Local Field Enhancement Research Articles

Tailored engineering of crystalline surface enabled by ion-irradiation-assisted femtosecond laser ablation

Ion irradiation and femtosecond laser ablation (FLA) are powerful technologies for micro-/nano-machining of transparent materials. In this work, we demonstrate selective surface engineering of optical crystal surface via ion irradiation and subsequent FLA, namely ion-irradiation-assisted FLA. Based on the material modification effects in the ion-irradiated layers, different types of surface structuring characterized by grooves, nanogratings or sub-micron tracks are selectively induced by FLA. It is revealed that the ion-electron interaction induced localized lattice defects and related property modulation in target crystal play important roles in the formation and evolution of laser ablation regimes. Furthermore, the formation process of high-spatial-frequency nanograting is illustrated with the periodical enhancement of local field through the excitation of surface plasmon polaritons, which is experimentally supported through the measurements of transmission electron microscope and energy-dispersive spectroscopy. Our findings further clarify the ion- and laser-matter interactions and the correlation between these processes and surface modifications. The approach proposed in this work shows potential applications in the rapid fabrication of hybrid and versatile surface structures on crystalline materials.

Self-assembly of smooth muscle myosin filaments: adaptation of filament length by telokin and Mg·ATP.

The contractile apparatus of smooth muscle is malleable to accommodate stress and strain exerted on the muscle cell and to maintain optimal contractility. Structural lability of smooth muscle myosin filaments is believed to play an important role in the cell's malleability. However, the mechanism and regulation of myosin filament formation is still poorly understood. In the present in vitro study, using a static light scattering method, length distributions were obtained from suspensions of short myosin filaments (SFs) formed by rapid dilution or long ones (LFs) formed by slow dialysis. The distributions indicated the presence of dynamic equilibriums between soluble myosin and the SFs; i.e.: trimers, hexamers and mini filaments, covering the range up to 0.75µm. The LFs were more stable, exhibiting favorable sizes at about 1.25, 2.4 and 4.5µm. More distinct distributions were obtained from filaments adsorbed to a glass surface, by evanescent wave scattering and local electric field enhancement. Addition of telokin (TL) to the suspensions of unphosphorylated SFs resulted in widening of the soluble range, while in the case of the LFs this shift was larger, and accompanied by reduced contribution of the soluble myosin species. Such changes were largely absent in the case of phosphorylated myosin. In contrast, the presence of Mg·ATP resulted in elongation of the filaments and clear separation of filaments from soluble myosin species. Thus, TL and Mg·ATP appeared to modify the distribution of myosin filament lengths, i.e., increasing the lengths in preparing for phosphorylation, or reducing it to aid dephosphorylation.

Transition from Phononic to Geometrical Mie Modes Measured in Single Subwavelength Polar Dielectric Spheres

Spherical dielectric resonators are highly attractive for light manipulation, thanks to their intrinsic electric and magnetic resonances. Here, we present measurements of the mid-infrared far-field thermal radiation of single subwavelength dielectric spheres deposited on a gold substrate, of radii ranging from 1 to 2.5 μm, which agree quantitatively with simulated absorption cross sections. For SiO2 microspheres, we evidence the excitation of both surface phonon-polariton (SPhP) modes and geometrical electric and magnetic Mie modes. The transition from a phonon-mode-dominated to a Mie-mode-dominated emission spectrum is observed, with a threshold radius of ∼1.5 μm. We also show that the presence of the metallic substrate augments the computed spheres absorption cross-section due to increased local field enhancement, arising from the near-field interaction of the spheres oscillating charges with their image in the metallic mirror. In contrast, measurements of single subwavelength SPhP-inactive PTFE spheres reveal that the mid-infrared response of such lossy spheres is dominated by their bulk absorption. Our results demonstrate how engineering the geometrical and dielectric properties of subwavelength scatterers can enable the control of thermal emission near room temperature, with exciting perspectives for applications such as radiative cooling.

Efficient Excitation and Tuning of Multi-Fano Resonances with High Q-Factor in All-Dielectric Metasurfaces.

Exciting Fano resonance can improve the quality factor (Q-factor) and enhance the light energy utilization rate of optical devices. However, due to the large inherent loss of metals and the limitation of phase matching, traditional optical devices based on surface plasmon resonance cannot obtain a larger Q-factor. In this study, a silicon square-hole nano disk (SHND) array device is proposed and studied numerically. The results show that, by breaking the symmetry of the SHND structure and transforming an ideal bound state in the continuum (BIC) with an infinite Q-factor into a quasi-BIC with a finite Q-factor, three Fano resonances can be realized. The calculation results also show that the three Fano resonances with narrow linewidth can produce significant local electric and magnetic field enhancements: the highest Q-factor value reaches 35,837, and the modulation depth of those Fano resonances can reach almost 100%. Considering these properties, the SHND structure realizes multi-Fano resonances with a high Q-factor, narrow line width, large modulation depth and high near-field enhancement, which could provide a new method for applications such as multi-wavelength communications, lasing, and nonlinear optical devices.

Local Field Enhancements of Periodical Metallic Powders in Microwaves

The temperature spike or sparking is vital to the catalytic effect with metallic powders in microwaves, but the mechanism is still unclear. In this article, the local field enhancement (LFE) of periodical metallic powders in microwaves is studied. It shows that microwaves can get through the separations among metallic powders with the LFE, which provides a preliminary understanding of the temperature spike or sparking among metallic powders. First, for simplicity, metallic powders are simplified to 2-D periodical metallic bars, which can be analyzed by the transmission line model and full-wave simulations. When the incident electric field is perpendicular to the separations among metallic bars, the electric field can be compressed. Specifically, even with the volume fraction of 99% metals, microwaves can pass through the separations well with intensive LFEs. Then, metallic powders are simplified to 3-D periodical spherical metallic powders. In this case, microwaves propagate at the slow wave mode with the LFE. Finally, the proposed models are validated by measuring reflectivity and observing the discharge in neon (Ne) gas at atmosphere in a rectangular waveguide at the TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sub> mode. This work provides a way of controlling LFEs for enhancing reaction reactivity and creating plasma.

Cap‐Like Ellipsoid Deposition on the ZnO Nanowires for Large Increase of Fluorescence Induced by Localized Field Enhancement Effect (Crystal Research and Technology 7/2022)

Cap‐Like Ellipsoid Deposition on the ZnO Nanowires for Large Increase of Fluorescence Induced by Localized Field Enhancement Effect (Crystal Research and Technology 7/2022)

Nanophysics in modern medicine

WNanophysics is rapidly developing for the recent couple of decades. Unique physical properties of materials at the nanoscale are the reason for this rapid development. Ideas, materials and structures of nanophysics have found their wide application in related fields of modern science, including biology and medicine. This short review is devoted to the application of nanophysics in modern medicine. The main focus was on application of ideas and physical phenomena of nanophysics in oncology and antiviral therapy. We have focused on the use of nanosystems both for tumor imaging and for the struggle against some types of tumors. The use of nanoparticles as nanocontainers for targeted drug delivery was briefly discussed. We also demonstrated how the effects of nanophysics can be used to develop new non-traditional methods of antiviral therapy. The focus of these methods was the idea of physical (field) action of nanoparticles on the viruses, which is based on the local-field enhancement effect that is the reason of ponderomotive forces acting on the viruses up to destruction of viral envelopes.

Flexible nanoparticle-on-mirror strategy for ultrasensitive molecule detection by directionally coupling surface plasmon polaritons

Applying local surface plasmon resonance (LSPR) to a nanoparticle-on-mirror (NPOM) configuration for realizing highly sensitive molecule detection using surface-enhanced Raman scattering (SERS) typically enhances the Raman signal by 106–1010 under limiting conditions. We propose a special nanoparticle-fishbone structure (NPFS) system, which incorporates the surfaceplasmon polariton (SPP) coupler of the fishbone structure into the NPOM configuration, to achieve ultra-high local electric field enhancement by plasmon hybridizing the LSPR mode of metallic nanoparticles and the SPP mode of the fishbone array structure. During the numerical simulation, the NPFS system exhibited a SERS enhancement factor of 1011, which is two orders of magnitude higher than that of the NPOM system; the SERS experimental results further demonstrated its outstanding performance. These findings indicate that introducing a well-designed SPP coupler on the metallic film can further improve the advantages of various NPOM configurations. The proposed approach can open new horizons for designing nanoscale photonic devices for ultrasensitive molecule detection.

A polarization-resolved ECL strategy based on the surface plasmon coupling effect of orientational Au nanobipyramids patterned structures

Here, we developed a polarization-resolved surface plasmon coupling electrochemiluminescence (ECL) strategy based on orientational Au nanobipyramids (Au NBPs) patterned structures. As the surface plasmonic materials, the Au NBPs with large localized electromagnetic field enhancement were used to construct the different patterned structures through the droplet evaporation induced assembly. The distinct morphology of the horizontal and vertical Au NBPs patterned structures provided the different plasmonic properties reflecting on resonance peak positions and polarized absorbance. As a result, the variations of the polarized ECL emission of SnS2 QDs have been regulated by the Au NBPs patterned structures via the surface plasmon coupling progress. Because of the superior polarization-resolved ECL performances, the sensing system was used to analyze the miRNA-21 and miRNA-205 expression level in tumor tissues of triple negative breast cancer patients. The satisfactory results indicated this polarization-resolved sensing strategy possessed great potential for clinical analysis.

Active metal–graphene hybrid terahertz surface plasmon polaritons

Abstract Surface plasmon polaritons (SPPs) are propagating electromagnetic surface waves with local electric field enhancement and nondiffraction limit at optical frequencies. At terahertz (THz) frequencies, a metal line with periodic grooves can mimic the optical SPPs with the same high cut-off response, which is referred to as designer SPPs. Here, by replacing metal grooves with graphene sheets, a novel active metal–graphene hybrid SPP device achieves significant phase modulation. Theoretically, the dispersion curves of THz SPPs are determined by the dimensions and periodicity of the grooves. Changing the chemical potential of graphene sweeps the effective groove depth, which correspondingly shifts the SPP cut-off frequency and modulates the slow-wave phase. A prototype device is fabricated and characterized under varying bias applied for graphene. The experiment demonstrates that the cut-off frequency red shifts from 200 to 177 GHz, and the phase variation is as large as 112° at 195 GHz under a low bias from −0.5 to 0.5 V. Simultaneously, the SPP transmittance is modulated by a factor of more than 3 dB from 140 to 177 GHz due to the graphene absorption. The proposed structure reveals a novel approach to study the nonreciprocal and topological SPPs with active modulation in the THz range.

Strong Field Enhancement and Unidirectional Scattering Based on Asymmetric Nanoantenna

Dielectric-metal nanostructures have lately emerged as one of the most promising approaches to modulating light at the optical frequency. Their remarkable electric and magnetic resonances give them a one-of-a-kind ability to augment local field enhancements with negligible absorption losses. Here, we propose a hybrid metal-dielectric-metal (MDM) nanoantenna that contains a dimer of three-layers of shell nanoparticles. In addition, we only theoretically and numerically show the optical properties of the hybrid dimer nanoantenna. We found that the nanoantenna sustained unidirectional forward scattering with narrow beamwidth (30.9 deg) and strong scattering intensity (up to 5 times larger than the single MDM particle). Furthermore, when the hybrid asymmetric dimer was excited by the plane wave with different electric polarization directions, our findings revealed that the hybrid nanoantenna boosted the gap’s electric near-field while also supporting unidirectional forward scattering. Finally, we analyzed the hybrid dimer with substrates of different materials. It supported strong electric high-order moments along the z-axis and x-axis in gaps between MDM nanoparticles and between MDM nanoparticles and the Ge substrate, owing to the intense displacement currents inside of the dielectric layer. We found that the local electric field of this MDM hybrid dimer nanoantenna with Ge substrate was well improved and attained 3325 v/m.

Ultra-narrow band perfect absorbance induced by magnetic lattice resonances in dielectric dimer metamaterials

Nanostructured dielectric metamaterials have received extensive attention in the field of nanophotonics owing to their low radiative losses and coexisting electric and magnetic lattice resonance features. Unfortunately, suffering from the poor electromagnetic field localization and weak magnetic response in the typical dielectric metamaterials, it remains challenging to simultaneously realize ultra-narrow band perfect absorbance and intensified electromagnetic field resonances. Herein, we theoretically demonstrate a kind of dielectric metamaterials formed by dielectric cylindrical dimer array that supports magnetic lattice resonances. Benefiting from the collective diffraction coupling among the powerful magnetic dipole resonance in the dielectric dimer array, the proposed dielectric metamaterials synchronously manifest ultra-narrow spectral characteristics with bandwidth less than 8 nm, perfect absorbance amplitude as high as 99.7% and strong electric/magnetic field enhancement factor. The effects of the structure parameters on the optical properties of the proposed nanostructure are investigated based on numerical simulations. The linewidth of absorbance spectrum can be narrowed down to approximately 3 nm with optimal design. These excellent optical features supported by the dielectric dimer metamaterials can be explored as a high-efficiency refractive index sensor with sensitivity of 824 nm/RIU and figure of merit as high as 242 RIU−1. This work paves an exciting way for narrow band perfect absorbance and localized field enhancement, exhibiting tremendous enormous potential in biochemical sensing, surface enhanced spectroscopy, and nonlinear nanophotonics.

Surface quality characterization of thin Nb films for superconducting radiofrequency cavities

Superconducting radiofrequency (SRF) cavities are vital components of particle accelerators nowadays. In order to minimise the energy dissipation, a perfect inner surface of the cavity, hindering the penetration of magnetic field, is required. In this work, we investigated ten planar samples differing in the surface quality of Nb film deposited on Cu substrate, and as a consequence exhibiting various levels of the first entry field, H en, at which the magnetic field starts to enter the film. The observed surface defects are categorised as hills, pits and cracks. For a practical range of dimensions of these features, the factor β, characterising the local magnetic field enhancement, was calculated by the numerical finite-element simulations. It is expected that the local field enhancement causes a premature penetration of the magnetic field, thus lowering H en. Then, for each investigated sample, the range of β values characterising defect type that cause the highest field enhancement, is identified and compared with the H en fields. We have found that the H en of the samples that contain multiple types of the surface features is indeed limited by those defects that cause the highest field enhancement. The H en vs β dependence has shown a good match with linear fit for the set of investigated samples. Thus, the main result is that the local magnetic field enhancement, computed in a straightforward way for the most significant defects, is a strong indicator of the surface quality that is relevant for the superconducting film intended for SRF cavity application.

Different coupling level induced mode shift of plasmon resonance in Au/ZnO heterojunction

Plasmon resonance properties of Au nanoparticle embedded in ZnO thin film are simulated by COMSOL Multiphysics software in Wave Optics Module. The absorption spectra, surface electric field distribution and the dielectric constant of heterojunction structures with semi and fully embedded Au nanoparticles are investigated. For the fully embedded situation, the local field enhancement caused by resonance is several times more than that of the semi-embedding case. We deduce the parameter A theoretically described size and interface effects in the dielectric constant equation to explain the coupling effect. The results confirm that ε′(ω)/ε″(ω) can be determined by function A, which illustrates that surface plasmon resonance (SPR) mode is transformed into local surface plasmon resonance (LSPR) mode. The resonance hot-spots are gradually formed at the surface of Au nanoparticle, which can be confirmed by calculating the increasingly penetration depth and reach its maximum value at thickness of 150 nm, indicating that the formation of LSPR can be verified. The exploration on the resonance reaction mechanism determined by the function A will promote the development of theoretical system and further application in semiconductor photoluminescence, photocatalysis and surface-enhanced Raman scattering devices.

Holographic Manipulation of Nanostructured Fiber Optics Enables Spatially‐Resolved, Reconfigurable Optical Control of Plasmonic Local Field Enhancement and SERS (Small 23/2022)

Plasmonics In article number 2200975, Liam Collard, Massimo De Vittorio, Ferruccio Pisanello, and co-workers use wavefront engineering to harness the resonant properties of plasmonic nanostructures on the tip of a multimode fiber and spotlight specific resonances across the dispersion diagram to enable spatially-resolved surface enhanced Raman spectroscopy and reconfigurable beam steering. [Cover image: artwork by Filippo Pisano].

Near-field nano-spectroscopy of strong mode coupling in phonon-polaritonic crystals

Strongly coupled phonon polaritons in patterned polar dielectric nano-resonators give rise to the formation of hybridized energy states with intriguing properties. However, direct observation of mode coupling in these periodic nanostructures is still challenging for momentum-matching-required far-field spectroscopies. Here, we explore the near-field response of strong coupling between propagating and localized polariton modes sustained in SiC phonon polaritonic crystals (PhPCs) to reveal the evolution of Rabi splittings with the change of lattice constant in the near-field perspective. The near-field nano-spectra of PhPCs show distinct Rabi splitting near the forbidden bands of ∼16 cm−1 in the band structures. In particular, an exotic three-polariton-coupling effect is observed with three splitting peaks in the nano-spectra induced by the interaction between local monopolar modes in nano-pillars and zone-folded phonon polaritons. Furthermore, sharp dips indicating weak near-field scatterings appear in nano-spectra at the intrinsic frequencies of the monopolar modes with strong local-field enhancement, which are estimated to be bright scattering peaks intuitively. These results would inspire the dispersion engineering and characterization of coupled phononic nano-resonators for diverse nanophotonic applications.

Wavelength Modulation Characteristics of Metal Gratings on Si-Based Blocked-Impurity-Band (BIB) Terahertz Detectors.

In this work, the wavelength selection characteristics of metal gratings on Si-based blocked-impurity-band (BIB) detectors in the terahertz band were studied by performing experiments and a finite difference time domain (FDTD) simulation. The transmission spectra of metal gratings with different periods on 130 μm intrinsic Si substrates were measured. When the metal grating period increased from 16 to 20 to 32 μm, the peak position of the spectrum moved from 21.71 to 24.50 to 36.59 μm, which is in good agreement with the FDTD simulation results. The structure with the period of 32 μm shows the best wavelength selective transmission characteristics. Then, the bare Si-based BIB devices and metal grating/Si-based BIB hybrid devices with different thicknesses of blocking layers of 2 and 5 μm were fabricated. By covering different periods of metal gratings for the devices with a thicker blocking layer of 2 μm, we obtained more effective wavelength selection characteristics and stronger response spectra enhancement ratios that were about 1.3, 2.4, or 1.9 times. This was mainly due to the localized optical field enhancement effect of the plasmons resonance in metal gratings, which decays exponentially in a vertical direction. Our results demonstrate a new approach for the Si-based BIB detector to realize multiband selective detection applications.

Challenges and prospects of plasmonic metasurfaces for photothermal catalysis

AbstractSolar-thermal technologies for converting chemicals using thermochemistry require extreme light concentration. Exploiting plasmonic nanostructures can dramatically increase the reaction rates by providing more efficient solar-to-heat conversion by broadband light absorption. Moreover, hot-carrier and local field enhancement effects can alter the reaction pathways. Such discoveries have boosted the field of photothermal catalysis, which aims at driving industrially-relevant chemical reactions using solar illumination rather than conventional heat sources. Nevertheless, only large arrays of plasmonic nano-units on a substrate, i.e., plasmonic metasurfaces, allow a quasi-unitary and broadband solar light absorption within a limited thickness (hundreds of nanometers) for practical applications. Through moderate light concentration (∼10 Suns), metasurfaces reach the same temperatures as conventional thermochemical reactors, or plasmonic nanoparticle bed reactors reach under ∼100 Suns. Plasmonic metasurfaces, however, have been mostly neglected so far for applications in the field of photothermal catalysis. In this Perspective, we discuss the potentialities of plasmonic metasurfaces in this emerging area of research. We present numerical simulations and experimental case studies illustrating how broadband absorption can be achieved within a limited thickness of these nanostructured materials. The approach highlights the synergy among different enhancement effects related to the ordered array of plasmonic units and the efficient heat transfer promoting faster dynamics than thicker structures (such as powdered catalysts). We foresee that plasmonic metasurfaces can play an important role in developing modular-like structures for the conversion of chemical feedstock into fuels without requiring extreme light concentrations. Customized metasurface-based systems could lead to small-scale and low-cost decentralized reactors instead of large-scale, infrastructure-intensive power plants.

Observation of in-plane exciton–polaritons in monolayer WSe2 driven by plasmonic nanofingers

Abstract The transition metal dichalcogenides (TMDs) have drawn great research attention, motivated by the derived remarkable optoelectronic properties and the potentials for high-efficient excitonic devices. The plasmonic nanocavity, integrating deep-sub wavelength confinement of optical mode with dramatic localized field enhancement, provides a practical platform to manipulate light–matter interaction. In order to obtain strong exciton–plasmon coupling effects, it’s crucial to match the vibration direction of exciton to the available strong localized in-plane electric field. Herein, we demonstrate the coupling effect of in-plane exciton in monolayer tungsten diselenide (WSe2) to deterministic gap-plasmon field which is produced by nanometrically gapped collapsed nanofingers. The gap-plasmon field which is completely parallel to the in-plane excitons in WSe2 will drive a strong exciton–plasmon coupling at room temperature. More interestingly, it is experimentally observed that the luminescence of exciton–polariton cannot be influenced by the temperature in the range from 77 K to 300 K due to the presence of nanofingers. According to the theoretical analysis results, we attribute this finding to the dielectric screening effect arising from the extremely strong localized electric field of plasmonic nanofingers. This work proposes a feasible way to harness and manipulate the exciton of low-dimensional semiconductor, which might be potential for quantum optoelectronics.

Dimensional Design for Surface-Enhanced Raman Spectroscopy.

Surface-enhanced Raman spectroscopy (SERS) is a vibrational spectroscopy technique that enables specific identification of target analytes with sensitivity down to the single-molecule level by harnessing metal nanoparticles and nanostructures. Excitation of localized surface plasmon resonance of a nanostructured surface and the associated huge local electric field enhancement lie at the heart of SERS, and things will become better if strong chemical enhancement is also available simultaneously. Thus, the precise control of surface characteristics of enhancing substrates plays a key role in broadening the scope of SERS for scientific purposes and developing SERS into a routine analytical tool. In this review, the development of SERS substrates is outlined with some milestones in the nearly half-century history of SERS. In particular, these substrates are classified into zero-dimensional, one-dimensional, two-dimensional, and three-dimensional substrates according to their geometric dimension. We show that, in each category of SERS substrates, design upon the geometric and composite configuration can be made to achieve an optimized enhancement factor for the Raman signal. We also show that the temporal dimension can be incorporated into SERS by applying femtosecond pulse laser technology, so that the SERS technique can be used not only to identify the chemical structure of molecules but also to uncover the ultrafast dynamics of molecular structural changes. By adopting SERS substrates with the power of four-dimensional spatiotemporal control and design, the ultimate goal of probing the single-molecule chemical structural changes in the femtosecond time scale, watching the chemical reactions in four dimensions, and visualizing the elementary reaction steps in chemistry might be realized in the near future.