A thin film structure for energy flow modulation based on multilayer films

Abstract

This invention relates to an energy flow modulation thin film structure based on multilayer films, comprising a substrate, multilayer films, and a metasurface arranged from bottom to top. The multilayer films include a high-reflectivity film, the metasurface includes a super-unit structure, and the multilayer films also include an aperiodic film, which is connected to both the high-reflectivity film and the metasurface. A wider range of energy flow modulation is achieved by adjusting the thickness of the aperiodic film layer. Compared with existing technologies, this invention achieves wider and stronger energy flow modulation by incorporating the aperiodic film layer; it offers greater freedom in metasurface design, reducing design complexity; it solves the problem of weak broadband modulation capability of multilayer films at Littrow incident angles; the use of more super-unit structures enhances the lateral energy flow transfer capability of the entire metasurface; and it achieves a broadband Littrow reflection efficiency of over 99% for laser wavelengths in the mid-infrared broadband range.

Description

Technical Field

[0001] This invention relates to the field of electromagnetic wave energy flow modulation, and in particular to an energy flow modulation thin film structure based on multilayer films. Background Technology

[0002] Metasurfaces, as a type of two-dimensional metamaterial, can control the phase, amplitude, and polarization of incident light through the design of unit structures, and have achieved a variety of functions, including lens focusing, polarization conversion, holographic imaging, vortex beams, and anomalous deflection.

[0003] Anomalous deflection is one of the fundamental functions of optical path manipulation. Anomalous reflection under Littrow incidence plays a crucial role in applications such as laser beam combining and spectral analysis, and a Littrow reflection efficiency of over 99% is the foundation for realizing advanced functions of metasurfaces. Reflective anomalous deflection devices mainly utilize metallic or dielectric metasurfaces to achieve high reflectivity and anomalous deflection. Due to the absorption characteristics of metals in the mid-infrared band, it is difficult to improve the efficiency of metallic metasurfaces to over 99%. Meanwhile, existing dielectric metasurfaces lack sufficient lateral energy flow transfer capability, causing their efficiency to drop rapidly at the bandwidth edge, enabling only narrowband Littrow perfect reflection, which is unfavorable for realizing more complex functions. Therefore, there is currently a lack of a suitable design method to achieve broadband perfect Littrow reflection in the mid-infrared band using all-dielectric metasurfaces.

[0004] Chinese Patent No. CN110727037A discloses a linear phase gradient metasurface structure based on a high-reflectivity film, comprising a substrate (1), a high-reflectivity film (2), and a metasurface (3) arranged sequentially from bottom to top. The high-reflectivity film (2) is a dielectric film stack composed of alternating high-refractive-index dielectric film layers H1 and low-refractive-index dielectric film layers L. The side of the high-reflectivity film (2) in contact with the metasurface (3) is the low-refractive-index dielectric film layer L. The metasurface (3) uses a high-refractive-index dielectric film layer H2. One period of the metasurface (3) is composed of multiple unit structures of different widths. This invention omits the traditional thick metal substrate and introduces an all-dielectric high-reflectivity film, which improves the reflectivity of the metasurface and reduces its loss. However, it lacks the ability to control energy flow over a wider range.

[0005] Currently, how to enhance the ability to regulate energy flow over a wider range and achieve perfect Littrow reflection in the mid-infrared broadband is an urgent problem to be solved for the practical application of metasurfaces with anomalous reflection. Summary of the Invention

[0006] The purpose of this invention is to overcome the defects of the prior art and provide an energy flow control thin film structure based on multilayer films.

[0007] The objective of this invention can be achieved through the following technical solutions:

[0008] A multilayer film-based energy flow control thin film structure includes a substrate, a multilayer film, and a metasurface arranged from bottom to top. The multilayer film includes a high-reflectivity film, the metasurface includes a meta-unit structure, and the multilayer film also includes an aperiodic film layer, which is connected to the high-reflectivity film and the metasurface respectively. A wider range of energy flow control can be achieved by adjusting the thickness of the aperiodic film layer.

[0009] As a preferred technical solution, the high-reflectivity film and the aperiodic film layer are dielectric film stacks composed of alternating high-refractive-index dielectric film layers H and low-refractive-index dielectric film layers L; the contact side between the aperiodic film layer and the high-reflectivity film and the metasurface is the low-refractive-index dielectric film layer L.

[0010] As a preferred technical solution, the high-reflectivity film structure is (LH). 7 The non-periodic film structure is LHL.

[0011] As a preferred technical solution, the high refractive index dielectric film layer H is made of Si, and the low refractive index dielectric film layer L is made of SiO2.

[0012] As a preferred technical solution, the method for selecting the thickness of the non-periodic film layer is as follows:

[0013] Based on the determined width of the supercell structure in the metasurface, the Littrow reflection efficiency of the non-periodic film layer at different thicknesses is scanned, and the thickness is locally fine-tuned so that the supercell structure of the metasurface achieves the highest broadband Littrow reflection efficiency.

[0014] As a preferred technical solution, the non-periodic film layer comprises a dielectric film stack consisting of three layers of high refractive index dielectric film H and low refractive index dielectric film L of different thicknesses, arranged alternately. The structure from bottom to top is the first low refractive index dielectric film L, the first high refractive index dielectric film H, and the second low refractive index dielectric film L.

[0015] As a preferred technical solution, the metasurface includes multiple super-unit periods with a period range of p, and each super-unit period includes n super-unit structures with different widths and the same height.

[0016] As a preferred technical solution, the supercell period range p and the number of supercell structures n are selected using the following formula:

[0017]

[0018] In the formula, λ is the Littrow reflection wavelength, and i is the Littrow incident angle.

[0019] As a preferred technical solution, the method for selecting the height of the meta-unit structure of the metasurface is as follows:

[0020] The width of a single supercell structure on the metasurface is scanned so that the reflection phase of each supercell structure can cover the range of 0 to 2π. Phase change is achieved using n supercell structures of different widths and a supercell period with a period range of p. Where n ≥ 4 and n is a positive integer.

[0021] As a preferred technical solution, the method for selecting the width of the meta-unit structure of the metasurface is as follows:

[0022] The theoretical values ​​of the Poynting vector inside the metasurface under different incident wavelengths are obtained through rigorous coupled-wave analysis. The width of n supercell structures in the supercell period of the metasurface is scanned to obtain the Poynting vector inside the metasurface at this time, so that it matches the target value obtained by theoretical calculation.

[0023] Compared with the prior art, the present invention has the following advantages:

[0024] 1. By setting an aperiodic membrane layer, the present invention can achieve wider and stronger energy flow regulation.

[0025] 2. By setting a non-periodic film layer, this invention provides greater freedom in metasurface design and reduces design difficulty.

[0026] 3. This invention solves the problem of weak broadband control capability of multilayer films at the Littrow incident angle by setting a non-periodic film layer.

[0027] 4. This invention uses more meta-unit structures, which enhances the transverse energy flow transfer capability of the entire metasurface.

[0028] 5. By setting an aperiodic film layer, the present invention can achieve a broadband Littrow reflection efficiency of over 99% in the mid-infrared broadband range of laser wavelengths. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the thin film structure of the present invention;

[0030] Figure 2 This is a schematic diagram of the metasurface metaunit structure of the present invention;

[0031] Figure 3 This is a graph showing the relationship between the metasurface of the present invention and different wavelengths in the broadband range and the required transverse energy flow amplitude;

[0032] Figure 4 The total energy flow amplitude scanning results of the meta-unit structure of the metasurface of the present invention at different widths are shown.

[0033] Figure 5 This is a diagram showing the Littrow reflection efficiency of the metasurface of the present invention.

[0034] 1. Substrate; 2. High-reflectivity film; 3. Aperiodic film layer; 4. Metasurface; 5. Supercell structure; H is a high-refractive-index dielectric film layer; L is a low-refractive-index dielectric film layer. Detailed Implementation

[0035] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0036] Anomalous deflection is one of the fundamental functions of optical path manipulation. Anomalous reflection under Littrow incidence plays a crucial role in applications such as laser beam combining and spectral analysis, and a Littrow reflection efficiency of over 99% is the foundation for realizing advanced functions of metasurfaces. Reflective anomalous deflection devices mainly utilize metallic or dielectric metasurfaces to achieve high reflectivity and anomalous deflection. Due to the absorption characteristics of metals in the mid-infrared band, it is difficult to improve the efficiency of metallic metasurfaces to over 99%. Meanwhile, existing dielectric metasurfaces lack sufficient lateral energy flow transfer capability, causing their efficiency to drop rapidly at the bandwidth edge, enabling only narrowband Littrow perfect reflection, which is unfavorable for realizing more complex functions. Therefore, there is currently a lack of a suitable design method to achieve broadband perfect Littrow reflection in the mid-infrared band using all-dielectric metasurfaces.

[0037] This invention achieves stronger energy flow control by setting non-periodic film layers, providing greater freedom in metasurface design and reducing design difficulty; it solves the problem of weak control capability of multilayer films under Littrow incident light, and enhances the transverse energy flow transfer capability of the entire metasurface; and it can achieve a broadband Littrow reflection efficiency of over 99% in the mid-infrared broadband laser wavelength range.

[0038] Example 1

[0039] A schematic diagram of the thin film structure is shown below. Figure 1 As shown, an energy flow control thin film structure based on a multilayer film includes a substrate 1, a multilayer film, and a metasurface 4 arranged from bottom to top. The multilayer film includes a high-reflectivity film 2, the metasurface 4 includes a meta-unit structure 5, and the multilayer film also includes an aperiodic film layer 3, which is connected to the high-reflectivity film 2 and the metasurface 4 respectively. A wider range of energy flow control can be achieved by adjusting the thickness of the aperiodic film layer 3.

[0040] The high-reflectivity film 2 and the aperiodic film layer 3 are dielectric film stacks composed of alternating high-refractive-index dielectric film layers H and low-refractive-index dielectric film layers L; the contact side of the aperiodic film layer 3 with the high-reflectivity film 2 and the metasurface 4 is the low-refractive-index dielectric film layer L.

[0041] The high-reflectivity film 2 has a structure of (LH). 7 The non-periodic film layer 3 has an LHL structure.

[0042] In this embodiment, the thin film structure is S|(LH). 7 LHL|G; where S and G are substrate 1 and metasurface 4, respectively.

[0043] In this embodiment, the high-reflectivity film 2 is composed of a dielectric film stack consisting of seven layers of high-refractive-index dielectric film H and seven layers of low-refractive-index dielectric film L arranged alternately; the substrate 1 is a silicon wafer; the high-refractive-index dielectric film H used in the high-reflectivity film 2 has a thickness of 258 nm and the low-refractive-index dielectric film L has a thickness of 596 nm.

[0044] The high-refractive-index dielectric film layer H is made of Si, and the low-refractive-index dielectric film layer L is made of SiO2.

[0045] The method for selecting the thickness of the non-periodic film layer 3 is as follows:

[0046] Based on the determined width of the supercell structure 5 in the metasurface 4, the Littrow reflection efficiency of the non-periodic film layer 3 at different thicknesses is scanned, and the thickness is locally fine-tuned so that the supercell structure 5 of the metasurface 4 achieves the highest broadband Littrow reflection efficiency.

[0047] The aperiodic film layer 3 comprises a dielectric film stack consisting of three layers of high refractive index dielectric film layer H and low refractive index dielectric film layer L of different thicknesses, arranged alternately. From bottom to top, the structure consists of a first low refractive index dielectric film layer L, a first high refractive index dielectric film layer H, and a second low refractive index dielectric film layer L.

[0048] In this embodiment, the thickness of the first low-refractive-index dielectric film layer L is 634 nm; the thickness of the first high-refractive-index dielectric film layer H is 335 nm; and the thickness of the second low-refractive-index dielectric film layer L is 119 nm. A wider range of energy flow control is achieved by adjusting the thickness of the aperiodic film layer 3. The aperiodic film layer 3 consists of three layers, and the number of layers can be increased to achieve a wider range of energy flow control.

[0049] Superunit structure such as Figure 2 As shown, the metasurface 4 includes multiple super-unit periods with a period range of p, and each super-unit period includes n super-unit structures 5 with different widths and the same height.

[0050] The supercell period range p and the five numbers n of the supercell structure are selected using the following formula:

[0051]

[0052] In the formula, λ is the Littrow reflection wavelength, and i is the Littrow incident angle.

[0053] In this embodiment, the metasurface 4 is composed of four super-unit structures 5 with the same height but different widths arranged in sequence, and the material of the super-unit structure 5 is Si.

[0054] The method for selecting the height of the super-unit structure 5 of the metasurface 4 is as follows:

[0055] The width of a single supercell structure 5 on the metasurface 4 is scanned so that the reflection phase of each supercell structure 5 can cover the range of 0 to 2π. Phase change is achieved by using n supercell structures 5 with different widths and a supercell period with a period range of p. Wherein, n≥4, and n is a positive integer.

[0056] In this embodiment, the supercell period range p of the metasurface 4 is 465 nm, and the height of the supercell structure 5 is 1100 nm. Under these structural parameters, the reflection phase coverage of 0~2π can be achieved by changing the width of the supercell structure 5.

[0057] The method for selecting the width of the super-unit structure 5 of the metasurface 4 is as follows:

[0058] The theoretical values ​​of the Poynting vector inside the metasurface 4 under different incident wavelengths were obtained through rigorous coupled-wave analysis. The widths of n super-unit structures 5 in the super-unit period of the metasurface 4 were scanned to obtain the Poynting vector inside the metasurface 4 at this time, so that it matches the target value obtained by theoretical calculation.

[0059] In this embodiment, Figure 3 The graph shows the relationship between the metasurface 4 and the required transverse energy flow amplitude over a wide bandwidth. The metasurface 4 requires a greater transverse energy flow transfer capability as the incident wavelength is further away from the Littrow wavelength.

[0060] By scanning the total energy flow amplitude corresponding to the metasurface 4 of the supercell structure 5 at different widths, such as... Figure 4 As shown, the widths of supercell structures 5, which are close to the target values, are 207 nm, 116 nm, 302 nm, and 419 nm from left to right.

[0061] The electromagnetic response of metasurface 4 was confirmed by electromagnetic simulation using the finite-difference time-domain method or rigorous coupled-wave analysis, such as... Figure 5As shown, the present invention can achieve broadband Littrow reflection with an efficiency of over 99% at a range of 3.12 to 3.52 μm.

[0062] The structural parameters of the metasurface 4 in this invention are not limited to these. More metaunit structures 5 can be used to achieve stronger lateral energy flow transfer capabilities, and more aperiodic films 3 can be used to achieve a wider range of energy flow modulation. The wavelength of the metasurface 4, the high-refractive-index material H, the low-refractive-index material L, the multilayer film system, and the height of the metaunit structure 5 can all be changed. The multilayer film only needs to provide a reflectivity greater than 99%. The period of the metaunit structure 5 needs to be designed based on the wavelength and the Littrow incident angle. The height of the metaunit structure 5 only needs to ensure that the reflection phase covers 0~2π when the width changes.

[0063] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in the present invention, and these modifications or substitutions should all be covered within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A multilayer film-based energy flow modulation thin film structure, comprising a substrate (1), a multilayer film, and a metasurface (4) arranged from bottom to top, wherein the multilayer film includes a high-reflectivity film (2), and the metasurface (4) includes a super-unit structure (5), characterized in that, The multilayer film also includes an aperiodic film layer (3), which is connected to the high-reflectivity film (2) and the metasurface (4) respectively; broadband energy flow regulation is achieved by adjusting the thickness of the aperiodic film layer (3); The high-reflectivity film (2) and the aperiodic film layer (3) are dielectric film stacks composed of alternating high-refractive-index dielectric film layer H and low-refractive-index dielectric film layer L; the contact side of the aperiodic film layer (3) with the high-reflectivity film (2) and the metasurface (4) is the low-refractive-index dielectric film layer L; The thickness of the non-periodic film layer (3) is selected as follows: Based on the determined width of the super unit structure (5) in the metasurface (4), the Littrow reflection efficiency of the non-periodic film layer (3) at different thicknesses is scanned, and the thickness is locally fine-tuned so that the super unit structure (5) of the metasurface (4) reaches the highest broadband Littrow reflection efficiency. The metasurface (4) includes multiple super-unit periods with a period of p, and the super-unit period includes n super-unit structures (5) with different widths and the same height. The method for selecting the height of the super-unit structure (5) of the metasurface (4) is as follows: The width of a single supercell structure (5) of the metasurface (4) is scanned so that the reflection phase of each supercell structure (5) can cover the range of 0~2π; the phase change is achieved by using n supercell structures (5) with different widths and a supercell period of p; where n≥4 and n is a positive integer; The method for selecting the width of the super-unit structure (5) of the metasurface (4) is as follows: The theoretical values ​​of the Poynting vector inside the metasurface under different incident wavelengths are obtained by rigorous coupled-wave analysis. The width of n super-unit structures (5) in the super-unit period of the metasurface (4) is scanned to obtain the Poynting vector inside the metasurface (4) at this time, so that it matches the target value obtained by theoretical calculation. The high-reflectivity film (2) has a structure of (LH). 7 The non-periodic film layer (3) has an LHL structure.

2. The energy flow modulation thin film structure based on multilayer films according to claim 1, characterized in that, The high-refractive-index dielectric film layer H is made of Si, and the low-refractive-index dielectric film layer L is made of SiO2.

3. The energy flow modulation thin film structure based on multilayer films according to claim 2, characterized in that, The non-periodic film layer (3) comprises a dielectric film stack consisting of three layers of high refractive index dielectric film layer H and low refractive index dielectric film layer L with different thicknesses, arranged alternately from bottom to top as the first low refractive index dielectric film layer L, the first high refractive index dielectric film layer H, and the second low refractive index dielectric film layer L.

4. The energy flow modulation thin film structure based on multilayer films according to claim 1, characterized in that, The period p and the number n of supercell structures (5) are selected using the following formula: In the formula, λ is the Littrow reflection wavelength, and i is the Littrow incident angle.

Images