Dipolar Plasmon Research Articles

Enhanced dual-band absorption of graphene mediated by an aluminum metastructure

Graphene absorption from the visible to infrared spectrum has great potential and broad applications in miniature of modern optoelectronic biosensors and photodetectors. However, graphene has zero bandgap energy, which limits its absorption to 2.3% in the visible and infrared spectrums. Here, we propose a metastructure to optimize graphene absorption in the visible to near-infrared frequency regions. The metastructure, comprising an array of aluminum square blocks (Al-SBs) on a graphene layer, a silica spacer, and an Al reflector, is investigated for absorption enhancement. This work deciphers the effect of the periodicity of decorated Al-SBs on the evolution of dual-band absorption in single-layer graphene under normal incidence. The electromagnetic signatures of two excited modes indicate that surface plasmons and magnetic dipole plasmons are mediators of absorption. The investigation into the impact of geometrical parameters illustrates that the coexisting phenomena of a relative broad peak and a relative sharp peak have been achieved simultaneously with high efficiency. The dynamic manipulation of surface plasmons and magnetic dipole plasmons presents great potential for a diverse range of applications, such as sensing and imaging. By controlling the periodicity of Al-SBs, it is possible to achieve active control of surface plasmon resonance, and a detection range of 300 nm is observed. Dynamic control of the magnetic dipole plasmon is successfully achieved by modifying the electrical environment of the graphene layer, which is realized by altering the underlying spacer material. Collectively, the findings of this study demonstrate the significant potential of the suggested metastructure for its prospective applications in optoelectronic devices, including biosensors, photovoltaics, and photodetectors that rely on the dynamic control of surface and magnetic plasmon resonances.

Dipol'naya plazmonnaya moda v nanorazmernykh poluprovodnikovykh kvantovykh tochkakh tipa "yadro-obolochka" s geteroperekhodom vtorogo roda

Excited states of nanosize two-component semiconductor core–shell crystals with a type II heterojunction are analyzed. It is demonstrated that the dipole plasmon resonance dominates in their photoabsorption spectra. It is found that the variation of the potential barrier height between the core and the shell in a comparatively narrow range leads to a fundamental change in the form of the collective mode from the surface plasmon resonance typical of the photoabsorption spectra of conducting nanosize particles to the rotational plasmon mode, for which only angular degrees of freedom are excited.

Polarization-selective modulation of meandered-line metamaterials with graphene surface plasmonics

Periodic metal lines are the first kind of metamaterials, which typically exhibit plasmonic resonances to the normally incident waves polarized perpendicularly to them, i.e., transverse magnetic (TM) waves. Meandered lines can obtain resonances for both TM and transverse electric (TE) incidences and have found important applications in chirality and polarization conversion. Recently, several stimuli have been investigated to dynamically modulate the optical properties of periodic lines, which simultaneously tune both TE and TM resonances and show little polarization dependence. In this work, metal-graphene hybrid meandered-line metamaterials are designed for polarization-selective modulation at terahertz frequencies for the first time. The periodic lines meandered in S-shape resonate at 103 GHz for TE waves and at 195 and 320 GHz for TM waves, respectively. Graphene strips are patterned in pairs right at the bending corners along the polarization direction of TE waves, which produce even plasmonic dipole mode for TE incidence. In contrast, they are asymmetric with respect to the TM waves and generate odd coupling mode under TM incidence. These modes with various field concentrations exhibit considerable modulation difference to the graphene conductivity variation. As the graphene chemical potential sweeps, TE transmittance is modulated from 0.7 to 0.47 at 100 GHz, and TM transmittance is modulated just from 0.4 to 0.39 at 190 GHz and from 0.57 to 0.52 at 289 GHz, respectively. The corresponding modulation axial ratio is as large as 23 and 4.6 under a low bias from -1.2 to 1.2 V. This work paves a new way to control terahertz waves polarization-selectively.

Multimode hybridizations in surface plasmon resonances excited in terahertz dipole cavities

Inventive design variations in subwavelength plasmonic structures can excite unconventional surface plasmon resonances (SPRs) that can enormously contribute to developing futuristic terahertz (THz) devices and components. Therefore, in this work, we demonstrate multimode resonance features in SPR characteristics through the introduction of split-ring resonators (SRRs) in plasmonic dipole cavities operating in the THz domain. Here, we propose a hybrid plasmonic system, where a single SRR is placed in the middle of a dipole cavity sustaining SPRs in the THz domain. Further, the SRR is displaced in orthogonal directions (horizontally as well as vertically) from the center of the cavity introducing asymmetry in the hybrid SRR–dipole cavity system. In such a way, introduced asymmetry resulted in diverse transmission responses with the realization of multiple SPR peaks. Such multi-SPR features are explicated using two-state and three-state mode hybridization schemes, which arise due to the near-field electric coupling between the SRR and the dipole cavity. Our findings based on multimode SPR characteristics excited in planar dipole cavities can contribute to realizing compact biosensor, absorber, and nonlinear THz devices operating simultaneously in multiple frequency bands.

Bandwidth-tunable absorption enhancement of visible and near-infrared light in monolayer graphene by localized plasmon resonances and their diffraction coupling

Bandwidth-tunable light absorption enhancement in monolayer graphene is practically important for graphene-based photoelectric devices. Especially, it is still a huge challenge to achieve high-efficiency graphene absorption with extremely narrow sub-nanometer bandwidth much smaller than one nanometer. In this work, both broadband and narrowband absorption peaks of monolayer graphene are numerically achieved in visible and near-infrared wavelength ranges. The broadband absorption peak is ascribed to localized dipolar plasmon resonances in individual silver nanodisks, and the narrowband absorption peak arises from collective first-order diffraction coupling effect in periodic array of silver nanodisks. By changing the array period, the full width at half maximum (FWHM) of the broadband absorption peak can vary from 100 nm to 50 nm. Correspondingly, the FWHM of the narrowband absorption peak is tuned from about 6.4 nm to only 0.25 nm, thus realizing an ultra-narrow sub-nanometer bandwidth.

Structurally Engineered Silica Shells on Gold Nanorods for Biomedical Applications

Owing to their unique optical and chemical properties, gold nanorods (AuNRs) are among the most frequently used nanomaterials for biomedical applications, including cancer therapy, imaging, and drug delivery. In particular, the longitudinal dipole plasmon wavelength of AuNRs can be verified from the visible to the near‐infrared (NIR) region, allowing AuNRs to be used as photodynamic/photothermal and imaging contrast agents. At the same time, the silica shell is important as it enhances stability and facilitates the functionalization and biocompatibility of AuNRs, offering numerous advantages in biomedical applications. In this review, silica‐coated AuNRs from a bioapplication perspective are focused. First, the importance of AuNRs for biomedical applications is explained and the purpose of silica coating on AuNRs is discussed. Then, recent studies on the development of silica‐coated AuNRs from a biomedical perspective are reviewed. Subsequently, various strategies for engineering silica coatings and their properties in the biomedical field are reviewed. This review is expected to promote further research on next‐generation silica‐coated AuNRs for biomedical applications.

Dipole plasmon vitalized efficient white light emission via charge transfer in all oxide-based heterojunctions

Solid-state white-light emission, which is rapidly becoming the standard in a wide range of lighting applications, can be regarded as the best possible light source for next-generation cost-effective lighting. Herein, we report a simple solution-based technique to get efficient white-light emission using all-oxide semiconductor-based heterojunction. In this work, vertically oriented CuO nanorods were used for outstanding white-light emission by depositing ZnO thin films over CuO nanorods. The resulted bi-layered ZnO/CuO heterojunction arrangement have shown excellent optoelectronic properties. Furthermore, the inclusion of Au layer between ZnO/CuO heterojunction has improved white-light emission significantly and as an integral, a white light photoluminescence (PL) with a chromaticity coordinate of (0.32, 0.34) was obtained. Ab-initio calculations have supported our mechanism of charge transfer from CuO to ZnO especially with Au insertion. The presented work will provide insights into the intriguing optoelectronic properties of oxide hetero-structures.

Boosting Light–Matter Interaction in a Longitudinal Bonding Dipole Plasmon Hybrid Anapole System

Achieving strong electromagnetic enhancement is critical for realizing strong light–matter coupling at the nanoscale. In this study, we constructed a hybrid anapole system composed of a nanohole silicon disk and a longitudinal bonding dipole plasmon mode-supported plasmonic dimer. Compared with the bare dimer plasmon, the hybrid system shows strong plasmonic resonance tuning ability, and its resonance peak can be tuned to the near-infrared region only by adjusting the radius of the silicon disk. Meanwhile, the E-field enhancement in the gap region can exceed four orders of magnitude without sacrificing the quality factor of the system. Furthermore, it is demonstrated that the emitter’s radiative decay rate enhancement in the hybrid system is much higher than that of a similar LBDP mode-supported plasmonic dimer nanodisk and the reported plasmonic nanocavity. In summary, our hybrid anapole systems combine the advantages of metal plasmonic nanodimers and conventional anapole mode-supported systems and avoid their disadvantages. This study provides a useful reference for the further exploration of single-photon emission sources, light harvesting, and other quantum nanophotonic applications.

Plasmons in a square of two-dimensional electrons

Microwave absorption spectra of a single square of two-dimensional electrons (2DESs) have been investigated using an optical detection technique. Fundamental dipole and harmonic quadrupole plasmon modes have been identified and compared to those in the disk geometry. In the square-shaped 2DES, a strong interaction is discovered between the neighboring plasmon modes, whereas no such hybridization is observed in the disk-shaped geometry. We establish a rigid theoretical platform to analytically describe the magneto-optical response of confined two-dimensional systems. The developed theory provides a proper description of the obtained experimental results.

BioMOF coupled with plasmonic CuNPs for sustainable CO2 fixation in cyclic carbonates at ambient conditions

To improve the actual environmental situation, in closing the loop of carbon dioxide (CO2) cycle by reutilization, the CO2 activation is necessary. The energy inputs for that should be economically viable and renewable, while the catalyst(s) should come from naturally abundant elements and building blocks. Important should be also technological simplicity of CO2 activation and environmental friendliness of used chemicals. In this work, we propose a promising solution for CO2 reutilization based on plasmon-assisted photocatalysis using CuNPs and MOF, based on L-glutamic acid and Zn ions (ZnGlu). ZnGlu crystals were hybridized with CuNPs to introduce plasmon-related (photo)absorption bands at visible wavelengths. ZnGlu is responsible for catalytic activity, triggered by the high-energy field of plasmonic dipoles of CuNPs. The proposed catalyst was used for CO2 cycloaddition with different reactivity forming corresponding carbonates and meets the following criteria: catalyst’s material-copper, light as a renewable energy source, functionality at normal temperature and pressure.

Spin texture and chiral coupling of circularly polarized dipole field

Abstract We show that a circularly polarized electric dipole harbors a near-field concentrated wave which orbits around with an energy flux significantly larger (five orders of magnitudes at ∼1 nm radial distance) than far-field radiation. This near-field wave is found to carry transverse spins and reveal skyrmion spin texture (Néel-type). By performing electromagnetic analysis and numerical simulation, we demonstrate chiral extraction of a near-field rotational energy flux: the confined energy flow is out-coupled to surface plasmons on metal surface, whose curvature is designed to provide orbital angular momentum matched to spin angular momentum of dipole field, that is, to facilitate spin–orbit interaction. Strong coupling occurs with high chiral selectivity (∼113) and Purcell enhancement (∼17) when both linear and angular momenta are matched between dipole field and surface plasmons. Existence of a high-intensity energy flux in the deep-bottom near-field region (r ∼ 1 nm) opens up an interesting avenue in altering fundamental properties of dipole emission. For example, extracting ∼1% of this flux would result in enhancing spontaneous emission rate by ∼1000 times.

Оптическая анизотропия пленок из поливилового спирта с металлическими наностержнями при одноосном растяжении

The possibility of obtaining macroscale anisotropic thin films from polyvinyl alcohol with included metal nanorods under uniaxial tension under conditions that do not lead to a change in the morphology of metal nanoparticles is shown. Silver and gold nanorods were obtained by directed growth from nuclei and then embedded in a polymer matrix based on polyvinyl alcohol. Initially isotropic films with absorption independent of the polarization of the probing light became anisotropic after stretching, which manifested itself in the dependence of the extinction spectra on the polarization of the probing radiation. The weakening of the longitudinal dipole plasmon resonance mode with the simultaneous enhancement of the transverse dipole plasmon resonance when the light polarization is rotated from 0 to 90 degree indicates the orientation of the nanorods in the film along the direction of its stretching. In addition to the change in absorption in the bands of dipole modes, a strong orientational dependence of absorption in the band of the quadrupole mode of the plasmon resonance of metal nanorods was found.

Miniaturized Spoof Plasmonic Antennas with Good Impedance Matching.

The ability of spoof surface plasmon polaritons (SSPPs) to confine electromagnetic fields in a subwavelength regime enables the design of miniaturized antennas. However, the impedance matching scheme for miniaturized spoof plasmonic antennas has not been studied systematically. In this paper, we propose a general method in the antenna design based on SSPPs, providing a feasible solution to impedance matching at the feeding point of miniaturized spoof plasmonic antennas. To verify the method, a prototype of a planar spoof plasmonic dipole antenna is simulated, fabricated and measured, of which the dipole arm length is reduced by 35.2% as compared with the traditional dipole antenna. A peak gain level of 4.29 dBi and the radiation efficiency of about 94.5% were measured at 6 GHz. This general method can be extended to solve the impedance matching problem in the design of other spoof plasmonic devices.

Facile fabrication of Al nanoparticle arrays with continuous tunable diameter and plasmonic band for UV–visible SERS detection

Al-based nanostructures have a huge potential for surface-enhanced Raman scattering (SERS) in area of life science, chemical and food safety because of its excellent plasmonic characteristics from UV to near-infrared ranges. By only controlling the deposited Al thickness onto the parabola-shaped alumina nanopores template, the Al nanoparticle arrays with tunable diameter from 55 nm to 94 nm and plasmonic bands ranging from 285 nm to 717 nm were obtained. With excitation at UV and visible plasmon resonance wavelength (325 nm, 633 nm), the experimental SERS enhancement factor (EF) reached 103 and relative standard deviation (RSD) below 10% on a large scale. Finite-difference time-domain (FDTD) simulations reveal electric field intensity and charge distribution of dipolar and quadrupolar plasmon mode. The simple and controllable fabrication strategy and superior SERS performance in UV–visible range make the Al nanoparticle arrays promising candidates for SERS-based sensor applications. And the method provides a new approach for developing an inexpensive, efficient, and mass-produced Al-based SERS active substrate.

Relevance of Rabi splitting effect for tunable enhancement of Raman scattering in self-assembled silver – Fullerene nanocomposite films

We report on detection of plasmon-enhanced Raman scattering (PERS) in self-assembled AgxC60 nanocomposite films and on evolution of this effect with increase of the Ag concentration, x, in the interval of 2<x < 25. Optical absorption spectra show that origin of the PERS effect is closely related with dipolar plasmonic mode generated by uniform 3D-ensemble of Ag nanoparticles, which arise in the C60-based matrix during the film deposition. Careful analysis of the absorption spectra designates features of strong coupling between dipolar plasmon mode and the C60 low-energy exciton suggesting energy exchange between the arisen high- and low-energy polaritonic states. Evaluation of dispersion spectra of the polaritons revealed anti-crossing effect as a sign of the strong plasmon-exciton coupling with Rabi splitting energy of about 300 meV. In frames of the recently-reported quantum-electrodynamics model, the detected PERS effect was ascribed to abundant population of the low-energy polaritonic states caused by asymmetric transitions between the low- and high-energy polaritons. The obtained PERS-spectra display sensitivity of the C60 vibrational modes to the x-driven plasmonic field strength, which demonstrates some reduction at higher x due to charge transfer through C60 molecular junctions. The evident tunability of PERS suggests an emerging potential of the self-assembled AgxC60 films for their application in molecular electronics and molecular photochemistry.

Broadband reflective dual-functional polarization convertor based on all-metal metasurface in visible region

In this paper, a broadband reflective dual-functional polarization convertor based on all-metal anisotropic metasurface was proposed and investigated numerically in visible region. The unit-cell of the all-metal anisotropic metasurface based polarization convertor is composed of the Au ellipse nanostructure adhered on an Au substrate. The proposed anisotropic metasurface can convert the incident linear polarization light into its orthogonal counterpart or circular polarization light by adjusting the geometric parameter of the ellipse nanostructure. When the all-metal anisotropic metasurface functioned as a linear-to-linear polarization convertor (LLPC), the conversion efficiency is over 90% operating from 356.5 THz to 536.5 THz. While the metasurface served as a linear-to-circular polarization convertor (LCPC), the absolute value of polarization extinction ratio (PER) is greater than 10 dB from 336.5 THz to 544.5 THz. The simulated electric field distributions on the unit-cell indicate that the higher polarization converion efficiency is mianly originated from the geometry anistropy and excitations of fundamental and higher-order dipolar plasmon resonance. Thus, our design can be found potential applications in many areas, such as remote nano-sensors, nano-antennas, and radiometers in visible region.

Plasmon-induced coherence, exciton-induced transparency, and Fano interference for hybrid plasmonic systems in strong coupling regime.

We present an analytical model describing the transition to a strong coupling regime for an ensemble of emitters resonantly coupled to a localized surface plasmon in a metal-dielectric structure. The response of a hybrid system to an external field is determined by two distinct mechanisms involving collective states of emitters interacting with the plasmon mode. The first mechanism is the near-field coupling between the bright collective state and the plasmon mode, which underpins the energy exchange between the system components and gives rise to exciton-induced transparency minimum in scattering spectra in the weak coupling regime and to emergence of polaritonic bands as the system transitions to the strong coupling regime. The second mechanism is the Fano interference between the plasmon dipole moment and the plasmon-induced dipole moment of the bright collective state as the hybrid system interacts with the radiation field. The latter mechanism is greatly facilitated by plasmon-induced coherence in a system with the characteristic size below the diffraction limit as the individual emitters comprising the collective state are driven by the same alternating plasmon near field and, therefore, all oscillate in phase. This cooperative effect leads to scaling of the Fano asymmetry parameter and of the Fano function amplitude with the ensemble size, and therefore, it strongly affects the shape of scattering spectra for large ensembles. Specifically, with increasing emitter numbers, the Fano interference leads to a spectral weight shift toward the lower energy polaritonic band.

Hysteresis in the Thermo-Responsive Assembly of Hexa(ethylene glycol) Derivative-Modified Gold Nanodiscs as an Effect of Shape.

Anisotropic gold nanodiscs (AuNDs) possess unique properties, such as large flat surfaces and dipolar plasmon modes, which are ideal constituents for the fabrication of plasmonic assemblies for novel and emergent functions. In this report, we present the thermo-responsive assembly and thermo-dynamic behavior of AuNDs functionalized with methyl-hexa(ethylene glycol) undecane-thiol as a thermo-responsive ligand. Upon heating, the temperature stimulus caused a blue shift of the plasmon peak to form a face-to-face assembly of AuNDs due to the strong hydrophobic and van der Waals interactions between their large flat surfaces. Importantly, AuNDs allowed for the incorporation of the carboxylic acid-terminated ligand while maintaining their thermo-responsive assembly ability. With regard to their reversible assembly/disassembly behavior in the thermal cycling process, significant rate-independent hysteresis, which is related to their thermo-dynamics, was observed and was shown to be dependent on the carboxylic acid content of the surface ligands. As AuNDs have not only unique plasmonic properties but also high potential for attachment due to the fact of their flat surfaces, this study paves the way for the exploitation of AuNDs in the development of novel functional materials with a wide range of applications.

Hybrid resonant cavities: A route towards phase engineered THz metasurfaces

SummaryCoupled resonant cavities can enable strong photon energy confinement to facilitate the miniaturization of functional photonic devices for applications in designs of sensors, modulators, couplers, waveguides, color filters etc. Typically, the resonances in subwavelength plasmonic cavities rely on the excitation of surface plasmons at specific phase-matching conditions, usually determined by the lattice parameters and constituent material properties. Contrary to this notion, we experimentally demonstrate the control and manipulation of cavity resonances via suitably modifying the split ring resonator geometry in hybrid plasmonic-metasurface (dipole cavity-SRR) configuration without altering the lattice parameters. This results to the excitation of dual resonance peaks. Such dual channel characteristics demonstrate high quality (Q) factor, multi-band resonances, not permissible with typical (unhybridized) plasmonic dipole cavities. We envisage such hybrid meta-cavity designs can become important ingredients for futuristic terahertz devices that can hold the key for sixth generation (6G) communications, designer filters, dual channel sensors etc.

Target-triggered core-satellite self-assemblies based on strong dipole plasmon Pt-tipped Au triangular nanoprism for dual-signal detection of telomerase and enhanced phototherapy

The detection and imaging of human telomerase in living cells is of great significance for early diagnosis and treatment of cancers. Herein, we designed a new promising target-induced self-assembled core-satellite (C-S) structure for label-free Raman detection and fluorescence imaging of telomerase in cancer cells and enhanced photothermal and photodynamic treatment of tumors. Nanoprobe would be triggered by telomerase to form C-S structure to improve the intensity of hot spots and thus to generate intense SERS signal change of adenine (A) for label free Raman detection of telomerase, then the signal of chlorin e6 (Ce6) would be released for in situ fluorescence imaging. Our experimental findings proved that such a strategy exhibited great sensitivity and selectivity for telomerase. In addition, the Pt nanozymes decorated on the tip of Au TNPs showed good catalase-like capacity and stability which can convert endogenous H2O2 to produce O2 to avoid cell hypoxia and promote the generation of cytotoxic singlet oxygen (1O2) for enhanced photodynamic therapy of tumors. This nanoprobe was successfully employed for in situ imaging of telomerase in different kinds of cells and for combination therapy of tumors. It further proves that this is a new biological strategy with far-reaching significance for early cancer diagnosis and treatment.