A novel light source for SICM–SNOM of living cells

We have developed a novel light source for use in a scanning near‐field optical microscope (SNOM or NSOM) based on a nanopipette whose distance from the sample surface is controlled using scanning ion conductance microscopy. The light source is based on the general principle of the chemical reaction between a fluorophore in the pipette and ligand in the bath, to produce a highly fluorescent complex that is continually renewed at the pipette tip. In these experiments we used fluo‐3 and calcium, respectively. This complex is then excited with an Ar⁺ laser, focused on the pipette tip, to produce the light source. This method overcomes the transmission problem of more traditional SNOM probes and has been used to acquire simultaneous high‐resolution topographic and optical images of biological samples in physiological buffer. A resolution of ～220 nm topographic and ～190 nm optical was determined through imaging fixed sea‐urchin sperm flagella. Live A6 cells were also imaged, demonstrating the potential of this system for SNOM imaging of living cells.     

Evanescent field excitation and measurement of dye fluorescence in a metallic probe near-field scanning optical microscope

We introduce a method of dye fluorescence excitation and measurement that utilizes a near-field scanning optical microscope (NSOM). This NSOM uses an apertureless metallic probe, and an optical system that contains a high numerical aperture (NA) objective lens (NA= 1.4). When the area which satisfies NA < 1 is masked, the objective lens allows for the rejection of possible transmitted light (NA < 1) through the sample. In such conditions, the focused spot consists of only the evanescent field. We found that this NSOM system strongly reduces the background of the dye fluorescence and allows for the measurement of the fluorescence intensity below the diffraction limit of the excitation source.     

Evanescent field excitation and measurement of dye fluorescence in a metallic probe near-field scanning optical microscope

We introduce a method of dye fluorescence excitation and measurement that utilizes a near-field scanning optical microscope (NSOM). This NSOM uses an apertureless metallic probe, and an optical system that contains a high numerical aperture (NA) objective lens (NA = 1.4). When the area which satisfies NA < 1 is masked, the objective lens allows for the rejection of possible transmitted light (NA < 1) through the sample. In such conditions, the focused spot consists of only the evanescent field. We found that this NSOM system strongly reduces the background of the dye fluorescence and allows for the measurement of the fluorescence intensity below the diffraction limit of the excitation source.     

Probing highly confined optical fields in the focal region of a high NA parabolic mirror with subwavelength spatial resolution

Parabolic mirrors with a high numerical aperture can be conveniently used to produce highly confined optical fields in the focal region. Furthermore, these fields can have interesting polarization behaviour due to the high numerical aperture. In particular, if the mirror is illuminated with a size matched radially polarized or azimuthally polarized doughnut mode, the electric field has in the focal region almost exclusively a longitudinal or a transverse polarization component. Such field distributions are interesting for applications in confocal or near‐field optical microscopy. Here we present experimental results where we have probed some of these field distributions by raster scanning a fine gold tip in nanometer steps through the focal region and detecting the scattered light intensity. The measured intensity patterns are compared with corresponding vector‐field calculations.     

Single molecule mapping of the optical field distribution of probes for near-field microscopy

The most difficult task in near-field scanning optical microscopy (NSOM) is to make a high quality subwavelength aperture probe. Recently, we have developed high definition NSOM probes by focused ion beam (FIB) milling. These probes have a higher brightness, better polarization characteristics, better aperture definition and a flatter end face than conventional NSOM probes. We have determined the quality of these probes in four independent ways: by FIB imaging and by shear-force microscopy (both providing geometrical information), by far-field optical measurements (yielding throughput and polarization characteristics), and ultimately by single molecule imaging in the near-field. In this paper, we report on a new method using shear-force microscopy to study the size of the aperture and the end face of the probe (with a roughness smaller than 1.5 nm). More importantly, we demonstrate the use of single molecules to measure the full three-dimensional optical near-field distribution of the probe with molecular spatial resolution. The single molecule images exhibit various intensity patterns, varying from circular and elliptical to double arc and ring structures, which depend on the orientation of the molecules with respect to the probe. The optical resolution in the measurements is not determined by the size of the aperture, but by the high optical field gradients at the rims of the aperture. With a 70 nm aperture probe, we obtain fluorescence field patterns with 45 nm FWHM. Clearly, this unprecedented near-field optical resolution constitutes an order of magnitude improvement over far-field methods like confocal microscopy.     

Near field optical microscopy in aqueous solution: implementation and characterization of a vibrating probe

Near field optical microscopy (NSOM) is one of the possible solutions to circumvent the diffraction limit, but the control of the optical probe in solution has been a technical challenge for practical applications. Most recently, it has been shown that the pipette used in the scanning ion conductance microscope can be modified to form a high resolution near field optical probe. When combined with a novel distance modulation mechanism, a robust near field microscope can be constructed for operation in aqueous solution. In this paper, we present technical details of this design and a further characterization of the NSOM system for imaging in solution. Fundamental limitations of this approach in comparison to other systems are also discussed. Based on the current technology, it is concluded that better than 50 nm resolution should be achievable with this technique for fluorescence, as well as fluorescence resonance energy transfer, imaging of biological specimens.     

Near-field optical patterning on chloromethylated polyimide

Near-field scanning optical microscopy (NSOM) coupled with laser is used in nano-scale processing to make nano-scale dots or nano-scale structure. Nano-sclae processing using NSOM coupled with laser can be applied to photo- chemical etching process on crstalline silicon, to additive processes on some polymers, to subtraction processes on SAMs and other polymers. And it can be used to change material’s optical properties in nano-scale geometry. As above, nano-scale processing using NSOM coupled with laser has an advantage that it can be applied to various processes. In this work, by using NSOM coupled with 266nm UV laser, nanoscale patterns were fabricated on chloromethylated polyimide (CMPI) films coated on silicon wafer. CMPI undergoes a fast photolysis under UV light. So, in the case of pattern fabrication on CMPI it is possible to fabricate patterns without development process. Possibilities for SMPI to be applied to nano-scale patterns fabrication were demonstrated. Compared to usual lithographic processes, the process proposed in this work is simple because development, one of steps to fabricate nano-scale patterns, is not needed. And the finite-difference-time-domain (FDTD) method was employed to simulate the energy intensity distribution in the near-field. The simulation was executed for NSOM tip and UV laser. The influence of aperture size and tip-sample distance on the resolution of the lithographic process is discussed from the simulation results. Comparison of some simulation results with corresponding experimental results could confirm the validity of the simulation model proposed.     

Single molecule mapping of the optical field distribution of probes for near-field microscopy

The most difficult task in near-field scanning optical microscopy (NSOM) is to make a high quality subwavelength aperture probe. Recently, we have developed high definition NSOM probes by focused ion beam (FIB) milling. These probes have a higher brightness, better polarization characteristics, better aperture definition and a flatter end face than conventional NSOM probes. We have determined the quality of these probes in four independent ways: by FIB imaging and by shear-force microscopy (both providing geometrical information), by far-field optical measurements (yielding throughput and polarization characteristics), and ultimately by single molecule imaging in the near-field. In this paper, we report on a new method using shear-force microscopy to study the size of the aperture and the end face of the probe (with a roughness smaller than 1.5 nm). More importantly, we demonstrate the use of single molecules to measure the full three-dimensional optical near-field distribution of the probe with molecular spatial resolution. The single molecule images exhibit various intensity patterns, varying from circular and elliptical to double arc and ring structures, which depend on the orientation of the molecules with respect to the probe. The optical resolution in the measurements is not determined by the size of the aperture, but by the high optical field gradients at the rims of the aperture. With a 70 nm aperture probe, we obtain fluorescence field patterns with 45 nm FWHM. Clearly, this unprecedented near-field optical resolution constitutes an order of magnitude improvement over far-field methods like confocal microscopy.     

Effective elimination of laser interference fringing in fluorescence microscopy by spinning azimuthal incidence angle

Laser illumination used in both conventional widefield epi-fluorescence as well as in total internal reflection fluorescence (TIRF) microscopy is subject to nonuniformities in intensity that obscure true image details. These intensity variations are interference fringes arising from coherent light scattering and diffraction at every surface in the laser light's optical path, including the lenses, mirrors, and coverslip. We present an inexpensive technique for effectively eliminating these interference fringes based upon introduction of the excitation laser beam by oblique through-the-objective incidence coupled with rapid azimuthal rotation of the plane of incidence. Although this rotation can be accomplished in several ways, a particularly simple method applicable to a free laser beam is to use an optical wedge, spun on a motor, which diverts the beam into a hollow cone of fixed angle. A system of lenses converts this collimated beam cone into a focused spot that traces a circle at the objective's back focal plane. Consequently, a collimated beam with fixed polar angle and spinning azimuthal angle illuminates the sample. If the wedge is spun rapidly, then the different interference patterns at every particular azimuthal incidence angle average out over a single camera exposure to produce an effectively uniform field of illumination.     

Phase separation in polyfluorene – polymethylmethacrylate blends studied using UV near-field microscopy

In this paper we present a near‐field microscopy study of thin films of a phase‐separated blend of the fluorescent conjugated‐polymer poly(9,9‐dioctylfluorene) [PFO] with the non‐fluorescent polymer polymethylmethacrylate [PMMA]. A scanning near‐field optical microscope (NSOM) was used to generate (blue) fluorescence from the PFO following UV excitation at 362 nm. A range of different concentrations of PFO in PMMA were studied ranging from 1 to 50% PFO in PMMA by mass. By studying both the shear force and fluorescence images we were able accurately to determine the distribution of PFO in the PMMA. We found that phase separation occurs over a number of different length‐scales between 5 µm and 250 nm. We show that at PFO concentrations of 1%, the PFO lies on top of the PMMA. At a PFO relative concentration of 50%, the PMMA phase extends through the whole thickness of the film to the underlying substrate. We use such samples to discuss the resolution of NSOM when imaging thick organic films. Furthermore, we confirm that the length‐scales of phase separation can be modified via control over spin‐casting protocols.     

Optical transition process in AgOx super-resolution near-field structure

A sandwiched 15 nm AgO_x thin film of the super‐resolution, near‐field optical disk was studied using a confocal Z‐scan system. Nonlinear optical properties of quartz glass/ZnS–SiO₂ (170 nm)/AgO_x (15 nm)/ZnS–SiO₂ (40 nm) were measured using a Q‐switch Nd : YAG pulse laser of wavelength 532 nm, pulse width 0.7 ns, and 15.79 kHz repetition rate. Transmittance and reflectance of the sandwiched AgO_x thin film show important optical responses at the focused position of Z‐scan. The dissociation processes of AgO_x, recombination of the silver and oxygen, and the resonance of the localized surface plasmon of the nano‐composites of the AgO_x thin film are correlated to transmittance and reflectance at the focused position of the Z‐scan for different input laser powers. An irreversible upper threshold intensity of 4.40 × 10⁶ mW cm^?2 at the focused position was found. A reversible working window of the focusing intensity between 1.86 × 10⁶ and 4.40 × 10⁶ mW cm^?2 was measured with sandwiched AgO_x thin film alone. The near‐field interactions of the AgO_x thin film and the recording layers of super‐resolution near‐field optical disk are also discussed.     

Large-area topography analysis and near-field Raman spectroscopy using bent fibre probes

We present a method for combined far‐field Raman imaging, topography analysis and near‐field spectroscopy. Surface‐enhanced Raman spectra of Rhodamine 6G (R6G) deposited on silver nanoparticles were recorded using a bent fibre aperture‐type near‐field scanning optical microscope (NSOM) operated in illumination mode. Special measures were taken to enable optical normal‐force detection for control of the tip–sample distance. Comparisons between far‐field Raman images of R6G‐covered Ag particle aggregates with topographic images recorded using atomic force microscopy (AFM) indicate saturation effects due to resonance excitation.     

拉盖尔-高斯光经超表面的聚焦特性

为了研究椭圆偏振拉盖尔-高斯光经超表面的聚焦特性,重点分析了聚焦光场的电场强度以及相位分布特性。利用时域有限差分(FDTD)方法仿真椭圆偏振拉盖尔-高斯光经超表面的聚焦光场。研究表明,椭圆偏振拉盖尔-高斯光经超表面聚焦后会在聚焦光场的纵向分量电场中出现自旋角动量转化为轨道角动量的现象,通过改变超表面的数值孔径大小、拉盖尔-高斯光的偏振类型等参数可以调控涡旋光场形态分布。该现象可应用于光镊、量子加密、光学扳手等技术中。     

Fundamental differences between micro- and nano-Raman spectroscopy

Electric field polarization orientations and gradients close to near-field scanning optical microscope (NSOM) probes render nano-Raman fundamentally different from micro-Raman spectroscopy. With x -polarized light incident through an NSOM aperture, transmitted light has x, y and z components allowing nano-Raman investigators to probe a variety of polarization configurations. In addition, the strong field gradients in the near-field of a NSOM probe lead to a breakdown of the assumption of micro-Raman spectroscopy that the field is constant over molecular dimensions. Thus, for nano-Raman spectroscopy with an NSOM, selection rules allow for the detection of active modes with intensity dependent on the field gradient. These modes can have similar activity as infra-red absorption modes. The mechanism can also explain the origin and intensity of some Raman modes observed in surface enhanced Raman spectroscopy.     

TEACO2非稳腔激光器远场光束质量的评价

研究了对虚共焦非稳腔结构的脉冲TEA CO2激光器远场光束质量进行评价的方法.首先将稳腔TEA CO2激光器加工改造成非稳腔结构;然后从实际设计加工的数据出发,用3种常用形式对其远场光束质量进行理论评价与分析,并与设计的2 kW非稳腔激光器的实测远场光强分布实验结果进行了对比.理论和实验结果表明,非稳腔能够获得接近衍射...     

激光直写系统焦斑整形的研究

针对激光直写光刻系统设计整形衍射光学元件(DOE),对写入焦斑的整形进行了研究。提出对不同焦距写入物镜下的焦斑,利用同一个DOE加以整形的方法。模拟表明:整形后焦斑的面形和边缘显著改善,衍射效率高于实用指标,提供一种改善系统分辨能力的有效途径。     

Diffraction of circularly polarized light from near-field optical probes

Diffracted fields from 100-nm aperture near-field scanning optical microscopy (NSOM) probes and uncoated tapered fibres are measured and analysed. Using a solid angle scanner, the two-dimensional intensity distribution and polarization state of the diffracted light are resolved experimentally. Polarization analyses show that circularly polarized input light does not maintain its polarization state for all diffraction angles, and is completely filtered into linearly polarized light at large polar diffraction angles. This drastic decomposition originates from the vector nature of light diffracted by the sub-wavelength aperture. There is a fundamental difficulty in generating circularly polarized light near the aperture of NSOM probes owing to polarization-dependent diffraction in the near-field regime. This is illustrated by the Bethe-Bouwkamp model using circularly polarized input light.     

基于旁瓣光束衍射反演的强激光远场焦斑测量方法

针对大型激光装置使用纹影法无法实现旁瓣光束弱信号区域光强分布精确测量的问题,提出了基于旁瓣光束衍射反演的纹影法强激光远场焦斑测量方法.采用逆向推演间接测量的研究方法,沿光路传播逆方向推导,以旁瓣光束衍射光强图像和相位图像作为输入,通过计算获得未遮挡前旁瓣光束远场焦斑分布.相比传统基于纹影的远场焦斑测量方法,本文的主要改...     

突破衍射极限的成像方法综述

"衍射极限"实际上不是一个真正的障碍,除非处理远场和定位精度。这种衍射障碍并不是坚不可摧的,可以利用一些智能技术来突破光学衍射极限。讨论了四种技术,近场扫描光学显微镜(NSOM)法,受激发射损耗(STED)显微镜法,光激活定位显微镜(PALM)法或随机光学重建显微镜(STORM)法和结构照明显微镜(SIM)法,并且介绍了各自的基本原则与优劣。NSOM利用纳米级探测器检测通过光纤的极小汇聚光斑,从而获得单个像素的分辨率;PALM和STORM利用荧光探针,实现暗场和荧光的转换,从而观察到极小的荧光团;SIM则是利用栅格图案与样品叠加成像来实现。其中,STORM具有相对较高的潜力,能够更为有效地突破衍射极限。     

Diffraction of circularly polarized light from near-field optical probes

Diffracted fields from 100-nm aperture near-field scanning optical microscopy (NSOM) probes and uncoated tapered fibres are measured and analysed. Using a solid angle scanner, the two-dimensional intensity distribution and polarization state of the diffracted light are resolved experimentally. Polarization analyses show that circularly polarized input light does not maintain its polarization state for all diffraction angles, and is completely filtered into linearly polarized light at large polar diffraction angles. This drastic decomposition originates from the vector nature of light diffracted by the sub-wavelength aperture. There is a fundamental difficulty in generating circularly polarized light near the aperture of NSOM probes owing to polarization-dependent diffraction in the near-field regime. This is illustrated by the Bethe-Bouwkamp model using circularly polarized input light.