Metal Hole Arrays Research Articles

Subwavelength B-shaped metallic hole array terahertz filter with InSb bar as thermally tunable structure

We propose a subwavelength B-shaped metallic hole array filter with an embedded thermally tunable InSb semiconductor bar in the terahertz regime. The resonance frequency of this filter can be actively tuned by controlling the temperature of InSb. The transmissions of the filter are calculated with the finite-difference time domain method at various temperatures. Narrowband THz wave with the full width at half maximum of 235 GHz can be selected in the frequency range from 0.74 to 2.02 THz at temperatures from 160 to 350 K.

Selective enhancement of infrared absorption with metal hole arrays

We use a surface-enhanced infrared absorption (SEIRA) spectroscopy, a useful sensing and surface analysis method complimentary to the Raman scattering spectroscopy, for the individual enhancement of specific molecular vibration bands and fingerprinting of molecular vibrations. SEIRA spectroscopic measurement using the metal hole array (MHA) is demonstrated with high spectral selectivity. The molecular IR absorption peaks are enhanced up to 10 times at the transmission peak of MHA structure when electromagnetic field enhancement is localized on the walls inside the holes. Experimental and numerical simulations results are in a good qualitative agreement. Selective IR band enhancement can be used for identification of specific molecules within complex mixtures and it can be extended to the longer wavelengths at THz molecular bands.

Subwavelength polarization rotators via double-layer metal hole arrays

We show that the polarization of linearly polarized light can be rotated an arbitrary angle by double-layer metal hole array structures in a subwavelength regime. The transmitted light with the rotated polarization, however, remains of nearly the same strength as the incident field at particular frequencies. The mechanism can be attributed to the subwavelength feature of the rectangular holes, and the tangential guiding modes between layers modulated by the orientation of the holes. The structures have potential applications as polarization rotators in a broad frequency range covering from terahertz (THz) to infrared frequencies.

Extraordinary transmission in metal hole array-photonic crystal hybrid structures

In this work we show that structures consisting of a metal hole array (MHA) lying on top of a 2D photonic crystal (PhC) exhibit the extraordinary transmission effect. In contrast to single MHAs, the extraordinary transmission in such hybrid structures is due to the coupling of an incident wave to eigenmodes of the PhC. Thus, the spectral positions of the transmission peaks are defined by the spectral positions of the corresponding PhC eigenmodes. Our results provide a novel powerful tool to manipulate light on a subwavelength scale.

Tunable wide-angle plasmonic perfect absorber at visible frequencies

We report a perfect absorber metallic hole array with wide-angle near unity absorbance in the visible regime. The periodicity of the structure enables excitation of surface-plasmon polaritons and results in a splitting of the absorbing peaks. An analytical formalism based on perturbation theory provides results in good agreement with experimental data and shows how the absorption peak can be tuned with hole geometry, periodicity, and material parameters. Being extendable to more complex hybrid structures, our result is important for the development of photon harvesting devices and thermal emitters.

Rational design of high performance surface plasmon resonance sensors based on two-dimensional metallic hole arrays

We have rationally designed two-dimensional Au and Ag hole arrays for high performing surface plasmon resonance (SPR) sensing. The figure-of-merit (FOM), which is defined as sensitivity/linewidth, is found to be highly geometry-dependent. For sensitivity, we find it is equal to the period of array when exciting low order surface plasmon modes at low incident angle. Therefore, increasing period improves sensitivity. On the other hand, narrow linewidth can be obtained from small hole size so that the radiative decay loss is minimized. By using a pair of orthogonally oriented polarizer and analyzer, the signal-to-noise ratio (SNR) can be greatly enhanced due to the elimination of the nonresonant reflection background. As a proof of our strategy, we have obtained FOM larger than 100/RIU and SNR higher than 110 from Au arrays. Our results show the importance of understanding the basic properties of surface plasmon polaritons in order to systematically optimize the performance of the plasmonic system for a given application.

Terahertz coherent control of surface plasmon polariton propagation in subwavelength metallic hole arrays

We demonstrate the terahertz coherent control of extraordinary transmission through subwavelength metallic hole arrays with double terahertz pulses. The interference of excitations of surface plasmon polaritons (SPPs) by two THz pulses sequence is employed to control the propagation of SPPs in the periodical structure. As a result, the THz wave transmission is controllable by adjusting the arriving time of the control THz pulse. Our results reveal that THz extraordinary transmission in subwavelength structure arises from the excitation of SPPs of the structures, which provides a universal ultrafast means to control the SPPs motion in various THz subwavelength structures.

Free carrier induced spectral shift for GaAs filled metallic hole arrays

For a GaAs filled metallic hole array on a pre-epi GaAs substrate, the free carriers, generated by three-photon absorption (3PA) assisted by strongly enhanced local fields, reduce the refractive index of GaAs in ~200-nm thick active area through band filling and free carrier absorption. Therefore, the surface plasma wave (SPW) resonance, and the related second harmonic (SH) spectrum blue shifts with increasing fluence; For the plasmonic structure on a substrate with surface defects, free carrier recombination dominates. The band gap emission spectral peak wavelength decreases 10-nm with increasing fluence, showing the transition from nonradiative-, at low excitation, to bimolecular-recombination at high carrier concentrations.

Erratum: Dispersion of spoof surface plasmons in open-ended metallic hole arrays [Phys. Rev. B84, 035418 (2011)

Received 20 February 2012DOI:https://doi.org/10.1103/PhysRevB.85.129901©2012 American Physical Society

Design and experimental verification of terahertz wideband filter based on double-layered metal hole arrays

A terahertz wideband filter based on double layer metal hole arrays is designed in this paper. A metal hole array is perforated on a metal layer with a square array of circular air holes. The transmission characteristics of the electromagnetic waves through the metal hole array can be determined by the accumulation of in-phase scattering, spoof surface plasmon polaritons, and waveguide modes. The transmission spectrum is tuned by adding another identical layer metal hole array, and a wideband filter can be formed accordingly. Samples containing double-layered metal hole arrays were fabricated by micromachining technology. A wideband filter with center frequency located at 0.8 THz and FWHM reaching 400 GHz was experimentally achieved.

Controlling extraordinary transmission characteristics of metal hole arrays with spoof surface plasmons

We experimentally controlled the spectral characteristics of the extraordinary transmissions observed in metal hole arrays by analogy with spoof surface plasmon-polaritons (spoof SPPs). We constructed doubly periodic metal hole arrays. The larger and smaller periodic holes are for inducing extraordinary transmissions and modifying the spoof SPPs’ characteristics, respectively. Introducing the smaller holes that surround the larger hole arrays can modify the dispersion curve of the surface waves excited on a structured metal surface analogous to the spoof SPP model, and consequently, the extraordinary transmission peak will show a redshift.

Understanding plasmon resonances of metal-coated colloidal crystal monolayers

Metal films deposited over two-dimensional colloidal crystals (MFoCC) constitute a low-cost periodic structure with interesting photonic and plasmonic properties. It has previously been shown that this structure exhibits a behaviour similar to the well-known Extraordinary Optical Transmission (EOT) of metallic hole arrays in planar films. Here, we explore the transmission characteristics of AgFoCC by systematic comparison with that of the bare CC. Furthermore with additional reflectivity measurements we evaluate the AgFoCC overall plasmonic response, which, notably, exhibits a strong plasmon absorption band at wavelengths larger than those of the transmitted maximum. By corroborating these results with finite-difference time-domain electromagnetic simulations, we identify a hybrid metal-dielectric propagative mode in the transmission mechanism. On the contrary a strongly localized mode is responsible for the maximum light absorption by this structure. These results shed new light on the current understanding of this highly promising plasmonic structure, being useful for the design of surface-enhanced Raman scattering and enhanced fluorescence substrates.

The Extraordinary Transmission Spectrum in Terahertz Regime: Combination of Shape Resonances and Rayleigh anomalies

We present the Terahertz wave extraordinary transmission through thin metal film with periodic arrays of subwavelength rectangular holes. By designing metallic hole arrays with different periods along x and y direction, the transmission spectrum with a non-Fano type is obtained. The roles of shape resonances and Rayleigh anomalies in extraordinary transmission are clarified. Our results demonstrate that the enhanced transmission can be attributed to the shape resonance, and the position of transmission minimum comes from the Rayleigh anomaly which is related to the period of the holes array. Shape resonance in hole arrays is different from the classical waveguide resonance, instead, the resonance behaves a surface plasmon polariton-like character but it is irrelevant to the period of the hole arrays. Our finding is applicable to exploit terahertz field localization which can be used to study terahertz nonlinear spectroscopy of materials.

Surface electromagnetic modes contribution to the anomalous terahertz transmission through double-layered metal hole array

Terahertz transmission spectra through both single-layered and double-layered sub-wavelength metal hole arrays were calculated using rigorous mode-matching method. The dispersion relations of the single-layered and double-layered metal hole arrays were obtained by FDTD method. There are three anomalous peaks in the spectrum of the double-layered metal hole array, which arise from the three surface electromagnetic modes appearing in the dispersion relation. By comparing the transmission spectra and the dispersion relations of the single-layered and double-layered metal hole arrays, it was found that the two surface electromagnetic modes at higher frequencies are introduced by the single-layered metal hole array, while the one at lower frequencies is introduced by the double-layered metal hole array. The validity of the analysis was qualitatively verified by the experimental results. This structure can be finely designed for potential applications, such as wideband filtering.

Understanding fractional-order surface plasmons

We show experimentally that diffraction-induced surface plasmon excitation can mimic enhanced transmission and cause a highly sensitive modulation by the coherent interference between zero-order and reflected first-order diffraction in select regions of the terahertz spectrum. Based on the study of a one-dimensional metallic grating, we obtain the physical mechanisms of the fractional-order surface plasmon resonances observed with the two-dimensional grating of the metallic hole array.

Experimental study on polarization lens formed by asymmetrical metallic hole array

A polarization bifocal lens based on the polarization effect caused by asymmetrical hole arrays had been designed, fabricated, and characterized experimentally. By considering the fact that the skin depth of an infrared electromagnetic field inside metal is much shorter than the incident wavelength, a polarization bifocal lens composed of high deep-width ratio metallic holes was realized by using a gold-coated silicon structure to replace the one directly formed on a thick metal film. An infrared optical experiment setup is built based on the secondary imagery method for characterizing the focal length of the designed bifocal lens. The measured focal lengths of the fabricated bifocal lens coincide well with the designed values, which proves the validity for realizing the polarization elements with the proposed structure and the feasibility of the fabrication process.

Enhanced transmission of THz radiation through sub-wavelength, asymmetry metallic hole arrays

Strongly enhanced transmission of terahertz radiation pulses through asymmetry rectangular hole arrays with various hole lengths are obtained experimentally. The transmission magnitudes increase and their positions shift to lower frequency with increasing hole length. The maximum transmittance peak is observed for the sample with a hole length of 400 μm. Apparent angle-dependent behavior is observed in the transmission spectra of the 200 × 400 μm2 hole array, which due to the size effect of the rectangular hole along the two orthogonal axes. The resonance frequencies are deduced according to the surface plasmon polaritons modes. Our experiment results closely match the theoretical calculations.

Dispersion of spoof surface plasmons in open-ended metallic hole arrays

Using phase sensitive microwave frequency measurements, we obtain the dispersion of spoof surface plasmon waves on a highly conducting sheet perforated with a two-dimensional array of subwavelength holes, and compare our results to an explicit analytical dispersion relation obtained by the modal matching method. We observe splitting into symmetric and antisymmetric surface modes, a behavior analogous to that of surface plasmons in thin metallic films at optical frequencies. We show that spoof surface modes play an important role in both near and far field transmissions. Specifically, we show that superfocussing effects which are present for surface plasmons in metallic films are absent for hole arrays (i.e., no amplification of near fields is possible for spoof surface plasmons). While many of the apparent properties of spoof surface plasmons resemble those of surface plasmons in thin metallic films, the analogy is therefore incomplete in the high frequency limit.

Propagation dynamics of femtosecond pulse through subwavelength metallic hole arrays

We have studied the frequency-dependent transmission time delay of femtosecond (fs) laser pulses through subwavelength hole arrays in aluminum films using the up-conversion technique. The pulse delays measured at different wavelengths mainly follow the far-field transmission profile of the surface-plasmon polariton (SPP) resonances. Temporal delays of 60 and 100 fs were found at the major SPP resonance in the single-layer and double-layer samples. A coupled-SPP transmission model is used to explain the temporal dynamics of the transmission process. Our experiment shows that the weak coupling between the SPP waves on different metal-dielectric interfaces leads to the large temporal delay of the transmitted pulses.

Saturation of the second harmonic generation from GaAs-filled metallic hole arrays by nonlinear absorption

Saturation of the enhanced second harmonic generation (SHG) based on the localized fields confined in a nanoscale periodic, GaAs-filled, plasmonic structure is presented. The SHG saturates at high pump intensities due to three-photon absorption and the resultant free-carrier absorption. Free carriers are monitored by photoluminescence (PL). A fifth power dependence of the PL is observed at 297 K, which results from the three photon absorption along with a transition from nonradiative carrier recombination at the defected GaAs surface at low excitation to bimolecular recombination at high carrier concentrations.