Method of applying a pattern and security device for an article

Abstract

A method of applying a pattern and a security device are disclosed. In one arrangement, a receiving member (10) having a layered structure (12) is provided. The layered structure (12) includes a layer of phase change material (PCM, 2). The phase change material (PCM, 2) is thermally switchable between a plurality of stable states having different refractive indices relative to one another. An embossing member (5) is pressed into the receiving member (10). The embossing member (5) heats selected portions of the layer of phase change material (PCM, 2) by contact with the receiving member (10) during the pressing. The heating thermally switches the phase change material (PCM, 2) in the selected portions and thereby applies a pattern having different refractive indices to the layer of phase change material (PCM, 2).

Description

Technical Field

[0001] This invention relates to a method of applying a pattern, and is particularly suitable for use with security devices incorporated into articles such as legal tender (e.g., banknotes). Background Technology

[0002] The increasing use of polymer banknotes in high-volume currencies like the British pound creates new opportunities for security products specifically designed for polymer substrates. Given that the public is the first line of defense against counterfeiters, overt features are of particular interest. Most commercially available security products are based on inks using traditional OVD (Optical Variable Device) technology, or on micro- and / or nano-structures based on holograms and / or lenses. Given their widespread availability over the years, there is now a need for new forms of security features that are difficult to replicate or simulate. Ideally, such security features should also be mass-producible. Summary of the Invention

[0003] The object of this invention is to provide a new way of applying patterns, especially in the context of safety devices.

[0004] According to one aspect of the invention, a method for applying a pattern is provided, comprising: providing a receiving member having a layered structure, the layered structure including a phase change material layer capable of thermally switching between a plurality of stable states having different refractive indices relative to each other; and stamping an imprinting member into the receiving member, wherein: the imprinting member heats a selected portion of the phase change material layer by contacting the receiving member during the stamping process, the heating being capable, for example, of thermally switching the phase change material in the selected portion and thereby applying a pattern with different refractive indices to the phase change material layer.

[0005] This method allows for the creation of visually appealing features (including a metallic appearance) that can be applied to both overt and covert security products. The layered structure, including phase change material (PCM) layers, enables precise switching between different states, allowing for precisely tunable colors and controlled viewing angle variability. High contrast and high reflectivity can be achieved. The pattern can be applied efficiently and on a large scale without the need for special inks or holographic techniques. The design of the layered structure and embossed components can be adjusted to provide an effect visible only under specific wavelengths of inquiry (by the human eye or optical instruments), which can be provided, for example, through specially selected inspection lasers or narrowband LEDs. This makes a reliable and difficult-to-counter method for verifying the authenticity of an item possible.

[0006] In one embodiment, the embossed member includes a stamped surface with a raised pattern, which, upon stamping, causes the raised areas to form a corresponding indentation pattern in the receiving member. Thus, the stamping process imparts two different types of patterns to the receiving member. The heating associated with the stamping alters the visual characteristics in localized areas by switching the PCM to different reflectivity states in these areas (e.g., by crystallizing the PCM in these areas but keeping it amorphous in others). Simultaneously, the indentation pattern changes the direction of reflection from the surface and provides enhanced viewing variability. Reflective behavior can be achieved where tilting the receiving member to a specific angle can result in two competing reflections from different surfaces, differing in color and brightness based on the light and the observer's viewing angle.

[0007] In one embodiment, the indentation pattern is spatially registered with a pattern in the PCM layer that has a different refractive index. Due to the nature of the stamping process, spatial registration can be achieved efficiently and precisely, as this process simultaneously applies two types of patterns (through PCM switching and indentation) and uses the same physical components (e.g., heated protrusions). Achieving similar results using two conventional, non-switchable, independent OVD inks requires a degree of feature registration that is currently beyond the capabilities of state-of-the-art printing technologies (e.g., <a few micrometers). Furthermore, the method of this embodiment remains difficult to replicate because it requires at least the following:

[0008] i. A deep understanding of the materials involved. Applicable PCMs have complex compositions, often including tri-element chalcogenide glasses with strictly defined relative elemental compositions.

[0009] ii. Obtain reliable suppliers of PCM materials for objects. The chemicals involved make object manufacturing a challenging task, and only a few suppliers are capable of producing high-quality objects.

[0010] iii. A comprehensive understanding of stack structures and their design principles. Specialized software and engineering skills are required to understand how to design these membranes.

[0011] In one embodiment, during the stamping process, at least a portion of the recessed area of ​​the stamping surface outside the protrusion in the stamping surface does not come into contact with the receiving member. This means that the stamping surface can be heated uniformly while still allowing for spatially uneven heating (through the protrusion) of the PCM.

[0012] Various optical effects can be achieved by changing the way the embossing member is pressed into the receiving member (e.g., pressing the embossing member into different sides or both sides of the receiving member), changing the form of the stamping member (e.g., providing different protrusion patterns, such as a pattern with individual protrusion elements having symmetrical or asymmetrical cross-sections), repeating the stamping process multiple times from different sides and / or using different stamping members at different locations, and / or providing other features in the indentation formed by stamping, such as transparent members that impart a retroreflective effect.

[0013] In one embodiment, the stamped surface of the embossed member has a non-uniform temperature distribution during the stamping process, which at least partially defines a selected portion of the phase change material layer that undergoes thermal switching during the stamping process. This approach is more complex to implement but allows for the definition of patterns with different refractive indices, which differ from (e.g., more complex) indentation patterns defined by protrusions.

[0014] In some embodiments, the method is used to form all or part of a security device for an article. The article may include legal tender items, such as banknotes, or any other items that will be used to install the device, such as other published documents, high-value documents, and / or pharmaceutical products.

[0015] According to an alternative, a mounting device for an article is provided, the device comprising a layered structure including a phase change material layer capable of thermally switching between multiple stable states having different refractive indices relative to each other, wherein: the phase change material layer includes patterns with different refractive indices, the patterns being at least partially defined by selected portions of the phase change material in one of the stable states and remaining portions in one or more other stable states; and the layered structure includes an indentation pattern located in a surface of the layered structure, the indentation pattern being spatially registered with patterns having different refractive indices in the phase change material layer. Attached Figure Description

[0016] The invention will now be further described with reference to the accompanying drawings and by way of example, wherein:

[0017] Figure 1 It is a schematic side cross-sectional view of a layered structure, and a pattern can be applied to the layered structure using the method of the present invention;

[0018] Figures 2 to 5 This illustrates the process of stamping an embossed component into a container. Figure 1 A schematic side cross-sectional view of the layered receiving member to apply registration patterns with different refractive indices and indentations;

[0019] Figure 6 It is shown schematically from Figure 4A side view of the reflection of the unindented portion of the receiving component;

[0020] Figure 7 It is an illustrative representation of... Figure 4 A side view of the reflection of the indentation in the receiving component;

[0021] Figure 8 It schematically shows a side cross-sectional view of a transparent member in an indentation of a receiving member, used to provide retroreflective functionality; and

[0022] Figure 9 The diagram shows a side cross-sectional view of an example stamped component, wherein the stamped surface has multiple asymmetrical protruding elements. Detailed Implementation

[0023] Throughout this specification, the terms "optical" and "light" are used because they are common terms in the field of electromagnetic radiation, but it should be understood that in the context of this specification they are not limited to visible light. It is envisioned that the invention can also be used for wavelengths beyond the visible spectrum, such as infrared and ultraviolet light.

[0024] like Figures 1 to 5 As illustrated, the present invention provides a method for applying a pattern to a receiving member 10. The receiving member 10 includes, for example... Figure 1 The layered structure 12 is shown. In some embodiments, the layered structure 12 includes a stack of thin films formed on a substrate 8. The substrate 8 may include a polymer material.

[0025] At least one layer in the layered structure 12 is a PCM2 layer. This PCM is thermally switchable in multiple states with different refractive indices relative to each other. Different refractive indices can include different imaginary components, thereby including different absorbances. Different refractive indices can allow the PCM2 to have different colors and / or provide different optical effects in different states.

[0026] All layers in the layered structure 12 are typically solid and configured to combine their thickness with reflectivity and absorption properties, such that different states of the PCM produce different (visible and / or measured) reflectance spectra. This type of optical device is described in Nature 511, 206-211 (July 10, 2014), WO2015 / 097468A1, WO2015 / 097469A1, EP3203309A1, and WO2017 / 064509A1.

[0027] In one example, the PCM comprises, is substantially composed of, or is composed of one or more of the following: oxides of vanadium (also known as VOx); oxides of niobium (also known as NbOx); 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, Alloys or compounds containing 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 the following compounds / alloys at any stable stoichiometry: GeSbTe, VOx, NbOx, GeTe, GeSb, GaSb, AgInSbTe, InSb, InSbTe, InSe, SbTe, TeGeSbS, AgSbSe, SbSe, GeSbMnSn, AgSbTe, AuSbTe, and AlSb). Preferably, PCM comprises Ge2Sb2Te5 and Ag3In4Sb. 76 Te 17 One of them. It's also understandable that these materials might have various stoichiometric forms: for example, Ge... x Sb y Te z Another suitable material is Ag3In4Sb. 76 Te 17 (Also known as AIST). Furthermore, any of the above materials may include one or more dopants, such as C or N. Other materials may be used.

[0028] It is known that the real and imaginary refractive indices of a PCM undergo drastic changes when switching between an amorphous and crystalline phase. The PCM is stable in each state. This switching can be achieved by any form of heating and, in principle, can be performed efficiently and infinitely, very rapidly. In the embodiment described below, the switching is achieved by transferring heat from the imprinting member 5 to the PCM through contact between the imprinting member 5 and the receiving member 10.

[0029] While some embodiments described herein mention that the PCM can switch between two states, such as a crystalline phase and an amorphous phase, it can switch between any two solid phases, including but not limited to: a crystalline phase to another crystalline or quasi-crystalline phase, or vice versa; an amorphous phase to a crystalline or quasi-crystalline / semi-ordered phase, or vice versa, and all forms in between. The embodiments are also not limited to only two states.

[0030] In one embodiment, the PCM comprises Ge2Sb2Te5 (GST) in a layer with a thickness of less than 200 nanometers (nm). In another embodiment, the PCM comprises GeTe (not necessarily an alloy in equal proportions) in a layer with a thickness of less than 100 nm.

[0031] See you again Figure 1 In some embodiments, the layered structure 12 includes a reflective layer 4. This reflective layer 4 can be fabricated to be highly reflective or only partially reflective. The reflective layer 4 may be omitted. In one embodiment, the reflective layer 4 comprises a reflective material, such as a metal. Metals are known to provide good reflectivity when sufficiently thick and also have high thermal and electrical conductivity. The reflective layer 4 may have a reflectivity of 50% or greater, optionally 90% or greater, or optionally 99% or greater for visible, infrared, and / or ultraviolet light. The reflective layer 4 may comprise, for example, a thin metal film composed of Au, Ag, Al, or Pt. If this layer is partially reflective, the selected thickness is in the range of 5 nanometers to 15 nanometers (nm); otherwise, the layer is fabricated to be thicker, such as 100 nm, for substantially complete reflection.

[0032] In some embodiments, the layered structure 12 further includes a spacer layer 3. The spacer layer 3 is located between the PCM2 and the reflective layer 4. In some embodiments, the layered structure 12 further includes a cover layer 1. The PCM2 is located between the cover layer 1 and the reflective layer 4. The upper surface of the cover layer 1 can represent the observation surface of the receiving member, and the reflective layer 4 serves as a back reflector. Light enters and exits the receiving member 10 through the cover layer 1, which serves as the observation surface. Interference effects, depending on the refractive index of the PCM2 and the thickness of the spacer layer 3, cause the reflectivity to vary significantly with wavelength. Both the spacer layer 3 and the cover layer 1 are translucent and ideally as transparent as possible.

[0033] Each of the capping layer 1 and the spacer layer 3 may consist of a single layer or comprise multiple layers with different refractive indices relative to each other (i.e., in the case where the capping layer 1 or the spacer layer 3 consists of multiple layers, at least two of these layers have different refractive indices relative to each other). The thickness and refractive index of one or more materials forming the capping layer 1 and / or the spacer layer 3 are selected to produce a desired spectral response (through interference and / or absorption). Materials that may be used to form the capping layer 1 and / or the spacer layer 3 may include, but are not limited to, ZnO, TiO2, SiO2, Si3N4, TaO, ITO, and ZnS-SiO2.

[0034] Any or all layers of the layered structure 12 can be formed by sputtering, which can be performed at a relatively low temperature of 100 degrees Celsius. Patterning can also be applied to the layer using conventional techniques known from photolithography or other techniques such as printing.

[0035] In a specific embodiment, the PCM2 layer includes a GST layer with a thickness of less than 100 nm, preferably less than 10 nm, such as 6 or 7 nm. Depending on the desired color and optical performance, the thickness of the spacer layer 3 is increased to typically range from 10 nm to 250 nm. The capping layer 1 is, for example, 20 nm thick.

[0036] like Figures 2 to 5 As shown, the method of forming a pattern includes pressing the embossing member 5 into the receiving member 10. Figure 2 This illustrates a stage of the stamping process when the stamping member 5 moves downward toward the receiving member 10 but has not yet contacted the receiving member 10. Figure 3 This illustrates the latter stage of the stamping process when the embossing member 5 comes into contact with the receiving member 10. Figure 4 The final stage of the stamping process is shown when the stamping member 5 leaves the receiving member 10. Figure 5 It shows the equivalent of Figure 3 The alternative stamping process stage, in addition to performing stamping from the opposite side of the receiving component 10.

[0037] like Figure 3 As shown, during the stamping process, the embossing member 5 heats a selected portion 2A of the PCM2 layer through its contact with the receiving member 10. Thus, before stamping begins, the embossing member 5 is hotter than the PCM2. This heating causes a thermal switch in the selected portion 2A of the PCM. The remaining portion (portion 2B) of the PCM2 layer remains in its initial refractive index state. The combination of portions 2A and 2B (which have different refractive indices relative to each other) defines a pattern with different refractive indices, which has been applied to the PCM2 layer by stamping.

[0038] In one embodiment, such as Figure 2As shown, all layers of PCM2 are provided in the same initial state before stamping. Therefore, the layers of PCM2 are patternless at this stage. In one embodiment, this initial state is amorphous. In one embodiment, the stamping of the embossed member 5 ( Figure 3 This causes a portion of 2A to change state (e.g., become crystalline), while the rest of the PCM2 layer retains its initial state (e.g., amorphous).

[0039] In one embodiment, the embossing member 5 includes a stamping surface ( Figures 2 to 4 The lower surface of the embossed component 5 and Figure 5 The upper surface of the stamping member 5. This stamped surface has a plurality of protrusions 6. Various shapes can be applied to the protrusions 6 to achieve corresponding optical effects. However, it is generally preferred that the protrusions 6 be constructed such that they can penetrate the receiving member 10 without excessive force. The protrusions 6 may thus be tapered (e.g., including tapered elements, such as tapered points and / or ridges).

[0040] In some embodiments, the protrusion 6 comprises a plurality of identical protruding elements (as shown in the example). Figures 2 to 5 In the diagram, protrusion 6 is shown as having three such protruding elements. When viewed along a direction perpendicular to the stamping direction (e.g., along a direction perpendicular to the plane of the paper in the figure), this protruding element may have a mirror-symmetrical cross-section. Figure 2 The example of one of the protruding elements is labeled 16 with a mirror symmetry line. This arrangement allows the same visual pattern to be observed in the resulting receiving element 10 from multiple directions. Alternatively, the cross-section of the protruding element is asymmetrical when viewed along a direction perpendicular to the stamping direction. Figure 9 An example of this arrangement is shown. This method can be used to provide a special visual pattern that is only observable within a narrow range relative to a selected direction of the observer, which can be used in security applications.

[0041] The stamping causes the protrusion 6 to form a corresponding indentation pattern 7 in the receiving member 10 (in Figure 4 (marked in the middle). The indentation 7 alters the reflection of light from the receiving element 10, thereby providing increased degrees of freedom to produce optical effects and / or variations in optical effects and / or variations in observable patterns related to the viewing angle. Figure 6 and Figure 7 Schematic illustration in Figures 2 to 4 The method shown in the figure illustrates how the indentation type 7 alters the reflection to provide retroreflective behavior. Figure 7 In this case, light incident from a specific angle is reflected back to the light source to a greater extent than when the reflecting surface is merely a flat surface. Figure 6Retroreflective behavior can be achieved, for example, through variations in the viewing angle relative to a single axis (2D retroreflectivity), such as elongated ridge-like indentations, or through variations in the viewing angle relative to multiple axes (3D retroreflectivity), such as indentations shaped like the interior corners of a cuboid. In some embodiments, such as Figure 8 As shown, a transparent member 14 is disposed in one or more indentations 7 formed by stamping. The transparent member 14 may be configured to provide a retroreflective effect. The transparent member 14 may, for example, be spherical and / or have a refractive index greater than 1. In some embodiments, the transparent member 14 is applied in a separate process after stamping has been performed. In other embodiments, the transparent member 14 and stamping are applied simultaneously. For example, the embossing member 5 may be provided with a pattern of protrusions 6, which includes one or more transparent members 14 (e.g., located at the respective tips of the respective protrusions in the pattern of protrusions). In this case, the stamping process presses the transparent member 14 into the receiving member 10 during stamping. The connection between the transparent member 14 and the embossing member 5 is arranged to be weaker than the connection between the transparent member 14 and the receiving member 10, such that when the embossing member 5 is pulled back, the transparent member 14 remains within the receiving member 10.

[0042] The pattern of the indentation 7 is spatially registered with a pattern in the layer of PCM2 that has a different refractive index. In the example shown, the spatial registration consists of a local area of ​​the portion 2A of the switched PCM2 located at the same position as the indentation (i.e., the location where the thermal protrusion penetrates the receiving member 10). The pattern of the indentation 7 can thus be aligned with the pattern that has a different refractive index (defined by the portion 2A of the switched PCM2). The pattern of the indentation 7 can be substantially identical to the pattern that has a different refractive index. This spatial registration and / or identity of the pattern can be effectively achieved relative to alternative methods for forming different types of patterns because, in the current case, both types of patterns are formed through contact between the same imprinting member 5 and the receiving member 10.

[0043] In one embodiment, at least a portion of the recessed area 9 located outside the protrusion 6 in the stamped surface does not come into contact with the receiving member 10 during the stamping process (see...). Figure 3 and Figure 5 This means that the stamped surface can be heated uniformly, while still allowing for spatially uneven heating of the PCM2 (through protrusion 6).

[0044] In other embodiments, the stamping surface of the embossing member 5 has a non-uniform temperature distribution during the stamping process. In this case, the non-uniform temperature distribution may at least partially define selected portions of the PCM2 layer that undergoes thermal switching during the stamping process. This non-uniform temperature distribution may be provided, for example, by a plurality of local heating elements. By addressing different combinations of heating elements and / or varying the energy output through these heating elements, different spatial and / or temporal heating profiles can be defined, thereby allowing the definition of patterns with different refractive indices that differ from (e.g., are more complex) the pattern of the protrusions 7 defined by the protrusions 6. In some embodiments, the embossing member 5 may be configured to allow individual control of the temperature of different portions of the pattern of the protrusions 6 (e.g., different individual protrusions).

[0045] The stamping member 5 can be pressed into the receiving member 10 from either side or both sides (at different times or simultaneously).

[0046] In some embodiments, as described in detail below, the layered structure 12 includes a reflective layer 4 located below the layers of the PCM2, extending from the side of the PCM2 opposite to the reflective layer 4 (e.g., from above, as shown in the image). Figures 2 to 4 (As shown in the structure) the stamping member 5 is pressed into the receiving member 10 at least once. Alternatively or additionally, such as Figure 5 As illustrated, in some embodiments, the embossing member 5 is pressed into the receiving member 10 at least once from the same side of the PCM2 as the reflective layer 4 (i.e., from the lower orientation shown in the figure). In this case, pressing the embossing member 5 into the receiving member 10 causes a change in the surface morphology on the side of the receiving member 10 opposite to the pressing (e.g., to form a raised region 18 spatially aligned with the protrusion 6 of the embossing member 5, such as...). Figure 5 (As shown).

[0047] In some embodiments, the stamping of the embossing member 5 into the receiving member 10 is performed multiple times. At least one sub-step of stamping can be performed with different embossing members 5 (e.g., embossing members 5 having stamping surfaces with different protrusion patterns). Multiple stampings (with or without different embossing members 5) can be employed to provide complex optical effects and / or adjust the visual effects at different times (e.g., modifying the mounting device to indicate a change in status, such as an upgrade or impending expiration).

[0048] The receiving member 10 can form all or part of a security device for an article. The article can be a legal tender (e.g., banknotes) or other articles. Thus, the security device can include a layered structure 12. The layered structure 12 includes layers made of PCM2. The PCM2 is thermally switchable between multiple stable devices with different refractive indices relative to each other. The layers of PCM2 include patterns with different refractive indices, defined at least partially by selected portions 2A of PCM2 in one of the stable states and remaining portions 2B of PCM2 in one or more other stable states. The layered structure 12 includes a pattern of indentations 7 in the surface of the layered structure 12. The pattern of indentations 7 is spatially registered with patterns with different refractive indices in the layers of PCM2. References above may be used. Figures 1 to 9 The methods discussed can be used to form patterns with different refractive indices. The above reference can be used as an example. Figures 1 to 9 The pattern of indentation 7 is formed by any method discussed.

Claims

1. A method of applying a pattern, comprising: providing a receiving member having a layered structure, the layered structure including a layer of phase change material capable of thermally switching between a plurality of stable states having different refractive indices relative to one another; and stamping a stamping member into the receiving member, wherein: the stamping member, by contact with the receiving member during stamping, heats selected portions of the layer of phase change material, the heating enabling the phase change material in the selected portions to thermally switch, and thereby imparting a pattern of different refractive indices to the layer of phase change material, wherein the stamping surface of the stamping member has a non-uniform temperature distribution during stamping, the non-uniform temperature distribution at least partially defining the selected portions of the layer of phase change material that thermally switch during stamping; wherein the stamping member includes a stamping surface having a pattern of protrusions, and the stamping causes the pattern of protrusions to form a corresponding pattern of indentations in the receiving member; the method further comprising providing a transparent member in one or more of the patterns of indentations, wherein the transparent member is shaped to provide a retroreflective effect; wherein the layered structure includes a reflective layer underlying the layer of phase change material; and at least one of the stamping of the stamping member into the receiving member is performed from a side of the layer of phase change material opposite the reflective layer.

2. The method of claim 1, wherein the pattern of indentations is spatially registered with the pattern of different refractive indices in the layer of phase change material.

3. The method of claim 2, wherein the pattern of indentations is aligned with the pattern of different refractive indices.

4. The method of claim 2 or 3, wherein the pattern of indentations is substantially identical to the pattern of different refractive indices.

5. The method of any one of claims 1-3, wherein at least a portion of a recessed region of the stamping surface, outside the pattern of protrusions on the stamping surface, does not contact the receiving member during stamping.

6. The method of claim 5, wherein the stamping surface has a uniform temperature distribution during stamping.

7. The method of any one of claims 1-3, wherein the pattern of protrusions includes tapered elements.

8. The method of any one of claims 1-3, wherein the pattern of protrusions includes a plurality of identical protrusion elements, each protrusion element being separated from other protrusion elements.

9. The method of claim 8, wherein the protrusion elements have a mirror-symmetric cross-section when viewed along a direction perpendicular to the direction of stamping.

10. The method of claim 8, wherein the protrusion elements have a mirror-asymmetric cross-section when viewed along a direction perpendicular to the direction of stamping.

11. The method of any one of claims 1-3, wherein: the layered structure includes a reflective layer underlying the layer of phase change material; and at least one of the stamping of the stamping member into the receiving member is performed from a side of the layer of phase change material identical to the reflective layer.

12. The method of claim 11, wherein stamping the stamping member into the receiving member from the same side of the phase change material layer as the reflective layer is capable of causing a change in surface topography on the side of the receiving member opposite the stamping.

13. The method of any one of claims 1-3, wherein stamping the stamping member into the receiving member is performed multiple times.

14. The method of claim 13, wherein at least one sub-step of stamping is performed with a different stamping member.

15. The method of any one of claims 1-3, wherein the phase change material consists essentially of one or more of: an oxide of vanadium; an oxide of niobium; an alloy or compound comprising Ge, Sb, and Te; an alloy or compound comprising Ge and Te; an alloy or compound comprising Ge and Sb; an alloy or compound comprising Ga and Sb; an alloy or compound comprising Ag, In, Sb, and Te; an alloy or compound comprising In and Sb; an alloy or compound comprising In, Sb, and Te; an alloy or compound comprising In and Se; an alloy or compound comprising Sb and Te; an alloy or compound comprising Te, Ge, Sb, and S; an alloy or compound comprising Ag, Sb, and Se; an alloy or compound comprising Sb and Se; an alloy or compound comprising Ge, Sb, Mn, and Sn; an alloy or compound comprising Ag, Sb, and Te; an alloy or compound comprising Au, Sb, and Te; and an alloy or compound comprising Al and Sb.

16. The method of any one of claims 1-3, wherein the layered structure includes a spacer layer disposed between the phase change material layer and the reflective layer, wherein the spacer layer consists of a single layer or includes multiple layers of material having different reflectivities.

17. The method of any one of claims 1-3, wherein the layered structure includes a capping layer, wherein the phase change material layer is disposed between the capping layer and the reflective layer, the capping layer consisting of a single layer or including multiple layers of material having different reflectivities.

18. The method of any one of claims 1-3, wherein the receiving member comprises a polymeric substrate.

19. The method of any one of claims 1-3, wherein the receiving member forms all or part of a security device for a legal tender item.

20. A security device for an item, the device comprising: a layered structure including a phase change material layer, the phase change material being thermally switchable between a plurality of stable states having different refractive indices relative to one another, wherein: the phase change material layer includes a pattern having different refractive indices defined at least in part by selected portions of the phase change material in one of the stable states and remaining portions of the phase change material in one or more of the other stable states; and the pattern is capable of being altered by stamping a stamping member into the receiving member. The layered structure includes an indentation pattern in a surface of the layered structure, the indentation pattern being spatially registered with a pattern having different refractive indices in the phase change material layer, wherein the indentation pattern provides a retro-reflective behavior by which incident light is reflected back to the light source to a greater extent than if the reflective surface were just planar; wherein the layered structure includes a reflective layer underlying the phase change material layer; and at least one stamping of a stamping member into a receiving member is performed from a side of the phase change material layer opposite the reflective layer.

21. The apparatus of claim 20, wherein the indentation pattern is aligned with the pattern having different refractive indices.

22. The apparatus of claim 20 or 21, wherein the indentation pattern is substantially identical to the pattern having different refractive indices.

23. The apparatus of claim 20 or 21, wherein the layered structure includes a polymeric substrate.

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