Fabrication of Optical Nanofibre-Based Cavities using Focussed Ion-Beam Milling -- A Review

Nanofibre-based optical cavities are particularly useful for quantum optics application, such as the development of integrated single-photon sources, and for studying fundamental light-matter interactions in cavity quantum electrodynamics (cQED). Although several techniques have been used to produce nanofibre-based optical cavities, focussed ion beam (FIB) milling is becoming popular; it can be used for the fabrication of complex structures directly in the nanofibre. This technique uses a highly accelerated ion beam to remove atoms from the target material with high resolution. However, it is challenging to mill insulating materials with highly-curved structures and large aspect ratios, such as silica nanofibres, due to charge accumulation in the material that leads to mechanical vibrations and misalignment issues. In this article, we highlight the main features of nanofibres and briefly review cQED with nanofibre-based optical cavities. An overview of the milling process is given with a summary of different FIB milled devices and their applications. Finally, we present our technique to produce nanofibre cavities by FIB milling. To overcome the aforementioned challenges, we present a specially designed base plate with an indium tin oxide (ITO)-coated Si substrate and outline our procedure, which improves stability during milling and increases repeatability.Comment: The manuscript has 16 pages, 9 figures, and 3 tables, and it will be submitted to Applied Physics B: Laser and Optic

Optofluidic waveguide as a transformation optics device for lightwave bending and manipulation

Author name used in this publication: Zhang X. M.2011-2012 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

Self-sensing composites: cure monitoring

The ability to monitor the cure kinetics at the fibre-matrix interface and the effects induced by the presence of silane coupling agents was the main focus of this study. E-glass reinforcing fibres were used as chemical sensors to eliminate the potential problems associated with implanting a foreign body into a composite structure in the form of an optical fibre. This study demonstrated conclusively that E-glass reinforcing fibres can be used to monitor the cure of an epoxy/amine resin system in-situ; the data generated via evanescent wave spectroscopy (EWS) using the reinforcing E-glass fibres showed excellent correlation with conventional transmission Fourier transform infrared spectroscopy over a range of temperatures. EWS via E-glass fibres was used to study the effects of the following coupling agents on the cure behaviour: (i) 3-glycidyloxypropyltrimethoxysilane (GPS); and (ii) 3-aminopropyltriethoxysilane (APS). The epoxy and amine functional groups in the resin system were affected by the presence of the silane coupling agents during cure. Increased GPS concentration induced faster rates of epoxy conversion, but reduced the final degree of epoxy conversion. The amine functional groups of the curing resin were converted at faster rates with APS treatment, but the final degree of amine conversion was reduced

Desenvolvimento e otimização de sensores em fibra ótica produzidos por laser de femtosegundo

In this work, optical fibre sensors were developed and optimized using a pulsed femtosecond laser. In addition to the inherent advantages of using femtosecond pulses, by emitting radiation in the NIR band, it was possible to modify the refractive index inside dielectric materials, namely silica and polymer optical fibres. Prior to the manufacturing of optical structures, a theoretical study was carried out on the peculiarities of writing-systems based on femtosecond lasers, as well as on the most common devices inscribed in optical fibres, namely Bragg gratings, long period gratings, and Fabry-Pérot interferometers. After assembling femtosecond NIR laser system, Bragg gratings, long period gratings, Fabry-Pérot interferometers, and interferometers based on the optical Vernier effect were manufactured using the direct-writing and phase mask methods. Using the micromachining setup, different structures were created in already existing optical fibre sensors, namely channels in hollow Fabry-Pérot cavities and laser etching around Bragg gratings inscribed in polymers optical fibres. The spectral responses of all devices were extensively characterized to, mainly, variations of temperature and strain, revealing unique sensitivity values, especially for the interferometers based on the optical Vernier effect (> 1 nm/°C and 0.1 nm/µε for temperature and strain, respectively). To demystify the thermal stability of fibre Bragg gratings, a theoretical and experimental study was carried out where several Bragg gratings were inscribed by different techniques, involving different lasers as well as silica and polymer optical fibres. The experimental results corroborated the theoretical predictions, where it was concluded that the gratings inscribed by the point-to-point method using a femtosecond laser have a greater thermal stability and lifetime, even when subjected to longer and higher temperature regimes. Finally, a bridge was stablished between the fundamental research developed during the manufacture of the elementary optical fibre sensors, and possible applications. Five different sensor concepts were demonstrated and tested, capable of detecting variations in magnetic fields, fluids refractive index, temperature, strain and humidity. As results, astonishing sensitivity values were attained, and several cross-sensitivity problems were mitigated, thus establishing the foundations for the development of new prototypes for the future.Neste trabalho foram desenvolvidos e otimizados sensores em fibra ótica através de um laser pulsado de femtosegundo. Para além das vantagens inerentes de usar pulsos da ordem do femtosegundo, ao emitir radiação na banda do infravermelho foi possível modificar o índice de refração no interior de materiais dielétricos, nomeadamente fibras óticas de sílica e polímero. Antes de proceder ao fabrico das estruturas óticas, foi realizado um estudo teórico sobre as peculiaridades dos sistemas de escrita baseados em lasers de femtosegundo, bem como sobre os principais dispositivos inscritos em fibra ótica, nomeadamente redes de Bragg, redes de período longo, e interferómetros de Fabry-Pérot. Após montado o sistema laser NIR de femtosegundo, através de inscrição direta e por máscara de fase foram fabricadas redes de Bragg, redes de período longo, interferómetros de Fabry-Pérot, e interferómetros baseados no efeito ótico de Vernier. Com a montagem de micromaquinação, diferentes estruturas foram criadas em sensores já existentes, nomeadamente buracos em cavidades Fabry-Pérot e remoção de material ao redor de redes de Bragg. As respostas espetrais de todos os dispositivos foram extensivamente caracterizadas, nomeadamente a variações de temperatura e tensão, revelando elevados valores de sensibilidades, especialmente para os interferómetros baseados no efeito ótico de Vernier (> 1 nm/°C e 0.1 nm/µε para temeprature e tensão, respetivamente). Para desmistificar a estabilidade térmica de redes de Bragg em fibra ótica, foi feito um estudo teórico e experimental onde várias redes de Bragg foram gravadas por diferentes técnicas, envolvendo diferentes lasers e fibras óticas de sílica e polímero. Os resultados experimentais corroboraram as previsões teóricas, onde se concluiu que as redes gravadas pelo método de ponto-a-ponto usando um laser de femtosegundo detêm uma maior estabilidade térmica e tempo de vida, mesmo quando submetidas a regimes longos de altas temperaturas. Por fim, foi feita a ponte entre a investigação fundamental desenvolvida durante o fabrico de dispositivos elementares em fibras óticas e possíveis aplicações. Foram demonstrados e testados cinco conceitos diferentes de sensores, capazes de detetar variações de campos magnéticos, índice de refração de fluídos, temperatura, tensão e humidade. Foram atingidos valores de sensibilidade surpreendentes, bem como mitigados problemas de sensibilidade cruzada, tendo sido assim estabelecidas as fundações para o desenvolvimento de novos protótipos para o futuro.Programa Doutoral em Engenharia Físic

Optical fibre sensors with applications in gas and biological sensing

This thesis describes the study of various grating based optical fibre sensors for applications in refractive index sensing. The sensitivity of these sensors has been studied and in some cases enhanced using novel techniques. The major areas of development are as follows. The sensitivity of long period gratings (LPGs) to surrounding medium refractive index (SRI) for various periods was investigated. The most sensitive period of LPG was found to be around 160 µm and this was due to the core mode coupling to a single cladding mode but phase matching at two wavelength locations, creating two attenuation peaks, close to the waveguide dispersion turning point. Large angle tilted fibre gratings (TFGs) have similar behaviour to LPGs, in that they couple to the co-propagating cladding modes. The tilted structure of the index modulation within the core of the fibre gives rise to a polarisation dependency, differing the large angle TFG from a LPG. Since the large angle TFG couple to the cladding mode they are SRI sensitive, the sensitivity to SRI can be further increased through cladding etching using HF acid. The thinning of the cladding layer caused a reordering of the cladding modes and shifted to more SRI sensitive cladding modes as the investigation discovered. In a SRI range of 1.36 to 1.40 a sensitivity of 506.9 nm/URI was achieved for the etched large angle TFG, which is greater than the dual resonance LPG. UV inscribed LPGs were coated with sol-gel materials with high RIs. The high RI of the coating caused an increase in cladding mode effective index which in turn caused an increase in the LPG sensitivity to SRI. LPGs of various periods of LPG were coated with sol-gel TiO2 and the optimal thickness was found to vary for each period. By coating of the already highly SRI sensitive 160µm period LPG (which is a dual resonance) with a sol-gel TiO2, the SRI sensitivity was further increased with a peak value of 1458 nm/URI, which was an almost 3 fold increase compared to the uncoated LPG. LPGs were also inscribed using a femtosecond laser which produced a highly focused index change which was no uniform throughout the core of the optical fibre. The inscription technique gave rise to a large polarisation sensitivity and the ability to couple to multiple azimuthal cladding mode sets, not seen with uniform UV inscribed gratings. Through coupling of the core mode to multiple sets of cladding modes, attenuation peaks with opposite wavelength shifts for increasing SRI was observed. Through combining this opposite wavelength shifts, a SRI sensitivity was achieved greater than any single observed attenuations peak. The maximum SRI achieved was 1680 nm/URI for a femtosecond inscribed LPG of period 400 µm. Three different types of surface plasmon resonance (SPR) sensors with a multilayer metal top coating were investigated in D shape optical fibre. The sensors could be separated into two types, utilized a pre UV inscribed tilted Bragg grating and the other employed a post UV exposure to generate surface relief grating structure. This surface perturbation aided the out coupling of light from the core but also changed the sensing mechanism from SPR to localised surface plasmon resonance (LSPR). This greatly increased the SRI sensitivity, compared to the SPR sensors; with the gold coated top layer surface relief sensor producing the largest SRI sensitivity of 2111.5nm/URI was achieved. While, the platinum and silver coated top layer surface relief sensors also gave high SRI sensitivities but also the ability to produce resonances in air (not previously seen with the SPR sensors). These properties were employed in two applications. The silver and platinum surface relief devices were used as gas sensors and were shown to be capable of detecting the minute RI change of different gases. The calculated maximum sensitivities produced were 1882.1dB/URI and 1493.5nm/URI for silver and platinum, respectively. Using a DFB laser and power meter a cheap alternative approach was investigated which showed the ability of the sensors to distinguish between different gases and flow rates of those gases. The gold surface relief sensor was coated in a with a bio compound called an aptamer and it was able to detect various concentrations of a biological compound called Thrombin, ranging from 1mM to as low as 10fM. A solution of 2M NaCl was found to give the best stripping results for Thrombin from the aptamer and showed the reusability of the sensor. The association and disassociation constants were calculated to be 1.0638×106Ms-1 and 0.2482s-1, respectively, showing the high affinity of the Aptamer to thrombin. This supports existing working stating that aptamers could be alternative to enzymes for chemical detection and also helps to explain the low detection limit of the gold surface relief sensor

Investigation of the colorimetric measurement of pH and metal ions by using reagents doped in sol-gel glasses for potential on-line monitoring

A thesis submitted to the University of Luton for the degree of Doctor of PhilosophySol-gel porous glasses have been doped with indicator molecules for colorimetric measurements of pH and metal ions in solution. pH measurements were made in real time (20 seconds) to a wide pH range 3-8 by using bromophenol blue doped in sol-gel thin films. New methods of sol-gel coating on the inside of test tubes and tubing have been introduced for simple, non invasive, on the spot chemical sensing. pH indicator doped films were also successfully autoclaved for biological applications without affecting their chemical and physical properties. The pore structure of thin films has been controlled for minimising the effect of ageing on response time by introducing dimethyl formamide (DMF). The effect of light, temperature and salt on thin films have been studied. The results show that they are relatively stable between 20-31' C and less affected « 0.03 absorbance unit decrease in 3 weeks) by light. However their response to pH is changed by adding salt in solution with concentration higher than O.OlM. Fourier transform infra red (FTIR) study of films has been conducted to elucidate the effect of ageing, DMF and autoclaving on their chemical structures. It was found that ageing continues after four weeks of fabrication and addition of DMF helps to reduce ageing and increase porosity. The long term stability of these pH indicator doped films in various solvents has been established. Thin films on microscope slides were deposited by using a newly designed spin coater and have been demonstrated as reusable pH slides. Sol-gel films were also doped by different metal reagents. Eriochrome cyanine doped thin films were found to be sensitive to copper ions in solution. Copper (Cu++) was measured to a low concentration of 0.6 ppm. The effect of light and temperature on Eriochrome copper complex was studied. Interferences of other metal ions were examined. A fibre-optic pH sensor has been demon.strated by coating an optical fibre with a sol-gel film (0.8 J.1m thick) doped with bromophenol blue. The sensor has shown fast response (5 seconds) to pH changes from pH 3 to 8 and no leaching or cracking during repeated use. It is simple to fabricate and easy to use as an interchangeable pH fibre probe. It has potential application in biological processes as an integral part of an online monitoring system

Investigations on the development of a novel hybrid sensor for environmental monitoring applications.

Heavy metal toxicity is a major environmental problem world-wide. Increased spreading and high concentration levels of the toxic heavy metals in water environments have posed a severe threat to human health and the ecosystem. Over the years, to improve the drinking water quality standards, safe threshold concentrations of these highly toxic pollutants are constantly being lowered by the governmental and environmental bodies. Current instrumental techniques used to detect these low levels of heavy metal ions are laboratory based, use sophisticated instruments, expensive, time consuming and require trained personnel. There is a constant need for the development of in-situ, rapid, highly sensitive and selective sensors to monitor the very low concentration levels. Various approaches for improving sensitivity and selectivity have been investigated over the years involving multiple detection techniques. In general, optical approaches provide higher sensitivity along with simplicity while electrochemical sensors provide better selectivity. In the last decade, nanomaterials have emerged as a key element in their sensitivity improvement. Combining all these advantages, a novel hybrid sensor has been envisaged integrating optical and electrical fields in addition to nanomaterials. This thesis reports investigations on enhancing the sensitivity/selectivity through optical, nanomaterials and electrochemical routes, and then integrating these to realise a hybrid sensor. A novel optical sensor has been developed using the phenomena of evanescent waves in optical fibre with dithizone to detect heavy metal ions. A U-bent sensor geometry has been investigated to enhance the optical sensitivity of the sensor through higher evanescent field near the surface. Further, optical field confinement to the surface has been investigated through thin film coating to improve the sensitivity. A new inverted trench design based sensor has been developed, and sensitivity enhancement has been achieved through this novel design and confirmed using modelling work accompanied by experimental results. Large surface to volume ratio of nanomaterials, such as ZnO nanowires, on the sensor surface can provide enhanced surface interactions leading to higher sensitivity. But, surfaces modified with ZnO nanostructures tend to be hydrophobic in nature. A new remote and non-contact method to tune the wettability of the ZnO nanostructures using LEDs has been developed. Higher sensitivity has been achieved by tuning the wettability of ZnO nanowires using the developed method. An electrochemical sensor has been developed in order to understand the potential effects of the electric field on the near surface molecular dynamics and thereby, effects on the optical detection. Effects of parameters such as deposition time, scan frequency, concentration, electrode materials and their surface area have been investigated to improve the sensitivity and selectivity. Multi-ions selectivity has been achieved by simultaneous detection of copper, mercury and lead ions in buffer solution. Higher sensitivity has been obtained by modifying the gold electrode using graphene flakes. Further, to integrate the optical field with this sensor to realize the hybrid sensor, thickness of the gold electrode has been optimised to allow the penetration of evanescent field onto the electrode surface. Under optimised conditions evanescent field resonantly couples to the surface plasmons of the gold electrode. Computational investigations have been carried out to study the effect of number of graphene layers on the sensitivity of the surface plasmon resonance (SPR) based optical sensor integrated with the electrochemical sensor. Preliminary investigations on the developed hybrid sensor show that the electric field complements the optical field. Investigations have shown that application of electric field enhances the sensitivity for optical detection by attracting more ions on the electrode and also, provides the multi-ion selectivity. These investigations have opened up new possibilities for the real-time monitoring of highly sensitive and selective molecular interactions, showing strong potential in a range of applications areas such as environmental sensing, biosensing and agricultural sensing

Characteristic and sensing properties of near- and mid-Infrared optical fibres

The work within this thesis investigates the characteristics and sensing properties of novel near- and mid-infrared tellurite and germanate glass fibres and their potential as sensing elements. An asymmetric splicing method for fusion-splicing tellurite and germanate glass fibres to standard silica fibre is demonstrated. The thermal and strain sensing properties of these glass fibres have been studied by analysing the properties of optical fibre Fabry-Perot cavities, which were formed when these high refractive index fibres were spliced to silica fibre, and fibre Bragg gratings. Using fibre F-P interferometer, the normalized thermal sensitivity of tellurite and germanate fibre was measured to be 10.76×10-6/°C and 15.56×10-6/°C respectively, and the normalized strain sensitivity of tellurite and germanate fibre was also measured with values of 0.676×10-6 /με and 0.817×10-6 /με respectively. These results show good agreement with measurements using fibre Bragg gratings in these fibres and are reasonably consistent with the values predicted using available published data for glasses of similar compositions. Tellurite and germanate glass fibres show potential as thermal sensing and load sensing elements compared with silica fibre. The design of an evanescent field gas sensor using tapered germanate fibre for methane gas species detection was investigated and modelled. This model shows the maximum gas cell length (sensing fibre length), detectable gas concentration range, and required gas cell length range for the expected minimum detectable gas concentration of a fibre evanescent field sensor, which gives guidance for the effective gas cell length choosen according to different minimum detectable gas concentration requirement in practise. The investigation of tellurite and germanate glass fibre characteristics and sensing properties offer guidance for their applications in sensing areas.Engineering and Physical Sciences Research Council (EPSRC