Near-field Illumination Research Articles

基于金属光栅近场衍射增强的结构光照明技术

基于金属光栅近场衍射增强的结构光照明技术

Optimization of a waveguide-mode sensing chip for an ultraviolet near-field illumination biosensor.

A waveguide-mode sensor with a planar sensing chip, consisting of two waveguiding layers and a glass substrate, is a promising candidate for a near-field illumination biosensor. Aiming at using fluorescent labeling induced by ultraviolet light, we optimize the structure of a waveguide-mode sensing chip, based on the mechanism for enhancing ultraviolet near-field light revealed by numerical calculations. Candidates of optimal materials are also presented. The chip optimized as above should be able to enhance the intensity of ultraviolet near-field light 25 times as high as an Al surface plasmon resonance sensing chip.

Mapping the aberrations of a wide-field spectrograph using a photonic comb.

We demonstrate a new approach to calibrating the spectral-spatial response of a wide-field spectrograph using a fibre etalon comb. Conventional wide-field instruments employed on front-line telescopes are mapped with a grid of diffraction-limited holes cut into a focal plane mask. The aberrated grid pattern in the image plane typically reveals n-symmetric (e.g. pincushion) distortion patterns over the field arising from the optical train. This approach is impractical in the presence of a dispersing element because the diffraction-limited spots in the focal plane are imaged as an array of overlapping spectra. Instead, we propose a compact solution that builds on recent developments in fibre-based, Fabry-Perot etalons. We introduce a novel approach to near-field illumination that exploits a 20cm aperture commercial telescope and the propagation of skew rays in a multimode fibre. The mapping of the optical transfer function across the full field is represented accurately (<0.5% rms residual) by an orthonormal set of Chebyshev moments. Thus we are able to reconstruct the full 4K × 4K CCD image of the dispersed output from the optical fibres using this mapping, as we demonstrate. Our method targets one of the largest sources of systematic error in multi-object spectroscopy, i.e. spectral distortion due to instrumental aberrations, and provides a comprehensive solution to their calibration and removal.

An off-axis, reflective system for uniform near-field illumination in optical microscopy

In traditional optical microsopy the uniformity of off-axis illumination is poor. In this paper we propose a novel LED optical system for near-field illumination in microscopy. The system includes a LED light source, an optical collimator and a freeform, micro-reflective lens array. The Monte Carlo method is used to simulate the system. The simulation results show that the illumination uniformity can be better than 85% in the observation zone. The optical system can be put at the side of the main optical axis of the microscope. The illumination uniformity is not varied when the observation distance changes so it has good stablilty.

Detection of Extremely Low Concentrations of Biological Substances Using Near-Field Illumination.

An external force-assisted near-field illumination biosensor (EFA-NI biosensor) detects a target substance that is propelled through an evanescent field by an external force. The target substance is sandwiched between an antibody coupled to a magnetic bead and an antibody coupled to a polystyrene bead. The external force is supplied by a magnetic field. The magnetic bead propels the target substance and the polystyrene bead emits an optical signal. The detection protocol includes only two steps; mixing the sample solution with a detection reagent containing the antibody-coated beads and injecting the sample mixture into a liquid cell. Because the system detects the motion of the beads, the sensor allows detection of trace amounts of target substances without a washing step. The detection capability of the sensor was demonstrated by the detection of norovirus virus-like particles at a concentration of ~40 particles per 100 μl in contaminated water.

Near-lighting Photometric Stereo for unknown scene distance and medium attenuation

Photometric Stereo in murky water is subject to light attenuation and near-field illumination, and the resulting image formation model is complex. Apart from the scene normals and albedo, the incident illumination varies per-pixel and it depends on the scene depth and the attenuation coefficient of the medium. When these are unknown, e.g. in a realistic scenario where a robotic platform explores an underwater scene (unknown shape and distance) within the dynamic subsea environment (unknown scattering level), Photometric Stereo becomes ambiguous. Previous approaches have tackled the problem by assuming distant-lighting and resorting to external hardware for estimating the unknown model variables. In our work, we show that the Photometric Stereo problem can be determined as soon as some additional constraints regarding the scene albedo and the presence of pixels with local intensity maxima within the image are incorporated into the optimization framework. Our proposed solution leads to effective Photometric Stereo and yields detailed 3D reconstruction of objects in murky water when the scene distance and the medium attenuation are unknown. We evaluate our work using both numerical simulations and real experiments in the controlled environment of a water tank and real port water using a remotely operated vehicle.

Interactive Near-Field Illumination for Photorealistic Augmented Reality with Varying Materials on Mobile Devices.

At present, photorealistic augmentation is not yet possible since the computational power of mobile devices is insufficient. Even streaming solutions from stationary PCs cause a latency that affects user interactions considerably. Therefore, we introduce a differential rendering method that allows for a consistent illumination of the inserted virtual objects on mobile devices, avoiding delays. The computation effort is shared between a stationary PC and the mobile devices to make use of the capacities available on both sides. The method is designed such that only a minimum amount of data has to be transferred asynchronously between the participants. This allows for an interactive illumination of virtual objects with a consistent appearance under both temporally and spatially varying real illumination conditions. To describe the complex near-field illumination in an indoor scenario, HDR video cameras are used to capture the illumination from multiple directions. In this way, sources of illumination can be considered that are not directly visible to the mobile device because of occlusions and the limited field of view. While our method focuses on Lambertian materials, we also provide some initial approaches to approximate non-diffuse virtual objects and thereby allow for a wider field of application at nearly the same cost.

Normal-Incidence Photoemission Electron Microscopy (NI-PEEM) for Imaging Surface Plasmon Polaritons

We introduce a novel time-resolved photoemission-based near-field illumination method, referred to as femtosecond normal-incidence photoemission microscopy (NI-PEEM). The change from the commonly used grazing-incidence to normal-incidence illumination geometry has a major impact on the achievable contrast and, hence, on the imaging potential of transient local near fields. By imaging surface plasmon polaritons in normal light incidence geometry, the observed fringe spacing directly resembles the wavelength of the plasmon wave. Our novel approach provides a direct descriptive visualization of SPP wave packets propagating across a metal surface.

Tip-based nanoscale selective growth of discrete silicon nanowires by near-field laser illumination

Growth of discrete silicon nanowires is reported with nanoscale location selectivity by employing near-field laser illumination. An uncoated dielectric atomic force microscope (AFM) tip provides a nanometer-scale light source that is sufficiently localized to induce nucleation and subsequent growth of a single nanowire under its optical near-field. Far-field laser-induced heating is additionally supplied to the substrate to both relieve the required near-field light budget and also assist stable epitaxial growth. Specific catalysts are selected for the nanowire growth by non-contact mode AFM imaging. Through real-time monitoring of the deflection of the AFM cantilever during the growth process, the gap between the tip and the sample and hence truly near-field illumination are maintained throughout the growth process. The study shows that tip-based near-field laser illumination could be an effective tool for the direct integration of semiconductor nanowires.

双自由曲面LED均匀照明准直透镜设计

A LED collimator with double freeform surfaces was designed for uniform illumination in both near field and far field. The collimator includes refractive part and reflective part, and each part has double freeform surfaces. Based on Malus Law, the constant optical path length equations that realize the rays collimating were derived. Then associating with tangent-plane iterative method, the date both up freeform surface and down freeform surface simultaneously could be calculated. The simulations show that, 84.55% of the energy is collected in 2 for 1 mm1 mm size white LED after considering the Fresnel loss, reflective surfaces loss and material absorbing. The uniformity in near field is 94.59%, and the uniformity in far field is 89.01%. The uniformity in near field and the optical efficiency in 4 are above 90% and 83.5% respectively, while the LED chip sizes are under 2 mm2 mm. It is illustrated that the LED collimator can realize uniform illumination in both near field and far field with high light efficiency and the tolerance meet the requirements of alignment.

Scanning near field optical microscopy of gold nano-disc arrays fabricated by electron beam lithography and their application as surface enhanced Raman scattering substrates

Gold nano-disc arrays were fabricated on glass substrates using electron beam lithography. Scanning near field optical microscopy imaging of the patterns in the near-field illumination and far-field collection (transmission and reflection) and far-field illumination with near-field collection of signal modes is reported. An analyzer was introduced before the detector to study the effect of polarization. High resolution and good contrast are obtained in reflection detection modes compared to that of transmission mode. Polarization of the beam in the transmission mode further improves the resolution and contrast of the images. Application of these arrays as surface enhanced Raman scattering substrates is demonstrated.

Decomposing intensity gradients into information about shape and material

Recent work has shown that the perception of 3D shapes, material properties and illumination are inter-dependent, although for practical reasons, each set of experiments has probed these three causal factors independently. Most of these studies share a common observation though: that variations in image intensity (both their magnitude and direction) play a central role in estimating the physical properties of objects and illumination. Our aim is to separate retinal image intensity gradients into contributions of different shape and material properties, through a theoretical analysis of image formation. We find that gradients can be understood as the sum of three terms: variations of surface depth conveyed through surface-varying reflectance and near-field illumination effects (shadows and inter-reflections); variations of surface orientation conveyed through reflections and far-field lighting effects; and variations of surface micro-structures conveyed through anisotropic reflections. We believe our image gradient decomposition constitutes a solid and novel basis for perceptual inquiry. We first illustrate each of these terms with synthetic 3D scenes rendered with global illumination. We then show that it is possible to mimic the visual appearance of shading and reflections directly in the image, by distorting patterns in 2D. Finally, we discuss the consistency of our mathematical relations with observations drawn by recent perceptual experiments, including the perception of shape from specular reflections and texture. In particular, we show that the analysis can correctly predict certain specific illusions of both shape and material.

Far-field super-resolution imaging using near-field illumination by micro-fiber

We propose an approach that uses a conventional optical microscope to achieve optical super-resolution. This approach will provide a direct, non-invasive, and far-field access in the observation of both metallic and non-metallic objects with a spatial resolution of tens-of-nanometres, achieved in a single snap shot. This method, combines near-field illumination by the micro-fiber and a passive spatial frequency shift by the sample itself, offering a promising approach to surface tomography imaging without localized field enhancement by surface plasmon polaritons. Our results present the potential of breaking the diffractive barrier using a conventional wide-field optical microscope within the visible spectrum.

In-situ monitoring of optical near-field material processing by electron microscopes

Lasers are efficient tools in a variety of micro/nanoscale material processing applications. Even though optical imaging techniques offer convenient in-situ monitoring, their spatial resolution is frequently not sufficient for inspecting the detailed phenomena occurring in micro/nano structures, hence requiring additional characterization tools. Besides the inconvenience, critical processing parameters cannot be readily determined ex situ. In this study, an example of an in-situ monitoring technique for micro/nanoscale laser processing is demonstrated by combining the optical near-field apparatus with a scanning electron microscopy (SEM). In-situ process monitoring under true optical near-field configuration is realized through orthogonal probe manipulation and combined probe-sample translation and tilting apparatus. Catalyst behavior under a chemical vapor deposition (CVD) gas environment coupled with near-field illumination is monitored in an environmental SEM.

Seismic Interferometry Using Non-Far-Field Sources and Removing the Spurious Arrival

Abstract Seismic interferometry using far-field correlation is a technique to obtain the Green’s function between two receivers using passive wave-field recordings, and often it is under the theoretical assumption that sources are in the far field when in fact they may not be. Using a heterogeneous medium enclosed by a closed boundary on which the sources are fired, we offer two views on the meaning of the far-field correlation. In the intrinsic view, the validity of the far-field correlation is based on the far-field approximation for wave scattering, and it is investigated by comparing various physical dimensions in the heterogeneous interior, regardless of the exterior. However, the extrinsic view centers on the medium properties in the exterior only without considering the interior. Previous studies showed that, with the correct scattering model and complete source coverage, no spurious arrivals should be generated if the illuminating sources are in the far field. We investigate the case of near-field cross correlation. This problem is considered in the context of two-dimensional space with a single embedded scatterer that is represented by a cylindrical inclusion with small radius. An analytical solution for the scattered wave is computed. However, if the sources are not in the far field, the cross-correlation kernel must be used to eliminate the spurious arrival, in addition to the correct scattering model and full source aperture. The kernel accounts for the near-field illumination of the region.

Ultra-high resolution resonant C-shaped aperture nano-tip

We report a new optical near-field transducer comprised of a metallic nano-antenna extending from the ridge of a C-shaped metallic nano-aperture. Finite-difference time domain simulations predict that the C-aperture nano-tip (CAN-Tip) provides high intensity (650x), high optical resolution (~λ/60), and background-free near-field illumination at a wavelength of 980 nm. The CAN-Tip has an aperture resonance and tip antenna resonance which may be tuned independently, so the structure can be made resonant at ultraviolet wavelengths without being unduly small. This near-field optical resolution of 16.1 nm has been experimentally confirmed by employing the CAN-Tip as an NSOM probe.

LED beam shaping lens based on the near-field illumination

This paper presents a novel method in near-field beam shaping based on the precise optical modeling of a gallium nitride light-emitting diode (GaN LED). A Monte Carlo ray tracing simulation has been utilized to calculate the spatial photon distribution near the LED's top. By analyzing the ray data in near-field, the miniaturized lens profile is created and machined with aspherical surfaces and total internal reflection (TIR) Fresnel facets. The prototype lens reduce the viewing angle of the LED from 150 degrees to 17.5 degrees at full width half maximum (FWHM) while increasing the peak luminous intensity 10 times. The array of proposed lens with CSP LEDs exhibits feasibility of ultra thin uniform illumination in near-field.

Far-Field Nonmodal Laser Emission for Low-Speckle Laser Projection

In this letter, we discuss how the nonmodal emission regime's far field of a broad-area vertical-cavity surface-emitting laser (BA-VCSEL) can be used for low-speckle laser projection. A microlens beam homogenizer is used to exploit the low spatial coherence of the VCSEL's far field in the nonmodal emission regime. Using far-field instead of near-field illumination of the homogenizer has some important advantages for a practical projection system: the field emitted by the VCSEL can be directly projected onto the homogenizer without the need for additional lenses or accurate alignment. Speckle contrast values as low as 2.5% are measured and in good agreement with modeled contrast values.

Confined detection volume of fluorescence correlation spectroscopy by bare fiber probes

A fiber-tip-based near-field fluorescence correlation spectroscopy (FCS) has been developed for confining the detection volume to sub-diffraction-limited dimensions. This near-field FCS is based on near-field illumination by coupling a scanning near-field optical microscope (SNOM) to a conventional confocal FCS. Single-molecule FCS analysis at 100 nM Rhodamine 6G has been achieved by using bare chemically etched, tapered fiber tips. The detection volume under control of the SNOM system has been reduced over one order of magnitude compared to that of the conventional confocal FCS. Related factors influencing the near-field FCS performance are investigated and discussed in detail. In this proof-of-principle study, the preliminary experimental results suggest that the fiber-tip-based near-field FCS might be a good alternative to realize localized analysis at the single-molecule level.

Strong tangential force within a small trapping volume under near-field Laguerre-Gaussian beam illumination

Near-field rotation of a trapped particle under focused evanescent Laguerre-Gaussian beam illumination is theoretically investigated by mapping the two-dimensional transverse trapping efficiency exerting on the particle. It is revealed that the severe focal field deformation associated with a focused evanescent Laguerre-Gaussian beam causes a significant impact on the transverse trapping performance of the microparticle. Compared with the far-field trapping force, strong tangential force components have been observed in the transverse efficiency mapping, which potentially lead to rotational motions to the particle within a small trapping volume in the optical near-field.