Near-field Transmission Research Articles

Scanning probe microscopies applied to the study of the domain wall in a ferroelectric crystal

Scanning near-field optical microscopy is capable of measuring the topography and optical signals at the same time. This fact makes this technique a valuable tool in the study of materials at nanometric scale and, in particular, of ferroelectric materials, as it permits the study of their domains structure without the need of chemical etching and, therefore, not damaging the surface (as will be demonstrated later). We have measured the scanning near-field optical microscopy transmission, as well as the topography, of an RbTiOPO(4) single crystalline slab, which exhibits two different of macroscopic ferroelectric domains. A chemical selective etching has been performed to distinguish between them, obtaining areas with a noticeable roughness (C(-) domain) in comparison with the original flat aspect of the other ones (C(+) domain). The effects of the selective chemical etching have been quantified in topographic images obtained by means of our fibre tip probe, and have been compared to topographic images obtained by Atomic Force Microscopy, with a better resolution. The near-field optical transmission images recorded have been obtained under different excitation wavelengths. These images are modulated by the light scattering due to the grains at the rough surface, which depends on the excitation wavelength used. In addition, they show a significant optical contrast due to the variations of the dielectric constant on the proximity of the ferroelectric domain wall.

Ballistic Electron Microscopy of Individual Molecules

We analyzed the transport of ballistic electrons through organic molecules on uniformly flat surfaces of bismuth grown on silicon. For the fullerene C60 and for a planar organic molecule (3,4,9,10-perylene-tetracarboxylic acid dianhydride), the signals revealed characteristic submolecular patterns that indicated where ballistic transport was enhanced or attenuated. The transport was associated to specific electronic molecular states. At electron energies of a few electron volts, this "scanning near-field electron transmission microscopy" method could be applied to various adsorbates or thin layers.

Reciprocity in scanning near-field optical microscopy: illumination and collection modes of transmission measurements

We experimentally investigated the reciprocity of scanning near-field optical microscopy between illumination and collection modes. Near-field transmission images of single gold spheres and nanorods observed by the two modes are found to be equivalent to each other in the region from visible to near infrared. This result shows that reciprocity holds for the near-field scattering problems. We found that the conventional optical selection rule for far-field excitations does not apply not only under illumination mode but also with collection-mode arrangements. The possible origin of this observation might be the near-field probe.

Plasmonic effects in near-field optical transmission enhancement through a single bowtie-shaped aperture

In this paper, the enhanced optical transmission through a special type of aperture of a bowtie shape is investigated through near-field imaging and finite-difference numerical analysis. Under linear polarizations in two orthogonal directions, the optical near fields of the bowtie aperture and comparable square and rectangular apertures made in gold and chromium thin films are measured and compared. The bowtie aperture is able to provide a nanometer-sized optical spot when the incident light is polarized across the bowtie gap and delivers a considerable amount of light. Localized surface plasmons are clearly observed in the near-field images for both bowtie and rectangular apertures in gold, but invisible in chromium. Finite-difference time-domain calculations reveal that, depending on the polarization of the incident light, the unique optical properties of the bowtie aperture are a result of either the optical waveguide and the coupled surface plasmon polariton modes existing in the bowtie gap or the coupling between the two open arms of the bowtie aperture.

Convenient near-field optical measurement and analysis of polystyrene spheres

The modification of a commercial tapping mode atomic force microscope (AFM) into a transmission mode near-field scanning optical microscope (NSOM) is presented and polystyrene spheres in the diameter of 100 nm are used in this experiment. The detection of near-field signals is based on photodiodes with lock-in technique, and resolutions of topography and near-field signals obtained are about 10 and 20 nm, respectively. Furthermore, it is discovered that the computer-simulated near-field energy distribution profile, obtained by scanning over polystyrene spheres under illumination mode, falls within 5% range as compared with experimental values. The near-field absorption coefficient can be determined by this way. This will be a useful theoretical model to analyze the near-field transmission effect from others.

Optical transmission through a near-field probe with a semiconducting matter in its core

The theory of light transmission through the aperture-type near-field optical probe with a semiconducting matter in its core is presented. It is based on the exact description of the transverse-magnetic (TM) and transverse-electric (TE) eigenmodes inside a conical waveguide with perfectly conducting metallic walls and a dissipative core described by a complex frequency-dependent dielectric function. We concentrate on evaluating the energy density distribution of the electromagnetic field inside a probe with a subwavelength aperture including the region near the tip exit, where it is mainly determined by the contribution of evanescent waves. Significant attention is paid to detailed calculations of the near-field transmission coefficient for semiconducting (GaP, GaN, GaAs, and Si) probes of mesoscopic length and to a comparison of the results obtained with those for the dielectric (SiO2, Si3N4, and diamond) probes. Our calculations indicate a strong enhancement in the transmission efficiency of the semiconducting near-field probes with a high refractive index both in the visible and near-infrared spectral ranges as compared to the conventional fiber or solid quartz tips. It is shown that the optical transmittance for the dominant transverse-electric (TE11) mode is significantly greater than that for the lowest-order transverse-magnetic (TM01) mode.

Imaging Nanometer Scale Optical Heterogeneities in Phospholipid Monolayers Deposited on Metal Island Films

Near-field scanning optical microscopy was applied to study the distribution of fluorescently labeled phospholipid monolayers deposited on the surface of gold island films by the Langmuir-Blodgett technique. Nanometer scale (approximately 50 nm) optical heterogeneities were observed in near-field fluorescence images of the monolayer deposited at 10 mN/m surface pressure. At higher surface pressure (30 mN/m) the heterogeneities became less pronounced. Overlaying of the near-field transmission and fluorescence images from the same area of the sample shows local transmission of gold island film is at a minimum where the fluorescence of the lipid monolayer is at a maximum. It was concluded that coverage of the metal island film by the Langmuir-Blodgett phospholipid monolayer is incomplete, and lipid molecules are preferentially localized in crevices of the film.

Imaging of Plasmons in Gold Nano-Particles by Dynamic Near-Field Spectroscopy

Optical properties of metal nanoparticles, especially gold nanorods in this review, are investigated by static and dynamic near-field microscopy and spectroscopy. We describe instrumentation for the dynamic near-field spectroscopy which enables less than 100-fs temporal resolution at the tip of the apertured near-field probe. We show that wavefunctions of the plasmon modes resonant with the incident light are imaged by near-field transmission imaging as well as by near-field two-photon induced photoluminescence measurements. We also show the results of the ultrafast near-field imaging of gold nanorods, where the dynamics after photoexcitation strongly depends on the position on the nanorod.

Imaging and dispersion relations of surface plasmon modes in silver nanorods by near-field spectroscopy

Surface plasmons of silver nanorods were investigated by using scanning near-field optical microscopy. Near-field transmission images showed spatially oscillatory patterns in nanorods. The oscillatory features of images are attributable to plasmon-mode wavefunctions. From the near-field images and spectra, dispersion relations of surface plasmon modes for various silver nanorods are obtained. It is found that dispersion relation of a nanorod is dependent on its diameter. The spectral features obtained are compared with those for gold nanorods.

Near-field optical imaging of plasmon modes in gold nanorods

We have investigated optical properties of single gold nanorods by using an apertured-type scanning near-field optical microscope. Near-field transmission spectrum of single gold nanorod shows several longitudinal surface plasmon resonances. Transmission images observed at these resonance wavelengths show oscillating pattern along the long axis of the nanorod. The number of oscillation increases with decrement of observing wavelength. These spatial characteristics were well reproduced by calculated local density-of-states maps and were attributed to spatial characteristics of plasmon modes inside the nanorods. Dispersion relation for plasmons in gold nanorods was obtained by plotting the resonance frequencies of the plasmon modes versus the wave vectors obtained from the transmission images.

Aktueller Stand der Entwicklung drahtloser Sensoren für medizinische Anwendungen / Development of wireless sensors for medical applications

Wireless near-field transmission has been a challenge for scientists developing medical sensors for a long time. Here, instruments which measure a patient's ECG, oxygen saturation, blood pressure, peak flow, weight, blood glucose etc. are to be equipped with suitable transmission technology. Application scenarios for these sensors can be found in all medical areas where cable connections are irritating for the doctor, patient and other care personnel. This problem is especially common in sport medicine, sleep medicine, emergency medicine and intensive care. Based on its beneficial properties with regard to power consumption, range, data security and network capability, the worldwide standard radio technology Bluetooth was selected to transmit measurements. Since digital data is sent to a receiving station via Bluetooth, the measurement pre-processing now takes place in the patient sensor itself, instead of being processed by the monitor. In this article, a Bluetooth ECG, Bluetooth pulse oximeter, Bluetooth peak flow meter and Bluetooth event recorder will be introduced. On the one hand, systems can be realized with these devices, which allow patients to be monitored online (ECG, pulse oximeter). These devices can also be integrated in disease management programs (peak flow meter) and can be used to monitor high-risk patients in their home environment (event recorder).

Transmission of visible and near-infrared radiation through a near-field silicon probe

We develop a theory of light transmission through the aperture-type near-field optical probe by taking into account the effects associated with light absorption inside its semiconducting core. Our model is based on the exact description of the transverse-magnetic (TM) eigenmodes inside a conical waveguide with perfectly conducting metallic walls. A dissipative matter of its core is described by a complex frequency-dependent dielectric function. Analytical formulas are derived for the energy density distributions of the electric and magnetic fields inside a probe. Particular attention is paid to the evaluation of the near-field transmission coefficient of a metallized silicon probe in the spectral region from $400\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ to $830\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. We study the dependences of the optical transmittance on the light wavelength, the aperture diameter, the taper angle as well as on the length of the probe. It is shown that the behavior of the electromagnetic fields and the transmission coefficient for the $\mathrm{Si}$ probe differ dramatically from the case of a loss-free dielectric core. In this work we point out that the use of a short $\mathrm{Si}$ probe instead of a glass one allows us to achieve a strong enhancement in the transmission efficiency in the visible and near-infrared spectral regions.

Optimal Transmission Frequency for Ultralow-Power Short-Range Radio Links

Analysis determining the optimal transmission frequency for maximum power transfer across a short-range wireless link is introduced, including a comparison of near-field transmission with far-field transmission. A new near-field power transfer formula has been derived, which allows direct comparison with the well-known far-field Friis transmission formula. Operating charts are presented, which provide the designer with the preferred transmission frequency as a function of distance and antenna dimensions, together with surface plots which show the power transfer for this frequency. The analysis, performed for loop antennas, has been used to evaluate the oscillator transmitter as a low-power topology. It is shown that the requirement of a high-Q factor to realize a low-power oscillator need not be contradictory to achieving optimal far-field radiation characteristics. Based on this fact an approach to sizing loop antennas for low-power oscillator transmitters is suggested.

Near-Field Infrared Imaging and Spectroscopy of a Thin Film Polystyrene/Poly(Ethyl Acrylate) Blend

The application of broadband, near-field infrared microscopy to the characterization of the mesoscale structure of a thin film polymer blend is described. Key features of this instrument, which couples the nanoscale spatial resolution of scanning probe microscopy with the chemical specificity of vibrational spectroscopy, include broad tunability and bandwidth, parallel spectral detection for high image acquisition rates, and infrared-transparent aperture probes. Nearfield spectral transmission images of a thin film of polystyrene/poly(ethyl acrylate) acquired in the C-H stretching region are reported. An assessment of the relative importance of transmission image contrast mechanisms is a significant aim of this work. Analysis of the near-field infrared spectra indicates that the image contrast in the C-H stretching region is largely due to near-field coupling and/or scattering effects. Identification and differentiation of the operative contrast mechanisms on the basis of their relative dependence on wavelength is discussed. Analysis of the contrast attributed to absorption is consistent with the chemical morphology of this sample derived from previous chemical modification/atomic force microscopy studies.

Optical fields inside a conical waveguide with a subwavelength-sized exit hole

Studies of the spatial distribution of time-harmonic optical fields inside a metallized cone, tapered to a subwavelength diameter, are reported. We consider the electric-type (TMmn)wave with the lowest-order indices m = 0 and n = 1. For these waves we obtain exact analytical results for electromagnetic fields inside a conical waveguide with a loss-free dielectric core and perfectly conducting metallic walls. The contributions of different field components to the energy density distributions are explicitly expressed as functions of the radial and angular coordinates. We outline a technique for the evaluation of the optical fields in the near-field zone beyond the exit aperture. This provides tools for the calculation of the amplitude reflection coefficient taking into account the influence of a plane interface between the truncated cone and free space. Particular attention is paid to the calculations of the energy density at the exit aperture and the near-field transmission coefficient of an optical probe with a glass core. We present a detailed analysis of the optical transmission of such a waveguide as a function of the wavelength for large taper angles and various values of the aperture diameter reaching . The results obtained yield the range of the cone parameters and wavelengths in which one can achieve a high spatial resolution capability and sufficiently high transmission efficiency.

Analysis of the Bloch mode spectra of surface polaritonic crystals in the weak and strong coupling regimes: grating-enhanced transmission at oblique incidence and suppression of SPP radiative losses

The Bloch mode spectrum of surface plasmon polaritons (SPPs) on a finite thickness metal film has been analyzed in the regimes of weak and strong coupling between SPP modes on the opposite film interfaces. The SPP mode dispersion and associated field distributions have been studied. The results have been applied to the description of the light transmission through thick and thin periodically structured metal films at oblique incidence. In contrast to normal incidence, all SPP Bloch modes on a grating structure participate in the resonant photon tunnelling leading to the transmission enhancement. However, at the angle of incidence corresponding to the crossing of different symmetry film SPP Bloch modes, the far-field transmission is suppressed despite the enhanced near-field transmission. The combined SPP mode consisting of the two film SPPs having different symmetries that is achieved at the crossing frequency exhibits no radiative losses on a structured surface.

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.

Near-field imaging of ultrathin magnetic films with in-plane magnetization.

A new approach to near-field magneto-optical imaging was developed capable of visualization of in-plane magnetization of ultrathin magnetic structures. The approach relies on the magneto-optical effect specific for thin magnetic layers and employs near-field transmission measurements of longitudinal and/or transverse magneto-optical effect arising from the presence of thin film interfaces. The near-field magneto-optical contrast of in-plane domain structure of ultrathin Co film has been demonstrated in different polarization configurations.

Optical properties and transmission features of J-aggregate in the merocyanine dye LB films by SNOAM

Abstract The changes of the absorption spectra by UV irradiation were measured with the optical spectrum of the J -band and the monomer band. We illustrated the topographical structure and optical structure of the merocyanine dye Langmuir–Blodgett (LB) films obtained by the scanning near-field optical/atomic force microscopy. The topographic image of the mutually mixed LB films shows more rough surface structure than expected and grains with average size of about 50 nm. In the optical transmission image, the elevated area means higher transmission than lower parts. In the continuous measurement of these dyes and the mutually mixed films, the appearance of near-field optical transmission images showed a certain dependence on the kinds of dyes with the shift and increase of transmission and the mutual mixing ratios of dyes. These experimental results suggest that there is a certain kind of interaction between these two dyes.

Spatial variation of photoluminescence and related defects in InGaN/GaN quantum wells

Spatially and spectrally resolved photoluminescence of InGaN/GaN quantum wells grown by metalorganic chemical vapor deposition is studied with near-field scanning optical microscopy (NSOM) and transmission electron microscopy (TEM). High-spatial-resolution NSOM images show bright blue quantum well emission around V defects and yellow emission inside the defects. TEM data suggest that the spatial distribution of blue luminescence is partly due to dislocation gettering by V defects. The yellow emission is attributed to the Ga vacancy-impurity complexes trapped inside V defects.