Optofluidic tunable filters using ionic liquid electrolyte capacitors

Abstract

Tunable optical filter is a basic component for most optical systems. This study reports a unique design of Fabry-Pérot (FP) tunable filter by using an ionic liquid solution. The tunable filter consists of two neighboring regions: capacitor region and FP region. The former is in the form of electrolyte capacitor and the latter remains transparent as an FP cavity for light transmission. When the capacitor region is applied with a bias voltage, it attracts the ions from the FP region and thus reduces the ion concentration of the FP region, resulting in a change of the refractive index and eventually a shift of transmission peak of the FP cavity. Among four electrolyte solutions studied, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6) exhibits the best overall performance, such as low insertion loss (3.2 dB), large side mode suppression ratio (23 dB) and high stability (drift <0.2 nm). Additionally, a wavelength tuning of 0.17 nm/V is achieved over 0–17 V, providing a tunable range of 3 nm. This device features low bias voltage, no mechanical movement, easy fabrication and seamless integration with microfluidics systems, and may find potential applications in spectral analyzers and lab-on-a-chip biosensing systems.