Infrared Absorption Enhancement Research Articles

Dimeric and monomeric conformation of SARS-CoV-2 main protease: New technical approaches based on IR radiation

The main proteases Mpro are a group of highly conserved cysteine hydrolases in β-coronaviruses. They have been demonstrated to play an unavoidable role in viral replication, and consequently they have been suggested as key targets for treating coronavirus-caused infectious diseases, mainly from the COVID-19 epidemic. Since the most functional form for Mpro enzymatic activity is associated to its homodimer, compounds inhibiting dimerization should also inhibit catalytic activity. We show how PIR-SEIRA (Plasmonic Internal Reflection-Surface Enhanced InfraRed Absorption) spectroscopy can be a noteworthy technique to study proteins subtle structural variations associated to inhibitor binding. Nanoantennas arrays can selectively confine and enhance electromagnetic field via localized plasmonic resonances, thus promoting ultrasensitive detection of biomolecules in close proximity of nanoantenna arrays and enabling the effective investigation of protein monolayers. By adopting this approach, reflection measurements conducted under back illumination of nanoantennas allow to probe anchored protein monolayers, with minimum contribution of environmental buffer molecules. PIR-SEIRA spectroscopy on Mpro was carried out by ad hoc designed devices, resonating in the spectral region of Amide I and Amide II bands. We evaluated here the structure of anchored monomers and dimers in different buffered environment and in presence of a newly designed Mpro inhibitor. Experimental results show that dimerization is not associated to relevant backbone rearrangements of the protein at secondary structure level, and even if the compound inhibits the dimerization, it is not effective at breaking preformed dimers.

Study of MXene Ti3C2Tx quantum dot thin film on quartz bulk acoustic wave uncooled infrared sensors.

The infrared absorption efficiency is essential for an infrared sensor. We propose a quartz bulk acoustic wave (BAW) uncooled infrared sensor coated with MXene quantum dot film. The infrared detection is realized by measuring the resonant frequency of a Y-cut quartz BAW sensitive unit. An infrared sensor is fabricated by MEMS process, then the MXene quantum dot film is coated through the spin coating technology. The mechanism of infrared absorption enhancement is analyzed. Test results show that after coating the film, the responsivity (R) of the sensor increased by nearly 41% at a wavelength of 830nm, from 10.88MHz/W to 15.28 MHz/W. The quartz BAW infrared sensor combined with MXene quantum dots film has the potential of high-performance infrared detection.

Structure-selected graphene/metallic surface plasmon coupling regime and infrared modulation application

Here we present a graphene-based long-wavelength infrared modulator characteristic of extra-high contrast, where the frequency detuning degree of magnetic and electric surface plasmons (SPs) is controllable by the gated graphene Fermi energy. If the device is designed to work in a strong SP-coupling regime by selecting an appropriate low-lossy gate dielectric thickness, a modulation depth (MD) up to ∼100% but insertion loss (IL) as low as ∼−0.37 dB is achievable. Moreover, a compromised MD > 90% with IL < −1.0 dB is still retainable in two broadband ranges. The disclosed underlying mechanism to the device working state in the strong, electromagnetic-induced transparency (EIT), or weak SP-coupling regime, indicates the coupling regime shows a strong dependence on the dielectric thickness, which is related to the magnetic-SP mode volume, while the working wavelength can be selected in a broader spectral range by scaling the device geometry. These findings are helpful to construct those optoelectronics for infrared absorption enhancement, EIT, and strong coupling spectral characteristic itself.

Strongly Absorbing Nanoscale Infrared Domains within Strained Bubbles at hBN-Graphene Interfaces.

Graphene has great potential for use in infrared (IR) nanodevices. At these length scales, nanoscale features, and their interaction with light, can be expected to play a significant role in device performance. Bubbles in van der Waals heterostructures are one such feature, which have recently attracted considerable attention, thanks to their ability to modify the optoelectronic properties of two-dimensional (2D) materials through strain. Here, we use scattering-type scanning near-field optical microscopy (sSNOM) to measure the nanoscale IR response from a network of variously shaped bubbles in hexagonal boron nitride (hBN)-encapsulated graphene. We show that within individual bubbles there are distinct domains with strongly enhanced IR absorption. The IR domain boundaries coincide with ridges in the bubbles, which leads us to attribute them to nanoscale strain domains. We further validate the strain distribution in the graphene by means of confocal Raman microscopy and vector decomposition analysis. This shows intricate and varied strain configurations, in which bubbles of different shape induce more bi- or uniaxial strain configurations. This reveals pathways toward future strain-based graphene IR devices.

Electromagnetic and optical responses of a composite material comprising individual single-walled carbon-nanotubes with a polymer coating

The composites and thin films comprising individual single-walled carbon nanotubes with a polymer coating (p-CNTs) have been prepared and their electromagnetic responses have been studied in a wide range from low-frequency (25–107 Hz) up to the infrared region. In spite of the high volume fraction of the nanotubes (up to 3.3%), the polymer coating prevents direct p-CNT contacts and the formation of the percolation network in those composites, so that p-CNTs interact only via the electromagnetic coupling. Thereby it is an ideal model system to verify experimentally the fundamental issues related to carbon nanotube electromagnetics, such as the influence of inter-tube electron tunneling on the localized plasmon resonance in the terahertz range, or the infrared absorption enhancement of polymer molecules attached to the nanotube surface. Along with addressing the fundamentals, applied carbon nanotube electromagnetics got insights important for the applications of p-CNT based composites as dielectric media in the terahertz regime. In particular, we found that the real part of the permittivity of the p-CNT film in the terahertz range is rather competitive, i.e. 8–13, however the loss tangent is not so small (0.4–0.6) as has been predicted. The way to increase p-CNT terahertz performance is also discussed.

Infrared epsilon-near-zero absorption excited by magnetic dipole resonance

In natural or artificial Epsilon-near-zero (ENZ) materials, excitation of the absorption at ENZ point requires large incident-angle or polarization dependence, and thus become limited in practical applications. In this paper, the magnetic dipole resonance in a metal-ENZ-insulator–metal structure breaks such restrictions on ENZ absorption, by providing a critical longitudinal electric field to the embedded ENZ material. In numerical and corresponding experimental realizations, a thin SiO2 film in the metal-ENZ-insulator–metal structure serves as the ENZ layer at mid-infrared and holds the ENZ absorption at both TE and TM polarizations under normal incidence. The cooperation of ENZ absorption, magnetic dipole mode and the loss of SiO2film further results in infrared absorption enhancement and band-width broadening.

Enhanced nanoplasmonic heating in standoff sensing of explosive residues with infrared reflection-absorption spectroscopy.

Various standoff sensing techniques employing optical spectroscopy have been developed to address challenges in safely identifying trace amounts of explosives at a distance. A flexible anodic aluminum oxide (AAO) microcantilever and a high-power quantum cascade laser utilized as the infrared (IR) source are used for standoff IR reflection-absorption spectroscopy to detect explosive residues on a metal surface. Standoff sensing of trinitrotoluene (TNT) is demonstrated by exploiting the high thermomechanical sensitivity of a bimetallic AAO microcantilever. Moreover, sputtering gold onto the fabricated AAO nanowells generates a strong scattering and absorption of IR light in the wavelength range of 5.18 µm to 5.85 µm resulting in enhanced nanoplasmonic heating. Utilizing the IR absorption enhancement in this wavelength range, the plasmonic AAO cantilever could detect TNT molecules 7 times better than could the bimetallic AAO cantilever.

Simulation of enhanced infrared absorption spectra by rigorous coupled wave analysis

The resonant-type contribution to the infrared absorption enhancement phenomenon was simulated using rigorous coupled wave analysis (RCWA) for well-defined periodic Au arrays. The Au arrays have size parameters compatible with those of vacuum-evaporated thin metal films, which are used conventionally for enhanced infrared absorption spectra measurements. Results obtained using RCWA technique show excellent quantitative agreement with both experimental observations of enhanced infrared absorption of adsorbed species and an electrostatic square column model calculation based on a non-resonant-type electromagnetic mechanism. This fact suggests that the resonant mechanism has a small contribution to the enhanced infrared absorption in an evaporated-film-like structure Au array.

Ultrasound delivery of Surface Enhanced InfraRed Absorption active gold-nanoprobes into fibroblast cells: a biological study via Synchrotron-based InfraRed microanalysis at single cell level

Ultrasound (US) induced transient membrane permeabilisation has emerged as a hugely promising tool for the delivery of exogenous vectors through the cytoplasmic membrane, paving the way to the design of novel anticancer strategies by targeting functional nanomaterials to specific biological sites. An essential step towards this end is the detailed recognition of suitably marked nanoparticles in sonoporated cells and the investigation of the potential related biological effects. By taking advantage of Synchrotron Radiation Fourier Transform Infrared micro-spectroscopy (SR-microFTIR) in providing highly sensitive analysis at the single cell level, we studied the internalisation of a nanoprobe within fibroblasts (NIH-3T3) promoted by low-intensity US. To this aim we employed 20 nm gold nanoparticles conjugated with the IR marker 4-aminothiophenol. The significant Surface Enhanced Infrared Absorption provided by the nanoprobes, with an absorbance increase up to two orders of magnitude, allowed us to efficiently recognise their inclusion within cells. Notably, the selective and stable SR-microFTIR detection from single cells that have internalised the nanoprobe exhibited clear changes in both shape and intensity of the spectral profile, highlighting the occurrence of biological effects. Flow cytometry, immunofluorescence and murine cytokinesis-block micronucleus assays confirmed the presence of slight but significant cytotoxic and genotoxic events associated with the US-nanoprobe combined treatments. Our results can provide novel hints towards US and nanomedicine combined strategies for cell spectral imaging as well as drug delivery-based therapies.

Experimental near infrared absorption enhancement of graphene layers in an optical resonant cavity

Graphene has recently emerged as a promising candidate for a wide range of photonic and optoelectronic applications, with a high application potential in devices using infrared radiation. The optical absorption of 2D materials and graphene can be uniquely enhanced when they are embedded in optical resonant cavities, since optically-thin atomic-thickness absorbers do not perturb the cavity itself. Despite the many theoretical studies, experimental validation is still lagging behind. Here, large near infrared (NIR) absorption of unpatterned chemical vapor deposition graphene is experimentally demonstrated for the first time in a large area (1 inch) passive optical device by exploiting the enhancement of the electric field at the center of a Fabry–Perot cavity. Test devices were fabricated with single layer, double layer and five layers graphene, sandwiched between two almost symmetric Bragg mirrors deposited by radio frequency sputtering and consisting of alternate layers of Si and SiO2. A thin evaporated MgF2 overlayer was used to reduce sputtering induced damage on graphene layers. Measured absorption values, in the range of 37%–45%, were found in very good accordance with simulated ones. A maximum absorption of 45% was measured at 2345 nm for the double-layer graphene.

Engineering the Infrared Luminescence and Photothermal Properties of Double-Shelled Rare-Earth-Doped Nanoparticles for Biomedical Applications.

The nascent field of theranostics, which couples targeted therapy with diagnostics, has catalyzed efforts toward improved nanoprobe designs that facilitate both localized treatment and diagnostic imaging. Rare-earth-doped nanoparticles (RENPs) have emerged as a leading candidate for theranostics because of their versatile synthesis and modification chemistries, photostability, and relative safety. Furthermore, their bright, tunable fluorescence using near-infrared (NIR) excitation enables multispectral imaging with high signal-to-background ratios. In this work, we have synthesized double-shelled RENPs with tunable properties for optimal fluorescent imaging, photoacoustic imaging, and photothermal therapy. The properties of the double-shelled RENPs were tailored by controlling the density of rare-earth ions (i.e., activator or sensitizer) by using either a functional amorphous organic or a crystalline outermost shell. This study systematically analyzes the effects of the functional organic or inorganic outermost shell on the imaging and photothermal conversion properties of our RENPs. Despite the weaker infrared absorption enhancement, the functional organic outermost shell impregnated with a low density of rare-earth ions led to minimal reduction of fluorescence emissions. In contrast, the higher density of rare-earth ions in the inorganic shell led to higher infrared absorption and consequently significant reduction in emissions arising from the undesired optical attenuation. Inorganic shell thickness was therefore modified to reduce the deleterious attenuation, leading to brighter emissions that also enabled the in vitro SWIR detection of ∼2500 cells/cluster. Using the enhanced infrared properties that arise from this functional inorganic layer, which could be engineered to respond to either NIR or SWIR, we demonstrated that (1) bright SWIR emissions allowed detection of small cell clusters; (2) strong PA signals allowed clear visualization of particle distribution within tumors; and (3) strong photothermal effects resulted in localized elevated temperatures. Collectively, these results highlight the utility of these double-shelled RENPs as theranostic agents that are compatible with both photoacoustic or fluorescent imaging platforms.

Verifying electromagnetic first layer effect on surface enhanced infrared absorption with evaporated gold nano island film

Infrared absorption enhancement is reported for a polyacrylic acid film deposited on evaporated gold film in normal transmission geometry. Comparison between absorption intensities measured with and without the self-assembled monolayer of p-nitrothiophenol demonstrates that the absorption intensities of polyacrylic acid are independent of the presence of a self-assembled monolayer. Results suggest that the first layer effect in the physical enhancement mechanism does not exist in this thermally evaporated island film.

Metasurface based on cross-shaped plasmonic nanoantennas as chemical sensor for surface-enhanced infrared absorption spectroscopy

Infrared spectroscopy is an effective technique extensively used in research and industry for the label-free and unambiguous identification of molecular species. However, the sensitivity of this technique is severely limited as a result of Beer’s law and, the small infrared absorption cross-section that make prohibitively weak the absorption signals, of minute amounts of analyte as those present in monolayers. This limitation can be overcome by enhancing the infrared vibration of molecules through the enhancement of the electromagnetic (EM) field. Surface Enhanced InfraRed Absorption (SEIRA) using resonant metal Nano-scale Antennas (NAs) can provide huge electromagnetic fields on the nanometer scale featuring localized collective oscillations of electrons, an effect named Localized Surface Plasmonic Resonances (LSPRsWe here report on a series of 2D arrays of cross-shaped NAs having several mm2 area coverage (metasurface) as SEIRA optimized antennas, which can be used in practical applications such as the vibrational sensing of chemical and biological analytes. Cross-shape designed NAs are insensitive to the polarization of the electromagnetic radiation impinging the active area. Due to the random orientation of the dipole moments of molecules they are particularly suitable for the construction of bio-molecular sensors. At the same time, the 2D-array configuration ensures a good near-field signal enhancement arising from the coupling between neighbour NAs Moreover, SEIRA NAs can be easily integrated with micrometre-sized channels and be suitable for the high sensitivity, real time analysis of IR emitting samples, in matrices where IR spectroscopy is severely limited due to absorption bands of liquid water. We present the design, fabrication and experimental characterization of large-area metasurfaces based on cross-shaped plasmonic NAs for the spectroscopic characterization of various types of compounds and for sensing applications in the mid-infrared range. The cross-shaped NAs we have designed exhibit SEIRA phenomena which are very sensitive to both refractive index changes in the surrounding medium and to the specific molecular vibration band emerging from surface adsorbed molecules. To test this effect on our device, we have used as model compounds small molecules (molecular weight (MW) < 500 g/mol) containing triple bond groups resonating at about 2100 cm−1 and a large polymer (MW ˜ 950,000 g/mol) containing carbonyl groups resonating at wavenumbers of about 1700 cm−1. We show a sensitivity of 600 nm/RIU at different wavelengths at a maximum amount of immobilized small molecule of 0.7 fmoles and a SEIRA enhancement factor of 48,000. We also show the device potential to reveal chemical reactions, occurring on the sensor surface at the same scale, where the nitrile group is converted to a triazole ring.

Gold nanoparticle mediated graphene plasmon for broadband enhanced infrared spectroscopy

Graphene plasmonics, with dynamic tunable resonance wavelength, has been successfully used in broadband plasmon-enhanced infrared spectroscopy. However, the requirement for external voltage loading makes the practical application sophisticated. In this work, the hybrid structure of graphene nanodot array (GNA) and gold nanoparticles (AuNPs) has been proposed as a passive platform for broadband infrared absorption enhancement. Numerical simulations show that the plasmon resonance peak of GNA becomes stiffer and broader when introducing AuNPs, and this is also proved by experimental results that the vibrational modes of polyethylene oxide molecule in a broad spectral range can be simultaneously enhanced. The metal-graphene hybrid plasmonic system has been proved to be a promising candidate for infrared sensing, which is significant for safety and healthy applications.

Enhancing the Responsivity of Uncooled Infrared Detectors Using Plasmonics for High-Performance Infrared Spectroscopy.

A lead zirconate titanate (PZT;Pb(Zr0.52Ti0.48)O3) layer embedded infrared (IR) detector decorated with wavelength-selective plasmonic crystals has been investigated for high-performance non-dispersive infrared (NDIR) spectroscopy. A plasmonic IR detector with an enhanced IR absorption band has been designed based on numerical simulations, fabricated by conventional microfabrication techniques, and characterized with a broadly tunable quantum cascade laser. The enhanced responsivity of the plasmonic IR detector at specific wavelength band has improved the performance of NDIR spectroscopy and pushed the limit of detection (LOD) by an order of magnitude. In this paper, a 13-fold enhancement in the LOD of a methane gas sensing using NDIR spectroscopy is demonstrated with the plasmonic IR detector.

Square fractal-like nanoapertures for SEIRA spectroscopy: An analytical, numerical and experimental study

Due to the sensitivity limitations, infrared (IR) spectroscopy systems, in which conventional sensing platforms are used, have some problems with detecting the small quantities of molecules. By the advantages coming from their aperture based nature, nanoaperture arrays increase the light-matter interaction and can be useful to overcome some of these limitations. In this work, we propose an analytical, numerical and experimental study for a novel nanoaperture array which has enhanced transmission and reflection characteristics with high surface enhanced IR absorption (SEIRA) distributions. We numerically analyze the square fractal-like nanoapertures through the finite difference time domain method. Thus, we present a fine tuning mechanism for adjusting the resonance frequency which is highly desirable for SEIRA applications. We then propose analytical formulas for the resonance frequencies of the structures. Moreover, we experimentally realize the structure by using a high-precision and lift-off free nanofabrication method based on electron-beam lithography. Correspondingly, refractive index sensitivity analyses are performed by embedding our structure into different cladding media with several refractive index values. Finally, in order to show the SEIRA performance of the proposed nanoaperture array, we analyze the sensing characteristic of carbonyl ν(CO) stretching mode in a thin poly(methyl methacrylate) (PMMA) film for a set of different film thicknesses. Due to the enhanced transmission and reflection behaviors with easily accessible ultra-high-field localization characteristics, the proposed sensing platform can be a good candidate for SEIRA spectroscopy and label-free bio sensing applications.

Broadband infrared absorption enhancement by electroless-deposited silver nanoparticles

Abstract Decorating semiconductor surfaces with plasmonic nanoparticles (NPs) is considered a viable solution for enhancing the absorptive properties of photovoltaic and photodetecting devices. We propose to deposit silver NPs on top of a semiconductor wafer by a cheap and fast electroless plating technique. Optical characterization confirms that the random array of electroless-deposited NPs improves absorption by up to 20% in a broadband of near-infrared frequencies from the bandgap edge to 2000 nm. Due to the small filling fraction of particles, the reflection in the visible range is practically unchanged, which points to the possible applications of such deposition method for harvesting photons in nanophotonics and photovoltaics. The broadband absorption is a consequence of the resonant behavior of particles with different shapes and sizes, which strongly localize the incident light at the interface of a high-index semiconductor substrate. Our hypothesis is substantiated by examining the plasmonic response of the electroless-deposited NPs using both electron energy loss spectroscopy and numerical calculations.

What is the Key Structural Parameter for Infrared Absorption Enhancement on Nanostructures?

The key structural parameter for greater enhancement of infrared absorption on metal nanostructures was identified by systematic and exhaustive surface-enhanced infrared absorption (SEIRA) measurements for well-defined periodic gold square column (SC) arrays on silicon wafers fabricated using electron beam lithography. The SC arrays have size parameters compatible with those of vacuum-evaporated thin metal films, which are used conventionally for SEIRA. The crucially important parameter determining the enhancement factor for infrared absorption is the ratio of the gap size to the nanoparticle size. An electrostatic SC model calculation based on a nonresonant-type electromagnetic mechanism shows good quantitative agreement with experimental observations of enhanced infrared absorption of adsorbed species. Furthermore, results show that the electrostatic SC model contains the previously proposed lightning-rod effect’s case. The clarified enhancement mechanism for infrared absorption of molecules adsorbed on...

Surface plasmonic contribution to SEIRA and asymmetrical band

The Surface Enhanced InfraRed Absorption (SEIRA) band reaches its the maximum intensity and the band shape becomes asymmetric at the just percolated metal thin films. To explain for them, various models (e.g. the Fano model) have been proposed. However, no model can explain them completely. In this paper, we conducted a classical model calculations (Fresnel’s formula and the Bruggemann effective medium theory) for SEIRA and asymmetric bands. The calculated SEIRA spectra were good agreement with the experimentally spectra. Therefore, the asymmetrical SEIRA band might not be attributable to Fano model. Moreover, we specifically addressed the relation between effective dielectric constants of a metal–insulator composite thin film and SEIRA band shape change. Although the sign of the real part switched from positive to negative and although the imaginary part became metallic, the asymmetrical band shape did not change. The sign of the real part is

Plasmonic Enhancement of Vibrational Excitations in the Infrared

Related to their size and shape, plasmonic excitations of metal particles occur in an extremely broad spectral range from visible down to radio frequencies. They can couple to other kinds of excitation with a similar energy, which in the infrared (IR) is origin of vibrational signal enhancement. In this study, we recollect main points of the current knowledge on such coupling and give examples with molecular vibrations and phonons. Plasmonic enhancement of various phononic signals gives rise to distinguish between surface enhanced IR absorption and surface enhanced IR scattering.