Molding of Plasmonic Resonances in Metallic Nanostructures: Dependence of the Non-Linear Electric Permittivity on System Size and Temperature

Mixed Nanosphere Assemblies at a Liquid-Liquid Interface

The in-plane packing of gold (Au), polystyrene (PS), and silica (SiO2) spherical nanoparticle (NP) mixtures at a water-oil interface is investigated in situ by UV-vis reflection spectroscopy. All NPs are functionalized with carboxylic acid such that they strongly interact with amine-functionalized ligands dissolved in an immiscible oil phase at the fluid interface. This interaction markedly increases the binding energy of these nanoparticle surfactants (NPSs). The separation distance between the Au NPSs and Au surface coverage are measured by the maximum plasmonic wavelength (λmax) and integrated intensities as the assemblies saturate for different concentrations of non-plasmonic (PS/SiO2) NPs. As the PS/SiO2 content increases, the time to reach intimate Au NP contact also increases, resulting from their hindered mobility. λmax changes within the first few minutes of adsorption due to weak attractive inter-NP forces. Additionally, a sharper peak in the reflection spectrum at NP saturation reveals tighter Au NP packing for assemblies with intermediate non-plasmonic NP content. Grazing incidence small angle X-ray scattering (GISAXS) and scanning electron microscopy (SEM) measurements confirm a decrease in Au NP domain size for mixtures with larger non-plasmonic NP content. The results demonstrate a simple means to probe interfacial phase separation behavior using in situ spectroscopy as interfacial structures densify into jammed, phase-separated NP films.

Probing Temperature-Induced Plasmonic Nonlinearity: Unveiling Opto-Thermal Effects on Light Absorption and Near-Field Enhancement

Precise measurement and control of local heating in plasmonic nanostructures are vital for diverse nanophotonic devices. Despite significant efforts, challenges in understanding temperature-induced plasmonic nonlinearity persist, particularly in light absorption and near-field enhancement due to the absence of suitable measurement techniques. This study presents an approach allowing simultaneous measurements of light absorption and near-field enhancement through angle-resolved near-field scanning optical microscopy with iterative opto-thermal analysis. We revealed gold thin films exhibit sublinear nonlinearity in near-field enhancement due to nonlinear opto-thermal effects, while light absorption shows both sublinear and superlinear behaviors at varying thicknesses. These observations align with predictions from a simple harmonic oscillation model, in which changes in damping parameters affect light absorption and field enhancement differently. The sensitivity of our method was experimentally examined by measuring the opto-thermal responses of three-dimensional nanostructure arrays. Our findings have direct implications for advancing plasmonic applications, including photocatalysis, photovoltaics, photothermal effects, and surface-enhanced Raman spectroscopy.

Ultrafast Laser-Induced Dynamics of Non-Equilibrium Electron Spill-Out in Nanoplasmonic Bilayers

Contemporary quantum plasmonics capture subtle corrections to the properties of plasmonic nano-objects in equilibrium. Here, we demonstrate non-equilibrium spill-out redistribution of the electronic density at the ultrafast time scale. As revealed by time-resolved 2D spectroscopy of nanoplasmonic Fe/Au bilayers, an injection of the laser-excited non-thermal electrons induces transient electron spill-out thus changing the plasma frequency. The response of the local electronic density switches the electronic density behavior from spill-in to strong (an order of magnitude larger) spill-out at the femtosecond time scale. The superdiffusive transport of hot electrons and the lack of a direct laser heating indicate significantly non-thermal origin of the underlying physics. Our results demonstrate an ultrafast and non-thermal way to control surface plasmon dispersion through transient variations of the spatial electron distribution at the nanoscale. These findings expand quantum plasmonics into previously unexplored directions by introducing ultrashort time scales in the non-equilibrium electronic systems.

Thermally tunable electromagnetic surface waves supported by graphene loaded indium antimonide (InSb) interface

The thermal agitation plays a vital role in tunability of optoelectronic, structural and chemical characteristics of the temperature sensitive materials. Graphene enables the THz optics, due to its unprecedent controlling characteristics over the traditional materials. The influence of temperature on the monolayer graphene is very negligible due to its low free charge carrier density, to enhance the thermal sensitivity of graphene, the graphene loaded temperature sensitive material interface has been proposed. A theoretical analysis has been carried out on temperature dependent propagation characteristics of electromagnetic surface waves supported by the graphene loaded semi-infinite indium antimonide (InSb). The InSb has been taken as temperature sensitive material. The Drude model has been used for the modeling of InSb in the THz region while the modeling of the graphene has been done by random phase approximation-based Kubo's formulism. To realize the graphene loaded indium antimonide interface, the impedance boundary conditions (IBCs) have been employed. The numerical analysis has been conducted to analyze the influence of temperature on the characteristics of electromagnetic surface waves i.e., dispersion curve, effective mode index (Neff), penetration depth (δ), propagation length (Lp), phase speed (Vp) and field profile, propagating along the graphene loaded InSb. In all the numerical results, the temperature variation has been considered from 200 to 350 K. It has been concluded that the graphene-InSb interface provides more temperature assisted tunability to the interfacial surface modes, commonly known as surface waves, as compared to monolayer graphene. Further, the graphene parameters can play a vital role in the dynamical tuning of electromagnetic surface waves in THz to IR frequency range. The numerically computed results have potential applications in designing of thermo-optical waveguides, temperature assisted communication devices, thermo-optical sensors and near field thermal imaging platforms.

Photonic crystal nanostructure as a photodetector for NaCl solution monitoring: theoretical approach

In this research, we have a theoretical simple and highly sensitive sodium chloride (NaCl) sensor based on the excitation of Tamm plasmon resonance through a one-dimensional photonic crystal structure. The configuration of the proposed design was, [prism/gold (Au)/water cavity/silicon (Si)/calcium fluoride (CaF₂)¹⁰/glass substrate]. The estimations are mainly investigated based on both the optical properties of the constituent materials and the transfer matrix method as well. The suggested sensor is designed for monitoring the salinity of water by detecting the concentration of NaCl solution through near-infrared (IR) wavelengths. The reflectance numerical analysis showed the Tamm plasmon resonance. As the water cavity is filled with NaCl of concentrations ranging from 0 g l^-1 to 60 g l^-1, Tamm resonance is shifted towards longer wavelengths. Furthermore, the suggested sensor provides a relatively high performance compared to its photonic crystal counterparts and photonic crystal fiber designs. Meanwhile, the sensitivity and detection limit of the suggested sensor could reach the values of 24 700 nm per RIU (0.576 nm (g l)^-1) and 0.217 g l^-1, respectively. Therefore, the suggested design could be of interest as a promising platform for sensing and monitoring NaCl concentrations and water salinity as well.

Influence of the temperature-dependent dielectric constant on the photoacoustic effect of gold nanospheres

Photoacoustic imaging techniques with gold nanoparticles as contrast agents have received a great deal of attention. The photoacoustic response of gold nanoparticles strongly depends on the far-field optical properties, which essentially depend on the dielectric constant of the material. The dielectric constant of gold not only varies with wavelength but is also affected by temperature. However, the effect of the temperature dependence of the dielectric constant on gold nanoparticles' photoacoustic response has not been fully investigated. In this work, the Drude-Lorentz model and Mie theory are used to calculate the dielectric constant and absorption efficiency of gold nanospheres in aqueous solution, respectively. Then, the finite element method is used to simulate the heat transfer process of gold nanospheres and surrounding water. Finally, the one-dimensional velocity-stress equation is solved by the finite-difference time-domain method to obtain the photoacoustic response of gold nanospheres. The results show that under the irradiation of a high-fluence nanosecond pulse laser, ignoring the temperature dependence of the dielectric constant will lead to large errors in the photothermal response and the nonlinear photoacoustic signals (it can even exceed 20% and 30%). The relative error of the photothermal and photoacoustic response caused by ignoring the temperature-dependent dielectric constant is determined from both the temperature dependence of absorption efficiency and the maximum temperature increase of gold nanospheres. This work provides a new perspective for the photothermal and photoacoustic effects of gold nanospheres, which is meaningful for the development of high-resolution photoacoustic detectors and nano/microscale temperature measurement techniques.

Temperature-resilient anapole modes associated with TE polarization in semiconductor nanowires

Polarization-dependent scattering anisotropy of cylindrical nanowires has numerous potential applications in, for example, nanoantennas, photothermal therapy, thermophotovoltaics, catalysis, sensing, optical filters and switches. In all these applications, temperature-dependent material properties play an important role and often adversely impact performance depending on the dominance of either radiative or dissipative damping. Here, we employ numerical modeling based on Mie scattering theory to investigate and compare the temperature and polarization-dependent optical anisotropy of metallic (gold, Au) nanowires with indirect (silicon, Si) and direct (gallium arsenide, GaAs) bandgap semiconducting nanowires. Results indicate that plasmonic scattering resonances in semiconductors, within the absorption band, deteriorate with an increase in temperature whereas those occurring away from the absorption band strengthen as a result of the increase in phononic contribution. Indirect-bandgap thin ([Formula: see text]) Si nanowires present low absorption efficiencies for both the transverse electric (TE, [Formula: see text]) and magnetic (TM, [Formula: see text]) modes, and high scattering efficiencies for the TM mode at shorter wavelengths making them suitable as highly efficient scatterers. Temperature-resilient higher-order anapole modes with their characteristic high absorption and low scattering efficiencies are also observed in the semiconductor nanowires ([Formula: see text] nm) for the TE polarization. Herein, the GaAs nanowires present [Formula: see text] times greater absorption efficiencies compared to the Si nanowires making them especially suitable for temperature-resilient applications such as scanning near-field optical microscopy (SNOM), localized heating, non-invasive sensing or detection that require strong localization of energy in the near field.

Hybrid Tamm plasmon resonance excitation towards a simple and efficient biomedical detector of NaI solution

This work presents a theoretical verification for the detection of Sodium iodide (NaI) solution with different concentrations in the vicinity of Tamm plasmon (TP) resonance. The proposed sensing tool is constituted of {prism/Ag/cavity/(GaN/CaF₂)¹⁵/air}. The essential foundation of this study is based on the displacement of the TP resonance by varying the concentration of an aqueous solution of sodium iodide (NaI) that fills the cavity layer. The resonant TP dip is shifted downwards the shorter wavelengths with the increment of the Ag layer thickness. Nevertheless, the resonant TP dip is shifted upwards to longer wavelengths with the increment of NaI refractive index/concentration. Also, the sensitivity of the sensing tool decreases with the increment of the NaI refractive index. However, the minimum result is not less than the value of 9913 nm RIU⁻¹ for a concentration of 25%. Finally, the performance of our sensor in the form of the quality factor, detection limit, and figure of merit showed significant improvements in designing a high-performance liquid and biosensor.

NaI sensor: the suggested design of a NaI photonic crystal sensor that is constituted.

Green Chemistry Based Gold Nanoparticles Synthesis Using the Marine Bacterium Lysinibacillus odysseyi PBCW2 and Their Multitudinous Activities

Green chemistry has paved an 'avant-garde avenue' in the production and fabrication of eco-friendly stable nanoparticles employing the utilization of biological agents. In the present study we present the first report on the potential of the marine bacterium Lysinibacillus odysseyi PBCW2 for the extracellular production of gold nanoparticles (AuNPs). Utilizing a variety of methods, AuNPs in the cell-free supernatant of L. odysseyi (CFS-LBOE) were identified and their antioxidant, antibacterial, and dye-degrading properties were examined. The visual coloring of the reaction mixture to a ruby red hue showed the production of LBOE-AuNPs; validated by means of XRD, TEM, SEM, XRD, DLS, TGA, and FT-IR analysis. Additionally, the 2,2-diphenyl-1-picrylhydrazyl technique and the well diffusion assay were used to examine their dose-dependent antioxidant and antibacterial activity. These biogenic LBOE-AuNPs showed 91% dye degradation efficiency during catalytic reduction activity on BTB dye, demonstrating their versatility as options for heterogeneous catalysis.

Noble classical and quantum approach to model the optical properties of metallic nanoparticles to enhance the sensitivity of optoplasmonic sensors

The bright light obtained from the quantum principle has a key role in the construction of optical sensors. Yet, theoretical and experimental work highlights the challenges of overcoming the high cost and low efficiency of such sensors. Therefore, we report a metallic nanoparticle-based metasurface plasmons polariton using quantum and classical models. We have investigated the material properties, absorption cross-section, scattering cross-section, and efficiency of the classical model. By quantizing light-matter interaction, the quantum features of light - degree of squeezing, correlation, and entanglement are quantified numerically and computationally. In addition, we note the penetration depth and propagation length from a hybrid model in order to enhance the optoplasmonic sensor performance for imaging, diagnosing, and early perception of cancer cells with label-free, direct, and real-time detection. Our study findings conclude that the frequency of incident light, size, shape, and type of nanoparticles has a significant impact on the optical properties of metallic nanoparticles and the nonlinear optical properties of metallic nanoparticles are dynamic, enhancing the sensitivity of the optoplasmonic sensor. Moreover, the resulting bright light shows the systematic potential for further medical image processing.

Simple, efficient and accurate method toward the monitoring of ethyl butanoate traces

We introduce in this research a simple, accurate, safe, and efficient design for the detection of ethyl butanoate that be present in the dry exhaled breath. In particular, the presence of ethyl butanoate in the dry exhaled breath could be utilized as a platform for the diagnosing of COVID 19. The main idea of this theoretical investigation is based on the inclusion of a cavity layer between a thin layer of Au and the well-known one-dimension photonic crystals. Accordingly, the cavity layer is filled with dry exhaled breath. The numerical results are investigated in the vicinity of the Drude model and transfer matrix method. The investigated results show the appearance of Tamm plasmon resonance in the reflectance spectrum of our design through the IR region. Such resonant mode provides very high sensitivity with the change in the concentration of ethyl butanoate. We have examined the performance of the proposed sensor by calculating its sensitivity, detection limit, detection accuracy, quality factor and figure of merit. The designed sensor could receive sensitivity of 0.3 nm/ppm or 260,486 nm/RIU, resolution of 7 ppm and quality factor of 969.

Thermal degradation of optical resonances in plasmonic nanoparticles

The dependence of plasmon resonance excitations in ultrafine (3-7 nm) gold nanoparticles on heating and melting is investigated. An integrated approach is adopted, where molecular dynamics simulations of the spatial and temporal development of the atoms constituting the nanoparticles generate trajectories out of which system conformations are sampled and extracted for calculations of plasmonic excitation cross sections which then are averaged over the sample configurations for the final result. The calculations of the plasmonic excitations, which take into account the temperature- and size-dependent relaxation of the plasmons, are carried out with a newly developed Extended Discrete Interaction Model (Ex-DIM) and complemented by multilayered Mie theory. The integrated approach clearly demonstrates the conditions for suppression of the plasmons starting at temperatures well below the melting point. We have found a strong inhomogeneous dependence of the atom mobility in the particle crystal lattice increasing from the center to its surface upon the temperature growth. The plasmon resonance suppression is associated with an increase of the mobility and in the amplitude of phonon vibrations of the lattice atoms accompanied by electron-phonon scattering. This leads to an increase in the relaxation constant impeding the plasmon excitation as the major source of the suppression, while the direct contribution from the increase in the lattice constant and its chaotization at melting is found to be minor. Experimental verification of the suppression of surface plasmon resonance is demonstrated for gold nanoparticles on a quartz substrate heated up to the melting temperature and above.

A 3D Plasmonic Antenna-Reactor for Nanoscale Thermal Hotspots and Gradients

Plasmonic nanoantennas focus light below the diffraction limit, creating strong field enhancements, typically within a nanoscale junction. Placing a nanostructure within the junction can greatly enhance the nanostructure's innate optical absorption, resulting in intense photothermal heating that could ultimately compromise both the nanostructure and the nanoantenna. Here, we demonstrate a three-dimensional "antenna-reactor" geometry that results in large nanoscale thermal gradients, inducing large local temperature increases in the confined nanostructure reactor while minimizing the temperature increase of the surrounding antenna. The nanostructure is supported on an insulating substrate within the antenna gap, while the antenna maintains direct contact with an underlying thermal conductor. Elevated local temperatures are quantified, and high local temperature gradients that thermally reshape only the internal reactor element within each antenna-reactor structure are observed. We also show that high local temperature increases of nominally 200 °C are achievable within antenna-reactors patterned into large extended arrays. This simple strategy can facilitate standoff optical generation of high-temperature hotspots, which may be useful in applications such as small-volume, high-throughput chemical processes, where reaction efficiencies depend exponentially on local temperature.

Nanoscale rotational optical manipulation

Light has momentum, and hence, it can move small particles. The optical tweezer, invented by Ashkin et al. [Opt. Lett. 11, 288 (1986)] is a representative application. It traps and manipulates microparticles and has led to great successes in the biosciences. Currently, optical manipulation of "nano-objects" is attracting growing attention, and new techniques have been proposed and realized. For flexible manipulation, push-pull switching [Phys. Rev. Lett. 109, 087402 (2012)] and super-resolution trapping by using the electronic resonance of nano-objects have been proposed [ACS Photonics 5, 318 (2017)]. However, regarding the "rotational operation" of nano-objects, the full potential of optical manipulation remains unknown. This study proposes mechanisms to realize rotation and direction switching of nano-objects in macroscopic and nanoscopic areas. By controlling the balance between the dissipative force and the gradient force by using optical nonlinearity, the direction of the macroscopic rotational motion of nano-objects is switched. Further, conversion between the spin angular momentum and orbital angular momentum by light scattering through localized surface plasmon resonance in metallic nano-complexes induces optical force for rotational motion in the nanoscale area. This study pieces out fundamental operations of the nanoscale optical manipulation of nanoparticles.

Nanostructured sapphire optical fiber embedded with Au nanorods for high-temperature plasmonics in harsh environments

Sensors for harsh environments must exhibit robust sensing response and considerable thermal and chemical stability. We report the exploration of a novel all-alumina nanostructured sapphire optical fiber (NSOF) embedded with Au nanorods (Au NRs) for plasmonics-based sensing at high temperatures. Temperature dependence of the localized surface plasmon resonance (LSPR) of Au NRs was studied in conjunction with numerical calculations using the Drude model. It was found that LSPR of Au NRs changes markedly with temperature, red shifting and increasing in transmission amplitude as the temperature increases. Furthermore, this variation is highly localized through tunneling by overlapping the near-field of thin cladding and sapphire optical fiber. The NSOF embedded with Au NRs has the potential for sensing in advanced energy generation systems.

Shape Effect on the Refractive Index Sensitivity at Localized Surface Plasmon Resonance Inflection Points of Single Gold Nanocubes with Vertices

Plasmonic gold nanoparticles with sharp tips and vertices, such as gold bipyramids (AuBPs) and gold nanocubes (AuNCs), have been widely used for high-sensitivity localized surface plasmon resonance (LSPR) sensing. However, conventional LSPR sensors based on frequency shifts have a major disadvantage: the asymmetry and broadening of LSPR peaks because of instrumental, environmental, and chemical noises that limit the precise determination of shift positions. Herein, we demonstrated an alternative method to improve the efficiency of the sensors by focusing on homogeneous LSPR scattering inflection points (IFs) of single gold nanoparticles with a single resonant mode. In addition, we investigated the effect of the shape and vertices of AuNCs on the refractive index (RI) sensitivity of homogeneous LSPR IFs by comparing with gold nanospheres (AuNSs) of similar size. The results show that for both AuNCs and AuNSs, tracking homogeneous LSPR IFs allows for higher RI sensitivity than tracking the frequency shifts of the LSPR peaks. Furthermore, single AuNCs with vertices exhibited higher RI sensitivity than single AuNSs of similar size in the homogeneous LSPR IFs. Therefore, we provided a deeper insight into the RI sensitivity of homogeneous LSPR IFs of AuNCs with vertices for their use in LSPR-based biosensors.

High Fluence Chromium and Tungsten Bowtie Nano-antennas

Nano-antennas are replicas of antennas that operate at radio-frequencies, but with considerably smaller dimensions when compared with their radio frequency counterparts. Noble metals based nano-antennas have the ability to enhance photoinduced phenomena such as localized electric fields, therefore-they have been used in various applications ranging from optical sensing and imaging to performance improvement of solar cells. However, such nano-structures can be damaged in high power applications such as heat resisted magnetic recording, solar thermo-photovoltaics and nano-scale heat transfer systems. Having a small footprint, nano-antennas cannot handle high fluences (energy density per unit area) and are subject to being damaged at adequately high power (some antennas can handle just a few milliwatts). In addition, given that nano-antennas are passive devices driven by external light sources, the potential damage of the antennas limits their use with high power lasers: this liability can be overcome by employing materials with high melting points such as chromium (Cr) and tungsten (W). In this article, we fabricate chromium and tungsten nano-antennas and demonstrate that they can handle 110 and 300 times higher fluence than that of gold (Au) counterpart, while the electric field enhancement is not significantly reduced.

Non-vertical optical transition in near-field enhanced spectroscopy of graphene

We theoretically investigate the optical transition of an electron in graphene that is excited by near-field around a conical Au tip. The interaction between the near-field and the electron is calculated by tight-binding method. In the case of near-field, the wavevector of the electron changes by the optical absorption from the valence band to the conduction band. We show that the change of the wavevector is inversely proportional to the localization width of the near-field, which is given as a function of the distance between the tip and graphene. We calculate the near-field absorption probability as a function of k in the Brillouin zone.

Gold as a Possible Alternative to Platinum-Based Chemotherapy for Colon Cancer Treatment

Due to the increasing incidence and high mortality associated with colorectal cancer (CRC), novel therapeutic strategies are urgently needed. Classic chemotherapy against CRC is based on oxaliplatin and other cisplatin analogues; however, platinum-based therapy lacks selectivity to cancer cells and leads to deleterious side effects. In addition, tumor resistance to oxaliplatin is related to chemotherapy failure. Gold(I) derivatives are a promising alternative to platinum complexes, since instead of interacting with DNA, they target proteins overexpressed on tumor cells, thus leading to less side effects than, but a comparable antitumor effect to, platinum derivatives. Moreover, given the huge potential of gold nanoparticles, the role of gold in CRC chemotherapy is not limited to gold(I) complexes. Gold nanoparticles have been found to be able to overcome multidrug resistance along with reduced side effects due to a more efficient uptake of classic drugs. Moreover, the use of gold nanoparticles has enhanced the effect of traditional therapies such as radiotherapy, photothermal therapy, or photodynamic therapy, and has displayed a potential role in diagnosis as a consequence of their optic properties. Herein, we have reviewed the most recent advances in the use of gold(I) derivatives and gold nanoparticles in CRC therapy.

Ultra-high sensitive 1D porous silicon photonic crystal sensor based on the coupling of Tamm/Fano resonances in the mid-infrared region

Porous silicon one-dimensional photonic crystals (PSi-1DPCs) are capable of sensing solutions and liquids based on the smallest variation of the refractive indices. In the present work, we present a novel metal/PSi-1DPC as a liquid sensor based on Tamm/Fano resonances. The operating wavelength range is from 6.35 to 9.85 μm in the mid-infrared (MIR) spectral region. Different metals (Al, Ag, Au, and Pt) are attached to the top surface of the PSi-1DPCs structure to show Tamm/Fano resonances more clearly. To the best of our knowledge, it is the first time that Tamm/Fano resonances exhibit simultaneously in PSi-1DPCs within the same structure. The reflection spectra were calculated for the metal/PSi-1DPC structure by using the transfer matrix method (TMM) and the Bruggeman’s effective medium approximation (BEMA). The simulations show that the Tamm/Fano resonances are red-shifted towards the higher wavelengths with increasing the refractive index of the pores. The Ag/PSi-1DPC sensor showed the highest performance. Its sensitivity can be reached to the value 5018 nm/RIU with a high-quality factor of about 2149.27. We predict the proposed sensors can be easily fabricated and we expect them to show higher performance than other reported sensors of this type. Therefore, it will be of interest in the field of optical sensing in different fields.

Tuning Chemical Interface Damping: Interfacial Electronic Effects of Adsorbate Molecules and Sharp Tips of Single Gold Bipyramids

The optimization of the localized surface plasmon resonance (LSPR)-decaying channels of hot-electrons is essential for efficient optical and photochemical processes. Understanding and having the ability to control chemical interface damping (CID) channel contributions will bring about new possibilities for tuning the efficiency of plasmonic hot-electron energy transfer in artificial devices. In this scanning electron microscopy-correlated dark-field scattering study, the CID was controlled by focusing on the electronic nature of disubstituted benzene rings acting as adsorbates, as well as the effects of sharp tips on gold bipyramids (AuBPs) with similar aspect ratios to those of gold nanorods. The results showed that the sharp tips on single AuBPs, as well as the electronic effects of the adsorbate molecules, increase the interfacial contact between the nanoparticles and adsorbate molecules. Electron withdrawing groups (EWGs) on the adsorbates induce larger homogeneous LSPR line widths compared to those of electron donating groups (EDGs). Depending on the location (ortho, meta, and para) of the EDG, the effect of benzene rings with an EDG, which was considered to be induced by sulfur atoms bound to the nanoparticle surface, is weakened by the back transfer of electrons facilitated by the difference in the availability of the electrons of the EDG. Therefore, this study reports that the CID in the LSPR total decay channels can be tuned by controlling the electron withdrawing and electron donating features of adsorbate molecules with the surface topology of metal.

Study of W/HfO2 grating selective thermal emitters for thermophotovoltaic applications

This paper explores the performance potential of gratings based on tungsten/hafnia (W/HfO₂) stacks for thermophotovoltaic thermal emitters via numerical simulations. Structures consisting of a W grating over a HfO₂ spacer layer and a W substrate are analyzed over a range of geometries. For shallow gratings (W grating thickness much smaller than the grating pitch), an emittance of 99.9% can be achieved for transverse magnetic (TM) polarization, but the transverse electric (TE) performance is appreciably lower. For deep gratings (W grating thickness on the order of the grating pitch), peak emittances of 97.8% and 99.7% for TE and TM polarizations, respectively, are achieved. We find that both surface plasmon polaritons and magnetic polaritons play a crucial role in shaping the emittance for TM radiation. On the other hand, cavity resonances are responsible for the almost perfect emittance in the case of TE polarization. These results suggest that by introducing an HfO₂ layer it is possible to reach high emittance for operating temperatures that match the absorption characteristics of GaSb and InGaAs photovoltaic cells.

Plasmon Controlled Shaping of Metal Nanoparticle Aggregates by Femtosecond Laser-Induced Melting

In this work, we show how to control the morphology of femtosecond laser melted gold nanosphere aggregates. A careful choice of both laser fluence and wavelength makes it possible to selectively excite different aggregate substructures to produce larger spherical nanoparticles, nanorods, and nanoprisms or necklace-like 1D nanostructures in which the nanoparticles are interlinked by bridges. Finite integral technique calculations have been performed on the near-field concentration of light in the nanostructures which confirm the wavelength dependence of the light concentration and suggest that the resulting localized high intensities lead to nonthermal melting. We show that by tuning the wavelength of the melting light it is possible to choose the spatial extension of the ensembles of NPs heated thus allowing us to exhibit control over the morphology of the nanostructures formed by the melting process. By a proper combination of this method with self-assembly of chemically synthesized nanoparticles, one can envisage the development of an innovative high-throughput high-resolution nanofabrication technique.

Homogeneous localized surface plasmon resonance inflection points for enhanced sensitivity and tracking plasmon damping in single gold bipyramids

The most polarizable localized surface plasmon resonance (LSPR) longitudinal mode of anisotropic metallic nanoparticles, such as gold bipyramids (AuBPs), is of high prominence. This optical response has tremendous applications from spectroscopy to photonics and energy devices to sensing. In conventional LSPR-based sensing, broadening and asymmetry in peaks due to chemical and instrument noise hinder obtaining a precise insight on shift positions, accordingly limiting the effectiveness and impact of LSPR sensors. Further, when investigating LSPR properties, utilizing more simplistic frequency dependent dielectric-type models can aberrantly impact the reliability of fundamental properties used for designing and fabricating efficient optical devices. For instance, more approximations can effectively limit screening intra-band and inter-band (IB) electronic transition contributions and other related optical properties. With an aim to find alternative methods to further improve their efficiency, as a first report, we devoted a particular focus on LSPR scattering inflection points (IFs) of single AuBPs. The findings reveal that tracking LSPR IFs exhibit high sensitivity over their counterpart LSPR peak shift locations. In addition, we newly detected IB transition contributions near the resonance energy in the range (1.50 eV-2.00 eV) dominated by intra-band transitions. A small increase in the local RI effectively enhances the LSPR quality factor due to IB transitions. Therefore, while neglecting IB transitions in the range below 2.4 eV can work for local air refractive index (RI), in high local RI media it can be aberrantly underestimated. Demonstrated by the use of the dielectric function based on Kramers-Kronig consistent Lorentz oscillators, our findings are in good agreement with the enhancing RI sensitivity effect. The results of this investigation support the idea that tracking curvature changes of an optical signal can be effectively used for LSPR longitudinal peak RI sensing as well as damping in the local RI environment of a single AuBP.

Synthesis of Gold Nanoparticles Using Leaf Extract of Ziziphus zizyphus and their Antimicrobial Activity

(1) Background: There is a growing need for the development of new methods for the synthesis of nanoparticles. The interest in such particles has raised concerns about the environmental safety of their production methods; (2) Objectives: The current methods of nanoparticle production are often expensive and employ chemicals that are potentially harmful to the environment, which calls for the development of “greener” protocols. Herein we describe the synthesis of gold nanoparticles (AuNPs) using plant extracts, which offers an alternative, efficient, inexpensive, and environmentally friendly method to produce well-defined geometries of nanoparticles; (3) Methods: The phytochemicals present in the aqueous leaf extract acted as an effective reducing agent. The generated AuNPs were characterized by Transmission electron microscopy (TEM), Scanning electron microscope (SEM), and Atomic Force microscopy (AFM), X-ray diffraction (XRD), UV-visible spectroscopy, energy dispersive X-ray (EDX), and thermogravimetric analyses (TGA); (4) Results and Conclusions: The prepared nanoparticles were found to be biocompatible and exhibited no antimicrobial or antifungal effect, deeming the particles safe for various applications in nanomedicine. TGA analysis revealed that biomolecules, which were present in the plant extract, capped the nanoparticles and acted as stabilizing agents.

Insight on agglomerates of gold nanoparticles in glass based on surface plasmon resonance spectrum: study by multi-spheres T-matrix method

The formation of a localized surface plasmon resonance (SPR) spectrum of randomly distributed gold nanoparticles in the surface layer of silicate float glass, generated and implanted by UV ArF-excimer laser irradiation of a thin gold layer sputter-coated on the glass surface, was studied by the T-matrix method, which enables particle agglomeration to be taken into account. The experimental technique used is promising for the production of submicron patterns of plasmonic nanoparticles (given by laser masks or gratings) without damage to the glass surface. Analysis of the applicability of the multi-spheres T-matrix (MSTM) method to the studied material was performed through calculations of SPR characteristics for differently arranged and structured gold nanoparticles (gold nanoparticles in solution, particles pairs, and core-shell silver-gold nanoparticles) for which either experimental data or results of the modeling by other methods are available. For the studied gold nanoparticles in glass, it was revealed that the theoretical description of their SPR spectrum requires consideration of the plasmon coupling between particles, which can be done effectively by MSTM calculations. The obtained statistical distributions over particle sizes and over interparticle distances demonstrated the saturation behavior with respect to the number of particles under consideration, which enabled us to determine the effective aggregate of particles, sufficient to form the SPR spectrum. The suggested technique for the fitting of an experimental SPR spectrum of gold nanoparticles in glass by varying the geometrical parameters of the particles aggregate in the recurring calculations of spectrum by MSTM method enabled us to determine statistical characteristics of the aggregate: the average distance between particles, average size, and size distribution of the particles. The fitting strategy of the SPR spectrum presented here can be applied to nanoparticles of any nature and in various substances, and, in principle, can be extended for particles with non-spherical shapes, like ellipsoids, rod-like and other T-matrix-solvable shapes.

Quantifying Remote Heating from Propagating Surface Plasmon Polaritons

We report a method to electrically detect heating from excitation of propagating surface plasmon polaritons (SPP). The coupling between SPP and a continuous wave laser beam is realized through lithographically defined gratings in the electrodes of thin film gold "bow tie" nanodevices. The propagating SPPs allow remote coupling of optical energy into a nanowire constriction. Heating of the constriction is detectable through changes in the device conductance and contains contributions from both thermal diffusion of heat generated at the grating and heat generated locally at the constriction by plasmon dissipation. We quantify these contributions through computational modeling and demonstrate that the propagation of SPPs provides the dominant contribution. Coupling optical energy into the constriction via propagating SPPs in this geometry produces an inferred temperature rise of the constriction a factor of 60 smaller than would take place if optical energy were introduced via directly illuminating the constriction. The grating approach provides a path for remote excitation of nanoconstrictions using SPPs for measurements that usually require direct laser illumination, such as surface-enhanced Raman spectroscopy.

Controlling the Heat Dissipation in Temperature-Matched Plasmonic Nanostructures

Heat dissipation in a plasmonic nanostructure is generally assumed to be ruled only by its own optical response even though also the temperature should be considered for determining the actual energy-to-heat conversion. Indeed, temperature influences the optical response of the nanostructure by affecting its absorption efficiency. Here, we show both theoretically and experimentally how, by properly nanopatterning a metallic surface, it is possible to increase or decrease the light-to-heat conversion rate depending on the temperature of the system. In particular, by borrowing the concept of matching condition from the classical antenna theory, we first analytically demonstrate how the temperature sets a maximum value for the absorption efficiency and how this quantity can be tuned, thus leading to a temperature-controlled optical heat dissipation. In fact, we show how the nonlinear dependence of the absorption on the electron-phonon damping can be maximized at a specific temperature, depending on the system geometry. In this regard, experimental results supported by numerical calculations are presented, showing how geometrically different nanostructures can lead to opposite dependence of the heat dissipation on the temperature, hence suggesting the fascinating possibility of employing plasmonic nanostructures to tailor the light-to-heat conversion rate of the system.

Surface plasmon resonance in gold nanoparticles: a review

In the last two decades, plasmon resonance in gold nanoparticles (Au NPs) has been the subject of intense research efforts. Plasmon physics is intriguing and its precise modelling proved to be challenging. In fact, plasmons are highly responsive to a multitude of factors, either intrinsic to the Au NPs or from the environment, and recently the need emerged for the correction of standard electromagnetic approaches with quantum effects. Applications related to plasmon absorption and scattering in Au NPs are impressively numerous, ranging from sensing to photothermal effects to cell imaging. Also, plasmon-enhanced phenomena are highly interesting for multiple purposes, including, for instance, Raman spectroscopy of nearby analytes, catalysis, or sunlight energy conversion. In addition, plasmon excitation is involved in a series of advanced physical processes such as non-linear optics, optical trapping, magneto-plasmonics, and optical activity. Here, we provide the general overview of the field and the background for appropriate modelling of the physical phenomena. Then, we report on the current state of the art and most recent applications of plasmon resonance in Au NPs.

Dynamics of Strong Coupling between CdSe Quantum Dots and Surface Plasmon Polaritons in Subwavelength Hole Array

We have investigated the strong coupling interaction between excitons of CdSe quantum dots (QDs) and surface plasmon polaritons (SPPs) of gold nanohole array by steady-state spectroscopic method and transient absorption measurements. Numerical and experimental steady-state measurements demonstrate that the SPP-QD system can indeed undergo strong coupling, characterized by a Rabi splitting up to 220 meV. In particular, it is found that in the transient absorption spectra, under resonant excitation, the 1S transition bleaching band from uncoupled CdSe QDs is completely separated into two distinctive bleaching bands, remarkably fingerprinting the hybrid SPP-QD state. It was also found that the lifetime of these hybrid bands is just slightly shorter than the lifetime of bare CdSe QDs, possibly caused by the phonon bottleneck effect due to the large Rabi splitting. These results could open a new avenue toward the development of novel nanoplasmon devices with strong SPP-QD interaction.

Suppression of surface plasmon resonance in Au nanoparticles upon transition to the liquid state

Significant suppression of resonant properties of single gold nanoparticles at the surface plasmon frequency during heating and subsequent transition to the liquid state has been demonstrated experimentally and explained for the first time. The results for plasmonic absorption of the nanoparticles have been analyzed by means of Mie theory using experimental values of the optical constants for the liquid and solid metal. The good qualitative agreement between calculated and experimental spectra support the idea that the process of melting is accompanied by an abrupt increase of the relaxation constants, which depends, beside electron-phonon coupling, on electron scattering at a rising number of lattice defects in a particle upon growth of its temperature, and subsequent melting as a major cause for the observed plasmonic suppression. It is emphasized that observed effect is fully reversible and may underlie nonlinear optical responses of nanocolloids and composite materials containing plasmonic nanoparticles and their aggregates in conditions of local heating and in general, manifest itself in a wide range of plasmonics phenomena associated with strong heating of nanoparticles.

Plasmonic Heating in Au Nanowires at Low Temperatures: The Role of Thermal Boundary Resistance

Inelastic electron tunneling and surface-enhanced optical spectroscopies at the molecular scale require cryogenic local temperatures even under illumination-conditions that are challenging to achieve with plasmonically resonant metallic nanostructures. We report a detailed study of the laser heating of plasmonically active nanowires at substrate temperatures from 5 to 60 K. The increase of the local temperature of the nanowire is quantified by a bolometric approach and could be as large as 100 K for a substrate temperature of 5 K and typical values of laser intensity. We also demonstrate that a ∼3-fold reduction of the local temperature increase is possible by switching to a sapphire or quartz substrate. Finite element modeling of the heat dissipation reveals that the local temperature increase of the nanowire at temperatures below ∼50 K is determined largely by the thermal boundary resistance of the metal-substrate interface. The model reproduces the striking experimental trend that in this regime the temperature of the nanowire varies nonlinearly with the incident optical power. The thermal boundary resistance is demonstrated to be a major constraint on reaching low temperatures necessary to perform simultaneous inelastic electron tunneling and surface-enhanced Raman spectroscopies.

The role of Rabi splitting tuning in the dynamics of strongly coupled J-aggregates and surface plasmon polaritons in nanohole arrays

We have investigated the influence of Rabi splitting tuning on the dynamics of strongly coupled J-aggregate/surface plasmon polariton systems. In particular, the Rabi splitting was tuned by modifying the J-aggregate molecule concentration while a polaritonic system was provided by a nanostructure formed by holes array in a golden layer. From the periodic and concentration changes we have identified, through numerical and experimental steady-state analyses, the best geometrical configuration for maximizing Rabi splitting, which was then used for transient absorption measurements. It was found that in transient absorption spectra, under upper band excitation, two bleaching peaks appear when a nanostructured polaritonic pattern is used. Importantly, their reciprocal distance increases upon increase of J-aggregate concentration, a result confirmed by steady-state analysis. In a similar manner it was also found that the lifetime of the upper band is intimately related to the coupling strength. In particular, we argue that with strong coupling strength, i.e. high J-aggregate concentration, a short lifetime of the upper band has to be expected due to the suppression of the bottleneck effect. This result supports the idea that the dynamics of hybrid systems is profoundly dependent on Rabi splitting.

Detection of single amino acid mutation in human breast cancer by disordered plasmonic self-similar chain

Control of the architecture and electromagnetic behavior of nanostructures offers the possibility of designing and fabricating sensors that, owing to their intrinsic behavior, provide solutions to new problems in various fields. We show detection of peptides in multicomponent mixtures derived from human samples for early diagnosis of breast cancer. The architecture of sensors is based on a matrix array where pixels constitute a plasmonic device showing a strong electric field enhancement localized in an area of a few square nanometers. The method allows detection of single point mutations in peptides composing the BRCA1 protein. The sensitivity demonstrated falls in the picomolar (10(-12) M) range. The success of this approach is a result of accurate design and fabrication control. The residual roughness introduced by fabrication was taken into account in optical modeling and was a further contributing factor in plasmon localization, increasing the sensitivity and selectivity of the sensors. This methodology developed for breast cancer detection can be considered a general strategy that is applicable to various pathologies and other chemical analytical cases where complex mixtures have to be resolved in their constitutive components.

Bimetallic 3D nanostar dimers in ring cavities: recyclable and robust surface-enhanced Raman scattering substrates for signal detection from few molecules

Top-down fabrication of electron-beam lithography (EBL)-defined metallic nanostructures is a successful route to obtain extremely high electromagnetic field enhancement via plasmonic effects in well-defined regions. To this aim, various geometries have been introduced such as disks, triangles, dimers, rings, self-similar lenses, and more. In particular, metallic dimers are highly efficient for surface-enhanced Raman spectroscopy (SERS), and their decoupling from the substrate in a three-dimensional design has proven to further improve their performance. However, the large fabrication time and cost has hindered EBL-defined structures from playing a role in practical applications. Here we present three-dimensional nanostar dimer devices that can be recycled via maskless metal etching and deposition processes, due to conservation of the nanostructure pattern in the 3D geometry of the underlying Si substrate. Furthermore, our 3D-nanostar-dimer-in-ring structures (3D-NSDiRs) incorporate several advantageous aspects for SERS by enhancing the performance of plasmonic dimers via an external ring cavity, by efficient decoupling from the substrate through an elevated 3D design, and by bimetallic AuAg layers that exploit the increased performance of Ag while maintaining the biocompatibility of Au. We demonstrate SERS detection on rhodamine and adenine at extremely low density up to the limit of few molecules and analyze the field enhancement of the 3D-NSDiRs with respect to the exciting wavelength and metal composition.