JIANG Libo 1, 2, QIAO Mengke 1, 2, LI Xiangcheng 1, 2
(1. State Key Laboratory of Advanced Refractories, Wuhan University of Science and Technology, Wuhan 430081,
Hubei, China; 2. Key Laboratory of High Temperature Electromagnetic Materials and Structure of MOE,
Wuhan University of Science and Technology, Wuhan 430081, Hubei, China)
Extended abstract:[Background and purposes] 5G base station is a critical infrastructure for the development of mobile communication networks. Materials with high infrared reflectivity and high microwave transmittance compatibility can improve the heat dissipation performance of base station radomes, while maintaining microwave transmission characteristics. Therefore, it has significant technical advantages and application significance in the development of mobile communication. However, high infrared reflectivity and high microwave transmittance have been difficult to be compatible. In this paper, a photonic crystal structure composed of Si/SiO2 is designed, basing on the transmission matrix theory in combination with the related theory of multilayer film system. In this periodic dielectric structure, the propagation characteristics of electromagnetic waves are modulated by the Bragg scattering effect, which results in the formation of a photonic bandgap, thus giving out high infrared reflectivity performance. In addition, the structure exhibits excellent transmittance in the microwave frequency range. The Si/SiO2 photonic crystal structure designed in this paper has a wide range of potential applications.[Methods] According to the transmission matrix theory and photonic forbidden band theory, the refractive index contrast of dielectric materials directly determines the width of the photonic forbidden band. The refractive index (n) of Si in the mid-infrared band of 3–5 μm shows good dispersion stability and its value does not change significantly over the wavelength range, with a stabilized value of about 3.5. The refractive index of SiO2 in the wide spectral range of 3–5 μm is stabilized at about 1.4. This excellent optical stability provides the basis for the precise tuning of the photonic band gap. Therefore, in this paper, the Si/SiO2 group photonic crystal structure was designed and prepared by magnetron sputtering using Si and SiO2 as a combination of high/low refractive index media.[Results] The designed Si/SiO2 photonic crystal structure has a photonic forbidden band in the range of 3.00–5.08 μm. The frequency of the bottom of the forbidden band is 5.9×1013 Hz, the frequency of the top of the forbidden band is 1.0×1014 Hz, while the width of the forbidden band is 4×1013 Hz. By simulating and analyzing the structure with different numbers of layers and layer thicknesses, the final Si/SiO2 photonic crystal structure has8 layers. The thicknesses of Si and SiO2 are 275 nm and 650 nm, respectively, while the total thickness is 3.7 μm. Infrared reflectance spectral properties of the Si/SiO2 photonic crystal structure are analyzed at different incidence angles. The reflectance spectra of the photonic crystals show an obvious blue-shift phenomenon when the angle of incidence is gradually increased from 0° to 80°, but the excellent and high reflectance characteristics (reflectance>98%) are always maintained at 3–5 μm. The surface of the prepared Si/SiO2 photonic samples consists of four substances: Si, SiO2 , Al2O3 and Co3O4.Si and SiO2 layers are stacked alternatively and the boundaries are smooth and clear. The infrared reflectance of the Si/SiO2 photonic crystals samples is tested by using Fourier infrared spectrometer and the average reflectance at 3–5 μm is 95%, which is close to the simulated data. At the same time, the microwave transmittance of the Si/SiO2 photonic crystal structure was tested by using a vector network analyzer and the average transmittance from 2 to 18 GHz was 94.5%, which is consistent with the simulation results and the measured data.[Conclusions] In this paper, a SiO2 /Si photonic crystal structure is designed based on the transmission matrix theory and photonic forbidden band theory, while the infrared reflectivity and microwave transmittance of the structure are analyzed. The SiO2 /Si photonic crystal structure is a periodic structure with a photonic forbidden bandwidth of 4×1013 Hz, which maintains a high reflectivity of >98% in the wavelength range of 3–5 μm, when the incident angle is varied from 0 to 80°. The average reflectivity of the Si/SiO2 structure is 95.0% in the wavelength of 3–5 µm, while the average transmittance is 94.5% in the of 2–18 GHz, which are consistent with the simulation results, thus verifying the validity of the design. The SiO2 /Si photonic crystal structure realizes the compatibility between the mid-infrared high reflectivity and high microwave transmittance domains, possessing a wide range of applications in the fields of heat dissipation efficiency improvement, signal transmission and multispectral compatibility.
Key words: photonic crystal; high infrared reflectivity; high microwave transmittance