Light transmission by subwavelength square coaxial aperture arrays in metallic films

Massively parallel direct writing of nanoapertures using multi-optical probes and super-resolution near-fields

Laser direct-writing enables micro and nanoscale patterning, and is thus widely used for cutting-edge research and industrial applications. Various nanolithography methods, such as near-field, plasmonic, and scanning-probe lithography, are gaining increasing attention because they enable fabrication of high-resolution nanopatterns that are much smaller than the wavelength of light. However, conventional methods are limited by low throughput and scalability, and tend to use electron beams or focused-ion beams to create nanostructures. In this study, we developed a procedure for massively parallel direct writing of nanoapertures using a multi-optical probe system and super-resolution near-fields. A glass micro-Fresnel zone plate array, which is an ultra-precision far-field optical system, was designed and fabricated as the multi-optical probe system. As a chalcogenide phase-change material (PCM), multiple layers of Sb₆₅Se₃₅ were used to generate the super-resolution near-field effect. A nanoaperture was fabricated through direct laser writing on a large-area (200 × 200 mm²) multi-layered PCM. A photoresist nanopattern was fabricated on an 8-inch wafer via near-field nanolithography using the developed nanoaperture and an i-line commercial exposure system. Unlike other methods, this technique allows high-throughput large-area nanolithography and overcomes the gap-control issue between the probe array and the patterning surface.

Modeling of second-harmonic generation in periodic nanostructures by the Fourier modal method with matched coordinates

We present an advanced formulation of the Fourier modal method for analyzing the second-harmonic generation in multilayers of periodic arrays of nanostructures. In our method, we solve Maxwell's equations in a curvilinear coordinate system, in which the interfaces are defined by surfaces of constant coordinates. Thus, we can apply the correct Fourier factorization rules as well as adaptive spatial resolution to nanostructures with complex cross sections. We extend here the factorization rules to the second-harmonic susceptibility tensor expressed in the curvilinear coordinates. The combination of adaptive curvilinear coordinates and factorization rules allows for efficient calculation of the second-harmonic intensity, as demonstrated for one- and two-dimensional periodic nanostructures.

Enhancement of magneto-optical Faraday effects and extraordinary optical transmission in a tri-layer structure with rectangular annular arrays

The properties of optics and magneto-optical Faraday effects in a metal-dielectric tri-layer structure with subwavelength rectangular annular arrays are investigated. It is noteworthy that we obtained the strongly enhanced Faraday rotation of the desired sign along with high transmittance by optimizing the parameters of the nanostructure in the visible spectral ranges. In this system, we obtained two extraordinary optical transmission (EOT) resonant peaks with enhanced Faraday rotations, whose signs are opposite, which may provide the possibility of designing multi-channel magneto-optical devices. Study results show that the maximum of the figure of merit (FOM) of the structure can be obtained between two EOT resonant peaks accompanied by an enhanced Faraday rotation. The positions of the maximum value of the FOM and resonant peaks of transmission along with a large Faraday rotation can be tailored by simply adjusting the geometric parameters of our models. These research findings are of great importance for future applications of magneto-optical devices.

Wedge and gap plasmonic resonances in double nanoholes

We study the plasmonic resonances of double nanoholes (DNHs) in metal films. These apertures exhibit the usual gap-mode Fabry-Pérot resonances, where the zeroth order resonance is determined by the waveguide cut-off and the first order resonance shows sensitivity to the film thickness. An additional wedge resonance is observed, which is sensitive to the curvature of the cusps in the DNHs, analogous to the wedge modes of single wedges. While the gap mode intensity increases dramatically with decreasing gap-width, the wedge mode intensity saturates since its field enhancement arises from the curvature of the metal film, like cylindrical Sommerfeld waves. Experimental transmission spectra agree well with finite-difference time-domain simulations showing these separate resonances. The controlled design of these resonances is critical for applications including optical tweezers, nonlinear conversion, sensing and spectroscopy.

Transmission properties of slanted annular aperture arrays. Giant energy deviation over sub-wavelength distance

This paper is devoted to the study of the transmission properties of Slanted Annular Aperture Arrays made in perfectly conducting metal. More precisely, we consider the transmission based on the excitation of the cutoff-less guided mode, namely the TEM mode. We numerically and analytically demonstrate some intrinsic properties of the structure showing a transmission coefficient of at least 50% of an unpolarized incident beam independently of the illumination configuration (angle and plane of incidence). The central symmetry exhibited by the structure is analytically exploited to demonstrate the existence of a polarization state for which all the incident energy is transmitted through the sub-wavelength apertures when the eigenmode is excited, whatever are the illumination and the geometrical parameters. For this state of polarization, the laminar flow of the energy through the structure can exhibit giant deviation over very small distances. An example of energy flow deviation of 220° per wavelength is presented for illustration. The results presented in this paper could be considered as an important contribution to the understanding of the enhanced transmission phenomenon based on the excitation of guided modes.

Transmission enhancement through square coaxial aperture arrays in metallic film: when leaky modes filter infrared light for multispectral imaging

The diffractive behavior of arrays of square coaxial apertures in a gold layer is studied. These structures exhibit a resonant transmission enhancement that is used to design tunable bandpass filters for multispectral imaging in the 7-13 μm wavelength range. A modal analysis is used for this design and the study of their spectral features. Thus we show that the resonance peak is due to the excitation of leaky modes of the open photonic structure. Fourier transform infrared (FTIR) spectrophotometry transmission measurements of samples deposited on Si substrate show good agreement with numerical results and demonstrate angular tolerance of up to 30 degrees of the fabricated filters.

A high throughput supra-wavelength plasmonic bull's eye photon sorter spatially and spectrally multiplexed on silica optical fiber facet

This paper presents a plasmonic bull's eye consisting of a micron-sized hole and a concentric nano-antenna metallic ring surrounded by periodic circular grooves on a thin gold film. The unique metallic nano-ring imbedded in the supra-wavelength-sized hole acts as an amplifying and filtering component to simultaneously provide a significantly lower spectral noise and a higher power transmission at the resonance wavelength, in comparison to prior sub-wavelength bull's eyes. Systematic numerical analyses based on finite-difference time-domain method were carried out to find the impacts of the structural parameters. Experimentally we integrated three proposed plasmonic structure on a cleaved facet of an optical fiber that can act as a spatially and spectrally multiplexed photon sorter. Transmission characteristics of the proposed devices were characterized in terms of the spectral response and signal to noise ratio. Potential applications of the fiber optic photon sorter were also discussed.

Broadband sharp 90-degree bends and T-splitters in plasmonic coaxial waveguides

We demonstrate numerically that sharp 90° bends and T-splitters can be designed in plasmonic coaxial waveguides at deep-subwavelength scale to operate without reflection and radiation over a broad range of wavelengths, including the telecommunication wavelength of 1.55 μm. We explain the principles of the operation using a transmission line model of the waveguide in the quasi-static limit. The compact bends and T-splitters open up a new avenue for the design of densely integrated optical circuits with minimal crosstalk.

Plasmon resonance tuning in metallic nanocavities

Nanocavities fabricated in a metallic surface have important and technologically useful properties of complete light absorption and strong field enhancement. Here, we demonstrate how a nanometerthick alumina deposition inside such a cavity can be used to gain an exquisite control over the resonance wavelength. This process allows achieving a precise control over the spectral response and is completely reversible allowing many tuning attempts to be made on a single structure until the optimum performance is achieved.

Field enhancement in metallic subwavelength aperture arrays probed by erbium upconversion luminescence

Upconversion luminescence from erbium ions placed in the near field of subwavelength aperture arrays is used to investigate field enhancement of incident near-infrared light in such nanostructures. We study field enhancement due to the excitation of both propagating and localized surface plasmon resonances in arrays of square and annular apertures in a Au film. The conversion of 1480 nm excitation light to 980 nm emission is shown to be enhanced up to a factor 450 through a subwavelength hole array. The effects of array periodicity and aperture size are investigated. It is shown that a Fano model can describe both far-field transmission and near-field intensity. The upconversion enhancement reveals the wavelength and linewidth of the surface plasmon modes that are responsible for extraordinary transmission in such arrays. Angle-dependent measurements on annular aperture arrays prove that the field enhancement due to localized resonances is independent of the incident angle. These experiments provide insight in the mechanisms responsible for extraordinary transmission and are important for applications that aim to exploit near-field enhancement in nanostructured metal films.

Gap plasmon mode of eccentric coaxial metal waveguide

The gap plasmon mode of an eccentric coaxial waveguide is analyzed by the effective index method. The results agree-well with fully-vectorial numerical calculations. In the eccentric structure, there is extreme subwavelength field localization around the narrowest gap due to the gap plasmon. Furthermore, the effective index of the lowest-order waveguide mode increases considerably, for example, to 3.7 in the structure considered with a 2 nm minimum gap. The nanostructure waveguide geometry and wavelength (4 microm) are comparable with recent experiments on coaxial structures, except that that position of the center island is shifted for the eccentric coaxial structure; therefore, the proposed structure is a good candidate for future fabrication and experiments. In the visible regime, the effective index increases to over 10 for the same structure. The influence of symmetry-breaking in the eccentric coaxial structure is discussed as a way to enhance the local field and improve optical coupling.

Single-scattering theory of light diffraction by a circular subwavelength aperture in a finitely conducting screen

A perturbation theory based on a single-scattering approximation is developed from the rigorous differential theory of diffraction in cylindrical coordinates. It results in analytical formulas in the inverse space for the field amplitudes providing results that are in quantitative agreement with the results of the rigorous method, in both the near- and far-field regions, when a proper correction to the incident field inside the aperture is made by using the renormalized Born approximation. When working in reflection by a screen having permittivity high in modulus, the method proposes an equivalence with the simple model consisting of the emission by a single magnetic dipole excited inside the pierced layer, emission that is then transferred back into the cladding following the Fresnel's coefficients of transmission from the layer into the cladding. The theory predicts a directivity of the radiation pattern that increases for smaller values of modulus of permittivity, both for dielectrics and metals, thus independently of the possibility of plasmon surface wave excitation along the interface. The theory can take into account such surface wave resonances, as well as the waveguide supported by a dielectric slab, but cannot implicitly recognize the modes carried out by the cylindrical waveguide corresponding to the aperture. This fact limits its domain of validity when used in transmission, although the far- and near-field maps can be reconstructed sufficiently well within a multiplicative factor corresponding to the enhanced transmission due to the excitation of these modes.

Optimization of plasmon excitation at structured apertures

Surface plasmon excitation that is due to a single or a structured circular aperture in a flat metallic screen is investigated theoretically and numerically with a view to enhancing the electric field close to the metallic surface. A systematic study of the homogeneous solution of the electromagnetic scattering problem is made with cylindrical coordinates, expanding Maxwell equations on a Fourier-Bessel basis. A perturbation analysis devoted to simple physical analyses of different types of cylindrical nanostructure is developed for the optimization of plasmon excitation by a normally incident linearly polarized monochromatic plane wave. The conclusions drawn from this analysis agree well with the results of rigorous electromagnetic calculations obtained with the differential theory of diffraction in cylindrical coordinates.

Microwave transmission through a single subwavelength annular aperture in a metal plate

The resonant transmission of a small annular aperture, with a diameter much smaller than the radiation wavelength, in a thin metal plate is studied at microwave frequencies. It transpires that such an annular aperture supports several resonant guided modes, including those that are not quantized in the azimuthal direction. Such modes have resonant frequencies that are largely independent of the diameter of the annular aperture, thus being supported by annular apertures that tend to zero radius. The transmittance of such a structure at microwave frequencies is detailed and compared with the predictions of a finite element method model.

Surface plasmon excitation on a single subwavelength hole in a metallic sheet

The diffraction of light by a single subwavelength hole in a highly conductive metallic sheet is analyzed with a recently developed differential theory that is able to plot the nearly electromagnetic field. Using rigorous electromagnetic and phenomenological analysis, we show that a single subwavelength hole can excite surface-plasmon resonance that contributes greatly to extraordinary transmission.

Enhanced infrared transmission through subwavelength coaxial metallic arrays

We report the observation of enhanced near-infrared transmission through arrays of subwavelength coaxial metallic structures compared with that through comparable diameter hole arrays as a result of localized electromagnetic modes supported by the complex coaxial unit cell. Polarization and angle-dependent transmission measurements clearly demonstrate the coupling between this localized mode and delocalized surface plasmon modes. A generalized, multiple discrete states Fano line shape provides a good fit to the experimental results.

Fractal extensions of near-field aperture shapes for enhanced transmission and resolution

Families of fractals are investigated as near-field aperture shapes. They are shown to have multiple transmission resonances associated with their multiple length scales. The higher iterations exhibit enhanced transmission, and spatial resolution exceeding the first order. Near-field enhancements of greater than 400 times the incident intensity and resolutions of better than ?/20 have been shown with apertures modeled after third iteration prefractals. Enhancements as large as 1011 have been shown, when compared with conventional square apertures that produce the same spot size. The effects of the complex permittivity values of the metal film are also addressed.

Transmission through single subwavelength apertures in thin metal films and effects of surface plasmons

The existing analyses on extraordinary optical transmission through apertures on a metal screen have been carried out assuming perfect conductivity or by examining arrays of closely spaced holes with subwavelength dimensions. We present an electromagnetic analysis of a single hole (modeled by use of an array of distant holes) in a finitely conducting metal membrane, applying no approximations. We demonstrate that finite conductivity is not of remarkable importance with small hole-diameter-to-wavelength ratios in the absence of strong resonances. However, if the angle of incidence of a plane wave is such that surface plasmons are excited, substantial enhancement of the transmittance can be observed, and the effect of finite conductivity will no longer be negligible. Our analysis also reveals that transmission of small apertures in highly conducting membranes can be described by approximate analytical formulas if surface waves are not excited, but with poor conductors the full electromagnetic analysis should be applied.