Exploiting Optical Asymmetry for Controlled Guiding of Particles with Light

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 nanoand microscale particles and objects has been a long-sought goal in science and engineering. 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. Thus far, external control of the position of such particles has been possible by methods that rely on chemical, electric, magnetic, acoustic, and temperature 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. Light has also been used to transport and guide wavelength and subwavelength sized particles in schemes that include optical tweezers and optical tractor beams. However, 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), 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. 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 of particle control is by switching on the thermophoretic action whenever the particle orientation is facing the target and off when it is not. This requires real-time optical imaging of the particle position and orientation and is limited to particles large enough to be optically resolved, imposing limitations on thermophoretic guiding. It would be desirable to develop a guiding method that eliminates such restrictions, while maintaining the robustness of the thermophoretic force. Here, we achieve this with a new type of asymmetric particle, and show that such a particle can be transported in space by only switching the incident beam’s frequency (this can, for example, be done by using two different monochromatic sources). We demonstrate this concept—which does not require focusing or manipulation of the light beam—on a particle with two counter faces (e.g. Figure 1). The two faces (in our case, gold and titanium-nitride) are designed to preferentially absorb light of different wavelength (500 and 800 nm in our case) regardless of the particle orientation, thus allowing for bidirectional motion. As an example, a guiding scheme that relies on a particle position (or distance to the target) can be used to transport a nano particle over distances greater than 100μm (see Figure 5 and, also, Supporting Information, Video). Because the scheme relies only on the particle location or distance to the target (and not its orientation), the proposed method can be used to guide subwavelength particles that are too small to be fully optically resolved or particles that cannot be visualized at all. Eliminating the need for focusing or shaping of the light beam, multiple particles or particles in strongly scattering environments can be guided. Finally, owing to biocompatibility and nontoxicity of light, this approach to particle guiding may find biological and in-vivo applications. We begin by examining a plane, linearly-polarized, incident light wave of the form Einc = x̂E0exp(ik0z − iωt) impinging on a composite asymmetric particle, as shown in Figure 1a. Because the particle is roughly spherical, its motion can be approximated by a set of differential equations for translation and rotation: m dx dt2 = Fopt(P) + 1