Narrow band omnidirectional reflectors and their use as structural colors
Disclosed is a multilayer structure wherein a first layer of a first material having an outer surface and a refracted index between 2 and 4 extends across an outer surface of a second layer having a refractive index between 1 and 3. The multilayer stack has a reflective band of less than 200 nanometers when viewed from angles between 0° and 80° and can be used to reflect a narrow range of electromagnetic radiation in the ultraviolet, visible and infrared spectrum ranges. In some instances, the reflection band of the multilayer structure is less than 100 nanometers. In addition, the multilayer structure can have a quantity defined as a range to mid-range ratio percentage of less than 2%.
1. A multilayer stack, said stack comprising:
a first layer of a first material having an outer surface, a first predefined thickness d H and a refractive index n H between 1.75 and 3.0; and
a second layer of a second material having an outer surface, a second predefined thickness d L and a refractive index n L between 1.0 and 3.0, said outer surface of said second layer extending across said outer surface of said first layer, said first layer in direct contact with and extending across said second layer;
said first predefined thickness d H and said second predefined thickness d L predefined such that said multilayer stack has a reflection peak of less than 200 nanometers and a dispersion of a center wavelength Δλ c of said reflection peak defined as:
Δ
λ
c
=
λ
c
θ
0
=
0
0
-
λ
c
θ
0
=
45
0
=
2
(
n
H
d
H
1
+
n
L
d
L
1
-
n
H
d
H
1
-
n
0
2
n
H
2
-
n
L
d
L
1
-
n
0
2
n
L
2
)
is less than 200 nm when said multilayer stack is viewed from 0 degrees and 45 degrees and where λ c at θ o =0° is a center wavelength of said reflection band when said multilayer stack is viewed from 0°, λ c at θ o =45° is a center wavelength of said reflection band when said multilayer stack is viewed from 45° and n o is a refractive index of a medium beyond said multilayer structure.
2. The multilayer stack of claim 1 , wherein said reflection peak is less than 75 nanometers when viewed from angles between 0 degrees and 45 degrees.
3. The multilayer stack of claim 1 , wherein said reflection peak is less than 50 nanometers when viewed from angles between 0 degrees and 45 degrees.
4. The multilayer stack of claim 1 , wherein said first layer has a refractive index between 2.2 and 2.4 and said second layer has a refractive index between 1.9 and 2.1.
5. The multilayer stack of claim 1 , wherein a difference between said refractive index of said first layer and said refractive index of said second layer is between 0.1 and 0.5.
6. The multilayer stack of claim 1 , wherein a difference between said refractive index of said first layer and said refractive index of said second layer is between 0.2 and 0.4.
7. The multilayer stack of claim 1 , wherein said stack has a range to mid-range ratio percentage between a value greater than zero and 10%.
8. The multilayer stack of claim 1 , wherein said stack has a range to mid-range ratio percentage between a value greater than zero and 5%.
9. The multilayer stack of claim 1 , wherein said stack has a range to mid-range ratio percentage between a value greater than zero and 2%.
10. The multilayer stack of claim 1 , wherein said stack has more than three total layers.
11. The multilayer stack of claim 1 , wherein said stack has more than seven total layers.
12. The multilayer stack of claim 1 , wherein said stack has more than fifteen total layers.
13. The multilayer stack of claim 1 , wherein said stack has more than twenty-five total layers.
14. The multilayer stack of claim 1 , wherein said first layer is TiO 2 having a reflective index of generally 2.3 and said second layer is HfO 2 having a reflective index of generally 2.0.
15. The multilayer stack of claim 1 , wherein said stack is in the form of a flake.
16. The multilayer stack of claim 15 , wherein said flake is mixed with a binder to make a paint.
17. The multilayer stack of claim 15 , wherein said flake is mixed with a binder to make a UV-protective coating.