A photochromic dye doped polymeric Mach-Zehnder interferometer for UV light detection

Owing to the compact size, immunity to electromagnetic interference, high sensitivity and real-time monitoring, optical waveguide sensors based on a refractive index change have become an emerging approach for smart sensing in the internet-of-things system. For this purpose, the synergetic efforts in material engineering (sensing materials) and optical engineering (waveguide design and fabrication) are both indispensable. Here we report a Mach-Zehnder interferometer (MZI) waveguide with photochromic dye-doped polymer sensing films for UV light detection. Firstly, UV light induced photochromism of a synthesized dye DAECHO in solution and in the solid state is investigated to understand the relationship between their structure and optical properties, especially the photochromism-induced refractive index change for waveguide sensors. Then the MZI structure is designed and simulated to optimize the performance of refractive index sensing. Following the standard microfabrication process of double photolithography-development, the device is fabricated with a DAECHO doped polymer as a sensing cladding material for UV light detection. The evanescent sensing of MZI is performed to detect weak UV light from 10 μW cm^-2 to 100 μW cm^-2 at 365 nm. Our demonstration of MZI for UV light detection indicates the potential of portable and highly sensitive UV light waveguide sensors for real-life applications.