Decay Of Evanescent Waves Research Articles

Deep-learning-based inverse design of phononic crystals for anticipated wave attenuation

Bandgaps of phononic crystals dominating the propagation of evanescent waves have received significant attention recently, which can be determined and tuned by the topology of a unit cell. Predicting a band structure and designing topological structures with desirable characteristics have become a research hotspot. In this study, a data-driven deep learning framework is applied to arrive at the prediction of the band structure and the inverse design of topology. A convolutional neural network is trained to predict band structures of phononic crystals. After training a generative adversarial network, the generator is concatenated with the convolutional neural network for inverse design. Meanwhile, a complex band structure of phononic crystals is computed by the periodic spectral finite element method to present the spatial decay of evanescent waves. The topology with the greater spatial attenuation is screened from the ground truth topology and the inversely designed topology. Finally, an optimized topological phononic crystal with an anticipated bandgap is obtained, which has the potential for better acoustic insulation and vibration isolation.

PVA/Tween 20 thin-film-based fiber optic humidity sensor with enhanced sensing performance.

A fiber optic humidity sensor based on polyvinyl alcohol (PVA)/Tween 20 film has been fabricated by modulating the intensity of light transmitted in optical fiber. PVA/Tween 20 film was used as the cladding and humidity-sensitive material of optical fiber. The logarithmic of output light intensity exhibited a linear increase with the increase of humidity (22%-82%RH). With the addition of Tween 20 in the formation of film, average sensitivity increased by 13-fold. Fast equilibrium on adsorption and desorption of water molecules were also achieved on the film. The response and recovery times were determined to be 11 s and 9 s, respectively. Moreover, the sensor possesses good repeatability. The sensing mechanism was probably based on the swelling of PVA after adsorbing water molecules, which affected scattering of evanescent waves in the cladding. The output light intensity varied with the decay of evanescent waves.

A waveguide metasurface based quasi-far-field transverse-electric superlens

The imaging capability of conventional lenses is mainly limited by the diffraction of light, and the so-called superlens has been developed allowing the recovery of evanescent waves in the focal plane. However, the remarkable focusing behavior of the superlens is greatly confined in the near-field regime due to the exponential decay of evanescent waves. To tackle this issue, we design a waveguide metasurface-based superlens with an extraordinary quasi-far-field focusing capability beyond the diffraction limit in the present work. Specifically, we analyze the underlying physical mechanism and provide experimental verification of the proposed superlens. The metasurface superlens is formed by an array of gradient nanoslits perforated in a gold slab, and supports transverse-electric (TE) waveguide modes under linearly polarized illumination along the long axis of the slits. Numerical results illustrate that exciting such TE waveguide modes can modulate not only optical phase but also evanescent waves. Consequently, some high-spatial-frequency waves can contribute to the focusing of the superlens, leading to the quasi-far-field super-resolution focusing of light. Under 405 nm illumination and oil immersion, the fabricated superlens shows a focus spot of 98 nm (i.e. <em>λ</em>/4.13) at a focal distance of 1.49 μm (i.e. 3.68<em>λ</em>) using an oil immersion objective, breaking the diffraction limit of <em>λ</em>/2.38 in the quasi-far field regime. The developed metasurface optical superlens with such extraordinary capabilities promises exciting avenues to nanolithography and ultra-small optoelectronic devices.

Far-Field Subwavelength Imaging Using Phase Gradient Metasurfaces

Imaging subwavelength features of an object is in close relationship with extracting information included in evanescent waves scattered from the object. These evanescent waves decay exponentially with distance; therefore, they cannot be captured at far field, resulting in a resolution limited image of the object. Here, we propose a far-field imaging technique based on gradiant metasurfaces, with the ability to provide an image of subwavelength features. In this technique, gradient metasurfaces are used to convert evanescent waves scattered by subwavelength features into propagating waves resulting in a resolution beyond the diffraction limit. The performance of the proposed imaging technique is evaluated using full-wave numerical analysis, and the results validate its capability for imaging beyond the diffraction limit.

One-step deconvolution for multi-angle TIRF microscopy with enhanced resolution.

Total internal reflection fluorescence microscopy (TIRF microscopy) uses a rapid decay of evanescent waves to excite fluorophores within several hundred nanometers (nm) beneath the plasma membrane, which can effectively suppress excitation of fluorescence signals in the deep layers. From image stacks obtained with a plurality of different incident angles, a three-dimensional spatial structure of the observed sample can be reconstructed by a Multi-Angle-TIRF (MA-TIRF) algorithm that provides an axial resolution of ~50 nm. Taking into account the point spread function (PSF) of the TIRF microscopes, we further increase its lateral resolution by introducing a fast deconvolution algorithm into the reconstruction of MA-TIRF data (DMA-TIRF), which is approached in just one step of minimizing the reconstruction function. We also introduce a TV regularization term in the deconvolution algorithm to suppress artifacts induced by the excessive noise. Therefore, based on the hardware of existing MA-TIRF microscopes, the proposed DMA-TIRF algorithm has achieved lateral and axial resolutions of ~200 and ~50 nm, respectively.

Maximizing spatial decay of evanescent waves in phononic crystals by topology optimization

The propagation of evanescent waves inside phononic band gaps is important for the design of phononic crystals with desirable functionalities. This paper extends the bi-directional evolutionary structure optimization (BESO) method to the design of phononic crystals for maximizing spatial decay of evanescent waves. The optimization objective is to enlarge the minimum imaginary part of wave vectors at a specified frequency. The study is systematically conducted for both out-of-plane and in-plane waves at various frequencies. Numerical examples demonstrate that the proposed optimization algorithm is effective for designing phononic crystals with maximum spatial decay of evanescent waves. Various topological patterns of optimized phononic crystals are given. The results also show that the proposed optimization algorithm can successfully overcome difficulties in opening band gap at desirable frequencies, especially for the complete band gap and multiple band gaps.

Evanescent spherical waves of scalar point source

It is well known that evanescent waves decay exponentially from a wave source and influence on a near field of the particular source. In a zero frequency limit these waves decay according to a power law around a spherical source. In the paper exact calculations of an evanescent and homogeneous field in the zero frequency limit has been performed. The calculations base on an angular spectrum representation of a spherical wave using Weyl’s expansion technique.

High resolution surface plasmon resonance imaging for single cells.

BackgroundSurface plasmon resonance imaging (SPRI) is a label-free technique that can image refractive index changes at an interface. We have previously used SPRI to study the dynamics of cell-substratum interactions. However, characterization of spatial resolution in 3 dimensions is necessary to quantitatively interpret SPR images. Spatial resolution is complicated by the asymmetric propagation length of surface plasmons in the x and y dimensions leading to image degradation in one direction. Inferring the distance of intracellular organelles and other subcellular features from the interface by SPRI is complicated by uncertainties regarding the detection of the evanescent wave decay into cells. This study provides an experimental basis for characterizing the resolution of an SPR imaging system in the lateral and distal dimensions and demonstrates a novel approach for resolving sub-micrometer cellular structures by SPRI. The SPRI resolution here is distinct in its ability to visualize subcellular structures that are in proximity to a surface, which is comparable with that of total internal reflection fluorescence (TIRF) microscopy but has the advantage of no fluorescent labels.ResultsAn SPR imaging system was designed that uses a high numerical aperture objective lens to image cells and a digital light projector to pattern the angle of the incident excitation on the sample. Cellular components such as focal adhesions, nucleus, and cellular secretions are visualized. The point spread function of polymeric nanoparticle beads indicates near-diffraction limited spatial resolution. To characterize the z-axis response, we used micrometer scale polymeric beads with a refractive index similar to cells as reference materials to determine the detection limit of the SPR field as a function of distance from the substrate. Multi-wavelength measurements of these microspheres show that it is possible to tailor the effective depth of penetration of the evanescent wave into the cellular environment.ConclusionWe describe how the use of patterned incident light provides SPRI at high spatial resolution, and we characterize a finite limit of detection for penetration depth. We demonstrate the application of a novel technique that allows unprecedented subcellular detail for SPRI, and enables a quantitative interpretation of SPRI for subcellular imaging.Electronic supplementary materialThe online version of this article (doi:10.1186/1471-2121-15-35) contains supplementary material, which is available to authorized users.

An accurate analysis of the radiation characteristics of a plane piston transducer with phase apodization for focusing

A plane piston transducer can be focused by a continuous variation of the excitation signal phase (or time delay) over the transducer surface. Prior analyses of this scheme used the Fresnel approximation, thereby limiting the validity. Using the angular spectrum method, an accurate radiation model of such a transducer has been developed that includes amplitude and phase apodization. The derivation includes the effects of diffraction and evanescent waves without using the Fresnel approximation. Moreover, this model develops insights into radiated field characteristics, including: (a) the spatial frequency bandwidth is constant over axial depth, suggesting that spatial resolution can be improved away from the focus; (b) the phase of the angular spectrum determines the spatial resolution for a given transducer configuration—a constant phase is optimal on any observation plane; (c) focusing can significantly increase the spatial frequency bandwidth; (d) the velocity potential on a plane parallel to the transducer is the Hankel convolution of the transducer surface velocity with the Green’s function; and (e) evanescent waves decay both with increasing spatial frequency and axial depth. The analytical model and associated insights enhance understanding of the radiated field characteristics, which can be of value in the development of signal processing techniques for image enhancement.

Super-resolution imaging via spatiotemporal frequency shifting and coherent detection

Diffraction limit is manifested in the loss of high spatial frequency information that results from decay of evanescent waves. As a result, conventional far-field optics yields no information about an object's subwavelength features. Here we propose a novel approach to recovering evanescent waves in the far field, thereby enabling subwavelength-resolved imaging and spatial spectroscopy. Our approach relies on shifting the frequency and the wave vector of near-field components via scattering on acoustic phonons. This process effectively removes the spatial frequency cut-off for unambiguous far field detection. This technique can be adapted for digital holography, making it possible to perform phase-sensitive subwavelength imaging. We discuss the implementation of such a system in the mid-IR and THz bands, with possible extension to other spectral regions.

Huygens–Fresnel diffraction and evanescent waves

Based on a modified Huygens–Fresnel principle valid in the near field, we analytically verify the exponential wave decay of evanescent waves. This field decay along the propagation direction can be considered as the result of interference between elementary oscillating dipole sources rather than point sources. Specific examples where the modified Huygens–Fresnel principle is applicable are also examined. In particular, the diffraction through a single nano-slit, as well as, the propagation of an engineered input leading to a subwavelength focus are presented.

Optical theorem for electromagnetic field scattering by dielectric structures and energy emission from the evanescent wave

We present an optical theorem for evanescent (near field) electromagnetic wave scattering by a dielectric structure. The derivation is based on the formalism of angular spectrum wave amplitudes and block scattering matrix. The optical theorem shows that an energy flux is emitted in the direction of the evanescent wave decay upon scattering. The energy emission effect from an evanescent wave is illustrated in two examples of evanescent wave scattering, first, by the electrical dipole and, second, one-dimensional grating with line-like rulings. Within the latter example, we show that an emitted energy flux upon evanescent wave scattering can travel through a dielectric structure even if the structure has a forbidden gap in the transmission spectrum of incident propagating waves.

Evanescent waves in the magnetic field of an electric dipole

The magnetic field of radiation emitted by an electric dipole contains travelling and evanescent waves when represented as an angular spectrum. The evanescent waves decay exponentially away from the xy-plane, and will therefore not contribute to the detectable radiation in the far field, in general. It is well known, however, that in a small region around the z-axis the evanescent waves of the electric field do end up in the far zone. We have studied the corresponding magnetic evanescent waves, and we have found that the evanescent waves of the magnetic field do not contribute to the far zone in the neighbourhood of the z-axis. When considering the neighbourhood of the xy-plane, it appears that both the electric and magnetic evanescent waves end up in the far field, and the travelling and evanescent waves contribute equally to the radiation in the far zone. Close to the dipole the radiation field diverges, and we have shown that this is entirely due to the evanescent waves.

Effect of energy emission from evanescent electromagnetic wave at scattering by a dielectric structure

We present an optical theorem for evanescent (near field) electromagnetic wave scattering by a dielectric structure. The derivation is based on the formalism of angular spectrum wave amplitudes. The optical theorem shows that an energy flux at scattering is emitted in the direction of incident evanescent wave decay.

Spatial separation of the traveling and evanescent parts of dipole radiation.

Electric dipole radiation consists of traveling and evanescent plane waves. When radiation is detected in the far field, only the traveling waves will contribute to the intensity distribution, as the evanescent waves decay exponentially. We propose a method to spatially separate the traveling and evanescent waves before detection. It is shown that when the radiation passes through an interface, evanescent waves can be converted into traveling waves and can subsequently be observed in the far field. Let the radiation be observed under angle theta(t) with the normal. Then there exists an angle theta(ac) such that for 0 < or = theta(t) < theta(ac) all intensity originates in traveling waves, whereas for theta(ac) < theta(t) < pi/2 only evanescent waves contribute. It is shown that with this technique and under the appropriate conditions almost all far-field power can be provided by evanescent waves.

Elastic and inelastic evanescent-wave mirrors for cold atoms

We report on experiments on an evanescent-wave mirror for cold 87Rb atoms. Measurements of the bouncing fraction show the importance of the Van der Waals attraction to the surface. We have directly observed radiation pressure parallel to the surface, exerted on the atoms by the evanescent-wave mirror. We analyze the radiation pressure by imaging the motion of the atom cloud after the bounce. The number of photon recoils ranges from 2 to 31. This is independent of laser power, inversely proportional to the detuning and proportional to the evanescent-wave decay length. By operating the mirror on an open transition, we have also observed atoms that bounce inelastically due to a spontaneous Raman transition. The observed distributions consist of a dense peak at the minimum velocity and a long tail of faster atoms, showing that the transition is a stochastic process with a strong preference to occur near the turning point of the bounce.

Tapping-mode tuning-fork near-field scanning optical microscopy of low power semiconductor lasers.

The newly developed inverted tapping-mode tuning-fork near-field scanning optical microscopy (TMTF-NSOM) is used to study the local near-field optical properties of strained AlGaInP/Ga0.4In0.6P low power visible multiquantum-well laser diodes. In contrast to shear-force mode NSOM, TMTF-NSOM provides the function to acquire the evanescent wave intensity ratio /I(2omega)/ / /I(omega)/ image, from which the evanescent wave decay coefficient q can be evaluated for a known tapping amplitude. Moreover, we probe the near-field stimulated emission spectrum, which gives the free-space laser light wavelength lambda(o) and the index of refraction nr of the laser diode resonant cavity. Once q, lambda(o), and n(r) are all measured, we can determine the angle of incidence theta(o) of the dominant totally internally reflected waves incident on the front mirror facet of the resonator. Determination of such an angle is very important in modelling the stability of the laser diode resonator.

Reproducible optical fiber tips for photon scanning tunneling microscopy with very small (&amp;gt;5°) cone angle

Sharp optical fiber tips for photon scanning tunneling microscopes (PSTMs) have been fabricated by employing a new alternative technique for etching multimode optical fibers. The tip diameter is less than 30 mm, while the cone full-angle can be as sharp as 3/spl deg/. To the knowledge of the authors, such tips are the sharpest reported up to now. Measurements, with 19 tips, of the evanescent wave decay distance produced by frustrated reflection of light on a same sample, show good reproducibility. Furthermore, the PSTM images, taken with the new tips, are very sharp and fit with images of the same sample obtained with an atomic force microscope (AFM).