Actively tunable nanoscale self-actuated surfaces for light manipulation

Abstract

Switchable and / or tunable surface topography is provided using a polymer having a crosslinking pattern formed within it. In operation the polymer is exposed to a liquid and it is switched or tuned between its various states by changing properties of the liquid. One of these states is a reference or " flat" state where the polymer thickness is substantially constant and the pattern is not apparent. In other states, the polymer swells in a pattern that corresponds ( inversely) to the crosslinking pattern. The less crosslinking there is at a point in the polymer, the more swelling there is at that point. The top and / or bottom surfaces of the polymer can be coated with metal to enhance the optical effects of these controllable thickness changes. By stacking such polymer layers, various combined functions are provided, such as independent control of texture and color.

Description

[0001] Actively tunable nanoscale sel f-actuated surfaces for light manipulation

[0002] FIELD OF THE INVENTION

[0003] This invention relates to devices having a tunable or switchable surface topography .

[0004] BACKGROUND

[0005] Controlling the topography of surfaces at the nanoscale is an important challenge across a variety of fields . A wide variety of approaches have been developed to achieve this , including nano-imprint lithography, grayscale lithography, three-dimensional ( 3D) patterning using scanning probes , nano-scale 3D printing via two-photon polymeri zation, and more . These approaches , which typically work with rigid materials , usually lead to surfaces which are static and fixed upon fabrication . Thus it would be an advance in the art to provide switchable or tunable topographic features .

[0006] SUMMARY

[0007] Providing a method to dynamically control the topography of surfaces could enable new applications in sensors , optical devices , bio-interfaces , tunable electronics and more . In particular, In particular, bringing a capacity for structural reconfiguration to micro and nanoscale photonic elements , which are highly sensitive to geometry, could enable new approaches to active light shaping, particularly for soft polymer based devices integrated with the human body .

[0008] We provide sel f-actuated surfaces capable of actively trans forming between three-dimensional patterns and demonstrate their utility as a platform for light manipulation . We find that electron-beam exposure modi fies the solubility and hence solvent uptake of thiophene based conj ugated-polymers . This includes polymers such as PEDOT : PSS poly ( 3 , 4-ethylenedioxythiophene ) polystyrene sul fonate ) , which take up water, and other polymers such as the polythiophene with triethylene glycol side chains (pg3T2 . We focus much of the discussion around processing of PEDOT : PSS films . This electron-beam induced solubility modi fication allows us to control the degree of solution induced swelling in the conducting polymer, PEDOT : PSS , with control over achievable swelling degrees up to 700 % . Electron-beam patterning is therefore used to control the height and prescribe our desired surface topography in the swollen state , with height contrasts of over 500 nm . This is also expected to locally modi fy the sti f fness . Furthermore , the surface topography depends on solution uptake , and this depends on the properties of the solution, the topography can be modi fied by changing the thermodynamic variables associated with the solution . This allows us to change the surface topography by application of chemical stimuli such as solvation energy and salt concentration . For example , by changing the solvent from water to isopropanol ( IPA) , we can switch the surface from completely swollen ( some topography with spatially modulated swelling degree contrasts of ~ 600% , leading to height contrasts > 500 nm for a ~ 100 nm dry thin film) to a de-swollen, flat topography with minimal swelling degree contrast (10%, or ~10 nm for a ~100 nm dry thin film) . Such a platform for control over nanoscale surface topography is expected to have a range of applications, including in optics, electronics, biology and biointerfaces .

[0009] Considering optics in particular, we can create complex optical elements through control of the shape. For example, by evaporating (e.g. electron beam or thermal evaporation) or depositing thin (<100 nm) metal films onto the polymer in the dry state allows the surface with metal to achieve a complex topography in the swollen state. This could enable various applications in optics. One application could involve creating surfaces with reversible switching of surface texture for control over visual appearances. At large feature scales (> 100 micron) , such topographical patterning can able visually apparent textures. At finer feature scales of 10-100 microns the distinctness of images and haze of images reflected from such surfaces can be modulated. At feature sizes of 0.5 - 10 microns, the bidirectional reflectance function of the surface can be modulated to allow for switching from specular reflection, appearing "glossy" to a diffuse reflection, appearing "matte". Other complex control of the bidirectional reflectance function may be possible by controlling the degree of order / disorder of the written pattern (e.g. writing differential swelling patterns with correlated disorder) .

[0010] By further using a metal film as the substrate and depositing a thin layer of metal on top of the surfaces, we can create complex optical cavities. By locally patterning the height of optical cavities, we can spatially modulate their reflection phase and amplitude at visible wavelengths. We use this method to demonstrate a range of dynamically tunable optical elements such as tunable color patterns and on-demand, "wavy" diffractive optical elements, including highly asymmetric blazed gratings and Fresnel lenses, along with elements such as phase plates. The method in general allows for switching between two phase profiles, one being a "flat" phase profile in the de-swollen state, and another being defined by the topographically complex cavity. Introduction of small amounts of surface roughness due to slight height changes at sub-wavelength scales can additionally allow for the creation of diffuse structural color and optical elements functioning with diffuse light. Application of chemical stimuli (e.g. changes in solvation energy, pH, osmolarity, electrochemical potential) allows us to dynamically modulate our optical elements. Our approach opens the door to integration of stimuli-responsive polymers in reconfigurable optical elements with applications in biosensing, display, and imaging.

[0011] This approach can further be implemented within multilayer, multifunctional systems. The switchable element can be written on top of an optical metasurface or a planarized diffractive optical element (DOE) . This therefore allows for combination of some static function that can be written with any metasurface or DOE (for example, a lens, or a Fourier filtering device for edge detection, a phase plate, and more) with some tunable additional function (for example, an additional tunable lens, or phase plate or other phase profile imprinted into the device) . This could allow for devices such as for example, an imaging system which switches from lens+edge detection to lens only, or for a tunable lens which compensates for chromatic aberration in base metasurface. Furthermore, by spin coating a different polymer, dispersed in an orthogonal dispersant (e.g. if aqueous PEDOT:PSS is the bottom layer, the second polymer layer should be dispersed in an orthogonal organic solvent such as chloroform) , a secondary polymer layer can be added to provide an intrinsic optical function, to have additional metal patterned on top to serve as another cavity, or be have patterned swelling to provide a second optical function. For example, if we spin coat pg3T2 onto a first textured PEDOT:PSS + metal layer, we can create colored textured surfaces, with color defined by polymer bandgap or absorption. This could control the absorption behavior of polymer by changing reflection from metal surface from specular to diffuse (essentially changing gloss as mentioned earlier) . This switches from weak absorption in the polymer to higher absorption in the polymer, changing absorption and emissivity of surface. Can allow change of color, from colored to "black", and also control of thermal emissivity by changing absorption properties of system. By depositing a layer of metal on top of the top polymer, a multi-layer cavity defined color can also be created. Finally, the top polymer layer (e.g. pg3T2) can itself be patterned to differentially swell before depositing the second metal layer. A second color pattern, or optical function such as lens can therefore be written onto the top cavity. The top layer can also be chosen such that it has a swelling response to orthogonal stimuli. For example, if the base layer is responsive to water, and the top layer is responsive to organic solvents (e.g. pg3T2) , complete switching of color patterns and / or optical function can be achieved by changing the solvent, allowing you to choose whether you access the pattern and optical function written into the corresponding layer. Therefore, one could write a convex lens pattern into polymer 1, and concave lens pattern into layer 2, and then switch between concave or convex lens, or other types of function, as in implementation 3 with static metasurface + tunable element. This allows for access to very different phase trajectories during the tuning process. Could also access both functions by a mix (e.g. water+acetone ) . Both functions could also be off (e.g. in IPA) .

[0012] We note that if the secondary layer is dispersed in the same solvent as the first layer, the secondary layer will have the texture or pattern of the bottom layer, as the bottom layer will swell during the spin-coating, resulting in spinning on a complex topography, leading to the imprinted complex topography in the dry state.

[0013] More broadly, such self-morphing structures may find diverse applications in areas ranging from tunable electronics to bio-interfaces. For example, surface stiffness and topography are known to affect stem cell differentiation, this could allow for creation of patterned cell communities. The usage of a conductive substrate (PEDOT:PSS) facilitates electrical and chemical monitoring of cells. In electronics, such a technique could tune RF electronics, e.g. by changing the height of inner and outer rings of a split ring resonator. This approach could create tunable organic electrochemical transistors, e.g. by tuning position of source and drain on OECT, or creating complicated position dependent gate separation.

[0014] We note that alternate fabrication methods can be used to achieve the systems and functions we describe here. For example, metal can be added onto the polymer film through the use of metal nanoparticles, which are appropriately functionalized to form covalent or physical bonds with the polymer. For example, functionalizing nanoparticles with surface charges opposite to that of the polymer film will allow for bonding between metal nanoparticles (NP's) and polymer (e.g. carboxylic acid functionalization of NP's provides negative charge, which would allow for bonding with positively charged surfaces.) The dose-dependent swelling can be achieved with approaches other than electron-beam writing through modification of polymer chemistry. For example, for PEDOT:PSS, previous works have described how the addition of the UV cross-linker polyethylene glycol dimethacrylate to commercially available PEDOT:PSS allows for photolithographic direct writing of PEDOT:PSS.

[0015] Modulating the UV exposure dose of the 2-5 multiples above the writing clearance dose would allow for modulation of the swelling degree by photolithographic exposure. A range of other photo-crosslinker chemistries could be used with PEDOT:PSS and other polythiophene based polymers to write complex, swelling encoded topographic patterns in a range of polymers with a photolithographic process.

[0016] Commercial Applications include, but are not limited to the following:

[0017] Wearable biosensors - for example, contact lens based sensors. A particular example relates to dry-eye monitoring and diagnosis. Currently tear films are sampled to check osmolarity, but at a specific location that doesn't necessarily correspond to tear behavior across the eye. This could be integrated with a contact lens to provide photonic read out across the eye as desired (e.g. with a glasses like wearable) . This is a specific example of a broader application area. This work could enable wearable monitoring (e.g. of hydration, for athletes) . Integration with other polymers could allow specific biosensing. Particular benefits including patterning of regions that could serve as self-referenced regions, integration with ion and target sensitive membranes.

[0018] Hyperspectral or other computational imagers for applications such as endoscopy. This work enables an approach to hyperspectral imaging involves random filter arrays for compressed imaging. This works when the scene has low Fourier components, like most natural scenes and allows for high speed single shot imaging. However, in more complex scenes, like in medical imaging, you want to gather more information, potentially by taking many shots that sample multiple basis. Here, for the first time, we could enable this in an integrated device that could be miniaturized. We can write an array of random filters that are also tunable, allowing for rapid imaging until reaching an area of interest (e.g. a potential lesion) , where we want more information to assist computer vision algorithms, so we tune all the filters spectrally.

[0019] Tunable diffractive or nano-optics (e.g. non-volatile, tunable lenses or filters) . Bio-interfaces - e.g. a substrate for cell monitoring that both detects force from cells, by monitoring of the cavity deformation, and electrical signals, via the conductive PEDOT polymer.

[0020] Significant advantages are provided, at least as follows :

[0021] Comparison to previously published academic literature: some recent papers have reported differentially crosslinking a humidity tunable hydrogel with e-beam in order to pattern cavities at different heights / with different swelling degrees. Firstly, these approaches do not allow for transformation from a flat to a complex topography, rather, they report transformation between two existing topographies. This tuning method is also difficult to control for photonics applications (e.g. for non-volatile, switchable optical components or security applications ) , not amenable to usage with established electrical or microfluidic control architectures , and does not discuss or show it can be used to create complex surfaces for di f fractive optical elements or metasurfaces . Furthermore , it does not allow for sensing in physiological solutions and finally, does not enable creation of general complex surfaces for bio-interfaces .

[0022] Other recent work has shown tuning of the topography of structured poly (N-isopropylacrylamide ) based hydrogels in water, in response to heat . Here again, such an approach does not allow for switching between a flat and structured state , but rather two structure states . This puts many applications , such as switching between specular and di f fuse reflection, or erasure of optical information out of reach, along with the possibility of writing multilayer systems with layers that show independent switching between flat to structured states . Furthermore , the heat-based switching method is slow and poses constraints on implementation, limiting usage in many applications , while not allowing for applications such as biosensing or electrochemical control of topography and function . The intrinsic optical response of these polymers are weak due to weak refractive index contrasts between water and the polymer . This limits the optical applications that can be enabled due to the low ef ficiency of any such structures . Furthermore , it is challenging to deposit or pattern metal on such materials (which have relatively poor adhesion) to enable metasurfaces or higher contrast , higher quality optical components .

[0023] Advantages and improvements relative to commercial products are mentioned above , but are also briefly summari zed here . We can enable soft sensors that could be part of combined microf luidic / photonic devices , or devices operating directly in physiological solution such as contact lenses . They can be patterned to be " sel f- referenced" , allowing for greater robustness and accuracy, a maj or problem in current wearables . For imaging devices , we can make use of the ability to pattern static structures for optical information processing to allow for high speed imaging, that can also be tuned to allow for a higher information content slower mode as needed . A particular example could be a miniaturi zed random filter array for hyperspectral imaging for use in applications such as endoscopy, that can then be tuned to gain more diagnostic information for a feature of interest .

[0024] Finally, our approach could allow transduction of mechanical force to be measured . Our use of a conductive polymer allows for a unique opportunity to create electrodes based on these optical cavities which simultaneously measure mechanical force and record electrically, allowing for opportunities to study say, the ef fect of drugs on the electrical and mechanical properties of cardiomyocytes while being cultured on regions of the polymer with di f ferent patterned boundary conditions ( geometry, sti f fness ) that recapitulate di f ferent micro-environments .

[0025] BRIEF DESCRIPTION OF THE DRAWINGS

[0026] FIGs . 1A-C show operating principles of several embodiments of the invention .

[0027] FIGs . 2A-E show examples of switchable surface topography between flat and topographic states .

[0028] FIGs . 3A-F show examples of switchable surface topography for texture control . FIGs. 4A-D show examples of switchable surface topography for color control.

[0029] FIGs. 5A-E show an example of switchable surface topography for independently controlling both texture and color .

[0030] FIGs. 6A-C show an exemplary e-beam patterning process, along with some characterization results of the e-beam processing .

[0031] FIG. 7 shows the dependence of swelling in water on e- beam dose, compared to data showing no swelling in IPA.

[0032] FIGs. 8A-B show an exemplary effect of a top metal layer on switchable surface topography.

[0033] FIG. 9 shows an example of different e-beam doses leading to different hues (i.e., different angular positions on a CIE diagram) using switchable surface topography for color control.

[0034] FIG. 10 shows an air-facing embodiment of the invention .

[0035] DETAILED DESCRIPTION

[0036] 1) Introduction

[0037] Surface textures influence many aspects of our sensory perception of objects. These range from tactile sensation to visual appearance, which is influenced by perception of both the spectral and angular distributions of light waves. Living organisms frequently employ the reversible deformation of soft tissue to modulate surface texture. This allows them to control interactions with their physical environment for camouflage and signaling. For example, actuation of muscle fibers in skin papillae allows cephalopods to rapidly trans form their skin texture between a smooth, planar state and topographically complex, three- dimensional ( 3D) states that resemble natural textures found in their environment . These extreme " flat-to-3D" textural changes , combined with the ability to change their skin' s coloration patterns through actuation of chromatophores , endow cephalopods with sophisticated control of their visual appearance .

[0038] Many optical studies that draw inspiration from biological systems have achieved control over the spectral component of visual appearance , i . e . color, by engineering nanoscale resonances and wave interference ef fects . However, the angular distribution of light is at least as important in determining visual appearance . Surface topography shapes this aspect of appearance by redistributing incident light waves based on the geometry and characteristic length scales of di f ferent surface features . This leads to the perception of properties ranging from visual texture , e . g . whether the surface appears bumpy or wavy, to surface finish, e . g . the degree of apparent gloss or haze . Dynamic control over surface textures , which are central in the way we perceive and interact with our environment , has remained elusive . Furthermore , independent control of texture and color, which is key to the camouflage abilities of cephalopods , has yet to be reali zed .

[0039] Mutable soft materials can be used to dynamically vary surface topography . Dynamic modulation of texture using soft materials at both the micro- and nano-scales could enable on-demand control over the scattering of light waves , providing new mechanisms for regulating visual appearance . However, current approaches to switchable surfaces , including recent demonstrations relying on elastic instabilities or patterning of hydrogels are limited in terms of either their degree of topographical contrast, their patternability, or their achievable feature size. There is a need for methods capable of encoding lithographically defined, naturalistic textures that can be hidden and revealed on-demand.

[0040] FIG. 1A is a schematic illustration of the working principle of polymer films with encoded flat-to-3D topographical transitions. Here 102 is the substrate, 104 is the polymer, 106 refers to the flat reference state, 108 refers to a topographic state (there can be one or more of these states) , and 110 is a liquid the device is exposed to. In this example, the swelling degree of the polymer PEDOT:PSS is patterned by electron-beam exposure. In 2- propanol (IPA) , which is an unfavorable solvent, PEDOT:PSS does not swell, therefore the surface remains essentially flat (as shown by 106) . In water, which is a favorable solvent, PEDOT:PSS swells, transitioning to a complex surface topography encoded by the previously written dose pattern (as shown by 108) .

[0041] FIG. IB shows optical micrographs of patterned PEDOT:PSS films coated with a thin layer of metal 112 (here substrate 114 is transparent) . They show switching of visible texture between a flat state in IPA and a textured state in water. Such metal layers are typically patterned, with a uniform layer counting as one of the possibilities for the layer pattern.

[0042] FIG. 1C shows optical micrographs of patterned Fabry- Perot cavities, with an Au-PEDOT : PSS-Au structure (here substrate 116 has a metal layer on its top to provide the bottom metal for the Fabry-Perot cavities) . Their degree of visual contrast switches between a contrast-free state in IPA and a high-contrast state in water. Here we develop programmable surfaces based on thin polymer films in which the degree of swelling is spatially varied through dose-dependent electron-beam exposure . Electron-beam exposure encodes complex topographies across a broad range of length-scales (mm- to pm-scale ) that appear or disappear upon immersion in di f ferent solvents ( FIG . 1A) . These " flat-to-3D" trans formations , which switch between the extrema of topographic contrast , allow us to reali ze various textural and colorful appearances as illustrated in FIGs . 1B-C . In this section, we start by showing how visible textures at mm-scales can be completely hidden and revealed on demand . Secondly, by creating switchable textures at pm- scales , we demonstrate switching between matte and mirrorlike reflection, providing a tuning knob for under-explored properties of visual appearance such as gloss . Thirdly, we encode dynamic patterns of structural color by creating topographically complex optical cavities that modi fy both the color and the angular scattering of light , imbuing patterns with a sense of textural richness . Finally, by combining these elements in a multi-layer device , we demonstrate independent , active control of both reflected color and texture . This platform enables functional surfaces for applications ranging from dynamic camouflage and display for soft machines to new types of photonic devices .

[0043] 2 ) Achieving local and arbitrary control over surface topography

[0044] FIG . 2A is a measured contrast curve for electron-beam exposed PEDOT : PSS , characteri zing the swollen height in water and IPA as a function of the electron dose . Measurements were made by sampling the edge height of isolated blocks in 5 locations . Data points shown are the mean, the standard deviation is not visible as it lies inside the plotted points.

[0045] FIG. 2B shows the achievable height contrast for a square wave pattern of alternating high (2500 pC / cm2) and low dose (500 pC / cm2) stripes at different periods.

[0046] FIG. 2C shows an example of in situ liquid AFM of patterned FEDOT :PSS. A replica of El Capitan is encoded into the FEDOT :PSS in the swollen state.

[0047] FIGs. 2D-E show in situ liquid AFM of patterned FEDOT :PSS in isopropyl alcohol (FIG. 2D) and water (FIG. 2E) . The topographical pattern is hidden in the flat state in isopropyl alcohol and becomes apparent in the swollen state in water.

[0048] The realization of complex, reversible textures hinges on the ability to induce large changes in surface topographies across multiple spatial scales. We demonstrate that this can be achieved with the widely used conductive polymer, poly ( 3 , 4-ethylenedioxythiophene ) : poly ( styrene sulfonate) (FEDOT :PSS) and quantify the swelling of thin FEDOT :PSS films modulated by exposure to electron-beam radiation. We use this property to create programmable surfaces by first spatially patterning the degree of swelling of our surfaces and then dynamically modulating it by applying different solvents (FIGs. 1A-C, FIGs. 6A-C) .

[0049] After electron-beam exposure, the degree to which the FEDOT swells in water is inversely proportional to the amount of exposure (FIG. 7) . The volumetric swelling of isolated structures in water can be modulated from over 800 % near the critical dose down to 200 % at higher doses (FIG. 2A) . For a film with a thickness of approximately 100 nm in the dry state, this difference leads to achievable height contrasts between isolated regions patterned with different doses of almost 550 nm. The swelling is driven by uptake of water, which is a favorable solvent for the hydrophilic PSS component of PEDOT:PSS. Previous reports have demonstrated that PEDOT:PSS is a negative electron-beam resist, suggesting that electron-beam exposure results reduces swelling through radiation induced cross-linking.

[0050] FIG. 2B shows the achievable height variation for a square wave pattern of alternating high (2500 pC / cm2) and low dose (500 pC / cm2) at different periods. As the period is reduced from 20 pm to 1.5 pm, the height variation is reduced from 420 nm to 140 nm.

[0051] The precise spatial control afforded by electron-beams allows intricate and complex topographies to be produced in the swollen state. As an example, we patterned a nanoscale replica of the topography of the rock formation, El Capitan (Yosemite National Park, CA, USA) (FIG. 2C) . The 3D topographical data was discretized into 256 layers with different applied doses. The achievable height variation for continuously patterned structures is limited by electronbeam proximity effects and mechanical constraints on the swelling of adjacent structures. These length-scales, which are within a few multiples of the wavelengths of visible and infrared radiation, are amenable to the creation of complex structures that shape light by controlling specular reflection and diffractive effects.

[0052] Applying liquids with different solvation energies for PEDOT:PSS allows the flat state to be retrieved. While the flat state can also be recovered by drying in air, controlling the topography while keeping the device in liquid allows for integration with fluidic control systems that are used widely in optofluidics and soft robotics. The height of isolated structures in 2-propanol (IPA) , which does not swell PEDOT:PSS, is essentially the same as in the dry state and varies by less than 11 nm across all doses (FIG. 2A) .

[0053] We highlight our ability to dynamically reveal virtually arbitrary, designer topographies by encoding different regions of the Stanford seal pattern to display different degrees of swelling. The written structure transitions from a hidden, flat state in IPA (FIG. 2D) to a structured state in water (FIG. 2E) . This approach realizes a lithographically defined flat-to-3D transition representing the extrema of textural contrast, suggesting the potential to dynamically control properties of reflected light using fine, wavelength scale structures.

[0054] 3) Dynamic control of visible appearance

[0055] FIGs. 3A-C show images illustrative of the macroscale appearance of textured surfaces taken with a digital camera, all with a scale bar of 2 mm. FIG. 3A shows a textured surface with millimeter scale features in its hidden, flat state in IPA. FIG. 3B shows the same textured surface with millimeter scale patterns swells in water to a state with visible wave-like textures. FIG. 3C shows a separate textured surface with micrometer scale features in its swollen state in water.

[0056] FIG. 3D shows the appearance of surfaces with different microscale textures taken with a bright-field microscope under white light illumination. Here (i-iii) are the textures. Optical micrographs shown have a scale bar of 20 pm. A reference image for the flat state of (i) is shown on the left.

[0057] FIG. 3E shows the angular reflectance spectra of the textures (i-iii) from FIG. 3D, as measured by back focal- plane imaging using a 600 nm laser source. FIG. 3F shows a printed photo of a walrus was held near a textured surface, encoded with the pattern in (FIG. 3E texture i) . The reflected image was captured using a digital camera in the flat state (302) and swollen state (304) . The scale bar is 4 mm.

[0058] The perception of visual textures arises from the interaction of light with surface topographies. Surface features with different length scales control different visual phenomena. We first patterned the swelling of our thin-films to create mm- scale visible surface textures in the swollen state. We enhanced the visibility of the encoded textures by depositing a thin (~20 nm) layer of metal on top of the polymer. Although the addition of metal reduced the swelling contrast, it allowed for easy observation of encoded textures with the naked eye. FIGs. 3A and 3B show a ripple-like texture pattern that is invisible in the flat state (FIG. 3A) and becomes visible in the swollen state (FIG. 3B) . Their swollen state surface topography, patterned at spatial frequencies over 100 times larger than the wavelength of visible light (~100's of pm) , specularly reflects incident light in a variety of directions. A region of the surface that redirects light rays in directions falling inside the collection solid angle of the eye (or other optical observation system) will appear brighter whereas regions that primarily reflect rays outside the collection solid angle will appear darker. These alternating regions of bright and dark create the visual appearance of texture .

[0059] Patterning structures at the microscale, with feature sizes of ~2-4 pm, affords control over the overall visual perception of the surfaces. In the swollen state, these micro-textured surfaces have a rough, matte-like appearance (FIG. 3C) due to a diffuse reflection. At these patterning length-scales, which are comparable to a few multiples of the wavelength of visible light, diffractive effects between waves reflected at different heights lead to diffuse scattering. FIG. 3D shows optical micrographs of a range of naturalistic textures written with microscale feature sizes. We investigate the angular distribution of reflected light through back-focal plane imaging. In the flat case, the reflected angular spectrum is concentrated within 1° of the specular direction (FIG. 3E) . In the swollen case, the reflected light becomes significantly more diffuse. The peak intensity in the specular direction is reduced by over 80%, and the light is redistributed to angles up to 10° around the specular direction (FIG. 3E textures i-iii) . The angular spectrum depends on the spatial frequency content of the texture patterns. This shows the possibility of engineering reflected light fields with desired angular distributions, such as uniform diffusers (FIG. 3E texture i) or grating like structures that can redirect light into preferred directions (FIG. 3E texture ii) . We illustrate how such microscale patterning provides control over visual perception of surfaces by demonstrating dynamic modulation of the haze of a surface (FIG. 3F) . Switching from a smooth to a hazy surface appearance degrades the quality of a reflected image of an ink-jet printed walrus, with significant blurring of key features such as its tusks.

[0060] In contrast to previous reports, which control the angular distribution of light with resonant nanostructures or reflective inks, our approach allows for control of angular distribution that i) can be dynamically switched on and off, and ii) that modifies the distribution without correlated color changes. 4) Dynamic spatial control over color with Fabry-Perot resonances

[0061] FIG. 4A is a schematic illustrating differential swelling of optical cavities with patterned swelling degree in different solvents.

[0062] FIG. 4B shows micrographs of a cavity patterned with stripes of different electron beam doses in IPA and water with a scale bar of 30 pm. From right to left, the dose is varied from 1175 pC / cm2to 4000 pC / cm2, in dose steps that begin at 100 pC / cm2and increase geometrically by 25%.

[0063] FIG. 4G shows the evolution of the coloration contrast, defined as the difference in greyscale intensity between highest and lowest intensity image pixels, when the solvent was switched from IPA to water (right) and then from water to IPA (left) . The dashed lines in the plots indicate the time period in which 90% of the full contrast change is achieved .

[0064] FIG. 4D is a schematic device cross-section for a cavity integrated with a flexible substrate to form a wearable device 402. Here flexible substrate 404 can be, for example, polyimide. The polymer layer 408 is sandwiched between a bottom gold layer 406 and a top gold layer 402 and can operate as described above.

[0065] In this section, we show how precise spatial control over swelling behavior can generate complex, dynamically switchable color patterns. To this end, we placed PEDOT:PSS in between two thin metallic layers to create Fabry-Perot resonators with a spectral reflectance that is controlled by the thickness of the polymer layer (FIG. 4A) . In the flat state, the surfaces display almost no observable color contrast, whereas in the swollen state the topography translates into a visible color pattern (FIG. 1C, FIG. 4B) . The large variations in the degree of swelling that can be achieved with the electron beam treatment allow for the creation of optical cavities with greatly varying mirror spacings and thus a broad range of colors ( FIG . 9 ) .

[0066] Upon switching the liquid from IPA to water using a liquid flow-cell , the surface switches between a low- contrast , "hidden" state , to a high-contrast state where adj acent structures display vivid and distinct hues ( FIG . 4G ) . Observing the switching dynamics reveals that 90% of the color change occurs in less than 10 seconds . While active control of structural color has been widely explored, it remains challenging to rapidly transition from a uni formly colored contrast- free state to a high-contrast state . The ability to ef fectively hide and display information on demand could provide new approaches to applications ranging from encryption to camouflage .

[0067] Repetitive switching between the two solvents slightly shi fts the resonance cavity wavelength by ~ 10 nm after 15 cycles as observed from the minima of the reflected spectra . This suggests that while our mechanism is largely reversible , there is a slight reduction in height contrast upon cycling . This could be related to inherent structural changes in the material , fatigue damage to the top metal layer, or a reduction in contrast due to residual water or IPA in the flow cell system after many cycles .

[0068] The fine control over cavity thickness af forded by our method allows us to achieve smooth gradations of color . This results in rich images capable of conveying a sense of depth . We demonstrate this ef fect by translating the grayscale of a naturalistic image of seagrass into a 256- level electron-beam dose pattern that is used to define the topography of an optical cavity . Blades of grass in the optical micrograph of this cavity exhibit subtle variations in hue that communicate features such as curvature . Intriguingly, the surface topography also af fects the angular distribution of light by scattering and redirecting reflected light at di f ferent angles . This ultimately lends textural richness to images and allows texture to be perceived even without cavity induced color gradients .

[0069] The broader uptake of our surfaces will be assisted by the industrial availability of PEDOT : PSS and its established integration in device fabrication processes in fields ranging from bioelectronics to optoelectronics . We highlight our ability to straightforwardly integrate our thin- film based system into a range of form factors by fabricating our device on a flexible polyimide substrate ( FIG . 4D) , demonstrating a prototype for structural color-based switchable photonic skin with potential applications ranging from wearable devices to coatings for soft machines .

[0070] 5 ) Independent control of texture and color

[0071] FIG . 5A is a schematic illustration of a bi-layer device structure 502 . The textured side 508 includes a 20 nm metal-coated, electron-beam patterned PEDOT : PSS film 504 fabricated directly on the substrate 518 . The color side 510 includes optical cavities that generate color patterns , formed by patterned PEDOT : PSS 506 in between two 20-nm-thick layers of Au . These are patterned on the backside of the substrate 518 , and Au layer 520 is one of the Au layers for these optical cavities . Separate solvents 512 and 514 can independently be applied to either side of the coverslip using a fluid cell . Top and bottom cover slips 516 can be used to confine the solvents .

[0072] FIGs . 5B-E show optical micrographs of the device of FIG . 5A. FIG . 5B shows the result for water on both sides of the device . FIG . 5C shows the result for IPA on both sides of the device . FIG . 5D shows the result for water on the texture side and IPA on the color side . FIG . 5E shows the result for IPA on the texture side and water on the color side . Scale bars for all optical micrographs are 200 pm .

[0073] The extraordinary camouflage abilities of cephalopods rely on simultaneous control of texture and color patterns that are independent of each other . By patterning optical cavities and textured surfaces on either side of a thin transparent substrate , we achieve independent control of distinct texture and color patterns by exposing each side to a solvent of choice ( FIG . 5A) . Exposure to water on both sides generates a colored, textured state ( FIG . 5B ) . The observed appearance is defined by both the pattern of dark and bright regions set by the texture pattern as well as the color pattern set by the optical cavities . Switching to IPA onto di f ferent sides of the substrates causes de-swelling of either the color or texture pattern which enables 3 appearance states : i ) a flat state with no color or texture pattern ( FIG . 5C ) , ii ) a texture-only state ( FIG . 5D) , and iii ) a color-only state ( FIG . 5E ) . The straightforward integration of multiple partially transmissive layers allows us to create devices that provide a new level of active control of visual appearance .

[0074] 6 ) Outlook

[0075] By providing a means to actively modulate texture and color simultaneously we enable new approaches to dynamic camouflage , along with other applications which may benefit from access to many complex visual appearance states , including encryption, dynamic artworks and displays . Currently we transition between a flat state and a single encoded pattern whereas cephalopods can achieve a diverse range of patterns . Recent studies have shown that PEDOT : PSS swells / de-swells reversibly in electrolyte solution in response to changing electrochemical potentials . This suggests the possibility to electrically tune appearance and achieve di f ferent texture patterns using addressable pixelated devices . Developing alternate methods of patterning the swelling degree of PEDOT : PSS , including novel chemistries compatible with photolithography, could enable scalable manufacture of our surfaces . Finally, integrating our surfaces into soft robotic systems capable of changing their bulk shape could enable truly cephalopod-like camouflage . These ef forts will be aided by the established integration of fluidic systems in soft machines for actuation and display .

[0076] Beyond the aesthetic possibilities , our programmable surfaces could enable new approaches to many applications where the micro- and nanoscale geometry dictates performance , ranging from programming of cell growth, control of friction, microfluidic control systems , 3D biointerfaces and tunable photonic devices .

[0077] 7 ) Methods

[0078] Sample Fabrication :

[0079] Programmable surfaces were prepared by exposing thin- films of spin-coated PEDOT : PSS to electron-beam radiation followed by development in water ( FIG . 6A) .

[0080] Prior to spin-coating, substrates are cleaned by sonication ( 5 minutes ) in acetone , and 2-propanol ( IPA) , followed by 13 minutes of UV-ozone treatment to improve the wettability for spin coating . Devices for process characterization and demonstrations of visible texture were prepared on bare quartz wafers. Devices demonstrating patterning of the Stanford logo and Yosemite topography were prepared on bare Si wafers. Devices demonstrating optical cavities were prepared on Au-coated Si substrates, prepared by electron-beam evaporation of Ti (3 nm) followed by Au

[0081] (100 nm) onto a bare Si wafer. Multi-layer devices were prepared using a 150 pm thick glass coverslip (purchased from VWR, #1 size) . In this instance, the metal cavity base layer was prepared by electron-beam evaporation of Ti (3 nm) followed by Au (100 nm) .

[0082] PEDOT:PSS was then spin-coated at 2000 RPM for 60 s. Samples are allowed to dry in an ambient environment without post-baking. Electron-beam patterning was performed by exposing the PEDOT:PSS films with a Raith Voyager electronbeam lithography (EBL) system using an acceleration voltage of 50 kV and an aperture of 60 pm. After the electron-beam exposure, samples were developed in deionized water (DI water) for 90 s followed by drying with a nitrogen gun. Samples can also be dried with air.

[0083] Thin metal films were subsequently deposited onto the exposed PEDOT:PSS surfaces by electron-beam evaporation. The metal top layer in all instances was a 20 nm Au film, except for the sample used to evaluate reflected image quality in FIG. 3F, where we used a 3 nm Au - 7 nm Ag - 3 nm Au trilayer film.

[0084] 8) Supplemental information

[0085] FIG. 6A shows a schematic of an exemplary fabrication process flow.

[0086] FIG. 6B is a comparison of the height of PEDOT:PSS in air and water at various e-beam doses. Measurements were made by sampling the edge height of isolated blocks in 5 locations. Measurements are plotted as the mean ± standard deviation. For data points where error bars are not visible, the standard deviation lies inside the plotted points. FIG. 6C is an expanded view of the height profile for blocks in the dry state.

[0087] Untreated PEDOT:PSS films are dispersible in water. Upon exposure above the critical electron-beam dose of 700 pC / cm2, exposed regions are no longer dispersible in water (FIG. 6A) . This allows for direct writing of PEDOT:PSS patterns through selective electron-beam exposure. As the dose is increased beyond the critical dose, the dry height of the structures remains constant (FIG. 6B) , as this is determined by the initial dry film thickness. However, the swelling reduces with increasing dose (FIG. 6B) . We hypothesise that this is due to increased film crosslinking .

[0088] Below the critical dose, the film is only partially stable in water and some material is washed away upon exposure to water. The height of the dry structures, which is lower than the initial film thickness, is no longer constant and depends on the applied dose. We hypothesise that the applied dose only partially crosslinks the film. The structures still swell significantly in water, but the swelling is spatially non-uniform due to the incomplete treatment .

[0089] FIG. 7 shows that swelling is inversely proportional to electron-beam dose. Height values are taken from FIG. 2A, and doses above the critical dose are re-plotted against 1 / Dose. The slopes, ksoivent, of linear fits to h oc ksolvent-^) , are given by kwater= 507020 2567 FIGs. 8A-B show the effect of metal thin-films on swelling of PEDOT:PSS. FIG. 8A shows height of electron-beam exposed PEDOT:PSS films in water with and without a 20 nm layer of Au as a function of dose. Measurements were made by sampling the edge height of isolated blocks in 5 locations. Data points shown are the mean, the standard deviation lies inside the plotted points. The PEDOT:PSS only curve re-plots data from FIG 2A. FIG. 8B shows the achievable height variation for a square wave pattern of alternating high (2500 pC / cm2) and low dose (500 pC / cm2) at different periods with and without a 20 nm layer of Au. The PEDOT:PSS only curve re-plots data from FIG. 2B.

[0090] FIG. 9 shows modulation of chromaticity of PEDOT:PSS based optical cavities, by showing the CIE 1931 chromaticity co-ordinates of different samples, determined from the measured spectra. A broad range of hues, as represented by the angular component of chromaticity, are accessible.

[0091] FIG. 10 shows an embodiment of the invention where the polymer layer is not immersed in the liquid. Instead, polymer layer 104 is exposed to liquid 1006 via a liquid- permeable membrane 1002 that is part of liquid chamber 1004. In this way, the tunable surface topography can be provided on an air-facing surface. Here liquid can diffuse through the membrane to switch the state of the polymer layer.

Claims

CLAIMS1 . Apparatus comprising : a substrate ; a top polymer layer disposed on top of the substrate and having a top surface ; wherein the top polymer layer includes a top crosslinking pattern formed by regions in the top polymer layer having di f ferent crosslinking; wherein the top polymer layer is configured to switch between a top reference state and one or more top topographic states ; wherein the top surface is substantially flat in the top reference state ; wherein the top surface has top surface features determined by the crosslinking pattern in any of the one or more topographic states ; and wherein the apparatus is configured to be switchable between its top states by changing one or more first properties of a first liquid the top polymer layer is exposed to .2 . The apparatus of claim 1 , wherein a height variation of at least one of the top topographic states is 50% or more of a maximum thickness of the top polymer layer .3 . The apparatus of claim 1 , wherein a height variation of the top reference state is 10% or less of a maximum thickness of the top polymer layer .

4. The apparatus of claim 1, wherein the first properties are selected from the group consisting of: solvation energy, pH, osmolarity, and electrochemical potential difference.

5. The apparatus of claim 1, wherein the crosslinking pattern is configured to provide a diffractive optical element selected from the group consisting of: lens, phase plate, spatial filter, and grating.

6. The apparatus of claim 1, wherein the apparatus is configured as a device selected from the group consisting of: optical display device, optical sensor device and diffractive optical device.

7. The apparatus of claim 1, further comprising: a patterned first metal layer disposed on top of the top polymer layer; wherein a visible texture of the apparatus or a metasurface function of the apparatus is tunable by switching between states of the top polymer layer.

8. The apparatus of claim 1, further comprising: a patterned first metal layer disposed on top of the top polymer layer and configured to act as a first metasurface; and a patterned second metal layer disposed between the substrate and the top polymer layer and configured to act as a second metasurface;wherein a combined metasurface function of the first and second metasurfaces is tunable by switching between states of the top polymer layer .9 . The apparatus of claim 8 , wherein the combined metasurface function is selected from the group consisting of : filters , spectral filters , polarimetric filters , and filters configured to encode or decode a sparse spectro- polarimetric basis .10 . The apparatus of claim 1 , further comprising : a bottom polymer layer disposed such that the substrate is sandwiched between the bottom polymer layer and the top polymer layer, the bottom polymer layer having a bottom surface facing away from the substrate ; wherein the bottom polymer layer includes a bottom crosslinking pattern formed by regions in the bottom polymer layer having di f ferent crosslinking; wherein the bottom polymer layer is configured to switch between a bottom reference state and one or more bottom topographic states ; wherein the bottom surface is substantially flat in the bottom reference state ; wherein the bottom surface has bottom surface features determined by the bottom crosslinking pattern in any of the one or more bottom topographic states ; wherein the apparatus is configured to be switchable between its bottom states by changing one or more properties of a second liquid the bottom polymer layer is exposed to .11 . The apparatus of claim 10 , further comprising : a patterned first metal layer disposed on top of the top polymer layer ; wherein a visible texture of the apparatus is tunable by switching between states of the top polymer layer .12 . The apparatus of claim 11 , further comprising : a patterned second metal layer disposed on the bottom surface ; and a patterned third metal layer disposed between the substrate and the bottom polymer layer wherein one or more optical resonators are formed by the patterned second metal layer and the patterned third metal layer ; wherein resonances of the one or more optical resonators are tunable by switching between states of the bottom polymer layer .13 . The apparatus of claim 12 , wherein the first liquid has a di f ferent composition than the second liquid .14 . The apparatus of claim 12 , wherein the first liquid and the second liquid have equal composition, and wherein the top polymer layer and the bottom polymer layer are independently electrochemically tunable by application of distinct tuning voltages .15 . The apparatus of claim 1 , wherein the substrate is a liquid-permeable membrane , and wherein the top polymer layeris exposed to the first liquid by diffusion of the first liquid through the liquid-permeable membrane.

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