Variable transmission frequency filter

Abstract

Provided is a variable transmission frequency filter comprising a Fabry-Perot resonator that incorporates, between two reflective layers parallel to each other, a variable pitch subwavelength grating layer which is independent of the reflective layers.

Description

Variable transmission frequency filter

[0001] The present invention relates to a variable transmission frequency filter.

[0002] Anticipating the arrival of a data-driven "ultra-smart society" in the 2030s, a promotion strategy is underway to realize "Beyond 5G." Taking into account the expected data communication volume and number of communication devices in the 2030s, "Beyond 5G" aims to achieve communication speeds 10 times faster than 5G in access networks and 100 times faster than current speeds in core networks. To achieve this goal, development is underway on communication technologies that efficiently and effectively utilize terahertz waves, which are higher-frequency electromagnetic waves.

[0003] Filters using a Fabry-Perot resonator (interferometer) are known as filters with wavelength-selective transmission (frequency-selective transmission), and filters utilizing the principles of the Fabry-Perot resonator have also been developed for terahertz waves. For example, Non-Patent Document 1 discloses a Fabry-Perot filter composed of a pair of subwavelength grating mirrors, and describes that the frequency of the terahertz waves that can be transmitted can be adjusted by changing the distance between the mirrors. Typically, terahertz-selective transmission filters using a Fabry-Perot resonator are designed to fix the resonant frequency to a specific value and transmit only terahertz waves of the desired frequency. Meanwhile, in recent years, technologies have been developed to turn such filters into frequency-tunable filters (wavelength-tunable filters) that can tune the frequency of the transmitted terahertz waves. For example, Non-Patent Document 2 discloses a liquid crystal Fabry-Perot device in which the cavity is filled with liquid crystal and the refraction index of the liquid crystal is controlled by applying a voltage, thereby shifting the resonant peak.

[0004] Goebel et al., “Tunable Fabry-Perot THz filter with sub-wavelength grating mirrors”, Proc. of SPIE, Vol. 6989, 698911, 2008Li et al., “Electrically Tunable Liquid-crystal Fabry-Perot Device for Terahertz Radiation”, Proc. of SPIE, Vol. 9795, 979539, 2015

[0005] Because the dimensions of terahertz wave filters are specified on the order of micrometers, extremely strict control is required to create a variable-passband filter. For example, mechanically varying the distance between mirrors (reflective layers) can easily reduce the parallelism of the reflective layers, potentially resulting in a decrease in the Q-factor and transmittance. Furthermore, in the technology described in Non-Patent Document 2, in which the resonator cavity is filled with liquid crystal and the refractive index is controlled by applying a driving voltage, the coating of the reflective layer is easily damaged by contact between the liquid crystal and the reflective layer, and the driving voltage can damage the reflective layer, potentially shortening the device's lifespan. Furthermore, because the change in refractive index caused by applying a voltage to the liquid crystal is relatively small, the applicable frequency range is limited.

[0006] An object of the present invention is to provide a new variable transmission frequency filter that can selectively transmit electromagnetic waves of a desired frequency with high efficiency and can change the frequency of the transmitted electromagnetic waves.

[0007] As a result of intensive research conducted by the inventors to solve the above-mentioned problems, they discovered that by using a filter equipped with a Fabry-Perot resonator in which a pitch-tunable subwavelength grating layer is incorporated between two reflective layers independently of the reflective layers and the period of the grating layer is mechanically controllable, it is possible to arbitrarily control the effective refractive index of the grating layer to a desired level, thereby controlling the transmitted electromagnetic wave to a desired frequency. The present invention was completed through further research based on these findings.

[0008] That is, the above-mentioned object of the present invention has been achieved by the following means. [1] A tunable transmission frequency filter including a Fabry-Perot resonator in which a pitch-tunable subwavelength grating layer independent of the reflective layers is incorporated between two reflective layers parallel to each other. [2] The tunable transmission frequency filter according to [1] above, in which the proportion of voids in the pitch-tunable subwavelength grating layer increases or decreases due to expansion or contraction in the periodic direction of the pitch-tunable subwavelength grating layer, thereby changing the period of the pitch-tunable subwavelength grating layer. [3] The tunable transmission frequency filter according to [1] or [2] above, in which the two reflective layers are made of silicon, and the pitch-tunable subwavelength grating layer is also made of silicon. [4] The tunable transmission frequency filter according to any of [1] to [3] above, in which the electromagnetic wave selectively transmitted by the tunable transmission frequency filter is a terahertz wave. [5] The tunable transmission frequency filter according to any of [1] to [4] above, in which the period of the pitch-tunable subwavelength grating layer is pitch-tunable within a range of 0.5 to 3 times. [6] The tunable transmission frequency filter according to any one of [1] to [5], wherein the pitch-variable sub-wavelength grating layer has a grating portion and spring portions formed on the left and right of the grating portion, and has a structure in which the grating portion can expand and contract in a periodic direction by the spring portions. [7] The tunable transmission frequency filter according to any one of [1] to [6], wherein one end of the pitch-variable sub-wavelength grating layer is fixed to a fixed substrate and the other end is fixed to a movable substrate, and the movable substrate is movable in the periodic direction. [8] A tunable transmission frequency device comprising the tunable transmission frequency filter according to any one of [1] to [7]. [9] A method for manufacturing a tunable transmission frequency filter, comprising obtaining the tunable transmission frequency filter according to any one of [1] to [7], by incorporating a pitch-variable sub-wavelength grating layer, independent of two parallel reflective layers, between the reflective layers.

[0009] The variable transmission frequency filter of the present invention can selectively transmit electromagnetic waves of a desired frequency with high efficiency, and can also vary the frequency of the transmitted electromagnetic waves.

[0010] FIG. 1 is a vertical end view schematically illustrating one example of a preferred embodiment of a tunable transmission frequency filter of the present invention. FIG. 2( a) is a graph illustrating the relationship between the period and effective refractive index of a pitch-variable subwavelength grating layer. FIG. 2( b) is a graph illustrating the relationship between the frequency of a transmitted electromagnetic wave and transmittance for a tunable transmission frequency filter having pitch-variable subwavelength grating layers with different effective refractive indices. FIG. 3 is a top view schematically illustrating one example of a pitch-variable subwavelength grating layer. FIG. 4 is an explanatory diagram illustrating expansion and contraction of the pitch-variable subwavelength grating layer. FIG. 5 is a vertical end view illustrating one preferred embodiment of a tunable transmission frequency filter of the present invention. FIG. 6 is an explanatory diagram schematically illustrating a manufacturing process for the pitch-variable subwavelength grating layer in the tunable transmission frequency filter of the present invention. FIG. 7 is an enlarged view of the pitch-variable subwavelength grating layer in an example. FIG. 8 is a graph illustrating the transmittance of a tunable transmission frequency filter in an experimental example.

[0011] A preferred embodiment of the present invention will be described below, but the present invention is not limited to the following embodiment except as defined by the present invention.

[0012] [Tunable Transmission Frequency Filter] The tunable transmission frequency filter of the present invention (hereinafter also referred to as "the filter of the present invention") includes a Fabry-Perot resonator in which a pitch-tunable subwavelength grating layer independent of two parallel reflective layers (reflectors) is incorporated between the reflective layers. In the filter of the present invention, the frequency of the electromagnetic wave passing through the Fabry-Perot resonator can be controlled by controlling the effective refractive index of the pitch-tunable subwavelength grating layer. Note that in the filter of the present invention, the reflective layer and the pitch-tunable subwavelength grating layer being independent means that the reflective layer and the pitch-tunable subwavelength grating layer are not in direct contact with each other. In other words, in the filter of the present invention, the grating period (hereinafter also simply referred to as "period") of the pitch-tunable subwavelength grating layer can be varied while leaving the reflective layer intact, thereby controlling the effective refractive index.

[0013] The electromagnetic waves that can be transmitted through the filter of the present invention are preferably terahertz waves. In this specification, "terahertz waves" refers to electromagnetic waves having a frequency in the range of 0.1 to 10 THz. The frequency range of the terahertz waves that can be transmitted through the filter of the present invention is not particularly limited and can be set appropriately depending on the application of the filter of the present invention. The frequency range is preferably 0.1 to 5 THz, more preferably 0.15 to 1 THz, and even more preferably 0.2 to 0.4 THz. The frequency range can be determined by controlling the refractive index of the reflective layer, the effective refractive index of the pitch-variable subwavelength grating layer, the distance between the reflective layers, etc.

[0014] The filter of the present invention can be incorporated into, for example, 6G communication equipment, automotive radar, 5G communication equipment, space communications, game consoles, LiDAR (Light Detection and Ranging) devices, terahertz wave-compatible cameras, terahertz wave-compatible imagers, terahertz wave-compatible scanners, and terahertz wave-compatible analysis and evaluation devices using these, terahertz wave-compatible medical devices (e.g., biosensors), high-speed robots, body check devices used at airports, non-destructive testing devices, various security devices, terahertz spectroscopy systems, and terahertz wave-compatible analysis and inspection devices using the same (e.g., systems for identifying chemical substances and pharmaceuticals), agricultural and medical application devices, TOF (Time of Flight) distance sensors, and high-speed communication and optical calculation circuit devices, to form a transmission frequency-tunable device having the filter of the present invention. By controlling the effective refractive index of the pitch-tunable subwavelength grating layer, the filter of the present invention can control the frequency of electromagnetic waves (preferably terahertz waves) that can pass through the filter of the present invention to any desired frequency. For example, in the 3.7 GHz band for 5G communications, frequency bands are assigned to each telecommunications carrier, such as NTT DOCOMO, INC. (3.6-3.7 GHz) and KDDI CORPORATION (3.7-3.8 GHz), and it is expected that similar frequency bands will be assigned for 6G communications in the future. Therefore, by manufacturing a radio wave receiving antenna using the filter of the present invention, it is possible to quickly tune the antenna's frequency band and selectively receive only the frequencies of the desired telecommunications carrier. Similarly, since automotive radars also have several frequency channels, active frequency tuning using the filter of the present invention can avoid interference. Furthermore, in terahertz imagers and body check devices, the filter of the present invention can be used to acquire two-dimensional images for each frequency, thereby improving the accuracy of substance identification and image diagnosis / analysis.

[0015] FIG. 1 is a vertical end view illustrating a mechanism for controlling the frequency of transmitted electromagnetic waves in a preferred embodiment of a filter according to the present invention. The filter 100 of the present invention illustrated in FIG. 1 includes two reflective layers 110 (a first reflective layer 111 and a second reflective layer 112) separated by an air cavity, and a pitch-tunable subwavelength grating layer 120 disposed in the center of the air cavity. As the pitch-tunable subwavelength grating layer 120 expands and contracts in the periodic direction (the horizontal direction in FIG. 1 ), the period p of the pitch-tunable subwavelength grating layer 120 changes. The changed period p increases or decreases the effective refractive index of the pitch-tunable subwavelength grating layer 120, thereby controlling the frequency of electromagnetic waves that can be transmitted through the electromagnetic wave transmission region 101 of the filter 100 according to the present invention. FIG. 2( a) is a graph illustrating the relationship between the period p of the pitch-tunable subwavelength grating layer 120 and the effective refractive index. Increasing the period p of the pitch-tunable subwavelength grating layer 120 can reduce the effective refractive index of the pitch-tunable subwavelength grating layer 120. Also, Figure 2(b) shows a graph illustrating the relationship between the frequency of the electromagnetic wave transmitted through the filter 100 of the present invention and its transmittance, for a filter 100 of the present invention having pitch-tunable subwavelength grating layers 120 with different effective refractive indices. By reducing the effective refractive index, the frequency (peak frequency) of the electromagnetic wave transmitted through the filter 100 of the present invention can be controlled to a higher frequency. In other words, changing the period p of the pitch-tunable subwavelength grating layer 120 can control the frequency of the electromagnetic wave transmitted through the filter 100 of the present invention.

[0016] The preferred embodiments of each component of the filter of the present invention will be described below.

[0017] (Reflective Layer) The filter of the present invention has two parallel reflective layers. The two reflective layers are preferably made of the same material, thickness, etc. The inner surfaces of these reflective layers function as reflectors, allowing electromagnetic waves incident on the filter of the present invention to undergo multiple reflections, thereby transmitting only electromagnetic waves of a specific frequency. The space (air cavity) between the two reflective layers is filled with air, and a pitch-variable subwavelength grating layer is disposed in the center. The material of the reflective layer is not particularly limited, and can be any known dielectric material, a metal commonly used for the mirror surface of a Fabry-Perot filter (e.g., gold, silver, copper, aluminum, etc.), or a dielectric multilayer film. Among these, a high-dielectric material is preferably used. Examples of high-dielectric materials include ceramics such as silicon, alumina, zirconia, silicon carbide, aluminum nitride, silicon nitride, yttria, barium titanate, lithium tantalate, and titanium oxide, as well as compounds containing such ceramics. Among these, silicon is preferably used as the material for the reflective layer. The reflectivity of the reflective layer is controlled to be higher than that of the pitch-variable subwavelength grating layer.

[0018] The thickness of the reflective layer (t shown in FIG. １ ) can be appropriately set depending on the purpose. For example, when the reflective layer is made of silicon, the thickness t １ can be 13 to 662 μm, or 66 to 442 μm, or 165 to 331 μm. The distance between the two reflective layers (h shown in FIG. 1 ) can also be set appropriately depending on the frequency of the electromagnetic waves to be transmitted. For example, the distance h between the reflective layers can be 108 to 5400 μm, or 540 to 3600 μm, or 1350 to 2700 μm.

[0019] The reflective layer may be a layer having a sub-wavelength grating structure. Even when the reflective layer has a sub-wavelength grating structure, the reflectance of the reflective layer is controlled to be higher than the reflectance of the pitch-variable sub-wavelength grating layer.

[0020] (Pitch-Variable Subwavelength Grating Layer) The filter of the present invention includes a pitch-variable subwavelength grating layer between the two reflective layers, independent of the reflective layers. In the filter of the present invention, the pitch-variable subwavelength grating layer is a layer having a subwavelength grating structure. The pitch-variable subwavelength grating layer can be adjusted to a desired refractive index (effective refractive index) by arbitrarily controlling the period (pitch) of the subwavelength grating structure. The subwavelength grating structure is not particularly limited as long as its period can be varied. For example, if the subwavelength grating structure is formed of multiple strips, expansion and contraction in the periodic direction increases or decreases the proportion of gaps between the strips of the pitch-variable subwavelength grating layer, thereby changing the period of the pitch-variable subwavelength grating layer. FIG. 3 shows a schematic top view of an example of a pitch-variable subwavelength grating layer whose subwavelength grating structure is formed of multiple strips. Note that the bold frame in FIG. 3 is drawn to a different scale (ratio) for structural explanation. The pitch-variable subwavelength grating layer 120 shown in Figure 3 has a grating portion 121, spring portions 122 formed on the left and right sides of the grating portion 121, and end portions 123 formed above and below the grating portion 121. The structure including the spring portions 122 allows the width of the gaps 121b between the strips 121a to be expanded and contracted more uniformly when the pitch-variable subwavelength grating layer 120 is expanded or contracted in the periodic direction (the up-and-down direction in Figure 3), thereby enabling the effective refractive index of the entire pitch-variable subwavelength grating layer 120 (grating portion 121) to be controlled more uniformly. Note that, when the pitch-variable subwavelength grating layer has components other than the grating portion, as in the example shown in Figure 3, the effective refractive index of the pitch-variable subwavelength grating layer 120 refers to the effective refractive index of the grating portion 121. FIG. 4 is a schematic diagram of an example of a pitch-variable sub-wavelength grating layer 120 consisting of a grating portion 121 and a spring portion 122, observed from above, and shows a schematic diagram of what happens when the pitch-variable sub-wavelength grating layer 120 is stretched in the periodic direction (the up-and-down direction in FIG. 4 ) to lengthen the period.

[0021] The pitch-variable subwavelength grating layer can be made of a material (pattern material) that is highly transparent to terahertz waves, such as COP (Cyclo Olefin Polymer), Teflon (registered trademark), polyethylene, polypropylene, polyimide, or Tsurupica (manufactured by Pax Co., Ltd.)) or a dielectric material (preferably silicon). When the reflective layer is a dielectric, it is preferably made of the same material as the reflective layer. The reflectance of the pitch-variable subwavelength grating layer is typically lower than that of the reflective layer. In particular, when each layer is made of the same material, the reflectance of the pitch-variable subwavelength grating layer is an average value that depends on the volume occupancy of the voids between each pattern. Therefore, the reflectance of the reflective layer is higher than that of the pitch-variable subwavelength grating layer.

[0022] The dimensions of the pitch-variable sub-wavelength grating layer can be set appropriately. ２ ) can be appropriately set depending on the frequency of the electromagnetic wave to be transmitted. For example, the thickness t ２can be 50 to 1000 μm, or alternatively 100 to 500 μm, or even 150 to 250 μm. In the pitch-variable subwavelength grating layer, the period (p, initial value shown in FIG. 3 ) before expansion / contraction (at the time of design) is designed to be smaller than the wavelength of the electromagnetic wave incident on the filter of the present invention. The period is preferably 6 to 334 μm, more preferably 33 to 223 μm, and even more preferably 83 to 167 μm. Furthermore, the grating width a is preferably 5 to 284 μm, more preferably 28 to 189 μm, and even more preferably 70 to 142 μm. The period p and grating width a can also be determined by simulation using, for example, RCWA (Rigorous Coupled-Wave Analysis) so as to maximize the transmittance of the electromagnetic wave of the target frequency. The period (initial value) of the pitch-variable subwavelength grating layer before expansion or contraction (at the time of design) may vary depending on the position of the pitch-variable subwavelength grating layer. For example, by setting the period (initial value) of the upper half of the grating portion of the pitch-variable subwavelength grating layer shown in FIG. 3 to 100 μm and the period (initial value) of the lower half to 200 μm, a filter can be obtained that can simultaneously control two electromagnetic waves of different frequencies. Furthermore, the period may vary continuously in the vertical and / or horizontal directions of the pitch-variable subwavelength grating layer shown in FIG. 3. By adopting such a structure in which the period varies continuously, the filter of the present invention can also be an active linear filter.

[0023] In addition, the pitch-variable sub-wavelength grating layer has a period (p ｉ ), the pitch can be varied preferably in the range of 0.2 to 5 times, more preferably in the range of 0.4 to 4 times, and even more preferably in the range of 0.5 to 3 times. For example, the period (p ｉWhen the period p is 100 μm, "the pitch is variable within a range of 0.5 to 3 times" means that the pitch is variable from 50 to 300 μm. Furthermore, the number of gratings (the number of repetitions of a structural unit, each consisting of a strip 121a and a gap 121b) in the pitch-variable subwavelength grating layer is preferably 50 or more, more preferably 80 or more, and even more preferably 100 or more. The number of gratings is typically 500 or less, preferably 300 or less, and more preferably 200 or less. By increasing the number of gratings, the period p during expansion and contraction can be more precisely controlled. For example, if the length of the grating section in the grating width direction (the length in the vertical direction in FIG. 3) is 10 mm, the period is 100 μm, and the number of gratings is 100, the period can be increased to 200 μm by extending the length of the grating section in the grating width direction to 20 mm.

[0024] 5 shows an example of a mechanism for expanding and contracting the pitch-variable subwavelength grating layer 120 in the filter 100 of the present invention. The second reflective layer 112 is bonded to a fixed substrate 140. The pitch-variable subwavelength grating layer 120 is stacked on the second reflective layer 112 via a spacer 130. The first reflective layer 111 is stacked on the pitch-variable subwavelength grating layer 120 via another spacer 130. One end of the second reflective layer 112 is fixed to the fixed substrate 140, while the other end of the second reflective layer 112 is not fixed to the movable substrate 150. In contrast, the other end of the pitch-variable subwavelength grating layer 120 is fixed to the movable substrate 150 via the spacer 130. Therefore, as the movable substrate 150 moves in the periodic direction (left and right in FIG. 5 ), the pitch-variable subwavelength grating layer is pulled in the periodic direction, changing the period of the grating section.

[0025] [Method for Manufacturing Variable Transmission Frequency Filter] The method for manufacturing the filter of the present invention is not particularly limited. For example, the filter of the present invention can be manufactured by manufacturing a variable-pitch subwavelength grating layer using semiconductor microfabrication technology and incorporating the variable-pitch subwavelength grating layer between two parallel reflective layers, independently of the reflective layers.

[0026] 6 is a schematic diagram illustrating an example of a manufacturing method for the pitch-tunable subwavelength grating layer 120 of the filter of the present invention, using a vertical end view of the pitch-tunable subwavelength grating layer 120. A resist (photoresist) film 202 is formed on a silicon substrate 201 ( FIGS. 6A and 6B ), and a grating periodic structure is formed by irradiating ultraviolet light from above a photomask 203 ( FIG. 6C ). The grating periodic structure is then formed on the silicon substrate 201 by dry etching such as plasma etching ( FIG. 6D ). The resist film 202 is then removed with a chemical or the like ( FIG. 6E ), yielding the pitch-tunable subwavelength grating layer 120 with the grating periodic structure formed therein.

[0027] The filter of the present invention can be obtained by laminating the reflective layer and the pitch-variable subwavelength grating layer independently. For example, a method for laminating the layers independently can be to place spacers between the layers to prevent direct contact between the layers. The material of the spacer is not particularly limited, and examples thereof include the above-mentioned dielectrics and metals.

[0028] The present invention will be described in more detail based on examples. The present invention is not to be construed as being limited to the following examples except as defined in the present invention.

[0029] <Preparation of pitch-tunable sub-wavelength grating layer> The pitch-tunable sub-wavelength grating layer was fabricated using a photolithography technique. ２A 200 μm silicon substrate (refractive index: 3.4) was cut into a 2 cm x 2 cm dimension and cleaned. A resist (OFPR-800LB 200 cp photoresist) film was coated onto the silicon substrate, and a pattern was formed on the resist film using a photomask with a grating structure. The silicon substrate was then etched using an ICP-RIE etching process, followed by removal of the resist film, resulting in a pitch-tunable subwavelength grating layer. The parameters of the pitch-tunable subwavelength grating layer were as follows: Design parameters: Number of gratings: 100 Length of grating section in grating width direction: 10 mm Length of grating section in grating length direction: 8 mm Grating period (p, initial value): 100 μm Grating width (a): 85 μm Spring length (l): 3600 μm Spring thickness (s): 400 μm Spring width (w): 40 μm

[0030] The resulting pitch-variable subwavelength grating layer was stretched in the periodic direction, and the increase in the period was observed using an optical microscope. The grating structure before stretching is shown in FIG. 7( a), and the grating structure after stretching is shown in FIG. 7( b). The period p of the grating structure before stretching shown in FIG. 7( a) was 100 μm, whereas after stretching the pitch-variable subwavelength grating layer by 5 mm in the periodic direction (the grating width direction of the grating portion), the period p of the grating structure after stretching shown in FIG. 7( b) was extended to 150 μm. It was also confirmed that the width of each gap in the grating portion increased uniformly by stretching the pitch-variable subwavelength grating layer.

[0031] <Manufacture of the Filter of the Present Invention> A filter of the present invention was manufactured with the configuration shown in Fig. 5. One end of the second reflective layer was adhered to a fixed substrate, and one end of the pitch-variable sub-wavelength grating layer prepared as described above was adhered to the reflective layer via a spacer. Furthermore, one end of the first reflective layer was adhered to the pitch-variable sub-wavelength grating layer via a spacer. The other end of the pitch-variable sub-wavelength grating layer was fixed to a movable substrate via a spacer. The material of the reflective layers (first reflective layer and second reflective layer) was silicon, and the material of the spacer was aluminum. The thickness t of the reflective layer was １ The distance h between the reflective layers was set to 1620 μm.

[0032] Using a terahertz spectrometer (Tera Prospector, manufactured by Nippou Precision Co., Ltd.), the transmission frequency of the filter of the present invention was measured when the period of the pitch-variable subwavelength grating layer was increased in 10 μm increments from 100 to 150 μm. The frequency resolution in this measurement was 6 GHz. The measurement data was interpolated using the Catmull-Rom Splines method. The results are shown in FIG. 8. As shown in FIG. 8, it was observed that the frequency of the terahertz waves transmitted through the filter of the present invention shifted to the higher frequency side by increasing the period of the pitch-variable subwavelength grating layer. Furthermore, it was demonstrated that all filters had high transmittance.

[0033] A summary of the results in Figure 8 is shown in Table 1 below. Before stretching (period: 100 μm), a sharp peak was observed at 0.303 THz in the transmission spectrum of the filter of the present invention. The Q-factor was 34, and the transmittance was 0.87. This demonstrates that the filter of the present invention is a narrow-band bandpass filter with high transmittance at the peak frequency (peak transmittance). Furthermore, by increasing the period of the pitch-variable subwavelength grating layer from 100 μm to 150 μm, the effective refractive index of the pitch-variable subwavelength grating layer could be changed from 2.08 to 1.50, and the resonant frequency could be controlled and shifted in the range from 0.303 THz to 0.320 THz (a difference of 17 GHz).

[0034]

[0035] It has been shown that the filter of the present invention can be made into a variable transmission frequency filter that can selectively transmit electromagnetic waves having a desired specific frequency by controlling the period of the pitch-variable subwavelength grating layer.

[0036] While the present invention has been described in connection with embodiments thereof, we do not intend to limit our invention to any of the details of the description unless otherwise specified, and believe that the claims should be construed broadly without departing from the spirit and scope of the invention as set forth in the appended claims.

[0037] This application claims priority based on Japanese Patent Application No. 2024-089511, filed on May 31, 2024, the contents of which are incorporated herein by reference as part of the present specification.

[0038] REFERENCE SIGNS LIST 100 Variable transmission frequency filter 101 Electromagnetic wave transmission region 110 Reflection layer 111 First reflection layer 112 Second reflection layer 120 Variable pitch sub-wavelength grating layer 121 Grating portion 121a Strip 121b Air gap 122 Spring portion 123 End portion 130 Spacer 140 Fixed substrate 150 Movable substrate 201 Silicon substrate 202 Resist film 203 Photomask

Claims

1. A tunable transmission frequency filter including a Fabry-Perot resonator incorporating a pitch-tunable subwavelength grating layer independent of two parallel reflective layers between the reflective layers.

2. The variable-frequency transmission filter according to claim 1, wherein the pitch-variable sub-wavelength grating layer expands or contracts in the periodic direction to increase or decrease the proportion of voids in the pitch-variable sub-wavelength grating layer, thereby changing the period of the pitch-variable sub-wavelength grating layer.

3. The variable transmission frequency filter according to claim 2, wherein the two reflective layers are made of silicon, and the variable pitch sub-wavelength grating layer is made of silicon.

4. The variable transmission frequency filter according to claim 3, wherein the electromagnetic waves selectively transmitted by the variable transmission frequency filter are terahertz waves.

5. The variable-frequency transmission filter according to claim 4, wherein the pitch of the variable-pitch subwavelength grating layer is variable in the range of 0.5 to 3 times the period.

6. A variable-frequency transmission filter according to claim 5, wherein the pitch-variable subwavelength grating layer has a grating section and spring sections formed on the left and right of the grating section, and has a structure in which the grating section can expand and contract in the periodic direction due to the spring sections.

7. The variable transmission frequency filter according to claim 6, wherein one end of the pitch-variable subwavelength grating layer is fixed to a fixed substrate and the other end is fixed to a movable substrate, and the movable substrate has a structure that allows movement in the periodic direction.

8. A variable transmission frequency device comprising the variable transmission frequency filter according to any one of claims 1 to 7.

9. A method for manufacturing a variable transmission frequency filter, comprising obtaining the variable transmission frequency filter according to any one of claims 1 to 7 by incorporating a variable pitch sub-wavelength grating layer between two parallel reflective layers, the variable pitch sub-wavelength grating layer being independent of the reflective layers.

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