Periodic Nanohole Arrays Research Articles

Periodic Nanohole Arrays with Enhanced Lasing and Spontaneous Emissions for Low-Cost Plasmonic Devices

Periodic arrays of air nanoholes in thin metal films that support surface plasmon resonances can provide an alternative approach for boosting the light–matter interactions at the nanoscale. Nanohole arrays have garnered great interest in recent years for their use in biosensing, light emission enhancement, and spectroscopy. Here, we employ a simple technique to fabricate nanohole arrays and examine their photonic applications including enhanced lasing and spontaneous emission of novel nanomaterials. In contrast to the complicated and most commonly used electron-beam lithography technique, hexagonal arrays of nanoholes are fabricated by using a simple combination of shadowing nanosphere lithography technique and electron-beam deposition. Through spectral and temporal characterizations, it was shown that these arrays offer an enhancement in the lasing emission of an organic dye liquid gain medium with a quality factor above 150 as well as an accelerated decay rate for CdSe quantum dots. The simple fabrication of nanohole arrays together with their excellent optical responses can therefore offer a great potential in the industrialization of plasmonic devices for use in various realms of emerging technologies such as gas sensing, biomedical imaging, and ultrafast on-chip coherent light sources.

Nano-patterning on multilayer MoS2 via block copolymer lithography for highly sensitive and responsive phototransistors

Indirect bandgap of multilayer molybdenum disulfide has been recognized as a major hindrance to high responsivity of MoS2 phototransistors. Here, to overcome this fundamental limitation, we propose a structural engineering of MoS2 via nano-patterning using block copolymer lithography. The fabricated nanoporous MoS2, consisting of periodic hexagonal arrays of hexagon nanoholes, includes abundant edges having a zigzag configuration of atomic columns with molybdenum and sulfur atoms. These exposed zigzag edges are responsible for multiple trap states in the bandgap region, as confirmed by photo-excited charge-collection spectroscopy measurements on multilayer nanoporous MoS2 phototransistors, showing that in-gap states only near the valence band can result in a photogating effect. The effect of nano-patterning is therefore to significantly enhance the responsivity of multilayer nanoporous MoS2 phototransistors, exhibiting an ultra-high photoresponsivity of 622.2 A W−1. Our nano-patterning of MoS2 for photosensing application paves a route to structural engineering of two-dimensional materials for highly sensitive and responsive optoelectronic devices.

Second harmonic generation by strongly coupled exciton-plasmons: The role of polaritonic states in nonlinear dynamics.

We investigate second harmonic generation (SHG) from hexagonal periodic arrays of triangular nano-holes of aluminum using a self-consistent methodology based on the hydrodynamics-Maxwell-Bloch approach. It is shown that angular polarization patterns of the far-field second harmonic response abide to threefold symmetry constraints on tensors. When a molecular layer is added to the system and its parameters are adjusted to achieve the strong coupling regime between a localized plasmon mode and molecular excitons, Rabi splitting is observed from the occurrence of both single- and two-photon transition peaks within the SHG power spectrum. It is argued that the splitting observed for both transitions results from direct two-photon transitions between lower and upper polaritonic states of the strongly coupled system. This interpretation can be accounted by a tailored three-level quantum model, with results in agreement with the unbiased numerical approach. Our results suggest that the hybrid states formed in strongly coupled systems directly contribute to the nonlinear dynamics. This opens new directions in designing THz sources and nonlinear frequency converters.

Electron energy loss spectroscopy on freestanding perforated gold films

We report on a combined far- and near-field study of surface plasmon polaritons on freestanding perforated gold films. The samples are fabricated by focused ion beam milling of {a periodic hole array into} a carbon membrane followed by thermal evaporation of gold and plasma ashing of the carbon film. Optical transmission spectra show a series of characteristic features, which can be attributed to the excitation of surface plasmon modes via the periodic nanohole array. The corresponding near-field distributions are mapped by electron energy-loss spectroscopy. Besides the optically bright surface plasmon modes, we observe in the near-field an additional dark plasmon mode, which is absent in the normal incidence far-field spectra. Our experimental results are in good agreement with numerical computations based on a discontinuous Galerkin time-domain method.

Aluminum Plasmonics Enriched Ultraviolet GaN Photodetector with Ultrahigh Responsivity, Detectivity, and Broad Bandwidth.

Plasmonics have been well investigated on photodetectors, particularly in IR and visible regimes. However, for a wide range of ultraviolet (UV) applications, plasmonics remain unavailable mainly because of the constrained optical properties of applicable plasmonic materials in the UV regime. Therefore, an epitaxial single‐crystalline aluminum (Al) film, an abundant metal with high plasma frequency and low intrinsic loss is fabricated, on a wide bandgap semiconductive gallium nitride (GaN) to form a UV photodetector. By deliberately designing a periodic nanohole array in this Al film, localized surface plasmon resonance and extraordinary transmission are enabled; hence, the maximum responsivity (670 A W−1) and highest detectivity (1.48 × 1015 cm Hz1/2 W−1) is obtained at the resonance wavelength of 355 nm. In addition, owing to coupling among nanoholes, the bandwidth expands substantially, encompassing the entire UV range. Finally, a Schottky contact is formed between the single‐crystalline Al nanohole array and the GaN substrate, resulting in a fast temporal response with a rise time of 51 ms and a fall time of 197 ms. To the best knowledge, the presented detectivity is the highest compared with those of other reported GaN photodetectors.

Color Thin-Film Transistors Employing Periodic Nanohole Structures

A thin-film transistor (TFT) with a color filter function achieved by designing periodic nanohole arrays in the metal electrodes was studied. Since this metal electrode is not opaque but transparen...

Chirality Breaking and Chirality Recovery in the Blue-Detune Trapping System Based on Surface Plasmons Involving a Chiral Molecule Analyzed by FDTD Simulation

We theoretically investigate the chirality breaking and the chirality recovery in the blue-detune trapping system based on surface plasmon polaritons (SPPs) involving a chiral molecule. The blue-detune trapping system is made up of a gold metal film with a periodic nanohole array. Single chiral molecule is trapped in each unit of the trapping system, which is described by two orthogonal dipoles in finite-difference time-domain (FDTD) simulations. The structure of the blue-detune trapping system is symmetric. Due to the chiral molecule in the trapping system, the symmetry of the blue-detune system is broken or recovered by the interactions between the chiral molecule and SPPs excited by the trapping system illuminated by the incident chiral beam. At the same time, the response of the system trapping a chiral molecule shows the chiral characteristic or the achiral characteristic, which can be called the chirality recovery and the chirality breaking. These phenomena are explained by the coherent superposition of the electric fields induced by the molecule and SPPs excited by Au structure. Our proposed structure has potential applications in the chiral molecule detecting.

Phase-Matched Mid-Infrared Difference Frequency Generation Using a Nanostructured Gallium Arsenide Metamaterial With Nanoholes

Phase-matched wavelength conversion is achieved in difference frequency generation (DFG) in a structure of gallium arsenide (GaAs) with periodic arrays of nanoholes. Linear properties (refractive indices) of the structure are determined from the S -parameters of the structure. Finite difference time domain (FDTD) simulation is used to calculate the S -parameters. The longest wavelength achieved is 16.2229 μm and the shortest is 3.2961 μm. The results of the FDTD simulation are compared with results obtained from the effective medium theory by using the Maxwell Garnett model. The comparison shows excellent agreement.

Four-wave mixing of topological edge plasmons in graphene metasurfaces.

We study topologically protected four-wave mixing (FWM) interactions in a plasmonic metasurface consisting of a periodic array of nanoholes in a graphene sheet, which exhibits a wide topological bandgap at terahertz frequencies upon the breaking of time reversal symmetry by a static magnetic field. We demonstrate that due to the significant nonlinearity enhancement and large life time of graphene plasmons in specific configurations, a net gain of FWM interaction of plasmonic edge states located in the topological bandgap can be achieved with a pump power of less than 10 nW. In particular, we find that the effective nonlinear edge-waveguide coefficient is about γ ≃ 1.1 × 1013 W-1 m-1, i.e., more than 10 orders of magnitude larger than that of commonly used, highly nonlinear silicon photonic nanowires. These findings could pave a new way for developing ultralow-power-consumption, highly integrated, and robust active photonic systems at deep-subwavelength scale for applications in quantum communications and information processing.

Plasmon enhanced second harmonic generation by periodic arrays of triangular nanoholes coupled to quantum emitters

Optical properties of periodic arrays of nanoholes of a triangular shape with experimentally realizable parameters are examined in both linear and nonlinear regimes. By utilizing a fully vectorial three-dimensional approach based on the nonlinear hydrodynamic Drude model describing metal coupled to Maxwell's equations and Bloch equations for molecular emitters, we analyze linear transmission, reflection, and nonlinear power spectra. Rigorous numerical calculations demonstrating second and third harmonic generation by the triangular hole arrays are performed. It is shown that both the Coulomb interaction of conduction electrons and the convective term contribute on equal footing to the nonlinear response of metal. It is demonstrated that the energy conversion efficiency in the second harmonic process is the highest when the system is pumped at the localized surface plasmon resonance. When molecular emitters are placed on a surface of the hole array line shapes, the second harmonic signal exhibits three peaks corresponding to second harmonics of the localized surface plasmon mode and upper and lower polaritonic states.

Large‐Area Fabrication of Highly Tunable Hybrid Plasmonic–Photonic Structures Based on Colloidal Lithography and a Photoreconfigurable Polymer

AbstractIn this work, based on colloidal lithography, a strategy is developed to produce a new type of hybrid plasmonic–photonic system comprising a periodic array of nanoholes in a photoreconfigurable polymer layer deposited on a gold surface. By controlling the experimental conditions, the geometric parameters of the nanoholes can be easily adjusted; hence, the optical properties of the systems are widely tunable. Importantly, the shape and topology of the round nanohole arrays can be easily reconfigured with proper photoirradiation, providing a means to modulate the optical properties and produce new functionalities, as exemplified by the formation of arrays with anisotropic or gradient structural features. Moreover, rubbing dielectric spheres into the formed nanoholes can further tune the topological structures of the dielectric components in the hybrid system, leading to a dramatic modification of the spatial energy distributions and inducing the formation of uniform electromagnetic fields confined near the embedded sphere surfaces, which are more beneficial for practical applications. All the results indicate that the combined use of the unique properties of both a photoreconfigurable polymer and colloidal lithography opens a new avenue with great potential for efficient and large‐area fabrication of hybrid plasmonic–photonic systems with a high degree of optical tunability.

Facile fabrication of periodic arrays of vertical Si nanoholes on (001)Si substrate with broadband light absorption properties

This study presents a facile and straightforward approach to fabricating periodic arrays of Au nanodisks and vertically aligned, depth- and morphology-controllable Si nanoholes on (001)Si substrates by combining self-assembled nanosphere lithography, selective Au wet etching, and Au nanodisk-catalyzed Si etching processes. All of the etching experiments were carried out in aqueous solutions at room temperature without using complex photolithography and hard-mask patterning processes. The shape, diameter, and spacing of the produced vertical Si nanoholes corresponded well to those of the catalytic Au nanodisks used, and the nanohole depth could be readily tuned from sub-micrometer to several micrometers by adjusting the Au nanodisk-catalyzed Si etching time. The Si substrates with nanohole-textured surfaces exhibited depth-dependent hydrophobic behaviors and broadband (400–1650 nm) light absorption properties. The measured integrated absorptance was found to increase with increasing the depth of Au nanodisk-embedded Si nanoholes and can reach up to 95% in the visible light region and 60% in the near-IR region. The resulting broadband absorption enhancement can be attributed to the combined effects of multiple scattering of light and localized surface plasmon resonance. The obtained results present the exciting prospects that the new combined approach proposed here would offer potential applications in constructing various high-efficiency nanohole array-based optoelectronic and photovoltaic devices.

Highly selective ultraviolet aluminum plasmonic filters on silicon

We report the use of aluminum patterning to make highly selective UV bandpass filters. We design and fabricate a periodic array of nanoholes in Al thin film on a bare silicon substrate as an analog for potential integration with a Si photodetector. Arrays were designed to operate in the wavelength range of 200-400 nm. Our results show that we can obtain a single dominant peak filter with a linewidth of 30 nm at normal incidence, in contrast to similar structures on glass substrates, where multiple modes influence the UV spectrum. Varying the angle of incidence is shown to split the plasmonic mode and further decrease the linewidth of the maximum wavelength mode down to 10 nm. Our results therefore show high potential for applications in solid-state image sensors for astronomy and planetary studies.

Scalable and Tunable Periodic Graphene Nanohole Arrays for Mid-Infrared Plasmonics.

Despite its great potential for a wide variety of devices, especially mid-infrared biosensors and photodetectors, graphene plasmonics is still confined to academic research. A major reason is the fact that, so far, expensive and low-throughput lithography techniques are needed to fabricate graphene nanostructures. Here, we report for the first time a detailed experimental study on electrostatically tunable graphene nanohole array surfaces with periods down to 100 nm, showing clear plasmonic response in the range ∼1300-1600 cm-1, which can be fabricated by a scalable nanoimprint technique. Such large area plasmonic nanostructures are suitable for industrial applications, for example, surface-enhanced infrared absorption (SEIRA) sensing, as they combine easy design, extreme field confinement, and the possibility to excite multiple plasmon modes enabling multiband sensing, a feature not readily available in nanoribbons or other localized resonant structures.

Weak enhanced resonant Faraday rotation in pure cobalt plasmonic lattices: Thickness dependent Faraday rotation studies

The thickness dependent magneto-optical Faraday rotation in Co ferromagnetic thin films and perforated periodic Co nano-hole arrays were investigated. It was observed that the optical response and Faraday rotation of Co nano-hole arrays were strongly dependent on the thickness of the film, and the Faraday rotation reached the maximum amplitude at an optimal thickness value of 30–50 nm, which contrasted with the Co thin films where the Faraday rotation increased with the film thickness. In addition, the Faraday rotation spectra of Co nano-hole arrays showed a weak resonantly enhanced peak at the anti-resonance modes (or Wood’s anomalies) on the transmission spectra, which correspond to the enhanced local electric field at the film and surrounding medium interfaces. All experimental results were confirmed by the finite-difference time domain calculations.

Nonlinear plasmonic behavior of nanohole arrays in thin gold films for imaging lipids

We demonstrate linear and nonlinear plasmonic behaviors of periodic nanohole arrays in thin gold (Au) films with varying periodicities. As expected, the linear optical transmission spectra of the nanohole arrays show a red-shift of the resonance wavelength and Wood's anomaly with increasing hole spacing. The optical transmission and electric near-field intensity distribution of the nanohole arrays are simulated using the finite element method. The nonlinear plasmonic behavior of the nanohole arrays is studied by using picosecond pulsed excitation at near-infrared wavelengths. The characteristic nonlinear signals indicating two-photon excited luminescence (TPEL), sum frequency generation, second harmonic generation, and four-wave mixing (FWM) are observed. A maximum FWM/TPEL signal intensity ratio is achieved for nanohole arrays with a periodicity of 500 nm. Furthermore, the significant FWM signal intensity and contrast compared to the background were harnessed to demonstrate the ability of surface-enhanced coherent anti-Stokes Raman scattering to visualize low concentrations of lipids deposited on the nanohole array with a periodicity of 500 nm.

Effect of Ga Implantation and Hole Geometry on Light Transmission through Nanohole Arrays in Al and Mg

The study of plasmonic nanostructures in the ultraviolet (UV) is a relatively uncharted field because of the challenges in both engineering and materials science. In this work, two-dimensional periodic nanohole arrays in aluminium (Al) and magnesium (Mg) films were fabricated using gallium (Ga) focused ion beam. Optical transmission through the arrays was obtained in the UV–visible range for varying array periodicity. Transmission results show strong resonance transmission enhancement and suppression with corresponding red shift as the period increases and the presence of stationary waveguide modes. Comparing Al and Mg, Al hole arrays enable greater transmission than those of Mg. Numerical analysis was carried out through the finite-difference time-domain method, which showed far-field transmission consistent with experiments. The simulation model was constructed based on transmission electron microscopy of cross-sectioned samples to take into account tapered sidewall geometry and undercut into the substrate. Energy-dispersive X-ray spectroscopy was used to assess elemental composition, including oxidation and alloying. The effect of Ga implantation was qualitatively studied, which indicates that Ga implants inside the hole, with greater implantation concentration within Mg than within Al, and affects the transmission through both Al and Mg arrays.

Plasmonic nanomeshes: their ambivalent role as transparent electrodes in organic solar cells

In this contribution, the optical losses and gains attributed to periodic nanohole array electrodes in polymer solar cells are systematically studied. For this, thin gold nanomeshes with hexagonally ordered holes and periodicities (P) ranging from 202 nm to 2560 nm are prepared by colloidal lithography. In combination with two different active layer materials (P3HT:PC61BM and PTB7:PC71BM), the optical properties are correlated with the power conversion efficiency (PCE) of the solar cells. A cavity mode is identified at the absorption edge of the active layer material. The resonance wavelength of this cavity mode is hardly defined by the nanomesh periodicity but rather by the absorption of the photoactive layer. This constitutes a fundamental dilemma when using nanomeshes as ITO replacement. The highest plasmonic enhancement requires small periodicities. This is accompanied by an overall low transmittance and high parasitic absorption losses. Consequently, larger periodicities with a less efficient cavity mode, yet lower absorptive losses were found to yield the highest PCE. Nevertheless, ITO-free solar cells reaching ~77% PCE compared to ITO reference devices are fabricated. Concomitantly, the benefits and drawbacks of this transparent nanomesh electrode are identified, which is of high relevance for future ITO replacement strategies.

Photoacoustic technique for the characterization of plasmonic properties of 2D periodic arrays of gold nanoholes

We apply photo-acoustic (PA) technique to examine plasmonic properties of 2D periodic arrays of nanoholes etched in gold/chromium layer upon a glass substrate. The pitch of these arrays lies in the near IR, and this, under appropriate wave vector matching conditions in the visible region, allows for the excitation of surface plasmon polaritons (SPP) guided along a dielectric – metal surface. SPP offered new approaches in light guiding and local field intensity enhancement, but their detection is often difficult due to the problematic discrimination of their contribution from the overall scattering. Here PA measures the energy absorbed due to the non-radiative decay of SPPs. We report on the absorption enhancement by presenting the spatial mapping of absorption under the incidence angles and wavelength that correspond to the efficient excitation of SPPs. Moreover, a comparison with optical transmission measurements is carried out, underlining the applicability and sensitivity of PA technique.

Near field enhancement of TiO2 nanoparticle array on different substrates for femtosecond laser processing

Optical near field enhancement on substrate can be achieved by localizing femtosecond laser energy with nanoparticles. The enhanced field is located in the region between nanoparticles and the substrate. The localized femtosecond optical field is of great significance for fabricating the micro/nano structure with characteristic size beyond the diffraction limit. Up to now, femtosecond processing nanohole assisted by particle array is only possible for metal particle (Au) and low-refractive-index dielectric polystyrene particle. However, previous research results show that it cannot be realized for metal particle arrays (Au) to form periodic nanohole arrays, and it is limited for polystyrene particle to choose the corresponding substrate. In this paper, a novel method is proposed, in which high refractive index TiO2 arrayed particles are placed on the substrate to achieve laser induced near field enhancement. This makes feasible the nanoscale processing beyond the diffraction limit. In this paper, near field distributions of TiO2 particle array on Si, Pt and SiO2 substrates are simulated by the finite-difference time-domain (FDTD) method. The results show that TiO2 particles concentrate the laser energy to a region with a diameter of 100 nm around the particle and the near field enhancement is 140 times higher than the incident laser intensity, which is beneficial to fabricating the nanostructure of super diffraction limit, such as sub-hundred nanometer nanohole ablation by femtosecond laser. For Si substrate, the near field enhancement is only about 30% lower for TiO2 particle array than that for single TiO2 particle. In order to explore the influence mechanism of the substrate material parameters on the near field enhancement of TiO2 nanoparticle array, we further simulate the enhancement factor for the substrates of different refractive indices. It is found that the near field is enhanced with the increase of substrate refractive index, and this is attributed to an increased interaction of the particle with the near field of substrate and the scattering effect in which the TiO2 particle supports forward near field intensity pattern. Moreover, the image charge model is introduced to analyze the effect of substrate optical parameters on local field enhancement. Results in this paper can be applied to most metals as well as dielectric substrate surfaces, and they open a new way for femtosecond laser near field nano-processing with characteristic size beyond the diffraction limit.