Novel optical sensors for chemical and biological applications

Abstract

Optical sensors have attracted a lot of interest due to their increased performance and ability to perform chemical identification through spectroscopy. Integrated sensors present the additional advantages of compactness and increased light-matter interactions. This thesis aimed at advancing the field of photonic sensing by demonstrating novel devices and applications, and improving the performance of current sensors. In particular, we studied flexible integrated photonic sensors and substrates for surface-enhanced Raman spectroscopy. We first propose and demonstrate a three-dimensional flexible photonic sensor array for stress mapping in soft materials systems such as cell cultures. Our device relies on stress-optical coupling to infer stress from optical measurements and uses a deterministic 3-D fabrication method to precisely position the sensors in space. We characterized the sensors' response to mechanical stimulation by measuring their strain-optical coupling coefficient.

Our device is amenable to measuring strains down to 0.001% or forces down to 1 nN in any matrix with a modulus greater than 300 Pa, with a spatial resolution of 100 æm, enabling the detection of the effects of about a dozen cells. Overall, our device provides fast, easy, and precise measurements even in opaque samples, in a greater range of volumes and geometries than previously available. More broadly, this platform prefigures the ability to perform multifunctional sensing and light delivery in three dimensions. In addition, we look at the efficiency of surface-enhanced Raman spectroscopy (SERS), a popular spectroscopy technique with a broad range of applications. Using a reasoning based on the local density of states (LDOS), we derived a limit for the enhancement provided by nanoantennas, which is shown to include factors relating to the antenna's material and to the antenna's geometry.

We then simulated the response of typical structures and found that they lie several orders of magnitude away from the bound. In the case of spheres, we showed that periodic structures can outperform isolated structures only under certain geometrical conditions. This study paves the way for the definition of performance metrics that can be used for further optimization of SERS substrates.