Bethe-Bouwkamp Model Research Articles

Near-field launching and mapping unidirectional surface plasmon polaritons using an automated dual-tip scanning near-field optical microscope

The spatial distribution of electromagnetic fields emitted from the aperture tip of a scanning near-field optical microscope (SNOM), which is called the emission pattern, depends on the geometry of the apex and the material composition of the tip’s coating. In previous works, experimental measurements of the emission pattern from the aperture tip were performed mostly in the far field. Moreover, the corresponding theoretical models were also developed based on these far-field measurements. Here, we have used the automated dual-tip SNOM to systematically characterize the emission from the aperture tip in the near field. In this regard, we have considered three different pairs of excitation and detection tips with distinct geometries. The emission patterns of the excitation tips were mapped using detection tips. Unidirectional surface plasmon polaritons (SPPs) at the surface of a gold platelet were launched by an excitation tip and measured in the near field by the detection tip. The experimental results were numerically reproduced by means of the Bethe–Bouwkamp model. This work puts into evidence the applicability of the automated dual-tip SNOM as the only available characterization technique to measure the emission from aperture tips in the near field. The reported asymmetric SPP radiation patterns can find applications in photonic integrated circuits or in biological and chemical sensing.

The origin of fine structure in near-field scanning optical lithography of an electroactive polymer

Near-field scanning optical lithography (NSOL) has been used to produce arbitrary structures of the electroactive polymer polyphenylenevinylene at sizes comparable to optical wavelengths, which are of interest for integrated optical devices. The structures are characterized using AFM and SEM and exhibit interesting fine structure. The characteristic size and shape of the lithographic features and their associated fine structure have been examined in the context of the electric field distribution at the near-field scanning optical microscope tip. In particular, the Bethe–Bouwkamp model for electric field distribution at an aperture has been used in combination with a recently developed model for precursor solubility dependence on UV energy dose to predict the characteristics of lithographic features produced by NSOL. The fine structure in the lithographic features is also investigated and explained. Suggestions for the further improvement of the technique are made.

Probing the transverse magneto-optical Kerr effect at the nanoscale

This article presents a theoretical formalism for a magneto-optical scanning near-field microscope (MO-SNOM) in the transverse magneto-optical Kerr effect configuration. Polarized light emitted by a nanosource is scattered by the magnetic object towards the detector. The nanosource is modelled as a circular aperture in an infinitely thin, perfectly conducting screen following the Bethe–Bouwkamp model. The theorem of reciprocity is used for calculating the detected signal in a constant-height scanning mode for a Co thin film pattern. Two magnetic distributions are investigated: the single domain and the Bloch wall. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Surface nanostructuring by femtosecond laser irradiation through near-field scanning optical microscopy

A near-field scanning optical microscopy (NSOM) and a double-frequency femtosecond laser (400 nm, 100 fs) were applied to push the optical resolution further down to sub-50 nm on thin UV photoresist. A 20-nm feature size can be obtained. It is at a resolution of λ/20 ( λ: laser wavelength) and a/2 ( a: NSOM probe aperture diameter), respectively. It is proved that laser power and exposure time can affect feature size of lithography patterns. In this paper, the effect of probe-to-sample distance on dot-pattern features is studied, and different dot-pattern shapes are generated: dumbbell-dot, ellipsoid-dot and circle-dot. The simulated light field spatial distributions across the nano-aperture based on Bethe–Bouwkamp model is found to agree with experimental results very well.

Local Probing of Photocurrent and Photoluminescence in a Phase‐Separated Conjugated‐Polymer Blend by Means of Near‐Field Excitation

AbstractIn this paper scanning near‐field microscopy is used to characterize polymer blends for photovoltaic applications, and fluorescence imaging and photoconductivity are combined to elucidate the spatial distribution and relative efficiency of current generation and photoluminescence in different domains of compositionally heterogeneous films. Focus is placed on a binary system consisting of poly[(9,9‐dioctylfluorene)‐alt‐benzothiadiazole] (F8BT) and poly[(9,9‐dioctylfluorene)‐alt‐(bis(N,N′‐(4‐butylphenyl))‐bis(N,N′‐phenyl‐1,4‐phenylenediamine))] (PFB), spun from xylene solutions, so as to obtain phase separation on micrometer and nanometer length scales. Protruding regions with diameters of about 5 μm in the topography image coincide with regions of high photocurrent (PC) and luminescence; these regions are identified as being F8BT‐rich. A general method to estimate the photoluminescence efficiency in the different domains of phase‐separated blends is proposed. As expected, lack of enhancement of the PC signal at the boundaries between protruding and lower‐lying phases indicate that these microscale boundaries play a small role in the charge generation by exciton splitting. This is consistent with the domains compositional inhomogeneity, and thus with finer phase separation within the domains. We also provide an analysis of the extent to which the metallized probe perturbs the near‐field photocurrent signal by integrating Poisson's equation. Finally, by using a Bethe–Bouwkamp model, the energy absorbed by the polymer film in the different regions is estimated.

Nonvolatile phase change memory nanocell fabrication by femtosecond laser writing assisted with near-field optical microscopy

The phase change memory cells were developed by using a combination system of a femtosecond laser with near-field scanning optical microscopy. The memory cells with feature size varying from 800nm down to 90nm were achieved. The cell functional performances were tested, and the scalability of the programming current as a function of the memory cell features was investigated. The optical near-field distance which is one of the critical factors to achieve high resolution nanostructures was studied experimentally with the consideration of the whole fabrication process for functional devices. The Bethe-Bouwkamp model was employed to study the effects of the optical near-field distance to the nanostructure geometry. The programming current of 0.8mA was observed for the memory nanocell at a feature size of 90nm.

Fabrication of conjugated polymers nanostructures via direct near-field optical lithography

We report our investigations into the fabrication of nanostructures of poly( p-phenylene vinylene) via direct scanning near-field lithography of its soluble precursor. Our technique is based on the spatially selective inhibition of the precursor solubility by exposure to the ultraviolet optical field present at the apex of commercially available, Au-coated near-field probes with aperture diameters between 40 and 80 nm (±5 nm). After development in methanol and thermal conversion under vacuum we obtain features with a minimum dimension of 160 nm. We analyse our results via tapping-mode atomic force microscopy, and find a clear phase contrast between the core and the centre of the lithographed features, corroborating the hypothesis that hard, fully insolubilised regions are surrounded by a gel-like phase, which we estimate of the order of 110–130 nm for the smallest features, by comparing our experiments with simulations carried out using a Bethe–Bouwkamp model. Use of such model also allows us to discuss the influence of probe size, tip–sample distance, and film thickness on the resolution of the lithographic process. We demonstrate the use of the technique for the direct writing of two-dimensional periodic structures with intentional defects and a periodicity relevant to applications in the visible range.

Near-field optical lithography of a conjugated polymer

We report the fabrication of poly(p-phenylene vinylene) nanostructures by direct scanning near-field lithography of its soluble precursor. The technique is based on the spatially selective inhibition of the precursor solubility by exposure to the ultraviolet optical field present at the apex of scanning near-field fiber probes with aperture diameters between 40 and 80 nm (±5 nm). After development in methanol and thermal conversion under vacuum we obtain features with a minimum dimension of 160 nm. We demonstrate the use of the technique for the direct writing of two-dimensional photonic crystals with intentional defects and a periodicity relevant to applications in the visible range. Using a Bethe–Bouwkamp model, we then discuss the influence of probe size, tip-sample distance, and film thickness on the resolution of the lithographic process. We also discuss limitations to the resolution that can arise from physical properties of the lithographic medium.

Modeling illumination-mode near-field optical microscopy of Au nanoparticles

We present a theoretical analysis of near-field scanning optical microscopy (NSOM) images of small Au particles made in the illumination mode. We model the metal-coated fiber tip as a thin disk consisting of a glass core and an aluminum coating. An external field locally illuminates the tip core. We solve for the local fields, including interactions between the tip and the Au particles, by use of the coupled dipole method and calculate the optical signal collected in the far field. We also determine the tip field, in the absence of the particle, for various tip sizes with different metal-coating thicknesses. Calculated tip fields and simulated images are compared with those obtained with the Bethe–Bouwkamp model, a commonly used simple model for the tip field. Calculated line scans of the NSOM images of Au particles depend strongly on the tip aperture size and metal-coating thickness. For blunt tips with a thick metal coating and sharp tips with a much thinner coating, our thin-disk model reproduces the key features of measured NSOM images. Line scans calculated with the Bethe–Bouwkamp model cannot describe the tip dependence of the experimental images. Tip fields obtained from the thin-disk model show significant enhancement beneath the metal coating and a broader field distribution perpendicular to the polarization. Tip fields obtained with the Bethe–Bouwkamp model do not show these effects. Differences in the line scans for these two models are correlated to the differences between the tip fields for the two models. These differences occur because only the disk model accounts for a finite metal coating.

Near-field photoconductivity imaging of a conjugated polymer blend

ABSTRACTWe report combined scanning near-field optical microscopy (SNOM) and near-field photocurrent (NPC) imaging of a binary conjugated polymer blend. We find phase separation on a scale of about 5 μm, with a good correspondence between topographic, fluorescence, and photocurrent images. We excited at 488 nm, a wavelength at which only one of the two polymers absorbs light. Under this illumination regions that are high in the topography image show high luminescence and photocurrent.The photoluminescence (PL) efficiencies in the different regions of the sample were determined by calculating the absorbed energy using the Bethe-Bouwkamp model, and knowledge about the chemical composition of the different phases of the polymer blend. The calculation also allowed us to conclude that the photocurrent generation efficiency (current/absorbed photons) of the different polymer phases is comparable within the limit of confidence of this experiment (±10 %).

Diffraction of circularly polarized light from near‐field optical probes

Diffracted fields from 100-nm aperture near-field scanning optical microscopy (NSOM) probes and uncoated tapered fibres are measured and analysed. Using a solid angle scanner, the two-dimensional intensity distribution and polarization state of the diffracted light are resolved experimentally. Polarization analyses show that circularly polarized input light does not maintain its polarization state for all diffraction angles, and is completely filtered into linearly polarized light at large polar diffraction angles. This drastic decomposition originates from the vector nature of light diffracted by the sub-wavelength aperture. There is a fundamental difficulty in generating circularly polarized light near the aperture of NSOM probes owing to polarization-dependent diffraction in the near-field regime. This is illustrated by the Bethe-Bouwkamp model using circularly polarized input light.

The use of a near-field probe for the study of semiconductor heterostructures

We present a calculation to determine the intensity profile and polarization state within a semiconductor sample of the electric field emission from a scanning near-field optical microscope (SNOM) probe. The propagation of radiation from the apex of a metal-coated tapered optical fibre to the sample, and the subsequent propagation of the electric field through the sample are calculated by using the Bethe-Bouwkamp model, considering the problem to be that of diffraction of linearly polarized monochromatic light by a subwavelength circular hole in a perfectly conducting metallic screen. The sample chosen in our calculation, GaAs, allows us to demonstrate that subwavelength imaging is possible on buried heterostructures.

A theoretical study of near-field detection and excitation of surface plasmons

Recently, optical near-field techniques have been applied to detect or to excite surface plasmons at vacuum–metal interface. In this paper we propose a theoretical study of those experiments. We firstly describe the formalism, the sample being a multilayered structure with flat interfaces. The incident field and the field in each layer are expressed as 3D plane-wave expansions. The spatial Fourier amplitudes in each layer are linearly connected to the incident ones by transmission matrices. We then apply this formalism to describe the excitation of a surface plasmon in a Kretschmann geometry and its detection by a near-field probe. In the second experiment, the tip was used in the emission mode to excite the surface plasmons, which were thus far-field detected. We describe the experiment using the Bethe–Bouwkamp model for tip emission.

Optical characterization of nanosources used in scanning near-field optical microscopy

We propose an optical method to characterize tips used in scanning near-field optical microscopy (SNOM). The tip is a nanosource, and its optical emission is studied by an inverted scanning tunneling optical microscopy (also called photon scanning tunneling microscopy) apparatus. Then we can obtain information about the spatial Fourier spectrum of the source. We give the formulas connecting the detected intensity to the properties of the nanosource and to geometric parameters of the apparatus. Our discussion is illustrated by a simple approximation for the nanosource: the Bethe–Bouwkamp model.