【作者】 王琳；

【导师】 丁卫强；

【作者基本信息】 哈尔滨工业大学 ， 物理学， 2021， 博士

【摘要】 微纳颗粒的非接触捕获和操控在分子生物学、生物化学、纳米制造等领域有重要应用价值。实现微粒捕获的光镊手段大致可分为传统光镊、金属等离激元光镊和电介质纳米天线光镊三种。传统光镊使用高度汇聚激光束形成的三维势阱捕获微粒,但受到衍射极限的制约;金属微结构利用表面等离激元的强局域场捕获微粒,但无法避免光吸收和光损伤;电介质纳米光镊能够实现与金属微结构相比拟的强局域场,并避免光毒性对微粒的损伤,因此引起人们的普遍关注。尽管目前对光学捕获的研究已经取得了重要成果,但对所捕获微粒在光场中的灵活精细可控移动研究还存在很多不足之处。本论文利用理论分析与数值模拟相结合的方法研究狭缝微腔中多模线性叠加、周期波导中法布里珀罗共振调制的布洛赫模式、光子晶体平板中的法诺共振、以及微环谐振腔的选择性激发等机制,深入研究周期性微纳结构中的稳定光捕获和可控精细移动。首先,在分布布拉格反射和狭缝局域增强两种机制结合所实现的光子晶体狭缝微腔中,实现对纳米金属和介质颗粒的稳定捕获。在此基础上,利用狭缝波导微腔中两个共振模式的线性叠加,实现光场和光势阱的灵活可控调节。通过改变两种共振模式的相对入射光功率,实现两个纳米颗粒间距在一定范围内的精细微调,并分析了调节范围与结构参数之间的关系。当两纳米颗粒间距需要在更大范围内调节时,我们提出周期分布的多狭缝微腔结构,该结构整体上具有一维镜面对称性,且每个周期包含三个不同的共振单元。通过入射波长调节的选择性激发方法,实现两个纳米颗粒间距在一维方向上的大范围可控调节,并利用郎之万方程模拟了在光势阱以及布朗运动共同作用下,两个纳米颗粒间距调控的动力学行为,模拟结果与理论分析结果相一致。其次,通过周期光子结构中本征模,即布洛赫模的研究,发现布洛赫模的光场分布本身就呈现亚波长强局域特性,且电场局域尺度受波长影响很小,主要与晶格结构有关,我们提出这一特性能够自然形成对微粒的高效率捕获。以往研究中通常都是利用周期结构中的缺陷模实现捕获,而我们此处提出利用无缺陷周期结构中的布洛赫模实现光学捕获。研究实际周期结构的有限尺寸效应产生的法布里珀罗共振及其对布洛赫模的影响,发现通过调节波长可以控制光场局域强度极大值在结构内微小移动,并在一维多层光子晶体结构中系统地分析了这种移动现象。根据这一结果,构造纳米孔排列形成的一维周期光子晶体波导结构,并得到了与一维层状光子晶体类似的光场局域特性。发现导带内临近波长所对应光场的分布规律;选择导带内三个临近波长,并将入射光波长按顺序依次调节为这三个波长,能够实现对纳米颗粒的亚波长精度位移控制。非常重要的一点是这三个波长的选择非常灵活,并且精度要求低,有利于实际应用。最后,研究电介质平板周期结构对微粒的捕获及在二维空间的位移。研究二维光子晶体平板的法诺共振,计算了不同入射角的偏振光对共振场强度的影响,利用特定入射角形成显著增强的法诺共振实现二维电介质平面的高效近场捕获。通过入射光场的调制,实现微粒在二维直角坐标系中的位移调控。研究具有纳米孔分布的电介质同心微环阵列,对微粒在二维极坐标中的高效捕获及汇集。基于微环中基于自旋轨道耦合而产生的单向传输特性,利用圆偏振光实现角向共振局域,进而控制微粒的角向捕获。研究不同圆环半径和其中纳米孔间距等参数对微环共振波长的影响,并以此形成径向上可控的共振模式。利用波长选择激发不同微环实现微粒捕获位置的径向调控,为低浓度纳米颗粒的高效汇聚提供了可行性方法。本论文对电介质微纳结构中可控光捕获特性的系统研究,为解决微纳尺度颗粒的精细捕获和定向移动问题提供全光解决思路和方法,对生物、化学及纳米等技术的应用有重要意义。更多还原

【Abstract】 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.更多还原