Cavity-Enhanced Lab-on-Fiber Technology: Toward Advanced Biosensors and Nano-Opto-Mechanical Active Devices

The integration of nanostructures able to manipulate light at the optical fiber tip is bringing the optical fibers to a renewed dimension since their beginnings. The past decade has seen unprecedented advancements in the lab-on-fiber technology field, pushed by the effective exploitation of nanoscale optical physics and supported by the improvement of nanofabrication techniques. In this context, here we report on a cavity enhanced lab-on-fiber optrode, which dramatically boosts the performances and widens the range of applications of the current lab-on-fiber systems, setting a new fundamental milestone along the roadmap of this technology. The "lab" integrated onto the fiber tip consists of a tunable optical cavity incorporating smart materials as the active layer. Specifically, the swelling/collapsing mechanism of multiresponsive microgels sandwiched in between gold layers induces the interplay between plasmonic resonances and cavity modes, according to optical cavity size. The combination of the optically resonant effects and microgels endows the optrode with the unique ability to work as both a sensor for detecting small molecules and a nano-opto-mechanical actuator triggered by light. In the specific case study here presented, we show that the device is able to detect glucose in solution, with a sensitivity improved by more than 1 order of magnitude compared to other state-of-the-art values. Moreover, we demonstrate that, by combining thermoplasmonics effects triggered by light coupled into the fiber and the microgel thermo-responsivity, it is possible to tune and control the optical cavity features, opening new avenues toward active nano-opto-mechanical actuators directly realized onto the fiber tip.