Photon Scanning Tunneling Microscope Research Articles

Near field diffraction grating for atoms

We propose to use the periodic light potential of an evanescent light wave produced on the surface of a dielectric prism and modulated through the absorption in parallel metal strips applied onto the dielectric surface as a diffraction grating and a coherent beam splitter for a precooled atomic beam. We have measured the spatial intensity distribution of the near field over the surface of such a microstructure using a photon scanning tunnelling microscope. The microstructure with a period of 10 μm was formed by nickel grooves 40 nm thick and 3 μm wide fabricated on the quartz surface. The possibility of coherent splitting of a precooled atomic beam into components of approximately the same intensity with an angular splitting of a few milliradians is shown using the semiclassical approach.

Polarization-dependent contrast in near-field optical microscopy

This article presents a simple modeling and simulation of experiments with a near-field optical microscope or a photon scanning tunneling microscope (PSTM) in the sub-100-nm range. The simulation employs a semimicroscopic and perturbative approach based on field propagator and linear response theory. A probe tip and sample are approximated as nanometric spheres in order to clarify the behavior of the near-field and far-field signal intensities, I, and the contrast, i.e., visibility, (Imax−Imin)/(Imax+Imin), for s and p polarization of incident light and three scanning methods: constant height, constant intensity, and constant distance. The signal intensity then becomes a function of the taper angle of the fiber probe tip θ or the numerical aperture of the collecting lens, in addition to the variables mentioned above. Note that the signal intensity I(θ=90°) corresponds to that for the near-field. The simulated polarization-dependent intensity and contrast are in good qualitative agreement with the experimental results. At the same time, for each polarization and scanning method, there is an optimal angle θ for maximizing the contrast and maintaining a high signal intensity. This result indicates that the taper angle and scanning method are very important factors in the polarization-dependent contrast and resolution of near-field optical microscopy.

High resolution photon scanning tunneling microscope

A photon scanning tunneling microscope with a lateral resolution of a few nanometers is successfully constructed. This is achieved by using a total internal reflection arrangement in combination with an uncoated optical fiber probe. A shear force technique for the tip-to-sample distance regulation is also employed. Chemically etched optical fiber probes have resonance frequency at , with a Q factor of more than 100. The intensity of the evanescent field picked up by a fiber tip is traced as a function of tip-to-sample separation. The intensity decay is exponential, therefore a typical example of an evanescent wave. An optical image of dispersed latex spheres is displayed. The image is in good correlation to the simultaneously obtained topographic image. In the optical image, each sphere is displayed as the dim area. Optical images obtained by Ar laser and Xe lamp are displayed. For Xe lamp the lateral resolution is comparable with the result for Ar laser.

Experimental study of surface-plasmon scattering by individual surface defects

A direct-write ablation technique has been implemented in a photon scanning tunneling microscope setup. This combination allows us to study surface-plasmon (SP) scattering by in situ created individual surface defects, while the sizes and shapes of the defects are varied continuously. It is found that within a certain range of its size, a ``hill'' on an otherwise flat surface can be the source of a very narrow plasmon beam. This effect is explained using the Huygens-Fresnel principle. Shadowing and refraction of the SP field by smaller defects has also been observed. In order to explain these results we introduce an effective SP refractive index for two classes of surface defects: shallow topographical defects and areas covered with absorbed molecular layers. This concept allows us to achieve a qualitative understanding of plasmon scattering in many practical cases. Some simple optical elements for the control of SP propagation are suggested and demonstrated. Our observations suggest numerous practical applications in multichannel chemical sensing, biosensing, and integrated optics.

Numerical study of optical transmission in sandwich slabs: Implication to photon scanning tunneling microscopy

In this work, we present a numerical simulation of scanning near-field optical microscopy. The simulation is based on a recently developed macroscopic theory which rigorously solves the electromagnetic field in a two-dimensional dielectric-air-dielectric sandwich system with arbitrary one-dimensional structures at its two interfaces. A tiny dielectric triangular is added on one interface to represent the probe tip in the device, while on the other interface, two identical topographic defects with various profiles are attached to represent the sample surface. The probe tip is assumed scanning at a constant height above the sample surface which is illuminated by a p-polarized light. The intensity of the local field averaged along a cross section inside the tip is calculated to represent the signal level of the microscope. The feasibility of the theoretical scheme is numerically demonstrated, and various aspects in the microscopy, such as the effects of the separation of the two objects, and of the shape differences between ridges and grooves are discussed.

Spectroscopic Contrast by Colour Photon Scanning Tunnelling Microscopy

A colour photon scanning tunnelling microscope (Colour-PSTM) using a white light illumination source has been designed in order to provide spectroscopic contrast with three colour channels. True colour images can be put together from independently measured topography and normalized red, green and blue signals. The measurement of tip–sample distance curves in the respective spectral ranges is consistent with theoretical considerations. Material contrast is demonstrated with two different dyes, which can be clearly distinguished. As an example for measuring intrinsic material properties, surface plasmon polaritons in thin silver films are recorded at different coupling conditions of the momentum vectors. This new Colour-PSTM offers one of the most desirable features in scanning probe microscopy, with a high potential of application: local material contrast by colour spectroscopy.© 1997 John Wiley & Sons, Ltd.

Spectroscopic Contrast by Colour Photon Scanning Tunnelling Microscopy

Spectroscopic Contrast by Colour Photon Scanning Tunnelling Microscopy

Study of large travel tube scanner for photon scanning tunneling microscopy and its investigation for soft materials

The tube scanner is a key part of the photon scanning tunneling microscope (PSTM). The sensitivity, resolution, scanning range and the linearity of PSTM are quite relevant to the study of the tube scanner. In this article, a new experimental method to measure the deflection sensitivity of the PZT tube is reported. The tube travel range, its linearity, and the coupling between x or y and z direction are analyzed. The theoretical formula for the coupling effects are also derived. With careful design the tube scanner coupling can be neglected and the experimental results show that the linearity of the large travel tube is satisfactory. Some images of PSTM demonstrate convincingly the system performance. The imaging of Langmuir–Blodgett (LB) films and fish scale plates is a significant application of PSTM in the fields of organic materials and living specimens.

Reconstruction of corrugated dielectric surfaces with a model of a photon scanning tunneling microscope: influence of the tip on the near field

Exact calculations of the scattering of light and other electromagnetic waves by a dielectric tip in front of corrugated dielectric samples are carried out by means of surface integrals derived from the extinction theorem and related equations. The configuration tackled is two dimensional, and the active part of the probing tip is simulated by a cylinder. The surface under study is illuminated from its dielectric side with a Gaussian beam at different angles of incidence. The transmitted near-field distribution is calculated inside the tip when it scans the surface at a constant distance from its mean plane. Different calculations are performed both for extended corrugated profiles (surface-relief gratings) and for flat dielectric interfaces with topographic subwavelength defects. The results show that the near fields scattered from these two kinds of surface corrugation have a notably different behavior with respect to the profile. Also, the influence of the angle of incidence, the polarization, and the shape of the incident beam on the near-field distribution is studied. It is shown that tips with diameters not larger than 0.1λ and a dielectric permittivity similar to that of glass do not appreciably perturb the near field transmitted by the sample.

真空蒸着誘電体薄膜のフォトン走査型トンネル顕微鏡による観察

真空蒸着誘電体薄膜のフォトン走査型トンネル顕微鏡による観察

A simple low-cost highly reproducible method of fabricating optical fiber tips for a photon scanning tunneling microscope

A simple low-cost highly reproducible method of fabricating optical fiber tips for a photon scanning tunneling microscope

Self-consistent model for photon scanning tunneling microscopy: implications for image formation and light scattering near a phase-conjugating mirror

A macroscopic self-consistent model for photon scanning tunneling microscopy with an uncoated fiber tip is developed by use of integral plane-wave representations of the total electric field in two-dimensional geometry. The model framework allows one to treat the incident field with an arbitrary angular spectrum and the presence of a thin-layer medium with a subwavelength structure on the sample surface. Imaging with the photon scanning tunneling microscope and light scattering near a phase-conjugating mirror are considered with our model by use of numerical simulations. We show that the near-field optical images provided by the microscope can be quite different from the light intensity distributions that exist near the sample surface in the absence of the fiber tip. We demonstrate that the self-consistent field at the site of a scatterer placed in front of a phase-conjugating mirror can be significantly enhanced as a result of multiple phase conjugation of the scattered light.

Photon tunneling scanning image separation method and instrument

A patent for the new photon scanning tunneling microscopy (PSTM) is described in this paper, called the Tunneling Scanning Image Separation Method and Instrument. We call the new instrument described here the Image Separated-PSTM (IS-PSTM). There may be some false image information with the first generation PSTM in the mixed image of the topography and refractive index of a general optical sample. With this new method and instrument we can eliminate this false image information and separate the surface topography image and the distribution image of varying refractive indices of a general optical sample from a mixed image of PSTM.

Near-field optical probing of fluorescent microspheres using a photon scanning tunneling microscope

We report on the direct near-field probing of fluorescent polystyrene microspheres using a Photon Scanning Tunneling Microscope. An optical fiber tip frustrates the evanescent part of the light existing in the near-field of the sample. We combine classical and fluorescence near-field probing, in the photon tunneling configuration and demonstrate a novel effect which, to our knowledge, has not been reported yet. An improvement of the discrimination of the microspheres is observed in near-field when fluorescence light is emitted owing to the presence of an evanescent part of the fluorescence light in the vicinity of the sample. This part which is normally compelled to remain confined to the near-field of the sample is converted into propagating light by the probe and this tunneling fluorescence light brings additional information about the sample. This effect is highly sensitive to the incident (i.e., excitation) wavelength and it is found that when the observed confinement is maximum, a slight change in the incident wavelength entails noticeable changes in the near-field images.

Virtual photon scattering at subwavelength sized tips

Silicon AFM tips can profitably be used as optical sensors for near field optical microscopy. In particular they are able to convert evanescent waves (also called virtual photons) into propagating ones which are conveniently guided in the tip and transferred to the air at the back of the cantilever. In this paper it is shown that virtual photons scattered at the tip end are also converted into real ones which are observed in the far field. Contrary to the previous observations of similar conversion at STM metal tips the scattered emission at dielectric silicon tips does not have a dependence with distance of a full exponential decay but rather follows the more complex dependence of the “dielectric capture”. This observation is consistent with Mie's theory of scattering of real photons by small particles. Experiments are performed with boths andp polarizations showing a regular dependence of the scattered intensity with the incidence angle. The same experiment is also performed with metallized silicon tips showing a metal scattering behaviour. This results can help to achieve a better control of the tip position in the close range of distances in a Photon Scanning Tunneling Microscopy (PSTM) experiment.

Imaging of Surface Plasmon Scattering by Lithographically Created Individual Surface Defects.

A new technique for direct-write ablation on a 100 nm scale has been implemented in a photon scanning tunneling microscope setup. This combination allows us, for the first time, to study surface plasmon scattering by in situ created individual surface defects, while the sizes and shapes of the defects are varied. It is found that, within a certain range of size, a hill on an otherwise flat surface can be a source of a micrometer wide plasmon beam. Another new observation is that the effective cross section of a defect can be substantially larger than its visible geometrical size. These results suggest possible practical applications of surface plasmons in two-dimensional optical arrangements.

Localization phenomena in elastic surface-polariton scattering caused by surface roughness.

Localization phenomena in the elastic scattering of surface plasmon polaritons (SPP{close_quote}s), i.e., in the SPP scattering in the surface plane, are related to the appropriate characteristics of SPP and surface roughness. Optical fields of SPP{close_quote}s, which are excited at two different wavelengths (488 and 633 nm) used in turn for each surface region of four different metal (silver and gold) films, are imaged simultaneously with surface topography by use of a photon scanning tunneling microscope with shear force feedback. Single scattering and weak and strong localization of SPP{close_quote}s are observed depending on the sample and the light wavelength. This is a direct demonstration of different regimes of the elastic SPP scattering realized at the same surface region for different light wavelengths. It is shown that the regime of SPP scattering and, consequently, near-field optical images, are determined by the topography of a local surface area with the size limited by the SPP propagation length. {copyright} {ital 1996 The American Physical Society.}

Tailoring a high-transmission fiber probe for photon scanning tunneling microscope

Transmission efficiency of a fiber probe used in photon scanning tunneling microscope is evaluated as a function of aperture diameter. The apertured probe has been fabricated by chemical etching technique and metal coating. By comparing two types of probes with different cone angles, we determine the most influential factor in the transmission property of the metal-cladding tapered waveguide. A long tip with high efficiency is developed by a multistep etching method so as to be suitable during actual scanning operation. Photoluminescence imaging of lateral p-n junctions on the GaAs substrate is demonstrated in the illumination-collection hybrid mode operation of photon scanning tunneling microscope.

Computation of the field diffracted by a local surface defect: application to tip–sample interaction in the photon scanning tunneling microscope

We use a method based on the Fourier transform of the electromagnetic field to compute the field diffracted by a local deformation of a plane boundary surface. We give a complete development of each step of the technique. To show the interaction that exists between the probe of a near-field optical microscope and the observed sample, we use the model of a truncated cone-shaped tip above a rectangular surface defect. We compute the electrical intensity along a line located between the tip and the local surface defect. We show the influence of the polarization of the incident wave and the effect of the position of the tip with respect to the position of the surface defect.

Numerical study of the tip-sample interaction in the photon scanning tunneling microscope

The differential method was used to model the scanning of a tip above a given surface defect in the photon scanning tunneling microscope illumination conditions. In order to estimate the influence of the tip-sample interaction, we consider two configurations. In the coupled configuration, the computation is complete and takes into account the tip-sample interaction. In the uncoupled configuration, we use an outgoing wave condition in order to forbid artificially the creation of a multiple scattering system between the tip and the surface. We show that, since the observed sample is dielectric, the tip can be regarded as a passive sensor even if the tip is coated with metal. We also investigate the case of a metallic sample for the two classical TE and TM states of polarization of the incident plane wave.