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

Contactless measurement of electric current using magnetic sensors

We review recent advances in magnetic sensors for DC/AC current transducers, especially novel AMR sensors and integrated fluxgates, and we make critical comparison of their properties. Most contactless electric current transducers use magnetic cores to concentrate the flux generated by the measured current and to shield the sensor against external magnetic fields. In order to achieve this, the magnetic core should be massive. We present coreless current transducers which are lightweight, linear and free of hysteresis and remanence. We also show how to suppress their weak point: crosstalk from external currents and magnetic fields

PCF Based Sensor with High Sensitivity, High Birefringence and Low Confinement Losses for Liquid Analyte Sensing Applications

In this paper, we report a design of high sensitivity Photonic Crystal Fiber (PCF) sensor with high birefringence and low confinement losses for liquid analyte sensing applications. The proposed PCF structures are designed with supplementary elliptical air holes in the core region vertically-shaped V-PCF and horizontally-shaped H-PCF. The full vectorial Finite Element Method (FEM) simulations performed to examine the sensitivity, the confinement losses, the effective refractive index and the modal birefringence features of the proposed elliptical air hole PCF structures. We show that the proposed PCF structures exhibit high relative sensitivity, high birefringence and low confinement losses simultaneously for various analytes

Topological engineering of interfacial optical Tamm states for highly-sensitive near-singular-phase optical detection

We developed planar multilayered photonic-plasmonic structures, which support topologically protected optical states on the interface between metal and dielectric materials, known as optical Tamm states. Coupling of incident light to the Tamm states can result in perfect absorption within one of several narrow frequency bands, which is accompanied by a singular behavior of the phase of electromagnetic field. In the case of near-perfect absorptance, very fast local variation of the phase can still be engineered. In this work, we theoretically and experimentally demonstrate how these drastic phase changes can improve sensitivity of optical sensors. A planar Tamm absorber was fabricated and used to demonstrate remote near-singular-phase temperature sensing with an over an order of magnitude improvement in sensor sensitivity and over two orders of magnitude improvement in the figure of merit over the standard approach of measuring shifts of resonant features in the reflectance spectra of the same absorber. Our experimentally demonstrated phase-to-amplitude detection sensitivity improvement nearly doubles that of state-of-the-art nano-patterned plasmonic singular-phase detectors, with further improvements possible via more precise fabrication. Tamm perfect absorbers form the basis for robust planar sensing platforms with tunable spectral characteristics, which do not rely on low-throughput nano-patterning techniques.Comment: 31 pages; 6 main text figures and 10 supplementary figure

MEG Upgrade Proposal

We propose the continuation of the MEG experiment to search for the charged lepton flavour violating decay (cLFV) \mu \to e \gamma, based on an upgrade of the experiment, which aims for a sensitivity enhancement of one order of magnitude compared to the final MEG result, down to the $6 \times 10^{-14}$ level. The key features of this new MEG upgrade are an increased rate capability of all detectors to enable running at the intensity frontier and improved energy, angular and timing resolutions, for both the positron and photon arms of the detector. On the positron-side a new low-mass, single volume, high granularity tracker is envisaged, in combination with a new highly segmented, fast timing counter array, to track positron from a thinner stopping target. The photon-arm, with the largest liquid xenon (LXe) detector in the world, totalling 900 l, will also be improved by increasing the granularity at the incident face, by replacing the current photomultiplier tubes (PMTs) with a larger number of smaller photosensors and optimizing the photosensor layout also on the lateral faces. A new DAQ scheme involving the implementation of a new combined readout board capable of integrating the diverse functions of digitization, trigger capability and splitter functionality into one condensed unit, is also under development. We describe here the status of the MEG experiment, the scientific merits of the upgrade and the experimental methods we plan to use.Comment: A. M. Baldini and T. Mori Spokespersons. Research proposal submitted to the Paul Scherrer Institute Research Committee for Particle Physics at the Ring Cyclotron. 131 Page

Surface-wave-enabled darkfield aperture for background suppression during weak signal detection

Sensitive optical signal detection can often be confounded by the presence of a significant background, and, as such, predetection background suppression is substantively important for weak signal detection. In this paper, we present a novel optical structure design, termed surface-wave-enabled darkfield aperture (SWEDA), which can be directly incorporated onto optical sensors to accomplish predetection background suppression. This SWEDA structure consists of a central hole and a set of groove pattern that channels incident light to the central hole via surface plasmon wave and surface-scattered wave coupling. We show that the surface wave component can mutually cancel the direct transmission component, resulting in near-zero net transmission under uniform normal incidence illumination. Here, we report the implementation of two SWEDA structures. The first structure, circular-groove-based SWEDA, is able to provide polarization-independent suppression of uniform illumination with a suppression factor of 1230. The second structure, linear-groove-based SWEDA, is able to provide a suppression factor of 5080 for transverse-magnetic wave and can serve as a highly compact (5.5 micrometer length) polarization sensor (the measured transmission ratio of two orthogonal polarizations is 6100). Because the exact destructive interference balance is highly delicate and can be easily disrupted by the nonuniformity of the localized light field or light field deviation from normal incidence, the SWEDA can therefore be used to suppress a bright background and allow for sensitive darkfield sensing and imaging (observed image contrast enhancement of 27 dB for the first SWEDA)

Cryogenic silicon detectors with implanted contacts for the detection of visible photons using the Neganov-Luke Effect

There is a common need in astroparticle experiments such as direct dark matter detection, 0{\nu}\b{eta}\b{eta} (double beta decay without emission of neutrinos) and Coherent Neutrino Nucleus Scattering experiments for light detectors with a very low energy threshold. By employing the Neganov-Luke Effect, the thermal signal of particle interactions in a semiconductor absorber operated at cryogenic temperatures, can be amplified by drifting the photogenerated electrons and holes in an electric field. This technology is not used in current experiments, in particular because of a reduction of the signal amplitude with time which is due to trapping of the charges within the absorber. We present here the first results of a novel type of Neganov-Luke Effect detector with an electric field configuration designed to improve the charge collection within the semiconductor.Comment: 6 pages, 5 figures, submitted to Journal of Low Temperature Physic

Self-adaptive loop for external disturbance reduction in differential measurement set-up

We present a method developed to actively compensate common-mode magnetic disturbances on a multi-sensor device devoted to differential measurements. The system uses a field-programmable-gated-array card, and operates in conjunction with a high sensitivity magnetometer: compensating the common-mode of magnetic disturbances results in a relevant reduction of the difference-mode noise. The digital nature of the compensation system allows for using a numerical approach aimed at automatically adapting the feedback loop filter response. A common mode disturbance attenuation exceeding 50 dB is achieved, resulting in a final improvement of the differential noise floor by a factor of 10 over the whole spectral interval of interest.Comment: 7 pages, 8 figures, 26 ref