Silicon chip-based dual Fabry–Pérot biosensor employing the Vernier effect for enhanced sensitivity

Abstract It is desirable to keep the cost, size, power and complexity of optical biosensors as low as possible to increase their suitability for mass manufacture and deployment outside of the lab in applications such as security, environmental sensing and medical diagnostics. Many optical biosensors suffer from low free spectral range and sensitivity, with strategies such as the Vernier effect being employed to combat this. Whilst the Vernier effect has been applied extensively in fibres or lateral chip-scale devices such as ring resonators and Mach Zehnder interferometers, limited attention has been given to biosensors based on vertical , dual-cavity chip-based Vernier structures. Here through simulations, we show a vertical dual Fabry Pérot/etalon stack utilising the high refractive index and mature processing of silicon combined with the Vernier effect to achieve theoretical sensitivity values in the range of 4200–93 155 nm RIU −1 and limit of detection values of order 10 −6 –10 −5 RIU using 1550 nm light. We simulate a number of different device dimensions and show that practical considerations such as thermal stability and fabrication tolerances, may necessitate operating the Verniers at lower sensitivity enhancements (e.g. ∼16 000 nm RIU −1 ) as a price for better stability. This enhancement is still significantly higher sensitivity than a single, non-Vernier cavity (∼1000 nm RIU −1 ). One of the key advantages of this approach versus lateral scale devices is the removal of waveguides, which greatly eases light coupling. The devices presented also do not require nano or even microscale features in the lateral plane, nor any multilayer Bragg mirrors to achieve these high sensitivity values. This work provides an initial design study to guide future fabrication and experimental verification.