Macroscale colloidal noble metal nanocrystal arrays and their refractive index-based sensing characteristics

Thermally and Electrically Regulated Plasmonic Devices Based on VO2-Covered Gold Nanoplate Arrays with SiO2 Interface Layer for Large Plasmon Shifts

Integrating metal nanoparticles with vanadium dioxide (VO2) is an effective means to realize active plasmonic regulation which has great application potential in optical devices that respond in real-time to external stimuli. However, the high temperature necessary for VO2 growth severely reshapes the metal nanoparticles, causing reduced refractive index (RI) sensitivity and degraded modulation performance. Herein, we construct a large-area dynamically tunable plasmonic system composed of a VO2-covered array of hexagonal gold nanoplates (AuNPLs). By introducing a SiO2 interface layer, the thermal tolerance of the AuNPLs is effectively improved, making the high RI sensitivity (∼368.3 nm/RIU at 855 nm) survive the subsequent VO2 deposition. Through tuning the localized surface plasmon resonance (LSPR) of the AuNPL array and the thickness of the VO2 film, the LSPR-related transmission dip can be tailored to the near-infrared region where VO2 shows a large two-phase RI contrast, a dip shift up to 272 nm is therefore realized upon VO2 phase transition. Furthermore, electro-optic modulation is demonstrated through electrically triggered VO2 partial phase transition which is accompanied by a gradually changed effective dielectric permittivity, and a continuous shift of the transmission dip from 1070 to 860 nm is achieved by varying the applied electrical current flowing through the film. This work provides a feasible route for controllably constructing stimuli-response optical devices with large wavelength modulation amplitude.

Bi-path color tunable plasmonic micro-nano hybrid structures for encrypted printing

Colored information is crucial for humans to perceive the world. Plasmonic spectra modulation can serve as an effective means to create different colors. Although several solutions for plasmonic color-printing have been proposed, further information encryption has not received any attention. Herein, we exhibit a fine color modulation strategy to construct noble-metal-based micro-nano hybrid structures in the bi-path of photo-thermal deformation and liquid-phase-chemical reaction. Ag/Ta2O5 bi-layer films are ablated at the center of the machined lines of nanosecond pulsed laser, while silver nanoparticles are formed in other regions by thermal radiation of the infrared laser, which can be further dissolved and shape-modulated in KCl solution under different periods. The variation of size and spacing of nano-Ag particles results in a precise shift of plasmonic spectra in visible region. Colored information can be hidden by adjusting the scan number and the energy density during laser processing, and will emerge after the subsequent chemical dissolution reactions. The bi-path color adjustment strategy is easy to operate and can play a role in key information protection and color image switching.

Gold Nanorods: The Most Versatile Plasmonic Nanoparticles

Gold nanorods (NRs), pseudo-one-dimensional rod-shaped nanoparticles (NPs), have become one of the burgeoning materials in the recent years due to their anisotropic shape and adjustable plasmonic properties. With the continuous improvement in synthetic methods, a variety of materials have been attached around Au NRs to achieve unexpected or improved plasmonic properties and explore state-of-the-art technologies. In this review, we comprehensively summarize the latest progress on Au NRs, the most versatile anisotropic plasmonic NPs. We present a representative overview of the advances in the synthetic strategies and outline an extensive catalogue of Au-NR-based heterostructures with tailored architectures and special functionalities. The bottom-up assembly of Au NRs into preprogrammed metastructures is then discussed, as well as the design principles. We also provide a systematic elucidation of the different plasmonic properties associated with the Au-NR-based structures, followed by a discussion of the promising applications of Au NRs in various fields. We finally discuss the future research directions and challenges of Au NRs.

Site-Selective Deposition of Metal-Organic Frameworks on Gold Nanobipyramids for Surface-Enhanced Raman Scattering

Site-selective deposition of metal-organic frameworks (MOFs) on metal nanocrystals has remained challenging because of the difficult control of the nucleation and growth of MOFs. Herein we report on a facile wet-chemistry approach for the selective deposition of zeolitic imidazolate framework-8 (ZIF-8) on anisotropic Au nanobipyramids (NBPs) and nanorods. ZIF-8 is selectively deposited at the ends and waist and around the entire surface of the elongated Au nanocrystals. The NBP-based nanostructures with end-deposited ZIF-8 exhibit the best surface-enhanced Raman scattering (SERS) performance, implying that molecules can be concentrated by ZIF-8 at the hot spots. In addition, the SERS signal exhibits good selectivity for small molecules because of the molecular sieving effect of ZIF-8. This study opens up a promising route for constructing plasmonic nanostructures with site selectively deposited ZIF-8, which hold enormous potential for molecular sensing, optical switching, and plasmonic catalysis.

Facet- and Gas-Dependent Reshaping of Au Nanoplates by Plasma Treatment

The reshaping of metal nanocrystals on substrates is usually realized by pulsed laser irradiation or ion-beam milling with complex procedures. In this work, we demonstrate a simple method for reshaping immobilized Au nanoplates through plasma treatment. Au nanoplates can be reshaped gradually with nearly periodic right pyramid arrays formed on the surface of the nanoplates. The gaseous environment in the plasma-treatment system plays a significant role in the reshaping process with only nitrogen-containing environments leading to reshaping. The reshaping phenomenon is facet-dependent, with right pyramids formed only on the exposed {111} facets of the Au nanoplates. The morphological change of the Au nanoplates induced by the plasma treatment leads to large plasmon peak redshifts. The reshaped Au nanoplates possess slightly higher refractive index sensitivities and largely increased surface-enhanced Raman scattering intensities compared to the flat, untreated nanoplates. Our results offer insights for studying the interaction mechanism between plasma and the different facets of noble metal nanocrystals and an approach for reshaping light-interacting noble metal nanocrystals.

Electrochemical coating of different conductive polymers on diverse plasmonic metal nanocrystals

Conductive polymers are attracting much attention for realizing active plasmonics on conventional static plasmonic nanostructures because of their variable dielectric functions. Combining organic conductive polymers with inorganic plasmonic nanostructures allows for the creation of active devices, such as active metasurfaces, reconfigurable metalenses and dynamic plasmonic holography. However, the complexity of such a combination, together with the poor control in polymer thickness and morphology, has limited the advancement of active plasmonics. Herein we report on the electrochemical coating of conductive polymers on pre-grown metal nanocrystals. Robust control of the polymer thickness and morphology is accomplished through the variation of the applied electrochemical potential. Various types of conductive polymers are coated on different metal nanocrystals, including Au, Pd and Pt. Active plasmonic color switching and H2O2 sensing are demonstrated with polyaniline-coated Au nanorods.

Plasmonically enabled two-dimensional material-based optoelectronic devices

Two-dimensional (2D) materials, such as graphene, transition metal dichalcogenides, black phosphorus and hexagonal boron nitride, have been intensively investigated as building blocks for optoelectronic devices in the past few years. Very recently, significant efforts have been devoted to the improvement of the optoelectronic performances of 2D materials, which are restricted by their intrinsically low light absorption due to the ultrathin thickness. Making use of the plasmonic effects of metal nanostructures and intrinsic plasmon excitation in graphene has been shown to be one of the promising strategies. In this minireview, recent progress in 2D material-based optoelectronics enabled by the plasmonic effects is highlighted. A perspective on more possibilities in plasmon-assisted 2D material-based optoelectronic applications will also be provided.

Gap-Induced Giant Third-Order Optical Nonlinearity and Long Electron Relaxation Time in Random-Distributed Gold Nanorod Arrays

The third-order optical nonlinearities and the hot electron relaxation time (τ) of random-distributed gold nanorods arrays on glass (R-GNRA) have been investigated by using Z-scan and optical Kerr effect (OKE) techniques. Large third-order optical susceptibility (χ⁽³⁾) with the value of 2.5 × 10^-6 esu has been obtained around the plamsonic resonance peak under the excitation power intensity of 0.1 GW/cm². Further decrease of the excitation power intensity down to 0.3 MW/cm² will lead to the significant increase of χ⁽³⁾ up to 6.4 × 10^-4 esu. The OKE results show that the relaxation time of R-GNRA around the plasmonic peak is 13.9 ± 0.4 ps, which is more than 4 times longer than those of the individual gold nanostructures distributed in water solutions. The Finite-difference time domain simulations demonstrate that this large enhancement of χ⁽³⁾ and slow down of τ are caused by the gap-induced large local field enhancement of GNRs dimers in R-GNRA. These significant results offer great opportunities for plasmonic nanostructures in applications of photonic and photocatalytic devices.

A self-assembled plasmonic optical fiber nanoprobe for label-free biosensing

The plasmonic optical fiber sensors have attracted wide attention for label-free biosensing application because of their high integration, small footprint and point-of-care measurement. However, the integration of plasmonic nanostructures on optical fiber probes always relies on the top-down nanofabrication approaches, which have several inherent shortcomings, including high cost, time-consuming, and low yields. Here, we develop a plasmonic nanohole-patterned multimode optical fiber probe by self-assembly nanosphere lithography technique with low fabrication cost and high yields. The multimode optical fiber possesses large facet area and high numerical aperture, which not only simplifies fabrication process, but also increases coupling efficiency of incident light. Originating from the resonant coupling of plasmonic modes, the plasmonic fiber nanoprobe has a distinct reflection dip in the spectrum and exhibits strong near-field electromagnetic enhancement. We experimentally investigate the sensing performances of plasmonic fiber nanoprobe, and further demonstrate it in real-time monitoring specific binding of protein molecules. The experimental results imply that the nanohole-patterned multimode optical fiber probe is a good candidate for developing miniaturized and portable biosensing systems.

Plasmonic and sensing properties of vertically oriented hexagonal gold nanoplates

Plasmonic metal nanocrystals, owing to their high sensitivity to the dielectric changes in the surrounding environment, can allow for the direct probing and monitoring of molecular binding on their surfaces. Anisotropic Au nanoplates possess high refractive index sensitivities, with their nanoscale sensing volumes located at their sharp tips and edges. One of the main challenges in molecular detection based on localized plasmon resonance is to increase the detection capability at the single-particle level. Vertically oriented Au nanoplates are more attractive candidates for developing ultrasensitive plasmonic sensors than horizontally oriented ones, as vertical Au nanoplates allow for more analyte molecules to access their sharp tips and edges. However, few reports have studied the sensing performance of vertically oriented, elongated, individual metal nanocrystals. Herein we report on the deposition of vertically oriented, hexagonal Au nanoplates on substrates and the study of their plasmonic and sensing properties. The vertically aligned nanoplates are compared with the horizontally oriented ones both experimentally and through numerical simulations. The vertically oriented nanoplates possess shorter plasmon resonance wavelengths and narrower peak widths than the horizontally oriented ones of similar sizes. The shorter plasmon wavelengths and smaller peak widths are also confirmed by knocking down the vertical Au nanoplates through mild perturbation and performing the scattering measurements beforehand and afterwards. Further scattering measurements of the Au nanoplates upon the adsorption of positively charged polyelectrolyte reveal that the vertical Au nanoplates are more sensitive to the polyelectrolyte molecules than the horizontal ones.

Advanced Plasmonic Materials for Dynamic Color Display

Plasmonic structures exhibit promising applications in high-resolution and durable color generation. Research on advanced hybrid plasmonic materials that allow dynamically reconfigurable color control has developed rapidly in recent years. Some of these results may give rise to practically applicable reflective displays in living colors with high performance and low power consumption. They will attract broad interest from display markets, compared with static plasmonic color printing, for example, in applications such as digital signage, full-color electronic paper, and electronic device screens. In this progress report, the most promising recent examples of utilizing advanced plasmonic materials for the realization of dynamic color display are highlighted and put into perspective. The performances, advantages, and disadvantages of different technologies are discussed, with emphasis placed on both the potential and possible limitations of various hybrid materials for dynamic plasmonic color display.

Understanding the roles of plasmonic Au nanocrystal size, shape, aspect ratio and loading amount in Au/g-C3N4 hybrid nanostructures for photocatalytic hydrogen generation

Hybrid photocatalysts made of pre-grown gold nanocrystals and graphitic carbon nitride nanosheets are synthesized, studied and optimized for boosting photocatalytic hydrogen production.

Morphology-selective synthesis of Cu(NO3)2·2.5H2O micro/nanostructures achieved by rational manipulation of nucleation pathways and their morphology-preserved conversion to CuO porous micro/nanostructures

A special Cu(NO₃)₂ thin solution layer was built on highly hydrophilic mica surfaces and subjected to rapid evaporation. Controlling the evaporation time can intentionally manipulate the supersaturation.

Refractive index sensing using the metal layer in DVD-R discs

In recent years, grating based bio-sensors have received much attention due to their promising applications in integrated sensing devices. However, production of high quality, large scale and low cost metal gratings is still challenging. Here, we introduce an extremely simple and low cost method to fabricate metal gratings by peeling off the metal layer from a DVD-R disc. An atomic force microscope image shows that the metal layer is a high quality grating, the period and depth of which are 740 nm and 86 nm, respectively. Based on the fabricated metal grating, refractive index sensing is experimentally achieved using two configurations, where either the resonant wavelength or the modulated laser power is measured. The sensitivity of the sensor by wavelength modulation reaches as high as 637 nmRIU⁻¹, which is comparable with or even higher than that of the existing grating coupled sensors. Our method largely reduces the cost to fabricate high quality metal gratings and will promote the development of grating based SPR sensors.

Here, we introduce an extremely simple and low cost method to fabricate metal gratings by peeling off the metal layer from a DVD-R disc.

Amplifying the signal of localized surface plasmon resonance sensing for the sensitive detection of Escherichia coli O157:H7

Gold nanorods (Au NRs) based localized surface plasmon resonance (LSPR) sensors have been widely employed in various fields including biology, environment and food safety detection, but their size- and shape-dependent sensitivity limits their practical applications in sensing and biological detection. In our present work, we proposed an approach to maximally amplify the signal of Au NRs based LSPR sensing by coating an optimized thickness of mesoporous silica onto Au NRs. The plasmonic peaks of Au NRs@SiO₂ with different shell thickness showed finely linear response to the change of surrounding refractive index. The optimized thickness of mesoporous silica of Au NRs@SiO₂ not only provided high stability for LSPR sensor,but also displayed much higher sensitivity (390 nm/RIU) than values of Au NRs from previous reports. The obtained Au NRs@SiO₂ based LSPR sensor was further used in practical application for selectively detection of the E. coli O157:H7, and the detection limit achieved 10 CFU, which is much lower than conventional methods such as electrochemical methods and lateral-flow immunochromatography.

Refractive Index Sensor Based on Fano Resonances in Metal-Insulator-Metal Waveguides Coupled with Resonators

A surface plasmon polariton refractive index sensor based on Fano resonances in metal–insulator–metal (MIM) waveguides coupled with rectangular and ring resonators is proposed and numerically investigated using a finite element method. Fano resonances are observed in the transmission spectra, which result from the coupling between the narrow-band spectral response in the ring resonator and the broadband spectral response in the rectangular resonator. Results are analyzed using coupled-mode theory based on transmission line theory. The coupled mode theory is employed to explain the Fano resonance effect, and the analytical result is in good agreement with the simulation result. The results show that with an increase in the refractive index of the fill dielectric material in the slot of the system, the Fano resonance peak exhibits a remarkable red shift, and the highest value of sensitivity (S) is 1125 nm/RIU, RIU means refractive index unit. Furthermore, the coupled MIM waveguide structure can be integrated with other photonic devices at the chip scale. The results can provide a guide for future applications of this structure.

An Optical Sensor with Polyaniline-Gold Hybrid Nanostructures for Monitoring pH in Saliva

Saliva contains important personal physiological information that is related to some diseases, and it is a valuable source of biochemical information that can be collected rapidly, frequently, and without stress. In this article, we reported a new and simple localized surface plasmon resonance (LSPR) substrate composed of polyaniline (PANI)-gold hybrid nanostructures as an optical sensor for monitoring the pH of saliva samples. The overall appearance and topography of the substrates, the composition, and the wettability of the LSPR surfaces were characterized by optical and scanning electron microscope (SEM) images, infrared spectra, and contact angles measurement, respectively. The PANI-gold hybrid substrate readily responded to the pH. The response time was very short, which was 3.5 s when the pH switched from 2 to 7, and 4.5 s from 7 to 2. The changes of visible-near-infrared (NIR) spectra of this sensor upon varying pH in solution showed that-for the absorption at given wavelengths of 665 nm and 785 nm-the sensitivities were 0.0299 a.u./pH (a.u. = arbitrary unit) with a linear range of pH = 5-8 and 0.0234 a.u./pH with linear range of pH = 2-8, respectively. By using this new sensor, the pH of a real saliva sample was monitored and was consistent with the parallel measurements with a standard laboratory method. The results suggest that this novel LSPR sensor shows great potential in the field of mobile healthcare and home medical devices, and could also be modified by different sensitive materials to detect various molecules or ions in the future.

Dual-band moiré metasurface patches for multifunctional biomedical applications

There has been strong interest in developing multi-band plasmonic metasurfaces for multiple optical functions on single platforms. Herein, we developed Au moiré metasurface patches (AMMP), which leverage the tunable multi-band responses of Au moiré metasurfaces and the additional field enhancements of the metal-insulator-metal configuration to achieve dual-band plasmon resonance modes in near-infrared and mid-infrared regimes with high field enhancement. Furthermore, we demonstrate the multifunctional applications of AMMP, including surface-enhanced infrared spectroscopy, optical capture and patterning of bacteria, and photothermal denaturation of proteins. With their multiple functions of high performance, in combination with cost-effective fabrication using moiré nanosphere lithography, the AMMP will enable the development of highly integrated biophotonic platforms for a wide range of applications in disease theranostics, sterilization, and the study of microbiomes.

Role of shape in substrate-induced plasmonic shift and mode uncovering on gold nanocrystals

A number of plasmonic devices and applications, such as chemical and biological sensors, plasmon-enhanced solar cells, optical nanoantennas, metamaterials and metasurfaces, require the deposition of plasmonic metal nanocrystals on various substrates. Because the localized plasmon resonance modes, energies and strengths are strongly dependent on the dielectric function of the surrounding environment, the substrate is expected to largely affect the plasmonic properties of supported metal nanocrystals. Therefore, understanding the effects of the substrate on the plasmonic properties of metal nanocrystals and the roles of the involved factors will be crucial for designing various plasmonic devices that are made of metal nanocrystals deposited on different substrates. Herein we report on our study and results of the effects of substrates with distinct dielectric functions on the plasmonic properties of three types of Au nanocrystals. A combination of experiments and numerical simulations shows that the presence of a substrate causes plasmonic shifts as well as the appearance of new plasmon modes. The plasmonic shifts and the emergence of new plasmon modes are found to be dependent on the particle shape of Au nanocrystals and in turn on the fractional particle surface area that is in contact with the supporting substrate. For Au nanospheres and nanorods, plasmonic shifts, less than 100 nm, are observed on the scattering spectra by changing the supporting substrate from indium tin oxide to silicon. In comparison, a giant spectral shift of more than 300 nm is obtained for Au nanoplates. Moreover, silicon substrates induce the emergence of an out-of-plane quadrupolar mode of Au nanoplates, which interacts with an out-of-plane octupolar mode to give rise to a distinct Fano resonance. The Fano resonance is found to become stronger as the thickness of Au nanoplates is decreased. These results are of great importance for understanding the plasmonic properties of noble metal nanocrystals supported on various substrates, and for designing novel plasmonic nanostructures with desired optical properties and functions.

Sensitivity Tuning through Additive Heterogeneous Plasmon Coupling between 3D Assembled Plasmonic Nanoparticle and Nanocup Arrays

Plasmonic substrates have fixed sensitivity once the geometry of the structure is defined. In order to improve the sensitivity, significant research effort has been focused on designing new plasmonic structures, which involves high fabrication costs; however, a method is reported for improving sensitivity not by redesigning the structure but by simply assembling plasmonic nanoparticles (NPs) near the evanescent field of the underlying 3D plasmonic nanostructure. Here, a nanoscale Lycurgus cup array (nanoLCA) is employed as a base colorimetric plasmonic substrate and an assembly template. Compared to the nanoLCA, the NP assembled nanoLCA (NP-nanoLCA) exhibits much higher sensitivity for both bulk refractive index sensing and biotin-streptavidin binding detection. The limit of detection of the NP-nanoLCA is at least ten times smaller when detecting biotin-streptavidin conjugation. The numerical calculations confirm the importance of the additive plasmon coupling between the NPs and the nanoLCA for a denser and stronger electric field in the same 3D volumetric space. Tunable sensitivity is accomplished by controlling the number of NPs in each nanocup, or the number density of the hot spots. This simple yet scalable and cost-effective method of using additive heterogeneous plasmon coupling effects will benefit various chemical, medical, and environmental plasmon-based sensors.

Tunable fluorescence lifetime of Eu-PMMA films with plasmonic nanostructures for multiplexing

A method to tune fluorescence lifetime of Eu-PMMA films is proposed, which consists of self-assembled gold nanorods on glass substrate covered by Eu-PMMA shell. The fluorescence lifetime is tunable in a wide range, and depends on aspect ratio and mutual distance of gold nanorods. In a single red color emission channel, more than six distinct fluorescence lifetime populations ranging from 356 to 513 μs are obtained. Through theoretical calculation, we attribute tunable fluorescence lifetime to the change of radiative and nonradiative decay rate and density of photon states. In addition, we use these as-prepared Eu-PMMA films for security data storage to demonstrate optical multiplexing applications. The optical multiplexing experiments show an interesting pseudo-information "8" and conceal the real messages "2" and "6".

Influence of Surfactant Bilayers on the Refractive Index Sensitivity and Catalytic Properties of Anisotropic Gold Nanoparticles

Shape-controlled synthesis of gold nanoparticles generally involves the use of surfactants, typically cetyltrimethylammonium (CTAX, X = Cl(-) , Br(-)), to regulate the nucleation growth process and to obtain colloidally stable nanoparticles. The surfactants adsorb on the nanoparticle surface making further functionalization difficult and therefore limit their use in many applications. Herein, the influence of CTAX on nanoparticle sensitivity to local dielectric environment changes is reported. It is shown, both experimentally and theoretically, that the CTAX bilayer significantly reduces the refractive index (RI) sensitivity of anisotropic gold nanoparticles such as nanocubes and concave nanocubes, nanorods, and nanoprisms. The RI sensitivity can be increased by up to 40% by removing the surfactant layer from nanoparticles immobilized on a solid substrate using oxygen plasma treatment. This increase compensates for the otherwise problematic decrease in RI sensitivity caused by the substrate effect. Moreover, the removal of the surfactants both facilitates nanoparticle biofunctionalization and significantly improves their catalytic properties. The strategy presented herein is a simple yet effective universal method for enhancing the RI sensitivity of CTAX-stabilized gold nanoparticles and increasing their potential as transducers in nanoplasmonic sensors, as well as in catalytic and biomedical applications.

Noble metal plasmonic nanostructure related chromisms

Chromisms related to noble metal nanostructures are classified and discussed.

Tunable multiband metasurfaces by moiré nanosphere lithography

Moiré nanosphere lithography (MNSL), which features the relative in-plane rotation between two layers of self-assembled monodisperse nanospheres as masks, provides a cost-effective approach for creating moiré patterns on generic substrates. In this work, we experimentally and numerically investigate a series of moiré metasurfaces by MNSL. Due to the variety of gradient plasmonic nanostructures in arrays, single moiré metasurfaces can support multiple localized surface plasmon (LSP) modes with a wide range of resonant wavelengths from ∼600 nm to ∼4200 nm. We analyze the origin of the LSP modes based on the optical spectra and near-field electromagnetic distributions. In addition, we fabricate and analyze the metasurfaces with high-density nanogap structures. These nanogap structures support plasmonic gap modes with significant field enhancements. With their tunable multiband optical responses from visible to near-infrared to mid-infrared regimes, these moiré metasurfaces are applicable for ultrabroadband absorbers, multiband surface-enhanced infrared and Raman spectroscopy, and broadband single-molecule spectroscopy.

Polarization state-based refractive index sensing with plasmonic nanostructures

Spectral-based methods are often used for label-free biosensing. However, practical implementations with plasmonic nanostructures suffer from a broad line width caused by strong radiative and nonradiative losses, and the sensing performance characterized by figure of merit is poor for these spectral-based methods. This study provides a polarization state-based method using plasmonic nanostructures to improve the sensing performance. Instead of the intensity spectrum, the polarization state of the transmitted field is monitored to analyze variations of the surrounding medium. The polarization state of incidence is strongly modified due to the excitation of surface plasmons, and the ellipticity of the transmitted field changes dramatically around plasmon resonances. Sharp resonances with line widths down to sub-nanometer are achieved by plotting the spectra of the reciprocal of ellipticity. Therefore, the sensing performance can be significantly improved, and a theoretical value of the figure of merit exceeding 1700 is achieved by using the polarization state-based sensing approach.

Engineering of parallel plasmonic-photonic interactions for on-chip refractive index sensors

Ultra-narrow linewidth in the extinction spectrum of noble metal nanoparticle arrays induced by the lattice plasmon resonances (LPRs) is of great significance for applications in plasmonic lasers and plasmonic sensors. However, the challenge of sustaining LPRs in an asymmetric environment greatly restricts their practical applications, especially for high-performance on-chip plasmonic sensors. Herein, we fully study the parallel plasmonic-photonic interactions in both the Au nanodisk arrays (NDAs) and the core/shell SiO2/Au nanocylinder arrays (NCAs). Different from the dipolar interactions in the conventionally studied orthogonal coupling, the horizontal propagating electric field introduces the out-of-plane "hot spots" and results in electric field delocalization. Through controlling the aspect ratio to manipulate the "hot spot" distributions of the localized surface plasmon resonances (LSPRs) in the NCAs, we demonstrate a high-performance refractive index sensor with a wide dynamic range of refractive indexes ranging from 1.0 to 1.5. Both high figure of merit (FOM) and high signal-to-noise ratio (SNR) can be maintained under these detectable refractive indices. Furthermore, the electromagnetic field distributions confirm that the high FOM in the wide dynamic range is attributed to the parallel coupling between the superstrate diffraction orders and the height-induced LSPR modes. Our study on the near-field "hot-spot" engineering and far-field parallel coupling paves the way towards improved understanding of the parallel LPRs and the design of high-performance on-chip refractive index sensors.

Novel silica surface charge density mediated control of the optical properties of embedded optically active materials and its application for fiber optic pH sensing at elevated temperatures

Silica and silica incorporated nanocomposite materials have been extensively studied for a wide range of applications. Here we demonstrate an intriguing optical effect of silica that, depending on the solution pH, amplifies or attenuates the optical absorption of a variety of embedded optically active materials with very distinct properties, such as plasmonic Au nanoparticles, non-plasmonic Pt nanoparticles, and the organic dye rhodamine B (not a pH indicator), coated on an optical fiber. Interestingly, the observed optical response to varying pH appears to follow the surface charge density of the silica matrix for all the three different optically active materials. To the best of our knowledge, this optical effect has not been previously reported and it appears universal in that it is likely that any optically active material can be incorporated into the silica matrix to respond to solution pH or surface charge density variations. A direct application of this effect is for optical pH sensing which has very attractive features that can enable minimally invasive, remote, real time and continuous distributed pH monitoring. Particularly, as demonstrated here, using highly stable metal nanoparticles embedded in an inorganic silica matrix can significantly improve the capability of pH sensing in extremely harsh environments which is of increasing importance for applications in unconventional oil and gas resource recovery, carbon sequestration, water quality monitoring, etc. Our approach opens a pathway towards possible future development of robust optical pH sensors for the most demanding environmental conditions. The newly discovered optical effect of silica also offers the potential for control of the optical properties of optically active materials for a range of other potential applications such as electrochromic devices.

Plasmon-enhanced light harvesting: applications in enhanced photocatalysis, photodynamic therapy and photovoltaics

This review article summarizes the recent progress on surface plasmon-enhanced light harvesting and its applications toward enhanced photocatalysis, photodynamic therapy, chemical transformations and photovoltaics.

Plasmon-enhanced light harvesting: applications in enhanced photocatalysis, photodynamic therapy and photovoltaics

This review article summarizes the recent progress on surface plasmon-enhanced light harvesting and its applications toward enhanced photocatalysis, photodynamic therapy, chemical transformations and photovoltaics.

Plasmon-enhanced light harvesting: applications in enhanced photocatalysis, photodynamic therapy and photovoltaics

This review article summarizes the recent progress on surface plasmon-enhanced light harvesting and its applications toward enhanced photocatalysis, photodynamic therapy, chemical transformations and photovoltaics.

Comparison of the plasmonic performances between lithographically fabricated and chemically grown gold nanorods

The plasmonic performances of lithographic and chemical gold nanorods are quantitatively examined and compared through both experiments and electrodynamic simulations.

Photocurrent enhancement of HgTe quantum dot photodiodes by plasmonic gold nanorod structures

The near-field effects of noble metal nanoparticles can be utilized to enhance the performance of inorganic/organic photosensing devices, such as solar cells and photodetectors. In this work, we developed a well-controlled fabrication strategy to incorporate Au nanostructures into HgTe quantum dot (QD)/ZnO heterojunction photodiode photodetectors. Through an electrostatic immobilization and dry transfer protocol, a layer of Au nanorods with uniform distribution and controllable density is embedded at different depths in the ZnO layer for systematic comparison. More than 80 and 240% increments of average short-circuit current density (Jsc) are observed in the devices with Au nanorods covered by ∼7.5 and ∼4.5 nm ZnO layers, respectively. A periodic finite-difference time-domain (FDTD) simulation model is developed to analyze the depth-dependent property and confirm the mechanism of plasmon-enhanced light absorption in the QD layer. The wavelength-dependent external quantum efficiency spectra suggest that the exciton dissociation and charge extraction efficiencies are also enhanced by the Au nanorods, likely due to local electric field effects. The photodetection performance of the photodiodes is characterized, and the results show that the plasmonic structure improves the overall infrared detectivity of the HgTe QD photodetectors without affecting their temporal response. Our fabrication strategy and theoretical and experimental findings provide useful insight into the applications of metal nanostructures to enhance the performance of organic/inorganic hybrid optoelectronic devices.

(Gold nanorod core)/(polyaniline shell) plasmonic switches with large plasmon shifts and modulation depths

(Gold nanorod core)/(polyaniline shell) nanostructures are prepared for functioning as active plasmonic switches. The single core/shell nanostructures exhibit a remarkable switching performance, with the modulation depth and scattering peak shift reaching 10 dB and 100 nm, respectively. The nanostructures are also deposited on substrates to form macroscale monolayers with remarkable ensemble plasmonic switching performances.