Objective This study introduces a new type of metal wire grid polarization element with high transmittance for transverse magnetic (TM) waves and a high extinction ratio. This polarization element has the ability to transmit incident light TM waves and to achieve the polarization effect of transverse electric (TE) wave reflection, which indicates an important application in stealth recognition and feature detection. Commonly used metal wire grid elements can be manufactured with single- or double-layer metal wire grid structures. The single-layer wire grid has high TM wave transmittance, although the extinction ratio is affected by the period, duty cycle, height, and other parameters. Although the double-layer metal wire grid structure has a higher extinction ratio, it suppresses the transmission of TM waves and causes a sharp drop in transmittance. In 2014, TM wave transmittance of more than 60% and the maximum extinction ratio of 22.2 dB were realized in a single-layer metal wire grid polarizer produced in Japan. In 2018, a single-layer metal wire grid polarization element manufactured by Shanghai Institute of Technical Physics of Chinese Academy of Sciences achieved TM wave transmittance of 83.3% and an extinction ratio of 10 dB. Presently, metal wire grid polarization elements reported at home and abroad cannot simultaneously meet the performance requirements of high TM wave transmittance and a high extinction ratio. Therefore, it is necessary to further study the structural design and preparation technology of these elements. In this study, a single-layer metal wire grid structure in the mid-infrared 3--5 μm band is developed, and the design and the preparation process of a polarization element are studied to improve the TM wave transmittance and the extinction ratio.
Methods Effective medium theory (EMT) and rigorous coupled wave analysis (RCWA) were used to design the metal wire grid structure. According to the design theory of optical thin films, a multilayer anti-reflection film was designed to reduce the residual anti-reflection rate of the Si substrate and to further increase the transmittance of the TM waves. The finite difference time domain (FDTD) method was used to simulate the TM wave transmittance and the extinction ratio of the new grating structure. The polarization component structure was manufactured by electron beam evaporation and interference lithography. The surface quality of the film was improved by optimizing the deposition process of thin film materials, and the intermittent coating method was used to prepare the high-performance metal wire grid polarization element and to reduce the radiation temperature of the photoresist. The results were tested by the Fourier-transform infrared spectrometry and the finite difference time domain method.
Results and Discussions A new type of polarization element structure is designed by combining a multilayer anti-reflection film with a metal wire grid to solve the requirements of high TM wave transmittance and high extinction ratios of a polarization element. In the mid-infrared 3--5 μm band, the average TM wave transmittance is 89.1% and the average extinction ratio is 21.9 dB. According to the design theory of optical film, the oxygen partial pressure is optimized during the SiO film deposition. At the center wavelength of 4 μm, the refractive index is 1.64, which reduces the refractive index of the SiO and improves the transmittance of the multilayer anti-reflection film. By changing the speed of the homogenizer to manufacture photoresists with different thicknesses and using the lift-off method to strip the metal wire grid, the thickness of the photoresist that can be completely stripped is determined. When using the intermittent coating method to vaporize metal Al with a certain thickness for several times at an interval of 10 min, the photoresist deformation and the shape error of the photoresist grid caused by radiant heat are both reduced, and the polarization performance of the manufactured metal wire grid element is improved.
Conclusions A new type of polarization element is designed by combining a multilayer optical anti-reflection film with a metal wire grid, which reduces the residual reflectivity of the substrate and improves the transmittance of TM waves. The application of this film reduces the resonance effect between the substrate and the metal film. Thus, the energy enhancement caused by multiple reflection of TE waves is reduced, resulting in reduced transmittance of TE waves and an improved the extinction ratio. According to the experiment results, the influence of radiant heat from film deposition on the deformation of the photoresist during the preparation process is analyzed, and the deposition process parameters of the film are optimized. The preparation accuracy is improved, and a new type of metal wire grid polarization element is developed. After testing, the average TM wave transmittance and the average extinction ratio of the manufactured polarization device is 89.1% and 21.9 dB in the 3--5 μm band, respectively.
