A quantum cascade laser-based Mach–Zehnder interferometer for chemical sensing employing molecular absorption and dispersion

Hayden, Jakob; Hugger, S.; Fuchs, F.; Lendl, Bernhard

doi:10.1007/s00340-018-6899-8

Cited by 21 publications

(15 citation statements)

References 24 publications

(29 reference statements)

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“…To a great extent, the inspiration for our method came from advanced laser-based gravitational wave detectors that use a Michelson-type interferometer operating close to a dark fringe, such that transmission of the "carrier" laser light to the output port is strongly suppressed [7]. Also, our method bears resemblance to the dual-beam interferometry, in which the lower spatial frequencies of a background field are interferometrically suppressed, thereby enhancing the detectability of localized sources [8], and is similar to prior demonstrations of single-frequency quasi-zerobackground tunable laser absorption spectroscopy performed in a narrow range of frequencies [9,10]. Likewise, we were motivated by the work of Ref.…”

Section: Introductionsupporting

confidence: 59%

Background-free broadband absorption spectroscopy based on interferometric suppression with a sign-inverted waveform

Tomberg¹,

Muraviev²,

Ru³

et al. 2019

Optica

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Background-free methods have potentially superior detection sensitivity because of their ability to take advantage of the full laser power; they are therefore attractive to spectroscopists. We implement background-free Fourier transform spectroscopy based on coherent suppression of the background using an interferometer, whereby the central peak of the interferogram is suppressed without losing molecular absorption signatures. This results in the appearance of peaks rather than dips in the measured spectrum. The technique can be used with a variety of broadband spectroscopies and features advantages such as a reduction in the required detector dynamic range, the capability to perform quantitative measurements, and strongly enhanced sensitivity down to the quantum limit. We validated our method experimentally by performing mid-infrared dual-comb spectroscopy with a mixture of multiple molecular species over a broad wavelength range of 3-5 μm.

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Section: Introductionsupporting

confidence: 59%

Background-free broadband absorption spectroscopy based on interferometric suppression with a sign-inverted waveform

Tomberg¹,

Muraviev²,

Ru³

et al. 2019

Optica

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“…[30][31][32] Further, the coherence of the emitted QCL light enabled to develop a Mach-Zehnder interferometer to simultaneously record absorption and dispersion spectra of liquid samples. 33 These manifold examples demonstrate that the unique properties of EC-QCLs led to the implementation of novel experimental approaches and applications in different fields. 34 However so far, their basic advantage of the provided high emission power could not effectively be translated to significantly higher signal-to-noise ratios than in FT-IR spectroscopy in the case of broadband transmission spectroscopy.…”

mentioning

confidence: 99%

Beyond Fourier Transform Infrared Spectroscopy: External Cavity Quantum Cascade Laser-Based Mid-infrared Transmission Spectroscopy of Proteins in the Amide I and Amide II Region

et al. 2018

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In this work, we present a setup for mid-IR measurements of the protein amide I and amide II bands in aqueous solution. Employing a latest generation external cavity-quantum cascade laser (EC-QCL) at room temperature in pulsed operation mode allowed implementing a high optical path length of 31 μm that ensures robust sample handling. By application of a data processing routine, which removes occasionally deviating EC-QCL scans, the noise level could be lowered by a factor of 4. The thereby accomplished signal-to-noise ratio is better by a factor of approximately 2 compared to research-grade Fourier transform infrared (FT-IR) spectrometers at equal acquisition times. Employing this setup, characteristic spectral features of three representative proteins with different secondary structures could be measured at concentrations as low as 1 mg mL. Mathematical evaluation of the spectral overlap confirms excellent agreement of the quantum cascade laser infrared spectroscropy (QCL-IR) transmission measurements with protein spectra acquired by FT-IR spectroscopy. The presented setup combines performance surpassing FT-IR spectroscopy with large applicable optical paths and coverage of the relevant spectral range for protein analysis. This holds high potential for future EC-QCL-based protein studies, including the investigation of dynamic secondary structure changes and chemometrics-based protein quantification in complex matrices.

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“…To prove this assumption and to show that Equation (6) is indeed applicable, we used a quantum cascade laser-based Mach-Zehnder interferometer which allows to determine absorption and dispersion spectra in the mid-infrared region. [13,14] With this instrument, we determined the index of refraction variation of solutions of ɛ-caprolactam in chloroform in the region between 1580 and 1730 cm À 1 , which are due to the C=O vibration located at 1659 cm À 1 , in dependence of the concentration. For the used concentration range (10-70 mmol L À 1 ) comparably small changes of Δn with the wavenumber are expected, and a pointwise linear dependence of Δn could be safely assumed.…”

Section: Beyond Beer's Law: Why the Index Of Refraction Depends (Almomentioning

confidence: 99%

Beyond Beer's Law: Why the Index of Refraction Depends (Almost) Linearly on Concentration

et al. 2020

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Beer's empiric law states that absorbance is linearly proportional to the concentration. Based on electromagnetic theory, an approximately linear dependence can only be confirmed for comparably weak oscillators. For stronger oscillators the proportionality constant, the molar attenuation coefficient, is modulated by the inverse index of refraction, which is itself a function of concentration. For comparably weak oscillators, the index of refraction function depends, like absorbance, linearly on concentration. For stronger oscillators, this linearity is lost, except at wavenumbers considerably lower than the oscillator position. In these transparency regions, linearity between the change of the index of refraction and concentration is preserved to a high degree. This can be shown with help of the Kramers-Kronig relations which connect the integrated absorbance to the index of refraction change at lower wavenumbers than the corresponding band. This finding builds the foundation not only for refractive index sensing, but also for new interferometric approaches in IR spectroscopy, which allow measuring the complex index of refraction function.The exact origin of the form of Beer's law, as we employ it nowadays [1,2] AðṽÞ ¼ e * ðṽÞ � c � dwherein e * ðṽÞ is the molar attenuation coefficient, c the concentration and d the sample thickness, remains unclear. It seems that it was not employed in this form before about 1900, as it was not discussed nor provided in this form in Kayser's handbook of spectroscopy, [3] which was the reference work at this time. When Max Planck derived his particular kind of dispersion theory and employed it to compare the results with Beer's law in 1903, [4] he was most probably unaware of Equation (1), otherwise he probably would have derived it from dispersion theory. Recently, this derivation was carried out, first from dispersion theory [5] and, subsequently, from simple electromagnetic theory. [6] One of the main results of the derivation is that the molar attenuation coefficient is inversely proportional to the index of refraction, which is itself a function of concentration. Concerning the nature of this dependence, as was already stated in Ref.[5], a law comparable to Beer's law, but for the index of refraction function, can be derived. In ref. [5] the nature of this dependence was not investigated in detail. As we will show in the following, the use of the index of refraction variation instead of the absorbance to investigate concentration may have several advantages. To derive the concentration dependence of the complex index of refraction, it is possible to start from simple electromagnetic theory. Accordingly, the macroscopic polarizationP is related to the dipole momentp induced in an atom or a molecule by the following relation:Here, N is the number of dipole moments per unit volume (which is equal to the molar concentration multiplied by Avogadro's constant, N = N A ·c). Inherent to Equation (2) is the assumption that there are no interactions between the microscopic dipoles, i....

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A quantum cascade laser-based Mach–Zehnder interferometer for chemical sensing employing molecular absorption and dispersion

Cited by 21 publications

References 24 publications

Background-free broadband absorption spectroscopy based on interferometric suppression with a sign-inverted waveform

Background-free broadband absorption spectroscopy based on interferometric suppression with a sign-inverted waveform

Beyond Fourier Transform Infrared Spectroscopy: External Cavity Quantum Cascade Laser-Based Mid-infrared Transmission Spectroscopy of Proteins in the Amide I and Amide II Region

Beyond Beer's Law: Why the Index of Refraction Depends (Almost) Linearly on Concentration

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