Photons carry momentum, and the light-matter interaction can transfer the momentum from the photons to the object. The engineering of a laser beam using high numerical aperture (NA) objective can trap a tiny object in 3D, which is proposed by Arthur Ashkin in 1980s and awarded by 2018 Nobel Prize in Physics. In the past 30 years, optical tweezers have provided a novel strategy in a lot of applications, such as drug delivery, early-disease diagnosis, and the manufacturing of functional nanomaterials. From the view of momentum conservation, it is easy to understand that light can push an object along the light path once the object "feels" the pressure of light. However, it is counter-intuitive that light can pull an object towards the light beam.
In a new paper published in Light Science & Application, scientists from Walker Department of Mechanical Engineering, The University of Texas at Austin, Austin, USA, and the State Key Laboratory of Precision Measurement Technology and Instruments, Department of Precision Instrument, Tsinghua University, China, developed a new technology, termed opto-thermoelectric pulling (OTEP), which can achieve the optical pulling of light-absorbing particles using a low-power laser beam. The optical pulling force stems from the directional optical heating effect, which creates an asymmetric thermoelectric field on the particle surface pointing against the laser beam. Using an optical fiber for light output and amorphous Si particles with diameters ranging from 500 nm to 1200 nm as light-absorbing materials, the scientists demonstrate that the opto-thermoelectric pulling of particles at a distance up to millimeter scale is achievable. This technique can overcome the short working distance in traditional optical tweezers, making it possible the trap particles far away from the fiber tip.
"Using technologies such as optical tweezers, we need to move the focused laser spot close to the target object if we want to trap and manipulate the object. We need to search around and the throughput is low. Now, using optothermelectric pulling, we simply shine the laser beam along one direction. The particles irradiated will move against the laser beam and finally get trapped." The scientists explain.
Of course, this is not the first demonstration that optical pulling of particles is achievable. In 2011, Scientists from The Hong Kong University of Science and Technology have proposed that optical pulling of particles is possible using a backward scattering force. In the past ten years, several other strategies have been proposed for particle pulling, e.g., photophoretic pulling. However, the design of both particle structure and pulling laser beam were complicated. It was anticipated that optical pulling using a plane wave is physically unachievable. The new technique proposed here overcome such limitation taking advantage of the thermoelectric force pointing from cold to hot.
More interestingly, taking advantage of the opto-thermoelectric pulling force, the scientists further demonstrate that 3D trapping of the light-absorbing particles can be achieved using a low-NA and low-power focused laser beam. In other words, 3D trapping of a particle can be realized using heat generated by the particle itself. They also extend the manipulation capability to non-absorbing particles using the light-absorbing particles as a shuttle.
The scientists believe that this new technology will inspire the design of optofluidic and lab-on-a-chip devices for many different applications.
"In traditional microfluidic devices, we use PDMS channel to confine the liquid and a mechanical pump to drive the flow. Now, we can use light as a channel for both confinement and pumping. Thus, all-optical and 3D microfluidic devices may be realized in the future" The scientists forecast.