Subwavelength Holes Research Articles

Ultrathin plasmonic frequency selective surface with subwavelength hole arrays

ABSTRACTManipulation of optical transmission using plasmonic nanostructures is a significant issue in applications related to ultra‐compact optoelectronic devices. In this paper, we present a type of plasmonic frequency selective surface constructed with an ultrathin gold film (∼20 nm) with arrays of subwavelength holes. By means of modified nanosphere lithography, we fabricate the nanostructured gold films with scalable sizes and tunable transmission in the visible and near‐infrared range. Moreover, electromagnetic simulations are performed and demonstrate good agreement with experimental results and further reveal the evolution of optical properties from the optically thick film to the ultrathin film. The research achievements will provide significant design guidance of ultrathin plasmonic frequency selective surface for future nanophotonic devices. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:2171–2176, 2016

Computationally efficient analysis of extraordinary optical transmission through infinite and truncated subwavelength hole arrays.

The authors present a computationally efficient technique for the analysis of extraordinary transmission through both infinite and truncated periodic arrays of slots in perfect conductor screens of negligible thickness. An integral equation is obtained for the tangential electric field in the slots both in the infinite case and in the truncated case. The unknown functions are expressed as linear combinations of known basis functions, and the unknown weight coefficients are determined by means of Galerkin's method. The coefficients of Galerkin's matrix are obtained in the spatial domain in terms of double finite integrals containing the Green's functions (which, in the infinite case, is efficiently computed by means of Ewald's method) times cross-correlations between both the basis functions and their divergences. The computation in the spatial domain is an efficient alternative to the direct computation in the spectral domain since this latter approach involves the determination of either slowly convergent double infinite summations (infinite case) or slowly convergent double infinite integrals (truncated case). The results obtained are validated by means of commercial software, and it is found that the integral equation technique presented in this paper is at least two orders of magnitude faster than commercial software for a similar accuracy. It is also shown that the phenomena related to periodicity such as extraordinary transmission and Wood's anomaly start to appear in the truncated case for arrays with more than 100 (10×10) slots.

Dynamics of Strong Coupling between J‐Aggregates and Surface Plasmon Polaritons in Subwavelength Hole Arrays

A prototypical hybrid system formed by strong coupled gold hole arrays and J‐aggregate molecules is investigated by using both steady‐state spectroscopic method and ultrafast pump‐probe approach. In particular, the plasmonic response of the device has been tuned by modifying its periodicity thus to achieve the strongest possible coupling regime. It is found that in the transient absorption spectra, under upper band excitation, the bleaching signal from uncoupled J‐aggregate molecules completely disappears. Instead, two distinctive period dependent bleaching bands are formed, clearly fingerprint of the hybrid exciton‐plasmon state. The dynamics of these bands is also directly analyzed. A remarkable long lifetime is found especially for the upper band, corresponding to the presence of a trap state in its transient absorption spectra under resonance excitation. Such unique feature should provide a new approach to control quantum‐mechanical states under coherent coupling.

Cloaking of solar cell contacts at the onset of Rayleigh scattering.

Electrical contacts on the top surface of solar cells and light emitting diodes cause shadow losses. The phenomenon of extraordinary optical transmission through arrays of subwavelength holes suggests the possibility of engineering such contacts to reduce the shadow using plasmonics, but resonance effects occur only at specific wavelengths. Here we describe instead a broadband effect of enhanced light transmission through arrays of subwavelength metallic wires, due to the fact that, in the absence of resonances, metal wires asymptotically tend to invisibility in the small size limit regardless of the fraction of the device area taken up by the contacts. The effect occurs for wires more than an order of magnitude thicker than the transparency limit for metal thin films. Finite difference in time domain calculations predict that it is possible to have high cloaking efficiencies in a broadband wavelength range, and we experimentally demonstrate contact shadow losses less than half of the geometric shadow.

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.

Quantum selection rule dependent plasmonic enhancement in quantum dot infrared photodetectors

In this paper, we analyze quantum selection rules of intersubband transitions in quantum dots (QDs) and determine their impact on plasmonic enhancement in quantum dot infrared photodetectors (QDIPs). Photoluminescence and photocurrent spectrum measurement were performed on QD samples with different doping levels to identify the QD energy levels and associate the photodetection peaks with the intersubband transitions. The quantum selection rules of the intersubband transitions are determined by the electric-dipole interaction. To determine the impact of quantum selection rules on the plasmonic enhancement, we fabricated metallic two-dimensional subwavelength hole array (2DSHA) plasmonic structures with different periods on QDIPs for specific plasmonic enhancement of individual intersubband transitions. We found that the plasmonic enhancement ratios of different intersubband transitions are not the same. The unequal enhancement ratios are attributed to the quantum selection rules in the intersubband transitions and the dominant electric field (E-field E→) vectors induced by the 2DSHA plasmonic structure.

Photon transport through a nanohole by a moving atom

We have proposed and investigated for the first time an efficient way of photon transport through a subwavelength hole by a moving atom. The transfer mechanism is based on the reduction of the wave packet of a single photon due to its absorption by an atom and, correspondingly, its localization in a volume is smaller than both the radiation wavelength and the nanohole size. The scheme realizes the transformation of a single-photon single-mode wave packet of the laser light into a single-photon multimode wave packet in free space.

A broadband sound absorber based on lossy acoustic rainbow trapping metamaterials

Acoustic rainbow trapping (ART) metamaterials offer spatial-spectral modulation and intensive trapping of broadband sound, with strong acoustic dispersion that is absent in naturally occurring materials. However, thermal and viscous losses stemming from the acoustic boundary layers within narrow resonance regions of ART metamaterials has not been thoroughly studied and effectively utilized. Here, we would like to propose a lossy model of ART metamaterials. By taking advantage of the thermal and viscous losses, such lossy metamaterials can effectively work as a broadband sound absorber. A rigid surface perforated with very narrow subwavelength holes that can provide boundary-layer losses and have gradient depth along the propagation direction is constructed to mimic the lossy ART model. Full wave numerical simulation results indicate that the system's thermal and viscous losses are governed by the geometry of the resonance unit cells. Together with the intrinsic trapping effect, this lossy ART metamaterials function as a high performance broadband sound absorber. It may contribute to the optimization of sound absorption materials and wedge design of anechoic chambers.

Quasiperiodic air hole arrays for broadband and omnidirectional suppression of reflection

Surfaces patterned with quasi-periodic array of sub-wavelength air holes have been studied for their effectiveness in suppressing air-substrate reflection in the wavelength range of 450–1350 nm. Superlattice structures formed by superposing two different quasiperiodic arrays with 450 nm deep holes showed reflectance of ∼2% (compared to 6% for unpatterned substrate) for all measured incidence angles up to 50° and also show very weak polarization dependence. Dense k-space of quasiperiodic array along with the graded index offered by tapered holes provide broadband, polarization independent, and omnidirectional antireflection property.

Field localization control in aperture-based plasmonics by Boolean superposition of primitive forms at deep subwavelength scale

Aperture-based nanoplasmonics deals with an important class of structures with subwavelength hole arrays that support surface plasmons polaritons, thus ensuring control over propagation and localization of electromagnetic fields. The electromagnetic field localization in aperture-based plasmonic structures can be tailored by modifying the structure geometry at the deep subwavelength scale, in this way generating field hotspots in a controlled way. We defined subwavelength primitive objects and combined them in Boolean manner by applying logical operations like AND or OR to their shapes to generate complex forms and thus modify the subwavelength unit cell geometry. We designed our structures and simulated their scattering parameters by the finite element method and fabricated the experimental samples for the mid-wavelength infrared range using the conventional silicon-based planar technologies with gold and aluminum as plasmonic materials. We show that one can readily use the conventional, non-subwavelength photolithography to generate strong field nonlocalities without increasing the complexity of the system or requiring finer resolutions. The approach can be used for multispectral operation of plasmonic chemical or biological sensors.

Dynamic coupling of plasmonic resonators.

We clarify the nature of dynamic coupling in plasmonic resonators and determine the dynamic coupling coefficient using a simple analytic model. We show that plasmonic resonators, such as subwavelength holes in a metal film which can be treated as bound charge oscillators, couple to each other through the retarded interaction of oscillating screened charges. Our dynamic coupling model offers, for the first time, a quantitative analytic description of the fundamental symmetric and anti-symmetric modes of coupled resonators which agrees with experimental results. Our model also reveals that plasmonic electromagnetically induced transparency arises in any coupled resonators of slightly unequal lengths, as confirmed by a rigorous numerical calculation and experiments.

Hybrid Magnetoplasmonic Crystals Boost the Performance of Nanohole Arrays as Plasmonic Sensors

We present here a theoretical study that shows how the use of hybrid magnetoplasmonic crystals comprising both ferromagnetic and noble metals leads to a large enhancement of the performance of nanohole arrays as plasmonic sensors. In particular, we propose using Au–Co–Au films perforated with a periodic array of subwavelength holes as transducers in magneto-optical surface-plasmon-resonance sensors, where the sensing principle is based on measurements of the transverse magneto-optical Kerr effect. We demonstrate that this detection scheme may result in bulk figures of merit that are 2 orders of magnitude larger than those of any other type of plasmonic sensor. The sensing strategy put forward here can make use of the different advantages of nanohole-based plasmonic sensors such as miniaturization, multiplexing, and its combination with microfluidics.

MEMS Tunable Mid-Infrared Plasmonic Spectrometer

We present a microelectromechanical systems (MEMS) tunable metamaterial, Fabry-Perot interferometer with a widely tunable mid-infrared response. An array of subwavelength holes in a gold film is suspended above a gold reflector, forming an interferometer cavity whose length can be modulated over a range of 1.7 to 21.67 μm using MEMS electrostatic actuation. Reflectance spectra exhibit the convolution of extraordinary optical transmission through the holes and Fabry-Perot resonances with free spectral ranges from 2900 to 230.7 cm–1. Measuring the free spectral range enables us to perform in situ interferometric calibration of the cavity length. We present a simple analytical model that describes the experimental and simulated results. This device shows promise as a surface-enhanced sensing substrate with a tunable spectral response.

Optical properties of high-quality nanohole arrays in gold made using soft-nanoimprint lithography

We present a novel soft-nanoimprint procedure to fabricate high-quality sub-wavelength hole arrays in optically thick films of gold on glass substrates. We fabricate 0.5 × 0.5 mm2 structures composed of a square array of 180 nm-diameter holes with a 780 nm pitch. Optical angular transmission measurements on the arrays show clear extraordinary transmission peaks corresponding to the dispersion of surface plasmon polaritons propagating on either side of the metal film. The transmission features can be strongly controlled by engineering the dielectric environment around the holes. As the nanoimprint procedure enables fabrication of nanoscale patterns over wafer-scale areas at low cost, these imprinted metal nanoparticle arrays can find applications in, e.g., optical components, photovoltaics, integrated optics, and microfluidics.

Spoof surface plasmon polaritons in terahertz transmission through subwavelength hole arrays analyzed by coupled oscillator model.

Both the localized resonance and excitation of spoof surface plasmon polaritons are observed in the terahertz transmission spectra of periodic subwavelength hole arrays. Analyzing with the coupled oscillator model, we find that the terahertz transmission is actually facilitated by three successive processes: the incident terahertz field first initiates the localized oscillation around each hole, and then the spoof surface plasmon polaritons are excited by the localized resonance, and finally the two resonances couple and contribute to the transmission. Tailoring the localized resonance by hole size, the coupling strength between spoof surface plasmon polaritons and localized resonances is quantitatively extracted. The hole size dependent transmittance and the coupling mechanism are further confirmed by fitting the measured spectra to a modified multi-order Fano model.

Investigation of plasmon resonance tunneling through subwavelength hole arrays in highly doped conductive ZnO films

Experimental results pertaining to plasmon resonance tunneling through a highly conductive zinc oxide (ZnO) layer with subwavelength hole-arrays is investigated in the mid-infrared regime. Gallium-doped ZnO layers are pulsed-laser deposited on a silicon wafer. The ZnO has metallic optical properties with a bulk plasma frequency of 214 THz, which is equivalent to a free space wavelength of 1.4 μm. Hole arrays with different periods and hole shapes are fabricated via a standard photolithography process. Resonant mode tunneling characteristics are experimentally studied for different incident angles and compared with surface plasmon theoretical calculations and finite-difference time-domain simulations. Transmission peaks, higher than the baseline predicted by diffraction theory, are observed in each of the samples at wavelengths that correspond to the excitation of surface plasmon modes.

The dynamic process and microscopic mechanism of extraordinary terahertz transmission through perforated superconducting films.

Superconductor is a compelling plasmonic medium at terahertz frequencies owing to its intrinsic low Ohmic loss and good tuning property. However, the microscopic physics of the interaction between terahertz wave and superconducting plasmonic structures is still unknown. In this paper, we conducted experiments of the enhanced terahertz transmission through a series of superconducting NbN subwavelength hole arrays, and employed microscopic hybrid wave model in theoretical analysis of the role of hybrid waves in the enhanced transmission. The theoretical calculation provided a good match of experimental data. In particular, we obtained the following results. When the width of the holes is far below wavelength, the enhanced transmission is mainly caused by localized resonance around individual holes. On the contrary, when the holes are large, hybrid waves scattered by the array of holes dominate the extraordinary transmission. The surface plasmon polaritions are proved to be launched on the surface of superconducting film and the excitation efficiency increases when the temperature approaches critical temperature and the working frequency goes near energy gap frequency. This work will enrich our knowledge on the microscopic physics of extraordinary optical transmission at terahertz frequencies and contribute to developing terahertz plasmonic devices.

An extraordinary transmission analogue for enhancing microwave antenna performance

The theory of diffraction limit proposed by H.A Bethe limits the total power transfer through a subwavelength hole. Researchers all over the world have gone through different techniques for boosting the transmission through subwavelength holes resulting in the Extraordinary Transmission (EOT) behavior. We examine computationally and experimentally the concept of EOT nature in the microwave range for enhancing radiation performance of a stacked dipole antenna working in the S band. It is shown that the front to back ratio of the antenna is considerably enhanced without affecting the impedance matching performance of the design. The computational analysis based on Finite Difference Time Domain (FDTD) method reveals that the excitation of Fabry-Perot resonant modes on the slots is responsible for performance enhancement.

Heptad Phase Vortex Array in Extremely Deep Fresnel Diffraction Region Generated by Asymmetrical Metal Subwavelength Holes Film**Supported by the National Natural Science Foundation of China under Grant No 11574185, and the Science and Technology Development Program of Shandong Province under Grant No 2009GG10001005.

We report a heptad vortex array structure in the wave fields in an extremely deep Fresnel diffraction region by asymmetrical subwavelength holes in a metal film illuminated with linearly polarized light. A Mach–Zehnder interferometer with a microscopic objective is used to record the wave fields at different distances, and the phase maps are extracted by Fourier transform of the interference intensities. We study the evolutions of the heptad vortex array with distance from the sample to the object plane. To explain the formations and the evolutions of the vortex array, we calculate the diffracted wave fields with Kirchhoff's diffraction theory. The calculations are basically consistent with the experimental results, and the properties of the heptad vortex array structure are reasonably explained.

The acoustic magician hat: Broadband acoustic cloaking within a cavity with hard boundaries

This work reports the design, fabrication, and experimental validation of a broadband acoustic cloak for the concealing of three-dimensional (3D) objects placed inside an open cavity with arbitrary surfaces. This 3D cavity cloak represents the acoustic analogue of a magician hat, giving the illusion that a cavity with an object is empty. Transformation acoustics is employed to design this cavity cloak, whose parameters represent an anisotropic acoustic metamaterial. A practical realization is made of perforated layers fabricated by drilling subwavelength holes on 1-mm-thick Plexiglas plates. In both simulation and experimental results, concealing of the reference object by the device is shown for airborne sound with wavelengths between 10 cm and 17 cm.