Characterization of Photonic Crystal Waveguides Based on Fabry-Pérot Interference

We present two methods based on the analysis of Fabry-Perot interference for a detailed characterization of a 90degrees corner in a two-dimensional photonic crystal waveguide fabricated in a thin Si membrane. These methods are a means of identifying the critical waveguide elements in the process of improving photonic crystal devices. The effects of the elements forming the photonic crystal waveguide are identified and quantified by means of a stage-by-stage analysis. By Fourier transforming the transmission spectra we observe the amount of light that is back reflected inside the waveguide and based on the fringe contrast of the Fabry-Perot modulation we calculate the loss contribution of each waveguide element, such as the tapers and the 90degrees corner.

Subwavelength Metallic Structures for Sensing

Qing Tan

Following rapid developments in biotechnology and medicine, optical sensing promises to be extremely important in various applications such as drug discovery, environmental monitoring, etc. Refractive index (RI) based optical sensing is straightforward to monitor or analyze the physical and chemical properties of substances. Among diverse configurations developed for RI detection, plasmonics devices using nanoscale patterned metals are particularly promising. Nano-structured metallic devices enable field confinement in a volume smaller than the diffraction limit. Moreover, they present possibilities for device miniaturization and sensor multiplexing on the same substrate. This thesis is dedicated to investigating a sensing platform combining nanostructured metallic cavities and dielectric waveguides. Periodically structured Au films have been explored for their application in local RI sensing. Two structures are considered: the slot waveguide cavity (SWC) and the annular aperture array (AAA). The SWC is composed of a Au film structured by a periodic slot array, which is deposited on a silicon (Si) waveguide. The evanescent field overlap correlates the resonant optical response of the cavity to the transmission spectrum of the Si waveguide. Theoretical studies, fabrication and characterization have been performed to verify the RI sensing ability for extremely subwavelength slot dimensions (30 nm slot width for 20 nm-thick Au film and 700 nm cavity length). The experimental sensitivity, 730 ± 10 nm/RIU, corresponds to the theory and an optimum resolution of 5.8 × 10-5 RIU is anticipated for RI detection. For the AAA device, the patterned Au film is directly embedded in a Si3N4 waveguide. The Fabry-Pérot-like resonance of the Au cavity results in a resonant reflection, enabling a device sensitive to RI variations. For a wide range of analytes, the theoretical sensitivity is 764 nm/RIU in the near-infrared wavelength range. Compared with other resonance based photonic sensors, the two devices exhibit comparable sensitivities to the RI variation. The small metallic cavity implies a local detection on nano scale and the sample volume can be on the order of a femtolitre. Furthermore, the planar configuration permits easy integration with other photonic devices. Parallel sensing using compatible sensors on the same substrate is anticipated for the realization of portable and low-cost sensor systems.

EPFL2012

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

Characterization of buried photonic crystal waveguides and microcavities fabricated by deep UV-lithography

Hans Peter Herzig, Iwan Märki, Ross Stanley

We present results of the optical characterization of silicon photonic crystal waveguides and microcavities that are completely buried in a silicon dioxide cladding and are fabricated by deep ultraviolet (UV) lithography. The advantages of buried waveguides and deep UV lithography are discussed. Transmission spectra and loss factors for photonic crystal waveguides, as well as quality factors for resonant microcavities, are obtained. The observed characteristics are in good agreement with three-dimensional simulations.