Extraordinary nonlinear absorption in 3D bowtie nanoantennas

Perfect optical absorption in a single array of folded graphene ribbons

Due to its one atom thickness, optical absorption (OA) in graphene is a fundamental and challenging issue. Practically, the patterned graphene-dielectric-metal structure is commonly used to achieve perfect OA (POA). In this work, we propose a novel scenario to solve this issue, in which POA is obtained by using free-standing folded graphene ribbons (FGRs). We show several local resonances, e.g. a dipole state (Mode-I) and a bound state in continuum (BIC, Mode-II), will cause very efficient OA. At normal incidence, by choosing appropriate folding angle θ, 50% absorptance by the two states is easily achieved; at oblique incidence, the two states will result in roughly 98% absorptance as incidence angle φ≈40^∘. It is also interesting to see that the system has asymmetric OA spectra, e.g. POA of the former (latter) state existing in reverse (forward) incidence, respectively. Besides the angles θ and φ, POA here can also be actively tuned by electrostatic gating. As increasing Fermi level, POA of Mode-I will undergo a gradual blueshift, while that of Mode-II will experience a rapid blueshift and then be divided into three branches, due to Fano coupling to two guided modes. In reality, the achieved POA is well maintained even the dielectric substrates are used to support FGRs. Our work offers a remarkable scenario to achieve POA, and thus enhance light-matter interaction in graphene, which can build an alternative platform to study novel optical effects in general two-dimensional (2D) materials. The folding, mechanical operation in out-of-plane direction, may emerge as a new degree of freedom for optoelectronic device applications based on 2D materials.

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.

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.

3D zig-zag nanogaps based on nanoskiving for plasmonic nanofocusing

We combine anisotropic wet etching and nanoskiving to create a novel three-dimensional (3D) nanoantenna for plasmonic nanofocusing, vertically aligned zig-zag nanogaps, constituted of nanogaps with defined angles. Instead of conventional lithography, we used the thickness of a self-assembled monolayer (SAM) to define nanogaps with high throughput, and anisotropic etching of Si V-grooves to naturally define ultra-sharp tips. Both nanogaps and sharp tips can synergistically squeeze the electro-magnetic (EM) field and excite 3D nanofocusing, enabling great potential applications in chemical sensing and plasmonic devices. The dependence of the EM field enhancement on structural features is systematically investigated and optimized. We found that the field enhancement and confinement are stronger at the tipped-nanogap compared to what standalone tips or nanogaps produce. The intensity of surface-enhanced Raman spectroscopy (SERS) recorded on the 70.5° tipped-nanogaps is 45 times higher than that recorded with linear nanogaps and 5 times higher than that recorded with tip-only nanowires, which is attributed to the integration of the tip and gap in plasmonic nanostructures. This proposed nanofabrication technique and the resulting structures equipped with a strongly enhanced EM field will promote broad applications for nanophotonics and surface-enhanced spectroscopy.

Hybrid lasing in a plasmonic cavity

Distributed feedback lasing and surface plasmon lasing were achieved in a single laser device. The laser cavity consisted of a four-layer structure including two metal films, a grating, and a gain material; the cavity was fabricated by combining interference lithography and metal evaporation. A hollow structure was employed to overcome the Joule losses of the metal film. The total thickness of the multilayer structure was 350 nm. The lasing threshold for this hybrid lasing was decreased significantly owing to the coupling between the SP mode in two metal films and the waveguide mode. The combination of SP lasing and distributed feedback lasing could benefit the design of biosensors, all-optical circuits, and electrically pumped devices.

Correlating Nanoscopic Energy Transfer and Far-Field Emission to Unravel Lasing Dynamics in Plasmonic Nanocavity Arrays

Excited-state interactions between nanoscale cavities and photoactive molecules are critical in plasmonic nanolasing, although the underlying details are less-resolved. This paper reports direct visualization of the energy-transfer dynamics between two-dimensional arrays of plasmonic gold bowtie nanocavities and dye molecules. Transient absorption microscopy measurements of single bowties within the array surrounded by gain molecules showed fast excited-state quenching (2.6 ± 1 ps) characteristic of individual nanocavities. Upon optical pumping at powers above threshold, lasing action emerged depending on the spacing of the array. By correlating ultrafast microscopy and far-field light emission characteristics, we found that bowtie nanoparticles acted as isolated cavities when the diffractive modes of the array did not couple to the plasmonic gap mode. These results demonstrate how ultrafast microscopy can provide insight into energy relaxation pathways and, specifically, how nanocavities in arrays can show single-unit nanolaser properties.

Nanoscale 2.5-dimensional surface patterning with plasmonic lithography

We report an extension of plasmonic lithography to nanoscale 2.5-dimensional (2.5D) surface patterning. To obtain the impulse response of a plasmonic lithography system, we described the field distribution of a point dipole source generated by a metallic ridge aperture with a theoretical model using the concepts of quasi-spherical waves and surface plasmon-polaritons. We performed deconvolution to construct an exposure map of a target shape for patterning. For practical applications, we fabricated several nanoscale and microscale structures, such as a cone, microlens array, nanoneedle, and a multiscale structure using the plasmonic lithography system. We verified the possibility of applying plasmonic lithography to multiscale structuring from a few tens of nanometres to a few micrometres in the lateral dimension. We obtained a root-mean-square error of 4.7 nm between the target shape and the patterned shape, and a surface roughness of 11.5 nm.

Metamaterial study of quasi-three-dimensional bowtie nanoantennas at visible wavelengths

In this paper, a novel array of quasi-three-dimensional (quasi-3D) bowtie nanoantennas has been investigated numerically and experimentally. A low-cost and facile method has been designed and implemented to fabricate the quasi-3D bowtie nanoantennas. The fabrication processes containing laser patterning and wet etching have demonstrated the advantages of easily tuning the periodic and diameter of microhole arrays. According to the simulated results, the electric and magnetic resonances at visible wavelengths are obtained in the tips and contours of the metamaterials made of the quasi-3D bowtie nanoantennas, respectively. The effects of the size and gap of quasi-3D bowtie nanoantennas on the array performance have also been studied. The underlying mechanism suggests that different electric and magnetic resonant ranges of the metamaterials could contribute to the broad resonant range for the monolithic metamaterials.

Dichroic nonlinear absorption response of silver nanoprism arrays

The dichroic nonlinear absorption of Ag nanoprism arrays is interpreted using FEM simulations of the polarization-dependent local electric field distribution.

Split-Wedge Antennas with Sub-5 nm Gaps for Plasmonic Nanofocusing

We present a novel plasmonic antenna structure, a split-wedge antenna, created by splitting an ultrasharp metallic wedge with a nanogap perpendicular to its apex. The nanogap can tightly confine gap plasmons and boost the local optical field intensity in and around these opposing metallic wedge tips. This three-dimensional split-wedge antenna integrates the key features of nanogaps and sharp tips, i.e., tight field confinement and three-dimensional nanofocusing, respectively, into a single platform. We fabricate split-wedge antennas with gaps that are as small as 1 nm in width at the wafer scale by combining silicon V-grooves with template stripping and atomic layer lithography. Computer simulations show that the field enhancement and confinement are stronger at the tip-gap interface compared to what standalone tips or nanogaps produce, with electric field amplitude enhancement factors exceeding 50 when near-infrared light is focused on the tip-gap geometry. The resulting nanometric hotspot volume is on the order of λ³/10⁶. Experimentally, Raman enhancement factors exceeding 10⁷ are observed from a 2 nm gap split-wedge antenna, demonstrating its potential for sensing and spectroscopy applications.

Tunable Optical Performances on a Periodic Array of Plasmonic Bowtie Nanoantennas with Hollow Cavities

We propose a design method to tune the near-field intensities and absorption spectra of a periodic array of plasmonic bowtie nanoantennas (PBNAs) by introducing the hollow cavities inside the metal nanostructures. The numerical method is performed by finite element method that demonstrates the engineered hollow PBNAs can tune the optical spectrum in the range of 400-3000 nm. Simulation results show the hollow number is a key factor for enhancing the cavity plasmon resonance with respect to the hotspot region in PBNAs. The design efforts primarily concentrate on shifting the operation wavelength and enhancing the local fields by manipulating the filling dielectric medium, outline film thickness, and hollow number in PBNAs. Such characteristics indicate that the proposed hollow PBNAs can be a potential candidate for plasmonic enhancers and absorbers in multifunctional opto-electronic biosensors.

Size-Reduction Template Stripping of Smooth Curved Metallic Tips for Adiabatic Nanofocusing of Surface Plasmons

We present a new technique to engineer metallic interfaces to produce sharp tips with smooth curved surfaces and variable tip angles, as well as ridges with arbitrary contour shapes, all of which can be integrated with grating couplers for applications in plasmonics and nanophotonics. We combine template stripping, a nanofabrication scheme, with atomic layer deposition (ALD) to produce the ultrasharp nanoscale tips and wedges using only conventional photolithography. Conformal ALD coating of insulators over silicon trench molds of various shapes reduces their widths to make nanoscale features without high-resolution lithography. Along with a metal deposition and template stripping, this size-reduction scheme can mass-produce narrow and ultrasharp (<10 nm radius of curvature) metallic wedges and tips over an entire 4 in. wafer. This size-reduction scheme can create metallic tips out of arbitrary trench patterns that have smooth curved surfaces to facilitate efficient adiabatic nanofocusing which will benefit applications in near-field optical spectroscopy, plasmonic waveguides, particle trapping, hot-electron plasmonics, and nonlinear optics.

3D nanostructures fabricated by advanced stencil lithography

This letter reports on a novel fabrication method for 3D metal nanostructures using high-throughput nanostencil lithography. Aperture clogging, which occurs on the stencil membranes during physical vapor deposition, is leveraged to create complex topographies on the nanoscale. The precision of the 3D nanofabrication method is studied in terms of geometric parameters and material types. The versatility of the technique is demonstrated by various symmetric and chiral patterns made of Al and Au.

Full three-dimensional power flow analysis of single-emitter-plasmonic-nanoantenna system

We present a full three-dimensional (3D) power flow analysis of an emitter-nanoantenna system. A conventional analysis, based on the total Poynting vector, calculates only the coupling strength in terms of the Purcell enhancement. For a better understanding of the emitter-nanoantenna system, not only the Purcell enhancement but also complete information on the energy transfer channels is necessary. The separation of the pure scattering and emitter output Poynting vectors enables the quantification of the individual energy transfer channels. Employing the finite-difference time-domain method (FDTD), we examine a nanodisk antenna that supports the bright dipole and dark quadrupole resonance modes for which the power flow characteristics are completely distinct, and we analyze the power flow enhancements to the energy transfer channels with respect to the wavelength, polarization, and position of the emitter coupled to the antenna. The 3D power flow analysis reveals how the constructive or destructive interference between the emitter and the antenna resonance mode affects the power flow enhancements and the far-field radiation pattern. Our proposed power flow analysis should play a critical role in characterizing the emitter-antenna system and customizing its energy transfer properties for desired applications.

Hetero-oligomer nanoparticle arrays for plasmon-enhanced hydrogen sensing

This paper describes how the ability to tune each nanoparticle in a plasmonic hetero-oligomer can optimize architectures for plasmon-enhanced applications. We demonstrate how a large-area nanofabrication approach, reconstructable mask lithography (RML), can achieve independent control over the size, position, and material of up to four nanoparticles within a subwavelength unit. We show how arrays of plasmonic hetero-oligomers consisting of strong plasmonic materials (Au) and reactant-specific elements (Pd) provide a unique platform for enhanced hydrogen gas sensing. Using finite-difference time-domain simulations, we modeled different configurations of Au–Pd hetero-oligomers and compared their hydrogen gas sensing capabilities. In agreement with calculations, we found that Au–Pd nanoparticle dimers showed a red-shift and that Au–Pd trimers with touching Au and Pd nanoparticles showed a blue-shift upon exposure to both high and low concentrations of hydrogen gas. Both Au–Pd hetero-oligomer sensors displayed high sensitivity, fast response times, and excellent recovery.

Nonlinear optical point light sources through field enhancement at metallic nanocones

A stable nonlinear optical point light source is investigated, based on field enhancement at individual, pointed gold nanocones with sub-wavelength dimensions. Exciting these cones with near-infrared, focused radially polarized femtosecond beams allows for tip-emission at the second harmonic wavelength (second harmonic generation, SHG) in the visible range. In fact, gold nanocones with ultra-sharp tips possess interesting nonlinear optical (NLO) properties for SHG and two-photon photoluminescence (TPPL) emission, due to the enhanced electric field confinement at the tip apex combined with centrosymmetry breaking. Using two complementary optical setups for bottom or top illumination a sharp tip SHG emission is discriminated from the broad TPPL background continuum. Moreover, comparing the experiments with theoretical calculations manifests that these NLO signatures originate either from the tip apex or the base edge of the nanocones, clearly depending on the cone size, the surrounding medium, and illumination conditions. Finally, it is demonstrated that the tip-emitted signal vanishes when switching from radial to azimuthal polarization.

Template-stripped asymmetric metallic pyramids for tunable plasmonic nanofocusing

We demonstrate a novel scheme for plasmonic nanofocusing with internally illuminated asymmetric metallic pyramidal tips using linearly polarized light. A wafer-scale array of sharp metallic pyramids is fabricated via template stripping with films of different thicknesses on opposing pyramid facets. This structural asymmetry is achieved through a one-step angled metal deposition that does not require any additional lithography processing and when internally illuminated enables the generation of plasmons using a Kretschmann-like coupling method on only one side of the pyramids. Plasmons traveling toward the tip on one side will converge at the apex, forming a nanoscale "hotspot." The asymmetry is necessary for these focusing effects since symmetric pyramids display destructive plasmon interference at the tip. Computer simulations confirm that internal illumination with linearly polarized light at normal incidence on these asymmetric pyramids will focus optical energy into nanoscale volumes. Far-field optical experiments demonstrate large field enhancements as well as angle-dependent spectral tuning of the reradiated light. Because of the low background light levels, wafer-scale fabrication, and a straightforward excitation scheme, these asymmetric pyramidal tips will find applications in near-field optical microscopy and array-based optical trapping.

Enhanced second harmonic generation by photonic-plasmonic Fano-type coupling in nanoplasmonic arrays

In this communication, we systematically investigate the effects of Fano-type coupling between long-range photonic resonances and localized surface plasmons on the second harmonic generation from periodic arrays of Au nanoparticles arranged in monomer and dimer geometries. Specifically, by scanning the wavelength of an ultrafast tunable pump laser over a large range, we measure the second harmonic excitation spectra of these arrays and demonstrate their tunability with particle size and separation. Moreover, through a comparison with linear optical transmission spectra, which feature asymmetric Fano-type lineshapes, we demonstrate that the second harmonic generation is enhanced when coupled photonic-plasmonic resonances of the arrays are excited at the fundamental pump wavelength, thus boosting the intensity of the electromagnetic near-fields. Our experimental results, which are supported by numerical simulations of linear optical transmission and near-field enhancement spectra based on the Finite Difference Time Domain method, demonstrate a direct correlation between the onset of Fano-type coupling and the enhancement of second harmonic generation in arrays of Au nanoparticles. Our findings enable the engineering of the nonlinear optical response of Fano-type coupled nanoparticle arrays that are relevant to a number of device applications in nonlinear nano-optics and plasmonics, such as on-chip frequency generators, modulators, switchers, and sensors.

Plasmonic bowtie nanolaser arrays

Plasmonic lasers exploit strong electromagnetic field confinement at dimensions well below the diffraction limit. However, lasing from an electromagnetic hot spot supported by discrete, coupled metal nanoparticles (NPs) has not been explicitly demonstrated to date. We present a new design for a room-temperature nanolaser based on three-dimensional (3D) Au bowtie NPs supported by an organic gain material. The extreme field compression, and thus ultrasmall mode volume, within the bowtie gaps produced laser oscillations at the localized plasmon resonance gap mode of the 3D bowties. Transient absorption measurements confirmed ultrafast resonant energy transfer between photoexcited dye molecules and gap plasmons on the picosecond time scale. These plasmonic nanolasers are anticipated to be readily integrated into Si-based photonic devices, all-optical circuits, and nanoscale biosensors.

X-shaped quasi-3D plasmonic nanostructure arrays for enhancing electric field and Raman scattering

We propose and demonstrate strongly enhancing electric field and Raman scattering with a large tolerance to the light incident angle and polarization by using x-shaped quasi-3D plasmonic nanostructure arrays (X-Q3D-PNAs). The finite-difference time-domain simulations were used to study the reflectance spectra and electric field profiles of X-Q3D-PNAs. Results show that both surface plasmon polaritons and localized surface plasmon polaritons (LSPPs) can be generated at the metal/dielectric interfaces of the top gold thin film with square grating x-shaped nanoholes. The resonance of the LSPPs generated at the gold islands formed between x-shaped nanoholes at the top gold thin film greatly enhance the electric fields at the tips of the cross-sectors of the x-shaped nanoholes. Both plasmon resonances and electric field enhancements are affected by the structural dimensions. The strong electric field enhancement and the large tolerance to the laser polarization were demonstrated by surface-enhanced Raman scattering experiments. This unique plasmonic property of X-Q3D-PNAs could be attractive for photovoltaics and biosensing applications.