Terahertz polarizer and terahertz polarizing system equipped therewith

Abstract

We provide a terahertz polarizer capable of polarization of terahertz waves across a wide bandwidth. [Solution] The terahertz polarizer 10, which polarizes and reflects electromagnetic waves from the millimeter wave band to the terahertz band, comprises a dielectric plate 11 with a tapered surface 12 relative to its back surface 13. An electrically conductive layer 14, sufficiently thinner than the wavelength of the electromagnetic wave, is formed on the back surface 13 of the dielectric plate 11. Electromagnetic waves that enter from the surface 12 of the dielectric plate 11, which is placed in a surrounding medium 30 having a lower refractive index than the dielectric plate 11, and propagate through the dielectric plate 11 are reflected by the electrically conductive layer 14.

Description

[Technical Field]

[0001] The present invention relates to a terahertz polarizer that polarizes and reflects electromagnetic waves from the millimeter wave band to the terahertz band, and to a terahertz polarizing system equipped with such a terahertz polarizer. [Background technology]

[0002] Terahertz waves (THz waves) are electromagnetic waves with frequencies ranging from 0.1 THz to 10 THz and wavelengths from 3 mm to 30 μm, falling between visible light and radio waves. While terahertz waves are penetrating to many materials, they are known to exhibit unique absorption and reflection properties at specific frequencies. For this reason, terahertz waves play an important role in non-destructive testing and material analysis.

[0003] When terahertz waves are used in various applications, polarization is employed, and it is known that the reflectivity of p-waves becomes zero due to the Brewster effect. On the other hand, it has been reported that when electromagnetic waves in the terahertz to visible range are irradiated from the high refractive index medium side of a dielectric with a graphene layer formed on its surface, and these electromagnetic waves are reflected by the graphene layer, the reflectivity of s-waves can be made zero (see, for example, Non-Patent Document 1). Furthermore, it has been shown that the reflection of electromagnetic waves in dielectrics with an electrically conductive layer formed on their surface can be simplified and explained using a surface current model (see, for example, Non-Patent Document 2). [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] Lin, Xiao, et al. "Transverse-electric Brewster effect enabled by nonmagnetic two-dimensional materials." Physical Review A 94.2 (2016): 023836. [Non-Patent Document 2] Nagai, M., Watanabe, S., Imamura, R. et al. "Characterization of Ultrathin Conductive Films Using a Simplified Approach for Terahertz Time-Domain Spectroscopic Ellipsometry." Journal of Infrared, Millimeter, and Terahertz Waves, 45, 949-966 (2024). [Overview of the project] [Problems that the invention aims to solve]

[0005] As a device for polarizing terahertz waves, a terahertz wire grid polarizer is known, which consists of ultra-thin tungsten wires stretched parallel to each other in a substrate-less holder. However, this polarizer can only polarize terahertz waves up to a maximum frequency of 2 THz (wavelength 150 μm), and polarizing terahertz waves at higher frequencies is difficult. Therefore, the present invention aims to provide a terahertz polarizer capable of polarizing terahertz waves across a wide bandwidth, and a terahertz polarizing system equipped with such a terahertz polarizer. [Means for solving the problem]

[0006] According to one aspect of the present invention, a terahertz polarizer is provided that polarizes and reflects electromagnetic waves from the millimeter wave band to the terahertz band, comprising a dielectric plate with a tapered surface relative to its back surface, wherein an electrically conductive layer sufficiently thinner than the wavelength of the electromagnetic wave is formed on the back surface of the dielectric plate, and the electrically conductive layer reflects the electromagnetic wave that enters from the surface of the dielectric plate and propagates within the dielectric plate, which is placed in a surrounding medium having a refractive index smaller than that of the dielectric plate.

[0007] According to another aspect of the present invention, there is provided a terahertz polarizing system including the above terahertz polarizer and an angle adjusting mechanism for adjusting the incident angle to the terahertz polarizer and / or the angle of the terahertz polarizer such that the electromagnetic wave is incident on the electrically conductive layer near the critical angle.

Advantages of the Invention

[0008] According to the present invention, a polarizer that reflects almost only p-waves in a wide band ranging from several tens of GHz to several THz can be realized.

Brief Description of the Drawings

[0009] [Figure 1] It is a cross-sectional schematic view showing the basic structure of the terahertz polarizer according to the present invention. [Figure 2] It is a graph showing the angle dependence of the terahertz polarizer according to Example 1 composed of polyimide and an organic semiconductor. [Figure 3] It is a graph showing the frequency dependence of the terahertz polarizer according to Example 1. [Figure 4] It is a graph showing the conductivity dependence of the terahertz polarizer according to Example 1. [Figure 5] It is a graph showing the angle dependence of the terahertz polarizer according to Example 2 composed of undoped silicon and an organic semiconductor. [Figure 6] It is a graph showing the frequency dependence of the terahertz polarizer according to Example 2. [Figure 7] It is a graph showing the frequency dependence of the terahertz polarizer according to Example 2 when the terahertz wave is incident at an angle slightly smaller than the critical angle. [Figure 8] It is a graph showing the conductivity dependence of the terahertz polarizer according to Example 2. [Figure 9] It is a graph showing the relationship between the thickness of the electrically conductive layer and the angle dependence of the terahertz polarizer according to Example 1. [Figure 10] It is a graph showing the relationship between the thickness of the electrically conductive layer and the frequency dependence of the terahertz polarizer according to Example 1. [Figure 11] This graph shows the relationship between carrier mobility and angle dependence in the electrical conductive layer of the terahertz polarizer related to Example 1. [Figure 12] This graph shows the relationship between the real-to-imaginary-to-real-to-imaginary-to-reflectance ratio and the reflectance of the electrical conductive layer of the terahertz polarizer in Example 1. [Figure 13] This graph shows the relationship between the carrier concentration in the electrical conductive layer of a terahertz polarizer in Example 3, which is composed of undoped silicon and doped silicon, and the real part of the p-wave and s-wave reflectance. [Figure 14] This is a schematic diagram of a terahertz polarizing system according to one embodiment of the present invention. [Figure 15] This is a cross-sectional view of a terahertz polarizer according to one embodiment of the present invention. [Figure 16] This graph shows the relationship between the angle of incidence of terahertz waves onto the surface of a dielectric plate and the reflectivity of p-waves and s-waves when the front and back surfaces of the dielectric plate are parallel. [Figure 17] This graph shows the relationship between the angle of incidence of terahertz waves on the surface of a dielectric plate and the reflectivity of p-waves and s-waves when the surface of the dielectric plate is tapered relative to the back surface. [Modes for carrying out the invention]

[0010] ≪Basic Structure of a Terahertz Polarizer≫ Figure 1 is a schematic cross-sectional diagram showing the basic structure of the terahertz polarizer according to the present invention. There is an electrically conductive layer in close contact with the surface of a dielectric material with refractive index n1. The thickness h of the electrically conductive layer is sufficiently thin compared to the wavelength λ of the terahertz wave in the electrically conductive layer, for example, a few micrometers, which is a fraction of a wavelength. The dielectric material is surrounded by a surrounding medium with a refractive index n3 that is smaller than that of the dielectric material. Generally, terahertz waves refer to electromagnetic waves in the terahertz band from 0.1 to 1 THz, but in this specification, terahertz waves refer to a slightly wider range, including electromagnetic waves from tens of GHz to several THz, with the lower end being the millimeter wave band or lower, and the upper end being the terahertz band and higher.

[0011] The terahertz wave propagating in the dielectric is reflected by the electric conduction layer. At this time, the reflectivities of the p-wave and s-wave are different. Specifically, the p-wave is well reflected, while for the s-wave, a phenomenon similar to the Brewster phenomenon occurs and the reflectivity becomes extremely small.

[0012] Now, let the incident angle of the terahertz wave be θ in , the reflection angle be θ out , and the conductivity of the electric conduction layer be σ dc . Then, the s-wave reflection coefficient r s is given by the following equation (1). In particular, when θ in is the critical angle of total reflection, r s is given by the following equation (2). The s-wave reflectivity R s is given by the square value of r s . Since the thickness of the electric conduction layer is sufficiently thin with respect to the wavelength of the terahertz wave and it is considered that the dielectric is in contact with the surrounding medium, the critical angle is determined by the relationship between the refractive index n1 of the dielectric and the refractive index n3 of the surrounding medium. TIFF2026097037000002.tif30128However, Z0 is the impedance of vacuum, which is approximately 377 Ω.

[0013] By appropriately selecting the materials of the dielectric and the electric conduction layer, the thickness and conductivity of the electric conduction layer, the numerator on the right side of equation (2) can be made zero, and theoretically, R s can be made zero. That is, a terahertz polarizer that absorbs the s-wave and reflects only the p-wave can be realized.

[0014] ≪Characteristics of Terahertz Polarizer≫ Next, while showing the simulation results of terahertz polarizers with several combinations of materials, their characteristics will be explained.

[0015] The terahertz polarizer according to Example 1 is composed of a dielectric made of polyimide and an electric conduction layer made of an organic semiconductor. The surrounding medium is air. The refractive index of polyimide n1 = 1.85, the refractive index of air n3 = 1, and the critical angle is approximately 32.7°. The thickness of the electric conduction layer is 2000 nm, and the carrier mobility μ of the electric conduction layer = 0.1 cm2 Fix to / Vs

[0016] Figure 2 is a graph showing the angle dependence of a terahertz polarizer in Example 1, which is composed of polyimide and an organic semiconductor. The graph on the left shows the electrical conductivity σ of the electrical conductive layer. dc This figure shows the reflectivity of p-waves and s-waves as a function of the incident angle of a 1 THz terahertz wave at 19, 20, and 21 S / cm. The graph on the right is an enlarged view of a portion of the scale of the graph on the left. In Figure 2 and other figures, the numbers in the legend represent the electrical conductivity of the electrical conduction layer, and the letters following the numbers indicate whether it is a p-wave or an s-wave.

[0017] As can be seen in the graph on the left, the p-wave reflectance has a steep angular dependence, reaching nearly 100% when terahertz waves are incident at a critical angle, regardless of the difference in electrical conductivity of the electrical conduction layer.

[0018] Although the S-wave reflectance is extremely small compared to the P-wave reflectance, the magnified graph on the right shows that the S-wave reflectance is minimized when the incident angle of the terahertz wave is near the critical angle, especially when it is slightly larger than the critical angle. Furthermore, as the electrical conductivity of the electrical conductive layer increases, the incident angle of the terahertz wave at which the S-wave reflectance is minimized shifts to a higher angle. In addition, differences in the minimum value of the S-wave reflectance are observed depending on the electrical conductivity of the electrical conductive layer.

[0019] Thus, the terahertz polarizer in Example 1 is highly sensitive to the incident angle of terahertz waves for both p-wave and s-wave reflection. Furthermore, the angle dependence of s-wave reflection is influenced by the electrical conductivity of the electrical conduction layer.

[0020] Figure 3 is a graph showing the frequency dependence of the terahertz polarizer related to Example 1. The graph on the left shows the electrical conductivity σ of the electrical conduction layer. dc The graph shows the reflectivity of p-waves and s-waves against the frequency of terahertz waves incident at an angle of 32.8°, which is 0.1° larger than the critical angle, when the current is 20, 21, and 22 S / cm. The graph on the right is an enlarged version of a portion of the scale of the graph on the left.

[0021] As can be seen in the graph on the left, the p-wave reflectance remains at around 95% regardless of the difference in electrical conductivity of the electrical conductive layer. In other words, the frequency dependence of the p-wave reflectance is low.

[0022] The s-wave reflectance is extremely small compared to the p-wave reflectance, but as can be seen in the magnified graph on the right, the s-wave reflectance is minimized at a terahertz frequency of approximately 0.9 THz. Furthermore, differences in the minimum value of the s-wave reflectance are observed depending on the electrical conductivity of the electrical conductive layer. In this example, the electrical conductivity of the electrical conductive layer is σ dc The minimum value of the s-wave reflectance occurs when the value is 21 S / cm.

[0023] Thus, in the terahertz polarizer related to Example 1, the reflection of p-waves has low frequency dependence, but the reflection of s-waves is sensitive to the frequency of the terahertz waves. Furthermore, the frequency dependence of the reflection of s-waves is affected by the electrical conductivity of the electrical conduction layer.

[0024] Figure 4 is a graph showing the conductivity dependence of the terahertz polarizer according to Example 1. This graph shows the reflectivity of p-waves and s-waves with respect to the electrical conductivity of the electrical conduction layer when a terahertz wave with a frequency of 1 THz is incident at an angle of 32.8°, which is 0.1° larger than the critical angle.

[0025] The graph shows that the p-wave reflectance tends to decrease as the electrical conductivity of the electrical conduction layer increases, but it generally remains in the 90% range. On the other hand, the s-wave reflectance is greatly affected by the electrical conductivity of the electrical conduction layer, σ dc The minimum value occurs when it is approximately 20.6 S / cm. This corresponds to the s-wave reflectance R s This satisfies the condition for making it zero, that is, the condition for making the numerator on the right side of equation (2) zero.

[0026] Thus, the terahertz polarizer in Example 1 is very sensitive to the electrical conductivity of the electrical conduction layer with respect to the reflection of s-waves.

[0027] Next, we will describe the properties of a terahertz polarizer made of a different material. The terahertz polarizer in Example 2 is made of undoped silicon as the dielectric and an organic semiconductor as the electrical conductive layer. The surrounding medium is air. The refractive index of silicon is n1 = 3.42, the refractive index of air is n3 = 1, and the critical angle is approximately 17.0°. The thickness of the electrical conductive layer is 2000 nm, and the carrier mobility of the electrical conductive layer is μ = 0.1 cm. 2 Fix to / Vs

[0028] Figure 5 is a graph showing the angle dependence of a terahertz polarizer in Example 2, which is composed of undoped silicon and an organic semiconductor. The graph on the left shows the electrical conductivity σ of the electrical conductive layer. dc This graph shows the reflectivity of p-waves and s-waves as a function of the incident angle of a 1 THz terahertz wave at 41, 43, and 45 S / cm. The graph on the right is an enlarged version of a portion of the scale of the graph on the left.

[0029] Looking at the graph on the left, the p-wave reflectance is equal to the electrical conductivity σ of the electrical conductive layer. dc Regardless of the differences, terahertz waves exhibit a steep angle dependence, reaching nearly 100% when incident at a critical angle.

[0030] The S-wave reflectance is extremely small compared to the P-wave reflectance, but as can be seen in the magnified graph on the right, the S-wave reflectance is minimized when the incident angle of the terahertz wave is near the critical angle. Furthermore, as the electrical conductivity of the electrical conduction layer increases, the incident angle of the terahertz wave at which the S-wave reflectance is minimized shifts to a higher angle. Also, differences in the minimum value of the S-wave reflectance are observed depending on the electrical conductivity of the electrical conduction layer. In this example, the electrical conductivity of the electrical conduction layer is σ dc The minimum value of the s-wave reflectance occurs when the value is 43 S / cm.

[0031] Thus, the terahertz polarizer in Example 2, like in Example 1, is highly sensitive to the incident angle of the terahertz wave for both p-wave and s-wave reflection. Furthermore, the angle dependence of s-wave reflection is influenced by the electrical conductivity of the electrical conduction layer.

[0032] Figure 6 is a graph showing the frequency dependence of the terahertz polarizer related to Example 2. The graph on the left shows the electrical conductivity σ of the electrical conduction layer. dc The graph shows the reflectance of p-waves and s-waves against the frequency of terahertz waves incident at a critical angle of 17.0° when the current is 41, 43, and 45 S / cm. The graph on the right is an enlarged version of a portion of the scale of the graph on the left.

[0033] The graph on the left shows that the p-wave reflectance has a loose dependence on frequency, decreasing to about 75% at a frequency of 2 THz. No difference in p-wave reflectance is observed due to differences in the electrical conductivity of the electrical conductive layer.

[0034] Although the S-wave reflectance is extremely small compared to the P-wave reflectance, the magnified graph on the right shows that the S-wave reflectance also has a loose dependence on frequency. A tendency is observed where the S-wave reflectance increases with increasing terahertz wave frequency. Furthermore, differences in S-wave reflectance due to differences in the electrical conductivity of the electrical conductive layer are also observed.

[0035] The s-wave reflectance when the electrical conductivity σ = 41 S / cm is approximately constant at 0.07% regardless of the frequency of the terahertz wave, and the electrical conductivity σ dc The s-wave reflectivity at 41 S / cm is approximately 0.04% when the terahertz wave frequency is 1 THz, and increases to approximately 0.07% when the frequency is 2 THz. Electrical conductivity σ dc The s-wave reflectivity is best at 43 S / cm, with a reflectivity of 0.007% when the terahertz wave frequency is 1 THz and less than 0.02% even at a frequency of 2 THz.

[0036] Thus, unlike Example 1, the terahertz polarizer in Example 2 is less affected by the frequency of terahertz waves.

[0037] Figure 7 is a graph showing the frequency dependence of a terahertz polarizer in Example 2 when terahertz waves are incident at an angle slightly smaller than the critical angle. The graph on the left shows the electrical conductivity σ of the electrical conduction layer. dcThe graph shows the reflectivity of p-waves and s-waves against the frequency of terahertz waves incident at a critical angle of 16.7°, which is 0.3° smaller than the critical angle, when the current is 40, 41, and 42 S / cm. The graph on the right is an enlarged version of a portion of the scale of the graph on the left.

[0038] As can be seen in the graph on the left, the p-wave reflectance does not have a frequency dependence, but remains at a low value of less than 20% due to the influence of angle dependence.

[0039] The S-wave reflectance is extremely small compared to the P-wave reflectance, and as can be seen in the magnified graph on the right, the S-wave reflectance has a loose dependence on frequency. There is a tendency for the S-wave reflectance to increase as the frequency of terahertz waves increases. Furthermore, differences in S-wave reflectance due to differences in the electrical conductivity of the electrical conductive layer can also be observed.

[0040] Electrical conductivity σ dc The s-wave reflectivity at 40 S / cm is approximately 0.016% when the terahertz wave frequency is 1 THz, and increases to approximately 0.03% when the frequency is 2 THz. Electrical conductivity σ dc The s-wave reflectivity at 42 S / cm is approximately 0.016% when the terahertz wave frequency is 1 THz, and increases to approximately 0.04% when the frequency is 2 THz. Electrical conductivity σ dc The s-wave reflectivity is best at 43 S / cm, with a reflectivity of 0.005% at a terahertz frequency of 1 THz and less than 0.02% at a frequency of 2 THz.

[0041] Figure 8 is a graph showing the conductivity dependence of the polarizer in Example 2. This graph shows the p-wave and s-wave reflectances with respect to the electrical conductivity of the electrical conduction layer when a terahertz wave with a frequency of 1 THz is incident at a critical angle of 17.0°.

[0042] The graph shows that the p-wave reflectance remains around 80%, largely unaffected by the electrical conductivity of the electrical conduction layer. On the other hand, the s-wave reflectance is greatly influenced by the electrical conductivity of the electrical conduction layer, reaching its minimum value when σ = 43.4 S / cm. This corresponds to the s-wave reflectance R sThis satisfies the condition for making it zero, that is, the condition for making the numerator on the right side of equation (2) zero.

[0043] Thus, the terahertz polarizer in Example 2, like Example 1, is very sensitive to the electrical conductivity of the electrical conduction layer with respect to the reflection of s-waves.

[0044] Next, we will explain the relationship between the thickness of the electrical conductive layer and its angular and frequency dependence for the terahertz polarizer according to Example 1. Figure 9 is a graph showing the relationship between the thickness of the electrical conductive layer and its angular dependence for the terahertz polarizer according to Example 1.

[0045] The graph in the upper left shows the electrical conductivity σ of the electrical conductive layer when the thickness of the electrical conductive layer is 4000 nm. dc The graph shows the reflectance of p-waves and s-waves as a function of the incident angle of a 1 THz terahertz wave at values ​​of 10.25, 10.3, 10.35, and 10.4 S / cm. The graph in the middle left shows the electrical conductivity of the electrical conductive layer σ when the thickness of the electrical conductive layer is 2000 nm. dc The graph shows the reflectance of p-waves and s-waves as a function of the incident angle of a 1 THz terahertz wave at values ​​of 20.5, 20.6, 20.7, and 20.8 S / cm. The graph in the lower left shows the electrical conductivity σ of the electrical conductive layer when the thickness of the electrical conductive layer is 1000 nm. dc The graphs show the reflectance of p-waves and s-waves as a function of the incident angle of a 1 THz terahertz wave at values ​​of 41.0, 41.2, 41.4, and 41.6 S / cm. The graphs on the right of each graph are magnified versions of a portion of the scale. Note that the difference in electrical conductivity values ​​due to the difference in thickness of the electrical conductive layer is because the carrier mobility μ of the electrical conductive layer is 0.1 cm. 2 This is because it is fixed to / Vs.

[0046] Looking at the magnified graph in the second column from the left, the angle dependence of the p-wave reflectance decreases as the thickness of the conductive layer increases. Conversely, the angle dependence of the p-wave reflectance increases as the thickness of the conductive layer decreases. The incident angle of the terahertz wave at which the p-wave reflectance is at its maximum value is a critical angle of 32.7°, regardless of the thickness and conductivity of the conductive layer. The maximum value of the p-wave reflectance decreases as the thickness of the conductive layer increases.

[0047] Looking at the magnified graph in the third column from the left, we can see that the angle dependence of the s-wave reflectance also decreases as the thickness of the electrical conductive layer increases. However, unlike the p-wave, the incident angle of the terahertz wave at which the s-wave reflectance is at its minimum value does not depend on the electrical conductivity of the electrical conductive layer. When the thickness of the electrical conductive layer is 1000 nm, it is 32.746°, which is slightly greater than the critical angle of 32.7°. When the thickness of the electrical conductive layer is 2000 nm, it is 32.824°, which is even greater. When the thickness of the electrical conductive layer is 4000 nm, it is 33.124°, which is even greater. Thus, the angle shifts to the higher angle side depending on the thickness of the electrical conductive layer.

[0048] Looking at the enlarged graph in the far right column, we can see that the minimum value of the S-wave reflectance varies slightly depending on the electrical conductivity of the electrical conduction layer. The minimum value of the S-wave reflectance is when the thickness of the electrical conduction layer is 4000 nm, and the electrical conductivity of the electrical conduction layer is σ dc =10.3, 10.35, 10.25, 10.4 S / cm in that order, when the thickness of the electrical conductive layer is 2000 nm, the electrical conductivity σ of the electrical conductive layer. dc =20.7, 20.6, 20.8, 20.5 S / cm in that order, when the thickness of the electrical conductive layer is 1000 nm, the electrical conductivity σ of the electrical conductive layer. dc The values ​​increase in the order of 41.4, 41.2, 41.6, and 41.0 S / cm.

[0049] The point at which the p-wave reflectivity is maximum is fixed when the incident angle of the terahertz wave is at the critical angle, whereas the point at which the p-wave reflectivity is minimum changes depending on the incident angle of the terahertz wave. Therefore, by making the thickness of the electrical conductive layer as thin as possible and bringing the point at which the p-wave reflectivity is minimum as close to the critical angle as possible, the ratio of p-wave to s-wave reflectivity can be maximized.

[0050] Figure 10 is a graph showing the relationship between the thickness of the electrical conductive layer of the terahertz polarizer in Example 1 and its frequency dependence. The graph in the upper left shows the electrical conductivity σ of the electrical conductive layer when the thickness of the electrical conductive layer is 4000 nm. dcThe graph shows the s-wave reflectance for terahertz waves incident at an incident angle of 33.124°, where the s-wave reflectance is minimum, when the S-wave reflectance is 10.25, 10.3, 10.35, and 10.4 S / cm. The graph in the middle left shows the electrical conductivity of the electrical conductive layer when the thickness of the electrical conductive layer is 2000 nm. dc The graph shows the s-wave reflectance for terahertz waves incident at an incident angle of 32.824°, where the s-wave reflectance is minimum, when the S-wave reflectance is 20.5, 20.6, 20.7, and 20.8 S / cm. The graph in the lower left shows the electrical conductivity σ of the electrical conductive layer when the thickness of the electrical conductive layer is 1000 nm. dc The graphs in the upper right show the s-wave reflectance against the frequency of terahertz waves incident at an incident angle of 32.746°, where the s-wave reflectance is minimized, for S / cm values ​​of 41.0, 41.2, 41.4, and 41.6. The graphs in the upper right show the s-wave reflectance against the frequency of terahertz waves when the thickness of the conductive layer is 4000 nm and the incident angle of the terahertz waves is slightly shifted to 32.824°.

[0051] Looking at the graph in the left column, the frequency dependence of the S-wave reflectivity, in contrast to the angular dependence, becomes steeper and stronger as the electrical conductive layer thickens. In other words, there is a trade-off relationship between angular dependence and frequency dependence.

[0052] In the graph on the left, the frequency of the terahertz wave at which the s-wave reflectivity is minimum is 1 THz. However, as shown in the graph on the upper right, the frequency of the terahertz wave at which the s-wave reflectivity is minimum can be shifted by changing the angle of incidence of the terahertz wave. For example, by reducing the angle of incidence by 0.3°, the frequency of the terahertz wave at which the s-wave reflectivity is minimum becomes 0.5 THz.

[0053] Previously, the carrier mobility μ of the electrical conductive layer was μ = 0.1 cm 2 The explanation was given with / Vs fixed, but next, the relationship between the carrier mobility of the electrical conduction layer and the angle dependence will be explained for the terahertz polarizer related to Example 1. Figure 11 is a graph showing the relationship between the carrier mobility of the electrical conduction layer and the angle dependence for the terahertz polarizer related to Example 1. Each graph shows the electrical conductivity σ of the electrical conduction layer. dcThe graphs show the s-wave reflectance as an incidence angle of a 1 THz terahertz wave for carrier mobility μ = 0.1, 1, 10, 20, 30 cm from left to right. 2 This is for / Vs, and the thickness of the electrical conductive layer is fixed at 2000 nm. The graph in the lower right shows carrier mobility μ = 30 cm 2 This is a configuration where the thickness of the electrical conductive layer is set to 4000 nm using / Vs.

[0054] Looking at the graphs side by side, as carrier mobility increases, the angle dependence of the s-wave reflectivity becomes stronger, and the incident angle of the terahertz wave at which it is minimum shifts to a lower angle. In particular, when carrier mobility is 1 cm 2 Beyond / Vs, the angular dependence becomes stronger. Therefore, it is thought that the frequency dependence also becomes stronger as the carrier mobility increases. Looking at the graph in the lower right, even with relatively high carrier mobility, increasing the thickness of the conductive layer mitigates the angular dependence but strengthens the frequency dependence.

[0055] Electrical conductivity includes both a real and an imaginary part, and in equation (2), σ dc The x-axis represents the real part of electrical conductivity, and the horizontal axis of the graphs in Figures 4 and 8 represents the real part of electrical conductivity. Even if the real part of electrical conductivity is adjusted according to equation (2) to make the s-wave reflectance zero, the s-wave reflectance will not become zero due to the existence of the imaginary part of electrical conductivity. In particular, the larger the imaginary part of electrical conductivity, the larger the minimum value of the s-wave reflectance will be.

[0056] Figure 12 is a graph showing the relationship between the real-imaginary part ratio and reflectance of the electrical conductivity layer of the terahertz polarizer according to Example 1. The graph on the left plots the minimum values ​​of p-wave reflectance and s-wave reflectance against the real-imaginary part ratio (Im[σ] / Re[σ]) of electrical conductivity when a terahertz wave with a frequency of 1 THz is incident at the optimal angle. The graph on the right is an enlarged view of a portion of the scale of the graph on the left. The real part (Re[σ]) of electrical conductivity is fixed at 20.7 S / cm, and the real-imaginary part ratio is changed by varying the imaginary part. This real part value is the value that makes the s-wave reflectance zero when a terahertz wave with a frequency of 1 THz is incident at an angle of 32.8°, which is 0.1° larger than the critical angle, as shown in the graph in Figure 4.

[0057] The p-wave reflectance remains high regardless of the real-to-imaginary ratio of electrical conductivity. On the other hand, the minimum value of the s-wave reflectance is when the real-to-imaginary ratio of electrical conductivity is approximately 10. -1.6 The value is almost zero up to (=0.025), but it rises sharply beyond that point.

[0058] The organic semiconductor constituting the electrical conductive layer has a μ = 0.1 cm 2 Organic semiconductors have very low carrier mobility, around / Vs. Because of their low carrier mobility, organic semiconductors can achieve the desired electrical conductivity by doping them with a large amount of carriers to increase the carrier density. The imaginary part of the electrical conductivity of such organic semiconductors is very small, with a real-to-imaginary part ratio of about 0.0002, and the corresponding minimum value of the s-wave reflectance is almost zero. Therefore, organic semiconductors are suitable materials for constituting the electrical conductive layer of a terahertz polarizer.

[0059] Graphene is a non-silicon material that possesses electrical conductivity. The imaginary part of graphene's electrical conductivity is very large, with a real-to-imaginary part ratio of about 10, and the corresponding minimum value of its S-wave reflectivity is quite large. Therefore, graphene is not suitable as a material for the electrical conductive layer of a terahertz polarizer.

[0060] The electrical conductive layer can also be constructed from carrier-doped silicon. Doped silicon has higher carrier mobility than organic semiconductors, but by reducing the amount of doped carriers and thus the carrier density, the same electrical conductivity as organic semiconductors can be achieved. The imaginary part of such a doped silicon electrical conductive layer is smaller than that of graphene but larger than that of organic semiconductors, and the real-to-imaginary part ratio is about 0.2, so the corresponding minimum value of the s-wave reflectance is somewhat greater than zero. Therefore, doped silicon is a material that can be used for the electrical conductive layer of a terahertz polarizer.

[0061] Figure 13 is a graph showing the relationship between the carrier concentration of the electrical conduction layer and the real parts of the p-wave and s-wave reflectance in a terahertz polarizer according to Example 3, which is composed of undoped silicon and doped silicon. It is assumed that a terahertz wave with a frequency of 1 THz is incident on the electrical conduction layer of doped silicon at an appropriate angle. The solid line represents the theoretical value of the real part of the p-wave reflectance, the dashed line represents the theoretical value of the real part of the s-wave reflectance, and the plotted points represent measured values. As can be seen from the graph, the real part of the s-wave reflectance can be made zero by adjusting the carrier concentration of the doped silicon in the electrical conduction layer. Therefore, in a terahertz polarizer, it is possible to construct the dielectric with undoped silicon and the electrical conduction layer with doped silicon.

[0062] <<Embodiment>> Next, embodiments of the present invention will be described in detail with reference to the drawings as appropriate. However, unnecessarily detailed explanations may be omitted. For example, detailed explanations of already well-known matters and redundant explanations of substantially identical configurations may be omitted. This is to avoid the following explanation becoming unnecessarily verbose and to facilitate understanding by those skilled in the art. The inventors provide the accompanying drawings and the following explanation so that those skilled in the art can fully understand the present invention, and do not intend to limit the subject matter described in the claims by these. Also, the dimensions and detailed shapes of each component depicted in the drawings may differ from those of the actual components.

[0063] Figure 14 is a schematic diagram of a terahertz polarizing system according to one embodiment of the present invention. The terahertz polarizing system 100 according to this embodiment is a device that extracts only p-waves from an input terahertz wave 200. Specifically, the terahertz polarizing system 100 comprises a terahertz polarizer 10 and an angle adjustment mechanism 20. The terahertz polarizer 10 is placed in an ambient medium 30. The ambient medium 30 is, for example, air.

[0064] The terahertz polarizer 10 is an embodiment of the terahertz polarizer described above. Figure 15 is a cross-sectional view of a terahertz polarizer according to one embodiment of the present invention. The terahertz polarizer 10 is composed of a dielectric plate 11 made of a dielectric material such as polyimide or undoped silicon. The area of ​​the dielectric plate 11 is arbitrary, and its shape may be rectangular or circular. The thickness of the dielectric plate 11 is several millimeters, which is sufficiently larger than the wavelength of terahertz waves.

[0065] The surface 12 of the dielectric plate 11 is tapered relative to the back surface 13, for example, forming a prism shape. An electrically conductive layer 14 is formed on the back surface 13 of the dielectric plate 11. The electrically conductive layer 14 can be formed, for example, by depositing an organic semiconductor film on the back surface 13 of the dielectric plate 11 using a spin coating method or a casting method, followed by chemical doping or annealing. Alternatively, if the dielectric plate 11 is made of undoped silicon, the electrically conductive layer 14 can be formed by depositing a doped silicon film on the back surface 13 of the dielectric plate 11.

[0066] The approximate thickness of the conductive layer 14 is a few micrometers, which is sufficiently thinner than the wavelength of terahertz waves in the conductive layer 14. The optimal thickness of the conductive layer 14 is determined based on equation (2), taking into account the refractive index of the dielectric plate 11, the refractive index of the surrounding medium 30, and the electrical conductivity of the conductive layer 14.

[0067] In this way, the terahertz polarizer 10 with the above configuration reflects terahertz waves that enter from the surface 12 of the dielectric plate 11 and propagate within the dielectric plate 11 at the electrical conductive layer 14. In particular, when terahertz waves are incident on the electrical conductive layer 14 at an angle near the critical angle, more preferably at an angle slightly larger than the critical angle, the s-wave reflectivity becomes almost zero and only p-waves are reflected.

[0068] The tapered surface 12 of the dielectric plate 11 is to prevent multiple interferences. Figure 16 is a graph showing the relationship between the terahertz wave incidence angle on the dielectric plate surface and the reflectance of p-waves and s-waves when the front and back surfaces of the dielectric plate are parallel. The same graph shows that in a terahertz polarizer 10 in which a 2000 nm thick organic semiconductor electrical conductive layer 14 is formed on the back surface 13 of a 2 mm thick polyimide dielectric plate 11, the carrier mobility μ = 0.1 cm 2 The electrical conductivity of the electrical conductive layer 14 is fixed at / Vs σ dc This shows the reflectance of p-waves and s-waves as a function of incidence angle for 10 GHz terahertz waves at 0, 10, 20, and 30 S / cm.

[0069] If the surface 12 of the dielectric plate 11 is not tapered, multiple interference occurs between the reflected wave at the surface 12 of the dielectric plate 11 and the reflected wave at the conductive layer 14. As a result, as shown in the graph of Figure 16, the reflectivity of p-waves and s-waves at the surface 12 of the dielectric plate 11 becomes relatively large regardless of the electrical conductivity of the conductive layer 14. Consequently, the s-waves reflected at the surface 12 become mixed with the p-waves reflected at the conductive layer 14. This multiple interference can change depending on the thickness of the dielectric plate 11 and the frequency of the terahertz waves, and as long as such multiple interference exists, it may not be possible to completely eliminate the s-waves, potentially preventing the polarizer from functioning properly.

[0070] Figure 17 is a graph showing the relationship between the angle of incidence of terahertz waves on the surface of a dielectric plate and the reflectance of p-waves and s-waves when the surface of the dielectric plate is tapered relative to the back surface. The graph on the right of Figure 17 shows that in a terahertz polarizer 10 in which a 2000 nm thick organic semiconductor electrical conductive layer 14 is formed on the back surface 13 of a polyimide dielectric plate 11 of appropriate thickness, the carrier mobility μ = 0.1 cm² of the electrical conductive layer 14 is... 2 The electrical conductivity of the electrical conductive layer 14 is fixed at / Vs σ dc The graphs show the reflectivity of p-waves and s-waves as a function of the incident angle of a 10 GHz terahertz wave at frequencies of 0, 5, 10, and 20 S / cm. The graph on the right is an enlarged version of a portion of the scale of the graph on the left.

[0071] When the surface 12 of the dielectric plate 11 is tapered, multiple interference between the reflected wave at the surface 12 of the dielectric plate 11 and the reflected wave at the conductive layer 14 is suppressed. Therefore, as shown in the graph of Figure 17, the electrical conductivity σ of the conductive layer 14 dc When the current is 10 S / cm, the reflectivity of p-waves and s-waves on the surface 12 of the dielectric plate 11 can be reduced to 0.75% or less in the range of incident angles to the surface 12 of the dielectric plate 11 from 0 to 50°. This makes it possible to extract only the reflected wave from the electrical conductive layer 14, i.e., only the p-wave.

[0072] Returning to Figure 14, the angle adjustment mechanism 20 adjusts the incidence angle of the terahertz waves 200 to the terahertz polarizer 11 so that the incoming terahertz waves 200 are incident on the electrical conductive layer 14 of the terahertz polarizer 10 near the critical angle. For example, the angle adjustment mechanism 20 includes a mirror 21 that adjusts the propagation angle of the terahertz waves 200 entering the terahertz polarizer 10, and a mirror 22 that adjusts the propagation angle of the p-waves that have been polarized and reflected by the terahertz polarizer 10. Mirrors 21 and 22 are controlled with high precision to the specified angles by actuators (not shown).

[0073] Mirror 21 reflects, for example, the terahertz wave 200 traveling through the surrounding medium 30 parallel to the terahertz polarizer 10, changing the propagation angle of the terahertz wave 200 so that it is incident on the electrical conductive layer 14 of the terahertz polarizer 10 near the critical angle. Mirror 22 reflects the p-wave of the terahertz wave 200 that is reflected by the electrical conductive layer 14 of the terahertz polarizer 10 and emerges from the terahertz polarizer 10 into the surrounding medium 30, making its propagation angle parallel to the terahertz polarizer 10. This makes it possible to align the propagation axis of the terahertz wave 200 before and after the terahertz polarizer 10.

[0074] ≪Effects≫ According to the terahertz polarizer 10 of this embodiment, terahertz waves can be polarized and reflected over a wide bandwidth from tens of GHz to several THz to extract only p-waves. Although the terahertz polarizer 10 is highly angle-dependent, the terahertz polarizing system 100 of this embodiment allows for high-precision adjustment of the propagation angle of the terahertz waves, so that terahertz waves are incident on the terahertz polarizer 10 at the optimal angle, thereby extracting p-waves from the terahertz polarizer 10 with maximum intensity.

[0075] <<Examples of Modifications and Applications>> The taper of the surface 12 of the terahertz polarizer 10 can be made prism-shaped, or it can simply be made diagonally to the back surface 13.

[0076] The angle adjustment mechanism 20 may adjust the angle relative to the terahertz wave by moving the terahertz polarizer 10. Alternatively, it may adjust both the incident angle of the terahertz wave 200 to the terahertz polarizer 10 and the angle of the terahertz polarizer 10.

[0077] The Terahertz Polarizer 10 can also be used in the millimeter-wave band, making it suitable for use as a polarizer in imagers that utilize the millimeter-wave band.

[0078] As described above, embodiments have been explained as examples of the technology in the present invention. For this purpose, accompanying drawings and a detailed description have been provided. Therefore, among the components described in the accompanying drawings and detailed description, there may be not only components that are essential for solving the problem, but also components that are not essential for solving the problem, in order to illustrate the above technology. For this reason, the mere fact that these non-essential components are described in the accompanying drawings and detailed description should not be immediately assumed to be essential. Furthermore, since the above embodiments are for the purpose of illustrating the technology in the present invention, various changes, substitutions, additions, omissions, etc., can be made within the scope of the claims or equivalents. [Explanation of Symbols]

[0079] 100 Terahertz Polarizing System 10 Terahertz Polarizer 11 dielectric plate 12 Surface 13 Back side 14 Electrical Conductive Layer 20 Angle adjustment mechanism 30 Ambient medium 200 terahertz waves

Claims

1. A terahertz polarizer that polarizes and reflects electromagnetic waves from the millimeter wave band to the terahertz band, It has a dielectric plate with a tapered surface relative to the back surface, An electrically conductive layer, sufficiently thinner than the wavelength of the electromagnetic wave, is formed on the back surface of the dielectric plate. The electromagnetic waves that enter from the surface of the dielectric plate and travel through the dielectric plate, placed in a surrounding medium having a lower refractive index than the dielectric plate, are reflected by the electrical conductive layer. A terahertz polarizer characterized by the following features.

2. The dielectric plate is a polyimide plate, The terahertz polarizer according to claim 1, wherein the electrical conductive layer is an organic semiconductor film.

3. The dielectric plate is an undoped silicon plate, The terahertz polarizer according to claim 1, wherein the electrical conductive layer is an organic semiconductor film.

4. The dielectric plate is undoped silicon, The terahertz polarizer according to claim 1, wherein the electrical conductive layer is a doped silicon film.

5. The terahertz polarizer according to any one of claims 2 to 4, wherein the surrounding medium is air.

6. A terahertz polarizer according to any one of claims 1 to 4, The device includes an angle adjustment mechanism that adjusts the angle of incidence of the electromagnetic wave to the terahertz polarizer and / or the angle of the terahertz polarizer so that the electromagnetic wave is incident on the electrical conduction layer near a critical angle, A terahertz polarizing system characterized by the following features.

7. The terahertz polarizing system according to claim 6, wherein the angle near the critical angle is slightly larger than the critical angle.

8. The terahertz polarizer according to claim 5, The device includes an angle adjustment mechanism that adjusts the angle of incidence of the electromagnetic wave to the terahertz polarizer and / or the angle of the terahertz polarizer so that the electromagnetic wave is incident on the electrical conduction layer near a critical angle, A terahertz polarizing system characterized by the following features.

9. The terahertz polarizing system according to claim 8, wherein the angle near the critical angle is slightly larger than the critical angle.

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