Periodic Nanohole Arrays Research Articles

Tunable high-Q resonances based on the asymmetric nanohole array of phase-change material

Dielectric metasurfaces have made great progress over the past decade to enhance light-matter interaction. Recently, dielectric nanohole structures have been employed for the creation of dielectric metasurfaces. However, the optical characteristics of most dielectric nanohole structures remain fixed once they are manufactured. This study investigates the optical properties of Ge2Sb2Te5 (GST) film perforated with a periodic dual nanohole array. The GST dual nanohole array is capable of supporting multiple guided resonances and bound states in the continuum (BICs). By introducing asymmetry in the radius, BICs can be transformed into quasi-BICs with high-Q resonances. The wavelengths of guided resonances and quasi-BICs can be dynamically controlled through the phase transition of GST. Furthermore, modifying the gap allows for the achievement of active high-Q electromagnetically induced transparency, resulting from the interaction between one guided resonance and one quasi-BIC mode. The GST asymmetric nanohole array holds potential for applications in optical modulators, slow-light devices, and nonlinear optical devices.

Enhanced trion emission from WS2 monolayers directly exfoliated on Ag nanohole arrays

The extraordinarily large exciton binding energies of transition metal dichalcogenides (TMDs) have attracted significant attention in both fundamental scientific research and innovative technology development. TMD-metal nanostructures have been fabricated to control excitonic behaviors as well as improve the light-matter interaction in TMDs. WS2 monolayers are integrated with periodic Ag nanohole (AgNH) arrays prepared using a template stripping method. The ultrasmooth and contamination-free surface of the template-stripped Ag layer allows for the direct exfoliation of WS2 flakes on the AgNHs. Surface plasmon excitation increases optical absorption in WS2/AgNH, as evidenced by reflectance, PL, and Raman measurements, along with numerical calculations. Furthermore, the AgNH structures significantly increase the trion-to-exciton emission ratios from the WS2 monolayers on them. Nanoscopic surface photovoltage mapping of WS2/AgNH using Kelvin probe force microscopy can visualize electron accumulation at the suspended WS2 region under light illumination, which triggers the formation of trions. All the results highlight the capability of WS2/AgNH to boost trion emission through plasmon-induced concentrated light and a built-in potential gradient. This work introduces a simple and efficient strategy for fabricating TMD-metal integrated systems to control their excitonic behaviors.

Transmission enhancement in coupled nanohole and nanodisk arrays for solar blind UV filter

Extraordinary optical transmission (EOT) based on metallic nanohole array has great potential for optical filtering, owing to its spectral selectivity and structure-dependent tunability. However the transmittance of EOT is relatively low owing to the large loss of the metal film, particularly in the UV waveband. Herein, we propose a high transmission narrowband ultraviolet filter based on aluminum compound nanostructures on a UV-grade fused silica substrate. These compound nanostructures are consisted of periodic nanodisk and nanohole arrays with the same period in a staggered rectangular arrangement. Numerical simulations using the finite-difference time-domain (FDTD) method have shown that the compound structures exhibit high transmittance of over 70% and a narrower bandwidth of less than 50 nm in the 200–300 nm spectral region compared with the conventionally EOT of pure metallic nanohole arrays. Moreover, a broad suppression in the wavelength ranges of 300 to 1100 nm was achieved. The enhanced performance is attributed to the coupling between the surface plasmon polariton (SPP) of nanohole arrays and the localized surface plasmon resonance (LSPR) of nanodisk arrays. The compound coupled nanostructures can be used in solar-blind ultraviolet detectors and the enhancement mechanism has potential for use in other spectral regions.

Enhanced Performance of Metal‐Semiconductor‐Metal UV Photodetectors on Algan/Gan Hemt Structure via Periodic Nanohole Patterning

AbstractAlGaN/GaN High Electron Mobility Transistor (HEMT) structures offer superior electrical and material properties that make them ideal for the fabrication of high‐performance Ultraviolet photodetectors (UV PDs), especially using the metal‐semiconductor‐metal (MSM) configuration. However, the metal layout of the MSM design and crystal defects in multi‐stack HEMTs can reduce photocurrent and degrade device performance. Nano‐structuring of the AlGaN/GaN surface with different nanofeatures is a promising approach to improve light absorption efficiency and increase device response. In this work, AlGaN/GaN HEMT MSM UV photodetectors with enhanced performance parameters by engineering the surface with periodic nanohole arrays are demonstrated. Optical simulations are used to optimize the design of the nanoholes' periodicity and depth. Unpatterned and nanohole‐patterned devices with varying nanohole depths are fabricated, and their performance shows substantial enhancement with the incorporation of nanoholes. The device with 40 nm deep nanoholes and 230 nm array periodicity shows the highest performance in terms of photocurrent (0.15 mA), responsivity (1.4 × 105 A W−1), UV/visible rejection ratio (≈103), and specific detectivity (4.9 × 1014 Jones). These findings present a HEMT‐compatible strategy to enhance UV photodetector performance for power optoelectronic applications, highlighting that nanohole patterning is a promising prospect for advancements in UV photodetection technology.

Novel insight into the physics of short-range ordered nanoholes: Newly defined lattice model and transmittance response related to lattice parameters and ordering evolution

This paper presents unforeseen conceptualization, methods and results on the physics of short-range ordered (SRO) gold nanohole (NH) distributions, fabricated by a recent protocol developed by the authors. The straightforward extension to SRONHs of the existing knowledge about periodic NH arrays is confuted and an alternative interpretative picture is proposed and discussed based on three main advancements. First, it is set up a so-called short-range lattice (SR-Lat) method to rigorously and fully characterize the coverage-dependent short-range ordering through the determination of local coordination and periodicity length (aSR) of the NH arrangement. Second, it is demonstrated the failure of the common assumption that the average center-to-center distance of nearest neighbor colloids/nanoholes (dNN) is the characteristic length-scale of SRONHs and aSR is set as the proper periodicity parameter. Third, a predictive relationship is formulated between the wavelength of the propagating plasmon modes and aSR that highlights inherent differences with respect to periodic NHs.The presented results lay rigorous foundation for studying systems with correlated diosrdering in general and for making predictions useful not only in design and applications of SRONHs, but also on the surface physics in the photonics and sensing fields.

Achieving high sensitivity by adding rings to a plasmonic metasurface with nano-holes

This report demonstrates a subwavelength plasmonic metasurface for a refractive index sensor at near-infrared (NIR) wavelengths. This sensor includes a two-dimensional periodic array of gold nano-rings and nano-holes in a gold film. The structure's potential for high-sensitivity sensing in reflection configuration has been demonstrated. We use a TiN layer in this structure. It prevents light transmission from the resonators, making the transmission spectrum near zero and causing a deeper reflection dip. This sensor can be suitable for biosensing applications like poliovirus sensing. Generally, high sensitivity is obtained for poliovirus of over 817 nm/RIU. It is shown that making ledges around nano-holes increases the resonance wavelength shift and, therefore, the sensitivity by more than 33 times.

Plasmonic Titanium Nitride Nanohole Arrays for Refractometric Sensing.

Group IVB metal nitrides have attracted great interest as alternative plasmonic materials. Among them, titanium nitride (TiN) stands out due to the ease of deposition and relative abundance of Ti compared to those of Zr and Hf metals. Even though they do not have Au or Ag-like plasmonic characteristics, they offer many advantages, from high mechanical stability to refractory behavior and complementary metal oxide semiconductor-compatible fabrication to tunable electrical/optical properties. In this study, we utilized reactive RF magnetron sputtering to deposit plasmonic TiN thin films. The flow rate and ratio of Ar/N2 and oxygen scavenging methods were optimized to improve the plasmonic performance of TiN thin films. The stoichiometry and structure of the TiN thin films were thoroughly investigated to assess the viability of the optimized operation procedures. To assess the plasmonic performance of TiN thin films, periodic nanohole arrays were perforated on TiN thin films by using electron beam lithography and reactive ion etching methods. The resulting TiN periodic nanohole array with varying periods was investigated by using a custom microspectroscopy setup for both reflection and transmission characteristics in various media to underline the efficacy of TiN for refractometric sensing.

Ultrathin Twisted Stacked Gap‐Plasmon Metasurface with Giant and Tunable Shortwave Infrared Chirality

AbstractUltrathin optical components with giant circular dichroism hold great promise for polarization‐sensitive nanophotonics. However, existing chiral metamaterials often rely on complex‐shaped nanoinclusions, and there remains a paucity of designs active in the shortwave infrared (SWIR) useful for telecommunication and night vision. Here a detailed numerical analysis of the chiroptical response of a simple twisted stacked variant of gap‐plasmon metasurface is presented based on periodic nanohole arrays. Impressively, the 100‐nm free‐standing plasmonic metasurface can attain a thickness‐normalized ellipticity of up to 74.2° µm−1 with tunable bands in the SWIR. Strong and robust optical activity is achieved through energy confinement by gap surface plasmons and effective inter‐nanohole coupling, resulting in an intense superchiral near field. The ultrathin plasmonic metasurface serves as an excellent platform for refractive index sensing with a sensitivity Sn of up to 648.1 nm RIU−1 and field‐enhanced enantiodiscrimination. The simple metasurface with strong SWIR chirality can enjoy a multitude of potential applications including polarization‐sensitive photodetection, optical telecommunication, machine vision, medical imaging, and spectroscopy. These findings also offer insights into some fundamental design principles for twisted stacked aperture‐based metasurfaces and should facilitate the development of new chiral metamaterials with exceptional performance.

Selective-area growth of β-Ga2O3 nanowire films on nano-patterned Si(111) substrate by metal-organic chemical vapor deposition

β-Ga2O3 nanowire films were prepared on Si(111) substrate by selective area growth (SAG) using metal-organic chemical vapor deposition (MOCVD). A large-area periodic nano-hole array TiN mask was fabricated. The effects of MOCVD process parameters on SAG were discussed by calculating Ga supersaturation. It was found that the SAG is closely related to the selective area nucleation and inhibition of lateral growth. Transmission electron microscopy confirmed that β-Ga2O3 nanowires have a preferential orientation of the (002) crystal plane. The sample after in-situ O2 annealing showed an x-ray diffraction intensity increase of (400), (002), and (-111) crystal planes. On this basis, P–Si/β‐Ga2O3 nanowire heterojunctions were fabricated and presented high resistance of the films. At a bias voltage of 20 V, the current decreased from 6.14 × 10−6 to 2.25 × 10−6 A. The MOCVD-based SAG paves a novel way to fabricate β-Ga2O3 nanowire films with high surface-to-volume ratio and homogeneity.

Directional amplified spontaneous emissions from Ag nanohole array with high diffraction orders.

Surface plasmon excitations in metallic hole arrays have been extensively studied in the context of light-matter interaction, since the generated Bloch surface plasmon polariton (Bloch-SPP) exhibits unique properties of nanoscale light confinement, near-field enhancements, and long-range metal surface propagation. In this work, we experimentally demonstrate a plasmonic device that exhibits highly directional emission in visible light; using Ag film with a thickness of 100 nm deposited on a subwavelength porous alumina array as a plasmonic cavity, four-level rhodamine 6G (R6G) is selected as the gain material. It is suggested that a Bloch-SPP with high diffraction orders on a Ag nanohole array can generate a strong local electric field and a high Purcell factor at a nanohole edge. Moreover, directional five-fold enhanced amplified spontaneous emission (ASE) with polarization dependence is observed under a low threshold of 199.9 W/cm2 in the visible light region, which comes from the optical feedback provided by the 2D periodic nanohole array. This work opens up a wide range of applications for real-time tunable wavelength, controlled multimode laser, fluorescence detection, etc.

Efficient parallel strategy for molecular plasmonics – A numerical tool for integrating Maxwell-Schrödinger equations in three dimensions

An efficient parallelization approach to simulate optical properties of ensembles of quantum emitters in realistic electromagnetic environments is considered. It relies on balancing computing load of utilized processors and is built into three-dimensional domain decomposition methodology implemented for numerical integration of Maxwell's equations. The approach employed enables directly accessing dynamics of collective effects as the number of molecules in simulations can be drastically increased. Numerical experiments measuring speedup factors demonstrate the efficiency of the proposed methodology. As an example, we consider dynamics of nearly 700,000 diatomic molecules with ro-vibrational degrees of freedom explicitly accounted for coupled to electromagnetic radiation crafted by periodic arrays of split-ring resonators and triangular nanoholes. As an application of the approach, dissociation dynamics under strong coupling conditions is scrutinized. It is demonstrated that the dissociation rates are significantly affected near polaritonic frequencies.

Nanostructured Transition Metal Dichalcogenide Multilayers for Advanced Nanophotonics

AbstractTransition metal dichalcogenides (TMDs) attract significant attention due to their exceptional optical, excitonic, mechanical, and electronic properties. Nanostructured multilayer TMDs were recently shown to be highly promising for nanophotonic applications, as motivated by their exceptionally high refractive indices and optical anisotropy. Here, this vision is extended to more sophisticated structures, such as periodic arrays of nanodisks and nanoholes with ultra sharp walls, as well as proof‐of‐concept all‐TMD waveguides and resonators. Specific focus is given to various advanced nanofabrication strategies, including careful selection of resists for electron beam lithography and etching methods, especially for non‐conductiven but relevant for nanophotonic applications substrates, such as SiO2. The specific materials studied here include semiconducting WS2, in‐plane anisotropic ReS2, and metallic TaSe2, TaS2, and NbSe2. The resulting nanostructures can potentially impact several nanophotonic and optoelectronic areas, including high‐index nanophotonics, plasmonics and on‐chip optical circuits. The knowledge of TMD material‐dependent nanofabrication parameters developed here will help broaden the scope of future applications of all‐TMD nanophotonics.

Freestanding Metal Nanomembranes and Nanowires by Template Transfer with a Soluble Adhesive.

The fabrication of nanostructures usually involves chemical processes that have in certain steps. Especially, it is necessary to use the chemical etching method to release the as-patterned structures from the substrate in most of the transfer techniques. Here, a novel scheme of template transfer as developed for the fabrication of freestanding Au nanomembranes and nanowires by using a soluble PVP adhesive. The nanomembranes feature the periodic nanohole arrays with high uniformity. Without the substrates, these plasmonic nanohole arrays show symmetric and antisymmetric resonance modes with bright and dark spectral features, respectively, in transmission. Through the spectral analysis for reflection, we have disclosed that the usual dark mode in transmission is not really dark, but it reveals a distinct feature in reflection. Two coupling modes present distinct spectral features in transmission and reflection due to their different loss channels. To show their versatility, the freestanding nanomembranes were also employed as secondary templates to form Si nanowire arrays by the metal-assisted chemical etching method.

Investigating the Absorption Spectra of a Plasmonic Metamaterial Absorber Based on Disc-in-Hole Nanometallic Structure.

In this work, we present and explore the characteristics of a plasmonic metamaterial absorber based on a metal–insulator–metal functional stack. The proposed structure consists of glass “sandwiched” between a silver reflector and a titanium metallic disc, embedded inside a Ti periodic nano-hole array, as an outside layer. In the visible and infrared regimes, the optical absorption spectra of such structures have been investigated using the finite difference time domain method. The impact of modifying nano-hole and embedded disc diameters on the absorber’s performance has been investigated. Changing these two distinct structural parameters tunes the coupling effect between the localized and propagating surface plasmons. The adequate bandwidth, average spectral absorption rate, and short circuit current density are calculated to determine the performance of the designated absorber. The proposed structure of the plasmonic metamaterial absorber reaches an average absorption of over 94% in a bandwidth of 0.81 µm and near-perfect absorption of 98% around the wavelength of 0.7 µm, with an almost 100% relative absorption bandwidth and 41 mA/cm2 short circuit current density. In addition, the results show that the disc-in-hole absorber’s structural parameters can be changed precisely and facilely to tailor to the absorption spectra.

Plasmonic crystals fabricated by nanosphere lithography for advanced biosensing.

Plasmonic nanostructures have attracted wide attention in the past few years for their promising applications such as surface-enhanced spectroscopies, chemical or biosensing, and so on. However, the fabrication of plasmonic nanostructures relies on traditional photolithography methods such as electron beam lithography and focused ion beam lithography, which have inherent shortcomings, such as high fabrication cost and being time-consuming. Here, using the nanosphere lithography approach, we fabricate large-area long-range ordered periodic Au nanohole arrays on an opaque Au substrate. The structure supports spectral-isolation and well-defined plasmonic resonances favorable to spectral monitoring at normal incidence of light. The bulk sensitivity of up to 403 nm/RIU is measured for the plasmon modes. Furthermore, we assess the surface-sensing performance of the system and obtain a near-field decay length of about 240 nm, meaning that it is desirable to detect the biological protein molecules. The suggested plasmonic-sensing platform has broad application prospects in the development of low-cost and high-throughput biosensor chips.

Tripling the light extraction efficiency of a deep ultraviolet LED using a nanostructured p-contact

Despite a wide array of applications, deep ultra-violet light emitting diodes offer relatively poor efficiencies compared to their optical counterparts. A contributing factor is the lower light extraction efficiency due to both highly absorbing p-contacts and total internal reflection. Here, we propose a structure consisting of a hexagonal periodic array of cylindrical nanoholes in the multi-layered p-contact which are filled with platinum. This nanostructure reduces the absorption of the p-contact layer, leading to a higher emission into the n-contact compared to a planar reference. An optimum geometry of the nanostructure allows a light extraction efficiency of 15.0%, much higher than the typical 4.6% of a planar reference. While the nanostructure strongly decreases the light absorption in the p-contact, it is still not able to considerably reduce the total internal reflection. Consequently, the nanostructured p-contact should be combined with other optical strategies, such as nanopatterned sapphire substrates to increase the efficiency even further. Despite this, the nanostructure described in this work provides a readily realizable path to enhancing the light extraction efficiency of state-of-the-art deep ultra-violet light emitting diodes.

Shape Modulation of Plasmonic Nanostructures by Unconventional Lithographic Technique.

Conventional nano-sphere lithography techniques have been extended to the fabrication of highly periodic arrays of sub-wavelength nanoholes in a thin metal film. By combining the dry etching processes of self-assembled monolayers of polystyrene colloids with metal physical deposition, the complete transition from increasing size triangular nanoprism to hexagonally distributed nanoholes array onto thin metal film has been gradually explored. The investigated nano-structured materials exhibit interesting plasmonic properties which can be precisely modulated in a desired optical spectral region. An interesting approach based on optical absorbance measurements has been adopted for rapid and non-invasive inspections of the nano-sphere monolayer after the ion etching process. By enabling an indirect and accurate evaluation of colloid dimensions in a large area, this approach allows the low-cost and reproducible fabrication of plasmonic materials with specifically modulated optical properties suitable for many application in biosensing devices or Raman enhanced effects.

Lattice Resonances of Nanohole Arrays for Quantum Enhanced Sensing

Periodic arrays of nanoholes perforated in metallic thin films interact strongly with light and produce large electromagnetic near-field enhancements in their vicinity. As a result, the optical response of these systems is very sensitive to changes in their dielectric environment, thus making them an exceptional platform for the development of compact optical sensors. Given that these systems already operate at the shot-noise limit when used as optical sensors, their sensing capabilities can be enhanced beyond this limit by probing them with quantum light, such as squeezed or entangled states. Motivated by this goal, here, we present a comparative theoretical analysis of the quantum enhanced sensing capabilities of metallic nanohole arrays with one and two holes per unit cell. Through a detailed investigation of their optical response, we find that the two-hole array supports resonances that are narrower and stronger than its one-hole counterpart, and therefore have a higher fundamental sensitivity limit as defined by the quantum Cram\'er-Rao bound. We validate the optical response of the analyzed arrays with experimental measurements of the reflectance of representative samples. The results of this work advance our understanding of the optical response of these systems and pave the way for developing sensing platforms capable of taking full advantage of the resources offered by quantum states of light.

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.