Near-field Optics Research Articles

Electron-driven photon sources for correlative electron-photon spectroscopy with electron microscopes

Abstract Electron beams in electron microscopes are efficient probes of optical near-fields, thanks to spectroscopy tools like electron energy-loss spectroscopy and cathodoluminescence spectroscopy. Nowadays, we can acquire multitudes of information about nanophotonic systems by applying space-resolved diffraction and time-resolved spectroscopy techniques. In addition, moving electrons interacting with metallic materials and optical gratings appear as coherent sources of radiation. A swift electron traversing metallic nanostructures induces polarization density waves in the form of electronic collective excitations, i.e., the so-called plasmon polariton. Propagating plasmon polariton waves normally do not contribute to the radiation; nevertheless, they diffract from natural and engineered defects and cause radiation. Additionally, electrons can emit coherent light waves due to transition radiation, diffraction radiation, and Smith-Purcell radiation. Some of the mechanisms of radiation from electron beams have so far been employed for designing tunable radiation sources, particularly in those energy ranges not easily accessible by the state-of-the-art laser technology, such as the THz regime. Here, we review various approaches for the design of coherent electron-driven photon sources. In particular, we introduce the theory and nanofabrication techniques and discuss the possibilities for designing and realizing electron-driven photon sources for on-demand radiation beam shaping in an ultrabroadband spectral range to be able to realize ultrafast few-photon sources. We also discuss our recent attempts for generating structured light from precisely fabricated nanostructures. Our outlook for the realization of a correlative electron-photon microscope/spectroscope, which utilizes the above-mentioned radiation sources, is also described.

Tunable and dual-broadband giant enhancement of second-harmonic and third-harmonic generation in an optimized graphene-insulator-graphene metasurface

We demonstrate a novel scheme to dramatically enhance both the second- and third-harmonic generation in a graphene-insulator-graphene metasurface. The key underlying feature of our approach is the existence of a double-resonance phenomenon, namely the metasurface is designed to possess fundamental plasmon resonances at both the fundamental frequency and the higher harmonic. In particular, this dual resonant field enhancement at the two optical frequencies, combined with a favorable spatial overlap of the optical near-fields, lead to the increase of the generated higher harmonic by several orders of magnitude. Remarkably, we demonstrate that by tuning the Fermi energy of the graphene gratings the dual-resonance property can be locked-in over a broad spectral range of ~20 THz, and equally important, the enhanced nonlinear frequency generation process can be readily switched in the same device between the second and third harmonic. This new type of graphene metasurface could open up new avenues towards the development of novel ultra-compact and multi-frequency active photonic nanodevices.

Few-cycle localized plasmon oscillations

The generation of few-cycle laser pulses proved to be a key enabling technology in strong-field physics and ultrafast science. The question naturally arises whether one can induce few-cycle localized plasmon oscillations in optical near-fields. Here, we perform a comparative study of different plasmonic nanoresonators illuminated by few-cycle pulses. We analyze the number of cycles (NOC) of the plasmonic field, the near-field enhancement (NFE) as well as the figure of merit NFE/NOC. The pulse length dependence of these quantities is also investigated. Throughout the inspected pulse-length interval silica-gold and silica-silver core–shell monomers have the potential to preserve the NOC of the incoming pulse, silver bow-ties result in the highest NFE, whereas gold core–shell dimers have the highest NFE/NOC. Based on the analysis, silver bow-ties, gold core–shell and silver nanorod dimers proved to be the most suitable for few-cycle near-field amplification.

Numerical evaluation of Sommerfeld-type integrals for reflection and transmission of dipole radiation

A radiating electric dipole is located near the interface with a layer of material. The electric and magnetic fields reflect off the interface and transmit through the material. The exact solution of Maxwell’s equations can be found in terms of Sommerfeld-type integrals. These integrals have in general a singularity on the integration axis, and the integrands are extremely complicated functions of the parameters in the problem. We present a method for the computation of these integrals, and the corresponding electric and magnetic fields. Key to the solution is the splitting of the incident field in its traveling and evanescent contributions. With a change of variables, the singularities can be transformed away, and the method also greatly improves the accuracy and efficiency of the integration. We illustrate the feasibility of our approach with the computation of the flow lines of electromagnetic energy in the system. For such flow diagrams, a large number of integrals needs to be computed with reasonable accuracy. We show that in our approach even the smallest details in flow diagrams can be revealed. Program summaryProgram titles: CPiP-Auxiliary-1, CPiP-Auxiliary-2, CPiP-Field lines-1, CPiP-Field lines-2.CPC Library link to program files:http://dx.doi.org/10.17632/476n5ffkvv.1Licensing provisions: GPLv3.Programming language: Mathematica.Nature of problem: In near-field optics and nano-photonics, exact solutions of Maxwell’s equations are needed. Of particular interest are the reflected and transmitted electric and magnetic fields of dipole radiation by a layer of material. These solutions involve a large number of integrals, which need to be computed numerically. In the literature, these integrals are known as Sommerfeld-type integrals.Solution method: We split the integration range in two parts. The first part corresponds to traveling dipole waves and the second part results from evanescent dipole waves. In each region we make a (different) change of variables. The result of this transformation is that it removes a possible singularity on the integration axis, and it also has a tendency to smoothen out the integrand. There are 38 different types of integrals. Our method applies to all of them, and is self-contained. There is no need for tweaking of the programs for each of these, and no adjustments need to be made for different values of the parameters. The method is developed for the near field. For large distances to the source (the far field), asymptotic methods are available, and there would be no need for numerical integration.

Electron Beam Aberration Correction Using Optical Near Fields.

The interaction between free electrons and optical near fields is attracting increasing attention as a way to manipulate the electron wave function in space, time, and energy. Relying on currently attainable experimental capabilities, we design optical near-field plates to imprint a lateral phase on the electron wave function that can largely correct spherical aberration without the involvement of electric or magnetic lenses in the electron optics, and further generate on-demand lateral focal spot profiles. Our work introduces a disruptive and powerful approach toward aberration correction based on light-electron interactions that could lead to compact and versatile time-resolved free-electron microscopy and spectroscopy.

Spin-resolved near-field scanning optical microscopy for mapping of the spin angular momentum distribution of focused beams

We proposed and built a near-field scanning optical microscope (NSOM) to enable the characterization of the spin angular momentum (SAM) distribution of electromagnetic fields with nanoscale resolution. The NSOM probe was composed of a circular nanohole formed in a thick gold film that was deposited on a tapered cone fiber. The near-field signal, when coupled through the nanohole to the fiber, was split and analyzed using a combination of a quarter-wave plate and a polarizer to extract the two circular polarization components of the signal. This allowed us to characterize the out-of-plane SAM component, which was determined using the relationship Sz ∝ IRCP − ILCP. Using the developed system, we mapped the SAM distributions of a variety of tightly focused cylindrical vector vortex beams and thus validated the system's effectiveness. The proposed spin-resolved NSOM could be a valuable tool for studies of both near-field spin optics and topological photonics.

Convergence and accuracy of near‐field‐corrected iterative physical optics for scattering by imperfectly conducting and dielectric objects

This communication investigates the convergence behaviour and accuracy of a near-field corrected iterative physical optics scheme for scatterings by imperfectly conducting and dielectric objects. The scatterings by large and complex impedance surface bodies such as fixed wing aircraft, rotatory wing aircraft, tank, vessel, and boat are computed and compared with those of a full-wave method. Several RF characteristics of a Von-Karman radome are also analysed and compared as a wave penetration application. Based on these comparisons, the near-field correction effect is examined on the accuracy of the iterative physical optics scheme.

Retraction Note to: Modelling and verification of white light oil immersion microsphere optical nanoscope

Microsphere has been demonstrated for imaging resolution improvement based on optical microscope, and applied in many engineering and biological fields successfully, while its mechanism is still inconclusive due to the complex physics of both near-field and far-field optics involved. In this paper, a comprehensive simulation model based on point spread function is built to develop a physical insight into microsphere optical nanoscope by numerical simulation. With this model, the Fresnel-region focusing of microsphere lens is studied, which shows the contradiction between focal spot size and focal length. Furthermore, the gap between microsphere and the sample surface is investigated by numerical simulation and experimental verification. As increasing of this gap, the image resolution declines rapidly, and the image becomes blur more and more. At last, a customized sample with a flower pattern etched on silicon substrate is demonstrated with microsphere optical nanoscope, and its feature sizes of 51 nm line width are imaged clearly. While it is generally accepted that microsphere optical nanoscope can be implemented in nano-imaging, these findings quantified the limit on image resolution of this setup, which serves as a general guideline in system design of the other microsphere or micro-particle assisted microscope/nanoscope.

Imaging of surface plasmon polaritons in low-loss highly metallic titanium nitride thin films in visible and infrared regimes.

Titanium nitride (TiN) has been identified as a promising refractory material for high temperature plasmonic applications such as surface plasmon polaritons (SPPs) waveguides, lasers and light sources, and near field optics. Such SPPs are sensitive not only to the highly metallic nature of the TiN, but also to its low loss. We have formed highly metallic, low-loss TiN thin films on MgO substrates to create SPPs with resonances between 775-825 nm. Scanning near-field optical microscopy (SNOM) allowed imaging of the SPP fringes, the accurate determination of the effective wavelength of the SPP modes, and propagation lengths greater than 10 microns. Further, we show the engineering of the band structure of the plasmonic modes in TiN in the mid-IR regime and experimentally demonstrate, for the first time, the ability of TiN to support Spoof Surface Plasmon Polaritons in the mid-IR (6 microns wavelength).

Strong Coupling of Carbon Quantum Dots in Plasmonic Nanocavities.

Confining light in extremely small cavities is crucial in nanophotonics, central to many applications. Employing a unique nanoparticle-on-mirror plasmonic structure and using a graphene film as a spacer, we create nanoscale cavities with volumes of only a few tens of cubic nanometers. The ultracompact cavity produces extremely strong optical near-fields, which facilitate the formation of single carbon quantum dots in the cavity and simultaneously empower the strong coupling between the excitons of the formed carbon quantum dot and the localized surface plasmons. This is manifested in the optical scattering spectra, showing a magnificent Rabi splitting of up to 200 meV under ambient conditions. In addition, we demonstrate that the strong coupling is tuneable with light irradiation. This opens new paradigms for investigating the fundamental light emission properties of carbon quantum dots in the quantum regime and paves the way for many significant applications.

Quantum Walk and Dressed Photon

A physical model called dressed photons, a composite system of photons and excitation of electrons in the nano-particles, is effectively used in the realm of near-field optics. Many interesting behaviors of dressed photons are known, especially the rapid energy transfer and the accumulation to singular points, e.g., points with strong dissipation. We propose a new modelling of dressed photons based on quantum walks, especially Grover walks on semi-infinite graphs which we call jellyfish graphs, and show a universal accumulation phenomena around the point with strong dissipation.

Plasmon Localization Assisted by Conformal Symmetry

Plasmonic systems have attracted remarkable interest due to their application to the subwavelength confinement of light and the associated enhancement of light-matter interactions. However, this requires light to dwell at a given spatial location over timescales longer than the coupling rate to any relevant loss mechanism. Here we develop a general strategy for the design of stopped-light plasmonic metasurfaces, by taking advantage of the conformal symmetry which underpins near-field optics. By means of the analytical technique of transformation optics, we propose a class of plasmonic gratings which is able to achieve ultra-slow group velocities, effectively freezing surface plasmon polaritons in space over their whole lifetime. Our method can be universally applied to the localization of polaritons in metallic systems, as well as in highly doped semiconductors and even two-dimensional conductive and polar materials, and may find potential applications in nano-focusing, nano-imaging, spectroscopy and light-harvesting.

<p style="text-align: left;">History, current developments, and future directions of near-field optical science

This paper reviews the science of the optical near-field (ONF), which is created and localized in a nanometer-sized material (NM) or on its surface. It is pointed out that work on near-field optics was started in order to break through the diffraction limit in optical microscopy and had already come to an end without giving answers to the essential questions on the origin of the near-field optical interaction. However, recent studies have reincarnated these studies and identified the ONF as an off-shell quantum field. Based on this identification, a novel science called off-shell science has started on the basis that the dispersion relation between energy and momentum is invalid for the ONF. This quantum field is called the dressed photon because it is created as a result of the interaction between photons and electrons (or excitons) in a NM and, thus, it accompanies the energies of electrons or excitons. In reviewing current developments, this paper presents fifteen novel phenomena that are contrary to the common views in conventional optical science. Novel technologies developed by applying these phenomena are also reviewed. These include: nanometer-sized optical devices, nano-fabrication technology, and energy conversion technology. High-power Si light emitting diodes, Si lasers, and SiC polarization rotators are reviewed as examples of electrical to optical energy conversion. For future directions, this paper also reviews novel theoretical studies that have commenced recently by relying on physical and mathematical bases.

Paradoxa in the Blueprint of the Visual Organ: Their Contribution to ‘Intelligent’ Vision

The development of the eyes is a product of the brain; the development of the central visual nerve tract which leads from the eyes to the brain is a product of the eyes and the development of the six extraocular muscles for the oculo-motoric control of the eyes is a further product of the brain. Therefore, the human visual system is a product of the brain and the eyes. Paradoxical constructions are part of the blueprint. These are not self-explanatory and are not immediately recognizable as intelligent solutions. Some of these solutions have been labelled as a ‘malperformance of nature’, others are paradoxically misinterpreted. Some paradoxa are discussed before the grating-optical 'cortical’ information processing in the three nuclear layers of the retina - the ‘brain in the eye’ - are looked at. The paradoxical construction of the visual nerve tract leading via the chiasm of the optic nerves (Chiasma Opticum) to the two CGL (Corpus Geniculatum Laterale) and to V1 (area 17) is based on this. At the same time, the ‘brain in the eye’ becomes the decisive basis of vision and the central visual nerve tract becomes a prominent organ of balance with a sensor and a motor function. Geometric optics and diffractive grating interference Near-field optics play an important part here as do the coordinate systems and the axis-centered symmetry operations.

Probing optical resonances of silicon nanostructures using tunable-excitation Raman spectroscopy.

Optical materials with a high refractive index enable effective manipulation of light at the nanoscale through strong light confinement. However, the optical near field, which is mainly confined inside such high-index nanostructures, is difficult to probe with existing measurement techniques. Here, we exploit the connection between Raman scattering and the stored electric energy to detect resonance-induced near-field enhancements in silicon nanostructures. We introduce a Raman setup with a wavelength-tunable laser, which allows us to tune the Raman excitation wavelength and thereby identify Fabry-Pérot and Mie type resonances in silicon thin films and nanodisk arrays, respectively. We measure the optical near-field enhancement by comparing the Raman response on and off resonance. Our results show that tunable-excitation Raman spectroscopy can be used as a complimentary far-field technique to reflection measurements for nanoscale characterization and quality control. As proof-of-principle for the latter, we demonstrate that Raman spectroscopy captures fabrication imperfections in the silicon nanodisk arrays, enabling an all-optical quality control of metasurfaces.

Visible light-induced thymine dimerisation based on large localised field gradient by non-uniform optical near-field

The localised excitations of several molecular reactions utilising optical irradiation have been studied in the field of molecular physics. In particular, deoxyribonucleic acid (DNA) strands organise the genetic information of all living matter. Therefore, artificial methods for freely controlling reactions using only light irradiation are highly desirable for reactions of these strands; this in regard with artificial protein synthesis, regional genetic curing, and stochastic analysis of several genetic expressions. Generally, DNA strands have strong absorption features in the deep ultra-violet (DUV) region, which are related to the degradation and reconstruction of the strand bonding structures. However, irradiation by DUV light unavoidably induces unintended molecular reactions which can damage and break the DNA strands. In this paper, we report a photo-induced molecular reaction initiated by the irradiation of DNA strands with visible light. We utilised photo-dissociation from the vibrational levels induced by non-uniform optical near-fields surrounding nanometric Au particles to which DNA strands were attached. The results were experimentally observed by a reduction in the DUV absorbance of the DNA strands during irradiation. There was a much higher yield of molecular reactions than expected due to the absorbance of visible light, and no defects were caused in the DNA strands.

Inhomogeneous Current Injection and Filamentary Lasing of Semipolar (2021¯) Blue GaN‐Based Vertical‐Cavity Surface‐Emitting Lasers with Buried Tunnel Junctions

Blue semipolar vertical‐cavity surface‐emitting lasers with a buried tunnel junction current aperture are demonstrated under continuous‐wave operation with a differential efficiency of 4% and a threshold current of 2.7 mA for a lasing mode at 452 nm. The effects of the aperture diameter on these 9λ cavity length devices are presented, showing that the differential efficiency increases with aperture size, whereas the threshold current density remains constant for apertures larger than 10 μm. Filamentary lasing is observed in the larger aperture sizes, and it is suggested that this mode behavior is due to current injection inhomogeneity across the aperture. This theory is supported by the correlation between optical nearfield images and thermal microscopy images.

A multiscale-multiphysics integrated model to investigate the coupling effects of non-uniform illumination on concentrated photovoltaic system with nanostructured front surface

Nanostructured front surface and concentrator can improve the photovoltaic performance by absorbing and concentrating more solar light. However, they lead to the non-uniform distribution of the absorbed solar radiation (Ia), which has a great influence on the concentrated photovoltaic (CPV) system. In CPV system with nanostructured front surface, there are the complicated multiscale-multiphysics processes, which include geometrical optics (concentrator), near-field optics (nanostructured front surface), photoelectric conversion (photovoltaics), and heat transfer (photovoltaic module). Hence, a multiscale-multiphysics integrated mathematical model is developed and applied in the physical model of a CPV system, which employs a linear Fresnel lens and a photovoltaics with the moth-eye nanostructured front surface. Six kinds of surface structures with different anti-reflection characteristics are chosen in this study. Investigation found that, with the decrease of the average reflectance of the nanostructure, the multiscale-multiphysics effects of the non-uniform Ia on the photovoltaic characteristics are enhanced. Moreover, the higher temperature difference caused by the nanostructure with lower reflectance is one of the most important reasons for the difference between photovoltaic characteristics for the non-uniform Ia and uniform Ia. Therefore, the nanostructures with lower reflectance are not definitely good for CPV system. In order to reduce the effects of photovoltaic temperature with non-uniform distribution, it has been studied that the variation of the CPV performance with the bottom convective heat transfer coefficient (hb). It is found that when the reflectance of nanostructure is too low, the multiscale-multiphysics effect of non-uniformity Ia is enhanced as the uniform hb increases, the “turning” point of temperature appears, which is caused by the non-uniform distribution of temperature. Finally, a normal distribution of the hb is applied for reducing temperature difference. It is found that, with the increase of the maximum in normal distribution of hb, the temperature difference does decrease, but the CPV performance decrease due to higher average temperature of CPV. Therefore, the design of cooling system for CPV system should focus on the cooling capacity rather than the distribution.

Dual-color near-field imaging by means of thin-film plasmonic waveguide with precise beam control of multiple wavelengths

We describe an improved optical beam control technique combined with a waveguide probe for imaging optical near-field using multiple wavelengths. The beam control technique varies the incident angle of pseudocollimated beams whose spot diameter is ∼10 μm such that it is equal to an arbitrary angle with ∼0.03° precision and the beam position to ∼0.1 μm precision. This helps resonantly excite surface plasmons using visible 660 nm and near-infrared 850 nm on an ∼10 μm width three-dimensional microwaveguide. We demonstrate the operation of this system by imaging the optical near-field and capturing the interference of surface plasmons in a gold microstructure. This system achieved dual-color imaging of the optical near-field of different modulation frequencies with submicrometer lateral spatial resolution.

All optical dynamic nanomanipulation with active colloidal tweezers

Manipulation of colloidal objects with light is important in diverse fields. While performance of traditional optical tweezers is restricted by the diffraction-limit, recent approaches based on plasmonic tweezers allow higher trapping efficiency at lower optical powers but suffer from the disadvantage that plasmonic nanostructures are fixed in space, which limits the speed and versatility of the trapping process. As we show here, plasmonic nanodisks fabricated over dielectric microrods provide a promising approach toward optical nanomanipulation: these hybrid structures can be maneuvered by conventional optical tweezers and simultaneously generate strongly confined optical near-fields in their vicinity, functioning as near-field traps themselves for colloids as small as 40 nm. The colloidal tweezers can be used to transport nanoscale cargo even in ionic solutions at optical intensities lower than the damage threshold of living micro-organisms, and in addition, allow parallel and independently controlled manipulation of different types of colloids, including fluorescent nanodiamonds and magnetic nanoparticles.