Excitation Of Localized Surface Plasmons Research Articles

Plasmon-Enhanced Multiphoton Polymer Crosslinking for Selective Modification of Plasmonic Hotspots.

A novel approach to selectively modify narrow subareas of metallic nanostructures adjacent to plasmonic hotspots, where strong electromagnetic field amplification occurs upon localized surface plasmon (LSP) excitation, is reported. In contrast to surface plasmon-triggered polymerization, it relies on plasmonically enhanced multiphoton crosslinking (MPC) of polymer chains carrying photoactive moieties. When they are contacted with metallic nanostructures and irradiated with a femtosecond near-infrared beam resonantly coupled with LSPs, the enhanced field intensity locally exceeds the threshold and initiates MPC only at plasmonic hotspots. This concept is demonstrated by using gold nanoparticle arrays coated with two specifically designed polymers. Local MPC of a poly(N,N-dimethylacrylamide)-based copolymer with an anthraquinone crosslinker is shown via atomic force microscopy. Additionally, MPC is tested with a thermoresponsive poly(N-isopropylacrylamide)-based terpolymer. The reversible thermally induced collapse and swelling of the MPC-formed hydrogel at specific nanoparticle locations are confirmed by polarization-resolved localized surface plasmon resonance (LSPR) spectroscopy. These hybrid metallic/hydrogel materials can be further postmodified, offering attractive characteristics for future spectroscopic/bioanalytical applications.

(Invited) Changes in Electric Double-Layer Structures Under Localized Surface Plasmon Excitation

For the visible light energy conversion, the localized surface plasmon resonance (LSPR), which is the collective oscillations of free electrons, should be promising. With the excitation of LSPR, the highly localized electric field generates in the vicinity of metal nanostructures. It is interesting that various unique photo-response properties, such as chemical reactions or molecular excitations, can be observed due to the enhancement of light-matter interaction.[1] To understand and control such mediated chemical reactions or electrochemical reactions, the precise understanding about the electrified interfaces of plasmonic nanostructures should be required.Especially in the electrochemical system, the electric double layer is formed due to the difference in electrochemical potential between the metal interface and solution. The study of the surface potential is important for the design of chemical reactions because the surface potential at the liquid-solid interface is dependent on the physical properties of the systems. The zeta potential which is the potential of the slipping surface near the Stern layer of the diffusion layer could provide such information. From this point of view, in this study, we investigated the electric double-layer structures through the examination of the zeta potential under LSPR excitation condition. To obtain the zeta potential, we have performed the streaming potential method using Au nanoisland structures under visible light illuminations to excite the LSPR. It was interesting that the zeta potential was drastically changes under the visible light illumination conditions. This zeta potential change indicates the changes in the electric double layer structures through the excitation of LSPR. Through the investigations of incident wavelength dependence or the substrate dependences, the effects for the LSPR excitations on the electric double layer structure have been revealed. The present work would provide the fundamental knowledge about the effect on the LSPR excitation at solid-liquid interfaces.[1] H. Minamimoto et al., Acc. Chem. Res., 2022, 55(6), 809.

Dynamic control of polarization conversion based on borophene nanostructures in optical communication bands

Polarized light has a number of potential applications in the communication bands, including optical communication, polarization imaging, quantum emission, and quantum communication. Nonetheless, there is a need to enhance the dynamic tunability, broadband operation, and flexibility of polarization control. Here, a borophene structure is proposed to dynamically control the polarization state of reflected light. The coherent excitation of localized surface plasmons (LSPs) empowers the achievement of a perfect linearly-circularly polarization conversion at the commercially important communication wavelength of 1550 nm. The dynamic tunability and switching, as well as the arbitrary polarization conversion, are enabled over a wide spectral range by modulating the carrier concentration of borophene. Moreover, by deforming the borophene array and alternately stimulating the upper and lower layers of the LSPs mode, the polarization rotation direction can be flexibly switched. Finally, the process of near-field coupling between the LSPs and dipole light source positioned at a chosen hotspot is demonstrated. This coupling enables polarization-tunable spontaneous emission enhancement, with a spontaneous emission enhancement exceeding 900. The proposed design contributes to enhancing the speed and efficiency of communication within the domain of quantum communication.

Unconventional Transverse Magneto‐Optical Kerr Effect in Cobalt Nanopillar Arrays

AbstractNovel magneto‐optical (MO) properties can be actualized through well‐designed nanostructures, greatly improving the application value of MO effects. As only p‐polarized light can generate transverse magneto‐optical Kerr effect (TMOKE) in traditional MO materials, it is quite challenging and valuable to effectuate TMOKE under s‐polarization incidence at optical frequencies. The constructed nanopillar structures in this work can realize a clear TMOKE under both s‐polarization and p‐polarization incidences, as proved by experiments and simulations. Under s‐polarized incident light with an elevation angle of 45°, electric dipole resonance (EDR) and electric quadrupole resonance (EQR) are excited, resulting in the significant enhancement of TMOKE (five orders of magnitude higher than for the planar Co film). The intensity of s‐polarized TMOKE shows an increase with the enhancing diameter and height of the nanopillar structure. The amplitude of p‐polarized TMOKE is also appreciably increased due to the excitation of surface plasmon polaritons (SPPs) and localized surface plasmons (LSPs). Moreover, the Fe and Ni nanopillars display similar reflectivity and s‐polarized TMOKE behaviors in simulations. The s‐polarized TMOKE is unexpectedly observed in ferromagnetic metals for the first time, which builds up a considerable platform for the development and application of novel MO phenomena.

Plasma amplifiers: multiscale light-enhanced uniform SERS composite substrates for breaking through resonance limitations.

A phenomenon known as plasmon resonance constitutes a unique optical effect that can induce an enhancement in localized electromagnetic fields, resulting in a substantial increase in the electromagnetic field intensity surrounding metallic nanostructures. In this work, the coupling effect of excitation of surface plasmon polaritons and local surface plasmons in nanoparticles is deeply studied under the background of nanoparticles/one-dimension grating composite structures through grating matching. By employing finite-difference time-domain simulations as our methodological approach, we discern gratings with a periodicity of 1.5 μm support surface plasmon bound states between the gratings. Furthermore, the modulation of SPs along the vertical sidewalls of the grating due to standing wave effects exhibits oscillatory behavior with varying grating heights. Experimental results obtained from the nanoparticle/grating composite SERS substrate validate theoretical predictions, demonstrating higher enhanced Raman signals at 633 nm compared to 532 nm. Remarkably, this structure exhibits good performance, with R6G detection sensitivity down to concentrations as low as 10-10 M and mapping achieving a relative standard deviation of 7.79%, underscoring its uniformity and capability of electromagnetic field enhancement.

Enhancement of NaYF<sub>4</sub>:Yb<sup>3+</sup>/Er<sup>3+</sup> up-conversion luminescence based on anodized alumina template

Up-conversion nanoparticle (UCNP) can collect near-infrared (NIR) light and convert it into visible light. Therefore, UCNP has potential applications in fields such as biomedicine, anti-counterfeiting, and solar cells. However, the efficiency of traditional UCNP in the above-mentioned fields is relatively low, greatly limiting its use in related fields. Therefore, enhancing the up-conversion luminescence intensity of up-conversion nanoparticles is particularly important and urgently needed. In this work, anodic alumina templates are used to enhance the luminescence intensity of up-conversion nanocrystals. NaYF<sub>4</sub>:Yb<sup>3+</sup>, Er<sup>3+</sup>with a diameter of 35 nm is prepared by using co-precipitation method. Single pass AAO templates with pore size and pore spacing of 88 nm and 100 nm are prepared by using two-step anodization method. Finally, NaYF<sub>4</sub>:Yb<sup>3+</sup>, Er<sup>3+</sup>/AAO composite structures are formed by using spin coating method. The red green light emission intensity of NaYF<sub>4</sub>:Yb<sup>3+</sup>, Er<sup>3+</sup>/AAO sample can increase 4.4 and 9.0 times that of NaYF<sub>4</sub>:Yb<sup>3+</sup>, Er<sup>3+</sup>/Al reference sample, respectively. The enhancement mechanism is explored by using the finite difference time domain method, and the results show that the primary source of enhancement is the localized surface plasmon resonance effect of the pores in the anodic alumina template. At the same time, the relationship between the up-conversion luminescence intensity of NaYF<sub>4</sub>:Yb<sup>3+</sup>, Er<sup>3+</sup>/AAO sample and the incident angle is investigated. The experimental results show that as the incident angle increases, the luminescence intensity of the red and green light of NaYF<sub>4</sub>:Yb<sup>3+</sup>, Er<sup>3+</sup>/AAO samples first decrease and then increase. Due to the coupling of the local surface plasmon resonance with the excitation wavelength and emission wavelength, the up-conversion luminescence intensity of the sample can be affected. The relationship of AAO channel enhancement factor with incident angle at excitation wavelength and emission wavelength is studied by using the finite difference time domain method. The results indicate that as the incident angle increases, the enhancement factor at the excitation wavelength decreases, while the enhancement factor at the emission wavelength increases after being illuminated at an incident angle of 15°. Therefore, when the incident angle is less than 20°, the electric field intensity at 980 nm dominates, but when it is greater than 20°, the electric field intensity at 540 nm and 650 nm takes precedence. The above results provide a reference for putting them into practical applications in the fields of anti-counterfeiting and solar cells.

Photoluminescence Enhancement of Monolayer WS2 by n-Doping with an Optically Excited Gold Disk.

In nanophotonics and quantum optics, we aim to control and manipulate light with tailored nanoscale structures. Hybrid systems of nanostructures and atomically thin materials are of interest here, as they offer rich physics and versatility due to the interaction between photons, plasmons, phonons, and excitons. In this study, we explore the optical and electronic properties of a hybrid system, a naturally n-doped monolayer WS2 covering a gold disk. We demonstrate that the nonresonant excitation of the gold disk in the high absorption regime efficiently generates hot carriers via localized surface plasmon excitation, which n-dope the monolayer WS2 and enhance the photoluminescence emission by regulating the multiexciton population and stabilizing the neutral exciton emission. The results are relevant to the further development of nanotransistors in photonic circuits and optoelectronic applications.

MMP simulation of coupled gold nanoparticles as nanoantennae: Efficient tuning of localized surface plasmon resonances

We study localized surface plasmon excitations on a set of two coupled two-dimensional (2D) gold nanoparticles as a nanoantenna by using the multiple multipoles (MMP) method. We investigate the plasmon spectrum as a function of the nanoparticle's size, the background index of refraction, and the separating gap between the nanoparticles in order to provide proper means to tune the underlying plasmon resonance efficiently. Accurate multiple multipoles (MMP) computations are conducted with precise control over error measures, particularly for configurations of coupled particles with varying inter-particle distances and illumination conditions. Our findings demonstrate that by selecting these parameters carefully, nanoantennae can amplify emitter radiation across a wide range of spectra from ultraviolet to near-infrared. Numerical calculations have revealed that the MMP method is accurate, easy to handle, and efficient even on a low-cost, state-of-the-art PC.

Size and crystallinity effect on the ultrafast optical response of chemically synthesized silver nanoparticles

The excited localized surface plasmon (LSP) in metallic nanoparticles is known to relax through several processes such as electron-electron scattering, electron-phonon coupling, and phonon-phonon scattering. In the current research, the ultrafast electron-phonon (e-ph) coupling relaxation processes for different average sizes and crystallinity of chemically synthesized silver nanoparticles were evaluated utilizing transient absorption spectroscopy. The nanoparticle size and crystallinity of similar linear dimension polycrystalline spherical and monocrystalline cubic nanoparticles ranging from ca. 30–60 nm was related to their electron relaxation time constants and revealed very different dependencies. For the monocrystalline nanocubes, the electron-phonon coupling was not dependent on the cube edge length, while for the polycrystalline nanospheres, it was linearly decreasing with diameter. We demonstrate that the e-ph coupling time constant could be used to evaluate crystallinity and crystallite size in plasmonic metal nanoparticles when the size (surface area) of the nanoparticle is known.

Circular dichroism enhancement induced by surface plasmons in the structure of a metallic grating coated by chiral TDBCs

We investigate the enhancement of the circular dichroism (CD) spectrum in a metallic grating by coating it with chiral TDBCs. Using the finite difference time domain method, we obtain reflection spectra and CD spectra. The amplification of TDBCs chirality is achieved through the excitation of both surface plasmon polaritons (SPPs) and localized surface plasmons (LSPs) in the Ag grating structure. Our results indicate that, compared to chiral TDBCs alone, the enhancement factor of chiral TDBCs coated on the metallic grating can reach up to 200 times. The coupling between chiral TDBCs and electromagnetic fields induced by SPPs and LSPs is tuned by both the period and groove thickness of the grating structure. Under certain conditions, strong coupling phenomena are observed, demonstrating a competitive relationship between the dissipation of our proposed structure and the coupling of electromagnetic fields and chiral TDBCs. The substantial amplification of the CD spectra suggests that our proposed structure provides a novel method to enhance the chirality of TDBCs experimentally.

Tunable reflective tri-band polarization converter based on graphene composite metasurface

A graphene composite metasurface-based reflective tri-band polarization converter working in the terahertz frequency regime is proposed and investigated by CST Microwave Studio software. The polarization converter achieves high polarization conversion ratios (above 99%) for linear-to-linear polarization conversion at 6.21, 9.11, and 11.66 THz, and for linear-to-circular polarization conversion at 5.14 THz, with a fixed Fermi energy level (Ef) of 1.0 ev due to the excitation of localized surface plasmons. Subsequently, the reason for cross-polarization conversion at three resonant frequencies and the incident angle sensitive property of the converter were discussed. Based on the graphene component, a tunable tri-band polarization converter was obtained and investigated; this converter could be applied in the active modulation area of wireless communication.

Plasmonic-hot-electron mediated room-temperature generation of charged biexciton in monolayer WS2

Transition-metal dichalcogenide (TMDC) monolayers exhibit strong many-body Coulomb interaction, which induces rich excitonic states, including neutral exciton, trion (charged exciton), biexciton, and even charged biexciton. Despite the fact that the biexcitons in pristine two-dimensional (2D) TMDCs have great potential in forming correlated exciton pairs at distinct valleys, they lack efficient emission and optical tunability at room temperature, hindering their applications in photonic and optoelectronic devices. Here, we report room-temperature strong emission of the charged biexcitons from a gold nanosphere capped ${\mathrm{WS}}_{2}$ monolayer through localized surface plasmon (LSP) induced hot-electron injection, and demonstrate all-optical control of the emission intensity through on- or off-resonant LSP excitation. Under on-resonant LSP excitation at 2.08 eV, the photoluminescence (PL) intensity shows a superlinear scaling with the laser power density, signifying the existence of charged biexcitons; On the contrary, the off-resonant LSP excitation at 2.33 eV results in no detectable PL signal of the charged biexcitons. By calculating the energy difference between the charged biexciton and trion, we further determine the binding energy of the former to be 25 meV. Our findings offer an opportunity for understanding the many-body effect associated with the charged biexcitons of 2D TMDCs, and could facilitate their potential quantum and optoelectronic applications under ambient environment.

Single-mode lasing by tailoring the excitation of localized surface plasmon resonances to whispering gallery modes in a microring laser.

Cavity mode manipulation in lasers is urgent for the stable single-mode operation of a microring laser. Here, we propose and experimentally demonstrate the plasmonic whispering gallery mode microring laser for strong coupling between local plasmonic resonances and whispering gallery modes (WGM) on the microring cavity to achieve pure single-mode lasing. The proposed structure is fabricated based on integrated photonics circuits consisting of gold nanoparticles deposited on a single microring. Additionally, our numerical simulation provides deep insight into the interaction between the gold nanoparticles and WGM modes. The manufacture of microlasers for the advancement of lab-on-a-chip devices and all-optical detection of ultra-low analysts may benefit from our findings.

Dynamic Control of Nonlinearly Generated Light Chirality with Nanostructured Graphene

The constant development of nanotechnology has facilitated significant improvements in the field of nonlinear optics. Metasurfaces and metamaterials, or nanostructured material systems, have allowed excellent control over a variety of linear and nonlinear optical processes, including manipulation of the amplitude, phase, and polarization of light. To facilitate this progress, there is an increasing demand for small footprint nonlinear material systems and metamaterial designs that offer diverse functionalities. Here, we demonstrate that this demand can be satisfied by utilizing the family of two-dimensional materials and more specifically graphene. In this work, we take advantage of the exceptionally large nonlinearity of graphene and demonstrate simultaneous dynamic control over the polarization state and nonlinear conversion efficiency of the third harmonic emission via the coherent excitation of localized surface plasmons in nanopatterned graphene rectangular arrays. Using finite-difference time-domain simulations and through electrostatic control over the Fermi level in graphene, we demonstrate broadband dynamic tuning between linear and circular polarization states for the third harmonic emission. The proposed method is not limited to third harmonic emission and is applicable to different nonlinear optical processes. As a result, the proposed metasurface constitutes an outstanding platform for commercially viable, CMOS-compatible, room-temperature devices for nonlinear conversion applications.

Purcell Factor for Plasmon-Enhanced Metal Photoluminescence

We present an analytical model for the plasmonic enhancement of metal photoluminescence (MPL) in metal nanostructures with a characteristic size below the diffraction limit. In such systems, the primary mechanism of MPL enhancement is the excitation of localized surface plasmons (LSP) by recombining carriers, followed by photon emission due to LSP radiative decay. For plasmonic nanostructures of arbitrary shape, we obtain a universal expression for the MPL Purcell factor that describes the plasmonic enhancement of MPL in terms of the metal dielectric function, LSP frequency, and system volume. We find that the lineshape of the MPL spectrum is affected by the interference between direct carrier recombination processes and those mediated by plasmonic antenna, which leads to a blueshift of the MPL spectral band relative to LSP resonance in scattering spectra observed in numerous experiments.

Absorption peak wavelength and bandwidth control in MoS2-based absorbers on the basis of SPPs and LSPs excitation

The nanometer-sized absorber that we introduced in this paper consists of a monolayer of atomically thin MoS2 on a silica substrate covered by inclined gold gratings. By selecting the period of the gold grating of the order of the incident light wavelength, both surface plasmon polaritons (SPPs) and localized surface plasmons (LSPs) can be excited in the structure at the visible spectrum. By selecting different values of the gold ribbon’s widths, we show that the excited modes can be adjusted to SPPs, LSPs, or SPPs and LSPs together. With the excitation of SPPs, the narrowband absorption peak wavelength, and with the excitation of LSPs, the FWHM of the absorption peak can be adjusted by the ribbon’s geometry parameters. In the case of simultaneous excitation of SPPs and LSPs, by increasing the ribbons’ inclination, the LSP peaks will redshift, while the SPP peaks blueshift, which leads to wider absorption peaks. The effect of increasing the ribbons’ height is opposite to that of the ribbons’ inclination, which is accompanied by the SPP peak red shift and LSP peak blue shift. This way, the more the height of the ribbons, the less the absorption peak bandwidth is. This paper’s results are a promising guide for designing plasmonic absorbers with desired bandwidths and peak wavelengths, which are widely demanded in designing photovoltaics and photonic sensors.

Dual-functional copper (Cu0/Cu2+)-modified SrTiO3-δ nanosheets with enhanced photothermal catalytic performance for CO2 reduction and H2 evolution

In recent years, metal–semiconductor heterojunctions have been intensively researched in the field of photocatalysis due to the effective spatial separation of photo-induced electron-hole pairs. Here, the dual-functional copper (Cu0/Cu2+)-modified SrTiO3-δ nanosheets (Cu/STCO) are successfully synthesized by a two-step route. First, Cu2+-doped SrTiO3-δ (STCO) nanosheets are synthesized by a hydrothermal method and then Cu0 nanoparticles (NPs) are in situ formed from the STCO under H2 atmosphere at 500 °C. The optimized ratio 6 %-Cu/STCO photocatalyst exhibits the best photothermal catalytic performance for CO2 reduction reaction (CO2RR) and H2 evolution performance. The detailed characterization demonstrate that the Cu2+ dopant extend the light absorption range and the in situ generated Cu0 NPs effectively improve the separation ability of photogenerated charge carriers by forming the Schottky contact with STCO. The excited localized surface plasmon resonance (LSPR) effect of metallic Cu0 in Cu/STCO shows a considerable catalysis performance even under 600 nm monochromatic light irradiation. The density functional theory (DFT) calculations suggest that the Cu/STCO diminishes the barrier of the rate-determining steps of the CO2RR and H2 evolution thus perform the outstanding catalysis ability in two filed. This work presents the synergistic effect of Cu0 and Cu2+ on enhancing the photothermal catalytic performance for CO2RR and H2 evolution and offers a novel strategy to synthesize the metal–semiconductor nanostructures for efficient harnessing of solar energy into fuel.

NiSe and CoSe Topological Nodal-Line Semimetals: A Sustainable Platform for Efficient Thermoplasmonics and Solar-Driven Photothermal Membrane Distillation.

The control of heat at the nanoscale via the excitation of localized surface plasmons in nanoparticles (NPs) irradiated with light holds great potential in several fields (cancer therapy, catalysis, desalination). To date, most thermoplasmonic applications are based on Ag and Au NPs, whose cost of raw materials inevitably limits the scalability for industrial applications requiring large amounts of photothermal NPs, as in the case of desalination plants. On the other hand, alternative nanomaterials proposed so far exhibit severe restrictions associated with the insufficient photothermal efficacy in the visible, the poor chemical stability, and the challenging scalability. Here, it is demonstrated the outstanding potential of NiSe and CoSe topological nodal-line semimetals for thermoplasmonics. The anisotropic dielectric properties of NiSe and CoSe activate additional plasmonic resonances. Specifically, NiSe and CoSe NPs support multiple localized surface plasmons in the optical range, resulting in a broadband matching with sunlight radiation spectrum. Finally, it is validated the proposed NiSe and CoSe-based thermoplasmonic platform by implementing solar-driven membrane distillation by adopting NiSe and CoSe nanofillers embedded in a polymeric membrane for seawater desalination. Remarkably, replacing Ag with NiSe and CoSe for solar membrane distillation increases the transmembrane flux by 330% and 690%, respectively. Correspondingly, costs of raw materials are also reduced by 24 and 11 times, respectively. The results pave the way for the advent of NiSe and CoSe for efficient and sustainable thermoplasmonics and related applications exploiting sunlight within the paradigm of the circular blue economy.

Recent Advances in Inflammatory Diagnosis with Graphene Quantum Dots Enhanced SERS Detection.

Inflammatory diseases are some of the most common diseases in different parts of the world. So far, most attention has been paid to the role of environmental factors in the inflammatory process. The diagnosis of inflammatory changes is an important goal for the timely diagnosis and treatment of various metastatic, autoimmune, and infectious diseases. Graphene quantum dots (GQDs) can be used for the diagnosis of inflammation due to their excellent properties, such as high biocompatibility, low toxicity, high stability, and specific surface area. Additionally, surface-enhanced Raman spectroscopy (SERS) allows the very sensitive structural detection of analytes at low concentrations by amplifying electromagnetic fields generated by the excitation of localized surface plasmons. In recent years, the use of graphene quantum dots amplified by SERS has increased for the diagnosis of inflammation. The known advantages of graphene quantum dots SERS include non-destructive analysis methods, sensitivity and specificity, and the generation of narrow spectral bands characteristic of the molecular components present, which have led to their increased application. In this article, we review recent advances in the diagnosis of inflammation using graphene quantum dots and their improved detection of SERS. In this review study, the graphene quantum dots synthesis method, bioactivation method, inflammatory biomarkers, plasma synthesis of GQDs and SERS GQD are investigated. Finally, the detection mechanisms of SERS and the detection of inflammation are presented.

Nanoscale Heating of an Ultrathin Oxide Film Studied by Tip-Enhanced Raman Spectroscopy

We report on the nanoscale heating mechanism of an ultrathin ZnO film using low-temperature tip-enhanced Raman spectroscopy. Under the resonance condition, intense Stokes and anti-Stokes Raman scattering can be observed for the phonon modes of a two-monolayer (ML) ZnO on an Ag(111) surface, enabling us to monitor local heating at the nanoscale. It is revealed that the local heating originates mainly from inelastic electron tunneling through the electronic resonance when the bias voltage exceeds the conduction band edge of the 2-ML ZnO. When the bias voltage is lower than the conduction band edge, the local heating arises from two different contributions, namely direct optical excitation between the interface state and the conduction band of 2-ML ZnO or injection of photoexcited electrons from an Ag tip into the conduction band. These optical heating processes are promoted by localized surface plasmon excitation. Simultaneous mapping of tip-enhanced Raman spectroscopy and scanning tunneling spectroscopy for 2-ML ZnO including an atomic-scale defect demonstrates visualizing a correlation between the heating efficiency and the local density of states, which further allows us to analyze the local electron-phonon coupling strength with ∼2 nm spatial resolution.