title: Scanning near-field optical microscopy as a tool for the characterization of multimode interference devices.
abstract: We report the scanning near-field optical microscopy (SNOM) characterization of a 4 x 4 multimode interference (MMI) device working at a wavelength of 1.55 microm and designed for astronomical signal recombination. A comprehensive analysis of the mapped propagating field is presented. We compare SNOM measurements with beam-propagation-method simulations and thus are able to determine the MMI structure's refractive-index contrast and show that the measured value is higher than the expected value. Further investigation allows us to demonstrate that good care must be taken with the refractive-index profile used in simulation when one deals with low-index contrast structures. We show evidence that a step-index contrast is not suitable for adequate simulation of our structure and present a model that permits good agreement between measured and simulated propagating fields.
authors.affiliation: Laboratoire de Physique de la Matière, Unité Mixte de Recherche-Centre National de la Recherche Scientifique 5511, Institut National des Sciences Apliqués de Lyon, Villeurbanne, France. matthieu.martin@insa-lyon.fr
authors.forename: Matthieu J
authors.initials: MJ
authors.lastname: Martin
authors.forename: Taha
authors.initials: T
authors.lastname: Benyattou
authors.forename: Régis
authors.initials: R
authors.lastname: Orobtchouk
authors.forename: Frédéric
authors.initials: F
authors.lastname: Rooms
authors.forename: Alain
authors.initials: A
authors.lastname: Morand
authors.forename: Isabelle
authors.initials: I
authors.lastname: Schanen
authors.forename: Pierre
authors.initials: P
authors.lastname: Benech