Étude expérimentale des propriétés optiques des cristaux photoniques bidimensionnels et de leur accordabillité

Photonic crystals are periodic dielectric structures, where the periodicity varies in one, two or three dimensions. Analogous to the periodic potential of electrons in semiconductors, the periodic variation of the dielectric constant influences the electromagnetic properties. The energy of the light is separated in bandgaps, energy ranges in which the propagation of the light is forbidden for certain directions and energies. These properties suggest that photonic crystals may be suitable for fabrication of the components needed for integrated optics. Since the fabrication of three dimensional photonic crystals is still limited by complex fabrication problems we have studied two dimensional photonic crystals. These photonic crystals can be fabricated by standard microelectronic technology. The photonic crystals studied in this thesis consist of GaAs and InP based low index vertical waveguides with a matrix of holes etched into it. The fabrication of photonic crystals with good optical properties is an important factor for photonic crystal devices in integrated optics. In the first part we studied the optical properties of photonic crystal slabs and of Fabry-Pérot cavities. These structures were measured by the internal light source technique, which allows quantitative normalized transmission measurements. The photonic crystals were etched by three different dry etching technologies. Out-of plane scattering is described by a 2D-FDTD fit of the transmission spectra and by a semi-analytical model. The finite hole depth and the conical shape of the holes are important structural parameters. The detailed study of their effects yields an important feedback for the critical parameters of the fabrication process. Photonic crystal waveguides are another group of components in integrated optics. We measured two types of straight photonic crystal waveguides by the endfire technique. Here the light is coupled by optical fibers and ridge waveguides in the photonic crystal waveguides. The transmission spectra reveal fringes which are due to multiple interference of the light in the sample. This interference contains information about propagation losses of the waveguides and an analysis based on the Fourier transform of the transmission spectra enabled us to deduce the propagation losses of the photonic crystal waveguides. The necessity of tuning or trimming the optical properties of photonic crystals is an important issue either compensating fabrication imperfections (trimming) or controlling the optical properties on demand (tuning) for devices like filters. We have shown that it is possible to tune the optical properties of planar photonic crystal cavities by temperature. The experimental results were validated by theoretical calculations. Also we have shown that is possible to tune the optical properties of photonic crystals by infiltrating liquid crystals in the holes. Liquid crystals are a birefringent material whose optical axis can be tuned by external fields (electric, temperature, photonic source, etc.). Phase transitions exhibiting abrupt changes in the refractive index are important characteristics of liquid crystals. The holes of the photonic crystal are infiltrated by a reversible and reliable infiltration process. The infiltration efficiency and the orientation of the molecules in the holes were determined by polarization resolved internal light source measurements. We have shown that the frequency of the Fabry-Pérot resonance changes at the phase transitions of the liquid crystal.

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

Near-field characterization of Bloch surface waves based 2D optical components

Richa Dubey

Bloch surface waves (BSWs) are surface electromagnetic modes that propagate at the interface between a multilayer substrate and a homogeneous external medium. The optical field of the surface mode is confined near the surface of the multilayer. This vertical confinement as well as the low absorption inherent to the dielectric materials make the BSWs an interesting candidate for the development of 2D optical systems and sensors. Such a periodic multilayer structure is introduced as a platform on which many optical functions can therefore be created. In this thesis, two-dimensional optical components based on the Bloch wave platform are studied, in particular: a disk resonator, a Bessel-like beam generator and a waveguide grating as a Bragg mirror engraved in a waveguide. The optical properties of the components such as the resonance inside the disc, the "quasi non-diffracting" behavior and the reflection properties are presented. The 2D optical components are designed from a commercial FDTD program (CST Microwave studio). They are then characterized by a near-field scanning microscope with multi-heterodyne detection (MH-SNOM). Thanks to the MH-SNOM, it is possible to map the field distribution locally at the surface of the structures with a resolution lower than the wavelength. Simultaneous measurement of amplitude and phase allows a detailed reconstruction of the complex amplitude of the electric field. In a first part, the influence of a device layer of material with a high refractive index (TiO2) is studied. The impact of the thickness of the TiO2 layer on the propagation properties of the BSWs is presented. It is demonstrated that by adapting the thickness of the device layer, the BSW dispersion curve position can be moved within the photonic band gap and consequently the BSW mode propagation properties can be adapted. The propagation properties of the BSWs include, for example, the propagation length and the effective refractive index. Thanks to the low losses and the design of our multilayer platform, propagation lengths in the range of millimeter are obtained. In a second part, 2D optical components fabricated in a 60 nm (wavelength/25) device layer of TiO2 are presented, and initially, disk resonators. The latter are key elements in integrated optics systems. For a disk with a radius of 100 µm, an experimental quality factor of the order of 10^3 is obtained. A 2D Bessel-like beam generator is also studied. An isosceles triangle is used to generate such a beams. The main expected property of Bessel-type beams is their "quasi non-diffracting" nature. The optical properties of non-diffracting beams are measured in the near-field for different base/height ratios of the isosceles triangle. It is demonstrated that the beam propagates without significant spreading for considerable propagation distance, approximately 50 µm. Finally, the gratings engraved in a waveguide are studied. It is demonstrated that they perform as a Bragg mirror at a wavelength of 1550 nm. Thanks to the MH-SNOM, the interference fringes between the incident wave and the reflected wave are measured. The experimental reflectivity is obtained from the contrast of the interference fringes. The presented waveguide grating can be used as a Bragg reflector at telecom wavelengths for 2D optics systems.

EPFL2017

Numerical investigations of two-dimensional photonic crystal optical properties, design and analysis of photonic crystal based structures

Vasily Zabelin

Two-dimensional (2D) photonic crystals (PhCs) operating in the near-infrared wavelengths are promising candidates for novel integrated optics applications. In this work the optical properties of the Bloch modes in 2D photonic crystals have been investigated theoretically using different numerical methods. The plane wave expansion (PWE) method have been formulated in a general form for arbitrary 2D photonic crystal and applied for detailed analysis of the properties of the Bloch modes. The k space based and the frequency based PWE methods have been derived and used, which allowed us to analyze both propagating and evanescent modes. The guided mode expansion method has been used to take into account three-dimensional distribution of the electromagnetic field in the planar photonic crystals and improve the accuracy of the analysis. A new method of calculation of the sensitivity of the photonic crystal optical properties to small variations of the photonic crystal parameters has been developed. This method is based on the perturbation theory and allows fast and accurate estimation of the optical properties corrections as a result of a change the parameters of the photonic crystal. The developed methods have been used for design and analysis of different photonic crystal based structures, such as cavities, waveguides, coupled-cavity waveguides and polarization beam splitter. The developed methods proved to be efficient and accurate tools for modelling two-dimensional photonic crystal properties and design of photonic crystal based devices.

EPFL2009

Near-field characterization of Bloch surface waves based 2D optical components

Richa Dubey

EPFL2017

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

Pascale El-Kallassi

EPFL2009