Bandpass optical filter based on metasurface, optical device and design method thereof

Abstract

The invention discloses a metasurface-based band-pass optical filter, an optical device and a design method thereof, the metasurface-based band-pass optical filter comprises at least one structural unit, each structural unit comprises a substrate layer, a dielectric layer and a metal array layer, the dielectric layer is located between the metal array layer and the substrate layer, and the thickness of each layer in each structural unit is the same; according to the metasurface structure unit design provided by the invention, the number of coating films is reduced to only three layers, and only three optical film materials are provided, so that the design and the process are relatively simple, and a narrow-band filtering effect can be realized.

Description

Technical Field

[0001] The present invention relates to the technical field of optical filters, and particularly to a metasurface-based band-pass filter, an optical device and a design method thereof. Background Art

[0002] Optical filters can be classified according to the filtering method into: transmissive filters (also called band-pass filters), reflective filters (also called band-stop / notch filters), and absorptive filters. The difference between a band-pass filter and a notch filter is that: a band-pass filter only allows light of a specific wavelength to pass through, while a notch filter is designed to block light of a specific wavelength, and they have very different applications. For different application scenarios, filters with different filtering methods each have their own advantages. Among them, transmissive filters can be applied to devices such as projection display systems, beauty devices, and imaging cameras. However, traditional filters are restricted in their tunability due to their inherent heating and low selectivity, and continuous exposure will cause performance degradation, thus limiting their application in devices such as display devices and spectrometers. Metasurface devices can achieve the design of any desired wavelength, and the spectral bandwidth is relatively narrow, with less crosstalk between each other, and can be used to design multi-channel filters. The narrowband tunable filter based on metasurface makes the structure of the device more compact and facilitates the integration of the entire system.

[0003] There are many structures for designing filters based on metasurface structures, such as MIM structures (metal-dielectric-metal), periodic hole or nanorod structures, and periodic nanograting structures, etc. MIM structures are often used in absorptive filters. For metasurface-based band-pass filters, there is still a lack of multi-channel designs with small bandwidth and highly consistent channels for visible light, which is more convenient for integrated applications in enhanced displays or display imaging.

[0004] In the related art, the shapes of band-pass filters designed based on metasurfaces are diverse, but less consideration is given to processing problems. Currently, micro-nano processing has many limitations on line width and aspect ratio. For example, the patent application document with publication number CN113296179A proposes a metal metasurface filter for thermophotovoltaics. Although this scheme is a band-pass filter, it is essentially a broadband optical filter. Moreover, when processing this metasurface filter, the multi-layer structure process of filling annular sub-wavelength holes is relatively complex for micro-nano processing, and it is difficult to ensure consistency. Summary of the Invention

[0005] The technical problem to be solved by the present invention is how to provide a metasurface-based band-pass filter that can achieve a narrow-band filtering effect and is convenient for processing.

[0006] The present invention solves the above technical problems by the following technical means:

[0007] A metasurface-based bandpass filter is proposed, which includes at least one structural unit. Each structural unit includes a base layer, a dielectric layer, and a metal array layer. The dielectric layer is located between the metal array layer and the base layer, and the thicknesses of the layers in each structural unit are the same.

[0008] Furthermore, the period, side length, and thicknesses of the layers of the structural unit satisfy the phase matching condition:

[0009]

[0010] where k is the in-plane wave vector of the incident light, β is the propagation constant of the guided mode, m is the diffraction order index, and P is the period.

[0011] Furthermore, the base layer is made of an aluminum oxide film.

[0012] Furthermore, the dielectric layer is made of a silicon oxide film.

[0013] Furthermore, the metal array layer includes a number of metal nanocolumns arranged in an array.

[0014] Furthermore, the metal nanocolumns are made of a metal aluminum film.

[0015] Furthermore, a metal protective layer is covered on the metal nanocolumns.

[0016] Furthermore, the base layer is located on the surface of the substrate layer.

[0017] Furthermore, the substrate layer is made of a quartz wafer.

[0018] In addition, the present invention also proposes an optical device, which includes the bandpass filter described above.

[0019] In addition, the present invention also proposes a design method for a metasurface-based bandpass filter, including:

[0020] Constructing a model of at least one structural unit, and determining design parameters through simulation of the model to generate a resonant transmission peak at the phase matching condition for guided mode resonance. The design parameters include structural parameters and material parameters;

[0021] Fabricating at least one structural unit according to the structural parameters and material parameters. Each structural unit includes a base layer, a dielectric layer, and a metal array layer. The dielectric layer is located between the metal array layer and the base layer.

[0022] Furthermore, the structural parameters include the thicknesses of the layers in each structural unit, the period of the structural unit, and the side length, and the material parameters include the materials of the layers.

[0023] Further, determining the design parameters by simulating the model to make the guided-mode resonance generate a resonance transmission peak at the phase matching condition includes:

[0024] When keeping the thickness of each layer fixed, adjusting the period and side length to make the guided-mode resonance generate a resonance transmission peak at the phase matching condition, and the phase matching condition is:

[0025]

[0026] wherein, k is the in-plane wave vector of the incident light, β is the propagation constant of the guided mode, m is the diffraction order index, and P is the period.

[0027] Further, fabricating at least one structural unit according to the structural parameters and material parameters includes:

[0028] Growing an aluminum oxide film on the surface of the substrate layer as the base layer;

[0029] Growing a silicon oxide film on the surface of the base layer as the dielectric layer;

[0030] Growing a metal aluminum film and a metal protective layer on the surface of the dielectric layer successively;

[0031] Coating a photoresist or a nanoimprint resist on the surface of the metal aluminum film and then etching out the metasurface micro-nano structure;

[0032] Removing the photoresist or the nanoimprint resist to obtain the structural unit.

[0033] Further, the metasurface micro-nano structure is a plurality of metal nano-columns arranged in an array.

[0034] The advantages of the present invention are as follows:

[0035] (1) The band-pass filter proposed by the present invention is composed of a three-layer structure, including the bottom base layer, the middle dielectric layer, and the top metal array. Using this structure, narrow-band filtering can be achieved in the visible light band and the near-infrared band, and the thickness of each layer in each structural unit is fixed to the same value, ensuring that multiple structural units can be prepared on one template for integration, which is convenient for processing. Moreover, since the thickness is fixed, only by changing the period and side length, the resonance wavelength of the surface plasmon can be effectively controlled, and thus the offset peak wavelength of the center wavelength can be controlled.

[0036] (2) The metal array layer provided by the present invention is composed of relatively simple metal nano-columns. For the micro-nano processing technology, it can be realized only by imprinting or lithography after coating, and the processing technology is relatively simple, which can ensure consistency.

[0037] Additional aspects and advantages of the present invention will be given in part in the following description, become apparent in part from the following description, or be understood through the practice of the present invention. Description of the Drawings

[0038] Figure 1 is a schematic structural diagram of a nine-channel bandpass filter based on metasurface proposed in an embodiment of the present invention;

[0039] Figure 2 is a schematic structural diagram of a structural unit in an embodiment of the present invention, where H is the height of the metal layer, h1 is the height of the metal protection layer, h2 is the height of the dielectric layer, h3 is the height of the substrate layer, P is the period, and D is the side length;

[0040] Figure 3 is a schematic flow diagram of a design method for a bandpass filter based on metasurface proposed in an embodiment of the present invention;

[0041] Figure 4 is a schematic diagram of the processing technology of a bandpass filter based on metasurface in an embodiment of the invention;

[0042] Figure 5 is a schematic diagram of the simulated nine-channel transmission intensity in an embodiment of the present invention;

[0043] Figure 6 is a schematic diagram of the electric field intensity at a wavelength of 580 nm in an embodiment of the present invention, where Figure 6 the left figure in is the electric field distribution diagram of a single-period structure at a wavelength of 580 nm, and the right figure is the electric field distribution diagram of an array structure of three periods of a single period at a wavelength of 580 nm;

[0044] Figure 7 is a schematic diagram of the electric field intensity at a wavelength of 632 nm in an embodiment of the present invention, where, Figure 7 the left figure in is the electric field distribution diagram of a single-period structure at a wavelength of 632 nm, and the right figure is the electric field distribution diagram of an array structure of three periods of a single period at a wavelength of 632 nm;

[0045] Figure 8 is a schematic diagram of the electric field intensity at a wavelength of 700 nm in an embodiment of the present invention, where, Figure 8 the left figure in is the electric field distribution diagram of a single-period structure at a wavelength of 700 nm, and the right figure is the electric field distribution diagram of an array structure of three periods of a single period at a wavelength of 700 nm;

[0046] Figure 9 is a schematic diagram of the transmission intensity at different side lengths in an embodiment of the present invention;

[0047] Figure 10 is a schematic diagram of the transmission intensity at different nano-column heights in an embodiment of the present invention;

[0048] Figure 11 is a schematic diagram of the transmission intensity at different substrate thicknesses in an embodiment of the present invention Detailed implementation manners

[0049] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0050] As Figures 1 to 2 shown, an embodiment of the present invention provides a metasurface-based bandpass filter, which includes at least one structural unit. Each structural unit includes a base layer, a dielectric layer, and a metal array layer. The dielectric layer is located between the metal array layer and the base layer, and the thicknesses of the layers in each structural unit are the same.

[0051] It should be noted that the bandpass filter proposed in this embodiment is composed of three layers of structures, including a base layer at the bottom, a dielectric layer in the middle, and a metal array at the top. Compared with traditional color filters based on organic or inorganic materials, the thickness of this filter is in the range of hundreds of nanometers, and the light beam is restricted within a very small volume. And the narrowband filtering achieved by using this periodic structure is called extraordinary optical transmission (EOT). The main cause of EOT can be attributed to surface plasmons, which are the collective oscillatory motion of free electrons on the metal surface. When this efficient extraordinary optical transmission appears in the visible range, it will be applicable to a color filter, which is a key component for enhancing the optical performance of electronic imaging devices such as enhanced displays and CMOS imaging sensors. And the multi-channel narrowband filter formed when multiple structural units are provided can divide the light radiated from the target object into specific narrowband wavelength bands, and the obtained spectral profile is formed by the resonant coupling of narrowband guided-mode resonance and surface plasmons, so as to facilitate the capture of spectral information. In addition, it can also be applied to the field of optical communication for wavelength division multiplexing, selectively transmitting light of a specific wavelength, and accurately realizing the transmission and reception of optical signals. In addition, the thicknesses of the layers in each structural unit are fixed to the same value, ensuring that multiple structural units can be prepared on a single template for integration, which is convenient for processing.

[0052] Specifically, a unit structure has only one nanorod, and the overall structure of the filter is a periodic array structure, that is, it extends infinitely in the x direction and the y direction in space. Because this structure is a relatively simple periodicity, the unit structure is given in this embodiment as Figure 2 shown.

[0053] As a further preferred technical solution, the period, side length, and thickness of each layer of the structural unit satisfy the phase matching condition:

[0054]

[0055] In the formula, k is the wave vector in the incident light plane, β is the propagation constant of the guided mode, m is the diffraction order index, and P is the period.

[0056] It should be noted that in this embodiment, software modeling and simulation are pre - carried out. By setting different structural parameters and material parameters for simulation, the optimal design parameters are determined when the guided - mode resonance generates a resonance transmission peak at the phase - matching condition for fabricating the band - pass filter.

[0057] As a further preferred technical solution, the base layer is made of an aluminum oxide film.

[0058] It should be noted that the thickness of the aluminum oxide film is set to 120 nanometers.

[0059] As a further preferred technical solution, the dielectric layer is made of a silicon oxide film.

[0060] It should be noted that the thickness of the silicon oxide film is set to 50 nanometers and is used as the waveguide layer.

[0061] As a further preferred technical solution, the metal array layer includes a number of metal nanocolumns arranged in an array.

[0062] It should be noted that the metal array layer set in this embodiment is composed of relatively simple metal nanocolumns. For the micro - nano processing technology, it can be realized only by imprinting or photolithography after coating. The processing technology is relatively simple and can ensure consistency.

[0063] As a further preferred technical solution, the metal nanocolumns are made of a metal aluminum film.

[0064] It should be noted that the thickness of the metal aluminum film is set to 20 nanometers, and the thickness of the metal protection layer is set to 10 nanometers.

[0065] As a further preferred technical solution, the metal nanocolumns are covered with a metal protection layer.

[0066] It should be noted that in this embodiment, the metal protection layer is set to protect the nanostructure.

[0067] As a further preferred technical solution, the base layer is located on the surface of the substrate layer.

[0068] As a further preferred technical solution, the substrate layer is made of a quartz wafer.

[0069] It should be noted that in this embodiment, a 12 - inch quartz wafer is used as the substrate layer.

[0070] In addition, this embodiment also proposes an optical device, and this optical device contains the band - pass filter described in the above - mentioned embodiment.

[0071] It should be noted that the use of the narrowband tunable metasurface-based filter proposed in this embodiment makes the structure of the optical device more compact and facilitates the integration of the entire system. In terms of technology, due to the advantages of structured light regulation, the complexity is simplified. Compared with the cumbersome processes of dozens or hundreds of layers of coatings and the use of various metal and non-metal optical materials in traditional filters, the design of the metasurface structural unit proposed in this embodiment reduces the number of coatings to only three layers, and there are only three types of optical film materials. Both the design and the process are relatively simple, which is conducive to replacing traditional filters and being applied to optical devices.

[0072] In addition, as Figure 3 shown, an embodiment of the present invention also proposes a design method for a band-pass filter based on a metasurface, including the following steps:

[0073] S10. Construct a model of at least one structural unit, and determine the design parameters by simulating the model to make the guided-mode resonance generate a resonance transmission peak at the phase-matching condition. The design parameters include structural parameters and material parameters;

[0074] Specifically, in this embodiment, the Lumerical FDTD software can be used to simulate the model.

[0075] S20. Fabricate at least one structural unit according to the structural parameters and material parameters. Each structural unit includes a substrate layer, a dielectric layer, and a metal array layer, and the dielectric layer is located between the metal array layer and the substrate layer.

[0076] It should be noted that the giant magnetoresistive (GMR) effect between the waveguide layer and the structure layer is the basic working principle of the proposed filter. When the phase matching between the transverse magnetic field (TM) and transverse electric field (TE) guided modes of the waveguide and the diffracted wave is satisfied, band-pass filtering can be achieved. Therefore, the thicknesses of each layer determined according to the simulation optimization in this embodiment.

[0077] As a further preferred technical solution, the structural parameters include the thickness of each layer in each structural unit, the period of the structural unit, and the side length, and the material parameters include the materials of each layer.

[0078] As a further preferred technical solution, in step S10, determining the design parameters by simulating the model to make the guided-mode resonance generate a resonance transmission peak at the phase-matching condition includes:

[0079] When keeping the thickness of each layer fixed, adjust the period and the side length to make the guided-mode resonance generate a resonance transmission peak at the phase-matching condition. The phase-matching condition is:

[0080]

[0081] In the formula, k is the wave vector in the incident light plane, β is the propagation constant of the guided mode, m is the diffraction order index, and P is the period.

[0082] It should be noted that the designed structural unit is a simple periodic structure. During simulation, the height of each layer of the structure is kept unchanged, and only the period and side length are changed to effectively control the resonance wavelength of the surface plasmon, thereby controlling the shift of the central wavelength.

[0083] To realize a multi-channel filter, the process difficulty needs to be ensured. Technologically, the thickness of each layer needs to be consistent. When processing multiple channels, the period and side length can be changed during lithography / nanoimprinting.

[0084] As a further preferred technical solution, as Figure 4 shown, the step S20: fabricate at least one structural unit according to the structural parameters and material parameters, including:

[0085] S21. Grow an aluminum oxide film on the surface of the substrate layer as the base layer;

[0086] It should be noted that the surface of a quartz wafer (12 inches) is cleaned as the substrate layer, and an aluminum oxide film with a thickness of 120 nanometers is grown on the surface of the quartz wafer as the base layer of the nanostructure unit.

[0087] S22. Grow a silicon oxide film on the surface of the base layer as the dielectric layer;

[0088] It should be noted that a silicon oxide film with a thickness of 50 nanometers is grown on the surface of the aluminum oxide as the dielectric layer of the nanostructure unit.

[0089] S23. Grow a metal aluminum film and a metal protective layer on the surface of the dielectric layer successively;

[0090] It should be noted that by successively growing a metal aluminum film with a thickness of 20 nanometers and a metal protective layer with a thickness of 10 nanometers on the surface of the silicon oxide, they are used as the processing layer and the protective layer of the nanostructure respectively.

[0091] S24. Coat a photoresist or a nanoimprinting resist on the surface of the metal aluminum film and then etch out the metasurface micro-nano structure;

[0092] S25. Remove the photoresist or the nanoimprinting resist to obtain the structural unit.

[0093] Specifically, coat a photoresist or a nanoimprinting resist on the surface of the metal aluminum film, use a photomask for lithography or a nanoimprinting template for imprinting and etching out the metasurface micro-nano structure, and then remove the photoresist or the nanoimprinting resist to fabricate the structural unit.

[0094] As a further preferred technical solution, the metasurface micro-nano structure is a number of metal nano-columns arranged in an array.

[0095] The band-pass filter prepared by the design method of this embodiment can achieve a narrow-band filtering effect in the visible light band and the near-infrared band, and the processing technology is simple, and it has great application potential in near-infrared and visible light detectors.

[0096] Taking the simulation analysis of a nine-channel band-pass filter as an example, in order to avoid interference between the nine channels, when selecting parameters, ensure that the crosstalk is below 0.3. The finally selected simulation curves of the nine channels are as Figure 5 shown.

[0097] In order to explore the optical response mechanism, taking the 6th channel as an example, the electric field distribution diagrams of the single-period structure and the three-period array structure at wavelengths of 580nm, 632nm and 700nm are plotted.

[0098] When the wavelength is 580nm, as Figure 6 shown, in addition to being distributed in the air above the structure, there is also an electric field distribution at the edge of the nano-column, making the transmission intensity there reach 0.2, which is higher than the transmission intensity after the wavelength of 700nm. As Figure 7 shown, the electric field intensity is mainly distributed in the gaps of the nano-array, and the field intensity reaches the maximum value at the edge of the nano-column. At 632nm, the transmission intensity is 0.674. This is because the electric field intensity of the lower layer structure cannot be ignored, so the total transmission efficiency cannot reach 100%. At the wavelength of 700nm, the electric field is mainly distributed in the air above the structure, see Figure 8 , resulting in a very low transmittance of only 0.02 here.

[0099] The influence of structural parameters on the results cannot be ignored. Therefore, the influence of several important structural parameters on the results is compared. Still taking the sixth channel as an example: using the control variable method, when the side length of the array structure decreases, as Figure 9 shown, the central wavelength will shift to the left, and the intensity of the transmission peak will also increase. As Figure 10 shown, only increasing the thickness of the metal nano-column, the central wavelength shifts to the right and the intensity decreases, and it has little impact on the overall transmission intensity. As Figure 11 shown, when the thickness of the base layer decreases, the central wavelength shifts to the left, and the influence on the overall transmission intensity is small. Therefore, the central wavelength of the filter can be accurately controlled by controlling the change of the thickness of the base layer. In summary, these results show that when the unit structure of the metasurface fluctuates within a small range, the filter can still maintain good performance, indicating that the structure has high robustness. The filter designed by the present invention independently tunes the central wavelength by adjusting the period and side length of the structure, thereby improving the flexibility in practical applications

[0100] In the description of this specification, the descriptions referring to terms such as "one embodiment", "some embodiments", "examples", "specific examples", or "some examples", etc., mean that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in a suitable manner in any one or more embodiments or examples.

[0101] In addition, the terms "first" and "second" are used only for descriptive purposes and cannot be construed as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, the features defined with "first" and "second" may explicitly or implicitly include at least one of these features. In the description of the present invention, the meaning of "a plurality" is at least two, such as two, three, etc., unless otherwise specifically defined.

[0102] Although the embodiments of the present invention have been shown and described above, it can be understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those of ordinary skill in the art can make changes, modifications, substitutions, and variations to the above embodiments within the scope of the present invention.

Claims

1. A bandpass filter based on a metasurface, characterized in that: It comprises at least one structural unit, each structural unit comprises a base layer, a dielectric layer and a metal array layer, the dielectric layer is located between the metal array layer and the base layer, and the thickness of each layer in each structural unit is the same.

2. The metasurface-based bandpass filter according to claim 1, characterized in that: The period, side length and thickness of each layer of the structural unit satisfy the phase matching condition: Where k is the incident light in-plane wave vector, β is the guided mode propagation constant, m is the diffraction order index, and P is the period.

3. The metasurface-based bandpass filter according to claim 1, characterized in that: The base layer uses an aluminum oxide film.

4. The metasurface-based bandpass filter according to claim 1, characterized in that: The dielectric layer uses a silicon oxide film.

5. The metasurface-based bandpass filter according to claim 1, characterized in that: The metal array layer includes a plurality of metal nanorods arranged in an array.

6. The metasurface-based bandpass filter according to claim 5, characterized in that: The metal nanocolumns are made of metal aluminum films.

7. The metasurface-based bandpass filter according to claim 5, characterized in that: The metal nanocolumns are covered with a metal protection layer.

8. The metasurface-based bandpass filter according to any one of claims 1 to 7, characterized in that: The base layer is located on the surface of the substrate layer.

9. The metasurface-based bandpass filter according to claim 8, characterized in that: The substrate layer is made of a quartz plate.

10. An optical device, characterized in that: Comprising the bandpass filter as described in any one of claims 1 to 9.

11. A design method for a bandpass filter based on a metasurface, characterized in that: include: Constructing a model of at least one structural unit, and determining design parameters by simulating the model so that the guided mode resonance generates a resonant transmission peak at a phase matching condition, wherein the design parameters include structural parameters and material parameters; At least one structural unit is manufactured according to the structural parameters and the material parameters. Each structural unit includes a base layer, a dielectric layer and a metal array layer. The dielectric layer is located between the metal array layer and the base layer.

12. The method for designing a bandpass filter based on a metasurface according to claim 11, characterized in that: The structural parameters include the thickness of each layer in each structural unit, the period and side length of the structural unit, and the material parameters include the material of each layer.

13. The design method of a bandpass filter based on a metasurface as claimed in claim 12, characterized in that: The method of determining the design parameters by simulating the model so that the guided mode resonance generates a resonant transmission peak at a phase matching condition includes: While keeping the thickness of each layer fixed, the period and side length are adjusted so that the guided mode resonance produces a resonant transmission peak at the phase matching condition. The phase matching condition is: Where k is the incident light in-plane wave vector, β is the guided mode propagation constant, m is the diffraction order index, and P is the period.

14. The design method of a bandpass filter based on a metasurface as claimed in claim 12, characterized in that: The step of manufacturing at least one structural unit according to the structural parameters and the material parameters comprises: Growing an aluminum oxide film as a base layer on the surface of the substrate layer; growing a silicon oxide film as a dielectric layer on the surface of the base layer; A metal aluminum film and a metal protective layer are successively grown on the surface of the dielectric layer; After coating the surface of the metal aluminum film with photoresist or nanoimprinting glue, an ultra-surface micro-nano structure is etched out; The photoresist or nanoimprinting glue is removed to obtain the structural unit.

15. The design method of a bandpass filter based on a metasurface as claimed in claim 14, characterized in that: The super surface micro-nano structure is a plurality of metal nano columns arranged in an array.

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