Terahertz wave diffuser and method for manufacturing the same

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 manufacturing the same

[0001] The present disclosure relates to a terahertz wave diffuser and a method for manufacturing the same, and more particularly to a terahertz wave diffuser in which the phase of an incident terahertz wave is disturbed, and a method for manufacturing the same.

[0002] Electromagnetic waves with frequencies of approximately 0.1 THz to 100 THz (wavelength in vacuum of 3 μm to 3 mm) are also known as terahertz waves (hereinafter also referred to as "THz waves") and are expected to have a wide range of applications. In fifth-generation communication systems and beyond (hereinafter referred to as "Beyond 5G / 6G") for high-speed communication, the use of electromagnetic waves exceeding 300 GHz, reaching the THz wave band, is also anticipated. For example, Non-Patent Document 1 discloses a transceiver using silicon CMOS technology capable of operating in the 300 GHz band. Furthermore, Non-Patent Document 2 discloses a 446 GHz RTD (resonant tunneling diode) radiation source with an active antenna array arranged in a 6x6 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 using THz waves in the 300 GHz band (Non-Patent Documents 3 and 4).

[0003] As the applications of THz waves expand, absorbers (including low-reflectors and transmission-shielding materials, the same applies below) are also being used. For example, Eccosorb (registered trademark) AN-72 is a typical absorber of electromagnetic waves, including THz waves, that is 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, and typical uses are to prevent reflection, transmission, and interference in order to prevent noise.

[0004] Recently, biotemplate technology using living organisms has also attracted attention. Patent Document 1 discloses an interference-type radio wave shielding or absorbing material characterized by a laminated structure consisting of a group of microhelical structures formed by forming a conductive surface layer on helical phytoplankton, the microhelical structures being electrically or magnetically connected to form an aggregate, and a normal layer of a right-handed radio wave propagation medium. Patent Documents 2, 3, and Non-Patent Document 5 disclose a method for manufacturing a microhelical structure using phytoplankton as a material suitable for mass-producible microhelical structures, and the microhelical structure itself. These technologies are examples of biotemplate technology in which the shape of an alga called Spirulina is used as a template to create a metal helical structure.

[0005] Patent No. 5234673 Patent No. 5274653 Patent No. 5606572

[0006] 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 / abdf24Y. Koyama et al., "A High-Power Terahertz Source Over 10 mW at 0.45 THz Using an Active Antenna Array With Integrated Patch Antennas and Resonant-Tunneling Diode", IEEE Trans. THz Sci. Tech. vol. 12, no. 5, pp. 510-519, (2022), DOI: 10.1109 / TTHZ.2022.3180492Chiko 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 single metal micro-helix at THz frequencies", Sci Rep 11, 3310 (2021). DOI: 10.1038 / s41598-020-80510-y.

[0007] One measure to suppress noise in the THz wave region is to use an absorptive 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 devices (Non-Patent Documents 3 and 4). However, methods that rely solely on the attenuation performance of an absorptive material are not necessarily sufficient in the THz wave region. An example of this is the adoption of quadrature amplitude modulation (QAM) in THz wave wireless communication technology being considered for Beyond 5G / 6G. Even in noise suppression for phase-based communication methods such as QAM, it is ideal to prevent the reflection or transmission of communication signal waves along unintended paths at either the transmission, transmission, or reception stages. However, good absorptive materials are limited in the THz wave region, making this not necessarily easy. If some signal waves that are not completely absorbed retain their phase, they can be highly harmful in phase-sensitive applications, even if their intensity is weak. In phase-sensitive applications, particularly high performance is required in the attenuation performance of the absorbent member. The present disclosure provides a novel method that can solve such problems, thereby contributing to improving the performance of various methods and products that utilize THz waves.

[0008] The present inventors have discovered that it is possible to effectively eliminate the noise-causing properties of reflected THz waves while employing fine particles such as those disclosed in Patent Documents 1 to 3, and have completed the invention of the present disclosure.

[0009] That is, one aspect of the present disclosure provides a terahertz wave diffuser (THz wave diffuser) comprising a dispersion medium that transmits incident terahertz waves, and a particulate dispersoid, each of whose individual particles has a material and a microstructure that responds to the incident terahertz waves, and which is dispersed and supported in the dispersion medium.

[0010] In such a THz wave diffuser, incident THz waves are converted into, for example, an electric current and absorbed in response to the waves, and a portion of the energy of the incident THz waves is converted into re-radiated THz waves. The re-radiated THz waves are then absorbed again by the surrounding THz wave diffusers, losing energy as this process is repeated. As a result, this THz wave diffuser attenuates the energy of the incident THz waves and extends the effective optical path length through diffusion. This attenuates the amplitude of the incident THz waves and disturbs their phase, making it less likely that the re-radiated THz waves will adversely affect the phase information of the incident THz waves as noise. In other words, in the THz wave diffuser of the present disclosure, by making the phase of the re-radiated THz waves disturbed relative to the incident THz waves, the adverse effect of noise, combined with absorption, can be suppressed.

[0011] Throughout this application, THz waves refer to electromagnetic waves in a frequency range of approximately 0.1 THz to 100 THz (wavelength in a vacuum of 3 μm to 3 mm). Furthermore, in this application, terms for light, including visible light, may be used to describe the phenomena and functions of THz waves. For example, terms such as "illumination" and "shading" may be used to describe the emission or blocking of THz waves. Furthermore, "random" refers to the position and orientation being disrupted to the extent that even the slightest degree of regularity is lost.

[0012] In any aspect of the present disclosure, a diffuser can be provided that, when irradiated with an incident THz wave, results in re-radiated THz waves having a perturbed phase relative to the diffuser.

[0013] FIG. 1 is a perspective view of a typical 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 according to an embodiment of the present disclosure. FIGS. 3A and 3B are a structural diagram of spirulina (FIG. 3A) used in an embodiment of the present disclosure, and a partially cutaway structural diagram of a metal microcoil in which a metal thin film is formed on spirulina (FIG. 3B). FIGS. 4A and 4B are explanatory diagrams illustrating the operation of the phase disturbance effect according to an embodiment of the present disclosure, with FIG. 4A showing the effect during transmission and FIG. 4B showing the effect during reflection. FIG. 5 is a flowchart showing a typical method for manufacturing a THz wave diffuser according to an embodiment of the present disclosure. FIGS. 6A and 6B are schematic diagrams showing an overview of the arrangement of optical elements in a reflectance measurement system according to an embodiment of the present disclosure. FIG. 7A is a graph showing a voltage waveform detected by a THz wave detector of an electromagnetic pulse emitted from a THz wave irradiator, and FIG. 7B is a graph of the power spectrum obtained from the voltage waveform. 8A to 8C are photographs of the appearance of each sample (Example Sample 1 ( FIG. 8A ), Example Sample 2 ( FIG. 8B )) actually produced in an embodiment of the present disclosure, and Comparative Example Sample 3 ( FIG. 8C ). FIGS. 9A to 9C are reflection signals actually measured in a reflection configuration using the configuration of FIG. 6A in an embodiment of the present disclosure, and are for Example Sample 1 ( FIG. 9A ), Example Sample 2 ( FIG. 9B ), and Comparative Example Sample 3 ( FIG. 9C ). FIGS. 10A and 10B are reflection spectra measured in a reflection configuration for samples in an embodiment of the present disclosure, and are the reflection spectra obtained from Example Sample 1 ( FIG. 10A ) and Comparative Example Sample 3 ( FIG. 10B ), respectively.

[0014] Hereinafter, embodiments of the diffuser according to the present disclosure will be described with reference to the drawings. In the description, common parts or elements are designated by common reference numerals unless otherwise specified. Furthermore, in the drawings, elements of each embodiment are not necessarily shown to scale.

[0015] 1. Overview The THz wave diffuser provided in this embodiment includes a dispersion medium and 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 external space. These THz waves are hereinafter referred to as "incident THz waves." The dispersion medium 2 does not have a strong effect on the incident THz waves as they pass through the THz wave diffuser 100. In other words, the dispersion medium 2 only slightly attenuates or refracts the incident THz waves, allowing them to pass through. In contrast, the particulate dispersoid 10 responds electromagnetically to the incident THz waves. To achieve this responsiveness, the individual particles 1 that make up the particulate dispersoid 10 are manufactured to have a material and microstructure that responds to the incident THz waves. Typical examples of the dispersion medium 2 are insulating materials such as polystyrene foam and urethane resin.

[0016] 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 individual particles 1 that constitute the particulate dispersoid. A typical example of the individual particles 1 is a metal microcoil 3. Such metal microcoils 3 are preferably formed by forming a metal (nickel, copper, etc.) film 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 microscopic object with a filamentary structure (spiral) that forms a helix like a screw thread. This spiral is determined depending on the type of spirulina and its growing conditions. In other words, the left-handed or right-handed spiral ("winding direction", also known as handedness or chirality), pitch, natural length of the coil, the length of the stretched filamentous portion 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 growing conditions. Details are disclosed in Non-Patent Document 5.

[0017] In a typical example using spirulina 4 as the micro-object, the metal microcoil 3 has a helical diameter of approximately 25 μm to 35 μm and a helical major axis length of approximately 40 μm to 300 μm. In another measured example, many of the micro-objects had an average length of 140 μm, and in another measured example, half of the micro-objects had an average length of 190 μm or more. When the oscillating electromagnetic field of incident THz waves acts on 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 waves. In particular, when the individual particles 1 are metal microcoils 3, they exhibit a characteristic response due to the electrical conductivity of the metal. That is, when incident THz waves act on the metal microcoil 3, the metal microcoil 3 acts as a receiving antenna, and an oscillating current is induced in the metal microcoil 3 itself. The current generated in the metal microcoil 3 by irradiation of the metal microcoil 3 with incident THz waves is an alternating current at the same frequency as the incident THz waves. However, AC currents of this frequency generally suffer from very large energy losses in the form of Joule heat due to electrical resistance ("Joule loss"). This situation is similar to that of AC currents in metal microcoils 3. In addition, unique phenomena due to the shape of the metal microcoils 3 may also occur. Even if a diffuser such as a fine metal rod of similar size were used instead of the metal microcoils 3 as the individual particles 1 constituting the particulate dispersoid 10, the diffuser would still cause scattering. However, the metal microcoils 3 are broader in bandwidth and higher in efficiency than fine metal rods of similar size and length, and the length of the path through which the AC current flows is longer, which is thought to result in stronger Joule loss. In other words, among the various configurations of the individual particles 1, the metal microcoils 3 are configured to be more susceptible to Joule loss. Furthermore, when the metal microcoils 3 are used as a receiving antenna for the individual particles 1, they are more efficient and broader in bandwidth than when using a separate antenna. They also function as a transmitting antenna, re-radiating THz waves at the same frequency as the incident THz waves while strongly attenuating the incident THz waves as Joule loss. Such a process of absorption and re-emission is efficiently repeated by a plurality of microcoils, resulting in a large contrast in the phase difference and randomness in the phase (i.e., disturbance of the wavefront).

[0018] The re-radiation effect can be explained in accordance with 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, in which the dipole moment is oriented in the axial direction of the spiral, and an antenna component of axial-mode radiation, in the axial direction of the spiral.

[0019] Dipole mode radiation exhibits strong radiation in a planar direction perpendicular to the spiral axis, similar to the radiation characteristics of a dipole antenna. In contrast, axial mode radiation exhibits 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 focusing on the re-radiation effect, these radiation characteristics also correspond to the response effect to incident THz waves, that is, the sensitivity (directivity) in each direction of reception when operating as a receiving antenna.

[0020] When a metal microcoil 3 is used for the particulate dispersoid 10, the response of each particle 1 to an incident THz wave can be anisotropic, 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. The dipole mode radiation and axial mode radiation have different radiation distributions, resulting in anisotropy in the intensity distribution. Dipole mode radiation and axial mode radiation are described in detail in Non-Patent Document 6.

[0021] 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 a variety of ways, from a position distribution of the dispersed individual particles 1 that is scattered in the dispersion medium 2 with at least some randomness and some bias, to a distribution that is random but unbiased and highly uniform. The directional distribution of the orientation of the dispersed individual particles 1 can also be set in a variety of ways, from a distribution that retains some degree of directional dependency to a distribution that exhibits high isotropy with no detectable directional dependency. The position and orientation distribution of the particulate dispersoid 10 in the dispersion medium 2 depend on how the particulate dispersoid 10 is mixed into the dispersion medium 2, and also depend on the material of the dispersion medium 2, the fluidity of the dispersion medium 2, and the solidification method.

[0022] 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 along the incident direction of the incident THz wave so as to have a dispersion range associated with the wavelength λ, repeated absorption and re-emission will be beneficial, as the phase of the re-emitted THz wave will have a phase difference from that of the incident THz wave. Specifically, it is preferable that the dispersion range be 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 be equal to or greater than 1 mm, and even 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 propagates in the z direction.

[0023] In the THz wave diffuser 100 of the present disclosure, it is preferable that the concentration of the particulate dispersoid be appropriately set. The concentration can be determined 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 using other indicators such as the weight of the particulate dispersoid 10 per unit volume of the THz wave diffuser 100 or the optical density of the THz wave diffuser 100. To specifically determine the concentration, the relationship with the incident THz waves that are scattered, reflected, or absorbed by the particulate dispersoid 10 is taken into consideration. The intensity of the incident THz waves propagating through the THz wave diffuser 100 attenuates as they propagate, and the degree of attenuation is related to the concentration. The penetration depth can be defined as 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)) of the intensity at the time of incidence. A higher concentration reduces the penetration depth because the incident THz waves attenuate over a shorter distance. However, 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 waves will attenuate immediately after penetrating the THz wave diffuser. Furthermore, even if the concentration is the same, uneven distribution of THz wave diffusers in certain locations will result in differences in characteristics. Therefore, the degree of dispersion (dispersibility) of the THz wave diffuser also affects the characteristics. The attenuation of incident THz waves in the THz wave diffuser of this embodiment is due not only to absorption, in which energy is converted into heat, but also to re-radiation. If the incident THz waves attenuate rapidly over a short distance, re-radiation occurs within that short distance. In this case, the re-radiation will occur without significant phase disturbance, potentially weakening the phase disturbance effect. Conversely, by appropriately reducing the concentration and causing spatial dispersion to cause re-radiation, the phase is effectively disturbed. Therefore, if the concentration of the particulate dispersoid 10 in the THz wave diffuser 100 is too high, it becomes difficult to cause phase disturbance.

[0024] As described above, in the THz wave diffuser 100 of this embodiment, the penetration depth increases as the concentration of the particulate dispersoid 10 decreases, allowing the incident THz waves to penetrate deeper. Therefore, it is advantageous to adjust the concentration and degree of dispersion of the particulate dispersoid 10 in order to spatially disperse the re-emission positions. Specifically, the concentration of the particulate dispersoid 10 is preferably set to a value smaller than the concentration at which the penetration depth of the incident THz waves in the THz wave diffuser 100 is equal to or greater than the wavelength of the incident THz waves, and more preferably equal to or greater than twice the wavelength of the incident THz waves. Furthermore, it is preferable that the degree of dispersion of the THz wave diffuser be such that the THz wave diffusers are dispersed over a wider range rather than concentrated in a specific area.

[0025] 4. Phase Disturbance Effect The phase disturbance effect of this embodiment will now be described. The phase when individual particles 1 of the particulate dispersoid 10 respond to and re-radiate incident THz waves is affected by the fact that both the response and re-radiation of the individual particles 1 exhibit anisotropy and that the individual particles are distributed in a positionally dispersed manner. Figures 4A and 4B are explanatory diagrams illustrating the operation of the phase disturbance effect, with Figure 4A showing the operation during transmission and Figure 4B showing the operation during reflection. Each figure depicts a cross section of the THz wave diffuser 100 having a thickness in the z direction, and schematically illustrates individual particles of the particulate dispersoid 10 in that cross section.

[0026] As shown in Figure 4A, during transmission, the phase difference that occurs mainly due to repeated multiple incidences and re-emissions at individual particles 1 is involved, 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 secondarily role, disrupting the phase of the transmitting THz wave.

[0027] 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 transmitted component of the incident THz wave itself, but also the re-radiated THz wave component re-radiated by another individual particle 1. Therefore, the THz wave transmitted 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 absorbed once by the individual particle 1 and re-radiated (i.e., interacted only once), the THz wave interacted twice by the individual particle 1, and so on. As described in relation to FIG. 4A , even THz waves that have interacted with individual particles 1 the same number of times have different phases due to differences in the orientation of the individual particles 1, and therefore the THz waves that have interacted multiple times have further different phases and are disturbed.

[0028] 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, can be explained as follows. Figure 4A shows particles 1A, 1B, and 1C. In Figure 4A, it is assumed that the incident THz wave Wi, which is a linearly polarized plane wave with an electric field in the x direction, is incident while heading in the z direction (to the right on the page). It is also assumed that both particles 1A and 1C have a helical axis along the x axis, which is perpendicular to the z direction, and particle 1B has a helical axis along the z axis. Figure 4A depicts the amplitude of the incident THz wave Wi at a certain time, including how it decays, at the top, and the re-emitted THz waves Wt of particles 1A to 1C. Ａ ~Wt Ｃ , only the portion in the transmission direction is shown by a wavefront. 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. THz waves of the same frequency are received at shifted timings, and THz waves are re-emitted while maintaining the shift, so the re-emitted THz waves Wt from particles 1A and 1C Ａ , Wt Ｃ The components in the +z direction are in phase with each other. Ａ , Wt ＣAlthough there is a phase difference due to the mechanism of interaction (reception and re-emission) when compared with the phase of the incident THz wave Wi, the positional shift of particles 1A and 1C cancels out the timing of incidence and emission. In contrast, in the case of the re-emitted particle 1B, since it has a different orientation (orientation) from particles 1A and 1C, the re-emitted THz wave Wt Ｂ is the re-radiated THz wave Wt from particles 1A and 1C Ａ , Wt Ｃ The intensity of the incident THz wave Wi is weakened as it propagates in the z-direction through the THz wave diffuser 100 due to interactions with the individual particles 1.

[0029] Thus, the THz wave diffuser 100 exhibits a phase disturbance effect even when used in a manner that transmits THz waves. In the explanation of Fig. 4A , the reason why 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 waves (z direction), and because the orientations of particles 1A and 1C are identical in relation to the polarization and incident direction. Because re-radiated THz waves generally have a directional distribution, and because the orientations of individual particles are distributed and there are almost no particles with the same orientation, a sufficient phase disturbance effect can be obtained even when the THz wave diffuser 100 is used in a transmission mode.

[0030] During reflection, as shown in Figure 4B, the phase disturbance is mainly caused by the effect of the optical path difference due to the positional distribution of individual particles, and secondarily caused by 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. Figure 4B shows the amplitude of the incident THz wave Wi at the top and the re-emitted THz wave Wr of particles 1A to 1C. Ａ ~Wr Ｃ The particle 1A and particle 1C, which are positioned differently in the z direction, do not produce a phase difference in transmission in the +z direction, but when reflected in the -z direction, the particle 1A and particle 1C themselves are misaligned in the z direction, resulting in a re-radiated THz wave Wr Ａ and Wr ＣThis is because the timing at which the incident THz wave Wi reaches the particles 1A and 1C differs, and the re-radiated THz wave Wr Ａ and Wr Ｃ This is due to the shift in the radiation position when the re-radiated THz wave Wr propagates in the -z direction. Ａ , Wr Ｃ has a phase difference due to the mechanism of interaction (reception, re-radiation) with the incident THz wave Wi, and in particle 1B, the re-radiated THz wave Wr is generated by particle 1B due to a different orientation from particles 1A and 1C. Ｂ is the re-radiated THz wave Wr from particles 1A and 1C. Ａ , Wr Ｃ and the incident THz waves Wi. Similarly, the intensity of the incident THz waves Wi weakens as the waves propagate through the THz wave diffuser 100 in the z direction. Furthermore, not only the phase but also the efficiency of reception and re-emission differs depending on the orientation (orientation) of the individual particles 1, so that the re-emitted THz waves exhibit an intensity that corresponds to the orientation of the individual particles, and a multiple disturbance effect also occurs. However, when the THz wave diffuser 100 is used in a reflection configuration, unlike transmission, the incident THz waves that have been attenuated by the reception action of the individual particles 1 are not directly reflected, and therefore do not need to be taken into consideration. Therefore, the THz waves reflected by the THz wave diffuser 100 are a superposition of THz waves, such as THz waves that have been absorbed once and re-emitted (interacted only once) by individual particles 1, THz waves that have interacted twice with individual particles 1, and so on.

[0031] In this way, the THz wave diffuser 100 exerts a phase disturbance effect even when used to reflect THz waves.

[0032] Note that the explanations 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 three-dimensional direction, thereby achieving phase disturbance by the individual particles 1. Furthermore, in the THz wave diffuser of the embodiment of the present disclosure, not only does the phase disturbance effect as described above occur, but conversion to Joule heat also occurs in the individual particles 1 at the same time, so the THz wave diffuser of this embodiment can also function as an absorber.

[0033] 6. Manufacturing Method FIG. 5 is a flowchart showing a typical method for manufacturing the THz wave diffuser 100 of this embodiment. First, coil-shaped micro-objects are formed (S02). To achieve this, for example, spirulina is grown to the 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, electroless plating, for example, can be used. At this stage, particulate dispersoids 10 are manufactured. Non-Patent Document 5 details a specific method for growing spirulina to the desired shape and a method for forming a metal layer on the surface of the grown spirulina. Next, the particulate dispersoids 10 are dispersed in a 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, which has been heated to exhibit fluidity, serves as the precursor of the dispersion medium. Therefore, the particulate dispersoids 10 can be mixed and dispersed therein, and then cooled to a temperature at which the resin loses its fluidity, thereby achieving dispersion and fixation. The specific dispersion and fixation techniques can be determined depending on the properties of the dispersion medium. If necessary, any means or technique for enhancing dispersibility can be employed, such as chemically modifying the surface of the material of the particulate dispersoid 10 or adding a dispersant to enhance dispersibility. However, the degree of dispersibility in the THz wave diffuser of this embodiment is not particularly limited.

[0034] 7. Example As an example of the THz wave diffuser of this embodiment, a sample was prepared and its actual reflection characteristics were measured. Figures 6A and 6B are schematic diagrams showing an outline of the optical element arrangement in 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.

[0035] Reflectance can be measured using a technique called terahertz time-domain spectroscopy (THz-TDS). For measurement, an electromagnetic pulse with a frequency component in the THz wave region is irradiated. For example, an electromagnetic pulse with a pulse width of approximately 1 psec is irradiated within a measurement window period with a time width of 160.0 psec. The spectrum of the measured waveform is obtained by FFT (fast Fourier transform) processing of data including the measured waveform during the measurement window period. In this embodiment, the band of 0.1 to 3.5 THz, which covers a portion of the THz wave, is used. Furthermore, a measurement dynamic range of approximately 80 dB can be ensured. 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 positioned, for example, so that their pupil positions are approximately confocal with the THz wave irradiator E. For this purpose, an appropriate half mirror HM is used, and an objective lens L made of a material that transmits THz waves is placed therein. Figure 6A shows a schematic diagram of an arrangement in which THz waves emitted from a THz wave irradiator E converge inside a sample S near its top surface, and the reflected waves converge on a THz wave detector D. This arrangement efficiently measures the reflected signals from inside the sample S near its top surface. Figure 6B shows a similar arrangement efficiently measuring the reflected signals from inside the sample S near its bottom 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 so that reflected waves from any position in the depth direction (z direction) of the sample S are incident on the THz wave irradiator E. In an actual measurement system, a delay path (not shown) with a variable optical path length is provided. 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, the optical path length of the delay path is adjusted to precisely determine the position in the z direction. A metal mirror MM is sometimes placed below the sample S and serves as a mirror that reflects the THz waves. The metal mirror MM is disposed as necessary to reproduce a state in which the THz wave diffuser 100 is attached to an object that reflects THz waves, such as a metal plate.

[0036] Figure 7A is a graph showing the voltage waveform detected by a THz wave detector D of an electromagnetic pulse emitted from a THz wave irradiator E, 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, the reflected time waveform was subjected to appropriate filtering to improve the visibility of the minute reflected pulse train from the metal microcoil in the sample.

[0037] Figures 8A to 8C are photographs of the appearance of each sample that was actually produced (Example Sample 1 (Figure 8A), Example Sample 2 (Figure 8B), and Comparative Example Sample 3 (Figure 8C)). Figures 9A to 9C are reflection signals actually measured in the reflection configuration of Figure 6A, and are for Example Sample 1 (Figure 9A), Example Sample 2 (Figure 9B), and Comparative Example Sample 3 (Figure 9C). Figures 10A and 10B are reflection spectra actually measured in the reflection configuration of the samples, and are the reflection spectra obtained from Example Sample 1 (Figure 10A) and Comparative Example Sample 3 (Figure 10B), respectively.

[0038] In Example Sample 1 of FIG. 8A , foamed polyethylene beads (so-called expanded polystyrene) were used as the dispersion medium 2, and metal microcoils 3 were dispersed in the dispersion medium 2 as particulate dispersoids 10. The metal microcoil 3 of Example Sample 1 was a left-handed spirulina 4 grown to an axial length of approximately 100 μm, on which nickel was formed by electroless plating as the metal thin film 5. In Example Sample 1, the concentration of the particulate dispersoids 10 was approximately 0.51 w / v %, and the thickness d was 20 mm. In Example Sample 2 of FIG. 8B , under the conditions of Example Sample 1, copper was formed by electroless plating as the metal thin film 5 of the metal microcoil 3, and the concentration of the particulate dispersoids 10 was 0.25 w / v % or less, and the thickness d was 15 mm. Comparative Example Sample 3 in FIG. 8C is a 0.6 cm thick Eccosorb AN-72 sheet (manufactured by Laird), which is a conventional standard absorber and low reflector in the terahertz band.

[0039] Figures 9A-9C show reflection amplitude signals actually measured in the reflection configuration shown in Figure 6A without the metal mirror MM. In each figure, the horizontal axis represents the delay time (unit: psec) corresponding to the position in the thickness direction of the sample (the vertical direction on the paper in Figures 6A and 6B), and is shown within the measurement window period (160 psec). Furthermore, for each sample, reflection measurements were performed with the focus focused on the incident surface of each sample, as shown in Figure 6A. The vertical axis in each figure represents the output voltage value of the THz wave detector D, and the voltage values ​​are shown in arbitrary units. However, the units are maintained so that the same values ​​represent the same voltage values ​​between the two curves in each figure and between Figures 9A-9C. In each figure, the smallest delay time corresponds to the top surface of each sample in Figure 6A, and the double-headed arrow indicates the delay time range corresponding to the positional range inside each sample. In each figure, the solid black line represents the background signal measured without a sample, and the curves with diagonal patterns overlaid represent the signals measured with each sample. The curves with diagonal lines overlap the solid black line for delay times outside the range indicated by the double-headed arrows, i.e., outside the sample. This indicates 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 represent actual reflected signals, and significant values ​​are distributed within the range indicated by the double-headed arrows for all samples.

[0040] Specifically, in Example Sample 1 in Figure 9A, among the delay times corresponding to the inside of the sample indicated by the double-headed arrow, the reflected signals were concentrated in the range of 5 psec to 50 psec, i.e., the delay times corresponding to the vicinity of the incident surface, and the reflected signals were very weak between 50 psec and 135 psec, despite being inside the sample. Furthermore, when compared with measurements (not shown) in which a metal mirror MM was placed, the reflected signals themselves were not significantly different at any of the delay times. For this reason, it is believed that in Example Sample 1, the incident THz waves were well absorbed within a short distance from the incident surface, and did not reach the rear surface of the sample (above and below the paper in Figures 6A and 6B).

[0041] In Example Sample 2 in Figure 9B, the reflected signals are distributed over a range of 10 psec to 140 psec, among the delay times corresponding to the inside of the sample indicated by the double-headed arrow, that is, over the entire delay time range from the incident surface to the back surface of the sample. Furthermore, in measurements (not shown) in which a metal mirror MM was placed, the reflected signals increased at positions exceeding 140 psec. Therefore, it is believed that Example Sample 2 achieved relatively weak absorption throughout the entire thickness of the sample, allowing the incident THz waves to reach the back surface of the sample.

[0042] In contrast, in Comparative Sample 3 of Figure 9C, a strong reflected signal is generated at delay times of 70 psec to 140 psec, corresponding to the sample interior indicated by the double-headed arrow, and at delay times of 70 psec to 90 psec, corresponding to a very narrow range within the sample from the incident surface. Furthermore, in Comparative Sample 3, no reflected signal is observed at positions exceeding 140 psec in measurements (not shown) where a metal mirror MM is placed. From these results, it can be said that in Comparative Sample 3, the incident THz wave is attenuated by the time it reaches the rear surface of the sample, but is reflected at a shallow position from the incident surface. Although the attenuation itself is sufficient in Comparative Sample 3, the reflectivity is relatively high. Furthermore, Comparative Sample 3 has high reflectivity in a narrow range in the thickness direction, and the waveform of the reflected signal strongly reflects the irradiated pulse waveform in Figure 7A. Focusing on the reflected signal alone, Comparative Sample 3 is stronger than Example Samples 1 and 2, and the phase appearing in the pulse waveform is preserved during reflection. From these results, the inventors believe that the effect of disturbing the phase of the incident THz wave is weak in Comparative Example Sample 3. Note that although the measurement results using the arrangement shown in Fig. 6A have been explained, the measurement results using the arrangement shown in Fig. 6B, in which the wave is converged on the rear surface of the sample, also showed no significant difference.

[0043] Figures 10A and 10B show reflection spectra measured in actual reflection configurations, obtained from Example Sample 1 (Figure 10A) and Comparative Example Sample 3 (Figure 10B). The horizontal axis in each figure represents frequency (unit: THz). The solid black line represents the reflection spectrum of the reference signal, i.e., the reflection signal itself, measured when no sample is placed, and only a metal mirror MM, as shown in Figure 7A, is placed as a reflector. In contrast, the curves with diagonal lines represent the reflection spectrum obtained from the reflection signal of each sample. Since no sample is placed in the solid black line, the drop from the solid black line to the diagonal line 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), reflection is slightly less than 1 / 100 (-20 dB). In contrast, at the same 0.3 THz, Example Sample 1 shown in Figure 10A suppresses reflection to 1 / 1000 (-30 dB). Although not shown, a reflection spectrum similar to that of Example Sample 1 was measured for Example Sample 2. A comparison of Figures 10A and 10B confirms 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.

[0044] As described above, the THz wave diffuser of this embodiment realizes a phase disturbance effect and exhibits properties suitable for phase-sensitive applications.

[0045] Anisotropy is not necessarily achieved when metal microcoils 3 are used for the particulate dispersoids 10; the individual particles 1 of the particulate dispersoids 10 can also be achieved with other 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 equivalent size. Metal microcoils have been put to practical use as broadband, compact antennas known as spiral antennas, and are used in mobile phone antennas, etc.

[0046] The embodiments of the present disclosure have been specifically described above. The above-mentioned embodiments and examples have been 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 within the scope of the present disclosure, including other combinations of the embodiments, are also included in the claims.

[0047] 100 THz wave diffuser 10 Particulate dispersoid 1, 1A, 1B, 1C Individual particles (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

1. A terahertz wave diffuser comprising: a dispersion medium that transmits incident terahertz waves; and a particulate dispersoid, each of whose individual particles has a material and a microstructure that responds to the incident terahertz waves, and which is dispersed and supported in the dispersion medium.

2. The diffuser according to claim 1, wherein the particulate dispersoid is dispersed and arranged in the dispersion medium over a certain dispersion range along the incident direction of the incident terahertz wave, and the dispersion range is equal to or greater than the wavelength of the incident terahertz wave.

3. The diffuser according to claim 1, wherein the concentration of the particulate dispersoid in the dispersion medium is smaller than the 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.

4. A diffuser according to any one of claims 1 to 3, wherein the individual particles of the particulate dispersoid are metal microcoils.

5. The diffuser according to claim 4, wherein the metallic microcoil is a coil-shaped micro-object having a metallic thin film formed on its surface.

6. The diffuser according to claim 5, wherein the micro-object is spirulina.

7. A diffuser according to any one of claims 1 to 3, wherein individual particles of the particulate dispersoid are responsive to the incident terahertz waves and re-radiate terahertz waves having the same frequency as the incident terahertz waves.

8. The diffuser of claim 7, wherein the response of the individual particles to the incident terahertz waves is anisotropic, and the particulate dispersoid is dispersed in the dispersion medium with the individual particles randomly oriented.

9. The diffuser according to claim 1, which gives a phase difference having a distribution to a reflected terahertz wave returning to the half space on the side from which the incident terahertz wave was incident, relative to the incident terahertz wave.

10. A method for manufacturing a terahertz wave diffuser, comprising: a particulate dispersoid manufacturing step for manufacturing particles which are metal microcoils by forming a thin metal film on the surface of a coil-shaped micro-object; and a dispersion step for dispersing the particulate dispersoid in a dispersion medium which transmits incident terahertz waves.

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