Near-field Illumination Research Articles

Plasmon dephasing time and optical field enhancement in a plasmonic nanobowl substrate studied by scanning near-field optical microscopy.

Plasmonic substrates have been extensively investigated due to their potential applications in fluorescence microscopy, chemical sensing, and photochemical reactions. The optical properties of the substrate depend on the spatial and temporal features of the plasmon excited. Hence, the ability to directly visualize plasmon dynamics is crucial. In this study, we investigated the spatial and temporal properties of plasmon excitation in a plasmonic nanobowl substrate consisting of a periodic hexagonal array of nanoscale bowl-like structures developed with self-assembly. Near-field transmission imaging revealed that multiple plasmon resonance bands are observed from visible to near-infrared spectral region, and the optical contrast of the image is dependent on the observed band. Near-field two-photon photoluminescence microscopy revealed that the probability of excitation inside each nanoscale bowl-like structure is greater than that in the surrounding area. Near-field time-resolved imaging revealed that the nanobowl substrate exhibited a substantially long plasmon dephasing time, exceeding 12fs. Based on the spectral features of the near-field and far-field spectra, we found that optically dark plasmon mode is excited by the near-field illumination and only partly contributes to the long dephasing time observed. This fact indicates that the dephasing time is extended by some other mechanism in the periodic substrate. We revealed from this study that the enhanced optical fields induced in the nanobowl structure originate from the photosynergetic effect of the cavity mode and plasmon mode excited.

Polarization Sensitivity in Scattering-Type Scanning Near-Field Optical Microscopy—Towards Nanoellipsometry

Electric field enhancement mediated through sharp tips in scattering-type scanning near-field optical microscopy (s-SNOM) enables optical material analysis down to the 10-nm length scale and even below. Nevertheless, the out-of-plane electric field component is primarily considered here due to the lightning rod effect of the elongated s-SNOM tip being orders of magnitude stronger than any in-plane field component. Nonetheless, the fundamental understanding of resonantly excited near-field coupled systems clearly allows us to take profit from all vectorial components, especially from the in-plane ones. In this paper, we theoretically and experimentally explore how the linear polarization control of both near-field illumination and detection can constructively be implemented to (non-)resonantly couple to selected sample permittivity tensor components, e.g., explicitly to the in-plane directions as well. When applying the point-dipole model, we show that resonantly excited samples respond with a strong near-field signal to all linear polarization angles. We then experimentally investigate the polarization-dependent responses for both non-resonant (Au) and phonon-resonant (3C-SiC) sample excitations at a 10.6 µm and 10.7 µm incident wavelength using a tabletop CO2 laser. Varying the illumination polarization angle thus allows one to quantitatively compare the scattered near-field signatures for the two wavelengths. Finally, we compare our experimental data to simulation results and thus gain a fundamental understanding of the polarization’s influence on the near-field interaction. As a result, the near-field components parallel and perpendicular to the sample surface can be easily disentangled and quantified through their polarization signatures, connecting them directly to the sample’s local permittivity.

Near-Field LED Illumination Strategies for Tonsillectomy and Adenoid Surgery.

Objective: Perform a pilot feasibility study to demonstrate viability, quality, and safety advantages of light-emitting diode (LED) illumination when performing tonsillectomy. Study Design: Prospective cohort. Setting: Children's Hospital and Community Multispecialty Hospital. Methods: We tested a commercially available LED light, held in position with a minimally modified mouth gag, for off-label use in a "cavernous wound." We assessed surgeons', residents', and nurses' perceptions of function, safety, as well as preferences compared to headlights. Results: The light was used in 30 cases. Noted advantages over traditional lighting methods included superior brightness, stability and consistency of illumination, and the ability for others to assist more quickly. The inability to adjust brightness and/or the angle of light was an observed disadvantage. A small oral cavity or large tonsillar pillars creating a shadow required the temporary addition of a headlight. However, LED light use was not discontinued. Surgeons and residents reported a preference for not wearing a headlight, and nurses expressed concerns about headlight cleanliness. Conclusion: LED lighting technology demonstrated utility for teaching surgeons, residents, and nurses, and was perceived as safe. Additional specifications may make the light applicable to a wider variety of cases and could potentially mitigate headlight use during procedures of the oral cavity and oropharynx.Level of Evidence: 4.

Research on spatial frequency shift super-resolution imaging based on evanescent wave illumination

<sec>In spite of the success of fluorescence microscopes (such as stimulated emission depletion microscopy, stochastic optical reconstruction microscopy and photoactivated localization microscopy) in biomedical field, which have realized nanometer scale imaging resolution and promoted the great development of bio-medicine, the super-resolution imaging method for non-fluorescent sample is still scarce, and the resolution still has a big gap to nanometer scale. Among existing methods, structured illumination microscopy, PSF engineering, super-oscillatory lens and microsphere assisted nanoscopy are more mature and widely used. However, limited by the theory itself or engineering practice, the resolutions of these methods are hard to exceed 50 nm, which limits their applications in many fields. Enlightened by synthetic aperture technique, researchers have proposed spatial frequency shift super-resolution microscopy through shifting and combining the spatial frequency spectrum of imaging target, which is a promising super-resolution imaging scheme, for its resolution limit can be broken through continually. Currently, owing to the limitation of the refractive index of optical material, the wavelength of illumination evanescent wave is hard to shorten when this wave is generated at prism surface via total internal reflection, which determines the highest resolution of this spatial frequency shift super-resolution imaging system. Another deficiency of this scheme is the difference in imaging resolution among different directions, for the image has the highest resolution only in the direction along the wave vector of illumination evanescent wave; while, the image has the lowest resolution in the direction perpendicular to the wave vector, which is the same as that obtained by far-field illumination.</sec><sec>In order to solve the above thorny questions, a new model of generating the evanescent wave is proposed, which can generates an omnidirectional evanescent wave with arbitrary wavelength based on the phase modulation of nano-structure, and solve the both problem in imaging system at the same time. To verify the our scheme, we set up a complete simulation model for spatial frequency shift imaging scheme, which includes three parts: the generation of evanescent wave and the interaction of the evanescent wave with the nano-structures at imaging target, which can be simulated with FDTD algorithm; the propagation of light field from near-field to far-field region, from the sample surface to the focal plane of objective lens, which can be calculated with angular spectrum theory; the propagation of light field from the focal place to the image plane, which can be worked out with Chirp-Z transform.</sec><sec>Firstly, with this complete simulation model, we compare the resolution of microscopy illuminated by evanescent wave with that by propagating wave. The experimental results verify the super-resolution imaging ability of evanescent wave illumination and the influence of prism refractive index. The higher the refractive index, the shorter the wavelength of evanescent wave is and the higher the resolution of spatial frequency shift imaging system. Secondly, we demonstrate the resolution difference in a series of directions with a three-bar imaging target rotated to different directions. The result shows that the highest imaging resolution occurs in the direction of illumination evanescent wave vector, and the lowest resolution appears in the direction perpendicular to the wave vector. Finally, we simulate the evanescent wave generated by nano-strcuture and demonstrate its properties of wavelength and vector direction. When applied to near-field illumination super-resolution imaging, the omnidirectional evanescent wave solves the both problems in the model of total internal reflection from the prism surface.</sec><sec>Therefore, the advantages of our scheme are higher imaging resolution and faster imaging speed, no need for multi-direction and multiple imaging, and also image post-processing. In this study, a new spatial frequency shift super-resolution imaging method is proposed, which lays a theoretical foundation for its applications.</sec>

Far-field sub-diffraction optical imaging using near-field illumination by optoplasmonic hybrid materials

Here, we propose a method to fabricate large-scale optoplasmonic hybrid materials (OHMs) as a near-field illumination source for sub-diffraction imaging through a traditional optical microscope. An OHM is comprised of a metal/dielectric multilayer coated substrate and a microcylinder (MC) placed on top of the multilayer. Under the dark-field illumination of a traditional optical microscope, the photonic nanojet (PNJ) generated by the MC and the electric field produced on the substrate surface by the photonic–plasmonic interaction work as the near-field illumination source for an object placed on the substrate near the PNJ. High spatial frequencies of the object can be shifted to the passband of the optical microscope and weak scattering signals of the object can be enhanced upon the near-field illumination. Low-contrast objects, such as a 300-nm-diameter SiO2 nanoparticle array, two 100-nm-diameter polystyrene nanoparticles spaced ∼86 nm apart, and Escherichia coli can be resolved using the optical microscope. In addition, the field of view (FOV) is rectangular, the length of the FOV depends on the length of the MC, and the width relates to the diameter of the MC. The proposed method is wide-field, label-free, direct, noninvasive, and far-field, and it presents the potential of super-resolution imaging of unlabeled biological samples using a traditional optical microscope.

Multi-moded high-index contrast optical waveguide for super-contrast high-resolution label-free microscopy.

The article elucidates the physical mechanism behind the generation of superior-contrast and high-resolution label-free images using an optical waveguide. Imaging is realized by employing a high index contrast multi-moded waveguide as a partially coherent light source. The modes provide near-field illumination of unlabeled samples, thereby repositioning the higher spatial frequencies of the sample into the far-field. These modes coherently scatter off the sample with different phases and are engineered to have random spatial distributions within the integration time of the camera. This mitigates the coherent speckle noise and enhances the contrast (2-10) × as opposed to other imaging techniques. Besides, the coherent scattering of the different modes gives rise to fluctuations in intensity. The technique demonstrated here is named chip-based Evanescent Light Scattering (cELS). The concepts introduced through this work are described mathematically and the high-contrast image generation process using a multi-moded waveguide as the light source is explained. The article then explores the feasibility of utilizing fluctuations in the captured images along with fluorescence-based techniques, like intensity-fluctuation algorithms, to mitigate poor-contrast and diffraction-limited resolution in the coherent imaging regime. Furthermore, a straight waveguide is demonstrated to have limited angular diversity between its multiple modes and therefore, for isotropic sample illumination, a multiple-arms waveguide geometry is used. The concepts introduced are validated experimentally via high-contrast label-free imaging of weakly scattering nanosized specimens such as extra-cellular vesicles (EVs), liposomes, nanobeads and biological cells such as fixed and live HeLa cells.

Wide-Field Super-Resolution Optical Fluctuation Imaging through Dynamic Near-Field Speckle Illumination.

Stochastic optical fluctuation imaging (SOFI) generates super-resolution fluorescence images by emphasizing the positions of fluorescent emitters via statistical analysis of their on-and-off blinking dynamics. In SOFI with speckle illumination (S-SOFI), the diffraction-limited grain size of the far-field speckles prevents independent blinking of closely located emitters, becoming a hurdle to realize the full super-resolution granted by SOFI processing. Here, we present a surface-sensitive super-resolution technique exploiting dynamic near-field speckle illumination to bring forth the full super-resolving power of SOFI without blinking fluorophores. With our near-field S-SOFI technique, up to 2.8- and 2.3-fold enhancements in lateral spatial resolution are demonstrated with computational and experimental fluorescent test targets labeled with conventional fluorophores, respectively. Fluorescent beads separated by 175 nm are also super-resolved by near-field speckles of 150 nm grain size, promising sub-100 nm resolution with speckle patterns of much smaller grain size.

Three-dimensional microscopic single-pixel imaging with chaotic light

Three-dimensional (3D) microscopic imaging is fundamentally important for microscale observations and manipulations. Here, we show that single-pixel imaging (SPI) is available for realizing superior 3D microscopic imaging, which relies on the near-field illumination with chaotic light. Like considering a two-point object to investigate two-dimensional microscopic imaging, we theoretically and experimentally studied the image formation process for a 3D object with two slices, demonstrating the excellent capability of the SPI for 3D microscopic imaging. The proposed 3D microscopic imaging via SPI would be useful for applications such as biological imaging and micromanipulations.

Super-Resolution Imaging with Patchy Microspheres

The diffraction limit is a fundamental barrier in optical microscopy, which restricts the smallest resolvable feature size of a microscopic system. Microsphere-based microscopy has proven to be a promising tool for challenging the diffraction limit. Nevertheless, the microspheres have a low imaging contrast in air, which hinders the application of this technique. In this work, we demonstrate that this challenge can be effectively overcome by using partially Ag-plated microspheres. The deposited Ag film acts as an aperture stop that blocks a portion of the incident beam, forming a photonic hook and an oblique near-field illumination. Such a photonic hook significantly enhanced the imaging contrast of the system, as experimentally verified by imaging the Blu-ray disc surface and colloidal particle arrays.

Investigation of far-field super-resolution imaging by microsphere-based optical microscopy

Optical microscopy techniques have recently been shown to have the capability to exceed the diffraction limit when a microsphere is used. The mechanism involved in super-resolution imaging is theoretically analysed in this paper. A microsphere is also known to generate a photonic nanojet with a waist smaller than the diffraction limit. This nanojet is used as near-field illumination, and interacts with the sample to give a passive spatial frequency shift for the sample, meaning that near-field evanescent waves containing high-frequency spatial information about the sample can be converted into far-field propagating waves. This method is simple and cost-effective, and offers a promising approach for achieving far-field super-resolution imaging. Our theoretical analysis of super-resolution imaging is verified based on numerical and experimental results.

Resolution improvement of dark-field microscopy via microparticle near-field illumination.

In this Letter, a novel, to the best of our knowledge, approach to improve the imaging resolution of dark-field microscopy is proposed and demonstrated. Inspired by an existing super-resolution imaging method based on near-filed illumination using a prism or microfiber, a microparticle-generated full-direction evanescent field for sample illumination was demonstrated to achieve a multi-orientation near-field illumination in one snapshot and to obtain a super-resolution image by spatial frequency shifting. The ultimate resolution and the additional magnification factor of this method were analyzed theoretically. Imaging experiments were carried on a standard microscope calibration target MetroChip and a Blu-ray disc characterized by subwavelength microstructures. High-imaging resolution was demonstrated experimentally, and two novel illumination modes were proposed to overcome imaging direction selectivity. Our work opened up a new perspective of super-resolution imaging with near-field illumination.

Double moiré localized plasmon structured illumination microscopy

Abstract Structured illumination microscopy (SIM) is a well-established fluorescence imaging technique, which can increase spatial resolution by up to a factor of two. This article reports on a new way to extend the capabilities of structured illumination microscopy, by combining ideas from the fields of illumination engineering and nanophotonics. In this technique, plasmonic arrays of hexagonal symmetry are illuminated by two obliquely incident beams originating from a single laser. The resulting interference between the light grating and plasmonic grating creates a wide range of spatial frequencies above the microscope passband, while still preserving the spatial frequencies of regular SIM. To systematically investigate this technique and to contrast it with regular SIM and localized plasmon SIM, we implement a rigorous simulation procedure, which simulates the near-field illumination of the plasmonic grating and uses it in the subsequent forward imaging model. The inverse problem, of obtaining a super-resolution (SR) image from multiple low-resolution images, is solved using a numerical reconstruction algorithm while the obtained resolution is quantitatively assessed. The results point at the possibility of resolution enhancements beyond regular SIM, which rapidly vanishes with the height above the grating. In an initial experimental realization, the existence of the expected spatial frequencies is shown and the performance of compatible reconstruction approaches is compared. Finally, we discuss the obstacles of experimental implementations that would need to be overcome for artifact-free SR imaging.

Real-time Approximation of Photometric Polygonal Lights

We present a real-time rendering technique for photometric polygonal lights. Our method uses a numerical integration technique based on a triangulation to calculate noise-free diffuse shading. We include a dynamic point in the triangulation that provides a continuous near-field illumination resembling the shape of the light emitter and its characteristics. We evaluate the accuracy of our approach with a diverse selection of photometric measurement data sets in a comprehensive benchmark framework. Furthermore, we provide an extension for specular reflection on surfaces with arbitrary roughness that facilitates the use of existing real-time shading techniques. Our technique is easy to integrate into real-time rendering systems and extends the range of possible applications with photometric area lights.

A Subwavelength-Laser-Driven Transmitting Optical Nanoantenna for Wireless Communications

Nanoantennas are efficient devices exhibiting large confined electric field enhancements. So far, they have been extensively researched mainly in the receiving mode, which means that the illuminating field is essentially a plane wave. In this article, we consider the problem of designing an efficient and highly directive transmitting nanoantenna where the system is energized by a nonplane wave field, a subwavelength laser excitation. The main objective of this article is to establish through an accurate computational model a basic understanding of how a dipole-type radiator can be made to resonate with such complex nonplane-wave type of near-field illumination. Including short-wavelength components allowed us to achieve a 200-nm spot radius, which is a quarter of its incident wavelength (800 nm). Near- and far-field antenna quantities are introduced and calculated using an efficient full-wave finite-element method-based multiphysics solver. A nanoscale optical antenna is then presented with optimized dimensions and material settings. Various design data and insights are also discussed in connection with issues such as how to define efficiency and determine whether the system is radiating properly.

Detection of norovirus-like particles with an external force-assisted near-field illumination biosensor

Optical near-field enhancement at a surface where a total internal reflection occurs is an effective phenomenon that can enhance optical signals from markers conjugated with biological materials on the surface. The enhanced signal can be an effective tool to detect trace amounts of a biological substance. An external force-assisted near-field illumination biosensor is based on a sandwich assay that uses magnetic particles instead of a sensor surface to capture the targets. This sensing method detects light signals from targets sandwiched between magnetic particles and optical markers. The signal of the sandwiched target can be easily distinguished from optical noise by observing the movement of the signal induced by an external magnetic field. In this study, we improved the detection ability by using 60 nm Ø gold nanoparticles as markers. Norovirus virus-like particles were added to a buffer or a contaminated water sample and successfully detected using this system.

Modeling and design of diffuse transmissive free-form surface based on LED collimation effect

An indirect illumination pattern with light-emitting diodes (LED) and diffuse transmissive free-form surface was proposed, and in this paper, we designed an ultra-compact rotational symmetric lens with double free-form surfaces based on the edge-ray principle. Based on the Lambertian characteristic of LEDs and ideal diffuse transmissive surface (e.g. frosted glass), a mathematical model was constructed. The coordinates of profile curve of the transmitting part are calculated by Matlab according to the Snell’s law and iterative method at first. Then the TIR lens model is established with Solidworks. The diffuse transmissive free-form surface is constructed by solving a series of equations numerically. The inner surface was elliptic and the outer surface was diffuse transmissive free-form. Furthermore, to keep the optical system compact and provide enough space for the LED, the size of the designed lens model was also greatly reduced. Finally, the results show better uniformity and higher efficiency comparing with the single diffuse transmissive free-form surfaces and flats, and the size of the lens was dramatically reduced. The Monte-Carlo ray-tracing result shows an illumination circular area with a clear cut-off at R = 150 mm at the target plane in a distance H = 600 mm for the near-field illumination. The uniformity within the target illumination area is greater than 86%.

A Novel Reciprocity Based Theory for Extraction of Scattered Field under Plane Wave Incidence from that of Near-field Illumination

ABSTRACT A novel theory based on the reciprocity theorem is presented for computing the plane wave scattered field of an arbitrary target, from its scattered field due to near-field illumination. The target is supposed to be illuminated by a transmitter probe on a cylindrical surface and the scattered near-field data is recorded in the near-field. The presented theory constructs a bridge between the scattered field due to the near-field illumination by a transmitter probe and that of plane wave illumination. Thus, referring to the definition of Radar Cross-Section (RCS) that assumes the incident field to be a plane wave, the RCS of the target can be analytically evaluated. For the verification of the proposed method, it is applied to an infinite conducting circular cylinder, which has an analytic solution for the infinite line source as a near-field source. Using the proposed theory the scattered field of such a target under plane wave incidence is evaluated, which is exactly identical to the same analytical scattered field under plane wave illumination.

Analytic spherical harmonic coefficients for polygonal area lights

Spherical Harmonic (SH) lighting is widely used for real-time rendering within Precomputed Radiance Transfer (PRT) systems. SH coefficients are precomputed and stored at object vertices, and combined interactively with SH lighting coefficients to enable effects like soft shadows, interreflections, and glossy reflection. However, the most common PRT techniques assume distant, low-frequency environment lighting, for which SH lighting coefficients can easily be computed once per frame. There is currently limited support for near-field illumination and area lights, since it is non-trivial to compute the SH coefficients for an area light, and the incident lighting (SH coefficients) varies over the object geometry. We present an efficient closed-form solution for projection of uniform polygonal area lights to spherical harmonic coefficients of arbitrary order, enabling easy adoption of accurate area lighting in PRT systems, with no modifications required to the core PRT framework. Our method only requires computing zonal harmonic (ZH) coefficients, for which we introduce a novel recurrence relation. In practice, ZH coefficients are built up iteratively, with computation linear in the desired SH order. General SH coefficients can then be obtained by the recently developed sparse zonal harmonic rotation method.

Site-Controlled Quantum Emitters in Dilute Nitrides and their Integration in Photonic Crystal Cavities

We review an innovative approach for the fabrication of site-controlled quantum emitters (i.e., single-photon emitting quantum dots) based on the spatially selective incorporation and/or removal of hydrogen in dilute nitride semiconductors (e.g., GaAsN). In such systems, the formation of stable N-H complexes removes the effects that nitrogen has on the alloy properties, thus enabling the in-plane engineering of the band bap energy of the system. Both a lithographic approach and/or a near-field optical illumination—coupled to the ultra-sharp diffusion profile of H in dilute nitrides—allow us to control the hydrogen implantation and/or removal on a nanometer scale. This, eventually, makes it possible to fabricate site-controlled quantum dots that are able to emit single photons on demand. The strategy for a deterministic spatial and spectral coupling of such quantum emitters with photonic crystal cavities is also presented.

Site-Controlled Single-Photon Emitters Fabricated by Near-Field Illumination.

Many of the most advanced applications of semiconductor quantum dots (QDs) in quantum information technology require a fine control of the QDs' position and confinement potential, which cannot be achieved with conventional growth techniques. Here, a novel and versatile approach for the fabrication of site-controlled QDs is presented. Hydrogen incorporation in GaAsN results in the formation of N-2H and N-2H-H complexes, which neutralize all the effects of N on GaAs, including the N-induced large reduction of the bandgap energy. Starting from a fully hydrogenated GaAs/GaAsN:H/GaAs quantum well, the NH bonds located within the light spot generated by a scanning near-field optical microscope tip are broken, thus obtaining site-controlled GaAsN QDs surrounded by a barrier of GaAsN:H (laterally) and GaAs (above and below). By adjusting the laser power density and exposure time, the optical properties of the QDs can be finely controlled and optimized, tuning the quantum confinement energy over more than 100 meV and resulting in the observation of single-photon emission from both the exciton and biexciton recombinations. This novel fabrication technique reaches a position accuracy <100 nm and it can easily be applied to the realization of more complex nanostructures.