Refractive index sensing in a monolithic micro-optofluidic lithium niobate chip

Lithium niobate is an outstanding material for the realization of fully integrated lab on a chip devices due to its variety of physical properties, such as high electro-optic, acousto-optic, pyro-electric, and nonlinear coefficients, which enable multiple functionalities. As a proof of principle for a monolithic optofluidic sensor, we report the fabrication of a refractive index sensor consisting of a femtosecond laser written waveguide crossing a microfluidic channel, which was fabricated in a lithium niobate sample. The microchannel was created by selective etching of a femtosecond laser inscribed structure and the surfaces of the etched channel were smoothed by subsequent annealing. Sensitivity and accuracy of the sensor were determined by refractive index measurements of sucrose solutions with different concentrations and the temperature dependency was investigated with an air-filled channel. The Fabry-Pérot interference spectrum recorded in reflection shows a high contrast of 24 dB, which indicates good optical quality of the cavity. Refractive index steps of 10^-3 were measured with an accuracy of 8.5 × 10^-5 and a sensitivity of 1215 nm RIU^-1 at a wavelength of 1554 nm. A very low repeatability error was determined by multiple measurements under stabilized temperature conditions.