WO2023099943A1

WO2023099943A1 - Micro- or nanostructured optical element

Info

Publication number: WO2023099943A1
Application number: PCT/IB2021/061213
Authority: WO
Inventors: Dorian HERLE; Niels QUACK
Original assignee: Ecole Polytechnique Federale De Lausanne (Epfl)
Priority date: 2021-12-02
Filing date: 2021-12-02
Publication date: 2023-06-08

Abstract

A micro- or nanostructured optical element (1) is proposed comprising: a membrane (3) with an array of holes (5), the membrane (3) comprising a membrane light wave facing surface; an array of pillars (7) sized and shaped such that a respective pillar (7) is configured to be at least partially received in a respective hole (5), the respective pillar (7) defining a central pillar axis, and comprising a pillar light wave facing surface; a support (9) configured to support the pillars (7); and an actuation mechanism configured to selectively generate a relative displacement between the membrane (3) and the pillars (7) along the central pillar axes to thereby define a first optical element reflectance state, where the membrane (3) is in an elevated position with respect to the support (9), and a second, different optical element reflectance state, where the membrane (3) is in a lowered position with respect to the support (9) such that the respective pillar (7) extends through the respective hole (5). In the first optical element reflectance state, the membrane light wave facing surface is substantially flush with the pillar light wave facing surfaces, or the membrane light wave facing surface is placed beyond the pillar light wave facing surfaces.

Description

MICRO- OR NANOSTRUCTURED OPTICAL ELEMENT

TECHNICAL FIELD

The present invention relates to a reflective opto-mechanical element that may for instance be used as a pixel element in a display device. More specifically, but not by way of limitation, the present invention proposes a new reflective display technology, where each pixel consists of a micro-electro-mechanical system (MEMS) enabled tuneable reflective surface, capable of switching between a high reflective state and low reflective state, thereby producing a monochrome display device. The present invention also relates to a process of fabricating the proposed optical element.

BACKGROUND OF THE INVENTION

Reflective displays have been developed since the 1970s, debuting with a technology called ’’Gyricon”, which is a type of electronic paper, at Xerox Palo Alto Research Center. Since then, a wide variety of reflective display technologies have emerged, based on electrophoretic, electrowetting, electrochromic, phase-change material, and MEMS technologies. Generally, most of these technologies suffer either from poor refresh-rate (electrophoretic, electrochromic), colour difficulties (phasechange), long-term stability (electrowetting) or incident-angle dependence (interferometric modulator (IMOD) MEMS displays).

Tuneable reflective surfaces have a great potential in light projection and display applications. One of the main impacting factors in reflection amplitude and spectrum of any reflective surface is the surface morphology. Nanostructured surfaces could reflect light in a wide range depending on the material and arrangement of the nanostructures. Based on this, recently, a nanoscale full colour printing is demonstrated by passive tuning of the geometrical parameters of the reflecting surface. For light projection and display applications, an active tenability is important which could be obtained implementing MEMS-based actuation approaches. For instance, light modulation by implementing electrostatic actuation of a surface with nano-hole grating was shown in a publication “A MEMS light modulator based on diffractive nanohole gratings” by Jack L. Skinner, A. Alec Talin, and David A. Horsley, Optics Express 3701 , Vol. 16, No. 6, 17 March 2008, Optical Society of America. However, these techniques do not entirely overcome the above shortcomings. SUMMARY OF THE INVENTION

It is an object of the present invention to overcome at least some of the problems identified above related to displays, or optical elements more broadly. More specifically, one of the objects of the present invention is to propose a micro- or nanostructured optical element that provides a good contrast ratio, incidence-angle independence, and fast switching speed. Nanostructured elements are understood to be elements with a microstructure, the characteristic length scale of which is on the order of a few to hundreds of nanometres. In other words, nanostructured elements are understood to be elements characterised by dimensions which are less than a micrometre. By analogy, microstructured elements are understood to be to be elements characterised by dimensions which are less than a millimetre.

According to the first aspect of the invention, there is provided an optical element as recited in claim 1 .

The proposed optical element is designed to overcome many of the above drawbacks, providing a good contrast-ratio, incidence-angle independence, and fast switching speed.

According to a second aspect of the invention, there is provided a display comprising the optical element according to the first aspect of the present invention.

According to a third aspect of the invention, there is provided a method of fabricating the optical element according to the first aspect of the present invention.

Other aspects of the invention are recited in the dependent claims attached hereto.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will become apparent from the following description of a non-limiting example embodiment, with reference to the appended drawings, in which:

• Figure 1a shows the proposed optical element according to one embodiment in a first reflectance state, which in this example is a high reflectance state;

• Figure 1 b shows the proposed optical element in a second reflectance state, which in this example is a low reflectance state; • Figure 1c shows a reflection spectrum for the high and low reflectance states shown in Figures 1a and 1b, respectively;

• Figures 2a to 2d illustrate different example designs for the optical element;

• Figures 3a to 3c illustrate different example suspension configurations for coupling the optical element to anchor elements;

• Figure 4 schematically illustrates an example actuation configuration of the optical element;

• Figure 5 illustrates how different dimensions of the optical element are measured;

• Figures 6a to 6c show different example pillar layouts;

• Figures 7a and 7b show in two different views an example design of an optical element, which is obtained by using a simplified fabrication process;

• Figures 8a to 8f schematically illustrate an example fabrication process of the optical element according to the present invention; and

• Figure 9 shows the optical element with optional stopper elements.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

An embodiment of the present invention will now be described in detail with reference to the attached figures. The embodiment is described in the context of an optical element for a display, but the teachings of the invention are not limited to this environment. For instance, instead of being used as an individual pixel or sub-pixel element of a display, the proposed optical element may serve as a variable reflector, optical modulator, or variable optical attenuator. Identical or corresponding functional and structural elements which appear in the different drawings are assigned the same reference numerals. It is to be noted that the use of words “first”, “second” and “third”, etc. may not imply any kind of particular order or hierarchy unless this is explicitly or implicitly made clear in the context.

Figures 1a and 1b illustrate the working principle of the proposed optomechanical element or switch 1 according to an example embodiment. A membrane, film, layer or covering 3 with holes 5 cross-sectionally through the membrane, and an array or a set of pillars 7, each defining a central or longitudinal pillar axis, are provided such that the membrane 3 and the pillars are arranged to be selectively displaced relative to each other thereby tuning the reflectivity of the incident light. The incoming light wave is indicated by the arrow in Figures 1a and 1b. The membrane is to be understood as a thin, relatively rigid or pliable sheet of material forming a barrier for incoming light waves. Thus, the membrane may be optically thick, which means that no light may pass through the membrane. However, the present invention is not limited to optically thick membranes. In other words, the membrane 3 may be e.g. translucent or semi-transparent, although this would not lead to equally good results as with nontransparent membranes.

In the present example, the pillars 7 are connected to a substrate 9 or support such that the pillars protrude from the substrate. More specifically, in this example, the pillars protrude from the substrate at an angle of 90° or substantially 90° with respect to the surface of the substrate. As shown, in this case the pillars are elongated elements, and in this example have a circular or substantially circular cross section orthogonally to the pillar central axis. However, other cross-sectional pillar shapes, such as oval, rectangular, and triangle shapes, would equally be possible as explained later. It is to be noted that the word pillar in the present description covers elongated elements, but which do not necessarily have a constant cross section. The pillars can be understood to form inlays or inserts that can be received or partially received in the holes in the membrane, and which are directly or indirectly mechanically supported by the substrate. Furthermore, in this example, the pillars are fixed, while the membrane is arranged to move or translate along the central pillar axes between a first position, and a second position. In this case, the first position is an elevated or raised membrane position, while the second position is a lowered membrane position.

As shown in the figures, the substrate 9 comprises or consists of two layers, namely a first substrate layer 11 and a second substrate layer 13. The pillars protrude from the first layer 11 , which in this case is a dielectric, i.e. electrically non-conductive. In this case, the second layer is in direct contact with the first layer and located below the first layer. In this example, the membrane 3 and the second layer 13 are electrically conductive, which allows electrostatic actuation of the membrane as explained later. More specifically, the membrane forms a first electrode, and the second layer forms a second electrode, while the first layer 11 electrically isolates the two electrodes from each other. In the present example, the second layer 13 is a crystalline silicon layer.

The optical element 1 defines a first operational, reflectance or reflective state, which is referred to as an ON state and a second operational, reflectance or reflective state, which is referred to as an OFF state. As shown in Figure 1a, in the ON state, the membrane top surface, which forms a membrane incoming light wave facing surface, and the pillar top surfaces, which form pillar incoming light wave facing surfaces, are aligned or substantially aligned, creating high reflectivity or colour. In other words, the membrane top surface and the pillar top surfaces are flush or substantially flush. The OFF state can be reached from the ON state by lowering the membrane 3 along the pillars’ axes. Thus, in the OFF state, which is shown in Figure 1b, the membrane 3 is in its lowered position, creating low reflectivity or a black state. Figure 1c illustrates an example reflection spectrum for the ON and OFF states. The x-axis shows the wavelength in nanometres, while the y-axis shows the reflectance value R, where the value 1 .0 corresponds to a reflectance value of 100%. In both states shown in Figures 1a and 1 b, the pillars are at least partially received in the holes 5. In the OFF state, the respective pillar 7 extends through the respective hole 5. However, it is to be noted that in the ON state, the membrane could alternatively be raised above the pillar top surfaces. In other words, in this case, the membrane would be raised or disposed beyond the top surfaces of the pillars towards the incoming light wave. Thus, in this case, the pillars 7 would not be received in the holes 5.

The materials for the substrate, pillars, and membrane can be selected to obtain broadband and incident angle tolerant ON and OFF states. The membrane 3 may be made of a first material, while the pillars may be made of a second material, where the first and second materials may or may not be the same. In the present context, the word material is to be understood broadly to cover also material compositions. The first material is characterised by a first light reflectance value, while the second material is characterised by a second light reflectance value. More broadly, the membrane light wave facing surface is made of a first material and characterised by a first light reflectance value, and the pillar light wave facing surfaces are made of a second material and characterised by a second light reflectance value, where the first material is different from the second material such that the first light reflectance value is different from the second light reflectance value. Advantageously, the first and second materials are selected so that the first light reflectance value is greater than the second light reflectance value. In other words, according to a possible embodiment, the pillars 7 could form a strongly absorbing pillar array, while the membrane could be made of a highly reflecting material. This would result in a monochrome black and white display. It is to be noted that colours could be introduced through the use of standard colour filters, such as a red, green, and blue (RGB) colour filter, as commonly used in liquid-crystal displays (LCDs). In this scenario, the colour filters would be placed above the membrane (but not necessarily physically touching the membrane) so that the incoming light waves would travel through these colour filters before reaching the membrane surface.

According to a variant of the invention, the membrane 3 could be made from a stack of materials e.g. according to the teachings of a publication by Chenying Yang et al., “Compact Multilayer Film Structure for Angle Insensitive Color Filtering”, Scientific Reports 5.1 (19 March 2015), Number: 1 Publisher: Nature Publishing Group, pp. 1-5, issn: 2045-2322, in order to obtain a colour reflection in the ON state, while an absorbing pillar array would create black. According to another variant of the invention, the membrane could also host dried ink (though direct inkjet printing) to produce colour through the pigments. In yet another variant of the invention, the role of the membrane and the pillars would be inversed. The membrane could be made to absorb light (through multi-layers or pigments), and the colours would be created from the pillars. Thus, in view of the above, the general configuration of a membrane with holes and pillars offers exciting optical opportunities.

The first and second materials, as well as the pillar geometry can be chosen so as to enhance the contrast between the ON state and the OFF state. In the present example, the membrane is a metal layer or more specifically an aluminium (Al) layer, while the pillars are made of amorphous silicon (aSi). Figures 2a to 2d illustrate different pillar designs or shapes. Figure 1a shows a pillar 7 with a round or circular cross section (when taken orthogonally to the central pillar axis). Figure 2b shows a pillar with a rectangular cross section, and more specifically a square-shaped cross section. Figure 2c shows a hollow pillar 7, which has a hollow interior, i.e. the pillar is in this example configured as a tubular or tube-like element. Figure 2d shows a multi-layered pillar 7, where the different pillar layers are characterised by different pillar cross-sectional diameters (the word diameter does not necessarily imply a circular cross section) and/or different materials. As shown in Figure 2d, in the present example, the uppermost layer forms a disc or a disc-like element. Thus, to improve optical performance, the pillars can be of a variety of shapes, can be structured (i.e. hollow pillar), made of multiple layers of material, or any combination of the above designs. The chosen design will often be a trade-off between fabrication constraints and optical performance.

Figures 3a to 3c illustrate different suspension elements or suspensions 15 for the membrane 3. The membrane is mechanically coupled to one or more suspension elements 15 such that the membrane is allowed to move between the elevated position and the lowered position during actuation of the membrane. The suspension arrangement should be designed in such a way as to minimise any vertical displacement non-uniformities of the membrane, allowing a suitable voltage operation (if actuated electrostatically) and be stiff enough to avoid any unwanted substantial bending of the suspension arrangement. In the configuration of Figure 3a, the membrane is coupled at its corners (e.g. at least at two corners) to a respective suspension element 15. The respective suspension element in this example comprises two arms or arm sections, such that a first arm is coupled to a membrane corner at one end, while the other end of the first arm merges into or is coupled to a second arm, which in the elevated position (or alternatively in the lowered position) of the membrane runs substantially parallel with the top surface of the membrane. The second arm then merges into or is coupled to an anchor 17 or anchor element, which connects the suspension element to the substrate 9. The suspension element 15 is in this example made elastic. This means that the suspension element is able to resume its normal or rest shape spontaneously after external forces applied to the membrane are no longer present. The suspension arrangement of Figure 3a can also be called a folded or meander suspension arrangement as the suspension elements are folded or bent at the junction between the first and second arms.

Figure 3b illustrates a configuration where the suspension elements 15 are configured as two or more membrane support elements 15, which are coupled to the membrane, such that the membrane support elements are clamped between two opposite anchors 17. The configuration of Figure 3b includes two membrane support elements, which are coupled to the substrate through the anchors 17. In the configuration of Figure 3b, the suspension may be made elastic similar to the configuration of Figure 3a so that it is able to resume its normal or rest shape spontaneously after external forces applied to the membrane are no longer present.

Figure 3c shows yet another suspension configuration. In the configuration of Figure 3c, the membrane is coupled at its corners (e.g. at least at two corners) to a respective suspension element 15, which in this case is straight or substantially straight element, which in turn is connected through the anchors 17 to the substrate 9. The suspension elements 15 run substantially parallel with the membrane top surface in one of the membrane states (in this case the ON state), while in the other state (in this case the OFF state), the suspension element central or longitudinal axes are angled (i.e. they have a non-zero angle) with respect to the membrane top surface. The suspension element 15 is also in this example, as in in the configurations of Figures 3a and 3b, made elastic. This means that the suspension element is able to resume its normal or rest shape spontaneously after external forces applied to the membrane are no longer present.

Figure 4 schematically illustrates an example actuation configuration of the membrane. The aim of the actuation mechanism is to allow for a relative displacement between the membrane 3 and pillars 7. In this example, the membrane is configured to be actuated electrostatically. For this purpose, the optical element 1 comprises a voltage source 19 to provide potential difference between the first and second electrodes formed by the membrane 3 and the electrically conductive layer 13 of the substrate, respectively. More specifically, the substrate may be kept at a constant electric potential, e.g. by grounding it, and a given voltage value is applied to the membrane 3. Alternatively, the membrane may be kept at a constant electric potential, e.g. by grounding it, and a given voltage value is applied to the substrate. Electrostatic actuation uses the attractive electrostatic force to move the membrane 3, which is one of the two electrodes. When a voltage is applied across the two electrodes 3, 13, electrostatic actuation moves the membrane against the other fixed electrode (which in this case is the electrically conductive layer 13 of the substrate), resulting in the compression of the space between the two electrodes. As soon as the applied voltage is removed, the elastic suspension elements 15 and thus also the movable membrane return to their original or rest position. It is to be noted that the voltage source 19 or another actuator, and means for coupling the membrane 3 to the substrate 9 or to another reference element may collectively be understood to form an actuation mechanism. In the above example, the coupling means comprise the suspension elements 15 and the anchors 17. Piezo-electric, electrothermal, mechanical, or electromagnetic actuation are further possible actuation mechanisms to obtain a relative displacement.

Typical ranges of dimensions of the optical element are given hereafter with reference to Figure 5. As can be seen, all the dimensions given below are submicrometre. This has to do with the fact that we target a sub-micrometre wavelength range (visible spectrum). The device can also be adjusted for other regions of the electromagnetic spectrum, such as ultraviolet (UV) or infrared (I R), and the dimensions would be scaled proportionally. The greatest cross-sectional diameter D of the pillars 7 may be in the range of 100 nm to 500 nm, or more specifically in the range of 200 nm to 400 nm. The longest dimension H, i.e. the height of the pillars 7 may be in the range of 100 nm to 800 nm, or more specifically in the range of 200 nm to 700 nm. The centre-to- centre distance or separation of the pillars 7 may be in the range of 200 nm to 800 nm, or more specifically in the range of 300 nm to 700 nm. The membrane thickness T may be in the range of 30 nm to 150 nm, or more specifically in the range of 70 nm to 130 nm. The gap G between the pillars 7 and the wall of the hole 5 may be in the range of 10 nm to 100 nm, or more specifically in the range of 30 nm to 80 nm. In other words, a clearance is present in the respective hole 5 between the respective pillar 7 and the respective hole wall at least in the lowered position of the membrane. It is to be noted that the minimum optical element size can be built well below Abbe’s classical diffraction limit, and is of the order of wavelength divided by 5 (2/5) (e.g. if the minimum wavelength equals 400 nm, the minimum spacing would be approximately 80 nm). The maximum dimensions depend on the final resolution requirements (typical value: 255 pixels per inch, thus the pixel size (consisting of e.g. two green pixel components or subpixels, one blue subpixel and one red subpixel) would be approximately 10 pm, and the subpixel size, i.e. in this case the optical element size would be approximately 3 pm). As an example, a rectangular array of circular nanopillars with a diameter of 290 nm, a period (centre-to-centre separation) of 612 nm, and a height of 600 nm showed a simulated average extinction coefficient of 1 :5 in the visible spectrum.

The position of the pillars can also be tuned to improve the optical performance. Figure 6a, as well as Figures 1a and 1 b show a periodic structure or pillar layout, but non-periodic or random pillar placement can also be envisioned. Figure 6b shows a hexagonal pillar placement, where the pillars in two adjacent lines are interleaved. Figure 6c shows a non-periodic or irregular pillar placement. It is also possible that irrespective of the pillar placement, any given optical element comprises different types of pillars, e.g. pillars of different cross-sectional shapes.

The array of pillars of the optical element 1 may comprise any appropriate number of pillars. In the example of Figures 1 a and 1 b, the array comprises a set of 2x2 pillars, while the examples of Figures 3a, 3, and 3c comprise a set of 4x4 pillars. The rows and columns in the array advantageously comprise an equal number of pillars, although this does not have to be the case. The total number of pillars in an array of a respective optical element may be between 4 and 10000, but in most applications the total number of pillars per optical element is in the range of 4 and 64. When used in displays, a plurality of optical elements are arranged side by side such that one optical element forms one picture element forming a pixel or a sub-pixel of the display. It is to be noted that every optical element is arranged to be selectively actuated by its own actuation mechanism.

Figures 7a and 7b show an example optical element 1 according to the present invention, which is obtained by using a simplified fabrication process. In other words, the design of the optical element is optimised to facilitate the fabrication process. In particular, the membrane and the uppermost part of the pillars are both made of the same material, in this case amorphous silicon, and they are chosen to have the same thickness/height. These two design choices allow for a single lithography step, in which the membrane and the pillars can be patterned at the same time. Furthermore, the membrane 3 comprises additional access or release holes 21 , such that after the sacrificial oxide release step (see below) the pillars would still be held in place by a strut or strut element 23 (in this case an oxide stand), while the membrane 3 would be fully released due to an increased number of access holes for the etchant to enter the space between membrane 3 and the substrate 9.

An example fabrication process is next explained with reference to Figures 8a to 8f. The process consists essentially of six steps as outlined below and in Figures 8a to 8f. As mentioned above, this example process flow is optimised as to only rely on a single lithography step, and no alignment of the pillars and the membrane is required in this example process. In a first step and as shown in Figure 8a, the different material layers are deposited on top of each other. The second substrate layer 13 (in this case crystalline silicon layer) is the bottom-most layer, followed by a silicon dioxide (SiO2) layer forming the first substrate layer 11 or an intermediate layer, a membrane/pillar layer 25 (which in this example is an amorphous silicon layer), and a lithography resist 27 on top of the stack. In this example, this step also comprises sputtering the amorphous silicon layer forming the membrane 3 and the pillars 7, and a subsequent step of spin coating of the lithography resist. In a second step and as shown in Figure 8b, the resist 27 is exposed, followed by a development step during which the resist is patterned to create patterns or structures in the resist. The exposure changes the solubility of the resist, enabling selective removal of either the exposed or non-exposed regions of the resist by immersing it in a solvent (developing).

In a third step and as shown in Figure 8c, directional etching of the amorphous silicon layer is carried out. In a fourth step and as shown in Figure 8d, directional etching of the silicon dioxide layer is carried out. In a fifth step and as shown in Figure 8d, the resist is removed, and the surface of the amorphous silicon layer cleaned. In a sixth step and as shown in Figure 8f, the membrane is released, in this case by isotropic etching. More specifically, a timed hydrogen fluoride (HF) vapour release may be carried out. For this purpose, the access holes 21 may advantageously be used. The membrane is now merely fixed at its different ends, but fully released otherwise. The disc-shaped pillar top portions are still mechanically supported by the remaining oxide thereby forming an array of strut elements 23. Thus, as shown in Figure 8f, the strut elements indirectly mechanically couple the pillar top portions to the substrate. In the example of Figures 1a and 1b, the pillars are directly coupled to the substrate.

Tuneable reflective surfaces have a great potential in light projection and display applications. One of the main impacting factors in reflection amplitude and spectrum of any reflective surface is the surface morphology. Nanostructured surfaces can reflect light in a wide range depending on the material and arrangement of the nanostructures. For light projection and display applications, an active tenability is important, which could be obtained by implementing MEMS-based actuation approaches. The present design offers some major advantages, namely a non-contact operation (no physical contact between the membrane and pillars) which allows for higher durability and yield, high contrast of at least 1 :6, as well as low incidence angle dependence. It is to be noted that the membrane and/or the substrate may comprise a micro- or nanostructured surface acting as stopper elements or stoppers 29 as shown in Figure 9 to minimise stiction in case of contact with each other. One or more stoppers 29 may be optionally introduced to avoid that the membrane 3 and the substrate 9 stick with each other upon contact (surface forces are very high with the present dimensions). Stoppers are small bumps, blocks, or protrusions on either one, or on both surfaces. When the surfaces get very close, only the tiny stoppers really touch the respective surface, thereby minimising the surface forces between the two surfaces, since (1) the contact surface of the stoppers is small, and (2) they keep the remaining surface at a specific distance from the other surface. If no stoppers are used, and if by mistake a too high voltage potential between the membrane and substrate is applied, then it might happen that the membrane gets pulled down onto the substrate. In that case, there is the potential risk of stiction. This risk can be minimised by introducing the stopper elements 29.

To summarise, according to one embodiment, the present invention proposes an optical element 1 that comprises fixed pillars 7 (made of a light absorbing material, such as amorphous silicon for example), and a movable membrane 3 with holes 5 (made of a highly reflective material, such as metal, and in particular aluminium for example). The pillar and membrane materials and dimensions of these elements are designed in such a way that the amount of reflected light can be modified by moving the membrane. The membrane can advantageously be displaced along the pillar axes such that the displacement distance is at least 20 nm, or more specifically at least 50 nm. All dimensions can be chosen to be compatible with typical microfabrication processes. The optical element can serve as a MEMS-enabled reflective light switch. In a continuous movement, the amount of light that is reflected from the element can be continuously adjusted. Particular features of the optical design include reflectivity over a broad spectral range (specifically the full visible spectrum for a particular design), and particular features of the mechanical design include a small pixel size (< 7 pm x 7 pm) and a fast response time (for example 10 microseconds or faster). The device can for example serve as a variable reflector, optical modulator, variable optical attenuator, or as an individual pixel element of a display. Due to the optical and mechanical properties, the display can be fast and thus show video capability, it has high contrast and reflective, i.e. it can be operated in bright daylight, and can be adapted for colour. Thanks to the MEMS actuation mechanism, the power consumption is very low, and the device is compatible with mass manufacturing techniques of the MEMS/semiconductor industry.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive, the invention being not limited to the disclosed embodiment. Other embodiments and variants are understood, and can be achieved by those skilled in the art when carrying out the claimed invention, based on a study of the drawings, the disclosure and the appended claims. Further variants may be obtained by combining the teachings of any of the designs explained above.

In the claims, the word ’’comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that different features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be advantageously used. Any reference signs in the claims should not be construed as limiting the scope of the invention.

Claims

1. A micro- or nanostructured optical element (1 ) comprising:

- a membrane (3) with an array of holes (5) cross-sectionally through the membrane (3), the membrane (3) comprising a membrane light wave facing surface;

- an array of pillars (7) sized and shaped such that a respective pillar (7) is configured to be at least partially received in a respective hole (5) of the array of holes (5), the respective pillar (7) defining a central pillar axis, and comprising a pillar light wave facing surface;

- a support (9) configured to directly or indirectly support the pillars (7); and

- an actuation mechanism (15, 17, 19) configured to generate a relative displacement between the membrane (3) and the pillars (7) along the central pillar axes to thereby define a first optical element reflectance state, where the membrane (3) is in an elevated position with respect to the support (9), and a second, different optical element reflectance state, where the membrane (3) is in a lowered position with respect to the support (9) such that the respective pillar (7) extends through the respective hole (5), wherein in the first optical element reflectance state, the membrane light wave facing surface is substantially flush with the pillar light wave facing surfaces, or the membrane light wave facing surface is placed beyond the pillar light wave facing surfaces.

2. The optical element (1 ) according to claim 1 , wherein the membrane light wave facing surface is made of a first material and characterised by a first light reflectance value, and the pillar light wave facing surfaces are made of a second material and characterised by a second light reflectance value, and wherein the first material is different from the second material such that the first light reflectance value is different from the second light reflectance value.

3. The optical element (1 ) according to claim 2, wherein the first light reflectance value is greater than the second light reflectance value.

4. The optical element (1 ) according to any one of the preceding claims, wherein the actuation mechanism (15, 17, 19) is configured to move the membrane between the elevated position and the lowered position.

5. The optical element (1 ) according to any one of the preceding claims, wherein the actuation mechanism (15, 17, 19) is configured to implement one or more of the following actuation operations to generate the relative displacement: electrostatic actuation, magnetic actuation, pneumatic actuation, mechanical actuation, piezoelectric actuation, or electrothermal actuation.

6. The optical element (1 ) according to any one of the preceding claims, wherein the membrane (3) is mechanically coupled to one or more suspension elements (15) such that the membrane (3) is allowed to move between the elevated position and the lowered position.

7. The optical element (1 ) according to claim 6, wherein a respective suspension element (15) is mechanically coupled to an anchor (17).

8. The optical element (1 ) according to claim 6 or 7, wherein the one or more suspension elements (15) are elastic.

9. The optical element (1 ) according to any one of the preceding claims, wherein a clearance is present in the respective hole (5) between the respective pillar (7) and a respective hole wall at least in the lowered position of the membrane (3).

10. The optical element (1 ) according to any one of the preceding claims, wherein a displacement distance along the central pillar axis between the elevated position and the lowered position equals at least 20 nm.

11. The optical element (1 ) according to any one of the preceding claims, wherein the membrane (3) comprises a first electrically conductive element forming a first electrode, and the substrate (9) comprises a second electrically conductive element (13) forming a second electrode.

12. The optical element (1 ) according to claim 11 , wherein the support (9) comprises a dielectric layer (11 ) between the second electrically conductive element (13) and the membrane (3) to electrically isolate the second electrically conductive element (13) from the membrane (3). 15

13. The optical element (1 ) according to any one of the preceding claims, wherein at least one of the pillars (7) is a multi-layered pillar comprising a first pillar section with a first cross-sectional thickness and/or made of the second material, and a second pillar section with a second, different cross-sectional thickness and/or made of a third, different material.

14. The optical element (1 ) according to claim 13, wherein the first pillar section is a disc-shaped element comprising the pillar light wave facing surface, and the second pillar section is configured as a strut element (23) coupling the first pillar section to the support (9).

15. The optical element (1 ) according to any one of the preceding claims, wherein the membrane light wave facing surface is made of metal, and the pillar light wave facing surfaces are made of light absorbing material.

16. The optical element (1 ) according to any one of the preceding claims, wherein the membrane (3) comprises one or more etchant access holes (21 ) to allow an etchant to enter a space between the support (9) and the membrane (3).

17. The optical element (1 ) according to any one of the preceding claims, wherein the membrane (3) comprises a stack of at least two different material layers.

18. The optical element (1 ) according to any one of the preceding claims, wherein the membrane (3) and/or the support (9) comprise(s) a micro- or nanostructured surface to minimise stiction in case of contact.

19. The optical element (1 ) according to any one of the preceding claims, wherein the greatest cross-sectional dimension of the respective pillar orthogonally to the central pillar axis is in the range of 100 nm to 500 nm.

20. The optical element (1 ) according to any one of the preceding claims, wherein the membrane thickness is in the range of 30 nm to 150 nm.

21. The optical element (1 ) according to any one of the preceding claims, wherein the centre-to-centre separation of the pillars (7) is in the range of 200 nm to 800 nm. 16

22. The optical element (1 ) according to any one of the preceding claims, wherein the height of the respective pillar (7) is in the range of 100 nm to 800 nm.

23. The optical element (1 ) according to any one of the preceding claims, wherein in the lowered position of the membrane (3), a gap in the range of 10 nm to 100 nm is present between the respective pillar (7) and a respective hole wall of the respective hole (5).

24. The optical element (1 ) according to any one of the preceding claims, wherein the number of pillars in the optical element (1 ) is in the range of 4 to 10000.

25. A display element comprising an array of optical elements (1 ) according to any one of the preceding claims placed side by side, wherein a respective optical element (1 ) forms a picture element of the display element.

26. A method of fabricating the optical element (1 ) according to any one of the preceding claims, wherein the method comprises the steps of:

- depositing on the support (9) an intermediate layer (11 ), a membrane layer (25) and a resist (27);

- patterning the resist (27) by exposing the resist (27), and subsequently creating desired patterns in the resist (27);

- etching the membrane layer (25) following the created patterns in the resist (27) to separate the pillars (7) from the membrane (3) both formed in the membrane layer (25);

- etching the intermediate layer (11 ) following the created patterns in the resist (27) and the etching results in the membrane layer (25);

- removing the resist (27); and

- further etching the intermediate layer (11 ) to release the membrane (3) from the intermediate layer (11 ), while leaving strut elements (23) in the intermediate layer (25) to mechanically support top portions of the pillars (7) comprising the pillar light wave facing surfaces.