Nanoscale chemical mapping using three-dimensional adiabatic compression of surface plasmon polaritons

Plasmon-mediated dynamics and lasing of nanoemitters enhanced by dispersing nanorings

We investigate the plasmon-mediated nonlinear dynamics and the optics of a laser emission of random nanoemitters (NEs) embedded in a two-dimensional (2D) lattice of conducting nanorings (NRs) enhanced by plasmon-polariton (PP) excitations. The interaction of quantum NEs with the PP field in the NRs perturbs the dynamics of the electronic populations in NEs, leading to a significant dependence of laser generation (dynamics) on the plasma frequency ωp of PP. This results in a strong coupling of NE field emission with the PP field and sharp variations of the average current in the NR lattice. The phase transition in the system was found when the macroscopic structures of PP fields are excited simultaneously in different regions of the system if ωp (control parameter) reaches critical value ωc. We have established the analytical dependence of the PP current I = I(ωp/ωc) on the plasma frequency, which is in excellent agreement with the results of numerical simulations. This effect may allow the design of new types of PP active devices with the use of conducting NRs in modern nanoelectronics.

Optimizing the Cell-Nanostructure Interface: Nanoconcave/Nanoconvex Device for Intracellular Recording of Cardiomyocytes

Nanostructures are powerful components for the development of high-performance nanodevices. Revealing and understanding the cell-nanostructure interface are essential for improving and guiding nanodevice design for investigations of cell physiology. For intracellular electrophysiological detection, the cell-nanostructure interface significantly affects the quality of recorded intracellular action potentials and the application of nanodevices in cardiology research and pharmacological screening. Most of the current investigations of biointerfaces focus on nanovertical structures, and few involve nanoconcave structures. Here, we design both nanoconvex and nanoconcave devices to perform intracellular electrophysiological recordings. The amplitude, signal-to-noise ratio, duration, and repeatability of the recorded intracellular electrophysiological signals provide a multifaceted characterization of the cell-nanostructure interface. We demonstrate that devices based on both convex and concave nanostructures can create tight coupling, which facilitates high-quality and stable intracellular recordings and paves the way for precise electrophysiological study.

Manipulating plasmonic vortex based on meta-atoms with four rectangular slits

In this paper, four rectangular slits with the same size and regular rotation angle are regarded as the meta-atom, arranged on circular contours, to create plasmonic vortex lenses (PVLs) solely based on the geometric phase. These PVLs can achieve the same purpose of exciting surface plasmon polariton (SPP) vortices with arbitrary combinations of topological charge (TC) when illuminated by circularly polarized (CP) light with different handedness as the traditional PVLs. Furthermore, they can generate SPP vortices with different TCs and specific constant or varying electric-field intensities when excited by linearly polarized (LP) light, which marks the first instance of this phenomenon solely through geometric phase manipulation. The TC can be dynamically altered by controlling the polarization order of the incident vector beam. These PVLs not only possess advantages in terms of device miniaturization and the creation of a more uniform vortex field, as compared to PVLs based on the transmission phase, but also offer a more straightforward design process in comparison to traditional structures that rely solely on the geometric phase.

Reference-less wavefront shaping in a Hopfield-like rough intensity landscape

This study introduces a new digital-micromirror based binary-phase wavefront shaping technique, which allows the measurement of the full coupling matrix of a disordered medium without a reference and enables to focusing transmitted light. The coupling matrix takes on a bi-dyadic structure, similar to a Hopfield memory matrix containing two memory patterns. Sequential wavefront optimization in this configuration often stalls due to a rough intensity landscape, resulting in a non-optimal state. To overcome this issue, we propose the Complete Couplings Mapping method, which consistently reaches the theoretically expected maximum intensity.

Rapid theoretical method for inverse design on a tip-enhanced Raman spectroscopy (TERS) probe

Tip-enhanced Raman spectroscopy (TERS) can provide correlated topographic and chemical information at the nanoscale, with great sensitivity and spatial resolution depending on the configuration of the TERS probe. The sensitivity of the TERS probe is largely determined by two effects: the lightning-rod effect and local surface plasmon resonance (LSPR). While 3D numerical simulations have traditionally been used to optimize the TERS probe structure by sweeping two or more parameters, this method is extremely resource-intensive, with computation times growing exponentially as the number of parameters increases. In this work, we propose an alternative rapid theoretical method that reduces computational loading while still achieving effective TERS probe optimization through the inverse design method. By applying this method to optimize a TERS probe with four free-structural parameters, we observed a nearly 1 order of magnitude improvement in enhancement factor (|E/E₀|²), in contrast to a parameter sweeping 3D simulation that would take ∼7000 hours of computation. Our method, therefore, shows great promise as a useful tool for designing not only TERS probes but also other near-field optical probes and optical antennas.

Ligand Size and Carbon-Chain Length Study of Silver Carboxylates in Focused Electron-Beam-Induced Deposition

Gas-assisted focused electron-beam-induced deposition is a versatile tool for the direct writing of complex-shaped nanostructures with unprecedented shape fidelity and resolution. While the technique is well-established for various materials, the direct electron beam writing of silver is still in its infancy. Here, we examine and compare five different silver carboxylates, three perfluorinated: [Ag₂(µ-O₂CCF₃)₂], [Ag₂(µ-O₂CC₂F₅)₂], and [Ag₂(µ-O₂CC₃F₇)₂], and two containing branched substituents: [Ag₂(µ-O₂CCMe₂Et)₂] and [Ag₂(µ-O₂C^tBu)₂], as potential precursors for focused electron-beam-induced deposition. All of the compounds show high sensitivity to electron dissociation and efficient dissociation of Ag-O bonds. The as-deposited materials have silver contents from 42 at.% to above 70 at.% and are composed of silver nano-crystals with impurities of carbon and fluorine between them. Precursors with the shortest carbon-fluorine chain ligands yield the highest silver contents. In addition, the deposited silver content depends on the balance of electron-induced ligand co-deposition and ligand desorption. For all of the tested compounds, low electron flux was related to high silver content. Our findings demonstrate that silver carboxylates constitute a promising group of precursors for gas-assisted focused electron beam writing of high silver content materials.

Near-unity Raman β-factor of surface-enhanced Raman scattering in a waveguide

The Raman scattering of light by molecular vibrations is a powerful technique to fingerprint molecules through their internal bonds and symmetries. Since Raman scattering is weak¹, methods to enhance, direct and harness it are highly desirable, and this has been achieved using optical cavities², waveguides^3-6 and surface-enhanced Raman scattering (SERS)^7-9. Although SERS offers dramatic enhancements^2,6,10,11 by localizing light within vanishingly small hot-spots in metallic nanostructures, these tiny interaction volumes are only sensitive to a few molecules, yielding weak signals¹². Here we show that SERS from 4-aminothiophenol molecules bonded to a plasmonic gap waveguide is directed into a single mode with >99% efficiency. Although sacrificing a confinement dimension, we find a SERS enhancement of ~10³ times across a broad spectral range enabled by the waveguide's larger sensing volume and non-resonant waveguide mode. Remarkably, this waveguide SERS is bright enough to image Raman transport across the waveguides, highlighting the role of nanofocusing^13-15 and the Purcell effect¹⁶. By analogy to the β-factor from laser physics^10,17-20, the near-unity Raman β-factor we observe exposes the SERS technique to alternative routes for controlling Raman scattering. The ability of waveguide SERS to direct Raman scattering is relevant to Raman sensors based on integrated photonics^7-9 with applications in gas sensing and biosensing.

Correlations between incident and emission polarization in nanowire-particle coupled junctions

Plasmonic nanostructures with subwavelength confinement are of great importance for the development of integrated nanophotonic circuits and devices. Here, we experimentally investigate how the polarization of the emitted light from nanowire-particle junction relies on the incident polarization. We demonstrate that the correlations can be effectively modulated by the particle position relative to the wire. By varying the wire-particle gap with only several nanometers, the nanowire-particle junction can be changed from polarization maintainer to rotator. Then, by moving the particle along the wire within half of the surface plasmon polariton (SPP) beat, the polarization behaviors can be tuned from positive to negative correlation. The mechanism can be well understood by the hybridization of wire-particle coupled mode and propagating SPP modes, which is verified by finite-difference time-domain simulations. These findings would provide a new degree of freedom for manipulating light polarization at the nanometer scale and additional flexibility for constructing nanophotonic devices.

Exciting Magnetic Dipole Mode of Split-Ring Plasmonic Nano-Resonator by Photonic Crystal Nanocavity

On-chip exciting electric modes in individual plasmonic nanostructures are realized widely; nevertheless, the excitation of their magnetic counterparts is seldom reported. Here, we propose a highly efficient on-chip excitation approach of the magnetic dipole mode of an individual split-ring resonator (SRR) by integrating it onto a photonic crystal nanocavity (PCNC). A high excitation efficiency of up to 58% is realized through the resonant coupling between the modes of the SRR and PCNC. A further fine adjustment of the excited magnetic dipole mode is demonstrated by tuning the relative position and twist angle between the SRR and PCNC. Finally, a structure with a photonic crystal waveguide side-coupled with the hybrid SRR–PCNC is illustrated, which could excite the magnetic dipole mode with an in-plane coupling geometry and potentially facilitate the future device application. Our result may open a way for developing chip-integrated photonic devices employing a magnetic field component in the optical field.

Plasmonic metal nanostructures with extremely small features: new effects, fabrication and applications

Surface plasmons in metals promise many fascinating properties and applications in optics, sensing, photonics and nonlinear fields. Plasmonic nanostructures with extremely small features especially demonstrate amazing new effects as the feature sizes scale down to the sub-nanometer scale, such as quantum size effects, quantum tunneling, spill-out of electrons and nonlocal states etc. The unusual physical, optical and photo-electronic properties observed in metallic structures with extreme feature sizes enable their unique applications in electromagnetic field focusing, spectra enhancing, imaging, quantum photonics, etc. In this review, we focus on the new effects, fabrication and applications of plasmonic metal nanostructures with extremely small features. For simplicity and consistency, we will focus our topic on the plasmonic metal nanostructures with feature sizes of sub-nanometers. Subsequently, we discussed four main and typical plasmonic metal nanostructures with extremely small features, including: (1) ultra-sharp plasmonic metal nanotips; (2) ultra-thin plasmonic metal films; (3) ultra-small plasmonic metal particles and (4) ultra-small plasmonic metal nanogaps. Additionally, the corresponding fascinating new effects (quantum nonlinear, non-locality, quantum size effect and quantum tunneling), applications (spectral enhancement, high-order harmonic wave generation, sensing and terahertz wave detection) and reliable fabrication methods will also be discussed. We end the discussion with a brief summary and outlook of the main challenges and possible breakthroughs in the field. We hope our discussion can inspire the broader design, fabrication and application of plasmonic metal nanostructures with extremely small feature sizes in the future.

Temperature-Induced Plasmon Excitations for the α-T3 Lattice in Perpendicular Magnetic Field

We have investigated the α-T3 model in the presence of a mass term which opens a gap in the energy dispersive spectrum, as well as under a uniform perpendicular quantizing magnetic field. The gap opening mass term plays the role of Zeeman splitting at low magnetic fields for this pseudospin-1 system, and, as a consequence, we are able to compare physical properties of the the α-T3 model at low and high magnetic fields. Specifically, we explore the magnetoplasmon dispersion relation in these two extreme limits. Central to the calculation of these collective modes is the dielectric function which is determined by the polarizability of the system. This latter function is generated by transition energies between subband states, as well as the overlap of their wave functions.

Low-Background Tip-Enhanced Raman Spectroscopy Enabled by a Plasmon Thin-Film Waveguide Probe

Tip-enhanced Raman spectroscopy (TERS) is a nano-optical approach to extract spatially resolved chemical information with nanometer precision. However, in the case of direct-illumination TERS, which is often employed in commercial TERS instruments, strong fluorescence or far-field Raman signals from the illuminated areas may be excited as a background. They may overwhelm the near-field TERS signal and dramatically decrease the near-field to far-field signal contrast of TERS spectra. It is still challenging for TERS to study the surface of fluorescent materials or a bulk sample that cannot be placed on an Au/Ag substrate. In this study, we developed an indirect-illumination TERS probe that allows a laser to be focused on a flat interface of a thin-film waveguide located far away from the region generating the TERS signal. Surface plasmon polaritons are generated stably on the waveguide and eventually accumulated at the tip apex, thereby producing a spatially and energetically confined hotspot to ensure stable and high-resolution TERS measurements with a low background. With this thin-film waveguide probe, TERS spectra with obvious contrast from a diamond plate can be acquired. Furthermore, the TERS technique based on this probe exhibits excellent TERS signal stability, a long lifetime, and good spatial resolution. This technique is expected to have commercial potential and enable further popularization and development of TERS technology as a powerful analytical method.

Plasmonic Gold Nanohole Arrays for Surface-Enhanced Sum Frequency Generation Detection

Nobel metal nanohole arrays have been used extensively in chemical and biological systems because of their fascinating optical properties. Gold nanohole arrays (Au NHAs) were prepared as surface plasmon polariton (SPP) generators for the surface-enhanced sum-frequency generation (SFG) detection of 4-Mercaptobenzonitrile (4-MBN). The angle-resolved reflectance spectra revealed that the Au NHAs have three angle-dependent SPP modes and two non-dispersive localized surface plasmon resonance (LSPR) modes under different structural orientation angles (sample surface orientation). An enhancement factor of ~30 was achieved when the SPP and LSPR modes of the Au NHAs were tuned to match the incident visible (VIS) and output SFG, respectively. This multi-mode matching strategy provided flexible controls and selective spectral windows for surface-enhanced measurements, and was especially useful in nonlinear spectroscopy where more than one light beam was involved. The structural orientation- and power-dependent performance demonstrated the potential of plasmonic NHAs in SFG and other nonlinear sensing applications, and provided a promising surface molecular analysis development platform.

Tuning temperature gradients in subwavelength plasmonic nanocones with tilted illumination

Inducing and controlling temperature gradients in illuminated subwavelength plasmonic structures is a challenging task. Here, we present a strategy to remotely induce and tune temperature gradients in a subwavelength metallic nanocone by adjusting the angle of incidence of linearly polarized continuous-wave illumination. We demonstrate, through rigorous three-dimensional numerical simulations, that properly tilting the incident illumination angle can increase or decrease the photoinduced temperature gradients within the nanostructure. We analyze the apex-base photoinduced temperature gradient for different illumination directions, resembling typical illumination schemes utilized in surface or tip-enhanced Raman spectroscopy.

Near Field Differential Interference Contrast Microscopy

Near field scanning optical microscopy exploiting differential interference contrast enhancement is demonstrated. Beam splitting in the near field region is implemented using a dual color probe based on plasmonic color sorter idea. This provides the ability to illuminate two neighboring points on the sample simultaneously. It is shown that by modulating the two wavelengths employed in exciting such a probe, phase difference information can be retrieved through measuring the near field photoinduced force at the difference of the two modulation frequencies. This difference in frequency is engineered to correspond to the first resonant frequency of the cantilever, resulting in improved SNR, and sensitivity. The effect of both topographical and material changes in the proposed near field differential interference (NFDIC) technique are investigated for CNT and silica samples. This method is a promising technique for high contrast and high spatial resolution microscopy.

Infrared and Raman chemical imaging and spectroscopy at the nanoscale

The advent of nanotechnology, and the need to understand the chemical composition at the nanoscale, has stimulated the convergence of IR and Raman spectroscopy with scanning probe methods, resulting in new nanospectroscopy paradigms. Here we review two such methods, namely photothermal induced resonance (PTIR), also known as AFM-IR and tip-enhanced Raman spectroscopy (TERS). AFM-IR and TERS fundamentals will be reviewed in detail together with their recent crucial advances. The most recent applications, now spanning across materials science, nanotechnology, biology, medicine, geology, optics, catalysis, art conservation and other fields are also discussed. Even though AFM-IR and TERS have developed independently and have initially targeted different applications, rapid innovation in the last 5 years has pushed the performance of these, in principle spectroscopically complimentary, techniques well beyond initial expectations, thus opening new opportunities for their convergence. Therefore, subtle differences and complementarity will be highlighted together with emerging trends and opportunities.

White nanolight source for optical nanoimaging

Nanolight sources, which are based on resonant excitation of plasmons near a sharp metallic nanostructure, have attracted tremendous interest in the vast research fields of optical nanoimaging. However, being a resonant phenomenon, this ideally works only for one wavelength that resonates with the plasmons. Multiple wavelengths of light in a broad range confined to one spot within a nanometric volume would be an interesting form of light, useful in numerous applications. Plasmon nanofocusing can generate a nanolight source through the propagation and adiabatic compressions of plasmons on a tapered metallic nanostructure, which is independent of wavelength, as it is based on the propagation, rather than resonance, of plasmons. Here, we report the generation of a white nanolight source spanning over the entire visible range through plasmon nanofocusing and demonstrate spectral bandgap nanoimaging of carbon nanotubes. Our experimental demonstration of the white nanolight source would stimulate diverse research fields toward next-generation nanophotonic technologies.

Focused Electron Beam-Based 3D Nanoprinting for Scanning Probe Microscopy: A Review

Scanning probe microscopy (SPM) has become an essential surface characterization technique in research and development. By concept, SPM performance crucially depends on the quality of the nano-probe element, in particular, the apex radius. Now, with the development of advanced SPM modes beyond morphology mapping, new challenges have emerged regarding the design, morphology, function, and reliability of nano-probes. To tackle these challenges, versatile fabrication methods for precise nano-fabrication are needed. Aside from well-established technologies for SPM nano-probe fabrication, focused electron beam-induced deposition (FEBID) has become increasingly relevant in recent years, with the demonstration of controlled 3D nanoscale deposition and tailored deposit chemistry. Moreover, FEBID is compatible with practically any given surface morphology. In this review article, we introduce the technology, with a focus on the most relevant demands (shapes, feature size, materials and functionalities, substrate demands, and scalability), discuss the opportunities and challenges, and rationalize how those can be useful for advanced SPM applications. As will be shown, FEBID is an ideal tool for fabrication / modification and rapid prototyping of SPM-tipswith the potential to scale up industrially relevant manufacturing.

Superstructure of silver crystals in a caged framework for plasmonic inverse sensing

Lowering the limit of detection is key to the design of sensors. Conventional transducers generate a signal proportional to the concentration of the molecule, but low concentrations are still difficult to detect with confidence. Here we present an inverse sensor that induces a signal that is larger when the target molecule is less concentrated. Each sensor consists of a micro-pot reactor with an inner volume for storing the reactants and the cage walls, over which many silver hotspots are spread, working as optical antenna to produce amplification of the signal. The aim is to attain inverse sensitivity during the enzymatic reaction, where reduction of the silver occurs. We demonstrate the sensitivity and robustness of this approach by detecting glucose concentrations down to 10^-12 M.

Photodegradable CuS SERS Probes for Intraoperative Residual Tumor Detection, Ablation, and Self-Clearance

Surface-enhanced Raman scattering (SERS) probes have exhibited great potential in biomedical applications. However, currently reported SERS probes are mainly fabricated by nondegradable Au or Ag nanostructures, which are not favorably cleared from the imaged tissues. This bottleneck hinders their in vivo applications. We herein explore a degradable SERS probe consisting of hollow CuS nanoparticles (NPs) to circumvent the current limitation. We identify, for the first time, the Raman enhancement effects of hollow CuS NPs as a SERS probe for Raman imaging of residual tumor lesions. Uniquely, CuS SERS probes are degradable, which stems from laser-induced photothermal effects of CuS NPs, leading to their disintegration from shell structures into individual crystals, thus facilitating their self-clearance from imaged tissues. This novel CuS SERS probe with photodegradation characteristics opens avenues for applying Raman imaging toward a myriad of biomedical applications.

Analytical analysis of spectral sensitivity of plasmon resonances in a nanocavity

Metallic nanocavities exhibit extremely high spectral sensitivity to geometrical variations and are promising for sensing applications. Here, the sensitivity of a cubic dimer cavity, to picometer gap variation, is analysed in a model, which takes into account the phase shift of scattering at the boundaries and the quantum tunnelling effect in the small gap limit. The resonance wavelengths are expressed in terms of the plasmon frequency, the medium dielectric function, and the geometry of the gap. The sensitivity of the resonance wavelength to the gap width variation is found to be as high as 1 nm pm-1. While the resonance wavelengths depend on the materials' dielectric functions, the sensitivity is found to scale universally as a function of gap distance. In the sub-nanometer regime, electron tunnelling across the gap starts to suppress the plasmonic field, setting the limit of sensitivity of such a dimer cavity. The results given by the analytical model are complemented by numerical simulations using Comsol. Our model reveals the origin and universal behaviours of the sensitivity of the cavity plasmon and provides guidance for the design of new sensitive rulers at the picometer scale.

A Review on Surface-Enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS) has become a powerful tool in chemical, material and life sciences, owing to its intrinsic features (i.e., fingerprint recognition capabilities and high sensitivity) and to the technological advancements that have lowered the cost of the instruments and improved their sensitivity and user-friendliness. We provide an overview of the most significant aspects of SERS. First, the phenomena at the basis of the SERS amplification are described. Then, the measurement of the enhancement and the key factors that determine it (the materials, the hot spots, and the analyte-surface distance) are discussed. A section is dedicated to the analysis of the relevant factors for the choice of the excitation wavelength in a SERS experiment. Several types of substrates and fabrication methods are illustrated, along with some examples of the coupling of SERS with separation and capturing techniques. Finally, a representative selection of applications in the biomedical field, with direct and indirect protocols, is provided. We intentionally avoided using a highly technical language and, whenever possible, intuitive explanations of the involved phenomena are provided, in order to make this review suitable to scientists with different degrees of specialization in this field.

Silver Nanoparticles: Synthesis and Application for Nanomedicine

Over the past few decades, metal nanoparticles less than 100 nm in diameter have made a substantial impact across diverse biomedical applications, such as diagnostic and medical devices, for personalized healthcare practice. In particular, silver nanoparticles (AgNPs) have great potential in a broad range of applications as antimicrobial agents, biomedical device coatings, drug-delivery carriers, imaging probes, and diagnostic and optoelectronic platforms, since they have discrete physical and optical properties and biochemical functionality tailored by diverse size- and shape-controlled AgNPs. In this review, we aimed to present major routes of synthesis of AgNPs, including physical, chemical, and biological synthesis processes, along with discrete physiochemical characteristics of AgNPs. We also discuss the underlying intricate molecular mechanisms behind their plasmonic properties on mono/bimetallic structures, potential cellular/microbial cytotoxicity, and optoelectronic property. Lastly, we conclude this review with a summary of current applications of AgNPs in nanoscience and nanomedicine and discuss their future perspectives in these areas.

Silver nano-needles: focused optical field induced solution synthesis and application in remote-excitation nanofocusing SERS

Tapered metallic nanostructures that harbor surface plasmons are highly interesting for nanophotonic applications because of their waveguiding and field-focusing properties. Here, we developed a focused optical field induced solution synthesis for unique crystallized silver nano-needles. Under the focused laser spot, inhomogeneous Ag monomer concentration is created, which triggers the uniaxial growth of silver nanostructures along the radial direction with decreasing rate, forming nano-needle structures. These nano-needles are several micrometers long, with diameter attenuating from hundreds to tens of nanometers, and terminated by a sharp apex only a few nanometers in diameter. Moreover, nano-needles with atomically smooth surfaces show excellent performance for plasmonic waveguiding and unique near-field compression abilities. This nano-needle structure can be used for effective remote-excitation detection/sensing. We also demonstrate the assembling and picking up of nano-needles, which indicate potential applications in intracellular endoscopy, high resolution scanning tips, on-chip nanophotonic devices, etc.

Toward High-Contrast Atomic Force Microscopy-Tip-Enhanced Raman Spectroscopy Imaging: Nanoantenna-Mediated Remote-Excitation on Sharp-Tip Silver Nanowire Probes

The tip-enhanced Raman spectroscopy (TERS) imaging technique is designed to provide correlated morphological and chemical information with a nanoscale spatial resolution by utilizing the plasmonic resonance supported by metallic nanostructures at the tip apex of a scanning probe. However, limited by the scattering cross sections of these nanostructures, only a small fraction of the incident light can be coupled to the plasmonic resonance to generate Raman signals. The uncoupled light then directly excites background spectra with a diffraction-limited resolution, which becomes the background noise that often blurs the TERS image. Here, we demonstrate how this problem can be solved by physically separating the light excitation region from the Raman signal generation region on the scanning probe. The remote-excitation TERS (RE-TERS) probe, which can be fabricated with a facile, robust and reproducible method, utilizes silver nanoparticles as nanoantennas to mediate the coupling of free-space excitation light to propagating surface plasmon polaritons (SPPs) in a sharp-tip silver nanowire to excite Raman signals remotely. With this RE-TERS probe, a 10 nm spatial resolution was demonstrated on a single-walled carbon nanotube sample, and the strain distribution in a monolayer molybdenum disulfide (MoS₂) was mapped.

Label-Free Optical Single-Molecule Micro- and Nanosensors

Label-free optical sensor systems have emerged that exhibit extraordinary sensitivity for detecting physical, chemical, and biological entities at the micro/nanoscale. Particularly exciting is the detection and analysis of molecules, on miniature optical devices that have many possible applications in health, environment, and security. These micro- and nanosensors have now reached a sensitivity level that allows for the detection and analysis of even single molecules. Their small size enables an exceedingly high sensitivity, and the application of quantum optical measurement techniques can allow the classical limits of detection to be approached or surpassed. The new class of label-free micro- and nanosensors allows dynamic processes at the single-molecule level to be observed directly with light. By virtue of their small interaction length, these micro- and nanosensors probe light-matter interactions over a dynamic range often inaccessible by other optical techniques. For researchers entering this rapidly advancing field of single-molecule micro- and nanosensors, there is an urgent need for a timely review that covers the most recent developments and that identifies the most exciting opportunities. The focus here is to provide a summary of the recent techniques that have either demonstrated label-free single-molecule detection or claim single-molecule sensitivity.

Low cost tips for tip-enhanced Raman spectroscopy fabricated by two-step electrochemical etching of 125 µm diameter gold wires

Tip-enhanced Raman spectroscopy (TERS) has become a well-applied technique for nanospectroscopy, allowing for single molecule sensitivity with sub-nanometer spatial resolution. The demand for efficient, reproducible and cost-effective probes for TERS is increasing. Here we report on a new electrochemical etching protocol to fabricate TERS tips starting from 125 µm diameter gold wires in a reproducible way. The process is reliable (50% of the tips have radius of curvature <35 nm, 66% <80 nm), fast (less than 2 min) and 2.5 times cheaper than the etching of standard 250 µm diameter wires. The TERS performance of the tips is tested on dyes, pigments and biomolecules and enhancement factors higher than 10⁵ are observed. TERS mapping with a spatial resolution of 5 nm is demonstrated.

Ultrashort pulse synthesis for energy concentration control in nanostructures

A waveform synthesis technique is introduced and applied to the femtosecond pulse excitation of plasmonic nanoantennas for temporal and spatial energy concentration control. The waveform synthesis process is based on phase and polarization shaping and an understanding of the electromagnetic response of the nanostructure. Linear and radial nano-dipole arrays are analyzed before the log-periodic toothed nanoantenna is investigated as a nanostructure capable of combining the benefits of the nano-dipole arrays. The consistent superiority of the log-periodic toothed nanoantenna is established by comparing its electromagnetic response to that of the radial nano-dipole array using a variety of synthesized excitation waveforms.

Nanoscale Optical Microscopy and Spectroscopy Using Near-Field Probes

Light-matter interactions can provide a wealth of detailed information about the structural, electronic, optical, and chemical properties of materials through various excitation and scattering processes that occur over different length, energy, and timescales. Unfortunately, the wavelike nature of light limits the achievable spatial resolution for interrogation and imaging of materials to roughly λ/2 because of diffraction. Scanning near-field optical microscopy (SNOM) breaks this diffraction limit by coupling light to nanostructures that are specifically designed to manipulate, enhance, and/or extract optical signals from very small regions of space. Progress in the SNOM field over the past 30 years has led to the development of many methods to optically characterize materials at lateral spatial resolutions well below 100 nm. We review these exciting developments and demonstrate how SNOM is truly extending optical imaging and spectroscopy to the nanoscale.

The Role of Membrane Curvature in Nanoscale Topography-Induced Intracellular Signaling

Over the past decade, there has been growing interest in developing biosensors and devices with nanoscale and vertical topography. Vertical nanostructures induce spontaneous cell engulfment, which enhances the cell-probe coupling efficiency and the sensitivity of biosensors. Although local membranes in contact with the nanostructures are found to be fully fluidic for lipid and membrane protein diffusions, cells appear to actively sense and respond to the surface topography presented by vertical nanostructures. For future development of biodevices, it is important to understand how cells interact with these nanostructures and how their presence modulates cellular function and activities. How cells recognize nanoscale surface topography has been an area of active research for two decades before the recent biosensor works. Extensive studies show that surface topographies in the range of tens to hundreds of nanometers can significantly affect cell functions, behaviors, and ultimately the cell fate. For example, titanium implants having rough surfaces are better for osteoblast attachment and host-implant integration than those with smooth surfaces. At the cellular level, nanoscale surface topography has been shown by a large number of studies to modulate cell attachment, activity, and differentiation. However, a mechanistic understanding of how cells interact and respond to nanoscale topographic features is still lacking. In this Account, we focus on some recent studies that support a new mechanism that local membrane curvature induced by nanoscale topography directly acts as a biochemical signal to induce intracellular signaling, which we refer to as the curvature hypothesis. The curvature hypothesis proposes that some intracellular proteins can recognize membrane curvatures of a certain range at the cell-to-material interface. These proteins then recruit and activate downstream components to modulate cell signaling and behavior. We discuss current technologies allowing the visualization of membrane deformation at the cell membrane-to-substrate interface with nanometer precision and demonstrate that vertical nanostructures induce local curvatures on the plasma membrane. These local curvatures enhance the process of clathrin-mediated endocytosis and affect actin dynamics. We also present evidence that vertical nanostructures can induce significant deformation of the nuclear membrane, which can affect chromatin distribution and gene expression. Finally, we provide a brief perspective on the curvature hypothesis and the challenges and opportunities for the design of nanotopography for manipulating cell behavior.

Near-Field Plasmonic Probe with Super Resolution and High Throughput and Signal-to-Noise Ratio

Near-field scanning optical microscopy (NSOM) enables observation of light-matter interaction with a spatial resolution far below the diffraction limit without the need for a vacuum environment. However, modern NSOM techniques remain subject to a few fundamental restrictions. For example, concerning the aperture tip (a-tip), the throughput is extremely low, and the lateral resolution is poor; both are limited by the aperture size. Meanwhile, with regard to the scattering tip (s-tip), the signal-to-noise ratio (SNR) appears to be almost zero; consequently, one cannot directly use the measured data. In this work, we present a plasmonic tip (p-tip) developed by tailoring subwavelength annuli so as to couple internal radial illumination to surface plasmon polaritons (SPPs), resulting in an ultrastrong, superfocused spot. Our p-tip supports both a radial symmetric SPP excitation and a Fabry-Pérot resonance, and experimental results indicate an optical resolution of 10 nm, a topographic resolution of 10 nm, a throughput of 3.28%, and an outstanding SNR of up to 18.2 (nearly free of background). The demonstrated p-tip outperforms state-of-the-art NSOM tips and can be readily employed in near-field optics, nanolithography, tip-enhanced Raman spectroscopy, and other applications.

Breakdown of Far-Field Raman Selection Rules by Light-Plasmon Coupling Demonstrated by Tip-Enhanced Raman Scattering

We present an experimental study on the near-field light-matter interaction by tip-enhanced Raman scattering (TERS) with polarized light in three different materials: germanium-doped gallium nitride (GaN), graphene, and carbon nanotubes. We investigate the dependence of the TERS signal on the incoming light polarization and on the sample carrier concentration, as well as the Raman selection rules in the near-field. We explain the experimental data with a tentative quantum mechanical interpretation, which takes into account the role of plasmon polaritons, and the associated evanescent field. The driving force for the breakdown of the classical Raman selection rules in TERS is caused by photon tunneling through the perturbation of the evanescent field, with the consequent polariton annihilation. Predictions based on this quantum mechanical approach are in good agreement with the experimental data, which are shown to be independent of incoming light polarization, leading to new Raman selection rules for TERS.

Hybrid photonic-plasmonic near-field probe for efficient light conversion into the nanoscale hot spot

In this Letter, we present a design and simulations of the novel hybrid photonic-plasmonic near-field probe. Near-field optics is a unique imaging tool that provides optical images with resolution down to tens of nanometers. One of the main limitations of this technology is its low light sensitivity. The presented hybrid probe solves this problem by combining a campanile plasmonic probe with the photonic layer, consisting of the diffractive optic element (DOE). The DOE is designed to match the plasmonic field at the broad side of the campanile probe with the fiber mode. This makes it possible to optimize the size of the campanile tip to convert light efficiently into the hot spot. The simulations show that the hybrid probe is ∼540 times more efficient compared with the conventional campanile on average in the 600-900 nm spectral range.

Resonant terahertz probes for near-field scattering microscopy

We propose and characterize a scattering probe for terahertz (THz) near-field microscopy, fabricated from indium, where the scattering efficiency is enhanced by the dipolar resonance supported by the indium probe. The scattering properties of the probe were evaluated experimentally using THz time-domain spectroscopy (TDS), and numerically using the finite-difference time-domain (FDTD) method in order to identify resonant enhancement. Numerical measurements show that the indium probes exhibit enhanced scattering across the THz frequency range due to dipolar resonance, with a fractional bandwidth of 0.65 at 1.24 THz. We experimentally observe the resonant enhancement of the scattered field with a peak at 0.3 THz. To enable practical THz microscopy applications of these resonant probes, we also demonstrate a simple excitation scheme utilizing a THz source with radial polarization, which excites a radial mode along the length of the tip. Strong field confinement at the apex of the tip, as required for THz near-field microscopy, was observed experimentally.

Chemical Functionalization of Plasmonic Surface Biosensors: A Tutorial Review on Issues, Strategies, and Costs

In an ideal plasmonic surface sensor, the bioactive area, where analytes are recognized by specific biomolecules, is surrounded by an area that is generally composed of a different material. The latter, often the surface of the supporting chip, is generally hard to be selectively functionalized, with respect to the active area. As a result, cross talks between the active area and the surrounding one may occur. In designing a plasmonic sensor, various issues must be addressed: the specificity of analyte recognition, the orientation of the immobilized biomolecule that acts as the analyte receptor, and the selectivity of surface coverage. The objective of this tutorial review is to introduce the main rational tools required for a correct and complete approach to chemically functionalize plasmonic surface biosensors. After a short introduction, the review discusses, in detail, the most common strategies for achieving effective surface functionalization. The most important issues, such as the orientation of active molecules and spatial and chemical selectivity, are considered. A list of well-defined protocols is suggested for the most common practical situations. Importantly, for the reported protocols, we also present direct comparisons in term of costs, labor demand, and risk vs benefit balance. In addition, a survey of the most used characterization techniques necessary to validate the chemical protocols is reported.

Tuning of nanofocused vector vortex beam of metallic granary-shaped nanotip with spin-dependent dielectric helical cone

We present the combined configuration of dielectric helical cone and metallic granary-shaped nanotip to produce three -dimensional vector vortex nanofocused optical field. The intensity and phase of the electric fields, and Povnting vector of the optical field generated by the combined configuration with linearly polarized illumination are studied with three-dimensional finite difference time-domain method. The localized vector electric field near the apex of the metallic granary-shaped nanotip is strongly depended on the chirality of the dielectric helical cone and the bottom radius of the metallic granary-shaped nanotip. The localized vector electric field is wavelength selective with the maximum intensity enhancement up to 10⁴ times and minimum size of about 900 nm², and the maximum radial electric field rotates 67.0° along z axis. This indicates the vector vortex beam generated by the combined configuration can be applied in nanofabrication, nano-sensing and nano-manipulation.

Combination of scanning probe technology with photonic nanojets

Light focusing through a microbead leads to the formation of a photonic nanojet functional for enhancing the spatial resolution of traditional optical systems. Despite numerous works that prove this phenomenon, a method to appropriately translate the nanojet on top of a region of interest is still missing. Here, by using advanced 3D fabrication techniques we integrated a microbead on an AFM cantilever thus realizing a system to efficiently position nanojets. This fabrication approach is robust and can be exploited in a myriad of applications, ranging from microscopy to Raman spectroscopy. We demonstrate the potential of portable nanojets by imaging different sub-wavelength structures. Thanks to the achieved portability, we were able to perform a detailed optical characterization of the resolution enhancement induced by the microbead, which sheds light into the many contradictory resolution claims present in literature. Our conclusions are strongly supported by rigorous data analysis and by numerical simulations, all in perfect agreement with experimental results.

Helicity locking of chiral light emitted from a plasmonic nanotaper

Surface plasmon waves carry an intrinsic transverse spin, which is locked to its propagation direction. Apparently, when a singular plasmonic mode is guided on a conic surface this spin-locking may lead to a strong circular polarization of the far-field emission. Specifically, a plasmonic vortex excited on a flat metal surface propagates on an adiabatically tapered gold nanocone where the mode accelerates and finally beams out from the tip apex. The helicity of this beam is shown to be single-handed and stems solely from the transverse spin-locking of the helical plasmonic wave-front. We present a simple geometric model that fully predicts the emerging light spin in our system. Finally, we experimentally demonstrate the helicity-locking phenomenon by using accurately fabricated nanostructures and confirm the results with the model and numerical data.

Campanile Near-Field Probes Fabricated by Nanoimprint Lithography on the Facet of an Optical Fiber

One of the major challenges to the widespread adoption of plasmonic and nano-optical devices in real-life applications is the difficulty to mass-fabricate nano-optical antennas in parallel and reproducible fashion, and the capability to precisely place nanoantennas into devices with nanometer-scale precision. In this study, we present a solution to this challenge using the state-of-the-art ultraviolet nanoimprint lithography (UV-NIL) to fabricate functional optical transformers onto the core of an optical fiber in a single step, mimicking the ‘campanile’ near-field probes. Imprinted probes were fabricated using a custom-built imprinter tool with co-axial alignment capability with sub <100 nm position accuracy, followed by a metallization step. Scanning electron micrographs confirm high imprint fidelity and precision with a thin residual layer to facilitate efficient optical coupling between the fiber and the imprinted optical transformer. The imprinted optical transformer probe was used in an actual NSOM measurement performing hyperspectral photoluminescence mapping of standard fluorescent beads. The calibration scans confirmed that imprinted probes enable sub-diffraction limited imaging with a spatial resolution consistent with the gap size. This novel nano-fabrication approach promises a low-cost, high-throughput, and reproducible manufacturing of advanced nano-optical devices.

Plasmonic hot electron transport drives nano-localized chemistry

Nanoscale localization of electromagnetic fields near metallic nanostructures underpins the fundamentals and applications of plasmonics. The unavoidable energy loss from plasmon decay, initially seen as a detriment, has now expanded the scope of plasmonic applications to exploit the generated hot carriers. However, quantitative understanding of the spatial localization of these hot carriers, akin to electromagnetic near-field maps, has been elusive. Here we spatially map hot-electron-driven reduction chemistry with 15 nm resolution as a function of time and electromagnetic field polarization for different plasmonic nanostructures. We combine experiments employing a six-electron photo-recycling process that modify the terminal group of a self-assembled monolayer on plasmonic silver nanoantennas, with theoretical predictions from first-principles calculations of non-equilibrium hot-carrier transport in these systems. The resulting localization of reactive regions, determined by hot-carrier transport from high-field regions, paves the way for improving efficiency in hot-carrier extraction science and nanoscale regio-selective surface chemistry.

Tip-enhanced Raman spectroscopy for surfaces and interfaces

TERS offers the high spatial resolution to establish structure-function correlation for surfaces and interfaces.

Hybrid plasmonic–photonic whispering gallery mode resonators for sensing: a critical review

In this review we present the state of the art and the most recent advances in the field of optical sensing with hybrid plasmonic–photonic whispering gallery mode (WGM) resonators.