Optical grating structure having stairstep blazed grating unit cells

Abstract

An optical grating structure comprises Stairstep Blazed Grating unit cells. Each unit cell includes a stairstep-shaped dielectric portion. Disposed on this dielectric is a thin layer of metal, thereby forming a floor level surface of metal, a bottom step level surface of metal, an optional middle step level surface of metal, and a top step level surface of metal. Either the bottom or the top metal surface is of a different shape than the other metal surface(s). The bottom step and its metal surface is triangular in one example, whereas the top step and its metal surface is rectangular. Disposed over the entire stairstep structure is transparent dielectric material. The unit cell is sized and fashioned to operate on visible light, suppressing m=0 light while maintaining diffraction efficiency for m=-1 order light.

Description

OPTICAL GRATING STRUCTURE HAVINGSTAIRSTEP BLAZED GRATING UNIT CELLSTECHNICAL FIELD

[0001] The described embodiments relate to optical grating structures having Stairstep Blazed Grating (SBG) unit cells, and more particularly to such structures that are compatible with scalable manufacturing processes such as Nanoimprint Lithography (NIL).BACKGROUND INFORMATION

[0002] The use of blazed gratings in anti-counterfeiting security features is known, as is the specific use of diffractive multi-level stairstep-type blazed gratings in such security features. FIG. 1 (Prior Art) is a diagram of a prior art multi-level stairstep-type blazed grating structure I. The structure 1 includes a dielectric material portion 2 that is repeated as stairstep-type unit cell. One of the unit cells is identified by reference numeral 3. The stairstep dielectric portion of the unit cell includes what is effectively a horizontally extending layer of dielectric material from which steps of dielectric material extend upward, when considered from the perspective of the diagram of FIG. I. Disposed on each step of this stairstep-type structure of dielectric material is a thin layer of metal 8 such as aluminum, gold, silver, or copper that coats upward facing surfaces of the dielectric material As illustrated in FIG. 1, the sidewalls of the steps of the dielectric material are not coated or covered with metal. Accordingly, a metal feature 4 forms a floor level surface of metal, a metal feature 5 forms a first step level surface of metal, a metal feature 6 forms a next step level surface of metal, and a metal feature 7 forms a top step level surface of metal. These metal features are all rectangular in shape when considered from the top-down perspective. The period of the unit cell 3 of the blazed grating structure 1 in the x direction is denoted by Px. The bottom step has a height of hi. The middle step has a height of In. The top step has a height of h. The run dimensions of the three steps in the x direction are denoted wi, ws, and W3, respectively.

[0003] FIG. 2 (Prior Art) illustrates an operation of the blazed grating structure 1 of FIG. 1. Incoming monochromatic light 9 of a given wavelength X is incident upon the blazed grating structure 1 with an angle of incidence 6k (with respect to the grating normal GN). The incoming monochromatic incident light is diffracted and reflected and otherwiseoperated upon by the grating structure. Diffracted light of orders m=-2, m=-1,is denoted by rays 10, 11 and 12, respectively. Reflected light of order m=0 is denoted by ray 13. In a case in which the incoming incident light is red light (has a wavelength of 600-700nm), there is a diffraction phenomenon as illustrated in FIG. 2. In a case in which the incoming incident light is green light (has a wavelength of 500-600nrn), there is another diffraction phenomenon resulting in different angles and intensities. Similarly, in a case in which the incoming incident light is blue (has a wavelength of 400-500nm), there is another diffraction phenomenon with different angles and intensities In polychromatic light, such as sunlight, there is a different diffraction phenomenon that operates on each different wavelength component of the polychromatic incoming light.

[0004] The blazed grating structure 1 exhibits a diffraction efficiency. Diffraction efficiency here is defined as the ratio between the optical power P of the diffracted light (diffracted into a particular order, for example into the desired order m=-l) and the optical power of the incident illuminating beam of light of a given wavelength A. In a case in which the blazed grating structure is being used as a security feature, such as for example a security feature incorporated into a banknote, it may be that the order m=- 1 diffracted light is the light to be perceived by the viewer. Light of the other auxiliary orders is undesired, including reflected light of the most prominent undesired auxiliary order m 0 Such undesired light may interfere with perception of the desired order m=-l light. Accordingly, a blazed grating structure is desired that exhibits a high diffractive efficiency (for m=- 1 order) and that suppresses undesired light including undesired light of the order m=0. When polychromatic white light is incident at a given angle upon the blazed grating structure, and when a viewer is looking at the blazed grating structure from a given perspective, the blazed grating structure may appear to be of a particular color because order m=-l light of that color (wavelength) is diffracted at the correct angle to reach the viewer, whereas order m=-l light of other colors (wavelengths) are diffracted at other angles. The steps of a blazed grating structure can be sized and positioned so that order m=-l light of a desired color diffracts at the correct angle for perception by the viewer. Accordingly, different blazed grating structures can be designed that, for a given angle of incidence of incoming light, and for a given positioning of the viewer, appear to be of different colors.SUMMARY

[0005] An optical grating structure comprises a two-dimensional array of Stairstep Blazed Grating (SBG) unit cells. The optical grating structure sees general utility in optics such as in a security feature and authentication, or as a beam steering element. It sees utility in diffraction-based spectroscopy, in holographic optics, in augmented reality, in virtual reality, and in authentication. Many of the optical grating structures can be made using a Nanoimprint Lithography (NIL) process so that the optical grating structures are embodied as parts of a roll of film, and so that an optical grating structure (a security feature, in this example) from the roll can then be separated from the remainder of the roll and can be attached to or incorporated into an object (for example, a security document such as a banknote). In the case of an optical grating structure used in authentication, the optical grating structure can be referred to as a security feature.

[0006] Each SBG unit cell of the optical grating structure includes a stairstep-shaped dielectric portion. The stairstep-shaped dielectric potion includes a base block portion, and a plurality of step portions that extend upward from the block base portion to form a plurality of ascending steps. There is a floor (a surface of the base block portion), a shortest bottom step, optionally one or more middle steps, and a tallest top step In addition to the stairstep-shaped dielectric portion, the SBG unit cell further includes thin metal features disposed on the floor and on the steps. In one example, the metal is disposed only on the tops of the steps and does not coat sidewalls of the dielectric material. In another example, the metal coats both the top surfaces of the steps as well as all sidewalls of the dielectric material of the stairstep dielectric portion. Due to the thin metal coating, the SBG unit cell has a floor level surface of metal, a bottom step level surface of metal, a mid-level step surface or surfaces of metal (optional), and a top step level surface of metal. One of the bottom step level surface of metal and the top step level surface of metal has a rectangular shape that has a width equal to the width Pyof the unit cell in a y dimension, whereas the other of the bottom and top step level surfaces of metal has a different shape taken from the group consisting of a rectangular shape having a width smaller than Py, a trapezoid, a triangle. Accordingly, either the top or the bottom step has a step surface that is different from the step surfaces of the other steps.

[0007] Disposed over the entire metal-coated stairstep structure is a layer of transparent dielectric material. The transparent dielectric material is conformal to themetal-coated stairstep structures, and fills the volumes between stairstep structures of the unit cells. But the upper surface of the transparent dielectric material (when the overall optical grating structure is considered) is substantially planar. The width Pyof the unit cell in the y dimension is less than 500nm, whereas the length Pxof the unit cell in an x dimension is less than 800nm. The floor level surface (top surface of the floor metal) is disposed in a first plane and the top step level surface (top surface of the top step metal) is disposed in a second plane. The first and second planes are separated by less than 400nrn The metal-coated stairstep structure is very small and is sized and fashioned to operate on visible light. The optical grating structure having the very small metal-coated stairstep structures is compatible with scalable manufacturing processes such as Nanoimprint Lithography (NIL).

[0008] In certain examples, the optical grating structure comprising the two-dimensional array of the SBG unit cells has an operational optical characteristic (for visible wavelength incident light) that suppresses order m=0 light (reduces the amount of order m=0 light returning from the array) better than does a stairstep blazed grating unit cell in which all the steps are of an identical rectangular shape. The array of novel SBG unit cells accomplishes this without substantially suppressing desired order m=-l light (without substantially reducing the amount of order m=-l light returning from the array).

[0009] Operation of the novel optical grating structure having SBG unit cells is described below in relation to a conventional diffraction grating A conventional diffraction grating relies on a certain phase gradient profile to diffract light toward the desired modes. For instance, a uniform phase profile will lead to diffraction of m=0 mode that is similar to a mirror reflection. To diffract light toward non-specular modes (miO), a sloped phase profile is needed. Typically, higher order mode (i.e m -2) diffraction requires a gradient phase profile that is sloped more than that of lower order (i.e m=-l). In addition to this phase engineering, the pitch dimension of a conventional blazed diffraction grating determines the number of diffraction orders that can be effectively excited within the grating structures. For the pitch dimension being used in the embodiments of the invention described in this patent document, diffraction modes of order other than (m=0 and m=-l) can be largely suppressed / cutoff within the visible spectrum ranging from 700nm to 400nm in a diffraction scheme For instance, the phase profile of m=-2 mode which neighbors m=-l mode is difficult to excite due to the steep phase profile required in the selected pitchsize dimension. On the other hand, the diffraction angle of m==-H mode which neighbors m=0 mode becomes very close to 90 degrees due to a 33 degree oblique angle of incidence Operation in the near-cutoff regime makes its efficiency minimal. With the two nearest neighboring modes impeded from excitation, other higher order modes can be considered to be completely cutoff. As such, in the design of the novel optical grating structure comprising SBG unit cells, the determination of the pitch dimension diffraction action is largely limited to m=0 and m=-1 modes.

[0010] Because the diffraction is predominated by m-1 and m=0 modes, the diffraction efficiency of the novel optical grating structure having SBG unit cells can be broken down into three components: diffraction efficiency of m=-1 mode, diffraction efficiency of m=0 mode, and absorption loss due to metal. Low-loss silver is used in the SBG unit cells to suppress metallic absorption so that a vast majority of energy can go to diffraction. The adoption of silver film is not provided to excite a strong plasmonic effect but rather is provided to achieve a sloped phase profile The silver does not induce strong plasmonic resonance loss. The decrease in m=0 mode achieved by the novel optical grating structure can be attributed to plasmonic resonance, while efficiency of m=-l remains largely unchanged. This plasmonic phenomenon may, however, lead to an increase in absorption loss, or an increase in transmission enabled by a plasmonic effect, as a form of energy transfer between m=0 and plasmonic behaviors. While a conventional blazed diffraction grating may exhibit a phase profile that favors m=-l diffraction, this phase profile is achieved by proper choice of the height and slope of the grating structure in the x dimension. If the height gradient is too small or too large, the efficiency of m=-l can be impacted The novel optical grating structure comprising SBG unit cells, however, provides another phase fine-tuning mechanism in that a physical structural width in the y dimension can be controlled and adjusted, independent of the height and slope of the structure in the x dimension. By narrowing the width of the bottom step of the SBG unit cell structure, even a steeper phase profile can be achieved (if the slope of the blaze phase profile achievable by adjustment of height or slope in the x dimension is insufficient). To achieve a more moderate phase profile if the slope of the blaze profile in the x dimension is too steep, the width of the top step of the SBG unit cell structure can be narrowed.

[0011] In a novel method, in a step (a) a stamp is used to emboss a blazed diffraction grating depression into a first layer of dielectric material. A first edge of the depression isa straight line, whereas a second edge of the depression opposite the first edge is not a straight line but rather has point extensions that point away from the first edge. The first edge and the second edge are disposed in the same plane. In a step (b), a thin layer of metal is deposited onto a dielectric surface of the depression thereby forming a metal-coated blazed diffraction grating depression. In a step (c), a second layer of dielectric material is deposited such that the metal-coated blazed diffraction grating depression is filled with dielectric material and such that the entire metal-coated blazed diffraction grating depression is overcoated with dielectric material of the second layer. In one example, the point extensions are spaced at intervals with a periodicity Py, with the periodicity Py being less than 500 nanometers. Carrying out steps (a), (b) and (c) results in a blazed diffraction grating structure, in which each of the elongated blazed raised grating structures has point extensions As described above, these point extensions cause complex plasmonic effects that change and improve optical performance of the overall grating structure.

[0012] In a novel method, in a step (a) a plurality of elongated blazed raised grating structures is formed. Each of these elongated blazed raised grating structures has a thin layer of metal deposited on it. The elongated blazed raised grating structures extend parallel to one another. Each of the elongated blazed raised grating structures extends upward from a floor surface of a dielectric material. In a step (b), a layer of transparent dielectric material is provided so that it is disposed over the thin layer of metal and so that it fills volumes between adjacent ones of the elongated blazed raised grating structures. A first boundary between one of the elongated blazed raised grating structures and the floor surface is a straight line, whereas a second boundary between that elongated blazed raised grating structure and the floor surface is not a straight line but rather has point extensions that point away from the first boundary. The first boundary and the second boundary are disposed in the same plane. In one example, the point extensions are spaced at intervals with a periodicity Py, with the periodicity Py being less than 500 nanometers Carrying out steps (a) and (b) results in a blazed diffraction grating structure, in which each of the elongated blazed raised grating structures has point extensions. As described above, these point extensions cause complex plasmonic effects that change and improve optical performance of the overall grating structure

[0013] Further details and embodiments and methods and techniques are described in the detailed description below. This summary does not purport to define the invention The invention is defined by the claims.DESCRIPTION OF THE DRAWINGS

[0014] The accompanying drawings, where like numerals indicate like components, illustrate embodiments of the invention.

[0015] FIG. 1 (Prior Art) is a is a diagram of a prior art multi-level stairstep-type blazed grating structure.

[0016] FIG. 2 (Prior Art) illustrates an operation of the blazed grating structure of FIG.1.

[0017] FIG. 3 is a perspective view diagram of a security document in accordance with one embodiment of the present invention.

[0018] FIG. 4 shows the two-dimensional array of super-pixels of the banknote of FIG. 3 in further detail.

[0019] FIG. 5A is a simplified perspective diagram of the stairstep dielectric portion of the SBG unit cell of FIG 4.

[0020] FIG. 5B is a simplified perspective diagram of the SBG unit cell of FIG. 4 showing the floor surface, the first step (bottom step) surface, the second step surface, and the third step (top step) surface.

[0021] FIG. 6A shows the dielectric material of the step portions of another embodiment 36 of an SBG unit cell.

[0022] FIG. 6B shows the floor surface, the first step (bottom step) surface, the second step surface, and the third step (top step) surface of the embodiment 36 of the unit cell of FIG. 6A.

[0023] FIG. 7 A shows the dielectric material of the step portions of another embodiment 37 of an SBG unit cell.

[0024] FIG. 7B shows the floor surface, the first step (bottom step) surface, the second step surface, and the third step (top step) surface of the embodiment 37 of the unit cell of FIG. 7 A.

[0025] FIG. 8A shows the dielectric material of the step portions of another embodiment 38 of an SBG unit cell.

[0026] FIG. 8B shows the floor surface, the first step (bottom step) surface, the second step surface, and the third step (top step) surface of the embodiment 38 of the unit cell of FIG. 8A.

[0027] FIG. 9A shows the dielectric material of the step portions of another embodiment 39 of an SBG unit cell.

[0028] FIG. 9B shows the floor surface, the first step (bottom step) surface, the second step surface, and the third step (top step) surface of the embodiment 39 of the unit cell of FIG. 9A.

[0029] FIG. 10A shows the dielectric material of the step portions of another embodiment 40 of an SBG unit cell.

[0030] FIG. 10B shows the floor surface, the first step (bottom step) surface, the second step surface, and the third step (top step) surface of the embodiment 40 of the unit cell of FIG. 10A.

[0031] FIG. 11 is a graph and corresponding diagram that shows how the diffraction efficiency for a conventional stairstep blazed grating structure changes as a function of the wavelength of incident light.

[0032] FIG. 12 is a table that summarizes diffraction efficiency data of the graph of FIG. 11.

[0033] FIG. 13 is a graph and corresponding diagram that shows how the diffraction efficiency for the novel stairstep blazed grating structure shown at the top of the diagram changes as a function of the wavelength of incident light

[0034] FIG. 14 is a table that summarizes diffraction efficiency data of the graph of FIG. 13.

[0035] FIG. 15 is a diagram that shows an SBG unit cell in perspective view and that also includes a top-down diagram of the SBG unit cell.

[0036] FIG. 16 is a graph that shows how the diffraction efficiency of the SBG unit cell structure of FIG. 15 varies as a function of width of the top step.

[0037] FIG. 17 is a table that sets forth dimensions of the SBG structure, the diffraction efficiency of which is plotted in FIG. 16.

[0038] FIG. 18 is a diagram that shows an SBG unit cell in perspective view and that also includes a top-down diagram of the SBG unit cell

[0039] FIG. 19 is a graph that shows how the diffraction efficiency of the SBG unit cell structure of FIG. 18 varies as a function of dimension of the trapezoidal top step of the structure.

[0040] FIG. 20 is a table that sets forth dimensions of the SBG structure, the diffraction efficiency of which is plotted in FIG. 19.

[0041] FIG. 21 is a diagram that show's an SBG unit cell in perspective view and that also includes a top-down diagram of the SBG unit cell.

[0042] FIG. 22 is a graph that shows how the diffraction efficiency of the SBG unit cell structure of FIG. 21 varies as a function of dimension of the trapezoidal bottom step of the structure.

[0043] FIG. 23 is a table that sets forth dimensions of the SBG structure, the diffraction efficiency of which is plotted in FIG 22.

[0044] FIG. 24A illustrates a step in a method of making an optical grating structure having a plurality of Stairstep Blazed Grating (SBG) unit cells

[0045] FIG. 24B illustrates a next step in the method of making the optical grating structure.

[0046] FIG. 24C illustrates a next step in the method of making the optical grating structure.

[0047] FIG. 24D illustrates a next step in the method of making the optical grating structure.

[0048] FIG. 24E illustrates a next step in the method of making the optical grating structure.

[0049] FIG. 24F illustrates a next step in the method of making the optical grating structure.

[0050] FIG. 24G illustrates a next step in the method of making the optical grating structure.

[0051] FIG. 24H illustrates a next step in the method of making the optical grating structure.

[0052] FIG. 241 illustrates a next step in the method of making the optical grating structure.

[0053] FIG. 24.1 illustrates a next step in the method of making the optical grating structure.

[0054] FIG. 24K illustrates a next step in the method of making the optical grating structure.

[0055] FIG. 24L illustrates a next step in the method of making the optical grating structure.

[0056] FIG. 24M illustrates a next step in the method of making the optical grating structure.DETAILED DESCRIPTION

[0057] FIG. 3 is a perspective view diagram of a security document 20 in accordance with one embodiment of the present invention. The security document 20 is a banknote to which a security feature 21 has been attached or into which the security feature 21 has been incorporated. Although the security document 20 pictured is a banknote, the security document 20 may be another type of security document such a check, a passport, an admission ticket, and identification card, a driver’s license. The exploded round portion 22 of FIG. 3 illustrates, from a top-down perspective, a part of an array of super-pixels 23 of the security feature 21. In the present example, the security feature is a multi-layer film that includes adhesive layer. The adhesive layer bonds the remainder of the multi-layer film to the substrate material of the banknote.

[0058] FIG. 4 show's the two-dimensional array 23 of row's and columns of superpixels in further detail. One of the super-pixels 24 is shown in further detail in schematic perspective view in the upper right of the diagram Super-pixel 24 is a two-dimensional eight-by-eight matrix of pixels. Reference numeral 25 identifies the left -bottom pixel Pixel 25 is shown in further expanded view below in the center of the diagram to include a red sub-pixel 26, a green sub-pixel 27, a blue sub-pixel 28, and a dark sub-pixel 29. The red sub-pixel 26 is shown in expanded view below that in the lower left of the diagram to include a two-dimensional matrix of Stairstep Blazed Grating (SBG) unit cells. The SBG cells are disposed in rows and columns as illustrated.

[0059] The optical grating structure of the red sub-pixel 26 includes a plurality of raised elongated blaze features that extend parallel to each other. From the perspective of FIG. 4, these raised elongated blaze features extend parallel to each other in the horizontal dimension. Each such raised feature extends up from the plane of the floor surface When considered from the top-down perspective of the diagram of FIG. 4, a corner boundary between a raised feature and the floor (from which the raised feature extends) defines aline. There is a first such floor boundary line on one side of a raised feature, and there is a second such floor boundary' line on the opposite side of the raised feature. In the case of raised feature 42, the floor boundary line 43 on the first side of the raised feature 42 is a straight line. The floor boundary line 44 that is on the second opposite side of the raised feature 42 is, however, not a straight line, but rather floor boundary line 44 has pointed projections or extensions. These pointed proj ections / extensions occur at periodic intervals as shown in FIG. 4. The pointed proj ections / extensions of floor boundary line 44 point outwardly and away from the opposite floor boundary line 43. These pairs of floor boundary lines are a characteristic feature of the red sub-pixel portion of the optical grating structure of FIG. 4.

[0060] One of the SBG unit cells 27 of the red sub-pixel 26 of FIG. 4 is shown in expanded view to the bottom right of the diagram. SBG unit cell 27 as seen from the top-down perspective has a floor surface 28, a first step (bottom step) surface 29, a second step (middle step) surface 30, and a third step (top step) surface 31. These surfaces 28, 29, 30 and 31 are surfaces of metal. The dashed rectangular line 32 represents the rectangular boundary (from the top-down perspective) of the unit cell 27. Pyrepresents the width of the unit cell extending the y direction. Pxrepresents the length of the unit cell extending in the x direction. The value Pyis also the pitch or periodicity of the unit cells in the y direction, and the value Pxis also the pitch or periodicity of the unit cells in the x direction.

[0061] FIG. 5A is a simplified perspective diagram of the steps of the stairstep dielectric portion of the SBG unit cell 27 of FIG. 4. The dashed line 32 represents the rectangular boundary (rectangular when viewed from the top-down perspective) of the unit cell. Although not pictured in FIG. 5 A, underlying the step portions is a rectangular block portion (rectangular when viewed from the top-down perspective) of the dielectric material. The step portions extend upward from this block portion of dielectric. As can be seen from FIG. 5A, the stairstep dielectric portion of dielectric material includes the base block portion (not shown), a triangular-shaped (bottom step) first step portion 33, a taller rectangular-shaped second step (middle step) portion 34, and a tallest rectangular-shaped third step (top step) portion 35. The underlying rectangular block portion and the step portions 33, 34 and 35 are integrally formed at the same time of the same material and constitute a single amount of the dielectric material.

[0062] FIG. 5B is a simplified perspective diagram of the SBG unit cell 27 of FIG. 4 showing the floor surface 28, the first step (bottom step) surface 29, the second step surface 30, and the third step (top step) surface 31. As explained above, these surfaces 28-31 are surfaces of planar metal. The metal may, for example, be aluminum, gold, silver, or copper. In the present example, the dielectric sidewalls are not covered with metal, but in other examples the dielectric sidewalls are covered in metal.

[0063] In one example, the structure of the multi-layer film of the security feature 21 that, includes the SBG unit cell 27 includes an adhesive layer (for example, a heat-activated adhesive) that bonds the remainder of the multi-layer film to the substrate of the banknote 20. A cladding layer of dielectric material is disposed on and over the adhesive layer. This cladding layer is actually a layer of dielectric material that includes all the upward-facing stairstep dielectric portions of all the unit cells of the entire security feature. This layer is referred to here as a “cladding layer” because it may be formed by filling embossed depressions that were previously stamped into another dielectric layer. Disposed on the upward-facing planar surfaces of the cladding layer (when the overall structure is considered in cross-section from the perspective of the adhesive layer being on the bottom) are the portions of the thin metal that form the floor surfaces, the bottom step surfaces, the middle step surfaces, and the top step surfaces of all the unit cells that make up the security feature. Next, disposed on and over the metal surfaces layer is another layer of transparent dielectric material that is conformal to the stairstep structures, and fills the volumes between stairstep structures, but whose upper surface (when the overall structure is considered in cross-section from the perspective of the adhesive layer being on the bottom) is substantially planar This layer of transparent dielectric material may be referred to as an “embossed layer” because depressions into it. may be stamped or embossed during manufacturing of the multi-layer film. This resulting entire structure may optionally be covered with yet another layer of transparent dielectric material that serves as a protective layer. In one example, the dielectric material of the cladding layer of which the stairstep dielectric portions is made, the dielectric material of the embossed layer that is conformal to the stairstep structures, and the dielectric material of the protective layer are the same UV-curable transparent polymeric dielectric resin material Actega RAVG00338 RadKote Coating 801LV or 801HV available from Actega North. America, Inc., 1450 Taylors Lane, Cinnaminson, New Jersey 08077. When the multi-layer film is in use on the banknote 20,incident light passes down into the film through the protective layer, and then through the embossed layer, to strike the metal surfaces of the various SBG unit cells. The light diffracts, and reflects, and interacts with the metal surfaces, and passes back upward through the embossed layer, the protective layer, and out of the film (for example, to a human viewer looking at the banknote 20 from above).

[0064] FIG. 6A and FIG. 6B are simplified perspective diagrams of another embodiment 36 of an SBG unit cell. FIG. 6 A shows the dielectric material of the step portions of the unit cell. FIG. 6B shows the floor surface, the first step (bottom step) surface, the second step surface, and the third step (top step) surface of the unit cell. The bottom step is a rectangular step (rectangular when considered from the top-down perspective) that is not as wide as the higher second and third steps.

[0065] FIG. 7A and FIG. 7B are simplified perspective diagrams of another embodiment 37 of an SBG unit cell. FIG. 7A show's the dielectric material of the step portions of the unit cell. FIG. 7B shows the floor surface, the first step (bottom step) surface, the second step surface, and the third step (top step) surface of the unit cell. The bottom step is a trapezoidal step (trapezoidal when considered from the top-down perspective).

[0066] FIG. 8A and FIG. 8B are simplified perspective diagrams of another embodiment 38 of an SBG unit cell. FIG. 8A shows the dielectric material of the step portions of the unit cell FIG 8B show's the floor surface, the first step (bottom step) surface, the second step surface, and the third step (top step) surface of the unit cell The top step is a triangular step (triangular when considered from the top-down perspective).

[0067] FIG. 9A and FIG. 9B are simplified perspective diagrams of another embodiment 39 of an SBG unit cell. FIG. 9A shows the dielectric material of the step portions of the unit cell. FIG. 9B shows the floor surface, the first step (bottom step) surface, the second step surface, and the third step (top step) surface of the unit cell. The top step is a rectangular step (rectangular when considered from the top-down perspective) that is not as wide as the lower first and second steps.

[0068] FIG. I0A and FIG. 10B are simplified perspective diagrams of another embodiment 40 of an SBG unit cell FIG. 10A shows the dielectric material of the step portions of the unit cell. FIG. 10B shows the floor surface, the first step (bottom step)surface, the second step surface, and the third step (top step) surface of the unit cell. The top step is a trapezoidal step (trapezoidal when considered from the top-down perspective)

[0069] FIG. 11 is a graph and corresponding diagram that shows how the diffraction efficiency for the conventional stairstep blazed grating structure 41 shown at the top of the diagram changes as a function of the wavelength of incident light. TE denotes transverse electric polarization. TM denotes transverse magnetic polarization. Dimensions of the structure 41 are as follows: hi~60nm, h2~120nm, hr::220nm, wi“160nm, W2”160nm, ww::160nm, wiv=240nm, W2y::::240nm, Px=608nm, Py::::24()nm, hAg::::40nm. The value h. Agis the thickness of the metal layer, with the “Ag” designating that the metal is silver. The second and third steps are 240nm wide as indicated by the Pyvalue of 240nm. The first step is a rectangular step 240nm wide as indicated by the wiyand w?yvalues of 240nm.

[0070] FIG. 12 is a table that summarizes diffraction efficiency data of the graph of FIG. 11.

[0071] FIG. 13 is a graph and corresponding diagram that shows how the diffraction efficiency for the novel stairstep blazed grating structure 39 shown at the top of the diagram changes as a function of the wavelength of incident light. This structure 39 is the same structure as is pictured in FIG. 9B. TE denotes transverse electric polarization. TM denotes transverse magnetic polarization. Dimensions of the structure 39 are as follows: h₁=60nm, h₂=120nm, h₃=220nm, w₁=160nm, w₂=160nm, w₃=160nm, w₁y=140nm, w₂y=140nm, Px=608nm, Py=240nm,The second and third steps are 240nm wide as indicated by the Pyvalue of 240nm. The first step is a rectangular step 140nm wide as indicated by the w₁y and w₂y values of 140nm.

[0072] FIG. 14 is a table that summarizes diffraction efficiency data of the graph of FIG. 13. Comparison of the tables of FIG. 12 and FIG. 14 reveals the beneficial m 0 suppression that the novel stairstep blazed grating structure 39 provides. High diffraction efficiencies are exhibited for order m=-1 for both TM and TE polarizations, although the diffraction efficiency for TM polarization drops in the blue wavelength regime. Similar m=-1 diffraction characteristics are observed as well, while efficiency for order m=0 is significantly reduced across the whole spectrum range. The averaged efficiencies are further tabulated in the tables of FIG. 12 and FIG. 14 based on three different spectrum regimes, integrated over wavelength domain between 700-600nm (Red), 600-500nm (Green), and 500-400nm (Blue), respectively. The novel structure 39 of FIG. 13 is seen toachieve significant suppression of order m=0 light in the red and blue regimes compared to the conventional stairstep structure 41 of FIG. 11. Concurrently, the high diffraction efficiency characteristics of m=-1 are not only preserved by the novel structure 39 of FIG.13, but are improved from 62 to 68 and from 18 to 23 in the red and blue regimes respectively (when efficiency of the novel structure 39 of FIG. 13 is compared to the efficiency of the conventional structure 41 of FIG. 11. Efficiency for the undesired order m=0 is one order of magnitude smaller than is the efficiency for the desired order m=-1.

[0073] FIG. 15 includes a top-down diagram of the stairstep blazed grating structure 39 as well as a perspective diagram of the structure.

[0074] FIG. 16 is a graph that shows how the diffraction efficiency of the structure 39 of FIG. 15 varies as a function of width of the top step. As indicated by the upward pointing arrow located at 140nm along the horizontal axis of the graph of FIG 16, efficiency for order m=0 light is seen to dip to a minimum at a step width of 140nm, and at this 140nm top step width the efficiency of order m=-1 light is relatively unaffected.

[0075] FIG. 17 is a table that sets forth dimensions of the stairstep blazed grating structure 39, the diffraction efficiency of which is plotted in FIG. 16. The widths (in the y direction) of the first and second steps is 240nm, whereas the width of the top step is 140nm.

[0076] FIG. 18 includes a top-down diagram of the stairstep blazed grating structure 40 as well as a perspective diagram of the structure.

[0077] FIG. 19 is a graph that shows how the diffraction efficiency of the structure 40 of FIG. 18 varies as a function of size of the top step (the length of the short base side of the trapezoid being varied, with the length of the long base side of the trapezoid being held constant). As indicated by the upward pointing arrow located at 80nm along the horizontal axis of the graph of FIG. 19, efficiency for order m=0 light is seen to dip to a minimum at a step width of 80nm, and at this 80nm top step width the efficiency of order m=-1 light is relatively unaffected.

[0078] FIG. 20 is a table that sets forth dimensions of the stairstep blazed grating structure 40, the diffraction efficiency of which is plotted in FIG. 19. The widths (in the y direction) of the first and second steps is 200nm. The top step has a trapezoidal shape that is 200nm on its long base side, and that is 80nm on its short base side.

[0079] FIG. 21 includes a top-down diagram of the stairstep blazed grating structure 37 as well as a perspective diagram of the structure.[0080 j FIG. 22 is a graph that shows how the diffraction efficiency of the structure 37 of FIG. 21 varies as a function of size of the bottom step (the length of the short base side of the trapezoid being varied, with the length of the long base side of the trapezoid being held constant). A triangularly shaped bottom step (w=0) can best suppress order m=0 light while affording high efficiency for order m=-1 light. A trapezoid with its short base side being of zero length (w=0) is a triangle.

[0081] FIG. 23 is a table that sets forth dimensions of the stairstep blazed grating structure 37, the diffraction efficiency of which is plotted in FIG. 22. The widths (in the y direction) of the second and third steps is 240nm. The bottom step has a trapezoidal shape that is 240nm on its long base side, and that is 0nm on its short base side. Because the short base side of the trapezoidal shape is zero in length, it is a triangle when considered from the top-down perspective

[0082] FIG. 24A thru FIG. 24M together are a sequence of diagrams that illustrates a method of making an optical grating structure having an array of Stairstep Blazed Grating (SBG) unit cells. A release layer 100 is deposited on a carrier layer 101. The carrier layer 101 may, for example, be a flexible layer of polyethylene terephthalate (PET). A layer 102 of transparent protective material is deposed on layer 100, and a layer 103 of transparent UV-curable polymeric dielectric material is deposited on the layer 102 FIG. 24A is a cross-sectional diagram showing the resulting stack of layers. Next, a hard stamp 104 is pressed down into layer 103 to form indentations into the layer 103 as shown in FIG. 24B. For more information on a hard stamp such as hard stamp 104 and how to make the hard stamp and how to use the hard stamp, see: U. S. Patent Application Serial Number 18 / 600,614, filed March 8, 2024, by Ripon Dey, Milad Khoshnegar Shahrestani, and Graham Beales (the entire subject matter of which is hereby incorporated into this application by reference). The layer 103 of dielectric material is exposed to UV radiation to cure and harden the layer FIG. 24C illustrates the resulting structure. Next, a layer 105 of positive photoresist is applied over the embossed layer resin layer 103 as illustrated in FIG. 24D. The layer 105 is selectively exposed to UV radiation to expose areas in which metal is later to be removed in a lift-off operation The selective exposure of photoresist layer 105 is illustrated in FIG. 24E. Next, areas of unexposed photoresist are removed.The result of this removal step is illustrated in FIG. 24F. The remaining exposed areas of photoresist form a mask 106. In some examples, the inverted stairstep structure of the exposed portion of dielectric material 103 is somewhat smoothed over such that sharp feature edges are not present, but rather are rounded off.

[0083] Next, a thin layer of metal 107 is deposited over the entire structure as illustrated in FIG. 24G. Next, the photoresist mask 106 is removed, and in the process any metal that was disposed on top of the photoresist mask is stripped off. Metal that was not disposed on the photoresist mask, however, remains. The resulting structure is shown in cross-section in FIG. 24H. The remaining metal is denoted by reference numeral 108. In this example, features of thin metal are left on upward-facing surfaces of the dielectric layer 103, but metal is not coated onto vertical sidewalls of the dielectric layer 103. Next, a layer 112 of dielectric material (sometimes called a cladding layer) is deposited on the structure of FIG. 24H so as to fill in the indentations, thereby to form what will be the stairstep dielectric structures. The resulting structure is shown in cross-section in FIG. 241 In one example, each of the transparent protective layer 102, the transparent dielectric material layer 103, and the cladding layer 112 is a layer of the UV-curable polymeric dielectric material Actega RAVG00338 RadKote Coating 801LV or 801HV available from Actega North America, Inc., 1450 Taylors Lane, Cinnaminson, New Jersey 08077, having a refractive index of 1.52. Next, a layer 113 of heat-activated adhesive is applied over the entire structure. The resulting structure is illustrated in cross-section in FIG 24J

[0084] It is understood that in a practical manufacturing process, so-called level surfaces of the steps of the SBG unit cells will not be perfectly flat or perfectly level or perfectly planar, and will not be of perfect rectangular, trapezoidal, or triangular shape when considered from the top-down perspective. Sharp edges and features will be smoothed, and rounded off. Sharp angular recesses will be somewhat filled in. The stairstep structure of the SBG unit cells is typically only approximated in the final actual manufactured film. The SBG unit cells are nonetheless described as having a stairstep structure.

[0085] The resulting structure of FIG. 24J is a film of optical grating structures that may be sold as a product. The film may take the form of a roll of such flexible film The roll carries and incorporates many copies of the optical grating structure. FIG 24K shows how the film structure of FIG. 24 J can be inverted (flipped) and attached, using the heat-activated adhesive layer 113, to a substrate 115 The substrate 115 may, for example, be a banknote, a passport, a check, an admission ticket, an article of commerce that is the subject of black market counterfeit copying, a driver’s license, an identification card. The substrate 115 may, for example, be a part of a waveguide into which the optical grating structure beam-steers or directs light. The substrate 115 may, for example, be a part of a waveguide from which the optical grating structure beam-steers or directs light. Next, the carrier layer 101 is removed by activating the release layer 100. This releasing is illustrated in FIG 24L. FIG. 24M illustrates the final optical grating structure disposed on and adhered to the substrate 115. Note that in this example, the upward facing planar surfaces of the dielectric stairstep structure of each SBG unit cell is coated with metal, but the perfectly vertical sidewalls of the dielectric stairstep structures are not coated with metal.

[0086] Although certain specific embodiments are described above for instructional purposes, the teachings of this patent document have general applicability and are not limited to the specific embodiments described above. Accordingly, various modifications, adaptations, and combinations of various features of the described embodiments can be practiced without departing from the scope of the invention as set forth in the claims.

Claims

CLAIMSWhat is claimed is:

1. A structure comprising:a two-dimensional array of Stairstep Blazed Grating (SBG) unit cells, wherein one of the unit cells has a floor level surface, a bottom step level surface, and a top step level surface, wherein the unit cell from a top-down perspective is rectangular and has a width Pyand a length Px, wherein one of the bottom step level surface and the top step level surface has a rectangular shape that has a width equal to Py, and wherein the other of the bottom step level surface and the top step level surface has a shape taken from the group consisting of: a rectangular shape having a width smaller than Py, a trapezoidal shape, a triangular shape, wherein the floor level surface, the bottom step level surface, and the top step level surface are surfaces of a layer of metal, wherein the layer of metal is disposed on a stairstep structure of a polymeric dielectric material, wherein the unit cell further comprises a layer of a transparent polymeric dielectric material that covers the floor level surface, the bottom step level surface and the top step level surface, wherein the width Pyis less than 500 nanometers, wherein the length Pxis less than 800 nanometers, wherein the floor level surface is disposed on a first plane and the top step level surface is disposed in a second plane, wherein the first and second planes are separated by less than 400 nanometers2. The structure of claim 1, wherein the stairstep structure of the polymeric dielectric material has a dielectric sidewall, and wherein metal of the layer of metal is disposed on and covers the dielectric sidewall.

3. The structure of claim 1, wherein the polymeric dielectric material of the stairstep structure is a UV-curable polymer, and wherein the transparent polymeric dielectric material is the same UV-curable polymer.

4. The structure of claim 1, wherein the structure is a roll of film, wherein the roll of film comprises a layer of carrier film, and wherein the two-dimensional array is disposed on the carrier film.

5. The structure of claim 1, wherein the structure is a security document, wherein the security document comprises a substrate layer, and wherein the two-dimensional array is disposed on the substrate layer.

6. The structure of claim 1, wherein the unit cell also has an additional step level surface disposed between the bottom step level surface and the top step level surface,7. The structure of claim 1, wherein the bottom step level surface of the unit cell is a triangle, and wherein the bottom step level surfaces together form a means for causing plasmonic effects when the structure is illuminated with incident visible light.

8. A method comprising:(a) using a stamp to emboss a blazed diffraction grating depression into a first layer of dielectric material, wherein a first edge of the depression is a straight line, and wherein a second edge of the depression opposite the first edge is not a straight line but rather has pointed extensions that point away from the first edge, wherein the first edge and the second edge are disposed in the same plane;(b) depositing a thin layer of metal onto a dielectric surface of the depression thereby forming a metal-coated blazed diffraction grating depression; and(c) depositing a second layer of dielectric material such that the metal-coated blazed diffraction grating depression is filled with dielectric material and such that the entire metal-coated blazed diffraction grating depression is overcoated with dielectric material of the second layer.

9. The method of claim 8, wherein the blazed diffraction grating depression that is embossed in step (a) is a multi-step blazed diffraction grating depression, and wherein the depression has a plurality of steps.

10. The method of claim 8, wherein the blazed diffraction grating depression is one of a plurality of parallel-extending depressions that are simultaneously embossed by the stamp into the first layer of the dielectric material in step (a).

11. The method of claim 8, wherein the pointed extensions are spaced at intervals with a periodicity Py, and wherein the periodicity Py is less than 500 nanometers.

12. A method comprising(a) forming a plurality of elongated blazed raised grating structures that have a thin layer of metal deposited on them, wherein the plurality of elongated blazed raised grating structures extend parallel to one another, wherein each of the elongated blazed raised grating structures extends upward from a floor surface of a dielectric material; and (b) providing a layer of transparent dielectric material that is disposed over the thin layer of metal and that fills volumes between adjacent ones of the elongated blazed raised grating structures, wherein a first boundary between one of the elongated blazed raised grating structures and the floor surface is a straight line, and wherein a second boundary between said one of the elongated blazed raised grating structures and the floor surface is not a straight line but rather has pointed extensions that point away from the first boundary, wherein the first boundary and the second boundary are disposed in the same plane.

13. The method of claim 12, wherein the pointed extensions are spaced at intervals with a periodicity Py, and wherein the periodicity Py is less than 500 nanometers.

14. The method of claim 12, wherein the layer of transparent dielectric material is an embossed layer, and wherein the dielectric material having the floor surface is a cladding layer.

15. The method of claim 12, wherein each of the elongated blazed raised grating structures is a stairstep structure that has a plurality of steps.

16. The method of claim 12, wherein the dielectric material that has the floor surface, the thin layer of metal, and the layer of transparent dielectric material are parts of a film, the method further comprising:providing an adhesive layer that is a layer of the film17. The method of claim 12, wherein the dielectric material that has the floor surface is a UV-curable polymer material.

18. The method of claim 12, wherein the transparent dielectric material is a UV-curable polymer material.

19. The method of claim 12, wherein each of the of elongated blazed raised grating structures has a height with respect to the floor surface, and wherein the height is less than 400 nanometers.

20. The method of claim 12, wherein the pointed extensions together form a means for causing plasmonic effects when elongated blazed raised grating structures are illuminated with incident visible light.

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