Fabrication des films microstructurés et leurs caractéristiques en spectroscopie de résonance des plasmons de surface

Cette thèse caractérise les propriétés optiques des matériaux plasmoniques microstructurés et procède à l’évaluation des paramètres analytiques afin de les employer comme plateforme de biodétection en spectroscopie de résonance des plasmons de surface (SPR). Aux dimensions micrométriques, les matériaux plasmoniques présentent des caractéristiques optiques propres aux nano- et macromatériaux. La cartographie physicooptiques en SPR de matériaux méso- et microscopiques s’est effectuée à l’aide de films structurés de motifs périodiques triangulaires et circulaires fabriqués par une technique modifiée de lithographie par nanosphères (nanosphere lithography, NSL). À partir de cette vue d’ensemble, quelques films structurés ont été sélectionné en fonction d’aspects analytiques tels que la sensibilité et la résolution face aux variations d’indice de réfraction (RI) pour déterminer le potentiel de ces matériaux comme plateforme de biodetection. Les propriétés optiques distinctes des films microstructurés proviennent d’interactions résonantes entre les modes de plasmons de surface (SP) localisé et délocalisé identifiés par la relation de dispersion en SPR ainsi que l’imagerie Raman. Les conditions de résonance des modes SP dépendant de paramètres expérimentaux (λ, θ, η) tel qu’observés numériquement par rigorous coupled wave analysis (RCWA) et empiriquement. Ces travaux démontrent la nature plasmonique distincte des micro-matériaux et leur potentiel d’intégration aux techniques analytiques SPR existantes. Les matériaux plasmoniques micrométriques furent également étudiés pour l’implémentation de la SPR à une pointe de microscopie à force atomique (atomic force microscopy, AFM) combinant ainsi la spectroscopie à l’imagerie topographique. Des travaux préliminaires se sont concentrés sur la signature spectroscopique de leviers en silicium (Si) et en nitrure de silicium (Si3N4), l’impact d’un revêtement d’or sur les pointes et l’influence de milieu environnant. Une image d’origine plasmonique a été obtenue avec des leviers en Si3N4 revêtus d’or en transmission dans un environnement aqueux, indiquant ainsi le potentiel de ces pointes comme micro-biocapteur SPR. Ces résultats préliminaires servent de fondement pour orienter les prochaines investigations dans ce projet.

Étude des propriétés plasmoniques des réseaux de nanotrous

Les réseaux de nanotrous sont des structures plasmoniques ayant un énorme potentiel en tant que transducteurs pour la conception de biocapteurs. De telles structures sont prometteuses pour l’élaboration de biocapteurs capable d’effectuer du criblage à haut débit. L’intérêt de travailler avec des réseaux de nanotrous est dû à la simplicité d’excitation des polaritons de plasmons de surface en transmission directe, à la sensibilité et à la facilité de fabrication de ces senseurs. L’architecture de tels réseaux métalliques permet la conception de nanostructures ayant de multiples propriétés plasmoniques. L’intensité, la signature spectrale et la sensibilité du signal plasmonique sont grandement affectées par l’aspect physique du réseau de nanotrous. L’optimisation du signal plasmonique nécessite ainsi un ajustement du diamètre des trous, de la périodicité et de la composition métallique du réseau. L'agencement de l'ensemble de ces paramètres permet d'identifier une structure optimale possédant une périodicité de 1000 nm, un diamètre des nanotrous de 600-650 nm et un film métallique ayant une épaisseur de 125 nm d'or. Ce type de transducteur a une sensibilité en solution de 500-600 nm/RIU pour des bandes plasmoniques situées entre 600-700 nm. L'intérêt de travailler avec cette structure est la possibilité d'exciter les plasmons de polaritons de surface (SPPs) selon deux modes d'excitation : en transmission exaltée (EOT) ou en réflexion totale interne par résonance des plasmons de surface (SPR). Une comparaison entre les propriétés plasmoniques des senseurs selon les modes d'excitation permet de déterminer expérimentalement que le couplage de la lumière avec les ondes de SPP de Bloch (BW-SPPs) en transmission directe résulte en un champ électromagnétique davantage propagatif que localisé. D'un point de vue analytique, la biodétection de l'IgG en SPR est 6 fois plus sensible par rapport au mode EOT pour une même structure. Une étude du signal plasmonique associé au BW-SPP pour un certain mode de diffraction démontre que la distance de pénétration de ces structures en EOT est d'environ 140 nm. La limite de détection de l'IgG humain pour un réseau de nanotrous de 1000 nm de périodicité est d'environ 50 nM en EOT. Ce mémoire démontre la viabilité des réseaux de nanotrous pour effectuer de la biodétection par criblage à haut débit lors de prochaines recherches. L'investigation de l'effet de l'angle d'excitation en transmission exaltée par rapport au signal plasmonique associé au mode (1,0) d'un réseau de nanotrous de 820 nm d'or démontre que la sensibilité en solution n'est pas proportionnelle à la sensibilité en surface du senseur. En fait, une optimisation de l'angle d'incidence pour le mode (1,0) de diffraction des BW-SPP permet d'amplifier la sensibilité en surface du senseur jusqu'à 3-fois pour un angle de 13,3°. Ce mémoire démontre ainsi la nécessité d'optimiser l'angle d'excitation et les propriétés physiques du senseur afin de développer un transducteur de grande sensibilité basé sur l'excitation en transmission de réseaux de nanotrous.

Plasma Polymerised Organic Thin Films-A Study on the Structural, Electrical and Nonlinear Optical Properties for Possible Applications

This proposed thesis is entitled “Plasma Polymerised Organic Thin Films: A study on the Structural, Electrical, and Nonlinear Optical Properties for Possible Applications. Polymers and polymer based materials find enormous applications in the realm of electronics and optoelectronics. They are employed as both active and passive components in making various devices. Enormous research activities are going on in this area for the last three decades or so, and many useful contributions are made quite accidentally. Conducting polymers is such a discovery, and eversince the discovery of conducting polyacetylene, a new branch of science itself has emerged in the form of synthetic metals. Conducting polymers are useful materials for many applications like polymer displays, high density data storage, polymer FETs, polymer LEDs, photo voltaic devices and electrochemical cells. With the emergence of molecular electronics and its potential in finding useful applications, organic thin films are receiving an unusual attention by scientists and engineers alike. This is evident from the vast literature pertaining to this field appearing in various journals. Recently, computer aided design of organic molecules have added further impetus to the ongoing research activities in this area. Polymers, especially, conducting polymers can be prepared both in the bulk and in the thinfilm form. However, many applications necessitate that they are grown in the thin film form either as free standing or on appropriate substrates. As far as their bulk counterparts are concerned, they can be prepared by various polymerisation techniques such as chemical routes and electrochemical means. A survey of the literature reveals that polymers like polyaniline, polypyrrole, polythiophene, have been investigated with a view to studying their structural electrical and optical properties. Among the various alternate techniques employed for the preparation of polymer thin films, the method of plasma polymerisation needs special attention in this context. The technique of plasma polymerisation is an inexpensive method and often requires very less infra structure. This method includes the employment of ac, rf, dc, microwave and pulsed sources. They produce pinhole free homogeneous films on appropriate substrates under controlled conditions. In conventional plasma polymerisation set up, the monomer is fed into an evacuated chamber and an ac/rf/dc/ w/pulsed discharge is created which enables the monomer species to dissociate, leading to the formation of polymer thin films. However, it has been found that the structure and hence the properties exhibited by plasma polymerized thin films are quite different from that of their counterparts produced by other thin film preparation techniques such as electrochemical deposition or spin coating. The properties of these thin films can be tuned only if the interrelationship between the structure and other properties are understood from a fundamental point of view. So very often, a through evaluation of the various properties is a pre-requisite for tailoring the properties of the thin films for applications. It has been found that conjugation is a necessary condition for enhancing the conductivity of polymer thin films. RF technique of plasma polymerisation is an excellent tool to induce conjugation and this modifies the electrical properties too. Both oxidative and reductive doping can be employed to modify the electrical properties of the polymer thin films for various applications. This is where organic thin films based on polymers scored over inorganic thin films, where in large area devices can be fabricated with organic semiconductors which is difficult to achieve by inorganic materials. For such applications, a variety of polymers have been synthesized such as polyaniline, polythiophene, polypyrrole etc. There are newer polymers added to this family every now and then. There are many virgin areas where plasma polymers are yet to make a foray namely low-k dielectrics or as potential nonlinear optical materials such as optical limiters. There are also many materials which are not been prepared by the method of plasma polymerisation. Some of the materials which are not been dealt with are phenyl hydrazine and tea tree oil. The advantage of employing organic extracts like tea tree oil monomers as precursors for making plasma polymers is that there can be value addition to the already existing uses and possibility exists in converting them to electronic grade materials, especially semiconductors and optically active materials for photonic applications. One of the major motivations of this study is to synthesize plasma polymer thin films based on aniline, phenyl hydrazine, pyrrole, tea tree oil and eucalyptus oil by employing both rf and ac plasma polymerisation techniques. This will be carried out with the objective of growing thin films on various substrates such as glass, quartz and indium tin oxide (ITO) coated glass. There are various properties namely structural, electrical, dielectric permittivity, nonlinear optical properties which are to be evaluated to establish the relationship with the structure and the other properties. Special emphasis will be laid in evaluating the optical parameters like refractive index (n), extinction coefficient (k), the real and imaginary components of dielectric constant and the optical transition energies of the polymer thin films from the spectroscopic ellipsometric studies. Apart from evaluating these physical constants, it is also possible to predict whether a material exhibit nonlinear optical properties by ellipsometric investigations. So further studies using open aperture z-scan technique in order to evaluate the nonlinear optical properties of a few selected samples which are potential nonlinear optical materials is another objective of the present study. It will be another endeavour to offer an appropriate explanation for the nonlinear optical properties displayed by these films. Doping of plasma polymers is found to modify both the electrical conductivity and optical properties. Iodine is found to modify the properties of the polymer thin films. However insitu iodine doping is tricky and the film often looses its stability because of the escape of iodine. An appropriate insitu technique of doping will be developed to dope iodine in to the plasma polymerized thin films. Doping of polymer thin films with iodine results in improved and modified optical and electrical properties. However it requires tools like FTIR and UV-Vis-NIR spectroscopy to elucidate the structural and optical modifications imparted to the polymer films. This will be attempted here to establish the role of iodine in the modification of the properties exhibited by the films

Optical and electrical properties of co-sputtered amorphous transparent conducting zinc indium tin oxide thin films

Highly conductive and transparent thin films of amorphous zinc indium tin oxide are prepared at room temperature by co-sputtering of zinc 10 oxide and indium tin oxide. Cationic contents in the films are varied by adjusting the power to the sputtering targets. Optical transmission study of 11 films showed an average transmission greater than 85% across the visible region. Maximum conductivity of 6×102 S cm−1 is obtained for Zn/In/ 12 Sn atomic ratio 0.4/0.4/0.2 in the film. Hall mobility strongly depends on carrier concentration and maximum mobility obtained is 18 cm2 V−1 s−1 13 at a carrier concentration of 2.1×1020 cm−3. Optical band gap of films varied from 3.44 eV to 3 eV with the increase of zinc content in the film 14 while the refractive index of the films at 600 nm is about 2.0.

An analytical study of the power distribution pattterns of silica waveguides with high paraboloic cylindrical deformation

An attempt is made to determine the relative power distribution in a step-index parabolic cylindrical waveguide (PCW) with high deformation across the direction of propagation. The guide is assumed to be made of silica. The scalar field approximation is employed for the analysis under which a vanishing refractive-index (RI) difference in the waveguide materials is considered. Further, no approximation for folds- is used in the analytical treatment. Due to the geometry of such waceguides, PCWs lose the well-defined modal discreteness, and a kind of mode bunching is observed instead, which becomes much more prominent in PCWs with high bends. However, with the increase in cross-sectional size, the mode-bunching tendency is slightly reduced. The general expressions for power in the guiding and nonguiding sections are obtained, and the fractional power patterns in all of the sections are presented for PCWs of various cross-sectional dimensions. It is observed that the confinement of power in the core section is increased for PCWs of larger cross-sectional size. Moreover, a fairly uniform distribution of power is seen over the modes having intermediate values of propagation constants

Power characteristics of dielectric optical piet hein waveguides

A fairly rigorous analytical treatment of the power characteristics of dielectric optical waveguides with Piet Hein core-cross sectional geometry is presented in this paper. This kind of wareguide structure would be advantageous owing to the absence of corners, which are found in rectangular guides, resulting in undesirable loss (hit to the scattering of light. In order to simplify this theoretical approach. em approximation of vanishing refractive index difference between the guiding and the non-guiding sections is implemented. The variation eJ logarithmic power is shown for different dimensions of the core, corresponding to different azimuthal modal indices. It is found that the nutlet with higher index values carry less logaritlunic power in the lower tail of the propagation 's constant range, and this feature affects the higher tail. A better kind of uniformity of the power distribution is observed near the higher tail of the range of propagation Constants

Luminescence tuning and enhanced nonlinear optical properties of nanocomposites of ZnO–TiO2

In this article we present the spectral and nonlinear optical properties of ZnO–TiO2 nanocomposites prepared by colloidal chemical synthesis. Emission peaks of ZnO–TiO2 nanocomposites change from 340 nm to 385 nm almost in proportion to changes in Eg. The nanocomposites show self-defocusing nonlinearity and good nonlinear absorption behaviour. The nonlinear refractive index and the nonlinear absorption increase with increasing TiO2 volume fraction at 532 nm and can be attributed to the enhancement of exciton oscillator strength. ZnO–TiO2 is a potential nanocomposite material for the tunable light emission and for the development of nonlinear optical devices with a relatively small limiting threshold

Physical and optical properties of phthalocyanine doped inorganic glasses

Physical and optical properties of various free base and metallic phthalocyanine (Pc) doped glass matrix are reported for the first time. Absorption spectral measurements of H2Pc, MnPc, NiPc, CoPc, CuPc, MoOPc, ZnPc and FePc doped borate glass matrix have been made in the 200–1100 nm region and the spectra obtained are analyzed in the 2.1–6.2 eV region to obtain the optical band gap (Eg) and the width of the band tail (Et). Other important optical and physical parameters viz. refractive index (n), molar extinction coefficient ("), density (½), glass transition temperature (Tg), molecular concentration (N ), polaron radius (rp), intermolecular separation (R), molar refractivity (Rm) are also reported

Physical and optical properties of phthalocyanine doped inorganic glasses

Physical and optical properties of various free base and metallic phthalocyanine (Pc) doped glass matrix are reported for the first time. Absorption spectral measurements of H2Pc, MnPc, NiPc, CoPc, CuPc, MoOPc, ZnPc and FePc doped borate glass matrix have been made in the 200–1100 nm region and the spectra obtained are analyzed in the 2.1–6.2 eV region to obtain the optical band gap (Eg) and the width of the band tail (Et). Other important optical and physical parameters viz. refractive index (n), molar extinction coefficient ("), density (½), glass transition temperature (Tg), molecular concentration (N ), polaron radius (rp), intermolecular separation (R), molar refractivity (Rm) are also reported.

Physical and optical properties of phthalocyanine doped inorganic glasses

Physical and optical properties of various free base and metallic phthalocyanine (Pc) doped glass matrix are reported for the first time. Absorption spectral measurements of H2Pc, MnPc, NiPc, CoPc, CuPc, MoOPc, ZnPc and FePc doped borate glass matrix have been made in the 200–1100 nm region and the spectra obtained are analyzed in the 2.1–6.2 eV region to obtain the optical band gap (Eg) and the width of the band tail (Et). Other important optical and physical parameters viz. refractive index (n), molar extinction coefficient ("), density (½), glass transition temperature (Tg), molecular concentration (N ), polaron radius (rp), intermolecular separation (R), molar refractivity (Rm) are also reported.

Spectral and Nonlinear Optical Characteristics of ZnO Nanocomposites

The spectral and nonlinear optical properties of ZnO based nanocomposites prepared by colloidal chemical synthesis are investigated. Very strong UV emissions are observed from ZnO–Ag, ZnO– Cu and ZnO–SiO2 nanocomposites. The strongest visible emission of a typical ZnO–Cu nanocomposite is over ten times stronger than that of pure Cu due to transition from deep donor level to the copper induced level. The optical band gap of ZnO–CdS and ZnO–TiO2 nanocomposites is tunable and emission peaks changes almost in proportion to changes in band gap. Nonlinear optical response of these nanocomposites is studied using nanosecond laser pulses from a tunable laser in the wavelength range of 450–650 nm at resonance and off-resonance wavelengths. The nonlinear response is wavelength dependent and switching from RSA to SA has been observed at resonant wavelengths. Such a change-over is related to the interplay of plasmon/exciton band bleach and optical limiting mechanisms. The observed nonlinear absorption is explained through two photon absorption followed by weak free carrier absoption, interband absorption and nonlinear scattering mechanisms. The nonlinearity of the silica colloid is low and its nonlinear response can be improved by making composites with ZnO and ZnO–TiO2. The increase of the third-order nonlinearity in the composites can be attributed to the enhancement of exciton oscillator strength. This study is important in identifying the spectral range and the composition over which the nonlinear material acts as an RSA based optical limiter. These nanocomposites can be used as optical limiters and are potential materials for the light emission and for the development of nonlinear optical devices with a relatively small limiting threshold.

Spectral and Nonlinear Optical Characteristics of ZnO Nanocomposites

The spectral and nonlinear optical properties of ZnO based nanocomposites prepared by colloidal chemical synthesis are investigated. Very strong UV emissions are observed from ZnO–Ag, ZnO– Cu and ZnO–SiO2 nanocomposites. The strongest visible emission of a typical ZnO–Cu nanocomposite is over ten times stronger than that of pure Cu due to transition from deep donor level to the copper induced level. The optical band gap of ZnO–CdS and ZnO–TiO2 nanocomposites is tunable and emission peaks changes almost in proportion to changes in band gap. Nonlinear optical response of these nanocomposites is studied using nanosecond laser pulses from a tunable laser in the wavelength range of 450–650 nm at resonance and off-resonance wavelengths. The nonlinear response is wavelength dependent and switching from RSA to SA has been observed at resonant wavelengths. Such a change-over is related to the interplay of plasmon/exciton band bleach and optical limiting mechanisms. The observed nonlinear absorption is explained through two photon absorption followed by weak free carrier absoption, interband absorption and nonlinear scattering mechanisms. The nonlinearity of the silica colloid is low and its nonlinear response can be improved by making composites with ZnO and ZnO–TiO2. The increase of the third-order nonlinearity in the composites can be attributed to the enhancement of exciton oscillator strength. This study is important in identifying the spectral range and the composition over which the nonlinear material acts as an RSA based optical limiter. These nanocomposites can be used as optical limiters and are potential materials for the light emission and for the development of nonlinear optical devices with a relatively small limiting threshold.

Comparative Performance of Two Fiber Optic Ammonia Sensors Employing Different Sensing Materials

The design and fabrication of fiber based ammonia sensors employing Bromothymol blue and Chitosan as sensing elements are presented in this paper. In the presence of ammonia gas the absorption of Bromothymol blue changes while in the case of Chitosan the refractive index changes which in turn modulates the intensity of light propagating through a fiber.

Nonlinear optical characteristics of self-assembled films of ZnO

In the present work, we have investigated the nonlinear optical properties of self-assembled films formed from ZnO colloidal spheres by z-scan technique. The sign of the nonlinear component of refractive index of the material remains the same; however, a switching from reverse saturable absorption to saturable absorption has been observed as the material changes from colloid to self-assembled film. These different nonlinear characteristics can be mainly attributed to ZnO defect states and electronic effects when the colloidal solution is transformed into self-assembled monolayers. We investigated the intensity, wavelength and size dependence of saturable and reverse saturable absorption of ZnO self-assembled films and colloids. Values of the imaginary part of third-order susceptibility are calculated for particles of size in the range 20–300 nm at different intensity levels ranging from 40 to 325MW/cm2 within the wavelength range of 450–650 nm.

Spectral and nonlinear optical characteristics of nanocomposites of ZnO-CdS

In this article, we present the spectral and nonlinear optical properties of ZnO–CdS nanocomposites prepared by colloidal chemical synthesis. The optical band gap (Eg) of the material is tunable between 2.62 and 3.84 eV. The emission peaks of ZnO–CdS nanocomposites change from 385 to 520 nm almost in proportion to changes in Eg. It is possible to obtain a desired luminescence color from UV to green by simply adjusting the composition. The nonlinear optical response of these samples is studied by using nanosecond laser pulses from a tunable laser at the excitonic resonance and off-resonance wavelengths. The nonlinear response is wavelength dependent, and switching from saturable absorption (SA) to reverse SA (RSA) has been observed for samples as the excitation wavelength changes from the excitonic resonance to off-resonance wavelengths. Such a changeover in the sign of the nonlinearity of ZnO–CdS nanocomposites is related to the interplay of exciton bleach and optical limiting mechanisms. The ZnO–CdS nanocomposites show self-defocusing nonlinearity and good nonlinear absorption behavior at off-resonant wavelengths. The nonlinear refractive index and the nonlinear absorption increase with increasing CdS volume fraction at 532 nm. The observed nonlinear absorption is attributed to two photon absorption followed by weak free carrier absorption. The enhancement of the third-order nonlinearity in the composites can be attributed to the concentration of exciton oscillator strength. This study is important in identifying the spectral range and composition over which the nonlinear material acts as a RSA based optical limiter. ZnO–CdS is a potential nanocomposite material for the tunable light emission and for the development of nonlinear optical devices with a relatively small limiting threshold.