Illumination apparatus comprising passive optical nanostructures

An illumination apparatus comprises a first substrate; an optical structure; an array of light emitting elements disposed between the first substrate and the optical structure and an array of passive optical nanostructures disposed between the first substrate and the optical structure. Each of the passive optical nanostructures are disposed on a respective one of the light emitting elements and each passive optical nanostructure comprises an air gap. Each passive optical nanostructure is disposed between its respective light emitting element and the optical structure, wherein each passive optical nanostructure is configured to receive light emitted by its respective light emitting element, pass the received light, and output the pass light towards the optical structure.

TECHNICAL FIELD

The present disclosure relates to illumination apparatuses including display apparatuses.

BACKGROUND

Illumination apparatuses in which micro-LEDs are used, e.g. for displays such as laptop or TV displays, are becoming increasingly commercially relevant. In these types of displays, it is typically desirable to prevent optical cross-talk. It is also typically desirable to provide a physically robust structure for the display.

BRIEF SUMMARY

According to a first aspect of the present disclosure, there is provided an illumination apparatus, comprising a first substrate, an optical structure, an array of light emitting elements disposed between the first substrate and the optical structure, an array of passive optical nanostructures disposed between the first substrate and the optical structure, each passive optical nanostructure being disposed on a respective one of the light emitting elements, each passive optical nanostructure comprising an air gap, and each passive optical nanostructure being disposed between its respective light emitting element and the optical structure, wherein each passive optical nanostructure is configured to receive light emitted by its respective light emitting element, pass the received light therethrough, and output the pass light towards the optical structure.

The optical structure may be arranged to direct light from a light emitting element and direct to a desirable location or angular range. Cross talk between adjacent optical elements of the optical structure is reduced and directionality of optical output from the illumination apparatus maintained. The passive optical nanostructures may be fabricated from monolithic wafers with high resolution optical features. The passive optical nanostructures may be provided with low area, reducing cost and increasing performance. High uniformity of passive optical nanostructures may be achieved.

Each passive optical nanostructure may comprise a plurality of spacers configured to separate its respective light emitting element from the optical structure. The air gap may comprise air which fills the space in between the plurality of spacers. Light from the light emitting element is passed by the passive optical nanostructure such that the light cone within the optical structure is within the critical angle of the light in the medium. Advantageously cross talk between adjacent optical elements of the optical structure is reduced.

A height of each of the plurality of spacers of each passive optical nanostructure may be greater than a wavelength of the light emitted by the respective light emitting element. Light from the light emitting element is passed by the passive optical nanostructure such that the light cone within the optical structure may be substantially the same as the critical angle of the light in the medium of the optical structure. Advantageously cross talk between adjacent optical elements of the optical structure is reduced. Light rays may be arranged to guide within the optical structure. Advantageously high output uniformity may be achieved over a wide area.

The illumination apparatus may further comprise a cup surrounding each of the plurality of passive optical nanostructures and its respective light emitting element. Advantageously cross talk may be further reduced.

At least some of the light emitted by each light emitting element may undergo total internal reflection at an interface with the air gap. Advantageously light rays may recirculate within the light emitting element to increase device efficiency.

The passive optical nanostructures may be hydrophobic. Advantageously, adhesive materials may not fill the gaps between the spacers of the passive optical nanostructures and optical output may be maintained within the critical angle within the optical structure.

The optical structure may be a catadioptric optical structure. Advantageously narrow output cone angles may be provided for output from the illumination apparatus with low thickness.

The illumination apparatus may further comprise an adhesive layer arranged to adhere the optical structure to the passive optical nanostructures. The first substrate and optical structure may be optically bonded to advantageously achieve increased resilience to variations in environmental conditions. Some light rays passing from the optical structure to the optical structure may be transmitted with low loss, increasing output efficiency.

The illumination apparatus may further comprise a colour conversion layer disposed between each light emitting element and its respective passive optical nanostructure. Advantageously output colour may be determined.

The light emitting elements may be micro-LEDs with a maximum dimension which is less than 300 μm. The light emitting elements may be micro-LEDs with a maximum dimension which is preferably less than 200 μm and most preferably less than 100 μm. Advantageously low thickness optical elements may be provided.

The illumination apparatus may further comprise a mask comprising a plurality of apertures therein, the mask being disposed on an opposite side of the first substrate to the array of light emitting elements. The optical structure may be configured to direct at least some of the light received from the array of light emitting elements through the apertures in the mask. Advantageously the illumination apparatus may be provided with high output efficiency. Low reflectivity of the illumination apparatus may be achieved. In a display apparatus high image contrast may be achieved.

According to a second aspect of the present disclosure there is provided a backlight apparatus comprising the illumination apparatus of the first aspect. Advantageously a low thickness high efficiency backlight may be provided with high resilience to environmental changes. The backlight may be flexible and have widely separated micro-LEDs, reducing micro-LED cost. High levels of collimation may be achieved.

According to a third aspect of the present disclosure, there is provided a display apparatus comprising the illumination apparatus of the first aspect or the backlight apparatus of the second aspect. A display apparatus with high image contrast in brightly lit ambient environments and with high efficiency with a low thickness may advantageously be achieved. A low stray light display such as a privacy display or a display for night-time operation may be provided.

According to a fourth aspect of the present disclosure, there is provided a method of manufacturing an illumination apparatus, comprising mounting an array of light emitting elements on a substrate, mounting a passive optical nanostructure on each of the light emitting elements, each passive optical nanostructure comprising an air gap, bonding an optical structure to the substrate such that each passive optical nanostructure is disposed between its respective light emitting element and the optical structure. Advantageously the illumination apparatus may be provided. High precision nanostructures may be provided on a monolithic wafer substrate and only those passive optical nanostructures that meet desirable performance levels may be transferred. High uniformity and low cost may be achieved.

The array of light emitting elements may be a non-monolithic array of light emitting elements. The method may further comprise extracting the non-monolithic array of light emitting elements from a monolithic wafer. Advantageously the light-emitting elements may be provided on a monolithic wafer substrate and only those light emitting elements that meet desirable performance levels may be transferred. High uniformity and low cost may be achieved.

DETAILED DESCRIPTION

In this specification, (except when qualified by the term “packaged”), “LED” or “micro-LED” refers to an unpackaged LED die chip extracted directly from a monolithic wafer, i.e. a semiconductor element. Micro-LEDs may be formed by array extraction methods in which multiple LEDs are removed from a monolithic epitaxial wafer in parallel and may be arranged with positional tolerances that are less than 5 micrometres. This is different from packaged LEDs. Packaged LEDs typically have a lead-frame and plastic or ceramic package with solder terminals suitable for standard surface-mount PCB (printed circuit board) assembly. The size of the packaged LEDs and limits of PCB assembly techniques means that displays formed from packaged LEDs are difficult to assemble with pixel pitches below about 1 mm. The accuracy of components placed by such assembly machines is typically about plus or minus 30 micrometres. Such sizes and tolerances prevent application to very high-resolution displays.

The structure and operation of various directional display devices will now be described. In this description, common elements have common reference numerals. It is noted that the disclosure relating to any element applies to each device in which the same or corresponding element is provided. Accordingly, for brevity such disclosure is not repeated.

FIG. 1Aillustrates a cross-sectional view of a section of an illumination apparatus100according to an embodiment. The illumination apparatus100comprises a light emitting element110, a colour conversion layer120, a passive optical nanostructure130, an adhesive layer206, and an optical structure220, stacked in that order.

The illumination apparatus100further comprises a first substrate200. An array of light emitting elements110is disposed between the first substrate200and the optical structure220. An array of passive optical nanostructures130is disposed between the first substrate200and the optical structure220, each passive optical nanostructure130being disposed on a respective one of the light emitting elements110, each passive optical nanostructure comprising an air gap133, and each passive optical nanostructure130being disposed between its respective light emitting element110and the optical structure220, wherein each passive optical nanostructure130is configured to receive light emitted by its respective light emitting element110, pass the received light170,180therethrough, and output the pass light towards the optical structure220. Each passive optical nanostructure130comprises a plurality of spacers132configured to separate its respective light emitting element110from the optical structure220. The air gap133comprises air which fills the space in between the plurality of spacers132. The light emitting element110is configured to emit light in a particular wavelength band (e.g. red, blue or green). In this embodiment, the light emitting element is a light emitting diode (LED). More specifically, in this embodiment, the light emitting diode is a micro-LED, i.e. an LED which has a maximum size or dimension of less than 300 μm, preferably less than 200 μm and most preferably less than 100 μm. Light rays160are emitted by excitations112within the emission layer of the micro-LED. The colour conversion layer120is disposed between each light emitting element110and its respective passive optical nanostructure130. The colour conversion layer120is configured to receive the light rays160emitted by the light emitting element110, absorb at least some of the light at absorption region122, and emit light rays170in a different wavelength band to the wavelength band of the light emitted by the light emitting element110. In other words, the colour conversion layer120effectively acts to convert the colour of at least some of the light emitted by the light emitting element110.

The colour conversion layer120may comprise a phosphor, quantum dot or other colour conversion material. It will be appreciated that, in some embodiments, the colour conversion layer120may be omitted and the coloured emission is provided directly by the light emitting element110.

The passive optical nanostructure130is configured to receive the light rays170emitted by the colour conversion layer120, convey the received light therethrough, and output the conveyed light.

The passive optical nanostructure130is configured to alter the direction of travel of at least some of the light which passes through it. The passive optical nanostructure130comprises a base layer131, a plurality of spacers132, and an air gap133.

Light rays170incident on the boundary with the air gap133are passed through the passive optical nanostructure130.

In this embodiment, the plurality of spacers132are pillars or columns. The plurality of spacers132extend perpendicularly from the base layer132. The plurality of spacers132are uniformly distributed on the base layer131such that the distance p between adjacent spacers132(also known as the pitch of the spacers132) is substantially the same for each pair of adjacent spacers132.

Each of the plurality of spacers132has substantially the same height h as each of the other spacers132. The air gap133is defined by the spacers132and the base layer131. More specifically, the air gap133comprises air which fills the space between the spacers132from the base layer131up to the height of the spacers132.

The base layer131is, for example, formed from an inorganic material such as silicon dioxide or may be a polymeric material. The spacers132are, for example, formed from a patterned inorganic material such as silicon dioxide or may be a polymeric material.

The adhesive layer206is arranged to adhere the optical structure220to the passive optical nanostructures130. The adhesive layer206bonds the passive optical nanostructure130to the optical structure220. More specifically, the adhesive layer206is bonded to the top of the spacers132and a bottom surface of the optical structure220. The adhesive layer206may be formed from any appropriate adhesive, for example an optically clear adhesive (OCA) or a pressure sensitive adhesive (PSA).

The optical structure220is an optical element which manipulates the light that it receives. In this embodiment, the optical structure220is a transparent substrate (e.g. a glass substrate) which allows the light that it receives to pass through it. The illumination apparatus ofFIG. 1Amay be further bonded to another optical structure, or the optical structure may be formed in or on to the optical structure. Examples of optical structures will be described below.

In operation, light rays180from the light emitting element110reaching the interface135between the base layer131and the air gap133is totally internally reflected if it hits the interface at an angle of incidence greater than the critical angle. Light rays180may be recirculated within the light emitting element110or colour conversion layer120. Some of the recirculated light rays182may be output through the passive optical nanostructure130, increasing efficiency.

Thus, only light which hits the interface135at an angle of incidence less than the critical angle at the interface passes through the interface135. A first light ray160is a ray emitted by the light emitting element110which has not been colour converted by colour conversion layer120. The first light ray160enters and passes through the air gap133as its angle of incidence at the interface is less than the critical angle. Second and third light rays170,180are rays which have been colour converted by colour conversion layer120. The second light ray170enters and passes through the air gap133as its angle of incidence at the interface is less than the critical angle. The third light ray180is totally internally reflected at the interface as its angle of incidence at the interface is greater than the critical angle. The second and third light rays170,180reach the interface between the air gap133and the adhesive layer206where they are refracted and pass into the adhesive layer206. The second and third light ray170,180then travel through the adhesive layer206and into the optical structure220.

In this embodiment, the adhesive layer206and optical structure220have substantially the same refractive index and so no refraction occurs at the interface between the adhesive layer206and the optical structure220. The above-described light propagation advantageously provides light reaching the optical structure220through the air gap has a restricted range of relatively small incidence angles within the optical structure220. The range within the optical structure220from the surface normal199direction is substantially determined by the critical angle θc.

This is because the light which would otherwise have reached the optical structure220at higher incidence angles does not reach the first substrate because it is instead totally internally reflected at the interface between the base layer131and the air gap133. Advantageously the light cones propagating within the optical structure220are not Lambertian but have a limited cone angle. As will be described further below, such limited cone angles enable collimation of light output by optical surfaces of the optical structure220.

Desirable dimensional properties of the passive optical nanostructure130for a nominal wavelength of 550 nm will now be described. The spacers132each have a height h greater than the wavelength λ of the light travelling through the air gap133and spacers132.

The height h of each of the plurality of spacers132of each passive optical nanostructure130is greater than a wavelength λ of the light160emitted by the respective light emitting element110. Further the height h may be greater than a wavelength λ of the light emitted by the respective light emitting element110and colour conversion layer120. Height h may be greater than the wavelength of the light which has been colour converted by the colour conversion layer120following emission by the respective light emitting element110.

The width w and pitch p of the spacers132is arranged to minimise diffractive light scatter from the spacers132of the light travelling through the air gap133, and to minimise guiding of light within the spacers132.

The pitch p may be less than 2λ, preferably less than λ, more preferably less than λ/2 and most preferably less than λ/5. The ratio w/p may be less than 0.5, and preferably less than 0.3 and more preferably less than 0.1.

In operation, the wave-like nature of light can be used to analyse propagation of light through the passive optical structure130, that is light propagation that may appear to be provided by rays160,170,180is in fact provided by propagating optical modes that pass through the air gap133and spacers132. In other words, a ray model of light propagation breaks down in the passive optical nanostructure130. At such scales, the passive optical structure130cannot be resolved using a ray model and a wave propagation interpretation is more appropriate, with the passive optical structure130appearing as approximately a uniform structure to incident waves.

Such passive optical elements130may provide high angle diffraction or zero order diffraction. Advantageously diffractive scatter from the spacers and gaps between the spacers may be reduced, minimising light scatter. In an illustrative example of the embodiment ofFIG. 5Ahereinbelow, cross talk between adjacent curved reflectors222A,222B may advantageously be reduced

Such elements may be provided by lithographic fabrication techniques on a monolithic wafer. The elements may be transferred from the monolithic wafer or may be arranged to provide a replication tool as described elsewhere herein.

By way of comparison with passive optical nanostructures a low effective index and small angle diffractive scatter may be achieved for example with passive optical microstructures with pitch p of 20 microns and width w of 5 microns. Such spacers guide incident light within the spacers, and provide Lambertian input to the optical structure220. Undesirable cross talk between the reflectors220A,220B may be provided.

The passive optical nanostructure130has an effective refractive index n1given by the equation:

n1=1+w2p2⁢(n-1)eqn.⁢1
where n1is the effective refractive index, n is the refractive index of the spacers132, p is the pitch of the spacers132and w is the width of each of the spacers132.

At least some of the light180emitted by each light emitting element110undergoes total internal reflection at an interface with the air gap133. w, p and n have values such that n1is a value which causes total internal reflection of at least some of the light reaching the passive optical nanostructure130. The critical angle θcof light within the optical structure220is then given by the equation:
θc=sin−1(n1/n2)   eqn. 2
where n2is the refractive index of the material of the optical structure220.

In other embodiments the spacers may have some distribution of spacings and distance p may have a distribution with an average pitch pavwhere pavis less than 1 micron, preferably less than 0.5 micron and most preferably less than 0.25 microns. Advantageously residual diffraction structures may be reduced in appearance in comparison to a fixed periodicity.

The above-described structure tends to allow light to be input into an optical element with controlled cone angle and high efficiency, while achieving bonding to the light emitting element by external substrates. Thus, said structure tends to allow optical cross talk in illumination systems to be reduced while also improving mechanical and thermal stability.

FIG. 1Billustrates a perspective view of a section of the illumination apparatus100ofFIG. 1A. Features of the embodiment ofFIG. 1Bnot discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

InFIG. 1B, the adhesive layer206is present but not shown for ease of illustration.FIG. 1Bfurther illustrates electrode pads210,212for driving the light emitting element110. The electrode pads210,212are in contact with a surface of the light emitting element110which is opposite to the surface of the light emitting element110that the passive optical nanostructure is in contact with. In comparison to the arrangement ofFIG. 1A, the colour conversion layer120is omitted.

FIG. 1Bfurther illustrates the structure of an inorganic micro-LED110. P-doped semiconductor layer124and n-doped semiconductor layer126is arranged on either side of a multiple quantum well structure122. Electrodes210,212are thus attached to p-doped and n-doped semiconductor layers124,126respectively.

The passive optical nanostructure130is provided on the output side of the light emitting element110. The first substrate200is provided upon which the light emitting element110is arranged. The first substrate200may further comprise reflective mask regions and control electrodes (not shown). In the region138a mask region may be provided so that all light is output from the light-emitting element110into the passive optical nanostructure130. Further the sides of the light emitting element110may be coated to prevent light emission other than through the passive optical nanostructure. In other embodiments the passive optical nanostructure130may be larger than the light emitting element110so that substantially all the light from the light emitting element is directed through the passive optical nanostructure130.

In an alternative embodiment the electrode pad212may be omitted. A transparent electrode (not shown) may be arranged on the optical substrate220and the spacers132may further be conductive such that electrical signals may be passed through the spacers132. Electrical control of the light emitting elements110may be achieved from top side electrical contacts. The current injection area is spread over the light emitting element110. Advantageously current crowding may be reduced and efficiency increased.

Advantageously a light emitting element may be provided suitable for illumination of optical structure220that is arranged to provide controlled illumination output in a thin structure.

FIG. 2illustrates a graph showing how the critical angle is dependent on parameters associated with the passive optical nanostructures130. Specifically,FIG. 2illustrates the critical angle as a function of the width w of the spacers132of the passive optical nanostructure divided by the pitch of the spacers132. Preferably the ratio of w/p is less than 0.5 which achieves a critical angle of less than 50° and more preferably the ratio of w/p is less than 0.3 which achieves a critical angle of less than 45°.

FIG. 3illustrates a perspective view of a monolithic wafer300that the passive optical nanostructure130is formed as part of. Features of the embodiment ofFIG. 3not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

Prior to being used in the illumination apparatus100described above, the passive optical nanostructure is formed (e.g. grown) as part of the monolithic wafer300of passive optical nanostructures130, and then extracted from the monolithic wafer300for use in manufacturing the illumination apparatus100.

Extraction of regions131corresponding to selected passive optical nanostructures130may be provided by known extraction methods. For example regions131may be illuminated with light such as UV light which at least partially separates the selected passive optical nanostructures130from the growth substrate monolithic wafer300. In other extraction methods, the regions131may be etched and extracted by means of mechanical detachment using mechanical stampers.

Multiple passive optical nanostructures130may be transferred in a single alignment step. Advantageously arrays of multiple passive optical nanostructures130may be transferred in a single step, reducing cost and complexity.

The selected passive optical nanostructures130corresponding to the extraction regions131are detached from the monolithic wafer300and adhered to a receiver that may be the light emitting element110. The receiver may comprise an adhesive such that the selected passive optical nanostructures130attach thereto when brought into contact with the receiver.

Advantageously high precision passive optical nanostructures may be provided by known semiconductor lithography manufacturing processes.

After growth, the monolithic wafer300may comprise regions of different performance as illustrated by boundaries302, and may further comprise scratches, debris and other defects that produce undesirable optical performance in operation. The passive optical nanostructures130of the present embodiment may advantageously be extracted only from desirable regions of the monolithic wafer. Advantageously uniformity of output may be increased. Further the yield of good passive optical nanostructures increased, achieving reduced cost. Arrays of light emitting elements with large numbers of light emitting elements may be provided with desirable optical output and low cost.

Multiple passive optical nanostructures may be provided between the light emitting element110and passive optical nanostructure130as will now be described.

FIG. 4illustrates how the passive optical nanostructure130is stacked on the light emitting element110according to an embodiment. Features of the embodiment ofFIG. 4not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

In this embodiment, following extraction from the monolithic wafer300, the passive optical nanostructure130is stacked on the light emitting element as part of a sequential order of different types of passive optical nanostructure all stacked on the light emitting element110. Specifically, in this embodiment, in bottom to top order, the stack comprises a moth eye structure410, a quantum rod structure420, a collimating nanostructure430, a wire grid polariser440, a form birefringence retarder450, and the passive optical nanostructure130described above with reference toFIGS. 1-4. In this way, the light reaching the passive optical nanostructure130from the light emitting element110will have been manipulated in various ways by the other types of passive optical nanostructure410,420,430,440,450in the stack before it is manipulated by the passive optical nanostructure130. It will be appreciated that, in other embodiments, the passive optical nanostructure130may be swapped in position with any of the other types of passive optical nanostructure410,420,430,440,450shown inFIG. 4, depending on at what point in the stack it is desired that the light is manipulated by the passive optical nanostructure130.

The operation of the passive optical nanostructure in an illumination apparatus will now be described.

FIG. 5AandFIG. 5Brespectively illustrate cross-sectional and perspective views of an embodiment of an illumination apparatus100. Features of the embodiment ofFIG. 5AandFIG. 5Bnot discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

The illumination apparatus100comprises a substrate200, an array of light emitting elements110disposed on the substrate200, and an array of passive optical nanostructures130each disposed on a respective one of the light emitting elements110.

In this embodiment, the light emitting elements110are arranged on reflective regions35such that light ray170is directed away from the substrate200by the light emitting elements110and reflective region35. The reflective region prevents light from being transmitted into the substrate200. Advantageously efficiency of transmission of light rays through the aperture152is increased.

In this embodiment, the illumination apparatus100further comprises an output mask150comprising a plurality of apertures152therein separated by opaque light absorbing region151, the output mask150being disposed on an opposite side of the substrate200to the array of light emitting elements110. The output mask150is configured to block light from passing therethrough, except for through its apertures152. In this embodiment, the optical structure220is configured to direct at least some of the light received from the array of light emitting elements110through the apertures152in the output mask150.

The output mask150opaque light absorbing region151may be formed from any appropriate opaque material, for example by printing a black material onto the substrate200. The opaque region151may alternatively or additionally comprise a nanostructured black absorber, a “nanoblack” or other similar material such as that marketed by Acktar, (Kiryat-Gat, Israel). Advantageously very low reflectivity may be achieved from the front of the illumination apparatus.

More specifically, in this embodiment, the optical structure220is a reflective optical structure configured to reflect at least some of the light it receives back towards the substrate200. In particular, the optical structure220comprises a plurality of concave curved reflective surfaces222A,222B, each aligned with a respective light emitting element110and configured to reflect at least some of the light received from that light emitting element110.

The optical structure220may be formed from a transparent main body and a reflective material disposed thereon constituting the curved reflectors222A,222B. The body of the optical structure220may be a glass or polymer material. The surface relief structure of the curved reflectors may be provided by a moulding or casting process material in a polymer material for example. A reflective layer may be formed on the curved reflectors222A,222B, for example a deposited metal coating that may comprise silver or aluminium materials as well as surface adhesion promoters and protective layers.

Each of the apertures152of the output mask150may comprise a diffuser disposed therein, the diffusers being configured to scatter light that travels through the apertures152to provide output light rays164. The diffuser in the aperture152provides rays164that may have a wide angular spread. Advantageously the illumination apparatus may provide illumination over wide illumination angles and in display applications the illumination apparatus may be visible from wide viewing angles.

The optical structure220may comprise a catadioptric optical structure. In the present embodiment, reflection at the curved reflective optical structure222A,222B and refraction at the output of the passive optical nanostructure130provides a catadioptric optical structure, that is the operation is provided by both refraction and reflection.

In operation light rays170are directed towards the curved reflective surface222B at an angle from the surface normal199that is less than the critical angle θc, where the surface normal199of the passive optical nanostructure130is illustrated inFIG. 1A.

Advantageously, the embodiment ofFIG. 5AandFIG. 5Btends to prevent light from a particular light emitting element110from reaching curved surfaces222A that the particular light emitting element110is not aligned with. This is due to the passive optical nanostructure130on each light emitting element110totally internally reflecting light. For example, in this embodiment, the maximum angle θc at which a light ray can be output from the passive optical nanostructure130is set by the critical angle associated with the interface at which light from the light emitting element110enters the air gap of the passive optical nanostructure130. Thus, this embodiment tends to prevent stray light associated with the reflective surfaces222A reflecting light from light emitting elements110that they are not aligned with. Advantageously stray light between adjacent apertures152is reduced. In display applications, image cross talk is reduced and image contrast increased.

Referring to the graph ofFIG. 2, the ratio of w/p may be provided so that the critical angle may be less than 45 degrees. The curved reflective surfaces222A,222B have no chromatic aberration and have geometric aberrations for light near the critical angle that may achieve efficient imaging of light from the light emitting element110to the respective aligned aperture152. Advantageously high coupling efficiency may be achieved with low stray light from adjacent light emitting element110. Cross talk between adjacent apertures152may be reduced.

The output mask150may be arranged to absorb incident light rays161from ambient illumination160(such as from room lighting or direct sunlight) such that the luminance of reflected light rays162is substantially lower than the luminance of light transmitted through the apertures152. Advantageously in a display application the contrast of the image may be substantially enhanced in environments with high ambient illuminance. Reduced luminance of light rays162may be provided to achieve desirable contrast in such high ambient illuminance environments. Display power consumption may be reduced.

The opaque region151may comprise a material that has similar reflectance to the surrounding decor materials. The display may be hidden within the decor, for example in an automotive application to advantageously improve aesthetic appearance of the display when not in use.

The illumination apparatus100also comprises electronics520for driving the light emitting elements110, the electronics520being disposed on the substrate200between adjacent light emitting elements110. Advantageously the present embodiment provides relatively large area for placement of control electronics520without loss. The control electronics520are not visible to an observer of the illumination apparatus.

In one embodiment, the control electronics520may be arranged to provide global control of all of the light emitting elements110. In another embodiment, the control electronics520and further external control electronics (not shown) may be arranged to provide a display function, that is each of the light emitting elements may be individually controllable with image data. In another embodiment, the control electronics520and further external control electronics (not shown) may be arranged to provide a backlight function for illumination of a transmissive spatial light modulator. Individual control of the light emitting elements110may be used to provide a high dynamic range function, and may be used to reduce display power consumption.

FIGS. 6A-Drespectively illustrate top views of embodiments of pluralities of spacers132. InFIG. 6Athe spacers132are arranged on a square grid and have circular cross sections, in comparison to the hexagonal grid ofFIG. 1Bfor example. Advantageously the area of the spacers132may be minimised. The visibility of residual diffraction may be adjusted in the design of the arrangement of spacers132. InFIG. 6Bthe spacers132are linear and are extended in at least one direction. Advantageously the visibility of residual diffraction may be eliminated in one axis. Mechanical strength may be increased. InFIG. 6Cthe spacers132are annular and inFIG. 6Dthe spacers132are box shaped. Advantageously mechanical strength may be increased.

It may be further desirable to limit off-axis propagation of light from the edge of the light emitting elements110.

FIG. 7illustrates a cross-sectional view of an embodiment of the illumination apparatus100. Features of the embodiment ofFIG. 7not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

In this embodiment, the illumination apparatus100further comprises a cup140surrounding each of the plurality of passive optical nanostructures130and its respective light emitting element110.

In this embodiment, each passive optical nanostructure130extends around the sides of its respective light emitting element110. In this embodiment, the illumination apparatus100comprises a plurality of cups140. Each cup140is disposed on the substrate200and surrounds the sides of a respective light emitting element110and passive optical nanostructure130. Each cup140is configured to block light from passing therethrough, and thus acts to prevent light being output from the sides of its respective passive optical nanostructure130. This helps to prevent wide angle light being output from the sides of the passive optical nanostructures130. Advantageously cross talk between adjacent elements may be reduced. Some collimation of light output may be achieved, and head-on efficiency may be increased.

A method to form the apparatus ofFIGS. 5A-Bwill now be described.

FIG. 8AandFIG. 8Billustrate cross-sectional views of a method of attaching the optical structure220to the substrate200during manufacture of the illumination apparatus100of the embodiment ofFIG. 5AandFIG. 5B. Features of the embodiment ofFIGS. 8A-Bnot discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

As shown inFIG. 8A, first the optical structure is positioned above the substrate200, the substrate200having the passive optical nanostructures130, light emitting elements110and output mask150mounted thereto. The optical structure220is positioned such that each of its curved reflective surfaces222A is aligned with a respective light emitting element110. Then, as shown inFIG. 8B, the adhesive layer206is formed between the optical structure and the substrate200to attach the optical structure to the substrate200. The adhesive layer206fills the space around the light emitting elements110and the passive optical nanostructures130. The adhesive of the adhesive layer206may be injected into the space between the optical structure220and the substrate200in liquid form, and then subsequently set into solid form to bond the optical structure220to the substrate200, for example by UV and/or thermal cure. Advantageously thermal and mechanical variations are minimised during operation. Fresnel reflections at the gap between the optical substrate220and the first substrate200are reduced, increasing optical efficiency and reducing cross talk between adjacent channels.

The passive optical nanostructures130may be hydrophobic so that their air gaps are not filled with adhesive if the adhesive is injected in liquid form. Advantageously, the surface tension may be arranged so that the adhesive material206does not fill the air gaps133between the spacers132of the passive optical nanostructure130and optical output may be maintained within the critical angle θc within the optical structure220.

Further methods to provide the optical apparatus of the embodiment ofFIGS. 5A-Bwill now be described.

FIG. 9andFIG. 10illustrate cross-sectional views of another method of attaching the optical structure220to the substrate200during manufacture of the illumination apparatus100of the embodiment ofFIG. 5AandFIG. 5B. Features of the embodiment ofFIGS. 9-10not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

As shown inFIG. 9, in this method, the optical structure220is positioned above the substrate200, with the substrate200having the light emitting elements110, reflective mask35and output mask150mounted thereto, and the optical structure220having the passive optical nanostructures130mounted thereto. Each passive optical nanostructure130is mounted to the optical structure220such that it is aligned with a respective curved reflective surface222A of the optical structure220. The optical structure is positioned such that each of its curved reflective surfaces222A is aligned with a respective light emitting element110. Then, as shown inFIG. 10, the adhesive layer206is formed between the optical structure and the substrate200to attach the optical structure to the substrate200. The adhesive layer206fills the space around the light emitting elements110and the passive optical nanostructures130. The adhesive of the adhesive layer206may be injected into the space between the optical structure220and the substrate200in liquid form, and then subsequently set into solid form to bond the optical structure220to the substrate200. The passive optical nanostructures130may be hydrophobic so that their air gaps are not filled with adhesive206if the adhesive206is injected in liquid form.

FIGS. 11A-Billustrate cross-sectional views of a method of attaching the optical structure to the substrate during manufacture of the illumination apparatus of an embodiment comprising a passive optical nanostructure arranged on the input side of an optical structure wherein the passive optical nanostructure is formed on the optical structure. Features of the embodiment ofFIGS. 12A-Bnot discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

In comparison to the method ofFIGS. 8A-B, the passive optical nanostructures130are arranged on the input side221of the optical structure220. The passive optical nanostructures130may be from a monolithic wafer as illustrated inFIG. 3. An adhesive may be formed on the optical structure220prior to attachment of the passive optical nanostructure130. Multiple passive optical nanostructures130may be transferred in a single alignment step. Advantageously the fewer process steps are provided for the light emitting elements110, reducing cost and complexity of the assembled first substrate200.

FIGS. 12A-Billustrate cross-sectional views of a method of attaching the optical structure to the substrate during manufacture of the illumination apparatus of an embodiment comprising a passive optical nanostructure arranged on the input side of an optical structure wherein the passive optical nanostructure is formed in the material of the optical structure. Features of the embodiment ofFIGS. 11A-Bnot discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

In comparison to the embodiment ofFIGS. 11A-B, as shown inFIG. 12A, the passive optical nanostructure130is formed in the material of the optical structure220on the input side and is not transferred from a monolithic wafer.FIG. 12Billustrates that the optical structure200is attached to the substrate200by means of adhesive200. Gaps133may be maintained by hydrophobic pillars132for example.

Advantageously the cost of the passive optical nanostructure may be reduced by incorporating the structure into a tool used to form the input side221of the optical structure.

FIG. 13andFIG. 14illustrate cross-sectional views of two more embodiments of the illumination apparatus100. Features of the embodiments ofFIG. 13andFIG. 14not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

FIG. 13illustrates an embodiment in which the optical structure220is transmissive and the substrate200is opaque so that light cannot travel through it. Light is transmitted through optical structure220so that light emitted from the light emitting elements110is output from the illumination apparatus100without travelling back towards the substrate200. Advantageously in comparison toFIG. 5A, higher density of electronic control components520and electrodes may be arranged on the substrate200without reduction of efficiency.

Lenses240A,240B are arranged to provide collimated output. A backlight apparatus for privacy display may be provided.

FIG. 14illustrates an embodiment in which the optical structure220is reflective and reflects light back towards the substrate200. In the embodiment ofFIG. 14, the substrate is transparent so that light can travel through it. Thus, in the embodiment ofFIG. 14, light emitted the light emitting elements110is output from the illumination apparatus100through the substrate200. In comparison to the arrangement ofFIG. 5A, the output is collimated and no mask150is provided. Advantageously a collimated output illumination may be achieved with high efficiency in a thin structure.

A display apparatus may comprise a backlight apparatus comprising the illumination apparatus100to illuminate transmissive spatial light modulator48. Such an apparatus may be provided for a display apparatus such as a privacy display.

In alternative embodiments the spatial light modulator48may be omitted and the illumination apparatus may be used for providing directional environmental illumination, for example for downlighting or for automotive headlights.

A thin backlight structure for a display apparatus comprising the passive optical nanostructures130of the present embodiment will now be described.

FIG. 15illustrates a further embodiment of the illumination apparatus100. Features of the embodiment ofFIG. 15not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

The substrate200is transparent and the optical structure220is reflective with regions222A,222B. In both of these embodiments, the optical structure220comprises a faceted reflective surface comprising light deflecting microstructures226for providing collimated output light rays190. In comparison to the embodiment ofFIG. 5A, light rays190that are input into the optical structure220at angles to the normal199are guided within the structure220. Such guided light spreads away from the respective light emitting element110. High levels of collimation may be achieved in thin packages. Further, the substrates200,220may be bonded to achieve thermal and mechanical stability.

As may be used herein, the terms “substantially” and “approximately” provide an industry-accepted tolerance for its corresponding term and/or relativity between items. Such an industry-accepted tolerance ranges from zero percent to ten percent and corresponds to, but is not limited to, component values, angles, et cetera. Such relativity between items ranges between approximately zero percent to ten percent.