Near-field Optics Research Articles

Unidirectional light propagation through two-layer nanostructures based on optical near-field interactions

We theoretically demonstrate direction-dependent polarization conversion efficiency, yielding unidirectional light transmission, through a two-layer nanostructure by using the angular spectrum representation of optical near-fields. The theory provides results that are consistent with electromagnetic numerical simulations. This study reveals that optical near-field interactions among nanostructured matter can provide unique optical properties, such as the unidirectionality observed here, and offers fundamental guiding principles for understanding and engineering nanostructures for realizing novel functionalities.

Possibilities of functionalized probes in optical near-field microscopy

Functionalization, bringing new functions to standard approaches, is used in different fields of science. Functionalized probes have found wide application in atomic force microscopy, especially in force spectroscopy and for the specific recognition of single molecules. At the same time they have found only minor application in scanning near-field optical microscopy in spite of the possibility to obtain new information. This article considers examples of the use of functionalized tips in near-field optics and emphasizes the new approach based on tip-enhanced Raman scattering (TERS) with functionalized tips. The concept of Raman probe is described considering some possible perspectives as well as the demonstration of an internal standard in TERS based on a functionalized tip.

Focus on spectroscopy and spectrometry

Focus on spectroscopy and spectrometry

Near-Field Mapping of Optical Modes on All-Dielectric Silicon Nanodisks

We measure, for the first time to our knowledge, the near-field amplitudes and phases of localized optical modes of high-index all-dielectric nanoparticles using apertureless near-field optical microscopy. For individual silicon nanodisks, we observe a four-lobed mode pattern and the formation of deep-subwavelength hot-spots. Our numerical calculations of the optical near-fields of the nanodisks in combination with a multipole expansion of the scattered field based on vector spherical harmonics reveal that the observed modes are dominated by electric quadrupole contributions. The observed mode is of particular interest for the design of low-loss all-dielectric metasurfaces and nanoantennas for a broad range of applications, such as directional and complex-polarization controlled emission, light extraction from multipolar atomic transitions, and coherent multiple-emitter-nanocavity interactions.

Randomness in highly reflective silver nanoparticles and their localized optical fields.

Reflection of near-infrared light is important for preventing heat transfer in energy saving applications. A large-area, mass-producible reflector that contains randomly distributed disk-shaped silver nanoparticles and that exhibits high reflection at near-infrared wavelengths was demonstrated. Although resonant coupling between incident light and the nanostructure of the reflector plays some role, what is more important is the geometrical randomness of the nanoparticles, which serves as the origin of a particle-dependent localization and hierarchical distribution of optical near-fields in the vicinity of the nanostructure. Here we show and clarified the unique optical near-field processes associated with the randomness seen in experimentally fabricated silver nanostructures by adapting a rigorous theory of optical near-fields based on an angular spectrum and detailed electromagnetic calculations.

Adjustable transmission properties through ring-shaped nanotube arrays using finite-difference time-domain method

Metallic ring-shaped nanotube arrays are proposed and its optical transmission properties are studied by using finite-difference time-domain (FDTD) method. Compared with the transmission spectra of conventional circular nanotube arrays, two photonic band gaps are emerged in the transmission spectra of ring-shaped nanotube arrays, the two band gaps and transmission spectra are adjusted by the length, inner radius, intertube spacing and the dielectric constants of the core and embedding medium, and magnitude modification, redshift and blueshift of the resonance modes are observed. A metallic ring-shaped nanotube arrays for subwavelength band-stop filter in the range of visible light can be achieved. To understand its physical origin, field-interference mechanism was suggested by the field distributions. The proposed nanostructures and results may have great potential applications in subwavelength near-field optics.

Nanoscale Optical and Mechanical Manipulation of Molecular Alignment in Metal–Molecule–Metal Structures

Equilibrium molecular orientations, monitored using surface enhanced Raman scattering, are controlled by both optical and mechanical means in metal–molecule–metal nanogap structures, formed on an atomically planar Au(111) surface. The spatially confined optical near-fields exert a torque on the molecules in the nanogaps and align the molecular orientations along the field vector. The mechanical compression from the metal nanogaps not only disturbs the molecular alignments but also perturbs the electronic structures at metal–molecule interfaces. The implications of such interactions are discussed with respect to control and detection of local electronic properties in molecular nanodevices.

Self-probing of metal nanotips by rescattered electrons reveals the nano-optical near-field

Self-probing of atoms or molecules by their own electrons is a powerful tool for exploring structural dynamics with high spatial and temporal resolution. Spectra from high-harmonic generation or photoelectron rescattering reveal the quantities of interest, such as the electronic structure of the system under scrutiny. Recently, we have observed the rescattering effect at solids, in particular at metal nanotips. Here we use the self-probing approach in order to investigate an effect inherent to nano-scale systems: laser light incident on nanotips generates strongly enhanced optical near-fields that crucially depend on their geometry and material. In our experiment, we determine the strength of the near-field of tungsten and gold tips from the high-energy cut-off of rescattered electrons. Detailed experimental results are compared with a numerical solution of the time-dependent Maxwell equations.

A Feasibility Study on Generation of Acoustic Waves Utilizing Evanescent Light

A new approach of generating acoustic waves utilizing evanescent light is presented. The evanescent light is a non-propagating electromagnetic wave that exhibits exponential decay with distance from the surface at which the total internal reflection of light is formed. In this research, the evanescent light during total internal reflection at prism surface is utilized for generating acoustic waves in aluminium and the feasibility for ultrasonic measurements is discussed. Pulsed Nd:YAG laser with 0.36 J/cm2 power density is used and the incident angle during the total internal reflection is arranged to be 69.0° for generating the evanescent light. It has been demonstrated that the amplitude of the acoustic waves by means of evanescent light is about 1/14 as large as the one generated by the conventional pulsed laser. This reveals the possibility of using a laser ultrasonic technique with near-field optics.

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.

The optical near-field: super-resolution imaging with structural and phase correlation

AbstractAn overview of near-field optics is presented with a focus on the fundamental advances that have been made in the field since its inception 30 years ago. A focus is placed on the advancements that have been achieved in instrumentation. These advances have led to a greater generality of use with ultra-low mechanical and optical noise and the ultimate in force sensitivity with near-field optical probes. An emphasis is placed on the importance of fully integrating near-field optics with other imaging and spectroscopic modalities including Raman spectroscopy and electron/ion beam imaging. Important directions in probe design, force feedback methods and scanner flexibility are described. These developing avenues provide considerable optimism for an ever increasing incorporation of near-field optics to help resolve critical problems in fundamental and applied science.

Numerical method for inverse scattering in two-layered background in near-field optics

The inverse scattering problem in two-layered background that arises in near-field optics is discussed. The reconstruction of the scatterer from inhomogeneous medium deposited on a homogeneous substrate is presented, where the measured data only lies on a limited aperture. An error bound of the Born approximation is given. A moment method with truncated SVD is developed to handle the exponentially ill-posed problem with noisy measured data. Numerical experiments are presented to illustrate the resolution of the method, which are satisfying even under the perturbed data.

Highly sensitive mode mapping of whispering-gallery modes by scanning thermocouple-probe microscopy

We propose a method for mapping optical near-fields with the help of a thermocouple scanning-probe microscope tip. As the tip scans the sample surface, its apex is heated by light absorption, generating a thermovoltage. The thermovoltage map represents the intensity distribution of light at the sample surface. The measurement technique has been employed to map optical whispering-gallery modes in fused silica microdisk resonators operating at near-infrared wavelengths. The method could potentially be employed for near-field imaging of a variety of systems in the near-infrared and visible spectral range.

Optical near-fields & nearfield optics

Optical near-fields & nearfield optics

Novel sub-100 nm surface chemical modification by optical near-field induced photocatalytic reaction

The surface modification is indispensable to facilitate new functional applications of micro/nanofluidics devices. Among many modification techniques developed so far, the photo-induced chemical modification is the most versatile method in terms of robustness, process simplicity, and feasibility of chemical functionality. In particular, the method is useful for closed spaces, such as post-bonded devices. However, the limitation by optical diffraction limit is still a challenging issue in scaling down the pattern sizes to nanoscale. Here, we demonstrated a novel surface modification on sub-100 nm scale utilizing the novel optical near-field (ONF) generated on nanostructures of photocatalyst (TiO2). The minimum pattern size of 40 nm, which was much smaller than diffraction limit, was achieved using a visible light source (488 nm) and a conventional irradiation setup. The controllability of pattern size by light intensity, the feasibility of functionality, and the non-contact working mode have impacts on surface patterning of post-bonded micro/nanofluidics devices. It is also worthy to note that our results verified for the first time the ONF on nanostructures of non-metal materials and its ability to manipulate the chemical reaction on nanoscale.

Numerical analysis of the temperature dependence of near-field polarization for nanoscale thermometry using a triple-tapered near-field optical fiber probe

A novel nanoscale temperature measurement method using near-field polarization, namely polarized near-field optics thermal nanoscopy (P-NOTN), has been developed. This method is performed in illumination–collection mode (I–C mode) using an Au-coated near-field fiber probe, and enables non-contact and nanoscale temperature measurement. The polarization change of the near-field light due to temperature change in the I–C mode is complicated. In order to confirm and understand the temperature dependence of the near-field polarization, and assess the validity of the temperature measurement by P-NOTN, numerical investigations were performed by the finite-difference time-domain (FDTD) method, which numerically solves Maxwell’s equations. Three-dimensional models of the Au-coated near-field fiber probe and the one-dimensional nanostructure as a sample (i.e. Au nanorod) were produced. The electromagnetic field between the probe tip and the nanoscale sample was calculated by the FDTD method in order to evaluate the polarization change in the I–C mode. The calculation results showed that the polarization plane in the near field changes as a function of the refractive index of the sample, which in turn is temperature-dependent. These calculation results verified the capability of P-NOTN to achieve nanoscale temperature measurement by detecting the temperature-dependent polarization rotation change in the near field.

Near-field optical effect of a core-shell nanostructure in proximity to a flat surface.

We provide an analytical solution for studying the near-field optical effect of a core-shell nanostructure in proximity to a flat surface, within quasi-static approximation. The distribution of electrostatic potential and the field enhancement in this complex geometry are obtained by solving a set of linear equations. This analytical result can be applied to a wide range of systems associated with near-field optics and surface plasmon polaritons. To illustrate the power of this technique, we study the field-attenuation effect of an oxidized shell in a silver tip in a near-field scanning microscope. The thickness of oxidized layer can be monitored by measuring the intensity of light. We also find a linear relation between resonant frequency and temperature in an Ag-Au core-shell structure, which provides insight for local temperature detection with nm scale resolution. Our results also show good agreement with recent finite element method results.

Nanophotonics for efficient solution searching and decision making

At present, there is great demand for novel computing devices and architectures that can overcome the limitations of conventional technologies based solely on electron transfer, including the need to reduce energy consumption and solve computationally demanding problems. In particular, conventional digital computers have difficulty in dealing with intractable problems in which the number of possible solutions increases exponentially as a function of the problem size (referred to as ‘combinatorial explosion’). It is also challenging for conventional computers to make efficient and adaptive decisions in the uncertain, dynamically changing environments seen in real-world applications. A promising solution is near-field nanophotonics, which has been extensively studied with the aim of unveiling and exploiting light-matter interactions that occur at a scale below the wavelength of light. Recent progress made in experimental technologies—both in nanomaterial fabrication, such as quantum dots (QDs), and in characterization—is driving further advancements in the field. We have shown that the dynamics of optical energy transfer mediated by near-field interactions can be exploited to solve solution-searching and decision-making problems.1–4 This suggests that computing systems based on near-field nanophotonics may one day be able to exhibit intellectual abilities. When QDs share common resonant energy levels mediated by optical near-field interactions, optical energy is transferred from smaller QDs to larger ones. This process has been experimentally demonstrated in various quantum nanostructures, such as those fabricated in indium gallium arsenide, zinc oxide, and cadmium selenide. In addition, we have shown this transfer of optical Figure 1. (a) Quantum-dot-based decision maker (QDM) consisting of five quantum dots (denoted QDLL, QDML, QDS, QDMR, and QDLR) interacting via optical near-fields. The subscripts 1, 2, and 3 denote the (1,1,1), (2,1,1), and (2,2,2) energy levels, respectively, while U represents the various optical near-field interactions. (b) Quick adaptation of the QDM to a dynamically changing environment, which in this case is the change of reward probabilities between two slot machines (PA and PB). Softmax: Best conventional algorithm.

二维三角晶格光子晶体波导的辐射特性

A photonic crystal waveguide structure with two-dimensional triangular lattice photonic crystals is proposed,in which emission efficiency can be enhanced. The composite structure is composed by cascading a coupled waveguide arrays to a single waveguide. The emission characteristics of light are analyzed via the finite-difference time-domain( FDTD) method. The results show that the emission efficiency of the cascading composite structure is improved. The device has been optimized further. When the distance between the single waveguide and the waveguide arrays is 1. 8a,the horizontal and vertical period number of the waveguide arrays are 6 and 5,the emission efficiency is the best. The emission efficiency is over 20%,when the distance is 100 a between the waveguide output and detector. This type structure of photonic crystal has potential applications in near-field optics and integrated optics.

Directional and enhanced spontaneous emission with a corrugated metal probe.

A three-dimensional corrugated metal tapered probe with surface corrugated gratings at the tip apex is proposed and investigated theoretically, which leads to an obvious emission beaming effect of spontaneous emission from a single emitter near the probe. In contrast with conventional apertureless metal probes, where only the enhancement of an optical near-field is concerned, the corrugated probe is able to manipulate local excitation intensity and far-field emission direction simultaneously. The angular emission from a single dipole source, being placed close to the corrugated probe, falls into a cone with a maximum directivity angle of ±11.6°, which improves the collection efficiency 25-fold. Such a probe simultaneously increases the localized field intensity to about twice as strong as the conventional bare tip. In addition, the radiation pattern is sensitive to the working wavelength and the dipole to tip-apex separation. These findings make a promising route to the development of plasmonic spontaneous emission manipulation based on corrugated tapered antenna-for instance, tip-enhanced spectroscopy, single-molecule sensing, and single-photon source.