In our co-pending British Patent Application No.8625513 (Publication No. 2197495), birefringent optical devices are described in which photochromic materials are incorporated into a plastics matrix. The devices can be switched between stable states having different refractive indices by increasing or decreasing the incident light intensity. Such devices can be used to perform logic functions, e.g. in optical signal processing and computing. The present invention is based on the realisation that if certain photochromic compounds are subjected to molecular orientation by alignment in an electric field while in a polar state, a layer of the photochromic material will be optically anisotopic. Furthermore, the molecularly ordered layer can be selectively switched between states in which it is optically isotropic, transmitting light in all planes, and an an sotropic one in which it transmits light polarised in only one orthogonal plane.

According to one aspect of the present invention there is provided an optical device having a polarisation state which is externally controllable and comprises a layer of a photochromic material which possesses a polar state in i ts bleached or coloured form and which has been

subjected to an electric or magnetic field or to mechanical stretching while in such state to cause molecular orientation of said photochromic material.

A photochromic material is characterised by its ability to undergo a reversible change between two different chemical species, each with its characteristic optical absorption spectrum; such a change is induced, in at least one direction, by the action of light. In many photochromic systems, absorption of light in the U.V. or blue region of the spectrum results in increased absorption in the visible region (colouring); this coloured state decays (bleaching) gradually in the dark or more rapidly on heating or exposure to visible radiation. Quantitatively, the change in optical absorption under either colouring or bleaching is proportional to the absorbed optical energy, or proportional to exposure time under conditions of constant illumination. In the latter case, colouring or bleaching proceeds continuously to saturation or complete bleaching respectively.

Certain organic photochromic materials possess planar heterocyclic molecular structures in both coloured and bleached states. The optical absorption is dominated by the presence of carbonyl (-C=0) groups together with a chain of conjugated double bonds, the position of the absorption maximum depending on the length of this chain. Essentially, the photochromic transition between the two optical states proceeds via pho t o- i duced bond- cleavage/ring-closure, with corresponding changes in the

length of the conjugated region.

The molecular asymmetry, particularly that associated with the planar structure, suggests that optical properties on the molecular scale will be anisotropic; in particular, if it is assumed that the polarisability of a particular bond is anisotropic, more specifically, exhibiting rotational symmetry parallel to the bond axis and varying with the angle between the bond axis and the applied electric field, then the overall molecular polarisability will show a related angular dependence. More generally, the degree of anisotropy of a molecule depends on the properties of the individual bonds and their spatial arrangement, and in a relatively planar structure, may be expected to exhibit a symmetry axis normal to the molecular plane.

This postulated structural behaviour leads to two important characteristics on the molecular scale, theoretically observable in single crystal form:

(a) Birefringence

The refractive index of the photochromic material exhibits anisotropy, typically with axial symmetry.

(b) Dichroi sm

The optical absorption properties of the material, in both coloured and bleached states, exhibits anisotropy, again generally with axial symmetry.

Previously considered photochromic materials, and optical devices derived from them, have been discussed exclusively in terms of planar layers formed by solvent-

casting, spin-coating or solvent-ass sted in-dif fusion (imbibing) techniques, as described in our British Patent Application No.86 25513 (Publication No.2197495). In each case, the resultant photochromic layer is formed of material at relatively low dilution in a suitable polymer matrix. Under such conditions, the photochromic molecules can be considered to be disordered, with molecular axes randomly and uniformly distributed in space. In order to exploit intrinsic optical anisotropies exhibited by these materials, a technique is required for preparation of molecularly ordered layers; since the preparation of single crystals in this context is generally impracticable.

In accordance with this invention, the preparation of highly aligned thin films of photochromic materials may be carried out by various methods described below, depending on the class of compound in question.

Examples of photochromic compounds which exhibit the required optical anisotropic properties are fulgides which possess a dipole in their bleached form. These include fulgides disclosed in British Patents Nos. 1442628; 1464603 and 1602755. Preferred fulgides may be represented by the general formula:

⁽ 1)

where R represents an aromatic group such as phenyl and R^, R ₂ and R ₄ represent hydrogen, alkyl, aryl or aralkyl groups or a heterocyclic ring, e.g. a furan, benzofuran, thiophene or benzoth opene ring. Specific examples are 2 , 5 - d i m e t h y 1 - 3 - f u r y 1- e t hy 1 i d ene (isopropylidene succinic anhydride and 2, 5-di methyl-3- furyl-ethylidene (adamantylide) succinic anhydride.

In practice, many photochromic materials exhibit extinction coefficients in excess of 10,000, with correspondingly low transmission through thin single crystal plates, where these are available. In physico- chemical terms, the dipole moment associated with the C=0 bonds in the fulgides lies parallel to the c-axis, while those relating to the aromatic bonds lie in the ab plane.

Also useful for the production of devices in accordance with the invention are photochromic benzo- or naphthopyrans which undergo thermally induced colouring to a polar form. Such pyran derivatives may be represented, for example, by the general formula (2):

(2)

where R and R ¹ may; be independently selected from hydrogen, alkyl, alkoxy, chloro, alkyl or dialkyl amino or hydroxy, and R ² and R ³ are preferably alkyl or aryl but may together represent a spiro heterocyclic ring. Photochromic pyrans and methods for their preparation are diclosed in our copending British patent applications Nos. 86 14680 and 86 11837 (Publication Nos: 2193005 and 2190379). Examples of specific compounds of this kind are shown in formulae (A), (B) , (C), & (D) below in which one of R* _| _ and r ₂ is hydrogen or methyl and the other is phenyl substituted with amino and R3 and R _* are methoxy.

In the case of photochromic compounds which undergo thermal colouring to a polar state, such as the naphthopyrans mentioned above, the compound may be crystallised from a melt while being subjected to an appropriate electric field. A suitable structure for carrying out the molecular alignment during cooling from the melt is shown diagrammatically m the accompanying Figures 1(a) and (b). As can be seen from Figure 1(a), a photochromic transparent layer 1 is sandwiched between a pair of opposed substrates 2 (e.g. a pair of glass slides). The inwardly facing surfaces of the glass slides are coated with a metal film 3. Perpendicular alignment of the molecules of photochromic compound within layer 1 is produced by establishing a potential difference between the metal films 3 forming a pair of electrodes. The molecular alignment is illustrated schematically by the arrows in Figure 1(a).

In Figure (1(b), the arrangement is similar to that shown in Figure 1(a), and the same reference numerals have been used to indicate equivalent parts of the structure. However, instead of coating the glass slides 2 overall with a metal film, the electrode forming film portion 3b is confined to opposite ends of the glass slides 2. When a potential difference is established between the electrodes, molecular alignment is encouraged generally parallel to the plane of the slides 2.

Photochromic compounds which exhibit a dipole or thermal colour at 100°C or less, including certain of those

mentioned above, may be aligned in a plastics matrix. A doped matrix may be prepared by dissolving the photochromic compound in a suitable polymer solution and solvent casting a thin film between substrates which are furnished with electrodes according to the alignment required. The film is heated to approxi ately 100°C and allowed to cool under the influence of the electric field. A particularly suitable range of polymers for photochromic compounds which show high solubility, such as the naphthopyrans, are polymers of dif luoroethy lene, tr i f luoroethy lene and vinylidene fluoride.

Apart from polarisation selectivity, the general properties of aligned photochromic materials, e.g. sensitivity, spectral response and spatial resolution, are typical of the unaligned materials. BIREFRINGENT PHOTOCHROMIC DEVICE APPLICATIONS

Nonlinear device applications associated with the use of materials exhibiting optical birefringence have been described in our British Patent Specification No. 2192071. In the configurations outlined therein, a family of nonlinear devices exhibiting polarisation selective properties was implemented by incorporating discrete nonlinear and birefringent photochromic elements into a Fabry-Perot etalon. The availability of a birefringent photochromic material permits implementation of polarisation selective nonlinear and related functions (with a single photochromic layer instead of the double element structure hitherto considered). Other devices and functions which

could be adapted using the polarisation techniques of this invention are described in our British Patent Speci ications

Nos: 2180360&2197495.

DICHROIC PHOTOCHROMIC DEVICE APPLICATIONS

Conventional plastic polarising sheet comprise arrays of dichroic molecules, selectively attached to aligned polymer molecules. In this arrangement light polarised in one plane is absorbed strongly while the orthogonal polarisation suffers relatively little attenuation; the resultant beam is linearly polarised. The present invention provides comparable materials, with the important difference that the selective absorption of one polarisation can be directly controlled by external optical means.

Figure 2 shows schematically an example of an optically controlled polariser based on an aligned photochromic layer in accordance with th s invention. In Figure 2(a), the bleached aligned photochromic layer 10 is optically isotropic, transmitting all polarisations equally. On colouring by means of external U.V. colouring beam 11, the layer 10 is converted to a coloured, aligned photochromic layer 10b (Figure 2(b). In this state, aligned molecules absorb one polarisation selectively, and the resultant transmitted light is linearly polarised in the orthogonal direction. Bleaching of the layer either via visible illumination or thermally restores the initial, unpolarismg state.

Figure 3 shows a modification of the arrangement shown in Figure 2 in which the structure is the same but the input

radiation is polarised in the ultimate photochromic extinction direction. This arrangement functions as an optically controlled switch, since conversion of the photochromic layer into its coloured form causes absorption of the single incident polarisation state. Figure 3(a) shows the 'on-state' and Figure 3(b) the Off-state".

The two-beam photochromic bistability described in our British Patent Application No.85 19711 (Publication No.2180360), operates via the absorptive properties of the photochromic layer.. The provision of an intrinsically polarisation-dependent photochromic layer according to this invention, permits the fabrication of new classes of polarisation-selective devices based on this principle.

The molecules of the photochromic material may be aligned by mechanical stretching of a film containing the photochromic material. Particularly suitable film materials are poly( v inyl idene fluoride), PVDF, and copolymers of vinylidene fluoride and trif luoroethylene (VDF:TrFE copolymers). PVDF is a semi-crystallme polymer having CF ₂ CH repeating units. Some of the properties of PVDF and techniques for polarization of PVDF films are described in 'Piezoelectricity in Polyvinylidene Fluoride', G.M. Sessler, J.Acoust.Soc.Am. 7f , 1596-1608 (1981). PVDF can exist in a number of crystalline forms which may be interconverted. In particular, drawing the trans-gauche- trans-gauche' material (Form II) yields the all trans planar zigzag Form I which transforma ion may be- utilised to orientate contained molecules of photochromic material.

Form II may also be converted to Form I by drawing. The conversion is reversible on high temperature annealing.

Form II can be converted to Form II and hence to Form I by the influence of electromagnetic fields. These transformations may also be used to effect alignment. Procedures for producing PVDF homopolymer and copolymer films and effecting molecular orientation of dipoles in the polymers are also described in British Patent No. 1589746. EXAMPLE

A VDF:TrFE copolymer (70:30 VDF:TrFE molar ratio) was dissolved in methyl ethyl ketone (M.E.K.) and the photochromic compound having the formula (C) above was added to the polymer solution to form a solution containing approximately 1% by weight of the photochromic compound. The solution was filtered and cast onto a glass plate onto which an aluminium electrode had previously been deposited by vacuum deposition. After drying at room temperature for 15 minutes and then in an oven for 1 hour at 60°C, an aluminium electrode was evaporated onto the exposed surface of the film. The film was poled by applying an electrical field of about 20 MVm while maintained at a temperature of 100°C to cause simultaneous alignment and colouring of the photochromic compound. In the coloured, aligned state the film is isotropic, but is reversibly converted to an anisotropic state by illumination with visible light.

Although the particular means for effecting alignment of the molecules of the photochromic material mentioned in the foregoing description has been restricted to electric or

magnetic fields, or mechanical stretching of a film containing the photochromic material, other methods are available for effecting such alignment.

For example, the photochromic material may be incorporated into a host polymer (e.g. by imbibition, as described in British Patent Specification No. 2197495, using as host polymers the fluorine containing polymers mentioned above or polymers described in British Patent Specification No. 2197495), in which the molecules are pre-aligned (e.g. by stretching), and the molecules of the photochromic material becoming orientated by proximity to aligned dipoles in the host polymer. Further alignment may be achieved by stretching of the doped host polymer or by use of an electric field.

Another possible method of achieving alignment of the photochromic material is by crystallisation of the material from a suitable solvent.