Design and Preparation of Infrared-microwave Compatible Si/SiO₂ Photonic Crystals

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.

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.

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 has 8 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.

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.