Photo-bleaching of optical waveguide polymers with dipolar chromophores to improve their sensitivity for explosive vapor detection
Optical polymers can easily be functionalized as versatile waveguide materials for chemical sensing. The ultimate endeavour of material innovation is to utilize a new material to functionalize a waveguide device, and meanwhile greatly simplify device processing to optimize the device performance. Herein, we show that a functional polycarbonate containing side-chain dipolar chromophores is sensitive to ultraviolet (UV) light and electron-deficient nitrobenzene (NB) explosive vapor. UV light induced photo-bleaching of the active cladding material polycarbonate is employed to selectively regulate the refractive index (RI) for the optimization of the waveguide evanescent field, which simply and greatly improves the sensitivity for RI sensing. In explosive vapor detection, polycarbonate acts as a sensitive cladding for NB vapor sensing, in which the dipole–dipole interactions between electron-deficient NB and side-chain dipolar chromophores result in the RI change of the polycarbonate cladding. Our efforts in materials and device engineering enable a significant change in the phase difference between the two arms of the fabricated asymmetric Mach–Zehnder interferometer (AMZI) waveguide in response to a very small change in concentration of NB and hence improve the sensitivity of the sensor. The fabricated photo-bleached (80 min) AMZI sensor exposed to NB vapor showed a wavelength shift of 6.8 nm, showing about 95% sensitivity improvement in comparison with the photo-bleached (40 min) AMZI. Our strategies leverage the photo-bleaching to improve the sensitivity for electron-deficient analyte detection of dipolar polycarbonate, which is simple, effective and does not require additional microfabrication for cost-effective and sensitive on-chip explosive detection.