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Broadband selective tailoring of spectral features with multiple-scale and multi-material metasurfaces
Optics Communications 467 (2020) 125691
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Broadband selective tailoring of spectral features with multiple-scale and multi-material metasurfaces Yuanpei Xu, Yimin Xuan ∗, Xianglei Liu School of Energy and Power Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China
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Keywords: Spectral features Tailoring Metasurface Composite period metastructure
ABSTRACT The tailoring of the electromagnetic wave plays a more and more critical role in various fields, such as infrared detection, invisibility, and telecommunication. Nowadays, the requirement for dual- and wide-band tailoring is more and more intense. In order to achieve infrared stealth property in addition to possessing the ability of radiative cooling, a composite periodic structure is proposed to meet the demand with multispectral tailoring. The metasurface is composed of circle-shaped lattices with different diameters and integrated insulator layers in one period. With this composite metasurface, low emission in the range of two atmospheric windows (3∼5 μm and 8∼14 μm) and broadband absorption from 5 to 8 μm are achieved. The suppressed emissivity enables the surface realize the infrared stealth. Radiative cooling is taken into account with the high emissivity from 5 to 8 μm. The inner mechanisms of the proposed composite periodic metasurface are revealed with intercepted magnetic and electric fields. Moreover, the spectral features with the change of incident angles are investigated, and the selective tailoring ability keeps almost unchanged even when the incident angle is up to 60 degrees. The obtained multifunctional characteristic through composite periodic metasurface demonstrates considerable prospect in the applications of detection, invisibility, radiative cooling, and other areas.
1. Introduction With the rapid development of detection techniques, the significance of the tailoring of electromagnetic waves in multi-band wavelength ranges is increasingly prominent, which cover visible light, infrared wave, and radio wave [1–4]. For these research activities, the metasurface is one of the most effective ways to tune these waves in such a broadband wavelength range with known surface plasmon polaritons (SPPs) [5–8], which is a coupling mode from the interaction between the surface electromagnetic wave and polarization of electrons. The excitation of SPPs mode needs to recur to various metal or electronic materials in the form of micro/nano-structures, and the structures can be diverse. The most popular type is the metal–insulator– metal (MIM) structures, which have extensive applications for the tailoring of electromagnetic waves [9–11]. This kind of metastructure can absorb incident energy at specific wavelengths with benefits of excited magnetic resonances to get strong absorption with developed localized surface plasmon polaritons (LSPPs). This is because that the excited plasmons can be attracted to the surrounding area of the MIM structure with the direct interaction between the incident electromagnetic wave and electrons instead of the type of surface wave in SPPs mode. Through the magnetic response between the up and bottom patterned metallic layers and electric responses, the impedance of the structure is coincident with that in the free space. In this way,
the wave inside the metasurfaces can be absorbed due to the loss provided by the imaginary part of materials [12]. There were various types of patterns for the tailoring through MIM structures, including square [13–16], circle [17–19], cross [20–22], ring [23,24], and other shapes [4,25–27]. Due to different application occasions [28–34], the structure exhibits different absorption properties from visible light to far-infrared radiation. Nevertheless, the traditional periodic MIM absorber is efficient only in a narrow band due to its specific resonant peak. Plenty of works were done to extend the tailoring wavelength range via the effects of sizes and materials for the required applications. Li and Bouchon et al. gave way for broadened absorption through the combination of different sizes of patterns with the advantage of the superposition of different resonant peaks [21,35]. Based on the sized-assembling structure, Guo et al. theoretically introduced defects to top metal layers to make the high-absorption region more broadband [36]. Hendrickson et al. achieved a more broadband absorption by adjusting the insulator layer with an extra thin indium tin oxide (ITO) layer [37]. The skill taken by Zhang et al. and Liu et al. was the stack of metastructures with different insulator layers [38,39]. For our purpose to realize infrared stealth, the emissivity in the regime of two atmospheric windows (3∼5 μm and 8∼14 μm) must be restrained. The suppressive infrared emission will make the dissipation of heat difficult. High emissivity from 5
∗ Corresponding author. E-mail address: [email protected] (Y. Xuan).
https://doi.org/10.1016/j.optcom.2020.125691 Received 17 November 2019; Received in revised form 2 March 2020; Accepted 6 March 2020 Available online 7 March 2020 0030-4018/© 2020 Elsevier B.V. All rights reserved.
Y. Xu, Y. Xuan and X. Liu
Optics Communications 467 (2020) 125691
only the tailoring of wavelengths will cast off the limitation of existing materials, but also the controllable wavelength ranges and flexibility will be developed. Fig. 1(b) gives the purpose of the tailoring in the mid- and farinfrared regime from 3 to 14 μm in this work. According to the atmospheric absorption, the aim is to restrain the emissivity in the wavelength range of two atmospheric windows (3∼5 μm and 8∼14 μm) and transfer the absorbed infrared radiation to the range of 5∼8 μm as much as possible, simultaneously. The enhanced emissivity in these wavelengths will also lead to the additional radiative cooling, which makes the proposed composite periodic metastructure to be multifunctional. In the simulation, the material of metal layers is chosen as silver. For the metal substrate, the thickness is 100 nm. The diameters of four lattices 𝐷1 to 𝐷4 are set as ranging from 1.65 μm to 2.1 μm. The thickness of the metal layers in lattices is 100 nm. The single MgF2 layer is 75 nm, and the thicknesses of MgF2 and ZnS layers in the equivalent insulator layer are 70 nm and 125 nm, respectively. The refractive index of MgF2 and ZnS are obtained from references [43,44], respectively, and the optical constant of silver is from Palik [45].
to 8 μm is urgent to the deduce of the surface temperature through cooling, simultaneously. Therefore, there is a strong requirement for the tailoring ability to cover dual bands. Many efforts have been made for the enhancement of infrared absorption [40–42]. However, the easy operation of the dual- and wide-band tailoring still faces challenges. The main problem is the difficulty for the fabrication of those specific structures. Thus, it is urgent to develop applicable modes for tailoring in wide and multiple bands with a more convenient and liable fabrication approach based on the existent simple fabrication methods, such as photolithography, magnetron sputtering and evaporation film deposition. Moreover, the angle-independent property of metasurfaces must be taken into account for practical utilization. In this work, we propose a composite periodic metastructure with the combination of coupling effects to realize dual-band and broader tailoring of infrared radiation. The specific goal is to achieve infrared stealth in the atmospheric windows and surface heat dissipation in other wavelengths through thermal radiation, which is of spectrally selective requirement being fully different from the so-called radiative cooling. Thus, low emissivity in the atmospheric windows must be guaranteed. Furthermore, the composite metasurface also gives rise to high absorption in the wavelengths of 5∼8 μm to take the advantage of radiative cooling, which makes the tailoring multiband and multifunctional. This work is devoted to the whole chain from the fundamental mechanism of realizing the spectral selectivity to designing and fabricating the metasurfaces with the above-mentioned spectral features. Both the theoretical analysis and experimental measurement are involved. The coupling effects of sizes, materials, and structure arrangements on the spectral properties of the structured surface are analyzed, which illustrates that the integration of generated resonant absorption peaks through different sizes and materials will cover the target wavelength range. Furthermore, the spectral features with different incident angles are investigated. Compared with traditional metasurfaces, the coupling parameters of composite metasurfaces, especially the integrated insulator layers, aim to make the tailoring more flexible. Therefore, the tailoring of wavelengths will cast off the limitation of existing materials. The fabrication process of the proposed structure is also very convenient through the regular photolithography and lift-off process. Thus, the obtained dual effects of the design concept in this type of metasurfaces display bright prospects for various possible applications, including infrared detection, infrared invisibility, and radiative cooling.
2.2. Optical simulation For the simulation in one period, the finite difference time domain (FDTD) method is used to get the spectral properties over the infrared wavelength range [46]. Thus, the source is a plane wave from 3 μm to 16 μm. In the direction perpendicular to the incident wave, periodic boundary conditions are set according to one unit periodic cell. Along the incident direction, the artificial boundary conditions of perfect match layers (PML) are employed. The reflected energy is obtained through a 2D monitor above the source. 2.3. Fabrication of the composite periodic metastructure The combination of photolithography, magnetron sputtering, and thermal vapor evaporation deposition is used to fabricate the microstructure. For the preparation of the light-off mask, a bilayer photoresist is coated on a silicon wafer with both speeds of 4000 rpm for 60 s on a 3 × 3 cm 100 nm-Ag-sputtered silicon wafer. The bottom lift-off photoresist is Lor 5A, which is non-photosensitive used as a sacrificial layer, and is baked on a hot plate at 160 ◦ C for 4 min. S1805 photoresist is used for the upper layer to get a high exposure precision and is baked at 115 ◦ C for 1 min. After the exposure for 1.6s, the patterns appear through image develop for 35 s. With the obtained holes, residual layers of the lattice are evaporated on the surface, and the final patterns are revealed through a lift-off process.
2. Simulation and experimental methods 2.1. The design of the composite periodic metastructure
2.4. Optical measurement
In this work, for both infrared control and radiative cooling in the specific wideband wavelength range, a unique composite periodic metasurface is designed. The proposed model is shown in Fig. 1. In one unit cell, four circle lattices are arranged at four center positions. Each lattice contains two kinds of metastructures with different insulator layers, and it has its specific diameter to give rise to its resonant peaks at the corresponding wavelengths. The materials of center dielectric layers are chosen as magnesium fluoride (MgF2 ) and zinc sulfide (ZnS) for integrated structures, which are common materials for infrared applications and photonic crystals. As is known to all, larger sizes of lattices and higher values of the refractive index of insulator materials will make redshifts of the resonant peaks. Firstly, with the set diameters, the combination of the lattice enables the absorption of the metastructure covers a wavelength range by benefits of different sizes. With further integration of upper and bottom insulator materials, the broadband infrared absorption is realized. For traditional MIM structures, the materials of center dielectric layers are selected single mediums. In our work, the material of the bottom insulator layer is MgF2 as ever. However, the combination of MgF2 and ZnS as an equivalent layer is adopted as the material of the upper insulator layer. In this way, not
The reflectivity R is measured using a Fourier transform infrared spectroscopy with an integrating sphere (FTIR, Bruker Tensor27) at the room temperature. The wavelength-dependent emissivity equal to absorption A, which is calculated from 𝐴 = 1 – R. 3. Results and discussion 3.1. Influences of the period and thickness for period metastructure The emissivity of single Ag-MgF2 -Ag metastructures is given in Fig. 2 with different diameters of circle lattices and the thickness of the center MgF2 layer, which are all set at the period of 2.5 μm. For singlesized lattice, absorption peaks are relatives to various parameters, especially sizes, the thickness of insulator layers and types of materials. The sizes and thicknesses of patterns attract the most concerns. The period of a single lattice is fixed at 1.65 μm when seeking for effects of thicknesses. In common, the thickness of the MgF2 layer is selected as 75 nm to while changing diameters of patterns. From the results shown 2
Y. Xu, Y. Xuan and X. Liu
Optics Communications 467 (2020) 125691
Fig. 1. (a) Disassembled model of the proposed composite periodic circle-patterned MIM structure. (b) Required ideal tailoring properties of the proposed MIM model.
Fig. 2. (a) Color figure of absorption for the periodic circle-patterned metastructure with a single MgF2 insulator layer in the mid-infrared wavelength range (3∼10 μm) The thicknesses of MgF2 change from 50 nm to 100 nm. The diameter of the circle pattern is 1.65 μm. (b) Emissivity of the periodic circle-patterned metastructure with a single MgF2 insulator layer in the mid- and far-infrared wavelength range (3∼16 μm) at different diameters of patterns. The thickness of MgF2 is locked at 75 nm. The period is 2.5 μm. The thicknesses of the bottom and upper silver are both 100 nm.. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
in Fig. 2(a), for the single periodic pattern, the excited resonant peak is nearly linear dependent on thicknesses and diameters. In a relatively larger variation range, absorption peaks are insensitive to the change of the thickness with the distribution of resonant peaks from 5 μm to 5.5 μm according to the thickness in the regime of 50∼100 nm. The relationship between thickness and absorption peaks is an apparently negative correlation. However, when the thickness is quite thin under 55 nm, the absorption peak stays around 5500 nm. Thus, the tailoring through thicknesses of the insulator layers has a limitation with the minimum thickness. Under this value, the absorption peak will have little development. It is different from the influence of thicknesses. The change of absorption peaks with the increasing of diameters is more humdrum with a simple positive linear relationship. This is why the tailoring of the electromagnetic wave by size effects can cover a wideband of wavelength range with few upper limitations. In this way, the design of composite periodic metastructures can be divided into two steps: (1) through the sketchy adjustment of sizes of patterns, the rough range of tailoring wavelengths is confirmed with specific sizes; (2) more accurate control is realized through inner parameters of lattices, such as the thickness of insulator layers, to position clear resonant wavelengths. In this section, the concept of the equivalent insulator layer is proposed in this section composed of the bilayer MgF2 _ZnS layer shown in Fig. 1(a) (the insulator layer 2). According to the results of periodic MIM structures with the single MgF2 insulator layer, it is clearly known
that the tuning ability of single insulator material is limited in a quite narrowband wavelength range as the only way is to adjust the thickness of the insulator layer. The integration of two different metastructures in a traditional lattice with different insulator materials is a very popular route to expand tuning wavelength ranges of absorption. However, employed insulator materials in this method are a single medium selected from available materials naturally, which makes the tailoring of the electromagnetic wave limited to the optical constants of these materials. Thus, instead of choosing another material as the second center insulator medium, the integration of the MgF2 and ZnS layer is combined into one equivalent insulator layer to break the limitation of the selection of existing materials to make the control more flexible. The ZnS layer is on the top side of the MgF2 layer. For the simulation of structures with single periods, the independent tuning parameters are the thickness of the ZnS layer and sizes of patterns. The thickness of the MgF2 layer is locked at 70 nm. According to the computational results, the integrated layer has the feature of the metastructure with a single insulator layer as the presented one absorption peak at a specific wavelength. This illustrates that the integrated insulator layer can be regarded as an equivalent layer with the equivalent refractive index between the refractive index of selected materials in the MIM structure. The shift of the resonant peaks is more complicated than that with single insulator material, as it concerns to more factors, such as the ratio of ZnS, the total thickness of the bilayer. Intuitively, since the thickness of MgF2 is settled, a thicker 3
Y. Xu, Y. Xuan and X. Liu
Optics Communications 467 (2020) 125691
Fig. 3. (a) Color figure of absorption for the periodic circle-patterned metastructure with the integrated MgF2_ ZnS insulator layer in the mid-infrared wavelength range (3∼10 μm). The thicknesses of MgF2 change from 50 nm to 100 nm. The diameter of the circle pattern is 1.65 μm. (b) Emissivity of the periodic circle-patterned metastructure with the integrated MgF2_ ZnS insulator layer in the mid- and far-infrared wavelength range (3∼16 μm) at different diameters of patterns. The thickness of MgF2 is locked at 75 nm. The period is 2.5 μm. The thicknesses of the bottom and upper silver are both 100 nm.. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Magnetic and vector distribution of periodic circle-patterned metastructures with different insulator layers at the x–z cross-sectional view: (a) a 75 nm single MgF2 insulator layer; (b) a 70_75 nm integrated MgF2 _ZnS insulator layer; (c) a 70_100 nm integrated MgF2 _ZnS insulator layer; (d) a 70_125 nm integrated MgF2 _ZnS insulator. The diameters of metastructures with single and integrated insulator layers are 1.65 μm and 2.1 μm, respectively. The thicknesses of two metal layers are both 100 nm.
ZnS layer will make a higher equivalent refractive index, which enables the redshift of the resonant peaks. However, the total thickness of the bilayer layer is also increasing, and the effects of this tendency are opposite to those caused by the increase of refractive index, which is proved by the investigation of the thickness of the single MgF2 layer. With the increase of the thickness of the ZnS layer, the absorption peak shifts apparently to longer wavelengths. When the thickness of the ZnS layer is under around 125 nm, the resonant peaks are dominated by its thickness, as the effects caused by higher equivalent refractive index are more important than those reverse influences of the higher total thickness of the MgF2 _ZnS bilayer. This characteristic disappears when the thickness of the ZnS layer is thick enough, which corresponds to the red mark in Fig. 3(a). Once the thickness of the ZnS layer exceeds
the value, the change of the total thickness will take the dominant part, which forces the resonant peak to shift to shorter wavelengths. Thus, an inflection point can be observed in the cloud picture. Moreover, a decline in the resonant density is exhibited with the redshift of resonant peaks, and the density reaches the lowest level corresponding to the highest resonant wavelength. In order to evaluate the behavior of the equivalent insulator layer further, the capacitor–inductor (LC) equivalent circuit model is utilized to predict the resonant frequency with different thicknesses of the ZnS layer. In the equivalent circuit, the magnetic inductance is 𝐿𝑚 = 0.5𝜇0 𝑙𝑒𝑓 𝑓 𝑑∕𝐷, and 𝐿𝑒 = 𝐷∕𝜀0 𝜔2 𝐴 is the kinetic inductance of the 𝑃 metal layer. 𝐶𝑚 = 𝑐1 𝜀0 𝜀𝑒𝑓 𝑓 𝐴𝑒𝑓 𝑓 ∕𝑑 represents the plate capacitor, and 𝐶𝑔 = 𝜀0 ℎ𝐷∕(𝛬 − 𝐷) stands for the influence of the neighbor lattice as 4
Y. Xu, Y. Xuan and X. Liu
Optics Communications 467 (2020) 125691
the gap capacitance. 𝑙𝑒𝑓 𝑓 = D and 𝐴𝑒𝑓 𝑓 = 𝜋𝐷2 /4 are the effective length and area of the lattice, respectively. In this work, equivalent permittivity 𝜀𝑒𝑓 𝑓 is applied to describe the equivalent dielectric layer, and the proposed 𝜀𝑒𝑓 𝑓 is given below 𝜀𝑒𝑓 𝑓 = (𝜀MgF2 𝑑MgF2 + c2 𝜀ZnS 𝑑ZnS )∕𝑑
layer. Overall speaking, the added influential parameters develop the dimensions for the tailoring of infrared radiation, which makes the tailoring process get rid of the limitation of a single material and be more adaptive to various applications.
(1)
3.2. Magnetic and electric behaviors in the metastructure
where 𝜀0 and 𝜇0 are the permittivity and permeability of the vacuum. 𝛬 and D are the period and diameter of the lattice, respectively. h is the thickness of the metal layer, and d is the total thickness of the integrated insulator layer. 𝜔𝑝 is the angular frequency of silver. 𝑐1 = 0.13 is a numerical factor which takes the charge-covered part along with the capacitor into account. 𝜀MgF2 and 𝑑MgF2 represent the permittivity and thickness of the MgF2 layer, respectively. 𝜀ZnS and 𝑑ZnS represent the permittivity and thickness of the ZnS layer, respectively. Here, we define a constant 𝑐2 = 2.5 to stand for the effect of the added ZnS layer. The total impedance is in the form [47] 𝑍𝑡𝑜𝑡 =
𝑖𝜔(𝐿𝑚 + 𝐿𝑒 ) 1 − 𝜔2 (𝐿𝑚 + 𝐿𝑒 )
−
2𝑖 + 𝑖𝜔(𝐿𝑚 + 𝐿𝑒 ) 𝜔𝐶𝑚
To further understand the light-matter interaction process inside the equivalent MIM structure, distributions of magnetic fields 𝐻𝑥 and the magnetic vector along the x–z cross-sectional view in a unit cell at resonant wavelengths are illustrated in Fig. 4(a)–(d) [48]. According to the theory of the equivalent circuit model of metastructures, the magnetic resonance is the dielectric response of the excited LSPPs controlled by insulator materials inside MIM structures. The electric distribution inside the metal and insulator layers can create a cloth path, which corresponds to the magnetic flux. From the comparison of magnetic fields between single MgF2 layers and integrated MgF2 _ZnS layers, it is clearly can be seen that the magnetic distribution inside the equivalent layer has no obvious boundaries between two different insulator materials. In combination with the distribution of magnetic fields at the y–z cross-sectional view in Fig. S2, the consistency in the distribution of magnetic fields between single and mixed insulator layers proves that the integrated MgF2 _ZnS layer can be regarded as one equivalent to be used. The attenuation of the intensity of magnetic fields with the increase of the thickness of the ZnS layers corresponds to the variation tendency of calculated spectral characteristics in Fig. 3. Moreover, the distribution of electric fields is presented at the x–z and y–z cross-sectional views. It can be seen from Fig. 5(a) and (b) that in the transfer of electric current, the lowest density of electrical energy appears inside the layer of MgF2 from the x–z view. In the
(2)
where 𝜔 stands for the angular frequency. The final resonant frequency 𝜔𝑅 is obtained from 𝑍𝑡𝑜𝑡 = 0 as the following [47] 1∕2 √ ⎡ 𝐶 + 𝐶 − 𝐶2 + 𝐶2 ⎤ 𝑔 𝑚 𝑔⎥ ⎢ 𝑚 𝜔𝑅 = ⎢ ⎥ ⎢ (𝐿𝑚 + 𝐿𝑒 )𝐶𝑚 𝐶𝑔 ⎥ ⎣ ⎦
(3)
The predicted resonant wavelengths from the LC equivalent model are consistent well with the calculated spectrum properties from the FDTD simulation as Fig. 3(a) shows. Fig. 3(b) also gives size effects on the emissivity of the equivalent metastructure with the change of diameters, and the results are similar to those of the single MgF2
Fig. 5. Electric distribution of periodic circle-patterned metastructure at the (a) x–z and (c) y–z cross-sections with a 75 nm single MgF2 insulator layer. Electric distribution of periodic circle-patterned metastructures at the (b) x–z and (d) y–z cross-sections with a 70_75 nm integrated MgF2 _ZnS insulator layer. The diameters of MIM structures with single and integrated insulator layers are 1.65 μm and 2.1 μm, respectively. The thicknesses of two metal layers are both 100 nm.
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Y. Xu, Y. Xuan and X. Liu
Optics Communications 467 (2020) 125691
Fig. 6. Electric distribution of periodic circle-patterned metastructure at the (a) x–z and (c) y–z cross-sections with the 70_75 nm integrated MgF2 _ZnS insulator layer. Electric distribution of periodic circle-patterned metastructure at the (b) x–z and (d) y–z cross-sections with the 75_70 nm integrated ZnS_MgF2 integrated MgF2 _ZnS insulator layer. The diameter of metastructures is 2.1 μm. The thicknesses of two metal layers are both 100 nm.
transfer process of electric charges in the MIM structure, the center insulator layer is regarded as a medium. The MgF2 -ZnS interface attracts the maximum electrical energy, which represents that this kind of coupling of different insulator materials will introduce loss at the interface of materials. The excitation of electromagnetic resonances can create reverse currents inside the upper and bottom metal dipoles. In this way, electric charges assemble at two terminals of dipoles with opposite electrical fluxes at the same side, resulting in the excitation of concentrated electric fields around the terminals of dipoles shown in Fig. 5(c) and (d). Overall speaking, unlike the transverse transfer of the magnetic flux, the transfer of electric charges suffers obstruction at the boundary of different insulator materials. Electric chargers more concentrate on the metal glued to MgF2 , which makes the stronger density of the electric field in the bottom MgF2 layer. For the sake of revealing the electrical behavior inside the integrated metastructure, the optical properties of a reverse arrangement of ZnS_MgF2 layer are presented in Fig. 6(a)–(d) with emissivity and electric fields at x–z and y–z cross-sections. Although the position of insulator materials is changed, the wavelengths of resonant peaks are almost coincident (Fig. S3). However, the distribution of electric fields is strongly dependent on the arrangement of insulator layers. The distribution also achieves a reversal. That is to say, the transfer of electric charges has the direction closely related to types of materials. However, the exchange of the positions of MgF2 and ZnS layers has scarcely any influences on the intensity of electric fields. In combination with the results of magnetic fields, it further comes to see that the integrated insulator layer can be regarded as an equivalent dielectric layer in this MIM structure. Through this integration of different dielectric materials as one equivalent layer, the broadband tailoring can also be realized by adjusting the ratio of different materials under the circumstance of
metasurface with a single size. Thus, another tailoring dimension can be applied to the design of metasurfaces besides sizes, and shapes, which also break the limitation of existing materials with inherent optical constants. 3.3. Influences of diameters on the emissivity profile To estimate the lineshape quality of the emissivity profile influenced by diameters, the full width at half maxima (FWHM), Q-factor, and dephasing time are chosen as the typical parameters for such estimation. The formula of Q-factor and dephasing time are given below [49,50] 𝑄 = 𝑓0 ∕𝛥𝑓
(4)
𝑡𝑑 = 2ℏ∕𝐹 𝑊 𝐻𝑀
(5)
where Q represents the value of Q-factor. 𝑓0 and 𝛥f stand for the resonant frequency and FWHM in a frequency form, respectively. 𝑡𝑑 is the dephasing time, and h is the Planck’s constant. The results are presented in Fig. 7. With the increase of diameter, the values of value for both kinds of metasurfaces are enhanced in an approximate linear profile. According to the results of Q-factor and dephasing time based on the FWHM, the increased FWHM will cause the reduction of Q-factor though the resonant frequency also keep increasing. Thus, the periodic metasurface obtains a relative higher quality with a smaller diameter, and this value decreases linearly when the diameter become larger. Moreover, the metasurface with a single MgF2 layer is superior to that with the integrated MgF2 _ZnS layer, as the circuit in the metasurface with the integrated MgF2 _ZnS layer will suffer the loss caused by the discontinuous interface between the MgF2 and ZnS layer. The dephasing time 𝑡𝑑 has a positive correlation with the change of diameters, and the value of metasurface with a single MgF2 layer is higher than that 6
Y. Xu, Y. Xuan and X. Liu
Optics Communications 467 (2020) 125691
Fig. 7. The influences of diameters on the (a) FFWM, (b) Q-factor, and (c) dephasing time (𝑡𝑑 ) for periodic MIM structures with MgF2 and MgF2 _ZnS insulator layers, respectively.
with the integrated MgF2 _ZnS layer according to the profile of FWHM. Thus, for both types of metasurfaces, the lineshape quality has similar profile with different diameters, and the quality parameters are almost linear dependent on diameters.
MIM structures, which makes the resonant peak be located between the rightmost peaks of these two metastructures. The further infrared emissive spectrum of composite periodic metastructure is investigated with the change of incident angle. The most disadvantage of the oblique incidence is the weakened reflection at wavelengths from 3 to 5 μm. However, apparent omni-directional resonant peaks can be observed in the incident range of 0∼60◦ from Fig. 8(b). There is a major decline in absorption intensity at two red-marked wavelengths when the incident angles are larger than 20 degrees. The center absorption peak almost keeps still at all incident angles. Thus, the infrared spectrum properties of the proposed metastructure possess angle-independent merit.
3.4. Simulated optical properties of the composite metastructure Based on the results of the single periodic metastructure, the range of size distribution in one period can be ensured with diameters from 1.65 μm to 2.1 μm. The period is 5 μm. The thickness of single MgF2 and integrated MgF2 _ZnS layers is 75 nm and 70_125 nm, respectively. The calculated emissivity of composite periodic MIM structures with MgF2 and MgF2 _ZnS insulator layers are given in Fig. 8(a). For the composite MIM structure with single MgF2 layers, the combination of resonant peaks is exhibited, giving rise to high emissivity in a wavelength range from 5 μm to 7 μm apparently due to the size effect. However, compared with the absorption intensity of MIM structures with single diameters, the intensity inside the composite MIM structure with the single MgF2 layer has a very apparent falloff. On the contrary, the composite MIM structure with the integrated MgF2 _ZnS layer has a slight enhancement in the absorption intensity with the same tuning effect for a broader wavelength range. The adding ZnS insulator layer is the source of the redshift of the high-absorption wavelength range. Thus, the MIM metastructure with the integrated insulator layer is more appropriate for the case with the combination of different sizes of patterns. When the two kinds of MIM structures are grouped into one, a broader tailoring wavelength range emerges. However, the leftmost resonant peak has an apparent decrease due to the MIM structure with single MgF2 is on the bottom. The energy into MgF2 is weakened by the upper MIM structure. Thus, the positions of the middle resonant peaks, which are dominated by the upper metastructure, are consistent with the metastructure with the integrated MgF2 _ZnS layer. The rightmost resonant peak embodies the synthetic action of two kinds of
3.5. Experimental performance of the composite periodic metastructure The cross-sectional scanning electron microscopre (SEM) images of the fabricated complex periodic metastructure are presented in Fig. 9(a) and (b). As shown in Fig. 9(c), the comparison of computational and experimental results indicates that the tailoring regime of experimental data corresponds well with those of the simulated results. The only difference is the absorption intensity of experimental data is that of simulated data. This difference is mainly due to distinction between the fabricated surface and designed perfect disks. This difference is mainly due to distinction between the fabricated surface and designed perfect disks. The dimensional discrepancy between the fabricated and designed disks result in the weakness of absorption peaks, which makes the curve of experimental results smoother. Although there exists dimensional discrepancy, the sizes of fabricated disks are in the range of the designed ones. Thus, the final range of emissivity is still consistent with the designed result. The reduction of absorption intensity is due to the final lift-off process. The breakage of the deposited films will have residual part at the margins of disks as the total thickness of deposited films is not much thinner than that of the bilayer photoresist, and this leads to the margins of the disks become warped. Thus, it is hard to
Fig. 8. Comparison of emissivity between composite periodic metastructures with single MgF2 (75 nm), MgF2 _ZnS (70_125 nm), and integrated of MgF2 and MgF2_ ZnS insulator layers. (b) Infrared omni-directional spectrum of the composite period MIM structure with integrated of MgF2 and MgF2_ ZnS insulator layers.
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Optics Communications 467 (2020) 125691
Fig. 9. Cross-sectional SEM images of the fabricated composited periodic metastructure from the view angles of (a) 20◦ and (c) 0◦ . (c) Comparison between computational and experimental results.
References
get a tight interface between deposited films, which leads to weakened absorption intensity. These two parts mainly gives rises to the distinct discrepancy between the experimental and numerical results. Although there are differences, the main tailoring effect and mutilband design concept through the coupling of different sizes and materials can be validated through the measured spectrum property.
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4. Conclusions In summary, a composite periodic metastructure is proposed with coupling effects, which enables dual- and wide-band infrared tailoring. The spectral features demonstrate low emissivity in the two atmospheric windows (3∼5 μm and 8∼14 μm) and high emissivity covering the regime of 5∼8 μm from theoretical analysis and experimental results. In this way, the infrared stealth property is realized with the obtained low emissivity. Moreover, high emissivity from 5 to 8 μm can bring down the surface temperature through the mechanism of radiative cooling, which makes the metasurface multifunctional. The performance benefits from the combination of different diameters of circle lattices with multilayer dielectric layers. The application of two dielectric materials as one equivalent insulator layer is an excellent improvement for the tailoring of electromagnetic waves. Further, this metasurface can hold the spectral characteristics within the range of incident angles from 0◦ to 60◦ . Thus, the tailoring of electromagnetic waves will be more convenient and flexible through tuning the combination of sizes and insulator layers. In this way, the composite periodic metasurface can meet more wide- and dual-band tailoring requirements and possesses a broader prospect and more practical significance for applications. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Yuanpei Xu: Methodology, Investigation, Data curation, Writing original draft, Writing - review & editing. Yimin Xuan: Conceptualization, Funding acquisition, Project administration, Writing - review & editing, Supervision. Xianglei Liu: Investigation, Writing - review & editing, Supervision. Appendix A. Supplementary data Supplementary material related to this article can be found online at https://doi.org/10.1016/j.optcom.2020.125691. Fabrication schematics of the designed composite periodic metasurface; distribution of the magnetic flux at the y–z cross section; emissivity of periodic metasurface with the reversed arrangement of the MgF2 _ZnS insulator layer. 8
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Optics Communications journal homepage: www.elsevier.com/locate/optcom
Broadband selective tailoring of spectral features with multiple-scale and multi-material metasurfaces Yuanpei Xu, Yimin Xuan ∗, Xianglei Liu School of Energy and Power Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China
ARTICLE
INFO
Keywords: Spectral features Tailoring Metasurface Composite period metastructure
ABSTRACT The tailoring of the electromagnetic wave plays a more and more critical role in various fields, such as infrared detection, invisibility, and telecommunication. Nowadays, the requirement for dual- and wide-band tailoring is more and more intense. In order to achieve infrared stealth property in addition to possessing the ability of radiative cooling, a composite periodic structure is proposed to meet the demand with multispectral tailoring. The metasurface is composed of circle-shaped lattices with different diameters and integrated insulator layers in one period. With this composite metasurface, low emission in the range of two atmospheric windows (3∼5 μm and 8∼14 μm) and broadband absorption from 5 to 8 μm are achieved. The suppressed emissivity enables the surface realize the infrared stealth. Radiative cooling is taken into account with the high emissivity from 5 to 8 μm. The inner mechanisms of the proposed composite periodic metasurface are revealed with intercepted magnetic and electric fields. Moreover, the spectral features with the change of incident angles are investigated, and the selective tailoring ability keeps almost unchanged even when the incident angle is up to 60 degrees. The obtained multifunctional characteristic through composite periodic metasurface demonstrates considerable prospect in the applications of detection, invisibility, radiative cooling, and other areas.
1. Introduction With the rapid development of detection techniques, the significance of the tailoring of electromagnetic waves in multi-band wavelength ranges is increasingly prominent, which cover visible light, infrared wave, and radio wave [1–4]. For these research activities, the metasurface is one of the most effective ways to tune these waves in such a broadband wavelength range with known surface plasmon polaritons (SPPs) [5–8], which is a coupling mode from the interaction between the surface electromagnetic wave and polarization of electrons. The excitation of SPPs mode needs to recur to various metal or electronic materials in the form of micro/nano-structures, and the structures can be diverse. The most popular type is the metal–insulator– metal (MIM) structures, which have extensive applications for the tailoring of electromagnetic waves [9–11]. This kind of metastructure can absorb incident energy at specific wavelengths with benefits of excited magnetic resonances to get strong absorption with developed localized surface plasmon polaritons (LSPPs). This is because that the excited plasmons can be attracted to the surrounding area of the MIM structure with the direct interaction between the incident electromagnetic wave and electrons instead of the type of surface wave in SPPs mode. Through the magnetic response between the up and bottom patterned metallic layers and electric responses, the impedance of the structure is coincident with that in the free space. In this way,
the wave inside the metasurfaces can be absorbed due to the loss provided by the imaginary part of materials [12]. There were various types of patterns for the tailoring through MIM structures, including square [13–16], circle [17–19], cross [20–22], ring [23,24], and other shapes [4,25–27]. Due to different application occasions [28–34], the structure exhibits different absorption properties from visible light to far-infrared radiation. Nevertheless, the traditional periodic MIM absorber is efficient only in a narrow band due to its specific resonant peak. Plenty of works were done to extend the tailoring wavelength range via the effects of sizes and materials for the required applications. Li and Bouchon et al. gave way for broadened absorption through the combination of different sizes of patterns with the advantage of the superposition of different resonant peaks [21,35]. Based on the sized-assembling structure, Guo et al. theoretically introduced defects to top metal layers to make the high-absorption region more broadband [36]. Hendrickson et al. achieved a more broadband absorption by adjusting the insulator layer with an extra thin indium tin oxide (ITO) layer [37]. The skill taken by Zhang et al. and Liu et al. was the stack of metastructures with different insulator layers [38,39]. For our purpose to realize infrared stealth, the emissivity in the regime of two atmospheric windows (3∼5 μm and 8∼14 μm) must be restrained. The suppressive infrared emission will make the dissipation of heat difficult. High emissivity from 5
∗ Corresponding author. E-mail address: [email protected] (Y. Xuan).
https://doi.org/10.1016/j.optcom.2020.125691 Received 17 November 2019; Received in revised form 2 March 2020; Accepted 6 March 2020 Available online 7 March 2020 0030-4018/© 2020 Elsevier B.V. All rights reserved.
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Optics Communications 467 (2020) 125691
only the tailoring of wavelengths will cast off the limitation of existing materials, but also the controllable wavelength ranges and flexibility will be developed. Fig. 1(b) gives the purpose of the tailoring in the mid- and farinfrared regime from 3 to 14 μm in this work. According to the atmospheric absorption, the aim is to restrain the emissivity in the wavelength range of two atmospheric windows (3∼5 μm and 8∼14 μm) and transfer the absorbed infrared radiation to the range of 5∼8 μm as much as possible, simultaneously. The enhanced emissivity in these wavelengths will also lead to the additional radiative cooling, which makes the proposed composite periodic metastructure to be multifunctional. In the simulation, the material of metal layers is chosen as silver. For the metal substrate, the thickness is 100 nm. The diameters of four lattices 𝐷1 to 𝐷4 are set as ranging from 1.65 μm to 2.1 μm. The thickness of the metal layers in lattices is 100 nm. The single MgF2 layer is 75 nm, and the thicknesses of MgF2 and ZnS layers in the equivalent insulator layer are 70 nm and 125 nm, respectively. The refractive index of MgF2 and ZnS are obtained from references [43,44], respectively, and the optical constant of silver is from Palik [45].
to 8 μm is urgent to the deduce of the surface temperature through cooling, simultaneously. Therefore, there is a strong requirement for the tailoring ability to cover dual bands. Many efforts have been made for the enhancement of infrared absorption [40–42]. However, the easy operation of the dual- and wide-band tailoring still faces challenges. The main problem is the difficulty for the fabrication of those specific structures. Thus, it is urgent to develop applicable modes for tailoring in wide and multiple bands with a more convenient and liable fabrication approach based on the existent simple fabrication methods, such as photolithography, magnetron sputtering and evaporation film deposition. Moreover, the angle-independent property of metasurfaces must be taken into account for practical utilization. In this work, we propose a composite periodic metastructure with the combination of coupling effects to realize dual-band and broader tailoring of infrared radiation. The specific goal is to achieve infrared stealth in the atmospheric windows and surface heat dissipation in other wavelengths through thermal radiation, which is of spectrally selective requirement being fully different from the so-called radiative cooling. Thus, low emissivity in the atmospheric windows must be guaranteed. Furthermore, the composite metasurface also gives rise to high absorption in the wavelengths of 5∼8 μm to take the advantage of radiative cooling, which makes the tailoring multiband and multifunctional. This work is devoted to the whole chain from the fundamental mechanism of realizing the spectral selectivity to designing and fabricating the metasurfaces with the above-mentioned spectral features. Both the theoretical analysis and experimental measurement are involved. The coupling effects of sizes, materials, and structure arrangements on the spectral properties of the structured surface are analyzed, which illustrates that the integration of generated resonant absorption peaks through different sizes and materials will cover the target wavelength range. Furthermore, the spectral features with different incident angles are investigated. Compared with traditional metasurfaces, the coupling parameters of composite metasurfaces, especially the integrated insulator layers, aim to make the tailoring more flexible. Therefore, the tailoring of wavelengths will cast off the limitation of existing materials. The fabrication process of the proposed structure is also very convenient through the regular photolithography and lift-off process. Thus, the obtained dual effects of the design concept in this type of metasurfaces display bright prospects for various possible applications, including infrared detection, infrared invisibility, and radiative cooling.
2.2. Optical simulation For the simulation in one period, the finite difference time domain (FDTD) method is used to get the spectral properties over the infrared wavelength range [46]. Thus, the source is a plane wave from 3 μm to 16 μm. In the direction perpendicular to the incident wave, periodic boundary conditions are set according to one unit periodic cell. Along the incident direction, the artificial boundary conditions of perfect match layers (PML) are employed. The reflected energy is obtained through a 2D monitor above the source. 2.3. Fabrication of the composite periodic metastructure The combination of photolithography, magnetron sputtering, and thermal vapor evaporation deposition is used to fabricate the microstructure. For the preparation of the light-off mask, a bilayer photoresist is coated on a silicon wafer with both speeds of 4000 rpm for 60 s on a 3 × 3 cm 100 nm-Ag-sputtered silicon wafer. The bottom lift-off photoresist is Lor 5A, which is non-photosensitive used as a sacrificial layer, and is baked on a hot plate at 160 ◦ C for 4 min. S1805 photoresist is used for the upper layer to get a high exposure precision and is baked at 115 ◦ C for 1 min. After the exposure for 1.6s, the patterns appear through image develop for 35 s. With the obtained holes, residual layers of the lattice are evaporated on the surface, and the final patterns are revealed through a lift-off process.
2. Simulation and experimental methods 2.1. The design of the composite periodic metastructure
2.4. Optical measurement
In this work, for both infrared control and radiative cooling in the specific wideband wavelength range, a unique composite periodic metasurface is designed. The proposed model is shown in Fig. 1. In one unit cell, four circle lattices are arranged at four center positions. Each lattice contains two kinds of metastructures with different insulator layers, and it has its specific diameter to give rise to its resonant peaks at the corresponding wavelengths. The materials of center dielectric layers are chosen as magnesium fluoride (MgF2 ) and zinc sulfide (ZnS) for integrated structures, which are common materials for infrared applications and photonic crystals. As is known to all, larger sizes of lattices and higher values of the refractive index of insulator materials will make redshifts of the resonant peaks. Firstly, with the set diameters, the combination of the lattice enables the absorption of the metastructure covers a wavelength range by benefits of different sizes. With further integration of upper and bottom insulator materials, the broadband infrared absorption is realized. For traditional MIM structures, the materials of center dielectric layers are selected single mediums. In our work, the material of the bottom insulator layer is MgF2 as ever. However, the combination of MgF2 and ZnS as an equivalent layer is adopted as the material of the upper insulator layer. In this way, not
The reflectivity R is measured using a Fourier transform infrared spectroscopy with an integrating sphere (FTIR, Bruker Tensor27) at the room temperature. The wavelength-dependent emissivity equal to absorption A, which is calculated from 𝐴 = 1 – R. 3. Results and discussion 3.1. Influences of the period and thickness for period metastructure The emissivity of single Ag-MgF2 -Ag metastructures is given in Fig. 2 with different diameters of circle lattices and the thickness of the center MgF2 layer, which are all set at the period of 2.5 μm. For singlesized lattice, absorption peaks are relatives to various parameters, especially sizes, the thickness of insulator layers and types of materials. The sizes and thicknesses of patterns attract the most concerns. The period of a single lattice is fixed at 1.65 μm when seeking for effects of thicknesses. In common, the thickness of the MgF2 layer is selected as 75 nm to while changing diameters of patterns. From the results shown 2
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Fig. 1. (a) Disassembled model of the proposed composite periodic circle-patterned MIM structure. (b) Required ideal tailoring properties of the proposed MIM model.
Fig. 2. (a) Color figure of absorption for the periodic circle-patterned metastructure with a single MgF2 insulator layer in the mid-infrared wavelength range (3∼10 μm) The thicknesses of MgF2 change from 50 nm to 100 nm. The diameter of the circle pattern is 1.65 μm. (b) Emissivity of the periodic circle-patterned metastructure with a single MgF2 insulator layer in the mid- and far-infrared wavelength range (3∼16 μm) at different diameters of patterns. The thickness of MgF2 is locked at 75 nm. The period is 2.5 μm. The thicknesses of the bottom and upper silver are both 100 nm.. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
in Fig. 2(a), for the single periodic pattern, the excited resonant peak is nearly linear dependent on thicknesses and diameters. In a relatively larger variation range, absorption peaks are insensitive to the change of the thickness with the distribution of resonant peaks from 5 μm to 5.5 μm according to the thickness in the regime of 50∼100 nm. The relationship between thickness and absorption peaks is an apparently negative correlation. However, when the thickness is quite thin under 55 nm, the absorption peak stays around 5500 nm. Thus, the tailoring through thicknesses of the insulator layers has a limitation with the minimum thickness. Under this value, the absorption peak will have little development. It is different from the influence of thicknesses. The change of absorption peaks with the increasing of diameters is more humdrum with a simple positive linear relationship. This is why the tailoring of the electromagnetic wave by size effects can cover a wideband of wavelength range with few upper limitations. In this way, the design of composite periodic metastructures can be divided into two steps: (1) through the sketchy adjustment of sizes of patterns, the rough range of tailoring wavelengths is confirmed with specific sizes; (2) more accurate control is realized through inner parameters of lattices, such as the thickness of insulator layers, to position clear resonant wavelengths. In this section, the concept of the equivalent insulator layer is proposed in this section composed of the bilayer MgF2 _ZnS layer shown in Fig. 1(a) (the insulator layer 2). According to the results of periodic MIM structures with the single MgF2 insulator layer, it is clearly known
that the tuning ability of single insulator material is limited in a quite narrowband wavelength range as the only way is to adjust the thickness of the insulator layer. The integration of two different metastructures in a traditional lattice with different insulator materials is a very popular route to expand tuning wavelength ranges of absorption. However, employed insulator materials in this method are a single medium selected from available materials naturally, which makes the tailoring of the electromagnetic wave limited to the optical constants of these materials. Thus, instead of choosing another material as the second center insulator medium, the integration of the MgF2 and ZnS layer is combined into one equivalent insulator layer to break the limitation of the selection of existing materials to make the control more flexible. The ZnS layer is on the top side of the MgF2 layer. For the simulation of structures with single periods, the independent tuning parameters are the thickness of the ZnS layer and sizes of patterns. The thickness of the MgF2 layer is locked at 70 nm. According to the computational results, the integrated layer has the feature of the metastructure with a single insulator layer as the presented one absorption peak at a specific wavelength. This illustrates that the integrated insulator layer can be regarded as an equivalent layer with the equivalent refractive index between the refractive index of selected materials in the MIM structure. The shift of the resonant peaks is more complicated than that with single insulator material, as it concerns to more factors, such as the ratio of ZnS, the total thickness of the bilayer. Intuitively, since the thickness of MgF2 is settled, a thicker 3
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Fig. 3. (a) Color figure of absorption for the periodic circle-patterned metastructure with the integrated MgF2_ ZnS insulator layer in the mid-infrared wavelength range (3∼10 μm). The thicknesses of MgF2 change from 50 nm to 100 nm. The diameter of the circle pattern is 1.65 μm. (b) Emissivity of the periodic circle-patterned metastructure with the integrated MgF2_ ZnS insulator layer in the mid- and far-infrared wavelength range (3∼16 μm) at different diameters of patterns. The thickness of MgF2 is locked at 75 nm. The period is 2.5 μm. The thicknesses of the bottom and upper silver are both 100 nm.. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Magnetic and vector distribution of periodic circle-patterned metastructures with different insulator layers at the x–z cross-sectional view: (a) a 75 nm single MgF2 insulator layer; (b) a 70_75 nm integrated MgF2 _ZnS insulator layer; (c) a 70_100 nm integrated MgF2 _ZnS insulator layer; (d) a 70_125 nm integrated MgF2 _ZnS insulator. The diameters of metastructures with single and integrated insulator layers are 1.65 μm and 2.1 μm, respectively. The thicknesses of two metal layers are both 100 nm.
ZnS layer will make a higher equivalent refractive index, which enables the redshift of the resonant peaks. However, the total thickness of the bilayer layer is also increasing, and the effects of this tendency are opposite to those caused by the increase of refractive index, which is proved by the investigation of the thickness of the single MgF2 layer. With the increase of the thickness of the ZnS layer, the absorption peak shifts apparently to longer wavelengths. When the thickness of the ZnS layer is under around 125 nm, the resonant peaks are dominated by its thickness, as the effects caused by higher equivalent refractive index are more important than those reverse influences of the higher total thickness of the MgF2 _ZnS bilayer. This characteristic disappears when the thickness of the ZnS layer is thick enough, which corresponds to the red mark in Fig. 3(a). Once the thickness of the ZnS layer exceeds
the value, the change of the total thickness will take the dominant part, which forces the resonant peak to shift to shorter wavelengths. Thus, an inflection point can be observed in the cloud picture. Moreover, a decline in the resonant density is exhibited with the redshift of resonant peaks, and the density reaches the lowest level corresponding to the highest resonant wavelength. In order to evaluate the behavior of the equivalent insulator layer further, the capacitor–inductor (LC) equivalent circuit model is utilized to predict the resonant frequency with different thicknesses of the ZnS layer. In the equivalent circuit, the magnetic inductance is 𝐿𝑚 = 0.5𝜇0 𝑙𝑒𝑓 𝑓 𝑑∕𝐷, and 𝐿𝑒 = 𝐷∕𝜀0 𝜔2 𝐴 is the kinetic inductance of the 𝑃 metal layer. 𝐶𝑚 = 𝑐1 𝜀0 𝜀𝑒𝑓 𝑓 𝐴𝑒𝑓 𝑓 ∕𝑑 represents the plate capacitor, and 𝐶𝑔 = 𝜀0 ℎ𝐷∕(𝛬 − 𝐷) stands for the influence of the neighbor lattice as 4
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Optics Communications 467 (2020) 125691
the gap capacitance. 𝑙𝑒𝑓 𝑓 = D and 𝐴𝑒𝑓 𝑓 = 𝜋𝐷2 /4 are the effective length and area of the lattice, respectively. In this work, equivalent permittivity 𝜀𝑒𝑓 𝑓 is applied to describe the equivalent dielectric layer, and the proposed 𝜀𝑒𝑓 𝑓 is given below 𝜀𝑒𝑓 𝑓 = (𝜀MgF2 𝑑MgF2 + c2 𝜀ZnS 𝑑ZnS )∕𝑑
layer. Overall speaking, the added influential parameters develop the dimensions for the tailoring of infrared radiation, which makes the tailoring process get rid of the limitation of a single material and be more adaptive to various applications.
(1)
3.2. Magnetic and electric behaviors in the metastructure
where 𝜀0 and 𝜇0 are the permittivity and permeability of the vacuum. 𝛬 and D are the period and diameter of the lattice, respectively. h is the thickness of the metal layer, and d is the total thickness of the integrated insulator layer. 𝜔𝑝 is the angular frequency of silver. 𝑐1 = 0.13 is a numerical factor which takes the charge-covered part along with the capacitor into account. 𝜀MgF2 and 𝑑MgF2 represent the permittivity and thickness of the MgF2 layer, respectively. 𝜀ZnS and 𝑑ZnS represent the permittivity and thickness of the ZnS layer, respectively. Here, we define a constant 𝑐2 = 2.5 to stand for the effect of the added ZnS layer. The total impedance is in the form [47] 𝑍𝑡𝑜𝑡 =
𝑖𝜔(𝐿𝑚 + 𝐿𝑒 ) 1 − 𝜔2 (𝐿𝑚 + 𝐿𝑒 )
−
2𝑖 + 𝑖𝜔(𝐿𝑚 + 𝐿𝑒 ) 𝜔𝐶𝑚
To further understand the light-matter interaction process inside the equivalent MIM structure, distributions of magnetic fields 𝐻𝑥 and the magnetic vector along the x–z cross-sectional view in a unit cell at resonant wavelengths are illustrated in Fig. 4(a)–(d) [48]. According to the theory of the equivalent circuit model of metastructures, the magnetic resonance is the dielectric response of the excited LSPPs controlled by insulator materials inside MIM structures. The electric distribution inside the metal and insulator layers can create a cloth path, which corresponds to the magnetic flux. From the comparison of magnetic fields between single MgF2 layers and integrated MgF2 _ZnS layers, it is clearly can be seen that the magnetic distribution inside the equivalent layer has no obvious boundaries between two different insulator materials. In combination with the distribution of magnetic fields at the y–z cross-sectional view in Fig. S2, the consistency in the distribution of magnetic fields between single and mixed insulator layers proves that the integrated MgF2 _ZnS layer can be regarded as one equivalent to be used. The attenuation of the intensity of magnetic fields with the increase of the thickness of the ZnS layers corresponds to the variation tendency of calculated spectral characteristics in Fig. 3. Moreover, the distribution of electric fields is presented at the x–z and y–z cross-sectional views. It can be seen from Fig. 5(a) and (b) that in the transfer of electric current, the lowest density of electrical energy appears inside the layer of MgF2 from the x–z view. In the
(2)
where 𝜔 stands for the angular frequency. The final resonant frequency 𝜔𝑅 is obtained from 𝑍𝑡𝑜𝑡 = 0 as the following [47] 1∕2 √ ⎡ 𝐶 + 𝐶 − 𝐶2 + 𝐶2 ⎤ 𝑔 𝑚 𝑔⎥ ⎢ 𝑚 𝜔𝑅 = ⎢ ⎥ ⎢ (𝐿𝑚 + 𝐿𝑒 )𝐶𝑚 𝐶𝑔 ⎥ ⎣ ⎦
(3)
The predicted resonant wavelengths from the LC equivalent model are consistent well with the calculated spectrum properties from the FDTD simulation as Fig. 3(a) shows. Fig. 3(b) also gives size effects on the emissivity of the equivalent metastructure with the change of diameters, and the results are similar to those of the single MgF2
Fig. 5. Electric distribution of periodic circle-patterned metastructure at the (a) x–z and (c) y–z cross-sections with a 75 nm single MgF2 insulator layer. Electric distribution of periodic circle-patterned metastructures at the (b) x–z and (d) y–z cross-sections with a 70_75 nm integrated MgF2 _ZnS insulator layer. The diameters of MIM structures with single and integrated insulator layers are 1.65 μm and 2.1 μm, respectively. The thicknesses of two metal layers are both 100 nm.
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Optics Communications 467 (2020) 125691
Fig. 6. Electric distribution of periodic circle-patterned metastructure at the (a) x–z and (c) y–z cross-sections with the 70_75 nm integrated MgF2 _ZnS insulator layer. Electric distribution of periodic circle-patterned metastructure at the (b) x–z and (d) y–z cross-sections with the 75_70 nm integrated ZnS_MgF2 integrated MgF2 _ZnS insulator layer. The diameter of metastructures is 2.1 μm. The thicknesses of two metal layers are both 100 nm.
transfer process of electric charges in the MIM structure, the center insulator layer is regarded as a medium. The MgF2 -ZnS interface attracts the maximum electrical energy, which represents that this kind of coupling of different insulator materials will introduce loss at the interface of materials. The excitation of electromagnetic resonances can create reverse currents inside the upper and bottom metal dipoles. In this way, electric charges assemble at two terminals of dipoles with opposite electrical fluxes at the same side, resulting in the excitation of concentrated electric fields around the terminals of dipoles shown in Fig. 5(c) and (d). Overall speaking, unlike the transverse transfer of the magnetic flux, the transfer of electric charges suffers obstruction at the boundary of different insulator materials. Electric chargers more concentrate on the metal glued to MgF2 , which makes the stronger density of the electric field in the bottom MgF2 layer. For the sake of revealing the electrical behavior inside the integrated metastructure, the optical properties of a reverse arrangement of ZnS_MgF2 layer are presented in Fig. 6(a)–(d) with emissivity and electric fields at x–z and y–z cross-sections. Although the position of insulator materials is changed, the wavelengths of resonant peaks are almost coincident (Fig. S3). However, the distribution of electric fields is strongly dependent on the arrangement of insulator layers. The distribution also achieves a reversal. That is to say, the transfer of electric charges has the direction closely related to types of materials. However, the exchange of the positions of MgF2 and ZnS layers has scarcely any influences on the intensity of electric fields. In combination with the results of magnetic fields, it further comes to see that the integrated insulator layer can be regarded as an equivalent dielectric layer in this MIM structure. Through this integration of different dielectric materials as one equivalent layer, the broadband tailoring can also be realized by adjusting the ratio of different materials under the circumstance of
metasurface with a single size. Thus, another tailoring dimension can be applied to the design of metasurfaces besides sizes, and shapes, which also break the limitation of existing materials with inherent optical constants. 3.3. Influences of diameters on the emissivity profile To estimate the lineshape quality of the emissivity profile influenced by diameters, the full width at half maxima (FWHM), Q-factor, and dephasing time are chosen as the typical parameters for such estimation. The formula of Q-factor and dephasing time are given below [49,50] 𝑄 = 𝑓0 ∕𝛥𝑓
(4)
𝑡𝑑 = 2ℏ∕𝐹 𝑊 𝐻𝑀
(5)
where Q represents the value of Q-factor. 𝑓0 and 𝛥f stand for the resonant frequency and FWHM in a frequency form, respectively. 𝑡𝑑 is the dephasing time, and h is the Planck’s constant. The results are presented in Fig. 7. With the increase of diameter, the values of value for both kinds of metasurfaces are enhanced in an approximate linear profile. According to the results of Q-factor and dephasing time based on the FWHM, the increased FWHM will cause the reduction of Q-factor though the resonant frequency also keep increasing. Thus, the periodic metasurface obtains a relative higher quality with a smaller diameter, and this value decreases linearly when the diameter become larger. Moreover, the metasurface with a single MgF2 layer is superior to that with the integrated MgF2 _ZnS layer, as the circuit in the metasurface with the integrated MgF2 _ZnS layer will suffer the loss caused by the discontinuous interface between the MgF2 and ZnS layer. The dephasing time 𝑡𝑑 has a positive correlation with the change of diameters, and the value of metasurface with a single MgF2 layer is higher than that 6
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Optics Communications 467 (2020) 125691
Fig. 7. The influences of diameters on the (a) FFWM, (b) Q-factor, and (c) dephasing time (𝑡𝑑 ) for periodic MIM structures with MgF2 and MgF2 _ZnS insulator layers, respectively.
with the integrated MgF2 _ZnS layer according to the profile of FWHM. Thus, for both types of metasurfaces, the lineshape quality has similar profile with different diameters, and the quality parameters are almost linear dependent on diameters.
MIM structures, which makes the resonant peak be located between the rightmost peaks of these two metastructures. The further infrared emissive spectrum of composite periodic metastructure is investigated with the change of incident angle. The most disadvantage of the oblique incidence is the weakened reflection at wavelengths from 3 to 5 μm. However, apparent omni-directional resonant peaks can be observed in the incident range of 0∼60◦ from Fig. 8(b). There is a major decline in absorption intensity at two red-marked wavelengths when the incident angles are larger than 20 degrees. The center absorption peak almost keeps still at all incident angles. Thus, the infrared spectrum properties of the proposed metastructure possess angle-independent merit.
3.4. Simulated optical properties of the composite metastructure Based on the results of the single periodic metastructure, the range of size distribution in one period can be ensured with diameters from 1.65 μm to 2.1 μm. The period is 5 μm. The thickness of single MgF2 and integrated MgF2 _ZnS layers is 75 nm and 70_125 nm, respectively. The calculated emissivity of composite periodic MIM structures with MgF2 and MgF2 _ZnS insulator layers are given in Fig. 8(a). For the composite MIM structure with single MgF2 layers, the combination of resonant peaks is exhibited, giving rise to high emissivity in a wavelength range from 5 μm to 7 μm apparently due to the size effect. However, compared with the absorption intensity of MIM structures with single diameters, the intensity inside the composite MIM structure with the single MgF2 layer has a very apparent falloff. On the contrary, the composite MIM structure with the integrated MgF2 _ZnS layer has a slight enhancement in the absorption intensity with the same tuning effect for a broader wavelength range. The adding ZnS insulator layer is the source of the redshift of the high-absorption wavelength range. Thus, the MIM metastructure with the integrated insulator layer is more appropriate for the case with the combination of different sizes of patterns. When the two kinds of MIM structures are grouped into one, a broader tailoring wavelength range emerges. However, the leftmost resonant peak has an apparent decrease due to the MIM structure with single MgF2 is on the bottom. The energy into MgF2 is weakened by the upper MIM structure. Thus, the positions of the middle resonant peaks, which are dominated by the upper metastructure, are consistent with the metastructure with the integrated MgF2 _ZnS layer. The rightmost resonant peak embodies the synthetic action of two kinds of
3.5. Experimental performance of the composite periodic metastructure The cross-sectional scanning electron microscopre (SEM) images of the fabricated complex periodic metastructure are presented in Fig. 9(a) and (b). As shown in Fig. 9(c), the comparison of computational and experimental results indicates that the tailoring regime of experimental data corresponds well with those of the simulated results. The only difference is the absorption intensity of experimental data is that of simulated data. This difference is mainly due to distinction between the fabricated surface and designed perfect disks. This difference is mainly due to distinction between the fabricated surface and designed perfect disks. The dimensional discrepancy between the fabricated and designed disks result in the weakness of absorption peaks, which makes the curve of experimental results smoother. Although there exists dimensional discrepancy, the sizes of fabricated disks are in the range of the designed ones. Thus, the final range of emissivity is still consistent with the designed result. The reduction of absorption intensity is due to the final lift-off process. The breakage of the deposited films will have residual part at the margins of disks as the total thickness of deposited films is not much thinner than that of the bilayer photoresist, and this leads to the margins of the disks become warped. Thus, it is hard to
Fig. 8. Comparison of emissivity between composite periodic metastructures with single MgF2 (75 nm), MgF2 _ZnS (70_125 nm), and integrated of MgF2 and MgF2_ ZnS insulator layers. (b) Infrared omni-directional spectrum of the composite period MIM structure with integrated of MgF2 and MgF2_ ZnS insulator layers.
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Fig. 9. Cross-sectional SEM images of the fabricated composited periodic metastructure from the view angles of (a) 20◦ and (c) 0◦ . (c) Comparison between computational and experimental results.
References
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4. Conclusions In summary, a composite periodic metastructure is proposed with coupling effects, which enables dual- and wide-band infrared tailoring. The spectral features demonstrate low emissivity in the two atmospheric windows (3∼5 μm and 8∼14 μm) and high emissivity covering the regime of 5∼8 μm from theoretical analysis and experimental results. In this way, the infrared stealth property is realized with the obtained low emissivity. Moreover, high emissivity from 5 to 8 μm can bring down the surface temperature through the mechanism of radiative cooling, which makes the metasurface multifunctional. The performance benefits from the combination of different diameters of circle lattices with multilayer dielectric layers. The application of two dielectric materials as one equivalent insulator layer is an excellent improvement for the tailoring of electromagnetic waves. Further, this metasurface can hold the spectral characteristics within the range of incident angles from 0◦ to 60◦ . Thus, the tailoring of electromagnetic waves will be more convenient and flexible through tuning the combination of sizes and insulator layers. In this way, the composite periodic metasurface can meet more wide- and dual-band tailoring requirements and possesses a broader prospect and more practical significance for applications. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Yuanpei Xu: Methodology, Investigation, Data curation, Writing original draft, Writing - review & editing. Yimin Xuan: Conceptualization, Funding acquisition, Project administration, Writing - review & editing, Supervision. Xianglei Liu: Investigation, Writing - review & editing, Supervision. Appendix A. Supplementary data Supplementary material related to this article can be found online at https://doi.org/10.1016/j.optcom.2020.125691. Fabrication schematics of the designed composite periodic metasurface; distribution of the magnetic flux at the y–z cross section; emissivity of periodic metasurface with the reversed arrangement of the MgF2 _ZnS insulator layer. 8
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