Patent ID: 7199928

Claim:
An optical element comprising at least one cavity which includes: a first composite stack layer having a first mirror stack layer formed by alternately stacking a plurality of first and second dielectric thin films having respectively different refractive indexes and a first conductive film formed on the first mirror stack layer; a second composite stack layer having a second mirror stack layer formed by alternately stacking a plurality of first and second dielectric thin films having respectively different refractive indexes and a second conductive film formed on the second mirror stack layer; a spacer layer placed between said first and second composite stack layers and made of a plane dielectric member having at least either primary or secondary electrooptic effect; and a conductive buffer layer formed between at least one of said first composite stack layer and said second composite stack layer and said spacer layer; wherein said first mirror stack layer is formed by stacking a first dielectric thin film which is made of a high refractive index material and has a physical film thickness of a ¼-wavelength of a design wavelength and a second dielectric thin film which is made of a low refractive index material having a refractive index lower than the high refractive index material and has a physical film thickness of the ¼-wavelength, alternately at least one time, said second mirror stack layer is formed by stacking a first dielectric thin film which is made of the high refractive index material and has the physical film thickness of the ¼-wavelength and a second dielectric thin film which is made of the low refractive index material having the refractive index lower than the high refractive index material and has the physical film thickness of the ¼-wavelength, alternately at least one time, said spacer layer is placed between said first and second mirror stack layers and is made of a dielectric thin film having a film thickness of positive even-number times the physical film thickness of the ¼-wavelength and having a primary or secondary electrooptic effect, said first conductive film is made of a conductive thin film which has a film thickness of positive odd-number times the physical film thickness of the ¼-wavelength, and said second conductive film is made of a transparent conductive thin film which has a film thickness of positive odd-number times the physical film thickness of the ¼-wavelength.