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

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.

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

Barbara Wild

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.

EPFL2006

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

Large evanescently-induced Brillouin scattering at the surrounding of a nanofibre

Flavien Gyger, Luc Thévenaz, Fan Yang, Li Zhang

Brillouin scattering has been widely exploited for advanced photonics functionalities such as microwave photonics, signal processing, sensing, lasing, and more recently in micro- and nano-photonic waveguides. Most of the works have focused on the opto-acoustic interaction driven from the core region of micro- and nano-waveguides. Here we observe, for the first time, an efficient Brillouin scattering generated by an evanescent field nearby a single-pass sub-wavelength waveguide embedded in a pressurised gas cell, with a maximum gain coefficient of 18.90 ± 0.17 m−1W−1. This gain is 11 times larger than the highest Brillouin gain obtained in a hollow-core fibre and 79 times larger than in a standard single-mode fibre. The realisation of strong free-space Brillouin scattering from a waveguide benefits from the flexibility of confined light while providing a direct access to the opto-acoustic interaction, as required in free-space optoacoustics such as Brillouin spectroscopy and microscopy. Therefore, our work creates an important bridge between Brillouin scattering in waveguides, Brillouin spectroscopy and microscopy, and opens new avenues in light-sound interactions, optomechanics, sensing, lasing and imaging.