Polymer waveguide Mach-Zehnder interferometer coated with dipolar polycarbonate for on-chip nitroaromatics detection

Abstract

The detection of nitroaromatic explosives is highly imperative for homeland security and environment protection. For in-field detection of explosives, the devices should be portable and inexpensive and, at the same time, offer fast response and high sensitivity. Optical waveguide sensor is one of the most promsing platforms to realize on-chip nitroaromatics detection due to their capability of large-scale integration, high sensitivity, and immunity to electronmagnetic interference. Meanwhile, polymeric waveguide materials are inexpensive and highly flexible for functionalization. In this study, we synthesize a functional polycarbonate with side-chain donor-π-acceptor dipolar chromophores, and design and fabricate an all-polymer waveguide sensor for the detection of nitrobenzene (NB) and 2, 4-dinitrotoluene (DNT) vapors. The waveguide sensor is based on an unbalanced Mach–Zehnder interferometer (MZI) with two different arms coated with the synthesized polycarbonate. The phase difference between the two arms of the MZI is modulated by the refractive-index change of the coating, which originates from the interaction between dipolar chromophores and electron-deficient NB or DNT vapor in polycarbonate. Our fabricated sensor can reversibly detect NB vapor up to its equilibrium concentration in air (∼300 ppm) with a maximum wavelength shift larger than 8 nm. It can irreversibly detect DNT at a much lower concentration (ppb level) with a maximum wavelength shift larger than 5 nm. The response time of the sensor is shorter than 100 s. Our all-polymer MZI, together with the new sensing material, provides a new approach for the realization of low-cost, high-performance explosive detection for in-field applications.