WO2006079821A1

WO2006079821A1 - Switchable phase grating

Info

Publication number: WO2006079821A1
Application number: PCT/GB2006/000274
Authority: WO
Inventors: Carl Vernon Brown; Glen Mchale; Michael Ian Newton
Original assignee: The Nottingham Trent University
Priority date: 2005-01-27
Filing date: 2006-01-27
Publication date: 2006-08-03
Also published as: GB2422680A; GB0501696D0; GB2422680B

Abstract

A diffractive optical element in the form of a switchable phase grating (1) for varying the phase and direction of light passing through the grating, comprising a fluid enclosure (2a, 2b) including first and second fluids (3, 4) arranged relative to each other and to an optical axis of the grating. The phase grating includes first and second electrodes (5a, 5b) operable to control the configuration of the fluids within the enclosure, wherein in response to a voltage applied across the electrodes, the fluids are configurable into an arrangement by electro-wetting processes to thereby vary an optical characteristic of the grating along a direction transverse to the optical axis, so as to change the phase and direction of the incident light. The phase grating is found to have particular application in optical display devices, telecommunications networks and microwave transmission applications.

Description

SWITCHABLE PHASE GRATING

The present invention relates to switchable optical elements, and in particular to electro-wetting based optical devices.

Diffractive optical elements such as phase gratings are used in a large number of applications, (e.g. in telecommunications networks, imaging devices and display systems) across a wide range of the electromagnetic spectrum. Conventional phase gratings, as illustrated in figure 1, consist of two materials Ml, M2 having different refractive indices arranged in an alternating pattern within a 1 or 2 dimensional array.

When plane parallel light is incident on the grating of figure 1, a ray of light travelling through material Ml experiences a retardation in its phase that is dependent on the refractive index and thickness of the material, along with the wavelength of the light. Similarly, a light ray travelling through material M2 also experiences a phase retardation, such that in the far field, the emergent light rays interfere to create a diffraction pattern where the angles θ_o...θ_n of the diffracted orders depend on the repeat distance, or pitch P, of the grating and the wavelength of the incident light.

A disadvantage of conventional gratings is that they have a constant grating pitch P, so that it is not possible to vary the angles θ_o...θ_n of the diffracted orders, unless the wavelength of the incident light is changed. In addition, for fixed refractive indices of materials Ml and M2, the fraction of the intensity of incident light that is directed into the different diffracted orders is also fixed.

Recently, switchable optical elements have been developed using the principle of microfluidic motion at low voltages. These optical elements rely on electro-wetting techniques where a difference in voltage between a hydrophobic material and a liquid produces a change in wettability. Such optical elements have found application in optical filters, adaptive lenses and display devices, such as described in International patent application WO2004/027489. Electro-wetting is an established technique by which the contact angle of a droplet of conducting liquid on an insulating layer is reduced when a voltage is applied between the liquid droplet and an electrode under the insulating layer. If the liquid is water, a larger initial contact angle can be obtained if the insulating layer is coated with or incorporates a hydrophobic coating.

Devices based on electro-wetting techniques typically include a pixelated array, in which each pixel contains two immiscible fluids, for instance water and oil. The water and oil are enclosed between two electrode layers, one of which is separated from the fluids by a hydrophobic insulator. In the absence of an applied voltage, the water and oil form separate layers, where the oil covers the hydrophobic insulator and the water is in contact with the other electrode. The oil is substantially opaque and/or coloured and prevents the transmission, or reflection, of incident light through, or from, the pixel. If however, a voltage is applied across the electrodes, it is found that the water moves into contact with the hydrophobic insulator, to thereby displace the oil.

Hence, by controlling the displacement of the oil, the transmission properties of the individual pixel, and hence overall array, can be switched between predetermined states. However, although electro-wetting based display devices are able to actively control the optical transmission properties of the display, the devices are unable to alter the direction of incident light, as in the manner of a conventional phase grating.

The present invention relates to a switchable phase grating based on electro-wetting techniques which can be switched between predetermined optical states, so as to actively control the phase and the fraction of the intensity of light incident on the grating that is directed into the different diffracted orders.

It is an object of the present invention to provide a switchable phase grating which allows the refractive index to be dynamically varied along the length of the grating transverse to the optical axis, to thereby change the phase and direction of light passing through the grating. It is a further object of the present invention to provide a switchable phase grating that can change the direction of incident light by changing the fraction of the intensity of light incident on the grating that is directed into the different diffracted orders, by varying the voltage applied to the grating.

It is a further object of the present invention to provide a switchable phase grating which allows the pitch of the grating to be dynamically varied to change the phase and direction of light passing through the grating.

It is a further object of the present invention to provide a switchable phase grating that can change the direction of incident light by varying the voltage applied to the grating.

It is a further object of the present invention to provide a switchable phase grating that has an effective refractive index dependent on the voltage applied to the grating.

It is a further object of the present invention to provide a switchable phase grating that has a diffraction efficiency that is independent of the polarisation of the incident light.

It is a further object of the present invention to provide a switchable phase grating that has bi-stable or multi-stable operating states.

Some or all of the above objects are provided by embodiments of the present invention as described hereinafter.

According to an aspect of the present invention, there is provided a switchable phase grating for varying phase and direction of light passing through the grating, comprising: a fluid enclosure including first and second fluids arranged relative to each other and to an optical axis of the grating; and first and second electrodes operable to control the configuration of the fluids within the enclosure; wherein in response to a voltage applied across the electrodes, the fluids are configurable into an arrangement to thereby vary an optical characteristic of the grating along a direction transverse to the optical axis, so as to change the phase and direction of the incident light.

Embodiments of the present invention will now be described by way of example and with reference to the accompanying drawings in which:

Figure 1 is a schematic diagram of a conventional phase grating.

Figure 2 is an exploded side cross-sectional view of an element of a switchable phase grating according to a preferred embodiment of the present invention.

Figure 3 is a side cross-sectional view of a switchable phase grating according to a preferred embodiment.

Figures 4 and 5 are side cross-sectional views of the switchable phase grating of figure 3 showing the effect of an increasing applied voltage.

Figure 6 is a side cross-sectional view of a switchable phase grating according to another preferred embodiment of the present invention. Figures 7a and 7b are side cross-sectional views of the switchable phase grating of figure 6 showing the effect of an increasing applied voltage.

Figures 8a and 8b are side cross-sectional views of the switchable phase grating of figure 6 showing the effect of a low applied voltage.

Figures 9a and 9b are side cross-sectional views of a switchable phase grating according to another preferred embodiment of the present invention, showing the effect of increasing applied voltage.

Figure 10 is a schematic diagram of a switchable phase grating according to another preferred embodiment of the present invention.

With reference to figure 2, there is shown an exploded side cross-sectional view of an element of a switchable phase grating 1 according to a preferred embodiment. The grating 1 comprises a fluid enclosure defined by substantially transparent substrates 2a, 2b. The substrates 2a, 2b are preferably planar and are arranged so as to be substantially parallel to each other.

The phase grating 1 is preferably operated in a "transmissive mode", such that both substrates 2a, 2b are transmissive to incident light. In alternative arrangements, one of the substrates may have a mirrored surface (i.e. coated with a reflective material), so that the grating operates in a "reflective mode" where the incident light is made to traverse the fluid enclosure twice, such that the light enters and exits the grating via the same substrate surface.

In the transmissive mode, the outer surfaces of the substrates 2a, 2b correspond to the "input faces" of the phase grating 1, and as such either, or both, outer surfaces are able to receive incident light.

In alternative embodiments, the substrates 2a, 2b may be arranged so as to be angled with respect to each other, so as to form a wedge shaped grating.

The substrates 2a, 2b may be made from any suitable material which allows confinement of the fluid, and is preferably an optically isotropic material (i.e. one which has no birefringence), such as glass. Alternatively, non-birefringent C-plane cut quartz, sapphire and diamond may also be used for the substrates 2a, 2b, and since it is envisaged that the present grating may have particular application in the telecoms industry, then non-birefringent plastics having low absorption at telecoms wavelengths (e.g. typically 1310 nm and 1550 nm) are also suitable materials.

However, it is to be appreciated that the substrates 2a, 2b should be formed from materials which are substantially transmissive over the wavelength band of operation of the grating (i.e. the particular region of the electro-magnetic spectrum), or in other words, materials which do not exhibit significant attenuation (i.e. high absorption) at such wavelengths of interest.

The thickness of the substrates is preferably in the range of about 0.5 mm to about 2 mm, and is most preferably about 1.1 mm. However, different thicknesses may be used depending on the particular application.

The fluid enclosure has an internal volume to receive first and second fluids, preferably in the form of immiscible liquids 3, 4, initially arranged relative to each other in separate layers. The first fluid is a conductive or polar liquid, preferably water 3, and the second fluid is a non-conductive liquid, preferably oil 4. However, as an alternative, two conductive and/or polar fluids may also be used provided one of the fluids is less conductive, and/or polar, relative to the other. In preferred embodiments, both the water 3 and oil 4 are substantially transparent over the wavelength band of operation, and each has a different refractive index. Suitable choices for the oil include minerals oils, such as silicon oil, and preferably polymethylsiloxane, which is commercially available with refractive index values of 1.447 (compared to water which is 1.33) and has a similar density to that of water.

Of course, it is to be appreciated that in all embodiments, the fluids for use in the phase grating are selected having regard to their absorption characteristics, such that they do not exhibit appreciable absorption over the particular wavelength band of operation.

The phase grating 1 further comprises first and second electrodes 5 a, 5b, preferably supported by a respective one of the substrates 2a, 2b so as to "sandwich" the water 3 and oil 4 therebetween. In preferred embodiments, the first and second electrodes 5a,

5b are substantially transparent and are operable to exert an electric field on the water

3 and oil 4 within the fluid enclosure. The first electrode 5a is disposed so as to be coupled to the water 3, either preferably by direct electrical contact with a surface of the water 3, or via capacitive coupling with the water 3.

In alternative embodiments, the first and second electrodes 5a, 5b may form part of, or be integrated with, a respective one of the substrates 2a, 2b.

As shown in the preferred embodiments of figure 2, the second electrode 5b is electrically insulated from the water 3 and oil 4 by an insulating layer 6. The insulating layer 6 is made from any suitable dielectric material and is preferably substantially transparent to light. The insulating layer 6 is preferably coated with a hydrophobic material 7, preferably an amorphous fluorocarbon or polyamide, having low dielectric constant and good optical transmission properties over the wavelength band of operation. Preferred examples include fluoroalkyltrichlorosilanes and Teflon AF - a family of amorphous fluoropolymers giving a high contact angle with water of 104-105 degrees. In other preferred embodiments, the insulating layer itself may be made from, or include, the hydrophobic material.

The first and second electrodes 5a, 5b may be made from any suitable transparent conductive material, preferably indium tin oxide (ITO) or Antimony Tin Oxide(ATO) or alternatively RuO₂ or PEDOT. The electrodes 5a, 5b preferably have a resistance per square below 100 ohms per square.

The phase grating as described herein may be constructed as a 1 -dimensional linear grating or a 2-dimensional grating array.

It is to be appreciated that references herein to "transparent" are to be taken as meaning that the material or substance permits most, if not all, of the light incident on the material or substance to pass therethrough without significant attenuation over the wavelength band of operation. Moreover, all references to "light", "incident light" and associated terms, e.g. "optical", are to be understood as referring, or relating, to in particular, visible light in the range of about 380 run to about 700 nm, but are also intended to include radiation from other regions of the electro-magnetic spectrum, such as, but not limited to, ultra-violet, infra-red, telecoms wavelengths 1310 nm and 1550 nm and microwave radiation in the wavelength band of about 30 to about 3000 microns.

Referring to figure 3, there is shown a particularly preferred embodiment of the switchable phase grating 1 in an equilibrium state. By "equilibrium state" we mean that there is zero voltage applied to the electrodes 5 a, 5b and the water 3 and oil 4 are in the form of distinct separate layers. In this state, any plane parallel rays of light incident on the grating 1 (not shown) experience substantially the same effective phase retardation at any position along the length of the grating (transverse to the optical axis O), as the effective refractive index of the grating is essentially the same at any position. Hence, any transmitted rays of light are in phase and so no interference takes place. It is to be appreciated in figure 3, that although equal volumes and thicknesses of water 3 and oil 4 are shown, this is not intended to be limiting, and differing respective volumes of the fluids are in accordance with the present invention.

In the preferred embodiment of figure 3, the first electrode 5a is in the form of a continuous layer in direct electrical contact with the water 3. The second electrode 5b is in the form of a structured electrode, preferably having a striped electrode pattern (shown in side cross-section). The effect of a structured electrode is to produce a spatially varying electric field between the first and second electrodes 5a, 5b when a voltage is applied across them. The electrodes therefore exert a spatially varying electric field on the water 3 and oil 4 within the fluid enclosure. A spatially varying electric field is important as this determines the particular arrangement of the water 3 and oil 4, and hence the effective refractive index of the grating 1. Therefore, by changing the spatial distribution of the water 3 and oil 4 within the fluid enclosure, it becomes possible to alter the refractive index variation along the length of the grating transverse to the optical axis O by varying the voltage applied to the grating 1.

It is to be appreciated that in all embodiments of the present invention, the voltages applied to the electrodes 5a, 5b of the grating 1 may be either d.c. or a.c. Advantages of using an a.c. voltage are that charging effects at the interface between the water 3 and insulating layer 6 can be minimised, or avoided, and the a.c. voltage prevents, or minimises, electrochemical degradation of the materials in the grating.

The second electrode 5b may be structured as a periodically repeating pattern, or as an irregular pattern, and may be in the form of interdigitated electrodes, which when used with a.c. voltages allow the phase of the applied voltage to alternate between adjacent electrodes. Spatial variations in the electric field can therefore arise as a result of the structure of the electrodes (i.e. due to the physical inter-electrode spacing), or else can be produced by selectively driving only specific electrodes within the pattern, so as to effectively vary the inter-electrode spacing. In this way, the periodicity of the variations in the electric field can be selected, thereby changing the spatial distribution of the water 3 and oil 4 within the fluid enclosure, to allow the pitch of the grating to be dynamically varied. In alternative arrangements, both the first and second electrodes 5 a and 5b of the grating 1 may be structured (e.g. striped or interdigitated) to allow further control of the spatial variation, and periodicity, in the electric field.

It is to be appreciated that in the preferred arrangements, each electrode in the striped or interdigitated electrode pattern may be driven independently of the others, depending on the particular electric field variation that is required within the grating.

For 2 dimensional grating arrays, either a passive matrix or an active matrix may be used in the phase grating 1. A passive matrix typically has row and column electrodes, with the row electrodes on one substrate and the column electrodes, orthogonally arranged with respect to the rows, on the other substrate.

Figure 4 illustrates the effect of applying a voltage to the first and second electrodes 5 a, 5b of the phase grating 1 of figure 3. When a voltage is applied, capacitive effects arise due to the structure of the water and oil layers, and so in order to minimise the energy of the fluid system, the water 3 moves towards the insulated second electrode 5b. The movement of the water 3 displaces the oil 4 such that the oil layer de- coalesces into discrete fluid regions, such as oil droplets having a typical width dimension of about 1 micron to 25 microns, which are distributed relative to one another along the length of the grating transverse to the optical axis O.

Due to the electro-wetting effect, application of a voltage to the first and second electrodes 5a, 5b increases the wettability of the insulating layer 6 by the water 3.

As a result of the applied voltage, the water 3 and oil 4 are now configured into an arrangement which produces an alternating sequence of varying refractive index, along the length of the grating 1. Hence, plane parallel light rays incident on the grating 1 experience different phase retardation, depending on the effective refractive index of the grating at the point of incidence of the light ray, and so interference occurs in the far field, leading to diffraction effects. By "effective refractive index" we mean the combined refractive index of the water and oil configuration along the direction of travel of the incident light ray.

By varying the magnitude of the voltage applied to the first and second electrodes 5a, 5b, the arrangement of water 3 and oil 4 can be controlled, such that the relative thicknesses of the complementary water/oil regions can be actively changed along the length of the grating 1. By "complementary" we mean that the surface contours of the water and oil regions are reciprocally shaped and are in contact along their interfaces, so that no voids are formed between the respective fluids. Since the phase retardation of an incident light ray is dependent on the refractive index and thickness of the material through which it passes, it follows therefore that the degree of phase retardation of the light ray can be actively controlled by varying the voltage applied to the grating 1.

The phase retardation δφ of light in traversing a medium (e.g. water and oil layers) is given by: δφ = 2 π t η / λ (1)

where λ is the wavelength of the incident light in air outside of the medium, t is the thickness of the medium and η is the refractive index of the medium. In use, the present phase grating typically needs to produce a phase difference between the water and oil regions of up to π radians, so that destructive interference occurs between light rays passing through the respective water and oil regions. In this way, the maximum amount of incident light is diffracted from the straight through zeroth order into the higher diffracted orders.

From equation 1, a phase difference of π radians can be produced between the water and oil regions for a wavelength of light of 1550 nm, if the thicknesses of the water and oil regions are 6.6 microns; whereas for light of wavelength 633 nm the thicknesses are 2.7 microns. In this calculation, the water and oil regions are assumed to be in the arrangement as shown in figure 5, and respective refractive indices of 1.33 and 1.447 (e.g. for polymethylsiloxane) have been used for the water and oil. As shown in figure 5, a higher voltage than that applied to the grating in figure 4 is applied to the first and second electrodes 5a, 5b of the grating. The higher voltage causes a larger portion of the hydrophobic coating 7 that is above the structured electrode 5b to be in contact with the water 3. This therefore displaces the oil regions even further, such that they are laterally offset with respect to the underlying second electrode pattern and are forced to extend away from the insulating layer 6 so that they may actually be in contact with both the insulating layer 6 and the first electrode 5 a. Hence, the water 3 and oil 4 are configured into a different arrangement than that shown in figure 4, to thereby produce a different sequence of alternating refractive index.

Hence, by varying the voltage applied to the grating 1 the relative sizes and positions of the oil regions can be controllably adjusted, so that the spatial distribution of the oil regions can be dynamically selected. Therefore, the fraction of the incident light that is directed into each of the diffracted orders may be actively controlled by varying the refractive index in a direction transverse to the optical axis O, as a result of varying the applied voltage to the grating.

In telecommunications applications for instance, it is necessary to route communication (light) signals between a multitude of fibre optics in a fibre array, and so the present grating would be required to diffract light between different fibre locations in the array. To prevent, or minimise, crosstalk (i.e. unwanted signals being directed into neighbouring fibres), the largest possible diffraction angles θ_o...θ_n are desirable which typically correspond to grating pitches p in the range of about 2 microns to about 50 microns for typical telecommunications wavelengths.

By way of example, the well known diffraction grating formula predicts that, in the far field, light of wavelength λ is diffracted to an angle θ by a grating of pitch p when:

p sin θ = nλ (2)

where integer n is the diffraction order. Therefore, to give a first order (n = 1) diffraction peak at an angle of 10 degrees for light of wavelength 1550 nm, the grating pitch would need to be 8.9 microns, and for a first order (n = 1) diffraction peak at an angle of 10 degrees for light of wavelength 633 nm the pitch would need to be 3.6 microns. In the present grating, the pitch p is defined as the minimum distance between like points on the repeating pattern of alternating oil droplets, and is determined by the physical, or effective, inter-electrode spacing. Therefore, referring to figure 4 by way of example, the pitch would be the distance between the peaks of adjacent oil droplets. Correspondingly, the pitch could also be taken to be the distance between the left hand edges of adjacent droplets etc.

Hence, if the structured electrode 5b is driven (i.e. biased) symmetrically (as shown in figure 4), the oil de-coalesces with reference to the driven electrode pattern, and therefore the pitch will be essentially the physical inter-electrode spacing. However, if the structured electrode 5b is driven asymmetrically (e.g. alternately biased), the oil de-coalesces with reference to the alternately driven electrode pattern, corresponding to twice the physical inter-electrode spacing, and therefore the pitch is doubled. It is clear therefore, that the grating pitch can be dynamically varied, simply by supplying different combinations of driving voltages to the structured electrode 5b and/or by using different structured electrode patterns in the grating.

Advantageously, by dynamically controlling the grating pitch, the angles of the diffracted orders θ_o...θ_n may be actively selected as a result of varying the voltage applied to the grating.

It is to be appreciated that the diffraction grating formula above gives the diffraction angle for light emerging from the water and oil layers, and that refraction of the light may occur at the interface of the fluid layers and substrate, and substrate and air interface.

Another significant advantage of the present phase grating is that the diffraction efficiency (i.e. the total fraction of the incident light intensity that is directed into all diffracted orders) of the grating is independent of the polarisation of the incident light, meaning that the angles of the diffracted orders are insensitive to polarisation. The polarisation insensitivity arises as none of the grating components exhibit be- refringence, as the water, oil, glass, insulator and electrodes are all substantially isotropic with respect to their refractive index. Hence, the grating can actively control the direction of incident light irrespective of the polarisation state of the light.

This feature is particularly important for telecommunications applications, where light signals emerging from fibres have unknown and/or temporally varying polarisation, hence optical routing elements are needed to direct light independently of polarisation, otherwise substantial insertion losses can result. At the present time, most routing elements are nematic liquid crystal (LC) devices having a quarter wave plate to make them polarisation insensitive. Ferroelectric LC devices are also used, but these are generally not polarisation insensitive as a result of the structure of the device.

It is to be appreciated that the application of different voltages to the grating will cause the diffraction efficiency of the grating to depend in a different manner on the wavelength of the incident light. This is because the resulting diffraction depends on the exact 2-dimensional refractive profile within the phase grating, as well as on the wavelength of the incident light, the refractive indices of the water and oil, and the thickness of the grating.

Referring to figures 4 and 5 again, due to the asymmetry of the water 3 and oil 4 arrangement along a direction parallel to the optical axis O of the grating, the present grating will produce a different diffraction pattern depending on which input face of the grating receives the incident light. Hence, incident light can be directed through varying angles or into different diffracted orders depending on which face of the grating the light emerges.

When the applied voltage on the first and second electrodes 5a, 5b is reduced to zero, the water 3 and oil 4 arrangement as shown in figures 4 and 5 reverts to the equilibrium state as shown in figure 3.

Another preferred embodiment of the phase grating is shown in figure 6. In this embodiment, both the first and second electrodes 5a, 5b are in the form of continuous layers, and the insulating layer 6 is structured to have a varying thickness profile along the length of the grating. By varying the thickness of the insulating layer 6, spatial variations in the electric field can be produced without the need for a structured electrode. The profiling of the insulating layer 6 gives rise to differing electric field strength at different areas of the layer, so as to produce a local increase in the electro- wetting effect.

In an alternative embodiment, the insulating layer 6 may be substantially planar (as described in earlier embodiments) and the second electrode 5b itself is made to have a profiled structure, so as to produce a variation in the electric field along the length of the grating 1. Alternatively, in other embodiments, a structured second electrode 5b (e.g. striped or interdigitated) may be used with a profiled insulating layer 6.

In the embodiment of figure 6, the thickness profile of the insulating layer 6 is substantially saw-tooth in cross-sectional profile, having corresponding "peaks" and

"troughs" arranged in a repeating pattern. In 3 -dimensions this profile would approximate to a layer having corrugations at least on one side. It is to be appreciated that the pattern as shown in figure 6, may also repeat along an axis normal to the page, so that in 3 -dimensions the insulating layer 6 may be in the form of an array of pyramidal structures.

The insulating layer 6 is coated with a hydrophobic material 7, as discussed in relation to earlier preferred embodiments.

Preferably, the refractive index of the oil 4 is substantially matched with the refractive index of the insulating layer 6, so that there are no diffraction effects when the grating is in the equilibrium state.

In figures 7(a) and 7(b), the effect of applying a voltage to the grating of figure 6 is illustrated. As before, the water will move towards the insulated electrode 5b, so as to come into contact with the hydrophobic coating 7 on the insulating layer 6. In figure 7(a), the water 3 makes contact with the troughs in the layer and displaces the oil 4 into regions towards the peaks, forming aperiodic distribution of oil droplets. As shown in figure 7(b), the water 3 makes contact with the peaks in the layer 6 and displaces the oil 4 into regions towards the troughs, forming a different periodic distribution of oil droplets. The exact arrangement of the water 3 and oil 4, and whether the water 3 contacts the troughs or the peaks of the insulating layer 6, depends on a number of factors, such as the thickness of the insulating layer 6 and relative thicknesses of the water and oil layers. Moreover, the exact shape of the fluid regions depend on the capacitance of the insulating layer 6, and the relationship to: the thickness profile of the layer, the capacitance of the oil, and the interfacial energies at the water/oil, water/insulating surface and oil/insulating surface interfaces.

An alternative fluid arrangement can be produced in the grating embodiment of figure 6, by lowering the applied voltage to a level whereby the movement of the water 3 is not quite able to cause the oil layer to de-coalesce into distributed fluid regions, so as to prevent the water 3 from making contact with the hydrophobic coating 7. In this example, the water/oil interface is "rippled" in a periodic structure, e.g. substantially sinusoidally, which may follow the periodicity of the insulating layer thickness profile. The rippled structure may be in anti-phase with the thickness profile, as shown in figure 8(a), or may be in phase as shown in figure 8(b). The exact arrangement and shape adopted by the fluids will again be dependent on the properties listed above.

Alternatively, the periodicity of the rippled structure could be selected to be a multiple of the spatial frequency of the insulating layer thickness profile.

In another preferred embodiment, the insulating layer may have a thickness profile which is substantially ramp-stepped in cross-section, as shown in figures 9(a) and 9(b). This embodiment is particularly advantageous as it allows the water 3 and oil 4 to be configured into an arrangement whereby incident light can be preferentially directed along a specific direction relative to the optical axis O of the grating 1. In this way, the switchable phase grating is operating as a "blaze grating", so that a particular order of the diffraction pattern can be produced at desired angles θ_o...θ_n. In figures 9(a) and 9(b), the effect of increasing voltage is shown on the blaze grating, so that higher voltages cause the water 3 to come into contact with more of the hydrophobic coating 7 on the "ramp" portions of the insulating layer, thereby displacing the oil regions further towards the "edges" of the ramp portions.

Although the method of varying the electric field within the fluid enclosure has been described in terms of structured electrodes 5b and profiled insulating layers 6, it is to be appreciated that any means suitable for producing a spatial variation in electric field across the grating may be used. For instance, it may also be possible to use a continuous electrode layer having portions of relatively lower and higher conductivity, arranged in a repeating pattern, or alternatively, to use an insulating layer having a spatial variation in its dielectric permittivity.

Another advantage of using a structured insulating layer, and/or profile structured electrode is that such structures are known to promote super-hydrophobicity or to enable bi-stable or hysteretic operation. It is known that micron scale surface relief structures (typically 0.3-0.5 microns) promote super-hydrophobicity, giving contact angles with water of greater than 160 degrees. Therefore, the present grating could be arranged so that the structure of the insulating layer 6 promotes super-hydrophobic characteristics, thereby enabling precise control of the oil/substrate and water/substrate interfacial surface tensions.

Moreover, the grating may also be configured to have bi-stable or multi-stable operation. Suitable insulating layer structures can be adapted to provide "pinning" of the oil droplets, so that the droplets remain in a predetermined arrangement with respect to the water when the voltage applied to the grating is switched off. The pinning is dependent on the restoring torque (resulting from the excess energy at the water/oil interface) on the oil droplets, which attempts to re-coalesce the oil droplets in the absence of an applied voltage. Certain insulating layer structures can impart a sufficient pinning energy to the oil droplets which can overcome the restoring torque, thereby allowing the grating to retain a particular fluid configuration after the voltage is removed. Furthermore, meta-stable states may also be incorporated into the operation of the grating, such that a "jogging" voltage pulse is required to reset the fluids in the grating, or a particular waveform can be applied to the second electrode 5b to create and re-set meta-stable states, in this way a number of possible fluid arrangements may he adopted without the need for a continuous applied voltage.

In another preferred embodiment of the phase grating, as shown in figure 10, the fluid enclosure is in the form of a substrate 8 having a plurality of substantially parallel channels 9. The fluid enclosure substrate 8 is made from any suitable substantially transparent material which provides fluid confinement, preferably glass, and is sandwiched between the transparent substrates 2a, 2b, so as to form the grating 1.

First and second fluids, preferably water 3 and oil 4, are maintained in respective reservoirs (not shown) in fluid communication with each of the plurality of laterally spaced channels 9, and the inside surfaces of the channels 9 are coated with a hydrophobic material, of the type as previously described. Preferably, the refractive index of the oil 4 is matched to the refractive index of the fluid enclosure substrate 8 to avoid diffraction effects in the equilibrium state.

Supported by the transparent substrates 2a, 2b are first and second electrodes 5a, 5b, preferably in the form of continuous layers and made from substantially transparent conductive material, preferably indium tin oxide (ITO). The second electrode 5b is electrically insulated from the water 3 and oil 4 by the substrate 8 of the fluid enclosure, whereas the first electrode 5a is able to come into direct contact with the water 3 by virtue of the open channels 9. Alternatively, the first electrode 5a and water 3 may be capacitively coupled.

In the equilibrium state (i.e. in the absence of an applied voltage), the channels 9 are filled with oil 4. However, if a voltage is applied to the electrodes 5a, 5b, it becomes energetically favourable within the fluid system for the water 3 to enter the channels 9 to displace the oil 4. As a result, the water 3 and oil 4 are now configured into an arrangement which produces an alternating sequence of varying refractive index, across the grating 1. Hence, plane parallel light rays incident on the grating experience different phase retardation and so interference occurs in the far field, leading to diffraction effects.

It is to be appreciated that although not shown in figures 1 to 9, the sides of the phase grating are sealed by any suitable means, e.g. another substrate or bonding agent etc., so as to prevent leakage of the fluids and to increase structural integrity of the grating.

In any of the preferred embodiments, the substrates 2a, 2b may be adapted so that they only pass certain wavelengths of incident light and block (i.e. absorb), other unwanted wavelengths. This is particularly advantageous in optical filtering applications, such as for colour filters in display devices. It is also possible for the substrates to be made from a photonic bandgap (PBG) material, so that the grating acts a highly selective tunable filter. For example, changes in the optical properties of the water and oil layers between the PBG substrates would tune the wavelengths of the sharp resonances that occur either side of the optical bandgap.

Additionally, it is also possible in other embodiments, to use fluids which have different spectral transmissivities, selected with regard to the wavelength band of operation, to allow the spectral characteristics of the incident light to also be controlled. Preferably, to this end, coloured water and/or coloured oil could be used in the grating.. In this way, the grating would not only permit the angles of the diffracted orders to be actively changed, but also enable dynamic filtering of spectral components of incident light. For example, if the water is coloured red (e.g. using a suitable dye agent etc.) and the oil is coloured blue, then in the equilibrium state, little or no transmission of incident red light is possible. However, when a voltage is applied to the grating the oil de-coalesces into droplets and allows regions of water to extend across the fluid enclosure, thereby permitting the incident red light to be transmitted through the grating.

The present grating may also be adapted to operate as an Optically Addressed Spatial Light Modulator (OASLM), if the optical surfaces are treated, by a suitable treatment agent, so that they change their interfacial properties under exposure to light of specific wavelengths. In addition, fluids or active additives may be added to the grating fluids to change their conductivity, their surface tensions, viscosities, or other physical properties when light of a specific wavelength is incident on the grating fluids.

The phase grating of the present invention has particular application in projection display devices, since a device having a grating at each pixel would be able to select a particular colour and transmit it towards an associated display screen. This is possible because, as recalling from equation 2, for a given diffraction angle (corresponding to the direction of the display screen), the pitch of the grating may be dynamically controlled so that only particular wavelengths (i.e. colours) are projected to the screen. Such a device would not require colour filters or polarisers, as it would be able to produce all necessary colours directly.

Although the preferred embodiments have been described in relation to water and oil, it is to be understood that other fluids may also be used in accordance with the present invention. For example, electrorheological fluids may be used, which have properties enabling their viscosity to change in response to an applied voltage. Such a property, combined with electro-wetting effects, would provide greater control of the fluid movement and subsequent fluid configuration.

Moreover, fluids in gaseous states may also be used, as these also provide suitable electro-wetting effects, hi other embodiments, it may also be possible to use dielectrophoresis effects to cause the conductive fluid to move within the fluid enclosure (e.g. a sphere of water in a body of oil) by applying an a.c. voltage, without the need for an insulating layer.

Although the preferred embodiments have been described in relation to a hydrophobic surface (i.e. the hydrophobic coating) opposite to a non-treated surface (i.e. first electrode), it is to be appreciated that the present invention would also work with an oliophobic surface opposite to the non-treated surface. Moreover, the principle of operation of the present invention would also work with a hydrophobic surface opposite to an oliophobic surface. Other embodiments are intentionally within the scope of the accompanying claims.

Claims

1. A switchable phase grating for varying phase and direction of light passing through the grating, comprising: a fluid enclosure including first and second fluids arranged relative to each other and to an optical axis of the grating; and first and second electrodes operable to control the configuration of the fluids "within the enclosure; wherein in response to a voltage applied across the electrodes, the fluids are configurable into an arrangement to thereby vary an optical characteristic of the grating along a direction transverse to the optical axis, so as to change the phase and direction of the incident light.

2. The phase grating of claim 1, wherein the optical characteristic of the grating is the refractive index.

3. The phase grating of claim 1 or claim 2, wherein at least one of the first and second fluids is substantially transparent to one of the following types of incident radiation: visible light, ultra-violet, infra-red, telecoms wavelengths 1310 nm and 1550 nm and microwaves in the range of about 30 to about 3000 microns.

4. The phase grating of any preceding claim, wherein the first and second fluids have different refractive indices.

5. The phase grating of any preceding claim, wherein the first and/or second fluids are coloured.

6. The phase grating of claim 1, wherein the first and second fluids have different spectral transmissivities.

7. The phase grating of any preceding claim, wherein in an equilibrium state the first fluid is disposed adjacent to the first electrode.

8. The phase grating of any preceding claim, wherein the first fluid is more electrically conductive or polar, than the second fluid.

9. The phase grating of any preceding claim, wherein the first and second electrodes are substantially transparent to the incident light.

10. The phase grating of any preceding claim, wherein the fluids are configurable into an arrangement of complementary respective fluid regions" distributed relative to each other along a direction transverse to the optical axis.

11. The phase grating of claim 10, wherein the fluid regions are formed by displacement of the first fluid towards the second electrode.

12. The phase grating of claim 10 or claim 11, wherein the fluid regions of the second fluid are periodically distributed within the fluid enclosure.

13. The phase grating of any claims 10 to 12, wherein the fluid regions of the second fluid are separate from each other and each extend in a direction away from the second electrode.

14. The phase grating of claim 12 or claim 13, wherein the fluid regions of the second fluid are laterally offset from one another with respect to the plane of the second electrode.

15. The phase grating of claim 1 or claim 10, wherein the fluid arrangement resulting from the applied voltage is dependent on the structure of the second electrode.

16. The phase grating of claim 1, wherein the second electrode is a continuous layer.

17. The phase grating of claim 15, wherein the second electrode is in the form of interdigitated electrodes.

18. The phase grating of any preceding claim, further comprising an insulating layer to insulate the first and second fluids from the second electrode.

19. The phase grating of claim 18, wherein the insulating layer is coated with a hydrophobic coating.

20. The phase grating of claim 19, wherein the hydrophobic coating has a higher wettability with respect to the second fluid than with respect to the first fluid.

21. The phase grating of any of claims 18 to 20, wherein the fluid arrangement is dependent on the structure of the insulating layer.

22. The phase grating of any of claims 18 to 21 , wherein the insulating layer has a non-planar structure.

23. The phase grating of claim 1, wherein the insulating layer is structured to have a varying thickness profile such that the separation between the first fluid and second electrode is varied along a direction transverse to the optical axis, so as to vary the electro-wetting effect.

24. The phase grating of claim 23, wherein the thickness profile of the insulating layer is substantially ramp-stepped or substantially saw-tooth.

25. The phase grating of claim 23 or claim 24, wherein the fluid regions of the second fluid are configured to be substantially coincident with peaks in the thickness profile.

26. The phase grating of claim 24 or claim 25, wherein the fluid regions of the second fluid are configured to be substantially coincident with troughs in the thickness profile.

27. The phase grating of any of claims 22 to 26, wherein the fluids are configurable into one or more stable arrangements.

28. The phase grating of claim 27, wherein the pinning energy of the second fluid maintains the fluids in a stable arrangement when the applied voltage is removed.

29. The phase grating of claim 28, wherein the pinning energy is dependent on the structure of the insulating layer.

30. The phase grating of any of claims 18 to 26, wherein the refractive index of the second fluid is substantially the same as the refractive index of the insulating layer.

31. The phase grating of any of claims 1 to 7, wherein the fluid enclosure is in the form of a substantially transparent substrate having a plurality of parallel channels therein to receive the first and second fluids, the channels being aligned along a direction substantially transverse to the optical axis.

32. The phase grating of claim 31, wherein the parallel channels are laterally spaced from each other and the refractive index of the substrate is substantially the same as the refractive index of the second fluid.

33. The phase grating of claim 31 or claim 32, wherein the fluids are configured by displacing the second fluid from one or more of the channels by introducing the first fluid into the respective channels.

34. The phase grating of any preceding claim, further comprising first and second substrates, each to support a respective one of the first and second electrodes.

35. The phase grating of claim 34, wherein the first and second substrates are substantially transparent.

36. The phase grating of any preceding claim, wherein the fluids are configured by an electro-wetting effect.

37. The phase grating of any preceding claim, wherein the first fluid is one of a conducting liquid and a polar liquid.

38. Apparatus substantially as described herein with reference to the accompanying drawings.