In optical systems a significant amount of light intensity is lost due to back reflection at the surfaces of the transmitting optical element. This loss increases as the angle of incidence of the light increases. The loss of transmission resulting from back reflection is cumulative in systems where multiple optical elements are used. This loss of energy could be largely reduced or eliminated by modifying the index of refraction of the surfaces of the transmitting optical element relative to the bulk material. Since reflection is a function of the refractive index the most conventional approach to improve the efficiency of the optical system has been to coat the surface of the optical element with a transparent material that has a refractive index equal to square root of that of the transmitting optical element. Theoretically, a quarter-wavelength-thick coating of this material reduces surface reflection to zero at that wavelength for one given angle of incidence. However, there is a need for broadband (i.e. a range of wavelengths) wide-angle (i.e. a range of angles of incidence) coatings for solar energy applications and for coatings that will withstand higher power densities in laser optics. This requirement cannot be fully met by the above coating method. A coating of tailored refractive index gradient across the coating-thickness is required to reduce reflection losses over a wide range of wavelengths and angles of incidence.
At present several methods are available for producing a graded-index antireflective surface. A brief outline of the reported methods and the limitations of each of those methods is given below.
A first method is to deposit multilayer coatings each made from different transparent materials. The choice of coating materials and the thickness of each layer of the coating should be such that a refractive index gradient is formed across the thickness of the multilayer coating. The main limitations of this method are: (1) the process is somewhat complicated and, as such, commercial production of this type of coating is difficult; and (2) such a coating when used in a high power pulsed laser shows poor laser-damage resistance.
A second method is to introduce graded porosity in a glass surface by selective leaching of phase separated glasses using an NH.sub.4 F.HF solution. Sodium-borosilicate and soda-lime-silica glasses with antireflective properties have been developed by this approach. The main limitation of this method is that the method is applicable to only those glasses which on heat treatment produce a leachable phase on a submicroscopic/microscopic scale.
A third method is to leach the glass directly with a solution of 0.034M Na.sub.2 HAsO.sub.4 and 0.013M Al.sup.+3 in distilled water at 87.degree. C. in a constant temperature bath. This process, which is claimed to be very controllable and reproducible, produces a broad reflectance minimum. Graded index antireflective films on Schott BK-7 borosilicate glass have been produced by this method. The applicability of this method on other glass systems has not yet been reported.
A fourth method is based on the sol-gel process of depositing a microporous gel-coating which, on thermal treatment and pore size tailoring, acquires the antireflective property. The gel-coating is deposited using a solution prepared by partial hydrolysis of an appropriate alkoxide precursor material. Subsequently, the gel-coating is subject to a thermal treatment to remove the volatiles and to partially densify the coating. Finally, the coated surface(s) is immersed in an NH.sub.4 F.HF solution for chemical etching in order to modify the pore-morphology of the coating across its thickness. This method has been used to deposit antireflective silica and borosilicate "glass-like" coatings on various glass substrates. The main advantage of this method is that by this method broadband antireflective coatings can be deposited on glasses of essentially any composition. However, this method has the following practical problems: (1) any minor change in the composition of the etching solution or the temperature of the etching solution or the duration of etching can influence the porosity grading of the coating quite considerably; (2) very often it becomes difficult to prevent severe enlargement of the surface pores relative to the substructure of the coating; and ( 3) if the etched surface is not properly cleaned by washing, particulate material may remain in the pores of the coating. Because of these problems it is extremely difficult to obtain reproducible antireflective properties with these coatings.
A fifth method is based on the sol-gel process. The main difference between this method and the fourth method is that in this method a suitable acid soluble dopant is added to the solution which is used for depositing a microporous gel-coating. Other processing steps of this method are similar to those of the fourth method. The main purpose of adding a dopant in the coating-solution is that when the thermally consolidated gel-coating derived from such a solution is chemically etched, a concentration gradient of the dopant element could develop across the coating thickness. This assists in developing a refractive index gradient. Modification of the pore morphology of the coating further contributes to the gradient. However, the practical problems of this method are similar to those of the fourth method. Moreover, sometimes additional problems arise due to the fact that it is extremely difficult to completely wash out the leached dopant. The presence of residual dopant may cause scattering problems. For the above reasons it is very difficult to obtain reproducible antireflective properties by this method.
Examples of prior art U.S. Pat. Nos. include 2,490,662; 2,707,899; 3,984,581; 4,004,851; 4,086,074; 4,190,321; 4,340,276; and 4,497,539.
In spite of the availability of the above methods there is a need for a more efficient, cost effective, reproducible, commercially acceptable method for producing broadband antireflective coatings. These coatings have applications in laser optics and solar cells, among others. The present invention meets this need.