Field

[0001] The present invention relates generally to an electrically tunable lens, an array of electrically tunable lenses and a compound eye with an array of electrically tunable lenses

Background

[0002] Small lenses and miniature camera modules are becoming increasingly utilised, for example, in webcams, medical devices such as endoscopes, mobile phones, tablets, laptops, surveillance cameras, wearable devices, home entertainment systems, and many other products. Depending upon the intended use, size constraints, and total track length (the distance between the image and the image sensor) of the lens may be critical factors. Processing capability of devices having such lenses can also be an issue in view of the proliferation of optimised embedded image processing algorithms of many devices making the speed of focus-tunability important.

[0003] Typically, for focus-tunability and size dependent uses, liquid crystals and mechanically focused lenses are used. However, the speed at which liquid crystals and mechanically focused lenses adjust their focal lengths is slow compared to the computing speed currently available for imaging, both in light projecting and light receiving applications. A mechanically focused lens' focal length adjustment operates in the millisecond scale, such being much slower than currently available computing speed. A liquid crystal lens also operates in the millisecond scale. Also mechanically focused lenses are complicated in design and structure. Furthermore, since liquid crystal is liquid, not solid, a lens is bulky and therefore integration into a variety of products is difficult, particularly those requiring flexibility. Most micro-lenses on the market have limited flexibility (bending radius typically less than 5 mm). Additionally, currently available micro- lenses have substantial voltage requirements.

Summary of Invention

[0004] It is an object of the present invention to substantially overcome, or at least ameliorate, one or more disadvantages of existing arrangements. [0005] Disclosed is a lens including a solid state lens component of at least one of two- dimensional (2D) materials and van der Waals heterostructures, in combination with an electrolyte gate having properties so that when subjected to an electric field the electrolyte gate generates Debye layers sufficient to vary the optical path length (OPL) of the lens component. Both the lens component and the electrolyte gate are held between electrodes that are arranged to provide an electric field across the electrodes, which may potentially operate at a low voltage of a few volts, such as, for example, 1 -5 volts. In addition to reduced voltage requirements, the disclosed lens is flexible and is particularly thin compared to most micro-lenses on the market. Also disclosed is an array of lenses including at least one of the described lenses. An array of lenses can be arranged, for example, on a flexible substrate so that the lens array is deformable. The disclosed lens can be used for light receiving and/or light projecting applications.

[0006] These 2D layered materials have electrical tunability because there are no chemical bonds between neighbouring layers in the material. Therefore, these layers outperform conventional bulk solid materials, such as Si. Conventional bulk solid materials do not possess useful usable electrical tunability, or at least have negligible tunability that is smaller than 0.01 %. Also, the electrical tunability in the 2D layered materials has intrinsically higher operational speed in terms of varying the optical path length than that of liquid crystal and mechanical lenses.

[0007] Compared with conventional tunable liquid and mechanical lenses, the disclosed lens can be approximately 1% the size in two dimensions and 0.1% in height. Furthermore, the disclosed lens can be extremely flexible (bending radius less than 0.1 mm) which is desirable for incorporation into wearable devices, LEDs and other flexible micro-optic components. The structure of the disclosed lens is suited to integration in many different products for either receiving and/or projecting light.

[0008] According to a first aspect, the present disclosure provides a lens, comprising: a positive electrode and a negative electrode arranged to provide an electric field between the positive electrode and the negative electrode when a variable voltage is applied across the positive electrode and the negative electrode; an electrically tunable solid state lens component comprising a layer of at least one of two-dimensional (2D) materials and van der Waals heterostructures, wherein the electrically tunable solid state lens component is capable of a variable optical path length in response to a varied electric field and is held proximal to one of the positive electrode and negative electrode; a substantially transparent electrolyte gate having properties so that when subjected to an electric field the electrolyte gate generates Debye layers sufficient to vary the optical path length of the lens component wherein the substantially transparent electrolyte gate is held between the positive electrode and the negative electrode and is held proximal to the electrically tunable solid state lens component; a substantially transparent substrate held proximal to one of the positive electrode and negative electrode; and circuitry configured to apply the variable voltage across the positive electrode and the negative electrode which when applied results in varying the variable optical path length of the electrically tunable solid state lens component.

[0009] Other aspects are also disclosed

Brief Description of Drawings

[00010] FIG. 1 depicts the disclosed lens;

[00011] FIG. 2 depicts the disclosed array including a plurality of lenses;

[0010] FIG. 3 depicts steps of an example method of fabricating the disclosed lens component.

[0011] FIGs. 4a-4c show optical microscope images of the growth by chemical vapour disposition of a disclosed MoS ₂ lens component starting from a nanoparticle dot and data pertaining to the same; and

[0012] FIGs. 5a-5c show experimental results of the optical lens fabricated using molybdenum disulphide (MoS ₂ ).

[0013] FIGs 6a-6c show an example of a MoS2 structure lens. Description of Embodiments

[0014] Disclosed is a lens including a solid state lens component in combination with an electrolyte gate having properties so that when subjected to an electric field, the electrolyte gate generates Debye layers sufficient to vary the optical path length of the lens component, such as a polymer electrolyte gate. For example, the electrolyte gate can be a substantially transparent polymer, such as a gel or rubbery polymer. The polymer may also be a solid polymer, or a substantially transparent solid polymer. Positive and negative electrodes are arranged to provide a variable electric field to the electrolyte gate and therefore the solid state lens component, both held between the electrodes. Circuitry to provide a low bias of a few volts to the electrodes can adequately provide a variable electric field to the solid state lens component sufficient to change its optical path length due to the electrolyte gate's propensity for generating Debye layers. The Debye layers are established by a Debye -length wherein the electric potential will decrease in magnitude by lie, where "e" is Euler's number, from the electrode, creating an accumulation of charge at the electrode. The high gate capacitance of the Debye layers at the electrodes can provide a sufficient electric field to vary the optical path length of the lens component proximal to an electrode even with a low bias of a few volts. Varying the electric field strength nearly simultaneously changes the charge at the electrodes and in turn, the OPL of the lens component changes nearly simultaneously making the lens focal length adjustment in the microsecond to nanosecond scale.

[0015] FIG. 1 depicts the disclosed lens 100. An electrically tunable solid state lens component 101 is formed from a layer 102 made up of at least one of two-dimensional (2D) materials and van der Waals heterostructures. The electrically tunable solid state lens component is capable of a variable optical path length in response to a varied electric field and is held proximal to a substantially transparent electrode 110, which in this example is a negative electrode. An atomically thin layer of material of the lens component 101 can include a single layer of atoms or molecules or a plurality of layers of atoms or molecules. In one example, a layer can include a single type of material. In another example, a layer can include a plurality of types of materials. Also, a plurality of layers can include a single or a plurality of layers of either a single type of material and/or a single or a plurality of layers of a plurality of materials. Positioning of a lens component with respect to an electrode so that it is held proximal to a substantially transparent electrode can occur during growth so that it is formed in such a position. Also positioning can occur at a different stage in manufacturing, for example, when layers or other components are bonded so that they are held in their respective positions. Any suitable manner in which different components are held with respect to one another is within the scope of this discussion.

[0016] 2D materials can include the semi-metallic graphene, semiconducting transition metal dichalcogenides (TMDs), TMD alloys, insulating hexagonal boron nitride (h-BN), group-IV monochalcogenides, gallium and indium monochalcogenides, 2D oxides and emerging semiconducting phosphorene with bulked structure and strongly anisotropic properties. The TMDs can have the formula MX2, where M is a transition metal and X is a chalcogen wherein M can be at least one of substances of the type Mo, W, Ti, Hf, Zr, V, Nb, Ta, and X can be at least one of S, Se, Te. The TMD alloys can have the formula ΜΧ1 ₂ χΧ22(ΐ- _χ ), or Ml _y M2( _1-y )X2, where M, Ml and M2 are a type of transition metal and X, XI, X2 are a type of chalcogens. The values of variables x and y are in the range from 0 to 1 . The phosphorene can be, for example, black phosphorus. The group-IV monochalcogenides can be SnS, GeS, SnSe, and GeSe. The gallium and indium monochalcogenides can have the formula of GaX and InX (where X is a chalcogen, such as S, Se, or Te), which are additional members of the family of hexagonal 2D materials. The 2D oxides can be oxide 2D crystals including mono- and few-layers of T1O2, M0O3, WO3, Mn02, V2O5, Ta03, Ru02, mica, hydroxides and perovskite-like crystals such as bismuth strontium calcium copper oxide, and LaNb20z, (Ca,Sr)2Nb30io, Bi ₄ Ti30i2, Ca2Ta2TiOio, Sr2Nb30io, Ni(OH)2, Eu(OH)2. All these materials are layered materials with van der Waals interactions between neighbouring layers, which enables the electrical tunability of their optical properties.

[0017] The layer 102 of two-dimensional (2D) materials and/or van der Waals heterostructures materials represents thin unsupported crystalline solids which can possess the electrically tunable optical properties. Also depicted in FIG. 1 are layers 104 and 106 which together with layer 102, in this example, form a dome-shaped component. A single layer and a plurality of layers are within the scope of this discussion.

[0018] In one example, the layer 102 of the lens component may be a few layers of M0S2 which may be less than 6.3 nm thick, as described below with reference to FIGs. 5a-5c. In atomically thin films of the disclosed lens component, the OPL value increases exponentially with the increase of the refractive index, in contrast to the bulk thin films where the OPL is linearly proportional to the refractive index. The thinness of the lens component in combination with the solid polymer electrolyte gate provides for a thin lens on the order of 10-100 nm thick in total.

[0019] Still referring to FIG. 1, a substantially transparent solid electrolyte gate 108 such as a transparent solid polymer can be held between the positive electrode 1 12 and the negative electrode 1 10 and held proximal to the electrically tunable solid state lens component 102. The material of the solid polymer electrolyte gate 108 can be, for example, poly (ethylene oxide )/LiC10 ₄ . Any other suitable dielectric material is within the scope of this discussion, for example, plasticized, gels, and rubbery micro/nano-composite polymer electrolytes. It is within the scope of this discussion that any material that provides Debye layers as described above may be used as the electrolyte gate.

[0020] The positive electrode 112 and the negative electrode 110 provide a variable electric field between the positive electrode and the negative electrode when a variable voltage 114 is applied across the positive electrode and the negative electrode. The positive electrode 1 12 and/or the negative electrode 110 can be substantially transparent. In another configuration, the positive electrode and/or the negative electrode may be small enough that the light need not pass through them and therefore, transparency is therefore the same as light bending around the electrode. FIG. 1 shows a substantially transparent substrate 1 16 that can be held proximal to one electrode, either the positive electrode 112 or the negative electrode 110. The configuration of the electrodes with respect to the electrically tunable solid state lens component, the substantially transparent solid polymer electrolyte gate, and the substrate of FIG. 1, is by way of example. Any suitable configuration is within the scope of this discussion.

[0021 ] The variable voltage 1 14 and the resultant variable electrical field in combination with the solid polymer electrolyte gate 108 can be sufficient to vary the optical path length by the lens component layer 102 or layers 102, 104 and/or 106. The response of the electrolyte gate 108, for example, a transparent solid polymer material, to a change in the electric field is sufficient to take advantage of the speed at which the OPL value exponentially changes with the change of the refractive index of the lens component layer 102 or layers 102, 104 and/or 106. The exponential change in the OPL of the lens component can be sufficient to respond to

substantially computational speeds.

[0022] In one example manner of fabricating the disclosed lens 100, a transparent bottom electrode (such as graphene or indium tin oxide (ITO)) can be deposited on a transparent substrate, of a material such as polydimethylsiloxane. Then ultra-thin two-dimensional (2D) materials and/or van der Waals heterostructures lens component can be fabricated in, for example, a manner described below. In one example, a polymer electrolyte layer 108 can be deposited by spin-coating and post-baking processes. A solid polymer electrolyte can include, for example, L1CIO4 and perchlorate/poly (ethylene oxide) (PEO) in the ratio 0.12: 1. A bottom electrolyte layer 108 can be deposited onto the polymer electrolyte layer 108. The top electrically-isolated transparent electrode 112 for each micro-lens can be patterned by thin-film deposition, photolithography and etching processes. The thickness of the electrolyte layer may be minimized to approximately 50 nm.

[0023] FIG. 2 depicts the disclosed lens array 200 including a plurality of lenses including at least one of the disclosed lenses. The array can include at least one of the described lens component 202 or a plurality of lens components 202, 204 and/or 206 held proximal to a described electrolyte gate 208. Such an array can be mechanically flexible and/or can be patterned onto curved surfaces. The negative electrode 210 is shown as a single electrode servicing all individual lens components 201a-e. A positive electrode can be as shown as well wherein individual positive electrodes 212a-e service each individual lens components 201a-e individually. It is within the scope of this discussion that the configuration of the positive and negative electrodes is any suitable configuration.

[0024] An array of lenses can be used for receiving or projecting light. As depicted in FIG. 2, in a light receiving application, an array of lenses can function, for example, as a compound eye. An image sensor 214a can be made, for example, of flexible graphene and transition metal dichalcogenides (TMD) transistor. Depending upon the intended use of the array, different types of image sensors may be utilised in the same array. For example, the array may be useful for both night vision where infrared light is detected and visible light detection.

[0025] Using, for example, a multi-aperture imaging system 220, instead of imaging through a single aperture, which transfers fractions of the full field of view through different optical channels, requires a single lens with a short focal length for each channel. With the architecture depicted in FIG. 2, the total track length of the multi-aperture setup can be reduced, for example, up to approximately 50% compared to single-aperture optics of the same resolution and sensitivity.

[0026] An array 200 for any suitable device can include a bus 222 that is connected to the positive electrodes 212a-e, where the bus is in communication with a CPU 224, data storage 226 and hardware 228 of the application. Individual controls for the individual disclosed lenses that make up an array 200, in combination with image sensors 214a-214e can be provided by the CPU 224, data storage 226 and hardware 228. Individual control of each lens and the flexibility of the array of lens 200 may be utilised in many applications. [0027] In an example method for fabricating a compound eye, an electrically focus-tunable lens can be based on the above-described two-dimensional (2D) materials and/or van der Waals heterostructures materials which can be patterned in one or more layers, 202, 204 and/or 206, using micro-/nano-fabrication techniques, which as mentioned, are described below. Optical lithography with backside alignment and metal thin film deposition can be used to pattern the diaphragm layer 220 on the backside of the target substrate 216a. Two-dimensional (2D) and van der Waals heterostructures materials held by an electrode can be transferred to another target substrate 216b. These two substrates 216a and 216b, with micro-lens/diaphragm arrays and image sensor arrays, respectively, can be bonded together with precise alignment. Since the polymer substrate 216a/b, atomically thin micro-lens and images sensors, are flexible, the whole unit can be curved to form the ultra-thin and flexible compound eyes, for a miniaturized optical system.

[0028] Flexible photonics generally refers to photonic devices fabricated on flexible polymer substrates and they can be mechanically deformed, such as bending, folding, rolling, twisting, stretching or compression, without substantially compromising their optical performance.

Providing mechanical flexibility to planar photonic structures such as display arrays, flexible arrays can be utilised in aberration-free cameras, wearable photonic textiles, foldable solar cells, optical systems integrated on curved surfaces or biological tissues, and so on. To achieve high flexibility, ultra-thin membrane layers (e.g. less than 10 nm thick and a bending radius less than 0.1 mm) can be fabricated from bulk materials, like Si and Ge, using conventional micro-/nano- machining technologies.

[0029] In one example method, fabrication of an atomically thin micro-lens array can be accomplished using micro-/nano-machining by Focused Ion Beam (FIB). A 10-micron radius MOSM micro-lens component can be formed using a FIB to shave off the layers atom by atom, until the layers have the dome shape of a lens. The disclosed micro-lens component can be fabricated in a FEI Helios 600 NanoLab FIB system (Gallium ions) using pre-calibrated dosage, optimized beam voltage (30 kV) and beam current (9.7 pA). The FIB etching technique can be used to fabricate micro-lens arrays from large-size few-layer TMDs.

[0030] Turning to FIG. 3, an example method of fabricating the disclosed lens component is depicted. In this example, fabrication of a micro-lens array by defect-assisted chemical vapour disposition (CVD) is shown. A thin layer of electron-beam resist can be spun on top of a piece of sapphire substrate, followed by a post baking process. Nano-hole arrays, with hole size in the range of approximately 10 nm, can be patterned on the resist layer, using electron beam lithography exposure and resist development processes (Fig. 3a). A thin layer (approximately 1 -3 nm thick) of, for example, M0O3 then can be deposited on top, using an electron beam evaporator (Fig. 3b). A lift off process can be used to remove the resist layer and an array of M0O3 nano-dots can be formed on the sapphire substrate (Fig. 3c). Then a high-temperature annealing process, at 600°C under Ar gas protection, can be used to convert M0O3 nano-dot array into M0O2 nano-dot array (Fig. 3d). Finally, a CVD growth process can be used to create the disclosed micro-lens array (Fig. 3e). For M0S2 micro-lens growth, the growth temperature can be controlled to be in the range of 800-840°C and the tube pressure level was controlled to be approximately l Pa. The growth can last 5-10 minutes, with the injection of sulfuric and M0O2 vapor.

[0031] FIGs. 4a-4c show optical microscope images of the growth by chemical vapour disposition of a disclosed M0S2 lens component starting from a nanoparticle dot and data pertaining to the same. FIG. 4a shows an optical microscope image of the growth of a disclosed M0S2 lens component starting from a nanoparticle dot by CVD. FIG. 4b shows a phase shifting interferometer (PSI) image of a CVD grown lens component. FIG. 4c is a graph with the scan distance in μιη in the horizontal axis and the optical path length in nanometers in the vertical axis. The graph shows a measured OPL profile versus position along the dashed line.

Depending upon the material or materials of the lens component, it can be transparent to visible and/or near infrared photons. The other components of the disclosed lens may also be transparent to visible and/or near infrared photons.

[0032] In the CVD growth, the defect sites (e.g. nanoparticle regions) can produce a thicker TMD layer than the flat substrate since the defect sites provide efficient nucleation centres for the growth. Raman spectroscopy can be used to confirm that those dome-shaped structures are TMD semiconductors. The measured OPL profile (FIG. 4b & c) shows that dome-shaped TMD can be utilised as the disclosed lens component. In other examples, Bragg-type grating shaped and meta-surface shaped lens components may be formed from the material layers. Any lens component form configuration is within the scope of this discussion.

[0033] The lens component can include van der Waals heterostructures layers which are made by stacking different 2D crystals on top of each other. The creation of a van der Waals heterostructure represents an artificial material assembled in a chosen sequence with blocks defined with one-atomic-plane precision, which is similar to building with Lego®. Van-der- Waals-like forces are sufficient to keep the stack together.

[0034] FIGs. 5a-5c show experimental results of the micro-lens fabricated using molybdenum disulphide (M0S2). FIG. 5a is a graph with the layer number in the horizontal axis and the optical path length in nm in the vertical direction where measured statistical data of OPL values from PSl for M0S2 and graphene samples. The inset is the schematic showing the PSl measured phase shifts of the reflected light from the M0S2 flake (φ _Μο5„ ) and the S1O2 substrate (φ ₅ . _0„ ).

[0035] FIG. 5b is a PSl image of a 6.3 nm thick M0S2 micro-lens. Inset is schematic of the micro-lens structure. The OPL value of a single layer 2D material could be in the range of 1 nm to 50 nm, depending on the refractive index the material. According to the experiments carried out by the inventor(s), it was discovered that single layers of molybdenum disulphide (M0S2), 0.7 nm thick, have giant optical path length (OPL) of 38 nm. For conventional bulk thin films, the OPL is linearly proportional to the refractive index n of the material (OPL = n* thickness). However, the inventor(s) found that in atomically thin films, the OPL value increases

exponentially with the increase of the refractive index, in great contrast to the bulk thin films. This giant OPL from M0S2 is created by relatively strong multiple reflections at the air-MoS2 and M0S2-S1O2 interfaces, appearing to a light beam to be 50 times thicker. Using such giant OPL to engineer the phase front of optical beams, the inventor(s) demonstrated the capability of manipulating the propagation of light using and controlling atomically thin high-index 2D materials. FIG. 5c is a measured lensing effect from the M0S2 micro-lens intensity distribution pattern of the M0S2 micro-lens measured by a scanning optical microscope (SOM).

[0036] Additional disclosure is provided in Figures 6A - 6C as follows.

[0037] An OPL of a single-layer MoS2 may be significantly modulated by an external electric field. Mechanical exfoliation was used to transfer a bi-layer MoS2 onto a SiO2/Si substrate (275 nm thermal oxide on n+-doped silicon). The MoS2 was placed near a gold electrode that was pre-patterned on the substrate. Another thick graphite flake was similarly transferred to electrically bridge the phosphorene flake and the gold electrode, forming a metal-oxide- semiconductor (MOS) device as shown in Fig 6A. In the proceeding measurements, the gold electrode was grounded, and the n+-doped Si substrate functions as a back gate to provide the external electric field. The measured OPL value of a single-layer MoS2 is highly dependent on the applied electric field as shown in Fig 6B. These observations match well with the theoretical prediction that the refractive index of MoS2 is sensitive to the applied electric field. The highly tunable OPL in MoS2 by electric field will enable the electrically tunable focal length of the ultra-thin micro-lens based on atomically thin semiconductors.

[0038] A MOS structure may be fabricated with independent and electrically tunable focus length for each of the 2D micro-lenses as shown in Figure 6C. The embedded transparent electrode, such as graphene or indium tin oxide (ITO), may be deposited onto a flexible polymer substrate, followed by the deposition of a high-k dielectric material Η1Ό2 on top by atomic layer deposition (ALD). After the 2D micro-lens fabrication, top electrodes may patterned by conventional optical lithography and metal deposition processes. The electric field applied may modulate the refractive index of the 2D materials, leading to the modulation of the focal length for the 2D micro-lens. Also, the integration of the uniform ALD high-k dielectric layer may offer large electric fields across the dielectric layer, enabling large range of focal length modulation for the 2D micro-lens.

[0039] As discussed above, the disclosed focus-tunable lens can be incorporated into a variety of products including for example, bimodal fingerprint capturing device, a 3D scanner, adaptive eyeglasses, a 3D light-sheet microscope and a tiny cameras for machine vision. The array of lenses can be incorporated in a variety of products including for example, a depth-tunable light- field 4D camera, a 3D display, a flexible 3D display and as a 3D camera, wherein capturing 3D images and videos is for the purposes of augmented (AR) and virtual (VR) reality visualization.