A light-emitting cavity device comprising: a pair of mirrors spaced apart to define a resonant cavity; a luminescent layer located in the cavity; and a control layer located in the cavity and controllable to adjust the resonance wavelength of the cavity and thereby spectrally redistribute the energy emitted by the luminescent layer.

BACKGROUND OF THE INVENTION 
This invention relates to tuneable microcavities, and especially their use 
in luminescent devices. 
FIG. 1 shows a planar microcavity. This is a Fabry-Perot resonator with two 
mirrors 1,2 spaced apart by a cavity 3 which contains a photon-emitting 
material 4. The mirror separation is of the order of the optical 
wavelength, so that the resonant frequency of the cavity corresponds to an 
optical frequency. Such a structure therefore has a narrow emission 
spectrum, allowing emission only at the resonance wavelength(s) of the 
cavity. It is also capable of enhancing the emission at a certain 
wavelength compared to the free-space emission of the luminescent material 
(see J. Gruner et al. J Appl. Phys. 80, 207 (1996)). These properties have 
proved to be useful for light-emitting devices based on broad bandwidth 
emitters such as organic molecular or polymeric materials, providing the 
improved colour purity and spectral tuneability required for multi-colour 
display applications (see U. Lemmer et al. Appl. Phys. Lett. 66, 1301 
(1996); H. F. Wittmann et al. Adv. Mater. 6, 541 (1995); and A. 
Dodabalapur et al. Electronics Letters 30, 1000 (1994)). 
One type of electroluminescent device is described in PCT/WO90/13148, the 
contents of which are incorporated herein by reference. The basic 
structure of this device is a light-emitting polymer film (for instance a 
film of a poly(p-phenylenevinylene)--"PPV") sandwiched between two 
electrodes, one of which injects electrons and the other of which injects 
holes. It is believed that the electrons and holes excite the polymer 
film, emitting photons. These devices have potential as flat panel 
displays. 
In more detail, such an organic electroluminescent device ("OLED") 
typically comprises an anode for injecting the positive charge carriers, a 
cathode for injecting the negative charge carriers and, sandwiched between 
the electrodes, at least one organoluminescent layer. The anode is 
typically a layer of indium-tin oxide ("ITO") which is deposited on a 
glass substrate. The organic layer(s) are then deposited on the anode and 
the cathode is then deposited on the organic layer(s) by, for example, 
evaporating or sputtering. The device is then packaged for protection. 
Organic light emitting devices have been incorporated in microcavity 
structures (see the papers by Lemmer et al. and Wittmann et al.). It has 
been shown that by using different thicknesses of a patterned inert filler 
it is possible to fabricate microcavity light-emitting devices with 
emission peaks from 490 nm to 630 nm from a single organic semiconductor 
(see the paper by Dodabalapur et al.). However, in such devices the 
resonance wavelength, and hence the emission colour, is fixed when the 
device is fabricated. Therefore, the colour (for instance of an individual 
pixel in a multi-pixel display) cannot be varied independently and/or 
externally after fabrication. 
SUMMARY OF THE INVENTION 
According to the present invention there is provided a light-emitting 
cavity device comprising: a pair of mirrors spaced apart to define a 
resonant cavity; a luminescent layer located in the cavity; and a control 
layer located in the cavity and controllable to adjust the resonance 
wavelength of the cavity and thereby spectrally redistribute the energy 
emitted by the luminescent layer. 
The mirrors preferably have a reflectivity high enough to support a 
resonant cavity mode in the spectral range where the luminescent material 
emits. This is most preferably in the visible light range. One or both 
mirrors are preferably non-absorbing or substantially non-absorbing 
mirrors, for example distributed Bragg reflectors ("DBR"). One or both of 
the mirrors may be conductive so as to serve as an electrode of the 
device. 
The resonance wavelength of the cavity is preferably adjusted by a change 
in refractive index or thickness of the cavity and/or of the control 
layer. Preferably the refractive index of the control layer is 
controllable to adjust the resonance wavelength of the cavity. The control 
layer preferably comprises a material whose refractive index is adjustable 
by the application of a stimulus. The stimulus may be an electric field, 
electromagnetic radiation (e.g. UV or IR radiation), pressure, or a 
chemical (especially a gas). One or both mirrors of the cavity may 
suitably be transparent to the relevant stimulus. The control layer most 
preferably comprises a liquid crystalline material, for instance a liquid 
crystal or a liquid crystalline polymer. The device may also comprise an 
alignment layer adjacent the liquid crystal for aligning (for example by 
means of its surface relief) the molecules of the liquid crystal. 
The control layer may be controlled by the application of an electric field 
across it. In this case the device preferably comprises a pair of 
electrodes disposed on either side of the control layer. One or both of 
these electrodes preferably lies outside the cavity. Where one or both of 
the mirrors are conductive they may also constitute one or both of those 
electrodes. If either of the electrodes lies in the cavity (between the 
mirrors) it is preferably transparent (or at least partially transparent), 
at least in the range of frequencies emitted by the luminescent layer. 
All material in the cavity preferably has low absorption in the range of 
emission of the luminescent material. 
The luminescent material could be a photo- or electroluminescent material. 
The luminescent material could comprise an inorganic semiconductor (e.g. 
ZnS, ZnSe or GaN), a glass doped with a fluorescent material or an organic 
material. The luminescent material could be a luminescent organic polymer 
(an organoluminescent polymer), especially a conductive or semi-conductive 
polymer such as a semiconductive conjugated polymer material. The 
luminescent material could be tris (8-hydroxyquinoline)aluminum, PPV, 
poly(2-methoxy-5(2'-ethyl)hexyloxyphenyleneviylene) ("MEH--PPV"), a 
PPV-derivative (e.g. a di-alkoxy derivative), a polyfluorene and/or a 
co-polymer incorporating polyfluorene segments, PPVs and/or related 
co-polymers. 
The term "conjugated" indicates a polymer for which the main chain is 
either fully conjugated, having extended pi molecular orbitals along the 
length of the chain, or is substantially conjugated, but with 
interruptions to conjugation at various positions, either random or 
regular, along the main chain. It includes within its scope homopolymers 
and copolymers. 
Where the luminescent material is an electroluminescent material there are 
preferably electrodes located on either side of it for applying an 
electric field to stimulate the luminescent material to emit light. One of 
these electrodes may lie outside the cavity or be provided by one of the 
mirrors. One of the electrodes may lie within the cavity (most preferably 
between the luminescent layer and the control layer) in which case it is 
preferably transparent or semi-transparent, at least in the range of 
frequencies emitted by the luminescent layer. One of these electrodes may 
be the same as one of the electrodes for applying an electric field across 
the control layer. 
The control layer is preferably substantially thicker than the luminescent 
layer. Preferably the ratio of the thickness of the control layer to the 
thickness of the luminescent layer is in the range from 50 to 300 nm, 
preferably in the range from 70 to 150 nm. 
The light emitted from the cavity is preferably fully, substantially fully 
or at least partially linearly polarised. To this end, the device may 
comprise a polariser for (to a suitable extent) polarising light emitted 
from the cavity. The polariser may suitably be in the form of a sheet, 
suitably located between the cavity and a viewer. Another suitable 
solution is for the luminescent layer to be adapted to emit at least 
partially polarised light.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 2 shows an embodiment of a microcavity device. The device comprises a 
pair of glass sheets 10,11 each of which bears a number of layers. Each 
glass sheet bears an electrode layer 12,13 which extends to the edge of 
the glass sheet to allow it to be connected to an external voltage. Over 
each electrode layer is a mirror layer 14,15. Over the mirror layer of one 
glass sheet is a layer 16 of rubbed polyimide. Over the mirror layer of 
the other glass substrate is a layer 17 of PPV. The glass sheets are 
arranged so that the polyimide layer faces the PPV layer across a cavity 
18, with a spacer 19 in the cavity holding the mirrors a fixed distance 
apart. The cavity is filled with liquid crystal material. 
To fabricate the device each glass substrate is first provided with its 
electrode layer. The electrode layers are of transparent indium tin oxide 
(ITO) deposited by sputtering to a thickness of between 30 and 100 nm. The 
sheet resistance of the ITO electrodes is around 30 Ohms/square. 
Then a mirror layer is deposited over each electrode layer. Each mirror 
layer is a 12 layer dielectric stack mirror (having alternating layers of 
MgF and ZnS) with a reflection band (higher than 97% reflectivity) from 
490 to 650 nm and a peak reflectivity of about 99%. 
PPV precursor is spun on to mirror 15 at 2000 rpm for 60 seconds and then 
baked at 200.degree. C. for 12 hours under vacuum to form an 80 nm PPV 
layer 17 on the mirror 15. 
On the other mirror 14 a layer of SN7212 polyimide is spun at 3000 rpm for 
40 seconds and then baked at 220.degree. C. for 3 hours. The polyimide 
layer 16 is then buffed at 500 rpm to provide an alignment surface for the 
liquid crystal. 
The cell is assembled using UV glue. A spacing between the mirrors of 
around 2 .mu.m is obtained by using 2 .mu.m epostar spacers, only one of 
which is shown in FIG. 2. The cell is then vacuum filled with positive 
uniaxial BL048 nematic liquid crystal (available from Merck Ltd as 
Licrilite TM) and sealed. A voltage source V is connected across the 
electrodes. 
The behaviour of the cell will now be discussed. The cell constitutes a 
planar microcavity. Three factors determine the effective length, 
L.sub.eff, and thus the resonance wavelengths .lambda..sub.Res of the 
cavity: the separation of the mirrors, 
L.sub.mirror.sbsb.--.sub.separation, the phase change on reflection of 
light from the mirrors, L.sub.phase.sbsb.--.sub.change, and the refractive 
indices, n, of the materials inside the cavity: 
EQU L.sub.eff =n.multidot.L.sub.mirror.sbsb.--.sub.separation 
+L.sub.phase.sbsb.--.sub.change 
and 
##EQU1## 
where q is an integer. 
A nematic liquid crystal typically lies flat on the rubbed surface, with 
its mesogenic groups orientated by the rubbing, and re-orientates itself 
to stand perpendicular to the surface if an electric field is applied. The 
liquid crystal in the cell cavity has a high birefringence. BL048 has a 
low refractive index n.sub.o of 1.5277 perpendicular to its optic axis and 
a high refractive index n.sub.c of 1.7904 along the optic axis. 
Consequently the two normal modes of polarisation in the liquid crystal 
will sample the same refractive index n.sub.o if the light propagates 
along the optic axis but two different refractive indices n.sub.o and 
n(.theta.) if the light propagates in any other direction, with n(.theta.) 
depending on the angle between the optic axis and the direction of light 
propagation and n.sub.o &lt;n(.theta.)&lt;n.sub.c. By applying an electric field 
across the electrodes the orientation of the liquid crystal molecules 
within the cell can be controlled, and this allows the refractive index 
within the cavity to be controlled. Thus the effective length of the 
cavity can be altered without changing its actual length. However, as 
explained above, only one polarisation samples a changed refractive index. 
The operation of the cell will now be described. To excite the 
photoluminescence ("PL") of the PPV a 458 nm laser is used. PPV can show 
efficient luminescence (with a PL efficiency of up to 80%) and has a broad 
emission spectrum which spans three cavity modes for the device of FIG. 2. 
The lower panel of FIG. 3 shows at 20 the free-space emission spectrum of 
PPV; its structure is due to vibronic coupling to the excitonic emission 
process. 
The upper panel of FIG. 3 shows the effect of applying electric fields 
across the electrodes of the cell whilst the PPV is being pumped by the 
laser. The spectra of FIG. 3 are measured with a polariser placed in front 
of the device, so as to pass light linearly polarised along the rubbing 
direction of the polyimide. (The polariser could polarise the pumping 
light or the emitted light). The electric fields are applied as a square 
wave of frequency 1 kHz and amplitude in the range from 0V to 16V. Lines 
21 to 25, corresponding respectively to fields of 0V, 4V, 6V, 8V and 16V 
are shown in the upper panel of FIG. 3. As shown in FIG. 3 the cavity 
modes are narrow (6 nm FWHM) and can be tuned in wavelength because the 
effective refractive index of the liquid crystal layer decreases as a 
function of bias, thus shifting the resonance wavelengths. For example, 
the twelfth mode is at 616 nm at 0V bias and can be moved to 560 nm at 16V 
bias. This shift of 56 nm corresponds to a change in refractive index from 
1.73 to 1.55 (including the thickness of all layers in the cavity) or a 
change in the optical thickness of the cavity from 3.3 .mu.m to 3.7 .mu.m. 
At 4V bias a relatively high fraction of the modes corresponding to the 
ordinary refractive index n.sub.o appear in the spectrum. This is because 
at low fields and at intermediate angles the forces on the liquid crystal 
molecules due to the alignment layer and the electric field are not high 
enough to keep them oriented in the plane perpendicular to the mirrors and 
along the rubbing direction. The light transmitted by the polariser has 
therefore sampled both refractive indices n.sub.o and n(.theta.). 
FIG. 4 shows the PL measured without the polariser for fields of 0V and 16V 
(lines 26 and 27 respectively). With the polariser removed, modes with the 
electric field perpendicular to the rubbing direction are also observed. 
These modes correspond to the low refractive index n.sub.o of the liquid 
crystal and do not shift with applied field. The mode numbers (shown in 
FIG. 4) and the mirror separation (here 2.13 .mu.m) can be calculated from 
the position and spacing of the peaks. 
This shift of the twelfth mode by 56 nm corresponds to a shift of 
wavelength from green to blue light or red to green. Even greater changes 
are achievable with further optimisation of the thicknesses of the layers 
present in this device, as is detailed in the modelling below, up to 90 
nm. In addition, still greater changes could be achieved by using liquid 
crystals with greater birefringence, and with improved alignment layer. 
The available shift can be calculated in more detail. For a liquid crystal 
of optical anisotropy .DELTA.n, the shift in resonance wavelength 
.DELTA..lambda. (assuming the thickness of the emissive layer is small 
compared to the thickness of the liquid crystalline layer) is given by: 
##EQU2## 
where .lambda..sub.0 is the resonance wavelength of the microcavity before 
the refractive index is altered and n is the refractive index for which 
the resonance wavelength of the cavity is .lambda..sub.0. Thus, even using 
BL048 liquid crystal a shift of around 90 nm is possible. For thicker 
cavities the mode spacing decreases. Then, increasing .DELTA.n optimises 
.DELTA..lambda. only to the point where the mode spacing becomes equal to 
.DELTA..lambda.. That is the case for: 
##EQU3## 
This yields a mirror separation of 1.23 .mu.m for BL048 and a wavelength 
separation of 90 nm in order to achieve a mode spacing of 90 nm. 
Some characteristics of the device may be noted, especially as regards the 
coupling of cavity modes to the electronic excited states in the polymer. 
The cavity resonances may be tuned through the peaks in the emission 
spectrum of the polymer. FIG. 3 shows that the heights of the microcavity 
peaks approximately scale with the free-space PPV PL spectrum. However, at 
wavelengths between 550 nm and 575 nm the height of the microcavity peaks 
decreases slightly faster with increasing wavelength than might be 
expected from the free-space spectrum. Also, at 16V 10% more power is 
radiated into the forward direction than at 6V, although the total 
radiated power remains constant. This is believed to be due to the angular 
dependence of the microcavity emission. The wavelengths of the cavity 
modes decrease (blue shift) with increasing viewing angle, as can be seen 
from FIG. 5. FIG. 5 shows spectra at viewing angles of 0.degree. to 
40.degree. for applied voltages of 6V (upper panel) and 16V (lower panel). 
If the modes at higher viewing angles overlap a peak in the PPV PL 
spectrum (as for the spectrum at 6V) they are coupled strongly (see FIG. 
5) thus reducing the power radiated into the forward direction and also 
causing the more rapid drop-off between 550 nm and 575 nm. 
Comparing the spectra of FIG. 3 for 0V and 8V it will be seen that both 
have cavity modes at 570 nm but since these have different mode numbers 
(differing by one) the adjacent modes are spaced more widely at 8V (95 nm 
as opposed to 85 nm). This leads to a reduction in intensity for the two 
outer peaks. As a result, the central peak at 8V is higher than that at 
0V. This is believed to demonstrate the modification of the spontaneous 
transition rate in a microcavity leading to the channelling of radiative 
power into the resonant modes. This is in addition to the spatial 
redistribution of the radiative power. 
The spontaneous emission rate of an optical emitter (such as PPV) can be 
altered if it is inside a resonant cavity because the cavity enhances the 
rate of emission at the resonance wavelengths of the cavity and suppresses 
the rate of emission at other wavelengths. In the presence of 
non-radiative decay channels as an increase in the total radiative rate 
can lead to an enhancement of the efficiency of light emission. The device 
of FIG. 2 affects the total emission rate only marginally. However, the 
spectral and spatial dependence of the radiative rate is strongly modified 
by the microcavity, which spectrally redistributes the energy emitted by 
the PPV layer. Thus, in contrast to the filter devices proposed in the 
prior art (see in particular U.S. Pat. No. 5,559,400 and T. Nakayama et 
al. Inorganic and Organic Electroluminescence 1996 Berlin p211), which 
select the emitted wavelength by rejecting unwanted frequencies from a 
luminescent layer, the device of FIG. 2 tunes wavelength emission by 
channelling radiative power (and therefore energy) into the resonant modes 
of the cavity. This provides a greatly increased efficiency. Providing the 
electrodes 12,13 outside the cavity also helps lift efficiency. 
Some alternative methods of constructing the device of FIG. 2 will now be 
mentioned. 
1. Electrode and mirror layers could be combined by providing a single 
conductive and reflective layer; for example a layer (say 20 to 50 nm 
thick) of a highly reflective metal--one example is a 35 nm thick layer of 
silver. Conductive mirror layers could also be distributed Bragg 
reflectors ("DBRs") with stopbands in the wavelength region where the 
luminescent polymer emits. 
2. The luminescent material could be any suitable photo- or 
electroluminescent material. Numerous materials could be used instead of 
PPV. Examples include luminescent molecular materials such as those used 
in molecular electroluminescent diodes, examples of which are described in 
"Doped Organic Electro-luminescent Devices with Improved Stability", J Shi 
and C W Tang, Appl. Phys. Lett. 70, 1665-1667 (1997). Other examples 
include luminescent materials selected from the class of conjugated 
polymers, of which the PPV described in the embodiment above is an 
example. Characteristics of conjugated polymers are described in our 
PCT/WO90/13148. The material should have a wide free-space emission 
spectrum because the output wavelength of the cavity can only be selected 
from within the range of the emission spectrum of the luminescent 
material. 
3. Additional layers could be used, especially to protect the luminescent 
material. For example, where the luminescent material is PPV a layer of 
PEDT/PSS (see EP 0 686 662) could be used between it and one or more 
adjacent layers. 
4. The mirror and electrode layers could be reversed so that at least one 
of the mirror layers is deposited directly on the glass sheet. However, 
this may be found disadvantageous because the electrode layer would then 
lie in the resonant cavity and may introduce additional losses. 
5. Instead of a liquid crystal in the cavity a liquid crystal polymer 
("LCP") (e.g. E63 or LCP105 available from Merck Liquid Crystals) could be 
used. This could allow the spacers to be dispensed with. The LCP could be 
prepared by a standard technique such as coating from solution. The 
thickness of the LCP film could be from 500 to 1000 nm. Higher voltages 
(e.g. 20V) generally need to be used with LCPs. Other alternatives are for 
the cavity to contain a material that exhibits a refractive index change 
on electrochemical doping, e.g. polythiophene (see P. G. Bruce "Solid 
State Electrochemistry", Cambridge University Press (1995)); exposure to 
radiation such as UV or IR, e.g. any suitable photorefractive material; or 
exposure to pressure or chemicals. In each case the device must be such as 
to allow the controlling medium to reach and act on the material in the 
cavity. 
6. The luminescent material could be arranged so as to emit partially or 
fully polarised light and thus avoid the need for polarisers. To achieve 
this the material could be deposited so that its dipoles are selectively 
oriented. Suitable techniques for achieving polarised emission from 
luminescent materials include Langmuir-Blodgett deposition (see M. Era et 
al. Thin Solid Films 179, 1 (1989)), vertical spin coating (see K. Misawa 
et al. Appl. Phys. Lett. 63, 577 (1993)), rubbing-induced molecular 
orientation (see M. Hamaguchi Jpn. J. Appl. Phys. 34, 712 (1995), 
stretching of the polymer (see P. Dyreklev et al. Adv. Mater. 7, 43 
(1995)) and vacuum deposition (M. Era et al. Appl. Phys. Lett. 67, 2436 
(1995)). Another approach is to incorporate the polariser into the cavity. 
7. Instead of the rubbed polyimide layer one of the other layers could be 
used to orientate the liquid crystal molecules. For instance the polyimide 
layer 16 could be omitted and the mirror 14 deposited by sputtering at an 
angle. 
8. Instead of glass sheet substrates other materials could be used, for 
example a sheet of an organic material. The sheet could still provide 
structural stability to the device. 
These are just non-limiting examples of the changes that could be made. 
The device of FIG. 2 absorbs light at short wavelengths (e.g. blue and UV) 
and emits light in a part of the spectrum where the emission wavelength 
can be controlled. As an alternative to laser stimulation of the PPV, it 
could be excited by an LED (e.g. a blue nitride LED) or a UV fluorescent 
lamp providing a high peak excitation intensity. Another alternative is 
the embodiment shown in FIG. 6. 
FIG. 6 shows an embodiment in which the PPV is stimulated electrically. 
(The electrical stimulation of PPV is described in PCT/WO90/13148). The 
device of FIG. 6 comprises a pair of glass sheets 30,31. Each glass sheet 
bears a mirror layer 32,33 which doubles as an electrode layer. Over the 
mirror/electrode layer 32 of one glass sheet is a layer 34 of rubbed 
polyimide. Over the mirror/electrode layer 33 of the other glass substrate 
is a layer 35 of PPV and over that a transparent electrode layer 36. The 
glass sheets are arranged so that the polyimide layer 34 faces the 
electrode layer 36 across a cavity 37, with a spacer 38 in the cavity 
holding the layers apart. The cavity is filled with liquid crystal 
material. 
To fabricate the device each glass substrate is first provided with its 
mirror/electrode layer 32,33. In this example the mirrors are layers of 
silver 35 nm thick. 
Over the mirror 32 the polyimide layer is deposited as for the device of 
FIG. 2. 
Over the mirror 33 the PPV layer is deposited as for the device of FIG. 2. 
Over that layer the electrode layer 36 is deposited. This layer is of ITO, 
deposited by electrodeposition or sputtering to a thickness of 30 to 100 
nm. The sheet resistance of the ITO layer is around 30 Ohms/square. 
Instead of ITO a transparent conductive polymer could be used for the 
electrode layer 36. 
The cell is then assembled using UV glue. A spacing between the mirrors of 
around 2 .mu.m is obtained by using 2 .mu.m epostar spacers. The cell is 
then vacuum filled with BL048 liquid crystal and sealed. 
In this embodiment two voltage sources are used, one (V1) for applying a 
voltage between electrodes 33 and 36 to stimulate the PPV and the other 
(V2) for applying a voltage between the electrodes 36 and 32 to control 
the liquid crystal. 
This device has similar properties to that of FIG. 2. The PPV can be 
stimulated by applying a voltage at V1 and the colour of the emitted light 
tuned by V2 by adjusting the resonance wavelength of the microcavity as 
for the embodiment of FIG. 2. However, in this example no external laser 
stimulation is needed. 
The principle of the devices described above may be used in several other 
applications: 
1. By patterning the luminescent layers and/or the electrodes a multi-pixel 
display device may be fabricated. For example, in the embodiment of FIG. 6 
the electrode layer 36 could be patterned to allow individual pixels and 
their associated cavity regions to be individually addressed. This 
overcomes the need in some prior devices for the luminescent layer to be 
patterned. 
2. Sensing devices may be fabricated. In one example the luminescent layer 
could be a photovoltaic cell. Then, by selecting the resonance wavelength 
of the microcavity the absorption in the absorbing layers of the 
photovoltaic cell (and therefore the current or field generated by the 
cell) could be controlled. The response of the photovoltaic cell as a 
function of resonance wavelength could then provide information on the 
spectrum of light incident on the device. 
3. An optical switch may be fabricated. The luminescent material could be 
chosen to have a narrow emission spectrum. When the resonance wavelength 
of the cavity lies in that spectrum the device will emit 
strongly--otherwise there will be low emission intensity (depending on the 
quality factor of the microcavity). 
4. There could be fabricated a sensor for a parameter that causes the 
resonance wavelength of the cavity to change. A photodetector could be 
used to monitor the emission intensity and/or wavelength of the device. In 
a particularly compact solution the photodetector could be integrated 
within the device. For instance, instead of the layers 35 and 36 in FIG. 6 
there could be an electrode of Al or Ca with an Al layer underneath it and 
on top a 100 nm PPV layer and an ITO top electrode. The Al layer would 
serve as the bottom mirror of the microcavity, and could be prepared as a 
DBR for maximum reflectivity. The overall device could then provide a 
sensor for electromagnetic radiation, humidity or pressure by detecting 
the change in emission wavelength, intensity, or current or voltage. An 
array of such devices could be used to detect spectral, spatial or 
time-dependent patterns of incident stimuli. 
5. A modulator could be provided in the case when the resonance wavelength 
of the cavity is dependent on incident electromagnetic radiation. 
6. In a multi-pixel display there is a need for uniformity of colour 
between the pixels. If a device has non-uniformity resulting from the 
production process this could be corrected by a tuneable microcavity with 
only a small change needed in refractive index. 
7. If the tuning layer of the microcavity is selected so that it can be 
switched quickly from one effective length to another then display devices 
can be made in this way whose pixels switch colour quickly to provide a 
wide range of apparent colours by temporal multiplexing. For these devices 
the cell should preferably be relatively thin; around 1 to 2 .mu.m. 
The present invention may include any feature or combination of features 
disclosed herein either implicitly or explicitly or any generalisation 
thereof irrespective of whether it relates to the presently claimed 
invention. In view of the foregoing description it will be evident to a 
person skilled in the art that various modifications may be made within 
the scope of the invention.