Photon Scanning Tunneling Microscope Research Articles

PSTM/NSOM modeling by 2-D quadridirectional eigenmode expansion

A two-dimensional (2-D) model for photon-scanning tunneling microscopy (PSTM) of integrated optical devices is evaluated. The simulations refer to a setup where the optical field in the vicinity of the sample is probed by detecting the optical power that is transferred via evanescent or radiative coupling to the tapered tip of an optical fiber close to the sample surface. Scanning the tip across the surface leads to a map of the local optical field in the sample. As a step beyond the mere analysis of the sample device, simulations are considered that include the sample as well as the probe tip. An efficient semianalytical simulation technique based on quadridirectional eigenmode expansions is applied. Results for a series of configurations, where slab waveguides with different types of corrugations serve as samples, allow assessment of the relation between the PSTM signal and the local field distribution in the sample. A reasonable qualitative agreement was observed between these computations and a previous experimental PSTM investigation of a waveguide Bragg grating.

Imaging standing surface plasmons by photon tunneling

We present a direct method for optically exciting and imaging delocalized standing surface plasmons in thin metal films. We show theoretically that when imaging the field of the plasmons with a photon scanning tunneling microscope, the presence of the dielectric probe has a negligible effect on the surface modes of the metal film. We demonstrate that plasmon interference can be sustained in arbitrarily large regions of the metal film in comparison to the excitation wavelength. This knowledge can be important when seeking the relative distance between two scattering centers such as the presence of micron or submicron structures.

A separation of the refractive index and topography in photon-scanning tunneling microscopy: Simulations and experiments

In order to separate the purely optical and topographic information from images in constant-gap mode simultaneously, we proposed the atomic force/photon-scanning tunneling microscopy (AF/PSTM). In this paper, we focus on the principle of separation of the refractive index image from the images of photon-scanning tunneling microscopy. We prove the formula of refractive index imaging by using a three-dimensional finite-difference time-domain method. The formula indicates that the refractive index of a sample is approximately proportional to photon tunneling information ( Δ I / I ¯ ) 2 . From the viewpoint of practical use, we simulated the refractive index images for the realistic experiments. We present line scans along two orthogonal directions and the transmitted intensity as a function of the tip position under the constant-gap mode. The experimental results are presented and are in good agreement with the numerical results.

Application of photon scanning tunneling microscope to measure optical near field for atom manipulation

Photon scanning tunneling microscope has been employed to measure the three-dimensional evanescent optical field of an atom funnel. A 3.8 neV repulsive optical potential has been estimated by a 300 μ m long probe with a tip radius of curvature of 21 nm. We have estimated limiting conditions for cold Rb atoms to reflect from the atom funnel. A two-dimensional doughnut-shaped optical near field has also been investigated. An aperture fiber probe is used to profile a focussed TEM 01 beam at the minimum beam waist and measure a dark center of about 10 μ m while it is focussed by a converging lens of focal length 8 cm.

Phase mapping of ultrashort pulses in bimodal photonic structures: A window on local group velocity dispersion

The amplitude and phase evolution of ultrashort pulses in a bimodal waveguide structure has been studied with a time-resolved photon scanning tunneling microscope (PSTM). When waveguide modes overlap in time intriguing phase patterns are observed. Phase singularities, arising from interference between different modes, are normally expected at equidistant intervals determined by the difference in effective index for the two modes. However, in the pulsed experiments the distance between individual singularities is found to change not only within one measurement frame, but even depends strongly on the reference time. To understand this observation it is necessary to take into account that the actual pulses generating the interference signal change shape upon propagation through a dispersive medium. This implies that the spatial distribution of phase singularities contains direct information on local dispersion characteristics. At the same time also the mode profiles, wave vectors, pulse lengths, and group velocities of all excited modes in the waveguide are directly measured. The combination of these parameters with an analytical model for the time-resolved PSTM measurements shows that the unique spatial phase information indeed gives a direct measure for the group velocity dispersion of individual modes. As a result interesting and useful effects, such as pulse compression, pulse spreading, and pulse reshaping become accessible in a local measurement.

Near-field characterization of Bragg mirrors engraved in surface plasmon waveguides

Surface plasmon waveguides (SPW's) are metal ridges featuring widths in the micrometer range and thicknesses of a few tens of nanometers. A focused ion beam has been used to carve microscatterers into gold SPW's and the near-field distributions around these microstructures are observed by means of photon scanning tunneling microscopy (PSTM). On the basis of near-field images, we show that a finite length periodic arrangement of narrow slits can reflect a surface plasmon mode propagating along a SPW. The reflection efficiency of the micrograting is found to depend upon the number of slits, the period of the grating, and the incident wavelength. The optimum reflection efficiency is obtained for a period of the micrograting equal to half the incident wavelength in vacuum. The PSTM images of the plasmon mirrors taken at different wavelengths allow us to measure the experimental dispersion curve of the SPW in the near-infrared. From this dispersion curve, we found that, in analogy with a surface plasmon (SP) excited on extended thin films, the group velocity of a SPW mode is close to the speed of light. For a given frequency in the near-infrared, the effective index of the SP mode supported by a $2.5\text{\penalty1000-\hskip0pt}\ensuremath{\mu}\mathrm{m}$-wide SPW is also found to be significantly larger than the effective index of an extended thin film SP. Finally, we show that the optical properties of microgratings engraved into a SPW can be qualitatively approached by a standard Bragg mirror model.

Surface Plasmon Near-Field Imaging of Very Thin Microstructured Polymer Layers

We report on the near-field imaging of microstructured polymer layers deposited on an homogeneous metal thin film on which a surface plasmon mode is excited. The microstructures in the polymer layers are designed by electron beam lithography, and the near-field imaging is performed with a photon scanning tunneling microscope (PSTM). We show that, despite their very small height, the microstructures can be conveniently imaged with a PSTM thanks to the field enhancement at the surface of the metal thin film supporting the surface plasmon. The influence of the illumination conditions on the contrast of the PSTM images is discussed. In particular, we show that both the field enhancement and the near-field intensity distribution around the microstructures depend dramatically upon the illumination conditions, leading to the conclusion that the PSTM is well suited for spatially resolved near-field surface plasmon sensing purposes.

Numerical simulations of photon scanning tunneling microscopy: role of a probe tip geometry in image formation

Numerical simulations of two-dimensional probe–object system emulating a photon scanning tunnelling microscope are presented. R-matrix propagation algorithm incorporated into the differential method was used to achieve an extended capability to rigorously model a realistic system consisting of both a probe and a sample. Influence of the probe tip parameters on image formation in scanning near-field microscopy has been investigated. Coupling of the near-field to a single-mode probe and formation of a guided fundamental mode in a probe were investigated for various probe widths and lengths. The influence of the probe taper shape and apex size on near-field images was studied for single-mode as well as multimode probes. The obtained results allow comparison of simulated images to unperturbed near-field distribution around the object in the absence of a probe and provide an understanding of the degree of perturbation introduced by probe tip in near-field. It is shown that whole taper of probe contributes to image formation, and thus, a realistic model cannot truncate the probe at distance of few wavelengths from the surface, as is done in the most numerical simulations. The developed model has no computational limitations on optical and geometrical parameters of probe and sample and can be used for realistic simulations of various near-field microscope configurations.

Two-Dimensional Numerical Simulations of Photon Scanning Tunneling Microscopy: Fourier Modal Method and R-Matrix Algorithm

Numerical simulation of a two-dimensional probe–object system for photon scanning tunneling microscope is presented. The R-matrix propagation algorithm incorporated into the Fourier modal method was used to achieve an extended capability for modeling of a realistic system consisting of both a probe and a sample. The type of the mode guided through the dielectric probe and the coupling of the near-field to fundamental guiding mode in the probe are discussed. The influence of the probe parameters on the near-field images is investigated. Three different probe shapes were simulated in the constant height scanning mode. The transmitted flux intensity through the probe was found to be strongly dependent on the tip shape. The analysis shows a good agreement of the obtained results with the available theoretical works and confirming experimental results. The proposed numerical scheme can find applications for near-field probe characterization and provides an understanding of the degree of perturbation introduced by a probe tip in the experiment.

Information content of the near field: two-dimensional samples.

Limits on the effective resolution of many optical near-field experiments are investigated. The results are applicable to variants of total-internal-reflection microscopy (TIRM), photon-scanning-tunneling microscopy (PSTM), and near-field-scanning-optical microscopy (NSOM) in which the sample is weakly scattering and the direction of illumination may be controlled. Analytical expressions for the variance of the estimate of the complex susceptibility of an unknown two-dimensional object as a function of spatial frequency are obtained for Gaussian and Poisson noise models, and a model-independent measure is examined. The results are used to explore the transition from near-zone to far-zone detection. It is demonstrated that the information content of the measurements made at a distance of even one wavelength away from the sample is already not much different from the information content of the far field.

Direct Measurement of Evanescent Wave Interference with aScanning Near-field Optical Microscope

Evanescent wave interference is studied theoretically andexperimentally. The interference patterns were directly measured with ascanning near-field optical microscope. The acquired image of theinterference pattern is clear and has better contrast than that previously acquiredwith a photon-scanning tunnelling microscope or laser-trapped particles. The spatial period of the interference fringes is 180 nm, whichagrees with the theoretical value. The results indicate that the probeof the scanning near-field optical microscope has a resolution beyond100 nm. The relation between the evanescent field intensity and thedistance is also measured. When the separation between the probe and theinterface is up to 180 nm, the intensity can decrease to 1/e of the maximum.

Computational lens for the near field.

A method is presented to reconstruct the structure of a scattering object from data acquired with a photon scanning tunneling microscope. The data may be understood to form a Gabor type near-field hologram and are obtained at a distance from the sample where the field is defocused and normally uninterpretable. Object structure is obtained by the solution of the inverse scattering problem within the accuracy of a perturbative, two-dimensional model of the object.

Probing large area surface plasmon interference in thin metal films using photon scanning tunneling microscopy

The interference of surface plasmons can provide important information regarding the surface features of the hosting thin metal film. We present an investigation of the interference of optically excited surface plasmons in the Kretschmann configuration in the visible spectrum. Large area surface plasmon interference regions are generated at several wavelengths and imaged with the photon scanning tunneling microscope. Furthermore, we discuss the non-retarded dispersion relations for the surface plasmons in the probe–metal system modeled as confocal hyperboloids of revolution in the spheroidal coordinate systems.

Tailoring the transmittance of integrated optical waveguides with short metallic nanoparticle chains

We study the ability of noble metal nanoparticle chains supporting localized surface plasmons to tailor the transmittance of channel waveguides on which they are deposited. The optical interaction between a microwaveguide (MWG) and various arrangements of nanoparticles is first analyzed by means numerical calculations based on the Green's tensor formalism. For specific geometries of the particle chains, the transmission spectra of the composite device (MWG and nanoparticles) exhibits strong modulations in the optical range with the appearance of a neat band gap. The results of an experiment inspired by this theoretical study are also discussed. The photon scanning tunneling microscope images recorded over a MWG decorated with a short chain of gold nanoparticles for two different incident frequencies are found to be in qualitative agreement with the theoretical proposal.

Addressing and imaging microring resonators with optical evanescent light

We show that optical evanescent light can be used to simultaneously address and image microring resonators. The optical addressing function is based on control of the overlap between a three-dimensional evanescent light spot and a linear dielectric ridge connected to the resonator. When detected with a Photon Scanning Tunnelling Microscope (PSTM), the evanescent light tailing off the device provides precise optical images of the light distribution inside and around the resonator. The complete phenomenon has been calculated numerically using the three-dimensional Green Dyadic Method.

Optical near-field distributions of surface plasmon waveguide modes

Thin gold stripes, featuring various widths in the micrometer range, were microfabricated to obtain surface-plasmon guides on a glass substrate. Each metal stripe (MS) was excited by an incident surface-plasmon polariton which was itself launched on an extended thin gold film by the total internal reflection of a focused beam coming through the substrate. The optical near-field distributions of the surface-plasmon (sp) modes sustained by the stripes were then recorded using a photon scanning tunneling microscope (PSTM). For a fixed frequency of the incident light, these field distributions are found to depend on the widths of the stripes. We first provide an experimental study of the various order modes which arise as a function of the decreasing width. Specifically, we show that the lateral confinement of a MS surface-plasmon mode is not related to a reflection of the SP on the edges of the stripe. On the basis of PSTM images recorded over a gold thin-film step discontinuity, we show that the metal stripe plasmon modes are hybrid modes created by the coupling of interface and boundary modes. Using MSs of various thicknesses, we finally demonstrate that, similarly to the symmetric mode of an extended metal thin film bounded by different dielectric media, the field of the MS modes is mostly localized at the interface between the metal and the dielectric medium with the lowest refractive index.

Tracking ultrashort pulses through dispersive media: experiment and theory.

We report on the direct visualization of a femtosecond pulse propagating through a dispersive waveguide at a telecom wavelength. The position of a propagating pulse is pinpointed at a particular point in space and time using a scanning probe based measurement. The actual propagation of the pulse is visualized by changing the reference time. Our phase-sensitive and time-resolved measurement provides local information on all properties of the light pulse as it propagates, in particular its phase and group velocity. Here, we show that the group velocity dispersion can be retrieved from our measurement by developing an analytical model for the measurements performed with a time-resolved photon scanning tunneling microscope. As a result, interesting and useful effects, such as pulse compression, pulse spreading, and pulse reshaping, become accessible in the local measurement.

Amplitude and phase evolution of optical fields inside periodic photonic structures

Optical amplitude distributions of light inside periodic photonic structures are visualized with subwavelength resolution. In addition, using a phase-sensitive photon scanning tunneling microscope, we simultaneously map the phase evolution of light. Two different structures, which consist of a ridge waveguide containing periodic arrays of nanometer scale features, are investigated. We determine the wavelength dependence of the exponential decay rate inside the periodic arrays. Furthermore, various interference patterns are observed, which we interpret as interference between light reflected by the substrate and light inside the waveguide. The phase information obtained reveals scattering phenomena around the periodic array, which gives rise to phase jumps and phase singularities. Locally around the air rods, we observe an unexpected change in effective refractive index, a possible indication for anomalous dispersion resulting from the periodicity of the array.

Imaging simulation of near‐field optical scanning microscope: Comparison between dielectric probe and metal‐coated aperture probe

AbstractIn this paper, a simulation code for a two‐dimensional photon scanning tunneling microscope (PSTM) is created by using the boundary element method based on the guided‐mode extracted integral equation. Imaging simulation is carried out for the cases of a dielectric probe or a metal‐coated probe and observation objects that are multiple dielectrics or metals. In the simulation, the transmission coefficient of the guided mode in a collection mode probe with an incident TE wave (s polarization) and the reflection coefficient of the illumination mode for the incident fundamental TE mode are derived. By a probe scan, the relationship between the output image and the observation object is studied. It is shown that the output image contrast decreases and the effect of interference pattern increases with a metal‐coated aperture probe, whereas the image characteristics on the output image are improved relative to those obtained with a dielectric probe. © 2002 Wiley Periodicals, Inc. Electron Comm Jpn Pt 2, 85(10): 7–16, 2002; Published online in Wiley InterScienc (www.interscience.wiley.com). DOI 10.1002/ecjb.10050

Determination of three-dimensional structure in photon scanning tunnelling microscopy

We present an analytic solution to the problem of determining three-dimensional object structure, described by the spatial dependence of the susceptibility, from data accessible through photon scanning tunnelling microscopy (PSTM) experiments. An analysis is presented of the scattering of evanescent waves with detection of the scattered field by a probe in the near zone of the scatterer. The results provide tomographic imaging capability in PSTM.