Anti-reflective, super-hydrophobic optical element having good mechanical strength

Abstract

 The invention relates to an optical element (10) which is transparent to at least one use wavelength (λ0), is anti-reflective and super-hydrophobic, and comprises a grating of sub-wavelength primary microstructures (MS1) which are made of a first material (M1) and are formed on a substrate (Sub), the primary microstructures having a conical or frustoconical shape, a first width (L1) and a first height (H1) such that a ratio of the first width to the first height is less than ½, and a pitch of the grating (p1) of less than the first height (H1) of the primary microstructure, the grating of primary microstructures thus having a structured surface (S), the structured surface being at least partially covered with a plurality of secondary microstructures (MS2) which have a nanopillar or nanocone shape and are made of a second material (M2) which is different from the first material, the second material having a hardness that is strictly greater than the hardness of the first material.Fig. 3

Description

Anti-reflective, super-hydrophobic optical element having good mechanical strength TECHNICAL FIELD[%3] The present invention lies in the field of optical windows, and more particularly of anti-reflective coatings dedicated to optical components or windows. The invention relates more particularly to a transmission optical element having anti-reflective (AR) and super-hydrophobic (SH) properties and exhibiting good mechanical strength. BACKGROUND[%3] Conventionally, anti-reflective coatings are obtained in various ways: a thin film, a thin film stack, or by surface structuring on a sub-wavelength scale.[%3] AR coatings are optimized for a used wavelength λ0 or even a given spectral band (containing λ0). The term sub-wavelength (referred to as sub-λ) is understood to mean structures having, as a first approximation, a size smaller than the ratio λ0 / ns, ns being the index of the substrate of the coating or window. These sub-λ structures will be referred to hereinafter as microstructures or pads, the term SWS standing for sub-wavelength structures.[%3] One highly relevant approach is the use of an array of conical or frustoconical microstructures MS, as illustrated in figure 1, which synthesize an index gradient perpendicular to the substrate so as to gradually adapt the index. The sub-λ microstructures form an artificial material having an effective index neff. An array of microstructures is understood to mean the pattern or spatial arrangement of the pads on the surface of a substrate Sub.[%3] The pitch of the array of pads is defined as p. The arrangement of the pads may be random or periodic. In the first case, the pitch p is defined as being the minimum distance between the center of any pad of the array and the center of its nearest neighbor. A "periodic" array of microstructures is understood to mean pads repeating with a fixed pitch to within ±10%.[%3] The parameters of the pad are its width L and its height H. It is made of a material Mat that may be identical to the material of the substrate or different therefrom, depending on the materials and manufacturing technology used.[%3] From a practical point of view, however, it is not easy to obtain a perfect gradient that varies from 1 (index of air) to the index of the substrate ns. Indeed, a surface fill rate of 100% is not possible with cones, even with a hexagonal mesh, which enables at best a theoretical fill rate of 0.9 when the cones are joined at their base. In practice, the maximum achievable rate is around 0.7. The use of pyramids enables a surface fill rate of 1, but realizing such structures is difficult and depends on the material, and it is preferred to realize cones.[%3] It is also desirable to produce components having a hydrophobic or super-hydrophobic surface. Such a property may be obtained by structuring the surface relatively finely. The roughness or structuring of the surface has the effect of trapping air in the structure, and a drop of water then rests on a composite surface made of solid and air. This effect, commonly called the "fakir" effect, makes it possible to achieve high contact angles (~160°) and a relatively low contact angle hysteresis (less than 10°).[%3] It is known to use periodic SWS to impart super-hydrophobic (SH) and anti-reflective (AR) properties on a glass transparent in the visible (Park KC et al. "Nanotextured silica surfaces with robust superhydrophobicity and omnidirectional broadband supertransmissivity" ACS Nano. 6(5):3789-99; 2012).[%3] Optical surfaces using SWS are therefore ideal candidates for realizing camera windows, binoculars or anti-reflective windows in observation systems in maritime environments, for example. However, these microstructures degrade rapidly under difficult conditions of use (rain, hail, sand erosion, etc.), thereby greatly limiting the duration of the SH and AR properties of these optical elements in these environments.[%3] In order to solve this problem, document WO2020 / 178304 proposes to conformally deposit a thin homogeneous layer CP of a material having a high hardness, greater than the hardness of the material Mat, as illustrated in figure 2. This layer CP has the role of making SWS resistant to severe environmental conditions. In that document, the hard material is chosen from: alumina, preferably in the sapphire phase, DLC (diamond-like carbon) or ZrO2. One exemplary embodiment is an AR structure in the 8-12 µm band with conical 3.2 µm-high germanium microstructures with a pitch of 1.6 µm, covered with an annealed alumina layer. The conformal layer protects the microstructures, and the SH and AR properties are considered to be maintained.[%3] However, it turns out that depositing a layer of a "hard" material on the array of cones tends, in some cases, to interfere with the optical properties of the surface and thus to reduce its anti-reflective capabilities.[%3] It should also be noted that, since the hard layer has a constant index and the hard materials compatible with a given array of cones are highly limited, the addition of this layer does not make it possible to improve the optical and hydrophobic performance of the array of cones, and at the very most it preserves the performance of the initial component without the layer.[%3] One aim of the present invention is to overcome the abovementioned drawbacks by providing an anti-reflective, super-hydrophobic optical element made of SWS and that still exhibits high mechanical strength and improved performance.SUMMARY[%3] The present invention relates to an anti-reflective and super-hydrophobic optical element that is transparent to at least one used wavelength, comprising an array of sub-wavelength primary microstructures that are made of a first material and formed on a substrate, the primary microstructures having a conical or frustoconical shape, a first width and a first height such that a ratio between the first width and the first height is less than ½, and a pitch of the array less than the first height of said primary microstructure, the array of primary microstructures thus having a structured surface, said structured surface being at least partially covered with a plurality of secondary microstructures in the form of nano-pillars or nano-cones and being made of a second material different from the first material, the second material having a hardness strictly greater than the hardness of the first material.[%3] According to one embodiment, the secondary microstructures cover substantially the entire structured surface.[%3] According to one embodiment, the array of primary microstructures is periodic, in a square or hexagonal mesh.[%3] According to one embodiment, a surface density of the secondary microstructures in the covered zones is determined such that an effective index of the plurality of secondary microstructures varies monotonically in a vertical direction perpendicular to the plane of the substrate.[%3] According to one embodiment, a surface density of the secondary microstructures in the covered zones is determined such that an effective index of the plurality of secondary microstructures is between the index of air and the index of the substrate.[%3] According to one embodiment, the second material is chosen from: diamond, alumina, DLC, SiN.[%3] According to one embodiment, the second material is boron-doped diamond.[%3] According to one embodiment, the optical element furthermore comprises a layer of second material arranged on the primary structures and on which the secondary microstructures are arranged.[%3] According to one embodiment, the hardness of the second material is greater than or equal to 1.3 times the hardness of the first material, hardness being measured on the Knoop scale.[%3] According to one embodiment, the used wavelength is in the visible or near-infrared, and the hardness of the second material is greater than or equal to 3 times the hardness of the first material, hardness being measured on the Knoop scale.[%3] According to one embodiment, the used wavelength is in the MWIR (3-5 µm) or LWIR (8-12 µm) band, and the hardness of the second material is greater than or equal to 5 times the hardness of the first material, hardness being measured on the Knoop scale.[%3] According to one embodiment, the secondary microstructures are arranged in a forest or grass.[%3] The invention also relates to a process for manufacturing an anti-reflective and super-hydrophobic optical element that is transparent to at least one used wavelength.[%3] According to a first variant, the process comprises the following steps:taking an array of sub-wavelength primary microstructures that are made of a first material and formed on a substrate, the primary microstructures having a conical or frustoconical shape, a first width and a first height such that a ratio between the first width and the first height is less than ½, and a pitch of the array less than the first height of said primary microstructure, the array of primary microstructures thus having a structured surface,depositing, on the structured surface, a layer of a second material different from the first material and having a hardness strictly greater than that of the first material,etching said structured surface covered with the layer of second material by way of plasma etching, ion etching, chemical etching or a combination of these techniques, such that the structured surface is at least partially covered with a plurality of secondary microstructures in the form of nano-pillars or nano-cones.[%3] According to one embodiment, the etching is reactive ion etching.[%3] According to a second variant, the process comprises the following steps:taking an array of sub-wavelength primary microstructures that are made of a first material and formed on a substrate, the primary microstructures having a conical or frustoconical shape, a first width and a first height such that a ratio between the first width and the first height is less than ½, and a pitch of the array less than the first height of said primary microstructure, the array of primary microstructures thus having a structured surface,depositing, by evaporation or cathode sputtering, a second material at an incidence that is oblique with respect to the normal to the substrate, such that the structured surface is at least partially covered with a plurality of secondary microstructures (MS2) in the form of nano-pillars or nano-cones, the array of primary microstructures rotating during the deposition step, the second material being different from the first material and having a hardness strictly greater than that of the first material.[%3] The following description presents a number of exemplary embodiments of the device of the invention: these examples do not limit the scope of the invention. These exemplary embodiments contain not only features that are essential to the invention but also additional features associated with the embodiments in question.BRIEF DESCRIPTION OF THE DRAWINGS[%3] The invention will be better understood and other features, aims and advantages thereof will become apparent from the following detailed description, which is provided with reference to the appended drawings, which are given by way of non-limiting examples and in which:[%3] Figure 1, already cited, illustrates anti-reflective and super-hydrophobic microstructures known from the prior art.[%3] Figure 2, already cited, illustrates anti-reflective and super-hydrophobic microstructures exhibiting good mechanical strength, known from the prior art.[%3] Figure 3 illustrates an optical element according to the invention with conical primary microstructures.[%3] Figure 4 illustrates an optical element according to the invention with frustoconical primary microstructures.[%3] Figure 4bis illustrates one embodiment of the optical element according to the invention in which the optical element comprises a layer made of second material arranged on the primary structures and on which the secondary microstructures are arranged.[%3] Figure 5 illustrates the simulated optical transmission as a function of wavelength of an element made of silica alone and for 4 angles of incidence 0°, 45°, 60° and 70°.[%3] Figure 6 illustrates the simulated optical transmission as a function of wavelength of a structure with cones alone according to the prior art for these same 4 angles of incidence 0°, 45°, 60° and 70° and for unpolarized light.[%3] Figure 7 illustrates the simulated optical transmission as a function of wavelength of a structure with cones covered with a 150 nm-thick homogeneous diamond layer according to the prior art for these same 4 angles of incidence 0°, 45°, 60° and 70° and for unpolarized light.[%3] Figure 8 illustrates the division of the structure into layers indexed i of thickness 25 nm, carried out for the simulation. Layer no. 1 is the layer of air, followed by 6 diamond layers (no. 2 to 7) for the case [cones + homogeneous diamond] or 6 layers of air for the case [cones alone], and then 20 layers (no. 8 to 27) for the cones themselves made of silica (and the diamond layer where applicable), and one layer (no. 28) for the substrate Sub, also made of silica.[%3] Figure 9 illustrates examples of layers for the structure comprising just the cones (horizontal cutting plane along the layer).[%3] Figure 10 illustrates examples of layers for the structure comprising the cones covered with a homogeneous diamond layer (horizontal cutting plane along the layer).[%3] Figure 11 illustrates the variation in the effective index for both cases of cones alone and cones with a diamond layer, as a function of the number of the layer.[%3] Figure 12 illustrates the transmission of the optical element according to the invention comprising a forest of pillars with an equivalent effective index of 1.3.[%3] Figure 13 illustrates the optical transmission of the structure comprising cones alone, for cones arranged with a pitch of 200 nm.[%3] Figure 14 illustrates the optical transmission of the structure comprising cones covered with a homogeneous diamond layer, for cones arranged with a pitch of 200 nm.[%3] Figure 15 illustrates the optical transmission of the structure comprising cones covered with a forest of pillars made of diamond with an effective index 1.8, for cones arranged with a pitch of 200 nm.[%3] Figure 16 illustrates the optical transmission of the structure comprising cones covered with a forest of pillars made of diamond with an effective index 1.3, for cones arranged with a pitch of 200 nm.[%3] Figure 17 illustrates a method for producing the optical element according to the invention.DETAILED DESCRIPTION[%3] The term "vertical" is understood to mean a direction Z perpendicular to the surface of the substrate.[%3] An "aspect ratio" R of an object is understood to mean the ratio between its width L and its height H. The term width is understood to mean the largest dimension in the plane tangent to the surface, and the term height H is understood to mean the largest dimension perpendicular to the tangent to the surface: R=L / H[%3] The term "transparent" is understood here to mean a transmission greater than 50%, preferably 75% and more preferably 95% at a wavelength or over an illumination wavelength range.[%3] The term "nano-pillars" is understood broadly to mean a nanostructure that is elongate in a direction D and may be in the shape of a solid or hollow cylinder, but also a zigzag, spiral, etc. The direction of elongation D is either the normal to the surface on which the nano-pillar is formed or a direction inclined with respect to the plane of this surface.[%3] The optical element 10 according to the invention is illustrated in figures 3 and 4. The optical element 10 is transparent to at least one used wavelength λ0, or in a used spectral band BS0 containing λ0. The element 10 is also anti-reflective for the used wavelength (or the band BS0) and super-hydrophobic.[%3] The element 10 comprises an array of primary microstructures MS1 made of a first material M1 and formed on a substrate Sub, the primary microstructures having a conical (figure 3) or frustoconical (figure 4) shape.[%3] The shape of the primary microstructures or pads is defined by a first width L1 and a first height H1, and they are separated by a first pitch p1.[%3] The microstructures are sub-λ and therefore, as a first approximation:[%3] L1<λ0 / ns, ns being the index of the substrate Sub.[%3] Strictly speaking, for a square mesh, L1<λ0 / (ns+sin⁡θ), where θ is the angle of incidence of the illumination.[%3] In the case of a hexagonal mesh, L1<~1.22 λ0 / (ns+sin⁡θ). .[%3] More generally, a condition for covering various types of mesh and illumination is:[%3] L1<1.5 λ0 / ns[%3] The conical or frustoconical shape, combined with the sub-λ character of the primary microstructures, ensures the anti-reflective function (see the prior art).[%3] The arrangement of the pads is random or periodic. In the first case, the pitch p1 is defined as being the minimum distance between the center of any pad of the array and the center of its nearest neighbor. In the second case, a "periodic" array of microstructures is understood to mean pads repeating with a fixed pitch to within ±10%. The periodic arrangement is typically in a square or hexagonal mesh (best fill rate).[%3] The aspect ratio R1 between the width L1 and the height H1 is less than ½, and the pitch of the array p1 is less than the first height H1:[%3] R1 = L1 / H1 ≤ 1 / 2[%3] p1 ≤ H1[%3] These two properties are necessary to obtain the hydrophobic character of the element. Indeed, the pitch p1 and the aspect ratio R1 have a direct impact on the angle of contact between the water and the structured surface (see the prior art).[%3] The array of primary microstructures thus has a structured surface S, consisting of the surface of the cones and the inter-cone surface. The set of cones constitutes the primary structure of the element 10. Overall, this primary structure is similar to the one described above in the prior art in figure 1.[%3] For good AR and SH behavior, a high pad density and a low aspect ratio (≤1 / 2, preferably ≤1 / 3, preferably ≤1 / 6) are sought. For a non-periodic array, the minimum distance from a cone to its nearest neighbor is preferably between H1 / 3 and 2.H1 / 3.[%3] According to one embodiment, the cones of the primary microstructures are contiguous.[%3] The first surface density DS1, or surface fill rate, of the first microstructures MS1 is defined as the proportion of the total surface area of the substrate occupied by the base of the cones MS1. In the case of a hexagonal mesh of cones and contiguous cones, this is 0.9 in theory, but is limited to 0.7 in practice.[%3] In the optical element 10 according to the invention, the structured surface S is furthermore at least partially covered with a plurality of secondary microstructures MS2 in the form of nano-pillars or nano-cones. These pillars or these cones are for example nanowires (NW) or nanospikes made of a material M2 different from the material M1. Hereinafter, the generic term "nano-pillar" will be used, this grouping together the various possible forms of the secondary micro / nanostructures.[%3] According to one embodiment, the arrangement of the microstructures MS2 is relatively compact and random, and is then commonly referred to as "forest" or "grass".[%3] The secondary microstructures constitute what is referred to as the secondary structure arranged on the primary structure. The element 10 thus has what is referred to as a hierarchical structure, the dimensions of the secondary structures being smaller than the dimensions of the primary structures.[%3] These nano-pillars typically have a width L2 of between 1 and 50 nm and a height H2 of between 50 nm and 500 nm, or even more (forests of nano-pillars with a height of the order of a micron are feasible). They preferably have an aspect ratio R2 ≤ 1 / 5, or even 1 / 10 and up to 1 / 100 or more. In the case of a "forest", the arrangement of the nano-pillars is not periodic but random, and the nano-pillars are not necessarily all identical and / or of the same height.[%3] Finally, the second material M2 has a hardness strictly greater than the hardness of the first material M1 of the pads. The hardness of the materials M1 and M2 is understood here to mean for the bulk material.[%3] Somewhat counter-intuitively, the inventors have established that replacing a homogeneous hard layer covering the element (see the prior art) with a structured assembly of smaller scale than the structures MS1, also called secondary microstructures, makes it possible to maintain a good hardness of the element. The presence of the forest of nano-pillars ensures good mechanical protection of the primary structure.[%3] It also turns out that this secondary structuring on a scale smaller than the primary structuring makes it possible to obtain better optical results than the homogeneous hard layer, and therefore to overcome the drawbacks thereof. Indeed, the secondary structuring behaves like a secondary artificial layer having an intermediate effective index between air and the material M1 and / or the substrate.[%3] Moreover, this second level of structuring improves fluidic performance, and notably increases hydrophobicity.[%3] The optical element according to the invention thus realizes a coating i) exhibiting improved mechanical strength in order to limit erosion / abrasion problems when the optics are implemented in a harsh environment; ii) exhibiting an increased AR property at the angle of incidence compared to the homogeneous hard layer; iii) exhibiting improved hydrophobicity.[%3] According to one embodiment, the first material M1 is identical to the material of the substrate. This is for example the case when the cones are produced directly on a substrate by masking and etching. According to another embodiment, M1 is different from the material of the substrate.[%3] The invention is applicable in used spectral bands such as the visible / near-infrared, the MWIR (3-5 µm) or LWIR (8-12 µm) band. The dimensions of the structures MS1 and MS2 and the pitch of the array of MS1 are adapted according to the used spectral band.[%3] For the visible / near-IR, the substrate and the structures MS1 are for example made of silica or BK7, and the material M2 is typically chosen from alumina (preferably sapphire-annealed alumina) or diamond.[%3] For the MWIR [3-5 µm] band, the substrate / MS1 is for example silicon, and the material M2 is typically chosen from alumina (preferably sapphire-annealed alumina), diamond, DLC (diamond-like carbon) and SiN.[%3] For the LWIR band, the substrate / MS1 is for example germanium, and the material M2 is typically chosen from diamond or DLC.[%3] Examples of hardness of the various materials are given in Table I below.MaterialKnoop hardness (kg / mm2).Molten silica500N-BK7610Silicon1150Germanium780Diamond7000Sapphire2200DLC1026100SiN1500-2000Table I[%3] For improved protection, the second material preferably has a hardness greater than or equal to 1.3 times the hardness of the material M1, hardness being measured on the Knoop scale (kg / mm2).[%3] With diamond on glass (silica or N-BK7), it is observed that the ratio of the hardnesses is greater than or equal to 10. For sapphire on glass, this ratio is greater than or equal to 3. Thus, in the visible / near-IR band, the ratio of the hardnesses is preferentially greater than or equal to 3.[%3] With diamond on germanium, it is observed that the ratio of the hardnesses is greater than or equal to 8. For DLC (with an accessible hardness of at least 4000) on germanium, this ratio is greater than 5. Thus, in the LWIR band, the ratio of the hardnesses is preferentially greater than or equal to 5.[%3] For the MWIR band, since silicon is relatively hard, the ratio of the hardnesses is preferably greater than or equal to 1.3 for the case of SiN and greater than or equal to 5 in the case of diamond.[%3] Thus, one material M2 that is particularly well suited to the production of MS2 is diamond, which has a very high hardness (very good mechanical strength) and with which it is possible to produce a forest of nano-pillars with compatible large-surface and curved-surface methods. Diamond is also transparent over all spectral bands of interest.[%3] According to one embodiment, the diamond is boron-doped, thereby giving the element a self-cleaning character.[%3] According to one preferred embodiment, the secondary microstructures MS2 cover the entire structured surface, that is to say the surface of the cones and the inter-cone surface. This is the best configuration for hardness (the cones are protected), hydrophobicity, or super-hydrophobicity.[%3] According to one embodiment illustrated in figure 4bis, for the non-limiting case in which the substrate and the primary microstructures are made of one and the same material M1, the optical element comprises a layer LM2 made of second material M2 arranged on the primary structures, and on which the secondary microstructures are arranged. The layer LM2 is for example a residual layer resulting from the process of manufacturing the secondary microstructures and that covers the entire surface S. It is typically between a few nanometers and a few hundred nm thick.[%3] The properties of the optical element according to the invention are highlighted in the example below.[%3] Consideration is given to an AR structure in the visible / near-IR in the band [400 nm – at least 1 µm], the cones and the substrate (M1) being made of silica (index ns = n1 = 1.45), the cones being arranged in a periodic hexagonal mesh with a period p1 = 150 nm. The cones are contiguous, and therefore L1 = p1 = 150 nm with DS1 = 0.9. The height H1 = 500 nm, and therefore R1 = 0.3. The forest consists of diamond nanowires (n2 = 2.4). The effective index of the forest neff / f is related to the surface fill density DS2 of the nanowires on the covered surface.[%3] The transmission as a function of wavelength (spectral band [0.4–1 µm]) of an element made of silica alone is given as a reminder in figure 5 for 4 angles of incidence 0°, 45°, 60° and 70° for unpolarized incident light. The incident light is unpolarized. At 70°, transmission drops below 85%.[%3] Figure 6 illustrates the simulated transmission of the structure with the cones MS alone for these same 4 angles of incidence. Transmission remains above 98% except for 70°, where it is at 92%. The AR character is clearly apparent compared to figure 5.[%3] Figure 7 illustrates the transmission with the cones MS covered with a 150 nm-thick homogeneous diamond layer CP (nd = 2.4) according to the prior art. Compared to figure 6, transmission drops, very notably, to 70°. The presence of the homogeneous diamond layer thus disrupts the AR function and degrades its performance.[%3] The simulation was carried out by dividing the structure into layers indexed i of 25 nm and by calculating the corresponding effective index for each layer. As illustrated in figure 8, layer no. 1 is the layer of air, followed by 6 diamond layers (no. 2 to 7) for the case [cones + homogeneous diamond] or 6 layers of air for the case [cones alone], and then 20 layers (no. 8 to 27) for the cones themselves made of silica (and the diamond layer where applicable), and one layer (no. 28) for the substrate Sub, also made of silica.[%3] Examples of layers for the two cases [cones alone] and [cones + homogeneous diamond] are illustrated in figures 9 and 10, respectively (horizontal cutting plane along the layer).[%3] Figure 11 illustrates the variation in the effective index for both cases as a function of the no. i of the layer. The effective index varies monotonically for the case of the cone alone (curve 10) and comprises a break point for the case [cones + homogeneous diamond] (curve 20) corresponding to the zone denoted 80 in figure 8.[%3] Figure 11 also illustrates the evolution of the effective index when the homogeneous diamond layer is replaced by a forest of pillars with an effective index related to the surface fill density of the pillars, so as to form an element 10 according to the invention. Three effective indices neff / f of 1.3 (curve 13), 1.5 (curve 15) and 1.8 (curve 18) are studied, corresponding to a surface fill density of 35%, 52% and 72%, respectively.[%3] Curve 13 is the one that makes it possible to obtain a monotonic variation in the effective index neff / f along a vertical axis (that is to say an axis perpendicular to the substrate plane), this resulting in the best optical performance. Thus, preferably, the density DS2 is determined such that the effective index neff / f exhibits a monotonic variation in the vertical direction.[%3] According to one embodiment that makes it possible to ensure a monotonic variation, the surface density DS2 of the secondary microstructures in the covered zones is determined such that its effective index is between the index of air and the index of the substrate ns: 1 < neff / f < ns[%3] Figure 12 illustrates the transmission of the element 10 according to the invention comprising a forest of pillars with an equivalent effective index neff / f of 1.3. Performance is very markedly improved compared to figure 7 (with a homogeneous diamond layer) but also slightly improved compared to figure 6 (cones alone).[%3] Thus, the presence of a forest of nano-pillars made of diamond makes it possible both to increase the mechanical strength of the component and to fully regain anti-reflective optical performance, or even to improve this compared to cones without a coating.[%3] Figures 13, 14, 15 and 16 illustrate another advantage of the forest of nanostructures.[%3] They show the optical transmission of the structure [cones alone] (figure 13), [cones + homogeneous diamond] (figure 14), [cones + forest / neff / f = 1.8] (figure 15) and [cones + forest / neff / f = 1.3] (figure 16), respectively, but with cones arranged with a pitch p1 of 200 nm (rather than 150 nm for figures 5-7). Cones that are spaced further apart are easier to manufacture in terms of manufacturing. Performance is maintained with the forest, and an effective index of 1.8 also produces good results. It may also be seen in figure 15 that, for this cone pitch, additional interference effects further degrade optical transmission for the case [cones + homogeneous diamond]. Thus, with the secondary micro / nanostructures, some constraints on the pitch of the arrangement of the cones with respect to the homogeneous layer have been relaxed.[%3] Methods for producing "grass" or a "forest" of nano-pillars have been described for planar surfaces, and are applicable to the structured surface S of the primary structure. Methods for producing primary and secondary microstructures according to the invention may also be carried out on curved substrates.[%3] The grass or forest of nano-pillars or nano-spikes may take different forms and structures depending on the production method used.[%3] According to another aspect, the invention relates to a process for manufacturing an anti-reflective and super-hydrophobic optical element that is transparent to a used wavelength.[%3] For this purpose, in a first step, it is necessary to take an array of primary microstructures MS1 made of a first material M1 and formed on a substrate Sub. The primary microstructures have a conical or frustoconical shape, a width L1 and a height H1 such that L1 / H1 ≤ ½, a pitch of the array p1≤H1, and L1<λ0 / ns. The array of primary microstructures thus has a structured surface S. Such a component is known, as are various processes for manufacturing it. See for example abovementioned document WO2020 / 178304.[%3] For the case in which the cones are formed in the material of the substrate, this step is tantamount to micro / nanostructuring the array of pads on the surface of the substrate.[%3] According to a first variant, a layer of a second material M2 different from the first material M1 and having a hardness strictly greater than that of the first material is then deposited on the structured surface. Various deposition techniques may be used, for example CVD (chemical vapor deposition) or ALD (atomic layer deposition). Preferably, the deposited layer is homogeneous and covers the entire structured surface of the cones (surface of the cones and inter-cone surface).[%3] Next, the structured surface covered with the layer of M2 is etched by way of plasma etching, ion etching, chemical etching or a combination of these techniques, such that the structured surface is at least partially covered with a plurality of secondary microstructures MS2 in the form of nano-pillars or nano-cones.[%3] Preferably, once the secondary microstructures have been produced, a "silanization" step is carried out in order to further increase hydrophobicity. This step typically consists in depositing a silane layer on the optical element. Hydrophobicity may also be increased by depositing a layer of a fluorinated compound.[%3] According to one embodiment, the etching is reactive ion etching.[%3] The document by Yoon et al. "Insights into the reactive ion etching mechanism of nanocrystalline diamond films as a function of film microstructure and the presence of fluorine gas", Journal of Applied Physics 107, 044313 (2010), describes a method for producing diamond grass on silicon. The diamond layer is deposited by CVD and is then etched by ICP (inductively coupled plasma) with an oxygen plasma. In that document, the process was implemented on a planar silicon substrate, but this method is applicable to a silicon substrate structured with cones, as illustrated in figure 17: A: deposition of the diamond layer CD on the structured surface S; B: etching to obtain nano-pillars. C illustrates the diamond nano-pillar grass obtained by the abovementioned publication.[%3] According to a second variant, what is known as glancing angle deposition (GLAD) technology is used. In this variant, the second material M2 is deposited at an incidence that is oblique with respect to the normal to the substrate by evaporation or cathode sputtering. During deposition, the array of primary microstructures rotates during the deposition step. Numerous publications have shown that it is thus possible to achieve nanostructuring of the deposited material. It is possible to obtain various elongated shapes using this technique: zigzag, spiral, solid or hollow cylinder with walls, etc. The direction of the elongated structure is either perpendicular or oblique with respect to the plane on which it is deposited.[%3] The form of the secondary nanostructures, their inclination, their density or their distribution (random or not) is not a dimensioning parameter for the invention. What is important is the effective index resulting from the arrangement of the nanostructures.

Claims

1. An anti-reflective and super-hydrophobic optical element (10) that is transparent to at least one used wavelength (λ0), comprising an array of sub-wavelength primary microstructures (MS1) that are made of a first material (M1) and formed on a substrate (Sub), the primary microstructures having a conical or frustoconical shape, a first width (L1) and a first height (H1) such that a ratio between the first width and the first height is less than ½, and a pitch of the array (p1) less than the first height (H1) of said primary microstructure, the array of primary microstructures thus having a structured surface (S),said structured surface being at least partially covered with a plurality of secondary microstructures (MS2) in the form of nano-pillars or nano-cones and being made of a second material (M2) different from the first material, the second material having a hardness strictly greater than the hardness of the first material. 2. The optical element as claimed in claim 1, wherein the secondary microstructures cover substantially the entire structured surface. 3. The optical element as claimed in claim 1, wherein the array of primary microstructures is periodic, in a square or hexagonal mesh. 4. The optical element as claimed in claim 1, wherein a surface density (DS2) of the secondary microstructures in the covered zones is determined such that an effective index of the plurality of secondary microstructures (neff / f) varies monotonically in a vertical direction perpendicular to the plane of the substrate. 5. The optical element as claimed in claim 4, wherein a surface density (DS2) of the secondary microstructures in the covered zones is determined such that an effective index of the plurality of secondary microstructures (neff / f) is between the index of air and the index of the substrate (ns). 6. The optical element as claimed in claim 1, wherein the second material is chosen from: diamond, alumina, DLC, SiN. 7. The optical element as claimed in claim 1, wherein the second material is boron-doped diamond. 8. The optical element as claimed in claim 1, furthermore comprising a layer (LM2) of second material arranged on the primary structures and on which the secondary microstructures are arranged. 9. The optical element as claimed in claim 1, wherein the hardness of the second material is greater than or equal to 1.3 times the hardness of the first material, hardness being measured on the Knoop scale. 10. The optical element as claimed in claim 1, wherein the used wavelength is in the visible or near-infrared, and wherein the hardness of the second material is greater than or equal to 3 times the hardness of the first material, hardness being measured on the Knoop scale. 11. The optical element as claimed in claim 1, wherein the used wavelength is in the [3-5 µm] spectral band or the [8-12 µm] spectral band, and wherein the hardness of the second material is greater than or equal to 5 times the hardness of the first material, hardness being measured on the Knoop scale. 12. The optical element as claimed in claim 1, wherein the secondary microstructures are arranged in a forest or grass. 13. A process for manufacturing an anti-reflective and super-hydrophobic optical element (10) that is transparent to at least one used wavelength (λ0), comprising the following steps:­ taking an array of sub-wavelength primary microstructures (MS1) that are made of a first material (M1) and formed on a substrate (Sub), the primary microstructures having a conical or frustoconical shape, a first width (L1) and a first height (H1) such that a ratio between the first width and the first height is less than ½, and a pitch of the array (p1) less than the first height (H1) of said primary microstructure, the array of primary microstructures thus having a structured surface (S),­ depositing, on the structured surface, a layer of a second material different from the first material and having a hardness strictly greater than that of the first material,­ etching said structured surface covered with the layer of second material by way of plasma etching, ion etching, chemical etching or a combination of these techniques, such that the structured surface is at least partially covered with a plurality of secondary microstructures (MS2) in the form of nano-pillars or nano-cones. 14. The process as claimed in claim 13, wherein the etching is reactive ion etching. 15. A process for manufacturing an anti-reflective and super-hydrophobic optical element (10) that is transparent to a used wavelength (λ0), comprising the following steps:­ taking an array of sub-wavelength primary microstructures (MS1) that are made of a first material (M1) and formed on a substrate (Sub), the primary microstructures having a conical or frustoconical shape, a first width (L1) and a first height (H1) such that a ratio between the first width and the first height is less than ½, and a pitch of the array (p1) less than the first height (H1) of said primary microstructure, the array of primary microstructures thus having a structured surface (S),­ depositing, by evaporation or cathode sputtering, a second material (M2) at an incidence that is oblique with respect to the normal to the substrate, such that the structured surface is at least partially covered with a plurality of secondary microstructures (MS2) in the form of nano-pillars or nano-cones, the array of primary microstructures rotating during the deposition step, the second material being different from the first material and having a hardness strictly greater than that of the first material. 

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