Electro-optically tunable barium–strontium–titanate filter for the 2 μm wavelength range

Abstract

We demonstrate a tunable optical bandpass filter with maximum transmittance of 67% based on a thin film substrate-transferred Ba0.5Sr0.5TiO3 (BST) layer integrated into a Fabry–Pérot cavity, designed for operation at 2 μm. The filter structure consists of a 450 nm BST film sandwiched between two distributed Bragg reflector mirrors composed of alternating TiO2 and SiO2 layers. Electrodes patterned directly onto the BST layer enabled the application of an in-plane electric field, inducing a blue shift of the central wavelength via the quadratic electro-optic effect. Optical characterization reveals a maximum spectral tunability of 3.8 nm under a 210 V DC bias. Our results highlight both the feasibility and design trade-offs of using paraelectric BST for tunable integrated photonic filters in the 2 μm wavelength range.