A polarizing situation: Taking an in-plane perspective for next-generation near-field studies

By enabling the probing of light–matter interactions at the functionally relevant length scales of most materials, near-field optical imaging and spectroscopy accesses information that is unobtainable with other methods. The advent of apertureless techniques, which exploit the ultralocalized and enhanced near-fields created by sharp metallic tips or plasmonic nanoparticles, has resulted in rapid adoption of near-field approaches for studying novel materials and phenomena, with spatial resolution approaching sub-molecular levels. However, these approaches are generally limited by the dominant out-of-plane polarization response of apertureless tips, restricting the exploration and discovery of many material properties. This has led to recent design and fabrication breakthroughs in near-field tips engineered specifically for enhancing in-plane interactions with near-field light components. This mini-review provides a perspective on recent progress and emerging directions aimed at utilizing and controlling in-plane optical polarization, highlighting key application spaces where in-plane near-field tip responses have enabled recent advancements in the understanding and development of new nanostructured materials and devices.     

Hierarchy in optical near-fields based on compositions of nanomaterials

Optical near-field interactions exhibit hierarchical responses in the nanometer scale allowing unique functions in nanophotonic systems. Such hierarchical properties in optical near-fields originate various physical entities in the nanometer scale. Engineering nanomaterial compositions, while maintaining geometrically equivalent conditions, leads to characteristic hierarchical responses. We experimentally demonstrate such material-dependent optical near-field hierarchy using core–shell-type nanostructures composed of gold and silver.     

Controllable and switchable chiral near-fields in symmetric graphene metasurfaces

A strong chiral near-field plays significant roles in the detection, separation and sensing of chiral molecules. In this paper, a simple and symmetric metasurface is proposed to generate strong chiral near-fields with both circularly polarized light and linearly polarized light illuminations in the mid-infrared region. Owing to the near-field interaction between plasmonic resonant modes of two nanosheets excited by circularly polarized light, there is a strong single-handed chiral near-field in the gap between the two graphene nanosheets and the maximum enhancement of the optical chirality could reach two orders of magnitude. As expected, the intensity and the response wavelength of the chiral near-fields could be controlled by the Fermi level and geometrical parameters of the graphene nanosheets, as well as the permittivity of the substrate. Meanwhile, based on the interaction between the incident field and scattered field, the one-handed chiral near-field in the gap also could be generated by the linearly polarized light excitation. For the two cases, the handedness of the chiral near-field could be switched by the polarized direction of the incident light. These results have potential opportunities for applications in molecular detection and sensing.     

Local fields close to the surface of nanoparticles and aggregates of nanoparticles

A refined discussion of the near-field scattering of spherical nanoparticles and the electromagnetic fields close to the particle surface is given. New results for the dependence on the distance from the surface and the angular distribution of the scattered light in the near-field are given. It will be shown that the radial component of the electric field leads to striking differences in the phase functions in the near-field and the far-field. Exemplary computations are presented for Ag and Au particles with different size. In a second part the discussion is extended to assemblies of spherical Ag and Au nanoparticles. It will be shown that large near-fields at wavelengths commonly used in SERS experiments are obtained for aggregates. In the near-field scattering intensity “hot spots” mark regions between particles in the aggregate where the near-field is particularly high. Received: 4 May 2001 / Revised version: 20 July 2001 / Published online: 19 September 2001     

Near-field magneto-optics and microscopy in resonant scattering of light from a linear nanoprobe

A theory is presented for the polar near-field magneto-optical Kerr effect in scattering of light from a linear nanoprobe. In the framework of Green’s function technique, a solution is obtained for the problem of near-field magneto-optics and apertureless scanning microscopy of lateral magnetic inhomogeneities (domains) with nanometer scale. The probe in the form of a nanowire and the sample with a near-surface magnetic nanolayer are considered to support surface plasmons. Electromagnetic coupling between a nanow-ire and a sample surface (polarizability of the complex “probe plus image charges”) is taken into account self-consistently. Magneto-induced polarization of an ultrathin near-surface layer is treated within linear approximation in magnetization which is perpendicular to the layer. The polarization, spectral and angular characteristics of light scattering modulated by magnetization and resonantly enhanced by surface plasmons are examined. Dependence of the near-field magneto-optical response on the probe-domain distance along the sample surface is obtained. The resolution power of scanning near-field microscopy is estimated and the factors to influence it are pointed out.     

Controlling near-field optical intensities in metal nanoparticle systems by polarization pulse shaping

Coherent control of near-fields in the proximity of nanoparticles is a promising tool for shaping the temporal and spatial properties of light at the nano-scale. Numerical simulations on a series of progressively more complex metallic nanoparticle systems were carried out to explore in detail the mechanisms of control. We find that an important element for the controllability of intensity localization is the spatial variation of the phase in the near-field that is due either to the interference between the plane incident wave and the near-field of the nanosystem or to the interference of fields generated by different parts of the nanosystem. For a collection of random nanoparticles we show that it is possible to select the appearance of highly localized hot spots.     

Dual mode near-field scanning optical microscopy for near-field imaging of surface plasmon polariton

In this paper, we theoretically and experimentally demonstrate that metal coated apertured probes are efficient near-field probes on surfaces with high reflectivity for the scattering as well as for the collection mode near-field scanning optical microscopy (NSOM). We show that a blunt apertured metal coated tip is very effective in suppressing image dipoles which affect strongly the signals scattered from frequently used sharp metal tips or gold nanoparticle attached probes. By using a simultaneous collection and scattering mode (dual mode) NSOM we measure the near-field images of surface plasmon polariton (SPP) launched from a slit. The collection mode measures propagating SPP along lateral distance in a long scan range with high signal-to-noise ratio, and the scattering mode measures the polarization resolved near-field of SPP. Comparisons of the measured data obtained in the dual mode enable to easily characterize SPP and to separate the measured near-field into the propagating SPP and the directly transmitted light.     

Surface-electrode Paul trap with optimized near-field microwave control

We describe the design of a microfabricated Paul trap with integrated microwave conductors for quantum simulation and entangling logic gates. We focus on an approach where near-field amplitude gradients of microwave fields from conductors in the trap structure induce the required spin-motional couplings. This necessitates a strong amplitude gradient of the microwave near-field at the position of the ions, while the field itself needs to be suppressed as much as possible. We introduce a single meander-like microwave conductor structure which provides the desired field configuration. We optimize its parameters through full-wave microwave numerical simulations of the near-fields. The microwave conductor is integrated with additional dc and rf electrodes to form the actual Paul trap. We discuss the influence of the additional electrodes on the field configuration. To be able to fine-tune the overlap of the Paul trap rf null with the microwave field minimum, our trap design allows relative tuning of trap rf electrode amplitudes. Our optimized geometry could achieve a ratio of sideband-to-carrier excitations comparable to experiments with focused laser beams.     

The 10 MHz ultrasonic near-field: calculations, hydrophone and optical diffraction tomography measurements

The near-field from a 10 MHz ultrasound transducer has been studied using three different techniques; theoretical calculations, miniature hydrophone measurements and optical diffraction tomography (ODT). The motive was twofold; to evaluate the measurement possibilities of ODT for high-frequency ultrasound and to verify the calculated complexity of near-fields from real ultrasound transducers designed for use in blood perfusion measurements. Calculations of the field from an ideal piston transducer were done with surface integrals of the Kirchhoff function and the measured transducer was designed to approximate this ideal. Contour maps, from the three methods, of pressure amplitude from 1 to 30 mm along the beam are presented with beam profiles at 5 and 45 mm. The results imply that ODT has an increased spatial resolution compared with the 0.5 and 1.0 mm hydrophones. However, even better spatial resolution is needed to draw definite conclusions regarding the complexity of the near-field of the 10 MHz transducer.     

Physics in plasmonic and Mie scattered near-field for efficient surface-enhanced Raman scattering template

We describe the physics of the SERS based on the optical near-field intensity enhancement on the metallic (plasmonic) and the nonmetallic (Mie scattering) nanostructured substrates with two-dimensional (2D) periodic nanohole arrays. The calculation by the Finite-Difference Time-Domain (FDTD) method revealed that the optical intensity enhancement increases with the increase of the thickness of a gold film coating on the nonmetallic (dielectric) nanostructured Si, GaAs, and SiC substrates. The resonance spectrum shifts with the changes in the geometrical structure of the void diameter and inter-void distance. It was clarified that the optical intensity enhancement obtained with the gold-coated substrate is equivalent to that with a gold substrate at 70-nm thick gold coating on the dielectric substrates in this structure. The resonance spectral bandwidth for Mie scattering and plasmonic near-fields is different. Therefore, if the Stokes line of the Raman scattering is located within the resonance bandwidth, the SERS signal is enhanced proportionally to the fourth power of the electric near-field. However, if the Stokes shift is located out of the resonance bandwidth, the SERS signal enhancement is only proportional to the square of the scattered near-field.     

Nanometer-scale probing of optical and thermal near-fields with an apertureless NSOM

We have used a home-made apertureless near-field scanning optical microscope (ANSOM) for mapping nanometric steps between SiC and gold regions under visible (λ=655 nm) and infrared (λ=10.6 μm) illumination. The images, obtained with a signal demodulation at the tip oscillation frequency and at higher harmonics, clearly show optical contrasts with a subwavelength resolution of about 30 nm. Other images recorded in the visible on a YBa₂Cu₃O₇ crystal indicate that the tip used in our experiments is able to reveal polarization effects. We also present a near-field thermal optical microscope (NTOM) which operates without any external illumination. In this new kind of microscope, the laser source which is usually used to excite the evanescent waves, is replaced by a simple heating of the sample. The electromagnetic radiation locally scattered by the tip comes from the thermal radiation. Our results with this new technique prove a 200 nm lateral resolution.     

Subwavelength optical imaging through a metallic nanorod array

We propose a subwavelength imaging system without a lens or a mirror but with an array of metallic nanorods. The near-field components of dipole sources were plasmonically transferred through the rod array to reproduce the source distribution in the other side. We calculated the field distribution at the different planes of imaging process using the finite-difference time-domain algorithm and found that the spatial resolution was 40 nm given by the rod size and spacing. A typical configuration is a hexagonal arrangement of silver rods of 50 nm height and 20 nm diameter. We also show that the image formation highly depends on the coherence and the polarization of the source distribution and the source-array distance.     

Near-field observation of plasmon excitation and propagation on ordered elliptical hole arrays

We report a near-field study of the excitation and propagation of surface plasmon on ordered Ag elliptical hole arrays with a scattering-type scanning near-field optical microscope. Strong dipole-like local plasmon is identified at each individual hole from near-field optical intensity and phase images. The excitation of the local plasmon at the elliptical hole is found to follow polarization excitation constraint. The coherent superposition of these local plasmon waves to form an extended surface plasmon wave propagating to an adjacent hole array is observed directly. The near-field results are consistent with the results obtained from far-field extraordinary transmission measurements. PACS 42.25.Bs; 42.25.Hz; 42.25.Ja; 42.25.Kb; 07.79.Fc     

Plasmonic laser nanoablation of silicon by the scattering of femtosecond pulses near gold nanospheres

We present the fabrication of nanostructures ablated on silicon(100) by the plasmonic scattering of 780 nm, 220 fs laser pulses in the near-field of gold nanospheres. We take advantage of the enhanced plasmonic scattering of ultrashort laser light in the particle near-field to ablate well-defined nanocraters. Gold nanospheres of 150 nm diameter are deposited onto a silicon surface and irradiated with a single laser pulse. We studied the effect of laser polarization on the morphology of ablated nanostructures and estimated the minimum fluence for plasmonic nanoablation. When the polarization of the incident radiation is directed at a 45° angle into the substrate surface, a near-field enhancement of 23.1±7.6 is measured, reducing the required silicon ablation fluence from 191±14 mJ/cm² to 8.2±2.9 mJ/cm². Enhancements are also measured for laser polarizations parallel to the substrate surface when the substrate is angled 0° and 45° to the incident irradiation, giving enhancements of 6.9±0.6 and 4.1±1.3, respectively. Generated nanocrater morphologies show a direct imprint of the particle dipolar scattering region, as predicted in our theoretical calculations. The measured near-field enhancement values agree well with the maximum field enhancements obtained in our calculations. The agreement between theory and measurements supports that the nanocraters are indeed formed by the enhanced plasmonic scattering in the near-field of the nanoparticles. PACS 42.62.-b; 52.38.Mf; 81.65.Cf; 81.16.-c; 78.67.Bf     

Phase interpretation for polarization-dependent near-field images of high-density gratings

It has been described that the near-field images of a high-density grating at the half self-imaging distance could be different for TE and TM polarization states. We propose that the phases of the diffraction orders play an important role in such polarization dependence. The view is verified through the coincidence of the numerical result of finite-difference time-domain method and the reconstructed results from the rigorous coupled-wave analysis. Field distributions of TE and TM polarizations are given numerically for a grating with period d = 2.3λ, which are verified through experiments with the scanning near-field optical microscopy technique. The concept of phase interpretation not only explains the polarization dependence at the half self-imaging distance of gratings with a physical view, but also, it could be widely used to describe the near-field diffraction of a variety of periodic diffractive optical elements whose feature size comparable to the wavelength.     

The Effect of a Silicon Substrate on the Directivity Pattern of Scattered Radiation of a Gold Nanospheroid and on Near-Field Polarization

We studied the radiation-directivity pattern and the near-field polarization of a spheroidal metallic nanoparticle located over a silicon substrate by interaction with a linearly and circularly polarized field. It is shown that the directivity pattern of the spheroidal particle near the silicon substrate becomes strongly asymmetric and forward scattering is predominant compared with the symmetric diagram of a particle in free space. The change of the near-field polarization of the nanoparticle in presence of the substrate is studied for different wavelengths in the vicinity of the plasmonic resonance. The near-field polarization is described using the generalized Stokes parameters, which allow pictorial visualization of results.     

Investigation of ultrafast plasmon control in silver block by PEEM

We carried out in situ imaging of ultrafast plasmonic near-field control within silver block through photoemission electron microscope (PEEM) combined with picosecond laser pulses. It is found that the near-field distribution can be confined at the edges of the block, and efficiently localized to different edges depending on the polarization direction of the illuminating laser light. In particular, the localized near-field will switch from the upper edge to the lower one in the block when the polarization angle of the laser is changed from 45 to 135°. Furthermore, we found that, due to the restriction to electron oscillation by the edge of the block, the photoemission yield is greatly enhanced in the edge that is perpendicular to the polarization direction of the laser. In addition, we also discussed the mechanism responsible for brightness difference between the left and upper/down edges in PEEM images under s- and p-polarized ultrafast laser illumination. Our findings pave the way for ultrafast plasmon application in the fields such as optical switches, logic operation and others.     

Reducing Background Light Interaction in Near-Field Optical Microscopy Using Lateral and Vertical Probe-Dithering

We investigate the effectiveness of differential detection, which is a combination of probe-dithering and synchronous detection, in discriminating near-field light interaction from background light interaction in apertureless near-field optical microscopy (NSOM). The lateral differential NSOM with a photocantilever is more effective than the vertical differential detection, which does not always provide sufficient discrimination. The V-dithering-based lateral differential detection provides apertureless NSOM that can image the optical coupling between sample and probe dipoles, which is an interaction through near-field light.This paper was originally presented at the 5th International Conference on NEAR FIELD OPTICS and RELATED TECHNOLOGIES (NFO-5), which was held on December 6–10, 1998 at Coganoi Bay Hotel, Shirahama, Japan, in cooperation with the Japan Society of Applied Physics and Mombusho Grant-in Aid for Scientific Research on Priority Areas “Near-field Nano-optics” Project, sponsored by Japan Society for the Promotion of Science.     

On chip chiral and plasmonic hybrid dimer or tetramer: Generic way to reverse longitudinal and lateral optical binding forces

For both the longitudinal binding force and the lateral binding force, a generic way of controlling the mutual attraction and repulsion (usually referred to as reversal of optical binding force) between chiral and plasmonic hybrid dimers or tetramers has not been reported so far. In this paper, by using a simple plane wave and an onchip configuration, we propose a possible generic way to control the binding force for such hybrid objects in both the near-field region and the far-field region. We also investigate different inter-particle distances while varying the wavelengths of light for each inter-particle distance throughout the investigations. First of all, for the case of longitudinal binding force, we find that chiral-plasmonic hybrid dimer pairs do not exhibit any reversal of optical binding force in the near-field region nor in the far-field region when the wavelength of light is varied in an air medium. However, when the same hybrid system of nanoparticles is placed over a plasmonic substrate, a possible chip, it is possible to achieve a reversal of the longitudinal optical binding force. Later, for the case of lateral optical binding force, we investigate a setup where we place the chiral and plasmonic tetramers on a plasmonic substrate by using two chiral nanoparticles and two plasmonic nanoparticles, with the setup illuminated by a circularly polarized plane wave. By applying the left-handed and the right-handed circular polarization state of light, we also observe the near-field and the far-field reversal of lateral optical binding force for both cases. As far as we know, so far, no work has been reported in the literature on the generic way of reversing the longitudinal optical binding force and the lateral optical binding force of such hybrid objects. Such a generic way of controlling optical binding forces can have important applications in different fields of science and technology in the near future.