Narrowband optical interactions in a plasmonic nanoparticle chain coupled to a metallic film

We study the coupling of localized surface plasmon (LSP) and surface-plasmon polariton (SPP) modes in a system composed of a metallic nanoparticle chain separated from a thin metallic film by a dielectric spacer. The thickness of such a spacer influences the level of interaction between LSP and SPP modes and controls the electromagnetic enhancement in this system. An enhancement with narrow resonances can be observed for appropriate parameters. The high-resonance quality factor and tunability of this system make it a very promising candidate for biosensing and surface-enhanced spectroscopy applications. (C) 2009 Optical Society of America

Mid-infrared surface plasmon resonance sensing applied to refractive index measurements of liquids

Sylvain Herminjard

There has been a very strong development of the sensors based on surface plasmon resonance during the last thirty years, mostly for biological and biomedical applications. If the first experiments in this field were carried out at the beginning of the 20t century, it would have been necessary to wait for the end of the sixties to see the first devices performing accurate measurements of the refractive indices of gas and liquid samples. Indeed, the surface plasmon resonance phenomenon allows to measure refractive index variations with a high sensitivity that is hardly obtained with other optical measurement techniques. The basic principle consists of exciting the free electrons of a metallic layer with light, which induces the formation of a surface wave called surface plasmon. This resonant phenomenon depends on the wavelength of the excitation light and the refractive indices of the metallic layer and the material located close to the metallic layer. A variation of the refractive index of the latter induces a variation of the resonance condition, which allows to realize optical sensors designed to measure determined samples. An appropriate calibration allows to set a relationship between the variation of the concentration of the sample and the measured refractive index value. Among the various parameters that describe the performances of the sensor, one of the most important is the sensitivity. It is possible to demonstrate that the excitation wavelength has an impact on the sensitivity of the system : the latter increases if the measured sample has an absorption peak at a wavelength close to the excitation wavelength. The main goal of this thesis is to explore this sensitivity enhancement technique based on the absorption properties of the probed medium. A proof-of-principle is experimentally demonstrated with an optical setup constituted of laser sources working in the visible range of the electromagnetic spectrum. However, many materials have absorption peaks located in the mid-infrared range due to their fundamental molecular vibrations. Therefore, the realization of a plasmonic sensor working in the mid-infrared could allow to measure these materials with an increased sensitivity. To the best of our knowledge, surface plasmon resonance based sensors have never been developed above the near-infrared. This thesis presents for the first time the design and realization of a plasmonic sensor working in the mid-infrared range able to measure refractive index variations of gas and liquids samples. Even if the optical sources, components and detectors designed for the mid-infrared are clearly different from the ones designed for the visible range, it has been possible to successfully realize two systems working in the mid-infrared range during this project. The first is able to perform gas measurements. As the excitation wavelength depends on the measured sample, it was necessary to select a laser source that had a wavelength located close to the absorption peak of the gas under measurement. For this purpose, we designed a plasmonic sensor working at a 4.4 µm wavelength able to measure carbon dioxide (CO2) absorbing at 4.3 µm. It has been demonstrated that the experimental sensitivity of this system is five times higher than the theoretical sensitivity obtained with a system working in the visible range at a 633 nm wavelength. The second has been conceived to measure liquid samples in order to measure variations of concentration of CO2 dissolved in water. If measurements of refractive index variations (without using the sensitivity enhancement technique) could be performed successfully with variations of concentration of alcohol in water, further work is necessary to get decisive results for CO2 dissolved in water. This project was carried out in collaboration with an industrial partner in order to take into account some industrial constraints during the development of the sensor. Therefore, particular care has been taken in order to design a system that is compact, mechanically robust, without any mechanical moving parts, cheap and easily reproducible. The technological development presented in this thesis finds many applications mainly in the chemical, biological and biomedical fields. Indeed, many proteins show absorption peaks in the mid-infrared. It could be possible to measure their concentration or the modification of their properties – like their composition or structure – with an increased sensitivity. The interest of surface plasmons is that the measurement is not a transmission-based technique, but rather a "reflection-based technique", as the light does not go through the gas or liquid sample. This measurement technique is also adapted for high volume samples that are present in industrial production processes like water or soft drink quality control.

EPFL2010

Longitudinally polarized fields in near-field imaging systems

The evolution path of nanotechnologies moves through the domain of microscopy. Among all the kinds of imaging tools available today, optical imaging systems are of primary importance. When using optical imaging systems, we take profit of the information remotely delivered by light to the detector, after interaction with the sample. Nevertheless, as light moves away from the place of interaction, a part of this information is inevitably lost. With the help of suitable probes, light in close proximity of the illuminated sample can be detected. In this way, an extraordinary richness of information about the sample becomes accessible and we can manage to "see the invisible". Light confined in proximity of the sample is called near-field and near-field microscopy is the branch of microscopy dealing with the detection of such an optical field. In the domain of near-field microscopy, the rigorous electromagnetic and the optical description of light meet together. When images are formed by detecting optical fields close to illuminated samples, the vectorial and electromagnetic nature of light can not be neglected. In this thesis we focused our attention on the special case of electromagnetic fields oriented parallel to the optical axis (longitudinal fields). The subject is particularly intriguing, as longitudinally polarized fields cannot be detected using standard microscope devices. We mainly considered the interaction of longitudinal fields with microfabricated, fully metal-coated quartz nano-probes, and managed to give a description of their role in the near-field imaging process. The work will be presented as follows. In the first chapter, an introduction to Scanning Near-field Optical Microscopy (SNOM) is given. Starting from the concept of super-resolution in paraxial imaging systems, we introduce the concept of near-field microscopy. For two of the best known SNOM techniques we demonstrate that a resolution beyond the capabilities of classical microscopes can be, in principle, achieved. Moreover, we present a rigorous electromagnetic description of a simplified SNOM system, and stress the necessity of adequate tools for the understanding and the interpretation of near-field images. The second chapter is devoted to the description of the SNOM system with which the main part of the experimental work has been performed. The near-field microscope is particularly interesting since it employs an innovative kind of probes: microfabricated, metal-coated quartz probes with no aperture at their apex. The presence of a full metal coating has strong effects in the near-field imaging characteristics of the probes. Results form standard imaging tests are also presented in this chapter. We demonstrate that fully-metalized probes can actually couple evanescent fields and are capable to attain super-resolution. Microfabricated probes can be used either as light sources or as light collectors. In the third chapter we study their optical response when 3D-oriented fields are collected. We find a non trivial selective collection behavior for longitudinally and transversely polarized fields. Moreover, the lateral resolution of our system shows a clear dependence on the polarization state of the collected field. A phenomenological explanation of this effect, involving a polarization-dependent coupling mechanism of light into the probes, is proposed. In the fourth chapter we study the behavior of fully-coated transparent probes as light emitters. With the help of a three-dimensional numerical model, we characterize the guiding properties of the probes and analyzed the near-field and far-field emitted patterns. A set of experimental measurements confirms the results expected by simulations and validates our phenomenological model describing the interaction of light with the probes. In addition, the essential role of longitudinal fields in an illumination-mode near-field scanning system is underlined. We managed to directly measure the longitudinally-polarized, ultra-small hot spot produced at the metallic apex, after injection of a radially polarized mode into the probe. An alternative way of producing longitudinally polarized light source for near-field imaging is proposed in the fifth chapter. With a set of theoretical calculations, we demonstrate that Surface Plasmon Polaritons (SPP) excited on structured metallic thin films can be exploited as non-radiating, strongly localized light sources. The study presented in this chapter is purely computational: no practical implementation of the proposed device has been performed yet. A conclusive summary of this work is presented in the sixth chapter.

Institut de Microtechnique, Université de Neuchâtel2006

Solving surface plasmon resonances and near field in metallic nanostructures: Green's matrix method and its applications

Olivier Martin

With the development of nanotechnology, many new optical phenomena in nanoscale have been demonstrated. Through the coupling of optical waves and collective oscillations of free electrons in metallic nanostructures, surface plasmon polaritons can be excited accompanying a strong near field enhancement that decays in a subwavelength scale, which have potential applications in the surface-enhanced Raman scattering, biosensor, optical communication, solar cells, and nonlinear optical frequency mixing. In the present article, we review the Green's matrix method for solving the surface plasmon resonances and near field in arbitrarily shaped nanostructures and in binary metallic nanostructures. Using this method, we design the plasmonic nanostructures whose resonances are tunable from the visible to near-infrared, study the interplay of plasmon resonances, and propose a new way to control plasmonic resonances in binary metallic nanostructures.

2010

Longitudinally polarized fields in near-field imaging systems

Institut de Microtechnique, Université de Neuchâtel2006

Mid-infrared surface plasmon resonance sensing applied to refractive index measurements of liquids

Sylvain Herminjard

EPFL2010