Numerical Investigations On The Enhancement Of Optical Force Via Surface Plasmons Of Two-Dimensional Materials

The traditional optical tweezers technology proposed by Ashkin provides the possibility of the effective capture and manipulation of mesoscopic particles,which was awarded the 2018 Nobel Prize in Physics.Surface plasmon(SP)optical tweezers broke the limitation of traditional optical tweezers,i.e.,the dependence on laser intensity and diffraction limit,which owes a good deal to the capability of surface plasmons in the control,enhancement,and confinement of surface optical fields.It is the extension and expansion of traditional optical tweezers.Surface plasmon optical tweezers are capable of trapping particles with subwavelength scale while weakening the effects of laser damage.The plasmon modes of two-dimensional materials push the manipulation of surface plasmon optical tweezers to deep subwavelength scale.Two-dimensional materials with the features of atomic thickness,tunable spectral response provide excellent conditions for the manufacture of high-strength,high-integrated optical tweezers.In this thesis,theoretical models of the enhancement of optical force via surface plasmons of graphene and black phosphorus are investigated,and the dynamic control of optical force is realized by virtue of the tunable carrier densities of graphene and black phosphorus.The enhancement of optical force via localized surface plasmon(LSP)of black phosphorus and acoustic graphene plasmon(AGP)of graphene are systematically investigated by combining Mie theory,Maxwell stress tensor(MST)method,optical gradient force(GF)theory and finite-difference time-domain(FDTD)solutions.In addition,plasmon-induced transparency(PIT)in borophene waveguide with strong absorption inhibition at critical-coupled state is also studied.The main results of this thesis are as follow:(1)A graphene-insulator-metal structure is proposed to investigate the enhancement effect of acoustic graphene plasmon cavity with ultra-small mode volume on optical force.The acoustic graphene plasmon mode can produce strong field confinement and higher momentum,and has great control ability of optical field,which can greatly enhance the total optical force and trapping force acted on particles.The scattering characteristics and the total optical force of acoustic graphene plasmon cavity are investigated by using the finite-difference time-domain solutions.The trapping potential and trapping force provided by acoustic graphene plasmon cavity acted on particles are analyzed by combining the theories of optical gradient force and trapping force.The trapping potential and force generated by acoustic graphene plasmon cavity are two and three orders of magnitude larger than the conventional coupled graphene strips system,and the trapping potential and force under ideal condition reach the maximum-13.6×10~5k _BT/W and F_x=2500 n N/W.Finally,the effect of the spacer between metal nanocubes and graphene sheet on the total light force and trapping force are also discussed.(2)A composite structure of black phosphorus coated dielectric cylinder is proposed,the electric and magnetic field around the black phosphorus coated dielectric cylinder is analyzed by Mie theory,and the optical force of the structure is studied by means of Maxwell stress tensor(MST)method.The results of theoretical analysis are verified by numerical calculation via the FDTD solutions.Finally,the extinction characteristics and optical force of dielectric cylinder pairs coated with black phosphorus are studied,as well as the trapping force acted on polystyrene nanoparticles.In the proposed structure,the total optical force between the two cylinders can reach over 1000 p N/m W/μm.Specifically for the polystyrene particle captured in the center of the structure,the optical trapping force up to 4000 p N/m W/μm can be achieved.These results provide new avenues for the successful manipulation and capture of nanoscale particles.(3)We propose a critical-coupled PIT borophene resonator-waveguide system in the communication band with strong absorption inhibition.Intrinsic loss of this scheme is strongly suppressed by coupling between critical-coupled bright mode and dark mode,and then the purpose of reducing absorption can be achieved,and the plasmon-induced transparency(PIT)effect with strong absorption inhibition is realized in the communication band.Based on numerical calculation by FDTD solutions and theoretical analysis via coupled mode theory(CMT),the transmission characteristics of this system are investigated.It turns out that this approach has minimized excess absorption losses and maximizes the amount of transmitted energy,and the results indicate that the structure meets our expectation.This scheme could realize low loss energy transmission in the communication band,and has the characteristics of switchable and tunable.Our results may provide an alternative way to design lab-ona-chip borophene devices.