Subwavelength Nanohole Research Articles

Mid-infrared plasmonic filter arrays using nanoholes in gold and silver films.

Plasmonic filters based on subwavelength nanohole arrays are an attractive solution for creating arrays of filters with varying passbands in a single lithography step. In this work, we have developed a fabrication method which allows fabrication of nanohole arrays in silver by use of a thin layer of aluminum oxide, which serves the dual purpose of both capping layer and hardmask for metal patterning. We demonstrate arrays of gold and silver mid-infrared plasmonic filters, fabricated on silicon, intended for use in optical filter blocks or for future integration with infrared imagers. The filter arrays are designed for the wavelength range 2-7 µm, and exhibit peak filter transmission efficiencies around 70%.

Dyadic Helmholtz Green’s Function for Electromagnetic Wave Transmission/Diffraction through a Subwavelength Nano-Hole in a 2D Quantum Plasmonic Layer: An Exact Solution Using “Contact Potential”-like Dirac Delta Functions

The dyadic Helmholtz Green’s function for electromagnetic (EM) wave transmission/ diffraction through a subwavelength nano-hole in a two-dimensional (2D) plasmonic layer is discussed here analytically and numerically, employing “contact potential”-like Dirac delta functions in 1 and 2 dimensions (δ(z) and δ(x)δ(y)≡δ(2)(r→)). This analysis is carried out employing a succession of two coupled integral equations. The first integral equation determines the dyadic electromagnetic Green’s function G^fs for the full non-perforated 2D quantum plasma layer in terms of the bulk 3D infinite-space dyadic electromagnetic Green’s function G^3D, with δ(z) representing the confinement of finite quantum plasma conductivity to the plane of the plasma layer at z=0. The second integral equation determines the dyadic electromagnetic “hole” Green’s function G^hole for the perforated 2D quantum plasma layer (containing the nano-hole) in terms of the dyadic electromagnetic Green’s function G^fs for the full non-perforated 2D plasma layer, with δ(2)(r→) describing the exclusion of the quantum plasma layer conductivity properties from the nano-hole region in the vicinity of r→=0 on the plane. Taking the radius of the subwavelength nano-hole to be the smallest length scale of the system in conjunction with the 2D Dirac delta function representation of the excluded nano-hole plasma conductivity, both of the successive coupled integral equations are solved exactly, and we present a thorough numerical analysis (based on the exact analytic solution) for the resulting dyadic “hole” Green’s function G^hole in full detail in both 3D and density plots. This result has been successfully applied to the determination of electromagnetic wave transmission/diffraction through the nano-hole of the perforated quantum plasmonic layer, jointly with the EM wave transmission through the rest of the plasma layer. This success necessarily involves spatial translational asymmetry induced by the use of spatial Dirac delta functions confining finite conductivity to the 2D quantum plasma sheet and the excision at a bit of it about the origin to represent the nano-hole perforation, thus breaking spatial translational invariance symmetry.

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.

T-Branch Waveguide Mirror for Multimode Optical Splitter With Arbitrary Power Ratios

Mode-division multiplexing is an enabling technique for large-capacity on-chip optical interconnects. Multimode power splitter becomes the subject of interests which is highly desirable. However, challenges still exist for multimode splitter with arbitrary power ratio. Inspired by geometric optics, we demonstrate a series of power splitters based on T-branch waveguide mirrors which consist of subwavelength nanoholes. Arbitrary split ratio can be achieved by modifying the transmittance and reflectance of the waveguide mirrors. The devices can support up to 9 modes from TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> -TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sub> . For a proof-of-concept demonstration, we fabricate dual-mode splitters with power ratios of 40:60, 30:70 and 20:80. For both TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> and TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> modes, the measured excess losses are lower than 0.92 dB, and the crosstalk are all below -14.42 dB within the wavelength range from 1540 to 1580 nm.

Optical Analysis of Hole Patterned Ag Nanoparticle Structure.

A structure with periodic sub-wavelength nanohole patterns interacts with incident light and causes extraordinary optical transmission (EOT), with metal nanoparticles leading to localized surface plasmon resonance (LSPR) phenomena. To explore the effects of metal nanoparticles (NPs), optical analysis is performed for metal NP layers with periodic hole patterns. Investigation of Ag NP arrangements and comparisons with metal film structures are presented. Ag NP structures with different hole configuration are explored. Also, the effects of increasing light incident angle are investigated for metal NP structures where EOT peak at 460 nm wavelength is observed. Moreover, electric field distributions at each transmittance peak wavelengths and optical noise are analyzed. As a result, optical characteristics of metal NP structures are obtained and differences in resonance at each wavelength are highlighted.

Plasmonic Diffraction Field Pattern Imaging Could Resolve Ultrasensitive Bioinformation

Subwavelength nanohole arrays have been very attractive for label-free biosensing applications, as they offer simplicity and flexibility in the read-out scheme. Recently, platforms employing imagin...

Efficient and Broadband Four‐Wave Mixing in a Compact Silicon Subwavelength Nanohole Waveguide

AbstractConfining light in a small volume offers an effective approach to enhance the four‐wave mixing (FWM) process. Recently, most efforts are devoted to improve the conversion efficiencies by using resonant structures. As a result, the bandwidths of the FWM conversions are typically limited to 1–2 nm. In this paper, a nonresonant silicon subwavelength nanohole waveguide is proposed to manipulate the field distribution of the propagating wave. The electromagnetic field of the Bloch mode can be engineered to concentrate in the silicon area where the FWM process occurs. Benefiting from the enhanced light intensity, an efficient and broadband FWM process is experimentally demonstrated. A conversion efficiency of −26.7 dB is achieved with a carefully designed silicon nanohole waveguide, showing a 12.5 dB improvement relative to a conventional silicon strip waveguide. The 3 dB conversion bandwidth of the silicon nanohole waveguide is ≈37 nm, limited by the optical amplifiers employed in the experiment.

Theoretical study for fabricating elliptical subwavelength nanohole arrays by higher-order waveguide-mode interference

A theoretical method is proposed for fabricating elliptical subwavelength nanohole arrays (ESNA) based on the higher-order waveguide mode (WM) interference in an asymmetric metal-cladding dielectric waveguide (AMDW). The finite-element method is used to simulate the optical field distributions in the AMDW. With excitation by 442-nm transverse magnetic (TM) polarized light, the metal layer is a 40-nm-thick silver film and the waveguide layer is a 9650-nm-thick photoresist that can support many guided modes by virtue of its thickness. The interference of TM51 WMs is used to demonstrate the proposed method for fabricating ESNA. The major and minor axes of the nanohole are obtained theoretically as 110 nm and 98 nm, respectively. With a certain photoresist thickness, ESNA with various periods can be fabricated by selecting different guided modes. The proposed method is a simple way to produce ESNA and could have wide applications in the field of nano-optics.

Rapid prototyping of highly ordered subwavelength silicon nanostructures with enhanced light trapping

High index nanostructures lead to unique light-matter interactions and strong optical resonances for potential light trapping applications and improved optical performances. In this work, Gaussian shaped subwavelength 3D nanostructures are introduced and rapid prototyping of these nanostructures is carried out on silicon (Si) in a single-step using focused ion beam (FIB) milling. The tapered geometry of Gaussian pillar structures provided reflection suppression from the smaller tip in contrast to its extended base, enabling trapping of light, confirmed through light reflection calculations and hot spots observed through absorption density distributions. The nature of accelerated ions in a FIB system is directly used and highly ordered Gaussian pillar like subwavelength nanostructures are fabricated in a single-step on Si. Simulations and experiments demonstrate nanostructure evolution from periodically spaced subwavelength nanoholes to Gaussian pillar structures with tuning of beam processing parameters, providing fabrication flexibility. The proposed light management structures, investigated through Finite Difference Time Domain (FDTD) calculations, exhibit 10–20% reflection for a broadband visible wavelength range against planar Si and previously reported Si structures (such as nano-cones, nano-wires, nano-pyramids etc.). Experimental results support theoretical predictions and provide evidence for antireflection properties from the fabricated nanostructures. Considering the rapid fabrication approach and tunable nature of Gaussian shaped nanostructures, exhibiting unique optical properties; this work provides realization of nanoscale light management 3D structures for localized light trapping and antireflection applications.

Enhancement of extraordinary optical transmission and sensing performance through coupling between metal nanohole and nanoparticle arrays

The extraordinary optical transmission (EOT) of sub-wavelength nanohole array is significantly enhanced through introducing nanoparticles, including nanospheres and nanocylinders, into the centers of the nanoholes. Maxima of 56% and 48% for nanocylinder and nanosphere matrices are achieved, compared with that of 37% for a none-in-hole nanohole array gold film. The mechanism behind the phenomena is discussed, indicating that surface plasmon mode coupling between nanoholes and nanoparticles rather than Fabry–Pérot resonance is the cause for the EOT enhancement. High near-field intensity enhancement also leads to the interaction between analytes and optical field increasing, therefore an improved sensitivity and figure of merit for biosensing. The modified structures are highly promising in practical sensing applications due to the incident angle independence. Similar results are obtained for a hexagonal array of nanohole gold film and nanoparticle modifications thereof.

High-Resolution Large-Ensemble Nanoparticle Trapping with Multifunctional Thermoplasmonic Nanohole Metasurface.

The intrinsic loss in a plasmonic metasurface is usually considered to be detrimental for device applications. Using plasmonic loss to our advantage, we introduce a thermoplasmonic metasurface that enables high-throughput large-ensemble nanoparticle assembly in a lab-on-a-chip platform. In our work, an array of subwavelength nanoholes in a metal film is used as a plasmonic metasurface that supports the excitation of localized surface plasmon and Bloch surface plasmon polariton waves upon optical illumination and provides a platform for molding both optical and thermal landscapes to achieve a tunable many-particle assembling process. The demonstrated many-particle trapping occurs against gravity in an inverted configuration where the light beam first passes through the nanoparticle suspension before illuminating the thermoplasmonic metasurface, a feat previously thought to be impossible. We also report an extraordinarily enhanced electrothermoplasmonic flow in the region of the thermoplasmonic nanohole metasurface, with comparatively larger transport velocities in comparison to the unpatterned region. This thermoplasmonic metasurface could enable possibilities for myriad applications in molecular analysis, quantum photonics, and self-assembly and creates a versatile platform for exploring nonequilibrium physics.

Nano/micro dual-textured antireflective subwavelength structures in anisotropically etched GaAs.

Light trapping by surface texturing is widely used to improve the performance of optoelectronic devices. In this Letter, we demonstrate nano/micro dual-scale textured GaAs by integrating triangular GaAs by orientation-dependent wet etching and subwavelength nanoholes by metal-assisted chemical etching (MacEtch). This is the first report on nano/micro dual-scale textured GaAs. The reflectance was adjusted by controlling the aspect ratio of the nanoholes by varying the MacEtch duration. The combination of the microstructure and subwavelength structures significantly reduced the solar-weighted reflectance of a bare GaAs substrate by 72%.

Extraordinary Figure‐of‐Merit of Magnetic Resonance from Ultrathin Silicon Nanohole Membrane as All‐Dielectric Metamaterial

Metamaterial allows novel nanophotonic applications such as negative permeability, negative refractive index, and near‐zero index. In particular, all‐dielectric metamaterials recently create new opportunities for manipulating electromagnetic fields, taking full advantage of low‐loss, bandwidth enhancement, and isotropic responses. Here a silicon dielectric metamaterial is reported in near‐infrared region, exhibiting extraordinarily figure‐of‐merit, defined by sensitivity (resonance shift/refractive index change) over full width at half maximum, from magnetic resonance shift depending on index changes of surrounding medium. This silicon dielectric metamaterial comprises subwavelength nanohole arrays in a square lattice on an ultrathin amorphous silicon membrane. The ultrathin silicon nanohole membrane is fabricated on a glass wafer by using e‐beam lithography, silicon reactive ion etching, and hydrogen fluoride wet etching. This all‐dielectric metamaterial successfully demonstrates exceptional figure‐of‐merit of 29, which is 7.6 times higher than those values of conventional metamaterials. This novel metamaterial enables not only the label‐free detection of chemical and biological molecules with different mass concentrations but also the in situ reaction monitoring of biochemical molecules.

Extraordinary Transmission Characteristics of Subwavelength Nanoholes with Rectangular Lattice

We investigate the extraordinary optical transmission (EOT) properties of nanohole arrays with a rectangular lattice for label-free refractive index sensing applications. We show that the deviation within the periodicities along the two axes at the nanohole plane leads to more advantageous spectral quality of EOT signal compared to the conventional square lattice geometries. We introduce a way to further improve the sensitivity of the aperture system by carefully choosing the periodicities. We introduce nanohole arrays with a rectangular lattice supporting EOT signals with larger figure-of-merit values as well as enabling much stronger light transmission. We also model a nanohole system covered with a thin dielectric layer, mimicking biomolecules captured on the gold surface, in order to show its biosensing capability. We also show that certain deviation amounts between periodicities create spectral splitting within the EOT signal leading to larger spectral shifts in the presence of a thin dielectric film.

High-resolution plasmonic structural colors from nanohole arrays with bottom metal disks.

We present transmissive plasmonic structural colors from subwavelength nanohole arrays with bottom metal disks for scaled-up manufacturing by nanoimprint lithography (NIL). Comprehensive theoretical and experimental studies are carried out to understand the specific extraordinary optical transmission behavior of the structures with such bottom metal disks. Distinctive colors covering the entire visible spectrum can be generated by changing the structural dimensions of hole arrays in Ag covered by the metal disks. The plasmonic energy hybridization theory is applied to explain the unstable color output with shallow holes so that a large processing window during NIL could be achieved for mass production. A high-resolution of 127,000 dots per inch is demonstrated with potential applications, including color filters and displays, high-resolution color printing, CMOS color imaging, and anti-counterfeiting.

Nonlocal effects in double fishnet metasurfaces nanostructured at deep subwavelength level as a path toward simultaneous sensing of multiple chemical analytes

Nanoplasmonic devices are among the most sensitive chemical sensors, with sensitivities reaching the single-molecule level. An especially convenient class of such sensors is that based on metasurfaces with subwavelength nanoholes, examples being extraordinary optical transmission arrays and double fishnet structures. Such structures ensure operation both in transmission and reflection mode and ensure high sensitivities and excellent coupling with external readout. In this paper we consider the possibility to tailor the response of aperture-based sensor structures by modifying the geometry of nanoholes at the deep subwavelength level through ensuring controlled use of nonlocal effects. We investigate the case where nonlocality is achieved by modifying the basic metamaterial fishnet structure (a metal-dielectric-metal sandwich with rectangular openings) by superposing additional subwavelength patterns, ensuring the appearance of new optical modes. The obtained unit cell superstructure will have multiple tailorable spectral peaks that will increase the selectivity at different wavelengths. The finite elements method was used for simulations of the proposed structures. As an example, we applied our results to the case of a benzene sensor, showing that its spectral properties and selectivity can be tuned by modifying geometry at a deep subwavelength scale. The obtained custom-designed spectral selectivity is convenient for multianalyte chemical sensing using a single structure.

Probing Graphene χ((2)) Using a Gold Photon Sieve.

Nonlinear second harmonic optical activity of graphene covering a gold photon sieve was determined for different polarizations. The photon sieve consists of a subwavelength gold nanohole array placed on glass. It combines the benefits of efficient light trapping and surface plasmon propagation to unravel different elements of graphene second-order susceptibility χ((2)). Those elements efficiently contribute to second harmonic generation. In fact, the graphene-coated photon sieve produces a second harmonic intensity at least two orders of magnitude higher compared with a bare, flat gold layer and an order of magnitude coming from the plasmonic effect of the photon sieve; the remaining enhancement arises from the graphene layer itself. The measured second harmonic generation yield, supplemented by semianalytical computations, provides an original method to constrain the graphene χ((2)) elements. The values obtained are |d31 + d33| ≤ 8.1 × 10(3) pm(2)/V and |d15| ≤ 1.4 × 10(6) pm(2)/V for a second harmonic signal at 780 nm. This original method can be applied to any kind of 2D materials covering such a plasmonic structure.

Study of flow rate induced measurement error in flow-through nano-hole plasmonic sensor.

Flow-through gold film perforated with periodically arrayed sub-wavelength nano-holes can cause extraordinary optical transmission (EOT), which has recently emerged as a label-free surface plasmon resonance sensor in biochemical detection by measuring the transmission spectral shift. This paper describes a systematic study of the effect of microfluidic field on the spectrum of EOT associated with the porous gold film. To detect biochemical molecules, the sub-micron-thick film is free-standing in a microfluidic field and thus subject to hydrodynamic deformation. The film deformation alone may cause spectral shift as measurement error, which is coupled with the spectral shift as real signal associated with the molecules. However, this microfluid-induced measurement error has long been overlooked in the field and needs to be identified in order to improve the measurement accuracy. Therefore, we have conducted simulation and analytic analysis to investigate how the microfluidic flow rate affects the EOT spectrum and verified the effect through experiment with a sandwiched device combining Au/Cr/Si3N4 nano-hole film and polydimethylsiloxane microchannels. We found significant spectral blue shift associated with even small flow rates, for example, 12.60 nm for 4.2 μl/min. This measurement error corresponds to 90 times the optical resolution of the current state-of-the-art commercially available spectrometer or 8400 times the limit of detection. This really severe measurement error suggests that we should pay attention to the microfluidic parameter setting for EOT-based flow-through nano-hole sensors and adopt right scheme to improve the measurement accuracy.

Electromagnetic wave transmission through a subwavelength nano-hole in a two-dimensional plasmonic layer.

An integral equation is formulated to describe electromagnetic wave transmission through a subwavelength nano-hole in a thin plasmonic sheet in terms of the dyadic Green's function for the associated Helmholtz problem. Taking the subwavelength radius of the nano-hole to be the smallest length of the system, we have obtained an exact solution of the integral equation for the dyadic Green's function analytically and in closed form. This dyadic Green's function is then employed in the numerical analysis of electromagnetic wave transmission through the nano-hole for normal incidence of the incoming wave train. The electromagnetic transmission involves two distinct contributions; one emanates from the nano-hole, and the other is directly transmitted through the thin plasmonic layer itself (which would not occur in the case of a perfect metal screen). The transmitted radiation exhibits interference fringes in the vicinity of the nano-hole, and they tend to flatten as a function of increasing lateral separation from the hole, reaching the uniform value of transmission through the sheet alone at large separations.

Enhanced optical transmission and Fano resonance through a nanostructured metal thin film.

Artificial and engineered nanostructures expand the degrees of freedom with which one can manipulate the intricate interplay of light and matter. Certain nanostructural arrangements in the excited state enable the efficient electromagnetic coupling of propagating light with localized fields. Here, we demonstrate that light transmitted through a nanostructured metal thin film without any apertures can be significantly enhanced. Distinct asymmetric Fano resonances are observed in the zero-order transmission spectra using an incoherent light source. The transmission efficiency surpasses that of a metal thin film with the same area and thickness at the resonance maxima. The transmission minima and the sharp resonance maxima bear a strong resemblance to the extraordinary optical transmission observed in sub-wavelength nanohole array structures The resonance wavelength closely matches the nanostructural periodicity. The sensitivity of the resonances to the surrounding medium and the transmission efficiency demonstrate the potential for use in energy harvesting, imaging, optical processing and sensing applications.