External Strain Enabled Post-Modification of Nanomembrane-Based Optical Microtube Cavities

Optical microtube cavities formed by self-rolling of pre-strained nanomembranes feature unique optical resonance properties for both fundamental and applied research. A post-fabrication treatment of the microcavities made of rolled-up nanomembranes is attractive in order to better manipulate and control the optical modes therein. Here, we report a new approach of modifying the resonant modes by applying external strain using a stretchable polymer substrate. The properties of both azimuthal and higher order axial modes are systematically investigated by varying external strain along the tube axial direction. The post-treatment process leads to a spectral redshift and improvement of quality factors, which is attributed to a modification of tube shape and interlayer compactness. For tubes with axial confinement, the measurements suggest that both the eigenenergies and mode spatial distributions of optical axial modes get significantly modified after applying the external strain. Our numerical calculation results show good agreement with the experimental results. This work reports a simple and robust strain-based modification scheme for manipulating the resonant mode energies, mode spacing, and mode field distributions

Silver Nanocap Enabled Conversion and Tuning of Hybrid Photon–Plasmon Modes in Microtubular Cavities

Hybrid photon–plasmon modes are promising for the study of enhanced light–matter interactions due to the formation of a unique plasmon-type evanescent field. Here, we demonstrate the tunability of photon–plasmon coupling enabled by a metal nanocap on a microtubular cavity. An angle-dependent tuning of the photon–plasmon hybridization is revealed, where the dominant polarization is transverse-magnetic (TM) polarized at the middle-top of the nanocap and gradually converts to be transverse-electric (TE) polarized at the sidewall of the microtube cavity. The intensity ratio of strongly hybridized TM and TE modes is extremely sensitive to nanoperturbations at the metal nanocap, thus providing a novel scheme for surface sensing. Theoretical calculations show that the sensitive intensity ratio change originates from the distinct tuning effect on the TM- and TE-polarized hybrid modes, which is particularly significant in thin-walled microtubular cavities. Our work reports photon–plasmon modes tuned by a metal nanostructure, which are promising for the fundamental studies of enhanced light–matter interactions and relevant applications

<i>In Situ</i> Generation of Plasmonic Nanoparticles for Manipulating Photon–Plasmon Coupling in Microtube Cavities

<i>In situ</i> generation of silver nanoparticles for selective coupling between localized plasmonic resonances and whispering-gallery modes (WGMs) is investigated by spatially resolved laser dewetting on microtube cavities. The size and morphology of the silver nanoparticles are changed by adjusting the laser power and irradiation time, which in turn effectively tune the photon–plasmon coupling strength. Depending on the relative position of the plasmonic nanoparticles spot and resonant field distribution of WGMs, selective coupling between the localized surface plasmon resonances (LSPRs) and WGMs is experimentally demonstrated. Moreover, by creating multiple plasmonic-nanoparticle spots on the microtube cavity, the field distribution of optical axial modes is freely tuned due to multicoupling between LSPRs and WGMs. The multicoupling mechanism is theoretically investigated by a modified quasipotential model based on perturbation theory. This work provides an <i>in situ</i> fabrication of plasmonic nanoparticles on three-dimensional microtube cavities for manipulating photon-plasmon coupling which is of interest for optical tuning abilities and enhanced light-matter interactions