Method for forming a pattern on an optical device

Abstract

The present disclosure relates to a method of forming a pattern on an optical device. The optical device includes a plurality of pixel elements. Each pixel element has a stack of layers, the stack of layers including a color control layer and a brightness control layer stacked on top of each other. For each of one or more of the pixel elements: energy is deposited into the color control layer to change the effect of the color control layer on the color of light; and energy is deposited into the brightness control layer to change the effect of the brightness control layer on the intensity of light.

Description

Technical Field

[0001] The present invention relates to a method of forming a pattern on an optical device, and more particularly to an optical device that uses a phase change material to control the spectral content of light reflected from the device. Background Art

[0002] Phase change materials (PCMs) are materials that can be reversibly switched between two stable, solid but structurally different states (usually an amorphous state and a crystalline solid state) by applying energy to the material, and these two states have different optical properties (such as complex refractive index) and electrical properties (such as conductivity). Industrial applications of such materials have been found in optical storage media (such as CD-RW and DVD-RW computer rewritable optical discs) and electronic "flash" type non-volatile storage devices (such as PC-RAM). In optical storage media, the energy for switching is provided by a laser directed at the PCM layer. In electronic "flash" type non-volatile storage devices, the energy is transferred through current and / or Joule heating.

[0003] Although writing, erasing, and rewriting data with a laser to a PCM-based optical storage device (such as a DVD-RW) has been well developed and commercialized, the data stored in these devices is written in sub-micron-sized dots, each dot representing a data bit, and a probing laser is required to detect the data using the difference in light reflectivity between the amorphous and crystalline regions at the wavelength of the probing laser. For example, in Japanese Patent Application JP2007095274A, there is also an aspiration to achieve laser writing of images visible to the naked eye using the same optical control. However, the industrial applicability of such a design is hindered by several disadvantages including the following.

[0004] i) PCM-based devices developed for optical storage have a low optical contrast between the two stable states when irradiated with broadband light under typical indoor and outdoor environmental conditions because the device was developed only to provide optical contrast at a specific wavelength of the expected probing laser.

[0005] ii) The PCM itself (i.e., the specific composition or alloy used, usually a chalcogenide glass, most commonly a Ge, Sb, and Te (GST) alloy) was developed to have a fast crystallization time to maximize the speed of laser writing and erasing data. The fast crystallization time limits the area of the material that can be re-amorphized with a single pulse. This is not a problem in data storage applications where a small spot size is beneficial for high data density, but it is a severe limitation for laser writing visible-sized images, where the size of each pixel should at least exceed the acuity limit of the human visual system at the expected viewing distance.

[0006] iii) A PCM device (such as a typical DVD - RW device) with a single PCM material layer (which forms part of a uniform optical stack across the device area) can only switch between two stable states with different optical reflectivities. Thus, an image formed using equivalent devices and methods will only consist of two colors.

[0007] EP3087562B1 and the publication “Hosseini, P. et al. “Optoelectronic frameworks enabled by low - dimensional phase - change thin films”, Nature, 511, pp. 206 - 211 (2014)” disclose a method for providing a rewritable PCM - based thin - film optical stack by introducing a spacer layer with a carefully controlled thickness between the PCM layer and the reflector layer, which effectively overcomes limitation i). This thin - film optical stack has a high optical contrast between two stable states, a wide range of selectable colors, and the ability to have bright, vivid reflected colors for each state.

[0008] WO2017134506A1 and the UK patent application numbered GB1821051.8 respectively disclose a method for electrically switching a large - area phase - change material and a PCM material composition compatible with such an increased switching area, which effectively overcomes limitation ii).

[0009] WO2016125491A1 and WO2017134506A1 disclose PCM - based optical devices using the stack design from EP3087562B1 to create an electrically updateable reflective display, where different regions (sub - pixels) of the display are provided with thin - film optical stacks having different layer thicknesses and / or material sets to provide a greater range of reflected color capabilities across the device. WO2016125491A1 discloses using an additional active, electrically controllable top shutter layer to selectively absorb light incident on and reflected by any given sub - pixel within a controllable range, thereby providing full - color control and grayscale of the resulting image. For the case of a device with built - in electronics for dynamically updating the image, this effectively overcomes limitation iii).

[0010] The publication “Kaino, Y. et al. “Laser - addressable full - color photo - quality rewritable sheet based on a thermochromic system and a colorless dye”, Journal of the Society for Information Display; 27: 295 - 303 (2019)” describes a laser - rewritable color - reflective image device, where three layers of optically switchable colorless dye materials are provided in each image region, and each layer has different light absorption in one or two states. Thus, a full - color image can be written and rewritten to the device using three different - wavelength lasers.

[0011] Publication “Talagrand, C. et al. “Solid State Reflective Displays for Video Rate, Full Color, Outdoor Readable Displays” ”, Journal of the Society for Information Display, 26, 10, pp. 619 - 624 (2018)” describes a reflective color still image reproduction device in which, by selective crystallization of the PCM layer in a thin - film optical stack, the color reflected by each image area (sub - pixel) of the device is set to one of two available states, and the brightness of each image area is determined by selectively obscuring a portion of the area of each sub - pixel with an overlapping black mask layer. However, the images produced by the device are not customizable or reconfigurable because the crystallization state of each PCM area and the area coverage of the black mask are permanently preset during manufacturing. Summary of the Invention

[0012] An object of the present invention is to provide an alternative method for forming patterns in optical devices.

[0013] According to one aspect, there is provided a method of forming a pattern on an optical device, wherein: the optical device includes a plurality of pixel elements, each pixel element including a stack of layers, the stack of layers including a color control layer and a brightness control layer that are stacked on top of each other in the viewing direction; and the method includes, for each of one or more of the pixel elements in the pixel elements: depositing energy into the color control layer of the pixel element to modify the color control layer so as to change, in use, the effect of the color control layer on the color of light leaving the optical device from the pixel element; and depositing energy into the brightness control layer of the pixel element to modify the brightness control layer so as to change, in use, the effect of the brightness control layer on the intensity of light leaving the optical device from the pixel element.

[0014] Thus, there is provided a method that enables highly customizable patterns to be formed in an optical device through two independently modifiable layers in each of a plurality of pixel elements. The customization can provide full - color images visible to the human eye or through magnifying devices. The method can be applied to form thin films with customizable images, rewritable signs (wherein the brightness control layer and the color control layer are configured to be rewritable), optical storage media, and optical security markings.

[0015] In an embodiment, the modification of the brightness control layer is performed differently in at least three different pixel elements such that the brightness of light from at least three different pixel elements has three corresponding different values. Thus, the method provides more than binary (on / off) control of the pixel elements, enabling a wide range of visual effects to be achieved.

[0016] In an embodiment, each different modification of the brightness control layer is performed by changing the transmittance of a corresponding different proportion of the brightness control layer in the pixel element relative to visible light. This method enables easy and effective implementation, including enabling the use of materials in the brightness control layer that only need to be switched (locally) in a binary manner.

[0017] In an embodiment, changing the transmittance of only a part of the brightness control layer in the pixel element includes removing that part of the brightness control layer. Providing local binary switching by removal (e.g., using laser-induced ablation) can be simply and effectively implemented. Laser ablation can generally be applied at a high spatial resolution.

[0018] In an embodiment, each modification of the brightness control layer in the pixel element includes a uniform modification of the entire brightness control layer in the pixel element. Providing three or more different transmittances uniformly on corresponding different pixel elements can be achieved using materials that can be tuned through three or more different transmittance states, and reduces the need for high-spatial-resolution deposition of energy compared to a method in which the transmittance level is controlled by the following step: in this step, the transmittance level is controlled by controlling the proportion of the brightness control layer modified in each pixel element (e.g., by removing only a selected part of the brightness control layer).

[0019] In an embodiment, depositing energy into the color control layer is performed by irradiating the color control layer, and depositing energy into the brightness control layer is performed by irradiating the brightness control layer. Irradiation enables the energy to be deposited quickly and accurately.

[0020] In an embodiment, for each pixel element in one or more pixel elements in the pixel element, the irradiation of the brightness control layer is spatially non-uniform within the pixel element. Providing spatially non-uniform irradiation within the pixel element enables the brightness control layer within the pixel element to be non-uniformly modified, such as by removing only a selected part of the brightness control layer in the pixel element.

[0021] In an embodiment, before modifying the brightness control layer, irradiating the color control layer is performed through the brightness control layer to modify the color control layer. In an embodiment, irradiating the color control layer is performed using radiation that modifies the color control layer but does not modify the brightness control layer. This method enables the color control layer and the brightness control layer to be modified by irradiating from the same side of the optical device, thereby enabling the method to be performed using simpler and / or more compact hardware.

[0022] In an embodiment, illumination of the color control layer is performed from a first side of the optical device; and illumination of the brightness control layer is performed from a second side of the optical device that is opposite the first side. Performing illumination from opposite sides reduces the risk of interference between modification of the color control layer and modification of the brightness control layer, thereby increasing the flexibility of illumination for two different operating configurations.

[0023] In an embodiment, the stack of layers in each pixel element further includes a reflective layer and a spacer layer between the reflective layer and the color control layer; the brightness control layer of each stack is disposed on a side of the color control layer opposite the reflective layer; and the first side of the optical device is on a side of the reflective layer opposite the color control layer. In an embodiment, illuminating the color control layer from the first side of the optical device includes depositing energy into the reflective layer, which is configured to laterally propagate the energy as heat within the reflective layer. Laterally propagating the energy reduces the risk of incomplete or otherwise misdriven pixel elements.

[0024] In an embodiment, the reflective layer is patterned to at least partially separate a reflective region corresponding to each pixel element among two or more pixel elements in the pixel element from a reflective region corresponding to pixel elements other than that pixel element. Patterning reduces the risk of crosstalk between different pixel elements during writing of the pattern to the optical device.

[0025] In an embodiment, in each of two or more pixel elements in the pixel element, the phase change material is provided as a plurality of phase change material sub-layers. In this type of embodiment, by selectively depositing energy into different combinations of sub-layers in different pixel elements, a variety of optical effects can be achieved. For example, a single pixel element type can be switched in this way to provide options that may only be available when using different pixel element types: for example, a single pixel element type can be selectively switched between white and each of a plurality of different colors (e.g., three different colors or more different colors), and / or between another color other than white and each of a plurality of different colors (e.g., three different colors or more different colors). Providing a single pixel element type with this greater flexibility reduces or eliminates the need for pixel elements with a variety of different pixel element types to achieve the desired color capabilities, thereby simplifying manufacturing.

[0026] In an embodiment, the pixel elements are arranged in a plurality of groups, and each group of pixel elements includes at least two pixel elements configured to provide different ranges of influence on the color of light exiting the optical device from the pixel elements. Providing such groups ensures that a wide range of effects can be achieved, such as forming full-color and high-contrast patterns.

[0027] In an embodiment, one or both of the color control layer and the brightness control layer are configured to be reversibly modifiable. Accordingly, a reconfigurable pattern can be formed.

[0028] According to an alternative aspect, there is provided a method of forming a pattern on an optical device including a plurality of pixel elements, the method comprising: providing an ink including a liquid medium and micro-stacked suspensions, the micro-stacked suspensions including a phase change material capable of switching between a plurality of stable states having different refractive indices relative to each other; and printing the ink in a pattern to form a color control layer in each of the pixel elements.

[0029] The phase change material is provided in the ink such that the phase change material can be deposited only where it is needed in the final device, thereby reducing waste compared to alternative patterning techniques that selectively remove or block the phase change material through a mask. The method enables more reliable and / or efficient coating of non-planar objects. The method enables the formation of different stacks close to each other without the need for lithography. For example, the stacks providing a full-color display can all be printed without the need for separate sputtering, lithography, and etching steps for each primary color.

[0030] According to an alternative aspect, there is provided a method of forming a pattern on an optical device, wherein: the optical device includes a plurality of pixel elements, each pixel element including a stack of layers including at least a color control layer; and the method includes: for each of one or more of the pixel elements, depositing energy into the color control layer of the pixel element to modify the color control layer so as to change, in use, the effect of the color control layer on the color of light exiting the optical device from the pixel element; the color control layer includes a phase change material capable of switching between a plurality of stable states having different refractive indices relative to each other; depositing energy into the color control layer of each pixel element is arranged to: switch the color control layer in the pixel element from one of the states to another of the states; and in each of two or more of the pixel elements, the phase change material is provided as a plurality of phase change material sub-layers.

[0031] The method enables energy to be selectively deposited into different combinations of sub-layers in different pixel elements to provide a wider range of visual effects than other feasible means. For example, the sub-layers can be configured such that a single pixel element type can be selectively switched into more than two different colors. Providing a single pixel element type with such greater flexibility reduces or eliminates the need for pixel elements having multiple different pixel element types to achieve the desired color capabilities, thereby simplifying manufacturing. Description of the Drawings

[0032] Now, the present invention will be further described by way of example with reference to the accompanying drawings, in which:

[0033] Figure 1 is a side cross-sectional view of a group of three pixel elements of an optical device;

[0034] Figure 2 is Figure 1 a front view of a color control layer in a 3×3 array of a group of three pixel elements of the type depicted;

[0035] Figure 3 is Figure 1 a front view of a brightness control layer in a 3×3 array of a group of pixel elements of the type depicted;

[0036] Figure 4 is a front view showing Figure 2 the superposition of the color control layer of Figure 3 with the brightness control layer of

[0037] Figures 5 to 7 is a side cross-sectional view of a group of three pixel elements of an optical device, which depicts an exemplary process flow for forming a pattern in the optical device by irradiating the color control layer and the brightness control layer starting from an opaque brightness control layer;

[0038] Figures 8 to 10 is a side cross-sectional view of a group of three pixel elements of an optical device, which depicts an exemplary process flow for forming a pattern in the optical device by irradiating the color control layer and the brightness control layer starting from a transparent brightness control layer;

[0039] Figure 11 and Figure 12 schematically depict micro-stacked suspensions in two different inks; and

[0040] Figure 13 is a side cross-sectional view depicting an exemplary pixel element printed with the inks of Figure 11 and Figure 12 ; DETAILED DESCRIPTION

[0041] Throughout the specification, the terms "optical" and "light" are used because these terms are commonly used in the art in relation to electromagnetic radiation, but it should be understood that in the context of this specification, the terms "optical" and "light" are not limited to visible light. It is contemplated that the present invention may also be used for wavelengths outside the visible spectrum, such as infrared light and ultraviolet light.

[0042] The present disclosure provides a method of forming a pattern on an optical device. The optical device includes a plurality of pixel elements. Figure 1Depicts an example group of three pixel elements 2A to 2C. Each pixel element 2A to 2C includes a stack of layers (e.g., a thin film optical stack) on a substrate 4. In some embodiments, the substrate 4 is a flexible film. The stack of each layer includes color control layers 13A to 13C and brightness control layers 15A to 15C. The color control layers 13A to 13C and the brightness control layers 15A to 15C of each pixel element 2A to 2C are stacked on top of each other in the viewing direction. Stacked means that light exits each pixel element 2A to 2C after passing through both the color control layers 13A to 13C and the brightness control layers 15A to 15C of the pixel element 2A to 2C.

[0043] In an embodiment, the following two steps are performed on each pixel element (optionally, on all pixel elements 2A to 2C) in one or more of the pixel elements 2A to 2C.

[0044] In the first of these two steps, energy is deposited into the color control layers 13A to 13C of the pixel elements 2A to 2C. In some embodiments, depositing energy into the color control layers 13A to 13C is performed by irradiating the color control layers 13A to 13C. In an embodiment, an external device (e.g., a laser) is used to deposit the energy. The deposition of energy modifies the color control layers 13A to 13C, thereby changing, in use, the effect of the color control layers 13A to 13C on the color (e.g., spectral shape) of the light exiting the optical device from the pixel elements 2A to 2C. For example, this modification can change the complex refractive index of the color control layers 13A to 13C, as discussed below.

[0045] In the second of these two steps, energy is deposited into the brightness control layers 15A to 15C of the pixel elements 2A to 2C. Depositing energy into the brightness control layers 15A to 15C can be performed at a time different from the time of depositing energy into the color control layers 13A to 13C. Thus, the second step can be performed before, during, or after the first step. In some embodiments, depositing energy into the brightness control layers 15A to 15C is performed by irradiating the brightness control layers 15A to 15C. In an embodiment, an external device (e.g., a laser) is used to deposit the energy. The deposition of energy modifies the brightness control layers 15A to 15C, thereby changing, in use, the effect of the brightness control layer on the intensity (or spectral power) of the light exiting the optical device from the pixel elements 2A to 2C.

[0046] In an embodiment, the color control layers 13A to 13B include PCMs that can switch (optionally, reversibly switch) between multiple stable states, and the multiple stable states have different complex refractive indices relative to each other. In this example, arrangements can be made to deposit energy into the color control layers 13A to 13C of each pixel element 2A to 2C, thereby switching the color control layers 13A to 13C in the pixel elements 2A to 2C from one stable state to another stable state. In "Hosseini, P. et al. "Optoelectronic frameworks enabled by low-dimensional phase-change thin films", Nature, 511, pp. 206-211 (2014)", detailed examples of how the switching of a PCM layer can be utilized to provide a change in the perceived color of a pixel are disclosed, and any of the mechanisms described therein can be used with the embodiments of the present disclosure.

[0047] In an embodiment, the PCM in each pixel element 2A to 2C can switch between an optical state group including at least two optical states, which enables the pixel elements 2A to 2C to have different colors. In an embodiment, the different colors include red and white, blue and white, or green and white. In an embodiment, Figure 1 Each pixel element 2A to 2C in a group is configured to be switchable between different color groups by switching the PCM in the pixel elements 2A to 2C. For example, switching the PCM in pixel element 2A can enable switching between white and a first color, switching the PCM in pixel element 2B can enable switching between white and a second color, and switching the PCM in pixel element 2C can enable switching between white and a third color. In an embodiment, the first color, the second color, and the third color are different colors. In an embodiment, the first color is red, the second color is blue, and the third color is green. Thus, Figure 1 Each pixel element 2A to 2C in a group can act as a sub-pixel of a single pixel. Independent control of the color control layers 13A to 13C and the brightness control layers 15A to 15C of each pixel element 2A to 2C provides full color and intensity control of a single pixel.

[0048] There may be variations to the above embodiments. For example, four or more pixel element types can be provided, which can enable a wider range of effects. The different colors can include red and white, blue and white, green and white, and a fourth color and white, and / or cyan, magenta, and / or yellow that each switch to white. Additionally, different from pure white, the secondary color states of each pixel element type can be lighter (i.e., more reflective, lower color saturation) colors with the same or similar hues as the initial state, or states with completely different color hues and / or brightness.

[0049] In some embodiments, such as in Figure 1 As illustrated in FIG. 1 , the stack of layers in each pixel element 2A to 2C includes a reflective layer 11A to 11C, and a spacer layer 12A to 12C between the reflective layer 11A to 11C and the color control layer 13A to 13C. In some embodiments, a cover layer 14A to 14C is further disposed on a side of the color control layer 13A to 13C opposite to the reflective layer 11A to 11C. In some embodiments, as in Figure 1 As illustrated in FIG. 1 , the pixel elements 2A to 2C are arranged in a plurality of groups (for example, as in FIG. Figures 2 to 4 Depicted in Figure 1 Multiple groups). Each group of pixel elements 2A to 2C may include at least two pixel elements, at least two pixel elements having spacer layers of different thicknesses. This modifies the interference effect with the stacking so that each pixel element in the group of pixel elements 2A to 2C provides a different range of effects on the color of light leaving the optical device from pixel elements 2A to 2C compared to the other one or more pixel elements in the group. In the examples discussed above, the spacer layer 13A in pixel element 2A may be configured to provide switching between white and red, the spacer layer 13B in pixel element 2B may be configured to provide switching between white and blue, and the spacer layer 13C in pixel element 2C may be configured to provide switching between white and green. More complex arrangements may also achieve additional optical effects and / or more types of different pixel element types, including: additional color control layers (including PCM), which are optionally arranged so that PCM layers and spacer layers are stacked alternately; additional spacer layers (the additional spacer layers are optionally of different thicknesses and / or at different locations); additional cover layers (the additional cover layers are optionally of different thicknesses and / or at different locations); and / or additional optically active layers, such as passive absorption layers (the additional optically active layers are optionally at different locations). Specific examples are further described below in which the PCM in each of two or more of the pixel elements is arranged as multiple PCM sublayers (optionally, the multiple PCM sublayers are separated from each other by non-PCM layers).

[0050] Figure 2 yes Figure 1 A front view of a color control layer in a 3×3 array of a group of pixel elements 2A to 2C of the type depicted. The distribution of different shades in the color control layer 13A to 13C in 81 pixel elements illustrates a possible configuration of the group after energy has been selectively deposited in the pixel elements 2A to 2C to form a desired pattern on the optical device.

[0051] Figure 3 yes Figure 1Front view of the brightness control layer in a 3×3 array of groups of pixel elements 2A to 2C of the type depicted. The areas shown in white represent regions of high transparency (e.g., having a transmittance of at least 90%, optionally at least 99% with respect to visible light), and the areas shown in black represent regions of low transparency (e.g., having a transmittance of less than 10%, optionally less than 1% with respect to visible light).

[0052] Modifications to the brightness control layers 15A to 15C can be made differently in at least three different pixel elements 2A to 2C (within the same group and / or between different groups) such that the brightness of the light from different pixel elements has correspondingly different values. In some embodiments, as exemplified in Figure 3 each different modification to the brightness control layers 15A to 15C can be performed by changing the transmittance of the corresponding different proportions of the brightness control layers 15A to 15C in the pixel elements 2A to 2C with respect to visible light. In the example of Figure 3 this results in the amount of light leaving the pixel elements 2A to 2C after reflection being determined by the relative proportions of high transparency regions and low transparency regions in each of the pixel elements 2A to 2C, with the relative proportions being defined by the modifications to the respective brightness control layers 15A to 15C. When the high transparency regions in the brightness control layers 15A to 15C of the pixel elements 2A to 2C are larger than the low transparency regions in the brightness control layers 15A to 15C of the pixel elements 2A to 2C (e.g., in the brightness control layer 15A of the pixel element 2A in the upper left corner in Figure 3 ) the observed brightness of each of the pixel elements 2A to 2C will be higher. When the high transparency regions in the brightness control layers 15A to 15C of the pixel elements 2A to 2C are smaller than the low transparency regions in the brightness control layers 15A to 15C of the pixel elements 2A to 2C (e.g., in the brightness control layer 15A of the pixel element 2A in the lower left corner in Figure 3 ) the observed brightness of each of the pixel elements 2A to 2C will be lower.

[0053] In other embodiments, each modification to the brightness control layers 15A to 15C in the pixel elements 2A to 2C comprises a uniform modification of the entire brightness control layers 15A to 15C in the pixel elements 2A to 2C. In this type of embodiment, the brightness control layers 15A to 15C are formed of a material that can be switched between more than two different transmittances. Allowing brightness control to be achieved by such a uniform modification avoids the need to provide switching at a spatial resolution finer than the size of each pixel element 2A to 2C. In some such embodiments, the transmittance can be continuously adjusted by a series of values.

[0054] In some embodiments, the brightness control layers 15A to 15C can be modified reversibly. When the color control layers 13A to 13C are also configured to be reversibly modifiable, the optical device can be fully rewritable. Examples of materials that can provide such a laser-rewritable bright-to-black transition are colorless dyes (such as the colorless dyes used in the devices of the aforementioned publication, which is Kaino, Y. et al., "Laser-Addressed Full-Color Photo-Quality Rewritable Sheet Based on a Thermochromic System and Colorless Dyes," Journal of the Information Display Society; 27: 295-303 (2019), but with alternative compositions for non-specific colors) and liquid crystal devices having a photosensitive alignment layer (which can be based on azo-dye materials).

[0055] In Figure 4 the stackings of the color control layers 13A to 13C and the brightness control layers in the pixel elements 2A to 2C schematically depicted in provide the desired combinations of color and brightness variations between the pixel elements 2A to 2C. As Figure 4 depicted, each group of three pixel elements 2A to 2C can form a pixel 21. Four exemplary pixels are highlighted by the dashed box in Figure 4 but it should be understood that Figure 4 the 3×3 array of groups of pixel elements 2A to 2C shown in includes nine individual pixels (each individual pixel includes three sub-pixels, which respectively correspond to the three pixel elements 2A to 2C). By selectively switching the states of the color control layers 13A to 13C and the brightness control layers 15A to 15C in each pixel element 2A to 2C of each pixel 21, a wide range of reflected colors and brightness can be generated. Thus enabling the optical device as a whole to reproduce high-quality, high-brightness, and high-spatial-resolution images with a large color gamut. For example, as shown for the top-left pixel 21 of the 3×3 pixel matrix in the example of Figure 4 , by switching the color control layers 13A to 13C of each pixel element 2A to 2C of the pixel 21 to their white-reflection states over the entire (or maximum) area of each pixel element 2A to 2C, and switching the brightness control layers 15A to 15C of each pixel element 2A to 2C to their bright-transmission states, the pixel 21 can be configured to have a high-brightness white reflectance. Alternatively, as Figure 4As shown by the upper right pixel, the right-centered pixel, and the lower right pixel of the example in [reference], by switching the color control layers 13A to 13C of each pixel element 2A to 2C of pixel 21 to their vivid color reflection states over the entire area, and only switching the brightness control layers 15A to 15C of the pixel elements 2A to 2C corresponding to the expected color type of pixel 21 (e.g., red, green, or blue) to their transmissive states, and keeping the brightness control layers 15A to 15C black (least transmissive) on the remaining two pixel elements 2A to 2C, pixel 21 can be configured to reflect a vivid, pure red, pure green, or pure blue spectrum. Alternatively, as Figure 4 shown by the lower left pixel 21 of the 3×3 pixel array of the example in [reference], by switching the color control layers 13A to 13C of each pixel element 2A to 2C of pixel 21 to the same state (all switched to the white state, or all switched to the vivid color state) over the same area or over the entire (or maximum) area of each pixel element 2A to 2C, and switching the brightness control layers 15A to 15C of each pixel element 2A to 2C to their absorptive dull states, pixel 21 can be configured to reflect a neutral gray or black appearance. These examples represent extreme examples of pixel color states (e.g., white, black, and primary colors), and thus can define the range of the color gamut capabilities of the optical device. However, by using two-layer color control described by different combinations of the pixel elements 2A to 2C in their colored and white states, and intermediate switching of the brightness control layers 15A to 15C, any intermediate color within the available color gamut can be produced. The publication Talagrand, C. et al. “Solid-State Reflective Displays for Video-Rate, Full-Color, Outdoor Readable Displays” ”, Journal of the Society for Information Display, 26, 10, pp. 619 - 624 (2018) illustrates and quantifies the high-quality image reproduction achievable using such two-layer control, and also uses PCMs containing thin-film optical stacks as color control layers and spatially patterned black mask layers as brightness control layers, although in non-customizable, non-reconfigurable static image reproduction devices.

[0056] Each modification to the brightness control layers 15A to 15C generally changes the brightness of the pixel elements 2A to 2C, but basically does not (or does not at all) change the color of the pixel elements 2A to 2C. Each modification to the brightness control layers 15A to 15C in the pixel elements 2A to 2C can be limited to, for example, changing the transmittance of the brightness control layers 15A to 15C in the pixel elements 2A to 2C with respect to visible light. Therefore, the mechanisms underlying the modification of the brightness control layers 15A to 15C and the composition of the brightness control layers 15A to 15C are generally different from the mechanisms underlying the modification of the color control layers 13A to 13C and the composition of the color control layers 13A to 13C, respectively.

[0057] Modifications to the brightness control layers 15A to 15C can be made differently in at least three different pixel elements 2A to 2C, such that the brightness of the light from at least three different pixel elements 2A to 2C has three corresponding different values. Thus, the control of brightness is not necessarily limited to binary changes. Various mechanisms can be used to provide brightness control. In Figure 3 and Figure 4 's example, as discussed above, brightness control is provided by controlling the relative sizes of the high transparency regions and the low transparency regions.

[0058] In some embodiments, the brightness control layers 15A to 15C in each of the pixel elements 2A to 2C are initially opaque (low transmittance), and the modification to the brightness control layers 15A to 15C includes increasing the transmittance per unit area with respect to visible light. Optionally, for at least one subset of the pixel elements 2A to 2C, the modification to the brightness control layers 15A to 15C includes increasing the transmittance per unit area of selected portions of the brightness control layers 15A to 15C with respect to visible light. For example, the brightness control layers 15A to 15C can be set as a black mask curable resist material. The increase in transmittance can be achieved by selectively evaporating / ablating the brightness control layers 15A to 15C. Thus, the transmittance of a portion of the brightness control layers 15A to 15C in a pixel element can be modified by removing only a portion of the brightness control layers 15A to 15C in the pixel element. Alternatively, the increase in transmittance can be achieved by fading the brightness control layers 15A to 15C to a transparent state. The spatial resolution when the brightness control layers 15A to 15C are evaporated / ablated or faded is high enough to allow intermediate control of the brightness (gray level) in the pixel element 2A to 2C by switching only a portion of the brightness control layers 15A to 15C of the selected pixel element 2A to 2C.

[0059] Figures 5 to 7 An exemplary process flow of this type is depicted. Figure 5 depicts three pixel elements 2A to 2C having the same layer sequence as the Figure 1 's pixel elements 2A to 2C, but the brightness control layers 15A to 15C have not been modified yet. In Figure 5 the depicted stage, the brightness control layers 15A to 15C are not uniformly transparent in each of the three pixel elements 2A to 2C. In Figure 6In the subsequent steps depicted, selected pixel elements 2A and 2C are irradiated in such a manner that in a part of the light control layers 15A to 15C of each of the pixel elements 2A and 2C, the transmittance per unit area with respect to visible light is increased (the white areas indicate where the transmittance has increased, and the black areas indicate where the transmittance has not increased). In an embodiment, the transmittance is increased by removing a part of the light control layers 15A to 15C (e.g., by evaporation / ablation) or otherwise locally damaging the light control layers 15A to 15C in a manner that increases the transmittance. By directing a higher concentration of radiation onto a part of the light control layer in the pixel element relative to other parts of the light control layer in the pixel element, the increase in transmittance can be limited to this part of the light control layer in the pixel element. Thus, for one or more of the pixel elements, the irradiation 20 may be spatially non-uniform within the light control layer of the pixel element. In Figure 7 In the subsequent steps depicted, the color control layer 13C in the pixel element 2C is modified by irradiating the pixel element 2C. The process flow can be changed to achieve any desired combination of modification of the light control layers 15A to 15C of the pixel elements 2A to 2C and modification of the color control layers 13A to 13C.

[0060] In other embodiments, the light control layers 15A to 15C in each of the pixel elements 2A to 2C are initially transparent (high transmittance), and the modification of the light control layers 15A to 15C includes reducing the transmittance per unit area with respect to visible light. Optionally, for at least one subset of the pixel elements 2A to 2C, the modification of the light control layers 15A to 15C includes reducing the transmittance per unit area of selected parts of the light control layers 15A to 15C with respect to visible light.

[0061] Figures 8 to 10 An exemplary process flow of this type is depicted. Figure 8 Depicted are three pixel elements 2A to 2C having the same layer sequence as the Figure 1 pixel elements 2A to 2C, but the light control layers 15A to 15C have not been modified. In Figure 8 the stage depicted, the light control layers 15A to 15C are uniformly transparent in each of the three pixel elements 2A to 2C. In Figure 9 In the subsequent steps depicted, selected pixel elements 2A and 2C are irradiated in such a manner that the color control layers are modified in the pixel elements 2A and 2C (e.g., switching the pixel element 2A from white to red and the pixel element 2C from white to green). In Figure 10In the subsequent steps depicted, the pixel elements 2A to 2C are irradiated in such a way that the transmittance with respect to visible light is uniformly reduced in the light control layer 15B of the pixel element 2B and reduced in selected portions of the light control layers 15A and 15C of the pixel elements 2A and 2C (the white areas represent where the transmittance has not been reduced and the black areas represent where the transmittance has been reduced). In an embodiment, the light control layers 15A to 15C are locally modified by oxidation, carbonization, or by any other energy-activated chemical reaction or degradation, thereby reducing the transmittance. The process flow can be varied to achieve any desired combination of modification of the light control layers 15A to 15C of the pixel elements 2A to 2C and modification of the color control layers 13A to 13C.

[0062] In the embodiments of the type discussed above with reference to Figures 8 to 10 where the light control layers 15A to 15C are initially transparent, radiation passing through the light control layers 15A to 15C can be used to perform modification of the color control layers 13A to 13C without modifying the light control layers 15A to 15C (e.g., by selecting wavelengths that make the light control layers 15A to 15C highly transparent and / or a radiation dose that is insufficient to cause significant modification of the light control layers 15A to 15C). Then radiation of different wavelengths and / or intensities can be used in subsequent steps to modify the light control layers 15A to 15C.

[0063] In an embodiment, deposition of energy into the color control layers 13A to 13C and deposition of energy into the light control layers 15A to 15C are performed by irradiating the color control layers 13A to 13C and the light control layers 15A to 15C. In an embodiment, detectable positioning marks (which can also be referred to as fiducial points) are provided to enable an automatic alignment of a radiation source (such as a laser). In this type of embodiment, using radiation for energy deposition can include: detecting the positioning marks and using the detected positions to direct the radiation to correctly deliver the energy to the desired locations.

[0064] In an embodiment, an energy pulse of radiation is applied with a spot size corresponding to the spatial resolution required to provide the intermediate brightness control discussed above (e.g., to modify less than all of the brightness control layers 15A to 15C for each pixel element in one or more of the pixel elements 2A to 2C). In an embodiment, a beam scanning device is provided such that the beam spot can be rasterized over all of the pixel elements 2A to 2C to be switched, thereby providing a desired image. Alternatively, a projection type instrument can be provided that delivers a spatially modulated "image" pulse that simultaneously switches multiple pixel elements 2A to 2C. In an embodiment, a different spatial distribution of radiation is applied to each pixel element 2A to 2C when the brightness control layers 15A to 15C are modified compared to when the color control layers 13A to 13C are modified. In particular, when the color control layers 13A to 13C are modified, the spatial distribution can be substantially uniform over the pixel elements 2A to 2C, while when the brightness control layers 15A to 15C are modified, the spatial distribution is non-uniform over the pixel elements 2A to 2C. For example, the non-uniform spatial distribution of the brightness control layers 15A to 15C can result in removing only a portion of the brightness control layers 15A to 15C in each pixel element of one or more of the pixel elements 2A to 2C and / or modifying (increasing or decreasing) the transmittance relative to visible light only in a portion of each pixel element of one or more of the pixel elements 2A to 2C. However, the non-uniform spatial distribution can also be used to modify the color control layers 13A to 13C, for example to achieve an intermediate state between different available color states controlled by the color control layers 13A to 13C. For example, if the pixel elements 2A to 2C are configured to be switched by a full switch between red and white by the color control layers 13A to 13C, the non-uniform spatial distribution can be used to create a pixel that appears red in a portion of the pixel element and white in another portion of the pixel element, thereby achieving a lighter red than the red achieved when all of the color control layers 13A to 13C are uniformly switched to the state corresponding to red.

[0065] In an embodiment, depositing energy into the color control layers 13A to 13C is performed by irradiating the color control layers 13A to 13C from a first side of the optical device (e.g., from Figure 1 the direction below) and depositing energy into the brightness control layers 15A to 15C is performed by irradiating the brightness control layers 15A to 15C from a second side of the optical device opposite the first side (e.g., from Figure 1 the direction above). The stack of layers in each pixel element 2A to 2C includes reflective layers 11A to 11C and spacer layers 12A to 12C between the reflective layers 11A to 11C and the color control layers 13A to 13C (as discussed below and for example in Figure 1In the case (illustrated in ) where each stacked brightness control layer 15A to 15C can be disposed on a side of the color control layers 13A to 13C opposite to the reflective layers 11A to 11C, and the first side of the optical device can be located on a side of the reflective layers 11A to 11C opposite to the color control layers 13A to 13C. In this type of embodiment, irradiating the color control layers 13A to 13C from the first side of the optical device can include depositing energy into the reflective layers 11A to 11C, where the reflective layers 11A to 11C are configured to laterally propagate the energy as heat within the reflective layers 11A to 11C, thereby increasing the spatial uniformity of the heating applied to the color control layers 13A to 13C due to the irradiation. This effect can mitigate the non-uniformity of the intensity within the laser spot. In some embodiments, the reflective layers 11A to 11C are patterned to at least partially separate the reflective regions corresponding to each pixel element among two or more pixel elements of the pixel elements 2A to 2C from the reflective regions corresponding to the other pixel elements 2A to 2C (e.g., by providing discontinuities in the material of the reflective layers 11A to 11C (such as lines of material having a lower thermal conductivity), thereby suppressing the discontinuous flow of heat). In the presence of such patterning, the heat propagation effect of the reflective layers 11A to 11C can be locally restricted to each pixel element 2A to 2C, thereby additionally reducing crosstalk between the driving of adjacent pixel elements 2A to 2C (e.g., where heating intended for one pixel element 2A to 2C propagates into one or more adjacent pixel elements 2A to 2C). The reflective layers 11A to 11C can be coated with a highly absorbent material or a pre-deposited layer on the lower side, or the lower surface thereof can be roughened, so that the layer can be reflective when viewed from above, but absorbent (black) when viewed from below, to assist in absorbing the switching energy when irradiated from below.

[0066] The PCM in each pixel element 2A to 2C can be switched between multiple stable states, which have different refractive indices relative to each other. In an embodiment, the switching is reversible. Each stable state has a different refractive index (optionally including different imaginary components of the refractive index, and thus different transmittance / absorbance) relative to each of the other stable states. In an embodiment, all the layers in each pixel element 2A to 2C are solid-state and are configured such that the thicknesses of all the layers, as well as the refractive index and absorption characteristics, are combined such that different states of the PCM result in different, visible, and / or measurably different reflection spectra. This type of optical device is described in "Nature 511, 206 - 211 (July 10, 2014)", WO2015 / 097468A1, WO2015 / 097469A1, EP3203309A1, and PCT / GB2016 / 053196.

[0067] In an embodiment, the PCM comprises one or more of the following, consists essentially of one or more of the following, or consists of one or more of the following: oxides of vanadium (also referred to as VO x ); oxides of niobium (also referred to as NbO x ); alloys or compounds containing Ge, Sb, and Te; alloys or compounds containing Ge and Te; alloys or compounds containing Ge and Sb; alloys or compounds containing Ga and Sb; alloys or compounds containing Ag, In, Sb, and Te; alloys or compounds containing In and Sb; alloys or compounds containing In, Sb, and Te; alloys or compounds containing In and Se; alloys or compounds containing Sb and Te; alloys or compounds containing Te, Ge, Sb, and S; alloys or compounds containing Ag, Sb, and Se; alloys or compounds containing Sb and Se; alloys or compounds containing Ge, Sb, Mn, and Sn; alloys or compounds containing Ag, Sb, and Te; alloys or compounds containing Au, Sb, and Te; and alloys or compounds containing Al and Sb (including any of the following compounds / alloys with stable stoichiometry: GeSbTe, VO x , NbO x , GeTe, GeSb, GaSb, AgInSbTe, InSb, InSbTe, InSe, SbTe, TeGeSbS, AgSbSe, SbSe, GeSbMnSn, AgSbTe, AuSbTe, and AlSb). Preferably, the PCM comprises one of Ge 2 Sb 2 Te 5 and Ag 3 In 4 Sb 76 Te 17 . It should also be understood that there may be various stoichiometric forms of these materials: for example, Ge x Sb y Te z ; another suitable material is Ag 3 In 4 Sb 76 Te 17 (also referred to as AIST). In addition, any of the above materials may include one or more dopants, such as C or N. Other materials may be used.

[0068] As is well known, when switching between the amorphous phase and the crystalline phase, both the real and imaginary refractive indices of the PCM change drastically. The switching can be achieved by heating caused by light pulses from a laser light source. When the material switches between the amorphous phase and the crystalline phase, the refractive index changes significantly. The material is stable in either state. The switching can be effectively performed an infinite number of times. However, the switching is not necessarily reversible.

[0069] Although some embodiments described herein mention that the PCM can switch between two states such as the crystalline phase and the amorphous phase, the transition can occur between any two solid phases, including but not limited to: the transition from the crystalline phase to another crystalline phase or a quasicrystalline phase, or the transition from a crystalline phase or a quasicrystalline phase to another crystalline phase; the transition from the amorphous phase to the crystalline phase or a quasicrystalline / semi-ordered phase, or the transition from a crystalline phase or a quasicrystalline / semi-ordered phase to the amorphous phase; and all forms of transitions between the above two transitions. The embodiments are also not limited to only two states.

[0070] The reflective layers 11A to 11C can be made highly reflective (e.g., for all visible wavelength radiation) or only partially reflective. In an embodiment, the reflective layers 11A to 11C include a reflective material, such as a metal. It is known that metals can provide good reflectivity (when thick enough). The reflective layers 11A to 11C can have a reflectivity of 50% or higher, optionally 90% or higher, optionally 99% or higher with respect to visible light, infrared light, and / or ultraviolet light. The reflective layers 11A to 11C can include a thin metal film, which is composed of, for example, Au, Ag, Al, or Pt. If the layer is partially reflective, a thickness in the range of 5 nm to 15 nm can be selected, otherwise the layer is made thicker (e.g., 100 nm) for substantially total reflection. In an embodiment, the reflective layers 11A to 11C uniformly span multiple pixel elements 2A to 2C, optionally spanning all pixel elements 2A to 2C. Alternatively, the reflective layers 11A to 11C can be patterned to have different reflective regions corresponding to each of two or more pixel elements (optionally, all pixel elements 2A to 2C) among the pixel elements 2A to 2C.

[0071] The spacer layers 12A to 12C and the cover layers 14A to 14C are both light-transmissive, ideally as transparent as possible. Each of the spacer layers 12A to 12C and the cover layers 14A to 14C can consist of a single layer or include multiple layers that have different refractive indices with respect to each other. The thickness and refractive index of one or more materials forming the spacer layers 12A to 12C and the cover layers 14A to 14C are selected to produce a desired spectral response (by interference and / or absorption). Materials that can be used to form the spacer layers 12A to 12C and / or the cover layers 14A to 14C can include (but are not limited to) ZnO, TiO 2 、SiO2 、 Si 3 N 4 、 TaO, ITO, and ZnS - SiO 2 。

[0072] In some embodiments, in each of two or more pixel elements among pixel elements 2A to 2C, the PCM is provided as a plurality of phase - change material sub - layers (optionally separated from each other by non - PCM layers). In such embodiments, energy can be selectively deposited into different combinations of sub - layers in different pixel elements 2A to 2C to provide a wider range of visual effects. For example, the sub - layers can be configured such that a single pixel element type can be selectively switched into more than two different colors. For example, a single pixel element type can be switched in this way to provide all of the options described above for different pixel element types: for example, a single pixel element type can be selectively switched between white and each of a plurality of different colors (e.g., three different colors or more), and / or between another color other than white and each of a plurality of different colors (e.g., three different colors or more). Providing a single pixel element type with such greater flexibility reduces or eliminates the need for pixel elements with multiple different pixel element types to achieve the desired color capabilities, thus simplifying manufacturing. This method can be implemented with or without a brightness control layer present in the pixel element.

[0073] The brightness control layers 15A to 15C can be directly deposited on the cover layers 14A to 14C (as depicted in the example) or deposited on an intermediate layer (e.g., a planarization layer or an encapsulation layer) disposed on top of the cover layers 14A to 14C. Optionally, a planarization layer or an encapsulation layer can be additionally added after the brightness control layers 15A to 15C (so as to be located on top of the brightness control layer).

[0074] In some embodiments, the color control layers 13A to 13C are deposited by ink. The ink is printed in a pattern to form the color control layers 13A to 13C. Inkjet printing technology, for example, can be used to print the ink. The ink can be printed on a flat surface or a non - flat surface. This method can be particularly advantageous when applied to non - flat surfaces where it may be difficult to achieve a high level of uniformity using alternative techniques such as sputtering. As in Figure 11 and Figure 12As illustrated in, the inks 30A, 30B can include a liquid medium 32 and suspensions of micro-stacks 34A, 34B, where the suspensions of micro-stacks 34A, 34B include a phase change material. Once printing is complete, the liquid medium dries and leaves behind the micro-stacks. The micro-stacks 34A, 34B can be provided by forming larger stacks (e.g., using sputtering) and processing the larger stacks to form micro-stacks (e.g., by crushing the larger stacks using known grinding techniques, for example). In some embodiments, the micro-stacks 34A, 34B are functionalized to promote uniform dispersion of the micro-stacks 34A, 34B in the liquid medium 32 (e.g., to prevent the micro-stacks from aggregating together). The composition and / or nature of the functionalization of the liquid medium 32 is not particularly limited. Each micro-stack 34A, 34B includes at least one phase change material layer. Each micro-stack 34A, 34B can include one or more additional layers, where the additional layer includes any of the layers described above with reference to Figures 1 to 10 (including a reflector layer (e.g., a metal layer), a capping layer, and / or a spacer layer). In some embodiments, as illustrated in Figure 11 and Figure 12 , each micro-stack 34A, 34B is mirror symmetric with respect to the reflector layer 36 in the micro-stack 34A, 34B, where at least one phase change material layer is disposed on each side of the reflector layer 36 in another stack 37. The another stack 37 can include any combination of the layers mentioned above (e.g., a spacer layer, a phase change material layer, and / or a capping layer). In one embodiment, each additional stack includes a phase change material layer sandwiched between two spacer layers, such as described above with reference to Figure 1 and Figures 5 to 10 . Configuring the micro-stacks to be mirror symmetric in this way means that the micro-stacks do not need to be deposited in a specific orientation (both orientations will provide the same optical effect).

[0075] In some embodiments, as illustrated in Figure 11 and Figure 12 , multiple inks 30A, 30B are provided. Each ink 30A, 30B includes a corresponding different type of micro-stack 34A, 34B. Each different type of micro-stack 34A, 34B is configured to provide a different range of effects on the color of light exiting the optical device. For example, different types of micro-stacks 34A, 34B can include layers with different compositions and / or thicknesses to provide different interference effects (e.g., providing different interference effects for spacer layers of different thicknesses). As Figure 13As depicted, each of the inks 30A, 30B can be printed in a different pattern to provide multiple groups 38 of pixel elements 2A, 2B. Each group 38 includes at least two pixel elements 2A, 2B, and the at least two pixel elements 2A, 2B include corresponding different types of microstacks (e.g., to provide corresponding different primary colors). As mentioned above, this can allow for the provision of a full-color display without the need for separate sputtering steps, lithography steps, and etching steps for each primary color.

Claims

1. A method of forming a pattern on an optical device including a plurality of pixel elements, the method comprising: providing an ink including a liquid medium and micro-stacked suspensions, the micro-stacked suspensions including a phase change material capable of switching between a plurality of stable states having different refractive indices relative to each other; and printing the ink in a pattern to form a color control layer in each of the pixel elements.

2. The method according to claim 1, the method further comprising depositing energy into the color control layer of each of one or more of the pixel elements to modify the color control layer so as to change, in use, the effect of the color control layer on the color of light exiting the optical device from the pixel elements.

3. The method according to claim 1 or 2, wherein, each micro-stack is mirror symmetric with respect to a reflector layer in the micro-stack, having at least one phase change material layer on each side of the reflector layer.

4. The method according to claim 3, wherein, each micro-stack includes a plurality of phase change material sub-layers on each side of the reflector layer.

5. The method according to claim 1 or 2, the method further comprising functionalizing the micro-stacks in the liquid medium to facilitate uniform dispersion of the micro-stacks in the liquid medium.

6. The method according to claim 1 or 2, wherein, the ink is printed using inkjet printing technology.

7. The method according to claim 1 or 2, wherein, the ink is printed onto a non-flat surface.

8. The method according to claim 1 or 2, wherein: a plurality of inks are provided, each ink including a respective different type of micro-stack, each different type of micro-stack being configured to provide a different range of effects on the color of light exiting the optical device; and each ink is printed in a different pattern to provide multiple sets of pixel elements, each set including at least two pixel elements, the at least two pixel elements including respective different types of micro-stacks.

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