Pronounced Fano Resonance in Single Gold Split Nanodisks with 15 nm Split Gaps for Intensive Second Harmonic Generation

Highly Efficient Ultraviolet Third-Harmonic Generation in an Isolated Thin Si Meta-Structure

Nonlinear nanophotonic devices have shown great potential for on-chip information processing, quantum source, 3D microfabrication, greatly promoting the developments of integrated optics, quantum science, nanoscience and technologies, etc. To promote the applications of nonlinear nanodevices, improving the nonlinear efficiency, expanding the spectra region of nonlinear response and reducing device thickness are three key issues. Herein, this study focuses on the nonlinear effect of third-harmonic generation (THG), and present a thin Si meta-sructure to improve the THG efficiency in the ultraviolet (UV) region. The measured THG efficiency is up to 10-5 at an emission wavelength of 309 nm. Also, the THG nanosystem is only 100 nm in thickness, which is two-five times thinner than previous all-dielectric nanosystems applied in THG studies. These findings not only present a powerful thin meta-structure with highly efficient THG emission in UV region, but also provide a constructive avenue for further understanding the light-matter interactions at subwavelength scales, guiding the design and fabricating of advanced photonic devices in future.

Plasmon-enhanced second harmonic generation of metal nanostructures

As the most common nonlinear optical process, second harmonic generation (SHG) has important application value in the field of nanophotonics. With the rapid development of metal nanomaterial processing and chemical preparation technology, various structures based on metal nanoparticles have been used to achieve the enhancement and modulation of SHG. In the field of nonlinear optics, plasmonic metal nanostructures have become potential candidates for nonlinear optoelectronic devices because of their highly adjustable physical characteristics. In this article, first, the basic optical principles of SHG and the source of surface symmetry breaking in metal nanoparticles are briefly introduced. Next, the related reports on SHG in metal nanostructures are reviewed from three aspects: the enhancement of SHG efficiency by double resonance structures, the SHG effect based on magnetic resonance and the harmonic energy transfer. Then, the applications of SHG in the sensing, imaging and in situ monitoring of metal nanostructures are summarized. Future opportunities for SHG in composite systems composed of metal nanostructures and two-dimensional materials are also proposed.

Exciton-induced Fano resonance in metallic nanocavity with tungsten disulfide atomic layer

Photon-exciton coupling behaviors in optical nanocavities attract broad attention due to their crucial applications in light manipulation and emission. Herein, we experimentally observed a Fano-like resonance with asymmetrical spectral response in an ultrathin metal-dielectric-metal (MDM) cavity integrated with an atomic-layer tungsten disulfide (WS₂). The resonance wavelength of an MDM nanocavity can be flexibly controlled by adjusting dielectric layer thickness. The results measured by the home-made microscopic spectrometer agree well with the numerical simulations. A temporal coupled-mode theoretical model was established to analyze the formation mechanism of Fano resonance in the ultrathin cavity. The theoretical analysis reveals that the Fano resonance is attributed to a weak coupling between the resonance photons in the nanocavity and excitons in the WS₂ atomic layer. The results will pave a new way for exciton-induced generation of Fano resonance and light spectral manipulation at the nanoscale.

Nanostructured surface plasmon resonance sensors: Toward narrow linewidths

Surface plasmon resonance sensors have found wide applications in optical sensing field due to their excellent sensitivity to the slight refractive index change of surrounding medium. However, the intrinsically high optical losses in metals make it nontrivial to obtain narrow resonance spectra, which greatly limits the performance of surface plasmon resonance sensors. This review first introduces the influence factors of plasmon linewidths of metallic nanostructures. Then, various approaches to achieve narrow resonance linewidths are summarized, including the fabrication of nanostructured surface plasmon resonance sensors supporting surface lattice resonance/plasmonic Fano resonance or coupling with a photonic cavity, the preparation of surface plasmon resonance sensors with ultra-narrow resonators, as well as strategies such as platform-induced modification, alternating different dielectric layers, and the coupling with whispering-gallery-modes. Lastly, the applications and some existing challenges of surface plasmon resonance sensors are discussed. This review aims to provide guidance for the further development of nanostructured surface plasmon resonance sensors.

Dual-Wavelength Forward-Enhanced Directional Scattering and Second Harmonic Enhancement in Open-Hole Silicon Nanoblock

Nanostructures with appropriate sizes can limit light-matter interaction and support electromagnetic multipole resonance. The interaction between light and nanostructures is intimately related to manipulating the direction of scattered light in the far field as well as the electromagnetic field in the near field. In this paper, we demonstrate dual-wavelength directional forward-scattering enhancement in an individual open-hole silicon nanoblock (OH-SiNB) and simultaneously achieve bulk and surface electromagnetic field localization. The second harmonic generation is enhanced using electromagnetic field localization on the square hole surface. Numerical simulations reveal that the resonance modes, at λ₁ = 800 nm and λ₂ = 1190 nm, approximately satisfy the Kerker condition. In the near field, the magnetic dipole modes at dual wavelength all satisfy the boundary condition that the normal component of the electric displacement is continuous on the square holes surface, thus obtaining the surface electromagnetic field localization. Moreover, highly efficient second harmonic generation can be achieved at dual wavelengths using the surface electromagnetic field localization and the increased surface area of the square holes. Our results provide a new strategy for the integration of nanoantennas and nonlinear optoelectronic devices in optical chips.

Chiral Third-Harmonic Metasurface for Multiplexed Holograms

Chiral nonlinear metasurfaces could natively synergize nonlinear wavefront manipulation and circular dichroism, offering enhanced capacity for multifunctional and multiplexed nonlinear metasurfaces. However, it is still quite challenging to simultaneously enable strong chiral response, precise wavefront control, high nonlinear conversion efficiency, and independent functions on spins and chirality. Here, we propose and experimentally demonstrate multiplexed third-harmonic (TH) holograms with four channels based on a chiral Au-ZnO hybrid metasurface. Specifically, the left- and right-handed circularly polarized (LCP and RCP) components of the TH holographic images can be designed independently under the excitation of an LCP (or RCP) fundamental beam. In addition, the TH conversion efficiency is measured to be as large as 10^-5, which is 8.6 times stronger than that of a bare ZnO film with the same thickness. Thus, our work provides a promising platform for realizing efficient and multifunctional nonlinear nanodevices.

Enhancement of third harmonic generation induced by surface lattice resonances in plasmonic metasurfaces

We investigate experimentally third harmonic generation (THG) from plasmonic metasurfaces consisting of two-dimensional rectangular lattices of centrosymmetric gold nanobars. By varying the incidence angle and the lattice period, we show how surface lattice resonances (SLRs) at the involved wavelengths are the major contributors in determining the magnitude of the nonlinear effects. A further boost on THG is observed when we excite together more than one SLR, either at the same or at different frequency. When such multiple resonances take place, interesting phenomena are observed, such as maximum THG enhancement for counter-propagating surface waves along the metasurface, and cascading effect emulating a third-order nonlinearity.

Enhancement of second-harmonic generation from Fano plasmonic metasurfaces by introducing structural asymmetries

Resonant plasmonic metasurfaces have attracted much attention for great potential in augmenting nonlinear optical conversion at the nanoscale and thus related sensing and integrated optics applications. In this work, we use the nonlinear scattering theory to numerically investigate enhanced second-harmonic generation (SHG) from Fano metasurfaces which consist of gold asymmetric double-bars. We find that the Fano resonance at the fundamental wavelength boosts the nonlinear response by more than a factor of 60. On this basis, by introducing translational and rotational structural asymmetries, the SHG signal is further amplified because of the broken mirror symmetry. More specifically, under the optimal condition, the previously suppressed SHG component can be greatly released and play a more important role compared to the original existing SHG component in an extra 6-fold enhancement in total SHG intensity. The 360-fold enhancement by tailoring both resonance quality and structural asymmetries indicates the clear and important roles of both linear resonance and local-field distribution in reaching the largest SHG emission. Our results are a step towards enlarging SHG responses of more complex plasmonic nanostructures.

Double Fano Resonance and Independent Regulation Characteristics in a Rectangular-like Nanotetramer Metasurface Structure

Fano resonance, which is based on a plasmonic metasurface, has many potential applications in various fields, such as biochemical sensors, slow light effect, and integrated optical circuits. In this study, a rectangular-like nanotetramer metasurface structure composed of four round-head nanorods was designed. The transmission spectrum, surface charge, and electrical field distributions of the proposed structure were simulated using the finite element method. A double Fano resonance profile was observed in the transmission spectrum. One of the Fano resonances was caused by the symmetry breaking and plasmon hybridization between the horizontal double rods, whereas the other resonance was due to the plasmonic modes' hybridization among four nanorods. These resonances could be independently tuned because of different formation mechanisms. The number of Fano resonances could be adjusted by changing the coupling distance between the horizontal and vertical rods. The results contributed to designing the highly sensitive sensors based on the plasmonic metasurface.

A planar plasmonic nano-gap and its array for enhancing light-matter interactions at the nanoscale

Gap surface plasmon (GSP) modes, the localized electromagnetic modes existing between two metal structures separated by a nano-gap, are able to support subwavelength confinement and enhancement of a light field upon resonance excitation. Such features can greatly facilitate various light-matter interactions at the nanoscale. Here, we demonstrate a planar nano-gap architecture existing between a pair of tip-shaped gold pads. The nano-gap gives rise to plasmon resonances with strong light confinement close to the tip surfaces in the visible to near-infrared spectral region. Accordingly, we showed that the plasmonic gold nano-gap can exhibit strong intrinsic second-harmonic generation (SHG) and significantly enhance the Raman scattering signal from small molecules. Furthermore, by arranging the nano-gap into arrays, a stronger SHG signal can be obtained. In addition, the surface enhanced Raman scattering (SERS) activity is also improved by two orders of magnitude compared to that of a single nano-gap. Overall, the findings in our study have demonstrated the potential applications of a plasmonic nano-gap and its arrays for signal generation and sensitive chemical sensing at the nanoscale.

Terahertz hybrid optical-plasmonic modes: tunable resonant frequency, narrow linewidth, and strong local field enhancement

Hybrid optical-plasmonic modes have the characteristics of low loss and small mode volume, which will result in the strong localization and enhancement of electromagnetic field. Such advantages of hybrid optical-plasmonic mode are important for the enhancement of light-matter interactions. Here, terahertz (THz) hybrid modes of Fabry-Perot resonances (FPRs) and spoof surface plasmon polaritons (SSPPs) in the modified Otto scheme are investigated both in theoretical and experimental aspects. The device structure is composed of a metal grating silicon waveguide (MGSW) and a metal slit grating (MSG). The two components are vertically stacked with a variable air gap between them. The THz hybrid modes are originated from the far-field coupling of the FPRs and the SSPP supported by the air gap and the MSG, respectively. By changing the thickness of the air gap, the resonant frequency of the FPR-SSPP modes can be tuned in a frequency range of about 0.1 THz. An anti-crossing behavior between two reflection dips corresponding to the guided-mode resonance in the MGSW and the FPR-SSPP mode is observed, which leads to the narrowing of the reflection dips in the anti-crossing region. Numerical simulations show that at the resonant frequencies of FPR-SSPP mode, there is a huge volume-averaged electromagnetic energy enhancement of about 1600 times in the grooves of the MSG, which is around 8.7 times larger than that induced by the SSPP directly launched by free-space electromagnetic field. The hybrid FPR-SSPP modes can be used to construct THz sensors and detectors with high sensitivity.

Third- and Second-Harmonic Generation in All-Dielectric Nanostructures: A Mini Review

Frequency conversion such as harmonic generation is a fundamental physical process in nonlinear optics. The conventional nonlinear optical systems suffer from bulky size and cumbersome phase-matching conditions due to the inherently weak nonlinear response of natural materials. Aiming at the manipulation of nonlinear frequency conversion at the nanoscale with favorable conversion efficiencies, recent research has shifted toward the integration of nonlinear functionality into nanophotonics. Compared with plasmonic nanostructures showing high dissipative losses and thermal heating, all-dielectric nanostructures have demonstrated many excellent properties, including low loss, high damage threshold, and controllable resonant electric and magnetic optical nonlinearity. In this review, we cover the recent advances in nonlinear nanophotonics, with special emphasis on third- and second-harmonic generation from all-dielectric nanoantennas and metasurfaces. We discuss the main theoretical concepts, the design principles, and the functionalities of third- and second-harmonic generation processes from dielectric nanostructures and provide an outlook on the future directions and developments of this research field.

Broadband Near-Field Near-Infrared Spectroscopy and Imaging with a Laser-Driven Light Source

The scattering-type scanning near-field optical microscope (s-SNOM) has become a powerful imaging and nano-spectroscopy tool, which is widely used in the characterization of electronic and photonic devices, two-dimensional materials and biomolecules. However, in the published literature, nano-spectroscopy is mainly employed in the mid-infrared band, and the near-infrared (NIR) nano-spectroscopy with broadband spectral range has not been well discussed. In the present paper, we introduce a home-built near-field NIR spectroscopy and imaging set-up that is based on a laser-driven light source (LDLS). By mapping the Ge-Au periodic grating sample and the photonic topology device, a ~30 nm spatial resolution and the excellent capability of characterizing complex samples are demonstrated. Spectra obtained by experiment reveal the optical band-gap of Ge with a spectral resolution of 25 cm−1, and a spectral range from 900 to 2000 nm. This technology is expected to provide a novel and unique approach for near-field NIR spectroscopy and imaging.

Nonlinear light amplification via 3D plasmonic nanocavities

Plasmonic nanocavities offer prospects for the amplification of inherently weak nonlinear responses at subwavelength scales. However, constructing these nanocavities with tunable modal volumes and reduced optical losses remains an open challenge in the development of nonlinear nanophotonics. Herein, we design and fabricate three-dimensional (3D) metal-dielectric-metal (MDM) plasmonic nanocavities that are capable of amplifying second-harmonic lights by up to three orders of magnitude with respect to dielectric-metal counterparts. In combination with experimental estimations of quantitative contributions of constituent parts in proposed 3D MDM designs, we further theoretically disclose the mechanism governing this signal amplification. We discover that this phenomenon can be attributed to the plasmon hybridization of both dipolar plasmon resonances and gap cavity resonances, such that an energy exchange channel can be attained and helps expand modal volumes while maintaining strong field localizations. Our results may advance the understanding of efficient nonlinear harmonic generations in 3D plasmonic nanostructures.

Synthesis of AuAg/Ag/Au open nanoshells with optimized magnetic plasmon resonance and broken symmetry for enhancing second-harmonic generation

The cooperation of magnetic and electric plasmon resonances in cup-shaped metallic nanostructures exhibits significant capability for second-harmonic generation (SHG) enhancement. Herein, we report an approach for synthesizing Au open nanoshells with tunable numbers and sizes of openings on a template of six-pointed PbS nanostars. The morphology of Au nanoshells is controlled by adjusting the amount of HAuCl₄, and the characteristic shapes of pointed nanocaps, open nanoshells, and hollow nanostars are obtained. Owing to the collaboration of electric and magnetic plasmon resonance modes, the Au nanoshells exhibit significantly broadened and highly tunable optical responses. Furthermore, the morphology-dependent SHG of the Au nanoshells shows two maximal SHG intensities, corresponding to four-opening and one-opening Au nanoshells with appropriate opening sizes. Ag/Au and AuAg/Ag/Au open nanoshells were further investigated to achieve enhanced SHG. By adjusting the thickness of the Ag shell, the SHG intensity of Ag/Au open nanoshells reaches a maximum due to the gradient field at the AuAg bimetallic interface. After replacing the Ag shells with Au shells, the SHG intensity of AuAg/Ag/Au open nanoshells reaches a maximum due to further symmetry breaking. These findings provide a strategy to prepare colloidal metal nanocrystals with prospective applications ranging from nonlinear photonic nanodevices to biospectroscopy and photocatalysis.

Integrated Terahertz Generator-Manipulators Using Epsilon-near-Zero-Hybrid Nonlinear Metasurfaces

In terahertz (THz) technologies, generation and manipulation of THz waves are two key processes usually implemented by different device modules. Integrating THz generation and manipulation into a single compact device will advance the applications of THz technologies in various fields. Here, we demonstrate a hybrid nonlinear plasmonic metasurface incorporating an epsilon-near-zero (ENZ) indium tin oxide (ITO) layer to seamlessly combine efficient generation and manipulation of THz waves across a wide frequency band. The coupling between the plasmonic resonance of the metasurface and the ENZ mode of the ITO thin film enhances the THz conversion efficiency by more than 4 orders of magnitude. Meanwhile, such a hybrid device is capable of shaping the polarization and wavefront of the emitted THz beam via the engineered nonlinear Pancharatnam-Berry (PB) phases of the plasmonic meta-atoms. The presented hybrid nonlinear metasurface opens a new avenue toward miniaturized integrated THz devices and systems with advanced functionalities.

Ultrafast, All Optically Reconfigurable, Nonlinear Nanoantenna

The enhancement of nonlinear optical effects via nanoscale engineering is a hot topic of research. Optical nanoantennas increase light-matter interaction and provide, simultaneously, a high throughput of the generated harmonics in the scattered light. However, nanoscale nonlinear optics has dealt so far with static or quasi-static configurations, whereas advanced applications would strongly benefit from high-speed reconfigurable nonlinear nanophotonic devices. Here we propose and experimentally demonstrate ultrafast all-optical modulation of the second harmonic (SH) from a single nanoantenna. Our design is based on a subwavelength AlGaAs nanopillar driven by a control femtosecond light pulse in the visible range. The control pulse photoinjects free carriers in the nanostructure, which in turn induce dramatic permittivity changes at the band edge of the semiconductor. This results in an efficient modulation of the SH signal generated at 775 nm by a second femtosecond pulse at the 1.55 μm telecommunications (telecom) wavelength. Our results can lead to the development of ultrafast, all optically reconfigurable, nonlinear nanophotonic devices for a broad class of telecom and sensing applications.

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.

Light-induced symmetry breaking for enhancing second-harmonic generation from an ultrathin plasmonic nanocavity

Efficient frequency up-conversion of coherent light at the nanoscale is highly demanded for a variety of modern photonic applications, but it remains challenging in nanophotonics. Surface second-order nonlinearity of noble metals can be significantly boosted up by plasmon-induced field enhancement, however the related far-field second-harmonic generation (SHG) may also be quenched in highly symmetric plasmonic nanostructures despite huge near-field amplification. Here, we demonstrate that the SHG from a single gold nanosphere is significantly enhanced when tightly coupled to a metal film, even in the absence of a plasmon resonance at the SH frequency. The light-induced electromagnetic asymmetry in the nanogap junction efficiently suppresses the cancelling of locally generated SHG fields and the SH emission is further amplified through preferential coupling to the bright, bonding dipolar resonance mode of the nanocavity. The far-field SHG conversion efficiency of up to [Formula: see text] W-1 is demonstrated from a single gold nanosphere of 100 nm diameter, two orders of magnitude higher than for complex double-resonant plasmonic nanostructures. Such highly efficient SHG from a metal nanocavity also constitutes an ultrasensitive nonlinear nanoprobe to map the distribution of longitudinal vectorial light fields in nanophotonic systems.

Intraband hot-electron photoluminescence of a silver nanowire-coupled gold film via high-order gap plasmons

We report a strong one-photon photoluminescence (PL) behavior of a silver nanowire directly coupled gold film. The PL peak position of the silver nanowire-coupled gold film deviates from the intrinsic interband transition of gold materials and is not sensitive to the diameter change of the silver nanowire. We attribute this strong PL behavior to the intraband transition of hot electrons dominated by high-order gap plasmons, which are excited in the ultra-small gap formed by an ultra-thin polyvinyl pyrrolidone (PVP) layer coated on the silver nanowire. The results show that the energy required for the strong PL of the heterogeneous system mainly comes from the gold film, acting as an incident energy absorber enhanced by the high-order gap plasmons, while the silver nanowire acts an efficient incident energy focusing antenna. In situ Raman scattering spectra and time-resolved PL intensity integral curves were used to record the carbonization and disappearance process of PVP. The understanding of the PL behavior of the silver nanowire directly coupled gold film proves the universality of plasmon-modulated PL theory and is also of great significance to improve the generation and utilization efficiency of hot electrons with high-order gap plasmons in the fields of catalysis and incident energy capture.

Seed-mediated growth of high yield Au nanoplates with in situ generated Au clusters through galvanic replacement

A photochemical method is used to grow Au nanoplates in high yield from in situ generated Au cluster seeds through the galvanic replacement reaction. The morphology of nanoplates can be further controllably tuned by adjusting the pH, and enhanced morphology determined non-linear optics performances are obtained.

Influence of InP coupling cavity on Fano resonance of sub wavelength MIM waveguide

In this paper, influence of InP coupling cavity on Fano resonance of sub wavelength MIM waveguide was studied by FDTD. It was observed that the resonant wavelengths of mode m_j (j = 1, 2, 3) were closely related with the height H₂ of InP coupling cavity. In addition, before and after the addition of air cavity, the relative farfield intensities I was a function of height H₂. Therefore, InP as discrete state could be used as the filling dielectrics of Fano resonance in the MIM waveguide.

Highly efficient second harmonic generation of thin film lithium niobate nanograting near bound states in the continuum

Bound states in the continuum (BICs) are ubiquitous physical phenomena where such states occur due to strong coupling between leaky modes in side lossy systems. BICs in meta-optics and nanophotonics enable optical mode confinement to strengthen local field enhancement in nonlinear optics. In this study, we numerically investigate second-harmonic generation (SHG) in the vicinity of BICs with a photonic structure comprising one-dimensional nanogratings and a slab waveguide made of lithium niobate (LiNbO₃, LN). By breaking the symmetry of LN nanogratings, BICs transition to quasi-BICs, which enable strong local field confinement inside LN slab waveguide to be supported, thereby resulting in improving SHG conversion with lower pump power of fundamental frequency (FW). With a peak intensity of 1.33 GW cm^-2at the FW, our structure features a second-harmonic conversion efficiency up to 8.13 × 10^-5at quasi-BICs. We believe that our results will facilitate the application of LN in integrated nonlinear nanophotonic.

Fabrication of single-nanometer metallic gaps via spontaneous nanoscale dewetting

Ultrasmall metallic nanogaps are of great significance for wide applications in various nanodevices. However, it is challenging to fabricate ultrasmall metallic nanogaps by using common lithographic methods due to the limited resolution. In this work, we establish an effective approach for successful formation of ultrasmall metallic nanogaps based on the spontaneous nanoscale dewetting effect during metal deposition. By varying the initial opening size of the exposed resist template, the influence of dewetting behavior could be adjusted and tiny metallic nanogaps can be obtained. We demonstrate that this method is effective to fabricate diverse sub-10 nm gaps in silver nanostructures. Based on this fabrication concept, even sub-5 nm metallic gaps were obtained. SERS measurements were performed to show the molecular detection capability of the fabricated Ag nanogaps. This approach is a promising candidate for sub-10 nm metallic gaps fabrication, thus possessing potential applications in nanoelectronics, nanoplasmonics, and nano-optoelectronics.

Recent advances in focused ion beam nanofabrication for nanostructures and devices: fundamentals and applications

The past few decades have witnessed growing research interest in developing powerful nanofabrication technologies for three-dimensional (3D) structures and devices to achieve nano-scale and nano-precision manufacturing. Among the various fabrication techniques, focused ion beam (FIB) nanofabrication has been established as a well-suited and promising technique in nearly all fields of nanotechnology for the fabrication of 3D nanostructures and devices because of increasing demands from industry and research. In this article, a series of FIB nanofabrication factors related to the fabrication of 3D nanostructures and devices, including mechanisms, instruments, processes, and typical applications of FIB nanofabrication, are systematically summarized and analyzed in detail. Additionally, current challenges and future development trends of FIB nanofabrication in this field are also given. This work intends to provide guidance for practitioners, researchers, or engineers who wish to learn more about the FIB nanofabrication technology that is driving the revolution in 3D nanostructures and devices.

Inch-Scale Ball-in-Bowl Plasmonic Nanostructure Arrays for Polarization-Independent Second-Harmonic Generation

Second-harmonic generation (SHG) in plasmonic nanostructures has been investigated for decades due to their wide applications in photonic circuit, quantum optics and biosensing. Development of large-scale, uniform, and efficient plasmonic nanostructure system with tunable modes is desirable for their feasible utilizations. Herein, we design an efficient inch-scale SHG source by a solution-processed method instead of traditional high-cost processes. By assembling the gold nanoparticles with the porous anodic alumina templates, multiresonance in both visible and near-infrared regions can be achieved in hexagonal plasmonic nanostructure arrays, which provide strong electric field enhancement at the gap region. Polarization-independence SHG radiation has been realized owing to the in-plane isotropic characteristic of assembled unit. The tilt-angle dependent and angle-resolved measurement showed that wide-angle nonlinear response is achieved in our device because of the gap geometry of ball-in-bowl nanostructure with nonlinear emission electric dipoles distributed on the concave surface, which makes it competitive in practical applications. Our progress not only makes it possible to produce uniform inch-scale nonlinear arrays through low-cost solution process; and also advances the understanding of the SHG radiation in plasmonic nanostructures.

Enhancement of the second harmonic signal of nonlinear crystals by a single metal nanoantenna

This work fundamentally investigates how the second harmonic generation (SHG) from commercial nonlinear crystals can be boosted by the addition of individual optical nanoantennas. Frequency conversion plays an important role in modern non-linear optics, and nonlinear crystals have become a widely used building block for non-linear processes. Still, SHG remains hampered by limited conversion efficiency. To strengthen SHG from the crystal surface, we investigate the interaction of LiNbO3 crystals with individual gold nanodiscs. The scattered intensities and resonance frequencies of the nanodiscs are analyzed by dark-field spectroscopy and simulations. Subsequently, the discs on LiNbO3 are excited by a pulsed femtosecond laser in a parabolic mirror setup. Comparing the SHG at the position of a single nanodisc at resonance on the crystal with that of the unstructured crystal and of gold nanodiscs on a reference substrate, local SHG enhancement of up to a factor of three was achieved in the focal volume through the presence of the antenna.

Large-Area Fabrication of Complex Nanohole Arrays with Highly Tunable Plasmonic Properties

By combining nanosphere lithography with oblique angle deposition, large-area asymmetric compound Ag nanohole arrays with nanorods inside the hole were patterned on substrates. The technique enabled the production of complex nanohole arrays with controlled hole diameter, thickness, and rod structure inside the hole. The compound asymmetric Ag nanohole structures showed strong polarization-dependent optical properties, and a new extraordinary optical transmission (EOT) mode with tunable resonance wavelength at the near-IR region was observed. The transmission at the new EOT wavelength region can increase from 27% of nanohole to 69% of the compound structure, and these structures can achieve a refractive index sensitivity as high as 847 nm RIU^-1. The tunable EOT wavelength and strong polarization-dependent optical properties make the structure ideal for ultrathin optical filters, polarizers, surface-enhanced spectroscopies, etc.

Strongly coupled evenly divided disks: a new compact and tunable platform for plasmonic Fano resonances

Plasmonic artificial molecules are promising platforms for linear and nonlinear optical modulation at various regimes including the visible, infrared and terahertz bands. Fano resonances in plasmonic artificial structures are widely used for controlling spectral lineshapes and tailoring of near-field and far-field optical response. Generation of a strong Fano resonance usually relies on strong plasmon coupling in densely packed plasmonic structures. Challenges in reproducible fabrication using conventional lithography significantly hinders the exploration of novel plasmonic nanostructures for strong Fano resonance. In this work, we propose a new class of plasmonic molecules with symmetric structure for Fano resonances, named evenly divided disk, which shows a strong Fano resonance due to the interference between a subradiant anti-bonding mode and a superradiant bonding mode. We successfully fabricated evenly divided disk structures with high reproducibility and with sub-20 nm gaps, using our recently developed sketch and peel lithography technique. The experimental spectra agree well with the calculated response, indicating the robustness of the Fano resonance for the evenly divided disk geometry. Control experiments reveal that the strength of the Fano resonance gradually increases when increasing the number of split parts on the disk from three to eight individual segments. The Fano-resonant plasmonic molecules that can also be reliably defined by our unique fabrication approach open up new avenues for application and provide insight into the design of artificial molecules for controlling light-matter interactions.

Double Fano resonances in hybrid disk/rod artificial plasmonic molecules based on dipole-quadrupole coupling

Fano resonance can be achieved by the destructive interference between a superradiant bright mode and a subradiant dark mode. A variety of artificial plasmonic oligomers have been fabricated to generate Fano resonance for its extensive applications. However, the Fano resonance in plasmonic oligomer systems comes from the interaction of all metal particles, which greatly limits the tunability of the Fano resonance. Besides, only a single Fano resonance is supported by many existing plasmonic oligomers, while multiple Fano resonances mostly occur in complex and multilayer structures, whose fabrication is greatly challenging. Here, a simple asymmetric plasmonic molecule consisting of a central metal disk and two side-coupled parallel metal rods is demonstrated. The simulation and experimental results clearly show that double Fano resonances appear in the transmission spectrum. In addition, the two Fano peaks can be independently tuned and single/double Fano peak switching can be achieved by changing one rod length or the gap distances between the rods and the disk. The modulation method is simple and effective, which greatly increases the tunability of the structure. The proposed asymmetric artificial plasmonic molecule can have applications in multi-channel optical switches, filters and biosensors. Moreover, the controllable plasmonic field intensity in the gap between the disk and rods also provides a new control means for plasmon-induced photocatalytic reactions and biosynthesis.

Overcurrent Electrodeposition of Fractal Plasmonic Black Gold with Broad-Band Absorption Properties for Excitation-Immune SERS

The dependence of plasmon resonance on the size, shape, and interparticle spacing of single, isolated nanostructures inherently limits their light-harvesting capability to a narrow spectral band. Here, we report a facile overcurrent electrodeposition strategy to prepare fractal plasmonic black gold (B-Au) with broad-band absorption properties (over 80% throughout the range of 300-1800 nm). The broad-band absorption properties are attributed to the excitation of multiple plasmons in the B-Au, which results in strong light-matter interaction over a broad-band spectral window. Consequently, the B-Au can produce strong broad-band surface-enhanced Raman scattering (SERS) regardless of the excitation light used. These findings demonstrate that the fractal B-Au allows efficient utilization of broad spectral photons and opens up exciting opportunities for highly sensitive SERS detection, photocatalysis, and photovoltaic devices.

Huge field enhancement and high transmittance enabled by terahertz bow-tie aperture arrays: a simulation study

Sub-wavelength aperture arrays featuring small gaps have an extraordinary significance in enhancing the interactions of terahertz (THz) waves with matters. But it is difficult to obtain large light-substance interaction enhancement and high optical response signal detection capabilities at the same time. Here, we propose a simple terahertz bow-tie aperture arrays structure with a large electric field enhancement factor and high transmittance at the same time. The field enhancement factor can reach a high value of 1.9×10⁴ and the transmission coefficient of around 0.8 (the corresponding normalized-to-area transmittance is about 14.3) at 0.04 µm feature gap simultaneously. The systematic simulation results show that the designed structure can enhance the intensity of electromagnetic hotspot by continuously reducing the feature gap size without affecting the intensity of the transmittance. We also visually displayed the significant advantages of extremely strong electromagnetic hot spots in local terahertz refractive index detection, which provides a potential platform and simple strategy for enhanced THz spectral detection.

Restoring the silenced surface second-harmonic generation in split-ring resonators by magnetic and electric mode matching

Surface second-harmonic generation (SHG) in plasmonic metal nanostructures provides a promising approach to design compact and ultrafast nonlinear nanophotonics devices. However, typical plasmonic nanostructures, such as those with tiny gaps that provide strong near-field-amplified nonlinear sources, often suffer from the cancellation of nonlinear fields in the gaps, which results in the so-called silenced SHG and consequently attenuates the overall nonlinear conversion efficiency. In this study, we propose and demonstrate that the silenced SHG in a gold split-ring resonator can be effectively restored by carefully tailoring its gap geometry to avoid the cancellation of nonlinear fields in the gap and simultaneously achieve both spatial and frequency mode matching between the magnetic and the electric dipolar resonances. As a result, the effective nonlinear sources in the gap can be dramatically amplified and the surface second-harmonic emissions can be efficiently coupled out, leading to an SHG intensity enhancement of 7 times compared to a conventional split-ring resonator. The overall SHG conversion efficiency can thus be enlarged to about 1.49 × 10^-8 in the near-infrared excitation region. Importantly, the restored surface second-harmonic emission exhibits the scattering characteristics of an ideal electric dipole, which can be very useful for nonlinear far-field manipulation such as beam steering and holograms.

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

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

Micromagnetic Configuration of Variable Nanostructured Cobalt Ferrite: Modulating and Simulations toward Memory Devices

Magnetic nanostructures with flux-closure state or single-domain state have widespread application in diverse memory devices. However, an insight into the modulation of these variable states within one specific magnetic material is rarely reported but still needed. Herein, these micromagnetic configurations within prototypical cobalt ferrite (CoFe₂O₄) nanostructures in different size and dimension were studied by modulating the assembly of CoFe₂O₄ building blocks. We find that the CoFe₂O₄ nanowire (NW) has a multidomain structure when the diameter is about 90 nm, in which the domain walls (DWs) locate preferentially at the grain boundary and can convert to single-domain state when the diameter is reduced. Alternatively, a flux-closure domain state is obtained when the CoFe₂O₄ nanostructure changes from NW to nanosheet (NS), where the DWs location depends on the overall shape of NS. In addition, we further confirm that the magnetic anisotropy and magnetostatic energy are two main factors affecting the micromagnetic configuration in CoFe₂O₄ nanostructures by crystallographic analysis and micromagnetic simulations. Our experimental and simulation results demonstrate that the modulation of morphology and dimension are efficient to tailor the micromagnetic configuration in magnetic nanostructures.

Enhanced second-harmonic generation assisted by breathing mode in a multi-resonant plasmonic trimer

Boosting the nonlinear conversion rate in nanoscale is pivotal for practical applications such as highly sensitive biosensors, extreme ultra-violate light sources, and frequency combs. Here, we theoretically study the enhancement of second-harmonic generation (SHG) in a plasmonic trimer assisted by breathing modes. The geometry of the trimer is fine-tuned to produce strong plasmonic resonances at both the fundamental and SH wavelengths to boost SHG intensity. Moreover, it is found that breathing modes show remarkable ability to augment SHG by increasing the enhancement area. In particular, these breathing modes ensure a substantial spatial mode overlap at the fundamental and SH wavelengths, resulting in further promotion of the SHG conversation rate. We envision that our findings could enable applications in nanoscale frequency converters with high efficiency.

Postdeposition UV-Ozone Treatment: An Enabling Technique to Enhance the Direct Adhesion of Gold Thin Films to Oxidized Silicon

We found that continuous films of gold (Au) on oxidized silicon (SiO₂) substrates, upon treatment with ultraviolet (UV)-ozone, exhibit strong adhesion to the SiO₂ support. Importantly, the enhancement is independent of micro- or nanostructuring of such nanometer-thick films. Deposition of a second Au layer on top of the pretreated Au layer makes the adhesion stable for at least 5 months in environmental air. Using this treatment method enables us to large-scale fabricate various SiO₂-supported Au structures at various thicknesses with dimensions spanning from a few hundreds of nanometers to a few micrometers, without the use of additional adhesion layers. We explain the observed adhesion improvement as polarization-induced increased strength of Au^δ-Si^δ+ bonds at the Au-SiO₂ interface due to the formation of a gold oxide monolayer on the Au surface by the UV-ozone treatment. Our simple and enabling method thus provides opportunities for patterning Au micro/nanostructures on SiO₂ substrates without an intermediate metallic adhesion layer, which is critical for biosensing and nanophotonic applications.

Coupling effects in single-mode and multimode resonator-coupled system

We have proposed a simple metal-dielectric-metal (MDM) waveguide system side-coupled with single-mode and multimode resonators. This proposed structure can achieve a typical dual plasmon-induced transparency (PIT) effect in the transmission spectra. The two PIT peaks exhibit opposite evolution tendencies with the increase in the depth of stubs. A multimode-coupled mode theory (M-CMT), confirmed by simulated results, is originally introduced to investigate the coupling effects of the proposed structure. Compared to the previous reported multichannel filters, the proposed structure includes obvious advantages, such as structural simplicity and ease of fabrication. In addition, the sensing characteristics of the proposed structure based on PIT effects are discussed numerically. The results demonstrate that the proposed structure is suitable for applications in multichannel filters, optical switches, and sensors.

Directional Fano Resonance in an Individual GaAs Nanospheroid

Fano resonance has been observed in a wide variety of nanophotonic structures such as photonic crystals, plasmonic structures, and metamaterials. It arises from the interference of discrete resonance states with broadband continuum states. As an emerging nanophotonic material, high-index all-dielectric nanomaterials provide a new platform to achieve Fano resonance by virtue of the simultaneous excited electric and magnetic resonances. However, to date, Fano resonance in the visible region has not been observed in individual high-index all-dielectric nanoparticles. Here, for the first time, the experimental observation of the directional Fano resonance is reported in an individual GaAs nanospheroid. The special geometry enables GaAs nanospheroids to generate spectrally overlapped electric and magnetic dipole resonances, which enhances their spectral coupling, giving rise to asymmetric-shaped backward scattering spectrum. This directional Fano resonance can be tuned by the aspect ratio of nanospheroids as well as excitation polarization. In addition, efficient directional light scattering is realized at the total scattering peak of the GaAs nanospheroid. The forward-to-backward scattering ratio can be largely enhanced due to Fano dip in the backward scattering spectrum. These findings suggest that high-index all-dielectric nanospheroid is a promising candidate for directional sources and optical switches.

All semiconductor enhanced high-harmonic generation from a single nanostructured cone

The enhancement and control of non-linear phenomena at a nanometer scale has a wide range of applications in science and in industry. Among these phenomena, high-harmonic generation in solids is a recent focus of research to realize next generation petahertz optoelectronic devices or compact all solid state EUV sources. Here, we report on the realization of the first nanoscale high harmonic source. The strong field regime is reached by confining the electric field from a few nanojoules femtosecond laser in a single 3D semiconductor waveguide. We reveal a strong competition between enhancement of coherent harmonics and incoherent fluorescence favored by excitonic processes. However, far from the band edge, clear enhancement of the harmonic emission is reported with a robust sustainability offering a compact nanosource for applications. We illustrate the potential of our harmonic nano-device by performing a coherent diffractive imaging experiment. Ultra-compact UV/X-ray nanoprobes are foreseen to have other applications such as petahertz electronics, nano-tomography or nano-medicine.

An ultranarrow SPR linewidth in the UV region for plasmonic sensing

Conventional surface plasmon resonance (SPR) modes based on gold and silver nanostructures only operate in the visible and near-infrared (NIR) regions. Nowadays, with the rapid development of strong coupling between molecules and plasmonic nanostructures and surface enhanced spectroscopy, it is highly desired to modulate the SPR modes with a narrow linewidth toward the ultraviolet (UV) wavelength region through a low cost and reproducible fabrication method. Herein, laser interference lithography is utilized to manufacture stable Al plasmonic arrays with well-controlled and tunable geometries. Importantly, an ultranarrow linewidth of SPR modes as narrow as 14 nm has been successfully obtained in the near UV region. The fabricated Al plasmonic arrays show a high sensitivity toward 485 nm RIU^-1 when it is used as a refractive index sensor. The results reported here make a valuable extension of plasmonic resonant modes spanning visible and NIR into the UV region, and it may provide a robust way to achieve alternative plasmonic materials for plasmon-enhanced molecular sensing, plasmonic nanolasers, non-linear optics, strong coupling and surface enhanced spectroscopy in the UV regions.

Symmetry-breaking induced magnetic Fano resonances in densely packed arrays of symmetric nanotrimers

Due to unique properties and great design flexibilities, Fano resonances represent one of the most promising optical features mediated by metallic nanostructures, while the excitation of some Fano modes is impossible due to symmetry reasons. The aim of this work is to show that dense lattice arrangements can have a profound impact on the optical properties of nanostructures and, in particular, can enable the excitation of otherwise dark modes. Here, we demonstrate this concept using the example of rectangular arrays of symmetric trimers packed so densely that the coupling between neighbouring unit cells imposes a symmetry break, enabling the excitation of magnetic Fano resonances. We found that in experiments as well as in simulations, electric and magnetic Fano resonances can be simultaneously formed in cases where the inter-trimer distances are sufficiently small. By analysing the transition from an isolated trimer mode into a regime of strong near-field coupling, we show that by modifying the rectangular unit cell lengths due to the symmetry mismatch between lattice and trimer, two types of Fano resonances can be found, especially magnetic Fano resonances with loop-type magnetic field distributions within the centre of each trimer, which can be either enhanced or suppressed. In addition, the influence of the refractive index environment was measured, showing sensitivity values of approximately 300 nm/RIU. Our work provides fundamental insights into the interaction of the lattice and nanostructure response and paves the way towards the observation of novel optical excitations.

Enhanced Second-Harmonic Generation from Fanolike Resonance in an Asymmetric Homodimer of Gold Elliptical Nanodisks

In this article, we have investigated the enhanced second-harmonic generation (SHG) from Fanolike resonance in an asymmetric homodimer of gold elliptical nanodisks using a three-dimensional finite element method. We have found that the broken symmetry will cause  Fanolike resonances in the extinction spectrum, resulting in the enhancement of SHG efficiency. When one of the gold elliptical nanodisks rotates, the SHG efficiency increases first and then decreases. In addition, we have also shown that the SHG signal blue-shifts with the reduction of efficiency when the separation between two nanodisks increases. Furthermore, when the nanodisks become thicker, the SHG signal also blue-shifts with the increase of efficiency. The SHG signal from this simple plasmonic structure with high efficiency and tunability may pave a way toward practical applications in sensing and generating a new light source.

Tailoring Fano lineshapes using plasmonic nanobars for highly sensitive sensing and directional emission

Plasmonic oligomers are one class of the most promising nanoclusters for generating Fano resonances. This study reveals that a nanobar-based heptamer concurrently sustains triple polarization-dependent Fano resonances, in sharp contrast to traditional nanodisk or nanosphere-based counterparts. Benefiting from the enhanced near field and reduced spectral linewidth, the gold heptamer exhibits a high refractive index sensitivity (940 nm per RIU) together with a figure of merit (FoM) value as large as 20.9, which outperforms that of most other gold oligomers. On the other hand, it is found that the spectral positions of hybridized eigenmodes depend strongly on the spatial configurations of the constituent nanobars. As a proof of concept, we design a simple heterodimer comprising a nanocross and a nanobar, where plasmonic modes with opposite radiative decay characteristics are excellently overlapped both spectrally and spatially by elaborate tailoring. Double strong Fano resonances appear on opposite sides of the spectrum as expected. More interestingly, the radiation main lobes all point to one direction at these two Fano resonances due to the spatial charge distributions and mode interferences with the maximal directivity ratio (DR) as high as 22.4, in a similar manner to the radio frequency (RF) Yagi-Uda antenna. Furthermore, the emission directions can also be easily switched by adjusting the orientations of the individual nanobar in the heterodimer. Our study demonstrates that the nanobar-based oligomers with tailored Fano lineshapes could serve as versatile and delicate platforms for the label-free biochemical sensing and directional transmission of optical information at the nanometre scale.

High-Q, low-mode-volume and multiresonant plasmonic nanoslit cavities fabricated by helium ion milling

Helium ion milling of chemically-synthesized micron-sized gold flakes is performed to fabricate ultra-narrow nanoslit cavities with a varying length and width down to 5 nm. Their plasmon resonances are characterized by one-photon photoluminescence spectroscopy. The combination of fabrication based on single-crystalline gold and resonant modes with low radiative losses leads to remarkably high quality factors of up to 24. Multiple Fabry-Pérot-type resonances in the visible/near infrared spectral range are observed due to the achieved narrow slit widths and the resulting short effective wavelengths of nanoslit plasmons. These features make nanoslit cavities attractive for a range of applications such as surface-enhanced spectroscopy, ultrafast nano-optics and strong light-matter coupling.

Field enhancement and spectral features of hexagonal necklaces of silver nanoparticles for enhanced nonlinear optical processes

The nonlinear properties of hybrid metallic-dielectric systems are attracting great interest due to their potential for the enhancement of frequency conversion processes at nanoscale dimensions. In this work, we theoretically and experimentally address the correlation between the near field distribution of hexagonal plasmonic necklaces of silver nanoparticles formed on the surface of a LiNbO₃ crystal and the second harmonic generation (SHG) produced by this nonlinear crystal in the vicinities of the necklaces. The spectral response of the hexagonal necklaces does not depend on the polarization direction and is characterized by two main modes, the absorptive high-energy mode located in the UV spectral region and the lower energy mode, which is strongly radiant and extends from the visible to the near infrared region. We show that the spatial distribution of the enhanced SHG is consistent with the local field related to the low energy plasmon mode, which spectrally overlaps the fundamental beam. The results are in agreement with the low absorption losses of this mode and the two-photon character of the nonlinear process and provide deeper insight in the connection between the linear and nonlinear optical properties of the hybrid plasmonic-ferroelectric system. The study also highlights the potential of hexagonal necklaces as useful plasmonic platforms for enhanced optical processes at the nanoscale.

Top-down fabrication of shape-controlled, monodisperse nanoparticles for biomedical applications

Nanoparticles for biomedical applications are generally formed by bottom-up approaches such as self-assembly, emulsification and precipitation. But these methods usually have critical limitations in fabrication of nanoparticles with controllable morphologies and monodispersed size. Compared with bottom-up methods, top-down nanofabrication techniques offer advantages of high fidelity and high controllability. This review focuses on top-down nanofabrication techniques for engineering particles along with their biomedical applications. We present several commonly used top-down nanofabrication techniques that have the potential to fabricate nanoparticles, including photolithography, interference lithography, electron beam lithography, mold-based lithography (nanoimprint lithography and soft lithography), nanostencil lithography, and nanosphere lithography. Varieties of current and emerging applications are also covered: (i) targeting, (ii) drug and gene delivery, (iii) imaging, and (iv) therapy. Finally, a future perspective of the nanoparticles fabricated by the top-down techniques in biomedicine is also addressed.

Zero-Mode Waveguide Nanophotonic Structures for Single Molecule Characterization

Single-molecule characterization has become a crucial research tool in the chemical and life sciences, but limitations, such as limited concentration range, inability to control molecular distributions in space, and intrinsic phenomena, such as photobleaching, present significant challenges. Recent developments in non-classical optics and nanophotonics offer promising routes to mitigating these restrictions, such that even low affinity (K D ~ mM) biomolecular interactions can be studied. Here we introduce and review specific nanophotonic devices used to support single molecule studies. Optical nanostructures, such as zero-mode waveguides (ZMWs), are usually fabricated in thin gold or aluminum films and serve to confine the observation volume of optical microspectroscopy to attoliter to zeptoliter volumes. These simple nanostructures allow individual molecules to be isolated for optical and electrochemical analysis, even when the molecules of interest are present at high concentration (μM - mM) in bulk solution. Arrays of ZMWs may be combined with optical probes such as single molecule fluorescence, single molecule fluorescence resonance energy transfer (smFRET), and fluorescence correlation spectroscopy (FCS) for distributed analysis of large numbers of single-molecule reactions or binding events in parallel. Furthermore, ZMWs may be used as multifunctional devices, for example by combining optical and electrochemical functions in a single discrete architecture to achieve electrochemical ZMWs (E-ZMW). In this review, we will describe the optical properties, fabrication, and applications of ZMWs for single-molecule studies, as well as the integration of ZMWs into systems for chemical and biochemical analysis.

Electric dipole-quadrupole hybridization induced enhancement of second-harmonic generation in T-shaped plasmonic heterodimers

In this work, we demonstrate computationally that electric dipole-quadrupole hybridization (EDQH) could be utilized to enhance plasmonic SHG efficiency. To this end, we construct T-shaped plasmonic heterodimers consisting of a short and a long gold nanorod with finite element method simulation. By controlling the strength of capacitive coupling between two gold nanorods, we explore the effect of EDQH evolution on the SHG process, including the SHG efficiency enhancement, corresponding near-field distribution, and far-field radiation pattern. Simulation results demonstrate that EDQH could enhance the SHG efficiency by a factor >100 in comparison with that achieved by an isolated gold nanorod. Additionally, the far-field pattern of the SHG could be adjusted beyond the well-known quadrupolar distribution and confirms that EDQH plays an important role in the SHG process.