Photosensitive and piezoelectric thin films for optical devices

Abstract

This thesis reports on the development of photosensitive and piezoelectric thin films for optical devices. The thesis is in two parts, the first concentrating on research into novel photosensitive materials for optical Bragg gratings, while Part II studies the development of piezoelectric ZnO thin films for acousto-optic devices.

Bragg gratings are an important element in the development of optical communications systems. Strongly photosensitive optical waveguiding materials are required to achieve more efficient Bragg gratings in both planar and fibre waveguides. In this thesis, metal oxides are studied for photosensitivity in their pure state as RF sputtered thin films. Photosensitivity is demonstrated for the first time in RF sputtered germanium oxide and tantalum oxide thin film waveguides via Bragg diffraction. A side-polished fibre coupler, capable of monitoring both the magnitude and sign of an index change, is then used to establish large, negative and permanent index changes in tantalum oxide thin films. UV-induced changes in absorption, measured in a similar tantalum oxide film, are contrasted with the observed index change via a Kramers-Kronig analysis, while a UV-induced reduction in film reflectivity consolidates the fibre coupler result.

ZnO, with a kt of 0.28, is an efficient piezoelectric material, and thin ZnO transducers permit operating frequencies of up to 3.5 GHz. In addition, ZnO transducers can be configured on curved surfaces, whose focusing properties greatly enhance device efficiency. In contrast, the operating frequency of LiNbO3 transducers is limited to approximately 0.8 GHz and curved transducers are a problem. Part II of this thesis discusses the fabrication and optimisation of ZnO transducers. Acousto-optic beam deflection by ZnO transducers on TeO2 crystals is demonstrated for the first time, and efficient in-line fibre phase modulation is achieved using a annular ZnO transducer on an optical fibre.

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