Polarization-maintaining reflection-mode THz time-domain spectroscopy of a polyimide based ultra-thin narrow-band metamaterial absorber

This paper reports the design, the microfabrication and the experimental characterization of an ultra-thin narrow-band metamaterial absorber at terahertz frequencies. The metamaterial device is composed of a highly flexible polyimide spacer included between a top electric ring resonator with a four-fold rotational symmetry and a bottom ground plane that avoids misalignment problems. Its performance has been experimentally demonstrated by a custom polarization-maintaining reflection-mode terahertz time-domain spectroscopy system properly designed in order to reach a collimated configuration of the terahertz beam. The dependence of the spectral characteristics of this metamaterial absorber has been evaluated on the azimuthal angle under oblique incidence. The obtained absorbance levels are comprised between 67% and 74% at 1.092 THz and the polarization insensitivity has been verified in transverse electric polarization. This offers potential prospects in terahertz imaging, in terahertz stealth technology, in substance identification, and in non-planar applications. The proposed compact experimental set-up can be applied to investigate arbitrary polarization-sensitive terahertz devices under oblique incidence, allowing for a wide reproducibility of the measurements

Selective detection of bacterial layers with terahertz plasmonic antennas

Current detection and identification of micro-organisms is based on either rather unspecific rapid microscopy or on more accurate complex, time-consuming procedures. In a medical context, the determination of the bacteria Gram type is of significant interest. The diagnostic of microbial infection often requires the identification of the microbiological agent responsible for the infection, or at least the identification of its family (Gram type), in a matter of minutes. In this work, we propose to use terahertz frequency range antennas for the enhanced selective detection of bacteria types. Several microorganisms are investigated by terahertz time-domain spectroscopy: a fast, contactless and damage-free investigation method to gain information on the presence and the nature of the microorganisms. We demonstrate that plasmonic antennas enhance the detection sensitivity for bacterial layers and allow the selective recognition of the Gram type of the bacteria

State-of-the-art terahertz sensing for food and water security – a comprehensive review

Background: Recently, there has been a dramatic change in the field of terahertz (THz) technology. The recent advancements in the THz radiation sector considering generation, manipulation and detection have brought revolution in this field, which enable the integration of THz sensing systems into real-world. The THz technology presents detection techniques and various issues, while providing significant opportunities for sensing food and water contamination detection. Scope and approach: Many researchers around the world have exploited the potential of invaluable new applications of THz sensing ranging from surveillance, healthcare and recently for food and water contamination detection. The microbial pollution in water and food is one the crucial issues with regard to the sanitary state for drinking water and daily consumption of food. To address this risk, the detection of microbial contamination is of utmost importance, since the consumption of insanitary or unhygienic food can lead to catastrophic illness. Key findings and conclusions: This paper presents a first-time review of the open literature covering the advances in the THz sensing for microbiological contamination of food and water, in addition to state-of-the-art in network architectures, applications and recent industrial developments. With unique superiority, the THz non-destructive detection technology in food inspection and water contamination detection is emerging as a new area of study. With the great progress, some important challenges and future research directions are presented within the field

Terahertz sensing with spoof plasmon surfaces

Terahertz (THz) radiation (≈ 0.1−10×10^12 Hz) is non-ionizing and its photon energies correspond to the rotational and vibrational modes of many complex molecules. Hence many substances of interest for biological and security applications can be detected using THz light; making THz spectroscopy an ideal tool for bio- and security sensing. However, a size mismatch between the photon wavelength and the size of many commonly sensed targets, and a lack of powerful sources hamper the progress of THz technology towards more widespread real-world applications. The focus of this thesis is to use novel concepts in the field of metamaterials to overcome or side step some of these challenges. In particular, the use of confined electromagnetic surface modes, such as lattice resonances and spoof plasmons, on metamaterial surfaces to conduct THz sensing is investigated. Different ways in which sensing information can be extracted from these specially structured metamaterial surfaces are explored so as to demonstrate the feasibility and versatility of metamaterial surfaces in THz sensing applications. The application of lattice resonances to detect refractive index changes caused by various fluids on an array of metallic rods is first reported in this thesis. This can be seen as a prelude to the work presented in later chapters where strongly confined spoof plasmons are employed for THz sensing. A metamaterial surface supporting spoof plasmons (simply termed as a Spoof Plasmon Surface (SPS)), consisting of a linear array of metallised sub-wavelength grooves, is filled with various fluids and shown to be capable of high performance refractive index sensing in an Otto prism setup. Sharp phase changes, readily available from THz time-domain spectroscopy (THz-TDS), associated with the coupling of THz radiation to spoof plasmons are used as the readout response in this case to indicate changes in the refractive indices of the fluids filling the grooves. Building upon the initial work on spoof plasmon sensing, further investigations demonstrate the feasibility of SPSs as a versatile platform on which various forms of sensing information can be extracted by using THz radiation that is coupled in and out of spoof plasmons via the scattering edge coupling scheme. The time-domain signal from the SPS is analysed using a short- time Fourier transform (STFT), enabling the extraction of the broadband spoof plasmon dispersion with a single measurement as well as the attenuation coefficient with a minimum of two measurements. Broadband sensing is demonstrated, again by filling the grooves with various fluids, which results in changes in the spoof plasmon dispersion and attenuation coefficients. In addition, the observation of the absorption peak of α-lactose monohydrate at 1.37 THz due to the enhanced light- matter interactions on an SPS is demonstrated and opens the door towards a more spectroscopic approach to THz sensing using SPSs.Open Acces

Terahertz dielectric study of bio-molecules using time-domain spectrometry and molecular dynamics simulations

PhDTerahertz frequency domain constitutes the least explored part of electromagnetic spectrum. At the same time plenty of physical phenomena occurs on picoseconds to nanosecond time-scale and have and can be monitored/controlled/studied by THz and sub-THz waves. Since the advent of photo-conductive generation followed by invention of the first THz-TDS system, research in this field made a huge progress, although still possess a considerable potential for growth. Alongside advances in generation and detection of THz radiation simulation tools are becoming increasingly important and facilitate interpretation of the experimental results. Thesis comprises three related subjects, namely the processing of THz-TDS raw data, analysis of protein solvation dynamics by simulations and experimental investigation of water-protein solution at different concentrations. Experimental works in this thesis is performed using THz-TDS (normally covers 0.1-4 THz domain) and quasi-optical bench which covers the 75-325 GHz frequency bands. Molecular dynamics simulations were conducted in Gromacs package with a purely mechanical force field. The thesis is organized in the following way: chapter 1 introduces THz frequency domain to the reader, by describing its location in the electromagnetic spectrum, the physical phenomena that falls to THz domain, the main applications of THz radiation and overview of the mechanism of interaction between THz waves and bio-molecules. Second chapter outlines the principles of operation, physical processes and areas of application of THz-TDS. It is completed with a detailed description of the THz-TDS available in our laboratory. Third chapter gives a general picture of data processing related to material parameter extraction from time-domain response of the sample recorded by THz-TDS. Then it goes into details of associated error analysis, introducing the uncertainty caused by utilization of approximated transfer function. The application of the accurate algorithm for sample thickness determination based on its THz response is also presented in the third chapter. The fourth chapter discusses the application of Gromacs molecular dynamics simulations for the study of solvation dynamics of four selected proteins, namely TRP-tail, TRP-cage, BPTI and lysozyme proteins. All the water molecules solvating protein are divided into buried in the protein interior structure and the ‘on-surface’ water molecules. The later is shown to have similar properties for all proteins, while the former serve as the origin for the differences in solvation dynamics of proteins. Further in this chapter the radius of hydration shell and its dependence on the protein structure is investigated using vibrational density of states of solvating water molecules. The experimental investigation of the lysozyme, myoglobin and BSA proteins solutions performed over 0.22-0.325 THz domain using the PNA-driven quasi-optical bench is described in chapter 5. The relative absorption of protein molecules in solution and the hydration shell depth is also estimated. The last chapter concludes the thesis and outlines some future prospects.Queen Mary University of London College Doctoral Training fund (now – Principal’s studentship

Terahertz Surface Plasmon Resonance Sensor for Material Sensing

Terahertz wave (THz) is comprised of electromagnetic waves carrying frequencies from 0.1 to 30 THz. Terahertz radiation has the ability to interact with a wide range of materials, such as plastic and paper, and to provide low-­‐energy probing of the system's electronic nature, including inter/intra-­‐molecular motions and Debye relaxation -­‐ these are not accessible by other wavelengths. Further appealing feature for THz ray is the nonionizing nature and the distinctive optical response of various materials are important for analyzing diverse applications such as material quality control, pharmaceutical, industrial production lines, and biological. Upon that THz waves have been utilized for imaging and spectroscopy, especially Terahertz Time-­‐Domain Spectroscopy (THz-­‐TDS) associated to its ability in measuring the change in the electric field with high sensitivity in time-­‐domain. Surface plasmon-­‐polaritons (SPPs) at metal-­‐dielectric interfaces have been proven for several decades as a reliable technique for surface analysis and investigation of thin films due to the two dimensional nature of SPPs and the strong electromagnetic field at the interface. Extraordinary transmission of light through subwavelength hole arrays has attracted many areas of applications including optical data storage, near field microscopy, optical displays, and thin film sensing. The enhancement in the tunneled transmission light stemming from the coupling with SPP by the surface configurations has been explored through the waveguide theory and the grating theory of the frequency-­‐selective characteristic of SPP resonances. At THz frequencies, the extraordinary transmission through thin metallic hole arrays has been demonstrated through the excitation of SPP on the metal–dielectric interface confining the incident THz pulse around the holes, hence precluding THz pulse from easily passing and attenuating into the conductor. Implementing THz SPP in thin film sensing has great potential for industrial applications because the two dimensional nature of SPPs and the strong electromagnetic field at the interfacewith the THz natural reaction with the material provides reliable measurements of thin film spectroscopy including optical and dielectric constants, film thickness, and inhomogeneities at interfaces with high precision. This motivates the investigation of the characteristics such as purity of thin organic film including PMMA and those used in organic light emitting diode (OLED) through THz SPR devices. Two SPR devices contain either 2D periodic circular or square hole array in 500 nm Al on an 5 mm-­‐thick intrinsic silicon, or a single subwavelength aperture surrounded by concentric periodic grooves of a set period in a metal plate (which is known as a Bull’s eye structure), and was fabricated by following the micro-­‐ fabrications process encompassed from UV photolithography and wet and dry etching to transfer the pattern into the Al film. The SPR device consisting of 2D periodic circular or square hole array with and without thin Poly(methyl methacrylate) (PMMA) film on it is placed at the focus of the THz beam in transmission THz-­‐TDS, where the spectrum is obtained from the Fourier-­‐ transformed sample and reference THz pulses. The transmission is obtained from the ratio between the sample spectrum and reference spectrum, whereas the phase change is the phase difference between the two spectra. To avoid overlap with water absorption lines, the optimal SPR device design has a period of 320 μm and square holes of 150 μm side length. We successfully confirmed the theoretical SPR frequencies for metal-­‐silicon mode and demonstrate a shift to 0.9211 THz due to 2 μm of PMMA layer on the surface

Engineering metal parallel plate waveguides as a 2-D plane for high resolution THz time domain spectroscopy

Scope and Method of Study: The research summarized in this dissertation is on the investigation of the application of metal parallel plate waveguides (PPWG) for performing high resolution spectroscopic measurements of molecular solids at THz frequencies. The dissertation also presents results on the incorporation of high Q periodic structures within a metal PPWG and their application in sensitive detection of materials by monitoring change in refractive index. The experimental results were obtained by measuring the transmission of metal PPWG with samples in a standard THz time domain spectroscopy system based on photoconductive switches in 4f geometry.Findings and Conclusions: The main finding of this endeavor is that waveguide THz time domain spectroscopy using metal PPWGs can be efficiently applied to extract high resolution vibrational resonances associated with molecular solids. This technique discovered as a part of this research is a new and novel method which for the first time facilitates high resolution spectroscopic measurements of solid microcrystalline films which are easy to make in comparison to single crystal samples, but allows us to extract high resolution vibrational modes of the molecules. This technique can be complimentarily applied with the standard THz-TDS and Fourier transform infra-red spectroscopy to resolve the complete vibrational response of any molecular solid. We have also shown that metal PPWG also allows the incorporation of periodic structures like a weak Bragg stack and be applied as a very high - Q frequency filter having application in chemical sensing. The Q factors obtained by us are among the highest obtained for resonant and periodic structures within waveguide structures. We can conclude that the PPWG acts as an efficient 2-D plane which allows for performing high resolution spectroscopy and for incorporating frequency filtering devices with in the sub wavelength gap

Plasmonic Nanoplatforms for Biochemical Sensing and Medical Applications

Plasmonics, the science of the excitation of surface plasmon polaritons (SPP) at the metal-dielectric interface under intense beam radiation, has been studied for its immense potential for developing numerous nanophotonic devices, optical circuits and lab-on-a-chip devices. The key feature, which makes the plasmonic structures promising is the ability to support strong resonances with different behaviors and tunable localized hotspots, excitable in a wide spectral range. Therefore, the fundamental understanding of light-matter interactions at subwavelength nanostructures and use of this understanding to tailor plasmonic nanostructures with the ability to sustain high-quality tunable resonant modes are essential toward the realization of highly functional devices with a wide range of applications from sensing to switching. We investigated the excitation of various plasmonic resonance modes (i.e. Fano resonances, and toroidal moments) using both optical and terahertz (THz) plasmonic metamolecules. By designing and fabricating various nanostructures, we successfully predicted, demonstrated and analyzed the excitation of plasmonic resonances, numerically and experimentally. A simple comparison between the sensitivity and lineshape quality of various optically driven resonances reveals that nonradiative toroidal moments are exotic plasmonic modes with strong sensitivity to environmental perturbations. Employing toroidal plasmonic metasurfaces, we demonstrated ultrafast plasmonic switches and highly sensitive sensors. Focusing on the biomedical applications of toroidal moments, we developed plasmonic metamaterials for fast and cost-effective infection diagnosis using the THz range of the spectrum. We used the exotic behavior of toroidal moments for the identification of Zika-virus (ZIKV) envelope proteins as the infectious nano-agents through two protocols: 1) direct biding of targeted biomarkers to the plasmonic metasurfaces, and 2) attaching gold nanoparticles to the plasmonic metasurfaces and binding the proteins to the particles to enhance the sensitivity. This led to developing ultrasensitive THz plasmonic metasensors for detection of nanoscale and low-molecular-weight biomarkers at the picomolar range of concentration. In summary, by using high-quality and pronounced toroidal moments as sensitive resonances, we have successfully designed, fabricated and characterized novel plasmonic toroidal metamaterials for the detection of infectious biomarkers using different methods. The proposed approach allowed us to compare and analyze the binding properties, sensitivity, repeatability, and limit of detection of the metasensing device