GB2197495A - Bistable optical device - Google Patents

Abstract

The invention relates to bistable optical devices comprising a light-transmitting plastics matrix 1 incorporating a photochromic compound disposed between parallel reflectors 2,3 to form a Fabry-Perot resonant cavity, the photochromic compound having at least two stable states and having a maximum absorption band in the visible to infra-red range. The light-transmitting plastics material has the property of changing its refractive index locally when the device is subjected to incident radiation at an intensity greater than a critical level. <IMAGE>

Description

SPECIFICATION Bistable optical device This invention relates to bistable optical devices, particularly optical resonant assemblies, which utilise photochromic compounds.

In our U.K. patent application No.85 19711, (Publication No. 2180360), a bistable Fabry Perot resonant cavity is described which comprises a layer of photochromic material supported on a transparent substrate and located between a pair of parallel reflectors. The photochromic material is illuminated with two light beams, the first beam having a wavelength which causes the photochromic compound to colour and the second beam having a wavelength which causes bleaching. By varying the relative intensities of the two beams, data may be written to and read from the layer of photochromic material.

We have now found that when certain lightsensitive materials are incorporated in a lighttransmitting plastics matrix, and the resulting device is illuminated with light at a wavelength within the absorption band of the material and at an intensity at which heating occurs, a significant refractive index change takes place in the device. Moreover, the device can be rapidly switched between stable states having different refractive indices by increasing or decreasing the incident light intensity for short intervals of time.

According to the present invention there is provided a method of operating a bistable optical device, wherein a photochromic compound having two or more stable states is incorporated in a light-transmitting plastics matrix which is capable of changing its refractive index on heating, which method comprises illuminating the device with a light beam having a wavelength within the absorption band of said compound and subjecting the device to a light pulse at a wavelength within said absorption band and at an intensity and duration sufficient to switch said device from one stable state to another.

The invention also includes a bistable optical device which comprises a light-transmitting plastics matrix incorporatiang a photochromic compound disposed between parallel reflectors to form a Fabry-Perot resonant cavity, the photochromic compound having at least two stable states and having a maximum absorption band in the visible to infra-red range and the light-transmitting plastics material having the property of changing its refractive index locally when the device is subjected to incident radiation at an intensity greater than a critical level.

The principle of operation of the bistable optical device of this invention is illustrated by the following experimental demonstration and accompanying drawings in which: Figure 1 is a schematic, perspective view of an optical device in accordance with this invention, Figure 2 is a graph showing the bistable response of one photochromic compound/plastics matrix combination in accordance with the invention, Figure 3 is a graph of intensity of incident radiation against time showing the switching between two stable states.

Figures 4 and 5 are schematic illustrations of the use of the optical device as a multiport device, Figure 4 illustrating an AND/OR logic gate and Figure 5 a NAND/NOR.

Figures 6a and 6b are graphs illustrating the output of the devices of Figures 4 & 5 respectively.

Figures 7 & 8 show the absorption profiles of 2,5-dimethyl-3-furylethylidene (isopropylidene) succinic anhydride (compound A), Figure 9 is a graph showing the absorption profile of 2-xanthylidene-indan-1,3-dione (compound B), Figure 10 shows the structural formulae of compounds A & B.

Figures 11(a) and 11(b) illustrate schematically the use of the devices in accordance with the invention as a single switch and as a bistable switch array.

A sheet of CR39 plastic (diethylene glycol bis-allyl carbonate) was treated with an organic solvent solution of the light absorbing compound (in this case 2-xanthylidene-1,3-dione) using the inbibition technique described in our copending application No. 8709761 (Publication No. 2189624). The resulting impregnated sheet 1 was disposed between parallel reflectors 2 and 3 as indicated schematically in Figure 1 to form a Fabry-Perot cavity.

The resulting Fabry-Perot cavity can be operated as a bistable optical device which can be switched between two stable states by increasing and decreasing the intensity of an incident beam. The characteristics of the device are illustrated by reference to Figures 2 and 3.

Figure 2 shows graphically the relationship between the transmitted intensity-and incident intensities when the device was illuminated with an Argon ion laser beam (514.5 nm).

The device was biased by applying an incident beam having an intensity within the bistable region shown in Figure 2 (indicated by the value 4). Switching of the device between the two stable states indicated by the loop in Figure 2 was achieved by increasing or decreasing the incident light intensity to values outside the bistable region, e.g. values 5 & 6, for short pulses. For maximum speed of switching the pulses should be as short as possible. In the case of the particular device described above, switching occurs readily with pulses of 1 millisecond duration and can be effected with pulses of shorter duration. The response of the device to these short periods of increased or decreased light intensity is shown directly in Figure 3. In Figure 3 the upper trace 7 is the incident light beam, while trace 8 shows the corresponding level of light transmitted through the cavity.

The bistable devices of this invention can be used as a logic gate. For example, one or more incident beams can be used to produce an input having a modulated intensity, thus creating a multiport device. Figure 4 illustrates the device operating as a AND/OR logic gate.

Similarly, one or more incident beams can be arranged to produce the inverse logic functions (NAND/NOR) utilising the light reflected from the device as the output and this arrangement is illustrated by Figure 5. As shown in Figure 6, the reflected output from the device displays the inverse of the light transmitted from the device.

Our observations indicate that the bistability phenomenon arises from local heating in the polymer matrix caused by absorption of the incident light. Two possible mechanisms take place, viz: 1) Absorption of the incident radiation by the absorbing molecule/polymer matrix combination causing heating of the polymer. This results in expansion of the matrix and a corresponding change in its refractive index, which gives rise to a change in the path length of the cavity.

2) Absorption of the incident radiation by the absorbing molecule/polymer matrix combination causing heating of the absorbing molecules. This temperature increase allows a decrease in molecular overcrowding by slight reorientation of the molecule resulting in a change in the absorption profile.

The effect of heating on the absorption bands of various photochromic compounds has been investigated and our results indicate that materials should be selected for use in the present invention whose absorption band show a reversible change in their absorption profile when heated to about 100"C. As an illustration of this behaviour, Figures 7, 8 and 9 show the absorption profiles of two light sensitive compounds at several different temperatures. Figures 7 & 8 show the absorption profile of 2,5-dimethyl-3-furylethylidene (isopropylidene) succinic anhydride (compound A) and Figure 9 the absorption profile of 2-xanthylidene-indan-1,3-dione (compound B). The structure of these compounds is shown in Figure 10. These absorbance measurements were made on samples of CR39 plastic sheet imbibed with a saturated solution of compound A or B.As can be seen, heating causes a progressive reduction in the absorbance in the longer wavelength absorption band, which reverts to its original level on cooling.

Calculations from the change in the absorption spectra shown in Figures 7, 8 & 9 suggests that in the case of these particular compounds the effect of mechanism 1 above is insufficient to cause the refractive index change which has been observed. However, it is likely that it contributes something to the overall effect.

Thus, it is believed that for optimum performance in bistable optical devices of this invention, radiation absorbing compounds should be selected which should suffer a reversible change in absorption profile due to conformational change or relaxation of steric overcrowding when heated by laser irradiation to a temperature below the temperature at which the plastic matrix becomes irreversibly damaged. It is also possible that a molecule that is capable of reversible absorption/refractive index change on heating but does not itself absorb the incident radiation wili yield the same results when held in an absorbing matrix (necessary for systems using IR lasers).

A requirement for optimum performance from a material making full use of mechanism 1 (thermal expansion) is that the molecule/polymer matrix strongly absorb some part of the incident radiation.

The preferred compounds for use in this invention are those having large absorptions in the visible to infra-red region of the spectrum.

Suitable preferred materials may be, for example, selected from those described in U.K. patent specifications Nos: 1,442,628; 1,464,603 and 1,602,755 and in our copending patent application No.86 11837.

The polymer matrix may be any lighttransmitting polymer matrix which changes its refractive index on heating. Preferred polymer materials are polycarbonates. A preferred method of incorporating the light-sensitive materials in the polymer matrix is by imbibition from solution, since this enables the compounds to be substantially homogenously dispersed in the matrix. Preferably, the polymer matrix is impregnated with the compound by immersion in a boiling saturated solution of the compound in a hydrocarbon or fluorocarbon solvent.

The nature of the device fabrication process implies that extended areas, limited solely by the size of available substrates, can be prepared with a high degree of uniformity. This technique is thus ideally suited to the fabrication of extended arrays of bistable optical devices; no physical pixellation is required, since the active device region is defined essentially by the thermal characteristics of the substrate and the spatial extent of the illuminating/switching beam, which is focussed to a small diameter typically, but not necessarily, of the order of tens of microns.

Figures 11(a) and 11(b) illustrate schematically the application of the devices according to the invention to single and multiple optical processing. In Figure 11(b) a bistable switch array is produced by focussing a plurality of laser beams onto a bistable optical device fabricated to provide an extended area. Alterna tively a single beam or smaller number of beams could be deflected to interogate different areas of the device.

Claims (7)

1. A method of operating a bistable optical device, wherein a photochromic compound having two or more stable states is incorporated in a light-transmitting plastics matrix which is capable of changing its refractive index on heating, which method comprises illuminating the device with a light beam having a wavelength within the absorption band of said compound and subjecting the device to a light pulse at a wavelength within said absorption band and at an intensity and duration sufficient to switch said device from one stable state to another.

2. A method according to claim 1 in which the photochromic compound absorbs light in the visible to infra-red regions of the spectra.

3. A method according to claim 1 or claim 2 in which the optical device comprises a plastics matrix incorporating the photochromic compound and disposed between parallel reflectors to form a Fabry-Perot cavity.

4. A method according to any one of the preceding claims which is operated as a logic gate, one or more incident light beams being directed onto the device to provide one or more inputs, the intensity of the input light beams being modulated to cause switching between bistable states, thus creating a multiport device.

5. A bistable optical device which comprises a light-transmitting plastics matrix incorporating a photochromic compound disposed between parallel reflectors to form a Fabry Perot resonant cavity, the photochromic compound having at least two stable states and having a maximum absorption band in the visible to infra-red range and the light-transmitting plastics material having the property of changing its refractive index locally when the device is subjected to incident radiation at an intensity greater than a critical level.

6. A device according to claim 5 wherein the plastics matrix is impregnated with the photochromic compound by immersion in a reflexing solution of the compound in a hydrocarbon or fluorocarbon solvent.

7. A device according to claim 5 or 6 wherein the plastics matrix is a polycarbonate.