Mid-Infrared Electro-Optical Modulation Using Monolithically Integrated Titanium Dioxide on Lithium Niobate Optical Waveguides

Tunable photonic circuits were demonstrated in the mid-Infrared (mid-IR) regime using integrated TiO(2)-on-LiNbO(3) (ToL) waveguides. The upper waveguide ridge was made by a sputtered TiO(2) thin film with broad transparency at λ = 0.4–8 µm and an optimized refractive index n = 2.39. The waveguide substrate is a z-cut single crystalline LiNbO(3) (LN) wafer that has strong Pockels effect, thus enabling the tunability of the device through electro-optical (E-O) modulation. A sharp waveguide mode was obtained at λ = 2.5 µm without scattering or mode distortion found. The measured E-O coefficient γ(eff) was 5.9 pm/V approaching γ(31) of 8.6 pm/V of LN. The ToL waveguide showed a hybrid mode profile where its optical field can be modified by adjusting the TiO(2) ridge height. Our monolithically integrated ToL modulator is an efficient and small footprint optical switch critical for the development of reconfigurable photonic chips.