2D Subwavelength Research Articles

Terahertz Magnetic Moiré Grating for Enhanced Chirality Manipulation and Nonreciprocal Spin Isolation

AbstractMagneto‐optical activity is a significant way to achieve asymmetric transmission for spin photons. However, it is severely limited by the weak light‐matter interaction in the terahertz (THz) region. In this work, a magnetic Moiré grating (MMG) is constructed by sandwiching an Yttrium iron garnet (YIG) single crystal in a pair of twisted 2D subwavelength gratings. The twisted angle between two gratings introduces a strong geometric Moiré chirality related to the Moiré pattern. When an external magnetic field is applied to the YIG layer, three different symmetries of spacial mirror image, time reversal, and spin conjugation are broken simultaneously under the chiral coupling between magneto‐optical activity and Moiré chirality, leading to both the enhanced magneto‐optical chiral transmission and the nonreciprocal spin isolation. Under the magnetic field varying from −0.26 to +0.26 T in the experiment, the MMG achieves an active modulation of circular dichroism from 39 to −37.5 dB and optical activity from −35° to 30°. Moreover, the maximum spin isolation over 32.6 dB is also obtained near 0.5 THz. This mechanism provide a new way to realize independent manipulation and active modulation of THz spin photons more flexibly.

Rigorous optical modelling of long-wavelength infrared photodetector with 2D subwavelength hole array in gold film

Rigorous optical modelling of long-wavelength infrared photodetector with 2D subwavelength hole array in gold film

High-Efficiency Unidirectional Dual-Wavelength-Band Grating Coupler With 1D Subwavelength Structure

High-Efficiency Unidirectional Dual-Wavelength-Band Grating Coupler With 1D Subwavelength Structure

Wide band UV/Vis/NIR blazed-binary reflective gratings for spectro-imagers: two lithographic technologies investigation

We report on subwavelength reflective gratings for hyperspectral applications operating in a very large spectral band (340–1040 nm). Our study concerns a blazed-binary grating having a period of 30 μm and composed of 2D subwavelength structures with size from 120 nm to 350 nm. We demonstrate the manufacturing of the gratings on 3″ wafers by two lithography technologies (e-beam and nanoimprint) followed by classical dry etching process. Optical measurements show that the subwavelength grating approach enables a broadband efficiency, polarization behaviour and wavefront quality improvement with respect to the requirements for the next generation of spectro-imagers for Earth observation missions. An outlook towards spherical substrate based on nanoimprint lithography is also reported with the results of mixed features replication (holes and pillars in the range of 160–330 nm) on a 540 mm concave substrate which demonstrate uniformity and accuracy capabilities over 3″ surface.

Semi-analytical model for the analysis of a magnetically biased 1D subwavelength graphene-strip-grating

We propose a simple semi analytical model that allows to compute the transmittance and reflectance of a one dimensional subwavelength graphene strip grating under an external static magnetic field. In this model graphene is treated as an anisotropic layer with atomic thickness and a frequency dependent complex permittivity tensor. The model is based on an effective medium approach (EMA) and a rigorous phase correction. The scattering matrix approach is also used to take into account the different resonant phenomena occurring in the structure. The approach is validated against the Polynomial Modal Method (PMM) through numerical examples.

Semi-analytical model for the analysis of a magnetically biased 1D subwavelength graphene-strip-grating.

Semi-analytical model for the analysis of a magnetically biased 1D subwavelength graphene-strip-grating.

A Hybrid Synthetic Aperture and Time-Reversal MUSIC Algorithm for Subwavelength Radar Imaging

A promising chipless RFID approach uses millimeter-wave synthetic aperture radar (SAR) to image metal ink-printed ID tags from a meter or more away. Due to printing cost, it is desirable to minimize the size and spacing of metal patches within a tag, preferably into the subwavelength regime. Although circular SAR (CSAR) has a sharply peaked point response in 2D, its side lobes of closely-spaced targets interfere strongly with each other to distort the image. An alternative 2D subwavelength imaging approach with minimal side lobes is Time-Reversal MUSIC (TR-MUSIC). Traditional TR-MUSIC, however, requires a large number of transmitters and receivers. We propose a hybrid synthetic aperture TR-MUSIC algorithm (SATR-MUSIC) that combines the benefits of both approaches. Using relatively few transceivers, SATR-MUSIC is able to resolve objects separated by approximately in 2D with minimal background artifacts. It does so by averaging TR-MUSIC’s imaging kernel incoherently over the synthetic aperture.

Ultranarrow and Wavelength-Scalable Thermal Emitters Driven by High-Order Antiferromagnetic Resonances in Dielectric Nanogratings.

Engineering wavelength-selective thermal emission is a promising technology associated with several advanced applications, including thermal imaging, gas sensing, far/near-field thermophotovoltaics, and so on. However, the majority of reported approaches suffer from low Q-factor emission due to intrinsic loss of metallic components or rely on thick structures like multilayers to ensure unitary emissivity, making it challenging to design compatible high-Q narrowband emitters. In this work, we propose a mechanism to tailor thermal emission by taking advantage of optically induced high-order antiferromagnetic (AFM) resonances in a simple subwavelength 2D Si nanobar. Such AFM modes, stemmed from hybrid magnetic dipoles and high-order Fabry-Perot modes, exhibit both pronounced resonant responses and superior light confinement ability. We first reveal its essential roles in ultranarrowband emission control with a sharp (Q ∼ 400) and near-perfect emissivity available. Especially, the measured angle-resolved emission spectra further indicate that the AFM-induced emission peak, being nearly immune to changes of nanogratings' periods and incident angle, is able to be flexibly engineered in a wide waveband by merely tuning the width-to-height ratio of nanobars. Our work provides a promising strategy to design extremely high-Q thermal emitters possessing robust narrowband performance, large spectral tunability and desirable compatibility with advanced planar nanofabrication techniques, which will be more favorable in practice compared with metallic counterparts. Besides, we anticipate that, the revealed mechanism of high-order AFM modes can also stimulate advanced applications in diverse research communities including but not limited to multipolar physics, nonlinear nano-optics, energy harvesting, etc.

2D beam shaping via 1D spatial light modulator using static phase masks.

Many emerging, high-speed, reconfigurable optical systems are limited by routing complexity when producing dynamic, two-dimensional (2D) electric fields. We propose a gradient-based inverse-designed, static phase-mask doublet to generate arbitrary 2D intensity wavefronts using a one-dimensional (1D) intensity spatial light modulator (SLM). We numerically simulate the capability of mapping each point in a 49 element 1D array to a distinct $7 \times 7$ 2D spatial distribution. Our proposed method will significantly relax the routing complexity of electrical control signals, possibly enabling high-speed, sub-wavelength 2D SLMs leveraging new materials and pixel architectures.

Wide band UV/Vis/NIR blazed-binary reflective gratings: two lithographic techniques investigation

We report on subwavelength reflective gratings for hyperspectral applications operating in the 340 nm-1040 nm spectral range. The blazed grating period is 30 μm and is composed of 2D subwavelength binary structures with sizes in-between 120 nm and 350 nm. We demonstrate the manufacturing of gratings on 3” wafers by two lithography technologies (e-beam or nanoimprint) followed by dry etching process. These subwavelength gratings enable broadband efficiency which is in average 15%-20% above the efficiency requirement for next generation of spectro-imagers for Earth observation missions and a wavefront error that is much smaller than the 100 nm requirement for space application.

Three-Dimensional Photoengraving of Monolithic, Multifaceted Metasurfaces.

Metasurfaces present a potent platform to manipulate light by the spatial arrangement of sub-wavelength patterns with well-defined sizes and geometries, in thin films. Metasurfaces by definition are planar. However, it would be highly desirable to integrate metasurfaces with diverse, spatially programmed sub-wavelength features into a 3D monolith, to manipulate light within a compact 3D space. Here, a 3D photoengraving strategy is presented; that is, generation of such composite metasurfaces from a single microstructure via the irradiation of multiple interference laser beams onto different facets of the parent azopolymeric microstructure. Through "photofluidization,"thistechnique enables independent inscription and erasing of metasurfaces onto and from individual facets of 3D monoliths with arbitrary shapes and dimensions, in a high-throughput fashion (over approximately a few cm2 at a time). By engraving discrete sub-wavelength 1D surface relief gratings of different pitches on different facets of an inverse pyramidal array, a multiplexing structure-color filter is demonstrated.

A tunable plasmonic resonator using kinetic 2D inductance and patch capacitance

We have studied the microwave response of a high-mobility two-dimensional electron system (2DES) contacted by two side electrodes. Using kinetic inductance of the 2DES and inter-electrode capacitance, we have constructed a subwavelength 2D plasmonic resonator. We have shown that the resonant frequency of this circuit can be controlled by 2D electron density, external magnetic field, or size of the electrodes. This opens up possibilities for using arrays of plasmonic circuits as tunable components in different frequency ranges.

Electrically tunable polarization‐independent visible transmission guided‐mode resonance filter based on polymer‐dispersed liquid crystals

AbstractA tunable polarization‐independent narrowband guided‐mode resonance filter (GMRF), consisting of a 2D subwavelength aluminum grating and a polymer‐dispersed liquid crystal (PDLC) layer is demonstrated. By taking advantage of 2D structural phase matching, the guided mode resonance occurs under both transverse electric and magnetic guided modes, we obtained polarization‐independent bandpass filtering without loss of polarization, as verified via finite‐difference time‐domain simulations. Meanwhile, we use PDLC as a property adjustable material, when the orientation of the PDLC was controlled with an applied voltage, the transform mode of the incident light changed, which enabled easy refractive index tunability. A finite element method was used to design and characterize a tunable GMRF with a transmission resonance wavelength that can be shifted from 492.31 to 505.94 nm, based on the PDLC refraction. The relationship between resonance wavelength and the PDLC refractive index exhibits a linear curve, the shape and peak of the resonance transmission curve is stable during adjustments. With the design of this structure parameter, the GMRF transmission efficiency was over 82% in the visible band.

Tailoring Dispersion of Room-Temperature Exciton-Polaritons with Perovskite-Based Subwavelength Metasurfaces.

Exciton-polaritons represent a promising platform for studying quantum fluids of light and realizing prospective all-optical devices. Here we report on the experimental demonstration of exciton-polaritons at room temperature in resonant metasurfaces made from a sub-wavelength two-dimensional lattice of perovskite pillars. The strong coupling regime is revealed by both angular-resolved reflectivity and photoluminescence measurements, showing anticrossing between photonic modes and the exciton resonance with a Rabi splitting in the 200 meV range. Moreover, by tailoring the photonic Bloch mode to which perovskite excitons are coupled, polaritonic dispersions are engineered exhibiting linear, parabolic, and multivalley dispersions. All of our results are perfectly reproduced by both numerical simulations based on a rigorous coupled wave analysis and an elementary model based on a quantum theory of radiation-matter interaction. Our results suggest a new approach to study exciton-polaritons and pave the way toward large-scale and low-cost integrated polaritonic devices operating at room temperature.

Periodic Ultranarrow Rods as 1D Subwavelength Optical Lattices

We report on ground state properties of a one-dimensional, weakly-interacting Bose gas constrained by an infinite multi-rods periodic structure at zero temperature. We solve the stationary Gross-Pitaevskii equation (GPE) to obtain the Bloch wave functions from which we give a semi-analytical solution for the density profile, as well as for the phase of the wave function in terms of the Jacobi elliptic functions, and the incomplete elliptic integrals of the first, second and third kind. Then, we determine numerically the energy of the ground state, the chemical potential and the compressibility of the condensate and show their dependence on the potential height, as well as on the interaction between the bosons. We show the appearance of loops in the energy band spectrum of the system for strong enough interactions, which appear at the edges of the first Brillouin zone for odd bands and at the center for even bands. We apply our model to predict the energy band structure of the Bose gas in an optical lattice with subwavelength spatial structure. To discuss the density range of the validity of the GPE predictions, we calculate the ground state energies of the free Bose gas using the GPE, which we compare with the Lieb-Liniger exact energies.

Design and fabrication of 1[formula omitted]N polarization-insensitive beam splitters based on 2D subwavelength gratings

Based upon the wave front control of transmitted light using 2D high index contrast subwavelength gratings, a kind of 1×N polarization-insensitive beam splitters are proposed and demonstrated, which could redistribute the incident light into N beams while each of them converges to a spot respectively in the relevant subregion. 1×3 and 1×4 beam splitters as typical instances are designed applying the rigorous coupled-wave analysis (RCWA) results, of which the characterizations are tested by numerical simulation and the samples are fabricated on a silicon-on-insulator (SOI) wafer. Both numerical and experimental results demonstrate that good splitting ability without polarization sensitivity can be achieved while simultaneously maintaining high transmissivity under normal incidence at a wavelength of 1.55μm.

Femtosecond laser induced periodic surface structures for the enhancement of field emission properties of tungsten

Direct femtosecond laser ablation enables the maskless fabrication of nano- and micro-scale structures on variety of materials. A typical example is the formation of femtosecond Laser Induced Periodic Surface Structures (fs-LIPSSs), which can lead to strong modification in electrical, optical, wetting, and field emission properties of materials. Here, we study the field emission properties of fs-LIPSSs. We created fs-LIPSSs and fs-LIPSSs covered with nano- and micro-scale structures at different laser fluences on Tungsten (W) and showed that these structures offer significant enhancement in electron field emission properties. We provide a phenomenological model to explain the enhancement of electron emission parameters. The enhancement in the field emission properties of laser irradiated W is explained based on the convergence of electric field lines at the ridges of the fs-LIPSSs and fs-LIPSSs covered with nanoscale structures, which in turn, enhances the local field intensity and the electron emission parameters. The direct fabrication of 1D subwavelength structures is an important step towards the creation of low-cost cathodes for various potential applications.

Subwavelength Field Shaping Approach Based on Time Reversal Technique and Defective Metasurfaces

The resolution limitation is a main problem in the time-reversed field shaping system. In this paper, a subwavelength field shaping approach is proposed by combining time reversal (TR) technique and defective resonant metasurfaces. The subwavelength 2D and 3D field patterns can be effectively realized in both single-target and multi-target field-shaping systems. The resolution in both single-target and multi-target field-shaping systems aided with defective metasurfaces is approximately $0.1~\lambda $ , which is corresponding to the dimension of the applied metasurface cell. The proposed technique can be used for wireless power transfer, industrial heating, and localized wireless communications.

Graphene surface plasmon off-axis superlens based on tilted one-dimensional Si/SiO2 gratings

Graphene surface plasmon (GSP) superlenses, induced from the negative refraction, have recently been demonstrated in various two-dimensional photonic crystal systems. However, inplane GSP superlenses have never been reported in a one-dimensional (1D) photonic crystal system. Here, we propose a graphene-Si/SiO2 system, by transferring a graphene sheet on the tilted 1D subwavelength silicon/silica gratings. By discussing the dispersion relations of the inplane GSP in this system, the GSP negative refraction is found in the mid-infrared region. When the tilted angle, working wavelength, and Fermi level are set to be 60°, 11.22μm, and 0.2eV, respectively, the off-axis subwavelength focusing has the best resolution, and the full width at half-maximum (FWHM) of the image is 0.0091λ (102.1nm). Further, we investigate the effects of the Fermi level on the superlens frequency range, and the image's FWHM, the broadband, and the deep subwavelength superlens are achieved. The full-wave numerical simulations are conducted by the finite element method. Our findings can be applied to the manipulation of inplane GSP propagation and biological imaging.

Optical focusing based on the planar metasurface reflector with application to trapping cold molecules

We demonstrate theoretically a 2D subwavelength silicon-grating reflector with strong focusing capability and the potential application to an optical dipole trap of cold molecules such as MgF. We study the dependence of the focusing properties of this reflector on its structural parameters, numerical aperture, and fabrication-error tolerance. Our study shows that the reflector delivers high reflectivity and strong focusing performances with the maximum intensity at the focal point over 200 times the incident one. Such a focusing field on the reflector can provide a deep potential to trap cold MgF molecules from a standard magneto-optical trap.