Diffraction grating

Abstract

According to an embodiment, diffraction grating (100) comprises: a plurality of grating features (101); and a plurality dielectric stacks (120), wherein each dielectric stack (102) in the plurality of dielectric stacks (120) is arranged onto a corresponding grating feature in the plurality of grating features (101); wherein each dielectric stack (102) comprises at least one layer of a first dielectric material (103) and at least one layer of a second dielectric material (104) in an alternating arrangement in a thickness direction of the dielectric stack (102); and the plurality of dielectric stacks (120) comprise a plurality of thicknesses for the dieletric stacks.

Description

DIFFRACTION GRATINGTECHNICAL FIELD

[0001] The present disclosure relates to the field of diffractive optics, and more particularly to a diffraction grating, a display structure, and a display device.BACKGROUND

[0002] Diffraction gratings can be utilized in various optical applications, such as in augmented reality (AR) applications. For example, diffraction gratings can be used to couple light into a waveguide, manipulate light coupled into a waveguide, and couple light out of a waveguide. When designing diffraction gratings for AR applications, for example, various challenges may arise, such as how to achieve sufficient image uniformity. Image uniformity can suffer due to, for example, interference effects.SUMMARY

[0003] This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

[0004] It is an obj ect to provide a diffraction grating, a display structure , and a display device . The foregoing and other obj ects are achieved by the features of the independent claims . Further implementation forms are apparent from the dependent claims , the description and the figures .

[0005] According to a first aspect , a diffraction grating comprises : a plurality of grating features ; and a plurality dielectric stacks , wherein each dielectric stack in the plurality of dielectric stacks is arranged onto a corresponding grating feature in the plurality of grating features ; each dielectric stack comprises at least one layer of a first dielectric material and at least one layer of a second dielectric material in an alternating arrangement in a thickness direction of the dielectric stack; and the plurality of dielectric stacks comprise a plurality of thicknesses for the dielectric stacks .

[0006] In an implementation form of the first aspect , the first dielectric material comprises titanium dioxide, aluminium oxide , silicon dioxide , and / or magnesium fluoride and / or the second dielectric material comprises titanium dioxide , aluminium oxide , silicon dioxide , and / or magnesium fluoride .

[0007] In another implementation form of the first aspect , the plurality of grating features comprise titanium dioxide .

[0008] In another implementation form of the first aspect , the diffraction grating further comprises a bottom dielectric layer arranged below the plurality of grating features .

[0009] In another implementation form of the first aspect , the diffraction grating further comprises a titanium dioxide layer arranged below the bottom dielectric layer .

[0010] In another implementation form of the first aspect , the plurality of grating features are arranged into a plurality of regions , wherein the thickness of the plurality of dielectric stacks is substantially constant in each region in the plurality of regions .

[0011] In another implementation form of the first aspect , the plurality of dielectric stacks comprises a plurality of thicknesses for the at least one layer of the first dielectric material ; the plurality of dielectric stacks comprises a plurality of thicknesses for the at least one layer of the second dielectric material ; a number of layers of the first dielectric material in a dielectric stack varies over the plurality of dielectric stacks ; and / or a number of layers of the second dielectric material in a dielectric stack varies over the plurality of dielectric stacks .

[0012] In another implementation form of the first aspect , a thickness of each layer of the f irst dielectric material is 10 - 500 nanometres , a thickness of each layer of the second dielectric material is 10 - 500 nanometres , and / or a thickness of each dielectric stackin the plurality of dielectric stacks is 10 500 nano- metres .1001 3] According to second aspect , a display structure comprises a planar waveguide and the diffraction grating according to the first aspect on the planar waveguide .

[0014] In an implementation form of the second aspect , the diffraction grating is arranged as an exit pupil expansion grating .

[0015] According to a third aspect , a display device comprises the display structure according to the second aspect .

[0016] In an implementation form of the third aspect , the display device is implemented as a see-through display device .

[0017] In another implementation form of the third aspect , the display device is implemented as a headmounted display device .

[0018] Many of the attendant features wil l be more readily appreciated as they become better understood by reference to the following detailed description considered in connection with the accompanying drawings .DESCRIPTION OF THE DRAWINGS

[0019] In the following, embodiments are described in more detail with reference to the attached f igures and drawings , in which :

[0020] Fig . 1 illustrates a schematic representation of a diffraction grating according to an embodiment ;

[0021] Fig . 2 illustrates a schematic representation of a diffraction grating according to another embodiment ;

[0022] Fig . 3 illustrates a schematic representation of a diffraction grating according to another embodiment ;

[0023] Fig . 4 illustrates a schematic representation of a diffraction grating according to another embodiment ;

[0024] Fig . 5 illustrates a schematic representation of a display structure according to an embodiment ;

[0025] Fig . 6 illustrates a schematic representation of a display device according to an embodiment ;

[0026] Fig . 7 illustrates a schematic representation of image uniformity according to an embodiment ;

[0027] Fig . 8 illustrates a plot representation of simulation results according to a comparative example ; and

[0028] Fig . 9 illustrates a plot representation of simulation results according to an embodiment .

[0029] In the following, identical reference signs refer to similar or at least functionally equivalent features .DETAILED DESCRIPTION

[0030] In the following description, reference is made to the accompanying drawings , which form part of thedisclosure , and in which are shown, by way of illustration, specific aspects in which the present disclosure may be placed . It is understood that other aspects may be utilised, and structural or logical changes may be made without departing from the scope of the present disclosure . The following detailed description, therefore , is not to be taken in a limiting sense , as the scope of the present disclosure is defined by the appended claims .

[0031] For instance , it is understood that a disclosure in connection with a described method may also hold true for a corresponding device or system configured to perform the method and vice versa . For example , if a specific method step is described, a corresponding device may include a unit to perform the described method step, even if such unit is not explicitly described or il lustrated in the f igures . On the other hand, for example , if a specific apparatus is described based on functional units , a corresponding method may include a step performing the described functionality, even if such step is not explicitly described or illustrated in the figures . Further, it is understood that the features of the various example aspects described herein may be combined with each other, unless specifically noted otherwise .

[0032] Fig . 1 illustrates a schematic representation of a diffraction grating according to an embodiment .

[0033] According to an embodiment , a diffraction grating 100 comprises a plurality of grating features 101 .

[0034] Herein, grating features may also be referred to as diffraction grating lines , diffraction grating features , diffraction grating elements , grating elements , diffraction grating cells , grating cells , or similar .

[0035] Herein, grating features may refer to grating features that have a spatial periodicity of the same order of magnitude or greater than the smallest wavelength of light incident onto the diffraction grating .

[0036] Alternatively or additionally, diffractive grating features may refer to grating features that have a spatial periodicity, which, in the used incidence mounting, allows propagating diffraction orders , in either reflected or transmitted light , to emerge .

[0037] In some embodiments , the diffraction grating 100 may comprise a one-dimensional ( ID) diffraction grating . A ID diffraction grating may comprise substantially parallel diffractive grating lines as the grating features 101 .

[0038] In some embodiments , the diffraction grating 100 may comprise a two-dimensional ( 2D) diffraction grating . A 2D diffraction grating may comprise a first plurality of substantially parallel diffractive grating lines and a second plurality of substantially parallel diffractive grating lines . The first plurality of substantially parallel diffractive grating lines and the second plurality of substantially parallel diffractive grating lines may be non-parallel .

[0039] The diffraction grating 100 may further comprise a plurality dielectric stacks 120 , wherein each dielectric stack 102 in the plurality of dielectric stacks 120 is arranged onto a corresponding grating feature in the plurality of grating features 101 .

[0040] It should be appreciated that although the embodiment of Fig . 1 illustrates various different dielectric stacks 102 , all of the dielectric stacks 102 illustrated in Fig . 1 may not be implemented in the plurality of dielectric stacks . Rather, the embodiment of Fig . 1 only illustrates various ways of implementing a dielectric stack 102 .

[0041] Each dielectric stack 102 may be in contact with a corresponding grating feature 101 .

[0042] Each dielectric stack 102 may be disconnected from other dielectric stacks in the plurality of dielectric stacks . Thus , the plurality of dielectric stacks may be disj ointed from each other . For example , in the embodiment of Fig . 1 , each dielectric stack 102 is disconnected from other dielectric stacks in the plurality of dielectric stacks .

[0043] Each dielectric stack 102 may comprise at least one layer of a first dielectric material 103 and at least one layer of a second dielectric material 104 in an alternating arrangement in a thickness direction of the dielectric stack 102 .

[0044] The layers of the dielectric stacks can be planar . Thus , dimensions of the layers can be larger in two directions than in the third direction . The thicknessdirection of the dielectric stack 102 may refer to the direction in which dimensions of the layers of the di electric stack 102 are smaller than in the other two directions . For example , in the embodiment of Fig . 1 , the thickness direction of the dielectric stack 102 is in the up-down direction . When the dielectric stack 102 is placed on a waveguide 110 , such as in the embodiment of Fig . 1 , the thickness direction of the dielectric stack 102 can be parallel with the normal direction of the surface of the waveguide 110 .

[0045] In some embodiments , dimensions of each dielectric stack 102 in a plane of the waveguide 110 may be substantially equal to dimensions of the corresponding grating feature 101 in the plane of the waveguide 110 . The plane of the waveguide 110 may be orthogonal to the thickness direction of the dielectric stacks 102 . For example , in the embodiment of Fig . 1 , dimensions of each dielectric stack 102 in the plane of the waveguide 110 may be substantially equal to dimensions of the corresponding grating feature 101 .

[0046] The first dielectric material may be different from the second dielectric material .

[0047] In the alternating arrangement , for all layers expect the bottommost layer and the topmost layer, each layer of the first dielectric material 103 may be between two layers of the second dielectric material 104 and each layer of the second dielectric material 104 may be between two layers of the first dielectric material 103 .

[0048] In some embodiments , each dielectric stack 102 may comprise at least two layers of a f irst dielectric material 103 and at least one layer of a second dielectric material 104 .

[0049] In some embodiments , each dielectric stack 102 may comprise at least one layer of a first dielectric material 103 and at least two layers of a second dielectric material 104 .

[0050] In some embodiments , each dielectric stack 102 may comprise at least two layers of a f irst dielectric material 103 and at least two layers of a second dielectric material 104 .

[0051] In some embodiments , each dielectric stack 102 may comprise at least one layer of a third dielectric material . The dielectric stacks 102 can comprise any number of layers of different dielectric materials .

[0052] The plurality of dielectric stacks may comprise a plurality of thicknesses for the dielectric stacks 102 .

[0053] The thickness of the dielectric stacks 102 may vary over the plurality of dielectric stacks 120 . In some embodiments , the plurality of dielectric stacks 120 may comprise at least three thicknesses for the dielectric stacks 102 . In some embodiments , the plural ity of dielectric stacks 120 may comprise at least four thicknesses for the dielectric stacks 102 .

[0054] The plurality of thicknesses for the dielectric stacks 102 may be implemented in various ways . For example , the number of layers in the dielectric stacks 102may vary over the plurality of dielectric stacks 120 . Alternatively or additionally, the thickness of the layers in the dielectric stacks 102 may vary over the plurality of dielectric stacks . In some embodiments , the number of layers in the dielectric stacks 102 and the thickness of the layers in the dielectric stacks 102 may vary over the plurality of dielectric stacks 120 .

[0055] According to an embodiment , a refractive index of the first dielectric material and / or of the second dielectric material is in the range 1 . 38 - 2 . 4 .

[0056] According to an embodiment , the first dielectric material comprises titanium dioxide ( TiCy ) , aluminium oxide (AI2O3 ) , silicon dioxide ( SiCy ) , and / or magnesium fluoride (MgF2 ) and / or the second dielectric material comprises titanium dioxide , aluminium oxide , silicon dioxide , and / or magnesium fluoride .

[0057] According to an embodiment , the first dielectric material comprises TiCy and the second dielectric material comprises AI2O3 .

[0058] According to an embodiment , the first dielectric material comprises TiCy and the second dielectric material comprises SiCy .

[0059] According to an embodiment , the first dielectric material comprises TiCy and the second dielectric material comprises MgF2 .

[0060] According to an embodiment , the first dielectric material compri ses AI2O3 and the second dielectric material comprises TiCy .

[0061] According to an embodiment , the first dielectric material compri ses I2O3 and the second dielectric material comprises SiCy .

[0062] According to an embodiment , the first dielectric material compri ses AI2O3 and the second dielectric material comprises MgF2 .

[0063] According to an embodiment , the first dielectric material comprises SiCy and the second dielectric material comprises TiCy .

[0064] According to an embodiment , the first dielectric material comprises SiCy and the second dielectric material comprises AI2O3 .

[0065] According to an embodiment , the first dielectric material comprises SiCy and the second dielectric material comprises MgF2 .

[0066] According to an embodiment , the first dielectric material comprises MgF2 and the second dielectric material comprises TiCy .

[0067] According to an embodiment , the first dielectric material comprises MgF2 and the second dielectric material comprises AI2O3 .

[0068] According to an embodiment , the first dielectric material comprises MgF2 and the second dielectric material comprises SiCy .

[0069] According to an embodiment , the plurality of grating features 101 comprise titanium dioxide .

[0070] For example , the plurality of grating features 101 may be made of TiCy .

[0071] According to an embodiment , the plurality of dielectric stacks 120 comprises a plurality of thicknesses for the at least one layer of the first dielectric material 103 .

[0072] The thicknesses of the at least one layer of the first dielectric material 103 may vary over the plurality of dielectric stacks 120 . For example , as illustrated in the embodiment of Fig . 1 , the thicknes ses of the at least one layer of the first dielectric material 103 may be different in different dielectric stacks 102 .

[0073] According to an embodiment , the plurality of dielectric stacks 120 comprises at least a first subset of dielectric stacks and a second subset of dielectric stacks , wherein the first subset of dielectric stacks comprises at least one layer of the first dielectric material 103 having a first thickness and the second subset of dielectric stacks comprises at least one layer of the first dielectric material 103 having a second thickness , different from the first thickness .

[0074] According to an embodiment , the plurality of dielectric stacks 120 comprises a plurality of thicknesses for the at least one layer of the second dielectric material 104 .

[0075] The thicknesses of the at least one layer of the second dielectric material 104 may vary over the plurality of dielectric stacks 120 . For example , as illustrated in the embodiment of Fig . 1 , the thicknes sesof the at least one layer of the second dielectric material 104 may be different in different dielectric stacks 102 .

[0076] According to an embodiment , the plurality of dielectric stacks 120 comprises at least a first subset of dielectric stacks and a second subset of dielectric stacks , wherein the first subset of dielectric stacks comprises at least one layer of the second dielectric material 104 having a first thickness and the second subset of dielectric stacks comprises at least one layer of the second dielectric material 104 having a second thickness , different from the first thickness .

[0077] According to an embodiment , a number of layers of the first dielectric material 103 in a dielectric stack varies over the plurality of dielectric stacks 120 .

[0078] For example , as illustrated in the embodiment of Fig . 1 , the number of layers of the first dielectric material 103 may be different in different dielectric stacks 102 .

[0079] According to an embodiment , the plurality of dielectric stacks 120 comprises at least a first subset of dielectric stacks and a second subset of dielectric stacks , wherein the first subset of dielectric stacks comprises a first number of layers of the first dielectric material 103 and the second subset of dielectric stacks comprises a second number of layers of the first dielectric material 103 , different from first number of layers of the first dielectric material 103 .

[0080] According to an embodiment , a number of layers of the second dielectric material in a dielectric stack varies over the plurality of dielectric stacks .

[0081] For example , as illustrated in the embodiment of Fig . 1 , the number of layers of the second dielectric material 104 may be different in different dielectric stacks 102 .

[0082] According to an embodiment , the plurality of dielectric stacks 120 comprises at least a first subset of dielectric stacks and a second subset of dielectric stacks , wherein the first subset of dielectric stacks comprises a first number of layers of the second dielectric material 104 and the second subset of dielectric stacks comprises a second number of layers of the second dielectric material 104 , different from first number of layers of the second dielectric material 104 .

[0083] According to an embodiment , a thickness of each layer of the first dielectric material 103 is 10 - 500 nanometres (nm) , a thickness of each layer of the second dielectric material 104 i s 10 - 500 nanometres , and / or a thickness of each dielectric stack 102 in the plurality of dielectric stacks 120 is 10 - 500 nanometres .

[0084] According to an embodiment , a thickness of each layer of the first dielectric material 103 is 10 - 400 nm, 10 - 300 nm, 50 - 400 nm, 50 - 300 nm, 150 - 300 nm, or 10 - 150 nm .

[0085] According to an embodiment , a thickness of each layer of the second dielectric material 104 is 10 - 400nm, 10 - 300 nm, 50 - 400 nm, 50 - 300 nm, 150 - 300 nm, or 10 - 150 nm .

[0086] According to an embodiment , a thickness of each dielectric stack 102 in the plurality of dielectric stacks 120 is 10 - 400 nm, 10 - 300 nm, 50 - 400 nm, 50 - 300 nm, 150 - 300 nm, or 10 - 150 nm .

[0087] The thickness variation to the dielectric stacks 102 over the diffraction grating 100 can allow to reduce interference effects and hence provide better image uniformity while keeping the image brightness at a good level .

[0088] Image uniformity can suffer due to , for example , interference strips / ef fects . The arrangement of the dif fraction grating 100 can help to mitigate these effects and improve image uniformity .

[0089] Fig . 2 illustrates a schematic representation of a diffraction grating according to another embodiment .

[0090] According to an embodiment , the diffraction grating 100 further comprises a bottom dielectric layer 201 arranged below the plurality of grating features 101 .

[0091] The bottom dielectric layer 201 may be in contact with the plurality of grating features 101 .

[0092] The bottom dielectric layer 201 may be on a different side of the plurality of grating features 101 from the plurality of dielectric stacks .

[0093] A thickness of the bottom dielectric layer 201 may be , for example , 10 - 50 nm, 10 - 40 nm, 20 - 40nm, 10 - 30 nm, 10 - 20 nm, 5 - 50 nm, 5 - 20 nm, or 20- 30 nm.

[0094] According to an embodiment, the bottom dielectric layer 201 comprises aluminium oxide, silicon dioxide, and / or magnesium fluoride.

[0095] For example, the bottom dielectric layer 201 may be made of aluminium oxide, silicon dioxide, and / or magnesium fluoride.

[0096] Fig. 3 illustrates a schematic representation of a diffraction grating according to another embodiment .

[0097] According to an embodiment, the diffraction grating further comprises a titanium dioxide layer 301 arranged below the bottom dielectric layer 201.

[0098] The titanium dioxide layer 301 may be in contact with the bottom dielectric layer 201.

[0099] The titanium dioxide layer 301 may be on a different side of the bottom dielectric layer 201 from the plurality of grating features.

[0100] A thickness of the titanium dioxide layer 301 may be, for example, 10 - 50 nm, 10 - 40 nm, 20 - 40 nm, 10 - 30 nm, 10 - 20 nm, 5 - 50 nm, 5 - 20 nm, or 20- 30 nm.

[0101] Fig. 4 illustrates a schematic representation of a diffraction grating according to another embodiment .

[0102] According to an embodiment, the plurality of grating features are arranged into a plurality of regions 350, wherein the thickness of the plurality ofdielectric stacks is substantially constant in each region 351 in the plurality of regions 350 .

[0103] The whole area of the dif fraction grating 100 or at least some subsection of the diffraction grating 100 may be divided into the plurality of regions 350 .

[0104] For example , in each region 351 , the structure of the dielectric stacks 102 can be constant and the structure of the dielectric stacks 102 can change between regions 351 . For example , in a first region, the dielectric stacks 102 may comprise two layers of the first dielectric material and two layers of the second dielectric material , and in a second region, the dielectric stacks 102 may compri se one layer of the first dielectric material and one layer of the second dielectric material . Alternatively or additionally, in a first region, the dielectric stacks 102 may comprise two layers of the first dielectric material and two layers of the second dielectric material , and in a second region, the dielectric stacks 102 may comprise two layers of the first dielectric material and two layers of the second dielectric material but the thicknes s of the layers of the first dielectric material and / or of the layers of the second dielectric material can be different in the first region and the second region .

[0105] The plurality of regions 350 and the structure of the diffraction grating 100 in each region 351 can be designed to , for example , compensate for interference effects observed in the diffraction grating 100 without the dielectric stacks 102 .

[0106] Fig. 5 illustrates a schematic representation of a display structure according to an embodiment.

[0107] According to an embodiment, a display structure 400 comprises a planar waveguide 110 and the diffraction grating 100 on the planar waveguide 110.

[0108] The planar waveguide 110 may comprise substantially planar sections. Alternatively or additionally, the planar waveguide 110 may also comprise curved sections. For example, planar waveguide 110 may correspond to a lens or a layer of a lens of augmented reality (AR) glasses .

[0109] According to an embodiment, the diffraction grating 100 is arranged as an exit pupil expansion grating .

[0110] According to an embodiment, the diffraction grating 100 is configured to perform exit pupil expansion.

[0111] In the embodiment of Fig. 5, a display structure 400 comprises a planar waveguide 110 and the diffraction grating 100 on the planar waveguide 110. The diffraction grating 100 is arranged as an exit pupil expansion (EPE) grating. The EPE grating may also be referred to as an EPE structure or similar.

[0112] The display structure 400 may further comprise an in-coupling (IC) grating 502 for coupling incident light 402 into the planar waveguide 110 as a set of incoupled beams 501. The set of in-coupled beams 501 may also be referred to as in-coupled light or similar. Thein-coupling grating 502 may also be referred to as an in-coupling structure or similar .[01 1 3] The EPE grating 100 may be configured to receive the set of in-coupled beams 501 and to diffract the set of in-coupled beams 501 in a plurality of directions , producing a set of diffracted beams 503 .

[0114] It should be appreciated that the set of di ffracted beams 503 illustrated in the embodiment of Fig .5 are only illustrative . In practical embodiments , the EPE structure 100 can diffract the set of in-coupled beams 501 in a plurality of directions in a more complex manner and the set of diffracted beams 503 can interact with the EPE structure 100 a plurality of times .[01 1 5] The display structure 500 may further comprise an out-coupling (OC) grating 505 configured to receive , from the EPE structure 100 , at least the set of diffracted beams 503 and to out-couple at least the set of diffracted beams 503 from the planar waveguide 110 as a set of output beams 504 .

[0116] The set of in-coupled beams 501 and the set of diffracted beams 503 can be guided inside the planar waveguide 110 via total internal reflection ( TIR) .[01 1 7] The set of output beams 504 may represent , for example, an expanded version of the image formed by the incident light 402 . Both the EPE grating 100 and the OC grating 505 can be conf igured to expand the image . For example , the EPE grating 100 may expand the image in one direction and the OC grating 505 can expand the image in a perpendicular direction . Alternatively, the EPEgrating 100 may be configured to expand the image in two perpendicular directions and the OC grating 505 may be configured to out-couple the light from the waveguide 110 .

[0118] For example , as the in-coupled beams 501 propagate in the area of the EPE grating 100 , each time the in-coupled beams 501 interact with the EPE grating 100 , the EPE grating 100 can diffract a part of the light in the in-coupled beams 501 towards the OC grating 505 as the set of diffracted beams 503 and a part of the light in the in-coupled beams 501 can continue propagating along the same direction . This can repeat as the incoupled beams 501 propagate in the area of the EPE grating 100 and the image can thus be expanded .

[0119] The IC structure 502 , the EPE structure 100 and / or the OC structure 505 may comprise , for example , a dif fractive grating on a surface of the planar waveguide 110 . The IC structure 502 may couple the incident light 402 into the planar waveguide 110 via diffraction . The EPE structure 100 may expand the image corresponding to the set of in-coupled beams 501 via diffraction . The OC structure 505 may out-couple the set of diffracted beams 503 from the planar waveguide 110 via diffraction .

[0120] The incident light 402 may be generated by, for example , a scanner-based optical engine . The incident light 402 may represent an image generated by, for example , such an optical engine . Thus , the incident light 402 may also be referred to as , for example , image-bearing light rays / beams, image-carrying light rays / beams, or similar.

[0121] It should be understood that the geometry of the display structure 400 illustrated in the embodiment of Fig. 5 is only exemplary and the display structure 400 may be implemented in various other ways.

[0122] Fig. 6 illustrates a schematic representation of a display device according to an embodiment.

[0123] According to an embodiment, a display device 600 comprises the display structure 400.

[0124] According to an embodiment, the display device 600 further comprises an optical engine 601 for directing the incident light 402 to the IC grating 502.

[0125] According to an embodiment, the display device 600 is implemented as a see-through display device.

[0126] According to an embodiment, the display device 600 is implemented as a head-mounted display device.

[0127] For example, in the embodiment of Fig. 6, the display device 600 is implemented as smart glasses. The planar waveguide 110 can correspond to a lens or a layer of a lens of such smart glasses. Such smart glasses may be used to, for example, implement augmented reality (AR) and / or virtual reality (VR) functionality.

[0128] In the embodiment of Fig. 6, the incident light 402 may be generated by, for example, an optical engine 601, such as a scanner-based optical engine. The incident light 402 may represent an image generated by, for example, such an optical engine. The display structure 400 of the display device 600 can direct the incidentlight 402 representing the image generated by the optical engine 601 into the eye of a user.

[0129] Fig. 7 illustrates a schematic representation of image uniformity according to an embodiment.

[0130] The left image 710 in the embodiment of Fig. 7 correspond to a diffraction grating without the dielectric stacks 120. A dark stripe 701 can be observed in the left image 710. The right image 720 in the embodiment of Fig. 7 correspond to a diffraction grating 100 comprising the dielectric stacks 120. Due to the dielectric stacks 120, the image uniformity is improved and there is no dark stripe in the right image 720.

[0131] Fig. 8 illustrates a plot representation of simulation results according to a comparative example.

[0132] In the comparative example of Fig. 8, the phase difference of first order reflection of a diffraction grating without dielectric stacks 120. Curve 801 corresponds to the phase difference between incident transverse electric (TE) light and reflected TE light. Curve 802 corresponds to the phase difference between incident TE light and reflected transverse magnetic (TM) light. Curve 803 corresponds to the phase difference between incident TM light and reflected TE light. Curve 804 corresponds to the phase difference between incident TM light and reflected TM light.

[0133] Herein, TE polarization may refer to a polarization the electric field of which is substantially parallel with the grating features 101 of the diffraction grating 100. Similarly, TM polarization may referto a polari zation the magnetic field of which is substantially parallel with the grating features 101 of the diffraction grating 100 .

[0134] Fig . 9 illustrates a plot representation of simulation results according to an embodiment .

[0135] In the embodiment of Fig . 9 , the phase difference of first order reflection of a diffraction grating dielectric stacks 120 . Simulation results for various materials and layer thicknesses are illustrated . Curves 901 correspond to the phase difference between incident TE light and reflected TE light . Curves 902 correspond to the phase difference between incident TE light and reflected TM light . Curves 903 correspond to the phase difference between incident TM light and reflected TE light . Curves 904 correspond to the phase difference between incident TM light and reflected TM light .

[0136] In the simulations of Fig . 9 , the dielectric stacks 120 comprise AI2O3 layers and SiO2 layers of various thicknesses in the range 0 - 300 nm .

[0137] Due to the phase differences introduced by the dielectric stacks 120 , interference effects can be reduced and hence image uniformity improved .

[0138] The simulations of Figs . 8 - 9 were performed using the Fourier Modal Method .

[0139] Any range or device value given herein may be extended or altered without losing the effect sought . Also any embodiment may be combined with another embodiment unless explicitly disallowed .

[0140] Although the subj ect matter has been described in language specific to structural features and / or acts , it is to be understood that the subj ect matter defined in the appended claims is not necessarily limited to the specific features or acts described above . Rather, the specific features and acts described above are disclosed as examples of implementing the claims and other equivalent features and acts are intended to be within the scope of the claims .

[0141] It will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments . The embodiments are not limited to those that solve any or all of the stated problems or those that have any or all of the stated benefits and advantages . It wil l further be understood that reference to ' an ' item may refer to one or more of those items .

[0142] Aspects of any of the embodiments described above may be combined with aspects of any of the other embodiments described to form further embodiments without losing the effect sought .

[0143] The term ' comprising ' is used herein to mean including the method, blocks or elements identified, but that such blocks or elements do not comprise an exclusive list and a method or apparatus may contain additional blocks or elements .

[0144] It will be understood that the above description is given by way of example only and that various modif ications may be made by those ski lled in the art .The above specification, examples and data provide a complete description of the structure and use of exemplary embodiments . Although various embodiments have been described above with a certain degree of particu- larity, or with reference to one or more individual embodiments , those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the spirit or scope of this specification .

Claims

CLAIMS :

1. A diffraction grating (100) comprising: a plurality of grating features (101) ; and a plurality dielectric stacks (120) , wherein each dielectric stack (102) in the plurality of dielectric stacks (120) is arranged onto a corresponding grating feature in the plurality of grating features (101) ; wherein each dielectric stack (102) comprises at least one layer of a first dielectric material (103) and at least one layer of a second dielectric material (104) in an alternating arrangement in a thickness direction of the dielectric stack (102) ; and the plurality of dielectric stacks (120) comprise a plurality of thicknesses for the dielectric stacks.

2. The diffraction grating (100) according to claim 1, wherein the first dielectric material comprises titanium dioxide, aluminium oxide, silicon dioxide, and / or magnesium fluoride and / or the second dielectric material comprises titanium dioxide, aluminium oxide, silicon dioxide, and / or magnesium fluoride.

3. The diffraction grating (100) according to claim 1 or claim 2, wherein the plurality of grating features (101) comprise titanium dioxide.

4. The diffraction grating (100) according to any preceding claim, further comprising a bottom dielectriclayer (201) arranged below the plurality of grating features (101) .

5. The diffraction grating (100) according to claim 4, further comprising a titanium dioxide layer (301) arranged below the bottom dielectric layer (201) .

6. The diffraction grating (100) according to any preceding claim, wherein the plurality of grating features (101) are arranged into a plurality of regions (350) , wherein the thickness of the plurality of dielectric stacks (120) is substantially constant in each region (351) in the plurality of regions (350) .

7. The diffraction grating (100) according to any preceding claim, wherein: the plurality of dielectric stacks (120) comprises a plurality of thicknesses for the at least one layer of the first dielectric material (103) ; the plurality of dielectric stacks comprises a plurality of thicknesses for the at least one layer of the second dielectric material (104) ; a number of layers of the first dielectric material in a dielectric stack (102) varies over the plurality of dielectric stacks (120) ; and / or a number of layers of the second dielectric material in a dielectric stack (102) varies over the plurality of dielectric stacks (120) .

8. The diffraction grating (100) according to any preceding claim, wherein a thickness of each layer of the first dielectric material is 10 - 500 nanometres, a thickness of each layer of the second dielectric material is 10 - 500 nanometres, and / or a thickness of each dielectric stack in the plurality of dielectric stacks is 10 - 500 nanometres.

9. A display structure (400) comprising a planar waveguide (110) and the diffraction grating (100) according to any preceding claim on the planar waveguide (110) .

10. The display structure (400) according to claim 9, wherein the diffraction grating (100) is arranged as an exit pupil expansion grating.

11. A display device (600) comprising the display structure (400) according to claim 9 or claim 10.

12. The display device (600) according to claim 11 implemented as a see-through display device.

13. The display device (600) according to claim 11 or claim 12 implemented as a head-mounted display de- vice .

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