An ultranarrow SPR linewidth in the UV region for plasmonic sensing

Crossed grating sensing refractive index change in the non-laboratory environment

Surface plasmon polaritons (SPPs) have been widely applied to refractive index (RI) sensing for their extremely high sensitivity to the surrounding RI change. Many efforts have been devoted to narrowing the linewidth of the SPP mode and enhancing the sensitivity of SPP sensors. However, most reported SPP-based RI sensing platforms could only operate in a laboratory environment for their bulky volume or sophisticated measuring systems. In this context, we have developed a miniaturized and portable RI sensing platform based on a 2D crossed grating coupled SPP sensor that can work under a non-laboratory environment. The crossed grating is fabricated by the laser interference lithography (LIL) method, which is cost-effective and reproductive. A series of glucose solutions with different concentrations have been used as analytes to verify the sensing performance of the fabricated crossed grating.

Extremely Ultranarrow Linewidth Based on Low-Symmetry Al Nanoellipse Metasurface

Plasmonic nanostructures with ultranarrow linewidths are of great significance in numerous applications, such as optical sensing, surface-enhanced Raman scattering (SERS), and imaging. The traditional plasmonic nanostructures generally consist of gold and silver materials, which are unavailable in the ultraviolet (UV) or deep-ultraviolet (DUV) regions. However, electronic absorption bands of many important biomolecules are mostly located in the UV or DUV regions. Therefore, researchers are eager to realize ultranarrow linewidth of plasmonic nanostructures in these regions. Aluminum (Al) plasmonic nanostructures are potential candidates for realizing the ultranarrow linewidth from the DUV to the near-infrared (NIR) regions. Nevertheless, realizing ultranarrow linewidth below 5 nm remains a challenge in the UV or DUV regions for Al plasmonic nanostructures. In this study, we theoretically designed low-symmetry an Al nanoellipse metasurface on the Al substrate. An ultranarrow linewidth of 1.9 nm has been successfully obtained in the near-UV region (400 nm). Additionally, the ultranarrow linewidth has been successfully modulated to the DUV region by adjusting structural parameters. This work aims to provide a theoretical basis and prediction for the applications, such as UV sensing and UV-SERS.

Low-cost and simple fabrication of hierarchical Al nanopit arrays for deep ultraviolet refractive index sensing

Herein, we proposed a simple non-lithographic way to fabricate hierarchical Al nanopit arrays performed as deep ultraviolet (DUV, 200-300 nm) refractive index sensing. Only by adjusting the Al deposition thickness on the Al nanopit array, the hierarchical Al nanopit arrays with tunable plasmonic properties in the DUV region were obtained. The prepared hierarchical Al nanopit arrays are of very good time stability and its RI sensitivity and concentration detection limit of adenine ethanol solution reach 311 nm/RIU and5×10-6M,respectively, as the Al deposition thickness is 60 nm. Furthermore, the electric field distribution simulation results show that high RI sensing characteristic are mainly attributed to the local surface plasmon resonance. This investigation provides a facile way to develop low cost, high efficient and easily fabricated Al-based RI sensor in the DUV region.

Enhanced sensing performance from trapezoidal metallic gratings fabricated by laser interference lithography

Nanograting-based plasmonic sensors are widely regarded as promising platforms due to their real-time label-free detection and ease of integration. However, many reported grating structures are too complicated to fabricate, which limits their application. We propose a 1D bilayer metallic grating with trapezoidal profile as a near-infrared plasmonic sensor in the spectral interrogation. Trapezoidal gratings tend to perform better than rectangular gratings as sensors, particularly as they can detect at oblique incidence to obtain higher performance. Furthermore, we have successfully fabricated such a grating with a period of 633 nm over a 2.25 cm² area using a two-step approach based on laser interference lithography. Glucose detection has been conducted to experimentally validate the sensing performance of the as-prepared grating. The measured sensitivity and figure-of-merit can reach up to 786 nm/RIU and 30, respectively. Our research sheds new light on the development of cost-effective sensing devices.

Quasi-Bragg plasmon modes for highly efficient plasmon-enhanced second-harmonic generation at near-ultraviolet frequencies

Boosting nonlinear frequency conversions with plasmonic nanostructures at near-ultraviolet (UV) frequencies remains a great challenge in nano-optics. Here we experimentally design and fabricate a plasmon-enhanced second-harmonic generation (PESHG) platform suitable for near-UV frequencies by integrating aluminum materials with grating configurations involved in structural heterogeneity. The SHG emission on the proposed platform can be amplified by up to three orders of magnitude with respect to unpatterned systems. Furthermore, the mechanism governing this amplification is identified as the occurrence of quasi-Bragg plasmon modes near second-harmonic wavelengths, such that a well-defined coherent interplay can be attained within the hot spot region and facilitate the efficient out-coupling of local second-harmonic lights to the far-field. Our work sheds light into the understanding of the role of grating-coupled surface plasmon resonances played in PESHG processes, and should pave an avenue toward UV nanosource and nonlinear metasurface applications.

Aluminium-Based Plasmonic Sensors in Ultraviolet

We theoretically investigate the surface plasmon polaritons (SPPs) generated on an Al film covered by an Al₂O₃ layer in the context of their application as refractive index sensors. The calculated reflection spectra indicate SPP resonance excited by ultraviolet light, which was affected by the thickness of both the metal and the oxide layers on the surface. With optimized geometry, the system can work as a tunable sensor with a wide UV wavelength range λ∼ 150–300 nm. We report a quality factor of up to 10 and a figure of merit on the order of 9, and these are comparable to the performance of more complicated UV plasmonic nanostructures and allow for the detection of a 1% change of the refraction index. The sensor can operate on the basis of either the incidence angle or wavelength changes. The effect of oxide surface roughness is also investigated with an emphasis on amplitude-based refraction index sensing.

Excitation of ultraviolet range Dirac-type plasmon resonance with an ultra-high Q-factor in the topological insulator Bi1.5Sb0.5Te1.8Se1.2 nanoshell

Excitation of ultraviolet (UV) range plasmon resonance with high quality (Q)-factor has been significantly challenging in plasmonics because of inherent limitations in metals like Au and Ag. Herein, we theoretically investigated UV-visible range plasmons in the topological insulator Bi_1.5Sb_0.5Te_1.8Se_1.2 (BSTS) nanosphere and nanoshell. In contrast to broad linewidth plasmon absorptions in the BSTS nanospheres, an ultra-sharp absorption peak with the Q-factor as high as 52 is excited at UV frequencies in the BSTS nanoshells. This peak is attributed to Dirac-type plasmon resonance originating from massless Dirac carriers in surface states of the BSTS. Furthermore, a tunable plasmon wavelength of the resonance is demonstrated by varying geometrical parameters of the BSTS nanoshells. This may find applications in surface enhanced Raman spectroscopies, nanolasers and biosensors in the UV regions.

Structural control and biomedical applications of plasmonic hollow gold nanospheres: A mini review

Hollow gold nanospheres (HGNs) are core/shell structures with a dielectric material core, usually composed of solvent, and a gold metal shell. Such structures have two metal/dielectric interfaces to allow interaction between the gold metal with the interior and external dielectric environment. Upon illumination by light, HGNs exhibit unique surface plasmon resonance (SPR) properties compared to solid gold nanoparticles. Their SPR absorption/scattering can be tuned by changing their diameter, shell thicknesses, and surface morphologies. In addition to the low toxicity, easy functionalization, resistance to photobleaching, and sensitivity to changes in surrounding medium of gold, the enhanced surface-to-volume ratio and tunable SPR of HGNs make them highly attractive for different applications in the fields of sensing, therapy, and theranostics. In this article, we review recent progress on the synthesis and structural control of HGNs and applications of their SPR properties in biomedical sensing and theranostics. This article is categorized under: Diagnostic Tools > Biosensing Diagnostic Tools > in vitro Nanoparticle-Based Sensing Diagnostic Tools > in vivo Nanodiagnostics and Imaging.

Plasmonic hybridization generation in self-aligned disk/hole nanocavities for multi-resonance sensing

Plasmonic nanostructures have proven an extensive practical prospect in ultra-sensitive label-free biomolecule sensing due to their nanoscale localization and large near-field enhancement. Here, we demonstrate a photonic plasmonic hybridization in the self-aligned disk/hole nanocavity array under two specific cases of nanogap and nanooverlap achieved by adjusting pillar height embedded into hole. The proposed disk/hole arrays in above two cases exhibit three hybridized modes with extremely high absorption, mainly arising from the in-phase (bonding) and out-of-phase (antibonding) coupling of dipolar modes of their parent disk and hole. Surprisingly, when the nanogap feature of the disk/hole array is transformed to the nanooverlap, crossing the quantum effect region, the bonding mode in the disk/hole array has an enormous transition in the resonant frequency. In comparison with the counterpart in the nanogap structure, the bonding mode in the nanooverlap structure supports strongest near-field localization (i.e., the decay length down to merely 3.8 nm), although charge transfer channel provided by the geometry connect between disk and hole quenches partial field enhancement. Furthermore, we systematically investigate the sensing performances of multiple hybridized modes in above two cases by considering two crucial evaluating parameters, bulk refractive index sensitivity and surface sensitivity. It is demonstrated that, in the nanogap structure, the bonding mode possesses both high bulk refractive index sensitivity and surface sensitivity. Dissimilarly, for the nanooverlap structure, the bonding and antibonding modes show different surface sensitivities in different regions away from the surface, which can be used to monitoring different bio-molecular sizes and achieve the most optimum sensitivity. Due to its unique sensing features, this disk/hole array mechanism is very valuable and promising for developing of high sensitivity sensing platform.

Grating-Coupled Surface Plasmon-Polariton Sensing at a Flat Metal-Analyte Interface in a Hybrid-Configuration

Surface plasmon resonance-based sensors have emerged as commercially fostering portable biodetectors. The scientific community is engaged in extensive research to improve their performance in terms of sensitivity, selectivity, and reproducibility for the recognition of specific biomolecules. Essentially, there is a need for miniaturizing the size of existing sensors with innovative designs without compromising their bioaffinity and sensitivity performance. In this work, we propose and demonstrate a grating-coupled surface plasmon polariton (SPP) sensor on a thin flat gold layer using a hybrid configuration. The proof of concept of the grating architecture has been realized through an innovative fabrication procedure, with experimental verification of its bulk sensitivity. The geometry is identical to the prism-coupling configuration, yet with miniaturization and compactness. Characteristics of the excited modes in the spectral regime of interest are investigated using the finite-difference time-domain simulations. The effective index calculation of the resonance conditions and the accompanying field distribution can identify the excited SPP and metal-assisted guided-mode resonance modes. Detailed probing of the electric field distribution of the desired SPP mode reveals an extended evanescent decay length of 1284 nm, close to the theoretical limit, and an extended propagation length of 270 μm. The experimental demonstration of the reflectance dip with two different analyte media perceived an increased bulk sensitivity of 1133 nm/RIU. Remarkably, this resonant mode exhibits sensing capabilities for a wide range of analyte refractive indexes. We believe that the fabricated configuration with observed high sensitivity and calculated ultradeep evanescent field penetration depth along with extended propagation length can lead to the designing of a hands-on biochip for detecting large biomolecules.

High-performance plasmonic oblique sensors for the detection of ions

Efficient optical sensing is desirable for a wide range of applications. For sensors, the spectral factors of the sensitivity (S) and the figure of merit (FoM) and the intensity change related figure of merit (FOM*) are all the key factors in sensing measurement. In this work, we propose and demonstrate a novel high-performance plasmonic sensor platform using a resonant cavity array grating under oblique excitation. An ultra-sharp absorption mode with a bandwidth down to 1.3 nm is achieved when the oblique angle is 7.5°. During the sensing of the Na+ (Cl-) ions in the solution, the spectral S and FoM factors reach 568 nm RIU-1 (refractive index unit) and 436 nm RIU-1, respectively. The minimum detection limit is as low as 3.521 × 10-6 RIU. The FOM* factor is simultaneously up to 907. Moreover, the spectral intensity change is up to 57% when only a 1% concentration change is introduced into the solution. The detection limit of the concentration of the ions can be as low as 0.002%. The sensor has great potential applications due to its ultrahigh S, FoM, and FOM*.

Plasmonic-Active Nanostructured Thin Films

Plasmonic-active nanomaterials are of high interest to scientists because of their expanding applications in the field for medicine and energy. Chemical and biological sensors based on plasmonic nanomaterials are well-established and commercially available, but the role of plasmonic nanomaterials on photothermal therapeutics, solar cells, super-resolution imaging, organic synthesis, etc. is still emerging. The effectiveness of the plasmonic materials on these technologies depends on their stability and sensitivity. Preparing plasmonics-active nanostructured thin films (PANTFs) on a solid substrate improves their physical stability. More importantly, the surface plasmons of thin film and that of nanostructures can couple in PANTFs enhancing the sensitivity. A PANTF can be used as a transducer for any of the three plasmonic-based sensing techniques, namely, the propagating surface plasmon, localized surface plasmon resonance, and surface-enhanced Raman spectroscopy-based sensing techniques. Additionally, continuous nanostructured metal films have an advantage for implementing electrical controls such as simultaneous sensing using both plasmonic and electrochemical techniques. Although research and development on PANTFs have been rapidly advancing, very few reviews on synthetic methods have been published. In this review, we provide some fundamental and practical aspects of plasmonics along with the recent advances in PANTFs synthesis, focusing on the advantages and shortcomings of the fabrication techniques. We also provide an overview of different types of PANTFs and their sensitivity for biosensing.

Hybrid metal-graphene plasmonic sensor for multi-spectral sensing in both near- and mid-infrared ranges

This paper proposes a hybrid metal-graphene plasmonic sensor which can simultaneously perform multi-spectral sensing in near- and mid-IR ranges. The proposed sensor consists of an array of asymmetric gold nano-antennas integrated with an unpatterned graphene sheet. The gold antennas support sharp Fano-resonances for near-IR sensing while the excitation of graphene plasmonic resonances extend the sensing spectra to the mid-IR range. Such a broadband spectral range goes far beyond previously demonstrated multi-spectral plasmonic sensors. The sensitivity and figure of merit (FOM) as well as their dependence on the thickness of the sensing layer and Fermi energy of graphene are studied systematically. This new type of sensor combines the advantages of conventional metallic plasmonic sensors and graphene plasmonic sensors and may open a new door for high-performance, multi-functional plasmonic sensing.

Tunable surface plasmon polaritons and ultrafast dynamics in 2D nanohole arrays

High-quality and unique surface plasmon resonance (SPR) with a narrow linewidth and controllable resonance energy plays a key role in wide applications including ultrahigh-resolution spectroscopy, on-chip sensing, optical modulation, and solar cell technology. In this work, the response of surface plasmon polariton (SPP) modes in Au nanohole arrays has been effectively tuned by properly adjusting the sample orientation without changing the geometrical parameters, and a very narrow linewidth down to 8 nm is achieved via the strong interference of two (0, -1) and (-1, 0) SPP modes in the Γ-M direction under transverse magnetic polarization. These results have been validated excellently by finite-element-method numerical simulations. More importantly, we have quantitatively investigated the contribution of conduction-band electron distribution to the SPP intensity of the array within a 20 ps timescale with ultrahigh sensitivity by utilizing home-built femtosecond transient absorption spectroscopy, and observed the minimum SPP intensity at ∼700 fs following excitation with a 0.2 μJ pulse. This study may help enhance the understanding toward the intrinsic micromechanism of SPR, thus offering opportunities for potential applications in strong coupling and new-style optical wave manipulations.

Tunable Perfect Narrow-Band Absorber Based on a Metal-Dielectric-Metal Structure

In this paper, a metal-dielectric-metal structure based on a Fabry–Perot cavity was proposed, which can provide near 100% perfect narrow-band absorption. The lossy ultrathin silver film was used as the top layer spaced by a lossless silicon oxide layer from the bottom silver mirror. We demonstrated a narrow bandwidth of 20 nm with 99.37% maximum absorption and the absorption peaks can be tuned by altering the thickness of the middle SiO2 layer. In addition, we established a deep understanding of the physics mechanism, which provides a new perspective in designing such a narrow-band perfect absorber. The proposed absorber can be easily fabricated by the mature thin film technology independent of any nano structure, which make it an appropriate candidate for photodetectors, sensing, and spectroscopy.