Local Field Enhancement Research Articles

High Q-factor mie lattice resonance generated from out-of-plane magnetic dipole in all-dielectric defected semi-cylinder metasurface array

Abstract Due to the weak far-field radiation and strong local field enhancement properties of Mie lattice resonances(MLRs), a transmission resonance with a significantly high Q factor is achieved, which can be used in applications such as refractive index sensing, filtering, and nanolasing. In this paper, we design a semi-cylinder with a cuboid defect metasurface array working in the near-infrared, where a resonance with extremely narrow linewidth in transmission spectrum has been demonstrated at the wavelength of 870.9408 nm. The full width at half-maximum of the resonance is as narrow as 0.0032 nm with a corresponding Q factor of 272169. According to distributions of the electric field and displacement current, it is confirmed that the narrow linewidth resonance is generated by the MLR, which originates from the coupling between the out-of-plane magnetic dipole mode and Rayleigh Anomalous diffraction. It is further verified that the out-of-plane magnetic dipole mode is excited due to the induce defect in the semi-cylinder. When applied to refractive index sensing, a sensitivity of 333 nm RIU‒1 and a figure of merit of 104063 RIU − 1 are numerically achieved simultaneously. Our study here provides a new approach for achieving resonances with high Q-factors for the required applications.

Improving the Efficiency of Bulk-heterojunction Solar Cells through Plasmonic Enhancement within a Silver Nanoparticle-Loaded Optical Spacer Layer.

We investigate the enhancement in the efficiency of organic bulk heterojunction solar cells enabled by plasmonic excitation of Ag nanoparticles (NPs) of different diameters (10, 20, and 30 nm), randomly incorporated within an optical spacing layer of TiO2 placed between the organic medium and the Ag cathode. Such structures significantly increase the optical absorption and the photocurrent within the device system, leading to a power conversion efficiency of more than 4%, over 2.5 times that of the control bulk heterojunction cell. This corresponds to a 61% increase in J SC and a 6.3% in fill factor. 3D Finite-difference time-domain simulations were utilized to investigate the plasmonic field coupling within the nanogap medium of TiO2. They show that coupling between the Ag nanoparticle and the Ag thin film cathode extends the wavelength range of the local field enhancement beyond that obtained for isolated NPs, providing a better overlap with the absorption spectrum of the organic medium.

High Q-factor Mie lattice resonance generated from out-of-plane magnetic dipole in all-dielectric defected semi-cylinder metasurface array

Abstract Due to the weak far-field radiation and strong local field enhancement properties of Mie lattice resonances, transmission resonance with significant high Q factor is achieved which could be used in applications such as refractive index sensing, filtering and nano lasing. In this paper, we design an semi-cylinder with cuboid defect metasurface array working in the near-infrared, where a resonance with extremely narrow linewidth in transmission spectrum has been demonstrated at the wavelength of 870.9408 nm. The full width at half-maximum of the resonance is as narrow as 0.0032 nm with a corresponding Q factor of 272169. According to distributions of the electric field and displacement current, it is confirmed that the narrow linewidth resonance is generated from the Mie lattice resonance which is originated from the coupling between the out-of-plane magnetic dipole mode with Rayleigh Anomalous diffraction. It is further verified that the out-of-plane magnetic dipole mode is excited due to the induced defect in the semi-cylinder. When being applied in refractive sensing, a sensitivity and a figure of merit of 333 nm/RIU and 104063 RIU-1 are numerically achieved simultaneously. Our study here provide a new way for achieving resonances with high Q-factors.

Rhodium nanospheres for ultraviolet and visible plasmonics.

The development and understanding of alternative plasmonic materials are crucial steps for leveraging new plasmonic technologies. Although gold and silver nanostructures have been intensively studied, the promising plasmonic, chemical and physical attributes of rhodium remain poorly investigated. Here, we report the synthesis and plasmonic response of spherical Rh nanoparticles (NPs) with sizes in the 20-40 nm range. Due to the high cohesive energy of this metal, synthesis and experimental investigations of Rh nanospheres in this size range have not been reported; yet, it becomes possible here using a green and one-step laser ablation in liquid method. The localized surface plasmon (LSP) of Rh NPs falls in the ultraviolet spectral range (195-255 nm), but the absorption tail in the visible region increases significantly upon clustering of the nanospheres. The surface binding ability of Rh NPs towards thiolated molecules is equivalent to that of Au and Ag NPs, while their chemical and physical stability at high temperatures and in the presence of strong acids such as aqua regia is superior to those of Au and Ag NPs. The plasmonic features are well described by classical electrodynamics, and the results are comparable to Au and Ag NPs in terms of extinction cross-section and local field enhancement, although blue shifted. This allowed, for instance, their use as an optical nanosensor for the detection of ions of toxic metals in aqueous solution and for the surface enhanced Raman scattering of various compounds under blue light excitation. This study explores the prospects of Rh NPs in the realms of UV and visible plasmonics, while also envisaging a multitude of opportunities for other underexplored applications related to plasmon-enhanced catalysis and chiroplasmonics.

Gold nanoparticle-modified single-walled carbon nanotube terahertz metasurface for ultrasensitive sensing of trace proteins.

Research on metasurface sensors with high sensitivity, strong specificity, good biocompatibility and strong integration is the key to promote the application of terahertz waves in the field of biomedical detection. However, traditional metallic terahertz metasurfaces have shortcomings such as poor biocompatibility and large ohmic loss in the terahertz frequency band, impeding their further application and integration in the field of biosensing detection. Here, we overcome this challenge by proposing a high-performance terahertz metasurface based on gold nanoparticles and single-walled carbon nanotubes composite film. Compared with metal materials, carbon nanotubes not only have better biocompatibility, which can reduce the potential adverse reactions between metasurfaces and biological samples, but also have strong tunability in electrical and optical properties. Experimentally, we reveal a method to adjust the dielectric properties of single-walled carbon nanotube films by doping with gold nanoparticles. Leveraging this mechanism, we designed and prepared a single-walled carbon nanotube terahertz metasurface composed of periodically arranged asymmetric open resonant rings. Compared with pure single-walled carbon nanotube films, this device based on a composite single-walled carbon nanotube films have better localized electromagnetic field enhancement characteristics. Through integration with microfluidic channels, this metasurface sensor can achieve direct detection of SAA proteins in solution environments at the fM level. In addition, the device also exhibits a detection sensitivity of 41GHz/fM. This work has not only made significant progress in the design and function of new high-sensitivity carbon-based terahertz metasurfaces, but also laid the foundation for its application in liquid environment detection of trace biological samples.

Broadband and large surface enhancements of the local electric field enabled by cross-etched hyperbolic metamaterials.

Hyperbolic metamaterials (HMMs) have recently attracted significant research attention due to their hyperbolic wavevector iso-frequency contour, which leads to substantial local electric field (EF) enhancements that benefit optical processes, such as the nonlinear generation, quantum science, biomedical sensing, and more. However, three main challenges hinder their practical implementation: the difficulty in exciting their resonant modes using free-space incidence, the weak enhancement of surface EF, and the narrow spectral range of EF enhancements. Herein, we proposed cross-etched HMMs (CeHMMs) as a novel type of HMM, addressing these issues. The CeHMMs can be easily fabricated by etching periodic cross-shaped arrays in conventional HMMs, and exhibited two resonant high-k modes within the spectral range of 700-1100 nm under linearly, circularly, or elliptically polarized incident light from free space. It was also calculated that the CeHMMs can provide a large surface EF enhancement across a broad spectral range (over 500 nm). After integrating a single layer of WSe2 onto the top surface, the photoluminescence (PL) enhancements of the CeHMMs and their hot spots, based on the emission resonance, were calculated to be 9.72 and 62 times, respectively. With their ability to provide broadband surface EF enhancement, CeHMMs are expected to offer considerable potential for a variety of nanophotonic applications, including nonlinear optics, integrated optics, and quantum photonics.

Gold nanorod in silver tetrahedron: Cysteamine mediated synthesis of SERS probes with embedded internal markers for AFP detection.

Plasmonic core-shell nanostructures with embedded internal markers used as Raman probes have attracted great attention in surface-enhanced Raman scattering (SERS) immunoassay for cancer biomarkers due to their excellent uniform enhancement. However, current core-shell nanostructures typically exhibit a spherical shape and are coated with a gold shell, resulting in constrained local field enhancement. In this work, we prepared a core-shell AuNR@BDT@Ag structure by depositing silver on the surface of Raman reporter-modified gold nanorods (AuNR). With cysteamine-driven specific deposition of Ag atoms, the internal standard nanoprobes with an ortho-tetrahedral morphology were obtained. Owing to its tetrahedral morphology and the effective plasmon coupling at the Ag-Au interface, this internal standard Raman probe exhibited excellent Raman enhancement. Also, with embedded Raman reporter, the probes of Au nanorod in Ag tetrahedron avoided the desorption of Raman reporter and competitive adsorption of interfering molecule, which greatly improved the stability and reproducibility of SERS signal and addressed the drawbacks of low reproducibility existing in SERS immunoassay. The feasibility of the AuNR@BDT@Ag probe was demonstrated by the sensitive detection of the liver cancer biomarker alpha-fetoprotein (AFP) in Eppendorf (EP) tubes and microfluidic chips. The results in EP tubes revealed a linear range of 1 fg/mL-1 ng/mL and a detection limit of 0.631fg/mL. When the detection was performed in microfluidic chips, the linear range was 10pg/mL to 0.1μg/mL, with a limit detection of 8.29pg/mL. The performance of AFP detection in EP tubes and microfluidic chips demonstrates that the tetrahedral core-shell AuNR@BDT@Ag nanostructures used as internal standard Raman probes have the potential to detect biomarkers in blood samples for cancer screening.

SERS-Based Local Field Enhancement in Biosensing Applications.

Surface-enhanced Raman scattering (SERS) stands out as a highly effective molecular identification technique, renowned for its exceptional sensitivity, specificity, and non-destructive nature. It has become a main technology in various sectors, including biological detection and imaging, environmental monitoring, and food safety. With the development of material science and the expansion of application fields, SERS substrate materials have also undergone significant changes: from precious metals to semiconductors, from single crystals to composite particles, from rigid to flexible substrates, and from two-dimensional to three-dimensional structures. This report delves into the advancements of the three latest types of SERS substrates: colloidal, chip-based, and tip-enhanced Raman spectroscopy. It explores the design principles, distinctive functionalities, and factors that influence SERS signal enhancement within various SERS-active nanomaterials. Furthermore, it provides an outlook on the future challenges and trends in the field. The insights presented are expected to aid researchers in the development and fabrication of SERS substrates that are not only more efficient but also more cost-effective. This progress is crucial for the multifunctionalization of SERS substrates and for their successful implementation in real-world applications.

Tungsten Diselenide Nanoparticles Produced via Femtosecond Ablation for SERS and Theranostics Applications.

Due to their high refractive index, record optical anisotropy and a set of excitonic transitions in visible range at a room temperature, transition metal dichalcogenides have gained much attention. Here, we adapted a femtosecond laser ablation for the synthesis of WSe2 nanoparticles (NPs) with diameters from 5 to 150 nm, which conserve the crystalline structure of the original bulk crystal. This method was chosen due to its inherently substrate-additive-free nature and a high output level. The obtained nanoparticles absorb light stronger than the bulk crystal thanks to the local field enhancement, and they have a much higher photothermal conversion than conventional Si nanospheres. The highly mobile colloidal state of produced NPs makes them flexible for further application-dependent manipulations, which we demonstrated by creating substrates for SERS sensors.

Light Absorption-Enhanced Ultra-Thin Perovskite Solar Cell Based on Cylindrical MAPbI3 Microstructure.

In order to promote power conversion efficiency and reduce energy loss, we propose a perovskite solar cell based on cylindrical MAPbI3 microstructure composed of a MAPbI3 perovskite layer and a hole transport layer (HTL) composed of PEDOT:PSS. According to the charge transport theory, which effectually increases the contact area of the HTL, promoting the electronic transmission capability, the local field enhancement and scattering effects of the surface plasmon polaritons help to couple the incident light to the solar cell, which can increase the absorption of light in the active layer of the solar cell and improve its light absorption efficiency (LAE). based on simulation results, a cylindrical microstructure of the perovskite layer increases the contact area of the hole transport layer, which could improve light absorption, quantum efficiency (QE), short-circuit current density (JSC), and electric power compared with the perovskite layer of other structures. In the AM 1.5 solar spectrum, the average light absorption efficiency is 93.86%, the QE is 80.7%, the JSC is 24.50 mA/cm2, and the power conversion efficiency (PCE) is 20.19%. By enhancing the efficiency and reducing material usage, this innovative design approach for perovskite solar cells is expected to play a significant role in advancing solar technology and positively impacting the development of renewable energy solutions.

Pixelated Toroidal Metasurface Sensor with Broadband Terahertz Fingerprint Enhancement for Biochemical Trace Detection.

Terahertz (THz) trace fingerprint detection is essential for identifying the characteristic biomolecular absorption fingerprints from inherent molecular rotational and vibrational modes. Plasmonic THz resonance shows a way to enhance the recognition of biomolecular absorption fingerprint spectra. In this research, we experimentally demonstrate a broadband THz fingerprint metasensor based on a pixelated toroidal metasurface, showcasing excellent capabilities in biochemical trace detection. The design employs mirrored asymmetric double split-ring resonators to excite the toroidal resonance with a high quality (Q) factor and strong localized field enhancement. This high Q-factor resonance exhibits exceptional spectral resolution with spectral resolution less than 50 GHz for broadband fingerprint sensing, thereby capturing characteristic information. More importantly, the design allows for linear modulation of the split-ring resonator radius, which can be extended to cover a larger spectral region by adjusting geometric parameters to meet multifingerprints detection of various biochemical analytes. Utilizing fingerprint enhancement and multiplexing technology, the metasensor precisely detects trace glucose with a high sensitivity. The introduction of the metasensor significantly enhances the absorption characteristics of glucose films, demonstrating an outstanding broadband fingerprint sensing performance. This high-performance and flexible fingerprint metasensor has potential applications in enhanced broadband THz fingerprint sensing, providing a powerful and low-cost approach for advanced biochemical trace detection.

Fabrication and Characterization of Supported Porous Au Nanoparticles

Porous plasmonic nanoparticles offer unique advantages for sensing and catalysis due to their high surface-to-volume ratio and localized electromagnetic field enhancements at nanoscale pores, or “hotspots.” However, current fabrication techniques, which are based on colloidal synthesis, face challenges in achieving precise control over particle size, shape, and porosity. Here, we present a robust nanofabrication method to produce supported arrays of porous Au nanoparticles with excellent dimensional and compositional control. By combining lithographically patterned AuAg alloy nanoparticles and selective dealloying via nitric acid, we achieve particle porosity without compromising particle morphology. Specifically, the method allows fabrication of supported porous nanoparticles with tunable dimension and porosity. Our approach demonstrates precise control of nanoparticle porosity by varying the initial Ag content in the alloy. Optical characterization reveals a blueshift in the extinction peak with increasing porosity, attributed to the reduced effective refractive index from intraparticle voids. Notably, a tunable shift of up to 100 nm in the plasmonic peak is observed, demonstrating the potential for fine-tuning optical properties. This study highlights the versatility of the proposed method in fabricating well-defined porous plasmonic nanoparticles and their ability to modulate optical properties through porosity control. These findings not only expand the toolkit for designing advanced plasmonic materials but also open pathways for applications in plasmon-mediated sensing, catalysis, and photonic devices.

Nonlinear LiNbO3 Local Electric Field Enhancement Photonic Crystal with Static Visible and Dynamic Asymmetric Laser Protection.

The rapid development of high-energy laser technology imposes heightened requirements on multifunctional laser protection. In this article, we report a study of static visible and dynamic asymmetric laser protection window, utilizing a one-dimensional photonic crystal comprising LiNbO3 and Nb2O5 defects fabricated by magnetron sputtering technique. The visible light transparency and asymmetric laser protection performance are attributed to photonic crystal energy band properties and significant local electric field enhancement. As 1064 nm laser energy levels are below 14.44 mJ/cm2, the forward and backward incident transmittances are 75.63 and 77.68%, respectively. With an increase in laser energy to 91.62 mJ/cm2, the forward and backward transmittances vary to 7.32 and 71.58% due to the combination of asymmetric electric field enhancement and the third-order nonlinear effect of LiNbO3, achieving dynamic asymmetric laser protection. Notably, the sample exhibits a lower optical protection threshold of 46.08 mJ/cm2 compared to that of traditional optical protection devices. Furthermore, the sample attains an average transmittance of 65.96% within the visible light band. This work presents inspirations for the preparation of multifunctional laser protection optical windows and advanced optical components.

Gap plasmonic properties of NPOM structures composed of gold nanoparticles and thin films

Metal nanoparticles have attracted great interest because of their unique near-field enhancement properties, and have important applications in many fields, such as surface-enhanced Raman scattering (SERS), fluorescence enhancement, solar cell efficiency enhancement and so on. In this paper, we use the finite element method to study the local field enhancement characteristics of the coupling system in which the gold ellipsoidal nanoparticles (GENP) on a mirror. In the coupling system, a hot spot will be formed at the gap between the GENP and the mirror under the appropriate incident light excitation. Based on the structural characteristics of GENP, we systematically study the local electric field produced by placing ellipsoids vertically and horizontally on the mirror. Under the excitation of the same polarized incident light, the maximum local electric field of the vertical GENP is 2200 (V/m), while that of the horizontal GENP is more than 9400 (V/m). Our proposed gap coupling system based on metal nanoparticles and thin films may open a new field of interest in the fields of plasmon, sensing, optical limitation and enhancement.

Oblique Deposited Ultra-Thin Silver Films on Polymer Gratings for Sensitive SERS Performance.

A small amount of silver was obliquely deposited onto a polymer subwavelength grating to form a metasurface that comprised silver split-tubes. An ultra-thin silver film with a monitor-controlled thickness of 20 nm at the corner of each ridge of the grating provided the most sensitive surface-enhanced Raman scattering (SERS) measurements. An excitation laser beam that was incident from the substrate provided similar or better SERS enhancement than did the general configuration with the laser beam incident directly on the surface of the nanostructure. Near-field simulations were conducted to model the localized electric field enhancement and to quantify the SERS performance, demonstrating the effectiveness of this novel deposition method.

Multiple Fano resonances in all-dielectric porous array structures

High Q metasurfaces play an important role in fields such as high-sensitivity sensing and nonlinear optics due to their strong localized electromagnetic field enhancement. Although ultra-high Q resonance has been developed in the field of optics, it is still a challenging task due to the loss of dielectric materials, design and fabrication of nanostructures. In this paper, we have designed an all-dielectric symmetric perforated array structure that supports multiple Fano resonances within the 0–1 THz range and realizes the anapole mode through this array perforation structure. By calculating the phase difference between different modes at the resonance frequencies, we explain the mechanism of the formation of resonance-coupled BIC. The Q-factors of the three modes have been calculated, where the highest Q-value can be up to 2703, it is excited by the MQ. Then, we analyzed the sensing performance and the highest sensitivity can reach 27,000 nm/RIU. Since the metasurface always maintains c_4v symmetry and mirror symmetry, all three resonances are polarization independent. The proposed metasurfaces can be applied to light-matter interactions, enhanced nonlinear response, and sensing.

Multiple surface lattice resonances in symmetric nanocuboid dimer arrays

Abstract Surface lattice resonances based on nanoparticle arrays have significant characteristics such as localized field enhancement and high quality factor, and can be applied in fields such as optical sensors and lasers. In this work, we propose a symmetric Si/SiO2 nanocuboid dimer array that can generate and regulate two surface lattice resonances. One of the surface lattice resonances (named SLR1) is mainly due to the coupling between the electric dipole resonance of single Si/SiO2 nanocuboid dimers and the diffraction waves perpendicular to the applied electric field. Another surface lattice resonance (named SLR2) mainly originates from the coupling between the magnetic dipole resonance of single Si/SiO2 nanocuboid dimers and the diffraction waves parallel to the direction of the applied electric field. The research results indicate that the polarization direction of the incident field, the period of the array, the gap between the nanocuboids in the dimer, particle size, and the medium environment are all important for regulating the two surface lattice resonances. The sensing application of multiple surface lattice resonances is also investigated. The results show that under appropriate structural parameters, SLR1 can provide good stability for sensing applications, its sensitivity and figures of merit are 472 nm RIU−1 and 104, respectively. However, SLR2 is very weak or suppressed when the refractive index of the medium environment is greater than or equal to 1.2. This characteristic limits the application range of SLR2 in sensing. This work is of great significance for the design of micro-nano photonic devices based on multiple surface lattice resonances.

Safety of non-invasive brain stimulation in patients with implants: a computational risk assessment.

Non-invasive brain stimulation (NIBS) methodologies, such as transcranial electric (tES) are increasingly employed for therapeutic, diagnostic, or research purposes. The concurrent presence of active/passive implants can pose safety risks, affect the NIBS delivery, or generate confounding signals. A systematic investigation is required to understand the interaction mechanisms, quantify exposure, assess risks, and establish guidance for NIBS applications. We used measurements, simplified generic, and detailed anatomical modeling to: (i) systematically analyze exposure conditions with passive and active implants, considering local field enhancement, exposure dosimetry, tissue heating and neuromodulation, capacitive lead current injection, low-impedance pathways between electrode contacts, and insulation damage; (ii) identify risk metrics and efficient prediction strategies; (iii) quantify these metrics in relevant exposure cases and (iv) identify worst case conditions. Various aspects including implant design, positioning, scar tissue formation, anisotropy, and frequency were investigated. At typical tES frequencies, local enhancement of dosimetric exposure quantities can reach up to one order of magnitude for deep brain stimulation (DBS) and stereoelectroencephalography implants (more for elongated passive implants), potentially resulting in unwanted neuromodulation that can confound results but is still 2-3 orders of magnitude lower than active DBS. Under worst-case conditions, capacitive current injection in the active implants' lead can produce local exposures of similar magnitude as the passive field enhancement, while capacitive pathways between contacts are negligible. Above 10 kHz, applied current magnitudes increase, necessitating consideration of tissue heating. Furthermore, capacitive effects become more prominent, leading to current injection that can reach DBS-like levels. Adverse effects from abandoned/damaged leads in direct electrode vicinity cannot be excluded. Safety related concerns of tES application in the presence of implants are systematically identified and explored, resulting in specific and quantitative guidance and establishing basis for safety standards. Furthermore,several methods for reducing risks are suggested while acknowledging the limitations(see Sec. 4.5).

Extraordinary terahertz transmission and field enhancement via hybrid surface plasmon coupling in rhombic and bow-tie composite aperture metasurface

Metasurface that achieves extraordinary terahertz transmission (ETT) and local electric field enhancement (FE) holds significant potential for terahertz studies involving extremely low concentrations of target materials. In this study, we explore a composite aperture metasurface capable of both ETT and local FE. By inserting bow-tie apertures in the “minimum-resonance” zone between four adjacent rhombic lattices, a local FE factor is achieved. Notably, adjusting the configuration of the bow-tie aperture enhances the coupling between surface plasmons, thereby expanding the transmission bandwidth. Through parameter optimization, the metasurface achieves a peak transmission exceeding 95% and a transmittance above 80% in the frequency range of 2.44–3.65 THz, while simultaneously exhibiting a maximum local FE factor of 1005 at 3.45 THz. This approach offers a promising avenue for the design of metasurfaces for spectroscopy and biosensor applications.

Three-dimensional composite substrate based on pyramidal pitted silicon array adhered Au@Ag nanospheres for high-performance surface-enhanced Raman scattering.

As a noninvasive and label-free optical technique, Raman spectroscopy offers significant advantages in studying the structure and properties of biomacromolecules, as well as real-time changes in cellular molecular structure. However, its practical applications are hindered by weak scattering responses, low signal intensity, and poor spectral uniformity, which affect the subsequent accuracy of spectral analysis. To address these issues, we report a novel surface-enhanced Raman scattering (SERS) substrate based on a pyramidal pitted silicon (PPSi) array structure adhered with Au-shell Ag-core nanospheres (Au@Ag NSs). By preparing a highly uniform PPSi array substrate with controllable size and arrangement, and constructing SERS-active Au@Ag NSs on this substrate, a three-dimensional (3D) composite SERS substrate is realized. The enhancement performance and spectral uniformity of 3D composite SERS substrate were examined using crystal violet (CV) and Rhodamine 6G (R6G) molecules, achieving a minimum detectable concentration of R6G at 10-9 M and the analytical enhancement factor (AEF) of 4.2 × 108. Moreover, SERS detection of biological samples with varying concentrations of Staphylococcus aureus demonstrated excellent biocompatibility of the SERS substrate and enabled quantitative analysis of bacterial concentration (R 2 = 99.7 %). Theoretical simulations using finite-difference time-domain (FDTD) analysis were conducted to examine the electromagnetic field distribution of the three-dimensional SERS composite substrate, confirming its local electric field enhancement effect. These experimental and theoretical results indicate that the Au@Ag NSs/PPSi substrate with a regulable pyramidal pitted array is a promising candidate for sensitive, label-free SERS detection in medical and biotechnological applications.