Abstract-In this paper, the design and measurement of a 3D-printed low-loss asymptotically single-mode hollow-core terahertz Bragg fiber is reported, operating across the frequency range from 0.246 to 0.276 THz. The HE11 mode is employed as it is the lowest loss propagating mode, with the electromagnetic field concentrated within the air core as a result of the photonic crystal bandgap behavior. The HE11 mode also has large loss discrimination compared to its main competing HE12 mode. This results in asymptotically single-mode operation of the Bragg fiber, which is verified by extensive simulations based on the actual fabricated Bragg fiber dimensions and measured material parameters. The measured average propagation loss of the Bragg fiber is lower than 5 dB/m over the frequency range from 0.246 to 0.276 THz, which is, to the best of our knowledge, the lowest loss asymptotically single-mode all-dielectric microstructured fiber yet reported in this frequency range, with a minimum loss of 3 dB/m at 0.265 THz.Index Terms-Bragg fiber, electromagnetic propagation, millimeter wave technology, photonic crystals, three-dimensional printing.
Abstract-In terahertz spectroscopy, multi-layered samples often need to be measured, for instance in a liquid flow cell, and this complicates the extraction of material parameters. We present a spectroscopic parameter extraction algorithm for multilayer samples that can also be used to extract the thickness of an unknown sample layer.
Abstract. The chemistry and reaction kinetics of reactive species
dominate changes to the composition of complex chemical systems, including
Earth's atmosphere. Laboratory experiments to identify reactive species and
their reaction products, and to monitor their reaction kinetics and product
yields, are key to our understanding of complex systems. In this work we
describe the development and characterisation of an experiment using laser
flash photolysis coupled with time-resolved mid-infrared (mid-IR) quantum
cascade laser (QCL) absorption spectroscopy, with initial results reported
for measurements of the infrared spectrum, kinetics, and product yields for
the reaction of the CH2OO Criegee intermediate with SO2. The
instrument presented has high spectral (< 0.004 cm−1) and
temporal (< 5 µs) resolution and is able to monitor kinetics
with a dynamic range to at least 20 000 s−1. Results obtained at 298 K
and pressures between 20 and 100 Torr gave a rate coefficient for the
reaction of CH2OO with SO2 of (3.83 ± 0.63) × 10−11 cm3 s−1, which compares well to the current IUPAC
recommendation of 3.70-0.40+0.45 × 10−11 cm3 s−1. A limit of
detection of 4.0 × 10−5, in absorbance terms, can be achieved,
which equates to a limit of detection of ∼ 2 × 1011 cm−3 for CH2OO, monitored at 1285.7 cm−1, based on
the detection path length of (218 ± 20) cm. Initial results, directly
monitoring SO3 at 1388.7 cm−1, demonstrate that SO3 is the
reaction product for CH2OO + SO2. The use of mid-IR QCL
absorption spectroscopy offers significant advantages over alternative
techniques commonly used to determine reaction kinetics, such as
laser-induced fluorescence (LIF) or ultraviolet absorption spectroscopy,
owing to the greater number of species to which IR measurements can be
applied. There are also significant advantages over alternative IR
techniques, such as step-scan FT-IR, owing to the coherence and increased
intensity and spectral resolution of the QCL source and in terms of cost.
The instrument described in this work has potential applications in
atmospheric chemistry, astrochemistry, combustion chemistry, and in the
monitoring of trace species in industrial processes and medical diagnostics.
The development and use of terahertz (THz) frequency spectroscopy systems for security screening has shown an increased growth over the past 15 years. In order to test these systems in real-world situations, safe simulants of illicit materials, such as Semtex-H, are required. Ideally, simulants should mimic key features of the material of interest, such that they at least resemble or even appear indistinguishable from the materials of interest to the interrogating technique(s), whilst not having hazardous or illicit properties. An ideal simulant should have similar physical properties (malleability, density, surface energy and volatility to the material of interest); be non-toxic and easy to clean and decontaminate from surfaces; be recyclable or disposable; and be useable in a public environment. Here, we present a method for developing such an explosive simulant (for Semtex-H) based on a database of THz spectra of common organic molecules, and the use of a genetic algorithm to select a mixture of compounds automatically to form such a simulant. Whilst we focus on a security application, this work could be applied to various other contexts, where the material of interest is dangerous, impractical or costly. We propose four mixtures that could then be used to test the spectral response of any instrument, working at terahertz frequencies, without the need for an explosive substance.
Abstract-We present a method of calculating the measurement variance-covariance matrix of a spectroscopic sample's complex refractive index from time-domain statistics in order to estimate uncertainty of a measurement. We compare this method to a numerical analysis and previously derived methodology, and show that our time-based estimate is both accurate and adaptable to complex extraction models.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.