Supercontinuum generation in titanium dioxide waveguides

International audienceOptical supercontinua are a fundamental topic that has stimulated a tremendous practical interest since the early works of Alfano et al. in the 70’s in bulk components. Photonic crystal fibers have then brought some remarkable potentialities in tailoring the dispersive properties of a waveguide while maintaining a high level of confinement over significant propagation distances. The next breakthrough is to further reduce the footprint of the nonlinear component and to achieve the generation of optical supercontinuum on a photonic chip. To reach this aim, several platforms have been successfully investigated such as silicon, silicon germanium, silicon nitride, chalcogenide waveguides to cite a few. Each material has obviously specific advantages but also limitations including strong two photon absorption and the associated free carrier absorption, low refractive index, low handling power, etc…We discuss here the design and fabrication a new component made on an alternative platform that remains relatively unexplored: titanium dioxide (TiO2). This material benefits from a transparency window spanning from the visible to the mid-infrared wavelengths. It combines a high linear and nonlinear refractive index with negligible TPA beyond 800 nm. Therefore, recent experimental studies have confirmed these features and reported the spectral broadening of a femtosecond pulse [1], the parametric wavelength conversion of a continuous wave [2] as well as the efficient generation of third harmonic [3].Our goal is to demonstrate the generation of an octave spanning supercontinuum in a TiO2 waveguide [4]. We consider here anatase TiO2 layers with a thickness of 450 nm. Pumping the component in the slightly anomalous dispersion regime is a major requirement and it therefore dictates the width of our waveguide. The dispersion profile of the fundamental TE mode obtained in a 1.5 µm wide waveguide is presented as well as structure that has been achieved. The input pulses are provided by a 1560 nm femtosecond laser source that is frequency shifted up to 1640 nm In order to optimize the light injection through butt-coupling, two 1-mm long tapers have been included in the TiO2 device. After propagation in the 2.2-cm long waveguide with subwavelength transverse dimensions, an octave-spanning supercontinuum is generated from 1050 nm up to 1900 nm, which represents an improvement by much more than one order of magnitude compared to [1]. The spectral extend of the supercontinuum is ruled by the location of the two zero dispersion wavelengths and we can note the emergence of a strong dispersive wave below 1200 nm. The observed dynamics can be reproduced by numerical integration of the generalized nonlinear Schrödinger equation. The third harmonic is also generated so that visible light is also clearly observed by the naked eye [3]