TEMPERATURE ACTIVATED OPTICAL FILMS
The present invention discloses a multilayer dielectric optical structure wherein one of the optical materials in the multilayer structure shows an optically isotropic state above and a birefringent state below a characteristic temperature Tc near the room temperature. The optical structure reflects a predetermined wavelength range of electromagnetic radiation above the Tc but allow the same to transmit through below the Tc. The predetermined wavelength can be the near infrared radiation from 700 nm to 2500 nm, and the optical structure rejects solar heat in warm summer days but admits the same to interior on a colder winter day.
1 . A multilayer dielectric optical structure, comprising:
a transparent substrate; and
a plurality of alternating first layers and second layers adjacent to the transparent substrate, wherein the first layers comprise a first optical material having a first and second optical axis and a first and second refractive index along the first and second optical axes, and wherein the second layers comprise a second optical material having a first and second refractive index along a first and second optical axis below a characteristic transition temperature (Tc) and a third refractive index along either the first or second optical axis above Tc, wherein the second optical material includes a liquid crystal material having an ordered-to-isotropic phase transition temperature between about 15 and 35 degrees Celsius, wherein the first optical axes of the first material and the second material are substantially parallel, the second optical axes of the first material and the second material are substantially parallel, and the first and second refractive indices of the first material are substantially equal to the first and second refractive indices of the second material, and wherein the third refractive index differs from the second refractive index and from the first refractive index,
wherein the optical thickness of each of the first layers is equal to ¼ times a predetermined wavelength of light and the optical thickness of each of the second layers is equal to ¼ times the predetermined wavelength when the temperature is above Tc.
2 . The optical structure of claim 1 , wherein the first optical material includes uniaxial birefringent optical plastic films.
3 . The optical structure of claim 1 , wherein the second optical material contains rod-shaped or discotic liquid crystals.
4 . The optical structure of claim 1 , wherein the first and/or second optical materials are formed by thermal or photochemical polymerization.
5 . The optical structure of claim 1 , wherein the transparent substrate comprises:
a support material; and
an optical alignment material having an optical axis that is determined upon exposure to polarized light.
6 . The optical structure of claim 5 , wherein the optical axis of a given layer is configured to be determined by irradiation of a previous layer with polarized light.
7 . The optical structure of claim 5 , wherein the optical axis is determined by a previous layer not adjacent to the given layer.
8 . The optical structure of claim 1 , wherein the number of first layers is between 1 and 1000.
9 . The optical structure of claim 1 , wherein the first optical material comprises a semi-crystalline polymer.
10 . The optical structure of claim 1 , wherein the first optical material comprises polyethylene terephthalate.
11 . An optical system, comprising:
a transparent substrate;
a plurality of dielectric pairs, wherein each pair comprises a first layer of a first optical material having a first refractive index and a second layer having a liquid crystal material having a second refractive index that is equal to the first refractive index when the temperature is below a transition temperature and different than the first refractive index when the temperature is above the transition temperature, wherein the optical thickness of the first layer (d 1 ) equals the optical thickness of the second layer which equals ¼ of a predetermined wavelength λ 0 , wherein the number of dielectric pairs is between 1 and 500; and
the optical thickness of each adjacent pair is in accordance with the formula:
d 1 =¼λ 0
d N+1 =r d N
wherein d N is the optical thickness of the Nth pair of layers, equal to the sum of the optical thicknesses of the two layers in the pair, and r is a number between 0.85 and 0.999.
12 . The optical system of claim 11 , wherein the second layer includes a polymer mesh at less than 20 weight % for maintaining mechanical stability and for limiting the mobility of the liquid crystal material.
13 . The optical system of claim 11 , wherein the second layer includes a discotic liquid crystal material.
14 . An optical structure, comprising:
a transparent substrate; and
a film stack having plurality of alternating first layers and second layers adjacent to the transparent substrate, the first layers having a first optical material and the second layers having a second optical material, the second optical material having an ordered-to-isotropic phase transition temperature (Tc) between about 15 and 35 degrees Celsius,
wherein below Tc the first refractive indices between the first and second layers are matched, and wherein above Tc the first refractive indices between the first and second layers are mismatched, and
wherein the optical thickness of each of the first layers is equal to ¼ times a predetermined wavelength of light and the optical thickness of each of the second layers is equal to ¼ times the predetermined wavelength when the temperature is above Tc.
15 . The optical structure of claim 14 , wherein the first layers include first and second refractive indices along first and second optical axes in the plane of each of the first layers, and the second layers include first and second refractive indices along first and second optical axes in the plane of each of the first layers.
16 . The optical structure of claim 14 , wherein the first and/or second layers are birefringent.
17 . The optical structure of claim 14 , wherein the second layers include a polymer mesh at less than about 20 weight %.
18 . The optical structure of claim 14 , wherein the second layer is formed of a liquid crystal material that is in a nematic liquid crystal phase below Tc and an isotropic phase above Tc.
19 . The optical structure of claim 14 , wherein the second layer includes a polymer mesh formed from substantially the same material as the first optical material.
20 . The optical structure of claim 14 , wherein the first layers are formed of a uniaxial birefringent optical plastic material.
21 . The optical structure of claim 14 , wherein the first layers include a stretch induced optical anisotropic material.
22 . The optical structure of claim 14 , wherein the first optical material is isotropic.