The perspectives and trends of THz technology in material research for future communication - a comprehensive review

Based on Drude model, the complex dielectric constant of THz band metal ε can be expressed as [151, 152]:

$$\begin{eqnarray}&&\varepsilon ={\varepsilon }_{r}+j{\varepsilon }_{i}={(n+jk)}^{2}\end{eqnarray} \tag{ 6 }$$

where, ε_r is the real part of the complex permittivity; ε_i is the imaginary part. There are:

$$\begin{eqnarray}&&{\varepsilon }_{r}={\varepsilon }_{\infty }-\displaystyle \frac{{\omega }_{p}^{2}}{{\omega }^{2}+{\omega }_{\tau }^{2}}\end{eqnarray} \tag{ 7 }$$

$$\begin{eqnarray}&&{\varepsilon }_{i}=\displaystyle \frac{{\varepsilon }_{\tau }{\varepsilon }_{p}^{2}}{\omega ({\omega }^{2}+{\omega }_{\tau }^{2})}\end{eqnarray} \tag{ 8 }$$

where ω_P is the plasma oscillation frequency.

Hollow metal waveguides use total internal reflection mechanism to guide THz wave. In 1999, McGowan and Gallot's team [153] successfully coupled THz waves of 0.8 ∼ 3.5 THz into a stainless steel circular metal waveguide with a diameter of 240 μm and a length of 24 mm, and its total absorption coefficient was less than 0.7 cm⁻¹. In 2000, Gallot et al [154] continued their research on THz circular and rectangular metal waveguides, in which the circular waveguide size is still 240 μm in diameter and 24 mm in length, and the rectangular waveguide size is 25 mm in length and the cross section is 125 μm × 250 μm, and the material is still stainless steel.

In 2007, Lu et al [155] from Zhengzhou University calculated the metal-coated waveguide with metal coating on the inner wall of plastic or glass tube. Harrington et al [156] reported fabrication of flexed empty polycarbonate waveguide with internal copper coatings for transmitting broadband terahertz with ordinary liquid phase chemistry technology. It is found that the loss of the optimal waveguide is less than 4 dB m⁻¹, and the single mode of the waveguide with smaller aperture can be preserved.

Zhang et al [157] fabricated dielectric metal film waveguides at microwave G band and 4.3 THz by plating metal film and dielectric film inside polycarbonate tube in 2019, and experimentally measured that waveguide loss in G band increases with the increase of dielectric film. In 2018, Seifert et al [158] proposed an all optical free contact method with large sample handling capacity that allowed to extract the spin current relaxation length and the spin Hall Angle.

After nearly 20 years of development, it can be seen that the loss of THz hollow metal waveguide has increased from 0.7 cm^-1 to 0.025 cm^-1, and the performance improvement is very obvious.

In 2001, Mendis et al [159] proposed THz parallel flat waveguide. They also glued two thin copper sheets together to produce a flexible parallel flat waveguide [160]. The 0.22ps THz pulse was widened to 0.33ps after passing through a 250mm waveguide, and its amplitude was attenuated 10 times.

In 2007, Mendis constructed a kind of dielectric-filled Metallic parallel-plate Waveguides (DF-PPWGs) [161] and found that although the absorption of high resistance silicon is smaller, compared with that of polyethylene, DF-PPWGs filled with high resistance silicon showed high transmission loss. Therefore, the key to realize low loss transmission in parallel planar waveguides is to use dielectric fillers with low absorption and refractive index.

In 2008, Cooke et al [162] realized the modulation of THz pulses by filling silicon with a transparent conductive Oxide film (Oxide is Tin fluoride Oxide, fluorine Oxide, FTO) in a parallel flat waveguide. There was not time delay between the air-filled waveguide and the confocal lens position, and the transmitted pulse keeps short with small reshaping. When an FTO-coated silicon plate of length L was inserted into a filled air copper waveguide, the pulses remained little and shifted Δt = L(n_Si − 1)/c, denoting that the transmitted pulses remained single mode and propagated with only a little number of group velocity dispersion, and the group index was near to the bulk silicon.

It can be known that the group velocity dispersion of THz parallel plate waveguide is very small, but the loss has no obvious advantage compared with THz hollow metal waveguide. At the same time, it can also be found that people have realized THz wave modulation by filling medium in THz parallel plate waveguide, instead of simply studying its guided wave properties.

In the field of THz wave transmission technology, wire waveguide is an important waveguide, which has the advantages of low dispersion and low loss. In 2004, Wang et al [163] proposed a bare-metal linear waveguide to solve the problem that THz wave cannot transmit over long distances, which has the advantages of low dispersion and low loss. In 2005, they [164] constructed a simple Y-type beam splitter on this basis.

In 2005, Jeon et al [165] carried out experimental and theoretical research on the propagation of terahertz wave on a single Cu wire, which shows THz pulse attenuation when copper wire is bent Δh in the experiment.

He et al [166] studied the transmission characteristics of THz wire waveguide in 2006, and analysed the influence of several common metal materials on the transmission performance.

THz wire waveguide possesses the advantages of simple structure, high flexibility, easy bending, low dispersion and low loss. However, as wire waveguide is a kind of bare structure, its anti-interference performance is not very good, so additional design is needed to compensate.

THz fibers are usually made of dielectric materials, especially polymer materials, because they have lower absorption losses and lower dispersion coefficients in the THz band. For example, polytetrafluoroethylene (PTFE, or Teflon), polymethyl methacrylate (PMMA), polyethylene (PE), polypropylene (PP) and cycloolefin copolymer (COC).

In 2000, Grischkowsky et al [167] studied the waveguide propagation characteristics of sub-picosecond THz pulses in single crystal sapphire fiber.

In 2006, Chen et al [168] studied THz fiber made of polyethylene. They used polyethylene fiber as the core of the fiber with high refractive index and directly used air as the cladding. They realized transmission of 0.3 THz wave in 200 μm diameter polyethylene fiber with loss as low as 0.01 cm⁻¹. THz dielectric fiber, different from the traditional quartz fiber, the performance of polyethylene fiber is very outstanding, its transmission of THz wave, loss can be as low as 0.01 cm⁻¹.

Dielectric tube waveguide is composed of dielectric tube and air core in the middle of tube. It has simple structure and low transmission loss. Its working mechanism is anti-resonance guiding mechanism. When operating at the antiresonance frequency, THz waves are fully reflected by the inner wall of the dielectric tube and thus transmitted through the inner air core.

In 2009, Lai et al [169] studied the THz dielectric tube waveguide made of PTFE. In 2010, Lu et al [170] carried out an experimental research on the bending loss of terahertz PTFE tube waveguide, and studied the influence of frequency, polarization, core diameter, cladding thickness and cladding material on the bending loss of the waveguide. The results show that the lower waveguide loss is due to the stronger mode constraint and the lower bending loss.

In 2015, Bao et al [171] measured three sizes of polymethyl methacrylate (PMMA) dielectric tube waveguides with hollow kernel diameter of 4 mm, and cladding thickness of 1.29 mm, 1.97 mm and 2.95 mm, respectively. The loss was measured in the range of 0.3 to 1 THz in the range of 0.05 to 0.5 cm⁻¹.

In THz dielectric tube waveguide, PTFE performs extremely well, its coupling efficiency can reach 80%, the lowest loss is far less than other materials dielectric tube waveguide, also far better than other types of THz waveguide, and has a broad application prospect.

In 1987, Yablonovitch [172] proposed the concept of Photonic Crystals (PCs) while studying how to suppress spontaneous radiation in solid physics and electronics. Almost at the same time, John [173] also proposed the concept of photonic crystal when discussing the photon local problem in disordered medium superlattices. It is thus possible to introduce concepts related to solid state physics into such superlattice problems, introducing the concept of band gaps. On this basis, Yariv [174] put forward the theory of photonic crystal waveguide in 1999.

Referring to Mr Huang solid State physics [175], the starting point of tight binding approximation is that when an electron is near an atom, it will be mainly affected by the action of the atomic field, and the action of other atomic fields is regarded as the perturbation, so obtaining the correlation between the crystal energy band and the atomic energy level of the electron. The main method is Linear Combination of Atomic Orbital Method (LCAO).

In terms of actual processing technology, the preparation of one-dimensional photonic crystal is relatively simple, but because its periodic structure is one-dimensional, relatively complex defect mode cannot be constructed, so it can realize not too many functions. The defect modes of 3D photonic crystal can be constructed are very rich, so many functions can be realized, but the preparation process is relatively complex and the cost is high. However, two-dimensional photonic crystals have both advantages, so the actual research is mainly focused on two-dimensional photonic crystals.

• (i)  
  THz photonic crystal waveguideIn 2006, Zhang et al [176] proposed a silica-based 2-D photonic crystal waveguide and numerically simulated the applicability of... THz multi-mode interference effect and self-imaging principle in 2-D Si photonic crystal waveguide. In 2009, Ponseca et al [177] analysed the transmission properties of lens conduit, Cytop planar photonic crystal waveguide and hollow micro-structured PMMA fiber in THz region. In 2012, Kitagawa et al [178] systematically studied the band structure of two-dimensional photonic crystal made of dielectric with a low dielectric constant, and proposed a flat waveguide of THz low-dielectric two-dimensional photonic crystal sandwiched between two parallel metal plates.In 2015, Tsuruda et al [179] proposed a photonic crystal waveguide based on high resistance silicon plate (resistivity of 20 kΩ/cm) in order to develop a photonic crystal waveguide with extremely low loss. In 2021, Si et al [180] designed a band-stop filter based on two-dimensional silicon photonic crystal waveguide and AAH (Aubry-Andr-Harper) resonator. Its stopband centre wavelength is 1668.3 μm, its stopband attenuation is 25.2 dB, and the passband insertion loss is 0.2 dB. With a high Q value of 1.5 × 10⁴, it is very convenient to build integrated optical path.THz photonic crystal waveguide has natural advantages in integrated optical path due to its extremely low loss and high Q value when conducting waves at large angles. Due to the suitability of semiconductor technology in THz band and the consideration of integrated optical path and integrated circuit, most THz photonic crystal waveguides are currently developed based on semiconductor materials [181].
• (ii)  
  THz photonic crystal fiber

Photonic crystal fibers are separated into Total Internal Reflection (TIR) type and Photonic Bandgap (PBG) type based on different light guiding mechanisms.

In 1996, Knight [182] reported at fiber-Optic Communications Conference that the first photonic crystal fiber was based not on photonic crystal band gap theory, but on total internal reflection theory like conventional fibers. PBG type photonic crystal fibers were fabricated by Knight et al in 1999 and published in Science [183].

THz band photonic crystal fiber did not appear until 2004 [184, 185]. In this year, Goto et al [184] from Japan produced low-loss THz photonic crystal fiber by using PTFE. Later, photonic crystal fibers with various structures were developed. In 2014, Li et al [186] developed a new triangulated lattice THz photonic crystal fiber with high birefringence and low dispersion over a wide spectral range. By tune-up the structural parameter of the photonic crystal fiber, up to 10^-3 birefringence can be obtained in the frequency range from 0.1 terahertz to 5 terahertz. In addition, the group velocity dispersion curve is close to zero and flat in the frequency range. So, it is adapted to transmit broadband terahertz pulse that remain polarized. Other photonic crystal fibers with various shapes, such as square [187] and hexagon [188, 189], have also attracted great attention.

The research on the working mode of photonic crystal fiber has also attracted great attention. In 2012, Chen et al [190] designed a new type of THz single-mode single-polarization photonic crystal fiber, which introduced asymmetric rectangular micro-hole array in its core region. In 2013, Han et al [191] carried out a numerical demonstration of 2× 6Gb/s mode multiplexing division transmission of 70 km long double mode photonic crystal fiber by using LP₀₁ and LP₁₁ acting as the mode converter between two orthogonal modes, and the mode crossing can be basically ignored and the power loss is less than 1.86dB. It reveals the possibility of combining mode multiplexing division and photonic crystal fiber technology to achieve large capacity transmission.

The best material of THz photonic crystal fiber and dielectric fiber is PTFE, and the dispersion and loss characteristics of THz photonic crystal fiber are further improved by introducing the photonic crystal structure into the fiber. There are also many other studies on THz photonic crystal fiber, such as THz ultra-wideband photonic crystal fiber [192, 193], THz photonic crystal coupling mode [194] and THz photonic crystal fiber preparation process [195, 196]. Literature [193] operation begins with two types of KF, which transverse cross sections are given.

The study of Surface Plasmon Polaritons (SPPs) began when Wood et al [197] observed an abnormal absorption peak after irradiating a grating with light in 1902, which is called the Wood anomaly. It was not until 1941 that Fano explained the anomaly from the electromagnetic wave mode of the metal-air interface [198]. In 1956, Pines was the first to theoretically explain wood anomalies through a model of collective free electron oscillations in metals, which he called 'Plasmons' [199]. In 1960, the concept of Surface Plasma resonance was proposed by Stern & Ferrell, which further promoted the theoretical research on Surface Plasma [200]. In 1968, Kretchmann & Otto respectively proposed the method of stimulating SPPs on metal [201, 202], so that comprehensive research on SPPs could be carried out widely. In 2003, Barnes 'Surface Plasmon Subwavelength Optics' summarized the development of this field and laid a solid foundation for it [203]. SPPs exist at the interface of various negative dielectric constant media and positive dielectric constant media, the most common is the metal-air interface [204–206]. When the electromagnetic wave is coupled to the metal surface, it will excite the collective oscillation of free electrons on the metal surface, forming a polarized wave. If the excited wave is a traveling wave propagating on the metal microstrip, it is called surface isoionization polariton. If they are standing waves, which exist on metal nanoparticles but cannot propagate, they are called local surface plasmons.

The plasma frequency of metal is generally in the ultraviolet band and has a high dielectric constant in THz band. Although metal can also realize surface plasmons (known as Sommerfeld-Zenneck waves [165, 207]) in THz band, the local ability is extremely weak. In 2004, Pendry et al [208] from Imperial College London realized SPPs in THz band by introducing sub-wavelength scale square hole array on metal Surface, which is called Spoof Surface Plasmon Polaritons. SSPPs). Pendry's team found that by diluting the concentration of free electrons on the metal surface, they increased the skin depth of electromagnetic waves on the metal surface, triggering SSPPs-like patterns on the metal surface, called SSPPs. The dispersion relationship can be obtained by solving the Maxwell equation. For simplicity, Pendry assumes a square array of side d and a square cross section a × a for the hole in figure 5. In 2008, Williams et al [209] changed the square holes into grooves and made the THz band SSPPs structure.

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In 2008, Fernandez-Dominguez et al [210] proposed a periodic metal linear waveguide. In this year, Fernandez-Dominguez et al [211] studied the propagation of tightly bound THz-SSPPs on spiral groove wires both theoretically and experimentally. The periodic structure of the wires can allow the formation of surface electromagnetic patterns with different azimuthal symmetry at wavelengths longer than the pitch. In 2009, Zhang et al [212] developed a subwavelength grating waveguide based on SSPPs, which consists of two parallel metal plates with periodic wavy inner boundaries to slow down the propagation speed of THz wave. In 2010, Gao et al [213] designed a structure consisting of a single row of metal wedges that periodically protrudes from the flat metal surface.

In 2013, Shen et al [214] presented the fabrication of ultra-thin metal gratings on ultra-thin flexible substrates, called Conformal Surface Plasmons (CSPs), realizing the two-dimensional structure of SSPPs. The team developed many functional devices based on this structure, such as SSPPs antenna [215], filter [216], second harmonic generator [217], etc.

In 2016, Liu et al [218] studied the dispersion characteristics of an ultra-thin metallic slotted SSPPs waveguide in THz band by applying the finite element method. They also studied the transmission characteristics of subwavelength planar SSPP waveguide ring resonator.

In 2016, Liu et al [219] presented and studied a THz electron source based on the effective generation of SSPPs mode in a double-corrugated metal waveguide. In 2017, Chowdhury et al [220] studied periodic fluted metallic THz waveguides filled with media. In 2018, Zhang et al [221] designed THz-SSPPs waveguide based on one-dimensional grating structure, and constructed S-bent waveguide, Y-type beam splitter and directional coupler on this basis. In 2018, Guo et al [222] proposed the realization of cotype SSPPs waveguide in THz band for the first time. In 2019, Jaiswal et al designed a bandpass filter based on a similar structure [223]. In 2020, Xu et al also designed a similar structure [224]. These studies are limited to numerical calculation in THz band, and experimental verification is carried out in microwave band. In 2021, Rahman et al [225] tested four samples of jet-deposited nickel layers on alumina layers by high-resolution THz time-domain spectroscopy. Once the correct spot is selected, the built-in motion screw is adjusted for maximum reflection, thus ensuring that the T-ray beam on the sample surface is incident vertically [225].

In a word, there are more and more researches on SSPPs, and various devices based on SSPPs have been developed, but most of them are limited to the microwave band. Although the research on THz band SSPPs waveguide is not rare, it is limited to theoretical and simulation work due to many practical reasons such as signal source and detector, and the experimental work is very rare, which is almost an unexplored direction and still has great potential.

Graphene, as the most representative new two-dimensional material, has excellent performance in force, heat, light, electricity and other aspects, and has a good application prospect in biology, medicine, materials and other directions. In recent years, it has been payed for wide attention and has been studied in depth continuously [226–230].

Graphene is typically only 0.2 to 0.5 nm thick, which is lower than the thickness of metal films that can be processed with existing techniques. Graphene exhibits an extremely high carrier mobility of 20 000 cm²/(V·s) at normal atmospheric temperature (when electron density is 2×10¹¹ cm⁻²) [231]. The light absorption of graphene is divided into two types: interband absorption and in band absorption [232]. In 2008, Hanson [233] proposed the THz Parallel Plate Waveguide (GPPWG) model for the first time. In 2011, Vakil & Engheta [234] used bias voltage to regulate the SPPs properties of graphene. In 2012, Yuan et al [235] designed a semiconductor-graphene cylindrical THz waveguide. In 2014, Wang et al [236] designed parallel and series couplers based on graphene SPPs waveguide. The dispersion curve of the graphene SPPs waveguide and the state transition of the coupler are able to be controlled by changing the bias voltage.

In 2018, Chen et al [237] proposed a novel Graphene /AlGaAs SPW structure combining Graphene Surface Plasmon Waveguides (GSPW) and nonlinear materials. It is used to generate terahertz wave difference frequency generation under near-IR pumping. In 2019, He et al [238] theoretically reported a novel graphene-based Hybrid Plasmonic Waveguide (Graphene-based Hybrid Plasmonic Waveguide) by integrating GaAs microtubes onto silica gasket supported by Graphene coating substrate. GHPW). In 2019, Zheng et al [239] also proposed a new TYPE of GHPW.

Compared with traditional materials, graphene exhibits superior characteristics such as stronger mode constraints and longer transmission distances while being workable with thinner thickness. It can be seen that graphene has broad prospects in the field of THz. At present, although the research on new 2D materials mainly focuses on the two directions of gold film and graphene, the research on other new 2D materials, such as WSe₂ [240], WS₂ [241, 242], MoS₂ [243] and black phosphorus [244], is also continuous. In order to further illustrate that 660 nm is a good modulation effect as a pump light excitation sample, the throughput curve of WS₂ film is calculated [240]. The thickness better than WS₂ film is only 0.65 nm, and the transmittance of the film cannot be measured directly. Therefore, the result is calculated from the transmittance of substrate Si and WS₂ Si samples. The unit of the terahertz absorber is given in figure 6 [243]. Figure 7 is a schematic representation of their presented plasmon sensing structure, where a single layer of BP is separated from the germanium prism by a TPX and crystalline quartz layer, and other TPX and crystalline layers are sequentially arranged between the BP and sensing medium layer [245].

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Metamaterial, also known as new artificial electromagnetic media and metamaterial materials, is characterized by arranging artificial unit structures (artificial atoms) with subwavelength scale in periodic or aperiodic order, thus obtaining electromagnetic properties beyond the limits of natural materials. For example, negative refractive index, zero refractive index, ultra-high refractive index, high-frequency magnetic response, etc [246–249]. Since the 1990s, researches on the basic theory, functional devices and engineering applications of electromagnetic metamaterials have attracted extensive interest in physics, information and materials fields. Based on the electromagnetic parameters and their distribution that can be designed by metamaterials, researchers have developed a variety of new devices such as metamaterial cloths, lenses and antennas [250–255]. According to the field-local and field-enhancement characteristics of metamaterials (this effect is particularly significant in surface plasmon metamaterials) and the characteristics of sub-wavelength scale, novel metamaterials sensing and imaging devices are developed, which can effectively improve the sensitivity and imaging resolution of sensors [244, 256, 257]. With the introduction and implementation of digital coding and programmable metamaterials, the characterization and design of metamaterials are carried out in binary digital mode (i.e., the numbers 0 and 1), which promotes the integration of metamaterials and information technology and makes it possible for new system metamaterials imaging system and communication system [246].

The terahertz frequency region is intermediate between far-IR and microwave in the electromagnetic spectra and is generally defined as electromagnetic radiation of 0.1 ∼ 10THz [258]. With the rapid developing THz sources and detectors and the continuous development of terahertz functional devices, terahertz science and technology have made vigorous development in recent years. Terahertz technology has a good application prospect in material characterization, security inspection, biomedical imaging and communication, etc [259, 260]. In terms of imaging, compared with microwave and millimetre wave frequencies, the terahertz imager will have higher spatial resolution, and compared with optical imaging, terahertz radiation has better penetration, so more depth information can be obtained [261]. At the same time, terahertz imaging applications have been receiving a lot of attention because it is a non-ionizing radiation with better biosecurity. Terahertz metamaterials and metamaterials have broad application prospects in terahertz imaging systems due to their advantages of subwavelength unit scale and flexible control of electromagnetic wave amplitude, phase and polarization characteristics [262, 263].

In the 1990s, Sir John proposed the method of equivalent media parameters to construct the negative permittivity and negative permeability of low frequencies [264, 265], and the team of Prof. Smith verified the experiment in microwave frequency [266]. Since then, electromagnetic metamaterials have been the hot topic to research for more than 20 years. With the application of micro-Nano processing technology, metamaterials in terahertz, infrared and visible wavelengths can also be realized in the laboratory [267–269]. In particular, terahertz magnetic metamaterials break through the limitations of natural materials and obtain magnetic responses at high frequencies [267]. Terahertz metamaterials can also be used to obtain negative or ultra-high refractive indices. At the same time, researchers have developed a variety of new terahertz metamaterial devices, including cloaks, absorbers, modulators and optical switches, to regulate the propagation properties of terahertz waves [270–272].

Electromagnetic metasurface, as a kind of two-dimensional metamaterial, has attracted extensive research interest. Compared with 3D metamaterials, metamaterials have deep subwavelength thickness, and their manipulation of electromagnetic waves does not depend on phase accumulation in space, but on the phase gradient (discontinuous distribution) between adjacent elements to realize the manipulation of electromagnetic wave front, amplitude, phase and polarization [273]. Therefore, adopting the metasurface scheme is beneficial to the design of ultra-light, ultra-thin and easily conformed terahertz devices, such as metasurface abnormal reflection/refraction devices, polarization converters and low scattering devices [179–276]. Through the combination of metasurface and a pair of orthogonal gratings, abnormal deflection and efficient polarization conversion of transmitted terahertz linearly polarized waves can be realized [276]. Therefore, terahertz metasurface devices can achieve flexible manipulation of terahertz beam and have the advantages of light weight and easy conformal.

Commonly used materials include superconducting materials, graphene, liquid crystal, phase change materials, titanate and magnetic materials, etc [263, 277].

Chen et al [278] designed terahertz SRR metamaterial on YBCO (YBa₂Cu₃O_7−δ , δ = 0.05) superconducting film with a thickness of 180nm, whose terahertz resonant frequency and transmission properties are adjustable with temperature. The metamaterial device covers a monolayer of graphene on the surface of a zero-refractive index metamaterial and realizes the adjustable Goos-Hänchen effect by electrically adjusting the Fermi energy levels of graphene [279]. For example, LC1852 has a THz normal and abnormal refractive index of 1.56 and 1.89 respectively, so it has potential applications in THz adjustable metamaterials, THz phase shifters and THz beam scanning devices [280]. The liquid crystal/metamaterial adjustable absorber structure consists of a resonant unit, a liquid crystal layer and a metal backplane layer [281]. The problems encountered in the design of LC-based terahertz components are systematically solved [282].

The conductivity of epitaxial VO₂ film can reach 10⁵ S m⁻¹, so the complex dielectric constant needs to be considered [283]. VO₂ has been widely used in tuneable metamaterials [284]. Zhu et al [285] gives a schematic diagram of the VO₂/metamaterial filter. Hillman et al [286] designed a broadband terahertz phase shifter using VO₂, which achieved a phase shift of more than 300° in the frequency range of 220 ∼ 250GHz, which could be applied to terahertz beam manipulation including beam scanning. Metamaterials and devices based on dielectric materials (such as titanate) are also adjustable with temperature [287].

Magnetic materials are also widely used in tuneable metamaterials. Bi et al [288] designed a magnetic metamaterial unit using Mn-Zn microwave ferrite material, and realized a thermally adjustable negative refractive index by recombination with metal wire element. In addition, magnetically tuneable wideband microwave filters can be realized by using ferrite-based metamaterials [289]. Liu et al [290] shows that when the intensity of magnetic field bias is 194mT, the modulation depth of terahertz transmission amplitude can reach 34%.

Reconfigurable metamaterials generally refer to the reconstruction or adjustment of metamaterials' properties through the reconstruction of their geometric structures. Common approaches to reconfigurable metamaterials include micro-electro-mechanical systems (MEMS) techniques and deformation techniques such as thermal and stress-induced deformation [291, 292]. It is also a common method in MEMS technology to change the height of the cantilever beam by driving voltage, and then adjust the geometric parameters and resonant frequency of metamaterial unit [293]. Metamaterials composed of materials with different thermal conductivity are usually bent by lifting and cooling, or substrates with good ductility are stretched to change metamaterials' geometric parameters [292].

In addition, active metamaterials can be constructed by introducing active components or functional materials, such as diodes, semiconductor materials and graphene, into metamaterials [294, 295]. Active control of THz electromagnetic induced transparency is able to be realized by changing the electrical conductivity of Si by changing the optical pumping intensity [295]. By dealing with the intensity distribution of pump light, the pattern reconstruction of the metasurface can be realized [296]. The cell of EIT metamaterial composes of cut wires (CW) and two SRRs, in which the silicon island is located in the gap in figure 8(a) [295]. The metamaterial structure was made of a silicon wafer on sapphire consisting of an undoped silicon film 500 nm thick and a sapphire substrate 495 microns thick. As shown in figure 8(c), electro-optic time-resolved OPTP system is used for measurement. The tested amplitude transmission for the first as continuous wave only and second samples as SRR pair are only given in figure 8(d).

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Tuneable, reconfigurable, and active metamaterials enable metamaterials to function not in a fixed state, but in an excitable manner, allowing them to operate at adjustable frequencies or switch between different functions, allowing them to manipulate the transmission properties of electromagnetic waves with greater freedom.

Prof. Cui presented the concepts of digitally coded metamaterials and field-programmable metamaterials [274]. Moreover, 1-bit, 2-bit and 3-bit digitally encoded metasurface can be got by selecting units of different sizes [297]. Single beam, double beam, multi-beam radiation and RCS reduction of terahertz wave can be obtained by optimizing the coding pattern.

The digital coding sequence is input into field programmable gate array, namely FPGA, and the bias voltage is controlled by FPGA to realize the real-time switching of the coding pattern of metamaterial array, to control the far-field radiation properties of metamaterial array [274]. Cong et al [298] designed a terahertz programmable metasurface scheme based on MEMS process.

As shown in figure 9(a), the authors think about special metasurfaces [274] consisting of binary numeric elements of '0' or '1'. Nevertheless, to achieve a wide band, they implement the binary element in figure 9(b) using a subwavelength square metal patch printed on the dielectric substrate. For example, on 010101... / 010101..., under a periodic coding sequence, the normal incident beam is mostly reflected by the metasurface to two symmetrical orientations, while in 010101... / 101010... / 01001... / 101010..., as given in figures 9(c) and (d).

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MEMS-based MIM systems typically consist of structured metal resonators designed as suspended cantilevers that can be reconfigured by applying electrostatic forces, and continuous metal planes separated by dielectric spacers on the substrate [298].

Digital coding metamaterials with field programmable metamaterials metamaterials, also called information through super material description method and the design method of digital, implements the metamaterial with the integration of information technology, so metamaterials can be directly to digital information processing based on information, and to develop a new system of wireless communication system and the imaging system.

The major effect of the insulation layer could be to make inferior thermal transmittance and thermal conductivity, which mainly relates to the thickness of the materials and several other factors in detail by Jelle [302]. Nevertheless, terahertz spectroscopy is used to detect most of the properties, such as gas, material moisture content, and thickness, infrared transmittance, temperature dependence, and material densities. High power terahertz imaging is able to discover the structure of traditional insulating material, including the content of fibers or fillers and the orientation of fibers, which decide the mechanical and physical characters of the final materials and is able to controlling in the process. In addition, since terahertz wave is also able to detect gas, it is used to estimate the presence of expanded gases captured in units of conventional polymer foam after production.

As the energy consumption of buildings and construction fields accounts for an important proportion of the total energy consumption in the world, new thermal insulation materials are proposed to promote the energy efficiency of building. Compared with the conventional building envelope, the new thermal insulation material has the advantage of thinning or forming multi-layer structure, which also is fit for terahertz sensing. Currently, the aerogel is the most prosperous high-performance thermal insulation material [303], which has a high porosity, making it easy for terahertz wave to penetrate the material. Therefore, terahertz systems are adapted to regulate the producting procedure of aerogels. As an example, the unbound form alkoxide group in the silica aerogel network, the water content in the pores over drying, and the distributing large pores in the materials, which could not be ideal for an insulation purpose. Terahertz TDS is utilized to research the adsorption procedure of silica aerogels on molecular vapours such as ammonia, water, chloromethane, and others [304]. Furthermore, terahertz imaging is also able to be used to visualize hidden flaws, such as water intrusion, cracks, etc in building envelope made of gas insulation panel or modern vacuum. The panel is composed of an overt porous kernel of fumed silica, which is wrapped by some layers of metal polymer, and even so a little puncture will raise the thermal conductivity and allow water and air to invade [302]. Phase varying material, such as non-paraffin, organic paraffin, metal materials or salt hydrate, added in the form of microcapsule or planar or cylindrical element has shown notable latent as insulating materials [305]. These main functions are to vary from a solid state to a liquid state when they are heated and vice versa when they are cooled. As for normal cases, the variation happens in the temperature varying from 18 to 32 °C [300] and is able to easily determined by terahertz spectroscopy combined with cryogenic systems. Figure 11 shows the development trend of THz in four fields.

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THz development trend mainly includes the following four aspects according to THz metamaterials as shown in figure 12.

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