Studies Of Optical Trapping In Plasmonic Tweezers

This dissertation aims to study the optical trapping of plasmonic tweezers.As is well-known,the invention of optical tweezers by A.Ashkin opened a new era in microscopic manipulation,which provides a useful tool in fields of medicine,biology,physics,etc.The fundamental physics underlying optical tweezers are the optical force and optical potential,which are the products of light-matter interaction,including the linear momentum and angular momentum exchange.Thanks to the laser,whose properties of high intensity,collimating,and coherence provide an opportunity to achieve noninvasive,high-precision optical manipulation.As the size of the object scales down from macroscope to microscope,optical force shows superiority over other forces,like gravity,buoyancy force,etc.That is why a single focused laser beam can trap microbeads and even deliver them.However,when the size of objects scales down to subwavelength,e.g.,nanoscale,the optical force decreases dramatically as it obeys the r3 rules and cannot provide enough magnitude to maintain the stable trapping of objects at the nanoscale.That is the drawback of traditional optical tweezers,due to the diffraction limit of the objective lens.Plasmonic tweezers,composed of metallic nanostructures that resonate in the visible or near-infrared,offer great potential for replacing traditional optical tweezers at the nanoscale.The advantages of plasmonic tweezers include:a).They support surface plasmon resonances(SPRs),which induce high field enhancement at the local hotspot.As a result,the optical force will gain several orders enhancement nearby the hotspot and overcome the disturb.b).The small volume of the hotspot will overcome the diffraction limit,so precise manipulation can be achieved.Therefore,the study of plasmonic tweezers can help to solve the problems in the field of nanoscale optical manipulations.In addition,the emergence of metasurfaces and metalens opened another way to overcome the difficulties of traditional optical tweezers.As one of the most promising candidates for integrated nanophotonic devices,metasurfaces are often regarded as the replacing or complementary part of traditional optical elements.Varieties of functionalities have been demonstrated before to replace the bulky optical elements,such as focusing lens,vortex plate,holography,etc.As a result,the flexibility of metasurfaces shows great potential in modern optical tweezers,i.e.,replacing bulky optical lenses with metasurfaces.In this dissertation,the plasmonic metasurfaces were studied to achieve some special functionalities,to realize optical trapping via plasmonic metasurfaces.The contents of this dissertation contain seven chapters,which are listed as follows:1.In the first chapter,we first summarized the concept of optical pressure,the invention of optical tweezers,and a brief introduction to their applications.Second,we made a discussion of optical force and optical potential,and the methodologies of optical tweezers were introduced.Third,we made an overview of the plasmonic tweezers and their applications.Finally,we made a brief remark about new optical tweezers based on metasurfaces and metalens.2.The skill of simulations and experimental methods were summarized in this chapter,including the full-wave simulation software,state-of-art fabrication methods,and optical measurements.3.The third chapter showed our first study on plasmonic tweezers.We focused on the double-disks plasmonic system,which supports the toroidal resonance.The method of Maxwell stress tensor was used to calculate the optical force acting on a 5-nm-diameter sphere.We demonstrated the annular trapping and potential capability of these plasmonic tweezers.4.The fourth chapter experimentally demonstrated the optical manipulation by plasmonic tweezers is composed of V-shaped antennas arrays.Two different trapping modes have been demonstrated for x-and y-polarized incidences,respectively.5.In the fifth chapter,we discussed the potential applications of metasurfaces-based optical tweezers.Then,a new method of phase design has been proposed,which can independently manipulate left-handed circularly polarized light(LCP)and right-handed circularly polarized light(RCP).Therefore,we have demonstrated the optical spin Hall effect.6.The sixth chapter discussed the ability to fully decouple phases of LCP and RCP.By using chiral nanostructures(enantiomers),the phase response of LCP and RCP can be fully decoupled.Moreover,the enantiomer metasurfaces have been fabricated and characterized to prove the decoupled phase.7.Finally,in the seventh chapter,we explored the counter-intuitive optical force,i.e.,optical pulling force via hyperbolic metamaterials.We numerically simulated the optical force acting on the gold nanowires when they are placed on the hyperbolic metamaterials(HMMs).