Multimode phase-matched third-harmonic generation in sub-micrometer-wide anatase TiO2 waveguides

Christopher C. Evans, Katia Shtyrkova, Orad Reshef, Michael Moebius, Jonathan D. B. Bradley, Sarah Griesse-Nascimento, Erich Ippen, and Eric Mazur

Third-harmonic generation (THG) has applications ranging from wavelength conversion to pulse characterization, and has important implications for quantum sources of entangled photons. However, on-chip THG devices are nearly unexplored because bulk techniques are difficult to adapt to integrated photonic circuits. Using sub-micrometer-wide polycrystalline anatase TiO2 waveguides, we demonstrate third-harmonic generation on a CMOS-compatible platform. We correlate higher conversion efficiencies with phase-matching between the fundamental pump mode and higher-order signal modes. Using scattered light, we estimate conversion efficiencies as high as 2.5% using femtosecond pulses, and thus demonstrate that multimode TiO2 waveguides are promising for wideband wavelength conversion and new applications ranging from sensors to triplet-photon sources.