Omnidirectional Antireflective Coatings Comprising of Single Material System

Significance 

There is an increasing desire to design and develop high quality and efficient optical devices. Unfortunately, various optical instruments like liquid crystal displays and monitors are associated with the unwanted light reflectance that reduces their performance efficiency. Therefore, researchers have been looking for alternatives for eliminating the unwanted light reflectance and have identified antireflective coating as a promising solution.

Presently, magnesium fluoride is the common antireflective coating component used in optical technology. However, it is mostly fit for materials with very low refractive indices which do not naturally exist. This is because it considers reflectivity at a specified and desired wavelength thus neglecting omnidirectionality. However, multilayers antireflective composite coatings can adversely improve the antireflective efficiency. Both single layers, multilayer or bilayer antireflective coating can be fabricated from different hybrid structures and their composites.

Generally, fabrication of multilayer antireflective coating is very complicated and expensive due to the mismatch of the properties, poor selection of the suitable refractive index as well as difficulty in adjusting a single layer in the multilayer stack. To this note, single inorganic materials have been used for antireflective coating fabrication. In a recently published literature, SiO2 have been extensively used for fabricating antireflective coating at desired wavelength due to their spatial attributes and large production volumes. Lowering the refractive indices is effective for enhancing the performance of the coatings. However, the limited choice of substrate materials is the major challenge of the silica antireflective coatings. To this end, various considerations have been initiated to achieve non-wetting properties in thin films antireflective tunability, thermal stability, suitable surface structure, and chemical modification.

In a recent research paper published in Advanced Engineering Material, Tsinghua University researchers: Dr. Sadaf Khan, Professor Hui Wu, Jianghao Li, Limin Chen and Professor Zhengjun Zhang formulated a multifunctional antireflective coating using a single material. They fabricated bilayer antireflective coating with periodically arranged SiO2 nanostructures looking like moth eyes over the SiO2 matrix. They used the glancing angle deposition method. The deposition angle was changed from 00-880 in order to tune the refractive index of the monolayer from 1.46 to 1.08. They managed to design the appropriate surface structure and surface chemical treatment and relate it to tune and artificially generate similar non-wetting and optical properties.

The authors observed that the proposed design is practically applicable to both the transparent and nontransparent substrates at different refractive indices. For instance, porosity in each layer can be controlled by parameter deposition to realize refractive index modulation on bilayer coating. Consequently, the fabricated bilayer film exhibited high omnidirectional antireflective performance with up to 300 °C thermal stability. Furthermore, the bilayer SiO2 antireflective film shows enhanced properties such as mechanical strength, hydrophobic capability and thermal stability which also enhanced its application in humid environments as shown in Figure 1.

The success of the developed approach will surely advance the use of single material antireflective coatings in optical technology to design and manufacture more efficient and high-performing optical devices. It also forms the basis for future research work in the related field.

Omnidirectional Antireflective Coatings Comprising of Single Material System - Advances in Engineering
Figure 1 a) represents the cross sectional SEM image of fabricated bilayer SiO2 AR coating on silicon substrate showing hydrophobic physiognomies, Inset in Figure 1(a) –i displays the schematic depiction of Bilayer hydrophobic SiO2 AR coating presenting hydrophobic characteristics b) shows the experimentally measured AR characteristics of bilayer SiO2 AR coating at different angle of light incidence fabricated on BK7 substrate.

Reference

Khan, S., Wu, H., Li, J., Chen, L., & Zhang, Z. (2018). Bilayer SiO2 Nanorod Arrays as Omnidirectional and Thermally Stable Antireflective Coating.  Advanced Engineering Materials, 20(5), 1700942.

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