Research on the Design and Preparation of Ultra Wideband Anti reflection Film

Abstract: This article focuses on the design and preparation of ultra wideband anti reflection films. Firstly, the key role and research background of ultra wideband anti reflection films in modern optical systems are elaborated. Then, the design principles are analyzed in detail, including traditional design methods based on multi-layer film interference theory and innovative design ideas introducing the concept of gradient refractive index. In terms of preparation technology, the application and advantages and disadvantages of various techniques such as electron beam evaporation, atomic layer deposition, and tilted deposition were discussed. Through experimental research, ultra wideband anti reflection films with specific properties were successfully prepared and their performance was comprehensively characterized. The research results provide important references for the widespread application of ultra wideband anti reflection films in high-power laser systems, solar photovoltaics, and other fields.

1. Introduction
In modern optical systems, such as high-power laser devices, high-performance optical imaging equipment, solar photovoltaic systems, etc., there is an extremely high requirement for the transmittance of optical components. Surface reflection can cause loss of light energy, reduce system efficiency, and at the same time, reflected light may also cause stray light interference, affecting the imaging quality or operational stability of the system. Ultra wideband anti reflection films can effectively reduce the surface reflectivity of optical components over a wide wavelength range, significantly improving the optical performance of the system, making them a research hotspot in the field of optical thin films.

2、 Design principle of ultra wideband anti reflection film
2.1 Multilayer Film Interference Theory
Traditional broadband anti reflection films are often designed based on the principle of multi-layer film interference, forming a film system by alternately stacking high and low refractive index materials. According to the Fresnel formula, when light propagates at the interface of media with different refractive indices, the amplitude and phase of reflected and refracted light will change. By precisely controlling the thickness and refractive index of each film layer, the reflected light from each layer interferes and cancels each other within a specific wavelength range, thereby achieving anti reflection effect. However, the anti reflection bandwidth that this method can achieve is limited by the refractive index of the material and the film structure, making it difficult to meet the anti reflection requirements of ultra wideband (such as visible light to near-infrared and even longer wavelength bands).

2.2 Gradient refractive index design
To overcome the limitations of traditional multilayer films, the concept of gradient refractive index is introduced. By preparing a film layer with a continuously changing refractive index, the refractive index gradually transitions when light propagates in the film layer, reducing reflections caused by sudden changes in refractive index. In an ideal situation, when the refractive index of the film layer gradually changes from the refractive index of air to the refractive index of the substrate material, theoretically zero reflection can be achieved across the entire wavelength range. In practice, gradient refractive index can be achieved through various methods, such as precise control of porosity during inclined deposition, allowing the membrane layer to gradually transition from high porosity (low refractive index) to low porosity (close to the intrinsic refractive index of the material); Or specific processes can be used to introduce nanoscale pores into the material, artificially reducing the effective refractive index of the material and achieving gradient distribution.

3、 Preparation technology of ultra wideband anti reflection film
3.1 Electron Beam Evaporation Technology
Electron beam evaporation is a commonly used technique for thin film preparation. In a high vacuum environment, the electron gun emits a high-energy electron beam to bombard the evaporation source material, causing it to evaporate and deposit on the substrate to form a thin film. The advantage of this technology is that it can accurately control the thickness of the film layer and evaporate various materials. When preparing ultra wideband anti reflection films, the thickness of each layer can be precisely controlled by controlling the evaporation rate and time. However, for the preparation of film layers with complex refractive index distributions, such as gradient refractive index film layers, there are certain difficulties in using electron beam evaporation technology alone. For example, researchers used electron beam evaporation technology to prepare Al ₂ O3 thin films, and changed their microstructure through subsequent processing to achieve broadband anti reflection performance.

3.2 Atomic Layer Deposition Technology
Atomic layer deposition (ALD) technology has the advantages of precise thickness control, high film quality, and the ability to achieve conformal growth on complex shaped surfaces. In the preparation of ultra wideband anti reflection films, ALD can accurately control the thickness and composition of the film layer, especially suitable for the preparation of nano layered film layers. By controlling the number of ALD cycles, the thickness of the nano layered sub layers can be precisely regulated. However, the ultra thin sub layer thickness and multi interface issues of nanostacks make the development of high optical performance thin films based on nanostacks extremely challenging, especially for broadband anti reflection thin films, which are sensitive to film thickness and refractive index.

3.3 Tilted Sedimentation Technology
During the inclined deposition process, due to the self shielding effect, the deposition angles of the deposited particles on the substrate surface are different, resulting in a porosity gradient in the film layer. This enables arbitrary control of the refractive index within the range of air and near material intrinsic refractive index, providing great flexibility for the development of ultra wideband anti reflection films. However, there is a significant problem of uneven film thickness in inclined deposition, which seriously reduces the practicality of the thin film. To solve this problem, a research group has designed a discontinuous gradient rotation scheme based on the refractive index control range of SiO ₂ film, combined with electron beam tilt deposition technology, and successfully developed a high-performance SiO ₂ gradient refractive index broadband anti reflection film with uniform thickness.

4、 Experimental research
4.1 Experimental Materials and Equipment
Fused silica is selected as the substrate material because of its good optical properties and chemical stability. The evaporation source materials include SiO ₂ (low refractive index material) and TiO ₂ (high refractive index material). The experimental equipment mainly includes high vacuum electron beam evaporation coating machine, atomic layer deposition system, spectrometer, scanning electron microscope (SEM), etc.

4.2 Membrane System Design and Preparation
According to the design concept of gradient refractive index, an ultra wideband anti reflection film system was designed, consisting of a bottom multi-layer interference stack and a top nanostructure layer with gradient low refractive index distribution. Firstly, the multilayer interference layers of SiO ₂ and TiO ₂ are alternately deposited on the fused silica substrate by electron beam evaporation technology, and the thickness of each layer is precisely controlled to meet the interference cancellation conditions. Then, the inclined deposition technique was used to prepare SiO ₂ nanostructure layers with gradient porosity on top of the multi-layer interference stack. The porosity gradient was controlled by controlling the deposition angle and time, thereby achieving a refractive index gradient distribution.

4.3 Performance characterization
Use a spectrometer to test the transmittance of the prepared ultra wideband anti reflection film in the wavelength range of 400-1800 nm. The results showed that the average transmittance of the double-sided coated samples in this wavelength range reached over 98%, and even approached 99% in some wavelengths, effectively verifying the anti reflection effect. By using SEM to observe the microstructure of the film layer, the uniformity of the bottom multi-layer interference stack and the gradient porosity characteristics of the top nanostructure layer are clearly displayed. The damage threshold of the film under 1064 nm laser was tested using a laser damage threshold testing system, and the results showed that the film has a high laser damage threshold and is suitable for high-power laser system applications.

5、 Conclusion
This article conducts an in-depth analysis of the design principles of ultra wideband anti reflection films, and combines various advanced preparation techniques to successfully prepare ultra wideband anti reflection films with excellent performance. This film achieves low reflectivity over a wide wavelength range, while possessing a high laser damage threshold and good microstructural stability. The research results have laid the foundation for the widespread application of ultra wideband anti reflection films in high-power laser systems, solar photovoltaics, high-performance optical imaging, and other fields. In the future, further optimization of membrane design and preparation processes to improve the performance stability of ultra wideband anti reflection films in more complex environments will be an important research direction in this field.