic range with averaging) comparably to state-of-the-art terahertz-TDS systems, yet with significantly reduced complexity. Thickness measurements of multilayer dielectric samples with layer-thicknesses down to 23 µm show its potential for real-world applications. Within only 0.2 s measurement time, an uncertainty of less than 2 % is achieved, the highest accuracy reported with continuous-wave terahertz spectroscopy. Hence, the optoelectronic FMCW approach paves the way towards broadband and compact terahertz spectrometers that combine fiber optics and photonic integration technologies.

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Dynamics of the Stratification Process in Drying Colloidal Dispersions Studied by Terahertz Time-Domain Spectroscopy

Cited by 13 publications

References 26 publications

Nelly: A User-Friendly and Open-Source Implementation of Tree-Based Complex Refractive Index Analysis for Terahertz Spectroscopy

Nelly: A User-Friendly and Open-Source Implementation of Tree-Based Complex Refractive Index Analysis for Terahertz Spectroscopy

Ultra-fast, High-Bandwidth Coherent cw THz Spectrometer for Non-destructive Testing

Optoelectronic frequency-modulated continuous-wave terahertz spectroscopy with 4 THz bandwidth

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