Exploiting Optical Asymmetry for Controlled Guiding of Particles with Light

Abstract

Abstract:Conventional methods of manipulating particles with light, such as optical tweezers and optical tractor beams, rely on beam-shaping to realize complex electromagnetic field profiles and are thus sensitive to scattering. Here, we show that by introducing tailored optical asymmetry in the particle, we can realize a novel guiding method that is controllable by the frequency of light, without regard to the direction or the shape of the light beam. With detailed stochastic simulations, we demonstrate guiding of a two-faced nanoparticle where the optically induced thermophoretic drift serves as the propulsion mechanism. Exploiting the difference in resonant absorption spectra of the two materials, we create a bidirectional local thermal gradient that is externally switchable. This is advantageous because the frequency of a light beam, unlike its shape or coherence, is preserved even in strongly scattering environments. Since this approach is insensitive to scattering and applicable to many particles at once as well as particles that cannot be optically resolved, it may enable useful applications in biology, microfluidics, in vivo tasks, and colloidal science.Controlling the motion of nano-and microscale particles and objects has been a long-sought goal in science and engineering.1 These functional particles-also referred to as microrobots, microswimmers and nanomotors-carry the potential for a wide range of applications across a variety of disciplines, including biology, medicine, microfluidics, colloidal science and many others.2 Thus far, external control of the position of such particles has been possible by methods that rely on chemical,and temperature 7-9 effects to power the transport on the nano and the microscales. However, many of the proposed schemes fail to meet the conditions for optimal particle guiding, which include controllable and high-speed movement, and biocompatibility as well as the ability to operate in biologically relevant environments. 10Light has also been used to transport and guide wavelength and subwavelength sized particles in schemes that include optical tweezers 11-15 and optical tractor beams. 16,17However, these approaches require focusing and shaping of the light beam and are thus particularly sensitive to scattering. 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. Currently, the only way to achieve a level ...

Light generated bubble for microparticle propulsion

“…Here, however, motion results from temperature gradients rather than directly from light-generated heat. As a result, forces are not much larger than the thermal fluctuations of the surrounding liquid 13,14 , or comparable to those found with radiation-pressure at best 15 . In the following we describe the conversion of light-generated heat into motion of micro-particle immersed in water and discuss the underlying mechanisms of such an event.…”

Recent Progress in Optical‐Resonance‐Assisted Movement Control of Nanomotors

“…Scientists have further developed more sophisticated nanomotors to achieve a better control of the directionality of their movement. For example, an asymmetric nanomotor with different materials deposited on its surface has been demonstrated to move bidirectionally with the movement direction tuned by changing the incident light wavelength (Figure b) . A polystyrene bead, half covered by gold and half covered by titanium‐nitride, has been designed to preferentially absorb light with wavelengths at 500 and 800 nm, respectively.…”

“…b) An asymmetric nanomotor consisting of a dielectric core, a titanium‐nitride cap (A) and an Au cap (B) on each side. The coating materials enable wavelength‐selective optical absorption so the nanomotor direction can be controlled by changing the wavelength of the incident light . Reproduced with permission .…”

External control strategies for self-propelled particles: Optimizing navigational efficiency in the presence of limited resources

“…To implement this strategy in our simulations, D R is replaced by D * R in Eq. (15). Figure 12 shows the dependence of the time duration to the target T as a function of the diffusional enhancement factor .…”