Imaging highly confined modes in sub-micron scale silicon waveguides using Transmission-based Near-field Scanning Optical Microscopy

Jacob T Robinson,Michal Lipson,Stefan F Preble

doi:10.1364/oe.14.010588

Abstract

We demonstrate a new technique for high resolution imaging of near field profiles in highly confining photonic structures. This technique, Transmission-based Near-field Scanning Optical Microscopy (TraNSOM), measures changes in transmission through a waveguide resulting from near field perturbation by a scanning metallic probe. Using this technique we compare different mode polarizations and measure a transverse optical decay length of lambda/15 in sub-micron Silicon On Insulator (SOI) waveguides. These measurements compare well to theoretical results.

Highlights

Confined optical modes, made possible by the large index contrast in Silicon On Insulator (SOI) or semiconductor air-bridge platforms, have allowed for the development of a variety of compact and efficient photonic devices [1]
Optical buffers based on the large group index in photonic crystals [6] and Stimulated Raman Scattering (SRS) [7] have been shown experimentally
Because light is confined to non-radiating guided modes and mode features are often smaller than the free space diffraction limit, conventional far field microscopy cannot resolve the local characteristics of guided modes

Summary

Introduction

Made possible by the large index contrast in Silicon On Insulator (SOI) or semiconductor air-bridge platforms, have allowed for the development of a variety of compact and efficient photonic devices [1]. Position creates an image of the local evanescent field This technique has been used to observe confinement and guiding in micron-sized semiconductor waveguides and transverse optical decay lengths as small as λ/7 in the near infrared have been reported [17]. In this paper we present a new type of apertureless NSOM: Transmission-based Nearfield Scanning Optical Microscopy (TraNSOM), which offers both high resolution imaging and high collection efficiency, eliminating the need for interferometric measurements or farfield collection optics We achieve this high resolution and improved collection efficiency by measuring the transmission of light through a device where the mode is disturbed by a metallic Atomic Force Microscope (AFM) probe. Using a commercial AFM as a TraNSOM we make the first quantitative measurements of highly confined mode profiles in SOI waveguides which show a transverse optical decay length of λ/15 for the fundamental quasi-TM mode

Experiment

TraNSOM signal

Measurement and Simulation

Conclusions