| Non-contact optical trapping of micro-and nano-particles(MNP’s)opens up new prospects in the fields of molecular biology,biochemistry,nanomanufacturing,etc.Generally speaking,there are there different kinds of optical trapping setups for optical tweezers(OTs),which are traditional OTs,metallic plasmonic nano tweezers and dielectric nano-antenna tweezers.Traditional OTs typically use the potential well of a tightly focused laser beam for the trapping of MNP’s.The precision of OTs,however,is limited by the diffraction limit of light.The metallic micro-structures use the strong localized field of surface plasmons to trap particles very precisely,but the drawback is the heating effect of metal,which may damage the samples.Dielectric nano-antenna OTs can achieve almost the same level of light localization as metallic structure,while it’s free from photo-damage on particles,and therefore,it has attracted much attention in recent years.Although great progresses have been made in optical trapping,the controllable movement of the trapped particles in precise and flexible manners is still an open question.In this dissertation,we systematically investigate the controllable and precise movement of trapped particles in periodic photonic structures using different mechanisms,including the linear superposition of modes in slot microcavities,the modulation of Bloch mode by the Fabry-Perot resonance in finite length periodic waveguides,Fano resonance in photonic crystal slabs and the selective excitation of concentric micro ring array.First,the stable trapping of nano metal and dielectric particles is investigated in a photonic crystal slot microcavity(PCSM),which is achieved by two kinds of light localization mechanisms,i.e.,the distributed Bragg reflection and slot induced light enhancement.Based on this result,the controllable shift of optical field localization and the potential well is realized by using the linear superposition of the multiple resonant modes in the PCSM.The interparticle spacing is tuned very precisely within some range by changing the relative powers of the resonant modes.We also analyze the relations between the structural parameters and the range of particles’ shift.In order to increase the separations between the particles further,a new structure formed by periodic multi-slot microcavity is proposed,which is mirror symmetric about its center,and there are three resonant subunits in each period.Using this structure,by selectively excitation of different subunits with corresponding source wavelength,the separation between the two MNP’s can be tuned gradually in a controllable manner within a very long range(which is infinite long in principle).The dynamics of the particle movement are simulated by using the Langevin equation with the driving of the optical force and the random Brownian motion force,and good agreement between simulation and theoretical analysis are obtained.Second,we find that the eigen mode of a periodic structurer,i.e.,the Bloch mode,can generate strong sub-wavelength localized fields,and the dimension of which is independent on the incident wavelength but related to the lattice structure only.This feature naturally ensures an efficient optical trapping.In previous investigations,defect modes are always used,but in this dissertation,we propose to use the Bloch mode of defect-free structure for optical trapping.The effect of Fabry-Perot resonance on the electric fields of finite length periodic photonic crystals is investigated,and it is found that the position of the electric field localization can be controlled by adjusting the incident wavelength.We analyzed the shift of the light localization systematically and precisely in a simplified multilayer structures.According to the results,a one-dimensional periodic photonic crystal waveguide is designed by introducing a line of nano-holes in a high refractive index strip waveguide,and light localization properties similar to those in multilayer structure is found.We find the distribution features of neighboring wavelength inside the guiding band,and three typical wavelengths are selected.When the wavelength of the incident source is tuned onto the three wavelengths in a predefined order,the MNP’s can be moved in control with a subwavelength precision.More importantly,the three wavelengths can be selected very flexible and robustly,which is beneficial to practical applications.Last,the trapping and transporting of particles are extended from onedimensional(1D)to two-dimensional(2D)cases.The effects of incident angle and polarization state on the Fano resonance in a 2D PC slab are studied insightfully.Results show that at some specific incident angle,significantly enhanced fields on the PC slab are achieved,and they are used to realize efficient near-field trapping.By tuning the incident configuration slightly,the Cartesian coordinates of the MNP’s can be changed arbitrarily on the surface of the 2D PC slab.The concentric dielectric micro ring array is designed to achieve efficient trapping and arbitrary tune of the polar coordinates of the MNP’s.Based on the unidirectional propagation in the micro ring due to the spin-angular coupling effect,handedness of circularly polarized light is used to achieve angularly controlled resonance field,which in turn controls the polar angle position of the particles.The effect of radii of the concentric rings and the separations between two neighboring holes(the arc length between them)on the resonance wavelength are investigated,and micro-rings with different parameters are designed to form stable trapping at different polar radius by selectively excited resonance mode in the radial direction.By continually tuning the incident wavelength,a trapped particle can be shifted from an outer ring position to the inner most ring step by step.This operation makes the concentration of extremely low abundant sample possible.The systematical investigation on the controllable optical trapping reported in this dissertation provides an all-optical routine for achieving precision trapping and directional movement of MNP’s.It is significant and will find potential applications in the investigation of biology,medical applications and nanotechnology. |
