GB2341973A - A laser screen for a flat panel display - Google Patents

Abstract

A laser screen 1 includes a panel, a cavity in the panel in which active material is located, the cavity extending substantially over the area of the panel; and a means of pumping the active material in such a way that lasing takes place, the pump means being arranged to cause lasing to take place simultaneously over the area of the screen. This results in a collimated source of light which, if in the UV or near-UV range, can be used as a source to be modulated by an LCD panel 3 on to a phosphor screen 7, thus forming a display. A polariser 5 may also be used, as may projection optics (fig 2). The active material may comprise an inorganic or organic semiconductor, a luminescent polymeric material, a metal chelate, another organic material, or any other phosphorescent or fluorescent material, either solid or liquid.

Description

--- 2341973 -1 LARGE-AREA LIGHT SOURCES Many display applications require

illumination which is collimated at some point. For example, in a liquid-crystal panel-type projection display the light has to be as near collimated when it passes through the panel as possible. This is generally achieved by using a very intense point source and collimating lenses, but even so it is not perfect and throughput is limited.

Standard LCD flat-panel displays also require collimation - both the brightness of the image in the viewing direction and the degree of multiplexing of the electro-optic effect improve with better collimation.

In particular the PL-LCD (photoluminescent liquid crystal display) architecture, where activating light is modulated by a liquid crystal so as to activate phosphors selectively, relies on good collimation to improve contrast and reduce pixel crosstalk.

For these reasons it would be advantageous to generate collimated light directly. This is, of course, precisely how a laser operates. However, the problem with lasers is that the cross-sectional area of the beam is usually very small and, although beam expanding is possible, it has so far not been practical to produce laser beams of the required size for most display applications. In the case of a flat-panel display the main reason for this is that beam expanding requires free space optics, for which there is no room in a flat panel architecture. Consequently it is desirable to construct a large-area laser directly, referred to as a laser screen, i.e. a laser emitting over the surface of more or less extensive panel.

According to one aspect of the invention there is provided a laser screen including a panel, a cavity in the panel in which active material is located, the cavity extending substantially over the area of the panel; and a means of pumping the active material in such a way that lasing takes place; in which the pump means is arranged to cause lasing to take place simultaneously over the area of the screen.

In a variation the lasing can take place simultaneously over a substantial part of the area of the screen, i. e. not merely at one point but, say, over a line of the screen from one side to the other. This can be particularly useful for line-addressed or raster-scanned displays. The pump means can be optical or electrical, or conceivably both.

In an alternative aspect the invention provides a liquid-crystal display including a source of activation light, a liquid-crystal modulator for modulating light is from the source and output means for giving an output when struck by the activating light, in which the source is provided by a laser screen as aforesaid, but not necessarily lasing simultaneously over the panel, the screen being arranged to act as a source of excitation light for the photoluminescent output material, the laser being pumped by the application of an electric field or optically.

A third aspect of the invention consists in having a laser screen laminated with a photoluminescent liquid-crystal assembly to provide a flat-panel display.

In display applications, if the screen does not lase simultaneously over its area then it must be scanned at a rate sufficient to avoid flicker, say at above about 50 Hz. In a proposed development the display is addressed in such a way that the image data at each pixel are only valid for a defined time. In this case it would be advantageous to synchronise the scanning of the backlight laser screen with the addressing of the pixels.

The active material could be any one of the following:

- an inorganic semiconductor (for example, prior art laser screens have been made with

A2E6 semiconductor materials); - an organic semiconductor, e.g. a conjugated polymer; - other luminescent polymeric materials; - metal chelates such as Alq3; - other organic materials i.e. polymers or perhaps laser dyes; - any other phosphorescent or fluorescent material, either solid or liquid.

At present a laser screen is most likely to be a solid-state device, although in principle at least it may be possible to construct a laser screen from a gaseous or liquid active material.

In order to achieve lasing the actual physical nature of the active material needs to be correct. In some cases this means that large mono-crystals have to be grown using for example a gas-phase technique such as Davydov-Markov. on the other hand it might be possible to grow large crystals of a semiconductor material directly and if necessary cut them into appropriately sized wafers. Phosphors are generally polycrystalline in nature and as such are scattering; this would prevent lasing and consequently needs to be prevented. This could be done by placing the phosphor material in an index-matching matrix or by directly growing the material on the substrate by atomic layer epitaxy or a similar method. Polymeric materials on the other hand would not suffer these sorts of disadvantages.

In the case of a solid-state laser screen, material can he laid down on an appropriate substrate 3S and a reflector of appropriate design deposited onto this substrate also, in order to form the cavity. This and the other side of the cavity could be formed in one of several ways:

a dielectric stack of the appropriate design could be deposited onto the active material or substrate, or alternatively such a stack could be made on a second substrate and this could then be laid on top of the active material; a thin layer of an appropriate metal could be deposited onto the active material, thin enough to act as a partly reflecting mirror without absorbing too much energy; or conceivably the refractive index change between one active material and the exit medium would be sufficient to provide the necessary lasing cavity.

In general, there are two main means by which an active material can be pumped so that it lases, namely electrical and optical pumping; moreover there are two different means of achieving electrical pumping. In laser diodes for example the active material is electrically pumped by providing electrical contacts across the material and applying a field that passes a current through the material. On the other hand active materials can be pumped by an electron beam and this is how prior-art laser screens are pumped (see "Full colour TV projector based on A.B. electron-beam pumped semiconductor lasers", Journal of Crystal Growth 117 (1992) pp. 1040-1045 (Nasibov et al.) or US 5339003 (Koslovsky et al.)). The crucial aspect of electron beam pumping is that the electron beam has to be scanned across the active material. Consequently the whole screen does not lase simultaneously. In addition the apparatus required for generating the electron beam is bulky and expensive since among other things electron beams can only travel through a vacuum.

Simultaneous lasing is not essential for some display requirements, though it is useful for backlight purposes and may be essential for others. In order to cause simultaneous lasing over the whole screen, therefore, alternative means to the electron beam are used, for instance pumping the screen optically by means of suitable illumination, or electrically through appropriate contacts and the application of a field.

One of the important aspects of constructing a laser screen is the uniformity of the screen dimensions and construction across its area. For example the characteristics of the lasing action are dependent on the size of the laser cavity. Thus in order to achieve uniform lasing action across the screen it will be necessary to control the width of the cavity across the area of the screen very accurately. This is a similar problem to the requirement for making flat-panel liquid-crystal displays, although the tolerances would be greater for a lasing screen.

one of the principal problems that non-uniform lasing causes is interference affects across the area of the beam. In applications where this laser beam is viewed directly, for example in a projection display architecture, these interference effects would be noticeable to the extent that they would seriously degrade the image. However in PL-LCD architectures the laser light is not directly viewed but is used to excite phosphors. The phosphors effectively integrate the total illumination falling on them and consequently 'average' out the effect of the interference fringes (provided the fringes are smaller than the phosphor pixels). For this reason the use of laser screens with PL-LCD architectures will be more practical than with other types of display architecture.

In order to avoid possible uniformity problems over the area of the screen, it might also be advantageous to pixellate the lasing material, probably matching this pixellation with that of the subsequent image generating device. As an alternative one can wedge' the screen cavity slightly in such a way that the spatial frequency of the fringes is greater than that of the pixellation (note this method does not necessarily imply that the laser screen itself is pixellated).

In a further alternative it is possible to alter the dimensions of the lasing cavity dynamically so that the position of the fringes is continually changing.

This could be achieved by an appropriate acoustic wave, and provided this dynamic variation is of a sufficiently high frequency - a few kHz is all that is required - then the eye will not be able to see the instantaneous position of the fringes and will therefore average them. Thus the fringes will not be visible. This approach is particularly advantageous in applications where the light from the laser screen is being viewed directly (rather than being used to excite phosphors in a PL-LCD architecture).

When a laser screen does lase it will probably do so with various modes. These modes may also include some transverse modes that would be useless for many applications. These transverse modes can be suppressed in various ways; for instance, the previously mentioned pixellation may prevent these modes being generated, especially if the inter-pixel gap is filled with an absorptive or other medium designed to reduce the gain, or one can place an absorptive or other medium around the edge of the screen also designed to reduce the gain in the transverse modes.

In addition various longitudinal modes may also be generated some of which might represent light propagating in directions other than strictly normal to the plane of the laser screen; these would also have to be suppressed for efficient operation. This could be done by incorporating some sort of refractive (i.e.

surface relief) or diffractive (i.e. holographic) pattern on the top surface of the laser screen or by designing the reflectors so that off-axis reflection is low. In this way the gain of any mode that does not propagate normal to the plane of the screen would be low and laser action is therefore less likely. In addition these unwanted modes could be tolerated, provided that patterning of the top surface allowed only the desired mode to be coupled out of the screen (although this would be a less efficient method).

Additionally it may be possible to design out, the unwanted modes, for example by using shaped cavity mirrors.

For a better understanding of the invention embodiments of it will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 shows an exploded view of a PL-LCD flat panel display using a laser screen as a backlight; Figure 2 is a schematic section of a projection display of the PLLCD type; and Figure 3 shows the principle of the laser screen.

In Figure 1 a PLLCD panel 11, broadly similar to those described in the PCT patent application WO 95/27920 or in GB 2291734 (Samsung), is made of an LCD panel 3, an output polariser 5 and a phosphor screen 7, all arranged in a laminated or flat-panel manner. UV or deep blue excitation light E is modulated by the LCD panel 3 with the polariser 5, and the resulting modulated light strikes the corresponding areas of the phosphor screen 7, preferably made of an array of RGB phosphor dots corresponding to the liquid crystal matrix, producing a display (output light 0).

This display is comparable to that of a CRT screen and does not suffer the usual disadvantage of an LCD display of having directional viewing characteristics.

Prior-art PLLCD architectures have generally required some degree of collimation of the excitation light E, partly to avoid crosstalk arising from oblique excitation light striking the wrong phosphor and partly because the response of the liquid-crystal material in terms of the available contrast is better at certain angles than others. In this embodiment of the present invention such collimated (and in many embodiments polarised) excitation light is produced by a laser screen 1 corresponding in area and rectangular shape to the LCD assembly.

The laser screen 1 is pumped by (incoherent) pump light P, which can be provided for instance by a high intensity UV lamp behind the screen. For flatpanel applications this arrangement might be bulky, so electrical pumping might be preferred. In a known manner the screen has electrodes and reflectors, not shown, in order to produce a preferentially directed polarised output beam constituting the excitation light E. If the output beam is not polarised then a polariser will have to be provided before or in the LCD panel 3.

Fig. 2 shows an alternative embodiment in which a laser screen pumped by an incoherent light source 21 is used in a projection display. Here, instead of the phosphor screen on the front plate of the LCD panel there is a much larger viewing screen 17 spaced from the panel. Projection optics 15 are provided in the usual way so that the image produced by the LCD assembly reaches the phosphor dots on the screen 17, as described in the applicant's earlier PCT/GB 98/1396.

For backlight applications ideally the whole screen lases simultaneously. However, one of the problems with the operation of a laser screen is the total power required in order for the whole area to lase. For example, a recent paper (A. Schlazgen et al, Near diffraction-limited laser emission from a polymer in a high finesse planar cavity, Applied Physics Letters 72(3), 19 January 1998, pp. 269-271) describes a polymer layer lasing between two dielectric mirrors.

However, the power threshold for lasing to occur was some 250MW/cm2. However, there are various techniques for reducing this threshold. For example see V.G.

Kozlov et al., "Laser action in organic semiconductor waveguide and double-heterostructure devices", Nature (date unknown).

This paper describes the use of heterostructures analogous to those used in LEDs and semiconductor diode lasers. They help to reduce the threshold by confining photons into the active area and/or by similarly confining the excited electrons (or whatever the excited species is).

Another method is described in Seng-Tiong Ho et al, "Recent advances in micro-cavity disk lasers and photonic-wire lasers", SPIE Vol 2891.

This paper describes how the spontaneous-emission coupling efficiency can be increased so that the lasing threshold is reduced. At the moment this technique seems limited to micro-structures but its incorporation into a laser screen seems feasible.

For some applications it may be preferable to operate the laser screen in a Q-switched mode rather than continuous-wave (CW) configuration. Q-switching lasers work in such a way that lasing is prevented by lowering the Q of the lasing cavity. In this way a large population can be built up, the Q is then quickly switched to a figure that allows lasing and a brief but very intense laser burst is emitted. Spontaneous emission limits the ratio of pump time to pulse time in practice to a ratio of about 103. Q-switching lasers have repetition rates that vary from a few Hz to tens or hundreds of kHz.

When a laser screen for a display is Q-switched in this way the repetition rate has to be sufficiently high that flicker is not observed - a figure of say 10OHz is typical. Additionally the laser burst may only last a few nanoseconds (say 100ns). A Q-switched laser screen is analogous to conventional AC-driven fluorescent backlights for FPDs; these flicker at a rate of around 50OHz.

An alternative approach is possible with electrical pumping. If the electrodes that control the pumping field are pixellated then it is possible, in principle, to address each pixel individually with the fields required for lasing to occur. In this way only single pixels within the entire backlight will lase.

The effect is to carry out a scan of the pixels within the screen in a way that is analogous to the prior-art CRT laser screen; the advantage is that this new scanned screen is still a flat-panel device, unlike the CRT-type prior art. In a similar way to the above, the scanned nature of the backlight may also be advantageous especially in that the operation of the display begins closely to resemble a CRT.

For completeness Figure 3 shows the basic operation of an optically pumped laser screen, by way of example. The incoherent source 21, which itself extends broadly over the area of the screen, produces pump light P. This light impinges on the active medium in a cavity 25 formed between two plane mirrors 23 of the laser. The resulting laser action causes the I emission of an output laser beam E.

AiDplications of Laser Screens The laser screens described above could have many uses, for instance simply as directional lamps, but -11 they have several advantages for the PLLCD display architecture.

The light generated from such a screen is monochromatic, collimated and largely polarised; thus a laser screen would be the perfect backlight for the PLLCD flat-panel display architecture. The application to other FPD architectures is less immediate since it is more difficult to generate colour. The theoretical efficiency of such a display would depend on the lasing efficiency of the screen but could be around 70'- This contrasts with 16% or so for a conventional FPD that has both a 50% polarisation loss and a 661-. loss due to colour filters.

In the projection architecture, the requirement is is also for a polarised, collimated and monochromatic light source. In conventional projection displays a collimated beam is generated from an intense, near point source such as a metal-halide lamp. As the source is not extended it is possible to produce a collimated beam using standard optics. However there are still geometric losses associated with this approach and furthermore the requirement for polarised light inserts a further loss of up to 50%.

Consequently a beam that is generated already polarised and collimated, such as that from a laser screen, is also the ideal source for a projection display. In a similar manner to the FPD architectures, the advantages of a laser screen seem restricted to photo-luminescent projection display architectures such as are described in PCT/GB 98/1396. However, the invention is not restricted to such applications.

Claims (4)

CIAIMS

1. A laser screen including a panel, a cavity in the panel in which active material is located, the cavity extending substantially over the area of the panel; and a means of pumping the active material in such a way that lasing takes place; in which the pump means is arranged to cause lasing to take place simultaneously over the area of the screen.

2. A liquid-crystal display including a source of activation light, a liquid-crystal or other electro optic modulator for modulating light from the source, and output means for giving an output when struck by the activating light; in which the source is provided by a laser screen arranged to act as a source of excitation light for the photoluminescent output material the laser being pumped by the application of an electric field or optically.

3. A flat-panel display comprising a laser screen laminated with and forming the activation light source for a photoluminescent liquid-crystal assembly.

4. A display apparatus as claimed in any preceding claim, in which the active material is or contains any one of the following:

- an inorganic semiconductor (for example an A2B6 semiconductor material); - an organic semiconductor, e.g. a conjugated polymer; - a luminescent polymeric material; - metal chelates such as Alq3; - other organic materials such as polymers or perhaps laser dyes; and - any other phosphorescent or fluorescent material, either solid or liquid.