Terahertz and Infrared Plasmon Polaritons in PtTe2 Type-II Dirac Topological Semimetal.

Surface plasmon polaritons (SPPs) are electromagnetic excitations existing at the interface between a metal and a dielectric. SPPs provide a promising path in nanophotonic devices for light manipulation at the micro and nanoscale with applications in optoelectronics, biomedicine, and energy harvesting. Recently, SPPs are extended to unconventional materials like graphene, transparent oxides, superconductors, and topological systems characterized by linearly dispersive electronic bands. In this respect, 3D Dirac and Weyl semimetals offer a promising frontier for infrared (IR) and terahertz (THz) radiation tuning by topologically-protected SPPs. In this work, the THz-IR optical response of platinum ditelluride (PtTe2) type-II Dirac topological semimetal films grown on Si substrates is investigated. SPPs generated on microscale ribbon arrays of PtTe2 are detected in the far-field limit, finding an excellent agreement among measurements, theoretical models, and electromagnetic simulation data. The far-field measurements are further supported by near-field IR data which indicate a strong electric field enhancement due to the SPP excitation near the ribbon edges. The present findings indicate that the PtTe2 ribbon array appears an ideal active layout for geometrically tunable SPPs thus inspiring a new fashion of optically tunable materials in the technologically demanding THz and IR spectrum.

Characterization of CuAg Alloys with Low Ag Concentrations.

Copper-based alloys designed to combine high electronic and thermal conductivities with high mechanical strength find a wide range of applications in different fields. Among the principal representatives, strongly diluted CuAg alloys are of particular interest as innovative materials for the realization of accelerating structures when the use of high-gradient fields requires increasingly high mechanical and thermal performances to overcome the limitations induced by breakdown phenomena. This work reports the production and optical characterization of CuAg crystals at low Ag concentrations, from 0.028% wt to 0.1% wt, which guarantee solid solution hardening while preserving the exceptional conductivity of Cu. By means of Fourier Transform Infrared (FTIR) micro-spectroscopy experiments, the low-energy electrodynamics of the alloys are compared with that of pure Cu, highlighting the complete indistinguishability in terms of electronic transport for such low concentrations. The optical data are further supported by Raman micro-spectroscopy and SEM microscopy analyses, allowing the demonstration of the full homogeneity and complete solubility of solid Ag in copper at those concentrations. Together with the solid solution hardening deriving from the alloying process, these results support the advantage of strongly diluted CuAg alloys over conventional materials for their application in particle accelerators.

Extraordinary optical transmittance generation on Si3N4 membranes.

Metamaterials are attracting increasing attention due to their ability to support novel and engineerable electromagnetic functionalities. In this paper, we investigate one of these functionalities, i.e. the extraordinary optical transmittance (EOT) effect based on silicon nitride (Si3N4) membranes patterned with a periodic lattice of micrometric holes. Here, the coupling between the incoming electromagnetic wave and a Si3N4 optical phonon located around 900 cm-1 triggers an increase of the transmitted infrared intensity in an otherwise opaque spectral region. Different hole sizes are investigated suggesting that the mediating mechanism responsible for this phenomenon is the excitation of a phonon-polariton mode. The electric field distribution around the holes is further investigated by numerical simulations and nano-IR measurements based on a Scattering-Scanning Near Field Microscope (s-SNOM) technique, confirming the phonon-polariton origin of the EOT effect. Being membrane technologies at the core of a broad range of applications, the confinement of IR radiation at the membrane surface provides this technology platform with a novel light-matter interaction functionality.

Secondary Structures of MERS-CoV, SARS-CoV, and SARS-CoV-2 Spike Proteins Revealed by Infrared Vibrational Spectroscopy.

All coronaviruses are characterized by spike glycoproteins whose S1 subunits contain the receptor binding domain (RBD). The RBD anchors the virus to the host cellular membrane to regulate the virus transmissibility and infectious process. Although the protein/receptor interaction mainly depends on the spike's conformation, particularly on its S1 unit, their secondary structures are poorly known. In this paper, the S1 conformation was investigated for MERS-CoV, SARS-CoV, and SARS-CoV-2 at serological pH by measuring their Amide I infrared absorption bands. The SARS-CoV-2 S1 secondary structure revealed a strong difference compared to those of MERS-CoV and SARS-CoV, with a significant presence of extended ß-sheets. Furthermore, the conformation of the SARS-CoV-2 S1 showed a significant change by moving from serological pH to mild acidic and alkaline pH conditions. Both results suggest the capability of infrared spectroscopy to follow the secondary structure adaptation of the SARS-CoV-2 S1 to different environments.

A new approach to deposit homogeneous samples of asbestos fibres for toxicological tests in vitro.

In this paper we describe the results obtained with a novel method to prepare depositions of asbestos fibres for toxicological tests in vitro. The technique is based on a micro-dispenser, working as an inkjet printer, able to deposit micro-sized droplets from a suspension of fibres in a liquid medium; we used here a highly evaporating liquid (ethanol) to reduce the experimental time, however other solvents could be used. Both the amount and spatial distribution of fibres on the substrate can be controlled by adjusting the parameters of the micro-dispenser such as deposition area, deposition time, uniformity and volume of the deposited liquid. Statistical analysis of images obtained by optical and scanning electron microscopy shows that this technique produces an extremely homogeneous distribution of fibers. Specifically, the number of deposited single fibres is maximized (up to 20 times), a feature that is essential when performing viability tests where agglomerated or untangled fibrous particles need to be avoided.

Spectroscopic evidence of the radioresistance of Chroococcidiopsis biosignatures: A combined Raman, FT-IR and THz-TDs spectroscopy study.

In the last decades, Mars has been widely studied with on-site missions and observations, showing a planet that could have hosted life in the past. For this reason, the recent and future space missions on the red planet will search for traces of past and, possibly, present life. As a basis for these missions, Space Agencies, such as the European Space Agency, have conducted many experiments on living organisms, studying their behavior in extraterrestrial conditions, learning to recognize their biosignatures with techniques remotely controllable such as Raman spectroscopy. Among these organisms, the radioresistant cyanobacterium Chroococcidiopsis was irradiated during the STARLIFE campaign with strong radiative insults. In this article we have investigated this cyanobacterium using Raman spectroscopy and extended the characterization of its biosignatures and its response to the radiative stress to the mid- Infrared and Terahertz spectral region using the Fourier Transform InfraRed (FT-IR) and Terahertz Time Domain spectroscopy (THz- TDs), which demonstrates the compatibility and suitability of these techniques for future space missions.

Terahertz continuous wave spectroscopy: a portable advanced method for atmospheric gas sensing.

Motivated by the increasing demand to monitor the air-quality, our study proved the feasibility of a new compact and portable experimental approach based on Terahertz (THz) continuous wave high resolution spectroscopy, to detect the presence of the air's contaminants as greenhouse gases (GHG) and volatile organic compounds (VOCs). In this specific work, we first characterized, determining their molar absorption coefficient in the spectral region (0.06-1.2) THz, the pure optical response of the vapor of five VOCs: methanol, ethanol, isopropanol, 1-butanol and 2-butanol. In particular, 1-butanol and 2-butanol are characterized for the first time in literature at THz frequencies. Then we studied the optical response of their mixtures achieved with ambient air and ethanol. The results show that it is possible for a differentiation of single components by describing their spectral absorption in terms of the linear combination of pure compounds absorption. This proof of concept for this apparatus study and set-up paves the way to the use of THz Continuous wave high resolution spectroscopy for the environmental tracking of air pollutants.

High Sensitivity Monitoring of VOCs in Air through FTIR Spectroscopy Using a Multipass Gas Cell Setup.

Human exposure to Volatile Organic Compounds (VOCs) and their presence in indoor and working environments is recognized as a serious health risk, causing impairments of varying severities. Different detecting systems able to monitor VOCs are available in the market; however, they have significant limitations for both sensitivity and chemical discrimination capability. During the last years we studied systematically the use of Fourier Transform Infrared (FTIR) spectroscopy as an alternative, powerful tool for quantifying VOCs in air. We calibrated the method for a set of compounds (styrene, acetone, ethanol and isopropanol) by using both laboratory and portable infrared spectrometers. The aim was to develop a new, and highly sensitive sensor system for VOCs monitoring. In this paper, we improved the setup performance, testing the feasibility of using a multipass cell with the aim of extending the sensitivity of our system down to the part per million (ppm) level. Considering that multipass cells are now also available for portable instruments, this study opens the road for the design of new high-resolution devices for environmental monitoring.

Real-time quantitative detection of styrene in atmosphere in presence of other volatile-organic compounds using a portable device.

Exposure to styrene is a major safety concern in the fibreglass processing industry. This compound is classified by the International Agency for Research on Cancer as a possible human carcinogen. Several types of analytical equipment exist for detecting volatile organic compounds (VOCs) in the atmosphere; however, most of them operate ex-situ or do not provide easy discrimination between different molecules. This work introduces an improved and portable method based on FTIR spectroscopy to analyse toxic gaseous substances in working sites down to a concentration of less than 4 ppm. Styrene and a combination of VOCs typically associated with it in industrial processes, such as acetone, ethanol, xylene and isopropanol, have been used to calibrate and test the methodology. The results demonstrate that the technique offers the possibility to discriminate between different gaseous compounds in the atmosphere with a high degree of confidence and obtain very accurate quantitative information on their concentration, down to the ppm level, even when different VOCs are present in a mixture.

Infrared Nanospectroscopy Reveals DNA Structural Modifications upon Immobilization onto Clay Nanotubes.

The growing demand for innovative means in biomedical, therapeutic and diagnostic sciences has led to the development of nanomedicine. In this context, naturally occurring tubular nanostructures composed of rolled sheets of alumino-silicates, known as halloysite nanotubes, have found wide application. Halloysite nanotubes indeed have surface properties that favor the selective loading of biomolecules. Here, we present the first, to our knowledge, structural study of DNA-decorated halloysite nanotubes, carried out with nanometric spatially-resolved infrared spectroscopy. Single nanotube absorption measurements indicate a partial covering of halloysite by DNA molecules, which show significant structural modifications taking place upon loading. The present study highlights the constraints for the use of nanostructured clays as DNA carriers and demonstrates the power of super-resolved infrared spectroscopy as an effective and versatile tool for the evaluation of immobilization processes in the context of drug delivery and gene transfer.

Fabrication and spectroscopic characterization of graphene transparent electrodes on flexible cyclo-olefin substrates for terahertz electro-optic applications.

We demonstrate graphene on flexible, low-loss, cyclo-olefin polymer films as transparent electrodes for terahertz electro-optic devices and applications. Graphene was grown by chemical vapor deposition and transferred to cyclo-olefin polymer substrates by the thermal release tape method as layers on an approximate area of 4 cm2. The structural and electromagnetic properties of the graphene samples as well as their spatial variation were systematically mapped by means of µRaman, terahertz time-domain and mid-infrared spectroscopy. Thanks to the small thickness and very low intrinsic absorption of the employed substrates, both high transmittance and conductivity were recorded, demonstrating the suitability of the technique for the fabrication of a new class of transparent and flexible electrodes working in the terahertz spectrum.

Characterization of volatile organic compounds (VOCs) in their liquid-phase by terahertz time-domain spectroscopy.

In this work the terahertz spectra of benzene, toluene, p-xylene and styrene-four volatile organic compounds (VOCs) of interest in environmental pollution studies-have been measured in their liquid phase at room temperature using terahertz time-domain spectroscopy (THz-TDS). Their frequency-dependent refractive index and absorption coefficient have been extracted and analyzed in the spectral range from 0.2 to 2.5 THz. The optical properties of bi-component VOCs mixtures have also been investigated and described in terms of a linear combination of pure VOCs optical components.

Integration of stimulated Raman gain and stimulated Raman losses detection modes in a single nonlinear microscope.

This work addresses some key challenges in the fields of bio and nanophotonics by stimulated Raman microscopy. We present the design and the implementation of a femtosecond stimulated Raman scattering microscope, equipped with three femtosecond laser sources, which allows the coexistence of stimulated Raman gain (SRG) and stimulate Raman losses (SRL) detection modes in a single microscopy setup and to generate images of the same region in succession, without adding or removing components. In order to demonstrate the switching between the two detection modes, SRL and SRG images of polystyrene beads are acquired and the images quality are evaluated and compared.

Subcellular chemical and morphological analysis by stimulated Raman scattering microscopy and image analysis techniques.

The visualization of heterogeneous morphology, segmentation and quantification of image features is a crucial point for nonlinear optics microscopy applications, spanning from imaging of living cells or tissues to biomedical diagnostic. In this paper, a methodology combining stimulated Raman scattering microscopy and image analysis technique is presented. The basic idea is to join the potential of vibrational contrast of stimulated Raman scattering and the strength of imaging analysis technique in order to delineate subcellular morphology with chemical specificity. Validation tests on label free imaging of polystyrene-beads and of adipocyte cells are reported and discussed.

Cooperative biological effects between ionizing radiation and other physical and chemical agents.

Exposure to ionizing radiation (IR), at environmentally and therapeutically relevant doses or as a result of diagnostics or accidents, causes cyto- and genotoxic damage. However, exposure to IR alone is a rare event as it occurs in spatial and temporal combination with several physico-chemical agents. Some of these are of known noxiousness, as is the case with chemical compounds at high dose, hence additive/synergistic effects can be expected or have been demonstrated. Conversely, the cellular toxicity of other agents, such as non-ionizing electromagnetic fields (EMFs), is only presumed and their short- and long-term cooperation on IR-induced damage remains undetermined. In this review, we shall examine evidence in support of the interplay between spatially and/or temporally related environmentally relevant stressors. In vitro or animal-based studies as well as epidemiological surveys have generally examined the combined action of no more than a couple of known or potentially DNA-damaging agents. Moreover, most existing research mainly focused on short-term effects of combined exposures. Hence, it is important that quantitative research addresses the issue of the possible cooperation between chronic exposure to environmental trace contaminants and exposure to EMFs, examining not only the modulation of damage acutely induced by IR but also long-term genome stability.