An optical fibre based polarisation modulation technique: Development and applications

This thesis undertakes an investigation of polarisation modulation techniques arriving from optical transmission in both free-space and optical fibres with the aim of determining relative performances for prospective applications. The system of optimal performance is then applied to make ellipsometric measurements. Polarisation modulation techniques considered allow control of the state of polarisation through an interferometric method and modulation of the phase difference. Precise state and orientation of polarised light at any instant of time are obtainable. These qualities match the specific requirement of a range of optical instruments, such as polarimeters and ellipsometers. The evolution and development of various polarisation modulation techniques are discussed in terms of important features such as the operating speed, the number of optical components required and methods of controlling the polarisation. After initial considerations, two configurations were chosen for more detailed experimental study. Their arrangement produces a rotating plane polarised output and are based on interferometric techniques. One configuration is characterised by free-space propagation (Michelson interferometer); whereas, the other uses highly biréfringent (HiBi) fibre as the light propagation medium. The theoretical and experimental performances of both these configurations are investigated. Full studies focusing on the output quality of the controlled light in terms of the degree of polarisation and ellipticity from the two arrangements were undertaken. These factors are used to determined which configuration offers better performance. Further extensive investigations, particularly on performance-related factors, for the chosen scheme (the optical fibre based system) are carried out to optimise the quality of the rotating plane polarised light emitted. Improvements through a range of the factors were undertaken and are clearly demonstrated. The achievements resulting from the improvement of the rotating plane polarised light include a reduction in ellipticity (in the range of 10-2), a much smaller effect due to the direction of rotation (a 1-2% asymmetry) and potentially high speed detection (up to 1 kHz). These achievements are considered to be appropriate enough to employ the improved optical fibre polarisation modulation technique to ellipsometric measurements. The optical fibre modulation scheme is applied to the ellipsometric measurements of a simple system consisting of interfaces of two semi-infinite media. Various materials are investigated and experimental results are shown in terms of the ellipsometric parameters Ѱ and which then allow an individual sample refractive index to be determined. The accuracy found in measuring the two ellipsometric parameters is within 2% of the parameter mean values. A broader application range is also demonstrated as the optical fibre polarisation modulated ellipsometer is used to characterise ambient-film-substrate samples differing in film thickness. The results gained are then compared with corresponding reference values, obtained from a commercial ellipsometers, in order to demonstrate the degree of sensitivity. The novel implementation of the technique as a thin film based sensor device is demonstrated by detecting the change of a thin film properties on exposure to external stimuli

Design and Characterization of Porous Core Polarization Maintaining Photonic Crystal Fiber for THz Guidance

An improved design of Teflon photonic crystal fiber with a porous air-core is presented for low-loss terahertz guidance. Optimization of total power confinement in the air-holes, together both in the cladding and core regions, is carried out for both quasi-TE and quasi-TM polarizations by using a full-vectorial finite element method. To achieve the polarization maintenance, modal birefringence is enhanced by destroying the circular symmetry with the introduction of unequal size air-holes in the first ring

Optical Fiber, Nanomaterial, and THz-Metasurface-Mediated Nano-Biosensors: A Review

The increasing use of nanomaterials and scalable, high-yield nanofabrication process are revolutionizing the development of novel biosensors. Over the past decades, researches on nanotechnology-mediated biosensing have been on the forefront due to their potential application in healthcare, pharmaceutical, cell diagnosis, drug delivery, and water and air quality monitoring. The advancement of nanoscale science relies on a better understanding of theory, manufacturing and fabrication practices, and the application specific methods. The topology and tunable properties of nanoparticles, a part of nanoscale science, can be changed by different manufacturing processes, which separate them from their bulk counterparts. In the recent past, different nanostructures, such as nanosphere, nanorods, nanofiber, core–shell nanoparticles, nanotubes, and thin films, have been exploited to enhance the detectability of labelled or label-free biological molecules with a high accuracy. Furthermore, these engineered-materials-associated transducing devices, e.g., optical waveguides and metasurface-based scattering media, widened the horizon of biosensors over a broad wavelength range from deep-ultraviolet to far-infrared. This review provides a comprehensive overview of the major scientific achievements in nano-biosensors based on optical fiber, nanomaterials and terahertz-domain metasurface-based refractometric, labelled and label-free nano-biosensors

Performance optimization of a metasurface incorporating non-volatile phase change material

Optical metasurface is a combination of manufactured periodic patterns of many artificial nanostructured unit cells, which can provide unique and attractive optical and electrical properties. Additionally, the function of the metasurface can be altered by adjusting the metasurface's size and configuration to satisfy a particular required property. However, once it is fabricated, such specific property is fixed and cannot be changed. Here, phase change material (PCM) can play an important role due to its two distinct states during the phase transition, referred to as amorphous and crystalline states, which exhibit significantly different refractive indices, particularly in the infrared wavelength. Therefore, a combination of metasurface with a phase change material may be attractive for achieving agile and tunable functions. In this paper, we numerically investigate an array of silicon cylinders with a thin PCM layer at their centers. The GST and GSST are the most well-known PCMs and were chosen for this study due to their non-volatile properties. This structure produces two resonant modes, magnetic dipole and electric dipole, at two different resonating wavelengths. We have numerically simulated the effect of cylinder's height and diameter on the reflecting profile, including the effect of thickness of the phase change material. Additionally, it is shown here that a superior performance can be achieved towards reduced insertion loss, enhanced extinction ratio, and increased figure of merit when a GST layer is replaced by a GSST layer

Recent Advances in Optical Hydrogen Sensor including Use of Metal and Metal Alloys: A Review

Optical sensing technologies for hydrogen monitoring are of increasing importance in connection with the development and expanded use of hydrogen and for transition to the hydrogen economy. The past decades have witnessed a rapid development of optical sensors for hydrogen monitoring due to their excellent features of being immune to electromagnetic interference, highly sensitive, and widely applicable to a broad range of applications including gas sensing at the sub-ppm range. However, the selection of hydrogen selective metal and metal alloy plays an important role. Considering the major advancements in the field of optical sensing technologies, this review aims to provide an overview of the recent progress in hydrogen monitoring. Additionally, this review highlights the sensing principles, advantages, limitations, and future development

Optical Fiber, Nanomaterial, and THz-Metasurface-Mediated Nano-Biosensors: A Review

The increasing use of nanomaterials and scalable, high-yield nanofabrication process are revolutionizing the development of novel biosensors. Over the past decades, researches on nanotechnology-mediated biosensing have been on the forefront due to their potential application in healthcare, pharmaceutical, cell diagnosis, drug delivery, and water and air quality monitoring. The advancement of nanoscale science relies on a better understanding of theory, manufacturing and fabrication practices, and the application specific methods. The topology and tunable properties of nanoparticles, a part of nanoscale science, can be changed by different manufacturing processes, which separate them from their bulk counterparts. In the recent past, different nanostructures, such as nanosphere, nanorods, nanofiber, core-shell nanoparticles, nanotubes, and thin films, have been exploited to enhance the detectability of labelled or label-free biological molecules with a high accuracy. Furthermore, these engineered-materials-associated transducing devices, e.g., optical waveguides and metasurface-based scattering media, widened the horizon of biosensors over a broad wavelength range from deep-ultraviolet to far-infrared. This review provides a comprehensive overview of the major scientific achievements in nano-biosensors based on optical fiber, nanomaterials and terahertz-domain metasurface-based refractometric, labelled and label-free nano-biosensors

Reflective Terahertz Metasurfaces Based on Non-Volatile Phase Change Material for Switchable Manipulation

Recently, metasurfaces have been investigated and exploited for various applications in the THz regime, including modulators and detectors. However, the responsive properties of the metasurface in THz stay fixed once the fabrication process is complete. This limitation can be modified when integrating the phase change material (PCM), whose states are switchable between crystalline and amorphous, into the metasurface structure. This characteristic of the PCM is appealing in achieving dynamic and customizable functionality. In this work, the reflective metasurface structure is designed as a hexagonal unit deposited on a polyimide substrate. The non-volatile PCM chosen for the numerical study is germanium antimony tellurium (GST). Our proposed phase change metasurface provides two resonant frequencies located at 1.72 and 2.70 THz, respectively; one of them shows a high contrast of reflectivity at almost 80%. The effects of geometrical parameters, incident angles, and polarization modes on the properties of the proposed structure are explored. Finally, the performances of the structure are evaluated in terms of the insertion loss and extinction ratio

Optical Fiber, Nanomaterial, and THz-Metasurface-Mediated Nano-Biosensors: A Review

The increasing use of nanomaterials and scalable, high-yield nanofabrication process arerevolutionizing the development of novel biosensors. Over the past decades, researches on nano-technology-mediated biosensing have been on the forefront due to their potential application inhealthcare, pharmaceutical, cell diagnosis, drug delivery, and water and air quality monitoring. Theadvancement of nanoscale science relies on a better understanding of theory, manufacturing andfabrication practices, and the application specific methods. The topology and tunable properties ofnanoparticles, a part of nanoscale science, can be changed by different manufacturing processes,which separate them from their bulk counterparts. In the recent past, different nanostructures, suchas nanosphere, nanorods, nanofiber, core–shell nanoparticles, nanotubes, and thin films, have beenexploited to enhance the detectability of labelled or label-free biological molecules with a high ac-curacy. Furthermore, these engineered-materials-associated transducing devices, e.g., optical wave-guides and metasurface-based scattering media, widened the horizon of biosensors over a broadwavelength range from deep-ultraviolet to far-infrared. This review provides a comprehensiveoverview of the major scientific achievements in nano-biosensors based on optical fiber, nano-materials and terahertz-domain metasurface-based refractometric, labelled and label-free nano-bi-osensor