Evaluation of nano-optical probe from scanning near-field optical microscope images

We studied a nanometre-sized optical probe in a scanning near-field optical microscope. The probe profile is determined by using a knife-edge method and a modulated transfer function evaluation method which uses nanometre-sized line-and-space tungsten patterns (with spaces 1 μm to 50 nm apart) on SiO₂ substrates. The aluminium-covered, pipette-pulled fibre probe used here has two optical probes: one with a large diameter (350 nm) and the other with a small diameter (10 nm). The small-diameter probe has an optical intensity ≈63 times larger than that of the large-diameter probe, but the power is about 1/25 of that of the large probe.     

Measurement of probe displacement to the thermal resolution limit in photonic force microscopy using a miniature quadrant photodetector

A photonic force microscope comprises of an optically trapped micro-probe and a position detection system to track the motion of the probe. Signal collection for motion detection is often carried out using the backscattered light off the probe-however, this mode has problems of low S/N due to the small backscattering cross sections of the micro-probes typically used. The position sensors often used in these cases are quadrant photodetectors. To ensure maximum sensitivity of such detectors, it would help if the detector size matched with the detection beam radius after the condenser lens (which for backscattered detection would be the trapping objective itself). To suit this condition, we have used a miniature displacement sensor whose dimensions makes it ideal to work with 1:1 images of micrometer-sized trapped probes in the backscattering detection mode. The detector is based on the quadrant photo-integrated chip in the optical pick-up head of a compact disc player. Using this detector, we measured absolute displacements of an optically trapped 1.1 μm probe with a resolution of ～10 nm for a bandwidth of 10 Hz at 95% significance without any sample or laser stabilization. We characterized our optical trap for different sized probes by measuring the power spectrum for each probe to 1% accuracy, and found that for 1.1 μm diameter probes, the noise in our position measurement matched the thermal resolution limit for averaging times up to 10 ms. We also achieved a linear response range of around 385 nm with cross talk between axes ?4% for 1.1 μm diameter probes. The detector has extremely high bandwidth (few MHz) and low optical power threshold-other factors that can lead to its widespread use in photonic force microscopy.     

Piezoresistor-equipped fluorescence-based cantilever probe for near-field scanning

Scanning near-field optical microscopes (SNOMs) with fluorescence-based probes are promising tools for evaluating the optical characteristics of nanoaperture devices used for biological investigations, and this article reports on the development of a microfabricated fluorescence-based SNOM probe with a piezoresistor. The piezoresistor was built into a two-legged root of a 160-microm-long cantilever. To improve the displacement sensitivity of the cantilever, the piezoresistor's doped area was shallowly formed on the cantilever surface. A fluorescent bead, 500 nm in diameter, was attached to the bottom of the cantilever end as a light-intensity-sensitive material in the visible-light range. The surface of the scanned sample was simply detected by the probe's end being displaced by contact with the sample. Measuring displacements piezoresistively is advantageous because it eliminates the noise arising from the use of the optical-lever method and is free of any disturbance in the absorption or the emission spectrum of the fluorescent material at the probe tip. The displacement sensitivity was estimated to be 6.1 x 10(-6) nm(-1), and the minimum measurable displacement was small enough for near-field measurement. This probe enabled clear scanning images of the light field near a 300 x 300 nm(2) aperture to be obtained in the near-field region where the tip-sample distance is much shorter than the light wavelength. This scanning result indicates that the piezoresistive way of tip-sample distance regulation is effective for characterizing nanoaperture optical devices.     

Optical microcantilever consisting of channel waveguide for scanning near-field optical microscopy controlled by atomic force

We develop a novel optical microcantilever for scanning near-field optical microscopy controlled by atomic force mode (SNOM/AFM). The optical microcantilever has the bent channel waveguide, the corner of which acts as aperture with a large tip angle. The resonance frequency of the optical microcantilever is 9 kHz, and the spring constant is estimated to be 0.59 N/m. The optical microcantilever can be operated in contact mode of SNOM/AFM and we obtain the optical resolution of about 200 nm, which is as same size as the diameter of aperture. We confirm that the throughput of optical microcantilever with an aperture of 170 nm diameter would be improved to be more than 10(-5).     

Microfabrication of a combined AFM-SNOM sensor

The objective of this work is to fabricate a scanning probe sensor that combines the well-established method for atomic force microscopy, employing a micro-machined Si cantilever and integrated tip, with a probe for the optical near field. A photosensitive pn-junction is integrated into the tip for that purpose and an Al coating is applied to the tip. It comprises an aperture of 50-70 nm in diameter at the apex of the tip in order to spatially limit the interaction of the tip to the optical near field of the sample. Characterization of the tip and first results of simultaneously recorded force and photon images are presented.     

Subpixel edge location method proposed to improve the performance of optical fiber spherical coupling probe during dimensional measurement of micro-cavities with high aspect ratio

A subpixel edge location method based on orthogonal Jacobi–Fourier moments is proposed in this paper to improve the performance of optical fiber spherical coupling probe during dimensional measurement of micro-cavities with high aspect ratio. The effectiveness of the proposed method is proved through the performance test of a micro-hole measuring machine with optical spherical coupling probe. Test results indicate that a blind micro-hole of 400 μm in diameter can be experimentally measured at the depth of 2000 μm with a repeatability of 40 nm and an extremity resolution of 42 nm.     

Optical microcantilever consisting of channel waveguide for scanning near-field optical microscopy controlled by atomic force

We develop a novel optical microcantilever for scanning near-field optical microscopy controlled by atomic force mode (SNOM/AFM). The optical microcantilever has the bent channel waveguide, the corner of which acts as aperture with a large tip angle. The resonance frequency of the optical microcantilever is 9 kHz, and the spring constant is estimated to be 0.59 N/m. The optical microcantilever can be operated in contact mode of SNOM/AFM and we obtain the optical resolution of about 200 nm, which is as same size as the diameter of aperture. We confirm that the throughput of optical microcantilever with an aperture of 170 nm diameter would be improved to be more than 10⁻⁵.     

Atomic contrast transfer in annular darkfield images

We examine some of the assumptions underlying the standard optical equation as used for modelling high-resolution annular darkfield (Z-contrast) images of thin specimens. The validity of the approximations (i) that scattering occurs as spherical waves (atoms are point potentials) and (ii) that the depth of focus is infinite is found to be resolution-dependent. The correction to the optical equation for non-spherical wave scattering is found to be important when the probe size is of the order of 0·1 nm or smaller, and does not vanish in the ideal case of a large annular detector with no hole. Depth of focus becomes an important consideration at 100 kV for probes smaller than about 0·2 nm when the specimen thickness exceeds 10 nm. Both corrections can significantly degrade the effective point-to-point resolution of high-resolution Z-contrast images relative to that predicted by the standard optical equation.     

Near-field fluorescence imaging of single molecules with a resolution in the range of 10 nm

We demonstrate fluorescence imaging of single molecules, by near-field scanning optical microscopy (NSOM), using the illumination-collection mode of operation, with an aperture probe. Fluorescence images of single dye molecules were obtained with a spatial resolution of 15 nm, which is smaller than the diameter of the aperture (20 nm) of the probe employed. Such super-resolution may be attributable to non-radiative energy transfer from the molecules to the coated metal of the probe since the resolution obtained in the case of conventional NSOM is limited to 30–50 nm due to penetration of light into the metal.     

Resolution enhancing using cantilevered tip-on-aperture silicon probe in scanning near-field optical microscopy

Scanning near-field optical microscopy (SNOM) achieves a resolution beyond the diffraction limit of conventional optical microscopy systems by utilizing subwavelength aperture probe scanning. A problem associated with SNOM is that the light throughput decreases markedly as the aperture diameter decreases. Apertureless scanning near-field optical microscopes obtain a much better resolution by concentrating the light field near the tip apex. However, a far-field illumination by a focused laser beam generates a large background scattering signal. Both disadvantages are overcome using the tip-on-aperture (TOA) approach, as presented in previous works. In this study, a finite difference time domain analysis of the degree of electromagnetic field enhancement is performed to verify the efficiency of TOA probes. For plasmon enhancement, silver is deposited on commercially available cantilevered SNOM tips with 20nm thicknesses. To form the aperture and TOA in the probes, electron beam-induced deposition and focused ion beam machining were applied at the end of the sharpened tip. The results show that cantilevered TOA probes were highly efficient for improvements of the resolution of optical and topological measurement of nanostructures.     

纳结构的连续激光复合微纳探针刻划加工

为了加工形貌稳定且尺寸尽可能小的纳结构,建立了一套连续激光复合微纳探针的加工系统,并研究了光纤探针导光的连续激光辐照微纳探针的近场增强效应以及该系统的加工性能。首先,根据表面等离子体激元理论仿真分析了激光辐照原子力显微镜(AFM)探针的近场增强因子,并研究了微纳探针的针尖温度场和针尖热膨胀。接着,搭建了基于光纤探针导光的连续激光复合微纳探针的纳结构加工系统。最后,对聚乙烯片状材料样品进行了纳结构加工。结果显示:加工得到的纳米点尺度为200nm左右;纳米线的尺度为30~40nm。结果表明:光纤探针导光连续激光复合微纳探针系统避免了复杂的空间光路结构,是一种成本低廉,结构简单的系统,能够实现纳结构的加工。     

Nanometer profile measurement of large aspheric optical surface by scanning deflectometry with rotatable devices: Uncertainty propagation analysis and experiments

High-accuracy mirrors and lenses with large dimensions are widely used in huge telescopes and other industrial fields. Interferometers are widely used to measure near flat surfaces and spherical optical surfaces because of their high accuracy and high efficiency. Scanning deflectometry is also used for measuring optical near flat surfaces with sub-nanometer uncertainty. However, for measuring an aspheric surface with a large departure from a perfect spherical surface, both of these methods are difficult to use. The key problem for scanning deflectometry is that high-accuracy autocollimators usually have a limited measuring range less than 1000″, so it cannot be used for measuring surfaces having a large slope. We have proposed a new method for measuring large aspheric surfaces with large slopes based on a scanning deflectometry method in which rotatable devices are used to enlarge the measuring range of the autocollimator. We also proposed a method to connect the angle data which is cut by the rotation of the rotatable devices. An analysis of uncertainty propagation in our proposed method was done. The result showed that when measuring a large aspheric surface with a diameter over 300 mm and a slope of 10 arc-deg, the uncertainty was less than 10 nm. For the verification of our proposed method, experimental devices were set up. A spherical optical mirror with a diameter of 35 mm and curvature radius of 5000 mm was measured. The measuring range of the autocollimator was successfully enlarged by our proposed method. Experimental results showed that the average standard deviation of 10 times measurement was about 20 nm.     

Photoluminescence imaging of phosphor particles using near-field optical microscope with UV light excitation

We have developed fibre probes suitable for 325 nm UV light excitation and a photoluminescence near-field scanning optical microscope (NSOM) and demonstrated the photoluminescence imaging of phosphor BaMgAl10O17:Eu2+ (BAM) particles. The probe was fabricated by a two-step-etching method that we developed. The probe had a large taper angle at the top of the probe and a small taper angle at the root. The NSOM image was different from the topographical structure but roughly reflected the corresponding features of the particles. The inhomogeneity of the photoluminescence intensity between BAM particles was observed in the NSOM image. The photoluminescence intensity with various bandpass filters showed differences between the individual particles, which means that they have different spectra.     

Microfabricated silicon dioxide cantilever with subwavelength aperture

We have developed a microfabricated SiO₂ cantilever with subwavelength aperture for scanning near-field optical microscopy (SNOM), to overcome the disadvantages of conventional optical fibre probes such as low reproducibility and low optical throughput. The microcantilever, which has a SiO₂ cantilever and an aperture tip near the end of the cantilever, is fabricated in a reproducible batch process. The circular aperture with a diameter of 100–150 nm is formed by a focused ion-beam technique. Incident light is directly focused on the aperture from the rear side of the cantilever using a focusing objective, and high optical throughput (10⁻² to 10⁻³) is obtained. The microcantilever can be operated as a SNOM probe in contact mode or in dynamic mode.     

An evanescent-field optical microscope

The classic diffraction limit of resolution in optical microscopy (～γ/2) can be overcome by detecting the diffracted field of a submicrometre-size probe in its near field. The present stage of this so-called scanning near-field optical microscopy (SNOM) is reviewed. An evanescent-field optical microscope (EFOM) is presented in which the near-field regime is provided by the exponentially decaying evanescent field caused by total internal reflection at a refractive-index transition. A sample placed in this field causes a spatial variation of the evanescent field which is characteristic for the dielectric and topographic properties of the sample. The evanescent field is frustrated by a dielectric probe and thus converted into a radiative field. In our case the probe consists either of an etched optical fibre or of a highly sharpened diamond tip. The probe is scanned over the sample surface with nanometre precision using a piezo-electric positioner. The distance between probe and sample is controlled by a feedback on the detected optical signal. The resolution of the microscope is determined by both the gradient of the evanescent field and the sharpness of the tip. Details of the experimental set-up are discussed. The coupling of the evanescent field to the submicrometre probe as a function of probe-sample distance, angle of incidence and polarization has been characterized quantitatively. The observed coupling is generally in agreement with presented theoretical calculations. Microscopy has been performed on a regular latex sphere structure, which clearly demonstrates the capacity of the evanescent-field optical microscope for nanometre-scale optical imaging. Resolution is typically 100 nm laterally and 10 nm vertically. The technique is promising for biological applications, especially if combined with optical spectroscopy.     

基于辐射压力控制的微型元件的加工与检测

We have been developing new fabrication tools based on optical radiation pressur e and related phenomena to develop aflexible and accurate microfabrication tec hnology. In this paper, the laser trapping probe for the nano-CMM for assessment, in addition to micromachining technique using a small particle controlled by optical radiation pressure and laser aggregation technique are discussed. As the positional detection probe for the nano-CMM, an optically trapped silica particle with 8 mm diameter in forced oscillation state is used. A probe sphere retains a stable position when applied with trapping force by Nd:YAG laser light formed an nu lar and is forced to oscillate by the driving force changed by modulating the in tensity of LD emission. Experintal results show that this vibrational microprobe h as the possibility to achieve positional sensing accuracy of less than 25 nm. As a new micromachining technique, nano-removal process using an optically trapped micro-grain is proposed. The laser trapping force enables not only to stably trap the diamond grain with asymmetrical shape but also to freely control the positi on with spinning. Using this micro machining tool, the machining experiments of h ydrocarbon film are performed. AFM observation confirmed that the fine groove wi th depths of about 3～4 nm can be fabricated. As an additive process based on ra diation pressure, a laser microstructure fabrication using laser agglomeration p h enomena of colloidal particles aided by radiation pressure is investigated. By c ontrolling laser beam scanning in slurry containing KOH solution and SiO₂ par ticles with a diameter of 140 nm, colloidal particles are aggregated and adhered firmly to a silicon wafer substrate. Using this laser agglomerating process, two-dimensional grid microstructures at the pitch of 5 mm can be fabricated.     

高精度光学表面磁流变修形技术研究

作为一种确定性子孔径的光学加工方法,磁流变抛光具有高精度、高效率、高表面质量以及无亚表面损伤的特点,有能力对各种形状的光学零件进行加工。本文系统的介绍了磁流变抛光高精度光学表面的关键技术,并采用自研的KDMRF－1000磁流变抛光机床和KDMRW-1水基磁流变抛光液对直径100mm的K4材料平面反射镜和直径200mm的K9材料球面反射镜进行加工实验。样件一面形收敛到PV值55.3nm,面形RMS值5.5nm;样件二面形收敛到PV值40.5nm,面形RMS值5nm。样件的表面粗糙度均有显著改善。     

石英音叉扫描探针显微镜

石英音叉是一种谐振频率稳定、品质因数高的时基器件,其音叉臂的谐振参数(谐振振幅和谐振频率)对微力极其敏感。利用石英音叉对外力的敏感性,与钨探针结合,构成一种新型的表面形貌扫描测头。该测头与xyz压电工作台结合,利用测头音叉臂谐振频率对扫描微力的敏感性,研制基于相位反馈控制的扫描探针显微镜。首先介绍石英音叉测头的构成、工作原理和特性测试,以及由该测头构建的扫描探针显微镜的结构和测试、分析。通过对测头和系统的测试结果分析,系统达到1.2 nm的垂直分辨率,并通过对一维栅的测量,给出扫描获得的试样表面微观形貌图以及相位图,证明系统的有效性。另外,由于采用大长径比的钨探针,该系统具有测量大深宽比微器件表面轮廓的能力。     

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.     

Photoluminescence imaging of phosphor particles using near-field optical microscope with UV light excitation

We have developed fibre probes suitable for 325 nm UV light excitation and a photoluminescence near-field scanning optical microscope (NSOM) and demonstrated the photoluminescence imaging of phosphor BaMgAl₁₀O₁₇:Eu²⁺ (BAM) particles. The probe was fabricated by a two-step-etching method that we developed. The probe had a large taper angle at the top of the probe and a small taper angle at the root. The NSOM image was different from the topographical structure but roughly reflected the corresponding features of the particles. The inhomogeneity of the photoluminescence intensity between BAM particles was observed in the NSOM image. The photoluminescence intensity with various bandpass filters showed differences between the individual particles, which means that they have different spectra.