Patent Publication Number: US-2022221617-A1

Title: Curved surface films and methods of manufacturing the same

Description:
This application is a divisional and claims the benefit of priority of U.S. patent application Ser. No. 16/401,703, filed May 2, 2019, which claims the benefit of priority to U.S. Provisional Application Ser. No. 62/670,187 filed on May 11, 2018, the content of which is relied upon and incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure generally relates to optical elements, and more specifically, to curved optical elements including films. 
     BACKGROUND 
     High numerical aperture (NA) lenses for optical systems may require many elements, some of which can have very steep surface curvatures. Steep surfaces create challenges for high-performance coatings over a wide angle range and/or a broad spectral bandwidth as the application of the films to curved surfaces may produce non-uniform thickness films and the like. Conventional films on lenses may be produced via physical vapor deposition (PVD) which is a line-of-sight deposition process. As coating material from the PVD process arrives at very large angles relative to the lens surface, the coating may exhibit thickness and mechanical properties which may be substantially different towards the edge compared to the center of the lens surface. The low coating uniformity leads to high spectral reflectance and polarization split at the edge of the lens. Several technical approaches have been explored to address the issue, such as tilting and masking. Both tilting and masking approaches can improve some coating uniformity on steep surfaces, but reduces coating packing density towards the center, leading to an increase scatter loss at the center. Accordingly, new optical films and methods of making them may be advantageous. 
     SUMMARY OF THE DISCLOSURE 
     According to at least one feature of the present disclosure, an optical element including an optically transparent lens which defines a curved surface having a steepness given by an R/# of from about 0.5 to about 1.0. A film is positioned on the curved surface. The film includes an index layer. A composite layer is positioned on the curved surface having a refractive index greater than the index layer. The composite layer includes HfO 2  and Al 2 O 3 . The composite layer has a mole fraction X of HfO 2 , wherein X is from about 0.05 to about 0.95 and a mole fraction of Al 2 O 3  in the composite layer is 1−X. 
     According to another feature of the present disclosure, an optical element includes a lens defining a curved surface. A film is positioned on the curved surface. The film includes a laminate layer positioned on the curved surface. The laminate layer having a plurality of first layers including HfO 2  and a plurality of second layers includes Al 2 O 3 . An index layer includes SiO 2 . The film has a variation in reflectance of from about 0% to about 4% over a wavelength band of from about 220 nm to about 500 nm as measured across the lens and between about a 0 clear aperture value and a 0.96 clear aperture value as measured by reflective spectral microscopy. 
     According to another feature of the present disclosure, a method of forming a film of an optical element, comprises the steps of: positioning a substantially transparent lens in a reactor chamber, wherein the lens defines a curved surface; exposing the lens to a first precursor comprising at least one of Al and Hf such that the first precursor is deposited on the curved surface of the lens; exposing the first precursor on the curved surface to a first oxidizer such that the first precursor present on the curved surface of the lens reacts with the first oxidizer to form a high refractive index layer of the film; exposing the high refractive index layer to a second precursor such that the second precursor is deposited on the high refractive index layer; and exposing the second precursor on the high refractive index layer to a second oxidizer such that the second precursor present on the high refractive index layer reacts with the second oxidizer to form a low refractive index layer of the film. 
     These and other features, advantages, and objects of the present disclosure will be further understood and appreciated by those skilled in the art by reference to the following specification, claims, and appended drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following is a description of the figures in the accompanying drawings. The figures are not necessarily to scale, and certain features and certain views of the figures may be shown exaggerated in scale or in schematic in the interest of clarity and conciseness. 
       In the drawings: 
         FIG. 1  is a schematic view of an optical element, according to at least one example; 
         FIG. 2A  is an enhanced view taken at section IIA of  FIG. 1 , according to at least one example; 
         FIG. 2B  is an enhanced view taken at section IIB of  FIG. 1 , according to at least one example; 
         FIG. 3  is a flowchart of an exemplary method of forming the optical element, according to at least one example; 
         FIG. 4  is a plot of index and extinction coefficient of an atomic layer deposition coating Al 2 O 3  and a physical vapor deposition coating of Al 2 O 3 ; 
         FIG. 5  is a plot of measured and calculated reflectance and transmittance vs. wavelength; 
         FIG. 6A  is a micrograph of a film coating on a lens at an apex of the lens; 
         FIG. 6B  is a micrograph of a film coating on a lens at an edge of the lens; 
         FIG. 7A  is a plot demonstrating reflectance (%) vs. wavelength of various points on an optical lens having a coating; 
         FIG. 7B  is a plot demonstrating reflectance (%) vs. wavelength of various points on an optical lens having a coating; 
         FIG. 8  is a measured reflectance spectral distribution taken at various points on an optical lens having a coating; and 
         FIGS. 9A-9G  are plots of S-polarization and P-polarization reflectance (%) vs angle of incidence for a variety of coatings having different compositions. 
     
    
    
     DETAILED DESCRIPTION 
     Additional features and advantages of the invention will be set forth in the detailed description which follows and will be apparent to those skilled in the art from the description, or recognized by practicing the invention as described in the following description, together with the claims and appended drawings. 
     As used herein, the term “and/or,” when used in a list of two or more items, means that any one of the listed items can be employed by itself, or any combination of two or more of the listed items can be employed. For example, if a composition is described as containing components A, B, and/or C, the composition can contain A alone; B alone; C alone; A and B in combination; A and C in combination; B and C in combination; or A, B, and C in combination. 
     In this document, relational terms, such as first and second, top and bottom, and the like, are used solely to distinguish one entity or action from another entity or action, without necessarily requiring or implying any actual such relationship or order between such entities or actions. 
     It will be understood by one having ordinary skill in the art that construction of the described disclosure, and other components, is not limited to any specific material. Other exemplary embodiments of the disclosure disclosed herein may be formed from a wide variety of materials, unless described otherwise herein. 
     For purposes of this disclosure, the term “coupled” (in all of its forms: couple, coupling, coupled, etc.) generally means the joining of two components (electrical or mechanical) directly or indirectly to one another. Such joining may be stationary in nature or movable in nature. Such joining may be achieved with the two components (electrical or mechanical) and any additional intermediate members being integrally formed as a single unitary body with one another or with the two components. Such joining may be permanent in nature, or may be removable or releasable in nature, unless otherwise stated. 
     As used herein, the term “about” means that amounts, sizes, formulations, parameters, and other quantities and characteristics are not and need not be exact, but may be approximate and/or larger or smaller, as desired, reflecting tolerances, conversion factors, rounding off, measurement error and the like, and other factors known to those of skill in the art. When the term “about” is used in describing a value or an end-point of a range, the disclosure should be understood to include the specific value or end-point referred to. Whether or not a numerical value or end-point of a range in the specification recites “about,” the numerical value or end-point of a range is intended to include two embodiments: one modified by “about,” and one not modified by “about.” It will be further understood that the end-points of each of the ranges are significant both in relation to the other end-point, and independently of the other end-point. 
     The terms “substantial,” “substantially,” and variations thereof as used herein are intended to note that a described feature is equal or approximately equal to a value or description. For example, a “substantially planar” surface is intended to denote a surface that is planar or approximately planar. Moreover, “substantially” is intended to denote that two values are equal or approximately equal. In some embodiments, “substantially” may denote values within about 10% of each other. 
     It is also important to note that the construction and arrangement of the elements of the disclosure, as shown in the exemplary embodiments, is illustrative only. Although only a few embodiments of the present innovations have been described in detail in this disclosure, those skilled in the art who review this disclosure will readily appreciate that many modifications are possible (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters, mounting arrangements, use of materials, colors, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter recited. For example, elements shown as integrally formed may be constructed of multiple parts, or elements shown as multiple parts may be integrally formed, the operation of the interfaces may be reversed or otherwise varied, the length or width of the structures, and/or members, or connectors, or other elements of the system, may be varied, and the nature or number of adjustment positions provided between the elements may be varied. It should be noted that the elements and/or assemblies of the system may be constructed from any of a wide variety of materials that provide sufficient strength or durability, in any of a wide variety of colors, textures, and combinations. Accordingly, all such modifications are intended to be included within the scope of the present innovations. Other substitutions, modifications, changes, and omissions may be made in the design, operating conditions, and arrangement of the desired and other exemplary embodiments without departing from the spirit of the present innovations. 
     Referring now to  FIG. 1 , an optical element  10  includes a lens  14  and a film  18 . As will be explained in detail below, the film  18  may be a multilayered structure which may provide one or more properties to the lens  14  such as mechanical properties (e.g., scratch resistance) and/or optical properties (e.g., anti-reflection and color neutrality). 
     The lens  14  may include a glass, a glass-ceramic, a ceramic material and/or combinations thereof. Exemplary glass-based examples of the lens  14  may include soda lime glass, alkali aluminosilicate glass, alkali containing borosilicate glass and/or alkali aluminoborosilicate glass. For purposes of this disclosure, the term “glass-based” may mean a glass, a glass-ceramic and/or a ceramic material. According to various examples, the lens  14  may be a glass-based substrate. In glass-based examples of the lens  14 , the lens  14  may be strengthened (e.g., alkali exchanged) or strong (e.g., polished to remove defects). The lens  14  may be substantially clear, transparent and/or free from light scattering. For example, the lens  14  may have a transmittance of from about 50% to about 100% at one or more wavelengths or wavelength bands over a wavelength range of from about 180 nm to about 700 nm. In glass-based examples of the lens  14 , the lens  14  may have a refractive index in the range from about 1.45 to about 1.55 at a wavelength of about 266 nm. Further, the lens  14  of the optical element  10  may include sapphire and/or polymeric materials. Examples of suitable polymers include, without limitation: thermoplastics including polystyrene (PS) (including styrene copolymers and blends), polycarbonate (PC) (including copolymers and blends), polyesters (including copolymers and blends, including polyethyleneterephthalate and polyethyleneterephthalate copolymers), polyolefins (PO) and cyclicpolyolefins (cyclic-PO), polyvinylchloride (PVC), acrylic polymers including polymethyl methacrylate (PMMA) (including copolymers and blends), thermoplastic urethanes (TPU), polyetherimide (PEI) and blends of these polymers with each other. Other exemplary polymers include epoxy, styrenic, phenolic, melamine, and silicone resins. 
     The lens  14  may define one or more curved surfaces  22 . The curved surfaces  22  aid in defining the lens  14  to have a generally curved shape. The curved surfaces  22  may form the lens  14  to have a generally biconvex, plano-convex, positive meniscus, negative meniscus, plano-concave, biconcave and/or combinations thereof. The curved surface  22  may have a steepness, or “speed,” which is expressed as an R/# value. The R/# value may be calculated as a radius of curvature (R) divided by the clear aperture of the lens  14 . For purposes of this disclosure, the radius of curvature may be defined as the distance between a vertex of the lens  14  and the center of curvature. For purposes of this disclosure, the clear aperture is defined as the diameter or size of the lens  14  through which light may pass. Clear aperture may be expressed herein as a fraction or decimal which indicates the distance from the center (e.g., 0.0 ca) of the clear aperture to the edge (1.0 ca) of the clear aperture. For example, halfway between the center of the clear aperture and the edge of the clear aperture is 0.5 ca. 
     The R/# of the curved surface  22  may be from about 0.5 to about 1.0, or from about 0.6 to about 1.0, or from about 0.7 to about 1.0, or from about 0.8 to about 1.0, or from about 0.9 to about 1.0. For example, the R/# value may be about 0.5, about 0.55, about 0.6, about 0.65, about 0.7, about 0.75, about 0.8, about 0.85, about 0.9, about 0.95, about 0.99, or any and all values and ranges therebetween. According to various examples, the curved surface  22  may have an R/# value of about 0.5 or greater. It will be understood that it is contemplated that one or more of the curved surfaces  22  of the lens  14  may have an R/# value of greater than 1 (e.g., 2 or greater, 5 or greater, 10 or greater, or 100 or greater) without departing from the teachings provided herein. 
     Still referring to  FIG. 1 , the film  18  is depicted as positioned directly on the curved surface  22  of the lens  14 , but it will be understood that one or more layers, coatings and/or films may be positioned between the film  18  and the lens  14 . For example, a crack mitigation layer, an adhesion layer, an electrically conductive layer, an electrically insulating layer, an optical layer, an anti-reflection layer, a protective layer, a scratch-resistant layer, a high hardness layer, other types of layers and/or combinations thereof may be positioned between the film  18  and the lens  14 . Further, the film  18  may be positioned on more than one surface of the lens  14 . For example, the film  18  may be positioned across multiple curved surfaces  22  and/or extend onto flat surfaces of the lens  14  without departing from the teachings provided herein. 
     The term “film,” as applied to the film  18  and/or other films incorporated into the optical element  10 , includes one or more layers that are formed by any known method in the art, including discrete deposition or continuous deposition processes. Such layers may be in direct contact with one another. The layers may be formed from the same material or more than one different material. In one or more alternative examples, such layers may have intervening layers of different materials disposed therebetween. In one or more examples, the film  18  may include one or more contiguous and uninterrupted layers and/or one or more discontinuous and interrupted layers (i.e., layers having different materials formed adjacent to one another). 
     The film  18  may be formed using various deposition methods such as vacuum deposition techniques, for example, chemical vapor deposition (e.g., plasma-enhanced chemical vapor deposition, low-pressure chemical vapor deposition, atmospheric pressure chemical vapor deposition, and plasma-enhanced atmospheric pressure chemical vapor deposition), physical vapor deposition (e.g., reactive or nonreactive sputtering or laser ablation), thermal or e-beam evaporation and/or atomic layer deposition. One or more layers of the optical film  18  may include nano-pores or mixed-materials to provide specific refractive index ranges or values. 
     The thickness of the film  18  may be in the range from about 0.005 μm to about 0.5 μm, or from about 0.01 μm to about 20 μm. According to other examples, the film  18  may have a thickness in the range from about 0.01 μm to about 10 μm, from about 0.05 μm to about 0.5 μm, from about 0.01 μm to about 0.15 μm or from about 0.015 μm to about 0.2 μm. In yet other examples, the film  18  may have a thickness from about 100 nm to about 200 nm. It will be understood that any and all values and ranges between above-noted values are contemplated. 
     According to various examples, the thickness of the film  18 , or any layers thereof as described in greater detail below, may have a high uniformity. For example, the thickness of the film  18  and/or any layers thereof may have a variance in thickness of from about ±0 nm to about ±100 nm as measured between any two points along the film  18  and/or layer. For example, the film  18  and/or any layers thereof may have a variance in thickness of about ±100 nm or less, about ±90 nm or less, about ±80 nm or less, about ±70 nm or less, about ±60 nm or less, about ±50 nm or less, about ±40 nm or less, about ±30 nm or less, about ±20 nm or less, about AO nm or less, about ±9 nm or less, about ±8 nm or less, about ±7 nm or less, about ±6 nm or less, about ±5 nm or less, about ±4 nm or less, about ±3 nm or less, about ±2 nm or less, about ±1 nm or less, about ±0.5 nm or less, about ±0.1 nm or less or any and all values and ranges therebetween. As will be explained in greater detail below, the high uniformity of the film  18  may be advantageous in ensuring consistent optical properties of the optical element  10  across various clear aperture locations. 
     According to various examples, the film  18  may have a low carbon content. For example, the film  18  may have a volume, mass and/or mole percent of carbon of from about 0.01% to about 0.5%, or from about 0.02% to about 0.4%, or from about 0.03% to about 0.3%, or from about 0.04% to about 0.2%, or from about 0.05% to about 0.5%. For example, the carbon content of the film  18  may be about 0.5% or less, about 0.45% or less, about 0.4% or less, about 0.35% or less, about 0.3% or less, about 0.25% or less, about 0.2% or less, about 0.15% or less, about 0.1% or less, about 0.09% or less, about 0.08% or less, about 0.07% or less, about 0.06% or less, about 0.05% or less, about 0.04% or less, about 0.03% or less, about 0.02% or less, about 0.01% or less or any and all values and ranges therebetween. 
     Referring now to  FIGS. 1, 2A and 2B , the film  18  may include one or more index layers  30  and one or more composite layers  34 . As will be explained in greater detail below, the composite layer  34  may include two or more layers. In such examples, the composite layer  34  may be referred to as a laminate layer. In the depicted example, the film  18  includes three index layers  30  and three composite layers  34 , but it will be understood that the film  18  may include one or more, two or more, four or more, five or more, or six or more index layers  30  and/or composite layers  34 . According to various examples, the index layers  30  and the composite layers  34  are positioned in an alternating manner. In other words, the film  18  may be composed of alternating layers of the index layers  30  and the composite layers  34 . It will be understood that other orientations of the film  18  are contemplated. For example, two or more index layers  30  or two or more composite layers  34  may be stacked on one another without departing from the teachings provided herein. Further, it will be understood that although the composite layer  34  is depicted as positioned on the curved surface  22 , the index layer  30  may be the layer placed on the curved surface  22  without departing from the teachings provided herein. 
     The index layers  30  may be composed of at least one of SiO 2 , GeO 2 , SiO, AlOxNy, AlN, SiN x , Si 3 N 4 , SiO x N y , Si u Al v O x N y , Ta 2 O 5 , Nb 2 O 5 , TiO 2 , ZrO 2 , Al-doped SiO 2 , TiN, MgO, MgF 2 , BaF 2 , CaF 2 , SnO 2 , Y 2 O 3 , MoO 3 , DyF 3 , YbF 3 , YF 3 , CeF 3 , Sc 2 O 3  and/or combinations thereof. According to various examples, the index layers  30  may include at least SiO 2 . Pure SiO 2  may be utilized in the index layers  30  in some examples where low reflectance of the film  18  is desired. 
     One or more of the index layers  30  may have a thickness of from about 1 nm to about 100 nm, or from about 1 nm to about 90 nm, or from about 1 nm to about 80 nm, or from about 1 nm to about 70 nm, or from about 1 nm to about 60 nm, or from about 1 nm to about 50 nm, or from about 1 nm to about 40 nm, or from about 1 nm to about 30 nm, or from about 1 nm to about 20 nm, or from about 1 nm to about 10 nm. For example, one or more of the index layers  30  may have a thickness of about 1 nm or greater, about 5 nm or greater, about 10 nm or greater, about 20 nm or greater, about 30 nm or greater, about 40 nm or greater, about 50 nm or greater, about 60 nm or greater, about 70 nm or greater, about 80 nm or greater, about 90 nm or greater, or about 100 nm or greater. For example, at least one of the index layers  30  may have a thickness of about 50 nm or greater. It will be understood that each of the index layers  30  present may have a different thickness than one or more of the other index layers  30 . A total thickness of the index layers  30  (e.g., for all layers added together) may be about 5 nm or greater, about 10 nm or greater, about 20 nm or greater, about 30 nm or greater, about 40 nm or greater, about 50 nm or greater, about 60 nm or greater, about 70 nm or greater, about 80 nm or greater, about 90 nm or greater, or about 100 nm or greater. According to various examples, each of the index layers  30  has a thickness of about 1 nm or greater. According to various examples, the index layers  30  account for about 5% or greater, about 10% or greater, about 20% or greater, about 30% or greater, about 40% or greater, about 50% or greater, about 60% or greater, or about 70% or greater of the thickness of the film  18 . According to various examples, one of index layers  30  may be substantially thicker than the rest of the index layers  30  of the film  18 . 
     According to various examples, the index layers  30  may have a refractive index lower than the composite layers  34 . For example, one or more of the index layers  30  may have a refractive index of about 1.2 or greater, about 1.25 or greater, about 1.3 or greater, about 1.35 or greater, about 1.4 or greater, about 1.45 or greater, about 1.5 or greater, about 1.55 or greater, about 1.6 or greater, about 1.65 or greater, about 1.7 or greater, about 1.75 or greater, about 1.8 or greater at a wavelength of about 266 nm. According to various examples, each of the index layers  30  has a refractive index of about 1.2 or greater at a wavelength of 266 nm. According to various examples, the refractive indexes of the index and composite layers  30 ,  34  may be different than one another such that the film  18  may function as an anti-reflective film. The difference in the refractive index of the index and composite layers  30 ,  34  may be about 0.01 or greater, about 0.05 or greater, about 0.1 or greater, about 0.2 or greater, about 0.3 or greater, about 0.4 or greater, about 0.5 or greater, about 0.6 or greater, about 0.7 or greater, about 0.8 or greater, about 0.9 or greater, or about 1.0 or greater. 
     As explained above, the film  18  also includes one or more composite layers  34 . Each of the composite layers  34  may have a thickness of from about 1 nm to about 100 nm, or from about 20 nm to about 90 nm, or from about 30 nm to about 80 nm, or from about 40 nm to about 70 nm, or from about 50 nm to about 60 nm. For example, the composite layers  34  may have a thickness of about 1 nm or greater, about 5 nm or greater, about 10 nm or greater, about 20 nm or greater, about 30 nm or greater, about 40 nm or greater, about 50 nm or greater, about 60 nm or greater, about 70 nm or greater, about 80 nm or greater, about 90 nm or greater, or about 100 nm or greater. For example, at least of the composite layers  34  has a thickness of about 50 nm or greater. It will be understood that each of the composite layers  34  present may have a different thickness than one or more of the other composite layers  34 . A total thickness of the composite layers  34  (e.g., for all composite layers  34  added together) may be about 5 nm or greater, about 10 nm or greater, about 20 nm or greater, about 30 nm or greater, about 40 nm or greater, about 50 nm or greater, about 60 nm or greater, about 70 nm or greater, about 80 nm or greater, about 90 nm or greater, or about 100 nm or greater. According to various examples, the composite layers  34  account for about 5% or greater, about 10% or greater, about 20% or greater, about 30% or greater, about 40% or greater, about 50% or greater, about 60% or greater, or about 70% or greater of a total thickness of the film  18 . According to various examples, one of composite layers  34  may be substantially thicker than the rest of the composite layers  34  of the film  18 . 
     According to various examples, the composite layers  34  may have a high refractive index, relative to the index layers  30 . The composite layers  34  may have a refractive index of about 1.7 or greater, about 1.75 or greater, about 1.8 or greater, about 1.85 or greater, about 1.9 or greater, about 1.95 or greater, about 2.0 or greater, about 2.05 or greater, about 2.1 or greater, about 2.15 or greater, about 2.2 or greater, about 2.25 or greater, about 2.3 or greater, about 2.35 or greater, about 2.4 or greater, about 2.45 or greater, about 2.5 or greater, or about 2.6 or greater at a wavelength of 266 nm. According to various examples, each of the composite layers  34  has a refractive index of about 2.0 or greater at a wavelength of 266 nm. It will be understood that the refractive index of each of the composite layers  34  may be different than the other composite layers  34 . 
     Referring now to  FIG. 2A , depicted is an example of the composite layer  34  where the constituents of the composite layer  34  are not, or are only minimally, segregated. In one aspect, the composite layer  34  is amorphous. In another aspect, the constituents of the composite layer  34  form a solid solution or homogeneous composition. Constituents of the composite layer  34  may include SiO 2 , Al 2 O 3 , GeO 2 , SiO, AlO x N y , AlN, SiN x , Si 3 N 4 , SiO x N y , Si u Al v O x N y , Ta 2 O 5 , HfO 2 , Nb 2 O 5 , TiO 2 , ZrO 2 , TiN, MgO, MgF 2 , BaF 2 , CaF 2 , SnO 2 , Y 2 O 3 , MoO 3 , DyF 3 , YbF 3 , YF 3 , CeF 3  and/or combinations thereof. The composite layer  34  may have a refractive index of about 2.1, about 2.15, about 2.2, about 2.25, about 2.3, about 2.35, about 2.4 or any and all values and ranges therebetween. According to various examples, the composite layer  34  may be composed of Al 2 O 3  and HfO 2 . In such examples, the mole fraction X of HfO 2  may range from about 0.001 to about 1, or from about 0.05 to about 0.95, or from about 0.10 to about 0.90, or from about 0.60 to about 0.90, or from about 0.70 to about 0.90, or from about 0.75 to about 0.85, or from about 0.55 to about 0.65. For example, the mole fraction of HfO 2  of the composite layer  34  may be about 0.001 or greater, about 0.005 or greater, about 0.01 or greater, about 0.05 or greater, about 0.10 or greater, about 0.015 or greater, about 0.20 or greater, about 0.25 or greater, about 0.30 or greater, about 0.35 or greater, about 0.40 or greater, about 0.45 or greater, about 0.50 or greater, about 0.55 or greater, about 0.60 or greater, about 0.65 or greater, about 0.70 or greater, about 0.75 or greater, about 0.80 or greater, about 0.85 or greater, about 0.90 or greater, about 0.95 or greater, about 0.99 or greater or any and all values and ranges therebetween. The mole fraction of Al 2 O 3  may be given by the mole fraction X of HfO 2  subtracted from 1. In other words, the Al 2 O 3  mole fraction is given by 1−X. As such, the Al 2 O 3  mole fraction in the composite layer  34  may range from about 0.001 to about 1, or from about 0.05 to about 0.95, or from about 0.10 to about 0.90. For example, the mole fraction of Al 2 O 3  of the composite layer  34  may be about 0.001 or greater, about 0.005 or greater, about 0.01 or greater, about 0.05 or greater, about 0.10 or greater, about 0.15 or greater, about 0.20 or greater, about 0.25 or greater, about 0.30 or greater, about 0.35 or greater, about 0.40 or greater, about 0.45 or greater, about 0.50 or greater, about 0.55 or greater, about 0.60 or greater, about 0.65 or greater, about 0.70 or greater, about 0.75 or greater, about 0.80 or greater, about 0.85 or greater, about 0.90 or greater, about 0.95 or greater, about 0.99 or greater or any and all values and ranges therebetween. According to various examples, one or more of the composite layers  34  are amorphous. The composition of a composite layer  34  composed of HfO 2  and Al 2 O 3  can be expressed as XHfO 2 -(1−X)Al 2 O 3  or (HfO 2 )x(Al 2 O 3 ) 1−X . In one aspect, a composite layer  34  composed of HfO 2  and Al 2 O 3  is amorphous. 
     Referring now to  FIG. 2B  depicted is a laminate example of the composite layers  34 . In such examples, the composite layers  34  each include a first plurality of layers  38  and a second plurality of layers  42 . It will be understood that examples of the film  18  using both the examples of  FIGS. 2A and 2B  of the composite layer  34  are contemplated. For example, one or more of the composite layers  34  may be a laminate while other composite layers may be homogeneous or non-segregated. According to various examples, the plurality of first and second layers  38 ,  42  of the composite layer  34  are stacked in an alternating order. In the depicted example, the composite layer  34  includes three first layers  38  and three second layers  42 , but it will be understood that the composite layer  34  may include one or more, two or more, four or more, five or more, or six or more first layers  38  and/or second layers  42 . Each of the plurality of first layers  38  may have a thickness of from about 1 nm to about 10 nm, or from about 2 nm to about 9 nm, or from about 3 nm to about 8 nm, or from about 4 nm to about 7 nm, or from about 5 nm to about 6 nm. For example each of the plurality of first layers  38  may have a thickness about 1 nm, about 2 nm, about 3 nm, about 4 nm, about 5 nm, about 6 nm, about 7 nm, about 8 nm, about 9 nm, about 10 nm or any and all values and ranges therebetween. According to various examples, each of the plurality of first layers  38  has a thickness of about 6 nm. It will be understood that the plurality of first layers  38  may have a uniform thickness or that one or more first layers  38  may have a different thickness than other first layers  38 . According to various examples, the first plurality of layers  38  alternate with the second plurality of layers  42 . According to various examples, the laminate layer is amorphous. 
     The plurality of first layers  38  may be composed of at least one of SiO 2 , Al 2 O 3 , GeO 2 , SiO, AlO x N y , AlN, SiN x , Si 3 N 4 , SiO x N y , Si u Al v O x N y , Ta 2 O 5 , HfO 2 , Nb 2 O 5 , TiO 2 , ZrO 2 , TiN, MgO, MgF 2 , BaF 2 , CaF 2 , SnO 2 , Y 2 O 3 , MoO 3 , DyF 3 , YbF 3 , YF 3 , CeF 3 , Sc 2 O 3  and/or combinations thereof. In one aspect, the first layer  38  is amorphous. According to various examples, the first plurality of layers  38  may include HfO 2 . According to various examples, the first plurality of layers  38  may have a refractive index of about 2.1 or greater, about 2.15 or greater, about 2.2 or greater, about 2.25 or greater, about 2.3 or greater, about 2.35 or greater, about 2.4 or greater, about 2.45 or greater, about 2.5 or greater, about 2.55 or greater, about 2.6 or greater, about 2.65 or greater, about 2.7 or greater at a wavelength of 266 nm. According to various examples, each of the first layers  38  has a refractive index of about 2.3 at a wavelength of 266 nm. According to various examples, the refractive indexes of the first and second layers  38 ,  42  may have a difference of about 0.01 or greater, about 0.05 or greater, about 0.1 or greater, about 0.2 or greater, about 0.3 or greater, about 0.4 or greater, about 0.5 or greater, about 0.6 or greater, about 0.7 or greater, about 0.8 or greater, about 0.9 or greater, or about 1.0 or greater. 
     Each of the plurality of second layers  42  may have a thickness of from about 1 nm to about 10 nm, or from about 2 nm to about 9 nm, or from about 3 nm to about 8 nm, or from about 4 nm to about 7 nm, or from about 5 nm to about 6 nm. For example each of the plurality of second layers  42  may have a thickness about 1 nm, about 2 nm, about 3 nm, about 4 nm, about 5 nm, about 6 nm, about 7 nm, about 8 nm, about 9 nm, about 10 nm or any and all values and ranges therebetween. According to various examples, each of the plurality of second layers  38  has a thickness of about 4 nm. It will be understood that the plurality of second layers  42  may have a uniform thickness or that one or more second layers  42  may have a different thickness than other second layers  42 . 
     The plurality of second layers  42  may be composed of at least one of SiO 2 , Al 2 O 3 , GeO 2 , SiO, AlO x N y , AlN, SiN x , Si 3 N 4 , SiO x N y , Si u Al v O x N y , Ta 2 O 5 , HfO 2 , Nb 2 O 5 , TiO 2 , ZrO 2 , TiN, MgO, MgF 2 , BaF 2 , CaF 2 , SnO 2 , Y 2 O 3 , MoO 3 , DyF 3 , YbF 3 , YF 3 , CeF 3  and/or combinations thereof. In one aspect, the second layer  42  is amorphous. According to various examples, the second plurality of layers  42  may include Al 2 O 3 . According to various examples, the second plurality of layers  42  may have a refractive index of about 1.5 or greater, about 1.55 or greater, about 1.6 or greater, about 1.65 or greater, about 1.7 or greater, about 1.75 or greater, about 1.8 or greater, about 1.85 or greater, about 1.9 or greater, about 1.95 or greater, about 2.0 or greater, about 2.05 or greater, or about 2.1 or greater at a wavelength of 266 nm. According to various examples, each of the second layers  42  has a refractive index of about 1.7 at a wavelength of 266 nm. 
     The first plurality of layers  38  may be from about 0.1% to about 99%, or from about 10% to about 90%, or from about 20% to about 80%, or from about 30% to about 75%, or from about 50% to about 70%, or from about 52% to about 68%, or from about 54% to about 66%, or from about 56% to about 64%, or from about 58% to about 62% of a thickness of the composite layer  34 . For example, the first plurality of layers  38  may account for about 50%, or about 51%, or about 52%, or about 53%, or about 54%, or about 55%, or about 56%, or about 57%, or about 58%, or about 59%, or about 60%, or about 61%, or about 62%, or about 63%, or about 64%, or about 65%, or about 66%, or about 67%, or about 68%, or about 69% or about 70% of the thickness of the composite layer  34 . The second plurality of layers  42  may be from about 30% to about 50%, or from about 32% to about 48%, or from about 34% to about 46%, or from about 36% to about 44%, or from about 38% to about 42% of a thickness of the composite layer  34 . For example, the second plurality of layers  42  may account for about 30%, or about 31%, or about 32%, or about 33%, or about 34%, or about 35%, or about 36%, or about 37%, or about 38%, or about 39%, or about 40%, or about 41%, or about 42%, or about 43%, or about 44%, or about 45%, or about 46%, or about 47%, or about 48%, or about 49% or about 50% of the thickness of the composite layer  34 . 
     Use of the first and second pluralities of layers  38 ,  42  may be advantageous in decreasing a roughness of the composite layer  34  and overall the film  18 . Such a feature may be advantageous in increasing optical properties of the film  18 . For examples, as HfO 2  containing examples of the first layers  38  are produced, crystallites or grains of the HfO 2  may grow, or increase in size, as the first layer  38  is formed. Such growth of the crystallites may result in grain coarsening which overall increases the roughness of the composite layer  34  and the film  18 . As such, by introducing the second layers  42 , which have a different microstructure (e.g., amorphous) than the first layers  38 , the growth of the large crystals may be interrupted, disrupted, or reset. Interrupting, or resetting, of the growth point for the first layers  38  allows the grain size for the first layers  38  to begin again with fine grains. Use of the second layers  42  may allow first layers  38  formed on top of the second layer  42  to have an average microstructural crystal size that is smaller than an average microstructural crystal size of first layers grown without the second layers  42 . According to various examples, each layer of the first plurality of layers  38  is amorphous and each layer of the second plurality of layers  42  is amorphous. 
     “Roughness,” “average surface roughness (Ra),” or like terms refer to, on a microscopic level or below, an uneven or irregular surface condition, such as an average root mean squared (RMS) roughness (Rq). Ra is calculated as the roughness average of a surface&#39;s measured microscopic peaks and valleys. Rq is calculated as the RMS of a surface&#39;s measured microscopic peaks and valleys. When described in terms of Rq, the roughness of the film  18 , the composite layer  34  and/or the index layer  30  may be about 20 nanometers or less, about 19 nm or less, about 18 nm or less, about 17 nm or less, about 16 nm or less, about 15 nm or less, about 14 nm or less, about 13 nm or less, about 12 nm or less, about 11 nm or less, about 10 nm or less, about 9 nm or less, about 8 nm or less, about 7 nm or less, about 6 nm or less, about 5 nm or less, about 4 nm or less, about 3 nm or less, about 2 nm or less or about 1 nm or less. When described in terms of Ra, the roughness may be about 20 nm or less, about 19 nm or less, about 18 nm or less, about 17 nm or less, about 16 nm or less, about 15 nm or less, about 14 nm or less, about 13 nm or less, about 12 nm or less, about 11 nm or less, about 10 nm or less, about 9 nm or less, about 8 nm or less, about 7 nm or less, about 6 nm or less, about 5 nm or less, about 4 nm or less, about 3 nm or less, about 2 nm or less or about 1 nm or less. 
     According to various examples, the individual layer thickness of the first and second plurality of layers  38 ,  42  may be much smaller than the quarter-wave thicknesses of their constituents. For example, the quarter-wave thickness of HfO 2  (e.g., the first layers  38 ) may be about 29.7 nm at a wavelength of 266 nm and the quarter-wave thickness of Al 2 O 3  (e.g., the second layers  42 ) may be about 38.4 nm at a wavelength of 266 nm. As such, the composite layer  34  may optically be considered to be homogenous despite the discrete layering of its constituents. Accordingly, the refractive index and/or quarter-wave thickness of the composite layer  34  may be a combination of the refractive index and quarter wave thickness of the first and second layers  38 ,  42  based on the relative proportions of the first and second layers  38 ,  42 . 
     According to various examples, the optical element  10  including the lens  14  and the film  18  (i.e., in either the non-segregated or laminate examples of the composite layer  34 ) exhibits a variance in reflectance between S-polarization and P-polarization of from about 0% to about 1% over an angle of incidence of from about 0° to about 45° at 266 nm, or from about 0° to about 58° at 266 nm as measured by spectroscopic ellipsometry at a normal angle of incidence. For example, the variance in reflectance between S-polarization and P-polarization may be about 0.9%, about 0.8%, about 0.7%, about 0.6%, about 0.5%, about 0.4%, about 0.3%, about 0.2%, or about 0.1% over an angle of incidence of from about 0° to about 45° at 266 nm, or from about 0° to about 58° at 266 nm as measured by spectroscopic ellipsometry at a normal angle of incidence. According to specific examples, the variance in reflectance between S-polarization and P-polarization may be about 0% to about 0.4% or from about 0% to about 0.2% over an angle of incidence from about 0° to about 45° at 266 nm as measured by spectroscopic ellipsometry at a normal angle of incidence. 
     The film  18  may exhibit a small variation in reflectance over a wavelength band of from about 220 nm to about 500 nm as measured between about a 0.0 ca clear aperture value and a 0.96 ca clear aperture value of the lens  14  using reflective spectral microscopy. According to various examples, the variation in reflectance may be from about 0% to about 10%, or from about 0% to about 9%, or from about 0% to about 8%, or from about 0% to about 7%, or from about 0% to about 6%, or from about 0% to about 5%, or from about 0% to about 4%, or from about 0% to about 3%, or from about 0% to about 2%, or from about 0% to about 1%, or from about 0% to a out 0.1%. For example, the variation in reflectance may be about 0%, about 0.1%, about 0.5%, about 1%, about 1.5%, about 2%, about 2.5%, about 3%, about 3.5%, about 4%, about 4.5%, about 5%, about 5.5%, about 6%, about 6.5%, about 7%, about 7.5%, about 8%, about 8.5%, about 9%, about 9.5%, about 10% or any and all values and ranges therebetween. 
     Referring now to  FIG. 3 , depicted is an exemplary method  50  of forming the optical element  10 . The method  50  may begin with a step  54  of positioning the substantially transparent lens  14  in a reactor chamber, wherein the lens  14  defines the curved surface  22 . According to various examples, the lens  14  may have a steepness given by an R/# value of from about 0.5 to about 1.0. According to various examples, the reactor may be an atomic layer deposition reactor. In such examples, the lens  14  may be positioned within the reactor such that one or more precursors and/or oxidizers which enter the reactor contact the curved surface  22 . Atomic layer deposition is a thin film growth technique based on the sequential exposure of a substrate to self-limiting surface half-reactions. 
     Next, a step  58  of exposing the lens  14  to a first precursor including at least one of Al and Hf such that the first precursor is deposited on the curved surface  22  of the lens  14  is performed. Although Al and Hf have been specifically called out, it will be understood that the first precursor may include any of the compounds noted above in connection with the composite layer  34  without departing from the teachings provided herein. The first precursor may be exposed to the curved surface  22  for a time period ranging from about 0.1 s to about 0.6 s, or from about 0.1 s to about 0.5 s, or from about 0.1 s to about 0.4 s, or from about 0.1 s to about 0.3 s, or from about 0.1 s to about 0.2 s. For example, the first precursor may be exposed to the curved surface  22  of the lens  14  for about 0.1 s, about 0.2 s, about 0.3 s, about 0.4 s, about 0.5 s, about 0.6 second and any and all values and ranges therebetween. The first precursor may be composed of compounds containing Al and/or Hf which may include organics and/or halides of Al and Hf. For example, the first precursor may include at least one of Trimethylaluminum, aluminum acetylacetonate, dimethylaluminum i-propoxid, Tetrakis(diethylamino)hafnium, Tetrakis(ethylmethylamino) hafnium, HfCl 4 , HfI 4 , HfClxH 1−x , HfCl x I 1−x , HFBr 4 , other Al and/or Hf containing compounds and/or combinations thereof. Once the first precursor has been exposed to the curved surface  22  for the predetermined time, the reactor may be purged for about 0.5 s, about 1 s, about 1.5 s, about 2 s, about 2.5 s, about 3 s, about 3.5 s, about 4 s or for about 5 s or greater. It will be understood that the first precursor may be used to form both the non-segregated and laminate examples of the composite layer  34 . For example, the first precursor may include both Al and Hf containing compounds to form the non-segregated composite layer  34 . In another example, the first precursor may include only one of Al or Hf to form either of the first or second plurality of layers  38 ,  42 . According to various examples, the first precursor includes Hf and a halide. 
     Next, a first oxidizer may be introduced to the reactor in a step  62  of exposing the first precursor on the curved surface  22  to the first oxidizer such that the first precursor present on the curved surface  22  of the lens reacts with the first oxidizer to form a high refractive index layer of the film  18  is performed. According to various examples, the high index layer may be either of the first or second plurality of layers  38 ,  42  or the non-segregated composite layer  34 . The first oxidizer may include water vapor, ozone, other materials which may oxidize the first precursor and/or combinations thereof. The first oxidizer may be exposed to the first precursor for a time period ranging from about 0.1 s to about 0.6 s, or from about 0.1 s to about 0.5 s, or from about 0.1 s to about 0.4 s, or from about 0.1 s to about 0.3 s, or from about 0.1 s to about 0.2 s. For example, the first oxidizer may be exposed to the first precursor for about 0.1 s, about 0.2 s, about 0.3 s, about 0.4 s, about 0.5 s, about 0.6 second and any and all values and ranges therebetween. It will be understood that steps  58  and  62  may be repeated until a desired thickness (e.g., of the composite layer  34 , the first layers  38  and/or second layers  42 ) is reached. 
     Next, a step  66  of exposing the high refractive index layer to a second precursor such that the second precursor is deposited on the high refractive index layer is performed. The second precursor may be exposed to the high refractive index layer for a time period ranging from about 0.1 s to about 0.6 s, or from about 0.1 s to about 0.5 s, or from about 0.1 s to about 0.4 s, or from about 0.1 s to about 0.3 s, or from about 0.1 s to about 0.1 s to about 0.2 s. For example, the second precursor may be exposed to the curved surface  22  of the lens  14  for about 0.1 s, about 0.2 s, about 0.3 s, about 0.4 s, about 0.5 s, about 0.6 second and any and all values and ranges therebetween. The second precursor may include tris(dimethylamino)silane, bis(diethylamino)silane, N-(diethylaminosilyl)-N-ethylethanamine, other silicon-containing compounds, other precursors of low index materials and/or combinations thereof. Once the second precursor has been exposed to the high refractive index layer for the predetermined time, the reactor may be purged for about 0.5 s, about 1 s, about 1.5 s, about 2 s, about 2.5 s, about 3 s about 3.5 s, about 4 s or for about 5 s or greater. 
     Next, a step  70  of exposing the second precursor on the high refractive index layer to a second oxidizer such that the second precursor present on the high refractive index layer reacts with the second oxidizer to form a low refractive index layer of the film  18  is performed. According to various examples, the low refractive index layer may be the index layer  30 . The second oxidizer may include water vapor, ozone, other materials which may oxidize the second precursor and/or combinations thereof. The second oxidizer may be exposed to the second precursor for a time period ranging from about 0.1 s to about 0.6 s, or from about 0.1 s to about 0.5 s, or from about 0.1 s to about 0.4 s, or from about 0.1 s to about 0.3 s, or from about 0.1 s to about 0.1 s to about 0.2 s. For example, the second oxidizer may be exposed to the second precursor for about 0.1 s, about 0.2 s, about 0.3 s, about 0.4 s, about 0.5 s, about 0.6 second and any and all values and ranges therebetween. 
     According to various examples, steps  54 - 70  may be performed at an elevated temperature. For example, steps  54 - 70  may be performed at a temperature of from about 20° C. to about 400° C., or from about 100° C. to about 400° C., or from about 200° C. to about 300° C. For example, steps  54 - 70  may be performed at a temperature of about 20° C., about 30° C., about 40° C., about 50° C., about 60° C., about 70° C., about 80° C., about 90° C., about 100° C., about 150° C., about 200° C., about 250° C., about 300° C., about 350° C., about 400° C. or any and all ranges and values therebetween. It will be understood that all the values and ranges disclosed above may be the temperature of the lens  14 , and layers formed on the lens  14  and/or the temperature at which the first and/or second precursors and/or oxidizers are introduced to the reactor. For example, the method  50  may include a step of heating the substantially transparent lens  14  to a temperature of from about 50° C. to about 350° C. 
     It will be understood that although the steps of the method  50  were described in a particular order, the method  50  may include additional steps, omit steps, be repeated or performed in any order where applicable without departing from the teachings provided herein. 
     Use of the present disclosure may offer a variety of advantages. First, as the composite layer  34  includes the first plurality of layers  38  and the second plurality of layers  42 , the roughness of the overall film  18  may be reduced. As explained above, by stacking the first and second layers  38 ,  42 , crystallite growth may be reduced which may decrease the roughness of the film  18 . As roughness of the film  18  may cause an increased scattering of light incident on the film  18 , reduction of the roughness of the film  18  may improve optical properties (e.g., scattering loss, reflection, polarization control, etc.) of the film  18  and optical element  10 . Second, use of atomic layer deposition to produce the film  18  may offer a variety of advantages. For example, use of atomic layer deposition provides a self-limiting film growth technology which enabling precise thickness control of various layers of the film  18  as well as the opportunity to simultaneously multiple surfaces of lens  14  as well as the ability to coat multiple lenses  14  simultaneously. Third, the use of atomic layer deposition allows high, or steep, curvature surfaces such as the curved surfaces  22  to be evenly coated while minimizing or eliminating conventional masking processes. Fourth, as the atomic layer deposition process may simplify tooling fixtures used to secure the lens  14  within the reactor, a reduced risk of mechanical damage to the lenses  14  may be realized as compared to conventional physical vapor deposition processes. Fifth, as the film  18  may be formed with a relatively low carbon impurity content, the film  18  may have a variety of beneficial optical properties. 
     Examples 
     Provided below are a number of non-limiting examples of the present disclosure. 
     Referring now to  FIG. 4 , provided is a plot of ellipsometry data of a layer of Al 2 O 3  formed via atomic layer deposition (e.g. one example of the second plurality of layers  42 ) and a layer of Al 2 O 3  formed via physical vapor deposition. As illustrated by  FIG. 4 , atomic layer deposition of Al 2 O 3  leads to an increase in the refractive index (n) and extinction coefficient (k) at lower wavelengths (e.g., about 175 nm to about 450 nm) as compared to physical vapor deposition examples. Such optical properties achieved by the use of atomic layer deposition to form layers of Al 2 O 3  in a coating (e.g., the film  18 ) may be advantageous in increasing the difference in refractive index between layers of the coating which may be advantageous for antireflective coatings. 
     Referring now to  FIG. 5 , provided is a plot of measured and calculated reflectance (Rx) and transmittance (Tx) spectra on a 1 in x 1 mm SiO 2  substrate with an antireflective coating having a layer of SiO 2  and a layer of Al 2 O 3  applied via atomic layer deposition simultaneously on both sides of the substrate. The thickness of the Al 2 O 3  coatings was 40.5 nm and the SiO 2  coatings had a thickness of 48.3 nm. The Al 2 O 3  was deposited using Trimethylaluminum (TMA) as the metal precursor and water as the oxidant at about 200° C. to about 300° C. The complete growth cycle was 0.2 s TMA, followed by a 3 s purge, followed by 0.3 s of H 2 O, followed by a 3 s purge. The growth rate was 1 Å/cycle. The SiO 2  was deposited using tris[dimethylamino] silane as the precursor and ozone as the oxidizer at about 100° C. to about 300° C. The measured and calculated values for the reflectance and transmittance are for a 5° angle of incidence of light on the substrate and coating. As can be seen from  FIG. 5 , the atomic layer deposition process of forming the coating allows for the formation of Al 2 O 3  and SiO 2  layers which nearly perfectly conform to optical models. As such, by utilizing atomic layer deposition to form the coating, optical lenses formed according to the present disclosure may closely match that of predicted models. 
     Referring now to  FIGS. 6A and 6B , provided are micrographs of a first example (Example 1) of the present disclosure illustrating an optical lens (e.g., the lens  14 ) having a coating (e.g. the film  18 ) disposed on a shaped surface (e.g., the curved surface  22 ). The shaped surface had an R/# value of about 0.5 representing a hemispherical lens. The optical lens had a diameter of about 4 mm and a radius of curvature of 2 mm. In Example 1, Al 2 O 3  was deposited on the optical lens during an atomic layer deposition process using trimethylaluminum as a metal precursor and water as the oxidant at a temperature of from about 200° C. to about 300° C. in a reactor. The growth of the coating was performed by supplying trimethylaluminum to the reactor for about 0.2 seconds, purging the trimethylaluminum from the reactor for about 3 seconds, supplying water vapor for about 0.3 seconds, and purging the water vapor for about 3 seconds. The growth rate of the coating was about 1 Å per cycle. The thickness of the Al 2 O 3  coating was about 36 nm.  FIG. 6A  is a micrograph of a peak, or apex, of a top of the optical lens at a clear aperture value of about 0 ca and  FIG. 6B  is a micrograph of an edge of the optical lens with a clear aperture value of about 0.9 ca. As can be seen in  FIGS. 6A and 6B , a thickness variation across of the coating across the optical lens between the 0 ca value and 0.9 ca value is less than 2% (36.33 nm vs. 36.70 nm) despite the high curvature of the hemispherically shaped surface. The micrographs of  FIGS. 6A and 6B  indicate that in addition to providing better optical qualities, use of the atomic layer deposition process may achieve highly uniform coatings across steeply curved surfaces (i.e., without traditional masking steps) which may be advantageous in ensuring consistent optical properties across the coating. 
     Referring now to  FIGS. 7A and 7B , provided is reflectance data of a second example (i.e., Example 2) at various points clear aperture (ca) along an optical lens. The optical lens was an SiO 2  hemispherical lens with a 2 mm radius of curvature. The optical lens included a 109 nm thick Al 2 O 3  coating. The Al 2 O 3  was deposited using Trimethylaluminum (TMA) as the metal precursor and water as the oxidant at about 200° C. to about 300° C. The complete growth cycle was 0.2 s TMA, followed by a 3 s purge, followed by 0.3 s of H 2 O, followed by a 3 s purge. The growth rate was 1 Å/cycle. The plots of  FIGS. 7A and 7B  provide percent reflectance vs. wavelength for points along the optical lens radiating from the center to the edge ( FIG. 7A ) and rotated ( FIG. 7B ) along 90° polar increments (e.g., r0 being north, r90 being east, r180 being south and r270 being west) at a constant clear aperture value of 0.96 ca.  FIG. 7A  indicates that there is about 2% or less variance in the reflectance between the various clear aperture values of the film.  FIG. 7B  indicates that there was essentially no asymmetry in reflectance between 90° separated points around the optical lens at a clear aperture value of about 0.96 ca. As reflection is a function coating uniformity, and as the reflectance across the various points of the optical lens are substantially uniform (e.g., have a variance of about 2% or less), the atomic layer deposition of the Al 2 O 3  coating provides a uniform coating not only with respect to clear aperture value of the optical lens, but also rotationally (azimuthally) around the optical lens. 
     Referring now to  FIG. 8 , provided is a plot of measured reflectance spectral distribution of a third example (i.e., Example 3) of the present disclosure. Example 3 includes a coating having a SiO 2  layer and an Al 2 O 3  layer to form an antireflection coating on a 4 mm diameter SiO 2  hemisphere lens with a 2 mm radius of curvature. The thickness of the Al 2 O 3  coatings was 40.5 nm and the SiO 2  coatings had a thickness of 48.3 nm. The Al 2 O 3  was deposited using Trimethylaluminum (TMA) as the metal precursor and water as the oxidant at about 200° C. to about 300° C. The complete growth cycle was 0.2 s TMA, followed by a 3 s purge, followed by 0.3 s of H 2 O, followed by a 3 s purge. The growth rate was 1 Å/cycle. The SiO 2  was deposited using tris[dimethylamino] silane as the precursor and ozone as the oxidizer at about 100° C. to about 300° C.  FIG. 8  provides the reflectance of Example 3 measured at a 0.61 clear aperture (0.61 ca) value and 0.96 clear aperture (0.96 ca) value and at various polar coordinates around Example 3 (e.g., r0 being north, r90 being east, r180 being south and r270 being west). As reflection is a function coating uniformity, and as the reflectance across the various points of the coating have approximately the same minimum reflectance around 285 nm, the data indicates a uniform and symmetrical antireflective coating across the hemisphere lens when measured near the circumference of the lens. 
     Referring now to  FIGS. 9A-9G  depicted are calculated plots of optical data for six different examples consistent with the optical element  10  of the present disclosure. Each of the examples, where indicated by relative proportions (e.g., in mole percent) of constituents, have nano-laminated examples of mixed layers (e.g., segregated examples of the composite layer  34 ). As explained above, mixed layers with a plurality of layers (e.g., the first and second layers  38 ,  42 ) with thicknesses much thinner than quarter wave thicknesses of the composition of the layers allow the mixed layer to optically be treated as a single layer with the optical properties of the mixed layer being based on the relative proportions of the layers within the mixed layer. Further, examples of the mixed layer where its constituents are homogenous, and non-segregated, may also exhibit optical properties based on the relative proportions of the constituents within the mixed layer. As such, the optical properties provided in  FIGS. 9A-9G  are consistent with all examples of the optical element  10 . The plots of  FIGS. 9A-9G  are provided as percent reflectance over an angle of incidence of from about 0° to about 45° at 266 nm as measured by spectroscopic ellipsometry. As can be seen from the plots of  FIGS. 9A-9G , the variation between the S-polarization and P-polarization is low indicating a low optical retardation. As optical retardation is generally caused by stress in the coating, non-uniformity of the coating, curvature of the optical lens and other deleterious factors, the low variation in polarization indicates a uniform and low-stress coating. 
     Referring now to  FIG. 9A , depicted is the S-polarization and P-polarization reflection of a six-layer antireflection coating positioned on an SiO 2  2 mm radius of curvature hemispherical lens at 266 nm according to a fourth example (i.e., Example 4). The antireflection coating of  FIG. 9A  has a layered structure, from the lens outward, of 5.27 nm of HfO 2 , 11.9 nm of SiO 2 , 70.28 nm of HfO 2 , 10.96 nm of SiO 2 , 33.46 nm of HfO 2  and 52.28 nm of SiO 2 . 
     Referring now to  FIG. 9B , depicted is the S-polarization and P-polarization reflection of a six-layer antireflection coating positioned on an SiO 2  2 mm radius of curvature hemispherical lens at 266 nm according to a fifth example (i.e., Example 5). The antireflection coating of  FIG. 9B  has a layered structure, from the lens outward given in thicknesses, of 5.33 nm of HfO 2 /Al 2 O 3 , 9.07 nm of SiO 2 , 70.17 nm of HfO 2 /Al 2 O 3 , 12.48 nm of SiO 2 , 36.94 nm of HfO 2 /Al 2 O 3  and 51.32 nm of SiO 2 . The relative molar proportions of HfO 2  to Al 2 O 3  for  FIG. 9B  are 90% HfO 2  and 10% Al 2 O 3 . 
     Referring now to  FIG. 9C , depicted is the S-polarization and P-polarization reflection of a six-layer antireflection coating positioned on an SiO 2  2 mm radius of curvature hemispherical lens at 266 nm according to a sixth example (i.e., Example 6). The antireflection coating of  FIG. 9C  has a layered structure, from the lens outward given in thicknesses, of 6.33 nm of HfO 2 /Al 2 O 3 , 9.07 nm of SiO 2 , 70.17 nm of HfO 2 /Al 2 O 3 , 12.48 nm of SiO 2 , 37.94 nm of HfO 2 /Al 2 O 3  and 51.32 nm of SiO 2 . The relative molar proportions of HfO 2  to Al 2 O 3  for  FIG. 9C  are 80% HfO 2  and 20% Al 2 O 3 . 
     Referring now to  FIG. 9D , depicted is the S-polarization and P-polarization reflection of a six-layer antireflection coating positioned on an SiO 2  2 mm radius of curvature hemispherical lens at 266 nm according to a seventh example (i.e., Example 7). The antireflection coating of  FIG. 9D  has a layered structure, from the lens outward given in thicknesses, of 6.8 nm of HfO 2 /Al 2 O 3 , 4.71 nm of SiO 2 , 69.9 nm of HfO 2 /Al 2 O 3 , 16.41 nm of SiO 2 , 40.24 nm of HfO 2 /Al 2 O 3  and 50.54 nm of SiO 2 . The relative molar proportions of HfO 2  to Al 2 O 3  for  FIG. 9D  are 70% HfO 2  and 30% Al 2 O 3 . 
     Referring now to  FIG. 9E , depicted is the S-polarization and P-polarization reflection of a six-layer antireflection coating positioned on an SiO 2  2 mm radius of curvature hemispherical lens at 266 nm according to an eighth example (i.e., Example 8). The antireflection coating of  FIG. 9E  has a layered structure, from the lens outward given in thicknesses, of 7.65 nm of HfO 2 /Al 2 O 3 , 1.36 nm of SiO 2 , 69.76 nm of HfO 2 /Al 2 O 3 , 20.6 nm of SiO 2 , 42.03 nm of HfO 2 /Al 2 O 3  and 49.61 nm of SiO 2 . The relative molar proportions of HfO 2  to Al 2 O 3  for  FIG. 9E  are 60% HfO 2  and 40% Al 2 O 3 . 
     Referring now to  FIG. 9F , depicted is the S-polarization and P-polarization reflection of a six-layer antireflection coating positioned on an SiO 2  2 mm radius of curvature hemispherical lens at 266 nm according to a ninth example (i.e., Example 9). The antireflection coating of  FIG. 9F  has a layered structure, from the lens outward given in thicknesses, of 6.49 nm of HfO 2 /Al 2 O 3 , 1.16 nm of SiO 2 , 71.98 nm of HfO 2 /Al 2 O 3 , 23.02 nm of SiO 2 , 42.36 nm of HfO 2 /Al 2 O 3  and 49.67 nm of SiO 2 . The relative molar proportions of HfO 2  to Al 2 O 3  for  FIG. 9F  are 50% HfO 2  and 50% Al 2 O 3 . 
     Referring now to  FIG. 9G , depicted is the S-polarization and P-polarization reflection of a six-layer antireflection coating positioned on an SiO 2  2 mm radius of curvature hemispherical lens at 266 nm according to a tenth example (i.e., Example 10). The antireflection coating of  FIG. 9G  has a layered structure, from the lens outward given in thicknesses, of 5.67 nm of HfO 2 , 1.73 nm of SiO 2 , 68.66 nm of HfO 2 /Al 2 O 3 , 21.28 nm of SiO 2 , 42.44 nm of HfO 2 /Al 2 O 3  and 49.44 nm of SiO 2 . The relative molar proportions of HfO 2  to Al 2 O 3  for  FIG. 9G  are 40% HfO 2  and 60% Al 2 O 3 . 
     Table 1 provides the averaged reflectance of the examples of  FIGS. 9A-9F  at 266 nm. As can be seen from  FIGS. 9A-9G  and table 1, use of the laminated layers of HfO 2  and Al 2 O 3  enables control of reflected polarization split with a reduced average reflection in addition to surface roughness reduction. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Example 
                 Averaged Reflectance (%) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Example 4 
                 0.25 
               
               
                   
                 Example 5 
                 0.21 
               
               
                   
                 Example 6 
                 0.221 
               
               
                   
                 Example 7 
                 0.19 
               
               
                   
                 Example 8 
                 0.20 
               
               
                   
                 Example 9 
                 0.19 
               
               
                   
                   
               
            
           
         
       
     
     Clause 1 of the description discloses: 
     An optical element, comprising: 
     an optically transparent lens defining a curved surface comprising a steepness given by an R/# of from about 0.5 to about 1.0; and 
     a film positioned on the curved surface, the film comprising: 
     an index layer; and
 
a composite layer positioned on the curved surface having a refractive index greater than the index layer, the composite layer comprising HfO 2  and Al 2 O 3 , wherein the composite layer comprises a mole fraction X of HfO 2 , wherein X is from about 0.05 to about 0.95 and a mole fraction of Al 2 O 3  in the composite layer is 1−X.
 
     Clause 2 of the description discloses: 
     The optical element of clause 1, wherein the mole fraction X of HfO 2  is from about 0.55 to about 0.65. 
     Clause 3 of the description discloses: 
     The optical element of clause 1 or 2, wherein the HfO 2  is segregated into a first plurality of layers and the Al 2 O 3  is segregated into a second plurality of layers, and wherein the first plurality of layers alternate with the second plurality of layers. 
     Clause 4 of the description discloses: 
     The optical element of clause 3, wherein each layer of the first plurality of layers is amorphous and each layer of the second plurality of layers is amorphous. 
     Clause 5 of the description discloses: 
     The optical element of clause 3 or 4, wherein the composite layer has a thickness, the thickness comprising about 60% of the first plurality of layers and about 40% of the second plurality of layers. 
     Clause 6 of the description discloses: 
     The optical element of any of clauses 1-5, wherein the index layer comprises SiO 2 . 
     Clause 7 of the description discloses: 
     The optical element of any of clauses 1-6, wherein the composite layer has a thickness of from about 30 nm to about 80 nm. 
     Clause 8 of the description discloses: 
     The optical element of clause 7, wherein the composite layer has a thickness of from about 40 nm to about 70 nm. 
     Clause 9 of the description discloses: 
     The optical element of any of clauses 1-8, wherein the index layer has a thickness of from about 1 nm to about 60 nm. 
     Clause 10 of the description discloses: 
     The optical element of clause 9, wherein the index layer has a thickness of from about 1 nm to about 30 nm. 
     Clause 11 of the description discloses: 
     The optical element of any of clauses 1-10, wherein the optical element exhibits a variance in reflectance between S-polarization and P-polarization of from about 0% to about 0.4% over an angle of incidence of from about 0° to about 45° at 266 nm as measured by spectroscopic ellipsometry. 
     Clause 12 of the description discloses: 
     The optical element of clause 11, wherein the optical element exhibits a variance in reflectance between S-polarization and P-polarization of from about 0% to about 0.2% over an angle of incidence from about 0° to about 45° at 266 nm as measured by spectroscopic ellipsometry. 
     Clause 13 of the description discloses: 
     The optical element of any of clauses 1-12, wherein the composite layer is amorphous. 
     Clause 14 of the description discloses: 
     An optical element, comprising: 
     a lens defining a curved surface; and 
     a film positioned on the curved surface, the film comprising: 
     a laminate layer positioned on the curved surface, the laminate layer comprising a plurality of first layers comprising HfO 2  and a plurality of second layers comprising Al 2 O 3 ; and
 
an index layer comprising SiO 2 , wherein the film has a variation in reflectance of from about 0% to about 4% over a wavelength band of from about 220 nm to about 500 nm as measured across the lens and between about a 0 clear aperture value and a 0.96 clear aperture value as measured by reflective spectral microscopy.
 
     Clause 15 of the description discloses: 
     The optical element of clause 14, wherein the film has a variation in reflectance of from about 0% to about 2% over a wavelength band of from about 220 nm to about 500 nm as measured across the lens and between about a 0 clear aperture value and a 0.96 clear aperture value as measured by reflective spectral microscopy. 
     Clause 16 of the description discloses: 
     The optical element of clause 14 or 15, wherein the plurality of first layers have a refractive index of about 2.3 at 266 nm. 
     Clause 17 of the description discloses: 
     The optical element of any of clauses 14-16, wherein the plurality of second layers have a refractive index of about 1.7 at 266 nm. 
     Clause 18 of the description discloses: 
     The optical element of any of clauses 14-17, wherein the plurality of first and second layers of the laminate layer are stacked in an alternating order. 
     Clause 19 of the description discloses: 
     The optical element of any of clauses 14-18, wherein the first plurality of layers comprise about 60% of the thickness of the laminate layer and the plurality of second layers comprise about 40% of the thickness of the laminate layer. 
     Clause 20 of the description discloses: 
     The optical element of any of clauses 14-19, wherein the laminate layer is amorphous. 
     Clause 21 of the description discloses: 
     A method of forming a film of an optical element, comprising the step of: 
     positioning a substantially transparent lens in a reactor chamber, wherein the lens defines a curved surface; 
     exposing the lens to a first precursor comprising at least one of Al and Hf such that the first precursor is deposited on the curved surface of the lens; 
     exposing the first precursor on the curved surface to a first oxidizer such that the first precursor present on the curved surface of the lens reacts with the first oxidizer to form a high refractive index layer of the film; 
     exposing the high refractive index layer to a second precursor such that the second precursor is deposited on the high refractive index layer; and 
     exposing the second precursor on the high refractive index layer to a second oxidizer such that the second precursor present on the high refractive index layer reacts with the second oxidizer to form a low refractive index layer of the film. 
     Clause 22 of the description discloses: 
     The method of clause 21, further comprising the step of:
 
heating the substantially transparent lens to a temperature of from about 50° C. to about 350° C.
 
     Clause 23 of the description discloses: 
     The method of clause 21 or 22, wherein the step of exposing the first precursor on the curved surface to a first oxidizer further comprises the step: 
     exposing the first precursor on the curved surface to water vapor. 
     Clause 24 of the description discloses: 
     The method of any of clauses 21-23, wherein the step of positioning the substantially transparent lens in the reactor chamber further comprises positioning the substantially transparent lens in the reactor chamber comprising a Steepness given by an R/# of from about 0.5 to about 1.0. 
     Clause 25 of the description discloses: 
     The method of any of clauses 21-24, wherein the first precursor comprises Hf and a halide. 
     Modifications of the disclosure will occur to those skilled in the art and to those who make or use the disclosure. Therefore, it is understood that the embodiments shown in the drawings and described above are merely for illustrative purposes and not intended to limit the scope of the disclosure, which is defined by the following claims, as interpreted according to the principles of patent law, including the doctrine of equivalents. 
     It will be understood by one having ordinary skill in the art that construction of the described disclosure, and other components, is not limited to any specific material. Other exemplary embodiments of the disclosure disclosed herein may be formed from a wide variety of materials, unless described otherwise herein. 
     It will be understood that any described processes, or steps within described processes, may be combined with other disclosed processes or steps to form structures within the scope of the present disclosure. The exemplary structures and processes disclosed herein are for illustrative purposes and are not to be construed as limiting. 
     It is also to be understood that variations and modifications can be made on the aforementioned structures and methods without departing from the concepts of the present disclosure, and, further, it is to be understood that such concepts are intended to be covered by the following claims, unless these claims, by their language, expressly state otherwise. Further, the claims, as set forth below,