An ultrawideband flexible THz absorber based on a structured water layer

Abstract

An ultrawideband flexible THz absorber based on a structured water layer is characterized by comprising a four-layer structure, from top to bottom: the first layer is a dielectric layer; the second layer is a structured water layer, which is a cuboid with a hemisphere on top; the third layer is the same dielectric layer as the first layer; and the fourth layer is a conductive glass layer. By introducing the structured water layer, the characteristic impedance of the absorber can be well matched with the free-space impedance across an extremely wide frequency band, thus maintaining extremely high absorption across this wide range. The interface between the first dielectric layer and the second structured water layer, and the fourth metal reflective layer, constitute a Fabry-Perot resonant cavity with a continuously varying cavity length. This causes electromagnetic waves with continuously varying wavelengths entering the absorber to undergo multiple reflections and destructive interference within the cavity until they are completely absorbed.

Description

Technical Field

[0001] This invention belongs to the field of terahertz metamaterial design technology, specifically relating to an ultrawideband flexible THz absorber based on a structured water layer. Background Technology

[0002] Terahertz (THz) waves are electromagnetic waves with frequencies ranging from 0.1 to 10 THz, located in the frequency gap between microwaves and infrared waves. They represent a transitional region from electronics to photonics, known as the "terahertz gap." Many characteristics of THz waves, such as low energy, broadband, and fingerprint spectrum, give them immense application potential in non-destructive testing, wireless communication, and biomedicine. Based on this, scholars both domestically and internationally have conducted a series of studies on THz technology and its applications. However, the extremely weak electromagnetic response of THz waves to traditional materials has led to a shortage of THz detectors, modulators, and other devices, severely limiting the research and application of THz waves. Metamaterials, artificial composite materials composed of subwavelength unit structures arranged in a specific pattern, have injected fresh blood into the development of THz devices, providing limitless possibilities. In recent years, THz devices based on metamaterials have sprung up like mushrooms after rain, with THz perfect absorbers, THz superlenses, and THz polarization converters receiving extensive research. Among them, THz absorbers have become one of the current research hotspots due to their important applications in electromagnetic compatibility, stealth, and thermal radiation collection. Currently, research on THz absorbers has become increasingly mature, but they still suffer from problems such as narrow bandwidth and overly demanding application scenarios. Therefore, further research is needed on absorbers with wider operating frequency bands.

[0003] The traditional approach to broadband THz absorbers involves stacking resonators of different sizes within a single unit structure. The coupling between the resonant peaks excited by these resonators achieves broadband absorption. There are two main approaches to stacking resonators: one is arranging resonators capable of exciting peaks at different frequencies on the same plane, such as in some topological fractal structures; the other is layering resonators of different sizes, separated by a dielectric layer. Undoubtedly, both approaches can broaden the absorber's bandwidth to some extent, but they also significantly increase the manufacturing complexity and cost. Researchers began seeking new solutions. Dispersive media came into focus. Using dispersive materials (such as doped silicon or water) as the absorber's medium allows for impedance matching over a wide spectral range, resulting in a broad operating bandwidth. However, using doped silicon as the medium in the absorber increases its cost. In contrast, adding water to the absorption structure not only significantly broadens the absorber's bandwidth but also greatly reduces costs due to water's abundance in nature. Currently, research on water-based THz absorbers is relatively limited, with most researchers focusing on the microwave band. Furthermore, the absorption bandwidth in these microwave bands is not ideal, hindering practical applications. Therefore, it is necessary to further develop ultra-wideband absorbers in the THz band to advance THz technology. Summary of the Invention

[0004] This invention designs an ultrawideband flexible THz absorber based on a structured water layer, which has the characteristics of ultrawideband absorption, large-angle absorption and polarization insensitivity.

[0005] The technical solution adopted in this invention is the design of an ultra-wideband flexible THz absorber based on a structured water layer. This absorber consists of four layers, from top to bottom: the first layer is a dielectric layer; the second layer is a structured water layer, which is a cuboid with a hemisphere on top; the third layer is the same dielectric layer as the first layer; and the fourth layer is a metallic reflective layer. The designed absorber introduces water as a dispersive medium, allowing its characteristic impedance to match well with the free-space impedance over an extremely wide frequency band, thereby effectively broadening the absorber's absorption bandwidth. Simultaneously, the interface between the first dielectric layer and the second water layer, and the fourth metallic reflective layer, constitute a Fabry-Perot resonant cavity, causing electromagnetic waves entering the absorber to undergo multiple reflections and destructive interference within the absorber until they are completely absorbed. Because the water layer is shaped like a cuboid with a hemisphere at the top, its thickness is not constant but continuously varying along the direction of electromagnetic wave incidence. This causes the cavity length of the Fabry-Perot cavity to also vary continuously, meaning that the wavelength of the electromagnetic wave satisfying the Fabry-Perot resonance is not unique but rather a continuous electromagnetic spectrum. In other words, any electromagnetic wave with a specific continuous frequency incident on the invented absorber can excite the Fabry-Perot resonance, thereby greatly widening the absorption bandwidth of the absorber. Therefore, the absorber can maintain extremely high absorption over a very wide frequency band.

[0006] The structured water layer top layer can be, but is not limited to, hemispherical or pyramidal shapes.

[0007] The material of the dielectric layer may be, but is not limited to, polydimethylsiloxane (PDMS).

[0008] The material of the metal reflective layer can be replaced with a conductive glass layer, which can also block the transmission of THz waves and has extremely high transmittance in the visible light band.

[0009] Furthermore, the dielectric constant of water in the THz band is described by the Liebe model as follows:

[0010] ε(ν,T)=ε M (ν,T)+ε R (ν,T) (1)

[0011] In the formula, ε M (ν,T) and ε R (ν,T) represent the Debye formula applied to 0.1-1THz and the Lorentzian resonance term applied to 1-30THz, respectively, and can be further described as:

[0012]

[0013]

[0014] In the formula, ε0=77.66–103.3θ, ε1=0.0671ε0, ε2=3.52+7.52θ are the static dielectric constant, the mid-frequency dielectric constant, and the high-frequency dielectric constant, respectively, and γ1=20.20+146.4θ+316θ 2 γ2 = 39.8γ1 are the first and second order relaxation frequencies, respectively. B1 = 25.03, ν1 = 5.11, γ3 = 0.00446, B2 = 282.4, ν2 = 18.2, γ4 = 0.0154, Debye parameter θ = 1-300 / T.

[0015] Furthermore, the two mirrors of the Fabry-Perot cavity are asymmetrical. One mirror, composed of the underlying metallic reflective layer, is perfectly reflective, while the other mirror, formed by the interface between the first medium and the water, is partially reflective. Simultaneously, due to the hemispherical structure of the water layer, a Fabry-Perot cavity with a continuously varying cavity length is achieved. According to the definition of a Fabry-Perot cavity, when the refractive index of the medium, the distance between the two mirrors, and the incident wavelength meet certain conditions, the incident wave will undergo multiple reflections between the two mirrors, resulting in destructive interference and thus perfect absorption.

[0016] The ultra-wideband absorber described in this invention uses water as the resonant medium and utilizes the dispersive properties of water to achieve impedance matching within an ultra-wideband frequency range. Furthermore, by optimizing the structure of the water layer, a Fabry-Perot cavity with continuously varying cavity length is constructed inside the absorber, ensuring that not only single-wavelength THz waves meet the condition of destructive interference within the cavity, thereby achieving broadband high absorption.

[0017] The ultra-wideband absorber of this invention uses flexible, bendable but not easily deformable polydimethylsiloxane as the water container, and combines it with easily bendable metals, such as gold and copper, as the metal back plate of the absorber, so that the absorber of the invention has flexible characteristics and is applicable to a wider range of scenarios.

[0018] The beneficial effects of the present invention are: compared with the prior art, the ultra-wideband absorber of the present invention has an ultra-wide absorption bandwidth, a simple structure, and a novel concept for achieving broadband high absorption. Attached Figure Description

[0019] To provide a clearer and more detailed description of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be noted that the drawings are merely some embodiments of the present invention and do not imply any limitation on the scope of protection of the present invention.

[0020] Figure 1 This is a schematic diagram of the absorber structure in Embodiment 1 of the present invention. The absorber consists of a medium layer, a water layer with a hemispherical top, a medium layer, and a metal reflective layer.

[0021] Figure 2 These are the absorption rate curves of the absorber in Embodiment 1 of the present invention at 300K as a function of frequency, and the absorption rate curves of the absorber with an unstructured water layer at water layer thicknesses of h2 and h2+r.

[0022] Figure 3 This is the curve showing the relative impedance of the absorber in Embodiment 1 of the present invention as a function of frequency.

[0023] Figure 4 This is a schematic diagram of the absorber structure in Embodiment 2 of the present invention. The absorber is composed of a dielectric layer, a water layer with a pyramid-shaped top, a dielectric layer, and a conductive glass layer.

[0024] Figure 5 These are the absorption rate curves of the absorber in Embodiment 2 of the present invention at 300K as a function of frequency, and the absorption rate curves of the absorber with an unstructured water layer at water layer thicknesses of h2 and h2+h3.

[0025] Figure 6 This is the curve showing the relative impedance of the absorber in Embodiment 2 of the present invention as a function of frequency. Detailed Implementation

[0026] The design scheme of the present invention will be clearly and completely described below with reference to the accompanying drawings. The described embodiments are only some embodiments of the present invention and are not intended to limit the scope of protection of the present invention in any way.

[0027] Example 1

[0028] An ultra-wideband flexible THz absorber unit structure based on a structured water layer consists of four layers, from top to bottom: a dielectric layer 1 made of polydimethylsiloxane, a hemispherical water layer 2, a dielectric layer 3 made of polydimethylsiloxane, and a reflective layer 4 made of metallic copper. Figure 1 As shown. In an embodiment, the period p of the unit structure is 75 micrometers, the height h2 of the cuboid water layer is 80 micrometers, the radius r of the hemispherical water layer is p / 2, i.e., 37.5 micrometers, and the thickness h1 of the dielectric layer 1, the thickness h3 of the dielectric layer 3, and the thickness t of the reflective layer 4 are 47.5 micrometers, 5 micrometers, and 1 micrometer, respectively.

[0029] Figure 2The absorption spectrum of the absorber in this embodiment is shown in the figure, considering both the presence and absence of a Fabry-Perot cavity with continuously varying cavity length. In this embodiment, an absorber with the same thickness of water as the invented absorber but without a top hemispherical water layer, consisting only of a cuboid water layer, is considered an absorber without a continuously varying cavity length Fabry-Perot cavity. As shown in the figure, the Fabry-Perot cavity, formed by the interface between the dielectric layer 1 and the water layer 2, and the bottom reflecting surface 4, effectively enhances the absorption rate by strengthening the destructive interference of the incident wave's reflection within the absorber. The absorber exhibits an absorption rate greater than 90% and a relative bandwidth of 196.6% in the frequency range of 0.26 THz to 30 THz. Since there is currently no well-fitting theoretical model for the dielectric constant of water above 30 THz, no further calculations are performed in this embodiment. However, it can be reasonably predicted that the absorption band of the absorber in this embodiment can extend from 30 THz to 50 THz or even higher.

[0030] Figure 3 The figure shows the relative impedance of the absorber as a function of frequency. Due to the high dielectric constant of water, it excites many absorption peaks. However, in this absorber, the fluctuations of the absorption peaks excited by water are very small, but the number of absorption peaks is still large, resulting in a very flat absorption curve. As can be seen from the figure, in the frequency band from 0.26THz to 30THz, the real part of the absorber's relative impedance is close to 1, and the imaginary part is close to 0. The absorber impedance is well matched with free space, achieving broadband high absorption.

[0031] Figure 4 This is a schematic diagram of the absorber unit structure in Example 2, which consists of four layers, from top to bottom: a dielectric layer 1 made of polydimethylsiloxane, a top-layer water layer 2 with a pyramid structure, a dielectric layer 3 made of polydimethylsiloxane, and a reflective layer 4 made of conductive glass, as shown below. Figure 4 As shown. In an embodiment, the period p of the unit structure is 75 micrometers, the height h2 of the cuboid water layer is 85 micrometers, the side length w of the top base of the pyramid-shaped water layer is 14 micrometers, the height h3 is 85 micrometers, the thickness h1 of the dielectric layer 1, the thickness h4 of the dielectric layer 3, and the thickness t of the reflective layer 4 are 95 micrometers, 5 micrometers, and 1 micrometer, respectively.

[0032] Figure 5The absorption spectrum of the absorber in Example 2 is shown with and without a Fabry-Perot cavity of continuously varying cavity length. In this example, an absorber with water of the same thickness as the invented absorber but without a top pyramid-shaped water layer, consisting only of a cuboid water layer, is considered as an absorber without a Fabry-Perot cavity of continuously varying cavity length. As shown in the figure, the Fabry-Perot cavity, formed by the interface between the dielectric layer 1 and the water layer 2 and the bottom reflecting surface 4, effectively enhances the absorption rate by strengthening the destructive interference of the incident wave reflection within the absorber. The absorber exhibits an absorption rate greater than 90% and a relative bandwidth of up to 194.6% in the frequency range of 0.41 THz to 30 THz.

[0033] Figure 6 The figure shows the relative impedance of the absorber in Example 2 as a function of frequency. As can be seen from the figure, in the frequency band from 0.26THz to 30THz, the real part of the relative impedance of the absorber is close to 1, and the imaginary part is close to 0. The absorber impedance is well matched with free space, achieving broadband high absorption.

[0034] In summary, this invention proposes an ultra-wideband flexible THz absorber based on a structured water layer. By using water as the dispersive medium for the absorber's resonant medium, impedance matching between the absorber and free space is achieved across an ultra-wide bandwidth, effectively broadening the absorption bandwidth. Simultaneously, by optimizing the water layer structure, a Fabry-Perot resonant cavity with continuously varying cavity length is formed between the interface between the top layer medium and water, and the bottom layer metal reflective surface. This resonant cavity structure can simultaneously support multiple reflections of electromagnetic waves of different wavelengths within the cavity, achieving destructive interference and thus improving the absorber's absorptivity. Furthermore, this proposed ultra-wideband absorption method can be applied not only to absorber design in the THz band but also to absorbers in the microwave and infrared bands. The absorber possesses advantages such as ultra-wide absorption bandwidth, high absorptivity, and flexibility.

[0035] The above describes the technical principles and specific examples of the application of this invention. Any equivalent or similar designs or improvements made based on the concept of this invention should be included within the scope of protection of this invention.

Claims

1. An ultrawideband flexible THz absorber based on a structured water layer, characterized in that: The absorber comprises a four-layer structure, from top to bottom: the first layer is a dielectric layer, the second layer is a structured water layer, which is a cuboid with a hemisphere on top, the third layer is the same dielectric layer as the first layer, and the fourth layer is a metal reflective layer. By introducing the structured water layer, the characteristic impedance of the absorber can be well matched with the free space impedance over an extremely wide frequency band, thus maintaining extremely high absorption over an extremely wide frequency band. The interface between the first dielectric layer and the second structured water layer, and the fourth metal reflective layer, constitute a Fabry-Perot resonant cavity with a continuously varying cavity length. This causes electromagnetic waves with continuously varying wavelengths entering the absorber to be reflected multiple times and undergo destructive interference inside until they are completely absorbed.

2. The ultrawideband flexible THz absorber based on a structured water layer according to claim 1, characterized in that: The structured water layer and the metal reflective layer can form a Fabry-Perot cavity with a continuously varying cavity length. The cuboid water layer forms a first resonant cavity with a uniform thickness, and the hemispherical water layer forms a second resonant cavity with a continuously varying thickness, thereby exciting Fabry-Perot resonance.

3. The ultrawideband flexible THz absorber based on a structured water layer according to claim 1, characterized in that: The dielectric layer and metal reflective layer of the absorber are made of soft and flexible materials. At the same time, the selected resonant material, water, can be arbitrarily changed in shape according to the shape of the container, thus realizing the flexible characteristics of the absorber.

Images