动态化学腐蚀法制备大锥角近场光纤探针

基于液压传递原理,设计出一种用动态化学腐蚀法制备大锥角近场光纤探针的装置.实验结果表明,在传统的化学腐蚀法制备光纤探针的过程中,通过控制腐蚀液液面的上升速度,可以有效地控制探针针尖的形状以及锥角的大小.在此基础上,还论述了分步控制光纤与腐蚀液液面的相对位移的方法在大锥角光纤探针的制备中所具有的独特优越性.利用所制备的大锥角近场光纤探针在扫描近场光学显微镜上对直径为200nm的小球进行探测,其力学像证实该探针具有较高的形貌分辨率(约为50nm). 关键词： 动态化学腐蚀法 大锥角 近场光纤探针     

一种高透过率光纤探针的制作

本文介绍了一种用于近场扫描光学显微镜(NSOM)中的高透过率光纤探针的制作方法。采用缓冲氢氟酸腐蚀低掺杂的普通单模石英光纤,选取适当比例的腐蚀液,可以得到直径、圆锥角均十分理想的针尖。这种方法不需要高掺杂的特殊光纤或是采用先熔拉后腐蚀的复合工艺。针尖经过镀膜后经测量为针尖直径100nm、透过率5.5×10^-4.     

A virtual optical probe based on evanescent wave interference

A virtual probe is a novel immaterial tip based on the near-field evanescent wave interference and small aperture diffraction, which can be used in near-field high-density optical data storage, nano-lithography, near-field optical imaging and spectral detection, near-field optical manipulation of nano-scale specimen, etc. In this paper, the formation mechanism of the virtual probe is analysed, the evanescent wave interference discussed theoretically, and the sidelobe suppression by small aperture is simulated by the three-dimensional finite-difference time-domain method. The simulation results of the optical distribution of the near-field virtual probe reveal that the transmission efficiency of the virtual probe is 10²－10⁴ times higher than that of the nano-aperture metal-coated fibre probe widely used in near-field optical systems. The full width at half maximum of the peak, in other words, the size of virtual probe, is constant whatever the distance in a certain range so that the critical nano-separation control in the near-field system can be relaxed. We give an example of the application of the virtual probe in ultrahigh-density optical data storage.     

Optimization of perpendicular magnetic anisotropy tips for high resolution magnetic force microscopy by micromagnetic simulations

Magnetic Force Microscopy (MFM) tip coated with perpendicular magnetic anisotropy film (PMA tip) is one of the choices for high resolution imaging at low scan height (SH), since it has negligible tip–sample interaction related to its stable magnetic state, sharp, and small tip stray field. In this work, detailed micromagnetic studies are carried out to understand the effect of geometrical and magnetic parameters including the cone angle θ of the PMA tip, intergrain exchange constant $A_{2}^{*}$ , saturation magnetization M _s and uniaxial crystalline anisotropy constant K ₁ of the tip coating on the MFM tip resolution. To evaluate the resolution performance of the optimized PMA tip, MFM images of high-density granular recording media and patterned media are simulated. We find that, for the PMA tip and its coating, a cone angle in a range of 36.9^° to 53.1^°, a saturation M _s of 700 emu/cm³, a large uniaxial crystalline anisotropy constant K ₁ (>4.9×10⁶ erg/cm³) and a high intergrain exchange constant $A_{2}^{*}$ of (0.3–1.0)×10^?6 erg/cm are optimized conditions for high resolution imaging. The optimized PMA tip has an excellent performance on imaging of high-density thin film media (bit size of 8×16 nm²) at low SH of 2–8 nm and bit pattern media with a pitch of 50 nm, edge-edge spacing of 5–15 nm at SH of 8–15 nm.     

Simulation on parameter optimization of gold nano-antenna attached to semiconductor ring resonator for heat-assisted magnetic recording device

Heat-assisted magnetic recording is a technology to improve recording density for hard disks. The authors’ group has proposed a device, in which a gold nano-antenna as a near-field transducer is attached to a semiconductor ring resonator as an integrated light source. Localized surface plasmon resonance at the tip of nano-antenna excites near-field light to form small recorded marks to increase recording density. In this study, to improve the device performance, the dependence of spot size and energy density of near-field light on tip curvature, length, and bottom diameter of nano-antenna was investigated through a numerical simulation. Cylinder type and cone type nano-antennas were considered. For both types, as the tip curvature of nano-antenna increased, the spot size decreased and the energy density increased. It was possible to reduce the spot size to 18 × 18 nm². For cylinder type, there was an optimal length of nano-antenna where the energy density became maximum, and the optimal length changed depending on the presence or absence of recording medium. This was because of the difference in plasmon resonance condition. Moreover, for cone type, there was an optimal bottom diameter of nano-antenna where the energy density became maximum, and the optimal bottom diameter changed depending on the length of nano-antenna.

     

制备扫描近场光学显微镜光纤探针的自动化腐蚀方法

提出了一种制备扫描近场光学显微镜光纤探针的自动化腐蚀方案.该方案利用静态腐蚀过程中光纤所形成的特殊结构,及动态腐蚀过程中光纤在氢氟酸中的移动所带来的新月形弯液面在光纤表面接触位置的变化,通过合理控制腐蚀时间来制备尖端锐利、大锥角或多锥体角等各种结构的探针.设计方案采用计算机控制整个装置实现了探针制备过程的自动化,保持了腐蚀光纤探针实验条件的一致性.实验结果表明,采用此方案可以制备出尖端孔径小于100 nm且锥体角高达70°的光纤探针,且重复性高.此外,该方案的装置结构简单,实现容易.     

Fabrication and Evaluation of Silica-based Optical Fiber Probes by Chemical Etching Method

A sensor probe with a microstructure, which is used for near-field scanning optical microscopes, plays an important role in the resolution and detection sensitivity of the microscope system. In this study, we devised a system for sharpening optical fibers, by which we could fabricate tapered tips of silica-based optical fibers reproducibly in a desired shape by controlling the solution temperature, etching time, and etching depth when applying chemical etching techniques using hydrofluoric acid. Furthermore, we evaluated the diameter of the core tip of an optical fiber by measuring the near-field pattern and comparing it with the Gaussian beam pattern of a non-edge-etched optical fiber. A strong correlation between the ratio of the 1/e²-value width of the beam profile of the near-field pattern to that of the Gaussian beam and the diameter of the extreme tip of the core became apparent. © 2005 The Optical Society of Japan     

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...     

Resolution enhancement in a reflection mode near-field optical microscope by second-harmonic modulation signals

The motion of the probe tip in a near-field scanning optical microscope, dithered by vibration of a tuning fork, can modulate the reflection signal from the sample surface not only at the fundamental dithering frequency but also at its second harmonic. By lock-in amplification of these modulated signals, enhanced optical images are obtained, even with an uncoated fiber probe. In particular, accurate optical images with higher resolution are obtained when the second-harmonic signal is detected, which results from the parametric modulation of the tip-sample separation at the double frequency of the horizontal dithering motion of the tip. Using a DVD ROM with a track pitch of 0.74 mum as a test sample, we observed that the sharp edges around the pits are clearly resolved with the second-harmonic signals and obtained enhanced resolution of ~70 nm full width at half-maximum.     

Polarization-dependent contrast of near-field and far-field-propagating signal intensities in C-mode scanning near-field optical microscopy/photon scanning tunneling microscopy

A recent paper by Nayaet al. (Opt. Commun.124 (1996) 9) presented high-resolution imaging results obtained in the sub-100-nm range with a collection-mode near-field optical microscope. The images exhibit apparent polarization dependence. A simple modeling and calculation based on the experiment, using a semi-microscopic and perturbative approach, showed that the far-field-propagating signal intensity converted from the near-field can qualitatively explain the polarization dependence of the experiment if the taper angle of the probe tip is taken into account.     

Near-field optical microscopy of nonlinear susceptibilities

Local probing of second-order susceptibilities with a near-field optical microscope is demonstrated for the first time. Using an uncoated fiber tip as a light source, near-field images of a surface of y-cut LiNbO₃ crystal and of a multilayer Langmuir–Blodgett film of 2-docosylamino-5-nitropyridine are obtained at the fundamental pump and second harmonic wavelengths while simultaneously recording surface topography. It is shown that optical second-harmonic images for different polarizations of the pump light reflect the difference in magnitude of the corresponding components of the second-order susceptibility tensor. Various degrees of correlation in contrast of optical (fundamental and second harmonic) and topographical images are observed and discussed.     

热拉伸和化学腐蚀相结合制备弯曲光纤探针

提出了原子力/光子扫描隧道显微镜(AF/PSTM)系统的关键部分——双功能弯曲光纤探针的制作方法.采用热拉伸与动态、静态两步化学腐蚀相结合的方法制作出AF/PSTM弯曲光纤探针，弯曲角度约为150°，尖端曲率半径优于100nm，锥角范围为60°—90°.将这种双功能弯曲光纤探针应用在新研制的AF/PSTM系统上，同时获得了样品的光学与形貌图像，实现了图像分解. 关键词： 原子力/光子扫描隧道显微镜 光纤探针 热拉伸 化学腐蚀     

Theoretical Analyses on the Resolution of Collection Mode Scanning Near-Field Optical Microscopy

Numerical simulations have been carried out in the framework of waveguide theory to model collection mode scanning near-field optical microscopy (SNOM). The theoretical model includes the optical fiber end and describes the metal coated aperture on the probe tip. The developed formalism goes beyond the existing Bethe-Bouwkamp theories for electromagnetic transmission of subwavelength apertures. The finite coating and optical fiber end are now taken into account. The new features enable us to simulate the near-field probes that are widely used in the collection mode SNOM. The emphases of the numerical analyses have been mainly on the resolution mechanism of the microscopy. Influence on the resolution from important parameters of the probe tips, such as the size of the apertures and the probe-sample distance, is extensively studied. The resolution dependence has been analyzed in the light of the near-field coupling efficiency of the probe tip. An optimum tip size has been found which is balanced between the significant signal transmission and the resolution of the device.     

The preparation of optical fibre nanoprobe and its application in spectral detection

As an important component of scanning near-field optical microscope (SNOM), optical fibre nanoprobe has been applied to many fields extensively. A melt-stretched etching method is proposed to produce optical fibre nanoprobe with low cost. Firstly, optical fibre tip with micrometer-sized diameter is created by the melt-stretched measure. Next, it is dipped into hydrofluoric acid (HF), and a fine optical fibre nanoprobe will be made after a short-time etching. Owing to the taper structure of tip, it can be etched again in acid if a nanoprobe is not constructed when the first etching is completed. In addition, optical fibre nanoprobe is applied to spectral investigation, and the fluorescence spectroscopy of rhodamine B (Rh B) solution is collected by an optical investigation system with a bifurcated fibre.     

新型抗弯曲大模场面积光子晶体光纤

研制出一种新型抗弯曲大模场面积石英光子晶体光纤. 利用光子晶体光纤结构设计的灵活性, 通过规划缺陷的位置及空气孔的尺寸, 实现了大模场面积单模及低弯曲损耗特性.应用建立的实际光子晶体光纤特性分析模型, 研究了研制光纤的模式特性和弯曲特性, 在波长1064 nm处, 平直状态下光纤的模场面积可以达到2812 μm², 基模限制损耗为0.00024 dB/m, 高阶模限制损耗高于1.248 dB/m. 基模和高阶模之间的高传输损耗差, 保证了在获得大模场面积的同时实现单模传输. 弯曲半径和弯曲方向角所带来弯曲损耗变化的研究结果显示, 即使在弯曲半径小到5 cm时, 弯曲损耗也在10^-3 dB/m量级以下, 而且在弯曲半径为30 cm时光纤可承受的弯曲方向角范围扩展至-60°–60°. 研制的光纤具有良好的低弯曲损耗特性, 可有效解决非对称结构所带来的光纤弯曲特性对弯曲方向角敏感的问题. 该光纤在高功率光纤激光器、放大器及高功率传输等技术领域具有重要的应用价值. 关键词： 光子晶体光纤 大模场面积 低弯曲损耗 弯曲方向角     

Experimental and Numerical Study of Free Convection on an Isothermal Downward Cone

Abstract

The laminar free-convection heat transfer from an isothermal downward cone in air is investigated experimentally and numerically. The experimental investigation is carried out by Mach-Zehnder interferometery technique and the numerical simulation was done by Fluent. The cone tip angle has been kept constant to 45° and it was suspended from its base throughout the experiment. This article focuses on the effect of Rayleigh number variation on the local and average free-convection heat transfer coefficient over the conical surface. The local and average Nusselt numbers were determined for the Rayleigh number range of 4.9×10⁵ to 1.1×10⁶. Also, the experiment and the numerical simulation were carried out on a vertical isothermal cylinder of circular cross section in order to compare results with other researchers for the verification of our experimental and numerical results. The significant influence of the upper end surface of the cone, both in the experimental and numerical studies, indicated a recirculation region above the upper end surface which affects the local convection heat transfer at the slant trailing edge and causes it to increase. Also, a correlation for the calculation of the local Nusselt number over the cone is proposed.     

Optical polarization response at gold nanosheet edges probed by scanning near-field optical microscopy

Optical properties of metallic edge-like structures known as knife-edges are a topic of interest and possess potential applications in enhanced Raman scattering, optical trapping, etc. In this work, we investigate the near-field optical polarization response at the edge of a triangular gold nanosheet, which is synthesized by a wet chemical method. A homemade scanning near-field optical microscope(SNOM) in collection mode is adopted, which is able to accurately locate its probe at the edge during experiments. An uncoated straight fiber probe is used in the SNOM, because it still preserves the property of light polarization though it has the depolarization to some extent. By comparing near-field intensities at the edge and glass substrate, detected in different polarization directions of incident light, the edge-induced depolarization is found,which is supported by the finite differential time domain(FDTD) simulated results. The depolarized phenomenon in the near-field is similar to that in the far-field.     

光通信中的玻璃无源器件的制备与研究

本文报道了以高质量的BK7光学玻璃为衬底,通过稀释Ag+-Na+离子交换技术,制备出低损耗的表面条波导,并用近场法测量出条波导的模场分布曲线。给出光纤与波导耦合的失配损耗为0.54dB,条波导传输损耗为0.21dB/cm.以及在此工作基础上,成功地研制了适用于大容量相于光通信系统的1×2和1×4单模波导分束器。     

Near-field scanning optical microscopy of single molecules by femtosecond two-photon excitation

We present a demonstration of near-field scanning optical microscopy of single molecules based on ultrafast two-photon-induced fluorescence. Measurements were performed by use of 100-fs pulses at 800 nm from a Ti:sapphire laser to excite the two-photon transition in Rhodamine B molecules. Although near-field probes are normally metal coated to achieve superresolution, we used uncoated tips to achieve sufficiently high optical powers to generate acceptable fluorescence emission rates. Images of single molecules demonstrate a resolution of ~175nm(< lambda/4) on a topographically smooth surface, which surpasses the apparent lambda/2 resolution limit for uncoated tips operating in the linear response regime.     

Enhanced light confinement in a near-field optical probe with a triangular aperture

We present a probe concept for scanning near-field optical microscopy combining the excellent background suppression of aperture probes with the superior light confinement of apertureless probes. A triangular aperture at the tip of a tetrahedral waveguide (full taper angle approximately 90 degrees ) shows a strong field enhancement at only one rim when illuminated with light of suitable polarization. Compared to a circular aperture of equivalent size, the resolution capability is doubled without loss of brightness. For a approximately 60 nm sized triangular aperture, we measured an optical resolution <40 nm and a transmission of approximately 10(-4).