Published papers

Dynamic tuning of nanoscale coloration by exploiting localized surface plasmon resonance of gold nanoparticles (AuNPs) combined with an interference coloration mechanism is demonstrated experimentally. When interference between the scattering field from AuNPs and the reflected field from the substrate is observed under back-scattering white-light microscopy, the AuNPs exhibit various colors depending on their distance to the substrate. When the numerical aperture of the microscope objective is optimized, much greater coverage of the color space than was achieved with previously reported plasmon-based approaches is attained. Also, color tunability is examined by exploiting the temperature-induced volume change of a temperature-responsive hydrogel with embedded AuNPs to dynamically modify the distance to the substrate.

Indirect interactions via shared memory deposited on the field (“field memory”) play an essential role in collective motions. Some motile species, such as ants and bacteria, use attractive pheromones to complete many tasks. Mimicking these kinds of collective behavior at the laboratory scale, we present a pheromone-based autonomous agent system with tunable interactions. In this system, colloidal particles leave phase-change trails reminiscent of the process of pheromone deposition by individual ants, and the trails attract other particles and themselves. To implement this, we combine two physical phenomena: the phase change of a Ge2Sb2Te5 (GST) substrate by self-propelled Janus particles (pheromone deposition) and the AC (alternating current) electroosmotic (ACEO) flow generated by this phase change (pheromone attraction). Laser irradiation causes the GST layer to crystalize locally beneath the Janus particles, owing to the lens heating effect. Under AC field application, the high conductivity of the crystalline trail causes a field concentration and generates ACEO flow, and we introduce this flow as an attractive interaction between the Janus particles and the crystalline trail. By changing the AC frequency and voltage, we can tune the attractive flow, i.e., the sensitivity of the Janus particles to the trail, and the isolated particles undergo diverse states of motion, from self-caging to directional motion. A swarm of Janus particles also shows different states of collective motion, including colony formation and line formation. This tunability enables a reconfigurable system driven by a pheromone-like memory field.