Exotic optical responses of designed metasurfaces, including non-Hermitian photonic systems exhibiting exceptional point (EP)-singularities, offer diverse applications in the field of quantum sensing, laser technology, gravitational-wave detection as well as biomedical instrumentation for weak signal detection. However, the sensitivity enhancement of such EP-sensors is limited by quantum/thermal noises. Here we propose a novel scheme of EP-based superior photonic sensing of any molecule, using suitably designed asymmetric coupled-lossy-resonators (ACLR) with a non-Hermitian Hamiltonian. Unlike conventional approach of EP-degeneracy lifting, the perturbing Hamiltonian of the molecular-vibron, in the present method, generates an EP-singularity by coupling with the ACLR that does not exhibit EP in the unperturbed condition. Raman spectroscopic measurements performed on such systems provide significantly enhanced peaks at the vibronic modes, thereby exploring a novel method for molecule detection with superior sensitivity. Such EP-sensing is experimentally demonstrated in the current work by detecting the relevant protein-vibrons of the recombinant Omicron strain of SARS-CoV-2 with 350%-2200% enhanced Raman intensities, using an optical-metasurface consisting of an array of Au-asymmetric split-ring-resonators as the ACLRs. This work will open up a novel field of EP-based superior photonic sensing of any molecule, in general, by appropriately designing the ACLR-structures for detecting molecular-vibrons with desired enhanced sensitivity.

This chapter explores the identification of groups of organic molecules using plasmonic resonances produced in arrays of asymmetric split-H (ASH) structures. It begins with consideration of the architecture of an ASH structure that involves conventional gold dipole nano-antennas and comprises two asymmetric arms and a "cross-bar" that has in its middle a nanometer-scale gap. The nanoscale regions, referred to as "hot spots", arise from the sharp edges, corners and narrow gap in the basic ASH structure. The chapter continues with analysis and design of the basic ASH structure using numerical modeling – and a description of nanofabrication of the ASH structure – with proper optimization of the geometry, together with the use of a periodic arrangement of array toward the production of a metasurface that exhibits high-sensitivity resonance peaks. Metasurface arrays based on the ASH structure derive their characteristics from the design geometry (of the structure) being tuned so that the two distinct resonance peaks occur in the mid-infrared (mid-IR) region of the electromagnetic spectrum. Emphasis is put on the fact that many important molecular vibrations occur in the mid-IR spectral region – coming from chemical bonds that are relevant to the detection of different estrogenic hormones and, for example, 17β-estradiol (E2), the most potent estrogen. This approach is crucial for a wide range of applications in spectroscopy, environmental sensing and bio-molecular sensing. The chapter also addresses the use of aptamers in the experimental study of various forms of estrogenic molecules via plasmonic sensing methods based on metasurfaces constructed from arrays of metamaterial elements – making them suitable for future applications of sensing technology.

By optimising the geometry of asymmetric split-H (ASH) resonators fabricated on zinc selenide, we have produced a total of four distinct plasmonic resonances that could be matched with six molecular vibration wavelengths (for O-H, C-H, C=O, C=C, CºC-H and C-C bonds) which are relevant to the detection of four estrogenic hormones: estrone (E1), 17β-estradiol (E2), estriol (E3) and synthetic estrogen; 17α-ethinyl estradiol (EE2). Specifically, sensitivities of 363 nm/RIU and 636 nm/RIU were achieved from the deposition of E2 on ASH1 (2 μm and 4 μm) and ASH2 (5 μm and 8 μm) respectively. A Fourier transform infrared (FTIR) spectrometer was used to measure the transmittance resonances of the fabricated ASH arrays. The amplitudes of the molecular vibrational resonances were also around 500 times greater when matched with the plasmonic resonances of the ASHs as compared with deposit on on bulk ZnSe substrates. Finally, when mixtures of two hormones were deposited on the nanoantennas, the molar ratio for each of the hormones could also be calculated by using the peak intensities for the different molecular vibration wavelengths. By engineering the spectral response of ASH resonators to match specific estrogenic fingerprints, the work paves the way for the development of metamaterial sensors with better specificity and enhanced functionalities.

In comparison with electromagnetic radiation at shorter wavelengths, infrared radiation is non-destructive for many organic molecules. Previous research has established the resonant behaviour of various nanostructures designed to operate in the mid-infrared region and observe the resonant behaviour of organic molecules, with applications in areas such as security screening, the environment and medicine. This paper considers some of the important features of a particular asymmetric nanostructure that is useful for such purposes. In addition, the paper will show how plasmon resonance hybridization can describe the resonance energy levels in asymmetric split ring resonators (A-SRRs).

lasmonic resonances are strong candidates for applications in the fields of biomedical and environmental sensing. Previous related work has not investigated fully the optimization of resonant behaviour based on the various metallic metamaterial structure designs now available. The performance of structures that exhibit two distinct plasmonic resonances can be improved by varying the design parameters e.g. the period of metamaterial arrays and asymmetry in the basic elements. In addition, selection of an appropriate substrate for the metamaterial array should be considered for the production of plasmonic resonances in regions of the electromagnetic spectrum that are of particular interest. In the present work, we report work on optimization of the resonances produced by gold asymmetric split-ring resonators (A-SRRs) fabricated on fused silica and silicon-oninsulator (SOI) substrates. The specific technological aspects of using a SOI substrate are also highlighted.

We demonstrate the fabrication and characterization of an array of plasmonic metamaterial nanostructures based on asymmetric split H (ASH) resonators on a zinc selenide substrate that produce plasmonic resonances matched with the molecular vibrations of an organic material. Estrogenic hormones; 17β-Estradiol (E2) and Estrone (E1) were chosen as analytes for coupling with the plasmonic resonances. The experimental results show there is a good match with the molecular bond resonances of the C-H, C=O and C=C observed in estrogen and we have also shown that it is possible to differentiate the molecular bond resonance spectrum of E2 in a mixture with E1.

One approach that has been demonstrated as viable in biomedical sensing is the use of fluorescent labelling, e.g. for carrying out competition immunoassays to identify the possible presence of specific analyte molecules in suitably prepared fluid samples. Many bio-materials exhibit a finite level of fluorescence that could possibly be exploited in sensing, but is also potentially problematic when it forms an undesired background, thereby limiting sensitivity. But the alternative approach of label-free biomedical sensing seems likely to be favoured in future applications. A 'standard' approach to label-free sensing exploits the detection of changes in the (complex) refractive index that occur when bio-material is added to the local environment of a designed resonant structure, thereby changing its resonance frequency. Designed tuning of reflection, transmission and absorption resonances can be used to help identify specific molecules, through selection of the known bond resonances of the molecules of interest. Since it is typically possible to organise resonant structures in arrays that consist of thousands of individual resonant 'atoms', thereby forming a metasurface, it has become possible to select and quantify various characteristic molecular bond resonances simultaneously - and to identify possible molecular compositions in composite bio-material.

In this study, atomic force microscopy (AFM) imaging has been used to reveal the preferential deposition of organic thin-films on patterned nanoantenna array surfaces-identifying the localised formation of both monolayer and multilayer films of octadecanethiol (ODT) molecules, depending on the concentration of the solutions used. Reliable identification of this selective deposition process has been demonstrated for the first time, to our knowledge. Organic thin-films, in particular films of ODT molecules, were deposited on plasmonic resonator surfaces through a chemi-sorption process-using different solution concentrations and immersion times. The nanoantennas based on gold asymmetric-split ring resonator (A-SRR) geometries were fabricated on zinc selenide (ZnSe) substrates using electron-beam lithography and the lift-off technique. Use of the plasmonic resonant-coupling technique has enabled the detection of ODT molecules deposited from a dilute, micromolar (1 mu M) solution concentration - with attomole sensitivity of deposited material per A-SRR - a value that is three orders of magnitude lower in concentration than previously reported. Additionally, on resonance, the amplitude of the molecular vibrational resonance peaks is typically an order of magnitude larger than that for the non-resonant coupling. Fourier-transform infrared (FTIR) spectroscopy shows molecule specific spectral responses - with magnitudes corresponding to the different film thicknesses deposited on the resonator surfaces. The experimental results are supported by numerical simulation. (C) 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license.

In this paper, we report on arrays of asymmetric split H-shape nanostructures tuned to produce two distinct resonances at wavelengths that range from 3 μm to 7 μm. The electric-field of the incident wave has been both polarized parallel to the vertical asymmetric dipole arms and polarized across the 50 nm gap in the asymmetric horizontal bar. We have produced resonance quality factors as large as 26 in the mid-infrared region.

Arrays of nanoantennas consisting of plasmonic dipole pairs have been widely used in surface-enhanced Raman spectroscopy (SERS). Fine-tuned structures that can efficiently convert incident electromagnetic energy to excite molecules and provide enhanced detection. However, this tuning mechanism also has its disadvantages. In order to prevent the cross coupling, the distance between each individual element must be increased. This leads to low packing density values which in turn results in a reduction of the overall enhanced Raman signal when these structures are compared to broadly tuned aggregates of particles such as those obtained through metal sputtering or colloidal deposition. In this work we demonstrate through simulations and experimental work that it is possible to increase the reflected signal of an array of nanoantennas by reducing the distance between them in the direction both perpendicular and parallel to the orientation of the incident electric field. It is shown the resonant wavelength shifts in two different spectral directions depending in how the intercell distance was reduced. These resultant shifts can reduce the tuning capabilities of the structures but also can increase the SERS intensity due to close coupling of the dipole pairs. We believe that these results will enable the design and fabrication of structures possessing a greater degree of tunability together with an overall enhanced Raman signal that can rival aggregated SERS substrates.

Proteins are essential biomolecules for living organism to regulate cellular growth and their sustainability in the crowded biological environments. Evolving proteins need to be correctly folded to embrace their appropriate structure to maintain proper functionality. Failure to do so or due to accumulation of misfolded proteins may contribute to the development of many neurodegenerative diseases like amyloidosis, diabetes and Alzheimer’s. Therefore, understating the specific adsorption behavior of proteins on gold nanoantennas is crucial towards the development of advanced biocompatible devices for improved medical diagnosis. Metallic-nanoantennas are subwavelength structures that interact with light and confine the optical intensity within a tiny volume. This research identifies the specific deposition nature of selective proteins on gold-nanoantenna surfaces using high resolution imaging tool - atomic force microscope (AFM) together with their molecular vibrational detection with FTIR spectroscopy. The specific nanoantennas are asymmetric-split ring resonators (A SRRs) based on a circular geometry and are fabricated using electron-beam lithography. The plasmonic resonance of A-SRRs were tuned to match the vibrational resonance of proteins molecules in the mid-infrared region, thereby enhancing the sensitivity of detection down to nanogram (10-9) per millilitre range. For higher concentrations - accumulated selective deposition of proteins were imaged on the functionalised nanoantenna surfaces and are shown in Figure 1. The specific nature of folded proteins may be significant because folded proteins are associated with the development of degenerative diseases, such as Alzheimer’s - but further experiments are required to draw firmer conclusions. The outcomes from this research will benefit significantly to the wider audience in understanding and development of new medical diagnostic devices with a faster response and enhanced sensitivity.

Detection and identification of biomedically significant molecules is an important application in infra-red (IR) spectroscopy. This presentation will consider some of the significant features of the different alternative building-block elements that can be used in array metasurfaces for enhanced detection sensitivity. The presentation will also address techniques and issues associated with the deposition and localisation of biological and organic chemical molecular material for detection and measurement using IR spectroscopy.

The deposition of organic molecules on gold nanoantennas is reported through chemisorption for sensing in the midinfrared (mid-IR) spectral range. The specific nanostructures are gold asymmetric-split ring resonators (A-SRRs) based on circular-geometry with two different ‘arc’ lengths. The plasmonic resonant coupling technique was used to match the vibrational responses of the targeted molecules for their enhanced detection. Gold nanostructures are functionalised through chemisorption of octadecanethiol (ODT) in ethanol solution. The molecular vibrational responses were measured using a microscope coupled Fourier Transform Infrared (FTIR) spectroscopy. The experimental findings are closely supported using FDTD simulation. The modified nanoantennas surfaces are capable of supporting wide range of organic-sensing applications.

Split ring resonator (SRR) based metamaterials have frequently been demonstrated for use as optical sensors of organic materials. This is made possible by matching the wavelength of the SRR plasmonic resonance with a molecular resonance of a specific analyte, which is usually placed on top of the metal structure. However, systematic studies of SRRs that identify the regions that exhibit a high electric field strength are commonly performed using simulations. In this paper we demonstrate that areas of high electric field strength, termed “hot-spots,” can be found by localizing a small quantity of organic analyte at various positions on or near the structure. Furthermore, the sensitivity of the SRR to the localized analyte can be quantified to determine, experimentally, suitable regions for optical sensing.

Metamaterial surfaces in the shape of Split Ring Resonators, Asymmetric Split Ring Resonators and H-shaped structures have been used as optical sensors for organic materials such PMMA and the hormone estradiol. Field enhancement by the plasmonic structures can be obtained by matching the wavelength of the structures plasmonic resonance with a molecular resonance of a specific analyte and results in an increased molecular signature. We consider the relative merits of the structures and their sensitivities.

In this paper, gold asymmetric-split ring resonators (A-SRRs) are used for proteins sensing in the mid -infrared (IR) spectral region. Self-assembled monolayers (SAMs) of octadecanethiol (ODT) in ethanolic solution were deposited on the resonator surfaces to immobilise protein molecules for their detection. Different diameters ASRRs were fabricated on zinc selenide (ZnSe) substrates using electron -beam lithography technique. Their plasmonic responses appear in the mid-IR spectral region and match with the vibrational responses of many organic molecules. After the formation of SAMs layer, one sample was immersed in bovine serum albumin (BSA) solution for proteins adsorption while other sample was immersed in hydroxyl terminated hexa-ethylene glycol (EG(6)-OH) solution to modify SAMs surfaces to resist immobilisation of proteins. The vibrational responses of these organic molecules, all samples were excited using an incident broadband mid-IR light source and their reflectance spectra were measured at normal incidence using a microscope coupled Fourier Transform Infrared (FTIR) spectrometer. This study highlights the capability of plasmonic structures (A-SRRs) fabricated on transparent and high refractive index ZnSe substrates allows the detection of BSA proteins with enhanced detection in the mid-IR spectral range, demonstrating their potential for a wide range of sensing applications, e.g. in biomedical engineering and food industries.

In this paper we report on a very sensitive biosensor based on gold asymmetric nanoantennas that are capable of enhancing the molecular resonances of C-H bonds. The nanoantennas are arranged as arrays of asymmetric-split H-shape (ASH) structures, tuned to produce plasmonic resonances with reflectance double peaks within the mid-infrared vibrational resonances of C-H bonds for the assay of deposited films of the molecule 17β-estradiol (E2), used as an analyte. Measurements and numerical simulations of the reflectance spectra have enabled an estimated enhancement factor on the order of 105 to be obtained for a thin film of E2 on the ASH array. A high sensitivity value of 2335 nm/RIU was achieved, together with a figure of merit of approximately 8. Our experimental results were corroborated using numerical simulations for the C-H stretch vibrational resonances from the analyte, superimposed on the plasmonic resonances of the ASH nanoantennas.