# Patent

## Title
Omnidirectional high chroma red structural color with combination metal absorber and dielectric absorber layers

## Abstract

A high-chroma omnidirectional red structural color pigment. The omnidirectional structural color pigment is in the form of a multilayer stack that has a reflective core layer, a metal absorber layer extending across the reflective core layer and a dielectric absorber layer extending across the metal absorber layer. The multilayer stack reflects a single band of visible light with a hue between 0-40°, and preferably between 10-30°, on an a*b* Lab color map. The single band of visible light has a hue shift of less than 30° on the a*b* Lab color map when viewed from all angles between 0-45° normal to an outer surface of the multilayer stack.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
The instant application is a continuation-in-part (CIP) of U.S. patent application Ser. No. 14/607,933 filed on Jan. 28, 2015, which in is a CIP of U.S. patent application Ser. No. 14/471,834 filed on Aug. 28, 2014, which in turn is a CIP of U.S. patent application Ser. No. 14/460,511 filed on Aug. 15, 2014, which in turn is a CIP of U.S. patent application Ser. No. 14/242,429 filed on Apr. 1, 2014, which in turn is a CIP of U.S. patent application Ser. No. 14/138,499 filed on Dec. 23, 2013, which in turn is a CIP of U.S. patent application Ser. No. 13/913,402 filed on Jun. 8, 2013, which in turn is a CIP of U.S. patent application Ser. No. 13/760,699 filed on Feb. 6, 2013, which in turn is a CIP of Ser. No. 13/572,071 filed on Aug. 10, 2012, all of which are incorporated in their entirety by reference.


FIELD OF THE INVENTION
The present invention is related to multilayer stack structures that exhibit a high chroma red color with a minimum or non-noticeable color shift when exposed to broadband electromagnetic radiation and viewed from different angles.
BACKGROUND OF THE INVENTION
Pigments made from multilayer structures are known. In addition, pigments that exhibit or provide a high-chroma omnidirectional structural color are also known. However, such prior art pigments have required as many as 39 thin film layers in order to obtain desired color properties.
It is appreciated that cost associated with the production of thin film multilayer pigments is proportional to the number of layers required. As such, the cost associated with the production of high-chroma omnidirectional structural colors using multilayer stacks of dielectric materials can be prohibitive. Therefore, a high-chroma omnidirectional structural color that requires a minimum number of thin film layers would be desirable.
In addition to the above, it is appreciated that the design of pigments with a red color face an additional hurdle to pigments of other colors such as blue, green, etc. In particular, the control of angular independence for red color is difficult since thicker dielectric layers are required, which in turn results in a high harmonic design, i.e. the presence of the second and possible third harmonics is inevitable. Also, the dark red color hue space is very narrow. As such, a red color multilayer stack has a higher angular variance.
Given the above, a high-chroma red omnidirectional structural color pigment with a minimum number of layers would be desirable.
SUMMARY OF THE INVENTION
An omnidirectional high-chroma red structural color pigment is provided. The omnidirectional structural color pigment is in the form of a multilayer stack that has a reflective core layer, a metal absorber layer extending across the reflective core layer and a dielectric absorber layer extending across the metal absorber layer. The multilayer stack reflects a single band of visible light with a hue between 0-40°, preferably between 10-30°, on an a*b* Lab color map. In addition, the single band of visible light has a hue shift of less than 30° on the a*b* Lab color map when viewed from all angles between 0-45° normal to an outer surface of the multilayer stack and as such provides a non-noticeable color shift to the human eye.
The reflective core layer has a thickness between 50-200 nanometers (nm), inclusive, and can be made from a reflective metal such as aluminum (Al), silver (Ag), platinum (Pt), tin (Sn), combinations thereof, and the like. The reflective core layer can also be made from a colorful metal such as gold (Au), copper (Cu), brass, bronze, and the like.
The metal absorber layer can have a thickness between 5-500 nm, inclusive, and be made from such materials such as colorful metals, e.g. copper (Cu), gold (Au), bronze (Cu—Zn alloys), brass (Cu—Sn alloys), amorphous silicon (Si) or a colorful nitride materials, e.g. titanium nitride (TiN). The dielectric absorber layer can have a thickness between 5-500 nm, inclusive, and be made from colorful dielectric materials such as but not limited to iron oxide (Fe2O3), . . . .
The reflective core layer, metal absorber layer and/or dielectric absorber layer can be dry deposited layers. However, the dielectric absorber layer can be a wet deposited layer. In addition, the reflective core layer can be a central reflective core layer and the metal absorber layer be a pair of metal absorber layers that extend across opposite sides of the central reflective core layer, i.e. the central reflective core layer is sandwiched between the pair of metal absorber layers. Furthermore, the dielectric absorber layer can be a pair of dielectric absorber layers such that the central reflective core layer and the pair of metal absorber layers are sandwiched between the pair of dielectric absorber layers.
A process for making such an omnidirectional high chroma red structural color includes manufacturing the multilayer stack by dry depositing the reflective core layer, dry depositing the metal absorber layer that extends across the reflective core layer and dry depositing or wet depositing the dielectric absorber layer that extends across the metal absorber layer. In this manner, a hybrid manufacturing process can be used to produce an omnidirectional high chroma red structural color that can be used for pigments, coatings, and the like.



BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 is a schematic illustration of an omnidirectional structural color multilayer stack made from a dielectric layer, a selective absorbing layer (SAL) and a reflector layer;
FIG. 2A is a schematic illustration of a zero or near-zero electric field point within a ZnS dielectric layer exposed to electromagnetic radiation (EMR) having a wavelength of 500 nm;
FIG. 2B is a graphical illustration of the absolute value of electric field squared (|E|2) versus thickness of the ZnS dielectric layer shown in FIG. 2A when exposed to EMR having wavelengths of 300, 400, 500, 600 and 700 nm;
FIG. 3 is a schematic illustration of a dielectric layer extending over a substrate or reflector layer and exposed to electromagnetic radiation at an angle θ relative to a normal direction to the outer surface of the dielectric layer;
FIG. 4 is a schematic illustration of a ZnS dielectric layer with a Cr absorber layer located at the zero or near-zero electric field point within the ZnS dielectric layer for incident EMR having a wavelength of 434 nm;
FIG. 5 is a graphical representation of percent reflectance versus reflected EMR wavelength for a multilayer stack without a Cr absorber layer (e.g., FIG. 2A) and a multilayer stack with a Cr absorber layer (e.g., FIG. 4) exposed to white light;
FIG. 6A is a graphical illustration of first harmonics and second harmonics exhibited by a ZnS dielectric layer extending over an Al reflector layer (e.g., FIG. 2A);
FIG. 6B is a graphical illustration of percent reflectance versus reflected EMR wavelength for a multilayer stack with a ZnS dielectric layer extending across an Al reflector layer, plus a Cr absorber layer located within the ZnS dielectric layer such that the second harmonics shown in FIG. 6A are absorbed;
FIG. 6C is a graphical illustration of percent reflectance versus reflected EMR wavelength for a multilayer stack with a ZnS dielectric layer extending across an Al reflector layer, plus a Cr absorber layer located within the ZnS dielectric layer such that the first harmonics shown in FIG. 6A are absorbed;
FIG. 7A is a graphical illustration of electric field squared versus dielectric layer thickness showing the electric field angular dependence of a Cr absorber layer for exposure to incident light at 0 and 45 degrees;
FIG. 7B is a graphical illustration of percent absorbance by a Cr absorber layer versus reflected EMR wavelength when exposed to white light at 0 and 45° angles relative to normal of the outer surface (0° being normal to surface);
FIG. 8A is a schematic illustration of a red omnidirectional structural color multilayer stack according to an aspect disclosed herein;
FIG. 8B is a graphical illustration of percent absorbance of the Cu absorber layer shown in FIG. 8A versus reflected EMR wavelength for white light exposure to the multilayer stack shown in FIG. 8A at incident angles of 0 and 45′;
FIG. 9 is a graphical comparison between calculation/simulation data and experimental data for percent reflectance versus reflected EMR wavelength for a proof of concept red omnidirectional structural color multilayer stack exposed to white light at an incident angle of 0°;
FIG. 10 is a graphical illustration of percent reflectance versus wavelength for an omnidirectional structural color multilayer stack according to an aspect disclosed herein;
FIG. 11 is a graphical illustration of percent reflectance versus wavelength for an omnidirectional structural color multilayer stack according to an aspect disclosed herein;
FIG. 12 is a graphical representation of a portion of an a*b* color map using the CIELAB (Lab) color space in which the chroma and hue shift of a conventional paint and a paint made from pigments according to an aspect disclosed herein are compared (Sample (b));
FIG. 13 is a schematic illustration of a red omnidirectional structural color multilayer stack according to another aspect disclosed herein;
FIG. 14 is a graphical representation of percent reflectance versus wavelength for the aspect shown in FIG. 13;
FIG. 15 is a graphical illustration of percent absorbance versus wavelength for the aspect shown in FIG. 13;
FIG. 16 is a graphical illustration of percent reflectance versus wavelength versus viewing angle for the aspect shown in FIG. 13;
FIG. 17 is a graphical illustration of chroma and hue versus viewing angle for the aspect shown in FIG. 13;
FIG. 18 is a graphical illustration of color reflected by the aspect shown in FIG. 13 with respect to an a*b* Lab color map; and
FIG. 19 is a schematic illustration of a process for manufacturing an omnidirectional red structural color multilayer stack according to an aspect disclosed herein.



DETAILED DESCRIPTION OF THE INVENTION
An omnidirectional high chroma red structural color pigment is provided. The omnidirectional high chroma red structural color is in the form of a multilayer stack that has a reflective core layer, a metal absorber layer and a dielectric absorber layer. The metal absorber layer extends across the reflective core layer and in some instances is located directly against or on top of the reflective core layer. The dielectric absorber layer extends across the metal absorber layer and in some instances is located directly against or on top of the metal absorber layer. The multilayer stack can be a symmetric stack, i.e. the reflective core layer is a central reflective core layer that is bounded by a pair of metal absorber layers and the pair of metal absorber layers are bounded by a pair of dielectric absorber layers.
The multilayer stack reflects a single band of visible light that has a red color with a hue between 0-40°, preferably between 10-30°, on an a*b* Lab color map. In addition, the hue shift of the single band of visible light is less than 30°, preferably less than 20°, and more preferably less than 10° on the a*b* Lab color map when the multilayer stack is viewed from all angles between 0-45° normal to an outer surface thereof. As such, the hue shift of the single band of reflected visible light can be within the 0-40° region and/or the 10-30° region on the a*b* Lab color map.
The reflective core layer can be a dry deposited layer with a thickness between 50-200 nm, inclusive. The term “dry deposited” refers to dry deposition techniques such as physical vapor deposition (PVD) techniques including electron beam deposition, sputtering, chemical vapor deposition (CVD), plasma assisted CVD, and the like. In some instances, the reflective core layer is made from a reflective metal such as Al, Ag, Pt, Sn, combinations thereof, and the like. In other instances, the reflective core layer is made from a colorful metal such as Au, Cu, brass, bronze, combinations thereof, and the like. It is appreciated that the terms “brass” and “bronze” refers to copper-zinc alloys and copper-tin alloys, respectively, known to those skilled in the art.
The metal absorber layer can also be a dry deposited layer that is deposited onto the reflective core layer. In the alternative, the reflective core layer can be deposited onto the metal absorber layer. The metal absorber layer can have a thickness between 5-500 nm, inclusive, and be made from a colorful metal such as Cu, bronze, brass, or a material such amorphous silicon (Si), germanium (Ge), TiN and the like. It is appreciated that for the purposes of the instant disclosure the term “metal absorber layer” includes materials that are typically not know as metals such the amorphous Si, Ge, TiN and the like.
The dielectric absorber layer can also be a dry deposited layer or a wet deposited layer that is deposited onto the metal absorber layer. In the alternative, the metal absorber layer can be deposited onto the dielectric absorber layer. The dielectric absorber layer can have a thickness between 5-500 nm, inclusive, and be made from a dielectric material such as iron oxide (Fe2O3) and the like. Also, the term “wet deposited” refers to wet deposition techniques such as sol-gel techniques, spin coating techniques, wet chemistry deposition techniques, and the like.
The overall thickness of the multilayer stack can be less than 3 microns, preferably less than 2 microns, more preferably less than 1.5 microns, and still more preferably less than or equal to 1.0 microns. In addition, the multilayer stack can have less than or equal to 9 total layers and preferably less than or equal to 7 total layers, and more preferably less than or equal to 5 total layers.
Referring to FIG. 1, a design is shown in which an underlying reflector layer (RL) has a first dielectric material layer DL1 extending thereacross and a selective absorbing layer SAL extending across the DL1 layer. In addition, another DL1 layer may or may not be provided and extend across the selective absorbing layer. Also shown in the figure is an illustration that all of the incident electromagnetic radiation is either reflected or selectively absorbed by the multilayer structure.
Such a design as illustrated in FIG. 1 corresponds to a different approach that is used for designing and manufacturing a desired multilayer stack. In particular, a zero or near-zero energy point thickness for a dielectric layer is used and discussed below.
For example, FIG. 2A is a schematic illustration of a ZnS dielectric layer extending across an Al reflector core layer. The ZnS dielectric layer has a total thickness of 143 nm, and for incident electromagnetic radiation with a wavelength of 500 nm, a zero or near-zero energy point is present at 77 nm. Stated differently, the ZnS dielectric layer exhibits a zero or near-zero electric field at a distance of 77 nm from the Al reflector layer for incident electromagnetic radiation (EMR) having a wavelength of 500 nm. In addition, FIG. 2B provides a graphical illustration of the energy field across the ZnS dielectric layer for a number of different incident EMR wavelengths. As shown in the graph, the dielectric layer has a zero electric field for the 500 nm wavelength at 77 nm thickness, but a non-zero electric field at the 77 nm thickness for EMR wavelengths of 300, 400, 600 and 700 nm.
Regarding calculation of a zero or near-zero electric field point, FIG. 3 illustrates a dielectric layer 4 having a total thickness ‘D’, an incremental thickness ‘d’ and an index of refraction ‘n’ on a substrate or core layer 2 having an index of refraction ns. Incident light strikes the outer surface 5 of the dielectric layer 4 at angle θ relative to line 6, which is perpendicular to the outer surface 5, and reflects from the outer surface 5 at the same angle θ. Incident light is transmitted through the outer surface 5 and into the dielectric layer 4 at an angle θF relative to the line 6 and strikes the surface 3 of substrate layer 2 at an angle θs.
For a single dielectric layer, θs=θF and the energy/electric field (E) can be expressed as E(z) when z=d. From Maxwell's equations, the electric field can be expressed for s polarization as:

E(d)={u(z),0,0}exp(ikαy)|z=d  (1)

and for p polarization as:





  
    
      
        
          
            E
            _
          
          ⁡
          
            (
            d
            )
          
        
        =
        
          
            
              {
              
                0
                ,
                
                  u
                  ⁡
                  
                    (
                    z
                    )
                  
                
                ,
                
                  
                    -
                    
                      α
                      
                        
                          ɛ
                          ~
                        
                        ⁡
                        
                          (
                          z
                          )
                        
                      
                    
                  
                  ⁢
                  
                    v
                    ⁡
                    
                      (
                      z
                      )
                    
                  
                
              
              }
            
            ⁢
            
              exp
              ⁡
              
                (
                
                  i
                  ⁢
                  k
                  ⁢
                  α
                  ⁢
                  y
                
                )
              
            
          
          ⁢
          
            |
            
              z
              =
              d
            
          
        
      
    
    
      
        (
        2
        )
      
    
  




where





  k
  =
  
    
      2
      ⁢
      π
    
    λ
  




and λ is a desired wavelength to be reflected. Also, α=ns sin θs where ‘s’ corresponds to the substrate in FIG. 5 and {tilde over (∈)}(z) is the permittivity of the layer as a function of z. As such,

|E(d)|2=|u(z)|2exp(2ikαy)|z=d  (3)

for s polarization and





  
    
      
        
          
            
            
              E
              ⁡
              
                (
                d
                )
              
            
            
          
          2
        
        =
        
          
            
              [
              
                
                  
                    
                    
                      u
                      ⁡
                      
                        (
                        z
                        )
                      
                    
                    
                  
                  2
                
                +
                
                  
                    
                    
                      
                        α
                        
                          n
                        
                      
                      ⁢
                      
                        v
                        ⁡
                        
                          (
                          z
                          )
                        
                      
                    
                    
                  
                  2
                
              
              ]
            
            ⁢
            
              exp
              ⁡
              
                (
                
                  2
                  ⁢
                  
                    i
                    ⁢
                    k
                    ⁢
                    α
                    ⁢
                    y
                  
                
                )
              
            
          
          ⁢
          
            |
            
              z
              =
              d
            
          
        
      
    
    
      
        (
        4
        )
      
    
  




for p polarization.

It is appreciated that variation of the electric field along the Z direction of the dielectric layer 4 can be estimated by calculation of the unknown parameters u(z) and v(z) where it can be shown that:




  
    
      
        
          
            (
            
              
                
                  u
                
              
              
                
                  v
                
              
            
            )
          
          
            z
            =
            d
          
        
        =
        
          
            (
            
              
                
                  
                    cos
                    ⁢
                    
                      
                    
                    ⁢
                    φ
                  
                
                
                  
                    
                      (
                      
                        i
                        ⁢
                        
                          /
                          q
                        
                      
                      )
                    
                    ⁢
                    sin
                    ⁢
                    
                      
                    
                    ⁢
                    φ
                  
                
              
              
                
                  
                    iq
                    ⁢
                    
                      
                    
                    ⁢
                    sin
                    ⁢
                    
                      
                    
                    ⁢
                    φ
                  
                
                
                  
                    cos
                    ⁢
                    
                      
                    
                    ⁢
                    φ
                  
                
              
            
            )
          
          ⁢
          
            
              (
              
                
                  
                    u
                  
                
                
                  
                    v
                  
                
              
              )
            
            
              
                z
                =
                0
              
              ,
              substrate
            
          
        
      
    
    
      
        (
        5
        )
      
    
  




Naturally, ‘i’ is the square root of −1. Using the boundary conditions u|z=0=1, v|z=0=qs and the following relations:

qs=ns cos θs for s-polarization  (6)

qs=ns/cos θs for p-polarization  (7)

q=n cos θF for s-polarization  (8)

q=n/cos θF for p-polarization  (9)

φ=k·n·d cos(θF)  (10)

u(z) and v(z) can be expressed as:





  
    
      
        
          
            
              
                
                  u
                  ⁡
                  
                    (
                    z
                    )
                  
                
                ⁢
                
                  |
                  
                    z
                    =
                    d
                  
                
              
              =
              
              ⁢
              
                u
                ⁢
                
                  |
                  
                    z
                    =
                    0
                  
                
                ⁢
                
                  
                    cos
                    ⁢
                    
                      
                    
                    ⁢
                    φ
                  
                  +
                  v
                
                ⁢
                
                  |
                  
                    z
                    =
                    o
                  
                
                ⁢
                
                  (
                  
                    
                      i
                      q
                    
                    ⁢
                    sin
                    ⁢
                    
                      
                    
                    ⁢
                    φ
                  
                  )
                
              
            
          
        
        
          
            
              =
              
              ⁢
              
                
                  cos
                  ⁢
                  
                    
                  
                  ⁢
                  φ
                
                +
                
                  
                    
                      i
                      .
                      
                        q
                        s
                      
                    
                    q
                  
                  ⁢
                  sin
                  ⁢
                  
                    
                  
                  ⁢
                  φ
                
              
            
          
        
      
    
    
      
        (
        11
        )
      
    
  




and





  
    
      
        
          
            
              
                
                  v
                  ⁡
                  
                    (
                    z
                    )
                  
                
                ⁢
                
                  |
                  
                    z
                    =
                    d
                  
                
              
              =
              
              ⁢
              
                iqu
                ⁢
                
                  |
                  
                    z
                    =
                    0
                  
                
                ⁢
                
                  
                    sin
                    ⁢
                    
                      
                    
                    ⁢
                    φ
                  
                  +
                  v
                
                ⁢
                
                  |
                  
                    z
                    =
                    o
                  
                
                ⁢
                
                  cos
                  ⁢
                  
                    
                  
                  ⁢
                  φ
                
              
            
          
        
        
          
            
              =
              
              ⁢
              
                
                  iq
                  ⁢
                  
                    
                  
                  ⁢
                  sin
                  ⁢
                  
                    
                  
                  ⁢
                  φ
                
                +
                
                  
                    q
                    s
                  
                  ⁢
                  cos
                  ⁢
                  
                    
                  
                  ⁢
                  φ
                
              
            
          
        
      
    
    
      
        (
        12
        )
      
    
  




Therefore:





  
    
      
        
          
            
              
                |
                
                  E
                  ⁡
                  
                    (
                    d
                    )
                  
                
                ⁢
                
                  |
                  2
                
              
              =
              
              ⁢
              
                
                  [
                  
                    
                      
                        cos
                        2
                      
                      ⁢
                      φ
                    
                    +
                    
                      
                        
                          q
                          s
                          2
                        
                        
                          q
                          2
                        
                      
                      ⁢
                      
                        sin
                        2
                      
                      ⁢
                      
                        
                      
                      ⁢
                      φ
                    
                  
                  ]
                
                ⁢
                
                  e
                  
                    2
                    ⁢
                    
                      i
                      ⁢
                      k
                      ⁢
                      αγ
                    
                  
                
              
            
          
        
        
          
            
              =
              
              ⁢
              
                
                  [
                  
                    
                      
                        cos
                        2
                      
                      ⁢
                      φ
                    
                    +
                    
                      
                        
                          n
                          s
                          2
                        
                        
                          n
                          2
                        
                      
                      ⁢
                      
                        sin
                        2
                      
                      ⁢
                      
                        
                      
                      ⁢
                      φ
                    
                  
                  ]
                
                ⁢
                
                  e
                  
                    2
                    ⁢
                    
                      i
                      ⁢
                      k
                    
                    ⁢
                    
                      
                    
                    ⁢
                    αγ
                  
                
              
            
          
        
      
    
    
      
        (
        13
        )
      
    
  




for s polarization with φ=k·n·d cos (θF), and:





  
    
      
        
          
            
              
                |
                
                  E
                  ⁡
                  
                    (
                    d
                    )
                  
                
                ⁢
                
                  |
                  2
                
              
              =
              
              ⁢
              
                [
                
                  
                    
                      cos
                      2
                    
                    ⁢
                    φ
                  
                  +
                  
                    
                      
                        n
                        s
                        2
                      
                      
                        n
                        2
                      
                    
                    ⁢
                    
                      sin
                      2
                    
                    ⁢
                    
                      
                    
                    ⁢
                    φ
                  
                  +
                  
                    
                      
                        α
                        2
                      
                      n
                    
                    ⁢
                    
                      (
                      
                        
                          
                            q
                            s
                            2
                          
                          ⁢
                          
                            cos
                            2
                          
                          ⁢
                          φ
                        
                        +
                        
                          
                            q
                            2
                          
                          ⁢
                          
                            sin
                            2
                          
                          ⁢
                          φ
                        
                      
                      )
                    
                  
                
                ]
              
            
          
        
        
          
            
              =
              
              ⁢
              
                [
                
                  
                    
                      (
                      
                        1
                        +
                        
                          
                            
                              α
                              2
                            
                            ⁢
                            
                              q
                              s
                              2
                            
                          
                          n
                        
                      
                      )
                    
                    ⁢
                    
                      cos
                      2
                    
                    ⁢
                    φ
                  
                  +
                  
                    
                      (
                      
                        
                          
                            n
                            s
                            2
                          
                          
                            n
                            2
                          
                        
                        +
                        
                          
                            
                              α
                              2
                            
                            ⁢
                            
                              q
                              2
                            
                          
                          n
                        
                      
                      )
                    
                    ⁢
                    
                      sin
                      2
                    
                    ⁢
                    φ
                  
                
                ]
              
            
          
        
      
    
    
      
        (
        14
        )
      
    
  




for p polarization where:





  
    
      
        α
        =
        
          
            
              n
              s
            
            ⁢
            sin
            ⁢
            
              
            
            ⁢
            
              θ
              s
            
          
          =
          
            n
            ⁢
            
              
            
            ⁢
            sin
            ⁢
            
              
            
            ⁢
            
              θ
              F
            
          
        
      
    
    
      
        (
        15
        )
      
    
  
  
    
      
        
          q
          s
        
        =
        
          
            n
            s
          
          
            cos
            ⁢
            
              
            
            ⁢
            
              θ
              s
            
          
        
      
    
    
      
        (
        16
        )
      
    
  
  
    
      and
    
    
      
        
      
    
  
  
    
      
        
          q
          s
        
        =
        
          n
          
            cos
            ⁢
            
              
            
            ⁢
            
              θ
              F
            
          
        
      
    
    
      
        (
        17
        )
      
    
  




Thus for a simple situation where θF=0 or normal incidence, φ=k·n·d, and α=0:




  
    
      
        
          |
          
            E
            ⁡
            
              (
              d
              )
            
          
          ⁢
          
            |
            2
          
          ⁢
          
            
          
          ⁢
          
            for
            ⁢
            
              
            
            ⁢
            s
            ⁢
            
              -
            
            ⁢
            polarization
          
        
        =
        
          
            |
            
              
            
            ⁢
            
              E
              ⁡
              
                (
                d
                )
              
            
            ⁢
            
              |
              2
            
            ⁢
            
              
            
            ⁢
            
              for
              ⁢
              
                
              
              ⁢
              p
              ⁢
              
                -
              
              ⁢
              polarization
            
          
          =
          
            [
            
              
            
            ⁢
            
              
                
                  cos
                  2
                
                ⁢
                φ
              
              +
              
                
                  
                    n
                    s
                    2
                  
                  
                    n
                    2
                  
                
                ⁢
                
                  sin
                  2
                
                ⁢
                
                  
                
                ⁢
                φ
              
            
            ]
          
        
      
    
    
      
        (
        18
        )
      
    
  
  
    
      
        =
        
          [
          
            
              
                cos
                2
              
              ⁡
              
                (
                
                  k
                  ·
                  n
                  ·
                  d
                
                )
              
            
            +
            
              
                
                  n
                  s
                  2
                
                
                  n
                  2
                
              
              ⁢
              
                
                  sin
                  2
                
                ⁡
                
                  (
                  
                    k
                    ·
                    n
                    ·
                    d
                  
                  )
                
              
            
          
          ]
        
      
    
    
      
        (
        19
        )
      
    
  




which allows for the thickness ‘d’ to be solved for, i.e. the position or location within the dielectric layer where the electric field is zero.

Referring now to FIG. 4, Equation 19 was used to calculate that the zero or near-zero electric field point in the ZnS dielectric layer shown in FIG. 2A when exposed to EMR having a wavelength of 434 nm. The zero or near-zero electric field point was calculated to be 70 nm (instead of 77 nm for a 500 nm wavelength). In addition, a 15 nm thick Cr absorber layer was inserted at the thickness or distance of 70 nm from the Al reflector core layer to afford for a zero or near-zero electric field ZnS—Cr interface. Such an inventive structure allows light having a wavelength of 434 nm to pass through the Cr—ZnS interfaces, but absorbs light not having a wavelength of 434 nm. Stated differently, the Cr—ZnS interfaces have a zero or near-zero electric field with respect to light having a wavelength of 434 nm and thus 434 nm light passes through the interfaces. However, the Cr—ZnS interfaces do not have a zero or near-zero electric field for light not having a wavelength of 434 nm and thus such light is absorbed by the Cr absorber layer and/or Cr—ZnS interfaces and not reflected by the Al reflector layer.
It is appreciated that some percentage of light within +/−10 nm of the desired 434 nm will pass through the Cr—ZnS interface. However, it is also appreciated that such a narrow band of reflected light, e.g. 434+/−10 nm, still provides a sharp structural color to a human eye.
The result of the Cr absorber layer in the multilayer stack in FIG. 4 is illustrated in FIG. 5 where percent reflectance versus reflected EMR wavelength is shown. As shown by the dotted line, which corresponds to the ZnS dielectric layer shown in FIG. 4 without a Cr absorber layer, a narrow reflected peak is present at about 400 nm, but a much broader peak is present at about 550+ nm. In addition, there is still a significant amount of light reflected in the 500 nm wavelength region. As such, a double peak that prevents the multilayer stack from having or exhibiting a structural color is present.
In contrast, the solid line in FIG. 5 corresponds to the structure shown in FIG. 4 with the Cr absorber layer present. As shown in the figure, a sharp peak at approximately 434 nm is present and a sharp drop off in reflectance for wavelengths greater than 434 nm is afforded by the Cr absorber layer. It is appreciated that the sharp peak represented by the solid line visually appears as a sharp/structural color. Also, FIG. 5 illustrates where the width of a reflected peak or band is measured, i.e. the width of the band is determined at 50% reflectance of the maximum reflected wavelength, also known as full width at half maximum (FWHM).
Regarding omnidirectional behavior of the multilayer structure shown in FIG. 4, the thickness of the ZnS dielectric layer can be designed or set such that only the first harmonics of reflected light is provided. It is appreciated that this is sufficient for a “blue” color, however the production of a “red” color requires additional considerations. For example, the control of angular independence for red color is difficult since thicker dielectric layers are required, which in turn results in a high harmonic design, i.e. the presence of the second and possible third harmonics is inevitable. Also, the dark red color hue space is very narrow. As such, a red color multilayer stack has a higher angular variance.
In order to overcome the higher angular variance for red color, the instant application discloses a unique and novel design/structure that affords for a red color that is angular independent. For example, FIG. 6A illustrates a dielectric layer exhibiting first and second harmonics for incident white light when an outer surface of the dielectric layer is viewed from 0 and 45° relative to the normal of the outer surface. As shown by the graphical representation, low angular dependence (small Δλc) is provided by the thickness of the dielectric layer, however, such a multilayer stack has a combination of blue color (1st harmonic) and red color (2nd harmonic) and thus is not suitable for a desired “red only” color. Therefore, the concept/structure of using an absorber layer to absorb an unwanted harmonic series has been developed. FIG. 6A also illustrates an example of the location of the reflected band center wavelength (λc) for a given reflection peak and the dispersion or shift of the center wavelength (Δλc) when the sample is viewed from 0 and 45°.
Turning now to FIG. 6B, the second harmonic shown in FIG. 6A is absorbed with a Cr absorber layer at the appropriate dielectric layer thickness (e.g. 72 nm) and a sharp blue color is provided. Also, FIG. 6C illustrates that by absorbing the first harmonics with the Cr absorber at a different dielectric layer thickness (e.g. 125 nm) a red color is provided. However, FIG. 6C also illustrates that the use of the Cr absorber layer can result in more than desired angular dependence by the multilayer stack, i.e. a larger than desired Δλc.
It is appreciated that the relatively large shift in λc for the red color compared to the blue color is due to the dark red color hue space being very narrow and the fact that the Cr absorber layer absorbs wavelengths associated with a non-zero electric field, i.e. does not absorb light when the electric field is zero or near-zero. As such, FIG. 7A illustrates that the zero or non-zero point is different for light wavelengths at different incident angles. Such factors result in the angular dependent absorbance shown in FIG. 7B, i.e. the difference in the 0° and 45° absorbance curves. Thus in order to further refine the multilayer stack design and angular independence performance, an absorber layer that absorbs, e.g. blue light, irrespective of whether or not the electric field is zero or not, is used.
In particular, FIG. 8A shows a multilayer stack with a Cu absorber layer instead of a Cr absorber layer extending across a dielectric ZnS layer. The results of using such a “colorful” or “selective” absorber layer is shown in FIG. 8B which demonstrates a much “tighter” grouping of the 0° and 45° absorbance lines for the multilayer stack shown in FIG. 8A. As such, a comparison between FIG. 8B and FIG. 7B illustrates the significant improvement in absorbance angular independence when using a selective absorber layer rather than non-selective absorber layer.
Based on the above, a proof of concept multilayer stack structure was designed and manufactured. In addition, calculation/simulation results and actual experimental data for the proof of concept sample were compared. In particular, and as shown by the graphical plot in FIG. 9, a sharp red color was produced (wavelengths greater than 700 nm are not typically seen by the human eye) and very good agreement was obtained between the calculation/simulation and experimental light data obtained from the actual sample. Stated differently, calculations/simulations can and/or are used to simulate the results of multilayer stack designs according to one or more embodiments disclosed herein and/or prior art multilayer stacks.
FIG. 10 shows a plot of percent reflectance versus reflected EMR wavelength for another omnidirectional reflector design when exposed to white light at angles of 0 and 45° relative to the normal of the outer surface of the reflector. As shown by the plot, both the 0° and 45° curves illustrate very low reflectance, e.g. less than 10%, provided by the omnidirectional reflector for wavelengths less than 550 nm. However, the reflector, as shown by the curves, provides a sharp increase in reflectance at wavelengths between 560-570 nm and reaches a maximum of approximately 90% at 700 nm. It is appreciated that the portion or region of the graph on the right hand side (IR side) of the curve represents the IR-portion of the reflection band provided by the reflector.
The sharp increase in reflectance provided by the omnidirectional reflector is characterized by a UV-sided edge of each curve that extends from a low reflectance portion at wavelengths below 550 nm up to a high reflectance portion, e.g. >70%. A linear portion 200 of the UV-sided edge is inclined at an angle (β) greater than 60° relative to the x-axis, has a length L of approximately 40 on the Reflectance-axis and a slope of 1.4. In some instances, the linear portion is inclined at an angle greater than 70° relative to the x-axis, while in other instances is greater than 75°. Also, the reflection band has a visible FWHM of less than 200 nm, and in some instances a visible FWHM of less than 150 nm, and in other instances a visible FWHM of less than 100 nm. In addition, the center wavelength λc for the visible reflection band as illustrated in FIG. 10 is defined as the wavelength that is equal-distance between the UV-sided edge of the reflection band and the IR edge of the IR spectrum at the visible FWHM.
It is appreciated that the term “visible FWHM” refers to the width of the reflection band between the UV-sided edge of the curve and the edge of the IR spectrum range, beyond which reflectance provided by the omnidirectional reflector is not visible to the human eye. In this manner, the inventive designs and multilayer stacks disclosed herein use the non-visible IR portion of the electromagnetic radiation spectrum to provide a sharp or structural color. Stated differently, the omnidirectional reflectors disclosed herein take advantage of the non-visible IR portion of the electromagnetic radiation spectrum in order to provide a narrow band of reflected visible light, despite the fact that the reflectors may reflect a much broader band of electromagnetic radiation that extends into the IR region.
Referring now to FIG. 11, a plot of percent reflectance versus wavelength is shown for another seven-layer design omnidirectional reflector when exposed to white light at angles of 0 and 45° relative to the surface of the reflector. In addition, a definition or characterization of omnidirectional properties provided by omnidirectional reflectors disclosed herein is shown. In particular, and when the reflection band provided by an inventive reflector has a maximum, i.e. a peak, as shown in the figure, each curve has a center wavelength (λc) defined as the wavelength that exhibits or experiences maximum reflectance. The term maximum reflected wavelength can also be used for λc.
As shown in FIG. 11, there is shift or displacement of λc when an outer surface of the omnidirectional reflector is observed from an angle 45° (λc(45°)), e.g. the outer surface is tilted 45° relative to a human eye looking at the surface, compared to when the surface is observed from an angle of 0° ((λc(0°)), i.e. normal to the surface. This shift of λc (Δλc) provides a measure of the omnidirectional property of the omnidirectional reflector. Naturally a zero shift, i.e. no shift at all, would be a perfectly omnidirectional reflector. However, omnidirectional reflectors disclosed herein can provide a Δλc of less than 50 nm, which to the human eye can appear as though the surface of the reflector has not changed color and thus from a practical perspective the reflector is omnidirectional. In some instances, omnidirectional reflectors disclosed herein can provide a Δλc of less than 40 nm, in other instances a Δλc of less than 30 nm, and in still other instances a Δλc of less than 20 nm, while in still yet other instances a Δλc of less than 15 nm. Such a shift in Δλc can be determined by an actual reflectance versus wavelength plot for a reflector, and/or in the alternative, by modeling of the reflector if the materials and layer thicknesses are known.
Another definition or characterization of a reflector's omnidirectional properties can be determined by the shift of a side edge for a given set of angle refection bands. For example, and with reference to FIG. 11, a shift or displacement of a UV-sided edge (ΔSUV) for reflectance from an omnidirectional reflector observed from 0° (SUV(0°)) compared to the UV-sided edge for reflectance by the same reflector observed from 45° (SUV(45°)) provides a measure of the omnidirectional property of the omnidirectional reflector. It is appreciated that the shift of the UV-sided edge (ΔSUV) is and/or can be measured at the visible FWHM.
Naturally a zero shift, i.e. no shift at all (ΔSUV=0 nm), would characterize a perfectly omnidirectional reflector. However, omnidirectional reflectors disclosed herein can provide a ΔSUV of less than 50 nm, which to the human eye can appear as though the surface of the reflector has not changed color and thus from a practical perspective the reflector is omnidirectional. In some instances, omnidirectional reflectors disclosed herein can provide a ΔSUV of less than 40 nm, in other instances a ΔSUV of less than 30 nm, and in still other instances a ΔSUV of less than 20 nm, while in still yet other instances a ΔSUV of less than 15 nm. Such a shift in ΔSUV can be determined by an actual reflectance versus wavelength plot for a reflector, and/or in the alternative, by modeling of the reflector if the materials and layer thicknesses are known.
The shift of an omnidirectional reflection can also be measured by a low hue shift. For example, the hue shift of pigments manufactured from multilayer stacks according an aspect disclosed herein is 30° or less, as shown in FIG. 12 (see e.g., Δθ1), and in some instances the hue shift is 25° or less, preferably less than 20°, more preferably less than 15° and still more preferably less than 10°. In contrast, traditional pigments exhibit hue shift of 45° or more (see e.g. Δθ2). It is appreciated that the hue shift associated with Δθ1 generally corresponds to a red color, however the low hue shift is relevant for any color reflected by a hybrid omnidirectional structural color pigment disclosed herein.
A schematic illustration of an omnidirectional multilayer stack according to another aspect disclosed herein is shown in FIG. 13 at reference numeral 10. The multilayer stack 10 has a first layer 110 and a second layer 120. An optional reflector layer 100 can be included. Example materials for the reflector layer 100, sometimes referred to as a reflector core layer, can include but is not limited to Al, Ag, Pt, Cr, Cu, Zn, Au, Sn and combinations or alloys thereof. As such, the reflector layer 100 can be a metallic reflector layer, however this is not required. In addition, exemplary thicknesses for the core reflector layer range between 30 to 200 nm.
A symmetric pair of layers can be on an opposite side of the reflector layer 100, i.e. the reflector layer 100 can have another first layer oppositely disposed from the first layer 110 such that the reflector layer 100 is sandwiched between a pair of first layers. In addition, another second layer 120 can be oppositely disposed the reflector layer 100 such that a five-layer structure is provided. Therefore, it should be appreciated that the discussion of the multilayer stacks provided herein also includes the possibility of a mirror structure with respect to one or more central layers. As such, FIG. 13 can be illustrative of half of a five-layer multilayer stack.
In contrast to the aspects discussed above, the first layer 110 can be an absorber layer, e.g. a metal absorber layer with a thickness between 5-500 nm, inclusive. Also, the second layer can be a dielectric absorber layer with a thickness between 5-500 nm, inclusive. The metal absorber layer 110 can be made a colorful metal material such as Cu, bronze, brass, or a material such as amorphous Si, Ge, TiN and combinations thereof. The dielectric absorber layer 120 can be made from Fe2O3.
An aspect as shown in FIG. 13 and having dimensions as shown in Table 1 below exhibited the reflectance spectrum shown in FIG. 14. As shown in this figure, a Cu or its Alloys, or other colored reflectors like TiN layer 110 and an Fe2O3 dielectric absorber layer 120 with the thicknesses shown in Table 1 provided a reflectance spectrum with wavelengths less than generally 550-575 nm having less than 10-15% reflectance and wavelengths greater than generally 575-600 nm corresponding to a hue between 0-40°, preferably between 10-30°, on the a*b* Lab color space map. In addition, the chroma for the reflected band of visible light is greater than 70, preferably greater than 80, and more preferably equal to or greater than 90.









TABLE 1





Layer
Material
Thickness (nm)

















100
Al
80.0


110
Cu or alloys such as brass,
184.5



bronze, etc.


120
Fe2O3
28.6









The reflective spectrum of such a multilayer stack as shown in FIG. 13 is illustratively shown in FIG. 14 for viewing angles. 0 of 0° and 45°. As shown in the figure, the shift of the UV-sided edge (ΔSUV=SUV(0°)−SUV(45°)) at FWHM is less than 50 nm, preferably less than 30 nm, still more preferably less than 20 nm and yet still more preferably less than 10 nm. In combination with the width of the band in the visible spectrum, the shift of the reflected band between angles of 0 and 45° corresponds to a non-noticeable color change to the human eye.
FIG. 15 shows an absorption versus wavelength for the design shown in FIG. 13. As shown in this figure, the multilayer stack 10 absorbs over 80% of the visible light spectrum for wavelengths up to approximately 575 nm. In addition, the aspect 10 absorbs more than 40% of all wavelengths up to approximately 660 nm. As such, a combination of the metal absorbing layer 110 and dielectric absorbing layer 120 provides for a visible reflection band with a hue between 0-40°, preferably between 10-30°, on the a*b* Lab color space, i.e. reflected wavelengths in the red color spectrum.
A graphical representation for the aspect 10 is shown in FIG. 16 as a function of percent reflectance, wavelength that is reflected, and angle of viewing. As shown in this 3D contour plot, reflectance is very low, i.e. less than 20% for wavelengths between 400-550-575 nm and viewing angles between 0 to 45-50°. However, a sharp increase in percent reflectance is present at wavelengths between approximately 550-600 nm.
Another method or technique for describing the omnidirectional properties of the inventive multilayer stacks disclosed herein is a plot of chroma and hue versus viewing angle as shown in FIG. 17. FIG. 17 illustrates the reflection characteristics of the aspect shown in FIG. 13 in which the hue for angles between 0 and 45° is between 20-30 and has a change or shift of less than 10°, preferably less than 5°. In addition, the chroma is between 80-90 for all viewing angles between 0-45° where the chroma (C*) is defined as C*=√{square root over (a*2+b*2)} with a* and b* being the coordinates on the Lab color space or map for the color reflected by multilayer stack when exposed to broadband electromagnetic radiation, e.g. white light.
FIG. 18 shows or plots the hue of the aspect shown in FIG. 13 (see data point with arrow pointing thereto) on an a*b* Lab color space map. Also shown on the map is the region between 15-40°. It is appreciated that these two points are illustrated for a 0° viewing angle with respect to the normal of an outer surface of the multilayer stacks. In addition, it is appreciated that between viewing angles of 0-45°, that the hue for the aspect shown in FIG. 13 does not move outside of the 15-40° hue region. Stated differently, the aspect demonstrates a low hue shift, for example less than 30°, preferably less than 20°, and still more preferably less than 10°. It is still further appreciated that the aspects shown in FIG. 13 can also be designed such that a single band of visible light with a hue between 0-40° hue is provided and can be plotted on FIG. 18, and preferably a single band of visible light with a hue between 10-30°.
Turning now to FIG. 19, a process for manufacturing an omnidirectional high-chroma red structural color is shown generally at reference numeral 20. The process 20 includes dry depositing a reflective core layer at step 202, followed by dry depositing a metal absorber layer onto the dry deposited reflective core layer at step 210. Then, a dielectric absorber layer is dry deposited or wet deposited onto the metal absorber layer at step 220. It is appreciated that steps 210 and 220 can be repeated in order to produce additional layers onto the dry deposited reflective core layer. In addition, the dry deposited reflective core layer can be deposited onto the metal absorber layer, as can the wet deposited dielectric layer.
The above examples and aspects are for illustrative purposes only and changes, modifications, and the like will be apparent to those skilled in the art and yet still fall within the scope of the invention. As such, the scope of the invention is defined by the claims and all equivalents thereof.

## Claims

1. An omnidirectional high chroma red structural color comprising:
a multilayer stack having:
a reflective core layer;
a metal absorber layer extending across said reflective core layer; and
a dielectric absorber layer extending across said metal absorber layer;

said multilayer stack reflecting a single band of visible light with a hue between 0-40° on an a*b* Lab color map, said single band of visible light having a hue shift within said 0-40° on said a*b* Lab color map when viewed from all angles between 0-45° normal to an outer surface of said multilayer stack.

2. The omnidirectional high chroma red structural color of claim 1, wherein said hue is between 10-30° and said hue shift is within said 10-30° on said a*b* Lab color map.

3. The omnidirectional high chroma red structural color of claim 1, wherein said reflective core layer has a thickness between 50-200 nanometers, inclusive.

4. The omnidirectional high chroma red structural color of claim 3, wherein said reflective core layer is made from a reflective metal selected from the group consisting of Al, Ag, Pt, Sn and combinations thereof.

5. The omnidirectional high chroma red structural color of claim 3, wherein said reflective core layer is made from a colorful metal selected from the group consisting of Au, Cu, brass, bronze and combinations thereof.

6. The omnidirectional high chroma red structural color of claim 3, wherein said metal absorber layer has a thickness between 5-500 nanometers, inclusive.

7. The omnidirectional high chroma red structural color of claim 6, wherein said metal absorber layer is made from the group consisting of Cu, bronze, brass, amorphous Si, Ge, TiN and combinations thereof.

8. The omnidirectional high chroma red structural color of claim 6, wherein said dielectric absorber layer has a thickness between 5-500 nanometers, inclusive.

9. The omnidirectional high chroma red structural color of claim 8, wherein said dielectric absorber layer is made from Fe2O3.

10. The omnidirectional high chroma red structural color of claim 6, wherein said reflective core layer is a central reflective core layer and said metal absorber layer is a pair of metal absorber layers extending across opposite sides of said central reflective core layer, said central reflective core layer sandwiched between said pair of metal absorber layers.

11. The omnidirectional high chroma red structural color of claim 10, wherein said dielectric absorber layer is a pair of dielectric absorber layers, said central reflective core layer and said pair of metal absorber layers sandwiched between said pair of dielectric absorber layers.

12. A process for making an omnidirectional high chroma red structural color, the process comprising:
manufacturing a multilayer stack by:
dry depositing a reflective core layer;
dry depositing a metal absorber layer that extends across the reflective core layer;
dry or wet depositing a dielectric absorber layer that extends across the metal absorber layer; and
the multilayer stack reflecting visible light with a hue between 0-40° on an a*b* Lab color map and having a hue shift within the 0-40° on the a*b* Lab color map when viewed from all angles between 0-45° normal to an outer surface of the multilayer stack.

13. The process of claim 12, wherein the multilayer stack reflects visible light with a hue between 10-30° on the a*b* Lab color map and has the hue shift between the 10-30° on the a*b* Lab color map.

14. The process of claim 12, wherein the reflective core layer has a thickness between 50-200 nanometers, inclusive.

15. The process of claim 14, wherein said reflective core layer is made from a reflective metal selected from the group consisting of Al, Ag, Pt, Sn and combinations thereof.

16. The process of claim 14, wherein the reflective core layer is made from a colorful metal selected from the group consisting of Au, Cu, brass, bronze and combinations thereof.

17. The process of claim 14, wherein the metal absorber layer has a thickness between 5-500 nanometers, inclusive.

18. The process of claim 17, wherein the metal absorber layer is made from the group consisting of Cu, bronze, brass, amorphous Si, Ge, TiN and combinations thereof.

19. The process of claim 17, wherein the dielectric absorber layer has a thickness between 5-500 nanometers, inclusive, and is made from Fe2O3.