Near-field optical trapping allows for contact-free control of objects, including nanoparticles and living cells. This is traditionally done using whispering-gallery-mode and waveguide-based platforms that rely on evanescent fields that only penetrate about 100 nanometers into the surrounding medium. This shallow interaction restricts trapping efficiency and makes the system highly sensitive to perturbations, limiting the practical applications that require dense, large-area or long-term manipulation.
Gradient-thickness-protected WGM microbottle resonator for large-scale particle trapping. Credit: Microsystems & Nanoengineering
Researchers from Fudan University and The Hong Kong Polytechnic University have developed a new optical trapping platform in Microsystems & Nanoengineering that overcomes this limitation.
The gradient-thickness-protected design
The team demonstrated a gradient-thickness-protected (GTP) microbottle resonator that enables large-scale, stable optical trapping via whispering-gallery modes. This method introduces a controlled wall-thickness gradient into a hollow microbottle geometry, supporting high-order axial modes that generate multiple optical trapping sites along its length. The design allows particles to be trapped over nearly 200 micrometers with ultra-low optical power.
The microbottle resonator’s wall is thinnest at the equator and gradually thickens toward both ends, fundamentally changing how optical fields are distributed inside the resonator. Instead of confining particles to weak evanescent fields near the surface, the device generates strong optical-field antinodes that extend several micrometers into the liquid core, creating deep, stable trapping potentials. The antinodes are the peaks of the light wave rather than the tail, making the trapping force significantly stronger and allowing it to reach farther into the liquid core.
The researchers showed that this configuration supports high-order axial whispering-gallery modes, forming dozens of discrete trapping “orbits” along the resonator axis. Experiments demonstrated stable trapping of 500-nanometer-radius polystyrene particles across an axial span exceeding 195 micrometers, with a trapping threshold power of 0.198 milliwatts.
Protection against performance degradation
The gradient-thickness design protects the strongest optical fields by confining them within the silica wall at the resonator ends. This minimizes the degradation of the optical quality factor when particles are trapped, ensuring consistent performance even during large-scale, multi-particle manipulation. The platform also supports localized, tunable trapping via standing-wave excitation, enabling precise repositioning of individual particles.
The microbottle resonator opens new possibilities for high-throughput and label-free particle manipulation. Its extended trapping range and multiple stable orbits make it suitable for parallel single-cell analysis, bioparticle sorting and real-time monitoring of microbial dynamics. The rapid orbital motion of trapped particles can also enhance micromixing, potentially accelerating biochemical reactions in microfluidic environments. The platform may also enable advanced sensing, targeted drug delivery and reconfigurable optofluidic devices. The study highlights how geometric design can unlock new regimes of light-matter interactions.
