FI20236225A1 - Display structure - Google Patents

Abstract

According to an embodiment, a display structure (100) comprises: a waveguide (101) configured to guide incoupled light (102) in the waveguide (101) via consecutive reflections from a first side (121) of the waveguide (101) and a second side (122) of the waveguide (101), where-in the in-coupled light (102) comprises a first polarization (131) and a second polarization (132); an out-coupling structure (111) on the first side (121) of the waveguide (101) configured to out-couple a part of the first polarization (131) of the in-coupled light (102) from the waveguide (101) when the in-coupled light (102) arrives to the first side (121) of the waveguide (101); and a polarization manipulating structure (112) on the second side (122) of the waveguide (101) configured to rotate at least a part of the second polarization (132) of the in-coupled light (102) into the first polarization (131) when the incoupled light (102) arrives to the second side (122) of the waveguide (101).

Description

DISPLAY STRUCTURE

TECHNICAL FIELD

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

BACKGROUND

[0002] In various optical applications, such as aug- mented reality (AR) applications, it is typically de- sirable to improve the quality of the image presented to the user. One aspect of image guality is image uni- formity, such as brightness and colour uniformity. When an image is presented to the user, it is usually desir- able that the image has a uniform brightness and colour over the whole image.

SUMMARY

[0003] This summary is provided to introduce a selec-

O 20 tion of concepts in a simplified form that are further

N

S described below in the detailed description. This sum- = mary is not intended to identify key features or essen-

N tial features of the claimed subject matter, nor is it

E intended to be used to limit the scope of the claimed a 10 25 subject matter.

O [0004] It is an object to provide a display structure

O and a display device. The foregoing and other objects are achieved by the features of the independent claims.

Further implementation forms are apparent from the de- pendent claims, the description and the figures.

[0005] According to a first aspect, a display struc- ture comprises: a waveguide configured to guide in-cou- pled light in the waveguide via consecutive reflections from a first side of the waveguide and a second side of the waveguide, wherein the in-coupled light comprises a first polarization and a second polarization; an out- coupling structure on the first side of the waveguide configured to out-couple a part of the first polariza- tion of the in-coupled light from the waveguide when the in-coupled light arrives to the first side of the wave- guide; and a polarization manipulating structure on the second side of the waveguide configured to rotate at least a part of the second polarization of the in-cou- pled light into the first polarization when the in- coupled light arrives to the second side of the wave- guide.

[0006] According to second aspect, a display device comprises a display structure according to the first

N aspect.

S [0007] Many of the attendant features will be more = readily appreciated as they become better understood by

N reference to the following detailed description consid-

I 25 ered in connection with the accompanying drawings. a

S

N DESCRIPTION OF THE DRAWINGS

N

&

[0008] In the following, embodiments are described in more detail with reference to the attached figures and drawings, in which:

[0009] Fig. 1 illustrates a schematic representation of a display structure according to an embodiment;

[0010] Fig. 2 illustrates a schematic representation of a display structure according to another embodiment;

[0011] Fig. 3 illustrates a schematic representation of an out-coupling structure according to an embodiment;

[0012] Fig. 4 illustrates a schematic representation of an out-coupling structure according to another em- bodiment;

[0013] Fig. 5 illustrates a schematic representation of an out-coupling structure according to another em- bodiment;

[0014] Fig. 6 illustrates a schematic representation of a polarization manipulating structure according to an embodiment; and

[0015] Fig. 7 illustrates a schematic representation of a display device according to an embodiment. & [0016] In the following, identical reference signs s refer to similar or at least functionally equivalent io features.

S

= a 25 DETAILED DESCRIPTION

S

N [0017] In the following description, reference is made

N to the accompanying drawings, which form part of the

N disclosure, and in which are shown, by way of illustra- tion, 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, there- fore, is not to be taken in a limiting sense, as the scope of the present disclosure is defined by the ap- pended claims.

[0018] For instance, it is understood that a disclo- sure 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 de- vice may include a unit to perform the described method step, even if such unit is not explicitly described or illustrated in the figures. On the other hand, for ex- ample, 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 s of the various example aspects described herein may be

N combined with each other, unless specifically noted oth- z 25 erwise. > [0019] Fig. 1 illustrates a schematic representation of a display structure according to an embodiment. & [0020] According to an embodiment, a display structure

N 100 comprises a waveguide 101 configured to guide in-

coupled light 102 in the waveguide 101 via consecutive reflections from a first side 121 of the waveguide 101 and a second side 122 of the waveguide 101, wherein the in-coupled light 102 comprises a first polarization 131 5 and a second polarization 132.

[0021] The in-coupled light 102 may also be referred to as a plurality of in-coupled beams, a plurality of in-coupled light beams, a plurality of in-coupled rays, a plurality of in-coupled light rays, or similar.

[0022] The in-coupled light 102 can be guided inside the waveguide 101 via total internal reflection (TIR).

[0023] The first polarization 131 may also be referred to as a first linear polarization. The second polariza- tion 132 may also be referred to as a second linear polarization.

[0024] The display structure 100 may further comprise an out-coupling structure 111 on the first side 121 of the waveguide 101 configured to out-couple a part of the first polarization 131 of the in-coupled light 102 from the waveguide 101 when the in-coupled light 102 arrives e to the first side 121 of the waveguide 101.

O [0025] In some embodiments, the out-coupling struc- = ture 111 may be further configured to not out-couple the

N second polarization 132 of the in-coupled light 102 from

E 25 the waveguide 101 when the in-coupled light 102 arrives o to the first side 121 of the waveguide 101. In other

N embodiments, the out-coupling structure 111 may be fur-

N ther configured to not substantially out-couple the sec- = ond polarization 132 of the in-coupled light 102 from the waveguide 101 when the in-coupled light 102 arrives to the first side 121 of the waveguide 101.

[0026] For example, in the embodiment of Fig. 1, the out-coupled light 103 comprises only the first polari- zation 131. In practical implementations, some of the second polarization 132 may be out-coupled also. How- ever, this may not be desirable.

[0027] In some embodiments, the out-coupling struc- ture 111 may be further configured to not substantially out-couple the second polarization 132 compared to the first polarization 131. For example, the out-coupling structure 111] may be configured out-couple the first polarization 131 with a first out-coupling efficiency 11 and to out-coupled the second polarization 132 with a second out-coupling efficiency N, wherein 1, > 12, for example 1; >2X1n,, 1, >5%Xn,, 1,> 10X172, 1,>20X72, n> 50 X 117, and/or n, > 100X1n,.

[0028] Thus, the outcoupling structure 111 may dif- fract only light that has a certain polarization direc- tion, and light with the other polarization direction

Q may continue propagating in the waveguide 101 via TIR.

N [0029] The display structure 100 may further comprise - a polarization manipulating structure 112 on the second

S side 122 of the waveguide 101 configured to rotate at

E 25 least a part of the second polarization 132 of the in-

O coupled light 102 into the first polarization 131 when

O the in-coupled light 102 arrives to the second side 122

O of the waveguide 101.

[0030] The polarization manipulating structure 112 can comprise, for example, a film or a subwavelength grating.

[0031] When the in-coupled light 102 hits the polar- ization manipulating structure 112, the reflected light can experience a polarization rotation. This rotation can then affect to the out-coupling once the light again hits the out-coupling structure 111. By tailoring the rotation strength of the polarization manipulating structure 112 one can, for example, balance the amount of out-coupled light 103 between consecutive hits. This can, for example, improve the brightness uniformity of throughout the outcouplina area.

[0032] In some embodiments, the polarization manipu- lating structure 112 may be further configured to rotate at least a part of the first polarization 131 of the in- coupled light 102 into the second polarization 132 when the in-coupled light 102 arrives to the second side 122 of the waveguide 101. Thus, in some embodiments, at least a part of the first polarization 131 of the in- n coupled light 102 may be rotated into the second polar-

S ization 132 and at least a part of the second polariza- = tion 132 of the in-coupled light 102 may be rotated into

N the first polarization 131. = 25 [0033] Although various light paths, such as for the > in-coupled light 102, are illustrated using single light

N rays in Fig. 1 and other embodiments disclosed herein, :

this is only for illustrative purposes. Any light dis- closed herein may comprise, for example, a plurality of light beams that may propagate in various directions.

[0034] The waveguide 101 may comprise, for example, a substantially planar waveguide. Alternatively or addi- tionally, the waveguide 101 may also comprise curved sections. For example, the waveguide 101 may correspond to a lens of augmented reality (AR) glasses. For exam- ple, the waveguide 101 may correspond to a layer of a lens of such AR glasses.

[0035] The second polarization 132 may be non-parallel with the first polarization 131.

[0036] According to an embodiment, the first polari- zation 131 and the second polarization 132 are substan- tially orthogonal.

[0037] The first polarization 131 and the second po- larization 132 may be substantially orthogonal when, for example, an angle between the first polarization 131 and the second polarization 132 is 80 — 100 degrees, 85 - 95 degrees, 87.5 — 92.5 degrees, 89 — 91 degrees, and/or < 89.5 — 90.5 degrees.

S [0038] In some embodiments, the first polarization 131 = and the second polarization 132 may be orthogonal.

S [0039] For example, in the embodiment of Fig. 1, the

E 25 first polarization 131 is perpendicular to the plane of 10 Fig. 1 and the second polarization 132 is parallel with

O the plane of Fig. 1.

O [0040] The positioning of the out-coupling structure 111 illustrated in the embodiment of Fig. 1 is only exemplary and the out-coupling structure 111 may be po- sitioned in various other ways. In some embodiments, the out-coupling structure 111] be positioned inside the waveguide 101.

[0041] According to an embodiment, the out-coupling structure 111 is configured to out-couple the part of the first polarization 131 of the in-coupled light 102 from the waveguide 101 via zeroth order and/or first order diffractions.

[0042] In any embodiment disclosed herein, the out- coupling structure 111 may comprise a reflective or a transmissive diffractive grating. For example, in the embodiment of Fig. 1, the out-coupling structure 111 can comprise a reflective diffractive grating and thus the out-coupled light 103 exits the waveguide 101 from the second side 122. In embodiments where the out-coupling structure 111 comprises a transmissive diffractive grat- ing, the out-coupled light 103 may exit the waveguide 101 from the first side 121.

[0043] It should be understood that the geometry of n the display structure 100 illustrated in the embodiment

S of Fig. 1 is only exemplary and the display structure = 100 may be implemented in various other ways. For exam-

N ple, the dimensions of the waveguide 101 and the dimen- z 25 sions of the out-coupling structure 111 have been chosen > for illustrative purposes.

N [0044] According to an embodiment, an out-coupling

S efficiency of the out-coupling structure changes along at least one direction on the first side of the wave- guide.

[0045] An out-coupling efficiency may quantify what portion of the in-coupled light 102 is out-coupled each time the in-coupled light 102 hits the first side 121 and interacts with the out-coupling structure 111.

[0046] According to an embodiment, an out-coupling efficiency of the out-coupling structure increases along a main propagation direction of the in-coupled light 102.

[0047] The main propagation direction of the in-cou- pled light 102 may refer to a direction in which the in- coupled light 102 propagates due to being guided by the

TIR. For example, in the embodiment of Fig. 1, the main propagation direction can be in the positive x direc- tion.

[0048] The in-coupled light 102 can be polarized in various ways. For example, in-coupled light 102 may be in an elliptical polarization state. This polarization state can comprise a linear combination of the first 131 e and second polarization 132. In such a case, the out-

S coupling efficiency at every interaction with the out- = coupling structure 111 depends on the amount of the

N first polarization 131 in the in-coupled light 102.

E 25 [0049] The polarization selectivity of the out-cou- o pling structure 111 can be achieved using various struc-

N tures, such as those disclosed in the embodiments

S herein. Different structures can produce polarization selectivity with varying strengths. Their general opti- cal responses may also be different, as well as their fabrication-suitability.

[0050] It should be appreciated that the polarization directions illustrated in the embodiment of Fig. 1 are only exemplary. In other embodiments, directions of the first polarization 131 and of the second polarization 132 may differ from what is illustrated in the embodi- ment of Fig. 1.

[0051] The outcoupling structure 111 can diffract and therefore outcouple only the first polarization 131.

Thus, the outcoupling efficiency can be tailored by ap- propriately configuring the outcoupling structure 111 and the polarization manipulating structure 112. The display structure 100 can allow better control for the outcoupling of the image as the outcoupling efficiency can depend on the amount of polarization rotation and the out-coupling efficiency of the out-coupling struc- ture 111. Thus, display structure 100 can provide addi- tional design freedom to improve brightness and colour uniformity.

O

S [0052] Fig. 2 illustrates a schematic representation = of a display structure according to another embodiment.

N [0053] According to an embodiment, the display struc-

E 25 ture 100 further comprises an exit pupil expansion (EPE)

LO structure 203 comprising a polarization sensitive grat-

N ing on one of: the first side 121 of the waveguide 101 and the second side 122 of the waveguide 101, and a second polarization manipulating structure on a differ- ent side of the waveguide 101 from the polarization sensitive grating, wherein the EPE structure 203 is con- figured to reduce the second polarization 132 in the in- coupled light 102 and guide the in-coupled light 102 towards the out-coupling structure 111.

[0054] The EPE structure 203 can reduce the second polarization 132 in the in-coupled light 102 in a sim- ilar fashion to the out-coupling structure 112 and the polarization manipulating structure 111 of the embodi- ment of Fig. 1. For example, the polarization sensitive grating of the EPE can diffract a part of the first polarization of the in-coupled light 102 when the in- coupled light 102 arrives to the polarization sensitive grating. Due to the diffraction, the part of the first polarization can be guided towards the out-coupling structure 111. The second polarization manipulating structure can rotate at least a part of the second po- larization of the in-coupled light 102 into the first polarization when the in-coupled light 102 arrives to the second polarization manipulating structure.

S [0055] In some embodiments, the polarization sensi- — tive grating may be on the first side 121 of the wave- & guide 101 and the second polarization manipulating

I 25 structure may be on the second side 122 of the waveguide > 101. In other embodiments, the polarization sensitive grating may be on the second side 122 of the waveguide 3 101 and the second polarization manipulating structure

N may be on the first side 121 of the waveguide 101.

[0056] According to an embodiment, the display struc- ture 100 further comprises an in-coupling structure 201 configured to couple an input light beam 202 into the waveguide 101 as the in-coupled light 102.

[0057] For example, in the embodiment of Fig. 2, the display structure 100 further comprises an in-coupling structure 201 configured to in-couple an input light beam 202 into the waveguide 101 as the in-coupled light 102 and direct the in-coupled light 102 towards the EPE structure 203. The EPE structure 203 can then reduce the second polarization 132 in the in-coupled light 102 and guide the in-coupled light 102 towards the out-coupling structure 111.

[0058] The EPE structure 203 may be further configured to expand the image corresponding to the in-coupled light 102 via diffraction.

[0059] It should be appreciated that the in-coupled light 102 expanded by the EPE structure 203 illustrated in the embodiment of Fig. 2 is only illustrative. In practical embodiments, the EPE structure 203 can dif- 2 fract the in-coupled light 102 in a plurality of direc- < tions in a much more complex manner and in-coupled light = 102 can interact with the EPE structure 203 a plurality

N of times.

E 25 [0060] The the out-coupled light 103 represent, for o example, an expanded version of the image formed by the ä input light beam 202.

O [0061] The input light beam 202 may also be referred to as an input light, a plurality of input beams, a plurality of input light beams, a plurality of input rays, a plurality of input light rays, or similar.

[0062] The input light beam 202 may be generated by, for example, a scanner-based optical engine. The input light beam 202 may represent an image generated by, for example, such an optical engine. Thus, the input light beam 202 may also be referred to as, for example, image- bearing light, image-carrying light, image-bearing light rays/beams, image-carrying light rays/beams, or similar. Alternatively or additionally, the input light beam 202 may be provided by some other optical compo- nents such as those disclosed in the embodiments herein.

[0063] The second polarization manipulating structure may comprise, for example, a quarter-wave plate. Herein, a quarter-wave plate may also be referred to as a 2/4 waveplate or similar.

[0064] Since the EPE structure 203 can reduce the sec- ond polarization 132 in the in-coupled light 102, out- coupling for every other interaction between the out- coupling structure 111 and the in-coupled light 102 can e be reduced or even removed because the OC structure 111

S is polarization sensitive and the polarization manipu- = lating structure 112 can rotate the first polarization

N 131 into the second polarization 132 and vice versa each

T 25 time the in-coupled light 102 hits the polarization ma- > nipulating structure 112. An example of this is illus-

N trated in the embodiment of Fig. 2, where the out-cou- :

pling for every other interaction between the out-cou- pling structure 111 and the in-coupled light 102 is reduced or removed as indicated by the dotted arrows.

[0065] Fig. 3 illustrates a schematic representation of an out-coupling structure according to an embodiment.

[0066] According to an embodiment, the out-coupling structure 111 comprises diffractive grating features and sub-wavelength grating features, wherein the diffrac- tive grating features are configured to out-couple the part of the first polarization 131 of the in-coupled light 102 from the waveguide 101 when the in-coupled light 102 arrives to the first side 121 of the waveguide 101 and the sub-wavelength grating features are config- ured to make the diffractive grating features at least partially transparent to the second polarization 132.

[0067] Herein, diffractive 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 the in-coupled light 102. Alternatively or additionally, sub-wavelength grating features may n refer to grating features that have a spatial periodic-

S ity of the same order of magnitude or less than the = smallest wavelength of visible light, such as less than

N 380 nanometres (nm). = 25 [0068] Alternatively or additionally, diffractive > grating features may refer to grating features that have

N a spatial periodicity, which, in the used incidence & mounting, allows propagating diffraction orders, in ei-

N ther reflected or transmitted light, to emerge.

[0069] Alternatively or additionally, sub-wavelength grating features may refer to grating features that have a spatial periodicity, which, in the used incidence mounting, does not allow diffraction orders to emerge.

[0070] The sub-wavelength grating features may also be referred to as zeroth order grating features.

[0071] For example, the embodiment of Fig. 3 illus- trates an out-coupling structure 111 comprising dif- fractive grating features along the x direction and sub- wavelength grating features along the y direction.

[0072] According to an embodiment, a grating period of the diffractive grating features, denoted by dy, is 300 - 400 nanometres (nm).

[0073] According to an embodiment, a grating period of the sub-wavelength grating features, denoted by dy, is 150 - 200 nm.

[0074] According to an embodiment, a grating heigh of the diffractive grating features and/or of the sub-wave- length grating features is 20 — 100 nm.

[0075] According to an embodiment, a line width of the & diffractive grating features and/or of the sub-wave- s length grating features is 60 — 200 nm.

N [0076] According to an embodiment, a refractive index = of the diffractive grating features and/or of the sub- s 25 wavelength grating features is 1.9 - 2.5. s [0077] The diffractive grating features and/or the 2 sub-wavelength grating features may be made of a die-

N lectric material, such as titanium dioxide (Ti0;). The refractive index of TiO, is approximately 2.4.

[0078] According to an embodiment, the diffractive grating features comprise a plurality of diffractive grating lines 301 and the sub-wavelength grating fea- tures comprise a plurality of sub-wavelength grating lines 302.

[0079] According to an embodiment, grating lines in the plurality of diffractive grating lines 301 are sub- stantially parallel with the first polarization 131.

[0080] According to an embodiment, each grating line in the plurality of diffractive grating lines 301 com- prises an air gap 303.

[0081] According to an embodiment, a grating period of the diffractive grating features is greater than 250 nanometres, a grating period of the sub-wavelength grat- ing features is less than 250 nanometres.

[0082] According to an embodiment, a width of each air gap 303 in the plurality of diffractive grating lines 301 is 30 — 100 nm.

[0083] The width of the air gap 303 is denoted by a, in the embodiment of Fig. 3.

D [0084] For example, in the embodiments of Fig. 3, Fig.

N 4, and Fig. 5, the plurality of diffractive grating - lines 301 and the plurality of sub-wavelength grating

S lines 302 are non-parallel. For example, the plurality = 25 of diffractive grating lines 301 and the plurality of

O sub-wavelength grating lines 302 may be substantially

O orthogonal such as in the embodiments of Fig. 3, Fig.

O 4, and Fig. 5. Alternatively, the plurality of diffrac-

tive grating lines 301 and the plurality of sub-wave- length grating lines 302 can be in any other non-paral- lel orientation.

[0085] According to an embodiment, a distance between grating lines in each consecutive grating line pair in the plurality of sub-wavelength grating lines 302 is 60 = 150 nm.

[0086] In the embodiments of Fig. 3 and Fig. 4, the distance between grating lines in each consecutive arat- ing line pair in the plurality of sub-wavelength grating lines 302 is denoted by ay.

[0087] According to an embodiment, a width of each grating line in the plurality of sub-wavelength grating lines is 50 - 160 nm.

[0088] According to an embodiment, grating lines in the plurality of diffractive grating lines are substan- tially parallel with the first polarization.

[0089] In the embodiment of Fig. 3, cross-sections of the out-coupling structure 111 along the dashed line 310 and along the dotted line 311 are also illustrated. In

S the out-coupling structure 111, the first polarization

N 131 can be along the dotted line 311 and the second - polarization 132 can be along the dashed line 310.

S [0090] Light experiences the sub-wavelength grating

E 25 features as a birefringent medium. Thus, the effective

O refractive indices for polarizations along the dashed

O line 310 and along the dotted line 311 are different.

O By tuning the dimensions of the subwavelength features,

refractive indices can be tuned such that the diffrac- tive grating becomes at least partially transparent to the polarization along the dashed line 310.

[0091] Herein transverse electric (TE) polarization may refer to a polarization the electric field of which is substantially parallel with the diffractive grating lines of the diffractive grating features of the out- coupling structure 111. Thus, a polarization the elec- tric field of which is along the y direction in the embodiments of Figs. 1 - 6 may be referred to as TE polarization. Similarly, transverse magnetic (TM) po- larization may refer to a polarization the magnetic field of which is substantially parallel with the dif- fractive grating lines of the diffractive grating fea- tures of the out-coupling structure 111. Thus, a polar- ization the electric field of which is in the xz plane in the embodiments of Figs. 1 - 6 may be referred to as

TM polarization.

[0092] In the embodiment of Fig. 3, for example, the diffractive grating features can be made at least par- n tially transparent to the second polarization 132 by

S tuning ay, dy, Ayr dy, the refractive index of the mate- = rial of the diffractive grating feature, and/or the re-

N fractive index of the material of the sub-wavelength

E 25 grating feature. Appropriate values for at least some > of these parameters can be found using, for example,

N optical simulations. In some cases, some of these pa-

N rameters can have predetermined values and the values

N of the rest of these parameters can be found using op- tical simulations. For example, the refractive index of the material of the diffractive grating feature, and/or the refractive index of the material of the sub-wave- length grating feature 302 may be pre-determined by the used material(s) and ay, dy, a, and/or dy can be found using optical simulations.

[0093] According to an embodiment, the sub-wavelength grating features are configured to make the diffractive grating features at least partially transparent to the second polarization 132 via a spatial refractive index average along a direction of the second polarization 132 being substantially constant.

[0094] According to an embodiment, a refractive index of a material of the diffractive grating features is in the range 1.9 - 2.4 and a refractive index of a material of the sub-wavelength grating features is in the range 1.9 — 2.4.

[0095] According to an embodiment dy is in the range 200 — 220 nm, dy is in the range 300 - 500 nm, ay is in e the range 60 - 150 nm, a, is in the range 30 - 100, a

N refractive index of a material of the diffractive grat- - ing features is substantially 2.4, and a refractive in-

S dex of a material of the sub-wavelength grating features

E 25 is substantially 2.4. o [0096] Fig. 4 illustrates a schematic representation

O of an out-coupling structure according to another em-

O bodiment.

[0097] According to an embodiment, the plurality of diffractive grating lines is made of a material with a first refractive index and the plurality of sub-wave- length grating lines is made of a material with a second refractive index different from the first refractive index.

[0098] The first refractive index may be denoted by n, and the second refractive index may be denoted by na.

[0099] In other embodiments, the plurality of dif- fractive grating lines and the plurality of sub-wave- length grating lines may be made of the same material.

[0100] A refractive index of a material may refer to a refractive index that light experiences when the light interacts with a substantially homogeneous piece of the material. The refractive index of a material may be wavelength dependent. It should be appreciated that an effective refractive index caused by, for example, the sub-wavelength grating features can differ from the re- fractive index of the material of which the sub-wave- length grating features are made of due to the sub-

O wavelength grating features having a sub-wavelength

O size. Since the sub-wavelength grating features have a = sub-wavelength size, the light experiences a spatially

N averaged effective refractive index that depends on the

E 25 relative orientation of the sub-wavelength grating fea-

LO tures and the polarization of the light. Thus, the ef-

N fective refractive index of the sub-wavelength grating features is anisotropic and polarization dependent.

[0101] In the embodiment of Fig. 4, cross-sections of the out-coupling structure 111 along the dashed line 410 and along the dotted line 411 are alsc illustrated. In the out-coupling structure 111, the first polarization 131 can be along the dotted line 411 and the second polarization 132 can be along the dashed line 410.

[0102] Light experiences the sub-wavelength grating features as a birefringent medium. Thus, the effective refractive indices for polarizations along the dashed line 410 and along the dotted line 411 are different.

By tuning the dimensions of the subwavelength features, refractive indices can be tuned such that the diffrac- tive grating structure become at least partially trans- parent to the polarization along the dashed line 410.

[0103] In the embodiment of Fig. 4, for example, the diffractive grating features can be made at least par- tially transparent to the second polarization 132 by tuning dy; ay, dy, the refractive index of the material of the diffractive grating features, and/or the refrac- tive index of the material of the sub-wavelength grating 2 feature 302. Appropriate values for at least some of < these parameters can be found using, for example, opti- = cal simulations. In some cases, some of these parameters

N can have predetermined values and the values of the rest =E 25 of these parameters can be found using optical simula- > tions. For example, the refractive index of the material

N of the diffractive grating feature 301, and/or the re-

N fractive index of the materia] of the sub-wavelength

N grating feature 302 may be pre-determined by the used material (s) and dy, a, and/or dy can be found using op- tical simulations.

[0104] According to an embodiment, each grating line in the plurality of diffractive grating lines comprises an air gap 303, the plurality of diffractive grating lines is made of a material with a first refractive index, and the plurality of sub-wavelength grating lines 202 is made of a material with a second refractive index different from the first refractive index. Thus, the embodiments of Fig. 3 and Fig. 4 may be combined into another embodiment.

[0105] According to an embodiment, the plurality of sub-wavelength grating lines are positioned between the plurality of diffractive grating lines and the plurality of diffractive grating lines and the plurality of sub- wavelength grating lines are non-parallel.

[0106] Fig. 5 illustrates a schematic representation of an out-coupling structure according to another em- bodiment.

[0107] In the embodiment of Fig. 5, the plurality of > diffractive grating lines 301 and the plurality of suk- & wavelength grating lines 302 are made of the same mate- = rial. For example, the plurality of diffractive grating

N lines 301 and the plurality of sub-wavelength grating

E 25 lines 302 can be made of titanium dioxide (Ti0,). The o refractive index of TiO, is approximately 2.4. The space

N between the plurality of diffractive grating lines 201 & and the plurality of sub-wavelength grating lines 202 = can be, for example, air.

[0108] According to an embodiment, a width of each grating line in the plurality of diffractive grating lines 201 is 80 nm and a width of each grating lines in the plurality of sub-wavelength grating lines 202 is 80 nm.

[0109] According to an embodiment, a grating period dy is in the range 300 - 400 nm.

[0110] According to an embodiment, a grating period dy is in the range 150 - 200 nm.

[0111] Fig. 6 illustrates a schematic representation of a polarization manipulating structure according to an embodiment.

[0112] According to an embodiment, the polarization manipulating structure 112 comprises a plurality of po- larization manipulating grating lines 601 that are non- parallel with the plurality of diffractive grating lines 301.

[0113] According to an embodiment, the polarization manipulating structure 112 comprises a sub-wavelength polarization manipulating grating. & [0114] According to an embodiment, a grating period s of the polarization manipulating grating is less than

S 250 nanometres.

I [0115] For example, the sub-wavelength polarization o 25 manipulating grating can be made of Ti0,. The space be- s tween grating lines in the sub-wavelength polarization 2 manipulating grating can be, for example, air.

N [0116] The embodiment of Fig. 5 illustrates an example of a polarization manipulating structure 112 comprising a sub-wavelength polarization manipulating grating 601.

The grating period of the polarization manipulating structure 112 is denoted by d, and the grating height of each grating line in the sub-wavelength polarization manipulating grating 601 is denoted by h. A cross sec- tion along the dotted line 611 is also illustrated.

[0117] According to an embodiment, a width of each grating line in the sub-wavelength polarization manip- ulating grating can be 70 nm.

[0118] According to an embodiment a grating period d, of the sub-wavelength polarization manipulating grating is in the range 150 - 200 nm. A width of each grating line in the sub-wavelength polarization manipulating grating can be 75 - 100 nm.

[0119] According to an embodiment a grating height h of the sub-wavelength polarization manipulating grating is in the range 50 - 250 nm.

[0120] According to an embodiment, a refractive index of the sub-wavelength polarization manipulating grating is in the range 1.9 - 2.5.

D [0121] In the embodiment of Fig. 5, for example, the

N polarization manipulating structure 112 can be config- - ured to rotate the polarization of the in-coupled light

S 102 by tuning h, dx, the width of each grating line in : 25 the sub-wavelength polarization manipulating grating

O 601, and/or the refractive index of each grating line 3 in the sub-wavelength polarization manipulating grating < 601. Appropriate values for at least some of these pa-

rameters can be found using, for example, optical sim- ulations. In some cases, some of these parameters can have predetermined values and the values of the rest of these parameters can be found using optical simulations.

For example, the refractive index of the material of the sub-wavelength polarization manipulating grating 601 may be pre-determined by the used material (s).

[0122] According to an embodiment, the polarization manipulating structure 112 comprises a one-dimensional the polarization manipulating grating.

[0123] The embodiment of Fig. 5 is only an example of the polarization manipulating structure 112 and the po- larization manipulating structure 112 may also be im- plemented in various other ways. For example, in some embodiments the polarization manipulating structure 112 may comprise a slanted grating.

[0124] Fig. 7 illustrates a schematic representation of a display device according to an embodiment.

[0125] According to an embodiment, a display device 800 comprises the display structure 100. © [0126] According to an embodiment, display device 800

S further comprises an optical engine 801 for directing = the input light beam 202 to the waveguide 101.

S [0127] According to an embodiment, the display device

E 25 800 is implemented as a see-through display device. 10 [0128] According to an embodiment, the display device

O 800 is implemented as a head-mounted display device.

O [0129] For example, in the embodiment of Fig. 7, the display device 800 is implemented as smart glasses. The waveguide 101 can correspond to a layer of a lens of such smart glasses. Such smart glasses may be used to, for example, implement augmented reality (AR), virtual reality (VR), and/or extended reality (XR) functional- ity.

[0130] In the embodiment of Fig. 7, the input light beam 202 may be generated by, for example, an optical engine 801, such as a scanner-based optical engine. The input light beam 202 may represent an image generated by, for example, such an optical engine. The display structure 100 of the display device 800 can direct the out-coupled light 103 representing the image generated by the optical engine 801 into the eye of a user.

[0131] 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 embod- iment unless explicitly disallowed.

[0132] Although the subject matter has been described in language specific to structural features and/or acts, it is to be understood that the subject matter defined n in the appended claims is not necessarily limited to the

S specific features or acts described above. Rather, the = specific features and acts described above are disclosed

N as examples of implementing the claims and other equiv-

I 25 alent features and acts are intended to be within the > scope of the claims.

N [0133] It will be understood that the benefits and

N advantages described above may relate to one embodiment

N 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 will further be un- derstood that reference to 'an' item may refer to one or more of those items.

[0134] Aspects of any of the embodiments described above may be combined with aspects of any of the other embodiments described to form further embodiments with- out losing the effect sought.

[0135] 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 exclu- sive list and a method or apparatus may contain addi- tional blocks or elements.

[0136] It will be understood that the above descrip- tion is given by way of example only and that various modifications may be made by those skilled in the art.

The above specification, examples and data provide a complete description of the structure and use of exem- plary embodiments. Although various embodiments have n been described above with a certain degree of particu-

S larity, or with reference to one or more individual = embodiments, those skilled in the art could make numer-

N ous alterations to the disclosed embodiments without

I 25 departing from the spirit or scope of this specifica- > tion.

N

S

&

Claims (15)

CLAIMS:

1. A display structure (100), comprising: - a waveguide (101) configured to guide in-cou- pled light (102) in the waveguide (101) via consecutive reflections from a first side (121) of the waveguide (101) and a second side (122) of the waveguide (101), wherein the in-coupled light (102) comprises a first polarization (131) and a second polarization (132); - an out-coupling structure (111) on the first side (121) of the waveguide (101) configured to out- couple a part of the first polarization (131) of the in- coupled light (102) from the waveguide (101) when the in-coupled light (102) arrives to the first side (121) of the waveguide (101); and - a polarization manipulating structure (112) on the second side (122) of the waveguide (101) configured to rotate at least a part of the second polarization (132) of the in-coupled light (102) into the first po- larization (131) when the in-coupled light (102) arrives to the second side (122) of the waveguide (101). N

O 2. The display structure (100) according to any — preceding claim, wherein an out-coupling efficiency of S the out-coupling structure (111) changes along at least x 25 one direction on the first side (121) of the waveguide E (101).

2 3. The display structure (100) according to any i preceding claim, wherein an out-coupling efficiency of the out-coupling structure (111) increases along a main propagation direction of the in-coupled light (102).

4. The display structure (100) according to any preceding claim further comprising an exit pupil expan- sion, EPE, structure (203) comprising a polarization sensitive grating on one of: the first side of the wave- guide (121) and the second side (122) of the waveguide (101), and a second polarization manipulating structure on a different side of the waveguide from the polariza- tion sensitive grating, wherein the EPE structure (203) is configured to reduce the second polarization in the in-coupled light (102) and guide the in-coupled light (102) towards the out-coupling structure (111).

5. The display structure (100) according to any preceding claim, wherein the out-coupling structure (111) comprises diffractive grating features and sub- wavelength grating features, wherein the diffractive grating features are configured to out-couple the part of the first polarization (131) of the in-coupled light = (102) from the waveguide (101) when the in-coupled light N (102) arrives to the first side (121) of the waveguide - (101) and the sub-wavelength grating features are con- S 25 figured to make the diffractive grating features at = least partially transparent to the second polarization O (132). 3

O 6. The display structure (100) according to claim 5, wherein the diffractive grating features comprise a plurality of diffractive grating lines (301) and the sub-wavelength grating features comprise a plurality of sub-wavelength grating lines (302).

7. The display structure (100) according to claim 6, wherein grating lines in the plurality of diffractive grating lines (301) are substantially parallel with the first polarization.

8. The display structure (100) according to any claim 6 or claim 7, wherein each grating line in the plurality of diffractive grating lines (301) comprises an air gap.

9. The display structure (100) according to any of claims 6 - 8, wherein the plurality of diffractive grat- ing lines (301) is made of a material with a first refractive index, and the plurality of sub-wavelength grating lines (302) is made of a material with a second refractive index different from the first refractive index. S N

10. The display structure (100) according to any - of claims 6 — 9, wherein the plurality of sub-wavelength S 25 grating lines (302) are positioned between the plurality E of diffractive grating lines (301) and the plurality of W diffractive grating lines (301) and the plurality of O sub-wavelength grating lines (302) are non-parallel. S

11. The display structure (100) according to any of claims 5 - 10, wherein a grating period of the dif- fractive grating features is greater than 250 nanome- tres, a grating period of the sub-wavelength grating features is less than 250 nanometres.

12. The display structure (100) according to any preceding claim, wherein the polarization manipulating structure (112) comprises a sub-wavelength polarization manipulating grating (601).

13. The display structure (100) according to any preceding claim, wherein the first polarization (131) and the second polarization (132) are substantially or- thogonal.

14. The display structure (100) according to any preceding claim, further comprising an in-coupling structure (201) configured to couple an input light beam (202) into the waveguide (101) as the in-coupled light (102). O S N

15. A display device (800) comprising the display - structure (100) according to any preceding claims. S 25 I a a LO N N © O N O N