Plasmonic Nanostructures Research Articles

Corner Reflector Plasmonic Nanoantennas for Enhanced Single-Photon Emission

The emission rate of atom-like photon sources can be significantly improved by coupling them to plasmonic resonant nanostructures. These arrangements function as nanoantennas, serving the dual purpose of enhancing light–matter interactions and decoupling the emitted photons. However, there is a contradiction between the requirements for these two tasks. A small resonator volume is necessary for maximizing interaction efficiency, while a large antenna mode volume is essential to achieve high emission directivity. In this work, we analyze a hybrid structure composed of a noble metal plasmonic resonant nanoparticle coupled to the atom-like emitter, which is designed to enhance the emission rate, alongside a corner reflector aimed at optimizing the angular distribution of the emitted photons. A comprehensive numerical study of silver and gold corner reflector nanoantennas, employing the finite difference time domain method, is presented. The results demonstrate that a well-designed corner reflector can significantly enhance photon emission directivity while also substantially boosting the emission rate.

Distinguishing Inner and Outer-Sphere Hot Electron Transfer in Au/p-GaN Photocathodes.

Exploring nonequilibrium hot carriers from plasmonic metal nanostructures is a dynamic field in optoelectronics, with applications including photochemical reactions for solar fuel generation. The hot carrier injection mechanism and the reaction rate are highly impacted by the metal/molecule interaction. However, determining the primary type of reaction and thus the injection mechanism of hot carriers has remained elusive. In this work, we reveal an electron injection mechanism deviating from a purely outer-sphere process for the reduction of ferricyanide redox molecule in a gold/p-type gallium nitride (Au/p-GaN) photocathode system. Combining our experimental approach with ab initio simulations, we discovered that an efficient inner-sphere transfer of low-energy electrons leads to an enhancement in the photocathode device performance in the interband regime. These findings provide important mechanistic insights, showing our methodology as a powerful tool for analyzing and engineering hot-carrier-driven processes in plasmonic photocatalytic systems and optoelectronic devices.

Near-Infrared Light-Driven CO2 Reduction on Cup-Stacked Carbon Nanotubes.

Carbon nanotubes feature one-dimensional nature of collective excitations, wherein strong confinement of surface plasmons severely hinders the liberation of hot electrons (HEs), posing grand challenges for their utilization in photochemistry. In this study, we prototypically achieved directed HEs flow and extraction in hybrid plasmonic CNN based on cup-stacked carbon nanotubes (CSCNTs), taking advantage of their privileged edge-plane sites. The localized pz electronic states and accessible intersubband plasmon excitations in the near-infrared (NIR) regime stands in striking contrast to the conventional concentric carbon nanotubes, as evidenced by combined photo-induced force microscopy (PiFM) and transient photocurrent response. The hybrid comprising intimately integrated CSCNTs-C3N4 effectively sustains interfacial electronic states and underlies the energy extraction out of plasmonic components. The CNN demonstrates almost near-unity NIR light-driven CO2 reduction to CO with a rate of 1.35 µmol g-1 h-1. This work sheds light on the exploitation of metal-free carbon-based plasmonic nanostructures for photocatalytic applications.

Near‐Infrared Light‐Driven CO2 Reduction on Cup‐Stacked Carbon Nanotubes

Carbon nanotubes feature one‐dimensional nature of collective excitations, wherein strong confinement of surface plasmons severely hinders the liberation of hot electrons (HEs), posing grand challenges for their utilization in photochemistry. In this study, we prototypically achieved directed HEs flow and extraction in hybrid plasmonic CNN based on cup‐stacked carbon nanotubes (CSCNTs), taking advantage of their privileged edge‐plane sites. The localized pz electronic states and accessible intersubband plasmon excitations in the near‐infrared (NIR) regime stands in striking contrast to the conventional concentric carbon nanotubes, as evidenced by combined photo‐induced force microscopy (PiFM) and transient photocurrent response. The hybrid comprising intimately integrated CSCNTs‐C3N4 effectively sustains interfacial electronic states and underlies the energy extraction out of plasmonic components. The CNN demonstrates almost near‐unity NIR light‐driven CO2 reduction to CO with a rate of 1.35 µmol g–1 h–1. This work sheds light on the exploitation of metal‐free carbon‐based plasmonic nanostructures for photocatalytic applications.

Unveiling multimodal hot carrier excitation in plasmonic bimetallic Au@Ag nanostars for photochemistry and SERS sensing

AbstractPlasmonic nanostructures stand at the forefront of nanophotonics research, particularly in sensing and energy conversion applications. Their unique ability to confine light energy at the nanoscale makes them indispensable for a wide array of technological advancements. The study of these structures often makes use of different materials and, even more extensively, explores new shapes and configurations to extend our common repertoire of useful nanophotonics tools. Exploring the creation of bimetallic plasmonic nanostructures combines these two dimensions determining the space of possible plasmonic resonators and opens the possibility of tailoring systems with behavior unavailable to single-metal plasmonic structures. In this paper, we delve into the exploration of bimetallic systems employing plasmonic nanostars. These structures have demonstrated remarkable capabilities for surface-enhanced Raman scattering (SERS) spectroscopy and photochemistry, due to the strong plasmonic response of their peaks, whose disposition following a spherical symmetry makes them largely polarization- and orientation-insensitive. Herein, we report the colloidal synthesis of two different water-stable Au@Ag nanostars, explore their performance as photocatalysts and SERS substrates, and provide an in-depth account of their non-trivial physical response.

On‐Demand Fabrication and Manipulation of Single Plasmonic Trimers for Ultrasensitive Enantiomer Detection

AbstractThe capability of plasmonic nanostructures in generating superchiral near‐fields holds great potential for a wide range of applications, including enantioselective sensing, medical diagnosis, and chirality‐based bioimaging. To implement high‐performance chiral nanophotonic devices, achieving in situ tuning of chiroptical activity in plasmonic nanostructures is highly desirable yet remains a formidable challenge. Here, a straightforward method is developed for deterministic assembly of plasmonic nanosphere trimers using spectroscopy‐assisted nano‐manipulation. The technique offers in situ, real‐time, and site‐specific control over the chiroptical response of trimers by adjusting their vertex angle and in‐plane orientation. The combination of numerical simulations with the Born‐Kuhn model reveals that oblique excitation effectively induces the symmetry breaking of the trimer structure, resulting in a preferential response of two distinct hybridized plasmonic modes to the handedness of light. Consequently, this yields a significant chiroptical response with the g factor up to 0.37. Remarkably, the trimer with an optimized obtuse angle exhibits a 193‐fold enhancement of optical chirality density, enabling the detection of molecular chirality with a record‐large spectral dissymmetric factor of 12 nm. The study facilitates the rational design of plasmonic nanostructures, offering promising prospects for chiral sensing at the single‐molecule level and asymmetric photocatalysis.

Design rules for catalysis in single-particle plasmonic nanogap reactors with precisely aligned molecular monolayers

Plasmonic nanostructures can both drive and interrogate light-driven catalytic reactions. Sensitive detection of reaction pathways is achieved by confining optical fields near the active surface. However, effective control of the reaction kinetics remains a challenge to utilize nanostructure constructs as efficient chemical reactors. Here we present a nanoreactor construct exhibiting high catalytic and optical efficiencies, based on a nanoparticle-on-mirror (NPoM) platform. We observe and track pathways of the Pd-catalysed C-C coupling reaction of molecules within a set of nanogaps presenting different chemical surfaces. Atomic monolayer coatings of Pd on the different Au facets enable tuning of the reaction kinetics of surface-bound molecules. Systematic analysis shows the catalytic efficiency of NPoM-based nanoreactors greatly improves on platforms based on aggregated nanoparticles. More importantly, we show Pd monolayers on the nanoparticle or on the mirror play significantly different roles in the surface reaction kinetics. Our data provides clear evidence for catalytic dependencies on molecular configuration in well-defined nanostructures. Such nanoreactor constructs therefore yield clearer design rules for plasmonic catalysis.

Double localized surface plasmon‒enhanced-second-harmonic generation behavior of equilateral triangular aluminum nanoprisms

Abstract This paper presents second-harmonic generation (SHG) behaviors for different-sized equilateral triangular aluminum nanoprisms. These behaviors were observed under the double localized surface plasmon (LSP) resonance condition. The SHG conversion efficiency was substantially enhanced when the excitation and SHG wavelengths were simultaneously tuned to the LSP-resonance wavelengths. The fundamental and higher-order LSP modes could be used to enhance the SHG conversion efficiency. The nonlinear light‒matter interaction associated with the double LSP resonance was numerically analyzed while focusing on the spatial overlap of the near-field distributions between the excitation and SHG wavelengths. The double LSP resonance enhanced SHG behaviors have been realized by using several elegantly designed plasmonic metal nanostructures consisting of the multi-elements. Our present study demonstrates this double LSP resonance enhanced SHG behavior from single, simple-shape triangular aluminum nanoprism.

Reliable fabrication of 3D freestanding nanostructures via all dry stacking of incompatible photoresist

Three-dimensional (3D) free-standing nanostructures based on electron-beam lithography (EBL) have potential applications in many fields with extremely high patterning resolution and design flexibility with direct writing. In numerous EBL processes designed for the creation of 3D structures, the multilayer resist system is pivotal due to its adaptability in design. Nevertheless, the compatibility of solvents between different layers of resists often restricts the variety of feasible multilayer combinations. This paper introduces an innovative approach to address the bottleneck issue by presenting a novel concept of multilayer resist dry stacking, which is facilitated by a near-zero adhesion strategy. The poly(methyl methacrylate) (PMMA) film is stacked onto the hydrogen silsesquioxane (HSQ) resist using a dry peel and release technique, effectively circumventing the issue of HSQ solubilization by PMMA solvents typically encountered during conventional spin-coating procedures. Simultaneously, a dry lift-off technique can be implemented by eschewing the use of organic solvents during the wet process. This pioneering method enables the fabrication of high-resolution 3D free-standing plasmonic nanostructures and intricate 3D free-standing nanostructures. Finally, this study presents a compelling proof of concept, showcasing the integration of 3D free-standing nanostructures, fabricated via the described technique, into the realm of Fabry-Perot cavity resonators, thereby highlighting their potential for practical applications. This approach is a promising candidate for arbitrary 3D free-standing nanostructure fabrication, which has potential applications in nanoplasmonics, nanoelectronics, and nanophotonics.

Mirror-coupled plasmonic nanostructures for enhanced in-plane magnetic dipole emission

Abstract Self-assembling of plasmonic nanoparticles into an oligomer can produce optical nanoantennas with resonant magnetic responses, which constitutes a promising platform to study magnetic light-matter interactions at the nanoscale. However, these self-assembled magnetic nanostructures typically feature a flat-lying geometric configuration, giving rise to out-of-plane magnetic dipole (MD) moments that cannot couple to the far-field light efficiently. Herein, we propose a new geometric configuration of plasmonic nanoantennas to realize in-plane MD responses. This is achieved by elegantly coupling a plasmonic nanoparticle oligomer to one or more adjacent metal mirrors, virtually forming a vertically standing nanoparticle tetramer or trimer, yet with in-plane MD moments. We verified this design strategy by numerically simulating the resonance responses of three typical mirror-coupled nanostructures, all exhibiting pronounced resonant MD characters. Furthermore, optical magnetic emitters can be readily coupled to these plasmonic nanoantennas and gain an emission enhancement of two orders of magnitude while simultaneously featuring a high emission directionality (collection efficiency up to ∼ 70 % evaluated with common microscopy optics). We believe these mirror-enabled magnetic nanoantennas could lead to novel nanophotonic devices fully exploiting the magnetic nature of light.

Quantifying the ultimate limit of plasmonic near-field enhancement

Quantitatively probing the ultimate limit of near-field enhancement around plasmonic nanostructures remains elusive, despite more than five decades since the discovery of surface-enhanced Raman scattering. Theoretical calculations have predicted an ultimate near-field enhancement exceeding 1000 using the best plasmonic material silver, but experimental estimations disperse by orders of magnitude. Here, we design a high-quality silver plasmonic nanocavity with atomic precision and precisely quantify the upper limit of near-field enhancement in ~1 nm junctions. A hot-spot averaged Raman enhancement of 4.27 × 1010 is recorded with a small fluctuation, corresponding to an averaged electric field enhancement larger than 1000 times. This result quantitatively delineates the ultimate limit of plasmonic field enhancement around plasmonic nanostructures, establishing a foundation for diverse plasmon-enhanced processes and strong light-matter interactions at the atomic scale.

Seed-Mediated Growth and Advanced Characterization of Chiral Gold Nanorods.

The controlled growth of gold nanostructures with complex shapes and reduced symmetry, exemplified by chiral gold nanorods and nanoparticles, is one of the most dynamic fields of nanochemistry. A timely summary of underlying concepts, including growth mechanisms and redefined chirality measures, would further promote this research area. In this perspective, we aim to establish qualitative connections between the chiral shapes and growth conditions, specifically for the seed-mediated synthesis of chiral gold nanorods as a convenient case of chiral morphogenesis. The crystallographic and morphological features of achiral nanorods used as seeds, the experimental conditions during chiral growth, and the symmetry of the chiral inducers, can all be exploited to obtain nanorods with intricate chiral shapes. Chirality characterization (such as electron tomography techniques) and quantification (including chirality measures) emerge as critical aspects to comprehensively explore and understand such structures, enabling optimization of their geometric and optical features. We conclude by discussing relevant challenges to be addressed toward a better controlled synthesis of chiral plasmonic nanostructures.

Ultrafast Laser Manipulation of In-Lattice Plasmonic Nanoparticles.

Plasmonic nanoparticles enable manipulation and enhancement of light fields at deep subwavelength scales, leading to structures and devices for diverse applications in optics. Despite hybrid plasmonic materials display remarkable optical properties due to interactions between components in nanoproximity, scalable production of plasmonic nanostructures within a single-crystalline matrix to achieve an ideal plasmon-crystal interface remains challenging. Here, a novel approach is presented to realize efficient manipulation of in-lattice plasmonic nanoparticles. Employing ultrafast-laser-driven plasmonic nanolithography, metallic nanoparticles with controllable morphology are precisely defined in the crystalline lattice of yttrium aluminum garnet (YAG) crystal. Through direct ion implantation, hybrid plasmonic material composed of nanoparticles embedded in a sub-surface amorphous YAG layer is created. Subsequently, femtosecond laser pulses guide formation and reshaping of plasmonic nanoparticles from the amorphous layer into the single-crystalline matrix along direction of light propagation, facilitated by a plasmon-mediated evolution of laser energy deposition. By tailoring resonance modes and optimizing the coupling between structured particle assemblies, a range of applications including polarization-dependent absorption and nonlinearity, controllable photoluminescence, and structural color generation is demonstrated. This research introduces a new approach for fabricating advanced optical materials featuring in-lattice plasmonic nanostructures, paving the way for the development of diverse functional photonic devices.

Ball-in-hole 3D nanoarchitectonics based on nanoporous Au film: Ultrasensitive, highly reliable, and reusable plasmonic platform

Hotspots have a locally enhanced electromagnetic field, effectively created by nanogaps in plasmonic metal nanostructures. The formation of uniform nanogaps, smaller than a few nanometers, allows for the detection of analytes at low concentrations and ensures reliable measurements across entire surface-enhanced Raman scattering (SERS) substrates. Despite the demand for SERS substrates in recent optical sensing applications, challenges remain in developing new techniques to maximize the number and optimize the size of nanogaps on large-area substrates. To address this issue, this study introduces a new ball-in-hole technique based on metal nanoarchitecture utilizing 3D nanoporous gold film (NPGF) and 0D gold nanoparticles (AuNPs). Using the ball-in-hole technique, 23 nm AuNPs are placed within the ∼ 28 nm mesopores of the metal film, creating uniform nanogaps of just a few nanometers that ensure consistent SERS signals across a large substrate area due to effective hotspots. The 23nmNP@NPGF substrate exhibited an enhancement factor (EF) of 1014 for 4-aminophenol (4-ATP), enabling detection at the attomole level. Moreover, the substrate retained its performance after five washing and reuse cycles, demonstrating excellent recyclability Finally, the substrate successfully detected femtomolar concentrations of key neurotransmitters like dopamine and glutamate, without requiring Raman reporters. This study presents a promising strategy for constructing metal nanoarchitectures with numerous hotspots, offering new possibilities for developing more efficient SERS substrates.

Silver shelled gold nanorods for sensitive detection of cholesterol and triglycerides

We report the synthesis of bimetallic plasmonic nanostructure of silver shelled gold nanorods (Ag–Au NRs), subsequently employed for the selective detection of cholesterol (Cho) and triglycerides (TGl) utilizing Surface Enhanced Raman Spectroscopy (SERS). In this direction, the gold nanorods (Au NRs) were synthesized via seed mediated growth method followed by forming a silver shell with reduction of AgNO3 onto Au NRs. Ag–Au NRs showed significantly augmented SERS property owing to hybridization of Localized Surface Plasmon Resonances (LSPR) of silver and gold. The enhanced plasmonic property was thus employed for biosensing. For this, two different Raman reporter molecules, 5,5′–dithiobis–(2–nitrobenzoic acid) (DTNB) and 4–aminothiophenol (4ATP) were immobilized separately on Ag–Au NRs to synthesize SERS active nanoprobes, before the attachment of the bioreceptors – cholesterol oxidase (ChOx) and lipase (Lp) to form ChOx–DTNB–Ag–Au NRs and Lp–4ATP–Ag–Au NRs. These nanoprobes were then utilized for the quantification of Cho and TGl via liquid mode Raman spectroscopic study. The change in SERS spectral intensity of DTNB and 4ATP were systematically recorded in reference to the baseline sample to mark the calibration for both Cho and TGl. Additionally, interference studies considering effects of ascorbic acid, glucose, sodium and potassium ion were performed to unveil excellent selectivity of the proposed method.

Design plasmonic nanostructure of silicon dioxide and titanium dioxide loaded on a nano surface for clean water production through photocatalysis and electrochemical techniques

The work is targeted to rebuild the framework of a nanocatalyst (NCat) that depends on the n/p-type heterojunction through the plasmonic structure of reduced graphene oxide (rGO), silicon oxide nanoparticles (SiO2 NPs), polyvinyl alcohol (PVA NPs), and titanium dioxide (TiO2 NPs). The removal of medical and organic contaminants occurs via photocatalysis operation which follows through the electrochemical technique and UV-spectrophotometer under visible light with nanocatalyst (rGO@SiO2@PVA@TiO2) for clean water and a safe medical environment. The Z-Scheme mechanism explains the development of electron maps in fabricated nanocatalyst engineering to increase the efficiency of the photocatalytic activity. The high activity of the NCat appeared, after 160 min, and the photocatalytic efficiency for methylene blue (MB), methyl orange (MO), rhodamine B (RhB), moxifloxacin (MFX), and colchicine, was 90 %, 76 %, 88 %, 84 %, and 91 % respectively. The stability of NCat was confirmed via the recyclability process, with the efficiency at 58 % after the fifth cycle. The high stability of NCat was proved by the electrochemical technique performed after 100 cycles. The safety of the NCat for the generation of clean water was highlighted by the cytotoxicity test using normal mouse liver cells. It is suggested to use the new NCat design because of its distinctive optical characteristics, which make it a viable candidate for use as a revolutionary nanomaterial with high efficiency and safety for clean water and removal of medical contaminants.

Horizontal aluminum magneto-plasmonic metasurface for efficient magneto-optical Kerr modulation and sensing in the ultraviolet range.

Most of the plasmonic nanostructures utilized for magneto-optical (MO) enhancement have been limited to noble metals with resulting enhancement in the visible and infrared spectral range. Here, we designed a horizontal aluminum magneto-plasmonic metasurface, with the ability to control the Kerr rotation angle and enhance the RI sensing performance based on magneto-plasmons, by exploiting the polarization degree of freedom in the ultraviolet range. The surface composes of L-shaped magnetic dielectric embedded in the Al film. The reflection spectrum and the Kerr rotation angle map are both symmetric about the polarization angle of 45° and 135°. It is demonstrated that the sign change of the two maximal Kerr rotation angles at polarization angle of 0° and 90°, originates from the relative contribution of the two mutually orthogonal oscillating electric dipoles. In addition, the RI sensing FoM based on Kerr reversal at 372 nm of this structure reaches 5000/RIU, which is superior to the result in the visible or infrared range (1735/RIU). The results of our investigation demonstrate the potential of Al-based magneto-plasmonic effect and offer opportunities to push the MO spectral response out of the visible range into the ultraviolet range.

Interference between multipolar two-photon transitions in quantum emitters near plasmonic nanostructures

In the vicinity of plasmonic nanostructures that support highly confined light fields, spontaneous emission processes, such as two-photon spontaneous emission (TPSE), exhibit higher-order multipolar emission pathways beyond the dipolar one. These multipolar emission channels occur simultaneously and can interfere with each other. We develop a novel framework that computes these interference effects for TPSE of a quantum emitter close to an arbitrary nanostructure. The model is based on the computation of Purcell factors that can be calculated with conventional electromagnetic simulations, which avoids complex analytic calculations for the environment. For a transition of a hydrogen-like emitter close to a graphene nanotriangle, we demonstrate a breakdown of the dipolar selection rule in the TPSE process. This breakdown is due to a huge enhancement of the two-electric dipole (2ED) and of the two-electric quadrupole (2EQ) transitions. We observe an important interference between these multipolar transitions, as it increases the total rate by 67%\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$67 \\, \\%$$\\end{document}. In the end, our framework is a complete tool to design emitters and nanostructures for TPSE, where the exploitation of previously ignored interference effects provides an additional degree of freedom, for example to boost desired transitions and to supress undesirable ones.

Ultrasensitive Analysis of Escherichia coli O157:H7 Based on Immunomagnetic Separation and Labeled Surface-Enhanced Raman Scattering with Minimized False Positive Identifications.

It is a big challenge to monitor pathogens in food with high selectivity. In this study, we reported an ultrasensitive method for Escherichia coli O157:H7 detection based on immunomagnetic separation and labeled surface-enhanced Raman scattering (SERS). The bacterium was identified by heterogeneous recognition elements, monoclonal antibody (mAb), and aptamer. E. coli O157:H7 was separated and enriched by magnetic nanoparticles modified by mAb, and then a plasmonic nanostructure functionalized by aptamers with embedded Raman tags and interior gaps was utilized for further discrimination and detection. The selectivity was enhanced by two binding sites. The higher Raman enhancement was obtained by strong local electromagnetic field oscillation in the gap and the firm embedment of 4-mercaptopyridine (4-Mpy). Optimum experiments created that SERS signals of 4-Mpy at 1010 cm-1 had a good linearity with E. coli O157:H7 at a large range of 10 to 107 CFU/mL with a limit of detection of 2 CFU/mL. This method has great potential for on-site food pathogenic bacterial detection.

Tracing the Background‐Suppressed Vibrational Decay Time of a Molecular Monolayer with Time‐Resolved Surface‐Enhanced Broadband Coherent Anti‐Stokes Raman Scattering

AbstractThe sensitivity of molecular spectroscopy based on surface‐enhanced coherent anti‐Stokes Raman scattering (SECARS) is limited by the spectrally overlapping background from nonresonant four‐wave mixing (FWM). While the SECARS signal is mediated by the molecular vibrational eigenstates and exhibits long lifetime (a few picoseconds), the FWM background stems from the instantaneous electronic polarization and decays rapidly with the vanishment of excitation. Therefore, CARS and FWM can be separated by time‐resolved CARS (trCARS) using ultrashort pulsed lasers. The broad spectral bandwidth also enables broadband CARS (BCARS) for simultaneous identification of multiple vibrational signatures. This work combines trCARS and BCARS with an optimized plasmonic system to demonstrate time‐resolved surface‐enhanced BCARS) and show its capability in obtaining background‐suppressed broadband vibrational spectra from a monolayer of 4‐Aminothiophenol (4‐ATP) molecules self‐assembled on a gold grating. The vibrational dephasing time of the ring‐breathing mode for the 4‐ATP monolayer on the gold grating (≈1.00 ± 0.17 ps) is significantly shortened compared to that for 4‐ATP powder on a glass substrate (2.10 ± 0.05 ps). This work presents an effective method to suppress FWM background and investigate the vibrational dynamics of molecules in the vicinity of plasmonic nanostructures.