Researchers demonstrate light-induced locomotion in a non-liquid environment and report a new type of liquid-like movement

Experimental observations of an Sb₂The₃ plate moving in a spiral around a microfiber. (a) Sketch of the experimental device implemented in a vacuum chamber. VOA, variable optical attenuator; FC, fiber coupler; MF, microfiber; and PM, power meter. (b) Temporal sequencing of SEM images of an Sb₂The₃ plate moving around a microfiber when the average attack laser power is below the threshold power. In this case, the asymmetrical reflection of the opto-thermo-elastic wave provides the driving force. (c) A liquid-like state can be observed at Sb₂The₃ plate-microfiber interface due to the Marangoni effect, when the average power of the pilot laser is higher than the threshold power. (d) In the case of high input optical power, the asymmetric deformation of the liquid-like state provides the driving force. Credit: Advanced Photonic Nexus (2022). DOI: 10.1117/1.APN.1.2.026005

Motion is everywhere in living systems and is necessary for the mechanical functions of artificial systems, such as robots and machines. Functional mechanical structures that can change volume and shape in response to external stimuli (such as light, heat, electricity, humidity, and chemistry) have a wide range of application prospects in the field. biomechanics and bionic robots. They have generated immense research interest, especially at micro- and nano-scales.

Many actuator proposals rely on light as the energy source. Optical force is commonly used to manipulate micro-objects, due to its unique advantages of precision, immediacy, and ability to miniaturize. The fundamental principle involved in optical manipulation is that photons carry momentum which can be transferred to objects during the processes of scattering and absorption, and hence allow their motion. But the optical force produced by momentum transfer is at a piconewton level, which is much smaller than the adhesive force on a solid interface, making it difficult to work in non-liquid environments.

Light-induced elastic waves present a solution. Induced by the rise in temperature by optical absorption in the micro-objects, they convey a sufficient mechanical displacement to allow the micro-objects to crawl on solid interfaces. The idea has been successfully illustrated with gold microplates in microfiber-based systems. These pioneering results encourage new perspectives for light-driven micromotors on solid interfaces, but there are still questions to explore. For example, the actuation principle is theoretically applicable to any micro-object capable of generating elastic waves by absorbing light, but it has not yet been extended to other elastic absorbing materials. Additionally, potential issues associated with thermal effects (such as thermal damage and melting) remain to be addressed.

As reported in Advanced Photonic Nexusresearchers from the Hangzhou Institute of Advanced Studies, the University of Chinese Academy of Sciences and Westlake University have addressed these outstanding questions by studying the movement of topological insulating antimony telluride plates two-dimensional on microfibers to which laser pulses are delivered.

Antimony Telluride, Sb₂The₃, is a unique quantum material harboring topologically protected boundary surface states that lead to several fascinating electrical and optical properties, such as electron spin momentum locking and ultra-broadband plasmon excitations. Researchers have exploited these properties to efficiently absorb light to generate elastic waves. since sb₂The₃ has a rather low thermal conductivity (~1 W/m/K, close to glass and two orders of magnitude smaller than gold), it can also dampen heat diffusion and intensify thermal effects.

Experimentally, the team implemented a microfiber-based actuation system in a scanning electron microscope (SEM) chamber, where light-induced actuation can be accurately characterized. Their successful observations of the continuous spiral motion of Sb₂The₃ the plates complement previous results with gold plates, providing new evidence in support of the principle of actuation based on light-induced elastic waves.

The team studied the thermal effects on actuation in their system by intentionally increasing the laser power. They observed a new kind of liquid-like motion that exhibits completely different characteristics from spiral motion based on elastic waves. They note that this phenomenon is caused by the formation of the micro-bumps induced by the Marangoni effect, which is a common thermal effect. The asymmetrical deformation of the thermally induced liquid-like state provides the driving force.

Many unique applications can be immediately envisioned with a light-activated actuator in a non-liquid environment. For example, mobile photonic modulation/switching by delivering micro-objects to targeted positions to control light output can be achieved by integrating this technique into an on-chip waveguide array. Additionally, multi-mode micro-robots operating in a vacuum system can be realized by carefully designing the driving lumen and actuator geometry.