保偏光纤模场直径和数值孔径测试研究

基于光纤测量远场法原理测量光纤远场光强分布。通过改进远场法得到光纤的远场光强分布后,同时计算得到保偏光子晶体光纤的模场直径和数值孔径,使测量装置实现集成化和简便化。通过光束测试仪测量光纤的出射光强分布,光束测试仪测量的是光纤中心最大光强点的两个垂直方向上的光强分布。保偏光子晶体光纤的出射光斑是椭圆的,每个方向的模场直径、数值孔径分布并不相同。传统的测试方法不能解决这个问题。通过旋转光纤测量光纤各个方向上的光强分布,然后计算各个方向上的模场直径,最后通过拟合各个方向上的模场直径得到保偏光子晶体光纤椭圆形的模场分布。实验测得保偏光子晶体光纤椭圆光斑长短轴的模场直径分别为7.5μm和4.2μm,数值孔径分别为0.159和0.276。     

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.     

Specimen-induced distortions in light microscopy

Specimen‐induced aberrations affect the imaging properties in optical 3D microscopy, especially when high numerical aperture lenses are used. Studies on aberrations are often properly concerned with the degradation of image quality such as compromised resolution or reduced signal intensity. Apart from these, aberration effects can also introduce geometric image distortions. The effects, discussed here are particularly strong when thick biological specimens are investigated. Using a high numerical aperture interferometer, we measured wavefront aberrations in transmission mode and quantified geometric distortions associated with specimen‐induced aberrations. This assessment for a range of biological specimens allows estimation of the accuracy of spatial measurements. The results show that high‐resolution spatial measurements can be significantly compromised by specimen‐induced aberrations.     

Study of the focused laser spots generated by various polarized laser beam conditions

In this work, three‐dimensional near‐field imaging of the focused laser spot was studied theoretically and experimentally. In the theoretical simulation, we use the electromagnetic equivalent of the vectorial Kirchhoff diffraction integral to calculate the intensity distribution of the focal region, and a high depolarization is found in high numerical aperture systems (NA = 0.85). The experimental set‐up is based on a near‐field scanning optical microscope (NSOM) system. A high‐NA objective lens is used to focus incident light of various polarizations, and a tapered near‐field optical fibre probe of the NSOM system is used to determine the intensity of the focal field. The results show an asymmetric distribution of the focused intensity with the linear polarized laser beam.     

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.     

多次函数拟合法计算光纤模场直径

光纤的模场直径是评价光纤性能的一个重要参数。测量单模光纤模场直径的常用方法是远场可变孔径法。通过测量给定单模光纤透过不同尺寸孔径光阑的多组远场辐射光功率数据并对实测数据进行处理,可得到被测光纤的模场直径。在实际测量过程中,为了消除被测光纤模场中心与孔径光阑中心偏离引起的测量误差,应用多次函数拟合法对远场可变孔径法测得的数据进行处理,以此自动识别并排除测量数据中的个别不合理的误差数据,从而有效提高模场直径的测量精度。     

Measurement of the width of thin,cylindrical, transparent objects by phase contrast light microscopy6

Measurement of the width of a thin, cylindrical, transparent object by phase contrast light microscopy has been frustrated by the absence of an established relationship between the true width of the object and its apparent width in the phase contrast image. We have solved this problem by devising a simple method by which individual glass fibres may be measured using both phase contrast light microscopy and electron microscopy. Using this method we have constructed calibration curves relating the diameter measured by phase contrast microscopy to the real diameter of the fibres. These curves are linear in the range 0.10-2.5 μm real diameter, with slopes close to unity and intercepts of about 0.2 μm. Thus widths of such objects are overestimated. The precise value of the intercept is related to the overall numerical aperture of the optical system. Each calibration curve permits the true width of a cylindrical object to be estimated by phase contrast microscopy with an accuracy of better than ±0.05 μm. We have found that greater precision is obtained by taking measurements of light micrographs subjectively using a microcomparator rather than objectively using a microdensitometer.     

Enabling the detection of UV signal in multimodal nonlinear microscopy with catalogue lens components

Using an optical system made from fused silica catalogue optical components, third‐order nonlinear microscopy has been enabled on conventional Ti:sapphire laser‐based multiphoton microscopy setups. The optical system is designed using two lens groups with straightforward adaptation to other microscope stands when one of the lens groups is exchanged. Within the theoretical design, the optical system collects and transmits light with wavelengths between the near ultraviolet and the near infrared from an object field of at least 1 mm in diameter within a resulting numerical aperture of up to 0.56. The numerical aperture can be controlled with a variable aperture stop between the two lens groups of the condenser. We demonstrate this new detection capability in third harmonic generation imaging experiments at the harmonic wavelength of ～300 nm and in multimodal nonlinear optical imaging experiments using third‐order sum frequency generation and coherent anti‐Stokes Raman scattering microscopy so that the wavelengths of the detected signals range from ～300 nm to ～660 nm.     

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.     

Photoplastic near-field optical probe with sub-100 nm aperture made by replication from a nanomould

Polymers have the ability to conform to surface contours down to a few nanometres. We studied the filling of transparent epoxy‐type EPON SU‐8 into nanoscale apertures made in a thin metal film as a new method for polymer/metal near‐field optical structures. Mould replica processes combining silicon micromachining with the photo‐curable SU‐8 offer great potential for low‐cost nanostructure fabrication. In addition to offering a route for mass production, the transparent pyramidal probes are expected to improve light transmission thanks to a wider geometry near the aperture. By combining silicon MEMS, mould geometry tuning by oxidation, anti‐adhesion coating by self‐assembled monolayer and mechanical release steps, we propose an advanced method for near‐field optical probe fabrication. The major improvement is the possibility to fabricate nanoscale apertures directly on wafer scale during the microfabrication process and not on free‐standing tips. Optical measurements were performed with the fabricated probes. The full width half maximum after a Gaussian fit of the intensity profile indicates a lateral optical resolution of ≈ 60 nm.     

Experimental statistics of near-field intensity distributions at nanostructured surfaces

Scanning near‐field optical microscopy is a technique in which the resolution is primarily determined by the size of a probe and not by the wavelength of illumination as in classical (far‐field) microscopy. However, the relationship between a sample and its near‐field optical image is usually rather complex. Typical factors responsible, at least partially, for such a complexity are the conditions of illumination and detection, sample characteristics (e.g. roughness and dielectric constant) and optical properties of the probe. Theoretical and experimental works conducted to improve our understanding of the relation between the object and the image have been reported ( Greffet & Carminati, 1997 ). Recently, with the help of a photon scanning tunnelling microscope we have carried out an extensive study of the resultant near‐field intensity distributions due to the elastic (in the plane) scattering of surface plasmon polaritons (SPPs) at metal film surfaces. We have also directly observed (in similar experimental conditions) localized dipolar excitations in silver colloid fractals ( Bozhevolnyi et al., 1998 ). In both cases, the studied phenomena are intimately related to the regime of multiple light scattering, in which the interference effects are rather complicated and therefore a proper interpretation of them was far from being trivial. Thus, even though a certain understanding of many features inherent to the subwavelength light interference phenomena was gained ( Bozhevolnyi & Coello, 1998 ; Bozhevolnyi et al., 1998 ; Coello & Bozhevolnyi, 1999 ), it is clear from the outcome of the investigations that more systematic studies in this context are still needed. A different and more powerful approach may be a statistical study of the recorded near‐field intensity distributions. In this work, we report what we believe to be the first results on experimental statistics of near‐field optical images exhibiting localized optical excitations (related to the regime of multiple scattering of light). We investigated optical images obtained with SPPs excited at different light wavelengths and scattered at different film surfaces, and with different polarizations and wavelengths of light scattered by silver colloid fractal structures. We have found significant differences in statistics between near‐field intensity distributions taken at rough and smooth metal film surfaces and fractal structures. Finally, our predictions seem to be in agreement with theoretical studies reported by other authors ( Sanchez‐Gil & Garcia‐Ramos, 1998 ).     

On the influence of lipid‐induced optical anisotropy for the bioimaging of exo‐ or endocytosis with interference microscopic imaging

Some implementations of interference microscopy imaging use digital holographic measurements of complex scattered fields to reconstruct three‐dimensional refractive index maps of weakly scattering, semi‐transparent objects, frequently encountered in biological investigations. Reconstruction occurs through application of the object scattering potential which assumes an isotropic refractive index throughout the object. Here, we demonstrate that this assumption can in some circumstances be invalid for biological imaging due to the presence of lipid‐induced optical anisotropy. We show that the nanoscale organization of lipids in the observation of cellular endocytosis with polarized light induces a significant change in far‐field scattering. We obtain this result by presenting a general solution to Maxwell's equations describing light scattering of core–shell particles near an isotropic substrate covered with an anisotropic thin film. This solution is based on an extension of the Bobbert–Vlieger solution for particle scattering near a substrate delivering an exact solution to the scattering problem in the near field as well as far field. By applying this solution to study light scattering by a lipid vesicle near a lipid bilayer, whereby the lipids are represented through a biaxial optical model, we conclude through ellipsometry concepts that effective amounts of lipid‐induced optical anisotropy significantly alter far‐field optical scattering in respect to an equivalent optical model that neglects the presence of optical anisotropy.     

Use of a night vision intensifier for direct visualization by eye of far-red and near-infrared fluorescence through an optical microscope

We describe the design, construction and testing of a prototype device that allows the direct visualization by eye of far‐red and near‐infrared (NIR) fluorescence through an optical microscope. The device incorporates a gallium arsenide (GaAs) image intensifier, typically utilized in low‐light or ‘night vision’ applications. The intensifier converts far‐red and NIR light into electrons and then into green light, which is visible to the human eye. The prototype makes possible the direct, real‐time viewing by eye of normally invisible far‐red and NIR fluorescence from a wide variety of fluorophores, using the full field of view of the microscope to which it is applied. The high sensitivity of the image intensifier facilitates the viewing of a wide variety of photosensitive specimens, including live cells and embryos, at vastly reduced illumination levels in both fluorescence and bright‐field microscopy. Modifications to the microscope are not required in order to use the prototype, which is fully compatible with all current fluorescence techniques. Refined versions of the prototype device will have broad research and clinical applications.     

A light-emitting diode light standard for photo- and videomicroscopy

A light calibration system consisting of a compact light-emitting diode (LED) source with feedback control of intensity is described. The source is positioned in the focal plane of the microscope objective and produces flat-field illumination of up to 31 μW. The source can be easily used to determine the performance of microscope optics and camera response. It can also be used as a standard light source for calibration of experimental systems. Selectable light intensities are produced by controlling the LED input power via a feedback circuit consisting of a photodiode that detects output light intensity. Spectral coverage extends between 550 and 670nm using green, yellow and red LEDs mounted side by side, which are selected individually. The LED chips are encapsulated in plastic diffusers which homogenize the light, and a flat field of illumination is obtained through a thin 1-mm-diameter aperture positioned directly over each chip. Provision is made for insertion of Ronchi rulings over the aperture to enable measurements of contrast modulation in a uniform field. The light may be pulse-modulated to assess camera response times and the device can be synchronized with video frames. Narrow bandpass interference filters can be placed between the objective lens and the LED source to produce monochromatic light without affecting the spacing of controlled light intensities since emission spectra do not shift appreciably over the range of LED powers chosen in this design. Results of tests using controlled light intensity and uniform illumination are presented.     

Near-field Raman spectroscopy using a sharp metal tip

Near‐field Raman spectroscopy with a spatial resolution of 20 nm is demonstrated by raster scanning a sharp metal tip over the sample surface. The method is used to image vibrational modes of single‐walled carbon nanotubes. By combining optical and topographical signals rendered by the single‐walled carbon nanotubes, we can separate near‐field and far‐field contributions and quantify the observed Raman enhancement factors.     

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.     

Measurement of the electric field distribution and potentials on the object surface in an emission electron microscope without restriction of the electron beams

An emission electron microscope without restriction of the electron beams was used to visualize and measure the distribution of electric fields and potentials on the surface under study. Investigations of this kind can be performed in an emission electron microscope without any aperture diaphragm. The potentialities of this method have been demonstrated using measurements with a silicon p–n junction to which a voltage has been applied in the reverse direction. The quantitative analysis becomes more complicated if the specimen is characterized by a heterogeneous intensity distribution of the electron emission from different areas of its surface. In the latter case two images obtained at different accelerating voltages (i.e. different voltages of the microscope extractor) provide the information necessary for an analysis of electric field and potential distributions.     

Spatial distribution and polarization dependence of the optical near-field in a silicon microfabricated probe

This paper reports on the spatial distribution and polarization behaviour of the optical near-field at the aperture of a Si micromachined probe. A sub-100 nm aperture at the apex of a SiO₂ tip on a Si cantilever was successfully fabricated by selective etching of the SiO₂ tip in a buffered-HF solution using a thin Cr film as a mask. The aperture, 10–100 nm in size, can be reproducibly fabricated by optimizing the etching time. The optical throughput of several apertures was measured. For a 100 nm aperture, a throughput of 1% was approved. The probe shows a very high optical throughput owing to the geometrical structure of the tip. The spatial distribution of the near-field light is measured and simulated using a finite difference-time domain method. The polarization behaviour of apertures with different shapes was analysed using a photon counting camera system.     

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.     

Calculation of the object edge position after its projection in a spatially noninvariant coherent optical system

Specific features of half-plane image formation in a spatially noninvariant (aberration-free) coherent optical system of the 2F–2F telecentric type with a limited aperture of the projection objective (in the absence of the spatial frequency filter) are studied. The dependence of the light intensity behavior at a point corresponding to the half-plane edge in the image on the object position is found in an analytical form on the basis of approximating the Fresnel functions by analytical functions. As the half-plane approaches the boundary of the field of vision of the system determined by the objective aperture diameter, the light intensity is demonstrated to deviate significantly from that in the case of the axial position of the half-plane, which may lead to noticeable measurement errors in inspecting the geometric parameters of objects by the projection method in transmitted light.