WO2025115663A1 - Terahertz wave diffuser and method for manufacturing the same - Google Patents

Abstract

This invention disrupts a phase of an incident THz wave. An embodiment of the present disclosure provides a terahertz wave diffuser 100 comprising a dispersion medium 2 and a particulate dispersoid 10. The dispersion medium transmits the incident terahertz wave. The particulate dispersoid is made up of particles 1 each comprising a microstructure and a material responding to the incident terahertz wave, and is supported by being dispersed in the dispersion medium. The present disclosure also provides a method for manufacturing the terahertz wave diffuser.

Description

Terahertz wave diffuser and method for producing same

The present disclosure relates to a terahertz wave diffuser and a method for manufacturing the same. More specifically, the present disclosure relates to a terahertz wave diffuser in which the phase of an incident terahertz wave is disturbed, and a method for manufacturing the same.

Electromagnetic waves with frequencies of 0.1 THz to 100 THz (wavelength in vacuum of 3 μm to 3 mm) are also called terahertz waves (hereinafter also written as "THz waves") and are expected to have a wide range of applications. In the fifth generation communication system and subsequent generations for high-speed communication (hereinafter referred to as "Beyond 5G/6G"), the use of electromagnetic waves exceeding 300 GHz and reaching the THz wave band is also expected. For example, Non-Patent Document 1 discloses a transceiver that uses silicon CMOS technology and can operate in the 300 GHz band. In addition, Non-Patent Document 2 discloses a 446 GHz band RTD (resonant tunneling diode) radiation source with an active antenna array arranged in a 6 x 6 square lattice. THz waves are expected to be used not only for communication but also for detection purposes. For example, some of the inventors of this application have developed a walk-through body scanner that uses 300 GHz band THz waves (Non-Patent Documents 3 and 4).

As the uses of THz waves expand, absorbers (including low reflectors and transparent shielding materials, the same applies below) are also coming into use. For example, Eccosorb (registered trademark) AN-72 is a typical absorber of electromagnetic waves, including THz waves, available on the market. The above-mentioned walk-through body scanner also uses a similar absorber on the inner walls of the device. THz wave absorbers are used for a variety of purposes, with typical uses being anti-reflection, anti-transmission, and anti-interference to prevent noise.

Recently, biotemplate technology that uses living organisms has also been attracting attention. Patent Document 1 discloses an interference-type radio wave shielding or absorbing body that is characterized by having a structure in which a left-handed radio wave propagation medium layer, which is made up of a group of micro-helical structures in which a conductive surface layer is formed on phytoplankton having a helical shape and in which the micro-helical structures are electrically or magnetically connected to form an aggregate, and a normal layer of a right-handed radio wave propagation medium are laminated. Patent Document 2, Patent Document 3, and Non-Patent Document 5 disclose a method for manufacturing a micro-helical structure that uses phytoplankton as a material suitable for mass-producible micro-helical structures, and the micro-helical structure itself. These technologies are examples of biotemplate technology that uses the shape of an algae called Spirulina as a template to create a metal helical structure.

Patent No. 5234673 Patent No. 5274653 Patent No. 5606572

M. Fujishima, “Future of 300 GHz band wireless communications and their enabler, CMOS transceiver technologies”, Jpn. J. Appl. Phys. 60, SB0803 (2021), DOI: 10.35848/1347-4065/abdf24 Y. Koyama et al., "A High-Power Terahertz Source Over 10 mW at 0.45 THz Using an Active Antenna Array With Integrated Patch Anten nas and Resonant-Tunneling Diode", IEEE Trans. THz Sci. Tech. vol. 12, no. 5, pp. 510-519, (2022), DOI: 10.1109/TTHZ.2022.3180492 Chiko Otani et al., “Development of 300 GHz walk-through body scanner for the security gate applications”, Proc. SPIE 11827, Terahertz Emitters, Receivers, and Applications XII, 118270N (1 August 2021), DOI: 10.1117/12.2594528 Tomoyuki Otani, "Terahertz Wave Sensing and Imaging Technology and Applications", Surface Technology, 2021, Vol. 72, No. 8, pp. 429-432, DOI: 10.4139/sfj.72.429 Kamata, K. et al., "Spirulina-Templated Metal Microcoils with Controlled Helical Structures for THz Electromagnetic Responses", Sci Rep 4, 4919 (2014). DOI: 10.1038/srep04919 Notake, T. et al., "Dynamical visualization of anisotropic electromagnetic re-emissions from a sin gle metal micro-helix at THz frequencies", Sci Rep 11, 3310 (2021). DOI: 10.1038/s41598-020-80510-y

One of the measures to suppress noise in the THz wave region is to use an absorbing material (including a shielding material; the same applies below) that can suppress the amplitude and intensity of reflected and transmitted waves, such as the black light-absorbing material used to prevent stray light in optical equipment (Non-Patent Documents 3 and 4). However, a method that relies only on the attenuation performance of an absorbing material is not necessarily sufficient in the THz wave region. An example of this is the case where quadrature amplitude modulation (QAM) is adopted in the THz wave wireless communication technology considered for Beyond 5G/6G. Even in noise countermeasures for communication methods that use phase such as QAM, it is ideal to prevent reflection or transmission of communication signal waves in unintended paths at either the transmission, transmission, or reception stage. However, there are only a limited number of good absorbing materials in the THz wave region, and this is not necessarily easy. This is because if some signal waves that are not completely absorbed retain the phase of the signal wave, they can be highly harmful in phase-sensitive applications even if their intensity is weak. In phase-sensitive applications, the attenuation performance of the absorbent material is required to be particularly high. This disclosure provides a new method that can solve such problems, thereby contributing to improving the performance of various methods and products that utilize THz waves.

The inventor discovered that it is possible to effectively eliminate the noise-causing properties of reflected THz waves while employing fine particles such as those shown in Patent Documents 1 to 3, and thus completed the invention disclosed herein.

In other words, one aspect of the present disclosure provides a terahertz wave diffuser (THz wave diffuser) that includes a dispersion medium that transmits incident terahertz waves, and a particulate dispersoid that is dispersed and supported in the dispersion medium, with each particle having a material and a microstructure that responds to the incident terahertz waves.

In such a THz wave diffuser, the incident THz wave is converted into, for example, an electric current and absorbed in response to the wave, and part of the energy of the incident THz wave is converted into a re-radiated THz wave. The re-radiated THz wave is absorbed again by the surrounding THz wave diffuser, and this process is repeated while losing energy. For this reason, this THz wave diffuser attenuates the energy of the incident THz wave, and the effective optical path length is extended by diffusion, and the amplitude of the incident THz wave is attenuated while the phase is also disturbed, so that the re-radiated THz wave is less likely to adversely affect the phase information of the incident THz wave as noise. In other words, in the THz wave diffuser of the present disclosure, the phase of the re-radiated THz wave is disturbed when viewed from the incident THz wave, so that, in addition to absorption, the adverse effect of noise can be suppressed.

Throughout this application, THz waves refer to electromagnetic waves in the frequency range of approximately 0.1 THz to 100 THz (wavelength in a vacuum of 3 μm to 3 mm). This application also uses terms for light, including visible light, to explain phenomena and functions of THz waves. For example, the emission or blocking of THz waves may be expressed using terms such as "illumination" and "shading." Additionally, "random" refers to positions and orientations that are disrupted to the extent that the regularity is broken even slightly.

In any of the aspects of the present disclosure, a diffuser can be provided that, when irradiated with an incident THz wave, produces re-radiated THz having a disturbed phase.

FIG. 1 is a perspective view of an exemplary THz wave diffuser according to an embodiment of the present disclosure. FIG. 2 is an enlarged view showing the structure of particles constituting a particulate dispersoid in an embodiment of the present disclosure. 3A and 3B are a structural diagram of spirulina used in an embodiment of the present disclosure (FIG. 3A) and a partially cutaway structural diagram of a metal microcoil in which a metal thin film is formed on spirulina (FIG. 3B). 4A and 4B are explanatory diagrams illustrating the operation of the phase disturbance effect in an embodiment of the present disclosure, with FIG. 4A showing the effect upon transmission and FIG. 4B showing the effect upon reflection. FIG. 5 is a flow chart illustrating an exemplary method of manufacturing a THz wave diffuser in accordance with an embodiment of the present disclosure. 6A and 6B are schematic diagrams showing an overview of the optical element arrangement of a reflectance measurement system in an embodiment of the present disclosure. FIG. 7A is a graph showing a voltage waveform obtained by detecting an electromagnetic pulse emitted from a THz wave irradiator using a THz wave detector, and FIG. 7B is a graph showing a power spectrum obtained from the voltage waveform. 8A to 8C are photographs showing the appearance of each sample actually produced in the embodiment of the present disclosure (Example sample 1 (FIG. 8A), Example sample 2 (FIG. 8B)) and Comparative example sample 3 (FIG. 8C). 9A-C show reflected signals in a reflection arrangement actually measured using the arrangement of FIG. 6A in an embodiment of the present disclosure, for example sample 1 (FIG. 9A), example sample 2 (FIG. 9B), and comparative example sample 3 (FIG. 9C). 10A and 10B show reflectance spectra measured in a reflection configuration for samples according to an embodiment of the present disclosure, and are the reflectance spectra obtained from Example Sample 1 (FIG. 10A) and Comparative Example Sample 3 (FIG. 10B), respectively.

Below, an embodiment of a diffuser according to the present disclosure will be described with reference to the drawings. Unless otherwise specified in the description, common parts or elements are given common reference symbols. Furthermore, in the drawings, the elements of each embodiment are not necessarily shown to scale.

1. Overview The THz wave diffuser provided in this embodiment includes a dispersion medium and a particulate dispersoid. FIG. 1 is a perspective view of a typical THz wave diffuser 100 of this embodiment. THz waves (not shown in FIG. 1) are incident on the THz wave diffuser 100 from an external space. These THz waves are hereinafter referred to as "incident THz waves." The dispersion medium 2 does not have a strong effect when the incident THz waves pass through the THz wave diffuser 100. In other words, the dispersion medium 2 only acts to slightly attenuate or refract the incident THz waves, and allows them to pass through. In contrast, the particulate dispersoid 10 responds electromagnetically to the incident THz waves. In order to obtain this responsiveness, the individual particles 1 constituting the particulate dispersoid 10 are manufactured so as to have a material and a fine structure that respond to the incident THz waves. Typical examples of the dispersion medium 2 are insulating materials such as polystyrene foam and urethane resin.

2. Particulate Dispersoid The particulate dispersoid 10 of the present disclosure is an aggregate of individual particles 1 having a material and a microstructure that electromagnetically responds to incident THz waves. FIG. 2 is an enlarged view showing the structure of an individual particle 1 that constitutes the particulate dispersoid. A typical example of the individual particle 1 is a metal microcoil 3, and such a metal microcoil 3 is preferably formed by forming a film of a metal (nickel, copper, etc.) on the surface of spirulina 4 by any method such as electroless plating. FIGS. 3A and 3B are respectively a structural diagram of spirulina 4 and a partially cutaway structural diagram of a metal microcoil 3 in which a metal thin film 5 is formed on spirulina 4. The spirulina 4 that serves as a biotemplate for the metal microcoil 3 is a micro-object that has a filamentary structure (spiral) that draws a helix like a screw thread. This spiral is determined according to the type of spirulina and the growing conditions. In other words, the left-handed or right-handed nature of the spiral ("winding direction", handedness or chirality), pitch, natural length of the coil, length of the stretched filamentous part of the coil, total number of turns, and diameter of the spiral can be adjusted to some extent depending on the type of Spirulina and the growing conditions. Details are disclosed in Non-Patent Document 5.

In a typical example in which spirulina 4 is used as a micro-object, the size of this metal microcoil 3 in its axial direction (left-right direction on the paper in FIG. 2) is about 25 μm to 35 μm in helical diameter and about 40 μm to 300 μm in helical major axis length. In another measured example, many of the individual pieces have an average length of 140 μm, and in yet another measured example, half of the individual pieces are 190 μm or more. When the electromagnetic field of the incident THz wave acts on the individual particles 1 constituting the particulate dispersoid 10 of the present disclosure, the individual particles 1 respond electromagnetically at the same frequency as the incident THz wave. In particular, when the individual particles 1 are metal microcoils 3, they show a characteristic response due to the electrical conductivity of the metal. That is, when the incident THz wave acts on the metal microcoil 3, the metal microcoil 3 acts as a receiving antenna, and a current that oscillates in response to the incident THz wave is induced in itself. The current in the metal microcoil 3 generated by irradiation of the incident THz wave to the metal microcoil 3 is an alternating current with the same frequency as the incident THz wave. However, this frequency of AC current generally has a very large energy loss in the form of Joule heat due to electrical resistance ("Joule loss"), and this situation is the same for the AC current in the metal microcoil 3. In addition, a unique phenomenon due to the shape of the metal microcoil 3 may also occur. Even if a diffuser such as a fine metal rod of a similar size is used instead of the metal microcoil 3 as the individual particles 1 constituting the particulate dispersoid 10, the diffuser will cause scattering. However, the metal microcoil 3 is broadband and highly efficient compared to a fine metal rod of a similar size and length, and the length of the path through which the AC current flows is long, so it is thought that the Joule loss will be strong. In other words, the metal microcoil 3 is configured to be prone to Joule loss among various aspects of the individual particles 1. In addition, when the metal microcoil 3 is used as a receiving antenna for the individual particles 1, it is highly efficient and broadband compared to using another one, and it also functions as a transmitting antenna that re-radiates THz waves of the same frequency as the incident THz waves while strongly attenuating the incident THz waves as Joule loss. This process of absorption and re-emission is then efficiently repeated by multiple microcoils, resulting in a large contrast in the phase difference and phase randomness (i.e., wavefront disturbance).

The re-radiation effect can be explained by corresponding to the direction, i.e. orientation, of the metal microcoil 3. The re-radiated THz waves can be explained by decomposing them into an antenna component of dipole-mode radiation, with the dipole moment oriented in the axial direction of the spiral, and an antenna component of axial-mode radiation in the axial direction of the spiral.

Dipole mode radiation, like the radiation characteristics of a dipole antenna, shows strong radiation in a planar direction perpendicular to the axis of the spiral. In contrast, axial mode radiation shows strong radiation in the axial direction of the spiral. The winding direction of the spiral also mainly affects the optical rotation in axial mode radiation. In other words, the intensity of the THz waves re-radiated from the incident THz waves differs between right-handed and left-handed circularly polarized light. Note that while the radiation characteristics as a transmitting antenna have been explained with a focus on the re-radiation effect, these radiation characteristics also correspond to the response effect to incident THz waves, that is, the sensitivity (directivity) for each direction of reception when operating as a receiving antenna.

When a metal microcoil 3 is used for the particulate dispersoid 10, the response of each individual particle 1 to an incident THz wave can have anisotropy depending on the orientation of the metal microcoil 3. This anisotropy is the anisotropy of dipole mode radiation and axial mode radiation for an incident THz wave of a single frequency. Since the radiation distributions of dipole mode radiation and axial mode radiation are different, anisotropy occurs in the intensity distribution. Dipole mode radiation and axial mode radiation are described in detail in Non-Patent Document 6.

3. Dispersion The particulate dispersoid 10 of the present disclosure is dispersed and supported in a dispersion medium 2 as shown in FIG. 1. The dispersion mode (dispersibility) of the particulate dispersoid 10 can be set in various ways, from a degree where the position distribution of the dispersed individual particles 1 is scattered in the dispersion medium 2 with at least some randomness and some bias, to a degree where the scattering is random but unbiased and highly uniform. In addition, the directional distribution of the orientation of the dispersed individual particles 1 can be set in various ways, from a degree where some directional dependency remains, to a degree where no directional dependency is found and the distribution of the position and orientation of the particulate dispersoid 10 in the dispersion medium 2 depends on how the particulate dispersoid 10 is mixed into the dispersion medium 2, and further depends on the material of the dispersion medium 2, the fluidity of the dispersion medium 2, and the solidification treatment method.

The positional range (dispersion range) in which the individual particles 1 of the particulate dispersoid 10 are arranged is preferably determined in relation to the wavelength λ of the incident THz wave. If the particulate dispersoid 10 is arranged so that the dispersion range is associated with the wavelength λ along the incident direction of the incident THz wave, it is useful that the phase of the re-radiated THz wave has a phase difference from that of the incident THz wave due to repeated absorption and re-radiation. Specifically, it is preferable that the dispersion range is equal to or greater than the wavelength λ, and more preferably equal to or greater than about 2λ. For example, since the wavelength λ of a THz wave with a frequency of 300 GHz (0.3 THz) is 1 mm in a vacuum, it is preferable that the dispersion range is equal to or greater than 1 mm, and more preferably equal to or greater than 2 mm. In the THz wave diffuser 100 of FIG. 1, this dispersion range can be determined by the thickness d when the incident THz wave is propagated in the z direction.

In the THz wave diffuser 100 of the present disclosure, it is preferable that the concentration of the particulate dispersoid is appropriately set. The concentration can be determined by using the number of particles of the particulate dispersoid 10, for example, by defining the number of particles per volume of the THz wave diffuser 100. Instead of the number of particles, the concentration can also be defined by other indicators such as the weight of the particulate dispersoid 10 per unit volume of the THz wave diffuser 100 or the optical density in the THz wave diffuser 100. To specifically determine the concentration, the relationship with the incident THz wave that is scattered, reflected, or absorbed by the particulate dispersoid 10 is taken into consideration. The intensity of the incident THz wave propagating through the THz wave diffuser 100 attenuates as it propagates, and the degree of attenuation is related to the concentration. The length measured from the incident surface to the position where the intensity at the time of incidence into the THz wave diffuser 100 is 1/e (where e is the base of the natural logarithm (Euler's number or Napier's number)) can be defined as the penetration length. When the concentration is high, the incident THz wave attenuates over a shorter distance, so the penetration length is small. Here, in the THz wave diffuser of this embodiment, it is not necessarily preferable to increase the concentration too much. If the concentration is too high, the incident THz wave will attenuate immediately after entering the THz wave diffuser. In addition, for these reasons, even if the concentration is the same, differences in characteristics will occur when the THz wave diffusers are unevenly distributed in some places. Therefore, the degree of dispersion (dispersibility) of the THz wave diffuser also relates to the characteristics. The attenuation of the incident THz wave in the THz wave diffuser of this embodiment is caused not only by absorption, in which energy is converted into heat, but also by re-radiation. If the incident THz wave attenuates rapidly over a short distance, re-radiation will occur within that short distance. In that case, the phase will be re-radiated without being disturbed much, and the phase disturbance effect may be weak. On the contrary, the phase is effectively disturbed by making the concentration moderately small and dispersing it spatially to cause re-radiation. Therefore, if the concentration of particulate dispersoid 10 in the THz wave diffuser 100 is too high, phase disturbance will be difficult to occur.

Thus, in the THz wave diffuser 100 of this embodiment, the lower the concentration of the particulate dispersoid 10, the greater the penetration depth, and the deeper the incident THz waves penetrate. For this reason, it is advantageous to adjust the concentration and degree of dispersion of the particulate dispersoid 10 in order to spatially disperse the positions of re-radiation. Specifically, it is preferable that the concentration of the particulate dispersoid 10 is a value smaller than the concentration at which the penetration length of the incident THz wave in the THz wave diffuser 100 is equal to or greater than the wavelength of the incident THz wave, and it is even more preferable that the concentration is equal to or greater than twice the wavelength of the incident THz wave. Furthermore, it is preferable that the degree of dispersion of the THz wave diffuser is distributed over a wider range than that of the THz wave diffuser being concentrated in one area.

4. Phase Disturbance Effect The phase disturbance effect of this embodiment will be described. The phase when each particle 1 of the particulate dispersoid 10 responds to the incident THz wave and re-radiates it is affected by the fact that both the response and re-radiation in each particle 1 are anisotropic, and that each particle is distributed in a positionally dispersed manner. Figures 4A and 4B are explanatory diagrams for explaining the action of the phase disturbance effect, with Figure 4A showing the action during transmission and Figure 4B showing the action during reflection. Each figure shows a cross section of a THz wave diffuser 100 having a thickness in the z direction, and each particle of the particulate dispersoid 10 in the cross section is shown diagrammatically.

As shown in Figure 4A, during transmission, the main factor is the phase difference that occurs at individual particles 1 due to repeated multiple incidences and re-emissions, and if a phase difference occurs between the incident THz wave and the re-emitted THz wave at an individual particle 1, this also plays a secondary role, disrupting the phase of the transmitted THz wave.

The main contributor to the phase disturbance effect is the effect of multiple interactions (multiple disturbance effect). The transmitted THz wave contains not only the incident THz wave itself that is transmitted, but also the re-radiated THz wave component that is re-radiated by another individual particle 1. For this reason, the THz wave that transmits through the THz wave diffuser 100 is a superposition of THz waves, such as the incident THz wave attenuated by the receiving action of the individual particle 1, the THz wave that is absorbed once by the individual particle 1 and re-radiated (i.e., has interacted only once), the THz wave that has interacted twice with the individual particle 1, and so on. As explained in relation to FIG. 4A, even THz waves that have interacted with individual particles 1 the same number of times have phase variations due to differences in the orientation of the individual particles 1, and therefore the THz wave that has interacted multiple times has further phase variations and is disturbed.

The phase difference that may occur in an individual particle 1 between the incident THz wave and the re-emitted THz wave, which may have a secondary contribution to the phase disturbance effect, is explained as follows. FIG. 4A shows particles 1A, 1B, and 1C. In FIG. 4A, it is assumed that an incident THz wave Wi, which is a linearly polarized plane wave having an electric field in the x direction, is incident while heading toward the z direction (to the right of the paper). In addition, it is assumed that both particles 1A and 1C have a helical axis along the x axis perpendicular to the z direction, and particle 1B has a helical axis along the z axis. In FIG. 4A, the amplitude of the incident THz wave Wi at a certain time is depicted at the top, including the attenuation state, and for the re-emitted THz waves Wt _A to Wt _C of particles 1A to 1C, only the portion in the transmission direction is shown by a wavefront. The particles 1A and 1C, which are shifted in position in the z direction, receive the incident THz wave at a timing corresponding to the shift, and each re-emits the THz wave. Since THz waves of the same frequency are received at shifted timings and THz waves that maintain the shift are re-emitted, the re-emitted THz waves Wt _A and Wt _C from the particles 1A and 1C have the same phase as each other in the components heading in the +z direction. Compared to the phase with the incident THz wave Wi, the re-emitted THz waves Wt _A and Wt _C have a phase difference due to the mechanism of interaction (reception, re-emission), but the shift in the positions of the particles 1A and 1C cancels the incident timing and emission timing. In contrast, in the case of the re-emitted particle 1B, since it has a different orientation (orientation) from the particles 1A and 1C, the re-emitted THz wave Wt _B from the particle 1B also has a phase difference with the re-emitted THz waves Wt _A and Wt _C from the particles 1A and 1C. The incident THz wave Wi weakens its intensity as it propagates in the z direction in the THz wave diffuser 100 due to the interaction with each particle 1.

Thus, the THz wave diffuser 100 exerts a phase disturbance effect even when used in a manner that transmits THz waves. In the explanation of FIG. 4A, the reason that no phase difference occurs in the re-radiated THz waves for particles 1A and 1C is because it is assumed that the re-radiated THz waves are directed forward in the direction of the incident THz wave (z direction), and furthermore, the orientation of particles 1A and 1C is the same in relation to the polarization and incident direction. Since re-radiated THz waves generally have a directional distribution, and the orientation of individual particles is distributed such that there are almost no particles with the same orientation, it is possible to obtain a sufficient phase disturbance effect even when the THz wave diffuser 100 is used in a transmitted manner.

As shown in FIG. 4B, even during reflection, the effect of the optical path difference due to the positional distribution of individual particles is primarily involved in the disturbance of the phase, and the phase difference between the incident THz wave and the re-emitted THz wave in each particle and the phase difference caused by the anisotropy of each particle are secondarily involved. FIG. 4B shows the incident THz wave Wi shown by the amplitude of the time at the top, and the transmission direction of the re-emitted THz waves Wr _A to Wr _C of the particles 1A to 1C in the wavefront. The particles 1A and 1C, which are at different positions in the z direction, did not produce a phase difference in the transmission in the +z direction, but when reflected in the -z direction, a phase difference occurs between the re-emitted THz waves Wr _A and Wr _C due to the shift in the z direction positions of the particles 1A and 1C themselves. This is due to both the shift in the timing at which the incident THz wave Wi reaches the particles 1A and 1C, and the shift in the emission position when the re-emitted THz waves Wr _A and Wr _C propagate from the particles 1A and 1C in the -z direction. As in the case of transmission, the re-radiated THz waves Wr _A and Wr _C have a phase difference due to the mechanism of interaction (reception, re-radiation) with the incident THz wave Wi, and the re-radiated THz wave Wr _B in the particle 1B has a phase difference with the re-radiated THz waves Wr _A and Wr _C from the particles 1A and 1C due to the different orientation from the particles 1A and 1C in the particle 1B. It is also the same that the intensity of the incident THz wave Wi is weakened as it propagates in the z direction in the THz wave diffuser 100. Furthermore, it is also the same that not only the phase but also the efficiency of reception and re-radiation differs depending on the orientation (orientation) of each particle 1, and the re-radiated THz wave shows an intensity according to the orientation of each particle, and a multiple disturbance effect occurs. However, when the THz wave diffuser 100 is used in a reflection arrangement, unlike transmission, the incident THz wave attenuated by the reception action of each particle 1 is not directly reflected and does not need to be considered. For this reason, the THz waves reflected by THz wave diffuser 100 are a superposition of THz waves, such as a THz wave that has been absorbed once by an individual particle 1 and re-emitted (interacted only once), a THz wave that has interacted twice with an individual particle 1, and so on.

In this way, the THz wave diffuser 100 exerts a phase disruption effect even when used to reflect THz waves.

Note that the explanations given for transmission and reflection are based on a typical arrangement, and the THz wave diffuser of this embodiment can achieve phase disturbance in various directions. The orientation of the individual particles 1 can also be oriented in any direction in three dimensions, thereby enabling phase disturbance by the individual particles 1. Furthermore, in the THz wave diffuser of the embodiment of the present disclosure, the phase disturbance effect as described above occurs, and at the same time, conversion to Joule heat occurs in the individual particles 1, so the THz wave diffuser of this embodiment can also act as an absorber.

6. Manufacturing method FIG. 5 is a flow chart showing a typical method for manufacturing the THz wave diffuser 100 of this embodiment. First, a coil-shaped micro object is formed (S02). For this purpose, for example, spirulina is grown to have a desired shape. Next, a metal microcoil is formed from the coil-shaped micro object (S04). Specifically, a metal layer is formed on the surface of the coil-shaped micro object. For this purpose, for example, electroless plating can be adopted. At this stage, the particulate dispersoid 10 is manufactured. Non-Patent Document 5 details a specific method for growing spirulina to have a desired shape and a method for forming a metal layer on the surface of the grown spirulina. Next, the particulate dispersoid 10 is dispersed in the dispersion medium 2 or a precursor of the dispersion medium 2 (S06), and the dispersion state is fixed (S08). If the dispersion medium is a thermoplastic resin, the thermoplastic resin that has been heated to exhibit fluidity becomes a precursor of the dispersion medium, so that the particulate dispersoid 10 is mixed and dispersed therein, and then cooled to a temperature at which the resin loses its fluidity, whereby dispersion and fixation can be realized. The specific method of dispersion and fixation can be determined depending on the properties of the dispersion medium. Any means or method for enhancing dispersibility can be adopted, such as chemically modifying the surface of the material of the particulate dispersoid 10 or adding a dispersant for enhancing dispersibility, as necessary. However, the degree of dispersibility in the THz wave diffuser of this embodiment is not particularly limited.

7. Example As an example of the THz wave diffuser of this embodiment, a sample was prepared and the actual reflection characteristics were measured. Figures 6A and 6B are schematic diagrams showing an outline of the optical element arrangement of the reflectance measurement system, Figure 7A is a graph showing the waveform of an electromagnetic pulse, and Figure 7B is the spectrum of the electromagnetic pulse calculated by FFT processing.

Reflectance can be measured by a method called terahertz time-domain spectroscopy (THz-TDS). For measurement, an electromagnetic pulse having a frequency component in the THz wave region is irradiated. For example, an electromagnetic pulse having a pulse width of about 1 psec is irradiated within a measurement window period of 160.0 psec. The spectrum of the measurement waveform is obtained by FFT (fast Fourier transform) processing of data including the measurement waveform during the measurement window period. In this embodiment, the band is 0.1 to 3.5 THz, which covers a part of the THz wave. In addition, the dynamic range of the measurement can be secured to about 80 dB. As shown in Figures 6A and 6B, the THz wave irradiator E is an epi-illumination system that illuminates the sample S from above, and the THz wave detector D is arranged, for example, so that the pupil positions of the THz wave irradiator E and the THz wave detector D are approximately confocal. For this purpose, an appropriate half mirror HM is used, and an objective lens L made of a material that transmits THz waves is arranged. FIG. 6A shows a schematic arrangement in which the THz wave from the THz wave irradiator E converges inside the sample S near its upper surface, and the reflected wave from there converges on the THz wave detector D. This is an arrangement for efficiently measuring the reflected signal from inside the sample near the upper surface of the sample S. FIG. 6B is an arrangement for efficiently measuring the reflected signal from inside the sample near the lower surface. However, the depth of field on the sample S side of the light receiving system including the objective lens L and the THz wave detector D is sufficiently large, and the reflected wave from any position in the depth direction (z direction) of the sample S is incident on the THz wave irradiator E. In the actual measurement system, a delay path (not shown) with a variable optical path length is provided, and by adjusting the optical path length, the reflected component of the THz wave from each position in the depth direction (z direction in the figure) of the sample S can be detected. Therefore, in order to precisely determine the position in the z direction, the optical path length of the delay path is adjusted. The metal mirror MM may be placed below the sample S, and serves as a mirror that reflects the THz wave. The metal mirror MM is positioned as necessary to reproduce the state in which the THz wave diffuser 100 is attached to an object that reflects THz waves, such as a metal plate.

Figure 7A is a graph showing the voltage waveform of the electromagnetic pulse emitted from the THz wave irradiator E and detected by the THz wave detector D, and Figure 7B is a graph of the power spectrum obtained from that voltage waveform. The measurement window period (160 psec in one example) is a length of time that can cover the time it takes for the THz wave to travel back and forth through the thickness of the sample. In addition, when measuring each sample, an appropriate filter was applied to the reflected time waveform to improve the visibility of the minute reflected pulse train from the metal microcoil in the sample.

Figures 8A-C are external photographs of each sample that was actually produced (Example Sample 1 (Figure 8A), Example Sample 2 (Figure 8B), Comparative Example Sample 3 (Figure 8C)). Figures 9A-C are reflection signals actually measured in the reflection arrangement of Figure 6A for Example Sample 1 (Figure 9A), Example Sample 2 (Figure 9B), and Comparative Example Sample 3 (Figure 9C). Figures 10A-B are reflection spectra actually measured in the reflection arrangement of the samples, and are the reflection spectra obtained from Example Sample 1 (Figure 10A) and Comparative Example Sample 3 (Figure 10B).

In Example Sample 1 of FIG. 8A, foamed styrene beads (so-called expanded polystyrene) are used for the dispersion medium 2, and metal microcoils 3 are dispersed in the dispersion medium 2 as particulate dispersoids 10. The metal microcoil 3 of Example Sample 1 is a left-handed spirulina 4 grown to have an axial length of about 100 μm, on which nickel is formed by electroless plating for the metal thin film 5. In Example Sample 1, the concentration of the particulate dispersoid 10 is about 0.51 w/v%, and the thickness d is 20 mm. Example Sample 2 of FIG. 8B uses the same conditions as Example Sample 1, but uses copper formed by electroless plating for the metal thin film 5 of the metal microcoil 3, and the concentration of the particulate dispersoid 10 is 0.25 w/v% or less, and the thickness d is 15 mm. Comparative example sample 3 in Figure 8C is a 0.6 cm thick Eccosorb AN-72 sheet (manufactured by Laird Corporation), a standard absorber and low reflector in the conventional terahertz band.

9A-C show the reflection amplitude signals in the reflection arrangement actually measured without placing the metal mirror MM in the arrangement of FIG. 6A. In each figure, the horizontal axis is the delay time (unit: psec) corresponding to the position in the thickness direction of the sample (the vertical direction on the paper in FIG. 6A and B), and is shown in the range of the measurement window period (160 psec). Also, for each sample, reflection measurement was performed in an arrangement focused on the incident surface of each sample as shown in FIG. 6A. The vertical axis of each figure shows the output voltage value of the THz wave detector D, and the voltage value is shown in arbitrary units. However, the units are maintained so that the same value shows the same voltage value between the two curves in each figure and between FIG. 9A-C. In each figure, the small value of the delay time corresponds to the top side of each sample in FIG. 6A, and the range of the delay time corresponding to the positional range inside each sample is shown by the double-headed arrow. The black solid line in each figure is the background signal measured without placing a sample, and the curve with the diagonal line pattern overlaid is the one with each sample placed. In all cases, the curves with diagonal lines overlap mostly with the solid black line for delay times outside the range of the double-headed arrows, i.e., outside the sample. This shows that when the metal mirror MM is not placed, no reflection occurs from that position, including below the sample. In the curves with diagonal lines, values that deviate from the solid black line are the actual reflected signals, and significant values are distributed within the range of the double-headed arrows for all samples.

Specifically, in Example Sample 1 in Figure 9A, of the delay times corresponding to the inside of the sample indicated by the double-headed arrow, the reflected signal was concentrated in the range of 5 psec to 50 psec, that is, the delay times corresponding to the vicinity of the incident surface, and from 50 psec to 135 psec, the reflected signal was very weak despite being inside the sample. Also, when compared with the measured value (not shown) in which a metal mirror MM was placed, the reflected signal itself did not differ significantly at any of the delay times. For this reason, it is believed that in Example Sample 1, the incident THz wave was well absorbed within a short distance from the incident surface, and did not reach the rear surface of the sample (above or below the paper in Figures 6A and B).

In Example Sample 2 in Figure 9B, the reflected signal is distributed over a range of 10 psec to 140 psec, out of the delay times corresponding to the inside of the sample indicated by the double-headed arrow, that is, delay times corresponding to the entire region from the incident surface to the back surface of the sample. Also, in the measured value (not shown) in which the metal mirror MM was placed, the reflected signal became large at positions exceeding 140 psec. For this reason, it is believed that in Example Sample 2, relatively weak absorption was achieved throughout the entire thickness of the sample, and the incident THz wave reached the back surface of the sample.

In contrast to these, in Comparative Example Sample 3 in FIG. 9C, a strong reflected signal is generated at a delay time of 70 psec to 90 psec, which corresponds to a very narrow range inside the sample from the incident surface, among the delay times of 70 psec to 140 psec corresponding to the inside of the sample indicated by the double-headed arrow. In Comparative Example Sample 3, no reflected signal is observed at a position exceeding 140 psec in the measured value (not shown) in which the metal mirror MM is placed. From these results, it can be said that in Comparative Example Sample 3, the incident THz wave is attenuated before reaching the back surface of the sample, but is reflected at a shallow position from the incident surface. In Comparative Example Sample 3, the attenuation itself can be said to be sufficient, but the reflectivity is relatively high. In Comparative Example Sample 3, the reflectivity is high in a narrow range in the thickness direction, and the waveform of the reflected signal also strongly reflects the irradiated pulse waveform in FIG. 7A. Focusing only on the reflected signal, it is stronger in Comparative Example Sample 3 than in Example Samples 1 and 2, and the phase appearing in the pulse waveform is preserved when it is reflected. From these results, the inventors believe that Comparative Example Sample 3 has a weak effect of disturbing the phase of the incident THz wave. Note that while the measurement results using the arrangement in Figure 6A have been explained, the measurement results using the arrangement in Figure 6B, in which the wave is focused on the rear surface of the sample, also showed no significant difference.

Figures 10A-B show reflection spectra in the reflection arrangement actually measured, obtained from Example Sample 1 (Figure 10A) and Comparative Example Sample 3 (Figure 10B). The horizontal axis of each figure is frequency (unit: THz). The black solid line is the reflection spectrum of the reflected signal itself in the measured value in which no reference signal, i.e. no sample, is placed, and only the metal mirror MM as in Figure 7A is placed as a reflector. In contrast, the curves with diagonal lines are the reflection spectrum obtained from the reflected signal of each sample. Since no sample is placed in the black solid line, the drop from the black solid line to the curve with diagonal lines reflects the contribution of reflection by the sample. Looking at Comparative Example Sample 3 shown in Figure 10B, for example, at 0.3 THz (300 GHz), only a little less than 1/100 (-20 dB) is reflected. In contrast, at the same 0.3 THz, reflection is suppressed to 1/1000 (-30 dB) in Example Sample 1 shown in Figure 10A. Although not shown, a reflection spectrum similar to that of Example Sample 1 was measured for Example Sample 2. Comparing Figures 10A and 10B, it was confirmed that the THz wave diffuser 100 of this embodiment is a low reflector that surpasses conventional absorbers in the relatively low frequency range of THz waves, with an upper limit of approximately 0.7 THz. This low reflectivity, combined with the phase disturbance effect described above, demonstrates the superiority of the THz wave diffuser 100 of this embodiment in phase-sensitive applications of THz waves.

As described above, the THz wave diffuser of this embodiment achieves a phase-disturbing effect and exhibits properties suitable for phase-sensitive applications.

Anisotropy is not necessarily achieved only when metal microcoils 3 are used for the particulate dispersoid 10, but can also be achieved when the individual particles 1 of the particulate dispersoid 10 have different materials and microstructures. For example, a microcoil in which the individual particles are made of only conductive metal is conceivable. However, current metal processing technology has not yet achieved the fabrication of a microcoil shape of a similar size. Metal microcoils are in practical use as so-called spiral antennas, which are broadband and compact antennas, and are used in mobile phone antennas, etc.

The above provides a specific description of the embodiments of the present disclosure. The above-mentioned embodiments and examples are described in order to explain the invention, and the scope of the invention of this application should be determined based on the description of the claims. In addition, modifications that exist within the scope of this disclosure, including other combinations of the embodiments, are also included in the claims.

100 THz wave diffuser 10 Particulate dispersoid 1, 1A, 1B, 1C Individual particle (of particulate dispersoid) 2 Dispersion medium 3 Metal microcoil 4 Spirulina 5 Metal thin film D THz wave detector E THz wave irradiator HM Half mirror L Objective lens MM Metal mirror S Sample

Claims (10)

a dispersion medium that transmits incident terahertz waves;
a particulate dispersoid, each of the individual particles having a material and a microstructure that responds to the incident terahertz waves, the particulate dispersoid being dispersed and supported in the dispersion medium. the particulate dispersoid is dispersed and disposed in the dispersion medium over a certain dispersion range along the incident direction of the incident terahertz wave,
The diffuser of claim 1 , wherein the dispersion range is equal to or greater than the wavelength of the incident terahertz wave. The diffuser according to claim 1 , wherein the concentration of the particulate dispersoid in the dispersion medium is smaller than a concentration at which the penetration depth of the incident terahertz wave into the diffuser is equal to or greater than the wavelength of the incident terahertz wave. The diffuser of any one of claims 1 to 3, wherein the individual particles of the particulate dispersoid are metal microcoils. The diffuser according to claim 4 , wherein the metal microcoil is a coil-shaped micro object having a metal thin film formed on the surface thereof. The diffuser according to claim 5, wherein the microscopic object is spirulina. 4. The diffuser according to claim 1, wherein individual particles of the particulate dispersoid are responsive to the incident terahertz wave and re-radiate terahertz waves having the same frequency as the incident terahertz wave. the response of the individual particles to the incident terahertz radiation is anisotropic;
The diffuser of claim 7 , wherein the particulate dispersoid is dispersed in the dispersion medium with the individual particles randomly oriented. The diffuser according to claim 1 , which gives a phase difference having a distribution between a reflected terahertz wave returning to the half space on the side where the incident terahertz wave is incident and the incident terahertz wave. A particulate dispersoid production step of producing particles that are metal microcoils by forming a metal thin film on the surface of a coil-shaped micro object;
a dispersing step of dispersing the particulate dispersoid in a dispersion medium that transmits incident terahertz waves.

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