Optically integrating pixel microstructure

An integrated optical microstructure includes a substrate carrying an optical waveguide and supporting a medium disposed to receive optical energy from the waveguide. The medium includes an optical re-radiator such as a phosphor, which reradiates optical energy in response to optical energy received from the waveguide. The structure further includes a reflector disposed to redirect some of the input optical energy emanating from the medium back into the medium, to achieve spatial confinement of the input light delivered by the input waveguide. The structure can thereby increase the efficiency of the light conversion processes of re-radiating materials. An aperture in the reflector permits optical energy emitted by the re-radiator to emerge from the structure and to propagate in a preferred direction, such as toward a viewer or sensor. The structure is useful for increasing the brightness of various kinds of small emissive elements which are excited by light delivered from an integrated optical waveguide, including pixels in an information display.

FIELD OF THE INVENTION
 The invention concerns means of improving the optical efficiency of
 optically excited light emitting structures such as pixels in flat-panel
 displays.
 BACKGROUND OF THE INVENTION
 Many commercial emissive display devices generate visible light using
 electron beam or ultraviolet radiation incident upon a phosphor, such as
 in cathode ray tube (CRT) or AC plasma visual displays. A less well known
 display technology, typified in Bischel et al. U.S. Pat. No. 5,544,268,
 incorporated herein by reference, uses optical waveguides to convey light
 from a light source onto a display screen. Waveguide-based flat panel
 displays generally utilize planar and/or channel waveguides. They
 typically include several parallel channel waveguides to be formed on a
 substrate. Optical switches are located either in or on the channel
 waveguides at predetermined matrix locations across the display screen.
 Optical energy injected into the channel waveguides is extracted at these
 predetermined positions by the optical switches and directed toward pixel
 structures which may, in certain embodiments described in the '268 patent,
 include re-radiators to emit light from the pixel structure towards a
 viewer. Such reradiators can include out-of-plane reflectors, scattering
 materials, or luminescent materials which emit at a wavelength which may
 differ from the wavelength of the input optical energy. Metal reflectors
 on or near the visible light emitting pixels are used in a variety of ways
 in different display architectures to redirect visible light emitted by
 phosphors into a preferred direction to achieve enhanced brightness at the
 viewer location. For example, Thomas U.S. Pat. No. 5,097,175, incorporated
 herein by reference, describes a pixel structure for CRT displays, where a
 material that emits visible light upon excitation with an electron beam is
 deposited on a transparent substrate in the form of a parabolic shaped
 cell that is coated with a reflective metal layer to redirect visible
 light emitted inside the cell through the substrate toward a viewer.
 In another example of the use of reflectors to direct light for a visual
 display, Murata U.S. Pat. No. 5,055,737, incorporated herein by reference,
 describes a luminescent screen which contains a material that emits
 visible light when excited by light incident from the viewing direction.
 This screen contains a reflective structure that redirects light from the
 emitting material, that otherwise would propagate in undesired directions,
 back toward the viewer, thereby enhancing brightness.
 Thus the conventional function of reflectors used in displays is to direct
 the light generated in the pixel toward the viewer. Such reflectors do not
 serve to enhance the efficiency of conversion to visible light of pump
 energy such as that from an electron beam in a CRT or from the
 ultra-violet light in a plasma display. Optical performance, including the
 conversion efficiency, brightness, and chromaticity, of display pixels
 containing certain optically activated luminescent materials such as
 phosphors, glasses, or crystals would benefit from increasing the amount
 of absorbed pump radiation. Therefore a different kind of reflective pixel
 structure is needed which confines the pump radiation while allowing for
 the emission of generated light.
 SUMMARY OF THE INVENTION
 The present invention provides a means of increasing the efficiency of the
 light conversion processes of re-radiating materials in integrated
 structures. Roughly described, this is achieved by using a reflective
 coating deposited on or near the light emitting material to achieve
 spatial confinement of the input light delivered by an integrated
 waveguide. The fraction of the input light that contributes to the
 generation of useful output light is thereby increased. It will be
 apparent to those skilled in the art that the structures described herein
 pertain not only to pixels in information displays but more generally to
 any reflectively enclosed light emitting structure that is optically
 excited by light from an integrated optical waveguide.
 In one embodiment, the optical performance, including conversion
 efficiency, of an upconversion phosphor contained in such a device is
 improved by the enhanced absorption of infrared pump light resulting from
 multiple passes through the absorber by reflection of otherwise unabsorbed
 pump light off the reflective coating of the confinement structure.
 Apertures in the reflective coating of the confinement structure allow for
 the visible light generated in the upconversion process to emerge from the
 pixel structure and to propagate in a preferred direction, such as toward
 a viewer or sensor. The efficiency of visible light generation upon
 infrared excitation of an upconversion phosphor material generally
 increases as the infrared power absorbed per unit volume is increased,
 because the probability of non-radiative energy transfer processes
 involved in the generation of the visible light increases as the average
 distance between excited optically active ions decreases.
 At a fundamental materials level, the invention enables the use of
 re-radiator materials that have small absorption coefficients for input
 light due to a small concentration of active dopant ions that absorb
 light. It is well known that the conversion efficiency of such phosphors
 often decreases as the concentration of absorbing ions is increased (see
 "Luminescent Materials", by G. Blasse and B. C. Grabmaier, 1994,
 incorporated herein by reference). The use of reflectors on a pixel
 designed to increase the absorption of input light helps ensure utility
 for such materials that may have low absorption but high conversion
 efficiency.
 The invention provides a means for increasing the brightness of small
 emissive structures such as pixels in information displays which are
 excited using light delivered by integrated optical waveguides. The
 advantage of the invention is significant in the example of an information
 display where the dimensions of the pixel feature are constrained
 according to display resolution requirements and the pixel cannot be
 increased to an arbitrarily large size to achieve larger single pass
 absorption of input light.
 Furthermore, in the case of upconversion phosphor grains in an optically
 transparent polymer binder, the mixture may comprise phosphor as perhaps
 only 5% by volume of the total volume enclosed by a pit/mound structure.
 Confinement of the input light by the reflective surfaces increases the
 total input light energy that is absorbed per upconversion phosphor
 particle. The resultant increase in excitation density within the
 phosphors can provide a higher efficiency of conversion of infrared to
 visible light within the phosphor grains.
 An additional advantage gained from the use of reflectors that confine the
 pump light delivered to a pixel is that the variation of efficiency with
 pump light wavelength of a wavelength-converting phosphor, for example,
 can be reduced. In such a structure, for example, a NaYF.sub.4
 upconversion phosphor doped with ytterbium and erbium ions having an
 absorption peak around 977 nm may be pumped at a variety of wavelengths
 in, say, the 960 nm to 990 nm range and produce comparable green light
 emission intensity despite a significant variation in the absorption
 coefficient of the phosphor over that wavelength range. This increase in
 wavelength tolerance relaxes the specification of lasers used with such a
 device and can improve device performance uniformity as the wavelengths of
 pump lasers tend to vary with temperature.
 This invention will be better understood upon reference to the following
 detailed description in connection with the accompanying drawings.

DETAILED DESCRIPTION
 FIG. 1 shows a cross-sectional view of a device comprising a medium 106
 which includes a re-radiator material 105 partially located in a pit 110
 which terminates an optical waveguide 115 that is integrated in an optical
 waveguide structure 125 on a substrate 120. The termination of an optical
 waveguide by the pit is defined as the pit intersecting with and capturing
 substantially all the light propagating towards the pit in the portion of
 the integrated optical waveguide that terminates at the pit. Even though
 the waveguide may not actually meet the pit, the pit may still be able to
 capture substantially all the light propagating in the waveguide in a
 downstream direction towards the pit. The optical waveguide structure 125
 may be a multilayer stack, or part of the substrate 120 incorporating
 ion-diffused or ion-exchanged channel or planar waveguides formed from,
 for instance, titanium, zinc or protons in lithium niobate; silver in
 glass; or one of many other techniques of waveguide fabrication which are
 well known in the art. The medium 106 in this example comprises a
 re-radiator material 105 which protrudes above the top surface of the
 optical waveguide structure 125 and can be of any shape and may extend
 laterally beyond the confines of the pit 110 onto the top surface of the
 waveguide structure 125 or onto any other layer deposited on this top
 surface of the waveguide structure 125. The medium may be added to the pit
 by many methods including stencil printing, photolithographic definition,
 or ink jet printing. A re-radiator material may be any single or
 multi-component material that alters the properties of input light and
 from which output light emanates. The input light and the output light
 emerging may be of single or multiple wavelengths. Generated light is that
 portion of output light whose wavelength is altered from that of the input
 light as a result of interaction with the re-radiator. For example, the
 reradiator material may include a luminescent material (refer to
 "Luminescent Materials" by Blasse and Grabmaier referenced earlier) or a
 phosphor that absorbs radiation at wavelengths shorter than the
 wavelength(s) of emission, henceforth referred to as down-conversion
 phosphor, or it may include a phosphor that generates light at wavelengths
 shorter than the input light wavelength at which it is excited, henceforth
 referred to as an upconversion phosphor. Examples of down-conversion
 phosphors include BaMgAl.sub.10 O.sub.17 :Eu.sup.2+ and SrS:Cu.sup.+.
 Examples of upconversion phosphors include BaY.sub.2 F.sub.8
 :Yb.sup.3+,Tm.sup.3+ and YF.sub.3 :Yb.sup.3+,Er.sup.3+. The wavelengths
 referred to above imply the spectral range extending from the far-infrared
 to the deep UV. Another example of a re-radiator material is a material
 that scatters input radiation without changing wavelength, in this case
 there is no generated light.
 An integrated optical waveguide 115 is any structure that provides for
 optical confinement of an input light beam in at least one dimension by
 careful choice of refractive index of composite materials and appropriate
 choice of physical dimensions. Examples include planar waveguides or
 channel waveguides. The parameters necessary to design an optical
 waveguide structure to guide light at a particular wavelength are well
 known in the art and may be found for instance in Nishihara et al.
 "Optical Integrated Circuits," McGraw Hill 1989, incorporated herein by
 reference in its entirety. The waveguide may be imbedded directly in the
 interior or top surface of a substrate or may be contained in a layered
 stack of materials of appropriate refractive index (core and/or top and/or
 bottom cladding) that is deposited, or otherwise attached, to the top
 surface of the substrate. It will be apparent that the bottom cladding may
 be the substrate in a case for example where the waveguide structure is a
 multilayered stack. A waveguide is considered herein to concentrate
 optical energy "primarily" within the core of the waveguide, although
 since evanescent tails extend out into the cladding layers, some energy
 nevertheless travels outside the core.
 The pit 110 may be of any shape that includes a surface that intersects the
 optical energy from the integrated optical waveguide 115. This surface of
 intersection includes a waveguide aperture 130 through which input light
 may be delivered from the integrated optical waveguide 115 into the medium
 106 comprising re-radiator material 105. The surfaces of the pit are
 coated with a reflector 135, 140, 145 such as a multi-layer dielectric
 film or a layer of a metal such as silver, gold, aluminum or any metal or
 metallic alloy reflector, or any other material or combination of
 materials that reflect the input wavelength. Preferably the reflector will
 exhibit a high degree of reflectivity for at least the input light. The
 waveguide aperture 130 is at least partially transparent at the wavelength
 of the input light to allow passage of light launched from the waveguide
 into the pit. In some cases, this aperture may also provide a means for
 escape of some of the light re-radiated from the re-radiator material 105.
 At least the portion of the top surface of the optical waveguide structure
 125 that is underneath the medium of re-radiator material 105 may also be
 coated with a metallic, dielectric or other reflector 150. Input light
 delivered to the pit by a waveguide 115 enters the pit 110 through the
 waveguide aperture 130. Subsequently it propagates through the medium 106
 containing re-radiator material 105, undergoing wavelength conversion
 and/or scattering. Input, scattered or generated light reaching the
 reflective coatings on the surface of the pit or optical waveguide
 structure at the base of the medium are reflected back into the medium, to
 be wavelength converted or to emerge from the top surface of the medium or
 to escape through the waveguide aperture 130.
 FIGS. 2A and 2B (sometimes referred to herein collectively as "FIG. 2")
 show a further embodiment of the invention with the addition of an optical
 reflector structure over part of the top surface of the medium. FIG. 2A is
 a perspective view of the structure, and FIG. 2B is a cross section in the
 plane defined by A-A' in FIG. 2A. FIGS. 2A and 2B also indicate the
 definitions of the bottom side and top side of all the embodiments and
 their variants shown in the figures hereof, in which it can be seen that
 all levels are described relative to a substrate at the "bottom" of the
 structure. The terms above, below, top, bottom and superposition as used
 herein are not intended to change their meanings if the structure is
 turned upside down or tilted. In addition, the term "superposition" refers
 to aboveness, and is not limited to super-adjacency.
 The preferred use of the embodiment of FIG. 2 is as an emissive pixel in a
 visual display, where the medium may comprise, for example, phosphors that
 generate optical radiation upon excitation with light at a different
 wavelength as described above. Generated light emitted from the medium is
 directed towards a viewer, a screen or perhaps an optical scanning device.
 For an embodiment that uses upconversion phosphor, infrared laser light
 propagates along an optical beam path 204 in an integrated optical
 waveguide structure 240, disposed on or integrated directly into a
 substrate 245, and enters the medium 210 through the waveguide aperture
 215. An optical beam path is defined as the direction of propagation of a
 light beam in a planar or channel waveguide, or in a bulk material. In a
 display, many pixels (such as any one or more of those described herein)
 are arranged in an array and are selectively excited with modulated input
 optical energy to produce an image.
 The top surface of the medium 210 is coated with a reflective material 230,
 henceforth referred to as a top reflector, that may have an optically
 transmissive aperture 235, henceforth referred to as the top aperture,
 which allows the emission of light from the structure. The top aperture
 substantially transmits at least the generated light. The top reflector
 can be any kind of reflective material including metallic or dielectric
 coatings that reflects radiation at one or more wavelengths. There may be
 more than one top aperture in the top reflector and the top aperture(s)
 may be at any location on the top surface of the medium 210 and have any
 shape. This allows for the design of the spatial intensity distribution of
 the output light. Preferably, the top reflector desirably should directly
 confront the bottom reflectors 250, 255 and 260 disposed on the surfaces
 of the pit 275 and optical waveguide structure 240, to minimize the number
 or area of unwanted optical apertures in the device. The top reflector may
 consist for instance of a multi-layer dielectric coating (as know in the
 art of optics) which is designed to preferentially reflect the input/pump
 light and transmit part or all of the generated light. Thus a defined
 aperture may not be necessary. Equally, the dielectric coating may highly
 reflect both input/pump and generated wavelengths and the aperture(s) may
 be formed by a hole in the coating layers at one or more position(s) on
 the medium. A further arrangement is a combination of the metallic and
 dielectric reflectors where the metallic reflector may provide the
 confinement for the generated light and the dielectric coating the
 confinement for the input light. In this way, an aperture may be opened in
 the metallic coating to allow emission of the generated light without
 allowing emission of the input/pump light, as the dielectric coating
 remains continuous across the surface of the medium.
 The presence of the top reflector results in un-wavelength-converted input
 light emanating from the medium, being reflected back into the medium
 instead of emerging. This increases the fraction of the input light that
 is confined to the medium and is available for wavelength conversion and
 thus increases the amount of generated light produced by the re-radiator
 medium, compared to the structure of FIG. 1 which has no top reflector.
 The optical performance of many re-radiator materials will be enhanced
 through increased absorption in such a structure. For example, input light
 at 300-400 nm will be absorbed by a re-radiator material comprising
 BaMgAl.sub.10 O.sub.17 doped with divalent europium ions resulting in the
 efficient generation of light in the 450 nm region. For some materials
 such as BaY.sub.2 F.sub.8 doped with trivalent ytterbium and trivalent
 thulium ions, an upconversion material that has an efficiency of
 infrared-to-blue conversion that increases super-linearly as the excited
 ytterbium ion density increases, the increase in the amount of light
 absorbed in the upconversion particles translates into an increased
 conversion efficiency per absorbed photon when using the pixel structure
 of FIG. 2.
 In one embodiment, the medium 210 comprises a polymeric binder material,
 such as an acrylate or epoxy that contains upconversion phosphor particles
 225 of, for instance, yttrium fluoride doped with ytterbium and erbium,
 that absorbs input light of wavelength around 980 nm and converts part of
 it to shorter wavelengths in the visible spectral region to create red,
 green, and blue light. As such, the binder material serves several
 functions including (i) being a host that binds together the upconversion
 phosphor particles 225 and providing a vehicle for deposition, (ii)
 providing a means to define a desired shape to the medium 210 and (iii)
 defining a refractive index boundary between the binder material and the
 upconversion phosphor which allows design of a specific optical scattering
 coefficient for the medium 210. In the latter function, for example, a
 binder can be chosen that has a refractive index that closely matches that
 of the phosphor material, glass or crystal so that input light and/or
 output light exhibits essentially no optical scattering. Input light
 propagates through the medium undergoing scattering and absorption due to
 interaction with the re-radiator material and undergoing reflection at the
 top reflector 230 or the reflective coatings 250, 255, and 260 at the base
 of the medium, until it is absorbed by the phosphor particles 225, or
 escapes through the top aperture 235, or escapes through the waveguide
 aperture 215, or is absorbed by any of the reflective coatings, or is
 absorbed by a residual absorption of the polymeric binder material.
 Optical absorption of the polymeric binder is preferably very low at the
 input and generated wavelengths. Preferably the decrease in input light
 intensity within the medium due to absorption by the upconversion phosphor
 should dominate other absorption and loss effects, such as due to the
 binder material and reflector material, in order to maximize the
 efficiency of emission of useful generated light by the structure. The
 output light generated by the re-radiator material then propagates through
 the medium, which preferably should have a small absorption coefficient at
 this wavelength relative to the absorption coefficient at the input light
 wavelength, and, after possibly many reflections inside the medium from
 the reflective coatings, emerges from the top aperture 235, or escapes
 through the waveguide aperture 215, or is absorbed by the reflective
 coatings or by the re-radiator or polymeric binder.
 Of course, the device of FIG. 2 may contain other re-radiator materials
 such as downconversion phosphors designed for example to absorb blue light
 and generate, say, red or green light. Alternatively, the re-radiator
 material may comprise a material that scatters light but does not have a
 significant absorption at the wavelength of the input light delivered via
 the waveguide.
 It will be apparent to those skilled in the art that the number, location,
 and shape of the top apertures 235, the type of reflector material, as
 well as the type of retomey radiator materials and the spatial
 distribution of the re-radiator particles 225 in the medium significantly
 influence the device performance as a pixel for a display application, and
 that different material choices and/or different applications lead to
 variations in embodiment. For example, if a reflector material of
 relatively low reflectivity at the generated light wavelength is used, the
 top aperture may need to be of larger total area than if a high
 reflectivity reflector is used so as to minimize cumulative losses at the
 reflector interface within the device, by minimizing the average number of
 reflections experienced by a ray of generated light before it escapes
 through the top aperture.
 A device of the type illustrated in FIG. 2 can in one example use as a
 reradiator an upconversion phosphor doped with ytterbium and erbium ions
 mixed with a polymeric binder. It can be fabricated by depositing a mound
 of the medium in a pit that intersects a channel waveguide on a substrate.
 The mound is cured then coated with a layer of metal to create the top
 reflector, and a top aperture is created on the top surface. Upon
 excitation with radiation of wavelength around 980 nm delivered through
 the waveguide, a four-fold increase in the optical performance (visible
 power emerging from the top aperture divided by power input into the
 mound) of the device can be observed relative to a device of similar
 design without the top reflector metal coating. Other designs will provide
 either more or less enhancement.
 The embodiment of FIGS. 2A and 2B may be fabricated as follows. A planar
 optical waveguide structure is disposed on the top surface of a substrate
 material. This structure may consist of a series of separately deposited
 layers, for instance glasses or UV curing polymers, each with a different
 refractive index dependent on their position in the structure. For
 instance, a low index lower cladding layer 265 may be deposited and cured
 (if necessary) on the substrate surface (for instance by spin/spray/dip
 coating, slot-die extrusion or vacuum deposition for polymer and
 spin-on-glass materials, sputtering, evaporation or chemical vapor
 deposition for hard oxides and glasses).
 A waveguide core layer 205 is deposited and cured on top of the lower
 cladding layer (or alternatively directly on top of the substrate), using
 a technique compatible with both the core layer and lower cladding
 materials. The refractive index of the core layer must be greater than
 that of the lower cladding layer (or the substrate if there is no lower
 cladding layer), and the combination of the refractive index difference
 and the core layer thickness should be sufficient to provide optical
 confinement for at least one transverse mode in the waveguide structure.
 This combination can readily be computed by a person skilled in the art
 based on the mathematical waveguide analysis found for instance in the
 "Optical Integrated Circuits" reference incorporated above. The thickness
 of the lower cladding layer (if used) should be great enough to ensure
 that the evanescent field of the guided mode has decayed to substantially
 zero before it reaches the substrate-lower cladding interface to prevent
 coupling of light into the substrate and potentially high optical
 propagation losses.
 A channel waveguide, or array of channel waveguides may be disposed over
 all or part of the planar optical waveguide structure by any of the
 methods known in the art compatible with the materials system chosen. For
 instance, for a polymer waveguide system, channel waveguides may be
 defined by reactive ion etching or laser ablation of the core or cladding
 layers to provide rib waveguides, photodefinition of the core or cladding
 layers or by photobleaching. For glass based, or other hard oxide
 materials (e.g. SiO.sub.2), dopants can be incorporated into the core
 layer (e.g. Ge) and wet or dry (e.g. RIE) etching used to pattern ridge
 waveguides. Patterned indiffusion of dopants (e.g. metals, Ti, Zn, Ag)
 into a uniform core layer, or into the surface of the substrate itself can
 also be used to provide a localized refractive index increase and a
 channel or planar waveguide structure. Channel waveguide segments may
 terminate at or upstream of the input waveguide aperture of a pit.
 The pattern of channel and planar waveguides may be registered to alignment
 marks to enable accurate relative placement of the waveguides and the
 pixel pit structure, and of other features not related to the pit, as
 necessary. The channel waveguide pattern provides a light distribution
 structure and may form an optical beam path to deliver light to the pixel
 structures.
 A third layer 270, termed the top cladding or buffer layer, may be disposed
 over the top surface of the core layer. The function of this layer is to
 isolate the optical mode in the core of the waveguide from features later
 deposited on the surface of the device, so that the optical mode
 propagates in the waveguide structure without interference except at
 carefully selected locations, such as the pixel pit. The refractive index
 of the top cladding must be less than that of the core layer, and the
 combination of the index difference (core-top cladding) and the cladding
 thickness should be sufficient to cause the evanescent field of the
 waveguide mode to have decayed substantially to zero before it reaches the
 top surface of the top cladding layer.
 Alternatively, the waveguide can be formed directly in the top surface of
 the substrate by an indiffusion technique, such as metal indiffusion (e.g.
 Ti, Zn) in lithium niobate or tantalate, or ion exchange (e.g. Ag) in
 glass. Other waveguide fabrication methods are well known in the art, and
 can be tailored for application to different substrate materials. Top
 cladding (buffer) layers may be applied as described above to protect the
 waveguide mode from unwanted interference from elements on the top surface
 of the device, above the cladding layer.
 The pit can be formed at pre-selected locations, aligned to the waveguide
 pattern described above, by one or a combination of several methods. The
 pit itself can be formed by any of the surface micro-machining or etching
 methods known in the art which are compatible with the materials used in
 the waveguide structure construction. For instance, a polymer optical
 structure may be etched by excimer laser ablation, RIE (Reactive Ion
 Etching, typically using fluorine based chemistries), or in some instances
 wet chemical etching. Other optical materials such as glasses and crystals
 can be etched using broadly the same techniques, but with detailed changes
 to the etching chemistry, or wavelength in the case of excimer laser
 ablation, to match the specific properties of the material. The exact
 parameters of the etch process will depend on the chemistry of the
 materials used in the waveguide structure. For instance, the wavelength
 used for laser ablation must be strongly absorbed in the material (e.g.
 wavelengths around 248nm or 193mn are commonly used), or the RIE process
 must use the appropriate chemistry to provide volatile by-products which
 can remove the etched material (e.g. fluorine based gases are often used
 for etching polymer materials).
 The shape and location of the pit can be defined by lithographic masking
 and patterning processes aligned to the waveguide structures. The depth of
 the pit may, if desired, vary across the width and length of the pit. The
 pit may extend completely or partially through the optical waveguide
 structure, and may even penetrate the substrate. In the embodiment of FIG.
 2 the pit cuts through the waveguide core layer in at least one point that
 intersects the optical path of light propagating in the waveguide
 structure, such that the waveguide delivers light to the re-radiator
 material disposed in the pit. For an etch process such as excimer laser
 ablation, the pit depth may be controlled by applying only a certain
 number of ablation pulses to the material. The pit shape in the depth
 direction, (e.g. the angles of the side walls) can be controlled by the
 detailed process parameters used to perform the etching. For instance with
 excimer laser ablation, the wall profile may be varied by altering the
 beam dimension and fluence as a function of etch depth into the structure.
 Vertical and angled walls (e.g. -45.degree.) can be created in this
 manner. The base of the pit may be flat or curved as desired for the pixel
 structure. For instance a curved base to the pit may be used to
 preferentially reflect generated light directly through the top aperture
 without undergoing further reflections from the reflector disposed at the
 surface of the medium. This may decrease the amount of generated light
 lost to absorption within the medium or on the reflectors, and provide
 some directionality to the output light emerging from the top aperture.
 The curved base may be fabricated via laser ablation in a manner analogous
 to the controlled profile of the pit walls, that is, by changing the laser
 beam dimension or intensity profile as a function of the depth of the pit,
 the amount of material ablated by each laser pulse will vary across the
 pit dimensions, resulting in a non-planar profile to the base.
 After etching the pit, a reflective coating is preferably deposited on the
 surface of the pit structure, for instance by sputtering of a thin metal
 layer (e.g. Ag), or by the deposition of a multi-layer-dielectric
 thin-film coating. Lithographic patterning (e.g. photoresist masking
 followed by a wet chemical etch or RIE, or excimer laser ablation) can be
 used to remove the reflector from unwanted locations, such as the
 waveguide aperture where the pit intersects the optical beam path of the
 waveguide structure. The lithographic pattern used is aligned to the
 intersection of the optical beam path and the pit so as to form the
 optically transparent waveguide aperture for delivering light to the
 medium in the pit.
 An alternative fabrication technique may use a directional deposition
 process such as e-beam evaporation (as opposed to the generally
 non-directional deposition obtained from a sputtering system) to deposit
 the reflective coating on the surfaces of the pit. Here, a deposition may
 be performed whereby the pit is oriented relative to the direction of the
 deposition source such that the face of the pit containing the waveguide
 aperture is substantially shadowed from the deposition and remains
 uncoated with reflective material. This avoids a later lithographic
 processing of the reflective material in order to define the optically
 transparent waveguide aperture and align it with the intersection between
 the optical path and the pit, thus reducing the overall number of steps in
 the fabrication process and in particular reducing the number of steps
 that require accurate alignment procedures.
 The medium which may for instance comprise an upconversion phosphor mixed
 with (suspended in) a curable polymer binder, is then deposited into the
 pit. The binder may be cured by exposure to UV, visible or IR light, by
 heat, or by evaporation of solvents. The medium comprising the re-radiator
 material should be chemically compatible with the materials used for the
 waveguide structure fabrication. The medium may be deposited into the pit
 by a variety of techniques including, but not limited to, stencil
 printing, volumetric dispensing with a syringe, inkjet printing or
 roto-gravure printing. The material (i.e. polymer binder) may then be
 cured to achieve structural integrity of the medium. The medium may
 overlie the top surface of the waveguide structure outside of the
 dimensions of the pit, or it may incompletely fill the pit. No constraint
 is set as to the shape of the medium in the vertical direction. The shape
 of the medium in the plane of the waveguide structure may be controlled by
 the deposition technique, for instance by the volume of medium dispensed
 from a syringe. Note that for some deposition techniques, additional
 components such as surfactants or fillers may be required in the binder to
 achieve uniform deposition. In some manifestations, the binder may
 comprise inorganic material, such as phosphoric acid or an alkaline metal
 silicate solution.
 In certain embodiments it may be preferable for the pit to be significantly
 smaller than the medium. In this case the pit can act primarily as an
 out-of-plane mirror to redirect the input light up into the body of the
 medium where it interacts with the majority of the re-radiator.
 A second reflector is preferably deposited on the top surface of the
 medium, for instance by the sputter or evaporation of a thin layer of
 metal (e.g. Ag). Lithographic patterning (e.g. wet etching or laser
 ablation) may again be performed to remove the reflector from unwanted
 areas, such as the top aperture on the top surface of the medium,
 providing an optically transparent emission aperture for output light
 created by the re-radiator material within the medium. Additionally, other
 coating methods may be used to apply the reflector, which does not have to
 be a thin film. The functionality of the reflector is to provide a high
 reflectivity at the interface between the medium and the reflector. Thus
 the reflector may be composed of a thin metallic film, but equally it may
 consist of a thick layer, of for instance solder or silver paste,
 deposited by dip coating, spray coating or stencil or screen printing, as
 indicated by reflector 305 in FIG. 3. The reflector 305 could even
 planarize the top surface 310 of the device, eliminating the surface
 topography of the medium 315 disposed on the waveguide structure 320.
 Whatever the form, thickness or material of the reflector, an aperture 325
 is preferably created at some position to enable output light to emerge
 from the medium.
 The use of laser ablation to define the top aperture in the reflector also
 enables the creation of a hole (a depression or a crevice) 405 in the top
 of the medium 410, as indicated in FIG. 4. This can be created simply by
 exposing the medium to further pulses from the excimer laser ablation
 system after the reflective layer has been removed. Provided that the
 medium 410 absorbs the excimer laser radiation the hole can be ablated in
 the same way as described above to create the pit. This hole in the top of
 the medium may be used to enhance the amount of output light emerging from
 the medium, while maintaining confinement of the input light. In the case
 of the generated light being of shorter wavelength than the input light,
 the generated light will be more strongly scattered at the surfaces of the
 etched hole and thus is more likely to exit the medium through the top
 aperture in the reflector than is the input light. In addition, a
 transparent material 415 could be applied to fill the hole thus providing
 a lensing effect to give directionality to the generated light.
 The optical performance of the structures described herein depends on a
 series of design parameters. For different uses of the structures,
 different choices for one or more of these parameters may be required to
 achieve the desired optical performance of the structure. These factors
 include, but are not limited to, device dimensions, absorption coefficient
 of the medium, the volume fraction of the re-radiator in the medium, the
 volume fractions of the phosphor/binder in the re-radiator, reflectivity
 at pump and generated light wavelengths of the reflectors, divergence of
 input light from the waveguide as it enters the pit, refractive index
 mismatch between the phosphor and binder, and the size of the waveguide
 aperture. The number, size, shape and position of top apertures in the
 reflector may be optimized for a given set of values for the device
 dimension and other aforementioned parameters.
 One embodiment includes a reflector material with high reflectivity at the
 wavelengths of both the input and generated light. The size of the input
 aperture is small compared to the top aperture(s), while still allowing
 the desired amount of input light to enter through it, in order to
 minimize input and generated light escaping through the input aperture and
 to maximize the fraction of generated light that escapes through the top
 aperture(s). The volume fraction of upconversion phosphor in the polymeric
 binder (or the re-radiator in the medium) is chosen such that the input
 light will be substantially absorbed after only a few passes through the
 re-radiator material. However, the choice of a phosphor volume fraction,
 for example, depends on the reflectivity of the reflector material. The
 use of a reflector with high reflectivity allows for more reflections,
 compared to the use of a reflector with lower reflectivity, for the same
 total optical loss to the reflector material surfaces, and it therefore
 allows for a lower phosphor volume fraction. This in turn may require
 smaller top aperture size(s) in order to maintain confinement of the input
 light. While the size of the phosphor pit and the aperture may vary
 significantly in different embodiments, in one example the pit has
 approximately a 200 micron diameter and the top aperture has a 50 micron
 diameter. It is apparent to those skilled in the art that achieving the
 desired function with optimal optical performance involves the
 optimization of one or more of the aforementioned parameters.
 Optical energy emitted by the pixel element has a primary direction of
 emission, defined by a weighted center of the solid angle of the emission.
 For display purposes at least, the primary direction of emission should be
 some direction that is not parallel to the plane of the substrate.
 Depending on the embodiment, the primary direction of emission might be
 away from the substrate or through the substrate, and need not be
 perpendicular to the substrate. A pixel element can also have a second
 primary direction of emission in certain embodiments, for example where
 the reflector includes more than one aperture.
 In a variation of FIG. 4 an optical fiber may be placed into the hole 405
 in the medium 410 in order to capture the output light from the medium.
 The optical fiber may be glued in place to provide permanent attachment to
 the medium. In this way the top reflector and fiber assembly may
 completely enclose the medium, preventing light loss from the medium
 around the edges of the fiber. Alternatively, a top reflective layer may
 be applied after the fiber attachment to cover the region around the hole
 admitting the fiber.
 The invention described herein has embodiments other than the preferred
 form as a visual display pixel. For example the devices of FIGS. 1 or 2
 may be used as a light source in a data storage device. Radiation
 delivered by the waveguide may be reradiated with different wavelength,
 emerge from the medium towards a collection lens (optional) and be
 directed onto a data storage medium, such as a hologram or compact disc.
 In order to avoid unnecessary repetition, it should be understood that the
 variations described in reference to FIG. 2 apply to the embodiments
 described below, and that the variations described in reference to the
 figures below also apply to FIG. 2.
 FIG. 5 indicates an embodiment of the invention where there is no aperture
 in the top reflector 505 disposed on the medium 510 comprising re-radiator
 material. Input light propagating along an optical beam path 515 in the
 optical waveguide structure 520 and entering the medium 510 through the
 waveguide aperture 525 is confined by reflection in this structure so that
 the only means for input light and/or output light from the re-radiator
 material to emerge from the structure is through the waveguide aperture
 525. In other embodiments, it may be desired that no light emerge from the
 waveguide aperture and all the input light may be absorbed.
 FIG. 6 indicates an embodiment of the invention where there is no top
 aperture, but that contains an optically transmissive aperture 605,
 henceforth referred to as a bottom aperture, in the reflector 610 disposed
 on the surface of the pit 615 at the bottom of the medium 620. There may
 be more than one bottom aperture, and such apertures can be of any shape
 and at any location on the surface of the pit 615. Alternatively these
 bottom apertures may be located in the reflector on the side walls 625 of
 pit 615 and/or in the reflective layer 630 on the top surface of the
 optical waveguide structure 635. Input light propagates along an optical
 beam path 640 in the optical waveguide structure and enters the medium 620
 through the waveguide aperture 645. The input light is highly confined by
 the reflector 650 disposed on the surface of the medium 620. Output light
 can emerge from the medium 620 either through the bottom aperture 605 or
 the waveguide aperture 645. The presence of the top reflector 650 will
 increase the efficiency of light re-radiation of the structure by the
 mechanisms described above, and at the same time provide for propagation
 of the generated light through the bottom aperture 605, the optical
 waveguide structure 635, and the substrate 655. This would be the
 preferred propagation direction in an embodiment of the invention used as
 a pixel in an emissive display that is viewed through the substrate.
 An alternative fabrication of the basic structure of FIG. 6 incorporates a
 multilayer dielectric coating reflector under the medium, which can be
 designed to provide high reflectivity at the input light wavelength and to
 transmit part or all of the generated light. The dielectric coating may be
 disposed on the bottom surface of the pit, or alternatively may be
 disposed directly on the substrate before the optical waveguide structure
 is deposited and patterned, or on some intermediate layer between the
 substrate and the optical waveguide structure. The dielectric coating may,
 if desired, be combined with a metallic reflector layer to independently
 control the reflectivity and emission apertures for the input light and
 the generated light, to optimize the efficiency of the pixel structure and
 maximize the emission of the generated light.
 In the context of using the device described here as a pixel in an emissive
 display, a manifestation which incorporates one or more bottom apertures
 605 in the bottom reflector 610 along with one or more top apertures 235
 (FIG. 2), can provide a structure with apertures on opposing sides of the
 medium. Such a structure will simultaneously provide high confinement of
 the input light and increased optical efficiency, compared to a structure
 with no reflector, and enable an emissive display that can be viewed from
 both sides of the substrate 655.
 FIG. 7 indicates a modification that can be applied to any of the
 embodiments described herein, with two optical beam paths 705, 710 in the
 optical waveguide structure 715, through which input light can be
 delivered to the medium 720 through waveguide apertures 725, 730,
 respectively disposed at the intersection of the pit 735 and the optical
 beam paths. Output light can also escape through the waveguide apertures
 725, 730 into the optical waveguide structure 715. There may be more than
 two such optical paths terminating at waveguide apertures in the pit 735,
 and these optical paths may come from any direction in the plane of the
 optical waveguide structure 715. Light from one or several light sources
 may be delivered along these optical beam paths to the medium. Thus, this
 embodiment may provide for further improvement of the optical performance
 by, for example, delivering additional input light from a second laser
 source (at the same or different wavelength) through waveguide 710 and
 waveguide aperture 730 to medium 720 for the purpose of increasing input
 power to the re-radiator material. This embodiment may also enable the use
 of two or more different types of input radiation, for example light at
 different wavelengths, for the purpose of enhancing the efficiency of the
 desired wavelength conversion process in the re-radiator material.
 FIG. 8 indicates another modification that can be applied to any of the
 embodiments described herein and is demonstrated as a variation applied to
 the device of FIG. 2 as an example. Input light propagating along an
 optical beam path 805 enters the pit 810 through waveguide aperture 815.
 In this device, the medium in the pit does not contain only the
 re-radiator material but rather, as an example, also contains a portion of
 material that is optically transparent to the input light. In this
 particular example, the re-radiator is disposed uniformly through only an
 upper volume of the medium, with substantially no re-radiator in the lower
 volume of the medium. It will be apparent that the re-radiator may
 alternatively be "clustered" in a predetermined location within the upper
 volume of medium or otherwise distributed. Optically transparent is
 defined as having minimal absorption at the wavelength of input and
 generated light. This "transparent" material may consist for instance of
 transparent glass, organic resin, or a gas such as air, and may in
 particular be the binder material containing no re-radiator material. It
 will be understood that the term "medium" in this context is intended to
 include media containing more than one substance, even if the substances
 are not intermixed, and even if the substances are deposited in different
 process steps.
 The top surface of the transparent material may be above or below the top
 surface of the optical waveguide structure and is not necessarily planar
 or flat. Preferably the optically transparent material is disposed
 adjacent to the waveguide aperture. After entering the pit 810 through the
 waveguide aperture 815 the input light propagates along the input plane
 within the optically transparent material without interacting with the
 re-radiator material, since there is substantially no re-radiator in this
 input plane. The light may enter the portion of the medium containing the
 re-radiator material 820 by means of scattering and/or refraction at the
 interface 825 between the optically transparent material and the
 re-radiator material 820. The interface 825 may be smooth or rough. If the
 interface is sufficiently smooth, internal reflection of at least part of
 the input light may occur as the light traverses the pit. In combination
 with a reflective surface 830 at the bottom of the pit 810, this allows
 for the input light to preferentially propagate toward the side of the pit
 that is opposite the waveguide aperture 815. If the refractive indices of
 the optically transparent material and the reradiator are chosen correctly
 (as described in any text on optical waveguide design, see for instance
 The Optical Integrated Circuits reference incorporated above), the
 combination of the reflector disposed on the surface of the pit and the
 interface 825 may act as an optical waveguide. A reflective or diffractive
 (for instance, a grating) outcoupling element on the distal side wall 835
 of the pit may be oriented (e.g. slanted) to direct input light from the
 pit into the portion of the medium containing the reradiator material 820.
 Such reflective or refractive outcoupling elements may also be located on
 the other walls or the bottom of the pit itself
 In an embodiment as a pixel that generates visible light for a display
 application and where the re-radiator material includes a binder material
 and/or a wavelength converting material such as an upconversion or
 downconversion phosphor, the scheme shown in FIG. 8 offers the advantage,
 relative to the case of FIG. 2, of generating less of the generated light
 in regions that are close to the waveguide aperture 815. Therefore the
 generated light has an increased probability of exiting the structure
 through the top aperture 840 from where it can be directed towards the
 viewer. The structure of FIG. 8 may also be fabricated using a multilayer
 thin film dielectric coating (e.g. a multilayer stack of alternating
 SiO.sub.2 /TiO.sub.2 layers) as a dichroic filter deposited at the
 interface 825 to allow input light at a first wavelength to enter the
 portion of the medium containing the re-radiator material 820 and designed
 to reflect generated light emitted from the re-radiator material back into
 the re-radiator material to emerge from the re-radiator through the top
 aperture 840. This type of interfacial reflector will prevent light
 generated in the re-radiator material from escaping through the waveguide
 aperture 815. For example, if the re-radiator material includes an
 upconversion material such as erbium-doped YF.sub.3 that generates green
 light when excited with infrared light around 1500 nm, 980 nm or 800 nm
 the interfacial dichroic filter may be designed to transmit the infrared
 wavelengths and to reflect green light so that the generated green light
 does not enter the transparent region and then escape through the
 waveguide aperture.
 The structure described above could be fabricated using a two stage
 deposition process. The pit may be located and fabricated as described
 above for the embodiment illustrated in FIG. 2. A first deposition step
 may be used to deposit the optically transparent material into the pit
 adjacent to the input aperture, the material may over-fill the pit and
 protrude above the surface and extend out onto the top surface of the
 optical waveguide structure, or the material may incompletely fill the pit
 and lie beneath or flush with the top surface of the waveguide structure.
 The top surface of the optically transparent material is not required to
 be planar or parallel to the top surface of the optical waveguide
 structure. This first deposition step could consist for instance of a
 screen or stencil printing process, or an inkjet or volumetric (via
 syringe) deposition of the transparent material. The material should then
 be cured if necessary before a second deposition process, which may be the
 same or different to the first, is used to add the second layer of
 material containing the re-radiator. Following curing of the second layer
 (if necessary) the top surface of the mound may be coated with a reflector
 as previously described and an aperture created to allow the emission of
 output light. If desired, a multi-layer dielectric coating may be
 deposited, by for instance electron-beam evaporation, on the surface of
 the transparent material before the deposition of the re-radiator medium.
 An alternative method of fabrication of the embodiment of FIG. 8 is shown
 in FIG. 9. Here the upper volume of the medium has been created in a
 separate layer structure 905 to the optical waveguide structure 910 and
 then attached (for instance glued or laminated) to the surface of the
 waveguide structure in alignment to the pit(s) 915 therein. This
 fabrication route offers the potential for the creation of taller medium
 920 structures that offer advantages in terms of single pass absorption
 efficiency of the input light, minimization of reflection losses on the
 surface of the medium, and increased directionality of the generated light
 emission from the top aperture. One route to fabricate the illustrated
 device would be as follows: A flexible (e.g. Mylar.RTM.) (or non flexible)
 substrate 905 with a thickness preferably greater than the desired height
 of the medium, is patterned to provide a depression 925 with the desired
 shape of the medium. This depression could be fabricated by embossing,
 molding, wet etching, reactive ion etching or excimer laser ablation,
 depending on the choice of substrate material (at least the latter two
 processes would be suitable for a plastic (e.g. a Mylar.RTM.)substrate). A
 reflective layer 930 is then disposed on the interior surface of the
 depression, for instance by the sputtering of a thin metal layer onto the
 structure. The non-directionality of the sputtering process enables the
 3-dimensional surface of the depression to be covered with a continuous
 layer of material. The reflective layer is then patterned to open an
 aperture 935 at the bottom of the depression, either using lithographic
 exposure and wet etching, or more simply by direct material removal using
 a projection excimer laser ablation system. After patterning the
 reflector, the medium 920 is deposited into the depression, for instance
 by screen or stencil printing, or inkjet or volumetric (syringe)
 deposition. Preferably the deposition process should leave the medium
 flush with the surface of the substrate or recessed slightly beneath the
 surface, rather than protruding from the surface of the substrate. The
 medium filled substrate 905 is then placed over the optical waveguide
 structure 910 as indicated in FIG. 9 and aligned so that the entrance to
 the medium filled depression is above the reflector 940 coated pit created
 in the optical waveguide structure (which may be fabricated as described
 in the embodiments detailed above). A suitable glue may be disposed
 between the two substrates, for instance it may be screen printed onto one
 or the other substrate before they are aligned and brought into contact.
 The glue may, if desired, form the optically transparent material
 described above and fill the pit adjacent to the waveguide aperture. Thus,
 light propagating along an optical beam path 950 within the optical
 waveguide structure 910 enters the pit through the waveguide aperture 945
 and is directed into the medium by the reflector 940 coated pit structure.
 Output light emerging from the medium is emitted from the top aperture 935
 opened in the reflector layer 930 surrounding the medium.
 FIG. 10 shows an embodiment of FIG. 2 in which the medium comprises a
 spatially non-uniform distribution of re-radiator material 1005, such as
 upconversion phosphor particles in a binder material, disposed between two
 optically transparent materials 1010 and 1015 (which may or may not be
 same material). The re-radiator may be placed in a particular location in
 the structure for several reasons. For example, the re-radiator may be
 more efficient if concentrated in a region within the structure where the
 input light excitation density is higher than at other locations in the
 medium. Alternatively, the re-radiator material 1005 may be preferentially
 located close to the top aperture 1020 where the generated light can more
 directly exit the top aperture 1020 in the top reflector 1025 thereby
 requiring fewer reflections from the reflective coatings before exit. A
 further embodiment may comprise a re-radiator material that itself
 comprises a spatially non-uniform distribution of phosphor particles for
 the reasons explained above.
 Several methods may create a spatially non-uniform distribution of
 re-radiator in the medium. For example, an optically transparent polymeric
 binder material might first be deposited by stencil printing, ink jet
 printing or spin coating, and a second material comprising, for example a
 polymeric binder containing upconversion phosphor might be stencil printed
 on top of the first layer. If desired, additional optically transparent
 material may be deposited on top of the re-radiator material and part or
 all of the structure may be covered with reflector.
 Of course, the spatial nonuniformity of re-radiator is not restricted to
 the use of layers or to the use of one type of re-radiator. For example,
 two or more small mounds of different re-radiator materials may for
 example, be deposited in a pit and an additional material may be deposited
 over the combination of re-radiator mounds. A reflector may be deposited
 over the entire structure. The reflector will ensure good absorption of
 light by the appropriate re-radiators. Multiple layers of materials, or
 small mounds of different re-radiator materials may be deposited serially
 in a sequence of deposition steps. For instance, screen or stencil
 printing and curing of the underlying transparent layer may be followed by
 individual volumetric depositions (or inkjet or stencil prints) to create
 layers or mounds of re-radiator materials, followed by a final deposition
 of transparent material to cap the re-radiator material.
 FIG. 10 also demonstrates that the structure may, if desired, contain only
 a small volume of one component and a larger volume of a second. For
 example, the upconversion phosphor particles in the re-radiator 1005 may
 occupy only a small fraction, say 5% of the total volume of the medium.
 Confinement of the input light by the reflective surfaces thereby
 increases the total input light energy that is absorbed per upconversion
 phosphor particle. The resultant increase in excitation density within the
 phosphors will provide a higher efficiency of conversion of infrared to
 visible light within the phosphor particles. This enhancement will occur
 whether the phosphor particles are distributed evenly, as a small volume
 fraction, or unevenly.
 It is also within the scope of the invention to utilize a specific shape to
 the medium and therefore the top reflector 1025 to create, by reflection,
 localized regions of higher intensity of light within the medium.
 Preferably the shape will maximize the intensity of input radiation at the
 same spatial location as the re-radiator material thus maximizing the
 excitation density and efficiency of the re-radiation process. Suitable
 shapes to perform this function would include parabolic profiles, or
 generally concave reflection surfaces.
 FIG. 11 shows a further embodiment of the invention using multiple
 depositions of different materials to create a mound of optical
 re-radiator in a medium.
 In this case a portion of optically transparent material 1105 is deposited
 and cured (if necessary) over and in the reflector coated pit 1110 formed
 in the optical waveguide structure 1115 (fabricated as described in the
 embodiments above). A depression 1120 is created in the top surface of the
 optically transparent material, for instance using excimer laser ablation
 or reactive ion etching to remove material from a desired area.
 The shape/profile of the depression may be controlled during the etch
 process as described for the embodiment of FIG. 2, leading preferably to a
 parabolic or near parabolic shape. Following the etching of the
 depression, a reflective layer 1125 is deposited over the mound of
 optically transparent material, and an aperture 1130 is opened as
 described above. The aperture should preferably be aligned relative to the
 reflector coated pit in the optical waveguide structure, such that input
 light from an optical beam path 1135 within the optical waveguide
 structure 1115 enters the pit 1110 and is redirected through the aperture.
 Re-radiator material 1140 is deposited into the depression formed in the
 top surface of the mound of optically transparent material, superposing
 the reflector layer, using any of the deposition methods previously
 described. The re-radiator material may protrude above the top surface of
 the transparent material, or it may be flush with or recessed below the
 top surface. Preferably, a multilayer dielectric mirror 1145 (e.g. a stack
 of alternating layers of SiO.sub.2 and TiO.sub.2) is deposited over the
 top surface of the mound of re-radiator to preferentially reflect the
 input light and transmit light generated within the re-radiator.
 FIG. 12 shows a structure which enables the design of a controlled delivery
 of input light to desired locations in the medium. At least part of the
 bottom of the pit 1205 does not extend further down than to the plane
 defined by the top surface 1210 of the integrated optical waveguide core
 1215, and at least another part of the bottom of the pit 1220 extends at
 least as far down as the plane defined by the bottom surface 1225 of the
 integrated optical waveguide core 1215. As a result, the optical waveguide
 core layer 1215 may extend partially into the medium 1230 and input light
 propagating through the optical waveguide core 1215 may be only partially
 confined within the waveguide core and may outcouple from the waveguide
 into the medium 1230. Optionally, a reflective coating 1235, such as a
 metallic or dielectric reflector, may be deposited onto the surface of the
 pit 1205 in the section where the waveguide extends into the medium 1230.
 Alternatively or additionally a metallic or dielectric reflector 1240 may
 be disposed directly on the surface of the substrate, or on some
 intermediate layer 1245 between the substrate 1250 and the optical
 waveguide structure 1255, before the deposition of the optical waveguide
 structure. This reflector layer can be designed to reflect one or both of
 the input light and generated light that emerges from the medium towards
 the substrate back into the medium for emission through the top aperture
 1260. The pit may extend from the top surface of the optical waveguide
 structure 1255 completely through the structure to reach the reflector
 disposed beneath the structure, such that a reflector 1265 disposed on the
 side walls of the pit abuts the reflector 1240 on the substrate leaving
 substantially no area for light to escape the medium at this location.
 Such a structure may be fabricated by several methods including the
 following multi-step process. Firstly, a lithographically defined etch
 technique is used to create a pit that extends into the top surface of the
 waveguide structure as described above. A multi-step etch process may be
 used to vary the depth of the pit at different locations, for instance
 using two lithographic masking steps for an RIE process, or using two
 different projection exposure masks for a laser ablation etch. The two
 masks used must of course be correctly aligned to ensure the desired
 overlap of the different depth regions of the pit. Deposition and
 patterning of a reflective layer 1235, 1265 on the bottom and side
 surfaces of the pit may be performed for instance by sputter coating and
 wet etching of a thin film metal layer. The medium 1230 may be deposited
 into the pit by any of the techniques previously described, and the top
 surface of the medium coated with a reflector 1270 and provided with a top
 emission aperture as previously described. Thus the structure of FIG. 12
 may be created, with an initial region where the pit is etched only
 partially into the upper cladding or core layer, followed by a second
 section where the pit extends deeper, to completely remove the core layer
 in at least one location within the pit.
 Additionally, FIGS. 13A and 13B show an alternative technique for
 controlling the delivery position of input light to desired locations in
 the medium 1305. In these embodiments, the medium 1305 is positioned off
 center from the pit 1310. By controlling the relative positions of the
 medium, input aperture and distal reflecting surface 1315, the path of
 input light 1320 can be controlled within the medium. Thus it is possible
 to preferentially direct the input light towards the front, center or back
 of the medium (and similarly in the lateral dimension not shown in the
 cross-sectional FIGS. 13A and 13B). As shown in FIG. 14 a further
 embodiment is such that the medium 1405 does not superpose all the
 surfaces of the pit 1410 but at least superposes the input face 1415
 comprising the input aperture 1420.
 Note that in all the embodiments described above there is no limitation on
 the relative dimensions of the pit and the medium. In certain embodiments
 it may be preferable for the pit to be significantly smaller than the
 medium such as in FIG. 13A. In this case the pit 1310 can act as an
 out-of-plane mirror to redirect the input light up into the body of the
 medium where it interacts with the majority of the re-radiator. In
 addition, consider the case of FIG. 15 in crossectional view where at
 least two pits 1505 and 1510 are disposed to deliver input light
 propagating in one or more optical beam paths 1515, 1520 to a common
 medium 1525.
 FIGS. 16 and 17 indicate embodiments in which the pit (1605 in FIG. 16 and
 1705 in FIG. 17) that terminates the optical energy path from the
 waveguide 1610, 1710 does not also support the medium containing the
 optical re-radiator. Instead the medium is supported in awell (1615 in
 FIG. 16 and 1715 in FIG. 17) substantially above the waveguide core, but
 close enough to the core such that input light energy propagating within
 the waveguide is transferred to the medium by evanescent coupling. The
 term "well", as used herein, does not itself imply any particular depth
 relative to the core layer. Optimizing evanescent coupling enables the
 design of distributed energy delivery into the re-radiator material. The
 separation between the bottom surface of the well and the core of the
 waveguide is determined by a combination of the following parameters: the
 desired strength of evanescent coupling, and the difference in refractive
 indices between the medium or the re-radiator material and the core and
 top cladding layer materials of the waveguide structure. The closer the
 well approaches the waveguide core, the stronger the optical coupling.
 In FIG. 16, the pit 1605 which intersects the optical beam path of light
 propagating within the waveguide core 1610 extends below the waveguide
 core at a surface 1620. The pit 1705 in FIG. 17 similarly extends below
 the waveguide core 1710 and intersects the optical beam path of the core
 1710. A reflector 1725 may be deposited on the intersecting surface as
 shown in FIG. 17, and/or, as shown in FIG. 16, an absorber material 1625
 may be deposited within the pit 1605 to prevent further propagation of
 light along the intersected optical path. An optical absorber material is
 one that is opaque to light of at least the wavelength of the input light
 and optionally the generated light. Note that if a reflector is disposed
 on the surfaces of the pit 1605 or 1705, then the pit 1605 or 1705 may in
 some embodiments be filled, or partly filled, with re-radiator material
 (rather than optical absorber) and may be enclosed by the top reflector
 surmounting the medium.
 In the particular embodiment of FIG. 16 an optional reflector 1630 with an
 top aperture 1635 is shown disposed on the top surface of the medium.
 Additionally, a dielectric reflector may be disposed on the bottom surface
 1640 of the well 1615 which allows the evanescent transfer from the
 waveguide to the medium but substantially reflects light generated by the
 re-radiator material into a direction away from the waveguide core.
 Alternatively a metallic or dielectric reflector may be disposed directly
 on the surface of the substrate, or on some intermediate layer between the
 substrate and the optical waveguide structure, before the deposition of
 the optical waveguide structure. This reflector layer can be designed to
 reflect one or both of the input light and generated light that emerges
 from the medium towards the substrate, back into the medium before
 emerging from the top aperture.
 A further embodiment based on the structure shown in FIG. 17 and described
 above is shown in FIGS. 18A, 18B, and 18C. FIG. 18A is a perspective view,
 FIG. 18B is a top view, and FIG. 18C is a cross section in the plane
 defined by B-B' in FIG. 18A. In this embodiment, as shown in FIG. 18A, the
 core termination surface 1805 of a pit 1810 describes a generally circular
 shape (a sidewall or an enclosing shape) that leaves at least one section
 1815 open for the delivery of input light along an optical beam path in
 the waveguide 1820 to the region 1825 of the waveguide that is bounded by
 the core termination surface 1805; we henceforth refer to this open
 section 1815 as the input aperture. There may be more than one input
 aperture and the core termination surface 1805 of the pit may describe any
 shape in the optical waveguide structure 1830 for example an oval,
 quadrilateral or any other polygonal shape and, optionally, may be coated
 with a reflector 1835. We henceforth refer to the area bounded by the core
 termination section as the confinement region 1825. The core termination
 surface 1805 may have a shape that minimizes the amount of input light,
 that propagates in the confinement region 1825, escaping through the input
 aperture 1815, thereby maximizing the fraction of input light that is
 confined to the confinement region by the core termination surface. This
 is achieved, for example, by a section of the core termination surface
 1805 acting as a baffle 1840 as shown in FIG. 18B. A well 1845 is located
 above the waveguide core layer in the confinement region 1825. The
 evanescent interaction section 1850 of the well 1845 is that portion of
 the well 1845 that is disposed above the waveguide core layer with a
 sufficiently thin top cladding layer such that light is coupled from the
 waveguide into the superposing medium 1855 by an evanescent coupling
 method. Deposited on top of the evanescent interaction section 1850 is a
 medium 1855. Optionally, a reflector may be added to the top surface of
 the medium 1855. Additionally or alternatively, a reflector 1860 may be
 added on the substrate or any surface between the substrate and the bottom
 surface of the core layer 1820.
 As shown by the rays 1860 in FIG. 18B, input light propagating in the core
 layer 1820 and entering the input aperture 1815, undergoes multiple
 reflections at the core termination surface 1805 and propagates in the
 plane of the waveguide core layer within the confinement region until it
 is coupled into the medium 1855 comprising reradiator material by the
 evanescent interaction, or escapes through the input aperture 1815, or is
 absorbed by the reflective coatings. Adjustment of the thickness or
 refractive index of the top cladding layer in the evanescent interaction
 region 1850 allows for control of the interaction process that couples
 input light into the re-radiator material. In an embodiment of FIGS.
 18A-18C where the re-radiator material contains upconversion phosphor, the
 evanescent interaction achieved by this device, for example, may allow for
 a very uniform excitation of upconversion phosphor particles near the
 surface 1850. Output light generated by the upconversion phosphor
 particles in the medium can then emerge from the structure and, in the
 context of using the structure as a pixel in emissive displays, provide
 for a uniform emission pixel on the display screen.
 For applications of any of the embodiments of the invention as pixels in
 emissive displays, it may be desirable to deposit a black material in the
 regions from which no light is emitted from the screen. One purpose of
 this so-called black matrix material is to absorb ambient light incident
 on the viewing area of the display and thereby to prevent ambient light
 reflected off the screen toward the viewer from reducing the contrast
 ratio of pixels on the display. FIG. 19 is an embodiment of the invention
 where a layer of black material 1905 is deposited on top of the top
 reflector 1910 on the medium 1915. This embodiment provides coverage of
 non-emissive areas with black material. In different embodiments these
 non-emissive areas could include just the reflector-coated top surface of
 the medium 1915 or the entire top surface of the device except the
 optically transmissive top apertures 1920 on the medium, or any other
 portion of the device top surface. In one mode, the black material is
 deposited after the deposition of the reflective material of the top
 reflector 1910 and before patterning of the top aperture 1920. An etching
 or ablation technique, such as laser-ablation, can then be used to remove
 the black material and the reflective material on the medium in order to
 create an optically clear aperture 1920 in a single manufacturing step.
 The black material may be deposited for instance by evaporating or
 sputtering an optically opaque (black) material after depositing the
 reflective layer, or the black material may consist of for instance a
 polymer binder material including a dye such as Sudan black or carbon
 black particles, and be deposited by screen or stencil printing, inkjet
 printing or spray or dip coating Note that the aperture in the black
 material may be larger than that formed in the reflective layer such that
 not all the reflective layer is covered by black material. Alternatively,
 the black material may be deposited and patterned if necessary after the
 aperture has been formed in the reflective layer. In this case the
 aperture in the black material may be smaller than, the same size as or
 larger than the aperture in the reflective layer.
 A further embodiment, shown in FIG. 20 allows for a simplified deposition
 process of the black matrix by coating the substrate 2005 with a layer of
 black material 2010 prior to creating the various optical layers of the
 optical waveguide structure 2015 and the components of the medium 2020.
 Alternatively the black material may be deposited on some intermediate
 layer between the substrate and the optical waveguide structure. FIG. 21
 shows a further embodiment where the optically transmissive aperture is
 created in the bottom surface of the medium through the reflector 2105 and
 black material layers 2110. In this embodiment, generated light is emitted
 through the substrate 2115.
 In a further embodiment, a layer of material may be added to a
 substantially planar medium in order to create an optically smooth layer.
 If the refractive indices of the materials are appropriately chosen it is
 possible to achieve a degree of optical confinement within the multilayer
 structure as a result of total internal reflection at the smooth top
 surface of the structure. In this way the input light can be confined
 within the medium until it is absorbed, without the requirement to add a
 further, highly reflective, layer over the top surface of the medium. A
 smooth layer may be created for instance by depositing a layer using one
 of the techniques previously described, where the material can undergo
 reflow to remove any surface topology induced by the deposition process or
 the underlying layer. Following reflow the layer should preferably be
 cured to provide a robust surface. A true reflow process may not be
 required to achieve the aim of this embodiment. The deposition of a low
 viscosity material will result in a substantial smoothing of the surface
 topology of an underlying layer as the low viscosity material is able to
 flow away from the high points and into any depressions that may be
 present. The degree of planarization achieved will be determined by a
 combination of the roughness of the underlying layers and the flow
 properties of the upper layer. It will be appreciated that there is a
 limit to the minimum material viscosity that can be used before the
 material simply flows over the entire surface of the structure.
 As shown in FIG. 20, an appropriately shaped lens element 2025 may be
 attached to the medium 2020 above the top aperture 2030. This lens will
 serve to control the direction and core angle of light emitted from the
 device towards a viewer or other sensor. Such a lens element may comprise
 for instance, a stencil printed or inkjet printed transparent epoxy in a
 three-dimensional ellipsoidal shape. Similarly, as shown in FIG. 21, for
 any embodiment comprising a lower aperture 2120, such a lens 2125 may be
 added to the substrate lower surface 2130.
 Unless otherwise specified, the term "substantially" is used herein to
 accommodate tolerances including manufacturing tolerances and optical
 tolerances (for example, dielectric reflectors physically can not reflect
 light at all angles). Omission of the word "substantially", however,
 should not be taken to require that such tolerances are not to be
 accommodated, since no real-world manufacturing process can be perfect.
 As used herein, the term "optical energy" is intended to include energy
 extending from far infrared to deep ultraviolet wavelengths.
 As used herein, a given event is "responsive" to a predecessor event if the
 predecessor event influenced the given event. If there is an intervening
 processing element, step or time period, the given event can still be
 "responsive" to the predecessor event. If the intervening processing
 element or step combines more than one event, the signal output of the
 processing element or step is considered "responsive" to each of the event
 inputs. "Dependency" of a given event upon another event is defined
 similarly.
 The specific embodiments of the invention described herein are intended to
 be illustrative only, and many other variations and modifications may be
 made thereto in accordance with the principles of the invention. All such
 embodiments and variations and modifications thereof are considered to be
 within the scope of the invention, as defined in the following claims.