Opto-thermoelectric pulling of light-absorbing particles

Lin et al. Light: Science & Applications (2020)9:34 https://doi.org/10.1038/s41377-020-0271-6 ARTICLE Official journal of the CIOMP 2047-7538 www.nature.com/lsa Open Access Opto-thermoelectric pulling of light-absorbing particles Linhan Lin1,2,3, Pavana Siddhartha Kollipara1, Abhay Kotnala1,2, Taizhi Jiang4, Yaoran Liu2,5, Xiaolei Peng2, Brian A. Korgel2,4 and Yuebing Zheng 1,2 1234567890():,; 1234567890():,; 1234567890():,; 1234567890():,; Abstract Optomechanics arises from the photon momentum and its exchange with low-dimensional objects. It is well known that optical radiation exerts pressure on objects, pushing them along the light path. However, optical pulling of an object against the light path is still a counter-intuitive phenomenon. Herein, we present a general concept of optical pulling—opto-thermoelectric pulling (OTEP)—where the optical heating of a light-absorbing particle using a simple plane wave can pull the particle itself against the light path. This irradiation orientation-directed pulling force imparts self-restoring behaviour to the particles, and three-dimensional (3D) trapping of single particles is achieved at an extremely low optical intensity of 10−2 mW μm−2. Moreover, the OTEP force can overcome the short trapping range of conventional optical tweezers and optically drive the particle flow up to a macroscopic distance. The concept of selfinduced opto-thermomechanical coupling is paving the way towards freeform optofluidic technology and lab-on-achip devices. Introduction A photon carries momentum, which can be transferred to low-dimensional objects to realize optical manipulation. Laser radiation can push a particle along the light path once the particle ‘feels’ the radiation pressure. Engineering of a laser beam using a high numerical aperture (NA) lens creates an intensity gradient that can efficiently trap an object at the beam centre, which was developed as optical tweezers by Arthur Ashkin1. However, the idea that light can pull an object against the flow of light, which is also known as an optical tractor beam, is counter-intuitive because the Poynting vector normally points along the propagation direction of incident light2. To maintain the conservation of momentum, the key to achieving optical pulling is to engineer the sign of the * momentum change Δ p during the light–matter Correspondence: Linhan Lin () or Yuebing Zheng () 1 Walker Department of Mechanical Engineering, The University of Texas at Austin, Austin, TX 78712, USA 2 Materials Science & Engineering Program and Texas Materials Institute, The University of Texas at Austin, Austin, TX 78712, USA Full list of author information is available at the end of the article. interaction. Over the past decade, some strategies have been proposed to achieve optical pulling, such as sign reversal of the Poynting vector3,4, amplification of the forward-to-backward scattering intensity to transfer backward momentum to the objects5–8, or the interaction between the object and the self-collimation mode from the photonic crystals9. However, it can be proven that optical pulling based on momentum transfer between an incident plane wave and low-dimensional objects is unachievable. From another perspective, photons also carry energy, which can be transferred to low-dimensional objects. Specifically, photon-to-phonon conversion, also known as the optothermal effect, is an entropically favourable process. Laser radiation on a light-absorbing object creates a temperature difference with asymmetric thermal energy densities, which provides an alternative strategy for optical manipulation10. Photophoresis, which arises from the asymmetric gas-dynamic force when a light-absorbing object is directionally irradiated in the gaseous medium, drives the object towards the low-thermal-energy side, where the force imparted by the gas molecules is weaker11. It is © The Author(s) 2020 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. Lin et al. Light: Science & Applications (2020)9:34 comprehensible that the photon-to-phonon energy transfer predominates at the illuminated side of light-absorbing objects. The key to pulling an object using photophoresis is to flip the thermal energy distribution, which requires a rigorous design for both the heating optics and the structures of the light-absorbing objects11–13. To date, pulling an object in free space using simple optics is still elusive, which hinders the development of the optical pulling force as a general manipulation technology. The essential approach to overcoming the rigorous design rule in the existing optical pulling systems is to obtain a light-directed force pointing from the side with low thermal energy density to the side with high thermal energy density. Herein, we propose that the directional irradiation of an incident plane wave on a light-absorbing particle can create a temperature gradient on the particle surface, which drives the thermophoresis of ionic species and induces a thermoelectric field to pull the particle consistently. Specifically, we demonstrate that the optothermoelectric pulling (OTEP) force can impart selfrestoring behaviour to the particle for low-power threedimensional (3D) manipulation. Moreover, we prove that OTEP can overcome the short working range of conventional optical tweezers and optically drive the particle flow up to a macroscopic distance without the need for mechanical confinement or pumping. Results Working principle Generally, thermophoresis describes the directional migration of an object along an external temperature gradient field, which can be generated optically or electrically14,15. It provides another possibility to convert photon energy into the mechanical energy of lowdimensional objects16–21. It is also possible to manipulate a low-dimensional object using the temperature gradient generated by the object itself without external heating sources, which is termed self-thermophoresis22. Herein, we adopt amorphous Si particles (see Fig. S1 for a scanning electron microscopic images), which possess high optothermal conversion efficiency and low thermal conductivity. Upon laser irradiation, a temperature gradient field is (...truncated)