Atomic layer deposited titanium dioxide in optical waveguiding applications

Aalto University, P.O. Box 11000, FI-00076 Aalto www.aalto.fi Author Tapani Alasaarela Name of the doctoral dissertation Atomic layer deposited titanium dioxide in optical waveguiding applications Publisher School of Electrical Engineering Unit Department of Microand Nanosciences Series Aalto University publication series DOCTORAL DISSERTATIONS 57/2011 Field of research Photonics Manuscript submitted 6 April 2011 Manuscript revised 13 June 2011 Date of the defence 29 June 2010 Language English Monograph Article dissertation (summary + original articles) Abstract This thesis introduces the use of atomic layer deposited (ALD) amorphous titanium dioxide (TiO2) in various waveguiding applications. The ALD of amorphous TiO2 is done by sequential pulsing of titanium tetrachloride (TiCl4) and water vapors into a nitrogen carrier gas flow at relatively low deposition temperatures of below 120°C and in a medium vacuum. The optical properties of the amorphous TiO2 slab waveguides are studied in the visible and near-infrared wavelengths, and a propagation loss of < 1 dB/cm at 1.55 micrometer wavelength is measured. The conformal growth mode of ALD is used to narrow down silicon slot and to smoothen waveguide edges with an amorphous TiO2 layer, which enables fabrication of silicon slot waveguides with thin air slots and relatively low losses of down to 7 dB/cm.This thesis introduces the use of atomic layer deposited (ALD) amorphous titanium dioxide (TiO2) in various waveguiding applications. The ALD of amorphous TiO2 is done by sequential pulsing of titanium tetrachloride (TiCl4) and water vapors into a nitrogen carrier gas flow at relatively low deposition temperatures of below 120°C and in a medium vacuum. The optical properties of the amorphous TiO2 slab waveguides are studied in the visible and near-infrared wavelengths, and a propagation loss of < 1 dB/cm at 1.55 micrometer wavelength is measured. The conformal growth mode of ALD is used to narrow down silicon slot and to smoothen waveguide edges with an amorphous TiO2 layer, which enables fabrication of silicon slot waveguides with thin air slots and relatively low losses of down to 7 dB/cm. ALD-TiO2 is also used to successfully fabricate novel absorbing polarization selective resonant gratings, and semi-wide bandwidth resonant waveguide reflectors. A novel way of fabricating a resonant waveguide grating by applying an ALD-TiO2 layer on a holographically written surface relief grating on an azobenzene polymer complex is introduced. Amorphous ALD-TiO2 shows an excellent performance in all the studied applications, and will very probably see use in various photonic devices in the future. The low growth temperature allows the fabrication of ALD-TiO2 waveguides on top of processed microchips, so ALD-TiO2 might also turn out to be a suitable material in building the optically interconnected microchips of the future.