Optical tweezers assisted coupling of nematic droplets to gold nanoparticle cluster: effect on whispering gallery modes

TL;DR

This work integrates dye-doped nematic liquid crystal microdroplets with plasmonic gold nanoparticle clusters to realize dynamically tunable whispering-gallery modes (WGMs) using optical tweezers for precise droplet parking. The plasmonic near-field enhances WGM emission and induces a spectral redshift, with tunability up to about $7.1\ \mathrm{nm}$ while maintaining a high quality factor ($Q \approx 310$–$325$); the extent of tuning scales with the size and morphology of the AuNP cluster. Importantly, the coupling is reversible: decoupling the droplet from the cluster restores the uncoupled WGM spectrum, enabling repeatable sensing cycles. The results point to a cost-effective, reconfigurable hybrid photonic–plasmonic platform suitable for sensitive chemical and biological detection, with tunable resonances governed by interparticle spacing and cluster geometry.

Abstract

Dye doped liquid crystal (LC) microdroplets exhibit tunable optical resonances modulated by size, shape, temperature, and external perturbations. When a dye-doped nematic microdroplet is coupled to a gold nanoparticle cluster, near-field interactions enhance local electric fields, boosting fluorescence emission. Optical tweezers serve as a tool for the parking of dye doped nematic microdroplets on gold nanoparticle clusters, enabling the dynamic coupling and excitation of whispering-gallery modes (WGMs). This configuration resulted in amplified WGMs, with a clearly detectable shift in the spectral position. Resonance mode red shifts confirmed efficient photonic plasmonic coupling, with up to seven nm tunability achieved without significant degradation of the Q-factor. The magnitude of tunability depends on the size of the gold nanoparticle cluster. Also, the WGM emission spectrum of the nematic microdroplet can be reversibly tuned by decoupling from the gold nanoparticle cluster.

Figures (5)

• Figure 1: (a) Schematic illustration of whispering gallery mode (WGM) resonance in a dye-doped nematic microdroplet in its decoupled and coupled states relative to a plasmonic gold nanoparticle cluster drop-casted on a silica substrate. The droplet is excited by a 532 nm laser (green arrow) focused at the droplet–medium interface. Red ellipsoids represent the electric field distribution of resonant WGMs. Bidirectional arrows represent the reversible coupling and decoupling process done by trapping with a 532 nm laser between the microdroplet and the gold nanoparticle cluster. The gold nanoparticle cluster, deposited on the substrate, is highlighted by the faded blue marking. (b) Time-sequence optical microscopy snapshots of a dye-doped nematic microdroplet under optical trapping, showing the controlled coupling–decoupling process. The cross symbol marks the focused laser beam position. At t = 0 s, the gold nanoparticle cluster (red circle) is positioned away from the droplet. As the trapped droplet is brought into proximity with the nanoparticle cluster (t = 7.6 s), it achieves parking over the cluster (t = 23 s). Subsequent nematic droplet moving away by trapping from the gold nanoparticle cluster leads to decoupling (t = 28 and 30 s).
• Figure 2: (a) Droplet as seen without polarisers. (b) A Nile-blue chloride dye-doped microdroplet illuminated by a focused 532 nm laser beam. The irradiation point is indicated by a green arrow. (c) Polarising optical microscope image of a 5CB nematic microdroplet in the presence of SDS solution at room temperature. A crossed polariser is shown on the bottom with red color. (d) Droplet visualised in dark field imaging. (e) Drop-casted nanoparticle cluster on silica substrate visualised in dark field. (f) Scanning electron microscopy (SEM) image of a gold nanoparticle cluster drop-casted on a silica substrate with a scale bar. The inset shows the size of an individual gold nanoparticle. All images of the same droplet are shown in figures (a), (b), (c), and (d).
• Figure 3: Whispering gallery mode (WGM) spectra of a dye-doped nematic microdroplet during coupling and decoupling interactions with a gold nanoparticle cluster. The green curve represents the WGM spectra of the isolated droplet before coupling. The red curve shows the WGM spectra when the droplet is coupled to the gold nanoparticle cluster, indicating enhanced fluorescence intensity due to plasmonic interaction. The blue curve corresponds to the decoupled state, where the droplet is once again isolated, showing the restoration of the almost original WGM spectrum.
• Figure 4: A normalised intensity versus wavelength plot of whispering-gallery modes (WGMs) of a dye-doped nematic microdroplet before coupling (green) and during coupling (red) with a gold nanoparticle cluster. The left panel shows whispering-gallery modes over the broad fluorescence emission spectrum (570–700 nm) before and during coupling. The shaded light purple region is enlarged as indicated by dotted lines in the central panel (590–650 nm), highlighting mode spacing and the non-overlap of the highest intensity modes in decoupled and coupled states. The dark light-purple band in the central panel is further enlarged, as indicated by dotted lines, to show the regions of spectral magnification presented in the rightmost panel. This right panel zooms in on the 600–620 nm range, clearly showing the red shift of the highest intensity mode in the coupled state. A horizontal grey dotted line marks the peak normalised intensity to guide the eye in comparing the spectral shift of modes before and during coupling.
• Figure 5: A normalised intensity versus wavelength plot of whispering-gallery modes (WGMs) of a dye-doped nematic microdroplet before (green) and after coupling (blue) with a gold nanoparticle cluster. The left panel shows whispering-gallery modes over the broad fluorescence emission spectrum (570–700 nm) before and after coupling. The shaded light sky-blue region is enlarged as indicated by dotted lines in the central panel (590–650 nm), highlighting mode spacing and the overlap of modes in decoupled states. The dark light sky-blue band in the central panel is further enlarged, as indicated by dotted lines, to show the regions of spectral magnification presented in the rightmost panel. This right panel zooms in on the 600–620 nm range, clearly showing the overlapping of individual modes before and after coupling. A horizontal grey dotted line marks the peak normalised intensity to guide the eye in comparing the reversibility of the highest intensity modes in decoupled states.