Broadband wave plate based on sub-wavelength structure and polarization grating

Abstract

The invention provides a broadband wave plate based on a sub-wavelength structure and a polarization grating, and relates to the technical field of optical elements. The broadband wave plate comprises a substrate; the anti-reflection layer is positioned on one side of the substrate; the columnar sub-wavelength structures are positioned on the other side of the substrate and are periodically arranged; wherein the columnar sub-wavelength structure comprises a plurality of medium sub-wavelength structure layers with different refractive indexes. Based on the broadband wave plate and the polarization grating provided by the invention, the columnar sub-wavelength structure comprises the multiple dielectric sub-wavelength structure layers with different refractive indexes, and the refractive index of the columnar sub-wavelength structure can achieve impedance matching with air, so that light reflection is effectively reduced, and the transmittance and the polarization conversion efficiency are improved.

Description

Technical Field

[0001] The present application relates to the technical field of optical elements, and in particular to a broadband wave plate and a polarization grating based on a sub-wavelength structure. Background Art

[0002] Broadband wave plates are optical elements used to adjust the phase of light. They can maintain the stability of phase delay over a wide wavelength range, and therefore play an important role in optical systems. By controlling the phase delay of light of different wavelengths, broadband wave plates can achieve effective regulation and control of light. Broadband wave plates can effectively reduce the dependence of phase delay on wavelength, making them important in many fields such as spectral shaping, polarization imaging, and broadband laser source research. However, existing broadband wave plates are mainly used in visible light and near-infrared bands.

[0003] In the long-wave infrared band (8-14 microns), dielectric silicon materials are widely used to design subwavelength structures because they are compatible with semiconductor processing technology. However, the reflection loss of existing silicon subwavelength structures is strong, which will affect the absolute polarization conversion efficiency of subwavelength structure wave plates. Summary of the invention

[0004] In order to at least overcome the above-mentioned deficiencies in the prior art, the purpose of the present application is to provide a broadband wave plate and a polarization grating based on a subwavelength structure, so as to solve the technical problem that the subwavelength structure of the existing silicon material (especially in the long-wave infrared band) has strong reflection loss, resulting in low polarization conversion efficiency of the corresponding broadband wave plate.

[0005] In a first aspect, an embodiment of the present application provides a broadband wave plate based on a subwavelength structure, the broadband wave plate comprising:

[0006] substrate;

[0007] an anti-reflection layer located on one side of the substrate;

[0008] a periodically arranged columnar sub-wavelength structure located on the other side of the substrate;

[0009] Wherein, the columnar sub-wavelength structure includes multiple layers of medium sub-wavelength structure layers with different refractive indices.

[0010] In the above implementation, since the columnar subwavelength structure includes multiple layers of dielectric subwavelength structure layers with different refractive indices, the refractive index of the columnar subwavelength structure can be adjusted to achieve impedance matching with air to effectively reduce light reflection, improve transmittance and polarization conversion efficiency. This solves the technical problem that the existing dielectric subwavelength structure of silicon material (especially in the long-wave infrared band) has strong reflection loss, resulting in low polarization conversion efficiency of the corresponding broadband wave plate.

[0011] In a possible implementation, the closer the dielectric sub-wavelength structure layer is to the substrate, the greater the refractive index is. In the above implementation, by setting the dielectric sub-wavelength structure layer closer to the substrate to have a greater refractive index, the columnar sub-wavelength structure can achieve impedance matching with the air, thereby effectively reducing light reflection and improving transmittance and polarization conversion efficiency.

[0012] In a possible implementation, the columnar sub-wavelength structure includes: a first dielectric sub-wavelength structure layer made of a silicon material, and a second dielectric sub-wavelength structure layer made of a zinc sulfide material.

[0013] In the above implementation, since the zinc sulfide material has a lower refractive index in the long-wave infrared band and is between silicon and air, silicon, zinc sulfide, and air can form impedance matching, thereby effectively reducing the interface reflection of the subwavelength structure and improving the absolute polarization conversion efficiency of the broadband wave plate and the polarization grating.

[0014] In a possible implementation, the anti-reflection layer includes: a first sublayer and a second sublayer; the first sublayer is located on a side of the second sublayer away from the substrate, the refractive index of the first sublayer is smaller than the refractive index of the second sublayer, and the refractive index of the second sublayer is smaller than the refractive index of the substrate.

[0015] In the above implementation, by providing multiple layers of anti-reflection layers, the transmittance on the substrate side can be further improved.

[0016] In a possible implementation, a plurality of the columnar subwavelength structures are arranged along the second direction, each of the columnar subwavelength structures extends along the first direction, and there is a gap between adjacent columnar subwavelength structures; wherein the first direction and the second direction are perpendicular to each other.

[0017] In a possible implementation, the period of the broadband wave plate includes 1 micron-3 microns; in the direction parallel to the substrate, the width of the columnar subwavelength structure in the second direction includes 0.5 microns-2.5 microns, and the width of the columnar subwavelength structure in the second direction is smaller than the period of the broadband wave plate; in the direction perpendicular to the substrate, the height of the first dielectric subwavelength structure layer includes 2 microns-5 microns, and the height of the second dielectric subwavelength structure layer includes 0.5 microns-2 microns.

[0018] In the above implementation, a quarter wave plate can be realized by setting "the period of the broadband wave plate includes 1 micron-3 microns; in the direction parallel to the substrate, the width of the columnar subwavelength structure in the second direction includes 0.5 microns-2.5 microns; in the direction perpendicular to the substrate, the height of the first medium subwavelength structure layer includes 2 microns-5 microns, and the height of the second medium subwavelength structure layer includes 0.5 microns-2 microns".

[0019] In a possible implementation, the period of the broadband wave plate includes 1 micron-3 microns; in the direction parallel to the substrate, the width of the columnar subwavelength structure in the second direction includes 0.5 microns-2.5 microns, and the width of the columnar subwavelength structure in the second direction is smaller than the period of the broadband wave plate; in the direction perpendicular to the substrate, the height of the first dielectric subwavelength structure layer includes 4 microns-7 microns, and the height of the second dielectric subwavelength structure layer includes 0.5 microns-2 microns.

[0020] In the above implementation, by setting "the period of the broadband wave plate includes 1 micron-3 microns; in the direction parallel to the substrate, the width of the columnar subwavelength structure in the second direction includes 0.5 microns-2.5 microns; in the direction perpendicular to the substrate, the height of the first medium subwavelength structure layer includes 4 microns-7 microns, and the height of the second medium subwavelength structure layer includes 0.5 microns-2 microns", a half wave plate can be realized.

[0021] In a possible implementation, the absolute polarization conversion efficiency of the broadband wave plate in the wavelength range of 8-14 microns comprises 92.4%, and the average relative polarization conversion efficiency comprises 99.8%.

[0022] In a second aspect, an embodiment of the present application further provides a polarization grating, wherein the polarization grating includes a plurality of broadband wave plates as described in any one of the first aspects above.

[0023] In a possible implementation manner, the arrangement directions of the plurality of columnar sub-wavelength structures in each of the broadband wave plates are consistent, and the arrangement directions of the columnar sub-wavelength structures in two adjacent broadband wave plates are different.

[0024] In a possible implementation manner, the arrangement angle intervals between the columnar sub-wavelength structures in any two adjacent broadband wave plates are consistent.

[0025] In a possible implementation, a period of the polarization grating comprises 500 micrometers.

[0026] Based on any of the above aspects, the broadband wave plate and polarization grating based on the subwavelength structure provided by the present application, because the columnar subwavelength structure includes multiple layers of dielectric subwavelength structure layers with different refractive indices, the refractive index of the columnar subwavelength structure can achieve impedance matching with air, so as to effectively reduce the reflection of light and improve the transmittance and polarization conversion efficiency. This solves the technical problem that the reflection loss of the existing dielectric subwavelength structure of silicon material (especially in the long-wave infrared band) is strong, resulting in low polarization conversion efficiency of the corresponding broadband wave plate.

[0027] In addition, the broadband wave plate and polarization grating based on the subwavelength structure provided in the present application can also realize the functions of a quarter-wave plate and a half-wave plate within the infrared broadband band range (8 microns-14 microns) by changing the geometric dimensions of the cylindrical subwavelength structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings required in the embodiments will be briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present application and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without creative work.

[0029] Figure 1 One of the structural schematic diagrams of the broadband wave plate provided in this embodiment;

[0030] Figure 2 The second schematic diagram of the structure of the broadband wave plate provided in this embodiment;

[0031] Figure 3 The third schematic diagram of the structure of the broadband wave plate provided in this embodiment;

[0032] Figure 4 A schematic diagram showing a comparison of polarization conversion efficiencies of the quarter-wave plates provided in this embodiment;

[0033] Figure 5 A schematic diagram showing a comparison of polarization conversion efficiencies of the half-wave plates provided in this embodiment;

[0034] Figure 6 A schematic diagram of the structure of the polarization grating provided in this embodiment;

[0035] Figure 7 A schematic diagram of the diffraction efficiency simulation of the polarization grating provided in this embodiment.

[0036] Icon: 100 - broadband wave plate; 110 - substrate; 120 - anti-reflection layer; 121 - first sublayer; 122 - second sublayer; 130 - columnar subwavelength structure; 131 - first dielectric subwavelength structure layer; 132 - second dielectric subwavelength structure layer. DETAILED DESCRIPTION

[0037] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. The components of the embodiments of the present application described and shown in the drawings here can be arranged and designed in various different configurations.

[0038] Therefore, the following detailed description of the embodiments of the present application provided in the accompanying drawings is not intended to limit the scope of the present application for which protection is sought, but merely represents selected embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in the field without creative work are within the scope of protection of the present application.

[0039] It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, further definition and explanation thereof is not required in subsequent drawings.

[0040] In the description of this application, it should be noted that the terms "upper", "lower", etc. indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship in which the product of the application is usually placed when in use, which is only for the convenience of describing this application and simplifying the description, and does not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on this application. In addition, the terms "first", "second", etc. are only used to distinguish the description, and cannot be understood as indicating or implying relative importance.

[0041] In addition, the terms "horizontal", "vertical", "overhanging" and the like do not mean that the components are required to be absolutely horizontal or overhanging, but can be slightly tilted. For example, "horizontal" only means that its direction is more horizontal than "vertical", and does not mean that the structure must be completely horizontal, but can be slightly tilted.

[0042] In the description of this application, it should also be noted that, unless otherwise clearly specified and limited, the terms "set", "install", "connect", and "connect" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two elements. For ordinary technicians in this field, the specific meanings of the above terms in this application can be understood according to specific circumstances.

[0043] It should be noted that, in the absence of conflict, different features in the embodiments of the present application may be combined with each other.

[0044] Among them, since the subwavelength structure has three-dimensional morphological characteristics, its optical performance is determined by the width and depth of the structural groove. The traditional anti-reflection film is composed of a multilayer film of different dielectric materials. If the anti-reflection film is directly plated on the dielectric subwavelength structure, the width and depth of the structural groove will change, and the change in the broadband will be uneven and difficult to control, affecting the optical performance. Therefore, it is impossible to use the traditional anti-reflection film method to reduce the reflection loss of the existing subwavelength structure.

[0045] After discovering this problem, the present application designs a columnar sub-wavelength structure including multiple layers of medium sub-wavelength structures with different refractive indices, and realizes a broadband wave plate and a polarization grating with high polarization conversion efficiency.

[0046] This embodiment provides a solution that can solve the above-mentioned problem. The specific implementation methods of this application are described in detail below in conjunction with the accompanying drawings.

[0047] Please refer to Figure 1 , Figure 1 FIG. 1 is a schematic diagram of the structure of the broadband wave plate 100 provided in this embodiment. Figure 1 As shown, the broadband wave plate 100 may include:

[0048] Substrate 110;

[0049] an anti-reflection layer 120 located on one side of the substrate 110;

[0050] A periodically arranged columnar sub-wavelength structure 130 located on the other side of the substrate 110;

[0051] The columnar sub-wavelength structure 130 includes multiple dielectric sub-wavelength structure layers with different refractive indices.

[0052] Among them, the broadband wave plate 100 can be an infrared broadband wave plate. The anti-reflection layer 120 can completely cover the surface of one side of the substrate 110, and the anti-reflection layer 120 can include a multilayer film structure to improve the transmittance on one side of the substrate 110. The anti-reflection layer 120 can be plated by a variety of thin film deposition techniques (such as physical vapor deposition, chemical vapor deposition, etc.), so that the thickness of the anti-reflection layer 120 can be flexibly adjusted to optimize its optical performance. The columnar subwavelength structure 130 can be located on the side of the substrate 110 away from the anti-reflection layer 120. The columnar subwavelength structure 130 can be periodically extended as a unit structure, and the shape of the columnar subwavelength structure 130 is columnar. The number of dielectric subwavelength structure layers included in the columnar subwavelength structure 130 can be 2 layers, or 3 layers, 5 layers or other reasonable values. The specific number of dielectric subwavelength structure layers can be adjusted according to the actual application scenario. In practical applications, the polarized light may be incident from a side of the anti-reflection layer 120 away from the substrate 110 , or may be incident from a side of the columnar sub-wavelength structure 130 away from the substrate 110 .

[0053] For example, Figure 1 As shown, the columnar sub-wavelength structure 130 can extend along the first direction D1, and the plurality of columnar sub-wavelength structures 130 can be arranged along the second direction D2. There is a gap between any two adjacent columnar sub-wavelength structures 130, and the gaps between the two adjacent columnar sub-wavelength structures 130 are generally equal. The first direction D1 is perpendicular to the second direction D2. The orthographic projections of the plurality of columnar sub-wavelength structures 130 on the substrate 110 can be located within the orthographic projection of the anti-reflection layer 120 on the substrate 110. It should be noted that, Figure 1 Only three columnar sub-wavelength structures 130 are shown as examples. In other examples, the number of columnar sub-wavelength structures 130 is not limited to three, but may be two or more, which is not specifically limited here. In addition, the widths of the plurality of columnar sub-wavelength structures 130 in the second direction D2 may be consistent; since the lateral dimensions of the columnar sub-wavelength structures 130 are consistent, the etching process may be directly used for processing.

[0054] In some optional embodiments, the closer the dielectric sub-wavelength structure layer is to the substrate 110 , the greater the refractive index is.

[0055] By setting the refractive index of the dielectric subwavelength structure layer closer to the substrate 110 to be larger, the columnar subwavelength structure 130 can achieve impedance matching with the air, thereby effectively reducing light reflection and improving transmittance and polarization conversion efficiency.

[0056] Please refer to Figure 2 , Figure 2 This is the second structural schematic diagram of the broadband wave plate provided in this embodiment.

[0057] like Figure 2 As shown, in some optional embodiments, the columnar subwavelength structure 130 may include: a first dielectric subwavelength structure layer 131 made of a silicon material, and a second dielectric subwavelength structure layer 132 made of a zinc sulfide material.

[0058] Among them, the columnar subwavelength structure 130 can be formed by first uniformly depositing the second dielectric subwavelength structure layer 132 on the surface of the first dielectric subwavelength structure layer 131, then coating the surface of the second dielectric subwavelength structure layer 132 with photoresist, then patterning the photoresist using laser direct writing technology, and finally forming it through an etching process. Since the zinc sulfide material has a lower refractive index in the long-wave infrared band and is between silicon and air; silicon, zinc sulfide, and air can form impedance matching, thereby effectively reducing the interface reflection of the subwavelength structure, and improving the absolute polarization conversion efficiency of the broadband wave plate and the polarization grating. It should be noted that the manufacturing material of the first dielectric subwavelength structure layer 131 can also include germanium material, and the manufacturing material of the second dielectric subwavelength structure layer 132 can also include ytterbium fluoride, which is not specifically limited in this application.

[0059] In this embodiment, since there is a large refractive index difference between the first dielectric subwavelength structure layer 131 made of silicon material and the air, it is easy to cause strong reflection loss. Therefore, a second dielectric subwavelength structure layer 132 made of zinc sulfide material can be added between the first dielectric subwavelength structure layer 131 and the air to form impedance matching, thereby reducing reflection, increasing transmittance, and effectively controlling the phase delay of light.

[0060] The second dielectric subwavelength structure layer 132 may be located on a side of the columnar subwavelength structure 130 away from the substrate 110. The orthographic projection of the second dielectric subwavelength structure layer 132 on the substrate 110 may completely overlap with the orthographic projection of the first dielectric subwavelength structure layer 131 on the substrate 110.

[0061] The refractive index of the substrate 110 may be the same as the refractive index of the first dielectric sub-wavelength structure layer 131, and the manufacturing material of the substrate 110 may also be the same as the manufacturing material of the first dielectric sub-wavelength structure layer 131. The manufacturing materials of the substrate 110 and the first dielectric sub-wavelength structure layer 131 may both be light-transmitting materials, for example, silicon or germanium (wherein the light-transmitting properties of germanium are mainly concentrated in the infrared spectrum range, and have high light transmittance to infrared light). Silicon has a high refractive index in the long-wave infrared band (8 microns-14 microns) and is compatible with semiconductor processing technology. The manufacturing material of the second dielectric sub-wavelength structure layer 132 is not limited to zinc sulfide, and may also include other materials, which are not specifically limited here, as long as the refractive index of the second dielectric sub-wavelength structure layer 132 is between the refractive index of the first dielectric sub-wavelength structure layer 131 and the refractive index of air.

[0062] In addition, the materials of the substrate 110 and the first dielectric subwavelength structure layer 131 are not limited to silicon, but may also include other materials, which are not specifically limited here, as long as the refractive index of the substrate 110 is greater than the refractive index of each film layer in the anti-reflection layer 120, and the refractive index of the first dielectric subwavelength structure layer 131 is greater than the refractive index of the second dielectric subwavelength structure layer 132.

[0063] In another possible implementation, the second medium subwavelength structure layer 132 can be further divided into multiple subwavelength structure layers, and the refractive index of the multiple subwavelength structure layers is different, and the refractive index of each subwavelength structure layer is between the first medium subwavelength structure layer 131 and air.

[0064] In some optional embodiments, the anti-reflection layer 120 includes: a first sublayer 121 and a second sublayer 122; the first sublayer 121 may be located on a side of the second sublayer 122 away from the substrate 110, and form a multilayer film structure. The refractive index of the first sublayer 121 is smaller than the refractive index of the second sublayer 122, and the refractive index of the second sublayer 122 is smaller than the refractive index of the substrate 110.

[0065] It should be noted that the film layers included in the anti-reflection layer 120 are not limited to the first sub-layer 121 and the second sub-layer 122. The anti-reflection layer 120 may also include a third sub-layer, a fourth sub-layer, etc., which is not specifically limited here, as long as the multiple film layers included in the anti-reflection layer 120 can achieve the effect of increasing the transmittance.

[0066] In some optional embodiments, a plurality of columnar subwavelength structures 130 are arranged along the second direction D2, each columnar subwavelength structure 130 extends along the first direction D1, and there is a gap between adjacent columnar subwavelength structures 130; wherein the first direction D1 and the second direction D2 are perpendicular to each other.

[0067] In the above structure, changing the geometric dimensions of the columnar subwavelength structure 130 can realize functions such as a quarter-wave plate and a half-wave plate within the infrared broadband band (8 microns-14 microns), and because the refractive index of the columnar subwavelength structure 130 can achieve impedance matching with air, it can effectively reduce light reflection and improve transmittance and polarization conversion efficiency.

[0068] Please refer to Figure 3 , Figure 3 This is the third structural schematic diagram of the broadband wave plate provided in this embodiment.

[0069] In one possible implementation, Figure 3As shown, when the broadband wave plate 100 is used to realize the function of a quarter wave plate within the infrared broadband band range (8 microns-14 microns), the period T of the broadband wave plate 100 may include 1 micron-3 microns, and in the direction parallel to the substrate 110, the width L of each columnar subwavelength structure 130 in the second direction D2 may include 0.5 microns-2.5 microns, and in the direction perpendicular to the substrate 110, the height H1 of each first dielectric subwavelength structure layer 131 may include 2 microns-5 microns, and the height H2 of the second dielectric subwavelength structure layer 132 may include 0.5 microns-2 microns. The quarter wave plate can cause a phase delay of π / 2 between the fast axis and the slow axis, and one of its main functions is to realize the conversion between different types of polarized light, including the conversion of linear polarized light to circular polarized light.

[0070] Among them, when the broadband wave plate 100 is used to realize the function of a quarter wave plate within the infrared broadband band range (8 microns-14 microns), the period T of the broadband wave plate 100 can be specifically 1 micron, 2 microns or 3 microns. The width L of each columnar subwavelength structure 130 in the second direction D2 can be specifically 0.5 microns, 1.6 microns or 2.5 microns. In the direction perpendicular to the substrate 110, the height H1 of each first dielectric subwavelength structure layer 131 can be specifically 2 microns, 3.2 microns or 5 microns. In the direction perpendicular to the substrate 110, the height H2 of each second dielectric subwavelength structure layer 132 can be specifically 0.5 microns, 1 micron or 2 microns. Exemplarily, when the broadband wave plate 100 is used to realize the function of a quarter wave plate within the infrared broadband band range (8 microns-14 microns), when the period T of the broadband wave plate 100 is 2 microns: in the direction parallel to the substrate 110, the width L of each columnar subwavelength structure 130 in the second direction D2 can be specifically 1.6 microns; in the direction perpendicular to the substrate 110, the height H1 of each first dielectric subwavelength structure layer 131 can be specifically 3.2 microns, and the height H2 of the second dielectric subwavelength structure layer 132 can be specifically 1 micron.

[0071] Please refer to Figure 4 , Figure 4 A schematic diagram showing a comparison of polarization conversion efficiencies of the quarter-wave plates provided in this embodiment. Figure 4A schematic diagram comparing the absolute polarization conversion efficiency and relative polarization conversion efficiency of a subwavelength structure quarter wave plate of a silicon-zinc sulfide combination and a full silicon subwavelength structure quarter wave plate is shown in the following example. The subwavelength structure of the silicon-zinc sulfide combination corresponds to the case where "the columnar subwavelength structure 130 includes: a first dielectric subwavelength structure layer 131 whose manufacturing material includes a silicon material, and a second dielectric subwavelength structure layer 132 whose manufacturing material includes a zinc sulfide material". In the wavelength range of 8-14 microns, the average absolute polarization conversion efficiency of the silicon-zinc sulfide combination structure quarter wave plate is 92.4%, the average relative polarization conversion efficiency is 99.8%, and the average transmittance is 92.6%. In the wavelength range of 8-14 microns, the average absolute polarization conversion efficiency of the full silicon structure quarter wave plate is 80%, the average relative polarization conversion efficiency is 98.7%, and the average transmittance is 81%. Among them, the relative / absolute polarization conversion efficiency is defined as the ratio of the energy of the outgoing target polarized light to the energy of the transmitted / incident light. It can be seen that the average absolute polarization conversion efficiency of the subwavelength structure of the silicon-zinc sulfide combination can be increased by more than 12% in the wavelength range of 8-14 microns compared with the traditional subwavelength structure.

[0072] In one possible implementation, please refer again to Figure 3 When the broadband wave plate 100 is used to realize the function of a half-wave plate in the infrared broadband band range (8 microns-14 microns), the period T of the broadband wave plate 100 may include 1 micron-3 microns, and in the direction parallel to the substrate 110, the width L of the columnar sub-wavelength structure 130 in the second direction D2 may include 0.5 microns-2.5 microns, and in the direction perpendicular to the substrate 110, the height H1 of the first dielectric sub-wavelength structure layer 131 may include 4 microns-7 microns, and the height H2 of the second dielectric sub-wavelength structure layer 132 may include 0.5 microns-2 microns. The half-wave plate can cause a phase delay of π between the fast axis and the slow axis, and one of its main functions is to realize the conversion between different types of polarized light, including the conversion from circular polarization to cross-circular polarization.

[0073] Among them, when the broadband wave plate 100 is used to realize the function of a half-wave plate within the infrared broadband band range (8 microns-14 microns), the period T of the broadband wave plate 100 can be specifically 1 micron, 2.4 microns or 3 microns. The width L of each columnar sub-wavelength structure 130 in the second direction D2 can be specifically 0.5 microns, 1.57 microns or 2.5 microns. In the direction perpendicular to the substrate 110, the height H1 of each first dielectric sub-wavelength structure layer 131 can be specifically 4 microns, 5.7 microns or 7 microns. In the direction perpendicular to the substrate 110, the height H2 of each second dielectric sub-wavelength structure layer 132 can be specifically 0.5 microns, 1 micron or 2 microns. Exemplarily, when the broadband wave plate 100 is used to realize the function of a half-wave plate within the infrared broadband band range (8 microns-14 microns), when the period T of the broadband wave plate 100 is 2.4 microns: in the direction parallel to the substrate 110, the width L of each columnar subwavelength structure 130 in the second direction D2 can be specifically 1.57 microns; in the direction perpendicular to the substrate 110, the height H1 of each first dielectric subwavelength structure layer 131 can be specifically 5.7 microns, and the height H2 of the second dielectric subwavelength structure layer 132 can be specifically 1 micron.

[0074] Please refer to Figure 5 , Figure 5 A schematic diagram comparing the polarization conversion efficiency of the half-wave plate provided in this embodiment. Figure 5 A specific example is a schematic diagram comparing the absolute polarization conversion efficiency and relative polarization conversion efficiency of the sub-wavelength structure half-wave plate of the silicon-zinc sulfide combination and the half-wave plate of the all-silicon sub-wavelength structure. In the wavelength range of 8-14 microns, the average absolute polarization conversion efficiency of the silicon-zinc sulfide combination structure half-wave plate is 91.3%, the average relative polarization conversion efficiency is 99.3%, and the average transmittance is 91.9%. In the wavelength range of 8-14 microns, the average absolute polarization conversion efficiency of the all-silicon structure half-wave plate is 82.5%, the average relative polarization conversion efficiency is 98.7%, and the average transmittance is 83.6%. It can be seen that the average absolute polarization conversion efficiency of the sub-wavelength structure of the silicon-zinc sulfide combination can be improved by more than 10% compared with the traditional sub-wavelength structure in the wavelength range of 8-14 microns.

[0075] It should be noted that the size of the broadband wave plate 100 can be adjusted according to actual needs and is not specifically limited here. For example, the geometric size of the columnar sub-wavelength structure 130 can be adjusted according to actual manufacturing materials and design band optimization.

[0076] like Figure 4 , Figure 5As shown, the average absolute polarization conversion efficiency of the existing broadband wave plate in the wavelength range of 8-14 microns is about 82%. Compared with the prior art, the average absolute polarization conversion efficiency and the average relative polarization conversion efficiency of the broadband wave plate 100 provided by the present application are both greater than those of the broadband wave plate in the prior art. The average absolute polarization conversion efficiency of the broadband wave plate 100 provided by the present application in the wavelength range of 8-14 microns can reach 92.4%, and the average relative polarization conversion efficiency can reach 99.8%.

[0077] Please refer to Figure 6 , Figure 6 A schematic diagram of the structure of the polarization grating provided in this embodiment.

[0078] An embodiment of the present application further provides a polarization grating, which may include a plurality of broadband wave plates 100 as described in any one of the first aspects above.

[0079] Specifically, the polarization grating may be formed by the half-wave plate provided in the above embodiment.

[0080] In a possible implementation, the polarization grating may include a plurality of broadband wave plates 100 , the plurality of columnar subwavelength structures on each broadband wave plate 100 are arranged in the same direction, and the columnar subwavelength structures on two adjacent broadband wave plates 100 are arranged in different directions.

[0081] like Figure 6 As shown, a plurality of broadband wave plates 100 may be arranged along a first direction D1, and the arrangement directions of the columnar sub-wavelength structures on adjacent broadband wave plates 100 are different.

[0082] In a possible implementation, the arrangement angle intervals between the columnar sub-wavelength structures in any two adjacent broadband wave plates 100 are consistent.

[0083] In a possible implementation, the polarization grating may be formed by rotating and arranging the half-wave plates. The length of the polarization grating in the first direction D1 may be 500 micrometers, and the length of the polarization grating in the second direction D2 may be 100 micrometers.

[0084] In a possible implementation, when the coverage phase of the polarization grating is 0-2π and the number of broadband wave plates 100 is 10, the arrangement angle interval between the columnar sub-wavelength structures in any two adjacent broadband wave plates 100 may be 18°. It should be noted that the number of broadband wave plates 100 is not limited to 10, and may also include 16, 8, etc., which is not specifically limited here.

[0085] In a possible implementation, the period of the polarization grating may include 500 microns.

[0086] In order to verify the effectiveness of this application, an experimental verification was carried out on this application.

[0087] Please refer to Figure 7 , Figure 7 A schematic diagram of the diffraction efficiency simulation of the polarization grating provided in this embodiment. After modulation by the polarization grating, the incident left-handed circularly polarized light (LCP) is converted into right-handed circularly polarized light (RCP) and diffracted to the -1 order. The absolute diffraction efficiency of the polarization grating in the wavelength range of 8-14 microns is 95%. The absolute diffraction efficiency is defined as the ratio of the energy of the -1 diffraction order to the energy of the incident circularly polarized light.

[0088] In summary, this embodiment provides a broadband wave plate and polarization grating based on a subwavelength structure, by setting an anti-reflection layer 120 on one side of the substrate 110 and setting a columnar subwavelength structure 130 on the other side of the substrate 110. Since the columnar subwavelength structure includes multiple layers of medium subwavelength structure layers with different refractive indices, the refractive index of the columnar subwavelength structure can achieve impedance matching with air, so as to effectively reduce light reflection and improve transmittance and polarization conversion efficiency. It is also possible to realize functions such as a quarter wave plate and a half wave plate within the infrared broadband band (8 microns-14 microns), and use a half wave plate to make a polarization grating.

[0089] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, the elements defined by the sentence "comprise a ..." do not exclude the existence of other identical elements in the process, method, article or device including the elements.

[0090] The above description is only the preferred embodiment of the present application and is not intended to limit the present application. For those skilled in the art, the present application may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A broadband wave plate, characterized in that: The broadband wave plate comprises: substrate; an anti-reflection layer located on one side of the substrate; a periodically arranged columnar sub-wavelength structure located on the other side of the substrate; Wherein, the columnar sub-wavelength structure includes multiple layers of medium sub-wavelength structure layers with different refractive indices.

2. The broadband wave plate according to claim 1, characterized in that: in, The closer the dielectric sub-wavelength structure layer is to the substrate, the greater the refractive index is.

3. The broadband wave plate according to claim 1, characterized in that: The columnar sub-wavelength structure includes: a first dielectric sub-wavelength structure layer made of a silicon material, and a second dielectric sub-wavelength structure layer made of a zinc sulfide material.

4. The broadband wave plate according to claim 1, characterized in that: The anti-reflection layer comprises: a first sub-layer and a second sub-layer; The first sublayer is located on a side of the second sublayer away from the substrate, the refractive index of the first sublayer is smaller than the refractive index of the second sublayer, and the refractive index of the second sublayer is smaller than the refractive index of the substrate.

5. The broadband wave plate according to any one of claims 1 to 4, characterized in that: in, A plurality of the columnar sub-wavelength structures are arranged along the second direction, each of the columnar sub-wavelength structures extends along the first direction, and there is a gap between adjacent columnar sub-wavelength structures; The first direction and the second direction are perpendicular to each other.

6. The broadband wave plate according to claim 3, characterized in that: The period of the broadband wave plate includes 1 micron-3 microns; In a direction parallel to the substrate, the width of the columnar sub-wavelength structure in the second direction includes 0.5 micrometers to 2.5 micrometers, and the width of the columnar sub-wavelength structure in the second direction is smaller than the period of the broadband wave plate; In a direction perpendicular to the substrate, a height of the first dielectric sub-wavelength structure layer is between 2 micrometers and 5 micrometers, and a height of the second dielectric sub-wavelength structure layer is between 0.5 micrometers and 2 micrometers.

7. The broadband wave plate according to claim 3, characterized in that: The period of the broadband wave plate includes 1 micron-3 microns; In a direction parallel to the substrate, the width of the columnar sub-wavelength structure in the second direction includes 0.5 micrometers to 2.5 micrometers, and the width of the columnar sub-wavelength structure in the second direction is smaller than the period of the broadband wave plate; In a direction perpendicular to the substrate, a height of the first dielectric sub-wavelength structure layer is between 4 micrometers and 7 micrometers, and a height of the second dielectric sub-wavelength structure layer is between 0.5 micrometers and 2 micrometers.

8. A polarization grating, characterized in that: The polarization grating comprises: a plurality of broadband wave plates as described in any one of claims 1 to 7.

9. The polarization grating according to claim 8, characterized in that: in, The arrangement directions of the plurality of columnar sub-wavelength structures in each of the broadband wave plates are consistent, and the arrangement directions of the columnar sub-wavelength structures in two adjacent broadband wave plates are different.

10. The polarization grating according to claim 9, characterized in that: in, The arrangement angle intervals between the columnar sub-wavelength structures in any two adjacent broadband wave plates are consistent.

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