利用光学天线裁剪光学矢量场
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摘要
在集成光学和纳米光子学领域,由于衍射极限的限制,传播场和局域场之间的高效转换是一个关键的基本物理问题。借鉴微波天线的概念,光学天线能够实现传播场和局域场的高效转换。随着现代纳米技术的快速发展,亚波长尺度的金属结构加工也更易于实现,极大地促进了光学天线领域的研究开展,光学天线在超衍射分辨成像、高分辨率光谱技术、高效率太阳能电池以及纳米光刻等领域都有着广泛的应用前景,被认为是现代光学发展的重要方向之一。矢量光场通过对光场偏振状态在空间上的裁剪从而可以与光学天线等亚波长金属结构更加有效地相互作用。圆柱矢量光束由于其特殊的空间变化的偏振分布,为光场的裁剪提供了新的思路。本论文以光学天线的设计和应用为研究目标,对圆盘形、牛眼形和螺旋形光学天线在近场光学中的应用进行了理论和实验的研究,通过矢量光场与不同结构天线的相互作用,可以根据特定应用的需求对光场进行裁剪。
本论文解决的主要科学问题及取得的学术成果总结如下：
问题一：根据相关报道,通过将量子点与牛眼天线相耦合,牛眼天线能够对荧光场的强度、方向性以及出射方向进行很好的调控。除此之外,在量子信息处理、单分子传感以及集成光子电路等应用中,纳米尺度的空间结构光场也有着极其重要的应用。然而,在现有的光学天线研究中,对发光体辐射场的角动量调制还未得到实现。
问题一的学术思想及解决方案：当纳米尺度发光体靠近光学天线时,辐射场的特性会因天线的光学共振而发生改变。当光束与均匀、各向同性的光学天线发生相互作用时,例如牛眼天线结构,自旋和轨道角动量之间的守恒是相互独立的。然而,如果天线的结构更为复杂,例如各向异性或非均匀,入射光子与天线之间将产生自旋-轨道相互作用。当辐射场与螺旋天线发生相互作用时,根据物理系统中的总角动量守恒规则,螺旋结构的手性将转化出射光子携带的角动量,从而实现对光子角动量的调控。
提出并设计了可用于调制纳米尺度发光体辐射场特性的阿基米德螺旋天线。通过将量子点与螺旋天线相耦合,能够获得方向性极好的圆偏振涡旋光束；通过改变螺旋结构的螺距和激发点源的位置,可以对出射光子的轨道角动量和出射方向进行有效的控制。实验中得到了与理论计算相符的结果,获得了半高全宽为10。、峰值强度增强为70倍、方向性为11.4dB、圆偏振的消光比为10的荧光场。当受激点源的位置偏离螺旋结构几何中心200nm和500nm时,观测到的荧光场峰值的偏移角度分别约为3。和6。。
问题二：在纳米光刻应用中,复杂图案的加工可以通过表面等离子体(SPPs)近场探针逐点扫描的方法实现。为了获得较高的SPPs场增强,通常采用径向偏振激发的轴对称探针结构。然而,由于激发光的偏振态在空间呈非均匀分布,径向偏振光的中心奇点需要与探针的结构中心准直。因此,该探针无法做成阵列而用于大面积的光刻应用。同时,在实际应用中需要一套扫描装置,从而降低了刻写的速度,限制了光刻的面积。
问题二的学术思想及解决方案：对于特定手性的阿基米德螺旋天线,它能够将具有相同手性的圆偏振光场聚焦为中空的光斑,而将相反手性的圆偏振光场聚焦为实心光斑。
提出并设计了一种锥形针尖整合在螺旋天线几何中心处的近场探针。螺旋天线的聚焦场分布具有自旋取向的空间分离特性,因此当入射光子的自旋态发生改变时,针尖尖端处的光场也会产生对比度很大的明暗变化。为了在获得高场强的同时保持相对较低的远场辐射损耗,探针采用了双层螺旋和复合型针尖的结构设计。数值模拟的结果表明,针尖处可获得的最高场增强因子为366,相对应的消光比为81。该探针采用的激励源为偏振态在空间中均匀分布的圆偏振光束,在使用时无需进行中心对准,因此可做成阵列的形式用于大面积的复杂结构的光刻加工。
问题三：在光学系统中,存在着很多具有轴对称性的光学器件,例如由同心环形狭缝构成的牛眼天线,以及近场扫描光学显微镜(NSOM)的探针。当普通的线偏振光束作用于这些轴对称结构时,SPPs只会在线偏振的方向上被激发,限制了在焦点处的局域场强度,同时焦场呈中空的两瓣分布,限制了光学器件所能达到的分辨率。
问题三的学术思想及解决方案：径向偏振光束的偏振态在横截面上呈径向的轴对称分布,当其作用于同样轴对称的金属结构时,SPPs会在全部的角向方向上产生,从而获得高效激发的SPPs场。基于这一特点,对现有的增强光学透射(EOT)结构和近场探针结构进行了改进,获得了更好的性能。
提出并设计了一种基于径向偏振光激发的EOT结构,该结构由金膜上的单圈环形狭缝和位于狭缝下方的圆盘形纳米金膜天线组成。纳米天线能够将入射光场的能量充分的收集,并高效的耦合至环形狭缝,继而传播到自由空间,获得极强的EOT效应。根据数值模拟的结果,该EOT结构最高可获得251倍的传输效率增强,远远高于之前相关报道中利用线偏振态激发所能达到的数值。
提出并设计了一种锥形针尖整合在牛眼结构中心的复合型近场探针。牛眼天线和锥形针尖都具有非常好的SPPs聚焦效应。在径向偏振光的激发下,这种探针结构有着比单独的牛眼或针尖结构更高的场增强因子。与入射场相比,针尖顶端附近能获得467倍的电场增强,比单独的锥形针尖结构提高了100%,比单独的牛眼结构提高了200多倍。同时,探针的场增强因子不会随针尖锥角的改变而发生剧烈的变化,大大降低了对加工精度的要求。
问题四：消逝贝塞尔光束在非线性光学、原子光学以及光学微操控等领域有着广泛的应用。在之前的报道中,通过将径向偏振光强聚焦在银膜表面,可以产生消逝的零阶贝塞尔光束。然而,在光镊应用中,希望能够产生多种阶次的消逝贝塞尔光束,应用于不同种类颗粒的光学捕获。
问题四的学术思想及解决方案：光子晶体有着可控的色散关系和传输特性,通过合理的选取参数,当光场处于特定的波段、入射角度和偏振时,才能够在光子晶体内部传播。
利用光子晶体的空间滤波特性,以及径向和角向偏振光束在强聚焦下的特殊场分布,提出并设计了利用光子晶体中缺陷模式和带边模式产生零阶和一阶消逝贝塞尔光束的两种装置。在不改变结构参数的前提下,仅通过调整入射场的偏振态为径向或角向,就可获得不同阶次的消逝贝塞尔光束。
本论文的创新点主要包括：
1.提出并设计了可用于调控纳米尺度发光体辐射光场的阿基米德螺旋型光学发射天线。通过将受激点源与螺旋天线相耦合,能够获得方向性极好的圆偏振涡旋光束；改变螺旋结构的螺距,以及激发点源的位置,可以对出射光子的轨道角动量和出射方向进行有效的控制。并用实验进行了验证,获得了方向性为11.4dB,圆偏振的消光比约为10的荧光场,与理论计算相符合。
2.根据螺旋式光学接收天线的聚焦特性,提出并设计了可用于阵列光刻加工的表面等离子体探针。探针采用双层螺旋以及复合型针尖的设计,能够获得极高的局域场增强和圆偏振消光比,数值模拟的结果表明,针尖处可获得的最高场增强因子为366,相对应的消光比为81。通过改变入射圆偏振光的手性,每个探针上的热点开关状态都可得到控制,适用于复杂结构的大面积光刻。
3.基于径向和角向偏振光在强聚焦下的特性和一维光子晶体的空间滤波特性,提出了产生零阶和一阶消逝贝塞尔光束的设计。数值计算结果表明,当结构中所产生增强的零阶消逝贝塞尔光束作用于金颗粒时,所产生的束缚力比目前其他方法有明显提高,从而获得稳定的三维捕获。
In the field of integrated optics and nanophotonics, due to the diffraction limit, the high conversion efficiency between propagating field and localized field is an important and fundamental physical problem. Based on the theory of microwave antenna, optical antenna is designed to efficiently convert free-propagating optical radiation to localized energy, and vice versa. Subwavelength metallic structures become increasingly accessible with the rapid development of modern nanofabrication techniques, assisting the rapid expansion of the research in optical antenna. Optics antenna is regarded as one of the major trend in the modern optics development, and it has great potential applications in supper resolution, high resolution spectrum technology, high efficiency solar cell and photolithography. The aim of the dissertation is studying the design and application of optical antenna, hi this dissertation, we mainly study the applications of disc-shaped, bull eye and spiral optical antennas in the field of near-field optics in theory and experiment. Through the interaction between optical vector field and optical antenna with different structure, the properties of optical field can be engineered according to specific applications.
The main scientific problems solved and academic achievements achieved in this dissertation have been summarized in the list below:
Problem1:Recently, a nanoaperture surrounded by five concentric circular corrugations has been demonstrated for significant enhancement of the fluorescence count rates per molecule and high emission directivity. Besides, the nanoscale spin photon source have high relevance for the application in quantum optical information processing and integrated photonic circuits. However, the modulation of angular momentum carried by the radiated field remains unsolved.
Academic idea and solution:Through controlling the optical resonance in the vicinity of emitters, the properties of emitted photons from the emitters can be engineered. The conservation of spin and angular momentum are independent. However, when the incident spin photons encounter a metallic structure with anisotropic inhomogeneous boundaries, a spin-dependent behavior of plasmonic field could be observed. This phenomenon is due to spin-orbit interaction that is manifested by a geometric Berry's phase.
We analytically, numerically and experimentally study the emission properties of a nanoscale emitter coupled to a plasmonic spiral structure. The spiral antenna would couple the emission into circularly polarized unidirectional emission in the far field, the spin carried by the emitted photons is determined by the handedness of the spiral antenna. Increasing number of turns of the spiral leads to narrower angular width of the emission pattern in the far field. By increasing the spiral pitch in the units of surface plasmon wavelength, these circularly polarized photons also gain orbital angular momentum with different topological charges. In the experiment, quantum dots are adopted as the nano-emitters. For a five-turn Archimedes'spiral antenna, field intensity increase up to70-fold simultaneously with antenna directivity of11.7dB has been measured in the experiment, and a circular polarization extinction ratio of10is obtainable. In addition, if the nanoscale emitter is displaced from the geometrical center of the spiral antenna, the emission peak will shift from the normal direction accordingly. The steering angle depends on the displacement of the nanoscale emitter. This steering phenomenon has been confirmed experimentally. For a3-turn Archimedes'spiral antenna, experimental results reveal that steering angles of3°and7°are obtainable when the excited quantum dots are moved horizontally from the center with a displacement of200nm and500nm, respectively.
Problem2:Nanoscale lithography with complex patterns can be fabricated through point by-point scanning in principle. Efficient plasmonic excitation and focusing can be achieved through matching the axially symmetric dielectric/metal plasmonic lens structure to the polarization symmetry of radially polarized illumination. However, the singularity center of the radially polarized beam needs to be aligned to the center of the plasmonic lens structure. This necessitates a scanning mechanism that leads to slow writing speed and limit the realistic size of lithography area.
Academic idea and solution:The plasmonic field distribution in the center relies on the handedness of incident circular polarization. It has been shown that RHC beam will be focused into a solid spot by a LHS structure, while the field distribution of LHC beam focused by a LHS structure is a doughnut with a dark center.
We study a high efficiency plasmonic near-field probe that integrates a spiral plasmonic lens and a sharp conical tip under circular polarized illumination. To achieve high field enhancement, two layers of spiral plasmonic lens and a composite tip design are adopted. The plasmonic probe exhibits optical spin dependence due to the use of spiral plasmonic lens. Under633nm wavelength excitation, an electric field enhancement factor of366and circular polarization extinction ratio of81can be achieved. Such a spin dependence enables the hot spot at the tip apex to be switched on and off by modulating the polarization handedness. The probe can be made in an array format that is suitable for large area parallel near-field optics applications such as lithography and microscopy.
Problem3:There are many axisymmetric optical devices in the optical system, such as bull-eye structure and near-field scanning optical microscope probe. Surface plasmon polaritons only could be excited in the direction of polarization when linearly polarized beam is used as excitation, which limit the intensity of localized field at the focus and the optical resolution.
Academic idea and solution:The local electrical field of radially polarized beam is linearly polarized along the radial directions. For the plasmonic structure with rotational symmetry, surface plasmons excited by the radial polarization at all azimuthal directions interfere constructively. Therefore, a tightly focused plasmonic field with strong field enhancement is obtained at the focus. According to this feature, we improve the performance of extraordinary optical transmission structure and near-field probe structure.
We numerically study the extraordinary optical transmission of a plasmonic structure that combines a circular nanoantenna and a vertical annular nanoslit etched into a gold film under radially polarized illumination. The nanoantenna collects the incident field and localized it in a horizontal Fabry-Perot cavity over the gold film. Due to the symmetry matching between the structure and the illumination polarization, surface plasmons can be excited effectively and enhanced the transmission. Through optimizing the structure parameters, the transmission efficiency can be greatly enhanced by251times. This axisymmetric extraordinary optical transmission setup may be fabricated on the facet of an optical fiber for optical sensing applications.
We numerically study a plasmonic near-field probe design that integrates a sharp metallic conical tip at the center of a multiple concentric rings plasmonic lens under radially polarized illumination. Due to the symmetry match between the plasmonic structure and the illumination polarization, surface plasmon waves can be efficiently excited and focused by the annular rings structure towards the conical tip at the center.
The metallic tip further localizes and enhances the plasmonic field at the tip apex. With a5μm tip height and5nm tip radius, spatial resolution with the full-width-at-half-maximum of5.97nm and electric energy enhancement of7.29×104can be achieved with632.8nm optical excitation. The enhancement factor of this probe design does not strongly depend on the tip cone angle and the excitation wavelength. The strong local field enhancement at the end of the tip and its less stringent fabrication requirements make this probe design very attractive for a broad range of application in near-field optical imaging.
Problem4:Evanescent Bessel beam has great potential applications in non-linear optics, atom optics and optical trapping. J1evanescent Bessel beam could be generated by highly focus the radially polarized beam onto the surface of silver film. However, in the application of optical trapping, various orders of evanescent Bessel beam are supposed to be generated which apply to optical trapping of different kinds of particles.
Academic idea and solution:Photonic crystal has controllable dispersion relation and transmission properties. By choosing the appropriate parameters, only optical wave with specific wavelength, incident angle and polarization could transmit inside the photonic crystal.
We propose simple setups for generating evanescent Bessel beams using one-dimensional photonic crystal, which can be a defect mode photonic crystal or a normal one. The angular selectivity provided by the multilayer structure mimics the role of an axicon for Bessel beam generation. When an azimuthally polarized beam is strongly focused onto the last interface of the1D photonic crystal, an evanescent Bessel beam of the first order is produced, while an evanescent Bessel beam of the zeroth order will be created under a radially polarized beam illumination. Switching between a donut shape and a solid focal distribution can be easily realized by controlling the polarization of the illumination.
Highlights of the dissertation:
1. We propose an Archimedes'spiral transmitting antenna which shows the capability of holistic controlling of photons radiated from nano-emitters, through coupling the emitters to a miniature plasmonic spiral antenna. This technique enables the engineering of photon emission in terms of the intensity, directivity, direction, polarization and angular momentum. The theoretical predictions have been experimentally confirmed. For a five-turn Archimedes' spiral antenna, field intensity increase up to70-fold simultaneously with antenna directivity of11.7dB has been measured in the experiment, and a circular polarization extinction ratio of10is obtainable.
2. Based on the unique focusing properties of Archimedes' spiral antenna for a circularly polarized illumination, we propose a high efficiency plasmonic near-field probe that integrates a spiral plasmonic lens and a sharp conical tip under circular polarized illumination. To achieve high field enhancement, two layers of spiral plasmonic lens and a composite tip design are adopted. It can be used in photolithography with an array format. Under633nm wavelength excitation, an electric field enhancement factor of366and circular polarization extinction ratio of81can be achieved. By modulating the handedness of the incident photon, the hot spot at the tip apex can be switched on and off.
3. Based on the unique focusing properties of radially and azimuthally polarized beam and the spatial filter fuction of photonic crystal, we propose the setups for generating evanescent J0and J1type Bessel beam by the use of one-dimensional photonic crystal. In the application of optical trapping of golden particle, the gradient forces provided by the generated zeroth-order evanescent Bessel beam are much larger compared to the cases using different methods, which leads to a stable three dimensional optical trapping.
本论文解决的主要科学问题及取得的学术成果总结如下：
问题一：根据相关报道,通过将量子点与牛眼天线相耦合,牛眼天线能够对荧光场的强度、方向性以及出射方向进行很好的调控。除此之外,在量子信息处理、单分子传感以及集成光子电路等应用中,纳米尺度的空间结构光场也有着极其重要的应用。然而,在现有的光学天线研究中,对发光体辐射场的角动量调制还未得到实现。
问题一的学术思想及解决方案：当纳米尺度发光体靠近光学天线时,辐射场的特性会因天线的光学共振而发生改变。当光束与均匀、各向同性的光学天线发生相互作用时,例如牛眼天线结构,自旋和轨道角动量之间的守恒是相互独立的。然而,如果天线的结构更为复杂,例如各向异性或非均匀,入射光子与天线之间将产生自旋-轨道相互作用。当辐射场与螺旋天线发生相互作用时,根据物理系统中的总角动量守恒规则,螺旋结构的手性将转化出射光子携带的角动量,从而实现对光子角动量的调控。
提出并设计了可用于调制纳米尺度发光体辐射场特性的阿基米德螺旋天线。通过将量子点与螺旋天线相耦合,能够获得方向性极好的圆偏振涡旋光束；通过改变螺旋结构的螺距和激发点源的位置,可以对出射光子的轨道角动量和出射方向进行有效的控制。实验中得到了与理论计算相符的结果,获得了半高全宽为10。、峰值强度增强为70倍、方向性为11.4dB、圆偏振的消光比为10的荧光场。当受激点源的位置偏离螺旋结构几何中心200nm和500nm时,观测到的荧光场峰值的偏移角度分别约为3。和6。。
问题二：在纳米光刻应用中,复杂图案的加工可以通过表面等离子体(SPPs)近场探针逐点扫描的方法实现。为了获得较高的SPPs场增强,通常采用径向偏振激发的轴对称探针结构。然而,由于激发光的偏振态在空间呈非均匀分布,径向偏振光的中心奇点需要与探针的结构中心准直。因此,该探针无法做成阵列而用于大面积的光刻应用。同时,在实际应用中需要一套扫描装置,从而降低了刻写的速度,限制了光刻的面积。
问题二的学术思想及解决方案：对于特定手性的阿基米德螺旋天线,它能够将具有相同手性的圆偏振光场聚焦为中空的光斑,而将相反手性的圆偏振光场聚焦为实心光斑。
提出并设计了一种锥形针尖整合在螺旋天线几何中心处的近场探针。螺旋天线的聚焦场分布具有自旋取向的空间分离特性,因此当入射光子的自旋态发生改变时,针尖尖端处的光场也会产生对比度很大的明暗变化。为了在获得高场强的同时保持相对较低的远场辐射损耗,探针采用了双层螺旋和复合型针尖的结构设计。数值模拟的结果表明,针尖处可获得的最高场增强因子为366,相对应的消光比为81。该探针采用的激励源为偏振态在空间中均匀分布的圆偏振光束,在使用时无需进行中心对准,因此可做成阵列的形式用于大面积的复杂结构的光刻加工。
问题三：在光学系统中,存在着很多具有轴对称性的光学器件,例如由同心环形狭缝构成的牛眼天线,以及近场扫描光学显微镜(NSOM)的探针。当普通的线偏振光束作用于这些轴对称结构时,SPPs只会在线偏振的方向上被激发,限制了在焦点处的局域场强度,同时焦场呈中空的两瓣分布,限制了光学器件所能达到的分辨率。
问题三的学术思想及解决方案：径向偏振光束的偏振态在横截面上呈径向的轴对称分布,当其作用于同样轴对称的金属结构时,SPPs会在全部的角向方向上产生,从而获得高效激发的SPPs场。基于这一特点,对现有的增强光学透射(EOT)结构和近场探针结构进行了改进,获得了更好的性能。
提出并设计了一种基于径向偏振光激发的EOT结构,该结构由金膜上的单圈环形狭缝和位于狭缝下方的圆盘形纳米金膜天线组成。纳米天线能够将入射光场的能量充分的收集,并高效的耦合至环形狭缝,继而传播到自由空间,获得极强的EOT效应。根据数值模拟的结果,该EOT结构最高可获得251倍的传输效率增强,远远高于之前相关报道中利用线偏振态激发所能达到的数值。
提出并设计了一种锥形针尖整合在牛眼结构中心的复合型近场探针。牛眼天线和锥形针尖都具有非常好的SPPs聚焦效应。在径向偏振光的激发下,这种探针结构有着比单独的牛眼或针尖结构更高的场增强因子。与入射场相比,针尖顶端附近能获得467倍的电场增强,比单独的锥形针尖结构提高了100%,比单独的牛眼结构提高了200多倍。同时,探针的场增强因子不会随针尖锥角的改变而发生剧烈的变化,大大降低了对加工精度的要求。
问题四：消逝贝塞尔光束在非线性光学、原子光学以及光学微操控等领域有着广泛的应用。在之前的报道中,通过将径向偏振光强聚焦在银膜表面,可以产生消逝的零阶贝塞尔光束。然而,在光镊应用中,希望能够产生多种阶次的消逝贝塞尔光束,应用于不同种类颗粒的光学捕获。
问题四的学术思想及解决方案：光子晶体有着可控的色散关系和传输特性,通过合理的选取参数,当光场处于特定的波段、入射角度和偏振时,才能够在光子晶体内部传播。
利用光子晶体的空间滤波特性,以及径向和角向偏振光束在强聚焦下的特殊场分布,提出并设计了利用光子晶体中缺陷模式和带边模式产生零阶和一阶消逝贝塞尔光束的两种装置。在不改变结构参数的前提下,仅通过调整入射场的偏振态为径向或角向,就可获得不同阶次的消逝贝塞尔光束。
本论文的创新点主要包括：
1.提出并设计了可用于调控纳米尺度发光体辐射光场的阿基米德螺旋型光学发射天线。通过将受激点源与螺旋天线相耦合,能够获得方向性极好的圆偏振涡旋光束；改变螺旋结构的螺距,以及激发点源的位置,可以对出射光子的轨道角动量和出射方向进行有效的控制。并用实验进行了验证,获得了方向性为11.4dB,圆偏振的消光比约为10的荧光场,与理论计算相符合。
2.根据螺旋式光学接收天线的聚焦特性,提出并设计了可用于阵列光刻加工的表面等离子体探针。探针采用双层螺旋以及复合型针尖的设计,能够获得极高的局域场增强和圆偏振消光比,数值模拟的结果表明,针尖处可获得的最高场增强因子为366,相对应的消光比为81。通过改变入射圆偏振光的手性,每个探针上的热点开关状态都可得到控制,适用于复杂结构的大面积光刻。
3.基于径向和角向偏振光在强聚焦下的特性和一维光子晶体的空间滤波特性,提出了产生零阶和一阶消逝贝塞尔光束的设计。数值计算结果表明,当结构中所产生增强的零阶消逝贝塞尔光束作用于金颗粒时,所产生的束缚力比目前其他方法有明显提高,从而获得稳定的三维捕获。
In the field of integrated optics and nanophotonics, due to the diffraction limit, the high conversion efficiency between propagating field and localized field is an important and fundamental physical problem. Based on the theory of microwave antenna, optical antenna is designed to efficiently convert free-propagating optical radiation to localized energy, and vice versa. Subwavelength metallic structures become increasingly accessible with the rapid development of modern nanofabrication techniques, assisting the rapid expansion of the research in optical antenna. Optics antenna is regarded as one of the major trend in the modern optics development, and it has great potential applications in supper resolution, high resolution spectrum technology, high efficiency solar cell and photolithography. The aim of the dissertation is studying the design and application of optical antenna, hi this dissertation, we mainly study the applications of disc-shaped, bull eye and spiral optical antennas in the field of near-field optics in theory and experiment. Through the interaction between optical vector field and optical antenna with different structure, the properties of optical field can be engineered according to specific applications.
The main scientific problems solved and academic achievements achieved in this dissertation have been summarized in the list below:
Problem1:Recently, a nanoaperture surrounded by five concentric circular corrugations has been demonstrated for significant enhancement of the fluorescence count rates per molecule and high emission directivity. Besides, the nanoscale spin photon source have high relevance for the application in quantum optical information processing and integrated photonic circuits. However, the modulation of angular momentum carried by the radiated field remains unsolved.
Academic idea and solution:Through controlling the optical resonance in the vicinity of emitters, the properties of emitted photons from the emitters can be engineered. The conservation of spin and angular momentum are independent. However, when the incident spin photons encounter a metallic structure with anisotropic inhomogeneous boundaries, a spin-dependent behavior of plasmonic field could be observed. This phenomenon is due to spin-orbit interaction that is manifested by a geometric Berry's phase.
We analytically, numerically and experimentally study the emission properties of a nanoscale emitter coupled to a plasmonic spiral structure. The spiral antenna would couple the emission into circularly polarized unidirectional emission in the far field, the spin carried by the emitted photons is determined by the handedness of the spiral antenna. Increasing number of turns of the spiral leads to narrower angular width of the emission pattern in the far field. By increasing the spiral pitch in the units of surface plasmon wavelength, these circularly polarized photons also gain orbital angular momentum with different topological charges. In the experiment, quantum dots are adopted as the nano-emitters. For a five-turn Archimedes'spiral antenna, field intensity increase up to70-fold simultaneously with antenna directivity of11.7dB has been measured in the experiment, and a circular polarization extinction ratio of10is obtainable. In addition, if the nanoscale emitter is displaced from the geometrical center of the spiral antenna, the emission peak will shift from the normal direction accordingly. The steering angle depends on the displacement of the nanoscale emitter. This steering phenomenon has been confirmed experimentally. For a3-turn Archimedes'spiral antenna, experimental results reveal that steering angles of3°and7°are obtainable when the excited quantum dots are moved horizontally from the center with a displacement of200nm and500nm, respectively.
Problem2:Nanoscale lithography with complex patterns can be fabricated through point by-point scanning in principle. Efficient plasmonic excitation and focusing can be achieved through matching the axially symmetric dielectric/metal plasmonic lens structure to the polarization symmetry of radially polarized illumination. However, the singularity center of the radially polarized beam needs to be aligned to the center of the plasmonic lens structure. This necessitates a scanning mechanism that leads to slow writing speed and limit the realistic size of lithography area.
Academic idea and solution:The plasmonic field distribution in the center relies on the handedness of incident circular polarization. It has been shown that RHC beam will be focused into a solid spot by a LHS structure, while the field distribution of LHC beam focused by a LHS structure is a doughnut with a dark center.
We study a high efficiency plasmonic near-field probe that integrates a spiral plasmonic lens and a sharp conical tip under circular polarized illumination. To achieve high field enhancement, two layers of spiral plasmonic lens and a composite tip design are adopted. The plasmonic probe exhibits optical spin dependence due to the use of spiral plasmonic lens. Under633nm wavelength excitation, an electric field enhancement factor of366and circular polarization extinction ratio of81can be achieved. Such a spin dependence enables the hot spot at the tip apex to be switched on and off by modulating the polarization handedness. The probe can be made in an array format that is suitable for large area parallel near-field optics applications such as lithography and microscopy.
Problem3:There are many axisymmetric optical devices in the optical system, such as bull-eye structure and near-field scanning optical microscope probe. Surface plasmon polaritons only could be excited in the direction of polarization when linearly polarized beam is used as excitation, which limit the intensity of localized field at the focus and the optical resolution.
Academic idea and solution:The local electrical field of radially polarized beam is linearly polarized along the radial directions. For the plasmonic structure with rotational symmetry, surface plasmons excited by the radial polarization at all azimuthal directions interfere constructively. Therefore, a tightly focused plasmonic field with strong field enhancement is obtained at the focus. According to this feature, we improve the performance of extraordinary optical transmission structure and near-field probe structure.
We numerically study the extraordinary optical transmission of a plasmonic structure that combines a circular nanoantenna and a vertical annular nanoslit etched into a gold film under radially polarized illumination. The nanoantenna collects the incident field and localized it in a horizontal Fabry-Perot cavity over the gold film. Due to the symmetry matching between the structure and the illumination polarization, surface plasmons can be excited effectively and enhanced the transmission. Through optimizing the structure parameters, the transmission efficiency can be greatly enhanced by251times. This axisymmetric extraordinary optical transmission setup may be fabricated on the facet of an optical fiber for optical sensing applications.
We numerically study a plasmonic near-field probe design that integrates a sharp metallic conical tip at the center of a multiple concentric rings plasmonic lens under radially polarized illumination. Due to the symmetry match between the plasmonic structure and the illumination polarization, surface plasmon waves can be efficiently excited and focused by the annular rings structure towards the conical tip at the center.
The metallic tip further localizes and enhances the plasmonic field at the tip apex. With a5μm tip height and5nm tip radius, spatial resolution with the full-width-at-half-maximum of5.97nm and electric energy enhancement of7.29×104can be achieved with632.8nm optical excitation. The enhancement factor of this probe design does not strongly depend on the tip cone angle and the excitation wavelength. The strong local field enhancement at the end of the tip and its less stringent fabrication requirements make this probe design very attractive for a broad range of application in near-field optical imaging.
Problem4:Evanescent Bessel beam has great potential applications in non-linear optics, atom optics and optical trapping. J1evanescent Bessel beam could be generated by highly focus the radially polarized beam onto the surface of silver film. However, in the application of optical trapping, various orders of evanescent Bessel beam are supposed to be generated which apply to optical trapping of different kinds of particles.
Academic idea and solution:Photonic crystal has controllable dispersion relation and transmission properties. By choosing the appropriate parameters, only optical wave with specific wavelength, incident angle and polarization could transmit inside the photonic crystal.
We propose simple setups for generating evanescent Bessel beams using one-dimensional photonic crystal, which can be a defect mode photonic crystal or a normal one. The angular selectivity provided by the multilayer structure mimics the role of an axicon for Bessel beam generation. When an azimuthally polarized beam is strongly focused onto the last interface of the1D photonic crystal, an evanescent Bessel beam of the first order is produced, while an evanescent Bessel beam of the zeroth order will be created under a radially polarized beam illumination. Switching between a donut shape and a solid focal distribution can be easily realized by controlling the polarization of the illumination.
Highlights of the dissertation:
1. We propose an Archimedes'spiral transmitting antenna which shows the capability of holistic controlling of photons radiated from nano-emitters, through coupling the emitters to a miniature plasmonic spiral antenna. This technique enables the engineering of photon emission in terms of the intensity, directivity, direction, polarization and angular momentum. The theoretical predictions have been experimentally confirmed. For a five-turn Archimedes' spiral antenna, field intensity increase up to70-fold simultaneously with antenna directivity of11.7dB has been measured in the experiment, and a circular polarization extinction ratio of10is obtainable.
2. Based on the unique focusing properties of Archimedes' spiral antenna for a circularly polarized illumination, we propose a high efficiency plasmonic near-field probe that integrates a spiral plasmonic lens and a sharp conical tip under circular polarized illumination. To achieve high field enhancement, two layers of spiral plasmonic lens and a composite tip design are adopted. It can be used in photolithography with an array format. Under633nm wavelength excitation, an electric field enhancement factor of366and circular polarization extinction ratio of81can be achieved. By modulating the handedness of the incident photon, the hot spot at the tip apex can be switched on and off.
3. Based on the unique focusing properties of radially and azimuthally polarized beam and the spatial filter fuction of photonic crystal, we propose the setups for generating evanescent J0and J1type Bessel beam by the use of one-dimensional photonic crystal. In the application of optical trapping of golden particle, the gradient forces provided by the generated zeroth-order evanescent Bessel beam are much larger compared to the cases using different methods, which leads to a stable three dimensional optical trapping.
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