Optical security device providing color switching or image switching effect
An optical security feature has pixels with faceted microstructures supporting nano-patterned optical filters. Each of the pixels includes a substrate and a nanostructure. The substrate is configured to emit light of at least a first wavelength and a second wavelength different than the first wavelength. The microstructure includes a first facet, a second facet non-parallel to the first facet, a first nano-patterned optical filter disposed on the first facet, and a second nano-patterned optical filter disposed on the second facet. The first nano-patterned optical filter includes a first stopband that includes the first wavelength and excludes the second wavelength. The second nano-patterned optical filter includes a second stopband that includes the second wavelength and excludes the first wavelength. For each pixel, the first nano-patterned optical filter inhibits emission of light at the first wavelength from the pixel via the first facet.
1. An optical security feature comprising:
a plurality of microstructure pixels, wherein at least one of the microstructure pixels comprises:
a substrate portion that emits light of at least a first wavelength and a second wavelength different from the first wavelength; and
a microstructure comprising a first facet and a second facet non-parallel to the first facet, the microstructure comprising:
a first nano-patterned optical filter provided on the first facet, the first nano-patterned optical filter having a first stopband that includes the first wavelength and does not include the second wavelength; and
a second nano-patterned optical filter provided on the second facet, the second nano-patterned optical filter having a second stopband that includes the second wavelength and does not include the first wavelength.
2. The optical security feature of claim 1 , wherein the substrate portion comprises a fluorescent material that when irradiated by UV light emits light of the first wavelength as well as light of the second wavelength.
3. The optical security feature of claim 1 , wherein the one microstructure pixel when irradiated with UV radiation and viewed from a first viewing angle normal to a major surface of the first facet emits more light from the first facet of the second wavelength than it emits light of the first wavelength, and wherein the one microstructure pixel when irradiated with UV radiation and viewed from a second viewing angle normal to a major surface of the second facet emits more light from the second facet of the first wavelength than it emits light of the second wavelength.
4. The optical security feature of claim 1 , wherein the one microstructure pixel when irradiated with UV radiation and viewed from a first viewing angle normal to a major surface of the first facet emits a first aggregate amount of light in a first wavelength range and emits a second aggregate amount of light in a second wavelength range, wherein the second aggregate amount is less than 60% of the first aggregate amount,
and wherein the one microstructure pixel when irradiated with UV radiation and viewed from a second viewing angle normal to a major surface of the second facet emits a first aggregate amount of light in a first wavelength range and emits a second aggregate amount of light in a second wavelength range, wherein the first aggregate amount is less than 60% of the second aggregate amount.
5. The optical security feature of claim 1 , wherein the one microstructure pixel when the viewed at a first viewing angle appears a first color, and when viewed at a second viewing angle appears a second color.
6. The optical security feature of claim 1 , wherein the microstructure comprises a transparent material substantially transparent to light of the first wavelength, light of the second wavelength, and to UV radiation.
7. The optical security feature of claim 6 , wherein the transparent material is a resin.
8. The optical security feature of claim 1 , wherein the first nano-pattered optical filter comprises a plurality of nanoholes.
9. The optical security feature of claim 1 , wherein the first nano-pattered optical filter comprises a plurality of nanoposts.
10. The optical security feature of claim 1 , wherein the first nano-patterned optical filter comprises a first lattice of nanostructures having a first lattice periodicity, P1, a first nanostructure dimension, D1, selected according to the first stopband, and wherein the second nano-patterned optical filter comprises a second lattice of nanostructures having a second lattice periodicity, P2, and a second nanostructure dimension, D2, selected according to the second stopband.
11. The optical security feature of claim 10 , wherein a first ratio D1/P1 and a second ratio D2/P2 are within the range of 0.1 to 0.9.
12. The optical security feature of claim 10 , wherein a first ratio D1/P1 and a second ratio D2/P2 are within the range of 0.25 to 0.75.
13. The optical security feature of claim 1 , wherein the angle between the surface normal of the first facet and the surface normal of the second facet is at least 25°.
14. The optical security feature of claim 1 , wherein the microstructure comprises more than two facets, each facet being non-parallel to the other facets, and wherein each facet comprises a nano-patterned optical filter that is configured with a stopband that is different than stopbands of the other facets such that a unique color is emitted along the viewing angle of each of the facets.
15. A method comprising:
providing a microstructure on a substrate, wherein the substrate includes a UV fluorescent material that when irradiated with UV radiation emits visible light of a first wavelength and a second wavelength, wherein the microstructure has a first facet and a second facet;
providing a first nano-patterned optical filter on the first facet, wherein the first nano-patterned optical filter has a first stopband that includes the first wavelength and does not include the second wavelength; and
providing a second nano-patterned optical filter on the second facet, wherein the second nano-patterned optical filter has a second stopband that includes the second wavelength and does not include the first wavelength.
16. The method of claim 15 , wherein the substrate is disposed on a banknote.
17. A method comprising:
embossing a microstructure into a resin layer, wherein the resin layer is disposed on a substrate, and wherein the substrate includes a UV fluorescent material that when irradiated with UV radiation emits visible light of a first wavelength and a second wavelength, wherein the microstructure has a first facet and a second facet;
forming a first nano-patterned optical filter on the first facet, wherein the first nano-patterned optical filter has a first stopband that includes the first wavelength and does not include the second wavelength; and
forming a second nano-patterned optical filter on the second facet, wherein the second nano-patterned optical filter has a second stopband that includes the second wavelength and does not include the first wavelength.
18. The method of claim 17 , further comprising:
attaching the substrate with the microstructure disposed on the substrate onto a banknote.
19. The method of claim 17 , wherein the first stopband is a first range of wavelengths of visible light, wherein the second stopband is a second range of wavelengths of visible light, and wherein the first and second stopbands do not overlap.
20. The method of claim 17 , wherein the first nano-patterned optical filter comprises a plurality of nanoposts.