Size-optimized polymeric whispering gallery mode lasers with enhanced sensing performance

Integration of optically active materials into whispering gallery mode (WGM) cavities enables low-threshold laser emission. In contrast to their passive counterparts, the WGMs of these microlasers can be pumped and read out easily via free-space optics. The WGMs interact with the cavity environment via their evanescent field, and thus lend themselves to label-free bio-sensing. The detection limit of such sensors, given as the ratio of the resolution of the whole measurement system to the sensitivity of the WGMs, is an important figure of merit. In this work we show that the detection limit of polymeric microdisk lasers can be improved by more than a factor of seven by optimizing their radius and thickness. We use the bulk refractive index sensitivity, the magnitude of the sensor reaction towards refractive index changes of the bulk environment, to quantify the sensing performance and show that it can be enhanced while the spectral resolution is maintained. Furthermore, we investigate the effect of the size of the cavity on the quality factor and the lasing threshold in an aqueous environment, hence allowing optimization of the cavity size for enhanced sensor performance. For all considered quantities, numerically computed expectations are verified by experimental results

Identification of Dielectric, Plasmonic, and Hybrid Modes in Metal-Coated Whispering-Gallery-Mode Resonators

Making available and accessing in a controlled manner optical modes with largely disparate properties in a given system constitutes a prime challenge for different applications. Here, we propose, realize, and optically characterize a high-<i>Q</i> polymeric wedge-like whispering-gallery-mode resonator coated with a thin silver layer that supports pure surface plasmon polariton modes, pure dielectric modes, and hybrid photonic–plasmonic modes with <i>Q</i>-factors larger than 1000 and modal volumes as small as only a few cubic micrometers. We demonstrate both theoretically and experimentally that all three distinct kinds of cavity eigenmodes can be efficiently excited in the infrared via evanescent coupling to a tapered fiber. Performing finite-element simulations and coupled-mode theory, we develop an experimental procedure based on mode filtering to unambiguously identify the resonances observed in fiber transmission spectra. By controlling both the position of the tapered fiber with respect to the resonator and the input laser polarization, we successfully demonstrate that dielectric, plasmonic, and hybrid modes can be selectively excited, allowing for an explicit classification of the distinct cavity eigenmodes. Experimental results are in excellent agreement with the simulations