Blue surface plasmon propagation along thin gold film-gas interface and its use for sensitive nitrogen dioxide detection

We demonstrate effective surface plasmon propagation along thin gold film in a blue spectral range: the measured 0.6 degrees angular width of the plasmon excitation peak corresponds to the propagation length around 7 mu m. For this end, we prepared a specially designed photonic crystal (PC; 18-layer 1D structure of altering transparent dielectric layers with relatively large and low refraction indices deposited onto a silica support) coated by a 8 nm thick gold layer. For given blue light wavelength (405 nm) and certain light incidence angle this 1D PC structure supports ultralong surface plasmon propagation along the gold nanofilm. This structure has been tested as a surface plasmon resonance sensor to detect small concentrations of nitrogen dioxide in air. This sensor exhibited the sensitivity similar to earlier works where surface plasmon resonance in red spectral range has been used for this purpose. (C) 2013 Elsevier B.V. All rights reserved.

Accordabilité de la réponse optique des cristaux photoniques par infiltration de matériaux organiques

Pascale El-Kallassi

Photonic crystals are stuctures in which the refractive index varies periodically on the wavelength scale in one, two or three dimensions. Due to interference effects, these structures can forbid light propagation over a large spectral range called "the photonic bandgap". Integrated optics is one of the potential applications of photonic crystals in which the information can be treated as a light signal that propagates through photonic crystal-based devices, such as filters, lasers, waveguides, bends and multiplexers. The photonic crystals structures studied for integrated optics applications consist of periodic arrays of holes etched through classical semiconducteur-based planar waveguides. For any practical application, the optical properties of the fabricated components have to be tuned by external means. In this thesis, we explore a possible approach to photonic crystal tuning : the infiltration of photoactive organic materials in the holes of these photonic structures. First we considered liquid crystals, which exhibit anisotropic optical properties like birefringence due to their long-range orientational order. This latter molecular organization can be modified by external factors, i.e. temperature and electric field. Nematic liquid crystals were infiltrated in the holes of the photonic crystals to tune their properties by changing the temperature and applying an electric field. Moreover, the organization of liquid crystals in the nanometric holes were studied by optical techniques. The demand for all-optical tunable devices in integrated optical circuits led us to consider the possibility of optically tuning the response of photonic crystal structures. For this purpose, we used photo-induced phase transitions of a liquid crystal blend doped with azobenzene photochromic molecules. Since the phase transitions in these mixtures depend on the concentration of the photochromic compound and on the temperature, we studied in detail the phase diagram of the blend. The last part of this thesis focused on one of the most innovative aspects in the domain of photonic crystal tuning : selective infiltration to locally modify the optical properties of the photonic structures. We used the photopolymerization of acrylate monomers by ultraviolet laser irradiation. Finally, with three clear examples, this thesis shows that organic photoactive materials are well-adapted to be combined with photonic crystals. We demonstrated that the obtained hybrid structures are very promising for integrated optics applications.

EPFL2009

Investigation of Surface Electromagnetic Waves with Multi-Heterodyne Scanning Near-Field Optical Microscopy

Tristan Sfez

In this thesis, we present the development of a tunable Multi-Heterodyne Scanning Near-Field Optical Microscope (MH-SNOM). This instrument has been built and evaluated for the investigation of optical near fields in amplitude, phase and polarization. With this microscope, the response of a structure illuminated with two orthogonally polarized beams can be simultaneously measured both in amplitude and phase. Moreover, the integral state of polarization at the surface of a specimen can be retrieved under specific conditions. We demonstrate the capabilities of the system through a series of measurements involving Surface Electromagnetic Waves (SEWs). We have mainly focused our attention on a particular class of SEWs known as Bloch Surface Waves (BSWs). The propagation of BSWs on the outer surface of a silicon nitride multilayer has been studied in detail. Furthermore, we show that this propagation is affected by the presence of shallow dielectric corrugations such as a subwavelength grating or at the straight interface with a coated portion of the multilayer. In particular, we demonstrate that ultra-thin (thickness < λ/10) dielectric ridges may act as BSW waveguides. Combining the detection capabilities of the MH-SNOM with a numerical treatment of the experimental data, we are able to separate the transverse and longitudinal field components of the three modes propagating within a specific BSW waveguide. This new structure provides interesting opportunities in waveguide-based biosensing schemes in which the ridge is realized with functionalized molecular layers of nanometric thickness. Finally, we investigate a structure sustaining another type of SEW: Surface Plasmon Polaritons (SPPs). This structure is designed for the asymmetrical coupling of SPPs at normal incidence. Through a detailed analysis of the spatial spectra, we show that, in addition to SPPs, the field contains other near-field components. All these experiments demonstrate the expected MH-SNOM capabilities of measuring the amplitude, phase and polarization of optical near fields. The MH-SNOM therefore serves as a powerful tool for the investigation with subwavelength resolution of optical near fields generated in structures such as integrated optics, photonic crystals, cavities, resonators, etc.

EPFL2010

Polymer multilayer photonic micro- and nanostructures with tailored spectral properties

Amazing and beautiful optical effects are present in Nature. Some examples are the iridescent colors produced by peacocks, butterflies and beetles. While the simulations of some of these structures have already been realized, only a few elements have been fabricated. We are interested to understand and to reproduce some of these amazing optical properties. Applications such as security or decorative elements in different fields like jewelery or horology can be imagined. SEM pictures of the structure of these animals show a frequent combination of periodic or pseudo-periodic elements such as interference filters, multilayer reflectors, microlenses and gratings. The different elements combine angular and spectral effects. This combination enables strong optical effects for a large range of viewing angles and spectra. Different technologies such as standard photolithography, recording of interference patterns, transfer of photoresist structures in glass substrates, replications, hot embossing and spincoating were used to fabricate a variety of optical elements. The fabricated elements were produced with polymer materials because of their ease of use, and the possibility to build multilevel structures with replication methods. A drawback is the small refractive index difference possible between polymers and compared to air. In the first part, different combinations of polymer micro- and nano-optical elements such as corrugated gratings, multilayer Bragg reflectors, microlens arrays, micro-prisms arrays and diffusers were successfully fabricated. Poly (vinyl alcohol) and poly (N-vinycarbazole) with refractive indices of 1.56 and 1.72, respectively, at a wavelength of 500nm were used to fabricate the polymer multilayer reflectors. The micro-optical elements used were: microlenses with diameter between 32 μm and 250 μm and around 20 μm height; prisms with 50 μm period and 25 μm height. The angularly dependent reflectivity of the different fabricated elements was studied. Thanks to the close combination of diffractive, refractive and reflective micro- and nano-optical elements, non-standard artificial visual color effects were produced. The fabricated elements were modeled with different tools according to the dimensions of the different optical elements. Ray tracing analysis was used in the case with micro-optical elements while the Fourier modal method permitted simulation of the interaction of the light with the periodic nano-structures. The specific effects of the variation of the different parameters were highlighted, the basic principles and the limitation of the polymer technology were identified. In the second part, the optical modeling tools and the fabrication technologies developed were used to model and fabricate polymer light emitting diodes in a distributed feedback regime. The optical properties of the different layers were modeled and the physical dimensions of elements with active conductive polymers me-LPPP and F8 were calculated, fabricated and tested. The sensitive dimensions and parameters were underlined. Polymer materials permitted rapid fabrication of complex and innovative optical elements with very simple technologies like spin-coating and replication methods. These technologies allow the combination of nano-optical elements with dimensions of the order of a hundred nanometers with micro-optical elements with dimension of the order of about ten to one hundred microns.

Institut de Microtechnique, Université de Neuchâtel2008