Fiber-interface directional coupler inscribed by femtosecond laser for refractive index measurements

A novel fiber-interface directional waveguide coupler was inscribed on the surface of a coreless fiber by femtosecond laser, and was successfully applied to highly sensitive refractive index (RI) measurements. The primary arm was first inscribed to couple light from a lead-in single mode fiber to the fiber interface, then back to a lead-out single mode fiber. A side arm was inscribed parallel and in close proximity to the primary arm. Light propagating in the primary arm could then be efficiently coupled into the side arm when a phase-matching condition was met, which produced a dramatic spectral dip at the coupling wavelength. The proposed device achieved a sensitivity as high as similar to 8249 nm/RIU over an RI range of 1.44-1.45, due to strong evanescent fields excited in fiber-interface waveguides. The proposed in-fiber directional coupler exhibits high mechanical strength, a compact configuration, and excellent RI sensitivity. As such, it has significant potential for practical applications in biochemical sensing. (c) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Single molecule detection and fluorescence correlation spectroscopy on surfaces

Kai Hassler

In this thesis a new approach for single molecule detection and analysis is explored. This approach is based on the combination of two well established methods, fluorescence correlation spectroscopy (FCS) and total internal reflection fluorescence microscopy (TIRFM). In contrast to most existing fluorescence spectroscopy techniques, the subject of primary interest in FCS is not the fluorescence intensity itself but the random intensity fluctuation around the mean value. Intensity fluctuations are induced by thermal noise in a minute observation volume, which is in classical FCS the confocal volume of a confocal microscope. E.g. FCS is commonly utilized to investigate diffusion. In this case, diffusing fluorescent molecules entering or leaving the observation volume cause intensity fluctuations, which are analyzed by calculating the temporal autocorrelation of the observed signal. The autocorrelation is a measure for the self-similarity of a signal and contains information about the average fluctuation strength and duration. The confocal observation volume, i.e. the measurement volume that is actually seen by the detector is approximately given by the product of the optical transfer function with the fluorescence exciting intensity distribution of a focused laser beam. To achieve a high signal-to-background ratio a small observation volume is absolutely essential, first of all because the background from e.g. scattered light increases with the size of the observation volume. Second, a small volume assures for a small average number of fluorophores inside the observation volume and therefore for a high fluctuation amplitude i.e. FCS signal. This thesis proposes and discusses an alternative to confocal FCS specially conceived for measurements on surface-bound molecular systems, such as biological receptors or immobilized enzymes. In contrast to confocal FCS, fluorescence is excited within an evanescent field generated by total internal reflection (TIR) of a laser beam at the interface between a microscope coverslip and the sample. This is achieved by focusing the laser beam off-axis at the back focal plane of a high NA oil-immersion objective. The collimated beam that emerges from the objective is incident at an oblique angle at the coverslip-sample interface and totally internal reflected. In contrast to confocal FCS, the generated observation volume is completely confined to the surface and background fluorescence as well as scattered light from the bulk is efficiently suppressed. Our method, called objective-type TIR-FCS in the following, features an increased collection efficiency compared to existing techniques that combine evanescent wave excitation and FCS. Existing techniques use total internal reflection on the surface of a prism to generate an evanescent field. This leads to a configuration where the choice of objectives is limited to air or water-immersion objectives. In our system we use a high NA oil-immersion objective, specially conceived for TIR applications, which collects light efficiently. The collection efficiency is further enhanced by a naturally occurring change of the emission properties of fluorophores close to interfaces between dielectric media. The presence of the interface favors emission into the optically denser medium so that about 60% of the emitted light can be collected. These factors, together with a reduced observation volume lead to a very sensitive method with a high potential for applications in single molecule detection and analysis. The performance of the proposed method was experimentally shown for measurements on molecules subject to Brownian motion and binding to modified coverslips. In particular, it was experimentally shown that objective-type TIR-FCS features high signal-to-background ratio on a single molecule level. In this thesis, concise derivations of analytical expressions for autocorrelation functions for diffusion and most important, the case of ligands reversibly binding to a single and localized binding site are presented. The derived model allows for the quantitative determination of binding rates for a single receptor. We strongly believe that the application of these results in the context of investigations of receptor-ligand binding kinetics will allow for deeper understanding of cellular signaling. Moreover, this thesis discusses the applicability of the proposed method in enzymology. Enzymes, as most proteins are subject to continuous changes of their structure or conformation. These conformational changes are correlated with the function of the enzyme. In the discussed example the enzyme catalyzes an oxidation where the product is fluorescent but the substrate is not. The function of the enzyme i.e. the recurring product formation leads to observable intensity fluctuations. Since function and conformational states are correlated, conformational fluctuations can be investigated by means of FCS as was already shown for confocal FCS. A technique closely related to FCS is fluorescence lifetime spectroscopy (where lifetime refers to the mean lifetime of the electronic excited state). Whereas in FCS the relaxation after random deviations from thermal equilibrium is investigated, relaxation of excited fluorophores towards their electronic ground-state is investigated in lifetime spectroscopy. The technique is used for e.g. discrimination between fluorophores with different lifetimes. Lifetime spectroscopy combined with imaging is used in many domains of life-science, including microarray reading and medical diagnostics. In preliminary work we developed a novel approach to perform lifetime imaging that is based on a multiplexing technique. The proposed method requires no mechanical scanning stage and only a single-point detector. Furthermore, noise is reduced under certain circumstances if the signal is low. Characteristics of this technique as well as advantages and disadvantages are shortly discussed.

EPFL2006

Couches minces nanoporeuses pour la réalisation de dispositifs optiques

Gaëtan Wicht

The physical-chemical properties of polymer films can be modified by embedding inorganic materials (e. g. inorganic nanoparticles) into the polymer matrix. Depending both on the material properties (e. g. nanoparticles concentration) and on the fabrication procedure, air pores can appear into such hybrid organic-inorganic matrices, thus creating nanoporous layers. At the end of the 90's, Ilford Imaging Switzerland GmbH was a pionner in the field with the development of a unique "roll-to-roll" process to coat such layers on large surfaces and on an industrial scale. First used as absorbing layers in the ink-jet printing of high quality photographs, the interest for such nanoporous films as optical films has been rapidly growing. In particular, the possibility of introducing air nanopores on demand in a polymer matrix make these layers the perfect candidates for the fabrication of optical films with a very low refractive index (n < 1.2). In optical devices based on thin film multilayers, such nanoporous materials can guarantee an excellent index matching between the air and a plastic substrate, thus acting as very good antireflection coatings. On the other hand, in a different approach, these nanoporous layers cans also yield a good index contrast between a guiding layer and its claddings, thus allowing the fabrication of good polymer waveguides. The goal of this thesis was then to demonstrate the optical potential of the industrial nanoporous layers fabricated by Ilford on flexible plastic substrates. First, both the morphology and the optical response of different nanoporous layers were thoroughly characterised with respect to the fabrication parameters. The impact of the fabrication process as well as of the nanoparticle type (silica (SiO2), bohemite (AlOOH) and titania oxyde (TiO2)), size and concentration were studied. While reflectance and transmittance measurements revealed the highly transparent nature of all these layers, ellipsometry measurements showed the large range of achievable refractive indices (1.15 < n < 2.0). With the possibility of coating several nanoporous layers one on top of the other, with independently adjusted thicknesses and indices, we designed and fabricated antireflection multilayers with graded-index, that, once coated on a plastic substrate successfully reduce its surface reflectance from 10 % to few 0.1 %. Besides the measurements, a theoretical model based on the transfer matrix method allowed us to analyse in details the response of our samples. We could thus show that the residual interface roughness is responsible for the loss of coherence of the light reflected at each interface, thus reducing the amplitude of the interference fringes in the reflectance spectrum and yielding an optical response almost constant over the entire visible spectrum. Finally, the nanoporous layers were also used as optically isolating layers for the fabrication of polymer optical waveguides. An original measurement method, based on the emission of a fluorescent dye dispersed inside the guiding layer, allowed us to clearly demonstrate the feasability of such guides. Propagation loss in the order of 1.5 dB/cm were measured, a very promising value that, after optimisation of both the material and the optical design, is likely to be further reduced in order to reach better performances than standard polymer waveguides and, eventually, to approach those of inorganic devices. Having highlighted the optical properties of our polymer nanoporous layers with the fabrication of two complementary optical devices based on polymer-nanoporous multilayers, this thesis clearly demonstrates their potential that, combined with their industrial nature, can guarantee them a very promising future, even brighter than that for similar prototype layers fabricated at a lab scale.

EPFL2010

Dispersion Properties of Photonic Crystals and Silicon Nanostructures Investigated by Fourier-Space Imaging

Jana Jágerská

State-of-the-art nanophotonic devices based on semiconductor technology use total internal reflection or the photonic bandgap effect to reduce the waveguide core dimensions down to hundreds of nanometers, ensuring strong optical confinement within the scale of the wavelength. Within the framework of this thesis, we investigate the light propagation in such devices by direct experimental reconstruction of their dispersion relation ω (k), where ω is the optical frequency and k the wave vector of the supported modes. Knowledge of the dispersion relation provides us with comprehensive information about the guided field, including the number of supported modes, their phase and group velocity as well as the higher order dispersion. As a principal characterization tool, an original experimental technique referred as Fourier-space imaging is used. It is based on far-field analysis of optical signal radiated out of the plane of the structure, which makes it possible to retrieve accurately, non-invasively and in one step the complex dispersion of both the leaky and the truly guided optical modes. The latter is feasible provided that the device is equipped with vanishingly weak grating probes that scatter a small part of the guided light into the light cone. The Fourier-space imaging technique was applied to study the optical properties of a large number of nanophotonic devices, ranging from simple nanowire waveguides to complex photonic crystal structures. In the first part of the work, silicon-on-insulator slot waveguides, coupled ridge waveguides and nanowire waveguide arrays are addressed. Besides the phase and group index dispersion, we investigate the phenomenon of mode splitting in coupled systems, being able to probe the coupling lengths with an accuracy of ±50 nm. In the case of waveguide arrays, beam steering using both thermo-optic effect and wavelength tuning was demonstrated. Concerning the photonic crystal devices, we primarily focus on the phenomenon of slow light propagation in line-defect and coupled-cavity photonic crystal waveguides. The latter represent a special type of a waveguide, which allows for substantial optical signal retardation by evanescent coupling along a chain of photonic crystal cavities. The main motivation was to accurately measure the group index of the slow light modes and recognize the main factors limiting its maximum achievable value. Among others, experimental observation of dispersion curve renormalization, enhanced out-of-plane and back-scattering as well as light localization due to residual disorder were reported. Finally, a detailed experimental study of hollow-core photonic crystal structures intended for optical sensing applications is presented.

EPFL2011