Polarization-independent liquid crystal micro-display

Abstract

The present disclosure relates to liquid crystal micro-displays (100). The micro-display comprises: a liquid crystal (LC) layer (101); a grating (102) that is integrated in the LC layer (101); a quarter-wave retarder layer (103) provided on the LC layer (101) having a retarder optic axis that is oriented at an angle of 45° to a LC optic axis; a reflective layer (104) provided on the quarter-wave retarder layer (103); an array of electrodes (105) provided on or in the reflective layer (104) and defining an array of pixels; and a controller configured to individually control each electrode of the array of electrodes (105) to regulate a respective effective refractive indices of at least some pixels of the array of pixels. The grating (102) is configured to diffract a first component of an input optical wave and a reflection from the reflective layer of a second component of the input optical wave based on the respective effective refractive index of the at least some pixels.

Description

[0001] POLARIZATION-INDEPENDENT LIQUID CRYSTAL MICRO-DISPLAY

[0002] TECHNICAL FIELD

[0003] The present disclosure relates to liquid crystal micro-displays. The disclosure proposes a microdisplay, and a corresponding method of operating the micro-display. The micro-display comprises a new configuration of layers.

[0004] BACKGROUND

[0005] Liquid crystal (LC)-based micro-displays are inherently polarization-dependent. Conventionally, LC-based displays are based on polarization transformation, which is converted into intensity by the use of polarizer / analyser pairs or polarizing beamsplitters (PBS). This causes larger size and low efficiency of such displays.

[0006] Conventionally, incident light of micro-displays is linearly polarized, and a liquid crystal on silicon (LCOS) layer comprised in the micro-displays is configured to only change a polarization state of the incident light and not the intensity, wherein, for example, a PBS is used to separate polarized waves and to convert polarization modulation into intensity modulation.

[0007] FIG. 2 shows the incident polarization of an exemplary input light for a LCOS micro-display (e.g. in projector or AR glasses), which has an orientation of 45° to the optic axis of the LC or an alignment direction. While passing through the LC layer, the input light is represented by two orthogonally polarized waves: ordinary (perpendicular to optic axis) and extraordinary (parallel to optic axis). The retardance between ordinary and extraordinary is dependent on the LC layer thickness and the effective birefringence, which may be set by an applied voltage. In FIG. 2, the retardance is close to a halfwave retardance, which causes the polarization to change by 90°. If a PBS is used (in a “normally black“ display) this will represent a bright state with almost maximum intensity after the polarizer. A normally black display refers to a display that displays a black image in an “off’ state, for example, in a state of zero voltage. SUMMARY

[0008] In view of the above, an objective of this disclosure is to provide a micro-display that is polarization-independent of an input optical wave. In particular, there may be no need to use polarizer / analyser pairs or a PBS to display an image with the micro-display.

[0009] These and other objectives are achieved by this disclosure as described in the enclosed independent claims. Advantageous implementations are further defined in the dependent claims.

[0010] A first aspect of this disclosure provides a micro-display comprising: a LC layer configured to receive an input optical wave having a central wavelength, wherein the LC layer has a LC optic axis and is divided into an array of pixels, each pixel having a respective effective refractive index at the central wavelength; a grating that is integrated in the LC layer; a quarter-wave retarder layer provided on, for example, provided below, the LC layer, wherein the quarterwave retarder layer is adapted to the central wavelength and has a retarder optic axis that is oriented at an angle of substantially 45° to the LC optic axis; a reflective layer provided on, for example, provided below, the quarter-wave retarder layer; an array of electrodes provided on, for example, provided below, or in the reflective layer; and a controller configured to individually control each electrode of the array of electrodes to regulate the respective effective refractive indices of at least some pixels, for example, all pixels, of the array of pixels; wherein the micro-display is configured to provide the input optical wave through the LC layer and through the quarter-wave retarder layer to the reflective layer, wherein the grating in the LC layer is configured to diffract a first component of the input optical wave in at least one pixel of the array of pixels based on the respective effective refractive index of the at least one pixel; wherein the reflective layer is configured to reflect the diffracted first component of the input optical wave and a second component of the input optical wave back through the quarter-wave retarder layer to the grating in the LC layer, and wherein the grating is further configured to diffract said reflection of the second component of the input optical wave in the at least one pixel of the array of pixels based on the respective effective refractive index of the at least one pixel. Each electrode of the array of electrodes may be associated with one respective pixel of the array of pixels. For example, each electrode may regulate, for example, only, the respective effective refractive index of the respective pixel that is associated with the electrode.

[0011] For example, the quarter- wave retarder layer may be sandwiched between the LC layer and the reflective layer.

[0012] The reflective layer may comprise the array of pixel electrodes.

[0013] The micro-display may comprise a circuitry layer provided on, for example, below, the array of electrodes and / or the reflective layer. The circuitry layer may comprise CMOS circuitry.

[0014] The quarter-wave retarder layer may be configured to provide a half-wave retardance on the input optical wave, for example, the first component and the second component, as the input optical wave passes twice through the quarter-wave retarder layer. Thus, the quarter-wave retarder layer may be considered to be an effectively half-wave retarder layer in the microdisplay. The reflective layer and the quarter-wave retarder layer may be combined into one layer that may be referred to as an effectively half- wave retarder layer.

[0015] The micro-device may not depend on the polarization of the input optical wave. The microdevice may function with any polarization of the input optical wave, wherein no additional optics for adjusting the polarization of the input optical wave or an output optical wave of the micro-display that is based on the input optical wave are required.

[0016] For example, the input optical wave may be one of unpolarised, linearly polarized, circular polarized, and elliptically polarized. Thus, the micro-display is flexible regarding the input optical wave.

[0017] The input optical wave may comprise, or for example, consist of, the first component and the second component.

[0018] The grating may be a diffraction grating. The micro-display being configured to provide the input optical wave through the LC layer and through the quarter-wave retarder layer to the reflective layer may refer to or include the second component of the input optical wave being provided to the reflective layer without being diffracted and / or the first component of the input optical wave being provided to the reflective layer while additionally being diffracted by the grating in the LC layer.

[0019] In an implementation form of the first aspect, further comprising a glass-based layer, wherein the LC layer is provided on, for example, provided below, the glass-based layer, and wherein the glass-based layer is configured to: receive the input optical wave and transmit the input optical wave to the LC layer, and / or receive the reflection of the diffracted first component of the input optical wave and the diffracted reflection of the second component of the input optical wave from the LC layer, and transmit the reflection of the diffracted first component of the input optical wave and the diffracted reflection of the second component of the input optical wave.

[0020] For example, the LC layer may be sandwiched between the glass-based layer and the quarterwave retarder layer. The glass-based layer may be for protecting the LC layer and / or the grating.

[0021] In a further implementation form of the first aspect, wherein the grating is configured to diffract the reflection of the second component of the input optical wave in the at least one pixel further based on a refractive index of the grating at the central wavelength, and / or wherein the grating is configured to diffract the first component of the input optical wave in the at least one pixel of the array of pixels further based on the refractive index of the grating at the central wavelength.

[0022] The refractive index of the grating at the central wavelength and the effective refractive index of the at least one pixel in the LC layer may determine if or how an optical wave, for example, the reflection of the second component of the input optical wave and / or the first component of the input optical wave, is diffracted in the at least one pixel.

[0023] In a further implementation form of the first aspect, wherein each pixel of the array of pixels has an ordinary refractive index at the central wavelength, wherein the ordinary refractive index at the central wavelength is substantially equal to the refractive index of the grating at the central wavelength. Thus, an ordinary component of any optical wave at the central wavelength, for example, the reflection of the diffracted first component of the input optical wave and / or the second component of the input optical wave, may not be diffracted.

[0024] In a further implementation form of the first aspect, wherein each electrode of the array of electrodes is associated with one respective pixel of the array of pixels, wherein, in a first state of the electrode, the effective refractive index of the respective pixel associated with the electrode is substantially equal to the refractive index of the grating at the central wavelength, and / or wherein, in a second state of the electrode, the effective refractive index of the respective pixel associated with the electrode is substantially different from the refractive index of the grating at the central wavelength.

[0025] In the first state of the electrode, the reflected second component of the input optical wave may propagate through the grating and / or the LC layer in the respective pixel associated with the electrode without being diffracted.

[0026] In the second state of the electrode, the reflected second component of the input optical wave may be diffracted by the grating and / or the LC layer in the respective pixel associated with the electrode.

[0027] In a further implementation form of the first aspect, wherein the controller is configured to control at least one electrode to be in the first state and at least one other electrode to be in the second state. The controller may individually control electrodes to different states such that an image can be generated.

[0028] In a further implementation form of the first aspect, wherein, for each electrode in the array of electrodes, the electrode is configured to: apply a voltage across the respective pixel associated with the electrode in the first state, and apply zero voltage across the respective pixel associated with the electrode in the second state, or wherein, for each electrode in the array of electrodes, the electrode is configured to: apply a voltage across the respective pixel associated with the electrode in the second state, and apply zero voltage across the respective pixel associated with the electrode in the first state. In a further implementation form of the first aspect, wherein the LC optic axis is parallel to an orientation of the array of pixels, and / or wherein the grating is oriented parallel to the LC optic axis.

[0029] In a further implementation form of the first aspect, wherein the grating is a binary grating, or wherein the grating is a blaze grating at the central wavelength for a lst-order of diffraction of the diffracted reflection of the second component of the input optical wave.

[0030] Alternatively, the grating may be a blaze grating at the central wavelength for a Ost-order of diffraction of the diffracted first component of the input optical wave.

[0031] In a further implementation form of the first aspect, wherein the grating is provided on, for example, provided below, a surface of the LC layer that faces away from the quarter-wave retarder layer or on, for example, above, another surface of the LC layer that faces towards the quarter-wave retarder layer.

[0032] For example, the grating may be sandwiched between the glass-based layer and the LC layer or the LC layer and the quarter-wave retarder layer.

[0033] In a further implementation form of the first aspect, wherein the grating and / or the glass-based layer comprises a first alignment layer for aligning at least some liquid crystal molecules of the LC layer in the vicinity of the grating and / or the glass-based layer.

[0034] In a further implementation form of the first aspect, wherein the quarter-wave retarder layer comprises a first passivation layer provided on, for example, provided below, the LC layer and / or a second alignment layer for aligning at least some other liquid crystal molecules of the LC layer in the vicinity of the quarter-wave retarder layer.

[0035] In a further implementation form of the first aspect, wherein the reflective layer comprises a second passivation layer provided on, for example, provided below, the quarter-wave retarder layer, and a reflective sub-layer provided on, for example, provided below, the second passivation layer, wherein the reflective sub-layer comprises the array of electrodes. In a further implementation form of the first aspect, wherein the quarter-wave retarder layer is a subwavelength grating, a metasurface, or comprises a reactive mesogene material.

[0036] In a further implementation form of the first aspect, wherein the micro-display comprises at least one of the first alignment layer that forms an interface between the glass-based layer and the LC layer; the first passivation layer and / or the second alignment layer that forms an interface between the LC layer and the quarter-wave retarder layer; and the second passivation layer that forms an interface between the quarter-wave retarder layer and the reflective layer. The first passivation layer may be a wafer for the LC layer. The second passivation layer may be a wafer for the quarter- wave retarder layer.

[0037] In a further implementation form of the first aspect, the glass-based layer is coated with an Index-matched Indium Tin Oxide (IMITO) layer and / or wherein the grating is coated with IMITO.

[0038] In a further implementation form of the first aspect, each electrode of the array of electrodes comprises aluminium.

[0039] In a further implementation form of the first aspect, the array of electrodes is provided on a circuitry layer that is electrically connected to the controller.

[0040] In a further implementation form of the first aspect, the circuitry layer is provided on, for example, above, a substrate of the micro-display.

[0041] The reflection of the diffracted first component of the input optical wave and the diffracted reflection of the second component of the input optical wave may form an output optical wave.

[0042] The micro display may comprise, be optically connectable, or be optically connected to an additional optical system.

[0043] The additional optical system may not comprise optics to modify the polarization of the output optical wave or the input optical wave.

[0044] The additional optical system may comprise or consist of an aperture and / or one or more lenses. The aperture may be configured to block at least some diffraction orders of the output optical wave. The one or more lenses may be configured to focus at least some other diffraction orders of the output optical wave on the aperture.

[0045] Alternatively, the aperture may be configured to block the at least some other diffraction orders of the output optical wave. The one or more lenses may be configured to focus the at least some diffraction orders of the output optical wave on the aperture.

[0046] The at least some other diffraction orders comprise a lst-order of diffraction. The at least some diffraction orders comprise a Oth-order of diffraction.

[0047] For example, the aperture may be configured to block the reflection of the diffracted first component of the input optical wave and at least some diffraction orders of the diffracted reflection of the second component of the input optical wave. The one or more lenses may be configured to focus at least some other diffraction orders of the diffracted reflection of the second component of the input optical wave on the aperture. Thus, the reflection of the at least some other diffraction orders of the diffracted reflection of the second component of the input optical wave may generate an image.

[0048] In another example, the aperture may be configured to block the at least some other diffraction orders of the diffracted reflection of the second component of the input optical wave, and / or the one or more lenses may be configured to focus the reflection of the diffracted first component of the input optical wave and the at least some diffraction orders of the diffracted reflection of the second component of the input optical wave on the aperture. Thus, the reflection of the diffracted first component of the input optical wave and the at least some diffraction orders of the diffracted reflection of the second component of the input optical wave may generate an image.

[0049] A second aspect of this disclosure provides a method of operating a micro-display, the micro display comprising: a LC layer, wherein the LC layer has a LC optic axis and is divided into an array of pixels, each pixel having a respective effective refractive index at a central wavelength; a grating that is integrated in the LC layer; a quarter-wave retarder layer provided on the LC layer, wherein the quarter-wave retarder layer is adapted to the central wavelength and has a retarder optic axis that is oriented at an angle of substantially 45° to the LC optic axis; a reflective layer provided on the quarter-wave retarder layer; an array of electrodes provided on or in the reflective layer; and a controller; wherein the method comprises: individually controlling, with the controller, each electrode of the array of electrodes to regulate the respective effective refractive indices of at least some pixels of the array of pixels; receiving, with the LC layer, an input optical wave having the central wavelength; providing, with the micro-display, the input optical wave through the LC layer and through the quarter-wave retarder layer to the reflective layer; diffracting, with the grating in the LC layer, a first component of the input optical wave in at least one pixel of the array of pixels based on the respective effective refractive index of the at least one pixel; reflecting, with the reflective layer, the diffracted first component of the input optical wave and a second component of the input optical wave back through the quarter-wave retarder layer to the grating in the LC layer; and diffracting, with the grating, said reflection of the second component of the input optical wave in the at least one pixel of the array of pixels based on the respective effective refractive index of the at least one pixel.

[0050] The method of the second aspect may have implementation forms that correspond to the implementation forms of the micro-display of the first aspect. The method of the second aspect and its implementation forms achieve the advantages and effects described above for the microdisplay of the first aspect and its respective implementation forms.

[0051] In this disclosure, providing a layer "on", for example, “above” or “below”, another layer may mean that this layer is formed after the other layer, but there may be formed one or more layers in between. Thus, surfaces of these layers may or may not be in contact.

[0052] “Below“ and “above“, as used herein, are defined with respect to a “body-fixed“ reference frame / coordinate system. The term “Body-fixed” refers to a reference frame that is fixed to the micro-display and rotates with the latter in case the latter is rotated. For example, the LC layer may be provided above the quarter-wave retarder layer.

[0053] Further, in this disclosure the term “substantially” can mean „within a certain range comprising the nominal value“. That range may be +- 10 %, +-5% or +-2 % of the nominal value. For example, the retarder optic axis being oriented at an angle of substantially 45° to the LC optic axis may refer to the fact that an angle of 45° may be a preferred value to achieve an improved effect or efficiency. The micro-display may still function if there is a deviation from 45°.

[0054] It has to be noted that all devices, elements, units and means described in the disclosure could be implemented in the software or hardware elements or any kind of combination thereof. All steps which are performed by the various entities described in the disclosure as well as the functionalities described to be performed by the various entities are intended to mean that the respective entity is adapted to or configured to perform the respective steps and functionalities.

[0055] Even if, in the following description of specific embodiments, a specific functionality or step to be performed by external entities is not reflected in the description of a specific detailed element of that entity which performs that specific step or functionality, it should be clear for a skilled person that these methods and functionalities can be implemented in respective software or hardware elements, or any kind of combination thereof.

[0056] BRIEF DESCRIPTION OF DRAWINGS

[0057] The above described aspects and implementation forms will be explained in the following description of specific embodiments in relation to the enclosed drawings, in which

[0058] FIG. 1 shows a micro-display according to this disclosure.

[0059] FIG. 2 shows the incident polarization of an exemplary input light for a LCOS micro-display.

[0060] FIG. 3 shows exemplary diffraction orders of an input optical wave based on the grating according to this disclosure.

[0061] FIG. 4 shows an exemplary micro-display according to this disclosure.

[0062] FIG. 5 shows the orientations of the LC optic axis and the quarter-wave retarder layer optic axis according to this disclosure. FIG. 6 shows an exemplary micro-display according to this disclosure, wherein the grating is located on the LCOS wafer.

[0063] FIG. 7 shows a micro-display comprising a blaze grating according to this disclosure.

[0064] FIG. 8 shows an exemplary additional optical system for the micro-display according to this disclosure.

[0065] FIG. 9 shows an exemplary additional optical system for the micro-display according to this disclosure.

[0066] FIG. 10 shows a method according to this disclosure.

[0067] DETAILED DESCRIPTION OF EMBODIMENTS

[0068] FIG. 1 shows a micro-display 100 according to this disclosure. The micro-display 100 comprises an LC layer 101, a grating 102 that is integrated in the LC layer 101, a quarter- wave retarder layer 103 provided on, for example, provided below, the LC layer 101, a reflective layer 104 provided on, for example, provided below, the quarter- wave retarder layer 103, an array of electrodes 105 provided on, for example, provided below, or in the reflective layer 104 (which is indicated in FIG. 1 by the overlap of the array of electrodes 105 with the reflective layer 104), and a controller 106. For example, the grating 102 may be provided on a surface of the LC layer 101 that faces towards the quarter- wave retarder layer 103. In another example, the grating 102 may be provided on a surface of the LC layer 101 that faces away from the quarter- wave retarder layer 103.

[0069] The LC layer 101 is configured to receive an input optical wave 107 having a central wavelength. The LC layer 101 has an LC optic axis and is divided into an array of pixels, wherein each pixel of the array of pixels has a respective effective refractive index at the central wavelength. The quarter- wave retarder layer 103 is adapted to the central wavelength and has a retarder optic axis that is oriented at an angle of substantially 45° to the LC optic axis. The input optical wave 107 comprises a first component 107a of the input optical wave 107 and a second component 107b of the input optical wave 107. For example, the first component 107a may be an extraordinary component of the input optical wave 107 in the LC layer 101 and the second component 107b may be an ordinary component of the input optical wave 107 in the LC layer 101. However, said components may change their polarization based on the quarterwave retarder layer.

[0070] The controller 106 is configured to individually control each electrode of the array of electrodes

[0071] 100 to regulate the respective effective refractive indices of at least some pixels of the array of pixels.

[0072] The micro-display 100 is configured to provide the input optical wave 107 through the LC layer

[0073] 101 and through the quarter-wave retarder layer 103 to the reflective layer 104. The grating 102 in the LC layer 101 is configured to diffract a first component 107a of the input optical wave 107 in at least one pixel of the array of pixels based on the respective effective refractive index of the at least one pixel. The reflective layer 100 is configured to reflect the diffracted first component 107c of the input optical wave 107 and a second component 107b of the input optical wave 107 back through the quarter- wave retarder layer 103 to the grating 102 in the LC layer 101. The grating 102 is further configured to diffract said reflection of the second component 107d of the input optical wave 100 in the at least one pixel of the array of pixels based on the respective effective refractive index of the at least one pixel.

[0074] FIG. 1 shows an exemplary optical path based on the second component 107b of the input optical wave 107 (including the reflection of the second component 107d of the input optical wave 107) and another exemplary optical path based on the first component 107a of the input optical wave 107 (including the diffracted first component 107c of the input optical wave 107).

[0075] The quarter- wave retarder layer 103 may be configured to provide quarter- wave retardance of any optical wave at the central wavelength that passes through the quarter-wave retarder layer 103. FIG. 1 exemplary shows that the optical path based on the second component 107b of the input optical wave 107 and the optical path based on the first component 107a of the input optical wave 107 respectively passes twice through the quarter- wave retarder layer 103. Thus, an effective half-wave retardance can be achieved, as the quarter-wave retardance effect of each of the two passes is combined to a half- wave retardance effect. This disclosure may provide a polarization-independent LCOS phase modulator. The polarization independence can be achieved by using a polarization flip based on the quarterwave retarder layer 103, which may, for example, be placed on a backplane of the micro-display 100. The phase modulation may be converted into intensity modulation by using the grating 102 (a diffractive element). The grating 102 may also be an alignment structure if oriented under 45° to the optic axis of the quarter- wave retarder layer 103.

[0076] The quarter- wave retarder layer 103 may be acting as a half- wave retarder layer, as any optical wave that is reflected at the reflective layer 104 passes two times through the quarter- wave retarder layer 103. For example, the quarter- wave retarder layer 103 may behave as a half- wave retarder layer. In this disclosure, the quarter- wave retarder layer 103 may be referred to as an effectively half-wave retarder layer.

[0077] The combination of the effectively half-wave retarder layer and the grating 102 enables the micro-display 100 to be polarization-independent of the input optical wave 107, wherein the grating 102 may convert a polarization-independent phase modulation into an intensity modulation.

[0078] The effectively half-wave retarder layer may be based on a subwavelength grating, a metasurface or a reactive mesogene. In this disclosure the “sub wavelength grating” does not refer to the “grating 102” or “diffraction grating 102” that is integrated in the LC layer 101.

[0079] In this disclosure, the grating 102 may be referred to as a diffraction grating 102.

[0080] FIG. 3 shows exemplary diffraction orders of an input optical wave 107 based on the grating 102 according to this disclosure. FIG. 3 shows the grating 102 being integrated in an LC layer 101 comprising a plurality of LC molecules, and a reflective layer 104 that reflects an input optical wave 107. At least a component of the reflected input optical wave 107 may be diffracted by the grating 102 mainly into a 0th, -1st, and 1storder of diffraction.

[0081] FIG. 4 shows an exemplary micro-display 100 according to this disclosure. The micro-display 100 comprises an LC layer 101, a grating 102 embedded in the LC layer 101, a quarter- wave retarder layer 103, for example, a subwavelength grating, metasurface, or reactive mesogene, a reflective layer 104, and an array of electrodes 105. The micro-display 100 may comprise a glass-based layer 108,

[0082] The glass-based layer 108 may comprise an index-matched indium tin oxide (IMITO) glass, which may contain antireflection (AR) layers on the front surface and / or on the inner surface to reduce respective reflections, and IMITO electrode on the inner side. The diffraction grating 102 which may be deposited on the IMITO side of the glass. The reflective layer 104, which may be a backplane layer and / or may comprise pixel electrodes 105 and underlying circuitry.

[0083] The diffraction grating 102 may contain a special alignment layer to improve alignment of the LC molecules in the vicinity of the glass-based layer 108. The glass-based layer 108 may be a coverglass. The quarter- wave retarder layer 103 may comprise passivation / embedding layers and an alignment layer to have LC alignment from the quarter- wave retarder layer 103 facing side. For example, the reflective backplane may comprise one or more passivation layers on top of two or more reflective Aluminum electrodes comprised in the array of electrodes to improve reflectivity and surface conditions.

[0084] The diffraction grating 102 may be coated with an IMITO layer instead of having the IMITO layer on the glass. This can improve the electrical field distribution in the LCOS cell.

[0085] FIG. 5 shows the orientations of the LC optic axis and the quarter- wave retarder layer 103 optic axis according to this disclosure. The quarter-wave retarder layer 103 optic axis has substantially 45° to the orientation of the LC optic axis. The diffraction grating 102 may be oriented along the LC axis, although it can work under any orientation.

[0086] In the following example, the refractive index of the diffraction grating 102 may be ngr = n0= 1.5, where n0is ordinary refractive index of the LC. The extraordinary refractive index nemay be ne= 1.8.

[0087] The input optical wave 107 of arbitrary polarization (e.g. unpolarized) may be going through the cover glass towards the reflective backplane. The input optical wave 107 may propagate in the LC layer 101 as an initially ordinary wave (perpendicular to the optic axis of the LC) and an initially extraordinary wave (parallel to the optic axis). The initially ordinary wave may face the identical refractive index by the diffraction grating 102 and the LC such that no diffraction occurs. The initially extraordinary wave may however be experiencing diffraction on the grating 102.

[0088] Further, the wavelength of the input optical wave 107 may be, e.g., 540 nm and the depth of the grating 102 may be d = 0.9 pm. The optical path difference may be d * (ne-n0) = 0.9 pm * 0.3 = 270 nm or half of the wavelength.

[0089] The initially ordinary wave may be the second component 107b of the input optical wave 107. The initially extraordinary wave may be the first component 107a of the input optical wave 107

[0090] The initially ordinary wave may be reflected from the reflective layer 104 and may pass twice through the quarter- wave retarder layer 103. Since the optic axis of the quarter- wave retarder layer 103 is making 45° with the polarization of the initially ordinary wave, the polarization may be flipped after its reflection or in other words rotated by 90° and will match the polarization of the initially extraordinary wave. The initially ordinary wave may then be referred to as a concluding extraordinary wave. During the propagation of the concluding extraordinary wave back through the LC layer 101 it will thus experience the diffraction grating 102 as an extraordinary wave.

[0091] The initially extraordinary wave may be reflected from the reflective layer 104 and may pass twice through the quarter- wave retarder layer 103. Since the optic axis of the quarter- wave retarder layer 103 is making 45° with the polarization of the initially extraordinary wave, the polarization may be flipped after its reflection or in other words rotated by 90° and will match the polarization of the initially ordinary wave. The initially extraordinary wave may then be referred to as a concluding ordinary wave. During the propagation of the concluding ordinary wave back through the LC layer 101 it will thus not experience diffraction on the grating 102 as an ordinary wave.

[0092] The grating 102 may be a binary grating 102. The optical wave coming back from the microdisplay may have minimum intensity in the Oth order (reflection) and maximum intensity in the + / -lstorders, for example, around 40% each.

[0093] A voltage can be applied through the LC layer 101. The LC layer 101 may comprise a plurality of LC molecules. By increasing a voltage across the LC layer 101, the LC molecules may increase their tilt towards the surface normal (aligning along the electric field). The initial extraordinary wave and the concluding extraordinary wave may thus experience not the neof 1.8, but a decreasing nee < 1.8. For example, the nemay match the n0= 1.5. In this case both ordinary and extraordinary waves may not experience diffraction on the grating 102, so the reflected light (0thorder diffraction) may be at maximum intensity, whereas the + / -lstorder diffraction may approach minimum (zero) intensity.

[0094] For example, if a voltage is high and ULC = ngr, then the Oth order diffraction is maximal, and the 1st order diffraction is minimal. In another example, if a voltage is 0 and nLC=L8 > ngr, then the Oth order diffraction is minimal, and the 1st order diffraction is maximal.

[0095] The examples above are implying a “positive” LC (positive dielectric anisotropy, the molecules are tending to align along the electrical field). In the case of a “negative” LC (negative dielectric anisotropy), the situation will inverse: by zero voltage (E-field off) the orientation of the LC optic axis would be 90° to a substrate (homeotropic or vertical alignment conditions), so that both ordinary and extraordinary waves may not see the diffraction grating (ULC = n0= 1.5) and the zero order diffraction (reflection) may be at a maximum, whereas the + / -lstorder diffractions may be minimal. By increasing a voltage across the LC layer 101, the LC molecules may tilt towards the substrates, so that the diffraction at the diffraction grating 102 may happen (either by an initial wave or a reflected wave) and the + / -lstorder diffractions may increase, whereas the 0thorder diffraction (reflection) may decrease.

[0096] The above can be controlled pixelwise by applying respective voltages. To visualize an image, either the 0thor + / -lstorders of diffraction may be spatially selected. Spatial selection of the said orders may be done using an aperture or an angular filter as part of an optical system.

[0097] The grating 102 of the micro-display 100 shown in FIG. 4 is located on the glass-based layer 108. It may be easier to fabricate the grating 102 on the glass-based layer 108, then on, for example, the LCOS wafer as shown in FIG. 6. The glass-based layer 108 may be a cover glass. FIG. 6 shows an exemplary micro-display 100 according to this disclosure, wherein the grating 102 is located on the LCOS wafer. The glass-based layer 108 may be an IMITO cover glass and may not comprise any special structuring.

[0098] For some fabrication and assembly processes of the micro-display 100, it may be preferred to have all or most structures (for example, metasurface 103, diffraction grating 102) on only one side of the LC layer 101. For example, only a glass-based layer 108 may be above the LC layer 101 and the other layers of the micro-display 100, for example, the grating 102, the quarterwave retarder layer 103, the reflective layer 104, and the array of electrodes 105, may be below (on the opposite side) the LC layer 101.

[0099] The micro-display 100 may be based on a binary grating 102 to convert phase modulation into intensity modulation. One advantage of a binary grating 102 may be a fast response time of the micro-display 100, as cell gaps may be thin, for example, thinner compared to a micro-display 100 comprising a blaze grating 102.

[0100] However if, for example, the response time is not a major objective, other grating 102 shapes may be used.

[0101] FIG. 7 shows a micro-display 100 comprising a blaze grating 102 according to this disclosure. A change of the voltage may redistribute the light between the 0thand the +lstorder of diffraction. Thus, additional optical systems, for example, for receiving said diffraction orders, may be simplified. Further, FIG. 7 shows the other layers 108, 101, 103, 104, 105 of the microdisplay as similarly shown in FIG. 6.

[0102] The quarter- wave retarder layer 103 may comprise, for example, instead of a metasurface, a reactive mesogene (RM) material (polymerized liquid crystal) with quarter-wave retardance (half-wave retardance in a double pass). The respective optic axis orientation may be as described above.

[0103] An advantage of using RM may be the simplicity of fabrication. RM may also be referred to as an LC polymer. The use of RM may have disadvantages. For example, an RM based quarter-wave retarder layer 103 is typically thicker than a metasurface based quarter- wave retarder layer 103. Thus, the drop of electrical field on the RM may be larger. Another disadvantage of RM may be a worse reliability against moisture and temperature cycling.

[0104] FIG. 8 shows an exemplary additional optical system for the micro-display 100 according to this disclosure. FIG. 9 shows another exemplary additional optical system for the micro-display 100 according to this disclosure. The micro-display 100 may be for display and illumination applications. For example, the illumination application may comprise high-resolution car headlights. Other illumination applications may be metrology applications like 3D measurements with pattern projection.

[0105] The additional optical system may comprise projection lenses and an aperture for projecting a particular diffraction order of the micro-display 100 onto a scene. Due to polarization independence of the micro-display 100 regarding the input optical wave 107, for example, no analyser or polarizer may be required in the additional optical system.

[0106] FIG. 8 shows an exemplary case, where the zero order is used to form the image (scene), whereas higher orders are blocked by the aperture.

[0107] FIG. 9 shows an opposite exemplary case, where the zero order is blocked by the aperture, whereas + 1SV- 1storders are used to form the image.

[0108] Further, the additional optical system may comprise illumination optics, for example, comprising lenses or mirrors, and a, for example, unpolarised, light source for providing the input optical wave 107 to the micro-display 100.

[0109] The controller 106 may be a processor 106.

[0110] Generally, the processor 106 may be configured to perform, conduct or initiate the various operations of the micro-display 100 described herein. The processor 106 may comprise hardware and / or may be controlled by software. The hardware may comprise analog circuitry or digital circuitry, or both analog and digital circuitry. The digital circuitry may comprise components such as application-specific integrated circuits (ASICs), field-programmable arrays (FPGAs), microcontrollers, digital signal processors (DSPs), or multi-purpose processors. The micro-display 100 may further comprise memory circuitry, which stores one or more instruction(s) that can be executed by the processor 106, in particular under control of the software. For instance, the memory circuitry may comprise a non-transitory storage medium storing executable software code which, when executed by the processor 106, causes the various operations of the micro-display 100 to be performed. In one embodiment, the microdisplay 100 may comprises one or more processors 106 and a non-transitory memory connected to the one or more processors 200. The non-transitory memory may carry executable program code which, when executed by the one or more processors 106, causes the micro-display 100 to perform, conduct or initiate the operations or methods described herein.

[0111] FIG. 10 shows a method 200 according to this disclosure. The method 200 is a method of operating a micro-display 100.

[0112] Generally, the micro display comprises: an LC layer 101, a grating 102, a quarter-wave retarder layer 103, a reflective layer 104, an array of electrodes 105, and a controller 106. The LC layer 101 has an LC optic axis and is divided into an array of pixels, each pixel having a respective effective refractive index at a central wavelength. The grating 102 is integrated in the LC layer 101. The quarter-wave retarder layer 103 is provided on the LC layer 101, adapted to the central wavelength, and has a retarder optic axis that is oriented at an angle of substantially 45° to the LC optic axis. The reflective layer 104 is provided on the quarter- wave retarder layer 103. The array of electrodes 105 is provided on or in the reflective layer 104.

[0113] The method 200 comprises a step 201 of individually controlling with the controller 106, each electrode of the array of electrodes 105 to regulate the respective effective refractive indices of at least some pixels of the array of pixels. Further, the method 200 comprises a step 202 of receiving with the LC layer 101, an input optical wave 107 having the central wavelength. Further, the method 200 comprises a step 203 of providing with the micro-display 100, the input optical wave 107 through the LC layer 101 and through the quarter- wave retarder layer 103 to the reflective layer 104. Further, the method 200 comprises a step 204 of diffracting with the grating 102 in the LC layer 101, a first component 107a of the input optical wave 107 in at least one pixel of the array of pixels based on the respective effective refractive index of the at least one pixel. Further, the method 200 comprises a step 205 of reflecting with the reflective layer 104, the diffracted first component 107c of the input optical wave 107 and a second component 107b of the input optical wave 107 back through the quarter- wave retarder layer 103 to the grating 102 in the LC layer 101. Further, the method 200 comprises a step 206 of diffracting with the grating 102, said reflection of the second component 107d of the input optical wave 107 in the at least one pixel of the array of pixels based on the respective effective refractive index of the at least one pixel.

[0114] The disclosure has been described in conjunction with various embodiments as examples as well as implementations. However, other variations can be understood and effected by those persons skilled in the art and practicing the claimed matter, from the studies of the drawings, this disclosure and the independent claims. In the claims as well as in the description the word “comprising” does not exclude other elements or steps and the indefinite article “a” or “an” does not exclude a plurality. A single element or other unit may fulfill the functions of several entities or items recited in the claims. The mere fact that certain measures are recited in the mutual different dependent claims does not indicate that a combination of these measures cannot be used in an advantageous implementation.

Claims

CLAIMS1. A micro-display (100) comprising: a liquid crystal, LC, layer (101) configured to receive an input optical wave (107) having a central wavelength, wherein the LC layer (101) has a LC optic axis and is divided into an array of pixels, each pixel having a respective effective refractive index at the central wavelength; a grating (102) that is integrated in the LC layer (101); a quarter-wave retarder layer (103) provided on the LC layer (101), wherein the quarter-wave retarder layer (103) is adapted to the central wavelength and has a retarder optic axis that is oriented at an angle of substantially 45° to the LC optic axis; a reflective layer (104) provided on the quarter-wave retarder layer (103); an array of electrodes (105) provided on or in the reflective layer (104); and a controller (106) configured to individually control each electrode of the array of electrodes (105) to regulate the respective effective refractive indices of at least some pixels of the array of pixels; wherein the micro-display (100) is configured to provide the input optical wave (107) through the LC layer (101) and through the quarter-wave retarder layer (103) to the reflective layer (104), wherein the grating (102) in the LC layer (101) is configured to diffract a first component (107a) of the input optical wave (107) in at least one pixel of the array of pixels based on the respective effective refractive index of the at least one pixel; wherein the reflective layer (100) is configured to reflect the diffracted first component (107c) of the input optical wave (107) and a second component (107b) of the input optical wave (107) back through the quarter-wave retarder layer (103) to the grating (102) in the LC layer (101), and wherein the grating (102) is further configured to diffract said reflection of the second component (107d) of the input optical wave (107) in the at least one pixel of the array of pixels based on the respective effective refractive index of the at least one pixel.

2. The micro-display (100) according to claim 1, further comprising a glass-based layer (108), wherein the LC layer (101) is provided on the glass-based layer (108), and wherein the glass-based layer (108) is configured to:receive the input optical wave (107) and transmit the input optical wave (107) to the LC layer (101), and / or receive the reflection of the diffracted first component of the input optical wave (107) and the diffracted reflection of the second component of the input optical wave (107) from the LC layer (101), and transmit the reflection of the diffracted first component of the input optical wave (107) and the diffracted reflection of the second component of the input optical wave (107).

3. The micro-display (100) according to claim 1 or 2, wherein the grating (102) is configured to diffract the reflection of the second component (107d) of the input optical wave (107) in the at least one pixel further based on a refractive index of the grating (102) at the central wavelength, and / or wherein the grating (102) is configured to diffract the first component (107a) of the input optical wave (107) in the at least one pixel of the array of pixels further based on the refractive index of the grating (102) at the central wavelength.

4. The micro-display (100) according to any one of the preceding claims, wherein each pixel of the array of pixels has an ordinary refractive index at the central wavelength, wherein the ordinary refractive index at the central wavelength is substantially equal to the refractive index of the grating (102) at the central wavelength.

5. The micro-display (100) according to any one of the preceding claims, wherein each electrode of the array of electrodes (105) is associated with one respective pixel of the array of pixels, wherein, in a first state of the electrode, the effective refractive index of the respective pixel associated with the electrode is substantially equal to the refractive index of the grating (102) at the central wavelength, and / or wherein, in a second state of the electrode, the effective refractive index of the respective pixel associated with the electrode is substantially different from the refractive index of the grating (102) at the central wavelength.

6. The micro-display (100) according to claim 5, wherein the controller (106) is configured to control at least one electrode to be in the first state and at least one other electrode to be in the second state.

7. The micro-display (100) according to claim 5 or 6, wherein, for each electrode in the array of electrodes (105), the electrode is configured to: apply a voltage across the respective pixel associated with the electrode in the first state, and apply zero voltage across the respective pixel associated with the electrode in the second state, or wherein, for each electrode in the array of electrodes (105), the electrode is configured to: apply a voltage across the respective pixel associated with the electrode in the second state, and apply zero voltage across the respective pixel associated with the electrode in the first state.

8. The micro-display (100) according to any one of the preceding claims, wherein the LC optic axis is parallel to an orientation of the array of pixels, and / or wherein the grating (102) is oriented parallel to the LC optic axis.

9. The micro-display (100) according to any one of the preceding claims, wherein the grating (102) is a binary grating (102), or wherein the grating (102) is a blaze grating (102) at the central wavelength for a Inorder of diffraction of the diffracted reflection of the second component of the input optical wave (107).

10. The micro-display (100) according to any one of the preceding claims, wherein the grating (102) is provided on a surface of the LC layer (101) that faces away from the quarter-wave retarder layer (103) or on another surface of the LC layer (101) that faces towards the quarter- wave retarder layer (103).

11. The micro-display (100) according to any one of the preceding claims, wherein the grating (102) and / or the glass-based layer (108) comprises a first alignment layer for aligning at least some liquid crystal molecules of the LC layer (101) in the vicinity of the grating (102) and / or the glass-based layer (108).

12. The micro-display (100) according to any one of the preceding claims, wherein the quarter- wave retarder layer (103) comprises a first passivation layer provided on the LC layer (101) and / or a second alignment layer for aligning at least some other liquid crystal molecules of the LC layer (101) in the vicinity of the quarter- wave retarder layer (103).

13. The micro-display (100) according to any one of the preceding claims, wherein the reflective layer (104) comprises a second passivation layer provided on the quarter- wave retarder layer (103), and a reflective sub-layer provided on the second passivation layer, wherein the reflective sub-layer comprises the array of electrodes (105).

14. The micro-display (100) according to any one of the preceding claims, wherein the quarter-wave retarder layer (103) is a subwavelength grating, a metasurface, or comprises a reactive mesogene material.

15. A method of operating a micro-display (100), the micro display comprising: a liquid crystal, LC, layer (101), wherein the LC layer (101) has a LC optic axis and is divided into an array of pixels, each pixel having a respective effective refractive index at a central wavelength; a grating (102) that is integrated in the LC layer (101); a quarter-wave retarder layer (103) provided on the LC layer (101), wherein the quarter-wave retarder layer (103) is adapted to the central wavelength and has a retarder optic axis that is oriented at an angle of substantially 45° to the LC optic axis; a reflective layer (104) provided on the quarter-wave retarder layer (103); an array of electrodes (105) provided on or in the reflective layer (104); and a controller (106); wherein the method comprises: individually controlling, with the controller (106), each electrode of the array of electrodes (105) to regulate the respective effective refractive indices of at least some pixels of the array of pixels; receiving, with the LC layer (101), an input optical wave (107) having the central wavelength;providing, with the micro-display (100), the input optical wave (107) through the LC layer (101) and through the quarter-wave retarder layer (103) to the reflective layer (104); diffracting, with the grating (102) in the LC layer (101), a first component (107a) of the input optical wave (107) in at least one pixel of the array of pixels based on the respective effective refractive index of the at least one pixel; reflecting, with the reflective layer (104), the diffracted first component (107c) of the input optical wave (107) and a second component (107b) of the input optical wave (107) back through the quarter-wave retarder layer (103) to the grating (102) in the LC layer (101); and diffracting, with the grating (102), said reflection of the second component (107d) of the input optical wave (107) in the at least one pixel of the array of pixels based on the respective effective refractive index of the at least one pixel.

Images