WO2002048802A9

WO2002048802A9 - Switchable holographic device

Info

Publication number: WO2002048802A9
Application number: PCT/US2001/048457
Authority: WO
Inventors: Jennifer Kong Colegrove; Sihai Gary G Q Qian
Original assignee: Digilens Inc; Jennifer Kong Colegrove; Sihai Gary G Q Qian
Priority date: 2000-12-15
Filing date: 2001-12-14
Publication date: 2002-09-06
Also published as: WO2002048802A1; AU2002236631A1

Abstract

A switchable holographic diffraction device comprises holographic fringes (14) formed by liquid crystal droplets (15) dispersed in a medium. The droplets are composed of at least partly of a non-nematic (preferably cholesteric) liquid crystal material. The fringes are switchable between a first condition in which the liquid crystal molecules are oriented randomly with respect to a direction of propagation of radiation through the device (Figure 3), and a second condition in which the liquid crystal molecules are oriented uniformly with respect to said direction of radiation propagation (Figure 4).

Description

Switchable Holographic Device

Field of the invention

This invention relates to a switchable holographic device, such as is used (for example) in optical displays or in optical telecommunications systems.

Background to the invention

Switchable holographic devices are typically composed of a holographic diffraction grating recorded in a polymer-dispersed liquid crystal material. During the hologram recordal process, the material undergoes a phase separation to create regions densely populated by liquid crystal microdroplets interspersed by regions of clear polymer. When an electric field is applied to the hologram by way of electrodes, the orientation of the liquid crystal molecules is changed. This in turn causes a reduction in the refractive index modulation of the holographic fringes, thereby reducing the diffraction efficiency of the fringes to a very low level, effectively erasing the hologram. In the case where the holographic diffraction grating is in the form of a Bragg grating, the resulting hologram can exhibit very high diffraction efficiencies and fast switching times.

The switchable holographic device can be used in either transmission or reflection mode. However, it is conventional for the polymer-dispersed liquid crystal (PDLC) material to utilise nematic liquid crystals. Under these circumstances, when the holographic device is used in transmission mode, it exhibits polarisation sensitivity. That is to say, the diffraction efficiency for radiation polarised (say) in the p direction is significantly greater than that for radiation polarised in the orthogonal (s) direction. This difference in diffraction efficiency can be as much as a factor of 50 to 100. One possible reason for this will be explained below. Figure 1 of the accompanying drawings shows (io sch'emalϊe side-view¹) a typical holographic device comprising holographic fringes recorded in a medium 10 which is sandwiched between two electrodes 11 and 12. The device receives a beam of radiation 13 in a direction generally perpendicular to the plane of the device (as indicated by arrow X), and the electrodes 11 and 12 are made of light-transmitting, electrically conductive material so as not to obstruct the passage of the radiation through the device. If the hologram is configured as a transmission grating, the Bragg surfaces 14 will be substantially parallel to the direction X of propagation of the radiation. The liquid crystal droplets 15 forming the fringes are elongated and are oriented with their major axes perpendicular to the Bragg surfaces 14. Because the liquid crystal molecule directors 16 tend to be aligned parallel to the major axis of each droplet 15, they will also exhibit a preferential orientation perpendicular to the Bragg surfaces 14. As a result, the liquid crystal droplets 15 will exhibit birefringence to radiation propagating in the direction of arrow X. More particularly, one component of the electric field vector of the radiation will "see" the ordinary refractive index of the liquid crystal droplets 15, whilst the orthogonal component will see the extraordinary index. As a consequence, the two components (i.e. the orthogonal polarisation states of the radiation) will be diffracted to different degrees.

Figure 2 of the accompanying drawings shows (in schematic side view) a device in which the hologram is instead configured as a reflection grating. The Bragg surfaces 14 are now arranged perpendicular to the direction X of propagation of the radiation. The liquid crystal droplets 15 will, as before, be oriented perpendicularly to the Bragg surfaces 14 and the liquid crystal molecule directors 16 will again exhibit a preferential orientation in that direction. Because the liquid crystal droplets 15 are now oriented generally parallel to the direction X of radiation propagation, the radiation will see only the ordinary refractive index of the droplets. As a result, the material will not show any sensitivity to the polarisation state of the radiation. Stated differently, both orthogonal polarisation states of the radiation will be diffracted to the same degree. If an electric field is applied to the hologram by way of the electrodes 11 and 12, this would normally cause the liquid crystal molecule directors 16 to reorient into a direction parallel to the electric field vector E. However, the directors 16 are already oriented in this direction, so they will remain essentially unaltered. As a result, it is not possible to create the necessary change in the refractive index modulation to switch the hologram on and off.

One possible solution to this problem is to tilt the electrodes 11 and 12 so that the electric field vector E lies at an angle to the molecule directors 16, or to construct at least one of the electrodes as two interdigitated components that can have a potential difference applied therebetween to produce an electric field extending generally parallel to the plane of the device. However, such electrodes are expensive and can give rise to a non-uniform field distribution.

Although Figure 2 is an accurate representation of certain types of reflection holograms, the applicants have found that other reflection holograms exhibit less elongated droplet shapes (i.e. as little as 1% anisotropy), in which the liquid crystal molecule orientations tend to follow a more complex distribution. They are certainly not linearly aligned as in the case of transmission holograms and it is possible to create changes in the refractive index modulation of such reflection holograms by applying an electric field. Since the distribution of molecules as seen by incoming light is to a rough approximation random, the polarisation sensitivity is very small at relatively low angles of incidence - say below 30-40 degrees. However, the polarisation sensitivity of these reflection holograms may be unacceptable for telecommunications applications.

In an ideal situation, the device should be capable of being switched between two conditions in which the holographic fringes are respectively switched on and off, and in both of which the material shows no sensitivity to the polarisation state of the radiation. For example, in a first condition the liquid crystal molecule directors 16 can be randomly oriented, such that the polarisation sensitivity of the individual liquid crystal droplets is evened out because of the overall randomness of their orientation. In a second condition, the molecule directors can all be oriented in a direction parallel to the direction of radiation propagation, such that the radiation sees only the ordinary refractive index of the droplets. Various ways of achieving this object are described in our pending International patent application PCT/GB01/03169 entitled "Switchable waveguide device".

Proposals have been made to produce PDLC material using cholesteric rather than cholesteric texture (PSCT). This offers the potential advantage of being able to produce polarisation-insensitive operation due (it is thought) to the randomness created by the helical structure of cholesteric liquid crystal molecules. However, known PSCT materials, in which there is a random dispersion of relatively large cholesteric liquid crystal molecules or agglomerates embedded within a polymer, suffer from the problem that their relaxation time (typically tens of milliseconds) is not fast enough for telecommunications and display applications.

It is an object of the present invention to obviate or mitigate the above-described problems and disadvantages.

Summary of the invention

According to the present invention, there is provided a switchable holographic diffraction device comprising holographic fringes formed by liquid crystal droplets dispersed in a medium, the droplets being composed of at least partly of a non- nematic (preferably cholesteric) liquid crystal material, the fringes being switchable between a first condition in which the liquid crystal molecules are oriented randomly with respect to a direction of propagation of radiation through the device, and a second condition in which the liquid crystal molecules are oriented uniformly with respect to said direction of radiation propagation.

Advantageously, the holographic fringes are in the form of a Bragg hologram.

Preferably, said medium is polymer-based.

Desirably, the fringes are composed of liquid crystal-rich layers and liquid crystal- poor layers, and the thickness of the liquid crystal-rich layers when divided by the pitch of the helical molecules of the cholesteric liquid crystal material, is at least equal to 0.78.

The liquid crystal material can comprise a mixture of nematic and non-nematic (preferably cholesteric) liquid crystal. Advantageously, the fringes are switched between ttøjrfhrstEβid second iconiditicrøs by means of an electric field.

Brief description of the drawings

The invention will now be further described, by way of example only, with reference to the remaining Figures of the accompanying drawings, in which:

Figure 3 is a schematic side view of a switchable holographic diffraction device according to the present invention, the device being shown in an active, diffracting state;

Figure 4 is a similar view to Figure 3 but showing the device in an inactive, non- diffracting state;

Figure 5A is a graph showing the power spectrum for p-polarised radiation transmitted by a first example of a holographic diffraction device according to the present invention;

Figure 5B is a similar graph but showing the power spectrum for s-polarised radiation; and

Figures 6A and 6B are graphs corresponding to those of Figures 5A and 5B, but showing the power spectrum for a device containing only nematic liquid crystal material.

Detailed description

By way of example, the holographic device of the present invention can utilise for its construction any of the recipes indicated below. These recipes essentially comprise a mixture of a pre-polymer and a cholesteric liquid crystal (CLC) material, although a nematic liquid crystal material can be included as well. The mixture is sandwiched between two substrates, which carry electrodes for switching the finished device. Where the device is to be used in transmission mode, these electrodes are designed to be light-transmitting so as not to obstBϋrcf ths passage ofciradiatton through the device: for example, the electrodes can be formed by ITO coatings on the substrates. The mixture is then subjected to two interfering UV laser beams, which are designed to produce holographic fringes of a desired configuration within the mix. The effect of the laser beams is to initiate polymerisation of the mixture, whilst at the same time causing migration of liquid crystal droplets to form regions of relatively high and relatively low concentration, corresponding in position to the interference fringes created by the laser beams. Stated differently, the migration of the liquid crystal droplets produces liquid crystal-rich and polymer-rich regions within the overall material. The interference fringes are arranged so as to be in the form of Bragg gratings, so that the overall device forms an electrically switchable Bragg grating (ESBG) device or cell. The resultant cell is then cured.

The recipes mentioned above are as follows:

Recipe A: 30% - 60% PN393 + 70% - 40% BL088

Recipe B: 30% - 60% PN393 + 20% - 40% BL088 + 20% - 40% BL087

Recipe C: 30% - 60% PN393 + 70% - 40% BL094

Recipe D: 30% - 60% PN393 + 70% - 40% BL095

Recipe E: 30% - 55% PN393 + 70% - 40% BL088 + 1 % - 5% SR399

PN393 is a pre-polymer system supplied by EM Industry Inc. BL088, BL094 and BL095 are cholesteric liquid crystal materials also supplied by EM Industry Inc., whose properties are listed in Table 1 below: BL088 and BL094 are right-handed (i.e. dextro-rotatory), whereas BL095 is left-handed (i.e. laevo-rotatory). BL087 is a nematic liquid crystal material supplied by EM Industry Inc. SR399 is a function 6 acrylate oligomer (a high density cross-linker) supplied by Satomer, Inc.

Recipes C and D are the same, but use different cholesteric liquid crystal materials, i.e. the right-handed BL094 or the left-handed BL095. Recipe E utilises the higher density cross-linker SR399, and is intended to give the resultant polymer-rich layers a high mechanical strength and a higher glass transition temperature.

The cholesteric liquid crystal materials mentioned above are typically resonant in the green wavelength band, but are found to give a pseudo-random twisted rest state with respect to infrared radiation at 1550 nm. However, most commercially-available cholesteric liquid crystal materials do not reflect light in the green wavelength band. The cholesteric liquid crystal material can be mixed with nematic liquid crystal material to form a mixture that reflects radiation at any desired wavelength from 522nm to above 1200nm. It is also possible to create different refractive indices and different relaxation times by using different combinations of monomer (pre-polymer) and cholesteric liquid crystal materials.

Figures 3 and 4 show a typical switchable holographic diffraction device produced by a method as described above. This device is generally similar to that described above with reference to Figure 1 , and similar parts are denoted by the same reference numerals. In this arrangement, however, the droplets 15 are formed either of cholesteric liquid crystal material alone, or of a mixture of cholesteric and nematic liquid crystal materials. As depicted in Figure 3, in a rest state of the device (i.e. with no electric field applied), the holographic fringes formed by the liquid crystal droplets act so as to diffract radiation propagating through the device in the direction of arrow X. In this condition, the molecules of the liquid crystal material are oriented randomly with respect to the direction X of propagation of radiation through the device. As a result, any polarisation sensitivity of the individual molecules is averaged-out throughout the diffracting medium. Stated differentl !, .the radia ion' i"seesl«an average of the ordinary and extraordinary refractive indices of the liquid crystal droplets. According to current understanding, the randomness of the liquid crystal molecule orientation arises from the helical structure of the molecules in the cholesteric liquid crystal material.

As depicted in Figure 4, when an electric field E is applied by way of the electrodes 11 and 12, the liquid crystal molecules tend to re-orient so that they lie parallel to the direction of the field E. If this direction is also arranged to be parallel to the direction X of radiation propagation, then this effectively switches off the holographic fringes, so that the incoming radiation is no longer diffracted thereby. In this condition, the radiation will "see" only the ordinary refractive index of the liquid crystal droplets, and as a result the diffracting medium will not be sensitive to the polarisation state of the radiation.

In practice, the quality and efficiency of the hologram can be optimised by adjusting the relative concentrations of the cholesteric liquid crystal material and the monomer (pre-polymer).

The invention will now be further described with reference to various examples of the switchable holographic diffraction device that have been constructed and tested. In each of these examples, the ITO-coated substrates measured 2.5 cm. x 2.5 cm. (1 in. x 1 in.) each and utilised spacers of 5 micron thickness. The final curing process was conducted using a UV lamp for 60 minutes at 700C.

In Example 1 , a mixture based on Recipe A above was prepared utilising 55% (by weight) of the cholesteric liquid crystal material BL088 and 45% (by weight) of the pre-polymer PN393. After processing as described above, the resultant cell was used in transmission mode as an electrically switchable Bragg hologram, and was found to give a high quality, bright hologram. The cell was analysed using a spectrophotometer to characterise its effect on both s- and p-polarised radiation with no electric field applied, and the results of this are summarised in Table 2 below and are also illustrated in Figures 5A and 5B (which show the power spectra for the p- and s-polarised radiation, respectively). It will be seen that the diffraction efficiency is the same for both the p- and s-polarised

B.®?qr) whie εimdiDajtes; that the orientation of the liquid crystal molecules in the cell in the rest state is completely random. This can be compared with Figures 6A and 6B, which show the corresponding spectra for a similar cell containing a nematic liquid crystal material only. The test results for such a cell are given at the bottom of Table 2, from which it can be seen that the diffraction efficiency for p-polarised light is around 60% but the diffraction efficiency for s-polarised light is virtually zero.

The test results for cells prepared using variants of Recipe B are given in Table 2 as Examples 2 and 3. It can be seen that such variants were found to give high quality, bright holograms and whilst they fail to achieve equal diffraction efficiencies for s- and p-polarised radiation, they do show a reduction in polarisation sensitivity over conventional devices comprising nematic liquid crystal material only.

Table 2

In this Table, the planar state reflection wavelength data (in nm.) are taken from the catalogue of EM Industry. The cholesteric helix pitch P is calculated using the formula:

P = λ/<n> where the parameter <n> is the average refractive index of the liquid crystal material, being given by:

<n> = (2ne + no)/3 where ne is the extraordinary refractive index and no is the ordinary refractive index of the liquid crystal material. For the purposes of the present calculations, the value of <n> has been estimated as being roughly equal to 1.6.

In the cases of Examples 1 , 2 and 3 the period of the Bragg grating in the holographic diffraction devices was approximately 510nm. The thickness of the liquid crystal-rich layers (LCRL) within which the cholesteric liquid crystal twist occurs, was about half the grating pitch, i.e. about 255 nm. The number of twist rotations that can exist in the liquid crystal-rich layers was computed by dividing the thickness of the layer by the helix pitch P, and Table 2 shows that a twist of about 0.78 is required in order to eliminate polarisation sensitivity completely. Partial polarisation sensitivity occurs below this value.

Examples of typical data on optical characteristics and switching voltages are given in Table 3 below. In general, the switching voltage for holographic diffraction devices employing cholesteric liquid crystal material appears to be slightly higher than that required for regular holographic diffraction devices (i.e. UV transmission devices utilising only nematic liquid crystal material). Also, different voltages are required for switching according to whether s- or p- polarised radiation is involved, and this may be due to residual anchoring preference at the interface between the liquid crystal-rich and polymer-rich layers.

Holographic diffraction devices utilising cholesteric liquid crystal material have turn- off decay times of about 22 μs and turn-on times of about 10 μs. These very fast switching times are thought to be attributable to the small liquid crystal domain (or droplet) sizes that result from the act of dispersing the liquid crystal material in the polymer to form the Bragg grating.

The invention offers a number of advantages over conventional switchable holographic diffraction devices, i.e. those utilising nematic liquid crystal material alone. It has been mentioned above that such conventional devices (when used in transmission mode) are sensitive to the polarisation state of the incident radiation. This means that it is necessary to use two such devices (one for the s-polarised component, the other for the p-polarised component) in order to ensure that all of the incident radiation is duly processed. This in turn requires a doubling-up not only of the holographic diffraction devices themselves, but also of their associated electrical control systems. As a result, such systems are both costly and bulky. In contrast, the switchable holographic diffraction device of the present invention is not sensitive to the polarisation state of the incident radiation, and accordingly only a single device is needed for operating on unpolarised radiation. A further benefit of using a cholestenc liquid crystal material in the

device of the invention, is that the switching speed is much less than that of corresponding systems that utilise only nematic liquid crystal materials.

Whereas the invention has been described in relation to what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not limited to the disclosed arrangements but rather is intended to cover various modifications and equivalent constructions included within the spirit and scope of the invention. In particular, although the invention is primarily intended for application in the fields of fibre optic communications systems and integrated optics, it is equally applicable to areas of technology such as optical display systems. For example, in the case of so-called colour sequential filtering, the device of the invention will allow higher throughput without the need for compensating optics, due to its polarisation insensitivity. The device of the invention can also be used in a fast colour-sequential filter or shutter in microdisplay applications. In the telecommunications sector, the invention can be used to provide a range of waveguide and free-space Bragg grating devices, such as add/drop multiplexers, attenuators and N x N cross-connects. The invention can also be used to provide sub-wavelength gratings.

Claims

1. A switchable holographic diffraction device comprising holographic fringes formed by liquid crystal droplets dispersed in a medium, the droplets being composed of at least partly of a non-nematic liquid crystal material, the fringes being switchable between a first condition in which the liquid crystal molecules are oriented randomly with respect to a direction of propagation of radiation through the device, and a second condition in which the liquid crystal molecules are oriented uniformly with respect to said direction of radiation propagation.

2. A switchable holographic diffraction device as claimed in claim 1 , wherein the non-nematic liquid crystal material is a cholesteric liquid crystal material.

3. A switchable holographic diffraction device as claimed in claim 2, wherein the fringes are composed of liquid crystal-rich layers and liquid crystal-poor layers, and the thickness of the liquid crystal-rich layers when divided by the pitch of the helical molecules of the cholesteric liquid crystal material, is at least equal to 0.78.

4. A switchable holographic diffraction device as claimed in claim 1 , 2 or 3, wherein the holographic fringes are in the form of a Bragg hologram.

5. A switchable holographic diffraction device as claimed in any preceding claim wherein said medium is polymer-based.

6. A switchable holographic diffraction device as claimed in any preceding claim, wherein the liquid crystal material comprises a mixture of nematic and non- nematic liquid crystal.

7. A switchable holographic diffraction device as claimed in any preceding claim, wherein the fringes are switched between their first and second conditions by means of an electric field.