Optical modeling of internal and external in-plane Fabry–Pérot interferometers for sensing polymer swelling

We present an optical model for polymer-functionalized in-plane Fabry–Pérot interferometers (FPIs), developed to enhance device sensitivity by exploiting polymer swelling as a micro-actuation mechanism for environmental and gas sensing applications. We investigate both internal and external optical cavity designs and fabricate external FPIs on silicon with adjustable cavity lengths. In the external configuration, the polymer interaction length is decoupled from the optical cavity length, allowing polymer swelling to be efficiently transduced into a change in resonant wavelength. This work is validated using temperature modulation to induce polymer swelling. The external configuration enables an increase in sensitivity by a factor of 2 and the measurement of polymers opaque to infrared light, broadening material options for future applications.