High spatial resolution imaging with near-field scanning optical microscopy in liquids.

The mechanism of tuning fork-based shear-force near-field scanning optical microscopy is investigated to determine optimal experimental conditions for imaging soft samples immersed in liquid. High feedback sensitivity and stability are obtained when only the fiber probe, i.e., excluding the tuning fork prongs, is immersed in solution, which also avoids electrical shorting in conductive (i.e., buffer) solutions. Images of MEH-PPV were obtained with comparable spatial resolution in both air and water. High optical resolution (approximately160 nm fwhm) was observed.     

Differential near-field scanning optical microscopy

We theoretically and experimentally illustrate a new apertured near-field scanning optical microscopy (NSOM) technique, termed differential NSOM (DNSOM). It involves scanning a relatively large (e.g., 0.3-2 mum wide) rectangular aperture (or a detector) in the near-field of an object and recording detected power as a function of the scanning position. The image reconstruction is achieved by taking a two-dimensional derivative of the recorded power map. Unlike conventional apertured NSOM, the size of the rectangular aperture/detector does not determine the resolution in DNSOM; instead, the resolution is practically determined by the sharpness of the corners of the rectangular aperture/detector. Principles of DNSOM can also be extended to other aperture/detector geometries such as triangles and parallelograms.     

Dual-optical-mode near-field scanning optical microscopy

The simultaneous operation of near-field scanning optical microscopy (NSOM) in reflection and transmission modes is demonstrated. In the transmission mode, a low-noise, large-area silicon photodetector was mounted between the piezoelectric transducer scanning stage and the sample. In the reflection mode, either a photomultiplier tube or two large-area silicon detectors was used for signal collection. The reflection-mode setup consisting of two silicon detectors provides a large numerical aperture of 0.9 as well as symmetrical detection of emitting photons. The dielectric thin films and the light-emitting polymers were used to demonstrate the capability of these two modes of NSOM. A comparison between these two different setups is also presented.     

Scanning near-field optical microscopy as a tool for the characterization of multimode interference devices

We report the scanning near-field optical microscopy (SNOM) characterization of a 4 x 4 multimode interference (MMI) device working at a wavelength of 1.55 microm and designed for astronomical signal recombination. A comprehensive analysis of the mapped propagating field is presented. We compare SNOM measurements with beam-propagation-method simulations and thus are able to determine the MMI structure's refractive-index contrast and show that the measured value is higher than the expected value. Further investigation allows us to demonstrate that good care must be taken with the refractive-index profile used in simulation when one deals with low-index contrast structures. We show evidence that a step-index contrast is not suitable for adequate simulation of our structure and present a model that permits good agreement between measured and simulated propagating fields.     

Scanning near-field optical data contrast measurement: a tomographylike near-field reconstruction

A method for the reconstruction of near-field optical signals is proposed to produce a tomographylike map of the field above the nanostructures being studied. The data obtained through lock-in detection are processed employing Chebyshev's polynomial function of the distance between the probe and the sample. The method is first applied to numerically generated near-field evanescent data, with three different decreasing lengths, and then applied to an experimental signal. Therefore the contrast of the signal above nanostructures is discussed to underline the discrepancy between the scanning near-field optical microscopy data and the reconstructions.     

Interferometric probe for near-field optical microscopy systems

We have studied the possibility of creating a new type of the interferometric near-field pinhole probe for near-field optical microscopy systems based on a Fabry-Perot fiber microresonator with a nanodimensional pinhole in one of its output mirrors. The dependence of the resonance wavelength shift in the Fabry-Perot interferometer on the distance from the output diaphragm to the object has been determined using the finite-difference method in the time domain. It is shown that the proposed technique ensures a spatial resolution of no worse than λ/15.     

Fluorescence lifetime imaging with near-field scanning optical microscopy

Near-field scanning optical microscopy (NSOM) is a high-resolution scanning probe technique capable of obtaining simultaneous optical and topographic images with spatial resolution of tens of nanometers. We have integrated time-correlated single-photon counting and NSOM to obtain images of fluorescence lifetimes with high spatial resolution. The technique can be used to measure either full fluorescence lifetime decays at individual spots with a spatial resolution of <100 nm or NSOM fluorescence images using fluorescence lifetime as a contrast mechanism. For imaging, a pulsed Ti:sapphire laser was used for sample excitation and fluorescent photons were time correlated and sorted into two time delay bins. The intensity in these bins can be used to estimate the fluorescence lifetime at each pixel in the image. The technique is demonstrated on thin films of poly(9,9'-dioctylfluorene) (PDOF). The fluorescence of PDOF is the results of both inter- and intrapolymer emitting species that can be easily distinguished in the time domain. Fluorescence lifetime imaging with near-field scanning optical microscopy demonstrates how photochemical degradation of the polymer leads to a quenching of short-delay intrachain emission and an increase in the long-delay photons associated with interpolymer emitting species. The images also show how intra- and interpolymer species are uniformly distributed in the films.     

Probe-sample interactions in reflection near-field scanning optical microscopy

Reflection near-field scanning optical microscopy with an asymmetric detector orientation is demonstrated. The effects of the probe-sample interactions are studied for different polarizations, detector orientations, and sample reflectivity. It is shown that the orientation of the detector can introduce shadowing in the images, which is opposite from the naive interpretation and which is dependent on the optical properties of the sample. Near-field optical images of metallic test patterns in reflection are shown that exhibit a lateral resolution of 40 nm.     

Fluorescence resonance energy transfer scanning near-field optical microscopy

The method of fluorescence resonance energy transfer scanning near-field optical microscopy (FRET SNOM) consists in the separation of a FRET pair between an SNOM tip and a sample. The donor (or acceptor) centre is located at the tip apex and scanned in the vicinity of a sample while acceptor fluorescence (or donor-fluorescence quenching) is detected. It is shown that the spatial resolution for such an approach is governed not by the aperture size but by the FRET characteristic radius (F?rster radius), and thus can attain the value of 2-7 nm with the same (or higher) sensitivity as characteristic for the aperture SNOM. The theoretical fundamentals of the method, its experimental realization and connections with other types of near-field optical microscopy are discussed. Coherent FRET SNOM, which can be realized at liquid helium temperatures, and its possible applications for quantum informatics, are briefly outlined.     

Field enhancement in apertureless near-field scanning optical microscopy.

The near field of an apertureless near-field scanning optical microscopy probe is investigated with a multiple-multipole technique to obtain optical fields in the vicinity of a silicon probe tip and a glass substrate. The results demonstrate that electric field enhancements of >15 relative to the incident fields can be achieved near a silicon tip, implying intensity enhancements of several orders of magnitude. This enhancement arises both from the antenna effect of the elongated probe and from a proximity effect when the probe is near the substrate surface and its image dipoles play a role.     

Phase separation in polyaniline with near-field scanning optical microscopy

We report the studies of conjugated polymers, polyaniline thin films, with a near-field scanning optical microscope. Because of the absorption variation in different oxidation states, transmission-mode near-field scanning optical microscope images were employed to map out the distribution of the oxidation states on a submicrometer scale. When the near-field wavelength is varied (between 632.8 and 543.5 nm), the phase separation between the oxidized and the reduced repeated units, with domain sizes on a nanometer-length scale, is observed.     

Image resolution in reflection scanning near-field optical microscopy using shear-force feedback: characterization with a spline and Fourier spectrum

Scanning near-field optical microscopes (SNOM's) actually lead to nanometric lateral resolution. A combination with shear-force feedback is sometimes used to keep the SNOM tip at a constant force from the sample. However, resolutions in shear-force and optical data are different. An estimation of both resolutions is important for characterizing the capabilities of such systems. The basic principle of the measurement is to compare a spline-fitted logarithm of the power spectra calculated with the optical image with that of the shear force image in which resolution is determined a priori. Quantitative results are given in the case of periodic or untested sample and simulated data. Moreover the accuracy and the stability of the method are discussed.     

Characterization of antiresonant reflecting optical waveguide devices by scanning near-field optical microscopy

Silicon-based antiresonant reflecting optical waveguide (ARROW) devices were studied by means of a scanning near-field optical microscope. Various structures such as a Y junction of a Mach-Zehnder interferometer and a directional optical coupler were characterized, showing the propagation of the light inside the devices simultaneously with the topography. Scattering on the splitting point of the Y junction was shown, as well as a partial coupling of the light between the two branches of the coupler. Measurements on the decay length of the evanescent field were also performed to study the use of the ARROW waveguide for sensor purposes.     

High-density optical data storage with one-photon and two-photon near-field fluorescence microscopy

A spatially localized photochemical reaction induced by near-field femtosecond laser pulses is demonstrated on a nanometer scale and used for high-density optical data storage. Recorded domains down to 120 and 70 nm are obtained with one-photon and two-photon excitation, respectively. It is shown that the local-field confinement that is due to the quadratic dependence of two-photon excitation on light intensity has the potential to increase the near-field optical storage density.     

Fabrication and characterization of optical-fiber nanoprobes for scanning near-field optical microscopy

The current scanning near-field optical microscopy has been developed with optical-fiber probes obtained by use of either laser-heated pulling or chemical etching. For high-resolution near-field imaging, the detected signal is rapidly attenuated as the aperture size of the probe decreases. It is thus important to fabricate probes optimized for both spot size and optical transmission. We present a two-step fabrication that allowed us to achieve an improved performance of the optical-fiber probes. Initially, a CO(2) laser-heated pulling was used to produce a parabolic transitional taper ending with a top thin filament. Then, a rapid chemical etching with 50% buffered hydrofluoric acid was used to remove the thin filament and to result in a final conical tip on the top of the parabolic transitional taper. Systematically, we obtained optical-fiber nanoprobes with the apex size as small as 10 nm and the final cone angle varying from 15 degrees to 80 degrees . It was found that the optical transmission efficiency increases rapidly as the taper angle increases from 15 degrees to 50 degrees , but a further increase in the taper angle gives rise to important broadening of the spot size. Finally, the fabricated nanoprobes were used in photon-scanning tunneling microscopy, which allowed observation of etched double lines and grating structures with periods as small as 200 nm.     

Reflection-mode scanning near-field optical microscopy using an apertureless metallic tip

Recently, a reflection-mode near-field optical microscope with an apertureless tungsten tip has been introduced and 100-nm resolution has been achieved [R. Bachelot, P. Gleyzes, and A. C. Boccara, Microsc. Microanal. Microstruc. 5, 389-397 (1994)]. The optical signal is recorded in parallel with a tapping-mode atomic force microscope signal. By showing several images here, we confirm the capabilities of this device and clearly demonstrate a 20-nm (~lambda/35) resolution that has been achieved with smaller tips. A study of these images shows that both the topography and the near electromagnetic field of the sample can be independently probed by this device. Additionally, we discuss the principle of our approach, notably on the basis of interference phenomena between a Rayleigh scatterer and its image through the reflecting surface, and some of the setup's experimental characteristics are presented.     

反射式近场光学显微镜样品近场光分布特性

建立了一种反射式近场光学显微镜中样品近场光分布特性的模型,应用矢量衍射理论,得到了系统的场方程。在弱波动条件下,采用微扰法对场方程进行了求解,能方便地得到样品表面的各阶近场光反射和透射模复振幅表达式。计算结果表明,一级场分布要比零级场小一个量级,各阶近场信号的强弱完全由面形函数的傅里叶变换决定。通过与零级结果的比较,证明了计算结果的正确性。提供了一种计算样品表面近场分布简便方法,对反射式近场光学显微镜中调制检测技术具有指导意义。     

Detection of probe dither motion in near-field scanning optical microscopy

The probe-to-sample separation in near-field scanning optical microscopes can be regulated by a noncontact shear-force sensing technique. The technique requires the measurement of a minute dither motion applied to the probe. We have characterized an optical detection method for measuring this motion to determine the optimum detection configuration in terms of sensitivity and stability. A scalar diffraction model of the detection method is developed for calculating sensitivity, and experimental results are found to be in good agreement with the theoretical predictions. We find that maximum sensitivity and stability cannot be achieved simultaneously, and it may be desirable in practice to trade sensitivity for enhanced stability.     

Absorption coefficient imaging by near-field scanning optical microscopy in bacteria

We present a method for obtaining a position-dependent absorption coefficient from near-field scanning optical transmission microscopy. We show that the optical transmission intensity can be combined with the topography, resulting into an absorption coefficient that simplifies the analysis of different materials within a sample. The method is tested with the dye rhodamine 6G, and we show some analysis in biological samples such as bacteria KIebsiella pneumoniae and Pseudomonas aeruginosa. The calculated absorption coefficient images show important details of the bacteria, in particular for P. aeruginosa, in which membrane vesicles are clearly seen.