An Optically Controlled Microscale Elevator Using Plasmonic Janus Particles

Abstract

In this article, we report how Janus particles, composed of a silica sphere with a gold half-shell, can be not only stably trapped by optical tweezers but also displaced controllably along the axis of the laser beam through a complex interplay between optical and thermal forces. Scattering forces orient the asymmetric particle, while strong absorption on the metal side induces a thermal gradient, resulting in particle motion. An increase in the laser power leads to an upward motion of the particle, while a decrease leads to a downward motion. We study this reversible axial displacement, including a hysteretic jump in the particle position that is a result of the complex pattern of a tightly focused laser beam structure above the focal plane. As a first application we simultaneously trap a spherical gold nanoparticle and show that we can control the distance between the two particles inside the trap. This photonic micron-scale “elevator” is a promising tool for thermal force studies, remote sensing, and optical and thermal micromanipulation experiments.

Guidable Thermophoretic Janus Micromotors Containing Gold Nanocolorifiers for Infrared Laser Assisted Tissue Welding

“…It is noted, that the total temperature is ≈135 °C but due to the absence of clear nucleation points, as well as the high Poisson pressure necessary to form a small bubble no boiling was observed. The finding that Janus particles offer (when they are in configuration like Figure 1B) uneven diffusion in X and Y dimension which is in line with the observation reported by He et al21 This finding seems to contradict reports22 of others reporting a shift of the particle in a way that the gold covered side of the Janus particles faces the laser. In such a system one should not be able to detect diffusion differences between X and Y dimensions.…”

“…In such a system one should not be able to detect diffusion differences between X and Y dimensions. The solution of this contradiction is that our particles are heavier than the ones reported in reference22 and therefore rotate due to surface friction at higher laser powers. This is indicated at 30 mW in Figure 2 B where the X and Y dimensions come into closer proximity than at lower laser powers, despite one expects the values to increase at higher laser powers.…”

Exploiting Optical Asymmetry for Controlled Guiding of Particles with Light

“…Light-induced thermal effects can exert force on a particle: for example, in a metal-dielectric particle (such as a Janus particle 18 ), the heat generated by the absorption of light in the metal side induces a local temperature difference, resulting in propulsion (thermophoresis) along the axis of the temperature gradient. 7,8,[19][20][21][22][23][24][25][26] Because the thermophoretic drift is based on absorption of light, it is robust † Massachusetts Institute of Technology ‡ University of Zagreb to scattering in the surrounding environment. However, this same feature is also a disadvantage: as the induced drift always points in the same direction, it is difficult to guide (steer) an object to the desired location.…”

Combined Optical and Chemical Control of a Microsized Photofueled Janus Particle

“…On the other hand, while, photoactivated self‐propelled particles require external macroscopic devices they present the advantage of temporal control and switchability of the motion. Recently, our group succeeded in using a tightly focused infrared laser beam to move a microscale Au–silica Janus particle along the self‐generated temperature gradient . The laser not only acts as the source for heat generation, but it also optically traps the Janus particle in three dimensions.…”