Polarization-insensitive Characteristics Research Articles

Far field ring beam generation based on 3-bit encoded metasurface

Abstract The encoding metasurface establishes a bridge between the physical and digital worlds, ushering in a new era of manipulating electromagnetic waves and realizing programmable metamaterials through digital coding sequences. This "digital metasurface," relying on binary logic, significantly simplifies the design process, thereby enhancing the flexibility and efficiency of controlling electromagnetic waves. While most encoding metasurfaces control beamforming for pencil-shaped beams, we propose a 3-bit encoding metasurface with a "well structure" in the microwave band.The 3-bit encoding metasurface features a more extensive encoding sequence, offering increased degrees of freedom and flexibility in manipulating the direction of electromagnetic wave propagation. Its symmetric design features polarization-insensitive characteristics, suitable for generating annular beams by varying gradient-encoded numbers radially. This approach enables the production of linearly polarized omnidirectional radiation within the desired elevation angle range. Additionally, adjustments in quantity and orientation of the produced annular beams are achieved through Fourier convolution theorem. This type of annular beams holds promise for applications in various fields, including wireless radio broadcasting and wireless local area networks.

Broadband low-loss polarization-insensitive graphene-integrated photonic crystal fiber modulators in the near-infrared range

The amplitude modulator with the function of converting electric signal to optical amplitude modulation is a key component in high-speed optical communication links. In this paper, we demonstrate a variety of two-dimensional graphene-integrated photonic crystal fiber modulators for efficient near-infrared amplitude modulation with ultra-low insertion loss. Taking the advantages of the perfect total internal reflection of the porous photonic crystal fiber and flexible tunability of the graphene conductivity, the modulation depth of 0.075 dB μm−1 with a low optical loss of 2.13 × 10−4 dB μm−1, a long propagation length of ∼20 mm and polarization-insensitive characteristics in a wide wavelength of 1.3 to 1.8 μm can be achieved. This high-performance modulator may have great potential applications in various high-speed telecommunication, interconnection, and graphene-based integrated photonic systems.

A Wideband Polarization-Insensitive Bistatic Radar Cross-Section Reduction Design Based on Hybrid Spherical Phase-Chessboard Metasurfaces

A wideband polarization-insensitive bistatic radar cross-section (RCS) reduction design under linear and circular polarization incidence is proposed based on spherical-chessboard metasurfaces. A new metasurface element with wideband characteristics was designed, including a double split-ring structure, single-layer media, and metal board. In the proposed RCS-reduction design, the Pancharatnam–Berry (P-B) phase theory is applied with the designed metasurface element to realize phase distribution mimicking the low-scattering sphere, and thus realizing RCS reduction. In addition, the chessboard configuration is combined with spherical phase distribution to further improve the performance of monostatic and bistatic RCS reduction. Finally, the proposed RCS reduction design can not only realize wideband RCS reduction but also exhibit polarization-insensitive characteristics. It realized 10 dB monostatic and bistatic RCS reduction in a frequency band ranging from 8.5 to 21 GHz (84.8% relative bandwidth) under linear polarization (LP) and circular polarization (CP) incidence. The straightforward and efficient design method of the hybrid spherical chessboard can effectively avoid the complex and time-consuming optimization process in RCS-reduction design.

Design and analysis of Minkowski fractal shaped metasurface absorber with broadband and polarization-insensitive characteristics using a tunable graphene layer for terahertz applications

The proposed research article’s main goal is to demonstrate a terahertz (THz) broadband absorber for high-speed wireless communication applications. The proposed structure is a compact one and possesses three layers. The ground layer acts as a metal reflector, lossy silicon acts as a dielectric material and finally a graphene layer in the shape of a minkowski fractal acts as a radiating patch for the proposed design. We have chosen a thickness of 5 μm for the lossy silicon which has a dielectric constant of 11.90. The bottom layer of the proposed design contains a good conductive material like gold with a conductivity of 4.561 e+007 s m−1 with a thickness of 0.2 μm. The thickness of a monolayer graphene is one nanometer, and the overall unit cell size of the proposed structure is 12 × 12 μm2. Because of its symmetrical nature, the proposed absorber offers a broad response to both TM and TE modes irrespective of any polarization angle. The proposed absorber can operate in the terahertz frequency range and has achieved two broad frequency bands from 3.34–3.98 THz and 4.6–5.30 THz, with an absorption percentage greater than 90. We can observe peak absorption frequencies at 3.60 THz and 5.04 THz, which exhibit an absorption percentage close to unity. Additionally, we validated the proposed broadband absorber using an equivalent circuit approach and verified it using the ADS tool.

A novel, mutual coupling independent, ultra-thin, and polarization-insensitive tetra band metamaterial absorber for microwave applications

This manuscript presents a novel, highly efficient, wide-angle, and polarization-stable symmetrical single-layered metamaterial absorber (MA) on a 0.8 mm thin dielectric layer that exhibits four distinct absorption peaks having 0.019 λ ∘ , 0.027 λ ∘ , 0.038 λ ∘ , and 0.044 λ ∘ , respectively. The amalgamation of two circular rings surrounded by a square-shaped four split ring resonators (RR) including a combined group of a circular patch and a square ring in a unit cell enabled mutually coupling independent environment amongst the inter-elements and resulted in a compact geometry. The inclusion of the relevant sub-elements in the proposed MA demonstrated polarization-insensitive characteristics over wide angles of incident waves for both transverse electric (TE) and transverse magnetic (TM) at four frequency bands, i.e. 7.16 GHz, 10.175 GHz, 12.92 GHz, and 16.82 GHz having 99.6%, 97.2%, 98.3%, and 94.8% absorptivity, respectively. The experimental validation of the prototype to validate the simulated results is performed in an anechoic chamber.

Thermally-stable solar energy absorber structure with machine learning optimization

Major problems that the world is currently confronting include global warming and the energy crisis, and the optimal absorption of solar spectrum radiation is a pivotal stage for a green future for the effective usage of solar thermal energy. The available solar absorbers are temperature sensitive and do not provide perfect absorption for a wide operation range. The major objectives of the proposed paper are to overcome these limitations and present an ultrawideband thermally stable solar absorber. The developed absorber utilizes a high melting point material comprising a Ti reflector element, a Si3N4 dielectric layer, and a pi-shaped top TiN metal layer. Within the spectral range spanning from 200 to 2500 nm, a notable average absorption rate of 97.69 % is attained. This structure has also demonstrated exceptionally under the AM1.5 conditions with a spectral absorption efficiency of 96 %. This work presents a novel optimization approach for solar absorbers, employing K-nearest neighbor (KNN) regression models to enhance design optimization by simulation time reduction with an R2 score of 0.999. The novelty resides in the absorber surface shape, utilization of cost efficient, sustainable, and thermally stable materials, as well as the KNN regression model for optimization of proposed solar absorber structure. Due to the angle and polarization insensitive characteristics, this structure can be potentially utilized for solar energy harvesting and shielding.

Monolayer actively tunable dual-frequency switch based on photosensitive silicon metamaterial

We propose a monolayer actively tunable dual-frequency switch in the terahertz (THz) range based on photosensitive silicon (PSi) metamaterial. The designed unit cell consists of hollow cross-shaped slots and four L-shaped slots made of a perfect electric conductor (PEC), with PSi embedded in both the cross-shaped and four L-shaped slots of identical dimensions. The PEC serves as the dielectric substrate. By varying the intensity of the pump beam to adjust the conductivity of the PSi, tuning of the transmittance at the resonant frequencies of the entire structure is achieved, enabling the switch to function in both ON and OFF states. Under vertical incidence of THz waves in the range of 1 THz to 3 THz, in the absence of pump beam, the PSi is in an insulating state with a conductivity of approximately 1 S/m. The proposed terahertz dual-frequency switch is in the ON state, with transmittances of 97.0% at the first resonance frequency (f1=1.736THz) and 98.3% at the second resonance frequency (f2=2.176THz), and group delays of 4.397 ps and 2.540 ps, respectively. When the pump beam intensity is 294.6 μj/cm2, the PSi is in a metallic state with a conductivity of approximately 1 × 105 S/m. The switch is in the OFF state, with zero transmittance in the range of 1 THz to 3 THz and a group delay of 0.327 ps. The calculated modulation degrees of amplitude at f1 and f2 for this terahertz dual-frequency switch are both 100%, demonstrating a high-performance switch. The two resonant frequencies f1 and f2 of the terahertz dual-frequency switch are generated by weakly coupled superposition between the embedded cross-shaped and four L-shaped PSi resonators (CSPR and FLSPR), exhibiting polarization-insensitive characteristics under vertical incidence of THz waves. We believe that this compact structure of terahertz metamaterial dual-frequency switch holds great application prospects in terahertz filtering, slow light devices, terahertz modulation, and the future 6G communication field.

Multiple Fano resonances inspired by bound states in all-dielectric double-opening resonant ring metasurfaces

Sensors are important tools in the fields of biology and chemistry. Highly sensitive, robust and low-cost sensors and their preparation methods have always constituted a research topic. In this paper, an all-dielectric metasurface with a double-opening resonant ring is proposed. The metasurfaces can support MD and ED modes to excite three q-BIC modes. And the Q-factor can reach 4.97×104, 2.02×105 and 1.39×106 respectively. For the asymmetric structure, the three excited peaks have certain polarization-insensitive characteristics. With the introduction of graphene, the dynamic regulation of transmission peak can be realized, specifically the regulation of resonance wavelength, and the intensity of transmittance. Since the all-dielectric metasurfaces assist in enhancing the interaction of the light field with dielectric materials, and the field enhancement effect is obvious, so we analyze the sensing properties of the metasurface. It has a sensitivity of up to 1340 nm/RIU and FOM can reach 1.17×106. Finally, we additionally analyze another asymmetric form, which enables the excitation of the q-BIC by changing the polarization angle, making the structure potentially valuable for optical switching as well.

A Reconfigurable Three-Dimensional Electromagnetically Induced Transparency Metamaterial with Low Loss and Large Group Delay

In this paper, a solid-state plasma (SSP) metamaterial for an analog of the electromagnetically induced transparency phenomenon is designed and investigated. This electromagnetically induced transparency metamaterial has the ability to interact with both incident electric and magnetic fields, and its low-loss characteristics, slow-wave effect, band reconfigurability, and polarization-insensitive characteristics are researched and explored. According to the tunable SSP, we have successfully implemented two modes of operation (mode 1 and mode 2) by whether the SSP resonance unit is excited or not. Low-loss characteristics and polarization-insensitive properties are achieved by rotating the split-ring resonator (SRR) by 180° in the plane and rotating the overall plane framework 90° to form a three-dimensional structure. After that, the maximum group delay of 261.51 ps and 785.09 ps as well as the delay bandwidth product of 17.51 and 62.96 at mode 1 and mode 2, respectively, are discussed respectively. This indicates a good slow-wave effect as well as a high efficiency of communication devices. After all, in mode 1, a transmission peak at 0.541 THz is observed for a transmission ratio of 92.05%; and in mode 2, a transmission peak at 0.741 THz is observed for a transmission ratio of 93.01%, resulting in a bandwidth shift of 0.2 THz. Due to the uniqueness of the developed metamaterial, it holds potential for a wide range of applications in slow-wave devices, modulators, sensors, and communications equipment.

Tri-band and high FOM THz metamaterial absorber for food/agricultural safety sensing applications

Multi-band based terahertz metamaterial absorber is of great importance in current research and applications because of its ability to achieve multi-band absorption of electromagnetic waves and multi-point matching of detection material information. In this paper, a three-band terahertz metamaterial absorber based on metal split-opening ring is proposed, which produces harp peaks with high Q values due to the resonant response of the metal resonator matched with the surface lattice resonance. By optimizing the structure parameters, the Q-factor is as high as 528 and the FOM value is 153.2, which is better than the FOM of the currently reported absorbers. The sensing performance of absorber as refractive index sensor and thickness sensor is studied, and the results show that it can be used for refractive index sensing and thickness sensing detection of analytes. Finally, five pesticides were simulated and tested. The maximum frequency shift of the absorber was 16ghz when the refractive index changed by 0.02 steps, which proved its ability to recognize different pesticides. The detection of proline solutions of 2–10 g/100 ml proves that the absorber could detect analytes with a refractive index difference of 0.003. These results show that the absorber has high sensitivity, high Q factor, ultra-high FOM, and polarization-insensitive characteristics, which can be used for detecting food/agricultural safety testing and other fields.

Partially decoupled polarization demultiplexing and phase noise equalizer for Alamouti code-based simplified coherent systems.

Alamouti space-time block code (STBC) combined with a simple heterodyne coherent receiver can realize polarization-insensitive phase-diversity detection to reduce the cost. In the receiver, a joint equalizer has been used for STBC's polarization demultiplexing and phase tracking. However, the joint equalizer requires two different step size parameters to update the tap weight coefficients for polarization demultiplexing and the phase noise estimation. This leads to the search process being complex so requiring more iterations for convergence. In this paper, we propose a partially decoupled equalizer that consists of a polarization and phase decoupled equalizer (PPDE) and a pilot-aided blind phase search (P-BPS) algorithm to accelerate the convergence and improve the phase noise tolerance. By theoretically calculating the phase noise, the PPDE can achieve polarization demultiplexing with only one single step size parameter, thus suppressing the searching space and greatly reducing the iterations required for convergence. In the carrier phase recovery stage, the P-BPS algorithm can effectively improve the phase noise tolerance and solve the cyclic slip problem of BPS. We conduct numerical simulations and an experiment to transmit a quadrature phase-shift keying (QPSK) signal. The results demonstrate that the number of iterations required for PPDE convergence is only half of that of the joint equalizer while maintaining polarization-insensitive characteristics in large phase noise. Meanwhile, the achievable linewidth tolerance of P-BPS is increased by three times compared with DD-LMS.

Silicon photonic broadband polarization-insensitive switch based on polarization-mode diversity conversion.

We present a 2 × 2 polarization-insensitive switch on a 220-nm silicon-on-insulator platform, employing a balanced Mach-Zehnder interferometer (MZI) structure. This design incorporates polarization-insensitive adiabatic couplers, polarization rotators based on mode hybridization and evolution, and thermo-optic mode-insensitive phase shifters with wide waveguides. The switch exhibits broadband polarization-insensitive characteristics, with extinction ratios larger than 15 dB, insertion losses less than 2.3 dB, and polarization-dependent losses less than 1 dB for wavelengths ranging from 1500 nm to 1600 nm. The power consumption required for simultaneously switching the fundamental transverse electric (TE0) and transverse magnetic (TM0) polarized modes is 29.1 mW. These results highlight the potential of the switch as a building block for on-chip polarization-division-multiplexed optical interconnects.

Ultra-broadband terahertz metamaterial absorber based on flexible wave-absorbing material

Metamaterial absorbers (MA) in terahertz (THz) based bands are receiving much attention for its potential applications. However, the limited bandwidth greatly reduces the further development and wide application of terahertz absorbers. To solve this problem, achieving ultra-broadband and efficient absorption characteristics in the terahertz band, we propose a nickel-based microstructure ultra-broadband MA that operates in the THz band. The proposed MA's electromagnetic wave absorption characteristics and physical absorption mechanism are modeled and numerically simulated. The research demonstrates that MA can achieve ultra-wideband efficient absorption covering the whole THz spectrum (0.1 THz-16 THz) with an overall absorption rate of over 95%. The proposed absorber has polarization-insensitive characteristics and achieves a broad-angle incidence with a range of almost 80°.allows a range of incidence angles up to 80°. The MA consists of only three classical metal-dielectric layer-metal structures, which greatly reduces the application cost. The proposed ultra-thin geometry super-surface scheme uses a flexible absorbing material (ECCOSORB AN-72), and the results show that the overall absorption efficiency are significantly improved after the application of the flexible absorbing material. This design presents a unique and innovative idea for the development and use of ultrawideband terahertz absorbers across various industries including terahertz stealth, sensor technology, imaging, and communication, as well as optoelectronics. The potential applications of this technology are vast and can greatly benefit these industries.

Design and Manufacturing of a Hexapattern Frequency Selective Surface Absorber for Aerospace Stealth Application.

Integrated frequency selective surface (IFSS) absorbers with larger bandwidth, effective reflection loss, polarization-insensitive characteristics, angular stability with compact/thin design, and ease of fabrication have captivated significant importance in stealth technology. Herein, we report on an IFSS absorber that has been designed, simulated, and implemented for manufacturing to achieve effective stealth properties. Initially, frequency selective surface (FSS) layers have been designed that comprise a closed centroid honeycomb structure surrounded with four annular hexagonal rings, splitted, alternatively, and enveloped with four L-shaped elements. The simulated pattern has been optimized on glass fabric for reflection loss (RC, dB) at a thickness of ∼0.1 mm by choosing sheet resistance of pattern 110 Ω/□. A FSS layer combined with interlayer lossy dielectric laminates (1.8 mm) and a carbon-fabric-reinforced-plastic ground has been simulated as an IFSS absorber. The performance of RC, in normal and angular configuration (0-60°), under transvers an electric/magnetic mode of polarization, including analysis of the displacement current, volume power loss distribution, and complex admittance has been carried on IFSS. Subsequently, the proposed absorber has been fabricated using customized carbon-based resistive ink imprinted on glass fabric by mask lithography compounded with laminates (a carbon black powder/epoxy composite) and ground. Their manufacturing details, including free space and anechoic chamber RC measurements, have been presented. The simulated and experimental RC performances of the absorber are found to be in good agreement, possessing minimal 10 dB reflection loss (90% absorption) with a sample thickness of 1.9 mm (0.05λL, where λL corresponds to a lower operating frequency), covering 76% fractional bandwidth in X and Ku bands. The proposed design architecture of the IFSS is ideally suitable for aerospace stealth platforms.

Triple broadband polarization-insensitive tunable terahertz metamaterial absorber on a hybrid graphene-gold pattern

In this paper, we propose a triple broadband metamaterial absorber with a composite hybrid structure of graphene and gold patch embedded in silica. Based on the analogy of a transmission line, this technique may yield polarization-insensitive triple-broadband terahertz absorption with adjustable active control by combining the benefits of gold and graphene. Simulation results indicate that when graphene’s electrochemical potential (Fermi energy) is adjusted to 0.9 eV, the absorber exhibits three broadband absorptions of more than 90% in the frequency ranges of (1.1-2.72) THz, (5.39-6.47) THz, and (9.25-10.97) THz, with a total bandwidth of 4.42 THz for the TE waves. The bandwidth for the TM waves is around 4.29 THz with absorbance greater than 90% in the frequency ranges of (1.14–2.64) THz, (5.51–5.5) THz, and (9.18–10.98) THz. Moreover, the proposed metamaterial absorber is polarization insensitive for both the TE and TM waves and maintains a strong absorption efficacy for both the TE and TM waves at incident angles of less than 40°. Therefore, a single layer of hybrid graphene-gold patch makes the proposed structure extremely simpler and provides triple broadband absorption with a much higher bandwidth with polarization insensitive characteristics, hence, could be used in soaking electromagnetic waves, modulators, detecting signals, tunable filtering, and other broadband devices.

Research on realizing high permeability and laser stealth compatibility in visible light band with dielectric/metal/dielectric film system

The “cat’s eye effect” in the optical window of all kinds of photoelectric equipment is the main basis of a laser active detection system, which poses a great threat to military equipment and combatants. However, under the condition of ensuring high visible transmittance, the sniper stealth scheme for anti-laser active detection remains to be discussed. In this paper, genetic algorithm is used to inverse design the metasurface anti-reflection film. The three-layer anti-reflection film are composed of Si<sub>3</sub>N<sub>4</sub> and Ag , and rectangular array of metal micro-nano structures is added on the top layer to form a wavelength selective absorber, so as to achieve the effect of low reflection and high absorption at laser wavelength. By combining the device design with genetic algorithm, the parameter combination that best possesses the target performance of the device is obtained. The average transmittance in a wavelength range of 380–780 nm is 88%, and a maximum transmittance peak is 94%. The reflectance and the absorption rate at 1550 nm are achieved to be 10% and 80%, respectively. In order to better meet the requirements for practical application, we further design the cross metal array to obtain polarization insensitive characteristics. The metasurface anti-reflective membrane with improved structure can achieve an average visible transmittance of 82% and a reflectance of 5% at 1550 nm. The two metasurface anti-reflection films designed in this work do not require additional devices, and the imaging quality can be guaranteed. At the same time, it can effectively reduce the laser echo energy, so as to achieve the effect of high-quality visible light transmittance and laser stealth compatibility.

Polarization-insensitive electromagnetically induced transparency and its sensing performance based on spoof localized surface plasmons in vanadium dioxide-based terahertz metasurfaces.

The multi-layer terahertz metasurfaces are designed to achieve polarization-insensitive electromagnetically induced transparency (EIT) effect and its sensing performance based on spoof localized surface plasmons (S-LSPs). The unit cell of the proposed metasurfaces is comprised of a metallic spiral (MS) structure, square metal frame (SMF) structure, and vanadium dioxide (VO2) layer. The EIT effect is realized by the bright-bright coupling between spoof electric localized surface plasmons (S-ELSPs) and electric dipole, which can be proved by the multipole scattering theory. The maximum value of transmission amplitude at the transparent window is 0.91, and the modulation depth can reach 51% by adjusting the conductivity of VO2. The theoretical results based on the two-particle model show excellent agreement with the simulated results. Moreover, the change of polarization angle has little effect on the EIT effect and the proposed metasurfaces show polarization-insensitive characteristics. The slow light effect of the proposed metasurfaces can also be dynamically controlled by tuning the conductivity of VO2. Due to the high Q value of the transparent window, the proposed metasurfaces exhibit excellent sensing performance, and the sensitivity is 0.172 THz RIU-1. Our study provides a method for the fabrication of EIT metasurfaces and has a broad application prospect in slow light devices, sensors, and modulators.

Optically Transparent Frequency-Tunable Microwave Absorber Based on Patterned Graphene-ITO Structure

In this work, the authors report the design, fabrication, and measurement of an optically transparent microwave absorber with tunable absorption frequency based on a patterned graphene-indium tin oxide (ITO) hybrid structure. This transparent tunable absorber consists of periodically patterned graphene sandwich structure (GSS), patterned ITO layer, polyvinyl chloride (PVC) substrate, and ITO ground plane from top to bottom. The frequency of absorption peak can be dynamically controlled from 13.1 to 10.9 GHz by regulating the bias voltage between the graphene sheets from 0 to 30 V, in which the peak absorption rates remain above 99.7%. An inhomogeneous doping model for graphene patterns is introduced to interpret the usage of relatively large tuning voltage. The engineered absorber also exhibits polarization-insensitive characteristics attributed to the symmetrical design of the hybrid structure. This tunable device with optical transparency achieved by integrating different transparent materials has broad prospects in the application of multispectral and smart stealth technology.

Design, development and characterization of resistive arm based planar and conformal metasurfaces for RCS reduction

This paper reports the design, development and characterization of a broadband, polarization insensitive metasurface absorber in planar, cylindrically bent and 90° dihedral surface geometry. Four metallic patches loaded with eight lumped resistors are used, which has been optimized numerically using CST MICROWAVE STUDIO, as a unit cell of developed metasurface absorber, to achieve 20 dB reflection reduction for 51.21% fractional bandwidth (13.42–22.66 GHz) under normal incidence with 0.12 {lambda }_{L} thickness (where {lambda }_{L} corresponds to lower operating frequency). The numerical findings are also verified analytically using equivalent circuit analysis, which exhibits very good agreement. Polarization-insensitive characteristics are achieved using fourfold rotation symmetry of the designed structure. The fabricated prototype of the designed absorber is experimentally characterized, using free space measurement method and ABmm vector network analyzer (VNA) system, and fairly good agreement with numerical-analytical findings are reported. The major novelty of this study is the design and development of a broadband (13.42–22.66 GHz), polarization insensitive metasurface absorber that provides 20 dB reflection reduction numerically as well as experimentally in the whole band, which to the author’s knowledge has not been observed till now. Also, keeping in mind the radar stealth applications, first time we have demonstrated both numerically and experimentally, different geometrical shapes of conformal metasurfaces that can be practically used in actual scenario.

Robust enhancement of high-harmonic generation from all-dielectric metasurfaces enabled by polarization-insensitive bound states in the continuum.

The emerging all-dielectric platform exhibits high-quality (Q) resonances governed by the physics of bound states in the continuum (BIC) that drives highly efficient nonlinear optical processes. Here we demonstrate the robust enhancement of third-(THG) and fifth-harmonic generation (FHG) from all-dielectric metasurfaces composed of four silicon nanodisks. Through the symmetry breaking, the genuine BIC transforms into the high-Q quasi-BIC resonance with tight field confinement for record high THG efficiency of 3.9 × 10-4 W-2 and FHG efficiency of 4.8 × 10-10 W-4 using a moderate pump intensity of 1 GW/cm2. Moreover, the quasi-BIC and the resonantly enhanced harmonics exhibit polarization-insensitive characteristics due to the special C4 arrangement of meta-atoms. Our results suggest the way for smart design of efficient and robust nonlinear nanophotonic devices.