Electromagnetic wave modulator

Abstract

An electromagnetic wave modulator, including: an electrically conductive base layer; a stack of layers disposed on and in direct contact with at least a portion of the base layer, wherein the stack of layers includes a plurality of layers of respective materials, including: (i) two or more two-dimensional (2D) material layers, at least one of the 2D material layers being a layer of graphene; or (ii) a 2D material layer and a bulk material layer sandwiched between and in direct contact with the 2D material layer and the base layer, the 2D material layer being a layer of graphene; wherein the stack of layers and the base layer are patterned with an array of openings therein to define a metasurface, the metasurface being configured such that interaction of an electromagnetic wave with the metasurface modulates at least one property of the electromagnetic wave in accordance with a voltage applied across the or a corresponding one of the 2D material layers or between corresponding ones of the 2D material layers.

Description

[0001] ELECTROMAGNETIC WAVE MODULATOR

[0002] TECHNICAL FIELD

[0003] The present invention relates to electromagnetic wave modulation, and in particular to a modulator for controllably modulating at least one property of electromagnetic waves, and methods for producing the modulator.

[0004] BACKGROUND

[0005] The ability to controllably modulate electromagnetic waves is important for many emerging applications, including target tracking, communications, sensing and imaging, to name but a few. For example, many research groups around the world are exploring ways to develop modulators for electromagnetic waves at terahertz (THz) frequencies.

[0006] Terahertz (THz) radiation refers to electromagnetic radiation with frequencies within the range of 0.1-10 THz. The THz region has gained significant interest in recent years as next generation (6G) wireless communications push carrier frequencies towards the millimetre wave (mmW) and THz bands. Owing to its ultra-high bandwidth, data rates aboveterabits per second are achievable for THz waves. THz technologies are expected to drive this push into 6G communications within the current decade. However, this generally requires THz devices to be compact, reconfigurable, programmable, and ultrafast. However, THz technologies are currently hampered by a lack of materials and device architectures that can perform beyond current electronics and photonics technologies in the so called 'THz Gap'. As such, new materials and architectures are highly sought after to empower THz technologies to realise ultrafast charge dynamics, reconfigurability and tuneability, enabling practical opto-electronics at THz frequencies.

[0007] It is desired to address or alleviate one or more disadvantages or limitations of the prior art, or to at least provide a useful alternative. SUMMARY

[0008] In accordance with some embodiments of the present invention, there is provided an electromagnetic wave modulator, including: an electrically conductive base layer; a stack of layers disposed on and in direct contact with at least a portion of the base layer, wherein the stack of layers includes a plurality of layers of respective materials, including:

[0009] (i) two or more two-dimensional (2D) material layers, at least one of the 2D material layers being a layer of graphene; or

[0010] (ii) a 2D material layer and a bulk material layer sandwiched between and in direct contact with the 2D material layer and the base layer, the 2D material layer being a layer of graphene; wherein the stack of layers and the base layer are patterned with an array of openings therein to define a metasurface, the metasurface being configured such that interaction of an electromagnetic wave with the metasurface modulates at least one property of the electromagnetic wave in accordance with a voltage applied across the or a corresponding one of the 2D material layers or between corresponding ones of the 2D material layers.

[0011] In some embodiments, the 2D material layers include one or more layers of hexagonal boron nitride.

[0012] In some embodiments, the 2D material layers include one or more layers of one or more transition metal dichalcogenides.

[0013] In some embodiments, the 2D material layers include one or more layers of graphene and one or more layers of a dielectric or semiconductor 2D material, wherein at least one of the one or more dielectric or semiconductor layers is in direct contact with at least one of the one or more layers of graphene.

[0014] In some embodiments, the bulk material layer is a dielectric layer. In some embodiments, the bulk material layer is a layer of SiO2. In some embodiments, the stack includes at least two dielectric layers and a layer of graphene sandwiched between and in direct contact with the two dielectric layers.

[0015] In some embodiments, the metasurface is configured to modulate a polarisation property of the electromagnetic wave. In some embodiments, the metasurface is configured to modulate a phase of the electromagnetic wave. In some embodiments, the metasurface is configured to modulate an amplitude of the electromagnetic wave.

[0016] In some embodiments, the base layer is further patterned to define a pair of electrical contacts, the electrical contacts being electrically coupled to one or more of the 2D material layers such that a voltage applied between the electrical contacts provides the voltage applied across the corresponding 2D material layer or between the corresponding 2D material layers.

[0017] In some embodiments, the modulator further includes a support layer having front and rear surfaces, and an electrically conductive rear layer disposed on the rear surface of the support layer such that the electromagnetic wave is reflected as an output electromagnetic wave with the at least one modulated property.

[0018] In some embodiments, the metasurface is configured to modulate the at least one property of electromagnetic waves having any frequency up to at least 2 Terahertz. In some embodiments, the metasurface is configured to modulate the at least one property of electromagnetic waves having any frequency up to 4 Terahertz.

[0019] In some embodiments, the stack of layers includes one or more bulk material layers in addition to the 2D material layers.

[0020] In accordance with some embodiments of the present invention, there is provided a method of producing an electromagnetic wave modulator, the method including the steps of: forming a stack of layers on at least a portion of an electrically conductive base layer, wherein the stack of layers includes a plurality of layers of respective materials, including:

[0021] (i) two or more two-dimensional (2D) material layers, at least one of the 2D material layers being a layer of graphene; or

[0022] (ii) a 2D material layer and a bulk material layer sandwiched between and in direct contact with the 2D material layer and the base layer, the 2D material layer being a layer of graphene; and forming an array of openings through the stack of layers and the base layer to define a metasurface configured such that interaction of an electromagnetic wave with the metasurface modulates at least one property of the electromagnetic wave in accordance with a voltage applied across a corresponding one of the 2D material layers or between corresponding ones of the 2D material layers.

[0023] In some embodiments, the 2D material layers include one or more layers of hexagonal boron nitride.

[0024] In some embodiments, the 2D material layers include one or more layers of one or more transition metal dichalcogenides.

[0025] In some embodiments, the 2D material layers include one or more layers of graphene and one or more layers of a dielectric or semiconductor 2D material, wherein at least one of the one or more dielectric or semiconductor layers is in direct contact with at least one of the one or more layers of graphene.

[0026] In some embodiments, the at least one property includes a polarisation property.

[0027] In some embodiments, the metasurface is configured to modulate the at least one property of electromagnetic waves having any frequency up to at least 2 Terahertz. In some embodiments, the metasurface is configured to modulate the at least one property of electromagnetic waves having any frequency up to 4 Terahertz.

[0028] In some embodiments, the metasurface is configured to modulate the at least one property of electromagnetic waves having any frequency from 0 to at least 2 Terahertz. the stack of layers includes one or more bulk material layers. One or more of the bulk material layers may be dielectric layers. One or more of the dielectric layers may be composed of SiO2.

[0029] In some embodiments, the metasurface is configured to modulate the at least one property of electromagnetic waves having any frequency from 0 to at least 2 Terahertz, the array of openings is formed by etching through the stack of layers and the base layer using pulsed ion milling while the stack of layers and the base layer is thermally coupled to a heat sink.

[0030] Also described herein is an electromagnetic wave modulator, including: an electrically conductive base layer; a stack of layers disposed on at least a portion of the base layer, wherein the stack of layers includes a plurality of layers of respective two-dimensional (2D) materials; wherein the stack of layers and the base layer are patterned with an array of openings therein to define a metasurface, the metasurface being configured such that interaction of an electromagnetic wave with the metasurface modulates at least one property of the electromagnetic wave in accordance with a voltage applied across a corresponding one of the 2D material layers or between corresponding ones of the 2D material layers. The electromagnetic wave may be a THz electromagnetic wave.

[0031] Also described herein is a method of producing an electromagnetic wave modulator, the method including the steps of: forming a stack of layers on at least a portion of an electrically conductive base layer, wherein the stack of layers includes a plurality of layers of respective two-dimensional (2D) materials; defining a metasurface by forming an array of openings through the stack of layers and the base layer, the metasurface being configured such that interaction of an electromagnetic wave with the metasurface modulates at least one property of the electromagnetic wave in accordance with a voltage applied across a corresponding one of the 2D material layers or between corresponding ones of the 2D material layers. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] One or more embodiments of the present invention are hereinafter described, by way of example only, with reference to the accompanying drawings, in which:

[0033] Figure 1 is a schematic cross-sectional side view of an electromagnetic wave modulator in accordance with an embodiment of the present invention;

[0034] Figure 2 is a flow diagram of a method for producing an electromagnetic wave modulator, in accordance with an embodiment of the present invention;

[0035] Figure 3 is a schematic diagram of a measurement apparatus for measuring the performance of the modulators described herein;

[0036] Figures 4 to 6 illustrate the effect of h-BN on polarisation modulation of a standalone layer or stack without a metasurface;

[0037] Figures 4 and 5 are graphs of time domain measurements of respectively a standalone graphene film (Figure 4) and a standalone stack consisting of a graphene film in direct contact with an h-BN film (Figure 5), with insets showing the corresponding power spectra, and for applied voltages of OV (solid line) and 24V (dashed line);

[0038] Figure 6 is a graph showing the polarisation tuneability (represented as the ratio of reflected power for applied voltages of 15V and OV) as a function of frequency for the standalone graphene film (solid line) and the standalone h-BN |graphene stack (dashed line);

[0039] Figure 7 is a schematic diagram illustrating successive stages during the production of electromagnetic wave modulators in accordance with some embodiments of the present invention;

[0040] Figure 8 is a plan view optical micrograph of a modulator configured to modulate a polarisation property of electromagnetic waves, showing the repeating metasurface pattern etched through the stack of layers and front contact layer to the underlying support;

[0041] Figure 9 shows the configuration and parameters of one element of the metasurface pattern of Figure 8;

[0042] Figures 10 and 11 show corresponding spatial distributions of simulated current for the metasurfaces of Figures 8 and 9, for: (i) the gold contact layer only (Figure 10), and (ii) the gold contact layer with a stack consisting of a graphene layer (with a conductivity of 246 mS) and an h-BN layer (Figure 11); Figure 12 shows simulated properties of a polarisation modulator with only a graphene layer on the gold contact layer, patterned with the metasurface configuration of Figures 8 and 9; the graphs show respective polarisation properties of the modulator as a function of electromagnetic wave frequency for graphene conductivities of 70 mS (corresponding to an applied voltage of 0V) and 246 mS (selected to be representative of an applied voltage of ~ 10 V), specifically: amplitudes of reflected polarisation components Ex and Ey (top-left), phase difference (top-right), polarisation angle (bottom left), and ellipticity (bottom right);

[0043] Figure 13 shows measurements corresponding to the simulations of Figure 12, and for applied voltages of 0 (corresponding to 70 mS) and 15V (which increases the conductivity to some value expected to be ~ 200-300 mS);

[0044] Figure 14 shows measurements corresponding to those of Figure 13, but for a polarisation modulator with an intermediate h-BN layer between the graphene layer and the front contact layer 104;

[0045] Figures 15 and 16 are graphs of the tuning performance (calculated as the ratio of the corresponding polarisation property at 0V and 15V) of polarisation angle and ellipticity, respectively, as a function of electromagnetic wave frequency;

[0046] Figures 17 to 22 are respective graphs of the measured power spectra of polarisation modulators with respective stack configurations of: graphene on h-BN on graphene, graphene on SiO2 on graphene, graphene on SiO2, SiO2 on graphene, SiO2 on graphene on SiO2, and M0S2 on graphene;

[0047] Figure 23 is a plan view optical micrograph of an amplitude modulator in accordance with an embodiment of the present invention;

[0048] Figure 24 is a graph of the power spectrum of the amplitude modulator as a function of THz frequency for bias voltages of 0 and 22V; and

[0049] Figure 25 is a graph of measured DC conductivity of the amplitude modulator as a function of applied voltage. DETAILED DESCRIPTION

[0050] Recognising that effective control and manipulation of electromagnetic waves is vital for next generation wireless communication and sensing technologies, the inventors have developed the electromagnetic wave modulators described herein to enable at least one property of electromagnetic waves to be controllably modified or modulated in accordance with a control voltage applied to the modulators.

[0051] In work leading up to the invention, the inventors were exploring modifications of their existing device for absorbing electromagnetic radiation, which is described in International Patent Application Publication WO 2022 / 236380, "Device for Interacting with Electromagnetic Radiation" ("the absorber PCT application"). The device described in the absorber PCT application has a metasurface formed by openings through two layers respectively composed of graphene and gold, and the inventors were researching the effects of adding additional layers when they made the surprising discoveries that the 'tuneability' of the device is significantly improved when device is modified by adding a layer of a two-dimensional ("2D") material in direct contact with the graphene layer, and in particular, but not only, when the 2D material is hexagonal boron nitride (h-BN). The word 'tuneability' as used herein refers to the ability to control a property of electromagnetic waves (e.g., polarisation, phase, amplitude), specifically the available range of values of that property.

[0052] This result is particularly surprising because the device described in the absorber PCT has the layer of graphene in direct contact with the gold / metal in the metasurface, not only to provide an electrical connection to the device, but also to directly couple the graphene to the electromagnetic wave / resonance. It was expected that adding another material (and in particular, a dielectric material such as h-BN) between the metal and graphene layers would inhibit or even prevent this coupling. Additionally, the additional layer also presents a fabrication challenge for 2D materials such as h-BN, which as indicated above, are notoriously fragile and subject to cracking and delamination during fabrication. It will be apparent that these difficulties are exacerbated when multiple layers of 2D materials are used to form the stack.

[0053] In the prior art, such devices were designed so that the electromagnetic field is highly confined or guided by metallic regions / resonators, which was assumed to require that the graphene is in direct contact with these metal regions to interact with the electromagnetic wave. Consequently, introducing an intermediate layer between the graphene layer and the metal layer / regions would be considered deleterious because it would physically separate the graphene from the electromagnetic field that it is intended to tune / control.

[0054] As described below, the inventors also explored the effects of including multiple 2D material layers, including graphene layers and / or multiple dielectric layers, and / or layers of other 2D materials, including transition metal dichalcogenides. In particular, the inventors determined that the provision of at least one layer of hexagonal boron nitride in direct contact with at least one layer of graphene enhances the ability to control or 'tune' a property of electromagnetic waves by expanding its available range, and without compromising other performance parameters or introducing parasitic absorption. In part, this is believed to result from the structural compatibility of h-BN and graphene, as described below.

[0055] A particular challenge that the inventors had to address was whether it would even be possible to form openings through a stack of layers with multiple layers of 2D materials, which are notoriously fragile and susceptible to damage, without adversely affecting their electromagnetic properties. Prior to the invention, 2D materials have been etched using 'gentle' methods such as reactive ion etching and low energy plasma etching. However, etching through a stack with multiple layers of 2D materials requires a relatively large amount of material to be removed, and moreover some 2D materials are particularly difficult to etch, precluding the use of such 2D etching methods. For example, the inventors found that neither reactive ion etching nor low energy plasma etching was even able to etch through a single layer of h-BN in isolation, let alone through a stack of layers including one or more h-BN layers. In view of this difficulty, the inventors explored various alternative methos to etch through the stack of layers. As a result of this research, the inventors were surprised to discover that, with careful selection of etching conditions, an etching method known as ion milling is able to etch through layer stacks without damaging the 2D materials in the stack. Although ion milling is a known subtractive method for bulk materials, to the inventors' knowledge it has not previously been applied to 2D materials. The inventors believe this is because one would expect that it would unacceptably heat and damage the 2D materials. However, after considerable experimentation, they found that it is possible to avoid such damage during ion milling by milling the layers in short bursts or pulses rather than continuously, while also providing a good thermal path for the heat generated during milling by attaching the stack support layer to a water cooled heat sink with thermal paste, as described further below.

[0056] By way of background, graphene is a 2D allotrope of carbon first isolated in 2004. It has a unique band structure that provides desirable electrical properties, including at THz frequencies. In particular, graphene's valence and conduction bands meet at the vertices of the First Brillouin Zone at so-called Dirac points. Accordingly, the charge carriers in graphene follow the Dirac equation, having a linear dispersion curve and behaving as relativistic massless fermions with ultrafast carrier mobilities (exceeding 100,000 cm2V-1s-1at room temperature). These carrier dynamics allow ultrafast (THz) responses to electric fields, beyond the capabilities of current electronics. Moreover, the Fermi level and intraband electrical conductivity of graphene can be tuned by external electrostatic doping. This ability can be applied to create electrically tuneable and reconfigurable devices, which are presently widely lacking in the THz regime in particular.

[0057] The utility of graphene is improved when coupled with metamaterials. As known by those skilled in the art, metamaterials imitate the periodicity of a crystal lattice through subwavelength dielectric or metallic unit cells. Metamaterials provide exceptional control of the polarisation, amplitude, and phase of electromagnetic radiation, which in turn can be tuned by incorporating graphene. Tuneable metamaterial devices are required for high value emerging applications, including target tracking, communications, sensing and imaging. However, despite some promising results to-date, the tuneability and ultrafast response of graphene-metamaterial devices has been compromised by the low quality of available graphene films, and the difficulty of fabricating metamaterial devices with graphene films. Graphene used in these devices has been produced by chemical vapour deposition (CVD) on copper or nickel foils, which generates large area (cm scale) films. However, CVD graphene films are polycrystalline, and suffer from structural defects and charged dopants that limit the carrier mobility to the order of 1000 cm2V'1s-1. Further, reasonable device tuning has required high bias voltages or ionic gels, which are impractical for real-world applications. One approach to improve the quality of graphene is to use exfoliated flakes. While these provide a drastic improvement in electrical performance, graphene flakes are limited to micron-scale sizes and arbitrary geometries, which are not appropriate for meta material arrays, and cannot be produced or controlled at scale. Hexagonal Boron Nitride (h-BN) is a large bandgap (5.9 eV) material with lattice parameters that closely resemble graphene. The structure of h-BN is such that it has no dangling bonds or charge traps, making it an excellent substrate for 2D material optoelectronics. Atomically thin layers of h-BN can be used to 'pick up' graphene flakes through the van der Waals interaction, resulting in atomically clean interfaces. When encapsulated in h-BN, graphene can exhibit carrier mobilities of 140,000 cm2V'1s’1, an order of magnitude higher than those of graphene on SiO2.

[0058] Figure 1 is a schematic cross-sectional side view of an electromagnetic wave modulator 100 in accordance with some embodiments of the present invention. The modulator 100 includes an electrically insulating support layer 102 between front and rear (or upper and lower as shown in Figure 1) electrically conductive contact layers 104, 106 disposed on the front and rear surfaces of the support layer 102, although in some embodiments the rear contact layer 106 is omitted. The front (or upper) contact layer 104 is divided into two mutually spaced portions defining respective front electrical contacts 108, 110 to the modulator 100. In the illustrated embodiment, the rear contact layer 106 is included, and acts as both a reflective plate and as an RF (radio frequency) ground plane. Although the illustrated modulator 100 operates in reflection, in other embodiments without the rear contact layer 106, the modulator can operate in transmission.

[0059] In the described embodiments, the support layer 102 is a thin (® 300p.m) and electrically insulating silicon wafer, but it will be apparent to those skilled in the art that other materials (e.g. sapphire) and / or forms may be used as the support layer 102 in other embodiments. In the described embodiments, the front and rear contact layers 104, 106 are composed of gold, selected both for convenience and because it provides a large and high-quality resonance. However, it will be apparent to those skilled in the art that other materials can be used instead of gold in other embodiments, provided that they are electrically conductive.

[0060] A stack of layers 112 including layers of respective two-dimensional (2D) materials is disposed on a portion of the front contact layer 104, bridging the gap between the two front contacts 108, 110. Each of these layers of 2D materials is extremely thin, generally only one or more monolayers. However, in some embodiments, the stack of layers 112 also includes one or more bulk material layers. Although the embodiment illustrated in Figure 1 has a stack 112 of six layers, in general the stack 112 can include any practical number of layers greater than one, including at least two layers of respective 2D materials.

[0061] In some embodiments, the layers of 2D materials includes one or more graphene layers and one or more dielectric layers, at least one of the dielectric layers being in direct contact with at least one of the graphene layers. In some embodiments, the stack 112 includes a graphene layer sandwiched between two dielectric layers so that the graphene layer is in direct contact with both dielectric layers, and may be encapsulated by them.

[0062] In addition to graphene, the stack 112 can include one or more other 2D materials. For example, in some embodiments, the stack 112 includes one or more layers composed of one or more transition metal dichalcogenides selected to provide a desired optical bandgap and enable optical tuning or switching. Transition metal dichalcogenides ("TMDC") have a molecular formula of MX2, where M is a transition metal (e.g., Mo or W) and X is a chalcogen (e.g., Se, S, or Te). As described above, in some embodiments the stack 112 can also include one or more bulk material layers.

[0063] The stack of layers 112 and the front contact layer 104 are patterned to define an array of openings 114 therein, extending through the stack of layers 112 and the front contact layer 104 ( / .e., through at least three layers in total) down to the support layer 102, thereby forming a metasurface. The metasurface and the stack of layers 112 are configured such that interaction of an electromagnetic wave with the metasurface modulates at least one property of the electromagnetic wave. That is, the at least one property (e.g., polarisation, phase, and / or amplitude) is changed by the interaction. In some embodiments, an electromagnetic wave incident upon the metasurface is reflected as an output electromagnetic wave with the at least one modulated property. Moreover, the modulation of the at least one property can be controlled by applying a voltage V between the electrical contacts 108, 110, as shown, and which in the embodiment of Figure 1 are also electrically connected to one or more graphene layers of the stack 112. Depending upon the configuration of the stack of layers 112 and their electrical connections to the front contacts 108, 110, the voltage applied between the contacts 108, 110 is applied across a corresponding 2D material (e.g., graphene) layer or between corresponding 2D material layers of the stack 112. The at least one property of the electromagnetic wave that is modulated is determined by the metasurface configuration, which is defined by the stack of layers 112 and the two-dimensional pattern that defines the plan-view arrangement of the openings through the stack of layers 112 and the front contact layer 104.

[0064] In some embodiments, the metasurface is configured to modulate a polarisation property of the electromagnetic wave. In some embodiments, the metasurface is configured to modulate the phase of the electromagnetic wave, and in some embodiments, the metasurface is configured to modulate the amplitude of the electromagnetic wave.

[0065] For example, in some embodiments described below, the two-dimensional pattern used to form the metasurface is a two-dimensional periodic array of repeating elements, each element being in the form of a pair of concentric annuli, as shown in Figure 9, with a break in each annulus so that it is incomplete or partial, and the breaks in the annuli being rotationally misaligned by 180° relative to each other. A two-dimensional array of instances of this element is created as openings through the complete stack of layers 112 and the front contact layer 104 to form a three-dimensional metasurface structure. Interaction of an electromagnetic wave with this metasurface configuration has the effect of modulating a polarisation property of the electromagnetic wave. In some embodiments, the two-dimensional pattern used to form the metasurface is again a two-dimensional periodic array of repeating elements, but each element has a shape known as a "cross potent" or "crutch cross", as shown in Figure 22. The resulting metasurface modulates the amplitude of electromagnetic waves interacting with it.

[0066] However, it will be apparent to those skilled in the art that the two-dimensional metasurface pattern may take other forms in other embodiments. In general, the metasurface pattern can be designed using metasurface design principles known to those skilled in the art, such as those described in H. Wang, J. Linghu, X. Wang, Q. Zhao, and H. Shen, "Angular-Dependent THz Modulator with Hybrid Metal-Graphene Metastructures ," (in English), Nanomaterials, vol. 13, no. 13, p. 1914, 2023 2023, doi: .https : / ZdpLorg / lQ.3390 / panol3131914, H. Park et al., "Electrically tunable THz graphene metasurface wave retarders," vol. 12, no. 13, pp. 2553-2562, 2023, doi: doi: 10.1515 / nanoph-2022-0812, and Q. S. Li et al., "Gate-tuned graphene metadevices for dynamically controlling terahertz wavefronts," NANOPHOTONICS, vol. 11, no. 9, pp. 2085-2096, MAY 11 2022, doi: 10.1515 / nanoph-2021-0801, for example. As described above, in some embodiments the stack of layers 112 includes at least one layer of h-BN in direct contact with a graphene layer. Surprisingly, the inventors found that this use of h-BN, a 2D dielectric material, in the metasurface stack 112 provides an immediate and significant improvement to the tuneability of graphene-based electromagnetic wave modulators, as described further below. The inventors believe there are two reasons for these improvements. Firstly, the similarity in crystal structure of h-BN and graphene results in a more optimised graphene transfer. The graphene films were observed to adhere much better to h-BN films than to gold or bare silicon substrates, presumably due to the strong van der Waals interaction between h-BN and graphene. This reduces both wrinkling and water retention beneath the graphene, both of which contribute to the degradation of electrical performance. This results in an improved production process, with more uniform spin coated films, more reliable and predictable photolithography, and ultimately better device performance.

[0067] Secondly, h-BN has lattice parameters that are similar to graphene, and an atomically smooth surface free of dangling bonds and charge traps. In devices incorporating exfoliated flakes of graphene, this translates to reduced roughness, lower intrinsic doping and chemical reactivity, and consequently better modulator performance. Also, this performance gain appears to scale to large area devices incorporating CVD thin films of graphene on h-BN, as shown for unpatterned graphene and polarisation converters as described above, although with the CVD process inherently leading to polycrystalline films and dopants, it is expected that this enhancement is less than that observed for exfoliated flakes.

[0068] Nonetheless, the tuneable electromagnetic wave modulators described herein demonstrate benchmark tuning performance at practically low applied voltages. For example, the ellipticity tuning of 0.73 of the polarisation modulator described in the Examples below is more than twice the maximum tuning of 0.31 reported in the literature (which is similar to the reference modulator without the h-BN layer, also described below). Further, the polarisation tuning of the described modulator is achieved at an order of magnitude lower voltage than prior art reports. Although one recent publication reported a similar ellipticity tuning of 0.69, it required an ion gel, which inherently involves a trade-off between graphene tuning and device speed. The inventors have found that the use of h-BN rather than ion gels completely avoids this trade-off, and that h-BN globally improves the electrical response of graphene. Although CVD h-BN has been used as described herein to improve the available tuning range of graphene-based electromagnetic wave modulators, the inventors recognise that it could also be used as a gating dielectric or substrate material, and that a wide variety of CVD graphene-based tuneable devices could immediately benefit from using a CVD h-BN film in direct contact with graphene to enhance their electrical tuning performance. This can be achieved using the transfer processes described below for both h-BN films and graphene films, without compromising other aspects of device electrical performance (unlike ion gels).

[0069] The modulators described herein can be fabricated by a modulator production process such as that shown in the flow diagram of Figure 2, using standard processing steps known to those skilled in the art. Starting with a suitable dielectric support layer or substrate (such as a high resistivity (e.g., undoped float-zone) silicon wafer), at step 202 the rear contact layer 106 is formed on the rear (lower in Figure 1) surface of the support layer 102 at step 202 by a physical deposition process such as radio-frequency (RF) sputtering or thermal evaporation, for example. However, this step is of course omitted in embodiments without a rear contact layer 106.

[0070] At step 204, the front contacts 108, 110 are formed on the front surface of the support layer 102. In the described embodiments, this involves using standard lithography methods known to those skilled in the art to form a photoresist layer over the front surface of the support layer 102, patterning the photoresist layer to define the front contact regions, and then depositing the front contact layer 104 (composed of gold in the described embodiments) over the patterned photoresist so that the gold contacts the support layer 102 only in the front contact regions. Lift-off is used to remove the other portion of the front contact layer 104. Alternatively, the gold can be deposited through openings in a hard mask to avoid the need for lithography and lift-off.

[0071] At step 206, the layers of the stack of layers 112 are sequentially deposited over the front contact layer 104. The specific method by which each layer is deposited depends upon the composition of that layer. In the described embodiments, any layers of h-BN, graphene, and transition metal dichalcogenides are formed ex-situ by chemical vapour deposition (CVD) and then wet-transferred to form the stack 112. However, other growth and / or transfer methods known to those skilled in the art may be used in other embodiments, and for other materials. In the described embodiments, electrical connections between the front contacts 108, 110 and one or more graphene layers of the stack 112 are achieved by depositing the one or more graphene layers so that each end of a graphene layer or opposite ends of respective graphene layers extend laterally beyond other layers of the stack 112 to contact the front contacts 108, 110, respectively, and making electrical connections thereto. However, in other embodiments the electrical connections may be made in other ways known to those skilled in the art.

[0072] After the stack of layers 112 has been formed, at step 208 the stack 112 and the underlying front contact layer 104 are patterned to form the metasurface. This involves forming an array of openings through the stack 112 and underlying front contact layer 104, down to the underlying support layer 102. As will be apparent to those skilled in the art, this involves selected area etching of corresponding regions of the stack 112 and underlying front contact layer 104 until the etched regions extend completely though the stack 112 and contact layer 104 down to the support layer 102.

[0073] As described above, a particular challenge is how to etch through all of these layers without damaging the 2D materials in the stack 112. In the described embodiments, the selected area etching is performed by ion beam milling, although it might be possible that other etching methods known to those skilled in the art could be used in other embodiments, depending on the specific materials in the stack 112. In the described embodiments, the selected area etching is achieved by forming an etch mask on the stack 112 with openings corresponding to the metasurface pattern. The regions of the stack 112 exposed though the mask openings are then etched by Ar ion beam milling at an Ar gas pressure of 9 x 10'3mbar, and an ion gun voltage of 500V, 100V acceleration voltage, 40V cathode voltage, and 4A neutraliser current, producing a beam current of 10 mA at the stack 112. To reduce heating of the 2D materials, the ion beam is operated in a pulsed mode. In the described embodiments, the durations of each ion beam pulse and the cool down period between successive pulses were both 60 seconds. The stack 112 is also thermally coupled to a water cooled metal heat sink with a thermally conductive paste. This combination of pulsed ion beam milling and heat sinking is sufficient to prevent damage to the 2D materials in the stack. However, it will be apparent to those skilled in the art that these parameters depend upon a range of factors, and different values may be sufficient in other embodiments. EXAMPLES

[0074] In the following examples, the performance of various modulators was simulated and / or measured. Full wave simulations were performed using the Frequency Domain Solver of the commercially available 3D EM analysis software package CST Studio Suite 2022.

[0075] As shown in Figure 3, THz measurements were performed with a Batop / Toptica hybrid time domain spectroscopy (TDS) system in reflection geometry. Photoconductive antennas (PCA) 302, 304 were used for THz generation and photodetection. The emitter PCA 302 was mounted onto a mechanical stage 306 to act as a delay line. The THz beam 308 was directed by off axis paraboloid mirrors (OAPM) 310, 312. THz radiation generated by the emitter PCA 302 is elliptically polarised, but heavily favours the axis parallel to the optical bench. As such, a wire grid polariser 314 was situated between the emitter PCA 302 and the first OAPM 310, with the grids orthogonal to the optical bench to ensure maximum transmission incident on the sample. Each modulator under test 316 was mounted at the focal position of the two OAPMs 310, 312, and was aligned to the emitter 302, 304 and detector before placement of the polarisers 314, 318. Due to the polarisation sensitivity of the detector 304 (favouring polarisation parallel to the optical bench also), this detector was mounted in a rotatable stage 320. Thus, a reference measurement was used to align the detector maximum signal with the second polariser 318. Throughout the measurements, the second polariser orientation and detector rotation were synchronised to ensure maximum signal and appropriate measurement of the polarisation components.

[0076] Effects of adding an h-BN layer to a standalone graphene layer

[0077] As described above, in work leading up to the invention the inventors were researching the effects of including different materials and additional layers. Part of this research involved measuring the effects of adding different materials to standalone graphene layers ( / .e., without any other layers or underlying gold layer or metasurface).

[0078] Figure 4 shows the measured polarisation modulation performance of a standalone graphene layer, namely the raw time-domain spectrum for the graphene layer alone, and for voltages of 0 V (solid line) and 24V DC (dashed line) applied across the graphene layer, with the corresponding power spectra shown in the inset graph. Figure 5 is the same as Figure 4, but where a dielectric layer composed of CVD h-BN is disposed on the graphene layer, with the graphene layers for Figures 4 and 5 obtained from adjacent sections of a Nickel foil to provide as direct a comparison as practicably feasible. A comparison of Figures 4 and 5 illustrates the dramatic improvement of the available range of output polarisations when the layer of h-BN is added. Clearly, the tuneability of polarisation conversion / modulation is vastly improved when a layer h-BN is added in direct contact with the graphene layer.

[0079] The inventors believe that a commercial need exists for optoelectronic modulators capable of modulating electromagnetic signals with frequencies up to 2 THz. Figure 6 is a graph of the ratio of the 24V and OV measurements for the standalone graphene layer (solid line) and the graphene / h-BN stack (dashed line), demonstrating that the addition of the h-BN layer improves the graphene tuning dramatically, by 20-40dB over the 0.2 - 4 THz range, twice the commercial target frequency of 2 THz. It should be noted that 0.2 THz is the lower limit of the measurement system, and other measurements show that effective modulation extends down to 0 THz, i.e. DC. Standalone graphene exhibits at best a lOdB modulation across the measured range, whereas the graphene / h-BN stack 112 provides a modulation ranging from 30dB to 50dB over the same frequency range. It is apparent that the inclusion of a CVD h-BN film significantly improves the tuning characteristics of a CVD graphene film. This enhancement translates into better functionality of bespoke THz electronics devices, including the polarisation converters described herein.

[0080] The simplest form of modulator described herein has a stack of only two layers 112 respectively composed of graphene and another material. If the other material is not a 2D material, such as SiO2, then it is in direct contact with the underlying front contact layer 104, which in the described embodiments is composed of gold. If the other layer is a 2D material, then it can be on either side of the graphene layer. In a first example, one such modulator was produced as described above to act as a polarisation modulator, and using h-BN as the other 2D material.

[0081] The polarisation modulator was produced by the process shown in Figure 2, with a 285 pm thick undoped float zone silicon wafer providing the support layer / substrate 102. As illustrated in Figure 7, the rear contact layer 106 was deposited by RF sputtering gold to a thickness of 220nm. On the front side, standard photolithography (involving spin coating of a 1.8 pm layer of photoresist, UV exposure of selected regions of the photoresist through a chrome mask to replicate the mask pattern in the photoresist layer, followed by development) was used to form a photoresist mask 702 defining the regions for the front electrical contacts 108, 110. The front contact layer 104 was then deposited over the photoresist mask, again by sputtering gold to a thickness of 220nm.

[0082] Then, a multi-monolayer CVD h-BN film 704 was wet transferred to the front contact layer 104, with sufficient coverage to ensure contact with the subsequently deposited graphene layer over all functional areas of the polarisation converter, and extending between the front contacts 108, 110 with a small overlap. The layer of (multi-monolayer CVD) graphene 706 was then wet transferred on top of the h-BN layer, with a small overlap at each end so that the graphene 706 overlapped the ends of the h-BN layer to contact the front contacts 108, 110. For comparison purposes, a reference set of modulators was prepared as described above, but omitting the h-BN layer 704 completely.

[0083] As described above, both the h-BN layer(s) 704 and the graphene layer(s) 706 were deposited using a wet transfer method. The CVD h-BN and CVD graphene films are commercially available, and were obtained from the Graphene Supermarket (see https : / / www .qraphene-su perma rket om) . The h-BN and graphene films are provided on Cu and Ni foils, respectively, and were prepared for wet transfer as follows. First, each film was spin coated with a sacrificial PMMA layer. The Cu or Ni foil was then etched away in Nitric acid, leaving the film attached to the PMMA layer. The film / PMMA composite layer was then transferred (floated) on de-ionised water and then transferred to the incomplete converter structure, and allowed to dry for a minimum of 24 hours. Finally, the PMMA layer was removed using anisole, before drying with N2 gas. The graphene and h-BN layers remain bonded together by Van der Waals attraction.

[0084] A second photolithography step was then used to define a photoresist etch mask 708 with a pattern of openings exposing corresponding regions of the stack 112 of layers. Ion beam etching with Argon gas was then used to selectively remove exposed regions of the stack layers 112 and the front contact layer 104 down to the support layer 102, thereby forming the desired metasurface structure 710, as described above and as shown in the plan view micrograph of the resulting polarisation converter of Figure 8. After etching, the resulting structure was cleaned in acetone solvent, and dried with N2 gas. External electrical connections to the front contacts 108, 110 were made by attaching Al wires to the gold pads with conductive Ag epoxy. For comparison purposes, the same process but omitting the h-BN deposition step was used to produce an otherwise identical reference polarisation modulator, but without the h-BN layer, in order to assess the relative performance of polarisation modulators with, and without, the h-BN layer 704.

[0085] The performance of these polarisation modulators was simulated by way of full wave simulations carried out using the Frequency Domain Solver of CST Studio Suite 2022. The metasurface used in the examples described below was designed to resonate at 0.24 THz, and consists of a two-dimensional array of repeating elements. Figure 9 shows the configuration of each element, which consists of two anisotropic and incomplete slotted annuli or rings with dimensions of D=0.23 mm, w=0.02 mm, oi=50°, □2=30°, ri=0.06 mm, and r2=0.1 mm. The two incomplete annuli are split at the directions of -45° and +135° with respect to x-axis, respectively.

[0086] Illumination of the element shown in Figure 9 by a y-polarized incident wave of frequency 238.5 GHz was first simulated for the gold layer only. Due to the anisotropy of the element, the outgoing wave is rotated by 90° to a x-polarized wave, as represented by arrows in the simulated spatial distributions of surface currents shown in Figure 10. The addition of the graphene and h-BN layers to the element ( / .e., with the same pattern in each of the three layers) changes the surface conductivity corresponding to different graphene chemical potentials, thereby varying the surface currents and the induced electric fields, as shown in Figure 11 for a graphene conductivity of 246 mS.

[0087] Comparison of Figures 10 and 11 shows that the current density is reduced along the edges of the metasurface openings as the surface conductivity decreases, thereby dampening the ideal y-to-x polarization conversion functionality. Correspondingly, the radiated components of y-polarization and the converted x-polarization are modified with different amplitudes and phases, thus enabling polarization variations among linear, elliptical, and circular states for different graphene conductivities.

[0088] Reference Metasurface Polarisation Converter with Graphene as the only Layer on the Contact layer

[0089] Figure 12 is a set of graphs of the simulated components of x- and y- polarisation, rotation angle and ellipticity as a function of frequency for a "reference" polarisation modulator with only a graphene layer on the front contact layer 104, these layers being patterned with the metasurface configuration of Figures 8 and 9. Simulations were performed using two different values for the conductivity of the graphene layer: 70 mS (solid line) and 246 mS (dashed line). These conductivity values were selected to be representative of the expected conductivities of the graphene film for applied voltages of 0V and 15 V. The simulations demonstrate that the output polarisation response can be tuned by altering the chemical potential of the graphene layer of the metasurface structure. Experimentally, this tuning can be achieved by controlling the voltage applied across the front contacts 108, 110 of the polarisation converter, and the addition of the h-BN layer provides an improved graphene electrical response, and thus enables access to a larger range of the simulated conductivity tuning shown in Figure 12.

[0090] Figure 13 shows the measured tuning performance of the reference metasurface polarisation converter for applied voltages of 0 and 15 V, corresponding to the simulations of Figure 12. The graph at top left shows that the y-polarisation decreases with applied voltage with a corresponding increase in the x-polarisation. This confirms the tuneability of the modulator, and the graph at top right shows rotation of the reflected polarisation state with voltage applied to the graphene / gold bilayer at the resonant frequency of 0.242 THz. This is reflected in variations of both the polarisation angle (bottom left graph) and ellipticity (bottom right graph) of the reflected beam.

[0091] Metasurface Polarisation Modulator with h-BN Dielectric Layer

[0092] The graphs in Figure 14 correspond to those of Figure 13, but for the polarisation modulator with the h-BN layer disposed between the front contact layer 104 and the graphene layer, in accordance with an embodiment of the present invention.

[0093] As predicted from the data shown in Figures 4 to 6, the addition of the h-BN layer in direct contact with the graphene layer results in significantly improved tuning performance.

[0094] Comparing Figure 14 with Figure 13, the top-left graph shows an enhanced shift in the resonant frequency, as well as a larger change in the amplitude of the x- and y- reflected polarisation components.

[0095] Figures 15 and 16 are graphs showing the tuning of angle and ellipticity, respectively, as a function of THz frequency for the modulators with (solid line) and without (dashed line) the h-BN layer, and for an applied voltage range of 15V, normalised to the corresponding values at 0V. The graphs demonstrate that the tuning bandwidth is increased when the h-BN layer is included. Figure 15 shows that the modulator with h- BN achieves polarisation angle tuning typically above 30° across a relatively broad 0.23- 0.26 THz frequency range (not including at resonance), whereas the device with h-BN is only able to tune over a limited range, mostly below 10°. Similarly, The left-hand graph Figure 16 shows that the ellipticity can be tuned more than 0.7 across a 0.242 - 0.251 THz range for the modulator with h-BN, compared to only ~ 0.4 for the modulator without h-BN. Thus, adding the h-BN layer improves the tuneable range of both the polarisation angle and ellipticity by a factor of about 2.

[0096] The addition of the h-BN layer also produces a considerable improvement in the frequency tuning range. In Figure 13, the top-right graph of phase difference as a function of THz frequency shows that the frequency position of the linear-polarized beam (0°) is shifted by around 17-20 GHz either side of resonance (0.220 - 0.240 THz and 0.253 - 0.270 THz). By comparison, the modulator without h-BN (Figure 12) can only tune the angle by 3-5 GHz either side of resonance. This is further reflected in the bottom-left graphs of polarisation angle vs frequency in Figures 13 and 14. In Figure 14, the metasurface with h-BN produces a linearly polarised state along -50° at 0.250 THz with no applied voltage (solid line). This state is frequency dependent with voltage, moving to a lower 0.243 THz at 15V (dashed line), corresponding to a frequency shift or tuning of 7 GHz. Without the h-BN layer (Figure 12), this shift of the linearly polarised mode is reduced to only 1-2 GHz. This four-fold increase in frequency tuning resulting from the addition of h-BN demonstrates that the improvement in graphene's electrical response by directly adhering an h-BN layer to the graphene layer can be applied to produce improved frequency-tuneable THz modulators.

[0097] In another example, a modulator with a stack 112 of graphene on h-BN was produced as described above, but using a metasurface pattern defined by an array of repeating "cross potents" or "crutch crosses", as shown in the optical micrograph of Figure 23, configured to modulate the amplitude of electromagnetic waves.

[0098] Figure 24 is a graph of the power spectrum of the amplitude modulator as a function of electromagnetic wave frequency for applied voltages of 0 and 22 V, which shows that the resonance frequency shifts with bias voltage.

[0099] Figure 25 is a graph showing the measured DC conductivity of the amplitude modulator as a function of applied DC voltage. These measurements show that the conductivity of the stack increases linearly with voltage from 0 to about 24 VDC, above which it increases dramatically. The ability to control the conductivity of the graphene layer at voltages down to OV allows the modulation to be correspondingly controlled or tuned. At low frequencies (e.g., < 0.2 THz), the conductivity becomes the dominant factor, and consequently the data of Figure 25 confirms the ability of the modulators to control the modulation(s) down to low frequencies, and down to 0 THz or DC.

[0100] Although the modulators described above are relatively simple in structure as the stack of layers 112 includes only two layers, of graphene and h-BN, respectively, some embodiments include additional layers in the stack 112. In one example, a second h- BN layer is disposed on the graphene layer, so that the graphene layer is disposed between and in direct contact with h-BN layers on opposite sides of the graphene layer. In another example, a second graphene layer is included and the h-BN layer is sandwiched between and in direct contact with the two graphene layers. The power spectrum of this modulator is shown in Figure 17 for applied voltages of 0 V (solid line) and 14 V (dashed line).

[0101] Although the use of h-BN in direct contact with graphene is particularly beneficial, some embodiments do not include any h-BN layer at all. For example, the power spectrum of the same modulator but with SiO2 instead of h-BN as the intermediate dielectric layer, is shown in Figure 18. This form of the stack 112, with a dielectric layer (e.g., h-BN or SiOz) sandwiched between graphene layers, allows the graphene to act as its own electrodes and become self-biasing. A voltage applied directly between the two graphene layers enables tuning without additional contacts or metal layers outside the stack 112. In such embodiments, it will be apparent that the voltage could be directly applied across the graphene layers, rather than via the front contacts 108, 110, which can therefore be omitted.

[0102] Similarly, Figures 19 and 20 show respective power spectra of two stack layer modulators with SiO2 instead of h-BN as the dielectric, with the SiO2 layer under and over the graphene layer, respectively.

[0103] In another example, a second SiO2 layer is deposited on the graphene layer so that the graphene layer is sandwiched between and in direct contact with the two SiO2 layers, resulting in the power spectrum of Figure 21. The effective encapsulation of graphene between dielectric layers protects the graphene and also increases the electric field to improve tuning. Although SiO2 is not as effective as h-BN at enhancing tuneability, it has the advantage that it is not a 2D material and can thus be applied as a much thicker layer if required. Additionally, the deposition of SiC is far more well established than the CVD 2D material transfer used for h-BN films.

[0104] In some embodiments, the stack 112 includes only two layers as above, but with a layer of a transition metal dichalcogenide instead of h-BN as the other 2D material, and disposed over the graphene layer. In one embodiment, the transition metal dichalcogenide is M0S2, an n-type semiconductor 2D material, resulting in the power spectrum of Figure 22.

[0105] In some embodiments, the stack 112 includes a dielectric layer (e.g., h-BN or SiC ) sandwiched between graphene layers, allowing the graphene to act as its own electrodes and become self-biasing. A voltage applied directly between the two graphene layers enables tuning without additional contacts or metal layers outside the stack 112. In such embodiments, it will be apparent that the voltage can be directly applied across the graphene layers, rather than via the front contacts 108, 110, which can therefore be omitted.

[0106] The effective encapsulation of graphene between dielectric layers protects the graphene, and also increases the electric field to improve tuning.

[0107] Although SiO2 is not as effective as h-BN at enhancing tuneability, it has the advantage that it is not a 2D material and can thus be applied as a much thicker layer if required. Additionally, the deposition of SiO2 is far more well established than the CVD 2D material transfer used for h-BN films.

[0108] It will be apparent that other configurations of the stack 112 and other metasurface patterns may be used in other embodiments in light of this disclosure. Many modifications will be apparent to those skilled in the art without departing from the scope of the present invention.

Claims

1. CLAIMS:

1. An electromagnetic wave modulator, including: an electrically conductive base layer; a stack of layers disposed on and in direct contact with at least a portion of the base layer, wherein the stack of layers includes a plurality of layers of respective materials, including:(i) two or more two-dimensional (2D) material layers, at least one of the 2D material layers being a layer of graphene; or(ii) a 2D material layer and a bulk material layer sandwiched between and in direct contact with the 2D material layer and the base layer, the 2D material layer being a layer of graphene; wherein the stack of layers and the base layer are patterned with an array of openings therein to define a metasurface, the metasurface being configured such that interaction of an electromagnetic wave with the metasurface modulates at least one property of the electromagnetic wave in accordance with a voltage applied across the or a corresponding one of the 2D material layers or between corresponding ones of the 2D material layers.

2. The modulator of claim 1, wherein the 2D material layers include one or more layers of hexagonal boron nitride.

3. The modulator of any one of claim 1 or 2, wherein the 2D material layers include one or more layers of one or more transition metal dichalcogenides.

4. The modulator of any one of claims 1 to 3, wherein the 2D material layers include one or more layers of graphene and one or more layers of a dielectric or semiconductor 2D material, wherein at least one of the one or more dielectric or semiconductor layers is in direct contact with at least one of the one or more layers of graphene.

5. The modulator of claim 1, wherein the bulk material layer is a layer of SiC .

6. The modulator of any one of claims 1 to 5, wherein the stack includes at least two dielectric layers and a layer of graphene sandwiched between and in direct contact with the two dielectric layers.

7. The modulator of any one of claims 1 to 6, wherein the metasurface is configured to modulate a polarisation property of the electromagnetic wave.

8. The modulator of any one of claims 1 to 7, wherein the metasurface is configured to modulate a phase of the electromagnetic wave.

9. The modulator of any one of claims 1 to 8, wherein the metasurface is configured to modulate an amplitude of the electromagnetic wave.

10. The modulator of any one of claims 1 to 9, wherein the base layer is further patterned to define a pair of electrical contacts, the electrical contacts being electrically coupled to one or more of the 2D material layers such that a voltage applied between the electrical contacts provides the voltage applied across the corresponding 2D material layer or between the corresponding 2D material layers.

11. The modulator of any one of claims 1 to 10, wherein the modulator further includes a support layer having front and rear surfaces, and an electrically conductive rear layer disposed on the rear surface of the support layer such that the electromagnetic wave is reflected as an output electromagnetic wave with the at least one modulated property.

12. The modulator of any one of claims 1 to 11, wherein the metasurface is configured to modulate the at least one property of electromagnetic waves having any frequency up to at least 2 Terahertz, and optionally up to 4 Terahertz.

13. The modulator of any one of claims 1 to 12, wherein the stack of layers includes one or more bulk material layers in addition to the 2D material layers.

14. A method of producing an electromagnetic wave modulator, the method including the steps of: forming a stack of layers on at least a portion of an electrically conductive base layer, wherein the stack of layers includes a plurality of layers of respective materials, including:(i) two or more two-dimensional (2D) material layers, at least one of the 2D material layers being a layer of graphene; or(ii) a 2D material layer and a bulk material layer sandwiched between and in direct contact with the 2D material layer and the base layer, the 2D material layer being a layer of graphene; and forming an array of openings through the stack of layers and the base layer to define a metasurface configured such that interaction of an electromagnetic wave with the metasurface modulates at least one property of the electromagnetic wave in accordance with a voltage applied across a corresponding one of the 2D material layers or between corresponding ones of the 2D material layers.

15. The method of claim 14, wherein the 2D material layers include one or more layers of hexagonal boron nitride.

16. The method of any one of claims 14 or 15, wherein the 2D material layers include one or more layers of one or more transition metal dichalcogenides.

17. The method of any one of claims 14 to 16, wherein the 2D material layers include one or more layers of graphene and one or more layers of a dielectric or semiconductor 2D material, wherein at least one of the one or more dielectric or semiconductor layers is in direct contact with at least one of the one or more layers of graphene.

18. The method of any one of claims 14 to 17, wherein the at least one property includes a polarisation property.

19. The method of any one of claims 14 to 18, wherein the metasurface is configured to modulate the at least one property of electromagnetic waves having any frequency up to at least 2 Terahertz and optionally up to 4 Terahertz.

20. The method of any one of claims 14 to 19, wherein the stack of layers includes one or more bulk material layers.

21. The method of any one of claims 14 to 20, wherein the array of openings is formed by etching through the stack of layers and the base layer using pulsed ion milling while the stack of layers and the base layer is thermally coupled to a heat sink.

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