Constant Height Mode Research Articles

Detection of the optical magnetic field by circular symmetry plasmons

We report on the influence of coating a sharpened optical fiber tip with Au when observing nanofabricated dielectric structures with a Photon Scanning Tunneling Microscope (PSTM) in constant-height mode. For well-defined incident wavelengths and coating thicknesses, we found that such tips detect the distribution of the magnetic field associated with the optical wave in the near-field zone. A simple tip model indicates that this phenomenon is related to the excitation of circular symmetry plasmons in Au coated tips.

Artifact-free near-field optical imaging by apertureless microscopy

A method for optical near field discrimination, leading to drastic artifact reduction in superresolved imaging by scanning interference apertureless microscopy is presented. The method relies on second harmonic detection of the modulated optical signal scattered by a vibrating silicon tip. An edge resolution of 15 nm, or 7 nm Rayleigh-type resolution, with optical contrast as high as 50%, has been obtained on aluminum projection pattern samples in the constant gap width mode. Our method has been determined not to be affected by topographical artifacts by constant height mode scans.

Field-enhanced scanning optical microscope

We present experimental results of an imaging technique that uses as a local probe the optical field enhanced at the junction of a scanning tunneling microscope illuminated by a p-polarized laser beam. Images of highly oriented pyrolithic graphite, recorded at a constant height mode, show a lateral optical resolution of as much as 10 nm. Approach curves exhibit sensitivity on a subnanometer scale of the optical signal to the tip-sample distance, yielding the ultrahigh vertical resolution reached in the images.

Facilitated tip-positioning and applications of non-electrode tips in scanning electrochemical microscopy using a shear force based constant-distance mode.

In scanning electrochemical microscopy (SECM) a microelectrode is usually scanned over a sample without following topographic changes (constant-height mode). Therefore, deconvolution of effects from distance variations arising from non-flat sample surface and electrochemical surface properties is in general not possible. Using a shear force-based constant distance mode, information about the morphology of a sample and its localized electrochemical activity can be obtained simultaneously. The setup of the SECM with integrated constant-distance mode and its application to non-flat or tilted surfaces, as well as samples with three-dimensional surface structures are presented and discussed. The facilitated use of non-amperometric tips in SECM like enzyme-filled glass capillaries is demonstrated.

Local signal processing to evaluate resolution in SNOM images, using 1D wavelets

The near-field optical images are known to be inhomogeneous due on the one hand to the piezo decay and on the other hand to the scanning process. Both can induce a variation of the tip-sample distance between the beginning and the end of the scanning in constant height mode and steps between consecutive scanning lines. Moreover, the tip often rubs on the sample and induces artifactual, local high variations of the recorded signal. Therefore, a local characterization of spectral information in the signal could be useful in defining the separation power of the setup. This paper proposes a convenient method to evaluate this local spectral information in terms of local characterization of spatial frequency power in the recorded signal. The location of the spatial frequencies is achieved by using the discrete wavelet transform method.

Imaging point defects in a liquid environment: A model AFM study

Effects of the liquid on atomic force microscopy (AFM) imaging are examined for a model system consisting of a hexagonal flake of seven Lennard-Jones (LJ) atoms as a multiatom tip, a monolayer of LJ crystal containing a single point defect as a substrate, and three-site model water as the liquid. A previous simulation [Koutsos et al., Europhys. Lett. 26, 103 (1994)] has shown that the true atomic resolution of a point defect cannot be achieved in vacuum by use of the multiatom tip. Here we examine the feasibility of such atomic resolution when both the tip and substrate are immersed in a liquid. The liquid-induced interaction between the tip and the substrate is evaluated in the framework of the integral equation theory of molecular fluids extended for the system consisting of an infinitely large crystal. It is found that the potential of mean force indeed manifests the point defect even when the tip and the substrate are a few Angstroms apart. This implies that in the liquid environment the point defect could be discerned in the constant-height mode of AFM measurement. For the constant-load mode of AFM, a characteristic potential can be used to determine the stable, metastable, and unstable vertical position (height) of the tip at any lateral position. The contour surface of the stable heights reveals periodic features of the underlying lattice of the substrate except in the vicinity of the defect, provided certain load is applied.

Near-field polarization states and optical images in transmission mode through different surface structures

We provide a theoretical study of the near-field optical image of a homogeneous dielectric surface with deterministic sub-wavelength structures. Numerical simulations for different shapes of the sample are performed at constant-height mode with normal incidence. Near-field polarization states have been calculated by analyzing the difference of the transmitted intensities of p- and s-polarized light. We show that the near-field intensity depends more stronger on the feature form than the feature size. However, the polarization property is more sensitive to the shape of the sample rather than the distance of detection.

Observation of the dynamics of live cardiomyocytes through a free-running scanning near-field optical microscopy setup

We report the observation of live-cell dynamics by noncontact scanning near-field optical microscopy (SNOM) modified to work with living biological samples that are fully immersed in liquid. We did not use the SNOM setup in strictly near-field conditions (we used 1-mum constant-height mode); however, we could examine the dynamics of rhythmically beating cardiac myocytes in culture with extremely high vertical sensitivity below the nanometric range. We could halt scans at any point to record localized contraction profiles of the cell membrane. We show that the contractions of the organisms changed shape dramatically within adjacent areas. We believe that the spatial dependency of the contractions arises because of the measurement system's ability to resolve the behavior of individual submembrane actin bundles. Our results, combining imaging and real-time recording in localized areas, reveal a new, to our knowledge, noninvasive method for using SNOM setups for studying the dynamics of live biological samples.

Piezoresistive silicon V-AFM cantilevers for high-speed imaging

A low stiffness silicon V-AFM cantilever with integrated piezoresistive sensors on its root is applied to an atomic force microscope (AFM) for high-speed piezoresistive imaging. The images are presented from a sample with 1.0 μm high microfeatures in constant-height (variable force) mode with a tip scan rate of 1000 μm/s. The experiment is performed in a DC Wheatstone bridge arrangement with a 3.0 V battery supply. Electrical hum pickup, 1/f noise, and magnetic effect affecting the image will be discussed.

Novel scanning near-field optical microscope (SNOM)/scanning confocal optical microscope based on normal force distance regulation and bent etched fiber tips

We report on a unique combination of a novel scanning near-field optical microscope/scanning confocal optical microscope based on normal force distance regulation with bent etched fiber tips. The use of normal force detection makes all classic atomic force microscope operation modes such as contact mode and tapping mode accessible to scanning near-field optical microscopy (SNOM), thus allowing near-field optical imaging at sample distances unavailable to shear force regulated distance control. The SNOM laser light is pulsed or detected synchronously to the fiber tapping amplitude to illuminate the sample at a predetermined height above the sample. Bent etched fiber tips are used, whose fiber diameters are additionally etched down to yield resonant frequencies between 60 and 300 kHz, Q factors around 180 and spring constants in the range of 80–400 N/m. The typical roughness of the tip cone surfaces is shown to be due to etching in a meniscus. For the first time, simultaneously acquired constant gap width mode topography and constant height mode near-field optical images are presented, allowing topography induced contrast to be separated from true optical contrast such as absorption and fluorescence.

Polarization properties of the near-field intensity reflected by metallic and dielectric one-dimensional structures.

We investigate the state of polarization and near-field intensity distribution in the vicinity of rectangular groove objects ruled on metallic and dielectric materials. The sample is illuminated from the vacuum side by a linear combination of p- and s-polarised waves. Two rigorous methods of solution are used and compared in calculations of the total intensity at constant height when the light is incident normally onto the surface. Some calculations of the total intensity in the 'follow-the-profile mode' are also presented. It is shown that in the constant height mode, the contrast in the image can be reversed as the plane of observation moves away from the mean plane of the sample. We also found that the state of polarization depends strongly on the material and the distance to the plane of detection.

Characterization of reflection scanning near-field optical microscopy and scanning tunnelling optical microscopy/photon scanning tunnelling microscopy working in preliminary approach constant height scanning mode.

The resolution in near-field images is currently determined by the visual inspection of recorded images. One of the major questions in near-field optical microscopy is 'what resolution can be reached, the tip-to-sample distance being known?' This knowledge is critical when choosing the scanning step and the distance between the tip and the sample, in a preliminary scan. This preliminary scan is often the only way to detect the interesting parts of the sample, with limited risk of tip crash and topographical artefacts. The method proposed here needs two scans of the same area, of the same sample, in constant height mode, recorded at two tip-to-sample distances. The pseudotransfer function is the ratio of the Fourier transform of these two data maps. This function enables the evaluation of the limit of resolution. Theoretical considerations are introduced to assess the method.

Ion beam-treated silicon probes operated in transmission and cross-polarized reflection mode near-infrared scanning near-field optical microscopy (NIR-SNOM)

We present aperture probes based on non-contact silicon atomic force microscopy (AFM) cantilevers for simultaneous AFM and near-infrared scanning near-field optical microscopy (SNOM). For use in high-resolution near-field optical microscopy, conventional AFM cantilevers are modified by covering their tip side with an aluminium layer to obtain an opaque coating. To fabricate an aperture, this metal layer is opened at the very end of the polyhedral probe using focused ion beams, thus allowing apertures of <200 nm in diameter to be created. We describe our concept and investigate the characteristics of these probes by discussing images in transmission and cross-polarized internal reflection mode. Apart from probe characterization, our attention is devoted to the dependence of both signal amplitude and resolution on the tip-to-sample distance and to the polarization effects observed in the course of these investigations. To exclude artefacts induced by distance control, the measurements analysed here are performed in constant-height mode. Copyright © 1999 John Wiley & Sons, Ltd.

Nano-slit probes for near-field optical microscopy fabricated by focused ion beams.

The near-field probes described in this paper are based on metallized non-contact atomic force microscope cantilevers made of silicon. For application in high-resolution near-field optical/infrared microscopy, we use aperture probes with the aperture being fabricated by focused ion beams. This technique allows us to create apertures of sub-wavelength dimensions with different geometries. In this paper we present the use of slit-shaped apertures which show a polarization-dependent transmission efficiency and a lateral resolution of < 100 nm at a wavelength of 1064 nm. As a test sample to characterize the near-field probes we investigated gold/palladium structures, deposited on an ultrathin chromium sublayer on a silicon wafer, in constant-height mode.

Readout characteristics of a near-field optical probe as a data-storage readout device: submicrometer scan height and resolution

A photodiode-embedded near-field scanning microscope cantilever (photocantilever) was used to scan in a noncontact, constant-height mode at a range common in hard disk drives to examine its readout capabilities when mounted on a flying-slider head. The intensity ratios of spatial frequencies that compose the obtained near-field image were analyzed by use of the fast Fourier transform. A simplified model was developed as a guiding principle for estimating the readout characteristics of the near-field optical probe in the above-proximity scan-height range.

Influence of detection conditions on near-field optical imaging

The process of image formation in transmission mode scanning near-field optical microscopy is analyzed both theoretically and experimentally. Changes in the dielectric and topographic properties of the sample influence not only the total transmitted intensity, but also its angular distribution in the far field. This opens up an additional source of optical information about the sample. Some of this additional information is retrieved by separate but simultaneous detection of the radiation emitted at angles smaller (allowed light) and larger (forbidden light) than the critical angle of total internal reflection, respectively. Different experimental setups and their respective advantages are discussed. High resolution, constant height mode optical images of test structures are compared with theoretical predictions. Forbidden-light optical images frequently provide enhanced resolution and/or contrast as compared to allowed light images. For small phase objects, in contrast to amplitude objects, a contrast reversal between forbidden and allowed light images is observed.

Two different domains in a cubic boron nitride (111) surface observed by friction force microscopy

A single-crystalline cubic boron nitride (c-BN) (111) surface has been studied by atomic force microscopy with simultaneous measurements of normal atomic force (constant height mode and constant force mode) and lateral (friction) force in air. On the friction force mode the different two domains observed can be attributed to the termination of the surface; boron (A side) or nitrogen (B side). 2.6 Å periodicity corresponding to the 1×1 surface unit cell as of c-BN (111) (as=a0/√=2.56 Å, where a0 is the lattice constant, 3.62 Å) has been observed also on the friction force mode, which is in agreement with our low-energy electron diffraction measurement.

Scanning Tunneling Microscopy Study of the Misfit Layer Compounds (LaSe)xNbSe2 and (PbSe)xNbSe2

Atomic images of the misfit layer compounds (LaSe)xNbSe2 and (PbSe)xNbSe2 were obtained with a scanning tunneling microscope (STM) operating in constant height mode in air. It was possible to record pictures of only the NbSe2 layers of both compounds, the LaSe and PbSe layers could not be observed. Formation of stage-2 portions embedded in the stage-1 crystal and instability of the LaSe and PbSe layers under the scanning conditions are discussed as possible causes. Comparing with the NbSe2 crystal, the lattice of the NbSe2 layers of the misfit layer compounds appear deformed. Nonperiodic Moiré-like structures have been observed in the (LaSe)xNbSe2 surface. We consider that this feature is caused by the STM tip pushing down the surface where it is softer. The nonperiodicity of the patterns might be due to the strain conditions of the crystal growth that would give rise to dislocations and defects.

Reflection scanning near-field optical microscopy (R-SNOM) in constant height mode with a dielectric probe Image interpretation and resolution for high topographic variations

The understanding of the correlation between the near-field images that are recorded by scanning near-field optical microscopy (SNOM) and the local optical properties of the sample surface (topography, index, etc.) is a condition for the development of such microscopes. The aim of this paper is to show that the “constant height imaging” (CHI) mode provides useful near-field characterizations, even in case of high-relief samples. Actually, the CHI near-field signal is free from perturbations brought by usual feedback regulation systems. Furthermore, we show a comparison between experimental and theoretical data to explain near-field image formation. Finally, we develop a specific method based on the Fourier spectral analysis to characterize the experimental SNOM setup working in CHI mode.

TEM moiré patterns explain STM images of bacteriophage T5 tails

A subtle combination of constant current and constant height modes in scanning tunnelling microscopy allowed the imaging of a non-flat uncoated biological specimen, namely the tail of the bacteriophage T5. In parallel, a reference three-dimensional structure of the T5 tail was calculated from cryo-transmission electron microscopy images, based on its helical symmetry. This three dimensional reconstruction was compared with scanning tunnelling microscopy data. The images of the tail obtained by transmission electron microscopy, as well as projections of the reconstructed model, show similar moiré patterns. Here we show that scanning tunnelling microscopy performed in an aqueous environment provides direct images which are remarkably similar to the projection of the three dimensional model obtained by transmission electron microscopy. We deduce that our scanning tunnelling microscopy images are the result of a transmission of electrons through the gap between the scanning tip and the conductive support across the biological speciment.