Near-field Investigation Research Articles

NOISE SOURCE LOCATION OF UNDEREXPANDED NONCIRCULAR SLOT JETS

It is generally understood that the non-circular jets exhibit better mixing enhancement and noise suppression properties compared to circular jet. The present paper is on the experimental investigation of near acoustic field and noise source location of non-circular underexpanded jets and comparison with circular jet. Square, triangular, rectangular slot jets are considered in this work. The near field overall sound pressure level contours obtained along transverse plane and spanwise plane of non-circular jets and compared with circular jet for nozzle pressure ratio R at 4 and 5 which is analyzed to get noise source location for various jets. The effect of geometry in acoustic field in near field is highlighted in this paper.

Near-field investigation of spatial variations of (202¯1¯) InGaN quantum well emission spectra

Scanning near-field optical spectroscopy was applied to semipolar (202¯1¯) InGaN/GaN quantum wells (QWs) to evaluate spatial homogeneity of QW band gap and its dependence on the growth conditions. In the most uniform QW, photoluminescence (PL) spectra were found to be narrow with small peak wavelength and spectral width variations. A QW grown at reduced temperature showed sub-micrometer size PL features aligned along the a axis and caused by nonuniform In incorporation at surface undulations. At extended defects, complex and strongly varying near-field spectra were observed and tentatively assigned to QW segments of different orientations around these defects.

Near-Field Microwave Investigation of Electrical Properties of Graphene-ITO Electrodes for LED Applications

We propose a method to evaluate the electrical properties of nanoscale layered materials. This study is important for the potential application of these structures in light emitting diode electrodes. For this purpose we measure the reflection coefficient of a microwave signal recorded by a near-field Scanning Microwave Microscope. This method allows the non-contact measurement of the sheet resistance of the material under analysis. It provides detailed maps of the electrical properties of a micrometric area under test to assay its uniformity. In particular, we have applied this technique to a multilayer material composed by an Indium–Tin–Oxide film and few layer graphene.

Near-Field Optical Imaging of a Porous Au Film: Influences of Topographic Artifacts and Surface Plasmons

In this work, near-field scanning optical microscopy is employed to study a porous Au film and the direct observation of topographic artifacts and surface plasmon influences is revealed. Under tip illumination, topographic artifacts are found to be present in a reflection mode optical image but not in a transmission mode image. A simple algorithm is used for filtering the topographic artifacts and extracting a correct near-field optical image approximately. On the other hand, surface plasmon influences are present in both modes. By using three exciting wavelengths of 488, 647.1, and 520.8 nm, it is confirmed that a suitable wavelength should be chosen for avoiding the surface plasmon interference in a near-field optical investigation of morphological or material dielectric contrast. Finally, plasmonic or nonplasmonic regions on the porous Au film can be identified from the observed optical intensity variation in the optical images obtained at incident polarizations of 0°, 90°, and 45°.

Near-field investigation of turbulence produced by multi-scale grids

We investigate grid turbulence behind two multi-scale grids and one conventional one, with particular emphasis on the properties of the near-field turbulence. All three grids are designed to produce the same integral scale, ℓ0, at a downstream distance of two meters (x = 2 m). We find that the turbulence is highly inhomogeneous for x < 50ℓ0, but approaches a homogeneous, fully developed state for x > 50ℓ0. The structure of the near-field turbulence for the present grids is dependent on the type of grid used, exhibiting a number of curious properties, which is consistent with earlier studies [D. Hurst and J. C. Vassilicos, “Scalings and decay of fractal-generated turbulence,” Phys. Fluids 19, 035103 (2007)]. However, the far-field is remarkably similar for all three grids. For example, the far-field energy decay rate is virtually identical for all three grids, as was shown by Krogstad and Davidson [“Freely-decaying, homogeneous turbulence generated by multiscale grids,” J. Fluid Mech. 680, 417 (2011)]. This ability of the turbulence to largely forget its initial conditions is consistent with numerical studies. There has been considerable recent interest in the possibility of an exponential decay of energy in homogeneous turbulence. We confirm a region of apparent exponential decay behind all three of our grids over a limited range of x. However, the range of x is narrow, and other functional forms fit the data equally well. Indeed, when the streamwise extent of the investigated region is increased, a classical power law fits the data even better. In any event, the region of apparent exponential decay lies in the highly inhomogeneous near-field, and so is not representative of homogeneous turbulence.

Near-field investigation of induced transparency in similarly oriented double split-ring resonators

We present near-field measurements of an induced transparency behavior using a double split-ring resonator geometry. Mapping the out-of-plane electric field component directly reveals that the induced transparency is linked to an asymmetric mode profile with the subunits oscillating in antiphase. The measurements are compared to complementary numerical simulations, and excellent agreement is found.

Near-field investigation of THz surface-wave emission from optically excited graphite flakes

THz emission from an unbiased graphite flake after optical sub-ps pulse inter-band excitation is measured using a novel micro-machined photoconductive probe-tip. Oscillatory behavior of the measured THz near-field emission is shown to originate from electromagnetic eigenmode resonances of the laterally limited graphite flake. The excitation efficiency of the lowest order eigenmode resonances strongly dependents on optical pump location. From this data a radial symmetric charge carrier translocation at the pump location is revealed as the dominating THz surface-wave emission effect. Measurements show good agreement with numerical field simulations of the eigenmode center frequencies and the spectral response expected for THz emission through basal plane oriented radial current surge excitation.

Bloch surface waves in ultrathin waveguides: near-field investigation of mode polarization and propagation

In this work, we use a multi-heterodyne scanning near-field optical microscope to investigate the polarization and propagation of Bloch surface waves in an ultrathin (∼λ∕10) ridge waveguide. First, we show that the structure sustains three surface modes, and demonstrate selective excitation of each. Then, by numerically processing the experimental data, we retrieve the transverse and longitudinal components of each of the modes, in good agreement with the calculated fields. Finally, we provide an experimental estimation of the effective indices and the dispersion relations of the modes.

Near-field investigations of nanoshell cylinder dimers

Metallic nanoparticles are known to exhibit strong particle size dependent localized surface plasmon resonances due to their specific optical response described via the complex dielectric function. Using the two-dimensional finite element method, the near-field behavior of core-shell nanocylinder dimers with either a dielectric or a gold core and a silver shell was investigated. With a detailed analysis the positions of maximum field enhancement usable for highly sensitive spectroscopy were unveiled and the surface charge distributions of the different kinds of resonances were visualized. It is shown that the usual far-field spectra do not give reliable estimates of local electric field peaks. Furthermore one observes a distinct mode at the natural plasma frequency of the silver shell which is independent of the core material. This mode is identified as a volume plasmon mode.

Direct near-field optical investigation of phase-change medium in blue-ray recordable and erasable disk

The authors report a direct examination of recorded marks in blue-ray recordable and erasable disks with scattering-type scanning near-field optical microscopy. The optical contrasts of the crystalline and amorphous regions of the phase-change layer match with the prediction based on their optical constants. The determined spatial optical variation in the recorded marks reflects the intensity profile of the recording laser beam. The identified nanometer-sized optical features are shown to correspond to 10 nm-sized crystalline domains within the amorphous recorded marks. The revealed near-field signatures show a potential influence on the carrier-to-noise ratio of this optical storage medium.

Nanowire Plasmon Excitation by Adiabatic Mode Transformation

We show with both experiment and calculation that highly confined surface plasmon polaritons can be efficiently excited on metallic nanowires through the process of mode transformation. One specific mode in a metallic waveguide is identified that adiabatically transforms to the confined nanowire mode as the waveguide width is reduced. Phase- and polarization-sensitive near-field investigation reveals the characteristic antisymmetric polarization nature of the mode and explains the coupling mechanism.

Polarization-sensitive near-field investigation of photonic crystal microcavities

We report on polarization sensitive imaging of two-dimensional photonic crystal microcavity modes. By using a near-field scanning optical microscope with a polarization sensitive setup, it is possible to selectively map, with a resolution beyond the diffraction limit, each electric field component in the plane of the sample. In addition, the simultaneous analysis of photoluminescence maps in different polarization channels allowed us to obtain important insight on near-field microscopy detection mechanism. Finite difference time domain simulations confirm the experimental results.

Cantilever Tip Near-Field Surface-Enhanced Raman Imaging of Tris(bipyridine)ruthenium(II) on Silver Nanoparticles-Coated Substrates

The near-field surface-enhanced Raman scattering (SERS) and surface-enhanced fluorescence (SEF) images of tris(bipyridine)ruthenium(II) adsorbed on a silver nanoparticles-coated substrate were obtained with a scanning near-field optical microscope (SNOM, or near-field scanning optical microscopy, NSOM) using a cantilever tip. In comparison with the most widely used fiber tip for SNOM, the cantilever tip has higher optical throughput and better thermal stability, making it more suitable for detecting the extremely low Raman signal in the near-field spectroscopic investigations. Our preliminary results show that the near-field SERS with the higher spatial resolution can provide richer fingerprint information than the far-field SERS. A comparison of the two types of images shows that there are more SERS than SEF hot spots, and the two types of hot spots do not overlap. More surprisingly, the near-field SERS spectra differ from the far-field SERS spectra obtained on the same sample in the band frequency and relative intensities of some major Raman bands, and some IR-active bands were observed with the near-field mode. These results are explained mainly by the electric field gradient effect and heterogeneous polarization character that operate only in the near-field SERS.

Near-field investigation of exciton dynamics under semiconductor surface

The near-field images calculation method for the semiconductor surface with the excitons generated by strong focused laser pulse was proposed. Calculation was performed using Green function method in the frame of concept of local field. The main characteristic of the proposed approach is maximal usage of the analytical calculations. The near-field images for the Si surface were studied. Developed approach is universal and could be able to find with experimental data on time-resolved near-field microscopy some parameters of exciton such as diffusion constant, relaxation time, and surface state density.

A Comparative Investigation of Round and Fan-Shaped Cooling Hole Near Flow Fields

This study presents velocity and turbulence data measured experimentally in the near field of a round and a laterally expanded fan-shaped cooling hole. Both holes are fed by a plenum inlet, and interact with a turbulent mainstream boundary layer. Flow is Reynolds number matched to engine conditions to preserve flow structure, and two coolant to mainstream blowing momentum ratios are investigated experimentally. Results clearly identify regions of high shear for the round hole as the jet penetrates into the mainstream. In contrast, the distinct lack of high shear regions for the fan-shaped hole points to reasons for improvements in cooling performance noted by previous studies. Two different computational fluid dynamics codes are used to predict the flow within and downstream of the fan-shaped hole, with validation from the experimental measurements. One code is the commercially available ANSYS CFX 10.0, and the other is the density-based solver with low Mach number preconditioning, HYDRA, developed in-house by Rolls-Royce plc for high speed turbomachinery flows. Good agreement between numerical and experimental data for the center-line traverses was obtained for a steady state solution, and a region of reversed flow within the expansion region of the fan-shaped hole was identified.

Gold nanoparticle tips for optical field confinement in infrared scattering near-field optical microscopy

We report on the implementation of metal nanoparticles as probes for scattering and apertureless near-field optical investigations in the mid-infrared (mid-IR) spectral regime. At these wavelengths, an efficient electric-field confinement is necessary and achieved here through a gold metal nanoparticle of 80 nm in diameter (Au80-MNP) acting as the optical antenna. The Au80-MNP is attached to a standard AFM cantilever used as the spatial manipulator. When approached to a sample surface while being illuminated with an infrared beam, the Au80-MNP produces a considerably improved spatial confinement of the electric field compared to an ordinary scattering AFM tip. We demonstrate here the confinement normal to the sample surface by making use of a sample-induced phonon polariton resonance in a ferroelectric lithium niobate sample. Our experimental findings are in very good agreement with the quasistatic dipole model and show improved optical resolution via well-selected antenna particles.

Near-Field Investigation of Chevron Nozzle Mechanisms

A detailed investigation into the effect of chevron nozzles on the near-field acoustics of a separate flow exhaust system was conducted at the University of Cincinnati Anechoic Test Facility. Chevrons with varying numbers of lobes and levels of penetration were selected to provide insight into the effects of these geometric parameters on the acoustic near field. Tests were conducted at two different nozzle operating conditions and the chevrons were shown to produce substantial modifications to the near field over a wide range of frequencies. The chevrons were most effective at lower frequencies where the peak noise region was reduced by 5-7 dB and dramatically reduced in size. At higher frequencies, the chevrons provided strong noise suppression downstream of approximately seven equivalent nozzle diameters with increases closer to the nozzle lip. The nozzle penetration was shown to have the most significant impact on the acoustic near field with more subtle differences being seen with respect to the number of chevron lobes.

Berry phase of two atoms in the same quantized light field

Investigation of near field in QED requires the refuse from an averaging of the Lorentz condition that smooths out some field peculiarities. Instead of it the Schwinger decomposition of 4-potential with the Bogoliubov method of interaction switching in time and in space regions is considered. At such approach near field is describable by the part of covariant Green function of QED, the fast-damping Schwinger function is formed by longitudinal and scalar components of Aμ none restricted by light cone. This description reveals possibility of superluminal phenomena within the near field zone as a “nonlocality in the small”. Some specification of the Bogoliubov method allows, as examples, descriptions of near fields of point-like charge and at FTIR phenomena. Precisely such possibilities of nonlocal interactions are revealed in the common QED expressions for the Van-der-Waals and Casimir interactions and in the Förster law.

Impact of optical in-plane anisotropy on near-field phonon polariton spectroscopy

The authors report a spectroscopic near-field investigation using a tunable free-electron laser in combination with a scattering near-field optical microscope. They excite optically uniaxial LiNbO3 close to a phonon resonance in the infrared regime, thereby exciting a phonon polariton resonance in the coupled tip-sample system. They find that the resonance shows a clear dependence on the orientation of the optical axis of the birefringent crystal within the surface plane. This provides evidence that in addition to the dominant contribution of the dipole moment parallel to the tip axis, also the component along the surface is sensed in such a scattering experiment.

Arrays of Well-Defined Size-Tunable Metallic Nano-Cones for Plasmonic Applications

ABSTRACTCones composed of noble metals with dimensions of the order of 100 nm, especially gold cones, are extremely well suited for optical near-field investigations using visible light. Their plasmon resonance frequency lies within the range of visible light frequencies, and upon illumination, strong electric field enhancement can be observed in the direct vicinity of the cone tip. For this purpose, a method to fabricate well-controlled nanocones with sharp tips is required. We present a top-down process in which arrays of cones are dry-etched from a metal stack on silicon. During the ion milling step, patterned hydrogen silsesquioxane resist is used as an etch mask. The resulting cones have tunable base diameters around 150 nm and tip radii down to less than 10 nm. Their optical characteristics are investigated by means of apertureless optical near-field microscopy, in which field enhancement at the cone tip is observed.