Design And Preparation Of High Reflective Films Composed Entirely Of Dielectric Materials For Multi-band Application

At present,optoelectronic devices working in a single band have become increasingly difficult to cope with the severe challenges brought about by the rapid development of modern instruments,the subsequent emergence of multi-band optoelectronic devices,which possess strong angular resolution and energy collection capabilities,and can integrate multiple functions to meet various requirements of each band.This paper mainly started from three aspects:the spectral performance of the multi-band high reflection films,the laser protection performance and the stress distribution of the large aperture film,the thermal stress of the multilayer film system was studied through modeling and theoretical in-depth study,and then the design and preparation of three-band compatible multilayer films were carried out,studies on the stress and damage properties of the films on the large aperture substrate were promoted.The models of thermal stress in the double-layer,multi-layer optical dielectric films on circular substrates were established on the basis of the theory of double-layer composite beams.The stress and strain distribution in the equivalent film-substrate system were analyzed by the boundary conditions,such as force balance and bending moment balance,and the theoretical relationship between the equivalent film layer and the thermal stress of the substrate was established.The derived formulae are not only effective in analyzing the stress and strain of the double-layer film system but also applicable in predicting the distribution of thermal stress in the periodic elastic multi-layer film.According to the actual radiuses of curvature of the substrates measured via the profilometer before and after the deposition of the HfO₂/SiO₂ double-layer,multi-layer films,the obtained residual stress of the films are-52.59 MPa,-101.65 MPa,respectively,the thennal stress of the films are calculated to be-79.33 MPa,-78.46 MPa using the theoretical formulae respectively.The thin films prepared at high temperatures have the low density of defects and small intrinsic stress during growth,thereby verifying the effectiveness of the film-substrate models.Based on the structure of G/（HL）^SH/A film system,TiO₂-SiO₂ were selected as the high and low refractive indices materials,and the basic experimental research was carried out on the K9 substrate of Φ30 mm.The center wavelengths of 500 nm～650 nm,808 nm,1064±40 nm were 530 nm,575 nm,and 1064 nm,respectively,according to the film systems G/（HL）8H/A,G/（1.5H1.5L）4H/A,G/（HL）8H/A,the high reflection films were prepared.The residual stress values of the three-band films are-31.52 MPa,-37.10 MPa,and-94.44 MPa respectively,and all of them are in compressive stress state.The laser-induced damage thresholds of 1064 nm high reflective films with TiO₂/SiO₂,HfO₂/SiO₂ and LaTiO₃/SiO₂ composites are 2.9 J/cm2,3.5 J/cm2 and 2.2 J/cm2,respectively.The former two film samples have good film-substrate adhesion,and the surface of the high reflective film of LaTiO₃/SiO₂ combination is wrinkled and peeled off,whose laser damage resistance is relatively weak.The three-band compatible dielectric high reflective film was designed and prepared,monitoring wavelength was 560 nm,and film structure was optimized to be G/（HL）⁸H（2L）⁴（1.4H1.4L）⁸H2L（1.9H1.9L）⁸1.9H/A,low refractive index layer was added between different film stacks to suppress the nominal problem,and the shortcomings of small bandwidth and low peak reflectivity at 808 nm wavelength were improved by increasing the number of cycles.Wide-band large-size multilayer high reflective film with good performance was successfully prepared on a large aperture substrate of Φ220 mm,whose spectral curve is relatively smooth,film thickness uniformity is good,the film is compact without wrinkle,crack and falling off phenomena,with higher fastness.In view of the fact that the compressive strength of glass is better than its tensile strength,the stress state of the film presents a good situation,showing a small compressive stress.Its spectral performance in the visible light range of 500 nm～650 nm,the average reflectivity is 99.5%,the peak reflectivity is 99.9%;within the scope of 778 nm～838 nm,the peak reflectivity is 99.9%,the average reflectivity is 99.8%;in the 1064±40 nm band,its average reflectivity can reach 99.7%,and the peak reflectivity reaches 99.9%.