Opto-thermoelectric pulling of light-absorbing particles

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 self-induced opto-thermomechanical coupling is paving the way towards freeform optofluidic technology and lab-on-a-chip devices. In a phenomenon termed opto-thermoelectric pulling (OTEP), the optical heating of a particle due to the absorption of a simple plane light wave can pull the particle against the direction of the light path. Linhan Lin, Yuebing Zheng and colleagues at the University of Texas at Austin, USA, demonstrate this counter-intuitive concept by manipulating particles of silicon 500 namometers across. The authors propose a mechanism allowing the effect of optical heating to oppose and overcome the optical force in the direction of the light beam. They explain how this can be achieved by manipulating the interaction of the energy of photons with the target particles. The phenomenon can trap and move particles using extremely low optical intensities. There are potential applications based on the interaction of light with fluids (optofluidics) especially in lab-on-a-chip devices.