Metallic Hole Research Articles

A complete multifluid model for bipolar semiconductors, with interacting carriers, phonons, and photons

If electrons (e) and holes (h) in metals or semiconductors are heated to the temperatures $$T_\mathrm{e}$$ and $$T_\mathrm{h}$$ greater than the lattice temperature, the electron–phonon interaction causes energy relaxation. In the non-uniform case a momentum relaxation occurs as well. In view of such an application, a new model, based on an asymptotic procedure for solving the kinetic equations of carriers, phonons, and photons, is proposed, which gives naturally the displaced Maxwellian at the leading order. Several generation–recombination (GR) events occur in bipolar semiconductors. In the presence of photons the most important ones are the radiative GR events, direct, indirect, and exciton-catalyzed. Phonons and photons are treated here as a participating species, with their own equation. All the phonon–photon interactions are accounted for. Moreover, carrier–photon (Compton) interactions are introduced, which make complete the model. After that, balance equations for the electron number, hole number, energy densities, and momentum densities are constructed, which constitute now a system of macroscopic equations for the chemical potentials (carriers), the temperatures (carriers and bosons), and the drift velocities (carriers and bosons). In the drift–diffusion approximation the constitutive laws are derived and the Onsager relations recovered, even in the presence of an external magnetic field.

New design of multi-band negative-index metamaterial and absorber at visible frequencies

A new negative-index metamaterial (NIM) structure is proposed by designing the metallic holes of traditional double-fishnet (DF) structures from uniform sizes to several different sizes. Numerical results demonstrate that the new metamaterial, as an improved variant of the DF structure, achieved a multi-band negative refractive index across a wide range of visible frequencies from 470 THz to 540 THz, which covers the red, orange, yellow, and green regions of the visible spectra. Meanwhile, a low-profile nanostrctured absorber was obtained when one side of the perforated metal layer of this multi-band NIM was substituted with a continuous metal film with the same thickness. The absorber showed the high absorption of more than 95% at multiple frequencies of 511, 520, 523, 525, and 527 THz. The behavior of multi-frequency response effectively broadened the working bandwidth. Finally, the physical mechanism of the multi-band operating characteristics of NIM and absorber was analyzed with the distributions of current intensity at different resonant frequencies.

Large area conductive nanoaperture arrays with strong optical resonances and spectrally flat terahertz transmission

Using simple and inexpensive nanosphere lithography, we produce large, centimeter-squared sized thin golden films patterned with a hexagonal array of nanoapertures with controllable dimensions on the order of 100–300 nm, spaced by a 350–375 nm pitch distance. The optical transmission spectra of our samples are dominated by the resonant plasmonic features in the spectral range 500–700 nm, caused by the nanostructure in the film. At the same time, the transmission at terahertz (THz) radiation is as high as ∼10% and is spectrally flat. Our measurements are in agreement with finite difference time domain simulations. Such thin metal hole array films allow for very efficient injection of optical energy, while at the same time maintaining reasonably high transparency of THz waves. These structures can be used in any application combining strong optical sensitivity and THz transparency, in optical biomolecular sensing, or as optically transparent electrodes.

Angle-Dependent Spoof Surface Plasmons in Metallic Hole Arrays at Terahertz Frequencies

We demonstrate the angle dependence of spoof surface plasmon on metallic hole arrays in terahertz regime. By varying polar angle and azimuthal angle, we have observed frequency shifting and splitting of resonance modes. The amplitude of extraordinary optical transmission also shows angle dependence and exhibits mirror-image or translational symmetries.

Angle resolved transmission through metal hole gratings.

We present the first angle resolved measurements of extraordinary optical transmission (EOT) through hole array gratings in a gold film. Varying the lattice spacing of the arrays and looking at higher diffraction orders, we retrieve the angular emission pattern of the constituent holes with better signal to noise ratio than with single-hole experiments. We present a method to determine separately the angular dependence of the direct and resonant contribution to EOT by using the spectral features of the diffraction orders together with an established model. The comparison of our results with the known angular transmission of a single hole in a metal film yields a good agreement for s-polarized light. Deviations are found for illumination with p-polarized light and we address the discrepancy with Coupled Mode Model calculations and Finite Difference Time Domain simulations. These measured deviations are currently not fully understood.

Fabricating a regular hexagonal lattice structure by interference pattern of six femtosecond laser beams

Interference of six countering femtosecond (fs) laser beams at a wavelength of 785nm has been utilized to fabricate nanostructures in a regular hexagonal lattice. A diffractive-optical element for six-beam splitting was introduced to a beam correlation system. The lattice structure was in accordance with the simulated structure based on the principle of superposition of electric fields. The unit structures fabricated on gold thin films were nanobit, nanodrop, and metallic hole array. The height and diameter of a representative nanodrop were 450 and 210nm, respectively. Molten structures such as nanodrops are believed to have been fabricated via a solid–liquid–solid (SLS) mechanism, as in the case of previous experiments using four beams. In addition, multi-shot processing is examined to fabricate through-holes at lower fluences.

太赫兹波段金属矩形孔高通滤波器

太赫兹波段金属矩形孔高通滤波器

Retraction: Effect of localized surface plasmon and surface plasmon polariton modes on properties of metamaterials

We present the effects of localized surface plasmon (LSP) and surface plasmon polariton (SPP) modes on transmission peaks and resonance frequency of a compound structure metamaterial consisting of periodic metallic particle and hole arrays. During simulation, the lateral interval (Lx) between the metal particle and hole arrays is increased but the longitudinal interval (L) remains unchanged. Simulated results indicate that the main transmittance peak is enhanced and the resonant frequency red-shifted when Lx is increased, while the high-frequency transmittance peak is reduced and its resonance frequency is unchanged. We find that the increase in the intensity of SPP modes leads to the variation of the main transmittance peak, and the increase of the intensity of LSP modes results in the reduction of the high-frequency transmittance peak with increasing Lx.

多重格子金属開口アレイのテラヘルツ領域での共鳴透過の結合モード法による解析

多重格子金属開口アレイのテラヘルツ領域での共鳴透過の結合モード法による解析

Widely Tuning Surface Plasmon Polaritons with Laser‐Induced Bubbles

The wide‐range tunable all‐optical plasmonic devices are of importance for the multifunctional photonic circuits of high densities. Here, the laser‐induced bubbles are utilized to widely tune the surface plasmon polaritons (SPPs) in the metallic hole array. Because of the bubbles generation under a pump beam, the equivalent refractive index of the dielectric medium adjacent to the metal surface decreases. This decrease blue‐shifts the transmission spectrum of the hole array. Experimentally, a modulation depth of 18 dB below the critical pump intensity and an index variation of Δn = −0.333 corresponding to a wavelength shift of about Δλ = −200 nm above the critical pump intensity are achieved. Moreover, the transmission intensity at λ = 870 nm can be tuned to cover the whole range from the peak intensity to the valley intensity of the hole array by controlling the size of the bubble. Such a large index variation (Δn = −0.333) and wavelength change (Δλ ≈ −200 nm) can greatly decrease the designing difficulty of the tunable plasmonic devices. Thus, the laser‐induced bubbles under low pump intensities may find important applications, such as efficient modulating as well as tunable color filter, focusing, and imaging with broadband responses.

Size Effect and Aperture Configuration Dependence of Extraordinary Optical Transmission through Penrose Metal Hole Arrays in the Terahertz Region

SUMMARYExtraordinary optical transmission (EOT) through Penrose metal hole arrays (PMHA) in the terahertz region is investigated numerically by using coupled‐mode analysis. Spectral change of the transmittance is studied at resonant frequencies by increasing the number of holes and comparing the spectra of PMHA and a periodic metal hole arrays (MHA). While the resonant mode of MHA is excited in periodic order along one direction, the resonant mode on PMHA has a complicated shape. It seems that the propagating length of surface wave of PMHA is shorter than that of the MHA. Therefore, when the number of holes is increased, increasing rate of the resonant transmittance through PMHA is smaller than that through MHA. In addition, the transmission spectra for three kinds of generalized PHMA are analyzed numerically. Although the distributions of holes in these PMHA are different from each other in real space, the transmission spectra have almost the same shape. Consequently, the structure given by the aperture configuration of PMHA in reciprocal lattice space has stronger effects on EOT through metal hole arrays, rather than that in real space.

Denitrification in an integrated bioelectro-photocatalytic system

Since nitrate causes severe ecological and health risks, nitrate contamination of drinking water sources has become one of the most important water quality concerns all over the world. Photocatalytic reduction of nitrate to molecular nitrogen presents a promising approach to remove nitrate from drinking water sources. However, harmful intermediates like NO2−, NO, NO2 and N2O are usually formed, and metal loading or hole scavengers are generally needed to reduce the recombination of photo-generated electrons and holes, which will cause secondary pollution to drinking water. In this work, an efficient, selective and sustainable bioelectro-photocatalytic nitrate-reducing system by utilizing commercial TiO2 nanoparticles P25 as the photocatalyst and bio-electrons from microbial metabolism as the hole scavenger is reported. In this system, bio-electrons extracted from organic substrates in bioanode are transferred to the photocathode through an external circuit for hole quenching. With the utilization of the residual photogenerated electrons, nitrate is completely reduced to nitrogen without accumulation of harmful nitrite or ammonium. The experimental results and the mechanistic analysis using the first-principles density functional theory calculations demonstrate that toxic by-products like nitrite or ammonium will not be accumulated in this system. Thus, this approach has a great potential for sustainable remediation of nitrate-contaminated drinking water sources.

Novel improved metamaterial transmission line and its application in wideband leaky-wave antenna with wide beam-scanning angle range and low cross-polarization

In this paper, a novel-improved metamaterial transmission line (MTM TL) is investigated and applied to design periodic leaky-wave antenna (LWA) with wide beam-scanning angle range and low cross-polarization. The improved MTM TL consists of metallic holes and interdigital fingers (which embedded in the main microstrip line). By introducing the metallic holes loaded in the interdigital fingers, the parasitic resonant modes of conventional MTM TL can be eliminated effectively. Considering the continuous phase constants changing from negative to positive values of the improved MTM TL under the balanced condition, a continuous beam-scanning property from backward to forward directions of the resultant periodic LWA can be obtained. For verification, a periodic LWA, which consists of 20 unit cells of the improved MTM TL, is fabricated and measured. The measured and simulated results are in good agreements with each other in all condition. According to the measured results, the fabricated periodic LWA operates from 3.38 to 4.60 GHz with a bandwidth of 30.57%, and achieves a continuous beam-scanning property from backward –65° to forward +71° (including broadside radiation) over the operating frequency band. Moreover, the measured cross-polarization remains at a level of more than 30 dB below the co-polarization across the entire radiation region. The presented LWA should find promising applications in modern wireless communication systems due to these wonderful electromagnetic performances.

A Low-loss Metasurface Antireflection Coating on Dispersive Surface Plasmon Structure.

Over the years, there has been increasing interest in the integration of metal hole array (MHA) with optoelectronic devices, as a result of enhanced coupling of incident light into the active layer of devices via surface plasmon polariton (SPP) resonances. However, not all incident light contributes to the SPP resonances due to significant reflection loss at the interface between incident medium and MHA. Conventional thin-film antireflection (AR) coating typically does not work well due to non-existing material satisfying the AR condition with strong dispersion of MHA’s effective impedances. We demonstrate a single-layer metasurface AR coating that completely eliminates the refection and significantly increases the transmission at the SPP resonances. Operating at off-resonance wavelengths, the metasurface exhibits extremely low loss and does not show resonant coupling with the MHA layer. The SPP resonance wavelengths of MHA layer are unaffected whereas the surface wave is significantly increased, thereby paving the way for improved performance of optoelectronic devices. With an improved retrieval method, the metasurface is proved to exhibit a high effective permittivity () and extremely low loss (tan δ ~ 0.005). A classical thin-film AR coating mechanism is identified through analytical derivations and numerical simulations.

Conversion of the optical orbital angular momentum in a plasmon-assisted second-harmonic generation

We experimentally demonstrate the plasmon-assisted second-harmonic generation of an optical orbital angular momentum (OAM) beam. Because of the shape resonance, the plasmons in a periodic array of rectangular metal holes greatly enhance the nonlinear optical conversion of an OAM state. The OAM conservation (i.e., 2l1 = l2 with l1 and l2 being the OAM numbers of the fundamental and second-harmonic waves, respectively) holds well under our experimental configuration. Our results provide a potential way to realize nonlinear optical manipulation of an OAM mode in a nano-photonic device.

One-way extraordinary optical transmission with dielectric/perforated metal/antiferromagnetic structure in the terahertz range

The one-way optical effect in a dielectric/perforated metal/antiferromagnetic (DPMA) sandwich structure in the terahertz range is studied. Theoretical modeling, which is based on the effective medium theory and the transmission matrix method, is conducted. The simulation results show that the one-way effect occurs within a wide frequency range and for various incident angles with a fixed external magnetic field. In addition, this behavior is examined for various DPMA structural characteristics, such as different dielectric materials, film thicknesses, and perforated metal hole sizes and spacings. The developed sandwich structures can aid the design of subminiature optical isolators for application in the terahertz range.

3D conductive coupling for efficient generation of prominent Fano resonances in metamaterials.

We demonstrate a 3D conductive coupling mechanism for the efficient generation of prominent and robust Fano resonances in 3D metamaterials (MMs) formed by integrating vertical U-shape split-ring resonators (SRRs) or vertical rectangular plates along a planar metallic hole array with extraordinary optical transmission (EOT). In such a configuration, intensified vertical E-field is induced along the metallic holes and naturally excites the electric resonances of the vertical structures, which form non-radiative “dark” modes. These 3D conductive “dark” modes strongly interfere with the “bright” resonance mode of the EOT structure, generating significant Fano resonances with both prominent destructive and constructive interferences. The demonstrated 3D conductive coupling mechanism is highly universal in that both 3D MMs with vertical SRRs and vertical plates exhibit the same prominent Fano resonances despite their dramatic structural difference, which is conceptually different from conventional capacitive and inductive coupling mechanisms that degraded drastically upon small structural deviations.

Experimental demonstration of a dual-band metamaterial absorber

We present the design, simulation and fabrication of a dual-band metamaterial absorber. The designed structure consists of periodic composite metallic holes array and dielectric layer. The availability of absorption enhancement is verified by our measured results. Cavity and electrical resonances lead to these two absorption peaks at λ1=1.8μm and λ2=4.3μm. Effects of structural parameters on absorption and resonant wavelengths have been experimentally surveyed. The average absorption can be increased by optimizing the structural parameters of the designed metamaterial absorber.

Survival of the orbital angular momentum of light through an extraordinary optical transmission process in the paraxial approximation.

We report on the extraordinary optical transmission (EOT) of an orbital angular momentum (OAM) state of light in the paraxial approximation. The OAM state transmits through a subwavelength metal hole array with a square structure, and then is analyzed by using a Mach-Zehnder interferometer. In the experiment, the transmitted light well conserves the OAM information while the transmission efficiency of the OAM mode is much greater than 1 (i.e., EOT works). Further study shows that the OAM mode has no significant influence on the transmission spectrum of the EOT paraxial process under our experimental configuration. Our work can be useful for future plasmon-based OAM devices.

Measurement of the Phase and Intensity Profile of Surface Plasmon Laser Emission

We study the near- and far-field radiation patterns of surface plasmon (SP) lasers in metal hole arrays and observe radially polarized vortex-vector laser beams in both near and far field. Besides the intensity profile, also the complementary phase profile is obtained with a beam block experiment, where we block part of the beam in the near field, measure the resulting changes in the far field, and retrieve the phase using an iterative algorithm. This phase profile provides valuable information on the feedback mechanisms and coherence of the laser and shows that our SP laser operates in a phase-slip mode instead of a pure dark mode. To explain our observations, we extend the standard model for distributed feedback lasers by introducing a position dependence in the optical gain and refractive index.