Polarization-independent Broadband Research Articles

All-silicon polarization-independent broadband achromatic metalens designed for the mid-wave and long-wave infrared.

Metasurfaces demonstrate excellent capabilities in manipulating the phase, amplitude and polarization of light. Metalens, as a typical kind of metasurface devices, shows great prospect in simplifying imaging systems. However, like diffractive optical elements, intrinsic dispersion of metasurfaces is high. Thus, significant chromatic aberration is present in common metalenses, deteriorating imaging quality under broadband illumination condition and limiting their applications. To tackle this problem, broadband achromatic metalenses have been proposed and demonstrated in the visible and near-infrared wavelength regions so far. However, broadband achromatic metalens working in the mid-wave and long-wave infrared is still rare. In this paper, thanks to the ingenious design of meta-units that provide the required local phase and phase dispersion, several all-silicon broadband achromatic metalenses working in the mid-wave infrared (3-5 µm) or long-wave infrared (8-14 µm) wavelengths are proposed. Numerical simulation results demonstrate that the designed broadband achromatic metalenses can provide a near-constant focal length with small deviations and an average focusing efficiency of about 70% over the whole operation bandwidths. In addition, these metalenses hold near diffraction-limited focusing capability and polarization-independent focusing features. The achromatic metalenses proposed here are beneficial for improving imaging quality under broadband illumination and increasing detection efficiency of mid-wave and long-wave infrared detection systems.

A switchable polarization-independent broadband GST-based metasurface infrared absorber in modulating applications

A switchable polarization-independent broadband GST-based metasurface infrared absorber in modulating applications

A functionality switchable meta-device: from perfect reflection to perfect absorption

Tunable meta-devices trigger immense interest in many applications like biosensing, thermal camouflage, radiative cooling, thermal emitting, infrared imaging, and energy harvesting. This paper presents a polarization-independent broadband tunable absorbing and reflecting meta-device using a phase change material Germanium-Antimony-Tellurium (GST). The proposed design manifests high absorptivity >80% over a broadband range of wavelength spanning the mid-IR regime. Furthermore, by changing the phase of GST material from crystalline to amorphous with the help of external stimuli, the absorbance of the device was reduced to almost zero in the range from 4 μm to 12 μm. To previse the angular constancy of the meta-absorber, the absorptivity was scrutinized under different incidence angles in both transverse magnetic (TM) and transverse electric (TE) polarizations which manifested the robustness of the proposed meta-absorber. As a result, the proposed absorber has tunable broadband absorbance and angular stability, which proclaim great potential in applications such as thermal camouflage, radiation detection, thermal emitter, and stealth technology, etc.

High performance infrared selective emissivity film tailored for thermal-stable camouflage

A polarization-independent broadband infrared selective absorber/emitter (ISAE) based on multilaminar architecture is proposed and demonstrated. The salient features are that it has both low average emissivity of less than 0.1 in two atmospheric windows (3–5μm and 8–14μm) and otherwise high average emissivity of more than 0.8 in two non-atmospheric windows (2.5–3μm and 5–8μm), which renders it tailored infrared camouflage performance with thermal stability. Meanwhile, there is sharp narrowband absorption around at 10.6μm, which allows it to additionally possess laser camouflage performance. The comprehensive dependence of multispectral selective emissivity properties on the structural parameters, the polarization and incident angle of incoming excitation are analyzed and the underlying physical mechanisms are explored. It is found that the selective absorption/emissivity in band 5–8μm is originated from the fundamental mode plasmonic resonances, while that in 2.5–5μm is originated from the high-order hybrid mode plasmonic resonances therein. Meanwhile, there does exist a coupled competition effect between the hybrid modes in 2.5–3μm and in 3–5μm. All the results construct the basic guideline for designing these kinds of ISAE materials. Furthermore, we reexamine the physical essence of infrared camouflage based on multispectral bands selective emissivity with thermal management and establish an optimized generalized method for evaluating the camouflage performance of ISAE. The proposed ISAE proves to have much better infrared camouflage property throughout 2.5–14μm than the existing designs reported. The proof-of-principle ISAE is prepared and the selective emission spectrum is characterized, which is in good agreement with the simulations.

Efficient solar absorber based on VO2-W-SiO2 hybrid metamaterials

The broad spectrum near unity solar absorber shows important significance in the solar energy conversion. In this work, a polarization independent broadband and wide-angle metamaterials solar absorber consisted of VO2 periodic arrays and alternately stacked SiO2/W layers is designed. To explore the absorption of solar radiation and proper geometric parameters, numerical studies are carried out on the proposed structure. And the absorption of the designed structure is above 90% in the spectra range of 300-2000 nm with an average absorption of 95%. Moreover, the proposed structure possesses excellent solar absorptivity and insensitivity to the incident angle and polarization of sunlight. The average integrated absorption remains 90% with the light incident angle of 50°. Hence, the proposed broadband solar absorber has potentially valuable applications in thermophotovoltaic conversion, thermal emitters, solar cell, and so on.

Polarization-Independent Broadband Angular Selectivity Based on Anisotropic Diamagnetic Metamaterial

Achieving broadband angular selectivity has been attracting much interest from both scientists and engineers in recent years. Previous experimental demonstrations have been limited to the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$p$ </tex-math></inline-formula> -polarized incidence. Here, we propose a universal approach to achieve polarization-independent broadband angular selectivity by introducing electrically and magnetically anisotropic diamagnetic medium into multiple 1-D photonic crystal stacks. Such stacks have a broadband angular photonic bandgap for both <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$s$ </tex-math></inline-formula> - and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$p$ </tex-math></inline-formula> -polarized waves. The anisotropic diamagnetic medium is artificially constructed with periodically arranged close metallic ring arrays, which exhibits a negligible frequency dispersion in an ultrawide band. As a proof of principle, one stack of photonic structures for the normal-incidence-selective case is fabricated and measured in the microwave region. The proposed approach is simple, robust, and scalable from near-zero frequencies to terahertz frequencies, showing promising practical applications such as spatial filters and anti-jamming communications.

Broadband achromatic mid-infrared metalens with polarization-insensitivity

Metasurfaces have shown excellent ability in manipulating light fields. However, the intrinsic chromatic aberration impairs the quality of their broadband operation, which is a serious problem restricting the application of metadevices, especially metalenses. In this paper, a polarization-independent broadband achromatic metalens composed of engineered structures that have spatially varying gradient sizes is devised to focus the light over the mid-infrared region from 3 to 5 µm. The proposed dielectric metalens is constructed by an array of meta-units with a hollowed square pillar. By tuning its structural parameters, the optical response of the meta-unit turns out to be broadband achromatic, insensitive to the polarization state, and of high efficiency. Numerical simulation results show that the designed metalens can significantly suppress chromatic aberrations over a broad mid-infrared and hold a focusing efficiency of nearly 75%. Our work is beneficial to improving the mid-infrared broadband imaging quality and detection efficiency.

Polarization-independent broadband achromatic metalens in the mid-infrared (3–5 μm) region

With superior capabilities for light manipulation and wavefront shaping, the metasurface recently has caught growing attention. However, the presence of chromatic aberration hinders metasurfaces, especially metalenses, from wider applications. Here, we design a polarization-independent broadband achromatic focusing metalens in the mid-infrared region, which covers continuous bands in 3–5 μm. Numerical simulation shows that different wavelengths can be focused to the same plane with a nearly diffraction-limited resolution, and can achieve an average focusing efficiency of nearly 70% in the whole bandwidth. We expect that our approach can underpin the development of integrated and mid-infrared imaging and detection.

Ultra-thin polarization independent broadband terahertz metamaterial absorber.

In this work, we present the design of a polarization independent broadband absorber in the terahertz (THz) frequency range using a metasurface resonator. The absorber comprises of three layers, of which, the top layer is made of a vanadium dioxide (VO2) resonator with an electrical conductivity of σ = 200000 S/m; the bottom layer consists of a planar layer made of gold metal, and a dielectric layer is sandwiched between these two layers. The optimized absorber exhibits absorption greater than 90% from 2.54-5.54 THz. Thus, the corresponding bandwidth of the designed absorber is 3 THz. Further, the thermal tunable absorption and reflection spectra have been analyzed by varying the electrical conductivity of VO2. The impact of the various geometrical parameters on the absorption characteristics has also been assessed. The physics of generation of broadband absorption of the proposed device has been explored using field analysis. Finally, the absorption characteristics of the unit cell has been studied for various incident and polarization angles.

Enhanced absorption based on gap-plasmon resonance and Fabry–Perot resonance in a refractory metasurface

In this paper, we report a polarization-independent broadband metasurface perfect absorber based on tunable gap magnetic resonance and Fabry–Perot (FP) resonance in a structure with consecutive size variation. By using the finite-difference time-domain method, the effects of size parameters are investigated. Due to the coexistence of the FP-like resonance and gap magnetic resonance, the near-unit absorption reaches as high as 99.46% with nanocone morphology throughout the visible-to-near infrared regime where most solar radiation is located. The structure raised in this paper is less complex and more thermally stable due to abandoning the spacer layer in traditional tri-layer structures. This method can be developed for other refractory materials and has great potential in solar energy related optoelectronics applications.

Solar thermal absorber based on dielectric filled two-dimensional nickel grating

In this paper, a solar thermal absorber based on a dielectric filled two-dimensional nickel grating is designed and numerically investigated for wide-angle and polarization-independent broadband absorption. The absorption of the proposed two-dimensional meta-surface absorber reaches nearly 100% in the whole visible region (400-800 nm). The physical mechanisms responsible for the high absorption including the impedance matching with the free space, the cavity resonances and the surface plasmonic resonances have been elucidated in detail. The strong resonances effectively trap the incident light in the nano-cavities and then dissipate it by the ohmic losses of the metal, giving rise to the high absorption of the proposed absorber. The meta-surface absorber may find applications in solar cells, photovoltaics, thermos-photovoltaics, thermal emitters, plasmonic sensors, and solar-energy harvesting.

Tunable Graphene-Based Metasurface for Polarization-Independent Broadband Absorption in Lower Mid-Infrared (MIR) Range

This paper proposes a tunable graphene-based wideband metasurface for high absorption in the lower mid-infrared frequency region. The unit cell of the proposed structure consists of slot generated out of modified fractal-shaped graphene layer deposited on top of amorphous silicon dioxide (SiO2) grown over 0.1-μm-thick gold layer. The structure exhibits distinctive high absorption greater than 90% absorptivity over the range of 3.69–9.77 THz covering a wideband of 6.08 THz. Incorporation of graphene in the metastructure can tune the absorption bandwidth owing to conductivity modulation of graphene by the control of impurity doping and mechanical stretching. Besides, the absorber is polarization independent under normal incidence because of the fourfold symmetry of the deposited graphene pattern. The designed prototype has been studied under oblique incidences for both transverse electric and transverse magnetic polarizations where wideband absorption has been experienced up to 40° incident angles for both cases. The prototype is found to be as thin as ∼ λ /11 with respect to the lowermost frequency of the absorption. The period of the unit cell is ∼ λ /13 supporting the effective homogeneity condition in subwavelength region. The structure can have applications toward sensing, spectroscopy, etc.

Polarization independent broadband metamaterial absorber for microwave applications

A new polarization independent broadband metamaterial absorber (MA) structure based on split ring resonators which are loaded with lumped elements and via connection lines is proposed. The designed structure shows a perfect absorption between 4 and 16 GHz which is validated by simulation studies. Experimental study is only made for the structure that has no via connections and no the lumped element resistors to show the importance of these entities in the proposed metamaterial structure. Both numerical and experimental study results show that broadband MA property depends on the resistors and via connections on the proposed structure. By having high absorption in a wideband range which is numerically demonstrated, the proposed structure can be used in energy harvesting or wireless power transfer applications with higher efficiencies.

Polarization-independent wide-angle ultrathin double-layered metamaterial absorber for broadband application

In this letter, a polarization-independent broadband metamaterial absorber (MMA) for wide incidence angle is proposed. The MMA is composed of double-layer symmetric split rings connected with two orthogonally arranged bars and the ground metallic plane separated by two equal-sized substrates. The whole structure is with an ultra-thin thickness of 2.4 mm, which is 0.031 $$\lambda$$ corresponds to the lowest frequency of absorption peaks. Multiple metallic layers and multi-scalability are employed to provide broadband absorptivity based on the cooperated mechanisms of the Ohmic loss and the Fabry–Perot interference. A desirable absorption is achieved with double strong absorption peaks in 2–6 GHz and a broadband absorption in 10–24 GHz. The absorptivity can reach up to 100% with nearly all absorption peaks over 90%. The symmetry enables the MMA of satisfactory stability for polarization and incidence angles. Numerical and experimental results prove the capability of the proposed MMA for broadband absorbance.

Flexible metasurface black nickel with stepped nanopillars.

We report on a monolithic, all-metallic, and flexible metasurface perfect absorber [black nickel (Ni)] based on coupled Mie resonances originated from vertically stepped Ni nanopillars homoepitaxially grown on an Ni substrate. Coupled Mie resonances are generated from Ni nanopillars with different sizes such that Mie resonances of the stepped two sets of Ni nanopillars occur complementarily at different wavelengths to realize polarization-independent broadband absorption over the entire visible wavelength band (400-760nm) within an ultra-thin surface layer of only 162nm thick in total. Two-step double-beam interference lithography and electroplating are utilized to fabricate the proposed monolithic metasurface that can be arbitrarily bent and pressed. A black nickel metasurface is experimentally demonstrated in which an average polarization-independent absorption of 0.972 (0.961, experiment) in the entire visible band is achieved and remains 0.838 (0.815, experiment) when the incident angle increases to 70°.

Polarization-independent broadband beam combining grating with over 98% measured diffraction efficiency from 1023 to 1080 nm.

We report a grating solution for achieving broadband and polarization-independent properties that brings a laser combining system to much higher power levels. The grating, with a high-refractive-index-contrast bilayer ridge, was designed and successfully fabricated based on high-power laser coatings, lithography, and ion-beam etching technology. The measured -1st order non-polarized reflective diffraction efficiency of the grating exceeds 98% over the wavelength range of 1.023-1.08μm, and the highest value is 99.15%.

Coding metasurface for broadband microwave scattering reduction with optical transparency.

Metasurfaces have promised great possibilities in full control of the electromagnetic wavefront by spatially manipulating the phase characteristics across the interface. Here, we report a scheme to realize broadband backward scattering reduction through diffusion-like microwave reflection by utilizing a flexible indium-tin-oxide (ITO)-based ultrathin coding metasurface (less than 0.1 wavelength thick) with high optical transparence. The diffusion-like scattering is caused by the destructive interference of the scattered far-field electromagnetic wave, which is further attributed to the randomly distributed reflection phases on the metasurface composed of pre-designed meta-atoms arranged with a computer-generated pseudorandom coding sequence. Both simulation and measurement on fabricated prototype sample have been carried out to validate its performance, demonstrating a polarization-independent broadband (nearly from 8 GHz to 15 GHz) 10 dB scattering reduction with good oblique performance. The excellent performances can also be preserved to conformal cases when the flexible metasurface is uniformly wrapped around a metallic cylinder. The proposed metasurface may create new opportunities to tailor the exotic microwave scattering features with simultaneously high transmittance in visible frequencies, which could provide crucial benefits in many practical uses, such as window and solar panel applications.

Polarization-independent broadband dielectric bilayer gratings for spectral beam combining system

We report on a polarization-independent all-dielectric trapezoidal bilayer grating with broadband and high diffraction efficiency. The bilayer trapezoidal grating ridge on a reflector consists of an HfO2 layer and a SiO2 layer. The theoretical −1st order efficiencies of the grating are more than 95% with wavelength range from 1010nm to 1080nm for both TE and TM polarizations. The fabrication tolerances depending on the HfO2 and SiO2 layer grating ridge depths are enough to obtain the designed grating using current craft. The fabricated grating with exceeding 94% efficiency from 1000nm to 1085nm measured by a non-polarization laser has been fabricated and applied in a spectral beam combining external cavity to combine eight beams into one beam output with 10.77kW.

Polarization-independent broadband absorber based on pyramidal metal-dielectric grating structure

An infrared broadband polarization-independent metamaterial absorber is designed and investigated. It consists of a pyramidal metal-dielectric multilayered rectangle grating structure. The absorber exhibits near-unit absorption at multiple adjacent wavelengths overlapping with each other, which results in a high absorption over a wide wavelength range. The absorbance at normal incidence is higher than 90% in a wavelength range of 2321 nm–4631 nm, and the broadband absorption performance can be maintained over a large incident angle range. Furthermore, the mechanism of such broadband absorption are investigated by illustrating the electric field distributions for TE polarization and magnetic field distributions for TM polarization at the resonant wavelengths. It is believed that the conclusions can be applied for developing polarization-independent broadband absorber.

Broadband and polarization-insensitive helix metamaterial absorber using graphene for terahertz region

A polarization-independent broadband metamaterial absorber having graphene layer is proposed in this study. It is found that the absorption spectrum of the absorber becomes more promising after using graphene layers. Graphene layer increases the device robustness and stability of the device. The proposed absorber works for a wide operating region covering near ultraviolet to near infrared, i.e., 200–1792 nm. Absorption characteristics including the operating wavelength and the polarization nature of various existing absorbers is compared with the proposed absorber. It is noticed that the proposed absorber is physically realizable since it has a sufficiently large geometry and resistivity toward change in the number of graphene layers.