Variable frequency filter

Abstract

To provide a novel variable frequency filter which is capable of selectively and efficiently passing an electromagnetic wave of a desired frequency and allows the frequency of a transmitted magnetic wave to be changed.SOLUTION: A variable frequency filter is provided, having a Fabry-Perot resonator consisting of a variable pitch sub-wavelength grating layer independently provided between a pair of parallelly arranged reflective layers.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[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 transparency (frequency-selective transparency), and filters that utilize 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 wave selective transmission filters using Fabry-Perot resonators are designed to have a fixed resonant frequency and transmit only terahertz waves of the desired frequency. In recent years, however, technologies have been developed to turn such filters into frequency-tunable filters (tunable wavelength filters) that can change the frequency of the terahertz waves they transmit. For example, Non-Patent Document 2 discloses a liquid crystal Fabry-Perot device in which the cavity is filled with liquid crystal and the refractive index of the liquid crystal is controlled by applying a voltage, thereby shifting the resonant peak. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] Goebel et al., “Tunable Fabry-Perot THz filter with sub-wavelength grating mirrors”, Proc. of SPIE, Vol. 6989, 698911, 2008 [Non-patent document 2] Li et al., “Electrically Tunable Liquid-crystal Fabry-Perot Device for Terahertz Radiation”, Proc. of SPIE, Vol. 9795, 979539, 2015 Summary of the Invention [Problem to be solved by the invention]

[0005] Because the dimensions of terahertz wave filters are specified on the order of micrometers, extremely strict control is required to create a tunable transmission frequency 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. [Means for solving the problem]

[0007] As a result of intensive research conducted by the inventors to solve the above-mentioned problems, they discovered that by incorporating a pitch-tunable subwavelength grating layer between two reflective layers, independently of the reflective layers, and using a filter equipped with a Fabry-Perot resonator with a structure in which the period of the grating layer can be mechanically controlled, 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 problems of the present invention have been solved by the following means. [1] A tunable transmission frequency filter including a Fabry-Perot cavity in which a pitch-tunable subwavelength grating layer independent of two parallel reflective layers is incorporated between the reflective layers. [2] The variable-frequency transmission filter according to [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 [1] or [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 any one of [1] to [3] above, wherein the electromagnetic wave selectively transmitted by the variable transmission frequency filter is a terahertz wave. [5] The variable-frequency transmission filter according to any one of [1] to [4], wherein the pitch-variable sub-wavelength grating layer has a variable pitch within a range of 0.5 to 3 times the period. [6] The variable-frequency transmission filter according to any one of [1] to [5], wherein the pitch-variable subwavelength grating layer has a grating portion and spring portions formed on the left and right sides of the grating portion, and has a structure in which the grating portion can expand and contract in a periodic direction due to the spring portions. [7] The variable 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 has a structure that is movable in a periodic direction. [8] [9] A variable transmission frequency device having the variable transmission frequency filter according to any one of [1] to [7]. A method for manufacturing a tunable transmission frequency filter, comprising: obtaining the tunable transmission frequency filter according to any one of [1] to [7] above by incorporating, between two parallel reflective layers, a pitch-variable sub-wavelength grating layer that is independent of the reflective layers. [Effects of the Invention]

[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. [Brief explanation of the drawings]

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

[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 passband 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 that incorporates a pitch-tunable subwavelength grating layer, independent of two parallel reflective layers (reflectors), between the reflective layers. In the filter of the present invention, the frequency of the electromagnetic wave that passes through the Fabry-Perot resonator can be controlled by controlling the effective refractive index of the pitch-tunable subwavelength grating layer. In the filter of the present invention, the reflective layer and the pitch-variable sub-wavelength grating layer being independent means that the reflective layer and the pitch-variable sub-wavelength grating layer are not in direct contact with each other. That is, in the filter of the present invention, the reflective layer remains unchanged, and only the grating period (hereinafter simply referred to as "period") of the pitch-variable sub-wavelength grating layer can be varied, 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 with 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 appropriately set 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, in-vehicle radar, 5G communication equipment, space communications, game machines, 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, etc., non-destructive testing devices, various security devices, terahertz spectroscopy systems, and terahertz wave compatible analysis and inspection devices using these (e.g., systems for identifying chemical substances and pharmaceuticals), agricultural and medical application devices, TOF (Time of Flight) distance sensors, high-speed communication and optical calculation circuit devices, etc., to form a transmission frequency variable device having the filter of the present invention. By controlling the effective refractive index of the pitch-variable subwavelength grating layer, the filter of the present invention can adjust the frequency of electromagnetic waves (preferably terahertz waves) that can pass through the filter 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 at 3.6-3.7 GHz and KDDI at 3.7-3.8 GHz. Similar frequency band assignments are expected for 6G communications in the future. Therefore, by using the filter of the present invention to manufacture a radio wave receiving antenna, it is possible to quickly tune the antenna's frequency band and selectively receive only the frequency of the desired telecommunications carrier. Furthermore, since automotive radars also have several frequency channels, active frequency tuning using the filter of the present invention can avoid crosstalk. Furthermore, the filter of the present invention can be used in terahertz imagers and body check devices 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 preferred embodiment of a filter according to the present invention, schematically illustrating the mechanism by which the frequency of transmitted electromagnetic waves can be controlled. The filter 100 according to the present invention illustrated in FIG. 1 includes two reflective layers 110 (first reflective layer 111 and 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 effective refractive index of the pitch-tunable subwavelength grating layer 120 increases or decreases due to the changed period p, 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) shows 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. FIG. 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. Reducing the effective refractive index can control the frequency (peak frequency) of the electromagnetic wave transmitted through the filter 100 of the present invention 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 reflective layers arranged parallel to each other. The two reflective layers are preferably made of the same material, thickness, etc. The inner surfaces of these reflective layers function as reflectors, so that electromagnetic waves incident on the filter of the present invention are repeatedly reflected multiple times, allowing only electromagnetic waves of a specific frequency to pass through. 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 (e.g., gold, silver, copper, aluminum, etc.) commonly used for the mirror surface of a Fabry-Perot filter, or a dielectric multilayer film. Among these, a high-dielectric material is preferable. 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 preferable as the material for the reflective layer. The reflectivity of the reflective layer is controlled to be higher than the reflectivity of the pitch-variable subwavelength grating layer.

[0018] The thickness of the reflective layer (t1 shown in FIG. 1) can be set appropriately depending on the purpose. For example, when the reflective layer is made of silicon, the thickness t1 of the reflective layer can be set to 13 to 662 μm, or alternatively, 66 to 442 μm, or alternatively, 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 set to 108 to 5400 μm, or alternatively, 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-tunable subwavelength grating layer) The filter of the present invention includes a pitch-variable subwavelength grating layer between the two reflective layers, the pitch-variable subwavelength grating layer being 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. By arbitrarily controlling the period (pitch) of the subwavelength grating structure, the pitch-variable subwavelength grating layer can be made to have a desired refractive index (effective refractive index). The subwavelength grating structure is not particularly limited as long as it has a structure that allows the period to be changed. For example, when the subwavelength grating structure is formed of multiple strips, the proportion of gaps between the strips of the pitch-variable subwavelength grating layer increases or decreases due to expansion and contraction in the periodic direction, 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 in which a subwavelength grating structure is formed by multiple strips. The bold frame in FIG. 3 has been altered in scale (ratio) for structural explanation. The pitch-variable subwavelength grating layer 120 shown in FIG. 3 includes 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 or 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 FIG. 3 ). This allows the effective refractive index of the entire pitch-variable subwavelength grating layer 120 (grating portion 121) to be more uniformly controlled. When the pitch-variable subwavelength grating layer includes components other than the grating portion, as in the example shown in FIG. 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 grating portions 121 and spring portions 122, observed from above, and schematically shows what happens when the pitch-variable sub-wavelength grating layer 120 is stretched in the periodic direction (the vertical direction in FIG. 4) to lengthen the period.

[0021] The constituent material of the pitch-variable subwavelength grating layer (constituent material of the pattern) can be a resin having high transmittance to terahertz waves (such as COP (Cyclo Olefin Polymer), Teflon (registered trademark), polyethylene, polypropylene, polyimide, Tsurupica (manufactured by Pax Co., Ltd.)), or the above-mentioned dielectric (preferably silicon). Furthermore, when the reflective layer is a dielectric, it is also preferable that the constituent material be the same as the constituent material of the reflective layer. Note that the reflectance of the pitch-variable subwavelength grating layer is usually lower than the reflectance 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, and therefore the reflectance of the reflective layer is higher than the reflectance of the pitch-variable subwavelength grating layer.

[0022] The dimensions of the pitch-variable subwavelength grating layer can be set appropriately. The thickness of the pitch-variable sub-wavelength grating layer (t2 shown in FIG. 1) can be set appropriately depending on the frequency of the electromagnetic wave to be transmitted. For example, the thickness t2 of the pitch-variable sub-wavelength grating layer can be set to 50 to 1000 μm, or alternatively, 100 to 500 μm, or 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. 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 on 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 made into an active linear filter.

[0023] In addition, the pitch-tunable subwavelength 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 i ) is 100 μm, "the pitch is variable in the range of 0.5 to 3 times the period" means that the pitch is variable in the range of 50 to 300 μm. Furthermore, the number of gratings in the pitch-variable subwavelength grating layer (the number of repetitions of a structural unit consisting of a pair of strips 121a and gaps 121b) 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. Increasing the number of gratings allows for more precise control of the period p during expansion and contraction. 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 sub-wavelength 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 sub-wavelength 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 sub-wavelength 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 sub-wavelength 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 (the left-right direction in FIG. 5), the pitch-variable sub-wavelength grating layer is pulled in the periodic direction, changing the period of the grating section.

[0025] [Manufacturing method for variable transmission frequency filters] The method for producing the filter of the present invention is not particularly limited. For example, the filter of the present invention can be produced by manufacturing a pitch-variable subwavelength grating layer using semiconductor microfabrication technology and incorporating the pitch-variable subwavelength grating layer between two parallel reflective layers, independently of the reflective layers.

[0026] FIG. 6 is an explanatory diagram illustrating an example of a manufacturing method for the pitch-variable sub-wavelength grating layer 120 in the filter of the present invention, using a vertical end view of the pitch-variable sub-wavelength grating layer 120. In FIG. A resist (photoresist) film 202 is formed on a silicon substrate 201 (FIGS. 6(a) and 6(b)), and ultraviolet light is irradiated onto the silicon substrate 201 from above a photomask 203 to form a grating periodic structure (FIG. 6(c)). The grating periodic structure is then formed on the silicon substrate 201 by dry etching such as plasma etching (FIG. 6(d)), and the resist film 202 is removed with a chemical or the like (FIG. 6(e)), thereby obtaining a pitch-variable sub-wavelength grating layer 120 on which the grating periodic structure is formed.

[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. [Example]

[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 subwavelength grating layers> The pitch-tunable subwavelength grating layer was fabricated using photolithography. A 2200 μm-thick silicon substrate (refractive index: 3.4) was cut into a 2 cm x 2 cm piece and cleaned. A resist film (OFPR-800LB 200 cp photoresist) 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, after which the resist film was removed to obtain the pitch-tunable subwavelength grating layer. The parameters of the pitch-variable sub-wavelength grating layer were as follows: -Design parameters- Number of grids: 100 Length of the lattice section in the lattice width direction: 10 mm Length of the lattice section in the longitudinal direction: 8 mm Grating period (p, initial value): 100 μm Grating width (a): 85μm Spring length (l): 3600 μm Spring thickness (s): 400 μm Spring width (w): 40 μm

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

[0031] <Production of the filter of the present invention> A filter of the present invention was manufactured with the configuration shown in Figure 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 reflective layers (first and second reflective layers) were made of silicon, the spacers were made of aluminum, and the thickness t1 of the reflective layers was 200 μm and the distance h between the reflective layers was 1620 μm.

[0032] Using a terahertz spectrometer (Tera Prospector, manufactured by Nippon 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 Figure 8. As shown in Figure 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 shown that all filters had high transmittance.

[0033] Table 1 below summarizes the results of Figure 8. 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-tunable subwavelength grating layer from 100 μm to 150 μm, the effective refractive index of the pitch-tunable 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] [Table 1]

[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. [Explanation of symbols]

[0036] 100 Variable passband filter 101 Electromagnetic wave transmission area 110 Reflective layer 111 1st reflective layer 112 Second reflective layer 120 pitch-tunable subwavelength grating layer 121 Lattice section 121a Strip 121b void 122 Spring part 123 End 130 spacer 140 Fixed board 150 Movable substrate 201 Silicon substrate 202 Resist film 203 Photomask

Claims

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

2. 2. The tunable transmission frequency filter according to claim 1, wherein the pitch-variable sub-wavelength grating layer expands or contracts in a 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. 3. The tunable transmission frequency filter according to claim 2, wherein the two reflective layers are made of silicon, and the pitch-variable sub-wavelength grating layer is made of silicon.

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

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

6. 6. The variable-frequency transmission filter according to claim 5, wherein the pitch-variable subwavelength grating layer has a grating portion and spring portions formed on left and right sides of the grating portion, and the spring portions enable the grating portion to expand and contract in a periodic direction.

7. 7. The variable-frequency transmission 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 is movable in a 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 tunable transmission frequency filter, comprising obtaining the tunable transmission frequency filter according to any one of claims 1 to 7 by incorporating a pitch-variable sub-wavelength grating layer, independent of the reflective layers, between two reflective layers that are parallel to each other.

Images