Reflective LC devices including thin film metal grating
View Patent ↗A sub-wavelength thin-film metal grating is placed inside a liquid crystal variable optical retarder at a selected distance from a reflective electrode to form a reflective half wave plate, thereby reducing polarization dependence of the optical retardation generated by the variable optical retarder. The approach enables to form within the device the reflective half wave plate that is suitably thin without modifying the reflective electrode of the device.
1. A liquid crystal (LC) device comprising:
a first electrode having a reflective top surface;
a separation layer disposed over the reflective top surface of the first electrode and between the first electrode and a sub-wavelength metal grating, wherein
the separation layer is substantially transparent at an operating wavelength of the LC device and
the sub-wavelength metal grating is disposed over the separation layer, the sub-wavelength metal grating comprising a plurality of parallel metal strips spaced apart from each other and extending along the reflective top surface of the first electrode at a pre-determined distance therefrom defined by the separation layer so as to form a reflective form-birefringent (FB) waveplate with the reflective top surface of the first electrode;
an LC layer disposed over the sub-wavelength metal grating; and
a second electrode disposed over the LC layer in opposition to the first electrode, wherein the second electrode is transparent at the operating wavelength so that the LC layer imparts a variable optical phase shift to light impinging on the second electrode when a voltage is applied between the first and second electrodes.
2. The LC device of claim 1 , further comprising:
a grating cap layer, disposed over the plurality of parallel metal strips, for providing a flat surface for the LC layer.
3. The LC device of claim 2 , wherein the grating cap layer comprises a dielectric material that extends into gaps between the plurality of parallel metal strips.
4. The LC device of claim 1 , wherein the plurality of parallel metal strips comprise thin film metal strips that are thinner than the separation layer.
5. The LC device of claim 1 , wherein the separation layer has a thickness providing a target optical retardance for light impinging upon the sub-wavelength metal grating from the second electrode.
6. The LC device of claim 1 , wherein the separation layer has an optical thickness of substantially a quarter of the operating wavelength.
7. The LC device of claim 1 , wherein the separation layer has an optical thickness in a range of 0.22*λ to 0.3*λ, wherein λ is the operating wavelength.
8. The LC device of claim 1 , wherein the sub-wavelength metal grating has a pitch in a range of 0.6 to 1.0 μm, and the plurality of parallel metal strips each have a width in a range of 0.2 to 0.4 μm and a thickness in a range of 0.03 to 0.07 μm.
9. The LC device of claim 1 , wherein the separation layer comprises an electrode passivation layer disposed over the first electrode and a spacer layer disposed over the electrode passivation layer.
10. The LC device of claim 1 , wherein a director of the LC layer forms an acute angle with the plurality of parallel metal strips.
11. The LC device of claim 1 , wherein the first electrode comprises a plurality of individually addressable pixel electrodes separated by electrode gaps.
12. The LC device of claim 11 , further comprising:
a substrate supporting the first electrode, wherein the substrate comprises circuitry for independently applying a voltage to each of the plurality of individually addressable pixel electrodes, and wherein the plurality of parallel metal strips have gaps located above the electrode gaps so as to electrically separate portions of the sub-wavelength metal grating located above adjacent pixel electrodes of the plurality of individually addressable pixel electrodes.