Subwavelength Rectangular Holes Research Articles

Transforming terahertz plasmonics within subwavelength hole arrays into enhanced terahertz mission via Smith-Purcell effect.

We illustrate the transformation of terahertz plasmonics within an array of rectangular sub-wavelength holes (RSHs) into coherent and enhanced terahertz emission via Smith-Purcell effect. The radiative plasmonic modes within each RSH of the array are successively excited by an free-electron beam, which then generate coherent radiation by constructive interference. Compared with the case without taking plasmonics into consideration, the radiation field intensity is enhanced by more than an order of magnitude, affording a promising way of developing high-power terahertz radiation. We perform detailed analysis of the plasmonic modes within the RSH by using the dielectric waveguide theory, and the results are verified by numerical simulations. The influences of the RSH parameters on the radiation properties are revealed and discussed.

A thermally tunable terahertz bandpass filter with insulator-metal phase transition of VO2 thin film

A terahertz bandpass filter with the sandwich structure consisting of thermally tunable vanadium dioxide (VO2) thin film, silica substrate and subwavelength rectangular Cu hole arrays is designed and theoretically analyzed. The results show that the transmittance of the filter can be actively tuned by controlling the temperature of VO2, the narrow band terahertz (THz) waves with the transmittance from 85.2% to 10.5% can be well selected at the frequency of 1.25 THz when the temperature changes from 50 °C to 80 °C, and the maximum modulation depth of this terahertz bandpass filter can achieve 74.7%.

Origin of Shape Resonance in Second-Harmonic Generation from Metallic Nanohole Arrays

Second-harmonic generation (SHG) from periodic arrays of subwavelength rectangular air hole with various aspect ratios perforated in gold thin films can get resonantly enhanced for some specific geometric shapes. Here we clarify the physical origin of this shape resonance effect. A nonlinear coupled-mode theory is set up to solve energy conversion from fundamental wave (FW) mode to second-harmonic wave (SHW) mode within the nanoscale air hole. It reveals that several physical mechanisms, including the FW mode excitation amplitude, FW-SHW modal spatial overlap, FW-SHW mode phase mismatch, and SHW mode attenuation, are all geometric shape sensitive and altogether act to induce the SHG shape resonance effect. The theory agrees well with experimental observations and provides an accurate and complete explanation for the long-emphasized but elusive shape effect. The study may stimulate deeper insights to visualize general nonlinear nanophotonic processes and pave the way to engineering high-efficiency nonlinear nanophotonic devices.

Polarization-Manipulated Dual-Band Enhanced Optical Transmission Through Subwavelength Rectangular Hole Array on Metallic Film

We proposed and experimentally demonstrated the polarization-controlled dual-band optical transmission through metallic hole array. It is utilized the anisotropic enhanced transmission through metallic array of subwavelength rectangular holes. The meaning is that the incident polarization state can decide the output wavelength. Two polarization-manipulated resonant transmissions at 632 nm (red) and 532 nm (green) have been demonstrated. It can be applied for achieving integrated polarization-controlled red-green light switching devices. Also, it can be used as a wavelength-dependent micropolarizer.

Metallic colour filtering arrays manufactured by NanoImprint lithography

Colour filters using two-dimensional sub-wavelength double-breasted rectangular hole, with a 250nm period, were proposed and manufactured. Using the cost-efficient wafer scale thermal NanoImprint lithography, a thin metallic aluminium silicon alloy layer was patterned into two dimensional structures onto 200mm wafer size. A fine tuning of the manufactured design was proposed with arm widths ranging from 30nm up to 60nm, and arm lengths ranging from 100nm up to 240nm, keeping the period constant at 250nm. Sub 20nm resolution 200mm silicon stamp, with aspect ratio larger than 5 were manufactured using electron beam lithography with proximity correction exposure strategy based on shape modification of the initial design. At the end, 864 different 100×100μm2 patterns were manufactured and etched in thin 40nm thick aluminium layer transferred to transparent glass to perform optical characterisations in transmission. Optical measurements of transmission spectra revealed that the optical transmission ranged up to 55%, slightly lower than the computed one. The short infra-red transmission levels were however larger than expected increasing the bandwidth response of the filtering function. A fine morphological characterisation revealed that the optical properties were very sensitive to the fine shape of the patterns etched in the metallic layer.

Spin-sensitive distribution of electromagnetic field via spin-orbit interaction in structured metamaterials

We investigate the spin-sensitive distribution of electromagnetic (EM) field from a kind of defective inhomogeneous anisotropic metamaterial, with azimuthally distributed subwavelength rectangular holes within a distribution angle of Φ<2π. This structure supports the spin-orbit interaction in the optical angular momentum, induced by the Pancharatnam-Berry phase from the manipulation of the spin states of polarization. Spin-sensitive distributions of electric field intensity, spin states of polarization, and transverse energy flow in the transmitted EM field are investigated. Importance of this investigation and the possible applications are discussed.

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.

Local investigation of the optical properties of subwavelength rectangular holes with a focused beam of electrons

The optical properties of rectangular subwavelength holes in a gold film are investigated using the light generated when a focused beam of electrons impinges on the sample close to the hole. Using this technique, multi-spectral maps of the holes are obtained with a resolution beyond the optical diffraction limit. The results show the influence of hole shape on the spectrum of locally scattered light. Rectangular holes of varying shape and size are investigated, and the spatial distribution of the polarization of the observed light is measured. The influence of neighbouring holes is investigated by measuring small clusters of holes.

Transmitted properties of terahertz wave through metallic hole arrays

Terahertz (THz) wave transmission through a thin metal film with periodic arrays of subwavelength rectangular holes is investigated systematically. The roles of the waveguide resonances and surface plasmon polaritons (SPPs) in extraordinary transmission are also clarified. The transverse waveguide resonances (hole-shape dependence) dominates the position and width of maximum transmission. In addition, the periodicity of the hole arrays dominates the position of minimum transmission due to the excitation of SPP, which acts as a band-stop filter in the THz transmission spectra. Our results clearly demonstrate that the extraordinary transmission in THz frequency is originated from the transverse waveguide resonances that behave with a SPP-like character. Simulation based on finite-difference time-domain (FDTD) agrees well with the experimental results.

Experimental studies of extraordinary light transmission through periodic arrays of subwavelength square and rectangular holes in metal films

We fabricate a series of periodic arrays of subwavelength square and rectangular air holes on gold films, and measure the transmission spectra of these metallic nanostructures. By changing some geometrical and physical parameters, such as array period, air hole size and shape, and the incident light polarization, we verify that both global surface plasmon resonance and localized waveguide mode resonance are influential on enhancing the transmission of light through nanostructured metal films. These two resonances induce different behaviours of transmission peak shift. The transmission through the rectangular air-hole structures exhibits an obvious polarization effect dependent on the morphology. Numerical simulations are also made by a plane-wave transfer-matrix method and in good consistency with the experimental results.

Increase of group delay and nonlinear effects with hole shape in subwavelength hole arrays

We investigate the influence of hole shape on the group delay of femtosecond laser pulses propagating through arrays of rectangular subwavelength holes in metal films. We find a pronounced dependence of the group delay on the aspect ratio of the holes in the arrays. The maximum group delay occurs near the cut-off frequency of the holes. These experimental results are found to be in good agreement with calculations. The slow propagation of light through the array gives rise to enhancement of the second harmonic generated in the structures. The observed behavior is consistent with the presence of a resonance at the cut-off frequency of the rectangular holes.

Optical hydrogen detection with periodic subwavelength palladium hole arrays

The extraordinary transmission of infrared light through subwavelength rectangular hole arrays of palladium is used to detect hydrogen. The main resonance peak of rectangular hole arrays is found to shift upon exposure to hydrogen. Experimental evidence of the change in the Pd phase, producing a shift toward longer wavelengths of the main resonance peak, is presented and supported by simulations that agree with experimental observation. The all-optical and selective detection scheme of hydrogen produces large peak shifts that enable the detection of hydrogen concentration near the lower flammability threshold in air.

Phase effects in the enhanced transmission through compound subwavelength rectangular hole arrays

We study the enhanced transmission through compound subwavelength rectangular hole arrays in perfect electronically conductive films. It is shown that phase effects become important in the enhanced transmission when extra rectangular holes are added per period. There are two types of resonance mechanisms due to the phase effects: one is the localized waveguide mode phase resonance and the other is the surface mode phase resonance. The localized waveguide mode phase resonances, which are related to the number of holes per period and their relative positions, split the waveguide resonance peak into several peaks. The surface mode phase resonances, which appear evidently only for some special arrangements of holes in a unit cell, lead to the suppression of some surface mode resonance peaks. Our approach is advantageous to modulate the enhanced transmission in metallic hole arrays in the terahertz or microwave frequency regime.

Localized surface plasmons-based transmission enhancement of terahertz radiation through metal aperture arrays

The extraordinary transmission spectrum of a copper film pierced by a periodic array of subwavelength rectangular holes is measured by the terahertz time-domain spectroscopy. The transmission coefficient is strongly dependent on the angle between the polarization of terahertz electric field and the latitudinal direction of the periodic apertures. When the angle increases from 0 o to 90 o, a peak becomes stronger and another peak reduces. The transmission is proposed to be the contributions of localized surface plasmons inside the apertures. The finite-difference-time-domain (FDTD) simulation results are in good agreement with experimental observations.

Effect of Wood’s anomalies on the profile of extraordinary transmission spectra through metal periodic arrays of rectangular subwavelength holes with different aspect ratio

The extraordinary transmission through silver film perforated with rectangular hole array with different aspect ratio was investigated. It was found that when the aspect ratio exceeded 7, the propagating surface plasmon polaritons (SPPs) transformed to localized resonance mode. The role of the Wood's anomaly on the shape of the transmission spectrum is investigated. By designing the rectangular hole arrays in a rectangular lattice, the Wood's anomaly can be shifted far apart from the transmission peak, the real localized resonance peak wavelength was identified and fitted well with the theoretical calculation using a simplified transmission-line model.

Metallic slit aperture as a near-field optical head for heat-assisted magnetic recording

Heat assisted magnetic recording (HAMR) technique requires an optical head that can efficiently generate a subwavelength optical spot. The results of finite difference time domain simulation shows that two subwavelength rectangular holes adjacent to a metallic slit aperture make an optical spot from the slit smaller and its peak intensity higher. A subwavelength spot of 82×46 nm2 (full width at half maximum) in the recording medium is obtainable when a pair of rectangular holes is flanked adjacent to the slit aperture with a much smaller distance than the optical wavelength. This configuration provides a high transmittance of the slit aperture and a high expectation for a high controllability of both a thickness of the slit and a distance between the slit and the rectangular hole by the use of the planar process. By using a plasmon waveguide to guide light into the metallic slit aperture, a thin and efficient optical head for HAMR is achieved.

Performance of the effective index method in the modeling of nanoscale rectangular apertures in a real metal

We examine the performance of the widely used effective index method (EIM) in the modeling of a sub-wavelength rectangular hole in a real metal. Two different ways of applying the EIM, depending on the dimension (shorter or longer) considered first, are found to give very much different results for the modal characteristics of the modes guided by such structures. It is shown that the EIM gives accurate results only if one starts with the surface plasmon polariton (SPP) mode irrespective of shorter or longer dimension. The present findings should be useful in applying the EIM to analyze apertures of other shapes also.

Modeling of a nanoscale rectangular hole in a real metal

We propose and implement a simple and accurate method to analyze a subwavelength rectangular hole in a real metal and obtain the modal characteristics of its fundamental mode. Our results are found to be in excellent agreement with those reported in the literature, obtained by the effective index method (EIM) and finite-element and finite-difference methods. Unlike the EIM, the present method has no ambiguity in its implementation and is able to predict the major field components also, which may be useful in understanding the extraordinary transmission characteristics of such structures.

Lightwave Propagation in Rectangular Subwavelength Holes

In this paper, a new method is presented for calculating dispersion relations for waves with wavelengths of the order of hundreds of nanometers propagating in rectangular waveguides with imperfectly conducting walls.

Resonant light transmission through a nanohole in a metal film

The maximum wavelength at which light can propagate through a rectangular hole in a metal is dramatically increased due to the existence of surface-plasmon (SP) waves along the edges of the hole. For example, a 15-nm-wide hole in silver can have a cutoff wavelength more than double that of a perfect electric conductor. Furthermore, we show that there is a Fabry-Pe/spl acute/rot (FP) resonance for light transmission close to the cutoff wavelength, which gives a peak in the transmission. Impedance mismatch between the hole and free space provides the reflection required for the FP resonance. Due to the SPs in the hole, the reflection coefficient is typically larger in amplitude and has a smaller phase-shift than what has previously been observed in microwave systems. Our findings using analytic theory, finite-difference mode calculations, and finite-difference time-domain simulations agree well with recent experiments on the transmission through subwavelength rectangular holes. The transmission resonances found in a single subwavelength hole are of interest to nanolithography, biosensors, subwavelength microscopy, and photonic integrated circuits.