Light-Actuated Micro/Nanoscale Mechanical Motions And Their Physical Mechanisms

Optical forces,as one of the most important technologies for manipulating micro/nanoobj ects in a non-contact way,are widely used in many research areas,including atomic physics,chemistry,and biology.By contrast,photophoretic forces have more advantages in manipulating or transporting light-absorbing objects in the air environment.Since light scattering and absorption during the light-matter interaction are so common,in most cases,these two kinds of light-induced forces co-exist,but usually only one dominates.In the air environment,photophoresis forces can be several orders of magnitude larger than that of the optical forces While in the liquid environment,due to the rapid heat dissipation of the liquid,it is difficult to form a considerable temperature gradient,and thus the photophoretic forces are usually neglected,and optical forces are dominant this time.Therefore,few studies discuss that these two kinds of forces work together to control objects since they work in different environmentsMost of the current optical manipulations are basically realized in the liquid environment When an object is in a non-liquid environment,there will be a giant Van der Waals adhesion between the object and the surface it contacts.For a micron-sized object,the adhesion can reach the order of ?N.However,the optical forces are usually in the order of pN.Therefore,the optical forces cannot overcome the resistance of van der Waals force to drive objects to move in non-liquid environments.That is why most of the previous optical manipulations are limited to low adhesive environments(immersed in liquids to eliminate the adhesion or suspended in air).How to increase the light-induced forces to overcome the surface adhesion resistance and realize the light-actuated motion in non-liquid(dry)environments has become an urgent scientific problem to be solved.In addition,the conventional optical tweezers use high numerical aperture(NA)lenses,which are relied on bulky and costly benchtop instruments.How to miniaturize the optical tweezers and make them easy to integrate to realize optical manipulations on the chip is a research project of great significanceTo solve the first problem abovementioned,we use tapered fiber-gold plate structures and synergize the optical forces and photophoretic forces to drive the gold plate on the tapered fiber to move back and forth.The optical force function as a pushing force to push the gold plate to move toward the tapered fiber tip.In contrast,the photophoretic force is manifested as a pulling force to pull the gold plate back and away from the tip,thereby making the gold plate oscillate back and forth on the tapered fiber.The moving speed of the gold plate can reach 28 ?m/s in experiments.Aiming at the second problem aforesaid,we use pulsed light to excite surface acoustic waves to drive objects to move.The light-induced forces are greatly improved and can reach the order of ?N.Therefore,it can overcome the huge surface adhesion in non-liquid environments to drive objects to move.We have experimentally realized light-driven rotation,swinging,and spiraling of a gold plate around a micro/nanofiber.The gold plate can perform stepping motion driven by a single light pulse,the resolution of light-driven locomotion reaches the sub-nanometer scale.The angle resolution of the light-driven rotation reaches 0.001 degrees.The rotating gold plate actuated by light is used as a micro-mirror to realize the precise deflecting and scanning of a laser beam.To solve the third problem mentioned above,we have designed and experimentally verified two kinds of on-chip optical tweezers that can achieve full three-dimensional optical trapping and manipulation of microspheres.The first kind of proposed on-chip optical tweezer is based on a high NA single-beam.Metasurface structures(i.e.,focusing gratings)are designed on the waveguide to scatter the light out from the waveguide and focus the light on the top.The designed depth of the potential wells in transverse and longitudinal directions reach 250 and 96 kBT/mW,respectively,which are high enough for the 3D stable trapping of microspheres.The second kind of proposed on-chip optical tweezers is based on crossed dual beams.Freeform focusing reflectors are 3D printed on the facet of the polymer waveguide.The light beams that come out from the waveguide will be total-internal-reflected and focused by the freeform focusing reflectors.Although the focusing of the dual beams is not as tight as that of the high NA single-beam,the trapping performances of the crossed dual beams are as good as that of the high NA single-beam.The designed three-dimensional potential well depths reach 519,574,and 330 kBT/mW,respectively.We have achieved single-particle 3D trapping on the chip with the crossed dual beams in the experiments.In addition,by using dual beams propagating in opposite directions,multiple particles are trapped on the chip at the same time,and the interaction between particles is also studied.