WO2016028642A1

WO2016028642A1 - Flexible and tunable reflective skin

Info

Publication number: WO2016028642A1
Application number: PCT/US2015/045328
Authority: WO
Inventors: Allan James Bruce; Michael Cyrus; Sergey Frolov
Original assignee: Sunlight Photonics Inc.
Priority date: 2014-08-22
Filing date: 2015-08-14
Publication date: 2016-02-25
Also published as: EP3183606A1

Abstract

Free-standing, flexible articles (skins), with settable or tunable reflectivity, for a defined range of electromagnetic frequencies are provided. The articles include a monolayer, or multi-layers, of ductile or elastic materials which retain mechanical integrity when the skin is stretched, flexed or otherwise altered in shape during deployment or use. The articles may further include an optical structure which exhibit changeable reflectivity when the skin is stretched, flexed or otherwise altered in shape. Methods of tuning the reflective characteristics of such skin through stretching, flexing or otherwise changing shape are also provided.

Description

FLEXIBLE AND TUNABLE REFLECTIVE SKIN

FIELD OF INVENTION

[0001] The present invention relates to the structure and functionality of a freestanding skin which is flexible and exhibits settable or tunable reflectivity when the skin is stretched, flexed or otherwise changed in shape. The skin may be used for a number of applications including dynamic camouflage, reflective tagging and optical filtering.

BACKGROUND

[0002] When light of given wavelength (lambda) is incident on a material or structure, it is reflected transmitted or absorbed to various, and complementing degrees. Reflection may be mirror-like (specular), with an angular dependence, or diffuse (scattered), without a specific angular dependence, or a combination of both. Reflection may occur at the surface without significant light penetration of the material, within a material layer immediate to the surface or at sub-surface interfaces or structures.

[0003] For some applications, it is desirable to minimize reflection from interfaces or structures. Anti-reflection (AR) technologies have been developed for this purpose which employ specific optical materials or engineered structures. A related Patent Application, entitled "Flexible and Tunable Anti-Reflection Skin," filed in the United States Patent and Trademark Office on even date herewith, addresses this area and is hereby incorporated by reference in its entirety.

[0004] For other applications it is desirable to have significant specular or diffuse reflection. Reflective coatings and structures typically have fixed dimensions and are used on fixed structures or devices. Examples include Multi-layer Interference Reflectors and Grating Reflectors. Multi-layer Interference Reflectors are used for both AR and reflection applications. The dimensionality of a multi-layer stack impacts the reflectivity and the wavelength dependence. Grating Reflectors are typically comprised of ID and 2D structures defined on the surface, or within, a device. If the grating period is much smaller than the wavelength of the incident light (sub-wavelength), the grating behaves like a homogeneous material.

[0005] Tunable reflectivity has been shown for a number of devices and structures. For example, tuning by thermal and electrical means in VCSELs has been

demonstrated. Other examples include electro-chromic mirrors where induced absorption in a surface layer is used to tune the intensity of reflection from underlying mirror surfaces.

[0006] Tunable reflectivity requires a capacity for altering reflectivity and a control mechanism._Applications in which it is desirable to be able to set or dynamically tune the reflective properties of a surface or structure include (i) dynamic camouflage, where reflections are used to mimic physical surroundings, (ii) dynamic optical tagging, where activated retro-reflectivity is used for identification purposes and (iii) optical switches, attenuators and filters for emitters, sensors and analogous devices.

SUMMARY

[0007] Various applications employ articles such as skins which have significant specular or diffuse reflection. As discussed below, the invention provides skins that are mechanically durable and which exhibit settable or tunable reflection when subject to changes in shape or physical dimensions. Embodiments include skins with stacked sub-wavelength layers and nano-, or micro-, structures which experience dimensional changes and exhibit changeable reflectivity when the skins are stretched, flexed or otherwise changed in shape. In their original state the skins may be transmitting, reflecting or partially both.

[0008] In a first aspect, the invention features a free-standing film or self-supporting film or skin which is flexible, stretchable and exhibits optical reflectivity in its original or stretched state.

[0009] The skin is distinguished from conventional reflectors in that it is specifically designed to be mechanically durable and alter its reflectivity when stretched or flexed. In this regard the materials, structure and optical design are selected, or engineered, such that any critical values of dimension, refractive index or other essential characteristics are achieved during deformation.

[0010] A variety of flexible and transparent base materials including polymers and fluoro-polymers and standard engineering and design methods may be employed to achieve the desired characteristics. The latter includes single- or multi-layer structures, doped, composite and nano-structured layers or surfaces. The achievable reflectivity characteristics are appropriate for a wide range of devices.

[0011] In a second aspect, the invention features tunable reflectivity characteristics achieved by stretching or flexing the skin. In this case the structure and reflectivity design of the skin is engineered to have a level of sensitivity to a defined deformation. This approach can be used to provide the ability to optimize performance at different wavelengths of light, filter light to, or from an underlying device, and implement retro reflective tagging or reflective camouflage.

[0012] Embodiments of the present invention, summarized above and discussed in greater detail below, can be understood by reference to the illustrative embodiments of the invention depicted in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

[0013] In many applications, reflectors, or reflecting surfaces have a fixed reflectivity, fixed dimension and are fixed or set in their spatial relationship to a substrate, structure or device. In the present invention the skin in its entirety can be considered a reflector, which exhibits adjustable reflectivity, variable dimension and as a stand-alone article, it need not have a fixed spatial relationship to a substrate, structure or device. These characteristics enable new applications.

BRIEF DESCRIPTION OF DRAWINGS

[0014] Figures 1(a) and 1(b) show a schematic representation of a multi-layer skin, with an optical interference stack. [0015] Figures 2(a) and 2(b) show another schematic representation of a skin, with a ID grating on the surface.

[0016] FIG. 3 shows a skin having a plurality of layers 202, 204 and 206.

DETAILED DESCRIPTION

[0017] In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of exemplary embodiments or other examples described herein. However, it will be understood that these embodiments and examples may be practiced without the specific details. In other instances, well- known methods, procedures, components and circuits have not been described in detail, so as not to obscure the following description. Further, the embodiments disclosed are for exemplary purposes only and other embodiments may be employed in lieu of, or in combination with, the embodiments disclosed.

Structure

[0018] The discrete skins may have physical structures which are resilient to significant mechanical or functional degradation or failure when stretched and, or flexed in the course of deployment or use. They may be comprised of ductile materials, if a single deployment is sufficient, or elastic materials if repeated or continuous bi-directional modification is required during use. In cases where the skin is comprised of multiple-layers the physical properties of the constituent layers should be sufficiently similar to maintain the integrity of the skin under conditions of use without mechanical, or functional, degradation. Examples of suitable skin materials include standard polymeric materials which meet the requirements of the application.

[0019] The preferred skins may comprise appropriate materials or surfaces to be able to function as discrete elements, such as structural skins, or to interface with supporting structures or surrounding media. Such interfacing may be achieved in a number of conventional ways by chemical, thermal, mechanical, electrical or other means and may include optional surface layers or materials to assist the interfacing process. Examples of such surface layers include layers comprised of standard adhesives.

[0020] The skins may have any of a number of transverse structures which enable the desired reflectivity. They may be comprised of mono-layers, which have constant, graded or varied refractive index. They may be comprised of a multi-layer where the layers have an engineered progression of refractive index. They may include nano- composite layers or nano-structured surfaces which may provide a wider range of engineered refractive index profiles than dense or single material layers. The skins may incorporate an interference stack of materials which change reflectivity when subject to flexing and stretching. The skins may also include grating structures, including sub-wavelength gratings, which exhibit varying reflectivity when stretched or flexed.

[0021] Figures 1(a) and 1(b) show a schematic representation of a multi-layer skin, with an optical interference stack. In the initial state, shown in FIG 1(a), incident light (Io) is substantially transmitted (To) through the skin with little or no reflection (Ro). When stretched, as shown in FIG. 1(b) the dimension of the interference stack is reduced, potentially also changing the refractive index of the materials, and the structure becomes more reflective and incident light (Ii) is substantially reflected (Ri) with little or no transmission (Ti). Conversely, the structure may be designed to be reflective in its initial state and transmitting when stretched. The structure can also be designed to produce intermediate levels of transmission and reflection for a defined range of deformation.

[0022] Figures 2(a) and 2(b) shows an analogous representation of a skin, with a ID grating on the surface. As shown in an un-stretched condition of FIG. 2(a) the skin may be designed to be transmitting. When, the skin is stretched in FIG. 2(b) the dimension and periodicity of the grating changes and may become more reflective. The skin may also be designed for the reverse behavior and intermediate performance. When the skin is flexed it can be simultaneously stretched or compressed in various regions which can produce more complex, but predictable, reflection characteristics.

Reflectivity [0023] Materials with suitable mechanical and optical properties include various polymers which are appropriately, elastic, or ductile. Depending on the optical structure employed they should also be suitable for chemical, or structural, modifiable to provide index variations, for patterning, or loading with a high index particles, including nano-particles. Examples include polymers which are polyethelene or polypropylene or fluoro-polymers such as ETFE and PVDF and structural or compositional modifications thereof, including composites, loaded with other materials or phases. Within a range the ETFE and modified derivatives can exhibit elastic behavior. The flexible skin should be sufficiently reflective under the conditions of use for the desired electromagnetic frequencies which may be in the ultra-violet, visible or infra red regions of the spectrum. Embodiments with specular or diffuse reflective performance may be preferred.

[0024] In some embodiments, the thickness, and reflectivity of the respective layers or structures in the flexible skin should be in a range that changes in these parameters induced by stretching or flexing during deployment, or use, are sufficient to significantly alter the reflectivity of the skin from its performance prior to deformation.

[0025] In another embodiment, a multi-layer skin, with an optical interference stack as shown in Figure 1 is provided. Such a skin may consist of alternating layers of ETFE, with different levels of high-index, nano-particle material loading to establish a desired index contrast. In the unperturbed state this skin may be substantially transparent. When stretched the periodicity of the interference stack will change and result in a change in the reflectivity of the skin. Conversely, the structure may initially be reflective and exhibit a change in transmission when stretched. The changes could be gradual or stepped in nature. If the skin is flexed, one surface may be stretched and the opposite surface compressed, producing more complex, but predictable, reflection characteristics. Asymmetric stretching could also be used to introduce, or change, the response to light of different polarizations.

[0026] In yet another embodiment, a skin with a reflective grating on the surface as shown in Figure 2 is provided. The grating could be ID, 2D or 3D in nature. Such a skin may consist of a nano-structured layer, or layers of ETFE. In an unperturbed state the skin could be transmitting. When stretched the dimension and periodicity of the grating will increase and could become more reflective. The skin may also be designed for the reverse behavior. These changes could be gradual or stepped in their response. When the skin is flexed it can be simultaneously stretched or compressed in various regions which can produce more complex, but predictable, reflection characteristics. Asymmetric stretching could also be used to introduce, or change, the response to light of different polarizations.

[0027] ] FIG. 3 shows a skin having a plurality of layers 202, 204 and 206, As shown, layers 204 and 206 include structured gratings. As further shown, layer 204 is subsurface layer.

Substrates and Platforms

[0028] Embodiments of the invention are directed to addressing reflective functionality, for applications involving non-planar, flexible, mobile and shape- changing substrates or platforms. In particular instances, embodiments with specular or diffuse reflectivity may be preferred.

[0029] Embodiments for mobile platforms including cars, ships and planes, for example, can include non-planar surfaces (e.g. a car top or fuselage) and variations in the relative angle of incidence from a defined source during operation.

[0030] Embodiments for platforms which change shape during use, including inflatable or wearable platforms. In such embodiments the skin should be elastic to allow for extended use.

Tunability

[0031] Some embodiments provide the ability to tune or adjust the intensity and/or wavelength of reflectivity of a skin. Such preferred embodiments may involve sensitizing the skin design in terms of tailoring thickness and optical characteristics to be near some critical point for changing reflective performance by stretching, flexing or otherwise changing the shape of a skin within a range which is accessible by the influence of an external stimulus, which could include a mechanical, thermal or electrical means of controlled flexing or stretching of the skin. For example by inflation, or by activating a piezo-electric element to induce stretching to reduce the index and/or layer thickness below a critical value.

[0032] In the simplest case some embodiments may be used to tune the skin for optimized reflective performance at a given wavelength. Other embodiments reduce transmission intensity by increasing reflectivity, for example, in order to prevent saturation of sensory devices. Another embodiment allows adjustment of the wavelength, intensity or modulation of reflected light for optical tagging or camouflage applications.

Claims

CLAIMS What is claimed is:

1. A self-supporting film or skin, comprising:

at least one layer at least partially reflective to optical energy at one or more optical wavelengths, the at least one layer being substantially flexible and/or stretchable and having a reflectivity to incident electromagnetic radiation of a given wavelength that is selectively variable when flexed and/or stretched

2. The film or skin of claim 1, wherein the at least one layer exhibits reflective behavior that is polarization dependent

3. The film or skin of claim 1„ further comprising a device, platform or structure on which the at least one layer is integrated.

4. The film or skin of claim 1, wherein the at least one layer includes a plurality of layers having common physical and mechanical properties.

5. The film or skin of claim 1, wherein the at least one layer includes a plurality of layers that include an interference stack with alternating refractive index layers.

6. The film or skin of claim 5, wherein the layer thickness and/or the refractive index profile of the interference stack change when the plurality of layers is stretched and/or flexed.

7. The film or skin of claim 5, wherein a dimensional or index change produces a change in reflectivity of the skin

8. The film or skin of claim 6, wherein the reflectivity increases, decreases or is unchanged.

9. The film or skin of claim 7, wherein a magnitude of the reflectivity change is determinable based on a degree of flexing and/or stretching within some definable range.

10. The film or skin of claim 1, wherein the at least one layer includes a structured grating.

1 1. The film or skin of claim 1, wherein the at least one layer includes a plurality of layers that includes a plurality of structured gratings.

12. The film or skin of claim 11, wherein at least one of the gratings is a sub-wavelength grating.

13. The film or skin of claim 11 , wherein at least one of the gratings is a 1, 2 or 3 dimensional.

14. The film or skin of claim 11, wherein at least one of the gratings is in a surface layer.

15. The film or skin of claim 11 , wherein at least one of the gratings is in sub-surface layer.

16. The film or skin of claim 11 , wherein the dimension of at least one of the gratings changes when the skin is stretched or flexed.

17. The film or skin of claim 15, wherein a dimensional change in the grating produces a change in reflectivity of the skin.

18. The film or skin of claim 16, wherein the reflectivity increases decreases or is unchanged.

19. The film or skin of claim 17, wherein a magnitude of the reflectivity change is determinable based on a degree of flexing or stretching within a definable range.

20. The film or skin of claim 1, wherein the degree of flexing and/or stretching is controlled by environmental, mechanical, electrical or thermal stimuli.

21. The film or skin of claim 8, wherein the reflectivity of the skin is tunable by initiation or control of external stimuli.

22. The film or skin of claim 21, wherein tunability is settable, intermittent or continuous.

23. The film or skin of claim 21, wherein tunability is localizable within regions of the skin.

24. The film or skin of claim 21 , wherein tunabilty allows reflective optical camouflage.

25. The film or skin of claim 21 , wherein tenability allows reflective optical tagging

26. The film or skin of claim 21 , wherein tenability is used for optical switching, filtering or attenuation