EP4690178A1

EP4690178A1 - Wave absorber devices and method

Info

Publication number: EP4690178A1
Application number: EP24715862.9A
Authority: EP
Inventors: Nikolay Ivanovich Zheludev; Eric Plum; Nina Vaidya; Anton VETLUGIN
Original assignee: University of Southampton; Nanyang Technological University
Current assignee: University of Southampton; Nanyang Technological University
Priority date: 2023-03-28
Filing date: 2024-03-26
Publication date: 2026-02-11
Also published as: GB202304486D0; WO2024201017A1

Abstract

A wave absorber device for absorbing energy from an incident wave that is at least partly spatially coherent comprises: a pair of reflective surfaces configured to reflect the incident wave and with an angle between the reflective surfaces of less than 180 degrees, the pair of reflective surfaces arranged to each intercept a different spatial portion of the wavefront of the incident wave and reflect the spatial portion along a direction away from a propagation direction of the incident wave; and an absorber configured to at least partly absorb the incident wave and having two opposite sides, the absorber arranged between the pair of reflective surfaces so as to receive, from the pair of reflective surfaces, a different spatial portion on each of the opposite sides such that the different spatial portions interfere with one another to form a standing wave at the absorber.

Description

TITLE OF THE INVENTION

WAVE ABSORBER DEVICES AND METHOD

BACKGROUND OF THE INVENTION

The present invention relates to wave absorber devices and a method of absorbing wave energy.

The efficient absorption of waves of all types, including electromagnetic (in particular optical/light), acoustic and water waves, is of importance for many technological applications, including wave detection, energy harvesting and quantum technologies. For light waves it has been shown that the absorption of a single propagating wave is probabilistic and depends on the strength and thickness of the absorbing element or absorber used to absorb the wave [1], Hence absorption depends on chance and cannot be perfect. In contrast, the absorption of light from a standing wave formed by two coherent counterpropagating lightwaves can be deterministic, even in a thin absorber (where the two counterpropagating waves are directed onto opposite side of the absorber), so that all or none of the wave energy that enters the absorber is absorbed, depending on whether the standing wave has an antinode or a node within the absorber [2], Hence, so-called “coherent perfect absorbers” are possible, in which total absorption or dissipation of the energy of an incident wave is theoretically achievable.

Optical implementations of absorber devices have been proposed, in which two copies of an incident beam of light are formed, and respectively directed onto opposite sides of an absorber so that the two beams interfere and form a standing wave within the absorber [2, 3, 4, 5], Similar arrangements for acoustic (sound) waves have also been proposed [6, 7],

Figure 1 shows a highly schematic representation of these known configurations of coherent absorbers. An incident coherent wave 1 is divided into two attenuated copies, in other words, the energy of the incident wave is distributed between two new waves 2 which are otherwise identical to, and therefore copies of, the incident wave 1. The new waves 2 are directed onto opposite sides of an absorber or absorbing element 3 so that the waves 2 are counterpropagating in the absorber 3 to form a standing wave at the absorber 3. Typically, such known arrangements rely on temporal coherence of the waves.

Figure 2 shows a schematic representation of a simple example of an optical implementation of the Figure 1 arrangement, using a typical interferometric setup. A coherent wave 1 is incident onto a beam splitter 4 which forms two copies of the coherent wave 1 in the usual manner, in that a first portion of the energy of the coherent wave 1 is reflected from a surface of beam splitter 4 and becomes a first copy 2a and a second portion of the energy of the coherent wave 1 is transmitted through the beam splitter 4 and becomes a second copy 2b, spatially separated from the first copy 2a. A mirror 5 is placed in the optical path of each of the copies 2a, 2b so as to direct the copies 2a, 2b respectively onto opposite sides of a thin film absorber 3 having appropriate optical properties for the coherent wave. Similar principles have been applied to microwaves and acoustic waves by generating two identical waves from the same signal wave, such as by splitting the driving signal of the signal (wave) generator.

Demonstrated systems of this type have used free space optical propagation or propagation along optical fibres to deliver the copies of the coherent wave to the absorber. The systems are necessarily bulky and relatively complex, and typically suitable only for narrow bandwidth laser light, requiring a plurality of optical components and being unstable in that it is necessary to achieve perfectly matched optical path lengths for the two copies of the coherent wave in order for the absorber to be properly located relative to the standing wave. These features make such devices unsuitable for many potential applications, so that the benefits of “perfect” absorption have not yet been practically realised.

Accordingly, improvements in coherent absorber devices are of interest.

SUMMARY OF THE INVENTION

Aspects and embodiments are set out in the appended claims.

According to a first aspect of certain embodiments described herein, there is provided a wave absorber device for absorbing energy from an incident wave that is at least partly spatially coherent, the device comprising: a pair of reflective surfaces configured to reflect the incident wave and with an angle between the reflective surfaces of less than 180 degrees, the pair of reflective surfaces arranged to each intercept a different spatial portion of the wavefront of the incident wave and reflect the spatial portion along a direction away from a propagation direction of the incident wave; and an absorber configured to at least partly absorb the incident wave and having two opposite sides, the absorber arranged between the pair of reflective surfaces so as to receive, from the pair of reflective surfaces, a different spatial portion on each of the opposite sides such that the different spatial portions interfere with one another to form a standing wave at the absorber.

According to a second aspect of certain embodiments described herein, there is provided a wave absorber array comprising a plurality of wave absorber devices according to the first aspect. According to a third aspect of certain embodiments described herein, there is provided a method of absorbing wave energy comprising: directing an incident wave that is at least partly spatially coherent onto a pair of reflective surfaces configured to reflect the incident wave and with an angle of less than 180 degrees between the reflective surfaces such that each reflective surface intercepts a different spatial portion of the wavefront of the incident wave and reflects the spatial portion along a direction away from a propagation direction of the incident wave; and receiving the different spatial portions on opposite sides of an absorber configured to at least partly absorb the incident wave, the absorber arranged between the pair of reflective surfaces such that the different spatial portions interfere with one another to form a standing wave at the absorber.

These and further aspects of certain embodiments are set out in the appended independent and dependent claims. It will be appreciated that features of the dependent claims may be combined with each other and features of the independent claims in combinations other than those explicitly set out in the claims. Furthermore, the approach described herein is not restricted to specific embodiments such as set out below, but includes and contemplates any appropriate combinations of features presented herein. For example, wave absorber devices and methods of absorbing waves may be provided in accordance with approaches described herein which includes any one or more of the various features described below as appropriate.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention and to show how the same may be carried into effect reference is now made by way of example to the accompanying drawings in which:

Figure 1 shows a highly schematic representation of a known coherent wave absorber;

Figure 2 shows a schematic representation of a known simple optical coherent wave absorber;

Figure 3 shows a highly schematic representation of a spatially coherent wave absorber according to the present disclosure;

Figure 4 shows a schematic side view representation of an example spatially coherent wave absorber device according to the present disclosure;

Figure 5 shows a schematic representation of an example spatially coherent wave absorber device annotated to show physical principles of the device operation; Figure 6 shows a schematic side view representation of a first example spatially coherent wave absorber device according to the present disclosure which is implemented using total internal reflection;

Figures 7A-7F shows schematic side view representations of a plurality of example spatially coherent wave absorber devices according to the present disclosure comprising absorbers formed from two or more absorbing layers;

Figure 8 shows schematic plan view representations of example configurations of four absorbing layers for forming a polarisation-sensitive absorber for a wave absorber device according to the present disclosure;

Figure 9 shows a schematic side view representation of an example spatially coherent wave absorber device according to the present disclosure which is implemented with fixed mirrored reflective surfaces;

Figures 10A and 10B show schematic perspective views of example three- dimensional spatially coherent wave absorber devices according to the present disclosure;

Figure 11 shows a schematic perspective view of an example array of spatially coherent wave absorber devices according to the present disclosure;

Figures 12A-12E show schematic side views of several further example arrays of spatially coherent wave absorber devices according to the present disclosure;

Figure 13 shows a simplified perspective view of an example array of spatially coherent wave absorber devices configured for operation as a photovoltaic converter;

Figure 14 shows a simplified perspective view of an example array of spatially coherent wave absorber devices configured for operation as an acoustic noise absorbing barrier;

Figure 15 shows a simplified perspective view of an example spatially coherent wave absorber device configured for operation as a water wave energy harvester;

Figures 16A-D show graphs of measured reflectivity for an incident light wave for an experimental electromagnetic wave absorber device according to the present disclosure, showing variations in absorption due to changes in alignment of the device (Figures 16A-C) and changes in the angle of incidence of the incident wave (Figure 16D);

Figure 17 shows a flow chart of steps in a method of absorbing wave energy according to the present disclosure;

Figures 18A and 18B show calculated absorption spectra with logarithmic and linear wavelength axes respectively for modelled example electromagnetic wave absorber devices according to the present disclosure having absorber layers of different materials and thicknesses.

DETAILED DESCRIPTION

Aspects and features of certain examples and embodiments are discussed I described herein. Some aspects and features of certain examples and embodiments may be implemented conventionally and these are not discussed I described in detail in the interests of brevity. It will thus be appreciated that aspects and features of devices and methods discussed herein which are not described in detail may be implemented in accordance with any conventional techniques for implementing such aspects and features.

In contrast to the known approach for achieving coherent “perfect” absorption explained above with reference to Figures 1 and 2, the present disclosure proposes a modified approach, where the modification lies in the manner in which the two coherent beams required for interference to form a standing wave in the absorbing element are formed from the original incident coherent wave. The known approach creates two attenuated copies of the incident coherent wave. The newly proposed approach takes two different spatial segments or portions of the incident coherent wave, which is at least partly spatially coherent, and directs each spatial segment onto an opposite side of the absorbing element, for the required counterpropagation and constructive (for absorption) or destructive (for reflection) interference. This enables compact and robust devices, and is fundamentally different from the known approach which directs the same spatial segment of an incident wave onto opposite sides of an absorber (in the form of two attenuated copies). For convenience herein, the term “spatially coherent” should be understood as indicating “at least partly or partially spatially coherent, including both partially spatially coherent and wholly spatially coherent”, unless the context implies a stricter interpretation as understood by the skilled person.

Figure 3 shows a highly schematic representation of a spatially coherent wave absorber device according to the present disclosure. An incident wave 1 , which has at least partial spatial coherence, is physically split or divided into two different spatial portions or spatial segments 6, which are directed onto opposite sides of an absorber 3 (absorbing element or absorbing layer). This can be achieved by interception of the incident wavefront so that a first spatial portion of the wavefront (where the spatial portion is defined as part of the transverse cross-sectional profile of the beam of the propagating incident wave, in other words, the wavefront) becomes incident on a first side of the absorber, and a second, different, spatial portion of the wavefront becomes incident on a second side of the absorber. As will become apparent from descriptions of proposed designs of wave absorber devices, this can be readily implemented by arranging a pair of reflectors relative to the incident wave so that a part of the beam cross-section is incident on one reflector and a different part of the beam cross-section is incident on the other reflector, the two reflected parts of the incident wave then being directed by the reflectors onto opposite sides of the absorber. This configuration can provide matching of the path lengths as a default feature of the device construction, so the device is inherently stable and requires no careful alignment to produce the required interference and standing wave at the absorber. Total deterministic absorption of a single incident spatially coherent wave beam is thereby enabled in a very simple device.

Usefully, the devices can operate across a broad bandwidth (wide range of wavelengths) of the type of wave for which any particular device is configured, and are applicable to any wave type, including transverse waves and longitudinal waves. In other words, all kinds of waves where particles, fields or media may be oscillating in any direction with respect to the direction of wave propagation can be perfectly or near- perfectly absorbed, in principle across all wavelengths. The devices rely on spatial coherence of the incident waves, however, all waves are spatially coherent on sufficiently small length scales. The reflectors and absorber are selected with reference to the wave type so as to be configured for reflection and absorption of the wave, as will be understood by the skilled person, and of an appropriate size for the expected transverse profile or wavefront of incident waves (where individual devices may be combined in an array to increase the total aperture size). Since reflectors and absorbers are available for essentially any type of wave, wave absorber devices are proposed for wave types including electromagnetic waves across the whole electromagnetic spectrum (optical (ultraviolet, visible and infrared) light) waves, terahertz waves, microwaves, radio waves), acoustic waves, water waves, plasmonic waves and polaritonic waves.

Accordingly, a very wide range of applications is expected for the proposed wave absorber devices. For electromagnetic applications, in particular optical applications, compact and robust devices are enabled, suitable for purposes including but not limited to:

• Up to 100% absorption of light with any wavelength(s) coming from a single source such as the sun, a light emitting diode or a laser, for applications in light detection, light sensing, power measurement and extreme telecommunications.

• High efficiency broadband absorption of electromagnetic waves that in principle can engage the entire electromagnetic spectrum, for applications in radar absorption, stealth technologies, bolometer detectors, electromagnetic energy harvesting and energy conversion such as solar energy to heat energy, solar electricity generation via heat and steam, photovoltaics and space-based solar power.

• Deterministic absorption of quantum light (single-photon and/or multi-photon states of light; coherent, squeezed and entangled, states of light) of any wavelength for broadband quantum detectors, high quantum efficiency detectors and photon-number resolving detectors with applications in quantum states tomography, quantum communication, quantum computing, and quantum metrology.

• Precision metrology of positions and orientations, with applications in platform position and tilt metrology. This is enabled by sensitivity of the absorption to any path length difference between the two spatial segments, according to general principles of interferometry, so that perturbations in incident wave direction and position of device components can be readily detected.

• Construction of spatial coherence measurement devices. This can be based on measuring the overall absorption (or reflection) of waves incident on a wave absorber device or detecting absorption (or reflection) of the incident waves as a function of position across the aperture of an individual wave absorber device (for example, using a beam splitter and a suitable spatially resolved detector, optionally with lenses in between).

• Construction of polarization-sensitive absorbers/reflectors and absorption-type reflective polarizers for applications in optical components, signal processing and passive and active displays. The polarisation sensitivity aspect is described further below.

• Construction of (non-detecting) devices for broadband quantum information processing, non-local quantum control and changing the quantum statistical properties of light.

• Construction of lightweight detectors and photovoltaic systems, useful in achieving low launch mass for space applications, for example.

Regarding non-electromagnetic waves, applications in sound engineering and detection and noise cancellation and control, including selective absorption of broadband noise/sound from a selected source/direction such as acoustic noise absorbing barriers/claddings/surface structures for highways/train lines/tunnels, headphone noisecancelling technologies, and locating of acoustic sources such as submarines are envisaged for sound waves. Energy harvesting (renewable electricity generation) and reflection prevention are envisaged for water waves, including ocean waves and river waves.

Other applications for various wave types for which absorption is of interest (including detection, energy harvesting and reflection prevention) will be apparent to the skilled person, and are not excluded.

As will become apparent from the description of specific example devices given below, the proposed devices provide technologically simple and practical designs which are manufacturable by a range of well-established high throughput fabrication techniques for producing reflective surfaces and absorbing elements, and substrates for supporting these components, including lithography, etching, spin coating, spray coating, vapour/plasma deposition, evaporation, 3D printing, nano-imprint, templating and moulding, exfoliation or laying of two-dimensional materials, rolling, etc., utilised with different and widely available materials. Hence, efficient, inexpensive and/or simple devices available at high unit volume for many and varied applications are anticipated.

Figure 4 shows a schematic side view representation of a generic example of a wave absorber device as proposed herein. As noted above, the two counterpropagating coherent beams or waves required for interference at an absorber are created from a single spatially coherent beam or wave 10 incident on the wave absorber device 20 by spatial division of the wavefront 11 of the incident wave 10 into two spatial portions or segments 10a, 10b. This is achieved by arranging two (a pair of) reflective surfaces 12a, 12b adjacently across the wavefront 11 so that one reflective surface 12a intercepts a first spatial portion 10a, and a second reflective surface 12b intercepts a second spatial portion 10b. In other words, a first part 10a of the wavefront 11 is incident onto the first reflective surface 12a and a second part 10b of the wavefront 11 is incident onto the second reflective surface 12b. Each spatial portion 10a, 10b is reflected from its respective reflective surface 12a, 12b. By tilting the reflective surfaces 12a, 12b towards each another, so that the reflective surfaces 12a, 12b are arranged at an angle 0 to one another which is less than 180 degrees, the reflective surfaces are also at an angle to the propagation direction of the incident wave 10, and the spatial portions are each reflected along respective propagation directions away from the propagation direction of the incident wave (so, the spatial portions 10a, 10b are directed away from the retroflection direction). Since the reflective surfaces 12a, 12b are at an angle less than 180 degrees to one another, the reflected propagation directions from each reflective surface 12a, 12b are towards each other. The two different spatial portions 10a, 10b are hence given propagation directions towards one another, and two counterpropagating waves are formed. The absorber 14 (which may comprise one or more absorbing layers as discussed further below) is therefore arranged between the two reflective surfaces 12a, 12b in order to receive the two spatial portions 10a, 10b. The absorber 14 comprises a thin or relatively thin (relative to the wavelength of the incident wave) absorbing element comprising a first side 14a and a second side 14b, with a thickness dimension extending between the first side 14a and the second side 14b. The first side 14a faces generally towards the first reflective surface 12a and the second side 14b faces generally towards the second reflective surface 12b. Hence, the first spatial portion 10a of the wavefront 11 of the incident wave 10 is incident onto the first side 14a of the absorber 14 and the second spatial portion 10b of the wavefront 11 of the incident wave 10 is incident on the second side 14b of the absorber 14. The two spatial portions 10a, 10b are hence able to interfere at the absorber 14 and form a standing wave.

As depicted, the absorber 14 is positioned at the midpoint of the angle 0 separating the first reflective surface 12a and the second reflective surface 12b (the angles a between the absorber 14 and each reflective surface 12a, 12b are therefore equal) so that the space between each reflective surface 12a, 12b and the absorber 14 is the same. This aids in giving equal optical path lengths within the device to each spatial portion within 10a and 10b that is directed to the same location within the absorber 14 so that the required constructive (or destructive) interference at the absorber 14 can be more readily achieved, but is not essential. The depicted example has a right angle between the two reflective surfaces (0 = 90 degrees), so that the angle a between the absorber 14 and each reflective surface 12a, 12b is 45 degrees. This is also not essential, and other smaller or larger angles may be used.

Also, the absorber 14 is shown with its innermost edge (the edge furthest from the incident wave 10) against the adjacent reflective surfaces where the angle 0 is defined. This allows all of each spatial portion 10a, 10b of the incident wave 10 that enters the device 20 to reach the absorber 14, for maximum absorption of the energy of the incident wave 10, and may also be most convenient for locating the absorber (since it can be mounted onto the reflective surfaces 12a, 12b or any support member for the reflective surfaces 12a, 12b), but is also not essential.

Depending on the location of the absorber within the standing wave, interference of the two spatial portions of the wave may form an antinode or a node at the absorber. Constructive interference (antinode) will enhance wave energy absorption, in principle up to 100% (perfect absorption), while destructive interference (node) will reduce wave energy absorption, in principle down to 0% (perfect transmission through the absorber and thus perfect reflection by the device). In the proposed implementations of this concept, the different spatial portions of an incident spatially coherent wave are directed or diverted by reflecting surfaces or boundaries onto the absorber within a single device (or an array of single devices), as described above. A significant benefit of using the proposed geometries is that the same type of interference on the absorber (one or more absorbing layers or elements) can be achieved for incident waves of any wavelength (or a broad range of wavelengths in the case of multiple absorbing layers or elements) simultaneously in a single simple and compact device. This can be utilised to achieve a large absorption bandwidth.

Coherent absorption (and transmission) by the absorber within wave absorber devices as proposed herein utilises spatial coherence of the incident waves across the aperture of individual wave absorber devices in the direction perpendicular to the (relevant) absorber. This is not a significant limitation since all waves (including sunlight) are spatially coherent on sufficiently small length scales [8], It implies that the dimension of the aperture of an individual wave absorber device (in the direction perpendicular to the relevant absorber, that is, the absorber that receives the spatial portions of the incident wave) should preferably be chosen to be appropriately small such that the incident wave has as least a partial, and optionally or preferably a moderate or high, degree of spatial coherence across the aperture (in said direction). In other words, the incident wave should be partly, mostly, substantially or wholly spatially coherent across the aperture in said direction, so that spatial portions of the incident wave interfere on the absorber in a partly, mostly or wholly predictable manner. The part of the incident wave which is spatially coherent (which may be part or all of the incident wave depending on the degree of spatial coherence) can, in principle, be perfectly absorbed (or transmitted) by the absorber, while any spatially incoherent part of the incident wave (present if the incident wave is only partially spatially coherent) will experience the normal level of absorption that the absorber exhibits for illumination from one side only (typically, any spatially incoherent part will be partially absorbed). Therefore, lower spatial coherence of the incident wave (on the relevant length scale) reduces the achievable level of absorption (and reflection) for the described wave absorber devices, but nevertheless can result in an improved and/or useful amount of absorption or reflection. Also, it follows that wave absorber devices as disclosed herein can be used to measure the spatial coherence of waves, either by measuring the overall absorption (or reflection) of waves incident on a wave absorber device, or by detecting absorption (or reflection) of the incident waves as a function of position across the aperture of an individual wave absorber device (for example, using a beam splitter and a suitably spatially resolved detector). For convenience herein, incident waves may be referred to as “a spatially coherent incident wave” or “an incident spatially coherent wave”, but this should be understood as indicating that the incident wave is at least partly or partially spatially coherent across the device aperture (in the direction perpendicular to the absorber that receives the spatial portions of the wave), including both partially spatially coherent incident waves and wholly spatially coherent incident waves.

Physical principles of the proposed wave absorber devices will now be described.

Figure 5 shows a schematic representation of an example wave absorber device 20 receiving an incident wave 10, similar to Figure 4. In this example, a pair of prisms is used to support the reflective surfaces 12a, 12b and the absorber 14; this design is discussed further below, and is not relevant to a description of the physical principles. As in Figure 4, two different spatial portions or segments 10a, 10b of incident wave 10 are respectively reflected onto the absorber 14 by the first and second reflective surfaces 12a, 12b. Wave propagation to the same location 14’ within the absorber 14 from locations within the first and second spatial portions 10a, 10b of the same incident wavefront 11 can be described by paths Pi and P2 through the device 20, which have effective path lengths Li and L2, and an effective path difference AL = Li - L2. Here, the effective path length is L = fn(s) ds with integration along the path and n(s) = Ao/A(s), where A(s) is the wavelength at position s and Ao is the incident wavelength. For example, for electromagnetic waves, n is the refractive index (the refractive index of the prism material in the present example), L is the optical path length, and AL is the optical path difference.

The path difference AL controls the bandwidth of the device, where use of an absorber 14 comprising a single absorbing layer with perfectly matched optical paths, that is, with a path difference of zero (AL = 0) can in principle provide infinite bandwidth as incident waves of any wavelength will interfere on the absorbing layer in the same way. In practice, large bandwidths can be obtained for AL< Ao.

Absorption (and reflection) by the proposed devices are governed by the total phase difference Acp = q>i - q>2 arising from wave propagation along said paths Pi and P₂. A > includes contributions from the effective path difference AL as well as phase changes upon reflection at the reflective surfaces, and geometric phase contributions due to changes in the wave propagation direction. Total phase differences (within ±TT/2) that are an even multiple of TT result in constructive interference, and therefore enhanced absorption. Total phase differences (within ±TT/2) that are an odd multiple of TT result in destructive interference, and therefore reduced absorption and thus enhanced reflection by the device. Note that these conclusions regarding absorption assume that A(p is calculated for the wave amplitude that interacts with the absorbing layer or element, in case of electromagnetic waves, this could be the electric field or the magnetic field of the waves.

The absorber may comprise one or more absorbing layers or elements (not shown in Figures 4 or 5). The layers can be separated by spacers or air or vacuum if there are two or more. Assuming that the absorbing layer(s) or element(s) are thin compared to the wavelengths A of the waves in the particular application for which the device is intended, where “thin” indicates a thickness of one quarter wavelength or less (thickness < A/4), the absorbing layer(s) or element(s) can be positioned at an anti-node or anti-nodes of the standing wave formed by the counterpropagating spatial portions of the incident waves (positions of constructive interference). In some examples, the layer(s) or element(s) may have a thickness very much less than A/4, for example of the order of A/10 or of the order of A/100 or of the order of A/1000 or of the order of A/10,000, or even thinner, depending on the purpose of the wave absorber device and the type of waves for which it is designed. An absorbing layer or element may be described in terms of complex amplitude reflection and transmission coefficients, r and t that relate the amplitudes of incident wave E', the reflected wave E^r and the transmitted wave E‘ according to E^r = r E' and E‘ = t E', where t = r + 1 for an infinitesimally thin (thickness « Ao) absorbing layer element. For electromagnetic waves the amplitudes are normally electric field amplitudes (rather than magnetic). Example reflection and transmission coefficients for optimal performance are given below for different implementations.

Various device designs with different reflective surfaces and absorbers to realize compact devices that (in principle, perfectly) absorb waves across a broad spectral range are proposed. For example, the solar spectrum of electromagnetic radiation from 300 nm to 1200 nm may be accommodated in a single device. Implementations with extremely thin (thickness « Ao) absorbers (comprising one or more absorbing layers) are an important special case.

As described above, the proposed wave absorber devices are based on an absorber placed between two reflective surfaces or reflecting boundaries that form an angle with each other and redirect two different spatial portions or segments of a single incident wave such that they interfere at or on the absorber. A wide variety of designs are possible, which include two- and three-dimensional geometries (referring to the number of dimensions in which wave propagation takes place within the device). The redirection of the waves is generally achieved at reflecting boundaries. In many cases a reflecting boundary may be thought of as a reflective surface, but, in some cases (such as in devices for surface waves such as surface plasmons) a reflecting boundary may also be a reflective line such as a surface modification or discontinuity along a line that reflects said surface waves. For electromagnetic waves, the reflective surface may be a boundary between a higher refractive index material and a lower refractive index material so that total internal reflection occurs, or may be a mirrored surface such as a supporting member or substrate with a reflective coating configured to reflect the wavelengths of interest.

The absorber is made from a material appropriate for absorption of the incident wave type for which the device is intended. In devices for electromagnetic waves, the material may be a metal (for example, chromium or nickel), a dielectric, a superconducting film (such as NbTiN, NbN or MoSi), a semiconductor (amorphous or crystalline), or a photoactive material that is organic, inorganic, or a mixture thereof (such as (multi-layer) graphene, a metamaterial, a wire grid, aligned polymer chains, a polarization-selective I anisotropic absorber, a spectrally-selective I dichroic absorber, perovskite, 3D/2D (di)chalcogenides, transition metal dichalcogenides, dye-sensitised organic cells, or quantum dot cells, all single or multijunction with charge separation and carrier layers). The reflectivity and transmissivity of the absorber can be optimized by adjusting the film thickness.

In devices for mechanical waves (such as acoustic waves), the absorber may be a piezoelectric material. Other materials are not excluded, and will be apparent to the skilled person according to the intended use of the device.

The simplest devices, such as shown in Figure 4, have an absorber comprising a single, thin (thickness « Ao) absorbing layer or element. In this arrangement and for incident longitudinal waves, matched effective path lengths yield broadband interference of one type, constructive or destructive, on the absorber. Here, we consider interference of the wave amplitude that interacts with the absorber. Broadband constructive interference results in broadband absorption of the waves. Broadband destructive interference results in reflection of the waves by the device. For incident transverse (e.g. electromagnetic) waves, matched effective path lengths yield broadband constructive interference for one polarization of the incident waves and destructive interference for the orthogonal polarization. Broadband constructive and destructive interference result in broadband absorption and reflection of said polarization components of the incident waves by the device, respectively. In the case of an absorber that interacts with the electric field of electromagnetic waves, the s-polarisation is absorbed and the p- polarisation is reflected, and for an absorber that interacts with the magnetic field of electromagnetic waves the opposite result occurs so that the s-polarisation is reflected and the p-polarisation is absorbed. Such a device may therefore be used as broadband polariser for electromagnetic waves, providing up to 100% absorption and 100% reflection for orthogonal polarisations of the incident waves.

Figure 6 shows a schematic side view representation of an example wave absorber device that utilises total internal reflection to implement the reflective surfaces, and is suitable for electromagnetic waves. Hence, the device comprises bulk material through which the incident wave propagates in order to reach a pair of reflective boundaries angled towards one another at which total internal reflection takes place. A convenient way to implement this configuration is to use a prism to provide a reflective boundary, so that two prisms can be abutted to provide the required relative angular position of the two reflective boundaries, with the absorber provided between the abutted surfaces of the prisms. This is the design depicted in Figure 6, although the invention is not limited in this way, and other bulk material elements may be used, either as an abutted pair (or additional pairs for more complex configurations discussed further below), or as a single element within the material of which the absorber is formed during device fabrication. Returning to Figure 6, two triangular prisms 16 (made of a material which is transparent or has low absorption for the wavelengths of intended incident waves, such as glass for visible light, silicon for infrared light, crystals or polymers) are used to provide the total internal reflection. In this example the prisms 16 are right- angled 45 degree triangular prisms, so that their short sides are of equal length (this is not essential). The prisms 16 are stacked together such that a short sided face of each is facing a short sided face of the other. The absorber 14 is formed, located or otherwise provided between the facing short faces, such as a separate element placed between the prisms, or as one or more layers deposited onto the short side surface of one or both prisms. The remaining short side faces 18 of the prisms 16 face outwardly adjacent to one another and form a continuous surface that defines an input face or aperture for the device 20, through which the incident wave enters the device 20. The hypotenuse side faces of the prisms 16 are therefore arranged at an angle less than 180 degrees to one another (in this example, 90 degrees) and provide the reflective surfaces 12a, 12b. The incident wave entering the device 20 through the outwardly facing prism short side faces 18 propagates through the prisms 16 to the hypotenuse side faces, where the two spatial portions experience total internal reflection at the boundary between the prism material and the environment outside the prisms 16 and are redirected towards the abutted short side faces of the prisms 16 and the absorber 14 at or between these faces. The outwardly facing short side faces defining the input face of the device 20 may be provided with antireflective or antireflection elements or structures (such as surface coatings) tailored for the wavelength(s) of the intended incident waves, in order to maximise transmission of the incident wave into and through the prisms 16 to the reflective surfaces 12a, 12b.

As noted above, if the absorber 14 comprises a single absorbing layer that interacts with one polarisation of the incident wave only, this device 20 can function as a polarisation-selective broadband absorber for transverse (e.g. electromagnetic) waves. For example the absorbing layer may be thin compared to the wavelength of the wave, with ideal reflection (r) and transmission (t) coefficients that are related by r=-t or r=t.

However, the proposed devices can be configured as polarization-insensitive broadband absorber devices for transverse (e.g. electromagnetic) waves by various modifications. In a first example, an absorber that interacts with both forms of the wave energy which a transverse wave oscillates between (the electric field and the magnetic field in case of electromagnetic waves) can provide polarization-insensitive absorption [9].

Alternatively, a birefringent element (layer or structure or component) may be added to the device in order to adjust the total phase differences for s-polarised and p- polarised waves, A(p_s and A(p_p, to approximate the ideal case of A(p_s = A(p_p = 0. This can be achieved with a birefringent element that acts as a (super-achromatic) half-waveplate and therefore introduces an additional TT phase difference between s-polarised and p- polarised waves which are incident on one side of the absorber only. Hence, the birefringence is applied to only one of the two spatial portions of the incident wave. The birefringent element can be placed anywhere along the propagation path of the spatial portion. In the case of devices from prisms, a convenient approach is to apply a birefringent coating to one surface of one of the prisms (the input face may be most convenient, but the internal reflection face or the face where the absorber is located may be used), but a separate birefringent element could alternatively be arranged in front of the device, in the path of the incident wave before it enters the device. This is suitable for prism-based devices or other implementations that use free-space wave propagation as described further below. Figure 6 shows optional birefringent coatings or elements 19 on or in front of the outwardly facing short side surfaces 18. As noted, it is sufficient to provide a birefringent element for one spatial portion only (so, a birefringent coating on a short side face of one prism only, for example) to achieve polarisation-insensitivity, but birefringence may be applied to both spatial portions in more complex implementations and for other effects. Overall, selective absorption of incident waves with any or all polarisations can be achieved by choosing the orientation and phase delay of one or more birefringent elements. A birefringent layer or layers may be used in conjunction with the anti refl ection coatings mentioned above. A single layer may be provided that combines both birefringent and anti refl ection properties.

As a further alternative to adjust the polarisation-based functionality of a device (applicable to both prism-based devices and free-space devices) the absorber may be implemented as two thin polarization-selective absorbing layers or elements separated by a transparent spacer, such that the s-polarization constructively interferes on an s- polarization absorbing layer or element and the p-polarization constructively interferes on a p-polarization absorbing layer or element. To maximize the bandwidth of the device, the optimal spacing of the absorbing layers or elements is A/4, where A is the wavelength of the wave in the transparent spacer medium.

The preceding two approaches, of a birefringent element and two absorbing layers, can be combined. In this case the optimal phase delay due to birefringence and the optimal spacing between the polarization-selective absorbing layers or elements will depend on each other. The choice of one will determine the other. The optimal spacing changes proportionally to the birefringence-induced phase difference. The optimal spacing changes by A/4 per birefringence-induced phase difference of TT. The spacing may be changed by A/2 without changing the device performance at the design wavelength; however, this will affect the bandwidth.

By adjusting the orientation of and phase delay in the birefringent element and/or the polarization-selectivity of the absorbing layers or elements, various polarization characteristics can be achieved, including polarizers (polarization-selective absorbers or reflectors) for waves with any chosen polarization.

Figure 7 shows schematic side view representations of a selection of wave absorber devices that comprise an absorber formed from two or more separate absorbing layers or elements. Figure 7 is intended to show examples of how absorbing layers may be arranged within the device, where the overall size of the device relative to the spacing between the absorbing layers as depicted in Figure 7 is chosen purely for clarity and ease of understanding, and is not intended to suggest or indicate actual sizes or relative dimensions of devices. The devices are shown as prism-based devices (the dotted lines indicating the boundary between the two prisms) but may be free-space devices. The individual absorbing layers may be thin compared to the wavelength, are spaced apart from one another by a distance d, and may be arranged symmetrically or asymmetrically within the device. Symmetrically indicates that the layers have the same arrangement on either side of the mid-point of the device, where the reflecting surfaces meet at the angle 0, and asymmetrically indicates that the layers have differing arrangements on either side of the mid-point, for example more layers on one side of the mid-point than on the other side.

Figures 7A and 7B show absorbers 14 comprising two absorbing layers 14c. The Figure 7A design is asymmetric, with a first absorbing layer 14c located at the midpoint between the reflecting surfaces 12a, 12b and a second absorbing layer 14c located spaced apart from the first absorbing layer. The layers are spaced apart by a distance d which is a multiple of A/2, where A is the characteristic wavelength of the waves (in the medium). The Figure 7B design is symmetric, with both absorbing layers spaced apart from the midpoint by the same distance, and a total spacing or separation between the layers d which is multiple of A. These example devices will absorb normally incident s- polarized (p-polarized) waves for absorbing layers with reflection and transmission coefficients of opposite (same) sign.

Figure 7C also shows an absorber 14 comprising two absorbing layers 14c, which are symmetrically arranged, one on either side of the midpoint, with a separation d which is an odd multiple of A/2. This example device absorbs normally incident p- polarized (s-polarized) waves for absorbing layers with negative (positive) reflection coefficients.

In all of the Figures 7A, B and C examples, the absorbing layers 14c are thin compared to the wavelength of the wave, where ideal combinations of reflection and transmission coefficients include r = ±1/3 and t = +2/3. These values and other examples are described further below.

Figures 7 D, E and F show absorbers 14 comprising multiple absorbing layers which are spaced apart by a separation d which is an odd multiple of A/4. In Figure 7D the absorber 14 comprises two absorbing layers 14d arranged asymmetrically, with one absorbing layer 14ds located at the midpoint and one absorbing layer 14dp spaced from it. The absorbing layers are thin compared to the wavelength of the waves, and the “s” and “p” suffixes indicate opposite characteristics in transmission and reflection for different polarisations: “s” indicates that the ideal reflection (r) and transmission (t) coefficients are related by r=-t for s-polarized waves and r=0 and t=1 for p-polarized waves, and “p” indicates that the ideal reflection (r) and transmission (t) coefficients are related by r=-t for p-polarized waves and r=0 and t=1 for s-polarized waves.

In Figure 7E the absorber 14 comprises three absorbing layers arranged symmetrically, with a first absorbing layer 14ds located at the midpoint and second and third absorbing layers 14cp spaced apart on either side of the first absorbing layer 14ds. Again, the absorbing layers are thin compared to the wavelength of the waves, and the “s” and “p” suffixes indicate opposite characteristics in transmission and reflection for different polarisations. The central, first absorbing layer 14ds has ideal reflection (r) and transmission (t) coefficients related by r=-t for s-polarized waves and r=0 and t=1 for p- polarized waves. The flanking second and third absorbing layers 14cp have ideal combinations of reflection and transmission coefficients that include r=-1/3 and t=+2/3 for p-polarized waves and r=0 and t=1 for s-polarized waves.

In Figure 7F the absorber 14 comprises four absorbing layers 14 arranged asymmetrically, with a first absorbing layer 14cs aligned with the midpoint, a second absorbing layer 14cp spaced apart on one side and third and fourth absorbing layers 14cp and 14cs spaced apart on the other side. Again, the absorbing layers are thin compared to the wavelength of the waves, and alternate as regards their transmission and reflection characteristics. The absorbing layers 14cs have ideal combinations of reflection and transmission coefficients that include r=-1/3 and t=+2/3 for s-polarized waves and r=0 and t=1 for p-polarized waves, and the absorbing layers 14cp have ideal combinations of reflection and transmission coefficients that include r=-1/3 and t=+2/3 for p-polarized waves and r=0 and t=1 for s-polarized waves.

The Figures 7D, E and F example devices absorb normally incident s- and p- polarized waves owing to the polarization-selective absorbing layers with the spacing d of an odd multiple of A/4 noted above. For absorbing layers with reflection coefficients of reversed sign and interchanged absorbing layer positions for s- and p-polarization, these devices would also absorb s- and p-polarized waves.

Figure 8 shows schematic plan views of four example absorbing layers 14cs, 14cp, 14ds and 14dp as described above. The absorbing layers have a structure that is an array of lines of sub-wavelength period. The examples have different duty cycles and orientations of the spaced-apart lines.

It will be apparent from the above discussion of Figure 7 that a range of transmission and reflection coefficients can be employed in the individual absorbing layer or layers that make up the absorber, and different absorbing layers can be combined to achieve different polarisation-related effects in the wave absorber device. This will now be discussed further.

Perfect (100%) absorption in a wave absorber device can, in principle, be obtained with an absorber having reflection (r) and transmission (t) coefficients related by r=t or r=-t, while perfect reflection can also be achieved if additionally, |r|=|t|=0.5. The latter is desirable in applications such as tilt/displacement sensors and polarizers (polarization-selective reflectors/absorbers). In case of perfect absorption, transmission of waves incident on one side of the absorber and reflection of waves incident on its opposite side will have the same amplitude but opposite phase and therefore cancel completely. Perfect absorption can be achieved with an absorber comprising absorbing layers or elements that are thin (compared to the wavelengths of the waves for which the device is intended) and which are located at positions of constructive interference to maximize absorption. The absorber may be made from one or more absorbing layers of elements separated by transparent spacers, as noted above. Perfect absorption can also be achieved with thick(er) absorbing layers or elements, where increased thickness comes at the cost of reduced bandwidth [10], Some examples of ideal absorbers for perfect absorption will now be discussed; some of these examples have already been mentioned above with regard to Figures 5, 6 and 7.

A first example comprises an absorber with reflection (r) and transmission (t) coefficients related by r = -t. For example, such a layer with r = -0.5 and t = +0.5 transmits 25%, reflects 25% and absorbs 50% of incident power when illuminated from one side only. In the case of transverse waves, these properties may apply to one or all polarizations. It can be made from a single thin absorbing layer. An example of such an absorbing layer is a chromium film. If the film has a thickness of 20 nm it will exhibit these reflection and transmission properties approximately for near-infrared electromagnetic waves, while a thickness of about 12 - 14 nm yields such properties in the visible part of the spectrum. Example results from an absorber of this type are presented later. The phenomenon is very broadband, for instance, a 14 - 20 nm thick chromium film is expected to exhibit >90% absorption in an ideal wave absorber device in the broad range of (free-space) wavelengths from 250 nm to 2000 nm, which covers almost the entire solar energy spectrum.

A second example comprises an absorber with reflection (r) and transmission (t) coefficients related by r = t. For example, such a layer with r = t = -0.5 transmits 25%, reflects 25% and absorbs 50% of incident power when illuminated from one side only. (In case of transverse waves, these properties may apply to one or all polarizations.)

A third example comprises an absorber formed from two thin polarization- selective absorbing layers or elements, one with reflection (r) and transmission (t) coefficients related by r = ±t for s-polarized waves and r = 0 and t = 1 for p-polarized waves, and the other with interchanged polarization characteristics. The two absorbing layers should ideally be separated by A/4, where A is the wavelength in the transparent spacer medium. Figure 7D shows this configuration. Spacings of 0.25A, 0.75A, 1.25A, 1.75A, ... are possible, but the bandwidth reduces with increasing spacing between the absorbing layers. Such a design enables simultaneous absorption of s- and p-polarized electromagnetic waves. In a fourth example, any absorber or absorbing layer in the previous three examples may be replaced with a sequence of absorbing layers. For example, reflection (r) and transmission (t) coefficients of -r = t = 0.5 (r = t = -0.5) can be achieved by two absorbing layers with coefficients n = ti - 1, r2 = t2 - 1 and t2 = ti / (3ti - 1) with free parameter ti and spaced by an even (odd) multiple of the half-wavelength in the spacer medium. The special case of identical absorbing layers corresponds to n = r2 = -1/3 and ti = t2 = 2/3. A larger number of M absorbing layers may be used and if they are identical their coefficients should be t_m = M / (M+1) and r_m = t_m - 1. The bandwidth reduces with increasing spacing between the absorbing layers, and with increasing M. A stack of different absorbing layers may also be used, and there is a group of solutions for spacing and coefficients of absorbing layers of such stacks with different total numbers of absorbing layers. Regarding n = r2= -1/3 and ti = t2= 2/3, such an individual absorbing layer transmits 4/9, reflects 1/9 and absorbs 4/9 of incident power when illuminated from one side only. A chromium film of about 10 - 14 nm thickness exhibits such properties for near-infrared electromagnetic waves. See Figures 7A, 7B, 7C, 7E and 7F for examples.

In reality, actual absorbers and absorbing layers will approximate these ideal conditions. Absorbers and individual absorbing layers do not have to be thin compared to the wavelengths of the waves for which a device is intended, but as noted above, the bandwidth of the device (that is, the range of wavelengths for which large absorption is achieved) will reduce with increasing thickness.

In devices for the absorption of electromagnetic waves, the reflective surfaces can be implemented by total internal reflection as described above, and this can be readily achieved by the use of prisms or similar bulk transparent elements where various faces of the prism provide an input face or aperture for receiving the incident wave, a boundary for the total internal reflection, and a face against or on which the absorber can be placed, deposited or formed, where the latter face abuts a corresponding face in a second prism that provides the second of the pair of reflective surfaces. A prism-based configuration offers the advantage of a robust construction in which the relative positions of the reflective surfaces and the absorber remain aligned with one another and the path lengths of the two spatial portions of the incident wave are fixed. However, prism-based devices are necessarily relatively heavy owing to the bulk of solid material, unsuitable for media-based wave types such as water waves, and effects on the wave transmission through the prism material may be unwanted.

Accordingly, in other designs, the pair of reflective surfaces is provided by a pair of mirrors or mirrored surfaces arranged at the relevant angle to one another, where the term “mirror” indicates that the surface has a characteristic that provides a reflecting property for the wave type of interest, such as an optical coating or use of a reflective or high refractive index material for light waves, or a physical barrier for water waves. These examples, in addition to being applicable to a wider variety of wave types, are also very useful for applications in which it is required to minimise mass, such as aerospace or space applications. Also, since mirror surfaces can be provided on thin and flexible substrates, foldable or collapsible designs are enabled that allow stowing away of a device for subsequent deployment into the required angled configuration of the mirror surfaces relative to one another and the absorber once at a final destination, such as in air or space, or another remote location. Folding can be achieved using flexible membranes or carbon-fiber-reinforced polymers (CFRP) that are themselves mirrored or act as a substrate to support a mirror coating and which can be folded or rolled, thin film mirrors supported by shaped brackets that spring into the required shape and/or position when the device is deployed, and rigid mirrors configured for folding via a hinged bending or flexing movement about the join or junction between the two reflective surfaces of the pair (or simply the midpoint if the reflective surfaces are formed from a single reflective element maintained in a bent configuration for use so as to define the pair of reflective surfaces).

Examples of suitable materials for forming the mirror reflective surfaces include, for electromagnetic wave devices, conventional silvered mirrors or similar, metals, thin film stacks, dielectrics, high-refractive-index films, reflecting (meta)material surfaces with two- or three-dimensional structures, all of which may be provided with or without a supporting substrate as required and as appropriate, as will be understood by the skilled person.

Returning to Figure 4, this shows a side view of an example wave absorber device implemented with mirrored reflective surfaces rather than total internal reflection. The device can be considered to have an aperture for receiving the incident wave which is bounded by the outer edges of the reflective surfaces 12a, 12b (rather than the solid input face of the prism-based designs), through which the incident wave passes before continuing propagation in the same medium as outside the device (air, water, vacuum, etc.) to reach the reflective surfaces. Drawing analogy with optical terminology, we can term this “free-space” propagation for all wave-types, as compared to propagation through a prism material. A non-rigid formation of reflective surfaces 12a, 12b at the angle 0 can allow folding or collapsing of the device such as by hinging or bending about the join between the reflective surfaces 12a, 12b, so that the reflective surfaces 12a, 12b can be moved towards or against the absorber to reduce the device size when the device is not in use, and folded out again to their intended operating position when the device has been deployed or is to be used, or to otherwise alter the angle 0. In other examples the reflective surfaces and optionally also the absorber may be fixed relative to one another, in order to ensure the device remains properly aligned.

Figure 9 shows a simplified side view of an example device in which the reflective surfaces are fixed. In this example, the reflective surfaces 12a, 12b are provided or formed on an appropriately shaped substrate comprising a block 21 of substrate material such as metal or polymer and having a shaped recess with sloped sides in one face corresponding to the shape and size of the reflective surfaces 12a, 12b, which are provided on the sloped sides of the recess by coating, polishing or affixing rigid mirrors. The absorber 14 is mounted to the base of the recess so as to lie between the reflective surfaces as required.

For both total internal reflection devices and free-space, mirrored devices, the angle between the pair of reflective surfaces need not be 90 degrees, as shown in the depicted examples thus far, but may be smaller or larger. A useful and practical range of angles is considered to be around 60 to 120 degrees, but other angles less than 180 degrees and more than 0 degrees can be used, such as in the larger range of 45 to 135 degrees. In many cases, an angle of or near 90 degrees will be practical, such as in the range of 85 to 95 degrees, or 80 to 100 degrees.

Also, it is not essential that the reflective surfaces (or the absorber) be perfectly flat or planar. Indeed, for microdevices and some folding or stretchable designs, the reflective surfaces are likely to be curved, at least to a small extent. Generally, any reflective surfaces may have roughness, surface structures (periodic or aperiodic, which may give the required reflective properties) and/or curvature in one or more dimensions. In determining the angle between the reflective surfaces, one can consider the overall average or general plane occupied by each reflective surface, so that the angle 0 is the angle between these two planes. Also, since it is challenging to fabricate perfectly sharp angles, any edges/corners/tips (for example of prisms, pyramids and cones) may be rounded or truncated.

Thus far, the example devices have been shown in side-view or cross-section only, as a two-dimensional device comprising a single pair of reflective surfaces receiving a wavefront extending along a single dimension (albeit that the reflective surfaces, the absorber, and the wavefront will often extend in the third dimension, which is into the plane of the page in the depicted examples). An incident wave may have a wavefront that extends in two dimensions, such as a beam of light or solar radiation. Absorber devices for such waves may be extended to be three-dimensional. For substantially planar reflective surfaces, this can be achieved by adding a second pair of reflective surfaces with an associated absorber between them, which are orthogonal to the first pair of reflective surfaces. To prevent loss of the incident wave passing straight through the device, the reflective surfaces can be in contact with one another along their edges to form a continuous four-sided reflector that completely surrounds the absorber(s).

Figure 10 shows schematic perspective views of some example three- dimensional devices. Each can be implemented using total internal reflection or mirror reflection, and in the latter case can be rigid or flexible/foldable/collapsible. Figure 10A shows an example device comprising two pairs of planar reflective surfaces. A first pair of reflective surfaces 12a, 12b and a first absorber 14 are arranged as described above, and shown in Figure 4, 5 or 6, for example. A second pair of reflective surfaces 12a’, 12b’ are arranged orthogonally to the first pair of reflective surfaces 12a, 12b. Since the reflective surfaces in each pair are tilted towards one another, the reflective surfaces each have a triangular shape so that the edges of adjacent reflective surfaces can be in contact or joined together, or otherwise placed adjacent to one another. This gives an overall square- or rectangular-based pyramidal shape to the device, where the base of the pyramid is defined by the free edges of the reflective surfaces (those not adjacent to other reflective surfaces) and forms the aperture or input face by which an incident wave enters the device 20. The absorber 14’ associated with the second pair of reflective surfaces 12a’, 12b’ is located between these reflective surfaces 12a’, 12b’, and bisects the absorber 14 of the first pair of reflective surfaces 12a, 12b. In operation, the wavefront of the incident wave is effectively divided into four spatial portions, pairs of which counterpropagate onto opposite faces of one or other of the absorbers 14, 14’. The two absorbers 14, 14’ could be considered as two absorbing elements that make up a single absorber.

As noted above, the reflective surfaces need not be flat, and may be curved. Figure 10B shows an example device that utilises this feature in order to shape the reflective surfaces as a cone, where the interior curved surface of the cone is reflective. The base of the cone forms the aperture or input face of the device 20. The first pair of reflective surfaces 12a, 12b can be considered to be two opposite quarters of the cone surface, and the second pair of reflective surfaces 12a’, 12b’ can be considered to be the remaining opposite quarters. A pair of bisecting planar absorbers could be provided as in the Figure 10A example. However, the depicted example comprises a single absorber 14 with an elongate shape that is located along the longitudinal axis of the cone. It is anticipated that cone-shaped devices may be particularly relevant for the absorption of longitudinal wave.

The full acceptance angle of wave absorber devices in the direction perpendicular to the (relevant) absorber is about arctan(A/D), where A is the incident wavelength and D is the smaller of the incident wave’s beam diameter and the aperture of the wave absorber device in the direction perpendicular to the absorber. Assuming a large beam diameter, it follows that this acceptance angle will be small if the device is much larger than the wavelength(s). The acceptance angle in the orthogonal direction is large. (For a design with multiple absorbers, such as the Figure 10A example, these considerations apply to each.) This has several implications.

Firstly, for applications that require a large overall aperture (compared to the wavelength) coupled with a large acceptance angle, arrays of small wave absorber devices can be used to attain both features. In particular, for broadband absorption of sunlight (for photovoltaic applications, for example), this indicates that arrays of wave absorber microdevices are of particular interest. An individual microdevice can have a size that is not significantly larger than the wavelength of light (for example, with dimensions less than twenty wavelengths), while coupling many microdevices together into a large spatial array gives a large aperture for collection of sunlight over a large area. Calculations show that wave absorber microdevices will allow simultaneous absorption of almost all direct sunlight.

Secondly, individual wave absorber devices that are large compared to the wavelength are suitable for precision metrology of orientations (applications in tilt metrology), since they will have a small acceptance angle and are therefore highly sensitive to variations in the propagation direction of the incident light. This is described further below with reference to Figure 16D.

Thirdly, the highly elongated acceptance profile of wave absorber devices that are large compared to the wavelength and contain a single absorber 14 (such as individual devices 20 or arrays of devices as illustrated in Figures 13 and 14, discussed further below) can be used to locate sources of waves, such as by scanning a region of the sky, sea/water or land with two wave absorber detector devices that have orthogonal orientations and rotate around orthogonal axes.

Where transparent substrates are used in wave absorber devices, such as the prisms in prism-based designs, the substrate material may have a graded phase velocity for the wave type of interest (so, graded refractive index for electromagnetic waves), and this may be used to increase the acceptance angle and/or reduce unwanted reflections. Spatial arrays of wave absorber devices offering an increased total aperture can be configured as a linear, one-dimensional array, or a two-dimensional array. For maximum wave collection, adjacent devices can be tessellated, that is located in contact or near contact with one another along the edges of the individual device apertures, in order to provide a continuous reflector surface that does not allow a significant proportion of the wavefront to pass between adjacent devices. For this reason, devices with planar reflective surfaces such as the pyramid device of Figure 10A may be more suitable than, for example, the cone device of Figure 10B. In other examples, gaps may be left between devices in an array.

Figure 11 shows a perspective view of an example two-dimensional array of individual wave absorber devices. The devices 20 have the pyramidal form of the Figure 10A example and are arranged in a plane with the edges of their individual apertures abutting against the edges of adjacent devices 20. The array 30 comprises nine devices in a 3x3 grid, but could comprise more or many more or fewer devices along either direction according to the total required aperture size for the array 30. The array 30 may be fabricated by assembling individual devices 20 together at the required location for the array 30, or by assembling the devices 20 together in advance before setting up the array 30 at the required location. If the devices 20 are flexible or foldable or collapsible, the array 30 may be collapsed along one or both dimensions in order to reduce its size for ease of transportation, before being unfolded to its intended size or a required size at the required location.

An array may be one-dimensional, comprising a linear assembly of individual devices.

Figures 12A-12E show schematic side views of several examples of linear arrays of wave absorber devices. Each array 30 is depicted as comprising three devices 20 only, but clearly any number of devices 20 may be coupled together in this way. Each device 20 abuts its neighbour along the edges of the individual apertures. Features of the various examples may also be implemented in two-dimensional arrays.

Figure 12A shows an example array 30 comprising a shaped substrate having multiple slope-sided recesses to define the reflective surfaces for each device 20, similar to the example of Figure 9. Each recess is filled with a medium 22 that is transparent to the intended wave type, which may be air in a simple case, or may comprise glass or a similar material as in the prism-based device of Figure 6. The interface between the transparent medium 22 and the substrate 21 may be coated with a material that is reflective for the waves, in order to form the reflective surfaces 12a, 12b. The substrate 21 may alternatively be formed from a material which is itself reflective for the waves. For devices intended for electromagnetic waves, the reflective material may be a mirror, a metal, a thin film stack, a dielectric, a high-refractive-index film or a reflecting (meta)material surface with a 2D or a 3D structure, for example. This array design gives a robust device with fixed alignment of the reflective surfaces 12a, 12b and the absorbers 14.

Figure 12B shows an example array 30 in which the devices 20 are supported and held together in an adjacent configuration by being fixed to a substrate 23 placed in front of the apertures of the devices 20. The substrate 23 is made of a material which is transparent to the wave and wavelength range of interest so that incident waves can enter the devices 20. Example materials include glass for visible light, silicon for infrared light, crystals and polymers. Also, the substrate 23 may have a graded phase velocity for the waves (that is, a graded refractive index for devices intended for electromagnetic waves). Anti-reflection and/or birefringent layers as described above may be applied to the substrate, or these properties provided by the substrate itself. The substrate material may or may not be the same transparent material that makes up the individual devices 20 in the case of prism-based devices. The devices 20 may alternatively be free-space devices so that air occupies the space between the substrate 23 and the reflective surfaces 12a, 12b.

Figure 12C shows an example array 30 which is lightweight, and comprises a zig-zag shaped element 24 that defines the pairs of reflective surfaces 12a, 12b by the sloped sides of each trough on one side of the zig-zag shape. The reflective property of the reflective surfaces 12a, 12b (mirrors, for free-space devices) may be provided by a reflective surface coating on the zig-zag shaped element 24, or the zig-zag shaped element may be formed from a reflective material. The absorber 14 of each device is mounted to and extends from the base of each trough of the zig-zag shaped element 24. The zig-zag shaped element may be formed as a unitary piece on which the individual devices are fabricated, or it may be assembled by connecting a plurality of individual devices edge-to-edge. If the angles of the zig-zag shaped element 24 are made hinged, bendable or flexible, the array 30 can be collapsed or retracted (extended) into a shorter (longer) overall length. This allows the array to be made more compact for ease of transportation and storage, and/or allows the angle between the pairs of reflective surfaces 12a, 12b in each device 20 to be adjusted, for example to provide tunable devices, or allow the array to be compressed or expanded to fill a required location.

Figure 12D shows the array 30 of Figure 12C in a slightly collapsed or shortened configuration, in which the angle between each pair of reflective surfaces has been made smaller. Figure 12E shows an alternative array 30 based on a zig-zag shaped element. In this example, the array is double-sided, in that a device 20 is formed in and by the troughs on both sides of the zig-zag shaped element 24.

Figure 13 shows a simplified perspective view of an example array of wave absorber devices configured for operation as a photovoltaic device or solar panel or solar cell or photodetetector for the conversion of light 26 such as sunlight 25 into electricity 27. The array 30 comprises six elongate wave absorber devices 20 which may be prism-based or of a free-space mirror-based design. However, such arrays could also comprise more than six or fewer than six wave absorber devices 20. By “elongate” it is meant that each device 20 has a much longer dimension along the direction parallel to its absorber 14 than perpendicular to the absorber. Adjacent devices 20 are connected or joined edge-to-edge along their long sides (sides of their individual apertures) in order to form a much larger overall aperture for the array 30 in order to harvest more incident light. The use of elongate devices 20 allows the size of the array 30 to be increased in an alternative manner to a two-dimensional grid of square (or rectangular) devices as in Figure 11, allowing a large overall aperture to be achieved from a smaller number of devices, so that fewer connections to the array for the purposes of electricity generation are required (as there is a lower number of absorbers).

Absorbers and absorbing layers in photovoltaic wave absorber devices can be made from a photoactive material so that the absorber not only absorbs all relevant incident electromagnetic radiation (typically light) but also produces electricity. Several permutations of optical designs, scaling, and various absorbing thin film materials can be used to optimise the concept for different applications. Photoactive materials include materials/structures such as thin film and 2D semiconductors (amorphous or crystalline), perovskites, 3D/2D transition-metal dichalcogenides (TMDs) including with dry method of fabrication, dye-sensitised, organic cells, or quantum dot cells - all single or multijunction with charge separation and carrier layers. Applications include photovoltaics and detectors (including quantum detectors). Additionally, the absorber layer can be a waveguide or part of a waveguide, with/without quantum dots and/or be part of a luminescent solar concentrator design permutation (cells at the edges).

Figure 14 shows a simplified perspective view of an example array of wave absorber devices configured as an acoustic absorber device. Again, the array 30 comprises a plurality of elongate wave absorber devices 20 adjacent to one another along their long edges. The reflective surfaces 12a, 12b and the absorbers 14 are configured for the reflection and absorption of sound energy from an incident sound wave 28. The array 30 could extend further in either or both dimensions (wider by comprising longer devices 20 or higher by comprising more devices), and as such can be considered as a fragment of an acoustic noise absorbing barrier such as are deployed next to roads and rail tracks, or a fragment of an acoustic noise absorbing cladding such as are used on the walls and ceiling of a road or rail tunnel.

Figure 15 shows a simplified perspective view of an example wave absorber device 20 configured for use as a water wave energy harvester to absorb energy from incident water waves 29 for conversion into electricity 27. A device 20 such as this could be constructed at the edge of the sea (individually or in one or more arrays), with the absorbing element 14 comprising of a floating device that is raised and lowered by the incident water waves on the surface 31 of the water, with additional components that transform this motion into another form of energy (e.g. electricity 27). The reflecting material for the reflecting surfaces 12a, 12b could be concrete or stone or a similar hard and resilient surface able to withstand prolonged exposure to water and weather.

In a wave absorber device, the wave-reflecting surfaces are ideally arranged such that the redirected waves originating from the two spatial portions of the incident wave are normally incident on opposite sides of the absorber. Assuming that the waves do not propagate along curved paths within the device, this will be achieved when the pair of reflecting surfaces and the associated absorber have normals in the same plane, with a 90° angle between the reflective surfaces and angles of 45° between a reflecting surface and the absorber in that plane, where the reflecting surfaces reflect waves onto opposite sides of the same absorber. Deviations from this ideal are expected to degrade device performance for longitudinal waves and for one or all polarization components (for example, s-polarization and/or p-polarization) of transverse waves, depending on the specifics of the deviations. Nevertheless, different angles may be chosen, particularly in devices for only one transverse polarization. Such different angles can arise from different types of 3D structures with different heights and the absorber placed at different planes/positions. Small deviations from ideal arrangements should be expected in realistic structures and will have only a small effect on device performance. Also, since the angle between the reflective surfaces does not have to be 90° for device operation, it may be changed in order to tailor the input aperture area, the weight/size, and/or the performance of the device.

Some experimental results are now presented to demonstrate the efficiency and broadband capability of the proposed wave absorber devices. The results were obtained for electromagnetic waves.

Figure 16A shows a graph of the variation with (free-space) optical wavelength of measured optical power reflected from an experimental device, as a percentage of the input power. Four sets of data are shown, corresponding to different positions of the reflective surfaces in the device. The device 20 comprised a pair of glass prisms 16 having between them an absorber layer 14 comprising a single layer of chromium film of 20 nm thickness, and reflectivity measurements were obtained over a wavelength range of 1540 nm - 1620 nm. The alignment of the device was altered by displacing (translating) one prism relative to the other along the propagation direction of the incident light wave 10, which entered the device normal to the input aperture face. This is indicated in the diagrams at the right-hand side of the Figure, where Case A shows a maximum displacement and misalignment of the prisms, and Case D shows a zero displacement in which the prisms are completely aligned along the propagation direction so that the effective optical path lengths for the two spatial portions of the incident wave are equal. Plots of the measured reflectivity are shown for Case A and Case D, plus two intermediate Cases B and C for intermediate amounts of misalignment (alignment improving from Case A through to Case D, as indicated by the arrow on the graph).

The graph shows that when the effective path lengths are not matched in a wave absorber device (Cases A-C), the reflectivity spectrum of the device oscillates between coherent absorption and coherent transmission. As the prisms are translated relative to each other to reduce the path difference, the period of the oscillation increases, representing a increase in the bandwidth of coherent absorption; the troughs in the oscillating curves are broadened as the path difference is reduced - compare Case A to Case B and then Case C, which show a progressive flattening of the reflectivity and hence a broadening of the absorption (since absorption is the complement of reflectivity, as device transmission is zero in all cases). When the prisms are fully aligned and the optical path lengths are matched, the curve becomes flat at a low level of reflectivity, indicating a very broad absorption bandwidth (Case D). In this example, there is only 9% reflectivity across the wavelength range, implying greater than 90% absorption. The data shown throughout Figure 16 has been processed to analytically apply an anti-reflection coating the input face of the device.

Figure 16B shows a graph of reflectivity measurements from the device over the same wavelength range for the cases of matched effective optical path lengths (line Ab), and one effective optical path length changed by about half a wavelength relative to the other by translation of one prism relative to the other (line Tr). The lines are substantially flat, and show how the device can be switched between broadband coherent absorption by the chromium absorber layer (device reflectivity only 9%) and broadband coherent transmission through the absorber layer (device reflectivity of 85%), that is, between device operation as an absorber and as a mirror. These results demonstrate not only the high effectiveness of the proposed devices for broadband wave energy absorption, but also indicate a high sensitivity to nanoscale displacements (in the case of devices for waves with nanoscale wavelengths such as light waves), which may be used for precision metrology, since a small spatial displacement within the device causes a large change in reflectivity performance so that displacement can be detected or measured (with suitable calibration) by monitoring the amount of incident wave power reflected from the device. An example use for this is for platform position metrology, by coupling the device to the platform such that movement of the platform causes a change in the path length difference within the device, even very small changes in platform position may be detected from changes in the reflected light level.

Figure 16C shows a graph of measured reflected power level (as a percentage of incident wave power level) from the experimental device for a range of relative prism displacements between 0 nm and 2700 nm, demonstrating a significant change in reflectivity for very small displacements, illustrating the high sensitivity to position that can be utilised for metrology. Two sets of data are shown, for an s-polarised incident light wave and a p-polarised incident light wave, indicating the different response of the device (which contains a single layer absorber) to different wave polarisations. As can be seen, due to phase changes upon reflection of s- and p-polarisations, absorption of s- polarised light is accompanied by transmission (reflection) of p-polarised light by the thin absorber layer (by the device), and vice versa. The device is hence sensitive to the polarisation of the incident wave. Polarisation-independent operation including broadband absorption may be achieved as described above, by the use of multiple absorbing layers in the absorber, and/or birefringent elements in the optical path.

In addition to sensitivity to displacement of elements within a device to alter the path difference, the proposed devices also show sensitivity to the angle of incidence of the incident wave, which can similarly be used for precision metrology and angular position (orientation) sensing or detection.

Figure 16D shows a graph of the variation of measured reflected light power level (as a percentage of incident light power level) with angle of incidence, for the same experimental device having an absorber comprising a single layer of 20 nm thickness chromium film. Here the prisms of the device are aligned but the angle of incidence of the incident wave was altered, as indicated by the diagrams at the right-hand side of the Figure. The upper diagram shows a zero degree angle of incidence (the incident wave 10 is normal to the input face of the device 20) and the lower diagram shows a non-zero degree angle of incidence. The incident wave was infrared light with a (free-space) wavelength of 1570 nm, and the beam diameter of the light was 7 mm, smaller than the input face of the device so that the whole wavefront is captured by the device. Measurements for both s-polarised light and p-polarised light are shown, again demonstrating the polarisation-sensitivity of the experimental device. The large slope of the data curves around normal incidence shows a high sensitivity to angular displacement. The device is sensitive to angle of incidence changes of 0.001 degree.

Figure 17 shows a flow chart of an example method of absorbing wave energy according to the approach presented herein. In a first step S1, an incident spatially coherent wave is directed onto a pair of reflective surfaces arranged to have an angle of less than 180 degrees between the reflective surfaces. In a second step S2, different spatial portions of the wavefront of the incident spatially coherent wave are intercepted by each reflective surface so that the different spatial portions are each reflected away from the propagation direction of the incident spatially coherent wave. In a third step S3, the different spatial portions are received on opposite sides of an absorber arranged between the pair of reflective surfaces. In a fourth step S4, the different spatial portions interfere to form a standing wave at the absorber, so that the energy of the incident spatially coherent wave is at least partly absorbed (or transmitted) by the absorber. As previously, “spatially coherent” includes both partly and wholly spatially coherent.

Figure 18A shows a graph of calculated absorption spectra as absorption A (y axis) as a function of wavelength A (x axis) for several modelled example devices configured for incident waves in the form of electromagnetic waves (light). The devices are formed from prisms with a refractive index of 1.5, have ideal anti refl ection coatings, and a selection of absorber layers of different materials and thicknesses. These are a 20 nm thickness of vanadium nitride (short dash line), a 20 nm thickness of chromium (solid line), a 40 nm thickness of graphite (long dash line), and a 99 nm thickness of pyrolytic carbon (dotted line). Results are shown for the cases of both coherent absorption (upper lines), produced from constructive interference of incident waves on the absorber layer, and coherent transmission (lower lines), produced from destructive interference of incident waves on the absorber layer, leading to reflection of light from the device. The calculations used complex refractive index data for the absorber materials. Data are shown across 11 octaves of the electromagnetic spectrum (noting that the wavelength on the x axis has a logarithmic scale) from extreme ultraviolet (31.25 nm) to far infrared (64000 nm).

Figure 18B shows individual graphs of the calculated absorption spectra for these example devices, plus a device with an absorption layer of 14 nm thickness molybdenum silicide (MoSi), a device with an absorption layer of 24 nm thickness niobium nitride (NbN) and a device with an absorption layer of 24 nm thickness niobium titanium nitride (NbTiN). These spectra are plotted with a linear scale for wavelengths on the x axis. Coherent absorption is shown in the graphs on the left of Figure 18B and coherent transmission is shown in the graphs on the right.

It can be seen from these graphs that coherent absorption of a 20 nm thick film of chromium is predicted to exceed 93% across the mid ultraviolet to near infrared spectral range (wavelengths <188 nm to >1937 nm; >3.4 octaves), indeed more than 98% absorption is predicted in the near infrared (676 nm to 1680 nm; 1.3 octaves). A 20 nm thickness of vanadium nitride should absorb more than 96% from the far ultraviolet to the near infrared (150 nm to 2500 nm; 4 octaves), while absorption of a 40 nm thickness of graphite should exceed 98% in the mid infrared (1730 nm to >10330 nm; >2.6 octaves), and 99 nm of pyrolytic carbon is expected to absorb more than 98% in the near and mid infrared (990 nm to 30500 nm; 5 octaves), even exceeding 99.98% in the mid-infrared (3580 nm to 14350 nm; 2 octaves).

Coherent transmission is predicted to yield negligible absorption for wavelengths that are large compared to the optical thickness of the absorbing film. Absorption drops below 5% at wavelengths above 582 nm for a 20 nm thickness of chromium, at wavelengths above 410 nm for a 20 nm thickness of vanadium nitride, at wavelengths above 1005 nm for a 40 nm thickness of graphite, and at wavelengths above 2280 nm for a 99 nm thickness of pyrolytic carbon. With increasing wavelength, absorption asymptotically approaches zero, for example, dropping below 1% for wavelengths longer than 1080 nm for the chromium absorber layer and below 1% and 0.1% for wavelengths longer than 2175 nm and 6825 nm for the graphite absorber layer.

As illustrated by Figure 18B, NbTiN, NbN and MoSi are promising absorber materials for the near ultraviolet to near infrared spectral range. Furthermore, doped silicon films may be considered for the far infrared, terahertz and microwave spectral ranges. It follows that devices as proposed herein can deliver near-complete absorption and/or reflection of electromagnetic waves across very large spectral ranges (spanning multiple octaves) when configured using existing absorber materials as an absorber layer.

The various embodiments described herein are presented only to assist in understanding and teaching the claimed features. These embodiments are provided as a representative sample of embodiments only, and are not exhaustive and/or exclusive. It is to be understood that advantages, embodiments, examples, functions, features, structures, and/or other aspects described herein are not to be considered limitations on the scope of the invention as defined by the claims or limitations on equivalents to the claims, and that other embodiments may be utilised and modifications may be made without departing from the scope of the claimed invention. Various embodiments of the invention may suitably comprise, consist of, or consist essentially of, appropriate combinations of the disclosed elements, components, features, parts, steps, means, etc., other than those specifically described herein. In addition, this disclosure may include other inventions not presently claimed, but which may be claimed in the future.

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[4] J Zhang et al, “Controlling light-with-light without nonlinearity”, Light: Science & Applications 1 , e18 (2012)

[5] E. Plum et al, “Controlling the optical response of 2D matter in standing waves”, ACS Photonics 4(12), 3000-3011 (2017)

[6] M Lanoy et al, “Broadband coherent perfect absorption of acoustic waves with bubble metascreens”, Applied Physics Letters 113(17), p.171907 (2018)

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Claims

1. A wave absorber device for absorbing energy from an incident wave that is at least partly spatially coherent, the device comprising: a pair of reflective surfaces configured to reflect the incident wave and with an angle between the reflective surfaces of less than 180 degrees, the pair of reflective surfaces arranged to each intercept a different spatial portion of the wavefront of the incident wave and reflect the spatial portion along a direction away from a propagation direction of the incident wave; and an absorber configured to at least partly absorb the incident wave and having two opposite sides, the absorber arranged between the pair of reflective surfaces so as to receive, from the pair of reflective surfaces, a different spatial portion on each of the opposite sides such that the different spatial portions interfere with one another to form a standing wave at the absorber.

2. A wave absorber according to claim 1, wherein the pair of reflective surfaces comprises a pair of surfaces having a characteristic providing a reflecting property for the incident wave.

3. A wave absorber according to claim 2, wherein the incident wave propagates through free space to be incident on the surfaces having a characteristic providing a reflecting property for the incident wave.

4. A wave absorber device according to any one of claims 1 to 3, wherein the pair of reflective surfaces are movable with respect to one another such that the wave absorber device can be collapsed, compressed or modified in size or shape, or that the size of the angle between the reflective surfaces can be altered.

5. A wave absorber device according to claim 1 , wherein the pair of reflective surfaces comprises a pair of boundaries at which total internal reflection of the incident wave occurs.

6. A wave absorber device according to claim 5, comprising two prisms formed from a material substantially transparent to the incident wave, a first face of each prism providing one boundary of the pair of boundaries.

7. A wave absorber device according to claim 6, wherein the absorber is located between facing second faces of the prisms.

8. A wave absorber device according to claim 6 or claim 7, wherein third faces of the prism together define an input face of the device.

9. A wave absorber device according to claim 8, comprising an anti refl ection element for reducing reflection of the incident wave on the input face.

10. A wave absorber device according to any preceding claim, further comprising a birefringent element in a propagation path of one of the different spatial portions.

11. A wave absorber device according to any one of claims 1 to 10, wherein the incident wave is an electromagnetic wave.

12. A wave absorber device according to any one of claims 1 to 9, wherein the incident wave is an acoustic wave.

13. A wave absorber device according to any one of claims 1 to 9, wherein the incident wave is a water wave.

14. A wave absorber device according to any one of claims 1 to 10, wherein the incident wave is a transverse wave.

15. A wave absorber device according to any one of claims 1 to 9, wherein the incident wave is a longitudinal wave.

16. A wave absorber device according to any one of claims 1 to 15, wherein the absorber comprises only a single absorbing layer.

17. A wave absorber device according to any one of claims 1 to 15, wherein the absorber comprises two or more absorbing layers spaced apart from one another.

18. A wave absorber device according to claim 17, wherein the two or more absorbing layers are spaced apart from one another by A/4, a multiple of A/4, A/2, a multiple of A/2, A or a multiple of A, where A is a wavelength of the incident wave.

19. A wave absorber device according to any one of claims 16 to 18, wherein the or each absorbing layer has a thickness along a direction between the two opposite sides of the absorber which is not more than A/4, where A is a wavelength of the incident wave.

20. A wave absorber device according to any one of claims 1 to 19, further comprising a second pair of reflecting surfaces and a second absorber arranged between the second pair of reflecting surfaces, the second pair of reflecting surfaces positioned substantially orthogonally to the pair of reflecting surfaces and the second absorber positioned to substantially bisect the absorber.

21. A wave absorber device according to any one of claims 1 to 5 or 10 to 19, wherein the pair of reflecting surfaces are two diametrically opposite portions of a cone- shaped reflecting surface, and the absorber has an elongate shape and is positioned along the longitudinal axis of the cone-shaped reflecting surface.

22. A wave absorber device according to any preceding claim, wherein the angle between the reflective surfaces is substantially 90 degrees.

23. A wave absorber array comprising a plurality of wave absorber devices according to any preceding claim.

24. A wave absorber array according to claim 23, wherein the plurality of wave absorber devices are tessellated within the array.

25. A method of absorbing wave energy comprising: directing an incident wave that is at least partly spatially coherent onto a pair of reflective surfaces configured to reflect the incident wave and with an angle of less than 180 degrees between the reflective surfaces such that each reflective surface intercepts a different spatial portion of the wavefront of the incident wave and reflects the spatial portion along a direction away from a propagation direction of the incident wave; and receiving the different spatial portions on opposite sides of an absorber configured to at least partly absorb the incident wave, the absorber arranged between the pair of reflective surfaces such that the different spatial portions interfere with one another to form a standing wave at the absorber.