Terahertz metasurface based on symmetric broken structure, terahertz metasurface device based on symmetric broken structure and application of terahertz metasurface device

Abstract

The invention discloses a terahertz metasurface based on a symmetric broken structure, a device and application, and belongs to the technical field of terahertz metasurfaces. The invention provides a terahertz metasurface based on a symmetric broken structure. The terahertz metasurface comprises a substrate and a metal structure layer arranged on the substrate. The metal structure layer comprises a plurality of sensing units, and each sensing unit is composed of a plurality of same structure units which are periodically arranged; each structural unit comprises a pair of oppositely arranged rectangular metal opening wireframes, and the opening sizes of the rectangular metal opening wireframes of the structural units are different; in different sensing units, the arrangement periods P of the structural units are different from one another so as to excite bound state resonance in the symmetrical protection type quasi-continuous domain at different frequency points. According to the terahertz metasurface, high-quality factor resonance of multiple frequency points is excited by introducing the symmetrical breaking structure, so that the sensitivity and broadband detection capability of the terahertz metasurface are improved, and wide-spectrum high-resolution detection is realized.

Description

Technical Field

[0001] This invention relates to the field of terahertz metasurface technology, specifically to a terahertz metasurface, device, and application based on a symmetry-broken structure. Background Technology

[0002] Terahertz waves, with their strong penetrating power into nonpolar substances, unique molecular fingerprint spectral recognition characteristics, and safety advantages due to low photon energy, have shown broad application prospects in fields such as non-destructive testing of biochemical macromolecules, drug quality monitoring, and biomolecular structure analysis. These characteristics enable terahertz technology to effectively distinguish the molecular vibrational modes of different substances, providing crucial support for the component identification of complex samples. However, the interaction strength between terahertz waves and matter is generally low, resulting in weak signal responses during sensing, which severely limits the sensitivity of detection and the effectiveness of practical applications.

[0003] To overcome this bottleneck, the development of micro- and nano-fabrication technologies has driven the widespread application of terahertz metasurfaces. By designing subwavelength-scale metallic structural units, metasurfaces can achieve localized enhancement and resonant modulation of electromagnetic fields, significantly improving the interaction strength with the analyte and thus enhancing sensing performance.

[0004] Despite this, existing metamaterial devices still face significant challenges. Due to the inherently high losses of the materials, the quality factors of these devices are generally low, leading to broadened resonance peaks and insufficient frequency resolution. Furthermore, traditional metasurface designs typically rely on a single resonance mode, resulting in a narrow operating bandwidth that struggles to cover a wide range of characteristic absorption frequencies. In practical applications, especially with analytes of complex or unknown composition, such as mixed liquids or biological samples, detection at a single frequency point cannot comprehensively capture all key spectral features, leading to decreased accuracy and limited applicability. Summary of the Invention

[0005] The purpose of this invention is to provide a terahertz metasurface, device, and application based on a symmetry-broken structure, to overcome the shortcomings of insufficient sensitivity and wideband detection capability in existing terahertz metasurfaces. The terahertz metasurface provided by this invention has the advantages of being able to excite multi-frequency resonances, improving sensing sensitivity, and achieving wideband coverage.

[0006] The present invention solves the above-mentioned technical problems through the following technical solution: The present invention provides a terahertz metasurface based on a symmetry-broken structure, comprising a substrate and a metal structure layer disposed on the substrate; The metal structure layer includes multiple sensing units, each of which consists of multiple identical structural units arranged in a periodic manner. The structural unit includes a pair of oppositely arranged rectangular metal opening frames, and the opening size of the rectangular metal opening frames of the structural unit in each sensing unit is different. In different sensing units, the arrangement period P of the structural units is different to excite bound state resonance in the symmetric protected quasi-continuous domain at different frequency points.

[0007] A further improvement of the present invention is that the rectangular metal opening wireframes are arranged axially symmetrically about the central axis of symmetry of the structural unit, and the opening positions are opposite.

[0008] A further improvement of this invention is that the opening sizes of a pair of rectangular metal opening wireframes in the structural unit are g1 and g2, respectively, wherein: , .

[0009] A further improvement of the present invention is that the value of the arrangement period P ranges from 50µm to 150µm.

[0010] A further improvement of the present invention is that the material of the metal structural layer is gold, silver or aluminum.

[0011] A further improvement of the present invention is that the substrate material is polyimide.

[0012] A further improvement of the present invention is that the dielectric constant of the polyimide is 3.5 and the loss tangent is 0.02.

[0013] The present invention also provides a terahertz metasurface device based on a symmetry-broken structure, including a cover plate and a terahertz metasurface based on a symmetry-broken structure as described above. The cover plate and the terahertz metasurface based on a symmetry-broken structure cooperate to form a microfluidic channel for containing a liquid analyte. The position of the microfluidic channel corresponds to the sensing unit.

[0014] The present invention also provides an application of the terahertz metasurface device based on the symmetry-broken structure described above, for fingerprint spectrum detection in the terahertz band.

[0015] A further improvement of this invention is that the terahertz band is 0.1THz-10THz.

[0016] Compared with the prior art, the positive and progressive effects of the present invention are as follows: The terahertz metasurface based on a symmetry-broken structure provided by this invention enhances the sensitivity and broadband detection capability of the terahertz metasurface by introducing a symmetry-broken structure to excite high-quality factor resonances at multiple frequency points. Specifically, the substrate and the metal structure layer disposed on the substrate provide stable mechanical support and an electromagnetic response basis, ensuring the reliability and consistency of the metasurface structure. The metal structure layer includes multiple sensing units, each composed of multiple periodically arranged identical structural units. This periodic arrangement enhances the localization effect of the electromagnetic field and strengthens the resonance intensity. Each structural unit includes a pair of oppositely arranged rectangular metal opening frames, which establish an initial symmetry framework. The opening size of the rectangular metal opening frames of each structural unit is different. The differentiated opening gaps disrupt the symmetry, exciting bound-state resonances in the symmetry-protected quasi-continuous domain and reducing radiation loss. In different sensing units, the arrangement period P of the structural units is different. By adjusting the period parameter, each sensing unit resonates at a specific frequency point, covering multiple characteristic absorption frequencies, solving the problem of insufficient single resonance points, and achieving broadband high-resolution detection. Attached Figure Description

[0017] The accompanying drawings are provided to further understand the invention and constitute a part of this invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0018] Figure 1 This is a schematic diagram of a terahertz array metasurface based on a symmetry-broken structure according to the present invention. Figure 2 This is a top view of the base of the present invention; Figure 3 This is a detailed schematic diagram of the structural unit of the present invention; Figure 4 The transmission spectrum image of this invention, taking an array with nine metasurfaces as an example, is shown without any analyte loaded. Figure 5 This invention uses an array with nine metasurfaces as an example to illustrate the fingerprint spectrum detection pattern after loading the analyte onto the upper surface. Figure 6 This is the terahertz fingerprint spectrum information of Cordyceps sinensis obtained from Cordyceps sinensis tablets.

[0019] The components are: 1. substrate; 2. metal structural layer; 3. sensing unit; 4. structural unit; and 5. cover plate. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0021] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0022] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0023] In the description of the embodiments of the present invention, it should be noted that if terms such as "upper," "lower," "horizontal," or "inner" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship commonly used when the product of the invention is in use, they are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the present invention. Furthermore, terms such as "first" and "second" are only used to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0024] Furthermore, it should be noted that, unless otherwise explicitly specified and limited, the terms "set," "install," "connect," and "link" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0025] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. This is an explanation of the present invention and not a limitation thereof.

[0026] The present invention provides a terahertz metasurface based on a symmetry-broken structure, comprising a substrate 1 and a metal structure layer 2 disposed on the substrate 1; The metal structure layer 2 includes multiple sensing units 3, each sensing unit 3 being composed of multiple periodically arranged identical structural units 4; The structural unit 4 includes a pair of rectangular metal opening frames arranged opposite each other, and the opening size of the rectangular metal opening frames of the structural unit 4 in each sensing unit 3 is different. In different sensing units 3, the arrangement period P of structural units 4 is different to excite bound state resonance in symmetric protected quasi-continuous domains at different frequency points.

[0027] The concept of bound states in a continuous domain (BIC) was first proposed in the quantum realm. The introduction of BIC provides metasurfaces with ultra-high Q-factors, enhanced local fields, and flexible control over radiation, offering a new theoretical framework and technical approach for optimizing metasurface performance. Combining continuous-domain bound states with arrayed metasurfaces solves the problem of single resonant metasurfaces failing to accurately identify different analytes when the analyte is unknown. Arrays can obtain multiple resonance peaks within a frequency range, while also exhibiting high sensitivity and Q-factor.

[0028] This invention enhances the sensitivity and broadband detection capability of terahertz metasurfaces by introducing a symmetry-breaking structure to excite high-quality factor resonances at multiple frequency points. Specifically, the substrate 1 and the metal structure layer 2 disposed on the substrate 1 provide stable mechanical support and electromagnetic response foundation, ensuring the reliability and consistency of the metasurface structure. The metal structure layer 2 includes multiple sensing units 3, each of which is composed of multiple periodically arranged identical structural units 4. This periodic arrangement enhances the localization effect of the electromagnetic field and strengthens the resonance intensity. The structural unit 4 includes a pair of oppositely arranged rectangular metal opening frames, which establish an initial symmetry framework. The opening size of the rectangular metal opening frames of each structural unit 4 is different. The symmetry is broken by the differentiated opening gaps, exciting bound state resonances in the symmetry-protected quasi-continuous domain and reducing radiation loss. In different sensing units 3, the arrangement period P of the structural units 4 is different. By adjusting the period parameter, each sensing unit 3 resonates at a specific frequency point, covering multiple characteristic absorption frequencies, solving the problem of insufficient single resonance points, and achieving broadband high-resolution detection.

[0029] Specifically, the rectangular metal opening wireframes are arranged symmetrically about the central axis of symmetry of structural unit 4, and the opening positions are opposite.

[0030] The structural unit 4 is arranged axially symmetrically about its central axis of symmetry, with opposite opening positions. This strengthens the symmetry-breaking structure, ensuring that while maintaining the overall axially symmetric framework, the opposite opening positions precisely introduce asymmetry, effectively stimulating bound-state resonance in the symmetry-protected quasi-continuous domain. The axially symmetric arrangement provides fundamental symmetry, establishing a stable foundation for the resonance mode; the opposite opening positions create specific defects, enhancing electromagnetic field localization, improving the sharpness and quality factor of the resonance, and solving the sensitivity reduction problem caused by insufficient symmetry breaking in traditional schemes.

[0031] Specifically, let the opening sizes of the pair of rectangular metal opening wireframes in structural unit 4 be g1 and g2, respectively, where: , .

[0032] By specifically defining the proportional relationship between the opening sizes g1 and g2 of the rectangular metal opening wireframe of structural unit 4 and the arrangement period P, precise control of the resonant frequency is achieved, thereby optimizing the symmetry breaking effect and improving the quality factor and sensing sensitivity. By correlating the opening size with the arrangement period P, optimized resonance performance is maintained under different arrangement periods.

[0033] Specifically, the permutation period P ranges from 50µm to 150µm.

[0034] By precisely limiting the range of the arrangement period P, it is ensured that the structural unit 4 can effectively excite resonance at the appropriate frequency point, avoiding excessively high or low frequencies caused by P being too small or too large, thereby improving the stability and detection accuracy of the resonance.

[0035] Specifically, the material of the metal structural layer 2 is gold, silver or aluminum.

[0036] By limiting the material of the metal structure layer 2 to gold, silver, or aluminum, the loss problem caused by improper material selection is solved, thereby optimizing the resonance performance. The use of gold, silver, or aluminum as the material for the metal structure layer 2, with its high conductivity and low loss characteristics, effectively reduces the energy loss of electromagnetic waves in the metal structure, improves the quality factor, and ensures that the sensing unit 3 maintains high sensitivity and stability during resonance excitation, thus enhancing the reliability and accuracy of terahertz fingerprint spectrum detection.

[0037] Specifically, the material of substrate 1 is polyimide.

[0038] Polyimide, as a low-loss dielectric material, possesses suitable properties such as dielectric constant and loss tangent, effectively reducing the energy attenuation of electromagnetic waves in substrate 1. By selecting this material, substrate 1 minimizes unnecessary electromagnetic interference while supporting the metal structure layer 2, ensuring the stability and strength of the resonant mode, thereby enhancing the sensitivity and reliability of the sensing unit 3. By specifying polyimide as the material of substrate 1, the loss problem caused by inappropriate substrate 1 material is solved, thus improving the metasurface performance.

[0039] Specifically, the dielectric constant of polyimide is 3.5 and the loss tangent is 0.02.

[0040] By specifically defining the dielectric constant and loss tangent of polyimide, the electromagnetic properties of substrate 1 were optimized, thereby reducing energy loss and improving the resonance quality and sensing effect of the metasurface. Specifically, a dielectric constant of 3.5 for polyimide helps maintain a uniform distribution and stable coupling of the electromagnetic field, reducing signal attenuation, as this dielectric constant better matches the metasurface structure and promotes efficient excitation of bound state resonances in the symmetry-protected quasi-continuous domain. A loss tangent of 0.02 reduces the dielectric loss of substrate 1, improves the quality factor, and thus enhances the local enhancement effect of terahertz waves, improving the accuracy and reliability of sensing.

[0041] The present invention also provides a terahertz metasurface device based on a symmetry-broken structure, including a cover plate 5 and a terahertz metasurface based on a symmetry-broken structure as described above. The cover plate 5 and the terahertz metasurface based on a symmetry-broken structure cooperate to form a microfluidic channel for containing a liquid analyte. The position of the microfluidic channel corresponds to the sensing unit 3.

[0042] By integrating a cover plate 5 with a metasurface to form a positioning microfluidic channel, the problems of liquid analyte containment and interaction are solved, improving the convenience and efficiency of detection. Specifically, the device includes a cover plate 5 and a terahertz metasurface based on a symmetry-broken structure. The cover plate 5 is used as an additional component to combine with the metasurface, constructing a complete device structure that is easy to operate and maintain. The cover plate 5 and the terahertz metasurface based on a symmetry-broken structure work together to form a microfluidic channel for containing liquid analytes. This dedicated channel stably contains liquid samples, preventing sample spillage or uneven distribution and ensuring a controllable detection environment. The position of the microfluidic channel corresponds to the sensing unit 3. Through alignment, the analyte is directly exposed to the resonance region, maximizing the enhancement effect of bound-state resonance in the symmetry-protected quasi-continuous domain, strengthening the interaction between terahertz waves and matter, thereby improving sensing sensitivity and detection accuracy. It has the advantages of simple structure and easy processing, and can identify unknown analytes over a wide frequency range.

[0043] The present invention also provides an application of the terahertz metasurface device based on the symmetry-broken structure described above, for fingerprint spectrum detection in the terahertz band.

[0044] By using the device for fingerprint spectrum detection, its high Q factor and wideband resonance characteristics enable high-resolution detection of molecular fingerprints, ensuring that the device can cover multiple characteristic absorption frequencies in applications, thereby improving detection accuracy and applicability.

[0045] Specifically, the terahertz band is 0.1THz-10THz.

[0046] A terahertz array metasurface based on a symmetry-broken structure is disclosed. Each array unit consists of a metal structure layer 2 and a substrate 1 from top to bottom. The metal structure layer 2 is composed of opposing rectangular metal opening wireframe structures. The metal pattern on the incident contact surface is a pair of opposing rectangular metal opening wireframes. The opening size of each pair of rectangular opening wireframes is different. The periodic parameters of the structure on each array unit are different. The metal structure of each metasurface has different geometric parameters, realizing multiple sharp resonance peaks at multiple different frequency points.

[0047] The array metasurface is composed of multiple metasurface array units, each consisting of metasurfaces with different unit period parameters. Each metasurface unit is a rectangular metal open wireframe arranged in opposite directions. The metasurfaces in different array units have different unit period parameters, thus enabling the excitation of symmetry-protected quasi-continuum bound-state resonances (SP-QBIC) at different frequencies, thereby achieving terahertz fingerprint spectrum detection over a wide frequency range. The array is composed of metal structures with different period parameters; the structural units 4 in different sensing units 3 within the array have different period parameters, allowing different sensing units 3 to excite SP-BIC resonances at different frequencies. Because the opening size of each rectangular open metal wireframe is different, bound states in a symmetry-broken quasi-continuum are excited on the metasurface, resulting in a high Q factor.

[0048] The main design used in this invention is an array-based design scheme. It combines the bound states in the symmetry-broken quasi-continuous domain to improve the sensing performance of the device. Rectangular metal opening wireframes with different opening sizes are used to break the symmetry of the structure, which excites the bound states in the symmetry-broken quasi-continuous domain, thereby improving the device sensitivity and Q factor.

[0049] Combination Figures 1-3 A terahertz array metasurface based on symmetry breaking consists of a top-down metal structure layer 2 and a substrate 1. The metal structure layer 2 is a rectangular metal opening wireframe with oppositely arranged structures, and the structure period is the same on each array unit. Within the array unit, two metal rings have openings of different sizes, the size of which is related to the period P. See [link to relevant documentation]. Figure 4 Taking an array of nine metasurfaces as an example, relatively rectangular metal opening wireframes with different opening sizes excite bound states in a symmetry-broken continuous domain, so that each metasurface excites a bound state in a quasi-continuous domain with a high Q value at the corresponding resonant frequency point. As the periodic parameters of the unit structure change, its resonant position also shifts in frequency, thus enabling the identification of the position of the test object in a wide frequency range.

[0050] Terahertz array metasurfaces can be loaded with trace amounts of liquid analytes using methods such as drop-drying and microfluidic channels. Taking nine metasurfaces as an example, the array metasurfaces are first placed in a terahertz time-domain spectroscopy system to obtain their transmission spectra. Cordyceps sinensis powder is compressed into tablets using a tablet press, and the transmission spectrum of the compressed Cordyceps sinensis tablets is obtained using a terahertz time-domain spectroscopy system. The absorption coefficient of the Cordyceps sinensis tablets is then extracted. Next, the Cordyceps sinensis powder is dissolved in alcohol, and after multiple dilutions, a 50 ppm Cordyceps sinensis alcohol solution is obtained. The solution is dropped onto each array unit, and after the alcohol dries, the transmission spectrum of the Cordyceps sinensis is obtained using a terahertz time-domain spectroscopy system. The fingerprint spectrum of the analyte is obtained by subtracting the transmission spectrum after loading the analyte from the transmission spectrum without loading the analyte. See [link to relevant documentation]. Figure 6 It can be seen that Cordyceps sinensis has four distinct absorption peaks, located at 0.47 THz, 0.59 THz, 0.7 THz, and 0.8 THz, respectively; (Compared to...) Figure 5 As can be seen from the envelope, obvious valleys also appear at the same four positions, which are highly consistent with the absorption spectrum characteristics of the tablet sample. This proves that the array metasurface and the bound state scheme in the symmetry-broken continuous domain adopted in this invention can effectively increase the identification and detection capability of unknown analytes over a wide spectral range.

[0051] A cover plate 5, capable of fixing microfluidic channels, is added to the top of the terahertz array metasurface based on symmetry breaking provided in this invention, positioning the microfluidic channels above each array unit. A prepared 50 ppm Cordyceps sinensis solution is then introduced into the channels and exited from the other end, distributing the liquid analyte on the upper surface of each array unit of the metasurface. The metasurface with the cover plate 5 is then placed in a terahertz time-domain spectroscopy system. During testing, the liquid analyte is introduced at a uniform rate. This method ensures that the liquid analyte is uniformly distributed in the strong field enhancement region of the metasurface, improving detection performance.

[0052] Finally, it should be noted that the embodiments listed above are merely one or more specific manifestations of the technical solution of this invention. Their purpose is to clearly illustrate the concept, principle, and application of this invention through specific examples, and is by no means intended to limit the scope of protection of this invention to these specific embodiments. In fact, the true value of this invention lies in its proposed technical ideas and innovations, rather than its manifestations or implementation methods.

[0053] For those skilled in the art, after thoroughly reading and understanding the technical solution of this invention, they are fully capable of making various changes, modifications, or equivalent substitutions to the specific implementation of the invention based on their own professional knowledge and skills. These changes may include, but are not limited to: adjusting the range of technical parameters, optimizing the algorithm flow to improve efficiency, and replacing some technical components to achieve better compatibility or reduce costs. As long as these modified technical solutions substantially retain the technical features claimed by the original invention, that is, they can still achieve the core functions and effects of this invention, then these changes should be considered to fall within the scope of protection of the pending claims of this invention.

[0054] Furthermore, with the continuous progress and development of technology, new technical means and methods are constantly emerging, which provides ample space for further improvement and perfection of this invention. Therefore, the scope of protection of this invention should also include reasonable and foresightful improvements and extensions based on existing technology. As long as these improvements and extensions do not depart from the basic principles and core concepts of this invention, they should be considered equivalents of this invention and are equally protected by patent rights.

Claims

1. A terahertz metasurface based on a symmetry-broken structure, characterized in that, It includes a substrate (1) and a metal structure layer (2) disposed on the substrate (1); The metal structure layer (2) includes multiple sensing units (3), each sensing unit (3) being composed of multiple periodically arranged identical structural units (4); The structural unit (4) includes a pair of rectangular metal opening frames arranged opposite each other, and the opening size of the rectangular metal opening frames of the structural unit (4) in each sensing unit (3) is different. In different sensing units (3), the arrangement period P of the structural units (4) is different to excite the bound state resonance in the symmetric protected quasi-continuous domain at different frequency points.

2. The terahertz metasurface based on a symmetry-broken structure according to claim 1, characterized in that, The rectangular metal opening wireframes are arranged axially symmetrically about the central axis of symmetry of the structural unit (4), and the opening positions are opposite.

3. The terahertz metasurface based on a symmetry-broken structure according to claim 1, characterized in that, Let the opening sizes of a pair of rectangular metal opening wireframes in structural unit (4) be g1 and g2, respectively, where: , .

4. A terahertz metasurface based on a symmetry-broken structure according to claim 1, characterized in that, The permutation period P ranges from 50µm to 150µm.

5. A terahertz metasurface based on a symmetry-broken structure according to claim 1, characterized in that, The metal structural layer (2) is made of gold, silver or aluminum.

6. A terahertz metasurface based on a symmetry-broken structure according to claim 1, characterized in that, The substrate (1) is made of polyimide.

7. A terahertz metasurface based on a symmetry-broken structure according to claim 6, characterized in that, The dielectric constant of polyimide is 3.5 and the loss tangent is 0.

02.

8. A terahertz metasurface device based on a symmetry-broken structure, characterized in that, Includes a cover plate (5) and a terahertz metasurface based on a symmetry-broken structure as described in any one of claims 1 to 7, wherein the cover plate (5) and the terahertz metasurface based on a symmetry-broken structure cooperate to form a microfluidic channel for containing a liquid analyte, and the position of the microfluidic channel corresponds to the sensing unit (3).

9. An application of a terahertz metasurface device based on a symmetry-broken structure as described in claim 8, characterized in that, Fingerprint spectrum detection for the terahertz band.

10. The application of the terahertz metasurface device based on the symmetry-broken structure according to claim 9, characterized in that, The terahertz band is 0.1THz-10THz.

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