Toward plasmonics with nanometer precision: nonlinear optics of helium-ion milled gold nanoantennas

The 3D Controllable Fabrication of Nanomaterials with FIB-SEM Synchronization Technology

Nanomaterials with unique structures and functions have been widely used in the fields of microelectronics, biology, medicine, and aerospace, etc. With advantages of high resolution and multi functions (e.g., milling, deposition, and implantation), focused ion beam (FIB) technology has been widely developed due to urgent demands for the 3D fabrication of nanomaterials in recent years. In this paper, FIB technology is illustrated in detail, including ion optical systems, operating modes, and combining equipment with other systems. Together with the in situ and real-time monitoring of scanning electron microscopy (SEM) imaging, a FIB-SEM synchronization system achieved 3D controllable fabrication from conductive to semiconductive and insulative nanomaterials. The controllable FIB-SEM processing of conductive nanomaterials with a high precision is studied, especially for the FIB-induced deposition (FIBID) 3D nano-patterning and nano-origami. As for semiconductive nanomaterials, the realization of high resolution and controllability is focused on nano-origami and 3D milling with a high aspect ratio. The parameters of FIB-SEM and its working modes are analyzed and optimized to achieve the high aspect ratio fabrication and 3D reconstruction of insulative nanomaterials. Furthermore, the current challenges and future outlooks are prospected for the 3D controllable processing of flexible insulative materials with high resolution.

Realising high aspect ratio 10 nm feature size in laser materials processing in air at 800 nm wavelength in the far-field by creating a high purity longitudinal light field at focus

In semiconductor and data storage device manufacturing, it is desirable to produce feature sizes less than 30 nm with a high depth-to-width aspect ratio on the target material rapidly at a low cost. However, optical diffraction limits the smallest focused laser beam diameter to around half of the laser wavelength (λ/2). The existing approach to achieving nanoscale fabrication is mainly based on costly extreme ultraviolet (EUV) technology operating within the diffraction limit. In this paper, a new method is shown to achieve materials processing resolution down to 10 nm (λ/80) at an infrared laser wavelength of around 800 nm in the far-field, in air, well beyond the optical diffraction limit. A high-quality longitudinal field with a purity of 94.7% is generated to realise this super-resolution. Both experiments and theoretical modelling have been carried out to verify and understand the findings. The ablation craters induced on polished silicon, copper, and sapphire are compared for different types of light fields. Holes of 10-30 nm in diameter are produced on sapphire with a depth-to-width aspect ratio of over 16 and a zero taper with a single pulse at 100-120 nJ pulse energy. Such high aspect ratio sub-50 nm holes produced with single pulse laser irradiation are rarely seen in laser processing, indicating a new material removal mechanism with the longitudinal field. The working distance (lens to target) is around 170 µm, thus the material processing is in the far field. Tapered nano-holes can also be produced by adjusting the lens to the target distance.

Complex 10-nm resolution nanogap and nanowire geometries for plasmonic metasurface miniaturization

Emerging electromagnetic inverse design methods have pushed nanofabrication methods to their limits to extract maximum performance from plasmonic aperture-based metasurfaces. Using plasmonic metamaterial-lined apertures as an example, we demonstrate the importance of fine nanowire and nanogap features for achieving strong miniaturization of plasmonic nanoapertures. Metamaterial-lined nanoapertures are miniaturized over bowtie nanoapertures with identical minimum feature sizes by a factor of 25% without loss of field enhancement. We show that features as small as 10 nm can be reliably patterned over the wide areas required of metasurfaces using the helium focused ion beam microscope. Under imperfect fabrication conditions, we achieve 11-nm-wide nanogaps and 12-nm-wide nanowires over an area of 13 µm², and successfully validate our results with optical characterization and comparable full-wave simulations.

Thermal-annealing-regulated plasmonic enhanced fluorescence platform enables accurate detection of antigen/antibody against infectious diseases

Plasmonic enhanced fluorescence (PEF) technology is a powerful strategy to improve the sensitivity of immunofluorescence microarrays (IFMA), however, current approaches to constructing PEF platforms are either expensive/time-consuming or reliant on specialized instruments. Here, we develop a completely alternative approach relying on a two-step protocol that includes the self-assembly of gold nanoparticles (GNPs) at the water-oil interface and subsequent annealing-assisted regulation of gold nanogap. Our optimized thermal-annealing GNPs (TA-GNP) platform generates adequate hot spots, and thus produces high-density electromagnetic coupling, eventually enabling 240-fold fluorescence enhancement of probed dyes in the near-infrared region. For clinical detection of human samples, TA-GNP provides super-high sensitivity and low detection limits for both hepatitis B surface antigen and SARS-CoV-2 binding antibody, coupled with a much-improved detection dynamic range up to six orders of magnitude. With fast detection, high sensitivity, and low detection limit, TA-GNP could not only substantially improve the outcomes of IFMA-based precision medicine but also find applications in fields of proteomic research and clinical pathology.

ELECTRONIC SUPPLEMENTARY MATERIAL

Supplementary material (UV-Vis absorption and transmission spectra of GNPs, SEM, microscopy and digital images of PEF platforms, and fluorescence images of IFMA on PEF platforms) is available in the online version of this article at 10.1007/s12274-022-5035-6.

Molecular Plasmonics with Metamaterials

Molecular plasmonics, the area which deals with the interactions between surface plasmons and molecules, has received enormous interest in fundamental research and found numerous technological applications. Plasmonic metamaterials, which offer rich opportunities to control the light intensity, field polarization, and local density of electromagnetic states on subwavelength scales, provide a versatile platform to enhance and tune light-molecule interactions. A variety of applications, including spontaneous emission enhancement, optical modulation, optical sensing, and photoactuated nanochemistry, have been reported by exploiting molecular interactions with plasmonic metamaterials. In this paper, we provide a comprehensive overview of the developments of molecular plasmonics with metamaterials. After a brief introduction to the optical properties of plasmonic metamaterials and relevant fabrication approaches, we discuss light-molecule interactions in plasmonic metamaterials in both weak and strong coupling regimes. We then highlight the exploitation of molecules in metamaterials for applications ranging from emission control and optical modulation to optical sensing. The role of hot carriers generated in metamaterials for nanochemistry is also discussed. Perspectives on the future development of molecular plasmonics with metamaterials conclude the review. The use of molecules in combination with designer metamaterials provides a rich playground both to actively control metamaterials using molecular interactions and, in turn, to use metamaterials to control molecular processes.

Nanoscale mapping of shifts in dark plasmon modes in sub 10 nm aluminum nanoantennas

In this work, we report the fabrication and spectroscopic characterization of subwavelength aluminum nanocavities-consisting of hexamer or tetramer clusters of sub 10 nm width Al nanorods-with tunable localized surface plasmon resonance (LSPR) energies on suspended SiN_xmembranes. Here the volume plasmon (VP) and LSPR modes of lithographically-fabricated Al nanocavities are revealed by low-loss electron energy-loss spectroscopy (EELS) in an aberration corrected scanning transmission electron microscope (STEM). We show that the existence of grain boundaries (GBs) in these nanocavities results in shifts in the VP energy and a reduction in the VP lifetime. We map the VP energy and lifetime across GBs and we observe a decrease in VP energy and lifetime at GBs that is consistent with a reduction in free carrier density and increased plasmon scattering at these locations. Dipolar LSPR modes resonant in the UV and blue regions of the electromagnetic spectrum as well as higher-energy optically dark quadrupolar and hexapolar LSPR modes are also observed and mapped by STEM and EELS. All LSPR modes are confirmed via electromagnetic simulations based on the boundary element method. Both tetramer and hexamer structures support the excitation of dipolar bright and dipolar dark modes. Finally, we find that asymmetries in fabricated nanorod hexamer and tetramer nanocavities result in a mode mixing leading to a shift in dipolar dark LSPR modes.

Ultra-bright and narrow-band emission from Ag atomic sized nanoclusters in a self-assembled plasmonic resonator

We have proposed, implemented and investigated a novel, efficient quantum emitter based on an atomic-sized Ag nanocluster in a plasmonic resonator. The quantum emitter enables the realization of: (1) ultra-bright fluorescence, (2) narrow-band emission down to 4 nm, (3) ultra-short fluorescence lifetime. The fluorescence cross-section of a quantum emitter is on the order of σ ∼ 10^-14 cm², which is comparable to the largest fluorescence cross-sections of dye molecules and quantum dots, and enables a light source with a record high intensity known only for plasmon nanolasers. The results presented suggest a unique method for fabricating nanoprobes with high brightness and wavelength-tunable spectrally narrow fluorescence, which is needed for multiplex diagnostics and detection of substances at extremely low concentrations.

High-Throughput Direct Writing of Metallic Micro- and Nano-Structures by Focused Ga+ Beam Irradiation of Palladium Acetate Films

Metallic nanopatterns are ubiquitous in applications that exploit the electrical conduction at the nanoscale, including interconnects, electrical nanocontacts, and small gaps between metallic pads. These metallic nanopatterns can be designed to show additional physical properties (optical transparency, plasmonic effects, ferromagnetism, superconductivity, heat evacuation, etc.). For these reasons, an intense search for novel lithography methods using uncomplicated processes represents a key on-going issue in the achievement of metallic nanopatterns with high resolution and high throughput. In this contribution, we introduce a simple methodology for the efficient decomposition of Pd3(OAc)6 spin-coated thin films by means of a focused Ga+ beam, which results in metallic-enriched Pd nanostructures. Remarkably, the usage of a charge dose as low as 30 μC/cm2 is sufficient to fabricate structures with a metallic Pd content above 50% (at.) exhibiting low electrical resistivity (70 μΩ·cm). Binary-collision-approximation simulations provide theoretical support to this experimental finding. Such notable behavior is used to provide three proof-of-concept applications: (i) creation of electrical contacts to nanowires, (ii) fabrication of small (40 nm) gaps between large metallic contact pads, and (iii) fabrication of large-area metallic meshes. The impact across several fields of the direct decomposition of spin-coated organometallic films by focused ion beams is discussed.

A Parabola-like Gold Nanobowtie on a Sapphire Substrate as a Nano-Cavity

Plasmonic, metallic nanostructures have attracted much interest for their ability to manipulate light on a subwavelength scale and for their related applications in various fields. In this work, a parabola-like gold nanobowtie (PGNB) on a sapphire substrate was designed as a nano-cavity for confining light waves in a nanoscale gap region. The near-field optical properties of the innovative PGNB structure were studied comprehensively, taking advantage of the time-resolved field calculation based on a finite-difference time-domain algorithm (FDTD). The calculation result showed that the resonance wavelength of the nano-cavity was quite sensitive to the geometry of the PGNB. The values that related to the scattering and absorption properties of the PGNB, such as the scattering cross section, absorption cross section, extinction cross section, scattering ratio, and also the absorption ratio, were strongly dependent on the geometrical parameters which affected the surface area of the nanobowtie. Increased sharpness of the gold tips on the parabola-like nano-wings benefited the concentration of high-density charges with opposite electric properties in the narrow gold tips with limited volume, thus, resulting in a highly enhanced electric field in the nano-cavity under illumination of the light wave. Reduction of the gap size between the two gold nano-tips, namely, the size of the nano-cavity, decreased the distance that the electric potential produced by the highly concentrated charges on the surface of each gold nano-tip had to jump across, therefore, causing a significantly enhanced field in the nano-cavity. Further, alignment of the linearly polarized electric field of the incident light wave with the symmetric axis of the PGNB efficiently enabled the free electrons in the PGNB to concentrate on the surface of the sharp gold tips with a high density, thus, strongly improving the field across the nano-cavity. The research provides a new insight for future design, nanofabrication, and characterization of PGNBs for applications in devices that relate to enhancing photons emission, improving efficiency for energy harvesting, and improving sensitivity for infrared detection.

Tesla-Range Femtosecond Pulses of Stationary Magnetic Field, Optically Generated at the Nanoscale in a Plasmonic Antenna

The inverse Faraday effect allows the generation of stationary magnetic fields through optical excitation only. This light-matter interaction in metals results from creating drift currents via nonlinear forces that light applies to the conduction electrons. Here, we describe the theory underlying the generation of drift currents in metals, particularly its application to photonic nanostructures using numerical simulations. We demonstrate that a gold photonic nanoantenna, optimized by a genetic algorithm, allows, under high excitation power, to maximize the drift currents and generate a pulse of stationary magnetic fields in the tesla range. This intense magnetic field, confined at the nanoscale and for a few femtoseconds, results from annular optical confinement and not from the creation of a single optical hot spot. Moreover, by controlling the incident polarization state, we demonstrate the orientation control of the created magnetic field and its reversal on demand. Finally, the stationary magnetic field's temporal behavior and the drift currents associated with it reveal the subcycle nature of this light-matter interaction. The manipulation of drift currents by a plasmonic nanostructure for the generation of stationary magnetic field pulses finds applications in the ultrafast control of magnetic domains with applications not only in data storage technologies but also in research fields such as magnetic trapping, magnetic skyrmion, magnetic circular dichroism, to spin control, spin precession, spin currents, and spin-waves, among others.

Nanostructured plasmonic chips employing nanopillar and nanoring hole arrays for enhanced sensitivity of SPR-based biosensing

We present a theoretical analysis of the different nanostructured plasmonic sensor chips-consisting of plasmonic nanostructures present on the surface of plasmonic thin films-interrogated using the Kretschmann configuration for highly sensitive localized sensing, with high tunability from the visible to the infrared regions. Rigorous coupled-wave analysis is performed to analyze all the proposed nanostructured sensor chips and compare their sensing performance. The sensitivity parameters are defined to focus on the detection of a thin layer of biomolecules on the surface of nanostructures. The dimensions of the nanostructures and the incident angle shift the plasmon resonance wavelengths and can be used to tune the operating wavelength. The nanostructured films create local regions of high electric fields, which results in enhanced sensitivity of the proposed structures. The proposed sensors can be used in surface plasmon resonance imaging to detect multiple biomolecules in a single measurement. An extremely high surface sensitivity and figure of merit (FOMS) of 91 nm nm-1 and 0.59 nm-1 has been found, respectively, for one of the proposed nanostructured sensing platforms. Moreover, we demonstrate a very high differential reflectance of 55% per nm thickness of the biolayer.

Scalable Fabrication of Metallic Nanogaps at the Sub-10 nm Level

Metallic nanogaps with metal-metal separations of less than 10 nm have many applications in nanoscale photonics and electronics. However, their fabrication remains a considerable challenge, especially for applications that require patterning of nanoscale features over macroscopic length-scales. Here, some of the most promising techniques for nanogap fabrication are evaluated, covering established technologies such as photolithography, electron-beam lithography (EBL), and focused ion beam (FIB) milling, plus a number of newer methods that use novel electrochemical and mechanical means to effect the patterning. The physical principles behind each method are reviewed and their strengths and limitations for nanogap patterning in terms of resolution, fidelity, speed, ease of implementation, versatility, and scalability to large substrate sizes are discussed.

Gold Nanorods: The Most Versatile Plasmonic Nanoparticles

Gold nanorods (NRs), pseudo-one-dimensional rod-shaped nanoparticles (NPs), have become one of the burgeoning materials in the recent years due to their anisotropic shape and adjustable plasmonic properties. With the continuous improvement in synthetic methods, a variety of materials have been attached around Au NRs to achieve unexpected or improved plasmonic properties and explore state-of-the-art technologies. In this review, we comprehensively summarize the latest progress on Au NRs, the most versatile anisotropic plasmonic NPs. We present a representative overview of the advances in the synthetic strategies and outline an extensive catalogue of Au-NR-based heterostructures with tailored architectures and special functionalities. The bottom-up assembly of Au NRs into preprogrammed metastructures is then discussed, as well as the design principles. We also provide a systematic elucidation of the different plasmonic properties associated with the Au-NR-based structures, followed by a discussion of the promising applications of Au NRs in various fields. We finally discuss the future research directions and challenges of Au NRs.

Synthesis, Assembly, Optical Properties, and Sensing Applications of Plasmonic Gap Nanostructures

Plasmonic gap nanostructures (PGNs) have been extensively investigated mainly because of their strongly enhanced optical responses, which stem from the high intensity of the localized field in the nanogap. The recently developed methods for the preparation of versatile nanogap structures open new avenues for the exploration of unprecedented optical properties and development of sensing applications relying on the amplification of various optical signals. However, the reproducible and controlled preparation of highly uniform plasmonic nanogaps and the prediction, understanding, and control of their optical properties, especially for nanogaps in the nanometer or sub-nanometer range, remain challenging. This is because subtle changes in the nanogap significantly affect the plasmonic response and are of paramount importance to the desired optical performance and further applications. Here, recent advances in the synthesis, assembly, and fabrication strategies, prediction and control of optical properties, and sensing applications of PGNs are discussed, and perspectives toward addressing these challenging issues and the future research directions are presented.

Patterning of Complex, Nanometer-Scale Features in Wide-Area Gold Nanoplasmonic Structures Using Helium Focused Ion Beam Milling

Meeting the evolving demands of plasmonics research requires increasingly precise control over surface plasmon properties, which necessitates extremely fine nanopatterning, complex geometries, and/or long-range order. Nanoplasmonic metasurfaces are representative of a modern research area requiring intricate, high-fidelity features reproduced over areas of several free-space wavelengths, making them one of the most challenging fabrication problems in the field today. This work presents a systematic study of the helium focused ion beam milling of gold for nanoplasmonic metasurface applications, using as its example a nanoplasmonic metasurface based on an array of nanometer-scale plasmonic-wire-loaded subwavelength apertures in a gold film. At each step, the pattern variations are compared to simulation to predict the experimental outcome. Our results show that even in a practical fabrication environment, helium ion beam milling can be used to reliably pattern 10 nm features into gold with 1:5 aspect ratio in complex geometries over a wide area.

Nanoscale Focus Pinspot for High-Purity Quantum Emitters via Focused-Ion-Beam-Induced Luminescence Quenching

Epitaxially grown quantum dots (QDs), especially embedded in photonic structures, play an essential role in various quantum photonic systems as on-demand single-photon sources. However, these QDs often suffer from adjacent unwanted emitters, which contribute to the background noise of the QD emission and fundamentally limit the single-photon purity. In this paper, a nanoscale focus pinspot (NFP) technique using focused-ion-beam-induced luminescence quenching enables us to improve single-photon purity from site-controlled QD as a proof-of-concept experiment. The optical quality of the QD emission is not degraded while the signal-to-noise ratio of the QD is improved. Moreover, the QD after the NFP technique reveals the single-photon nature at further elevated temperatures owing to the reduced background noise. As the NFP technique is nondestructive, it retains the apparent physical structures and photonic functions, thereby indicating its promising potential for applying diverse high-purity quantum emitters, particularly integrated in photonic devices and circuits.

Direct visualization of beam-resist interaction volume for sub-nanometer helium ion beam-lithography

Interaction volume of beam-resist is a basis unit of beam-lithography, directly determines the critical parameters of beam-lithography. We have visualized the interaction volume at the state-of-the-art sub-10 nm scale by a spot irradiation of sub-nanometer helium ion beam into an approximately free-standing resist. The visualized interaction volume suggests helium ion beam has an excellent capability in nanofabrication. Specifically, helium ion beam-lithography is 1000 times more efficient than electron beam-lithography (EBL), owns a sub-4 nm resolution, can achieve a large pattern aspect ratio (greater than 8), and does not suffer from backscattering effect at a normal exposure dose. Furthermore, the interaction volume has been theoretically studied by considering the spatial distribution of energy deposited in the resist, and eventually lead to a model for pattern prediction and proximity effect corrections. We expect that, our approach to visualize the interaction volume may be applied to study other high resolution lithographic techniques such as x-ray lithography and EBL, and it may open new possibilities in other applications, like beam-imaging, beam-milling, and beam-modification.

A review of defect engineering, ion implantation, and nanofabrication using the helium ion microscope

The helium ion microscope has emerged as a multifaceted instrument enabling a broad range of applications beyond imaging in which the finely focused helium ion beam is used for a variety of defect engineering, ion implantation, and nanofabrication tasks. Operation of the ion source with neon has extended the reach of this technology even further. This paper reviews the materials modification research that has been enabled by the helium ion microscope since its commercialization in 2007, ranging from fundamental studies of beam-sample effects, to the prototyping of new devices with features in the sub-10 nm domain.

Coherent Excitation and Control of Plasmons on Gold Using Two-Dimensional Transition Metal Dichalcogenides

The hybrid combination of two-dimensional (2D) transition metal dichalcogenides (TMDs) and plasmonic materials open up novel means of (ultrafast) optoelectronic applications and manipulation of nanoscale light-matter interaction. However, control of the plasmonic excitations by TMDs themselves has not been investigated. Here, we show that the ultrathin 2D WSe₂ crystallites permit nanoscale spatially controlled coherent excitation of surface plasmon polaritons (SPPs) on smooth Au films. The resulting complex plasmonic interference patterns are recorded with nanoscale resolution in a photoemission electron microscope. Modeling shows good agreement with experiments and further indicates how SPPs can be tailored with high spatiotemporal precision using the shape of the 2D TMDs with thicknesses down to single molecular layers. We demonstrate the use of WSe₂ nanocrystals as 2D optical elements for exploring the ultrafast dynamics of SPPs. Using few-femtosecond laser pulse pairs we excite an SPP at the boundary of a WSe₂ crystal and then have a WSe₂ monolayer wedge act as a delay line inducing a spatially varying phase difference down to the attosecond time range. The observed effects are a natural yet unexplored consequence of high dielectric functional values of TMDs in the visible range that should be considered when designing metal-TMD hybrid devices. As the 2D TMD crystals are stable in air, can be defect free, can be synthesized in many shapes, and are reliably positioned on metal surfaces, using them to excite and steer SPPs adds an interesting alternative in designing hybrid structures for plasmonic control.

The patterning toolbox FIB-o-mat: Exploiting the full potential of focused helium ions for nanofabrication

Focused beams of helium ions are a powerful tool for high-fidelity machining with spatial precision below 5 nm. Achieving such a high patterning precision over large areas and for different materials in a reproducible manner, however, is not trivial. Here, we introduce the Python toolbox FIB-o-mat for automated pattern creation and optimization, providing full flexibility to accomplish demanding patterning tasks. FIB-o-mat offers high-level pattern creation, enabling high-fidelity large-area patterning and systematic variations in geometry and raster settings. It also offers low-level beam path creation, providing full control over the beam movement and including sophisticated optimization tools. Three applications showcasing the potential of He ion beam nanofabrication for two-dimensional material systems and devices using FIB-o-mat are presented.

Scanning transmission helium ion microscopy on carbon nanomembranes

A dark-field scanning transmission ion microscopy detector was designed for the helium ion microscope. The detection principle is based on a secondary electron conversion holder with an exchangeable aperture strip allowing its acceptance angle to be tuned from 3 to 98 mrad. The contrast mechanism and performance were investigated using freestanding nanometer-thin carbon membranes. The results demonstrate that the detector can be optimized either for most efficient signal collection or for maximum image contrast. The designed setup allows for the imaging of thin low-density materials that otherwise provide little signal or contrast and for a clear end-point detection in the fabrication of nanopores. In addition, the detector is able to determine the thickness of membranes with sub-nanometer precision by quantitatively evaluating the image signal and comparing the results with Monte Carlo simulations. The thickness determined by the dark-field transmission detector is compared to X-ray photoelectron spectroscopy and energy-filtered transmission electron microscopy measurements.

Periodic arrays of plasmonic crossed-bowtie nanostructures interspaced with plasmonic nanocrosses for highly sensitive LSPR based chemical and biological sensing

In this paper, we present novel localized surface plasmon resonance (LSPR) sensors based on periodic arrays of gold crossed-bowtie nanostructures interspaced with gold nanocross pillars. Finite difference time domain (FDTD) numerical simulations were carried out to model bulk sensors as well as localized sensors based on the plasmonic nanostructures being proposed. The geometrical parameters of the plasmonic nanostructures are varied to obtain the best possible sensing performance in terms of sensitivity and figure of merit. A very high bulk sensitivity of 1753 nm per unit change in refractive index (nm RIU-1), with a figure of merit for bulk sensing (FOMbulk) of 3.65 RIU-1, is obtained for these plasmonic nanostructures. This value of bulk sensitivity is higher in comparison to previously proposed LSPR sensors based on plasmonic nanopillars and nanocrosses. Moreover, the optimized LSPR sensors being proposed in this paper provide maximum sensitivity of localized refractive index sensing of 70 nm/nm with a FOMlocalized of 0.33 nm-1. This sensitivity of localized refractive index sensing is the highest reported thus far in comparison with previously reported LSPR sensors. It is also demonstrated that the operating resonance wavelengths of these LSPR sensors can be controllably tuned for specific applications by changing the dimensions of the plasmonic nanostructures.

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.

Near-field enhancement in oxidized close gap aluminum dimers

Aluminum bowtie nanoantennas represent a possibility to confine and enhance electromagnetic (EM) field at optical frequencies in subwavelength regions by using an abundant and inexpensive metal. The native oxidation process of this metal is often viewed as a limitation for its application in plasmonics. Here, we show that in close gap configurations, the high refractive index of the native aluminum oxide helps in squeezing the plasmonic mode in extremely reduced size volumes, providing a higher EM near-field confinement and enhancement in the bowtie antenna gaps than achieved in the pure aluminum counterpart. Hence, the study provides new perspectives in the use of such a plasmonic antenna geometry within this aluminum system, which can be useful for improving plasmonics-enabled effects such as surface-enhanced Raman scattering- and light-matter interaction in strong coupling regime.

Nanoaperture fabrication in ultra-smooth single-grain gold films with helium ion beam lithography

We demonstrate a simple three-step gold thin-film sample preparation process to enhance the morphology and lithographic precision using helium ion based direct-writing. The procedure includes metal deposition, heat treatment and template stripping, which produce smooth monocrystalline gold grains with sizes up to 500 nm and an average surface roughness of 0.267 nm. By using a helium ion microscope, we can fabricate structures with feature sizes less than 20 nm in a 100 nm thick gold film with high-quality sidewalls. We demonstrate the efficacy of this technique by producing high-quality double nanohole (DNH) nanoapertures for single nanoparticle trapping in a single grain of 100 nm thick gold. This procedure can be applied to a wide range of antenna geometries and features that need to be fabricated producing optical and or electronic devices.

Nanobowtie arrays with tunable materials and geometries fabricated by holographic lithography

We introduce a highly efficient method for the fabrication of large area nanobowtie arrays (NBAs) based on a home-built tunable holographic lithography (THL) technique. By elaborately designing pattern templates, NBAs with different materials and geometric parameters can be easily obtained by a two-step approach. Both the plasmonic and semiconductor NBAs with tunable gap sizes and a high uniformity over an area of one square centimetre can be conveniently fabricated. Surface-enhanced Raman spectroscopy (SERS) performance and photoelectric properties have been demonstrated on the gold and TiO₂ NBAs, respectively. This THL technique shows unique advantages in fabricating well-defined and large-area nanostructures in a high throughput way, facilitating practical applications in a broad range of fields of optoelectronics.

Nano-patterning of a monolayer molybdenum disulfide with sub-nanometer helium ion beam: considering its shape, size and damage

We have studied nano-patterning of a two-dimensional (2D) material with an ultrafine helium ion beam considering shape-, size- and damage-control. The study reveals that the crystalline structure plays an important role in shape-control. Instead of commonly circular-shaped nanopores, spot irradiation onto a single layer of molybdenum disulfide (MoS₂) gives rise to a rhombus-shaped nanopore, which is well explained by the sub-rhombus crystalline structure of MoS₂. Helium ion beams also show promising capability to precisely control size using a delivered dose. However, the size of the nanopores is not linear with the delivered dose, due to the Gaussian distributed intensity profile of the helium ion beam. The intensity profiles are further estimated by considering aperture size, those results could be taken as a significant reference for size-control. In addition, we clarify that most of the damage is a result of re-deposition, thus controlling re-deposition might be a useful way to alleviate the damage.

Ultrafast Photoemission Electron Microscopy: Imaging Plasmons in Space and Time

Plasmonics is a rapidly growing field spanning research and applications across chemistry, physics, optics, energy harvesting, and medicine. Ultrafast photoemission electron microscopy (PEEM) has demonstrated unprecedented power in the characterization of surface plasmons and other electronic excitations, as it uniquely combines the requisite spatial and temporal resolution, making it ideally suited for 3D space and time coherent imaging of the dynamical plasmonic phenomena on the nanofemto scale. The ability to visualize plasmonic fields evolving at the local speed of light on subwavelength scale with optical phase resolution illuminates old phenomena and opens new directions for growth of plasmonics research. In this review, we guide the reader thorough experimental description of PEEM as a characterization tool for both surface plasmon polaritons and localized plasmons and summarize the exciting progress it has opened by the ultrafast imaging of plasmonic phenomena on the nanofemto scale.

5 nm Nanogap Electrodes and Arrays by Super-resolution Laser Lithography

The development of reliable, mass-produced, and cost-effective sub-10 nm nanofabrication technology leads to an unprecedented level of integration of photonic devices. In this work, we describe the development of a laser direct writing (LDW) lithography technique with ∼5 nm feature size, which is about 1/55 of the optical diffraction limit of the LDW system (405 nm laser and 0.9 NA objective), and the realization of 5 nm nanogap electrodes. This LDW lithography exhibits an attractive capability of well-site control and mass production of ∼5 × 10⁵ nanogap electrodes per hour, breaking the trade-off between resolution and throughput in a nanofabrication technique. Nanosensing chips have been demonstrated with the as-obtained nanogap electrodes, where controllable surface enhancement Raman scattering of rhodamine 6G has been realized via adjusting the gap width and, especially, the applied bias voltages. Our results establish that such a low-cost and high-efficient lithography technology has great potential to fabricate compact integrated circuits and biochips.

Hotspots on the Move: Active Molecular Enrichment by Hierarchically Structured Micromotors for Ultrasensitive SERS Sensing

Surface-enhanced Raman scattering (SERS) is recognized as one of the most sensitive spectroscopic tools for chemical and biological detections. Hotspots engineering has expedited promotion of SERS performance over the past few decades. Recently, molecular enrichment has proven to be another effective approach to improve the SERS performance. In this work, we propose a concept of "motile hotspots" to realize ultrasensitive SERS sensing by combining hotspots engineering and active molecular enrichment. High-density plasmonic nanostructure-supporting hotspots are assembled on the tubular outer wall of micromotors via nanoimprint and rolling origami techniques. The dense hotspots carried on these hierarchically structured micromotors (HSMs) can be magnet-powered to actively enrich molecules in fluid. The active enrichment manner of HSMs is revealed to be effective in accelerating the process of molecular adsorption. Consequently, SERS intensity increases significantly because of more molecules being adjacent to the hotspots after active molecular enrichment. This "motile hotspots" concept provides a synergistical approach in constructing a SERS platform with high performance. Moreover, the newly developed construction method of HSMs manifests the possibility of tailoring tubular length and diameter as well as surface patterns on the outer wall of HSMs, demonstrating good flexibility in constructing customized micromotors for various applications.

Synthesis of Monolayer Gold Nanorings Sandwich Film and Its Higher Surface-Enhanced Raman Scattering Intensity

Most previous studies relating to surface-enhanced Raman spectroscopy (SERS) signal enhancement were focused on the interaction between the light and the substrate in the x-y axis. 3D SERS substrates reported in the most of previous papers could contribute partial SERS enhancement via z axis, but the increases of the surface area were the main target for those reports. However, the z axis is also useful in achieving improved SERS intensity. In this work, hot spots along the z axis were specifically created in a sandwich nanofilm. Sandwich nanofilms were prepared with self-assembly and Langmuir-Blodgett techniques, and comprised of monolayer Au nanorings sandwiched between bottom Ag mirror and top Ag cover films. Monolayer Au nanorings were formed by self-assembly at the interface of water and hexane, followed by Langmuir-Blodgett transfer to a substrate with sputtered Ag mirror film. Their hollow property allows the light transmitted through a cover film. The use of a Ag cover layer of tens nanometers in thickness was critical, which allowed light access to the middle Au nanorings and the bottom Ag mirror, resulting in more plasmonic resonance and coupling along perpendicular interfaces (z-axis). The as-designed sandwich nanofilms could achieve an overall ~8 times SERS signals amplification compared to only the Au nanorings layer, which was principally attributed to enhanced electromagnetic fields along the created z-axis. Theoretical simulations based on finite-difference time-domain (FDTD) method showed consistent results with the experimental ones. This study points out a new direction to enhance the SERS intensity by involving more hot spots in z-axis in a designer nanostructure for high-performance molecular recognition and detection.

Miniaturized fractal optical nanoantennas defined by focused helium ion beam milling

It has been shown in the past that fractal geometries are beneficial for radio and communication antenna designs in terms of bandwidth and gain. Recently, this concept was extended to plasmonic nanoantennas. Here, we present a fabrication method based on electron beam lithography and focused helium ion beam milling to further miniaturize dimer nanoantennas of 0th, 1st and 2nd order Sierpiński fractals. With this state-of-the-art approach, it becomes feasible to experimentally move their resonance conditions into the sub-micron wavelength regime, while maintaining excellent pattern definition and achieving sub-10 nm gap sizes for high near-field enhancement. These highly sophisticated nanostructures are numerically simulated and analyzed by dark-field scattering spectroscopy to monitor the effects of the fractal structuring on the scattering spectra and near-field enhancement.

Capillary-force-induced collapse lithography for controlled plasmonic nanogap structures

The capillary force effect is one of the most important fabrication parameters that must be considered at the micro/nanoscale because it is strong enough to deform micro/nanostructures. However, the deformation of micro/nanostructures due to such capillary forces (e.g., stiction and collapse) has been regarded as an undesirable and uncontrollable obstacle to be avoided during fabrication. Here, we present a capillary-force-induced collapse lithography (CCL) technique, which exploits the capillary force to precisely control the collapse of micro/nanostructures. CCL uses electron-beam lithography, so nanopillars with various shapes can be fabricated by precisely controlling the capillary-force-dominant cohesion process and the nanopillar-geometry-dominant collapse process by adjusting the fabrication parameters such as the development time, electron dose, and shape of the nanopillars. CCL aims to achieve sub-10-nm plasmonic nanogap structures that promote extremely strong focusing of light. CCL is a simple and straightforward method to realize such nanogap structures that are needed for further research such as on plasmonic nanosensors.

Algorithmic approach for designing plasmonic nanotweezers

We use machine learning (simulated annealing) to design plasmonic nanoapertures that function as optical nanotweezers. The nanoapertures have irregular shapes that are chosen by our algorithm. We present electromagnetic simulations that show that these produce stronger field enhancements and extraction energies than nanoapertures comprising double nanoholes with the same gap geometry. We show that performance is further improved by etching one or more rings into the gold surrounding the nanoaperture. Lastly, we provide a direct comparison between our design and work that is representative of the state of the art in plasmonic nanotweezers at the time of writing.

Time-effective strategies for the fabrication of poly- and single-crystalline gold nano-structures by focused helium ion beam milling

Milling with the focused helium ion beam of a helium ion microscope is one of the most accurate ways to produce nano-structures such as plasmonic nanoantennas. In addition to good and immediate control of the dimensions, features in the sub-10 nm regime are achievable. Especially small gaps and sharp tips in this regime may lead to very high field enhancement under excitation. However, the milling rate of 30 keV helium ions is rather low, making it time-consuming to cut nano-structures out of a gold film. We present two processes to work around the low milling rate to obtain arrays of nano-structures with maximum precision within a reasonable time. These strategies can both be adapted to either poly-crystalline gold films or single-crystalline gold flakes. Using single crystals from a fabrication point of view enables even higher precision due to constant etch rates over the whole crystal as well as straight edges and vertical side-walls due to the uniform crystalline structure.

Nano-colorimetrically determined refractive index variation with ultra-high chromatic resolution

We develop a front-to-end solution where the shift of chromaticity from scattering of plasmonic nanoparticles is used as the reporter for nano-environmental refractive index variation. By co-projecting possible power combinations of RGB LEDs and digitized color grid density of CCD with various luminance onto the CIE 1931 chromaticity diagram, optimum condition for nanoenvironment sensing can be achieved. The highest resolution for local refractive index change is 0.0021 per distinguishable color, which is higher than that of a typical handheld spectrometer by 4.8 times. This result shows great potential in simplifying nano-environment sensing instruments and is particularly useful for multi-point dynamical process.

Improvement of High Affinity and Selectivity on Biosensors Using Genetically Engineered Phage by Binding Isotherm Screening

The genetically engineered M13 bacteriophage (M13 phage), developed via directed evolutionary screening process, can improve the sensitivity of sensors because of its selective binding to a target material. Herein, we propose a screening method to develop a selective and sensitive bioreporter for toxic material based on genetically engineered M13 phage. The paraquat (PQ)-binding M13 phage, developed by directed evolution, was used. The binding affinities of the PQ-binding M13 phage to PQ and similar molecules were analyzed using isothermal titration calorimetry (ITC). Based on the isotherms measured by ITC, binding affinities were calculated using the one-site binding model. The binding affinity was 5.161 × 10⁻⁷ for PQ, and 3.043 × 10⁻⁷ for diquat (DQ). The isotherm and raw ITC data show that the PQ-binding M13 phage does not selectively bind to difenzoquat (DIF). The phage biofilter experiment confirmed the ability of PQ-binding M13 bacteriophage to bind PQ. The surface-enhanced Raman scattering (SERS) platform based on the bioreporter, PQ-binding M13 phage, exhibited 3.7 times the signal intensity as compared with the wild-type-M13-phage-coated platform.

Sub-5 nm Metal Nanogaps: Physical Properties, Fabrication Methods, and Device Applications

Sub-5 nm metal nanogaps have attracted widespread attention in physics, chemistry, material sciences, and biology due to their physical properties, including great plasmon-enhanced effects in light-matter interactions and charge tunneling, Coulomb blockade, and the Kondo effect under an electrical stimulus. These properties especially meet the needs of many cutting-edge devices, such as sensing, optical, molecular, and electronic devices. However, fabricating sub-5 nm nanogaps is still challenging at the present, and scaled and reliable fabrication, improved addressability, and multifunction integration are desired for further applications in commercial devices. The aim of this work is to provide a comprehensive overview of sub-5 nm nanogaps and to present recent advancements in metal nanogaps, including their physical properties, fabrication methods, and device applications, with the ultimate aim to further inspire scientists and engineers in their research.

Probing hotspots of plasmon-enhanced Raman scattering by nanomanipulation of carbon nanotubes

We present a two-step procedure to probe hotspots of plasmon-enhanced Raman scattering with carbon nanotubes (CNTs). Dielectrophoretic deposition places a small CNT bundle on top of a plasmonic Au nanodimer. After 'pre-characterizing' both the nanotubes and dimer structure, we subsequently use the tip of an atomic force microscope to push the bundle into the plasmonic hotspot located in the 25 nm wide dimer gap, characterize its location inside the gap, and observe the onset of plasmon-enhanced Raman scattering. Evidence for the activation of the CNT's double-resonant D-mode by the near-field of the plasmonic hotspot is discussed.

Quantitative Nanoplasmonics

Plasmonics, the study of the interactions between photons and collective oscillations of electrons, has seen tremendous advances during the past decade. Controllable nanometer- and sub-nanometer-scale engineering in plasmonic resonance and electromagnetic field localization at the subwavelength scale have propelled diverse studies in optics, materials science, chemistry, biotechnology, energy science, and various applications in spectroscopy. However, for translation of these accomplishments from research into practice, major hurdles including low reproducibility and poor controllability in target structures must be overcome, particularly for reliable quantification of plasmonic signals and functionalities. This Outlook introduces and summarizes the recent attempts and findings of many groups toward more quantitative and reliable nanoplasmonics, and discusses the challenges and possible future directions.

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.

Magneto-Optical properties of noble-metal nanostructures: functional nanomaterials for bio sensing

Metallic nanostructures supporting Localized Surface Plasmon Resonances (LSPR) are characterized by their unique ability to control and manipulate light at the nanoscale. Noble metal nanostructures, such as gold nanostructures, are demonstrating to exhibit magneto-optic activity in the presence of modulated magnetic field of low intensity in transversal configuration (T-MOKE). Validation of experimental findings was achieved by numerical simulations based on Finite Element Method (FEM) techniques. The developed numerical models allowed studying the combination of the T-MOKE effect with the localized surface plasmon resonance of metal nanoparticles. Numerical optical and magneto-optical spectra provided a deep insight on the physical aspects behind the magneto-optical activity of metal nanostructures strictly related to direction of oscillations electrical dipoles generated in resonance conditions. Additionally the MO signal was characterized as a transducing signal for refractive index sensing in liquid conditions. The outcome is an increase in the limit of detection of magneto optical transducer with respect to traditional plasmonic sensors. A new strategy for magneto-plasmonic sensing based on the use of glass supported -Au nanostructures based on their MO properties has put forward.

Continuous reversible tuning of the gap size and plasmonic coupling of bow tie nanoantennas on flexible substrates

As a multifunctional device for sensing experiments and fundamental research, tailor-made plasmonic nanostructures with continuously tunable resonances are created by preparing bow tie-shaped nanostructures on a flexible substrate. The bow ties are fabricated by electron beam lithography on a chromium sacrificial layer and transferred to a polydimethylsiloxane (PDMS) substrate. The structures on PDMS are analyzed by reflection dark-field spectroscopy and scanning electron microscopy. Dark-field spectra of individual nano-antennas are obtained while the substrate is relaxed, and while strain is applied and the substrate is elastically stretched. Depending on the alignment of the bow ties relative to the direction of the strain, the deformation of the substrates leads to an increase or decrease of the nanostructure gaps, and therefore to a fully reversible decrease or increase of the antenna coupling, respectively. The continuous change in coupling is visible as a blue-shift in the resonance of the coupling mode for increasing gap widths, and a red-shift for decreasing gap widths. This configuration offers interesting perspectives for molecular transport and sensing investigations under variable coupling conditions as well as for tunable SERS substrates and optical strain sensor applications. In particular, very narrow gaps are within reach in the transversal configuration.

Comparative study of plasmonic antennas fabricated by electron beam and focused ion beam lithography

We present a comparative study of plasmonic antennas fabricated by electron beam lithography and direct focused ion beam milling. We have investigated optical and structural properties and chemical composition of gold disc-shaped plasmonic antennas on a silicon nitride membrane fabricated by both methods to identify their advantages and disadvantages. Plasmonic antennas were characterized using transmission electron microscopy including electron energy loss spectroscopy and energy dispersive X-ray spectroscopy, and atomic force microscopy. We have found stronger plasmonic response with better field confinement in the antennas fabricated by electron beam lithography, which is attributed to their better structural quality, homogeneous thickness, and only moderate contamination mostly of organic nature. Plasmonic antennas fabricated by focused ion beam lithography feature weaker plasmonic response, lower structural quality with pronounced thickness fluctuations, and strong contamination, both organic and inorganic, including implanted ions from the focused beam. While both techniques are suitable for the fabrication of plasmonic antennas, electron beam lithography shall be prioritized over focused ion beam lithography due to better quality and performance of its products.

Optical Properties of Low-Loss Ag Films and Nanostructures on Transparent Substrates

We demonstrate the fabrication of a low-loss single-crystalline Ag nanostructure deposited on transparent substrates. Our approach is based on an epitaxial growth technique in which a NaCl(001) substrate is used. The NaCl substrate is dissolved in water to allow the Ag film to be transferred onto the desired substrates. Focused ion beam milling is subsequently employed to pattern a nanoarray structure consisting of 200 nanorods. The epitaxial Ag films with nanoarray structures grown in the study exhibited very flat and smooth surfaces having excellent crystallinity and local misorientation of less than 1°. Further, spectroscopic ellipsometry measurements indicated that the imaginary part of the dielectric constant of the single-crystalline film was smaller than that of a conventional polycrystalline film. Moreover, we used the three-dimensional finite-difference time-domain method to analyze the plasmonic properties of the nanoarray structure by considering the actual processed structure. Characteristically, when the SiO₂ substrate was etched by ion beam milling to a depth of 30 nm, the spectrum showed a spectral shape 20% sharper than that of the substrate with no etching (depth: 0 nm). The plasmonic performance of the single-crystalline Ag nanostructure was largely determined by its structural precision and the dielectric properties of the metal.

Ultra-high-frequency microwave response from flexible transparent Au electromagnetic metamaterial nanopatterned antenna

Flexible transparent materials are a hot spot in current research but also a key technical difficulty in industry. They are playing an increasingly important role in flexible transparent display applications such as organic light-emitting diodes, transparent electrodes, and so on. On the other hand, the present research on nanopatterned antennas is mainly concentrated on the optical frequency but rarely on the microwave (such as 3G, 4G, and 5G) and terahertz frequency band communications, where nanopatterned antennas can have many novel applications. To the authors' knowledge, this is the first paper that presents a method for preparing a flexible transparent Au electromagnetic metamaterial nanopatterned antenna. We study its free-space performance at ultra-high frequency and its application in electronic products such as smartphones, tablets, personal computers, and wearable devices (such as smart watches) which have the function of mobile communication. The experimental results showed that the transparency of the antenna designed and fabricated in this work can be as high as 94%, and its efficiency can reach 74.5%-91.9% of antennas commonly seen at present in academia and industry. By adjusting the capacitive and inductive reactance of the nanopatterned antenna's matching circuit, combined with its measured efficiency and 3D electromagnetic simulation results, we speculate on the mechanism of the Au electromagnetic metamaterial nanopatterned antenna with good performance.

Subnanometer structure and function from ion beams through complex fluidics to fluorescent particles

The vertical dimensions of complex nanostructures determine the functions of diverse nanotechnologies. In this paper, we investigate the unknown limits of such structure-function relationships at subnanometer scales. We begin with a quantitative evaluation of measurement uncertainty from atomic force microscopy, which propagates through our investigation from ion beam fabrication to fluorescent particle characterization. We use a focused beam of gallium ions to subtractively pattern silicon surfaces, and silicon nitride and silicon dioxide films. Our study of material responses quantifies the atomic limits of forming complex topographies with subnanometer resolution of vertical features over a wide range of vertical and lateral dimensions. Our results demonstrate the underutilized capability of this standard system for rapid prototyping of subnanometer structures in hard materials. We directly apply this unprecedented dimensional control to fabricate nanofluidic devices for the analytical separation of colloidal nanoparticles by size exclusion. Optical microscopy of single nanoparticles within such reference materials establishes a subnanometer limit of the fluidic manipulation of particulate matter and enables critical-dimension particle tracking with subnanometer accuracy. After calibrating for optical interference within our multifunctional devices, which also enables device metrology and integrated spectroscopy, we reveal an unexpected relationship between nanoparticle size and emission intensity for common fluorescent probes. Emission intensity increases supervolumetrically with nanoparticle diameter and then decreases as nanoparticles with different diameters photobleach to similar values of terminal intensity. We propose a simple model to empirically interpret these surprising results. Our investigation enables new control and study of structure-function relationships at subnanometer scales.

Metasurface-assisted phase-matching-free second harmonic generation in lithium niobate waveguides

The phase-matching condition is a key aspect in nonlinear wavelength conversion processes, which requires the momenta of the photons involved in the processes to be conserved. Conventionally, nonlinear phase matching is achieved using either birefringent or periodically poled nonlinear crystals, which requires careful dispersion engineering and is usually narrowband. In recent years, metasurfaces consisting of densely packed arrays of optical antennas have been demonstrated to provide an effective optical momentum to bend light in arbitrary ways. Here, we demonstrate that gradient metasurface structures consisting of phased array antennas are able to circumvent the phase-matching requirement in on-chip nonlinear wavelength conversion. We experimentally demonstrate phase-matching-free second harmonic generation over many coherent lengths in thin film lithium niobate waveguides patterned with the gradient metasurfaces. Efficient second harmonic generation in the metasurface-based devices is observed over a wide range of pump wavelengths (λ = 1580-1650 nm).