Dual frequency switchable cholesteric liquid crystal light shutter and driving waveform

• US 5,877,826 A
• Filed: 02/06/1997
• Issued: 03/02/1999
• Est. Priority Date: 02/06/1997
• Status: Expired due to Term

• Current Assignee: Kent State University
• Original Assignee: Kent State University
• Inventors: Xu, Ming, Yang, Deng-Ke
• Primary Examiner(s): Sikes, William L.
• Assistant Examiner(s): CHOWDHURY, TARIFUR RASHID
• Application Number:
  US08/796,228
• Time in Patent Office: 754 Days
• Field of Search: 349/36, 349/170, 349/98
• US Class Current: 349/36
• CPC Class Codes: C09K 19/0208 Twisted Nematic (T.N.); Super Twisted Nematic (S.T.N.); Optical Mode Interference (O.M.I.)
• CPC Class Codes: G02F 1/13473 for wavelength filtering or for colour display without the use of colour mosaic filters
• CPC Class Codes: G02F 1/13718 based on a change of the texture state of a cholesteric liquid crystal

Abstract

A liquid crystal shutter and a drive waveform for controlling the same is presented by this invention. The liquid crystal shutter includes two distinct layers of liquid crystal material where one layer has a right hand twist sense and the other layer has a left hand twist sense. An electric field is selectively applied to the layers of liquid crystal material which have dual frequency properties. Accordingly, when a low frequency is applied to the liquid crystal materials they exhibit a positive dielectric anisotropy and a homeotropic texture which is substantially transparent. When a high frequency electric field is applied to the liquid crystal materials they exhibit a negative dielectric anisotropy and a planar texture which is substantially reflective. The liquid crystal material may be selected to selectively reflect predetermined spectrums of light. By alternatingly applying low and high frequencies to the liquid crystal shutter, the transition time between the transparent and reflective states is minimized.

Claims

1. A liquid crystal shutter, comprising:

a cholesteric liquid crystal material disposed between first and second substrates comprising a first layer having a first twist sense and a second layer having a second twist sense opposite said first twist sense, wherein a third substrate is interposed between said first and second substrates to separate said first and second layers; and

means for simultaneously addressing said first and second layers with at least two distinct ranges of frequency of an electric filed, wherein application of a low range of frequency between said first and third substrates and between said second and third substrates causes said layers to exhibit a positive dielectric anisotropy and wherein application of a high range of frequency between said first and third substrates and between said second and third substrates causes said layers to exhibit a negative dielectric anisotropy, wherein said negative dielectric anisotropy causes said material to be switched to a state which reflects both right and left circularly polarized light incident thereto within a predetermined spectrum of light while allowing remaining spectrums of light to be transmitted through said substrates, and wherein said positive dielectric anisotropy causes said material to be switched to a state which is transparent.

2. The liquid crystal shutter according to claim 1, wherein said material exhibits a homeotropic texture at said range of low frequency.

3. The liquid crystal shutter according to claim 1, wherein said material exhibits a planar texture at said range of high frequency.

4. The liquid crystal shutter according to claim 1, wherein the transition time of said material from transparent to reflective is less than 1 second.

5. The liquid crystal shutter according to claim 1, wherein said means for simultaneously addressing alternatingly applies a range of different low and high frequencies that causes said material to evolve from a transparent to a selectively reflective state.

6. The liquid crystal shutter according to claim 5, wherein the transition time of said material from transparent to reflective is less than 100 milliseconds.

7. The liquid crystal shutter according to claim 1, wherein said means for addressing provides a transition time between positive and negative dielectric anisotropies of between about 50 milliseconds and about 1 second.

8. A method for addressing a liquid crystal shutter comprising the steps of:

providing a cholesteric liquid crystal material between first and second substrates, said material having a first layer with a first rotational twist sense and a second layer with a second rotational twist sense;

providing a third substrate between said first and second substrates to separate said first layer from said second layer;

simultaneously applying a first frequency electric field between said first and third substrates and between said second and third substrates to cause said first and second layers to obtain a first texture;

simultaneously applying a second frequency electric field between said first and third substrates and between said second and third substrates to cause said first and second layers to obtain a second texture.

9. The method according to claim 8, wherein said step of simultaneously applying a first frequency comprises the step of;

applying a range of low frequency to said first and second layers to cause both said layers to become substantially transparent.

10. The method according to claim 9, further comprising the step of exhibiting a homeotropic texture when said range of low frequency is applied to said material.

11. The method according to claim 8, wherein said step of simultaneously applying a second frequency comprises the step of:

applying a range of high frequency to said first and second layers to cause both said layers to reflect both right and left circularly polarized light within a predetermined spectrum of light whiled allowing remaining spectrums of light to be transmitted.

12. The method according to claim 11, further comprising the step of exhibiting a planar texture when said range of high frequency is applied to said material.

13. The method according to claim 8, wherein said steps of simultaneously applying further comprise the steps of:

applying said range of low frequency electric field to cause both said layers to exhibit a positive dielectric anisotropy and appear substantially transparent; and

applying said range ofhigh frequency electric field to cause both said layers to exhibit a negative dielectric anisotropy and to reflect both right and left circularly polarized light within a predetermined spectrum of light while allowing remaining spectrums of light to be transmitted.

14. The method according to claim 13, further comprising the step of:

providing a transition time of about 1 second or less between said positive dielectric anisotropy and said negative dielectric anisotropy.

15. The method according to claim 8, wherein said steps for simultaneously applying further comprise the steps of:

applying a first low frequency electric field to said layers to obtain a substantially transparent homeotropic texture;

applying a first high frequency electric field to said layers to obtain a transient planar texture;

applying a second different low frequency electric field to said layers to initiate transition from a transient planar texture; and

applying a second different high frequency electric field to said layers to obtain a substantially reflective planar texture.

16. The method according to claim 15, further comprising the step of:

providing a transition time of about 100 milliseconds or less between said substantially transparent homeotropic texture and said substantially reflective planar texture.

17. The method according to claim 8, further comprising the step of:

providing a transition time of about 50 milliseconds or less between said first texture and said second texture.