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Light makes a gold plate dance gracefully in air

2019-03-15

First look at this footage may make you feel bored. If it is light that powers this micrometer-sized single-crystalline gold plate “dancing” around a microfiber about 1/50th as thick as a hair, will you be impressed?

A circular dance, born in the State Key Laboratory of Modern Optical Instrumentation, made its debut on the cover of Science Advances. Driven by light, the gold plate revolves ceaselessly around the microfiber in a graceful manner. It is the first time that a micrometer-sized gold plate has rotated around a microfiber due to light in air and vacuum. Previously, light-driven motions could only be achieved in liquid environments.This research, led by QIU Min, a professor of optical science and engineering, opens up a novel approach into the development of nanorobotics, which has great potential in many applications.

Light is the ideal “power” for these nanorobots because of the momentum and thrust when applied to an object. However, in the microscopic world, there is a vigorous opponent against light—dry adhesion force, also known as the van der Waals force, in a dry environment. 

“If a person were to shrink down to a nanoscale version, he or she would stick to the floor, thus unable to move a step. This will tell you how strong the adhesion force would appear,” said LU Jinsheng, the lead author of this research and a PhD Candidate of the college of optical science and engineering. In contrast with considerable adhesion force, light-induced force is several orders of magnitude smaller. If you want to use light to drive an object, it seems to be a drop in the bucket. Previously, attempts have been made to eliminate this adhesion force by means of liquid infiltration, thereby enabling light to push an object. But in air or vacuum, people are still at their wits’ end as to how to deal with adhesion.  

 “Inspired by the inchworm motor, we crack this hard nut,” says LU Jinsheng. A worm that wriggles on the surface of a leaf will stretch and contract, and move forward due to friction. The gold plate absorbs light and generate heat. When pulsed light applied to the gold plate, elastic waves will form at the surface of the plate as it expand and shrink. The adhesion force makes the plate stick to the fiber. “Like a nano-reptile, this gold plate can crawl around the microfiber by utilizing elastic waves and adhesion force,” he explains.  

“In our research, we enable nanomaterials to dance with ‘handcuffs’,” observes QIU Yu. Their idea transforms this adhesion force that was previously thought to bind the motion of nanodevices into the power of “dancing.”

Furthermore, researchers can control rotation velocity and step resolution by varying the repetition rate and pulse power respectively. A light-actuated micromirror scanning with 0.001° resolution is then demonstrated on the basis of this motor.  

This nanoscale motor can potentially be used in various fields, including prospective micro-opto-electromechanical systems in outer space, energy conversion, and vacuum high-precision mechanics. The rotating plate could serve as a scanning micromirror to deflect a laser beam. The typical applications of this include laser scanning for miniature lidar systems or laser display systems and optical modulating/switching for integrated microsystems. It could also function as a paddle for inducing gas motion in microfluidics systems. Furthermore, the discovery of this novel light-actuated locomotion phenomenon could open a new era of optical driving and manipulation, with subnanometer locomotion resolution and controllable motion duration. It renders it possible to explore the new landscape of optical nanomanipulation in environments requiring a new paradigm for liquid-based manipulation.

“(By) doing so, they bring light to a long-standing problem of controlling the rotational degree of freedom in a dry environment,” commented by the reviewer of this article.