基于半导体纳米线/锥形微光纤探针的被动式近场光学扫描成像

本文结合近场扫描结构和纳米线-微光纤耦合技术,提出了一种基于硫化镉纳米线/锥形微光纤探针结构的被动近场光学扫描成像系统.该系统采用被动式纳米探针,保留了纳米探针对样品表面反射光的强约束优势.其理论收集效率为4.65‰,相比于传统的金属镀膜近场探针收集效率提高了一个数量级,可有效地提高扫描探针对样品形貌信息的检测能力;而后通过硫化镉纳米线与微光纤之间高效的倏逝场耦合,将检测的光强信号传输到远场进行光电探测,最终实现对目标样品形貌的分析成像,其样品宽度测量误差在7.28%以内.该系统不需要外部激发光路,利用显微镜自身光源进行远场照明,被动扫描探针仅作为样品表面反射光的被动收集系统.本文基于半导体纳米线/锥形微光纤探针的被动式近场光学扫描成像方案,可有效地降低探针的制备难度和目标光场的检测难度,简化扫描成像的结构,为近场光学扫描显微系统之后的发展提供新的思路.     

近场研究表面等离子体在银纳米线上的传输

通过双探针近场光学扫描显微镜在银纳米线上实现近场激发和近场收集表面等离子体,用一个探针在银纳米线的一端近场激发表面等离子体,另一个探针近场探测银纳米线上的表面等离子体强度分布,得到强度分布图.强度分布图显示表面等离子体在银纳米线的一端被有效激发并且有一部分表面等离子体沿着银纳米线和基底的界面传播到了另一端.用有限元法对银纳米线内的传播模式进行数值模拟,结果显示银纳米线内存在两种表面等离子体传播模式,分别为基模和高阶模.沿着银纳米线和基底介质之间传输的基模表面等离子体由于传输环境稳定,散射损耗小,实际传输长度接近模式传输长度,达10μm以上;而高阶模表面等离子体由于部分裸露在空气中受表面缺陷散射的影响,散射损耗大,实际传输长度远小于模式传输长度.研究表明:以能量高度束缚的基模表面等离子体作为载体,不仅可以实现低损耗传输,还可以减小集成器件之间的信号串扰,有效提高信息传输的安全性,在集成光学中具有重要应用.     

基于光导微探针的近场/远场可扫描太赫兹光谱技术

太赫兹技术已经成为涉及公共安全、军事国防和国民经济等国家核心利益的前沿研究领域.以往太赫兹测量技术中通常以远场测量为主,如常用的太赫兹时域光谱仪.近年来太赫兹近场技术得到了迅猛的发展,特别是基于光导天线的探针技术的发展,为可扫描的太赫兹近场测量提供了可能.本文详细报道了我们近期在可扫描太赫兹近场光谱仪研究中的进展.采用光纤耦合的光导微探针实现了方便灵活的太赫兹近场/远场三维扫描,并同时获得振幅和相位信息.该系统将有可能广泛应用于人工微结构、石墨烯、表面等离子激元、波导传输、近场成像、生物样品检测、芯片检测等研究领域.     

扫描近场光学显微镜的光耦合偶极子模型

为了研究扫描近场光学显微镜中探针和粗糙样品表面的耦合相互作用,提出了一种光耦合偶极子模型.在该模型中,探针和样品突起都由光极化偶极子表示,在准静态电磁场近似的情况下样品表面的诱导极化效应由影像偶极子表示,应用偶极子辐射理论可以得到系统的自洽场方程.此模型提供了一种直观分析扫描近场光学显微镜中探针和样品相互作用机理的方法.在此基础上,进一步讨论了金属样品的近场成像特点和其特有的局域光学共振现象.数值结果表明：不同于一般的介质样品,金属样品的近场图像与入射光频率直接相关,改变入射光的频率,获得的样品近场图像的形状和对比度都会发生变化.特别是当入射光频率处于样品极化共振范围内时,金属纳米粒子的极化率会出现光极化共振,这样就可以获得样品粒子的最大有效尺寸,为提高系统的分辨率提供了一条重要途径.     

用时阈有限差分方法研究光纤微探针近场分布特性

采用三维时间域有限差分（FDTD）方法研究了扫描近场光学显微镜中光纤微探针的近场分布特性,研究结果表明,锥形裸光纤探针的近场光中,以从探针锥变区侧面泄露的光即传播场为主,当在探针的侧面镀上一层金属膜对于从侧面泄露的光具有较好屏蔽效果,因而比裸光纤探针具有更好的空间束缚能力,无论是裸光纤探针还是金属膜探针,入射偏振光从针尖出射手发生了退极化现象,近场分布中产生与入射光偏振方向垂直的其它两个电场分量,而且金属膜探针的退极化分量对近场分布的影响比裸探针显著得多,如果在近场成像中接收退极化分量而不是总场则可以提高近场成像的对比度,还详细地分析了微探针的近场分布特征与退极化之间的关系。     

采用DDS的近场扫描光学显微镜探针-样品的纳米距离检测

近场扫描光学显微术中, 近场距离的检测和控制是需要解决的核心技术之一. 本文研究了基于DDS驱动的压电传感器, 在一个压电陶瓷片上, 电极被分成相同的两部分, 分别用于振动驱动和振幅检测. 近场扫描的光纤探针固定于此压电陶瓷片上. 振动驱动信号采用DDS, 在样品的远场时, 可以通过频率扫描得到误差在0.006 Hz以内的压电陶瓷片谐振频率驱动信号, 而当光纤探针处于样品的近场距离之内时, 压电陶瓷片的谐振频率偏离驱动信号频率, 振幅明显减小, 从而检测出近场距离. 高精度振动驱动源DDS和高灵敏度压电传感器的采用提高了检测灵敏度和工作稳定性.     

影响近场光纤探针传输效率的若干因素

近场光学技术在高分辨率成像、光谱探测和纳米加工等领域有广泛应用,而光纤探针是其中一个关键部件.如何提高光纤探针的传输效率是近场光学技术应用中的一个重要问题.本文采用三维时域有限差分方法,计算了锥形有孔探针的传输效率,分析了锥角、针尖孔径、波长和金属层厚度等因素对光纤探针传输效率的影响,并比较了裸光纤探针和有金属涂层的光...     

一种探针-样品距离的切变力控制新方法

报道了近场扫描光学显微镜(Near-filed Scanning Optics Microscope NSOM)中一种基于切变力的探针 样品间距控制新方法.条形压电蜂鸣器片的上表面电极被沿着中心线分成两半,一半用于驱动,其上施加振荡源谐振频率进行激励,另一半上粘附光纤探针,并利用压电效应作为光纤探针的振幅传感器.当受振动激励的光纤探针由远处逐渐接近样品表面时,由于样品与探针之间的切变力阻尼作用使得探针的振幅减小,通过检测探针振幅的变化可以控制探针与样品的间距.我们建立了一套探针 样品间距探测控制系统并利用该系统获得了CD盘片表面8μm×8μm范围内的切变力形貌图.     

照明模式SNOM中样品对近场光场分布的影响

在同时考虑样品的形貌及材料光学参量和入射光偏振模式的情况下,利用基于边界元方法编写的二维矢量电磁场计算程序,对工作在照明模式下的扫描近场光学显微镜（Scanning Near-field Optical Microscope,SNOM）的近场矢量电磁场分布进行了数值计算模拟研究.结果表明,在没有表面形貌特征时,探针的光能量透射率随样品材料的折射率和损耗角的增加而增大,而样品表面光斑尺寸受折射率和损耗角的影响很小；对有形貌特征的探针扫描像研究结果表明,SNOM的分辨率随着样品的折射率和损耗角的增加而提高；对SNOM不同的工作模式的扫描成像信号进行的计算结果表明,恒定间距扫描方式比恒定高度扫描方式对样品表面的细节有较强的分辨能力.     

一种探针-样品距离的切变力控制新方法

报道了近场扫描光学显微镜(Near-filed Scanning Optics Microscope-NSOM)中一种基于切变力的探针-样品间距控制新方法.条形压电蜂鸣器片的上表面电极被沿着中心线分成两半,一半用于驱动,其上施加振荡源谐振频率进行激励,另一半上粘附光纤探针,并利用压电效应作为光纤探针的振幅传感器.当受振动激励的光纤探针由远处逐渐接近样品表面时,由于样品与探针之间的切变力阻尼作用使得探针的振幅减小,通过检测探针振幅的变化可以控制探针与样品的间距.我们建立了一套探针-样品间距探测控制系统并利用该系统获得了CD盘片表面8μm×8μm范围内的切变力形貌图.     

Near-field magneto-optics and microscopy in resonant scattering of light from a linear nanoprobe

A theory is presented for the polar near-field magneto-optical Kerr effect in scattering of light from a linear nanoprobe. In the framework of Green’s function technique, a solution is obtained for the problem of near-field magneto-optics and apertureless scanning microscopy of lateral magnetic inhomogeneities (domains) with nanometer scale. The probe in the form of a nanowire and the sample with a near-surface magnetic nanolayer are considered to support surface plasmons. Electromagnetic coupling between a nanow-ire and a sample surface (polarizability of the complex “probe plus image charges”) is taken into account self-consistently. Magneto-induced polarization of an ultrathin near-surface layer is treated within linear approximation in magnetization which is perpendicular to the layer. The polarization, spectral and angular characteristics of light scattering modulated by magnetization and resonantly enhanced by surface plasmons are examined. Dependence of the near-field magneto-optical response on the probe-domain distance along the sample surface is obtained. The resolution power of scanning near-field microscopy is estimated and the factors to influence it are pointed out.     

Magnetooptical Kerr effect in the near field of a nanowire supporting surface plasmons

In terms of Green’s functions, a theory is developed describing the resonant magnetooptical Kerr effect in light scattering by a linear probe that is parallel to the surface of a magnet and placed at a subwavelength distance from it. The probe is supposed to be a metal nanowire supporting long-lived surface plasmons and forming the near field of the “probe + image” complex. The resonant interaction between the probe and the sample is taken into account within a self-consistent approximation of multiple-scattering theory, and the magnetooptical interaction is included in the linear approximation in magnetization. The problem of scanning near-field magnetooptical microscopy with a linear probe is solved analytically in the case where the magnetization is parallel to both the magnet surface and the plane of incidence of light (longitudinal magnetooptical Kerr effect). The polarization, spectral, and angular characteristics of scattered light modulated by magnetization are discussed. It is shown that the magnetooptical modulation of the scattered light intensity is significantly enhanced when surface plasmons are resonantly excited in the nanowire.     

THz near-field imaging

We present first results of near-field imaging with ultrashort, broadband far-infrared pulses. By focusing the radiation into a tapered metal tip with a small exit aperture and scanning a sample in the near field of this aperture, sub-wavelength spatial resolution better than λ/4 is demonstrated.     

Efficient light confinement with nanostructured optical microfiber tips

Nanostructured optical microfiber tips are proposed and experimentally demonstrated to efficiently confine light beyond the diffraction limit at high powers. Focused ion beam milling was used for the nanostructuring of gold-coated optical microfiber tips with both single-ramp and wedge geometries. Small apertures were formed by flat cutting or hole drilling and optical spot sizes of ∼λ/10 with high transmission efficiency were achieved. Numerical simulations were carried out to optimize the device design with circularly polarized light. Enhanced transmission efficiencies (higher than 10⁻²) were recorded by optimizing the overall light throughput along the fiber tips. The tip thermal behavior was investigated by launching high powers into the device and recording the tip position in a scanning near-field optical microscopy set-up. This nanostructured optical microfiber tip has the potential for applications in optical recording, scanning near-field optical microscopy and lithography.     

有孔探针近场拉曼光谱和成像技术进展

基于有孔探针SNOM的近场拉曼光谱和成像技术的出现使得拉曼光谱的分辨率突破了光学衍射极限,从而提供了一个有力的工具对样品亚波长尺度之下的化学信息进行表征。文章讨论了探针性质对实现近场拉曼光谱的影响,并全面地介绍了有孔探针近场拉曼光谱发展十余年来在纳米尺度化学分辨成像、液-液界面性质研究、微观层面解释SERS增强机理、图像化反映SERS热点分布等诸多领域的研究进展。     

扫描近场光学显微镜光纤锥中导波反射特性

利用局部模耦合模型理论上分析了扫描近场光学显微镜光纤的光场性质,给出光纤维中存在的正,反向传播的基模场微分方程,以及基模反向耦合系数的数值计算结果,其最高反射系数达１％左右。这种反射光可起着光纤维激光器谐振腔输出端镜的作用。     

Optically-controlled resistive switching effectsof CdS nanowire memtransistor

Since it was proposed, memtransistors have been a leading candidate with powerful capabilities in the field of neural morphological networks. A memtransistor is an emerging structure combining the concepts of a memristor and a field-effect transistor with low-dimensional materials, so that both optical excitation and electrical stimuli can be used to modulate the memristive characteristics, which make it a promising multi-terminal hybrid device for synaptic structures. In this paper, a single CdS nanowire memtransistor has been constructed by the micromechanical exfoliation and alignment lithography methods. It is found that the CdS memtransistor has good non-volatile bipolar memristive characteristics, and the corresponding switching ratio is as high as 10⁶ in the dark. While under illumination, the behavior of the CdS memtransistor is similar to that of a transistor or a memristor depending on the incident wavelengths, and the memristive switching ratio varies in the range of 10 to 10⁵ with the increase of the incident wavelength in the visible light range. In addition, the optical power is also found to affect the memristive characteristics of the device. All of these can be attributed to the modulation of the potential barrier by abundant surface states of nanowires and the illumination influences on the carrier concentrations in nanowires.     

金属覆层光纤探针近场特性研究

采用有限积分法对用于近场光学显微镜的旋转对称光纤探针进行了数值模拟计算,研究了光纤探针开口附近亚波长范围的电磁场及其能量密度的分布.计算结果表明,在探针外,光场为沿探针轴线方向的近逝波,其空间光场的分布与激励光场及其极化方向有关,在激励光场的极化方向出现了由感应电荷引起的边缘增强效应.同时研究了近场光纤探针的空间分辨率和样品处的电磁场能量,结果表明光纤探针孔的尺寸及其与样品的作用距离是影响探针的分辨率和样品内电磁场能量的重要因素.