REFLECTIVE LC DEVICES INCLUDING THIN FILM METAL GRATING

• US 20150234221A1
• Filed: 02/19/2015
• Published: 08/20/2015
• Est. Priority Date: 02/19/2014
• Status: Active Grant

• Current Assignee: Lumentum Operations, LLC (Lumentum Holdings Inc.)
• Original Assignee: Viavi Solutions, Inc. (Lumentum Holdings Inc.)
• Inventors: MILLER, John Michael, ANDERSON, Keith, DJIE, Hery, TIAN, Lu
• Granted Patent: US 9,588,374 B2
• Time in Patent Office: Days
• US Class Current: 1/1
• CPC Class Codes: G02B 5/3058 comprising electrically conductive elements, e.g. wire grids, conductive particles
• CPC Class Codes: G02F 1/133553 Reflecting elements associated to illuminating devices G02F1/133605
• CPC Class Codes: G02F 1/133567 on the back side
• CPC Class Codes: G02F 1/13363 Birefringent elements, e.g. for optical compensation
• CPC Class Codes: G02F 1/133638 Waveplates, i.e. plates with a retardation value of lambda/n
• CPC Class Codes: G02F 1/134309 characterised by their geometrical arrangement
• CPC Class Codes: G02F 1/13439 characterised by their electrical, optical, physical properties; materials therefor; method of making
• CPC Class Codes: G02F 2201/305 diffraction grating
• CPC Class Codes: G02F 2201/307 Reflective grating, i.e. Bragg grating
• CPC Class Codes: G02F 2203/50 Phase-only modulation

Abstract

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.

Claims

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, wherein the separation layer is substantially transparent at an operating wavelength of the LC device;

a sub-wavelength metal grating disposed over the separation layer, the sub-wavelength metal grating comprised of 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, including a grating cap layer disposed over the parallel metal strips of the grating and providing a flat surface for the LC layer.

3. The LC device of claim 2, wherein the grating cap layer comprises dielectric material that extends into gaps between the parallel metal strips.

4. The LC device of claim 1, wherein the plurality of 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 the 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 the range of 0.6 to 1.0 μ

m, and the metal strips each have a width in the range of 0.2 to 0.4 μ

m and a thickness in the range of 0.03 to 0.07 μ

m.

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 metal strips of the sub-wavelength metal grating, whereby sensitivity of the LC device to a state of polarization of the light is lessened.

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 pixel electrodes, wherein the 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.

13. A method for fabricating a liquid crystal (LC) device, comprising:

a) depositing a spacer layer of an optically transparent material over a reflective top face of a substrate, the substrate comprising a first electrode having a flat top surface, and an electrode passivation layer disposed over the flat top surface of the first electrode so as to form the reflective top face of the substrate;

b) forming a sub-wavelength metal grating over the spacer layer, the sub-wavelength metal grating comprising a plurality of parallel, spaced apart metal strips;

c) depositing a grating cap layer over the sub-wavelength metal grating;

d) disposing an LC layer having a director over the grating capping layer; and

e) disposing a transparent second electrode over the LC layer in opposition to the first electrode, 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.

14. The method of claim 13, wherein a) comprises depositing the spacer layer of a thickness that is selected so that the spacer layer and the electrode passivation layer have a combined optical thickness that provides a substantially half wave plate double-pass retardance for the light impinging upon the sub-wavelength metal grating from the second electrode.

15. The method of claim 13, wherein b) comprises:

depositing a metal film over the spacer layer, and

patterning the metal film to form a plurality of parallel spaced apart metal strips.

16. The method of claim 13, wherein b) comprises forming the plurality of parallel, spaced apart metal strips using a lift-off process.

17. The method of claim 13, wherein the electrode passivation layer and the spacer layer are of the same optically transparent material selected from a group consisting of:

SiO2, Si3N4, Al2O3, TiO2, Ta2O5, and HfO2.

18. The method of claim 14, wherein a) comprises determining a target thickness of the spacer layer based on a thickness of the electrode passivation layer and a target retardance value of a reflective waveplate formed by the first electrode and the sub-wavelength metal grating.

19. The method of claim 13, wherein the first electrode comprises a plurality of pixel electrodes separated by electrode gaps, wherein the substrate comprises a CMOS substrate comprising electrical circuitry for individually addressing the pixel electrodes, and wherein b) comprises forming gaps across the metal strips over the electrode gaps so that portions of the metal strips that are disposed directly over adjacent pixel electrodes are electrically separate from each other.

20. The method of claim 13, wherein b) comprises forming the plurality of parallel, spaced apart metal strips with a pitch in the range of 0.6 to 1.0 μ

m, each of the metal strips having a width in the range of 0.2 to 0.4 μ

m and a thickness in the range of 0.03 to 0.07 μ

m.

21. The method of claim 13 wherein the electrode passivation layer has a thickness in the range of 0.05 to 0.15 μ

m, and wherein a) comprises depositing the spacer layer having a thickness in the range of 0.1 to 0.25 μ

m, and b) comprises depositing a metal film having a thickness in the range of 0.03 to 0.07 μ

m.