A packaged sub-wavelength grating polarization filter and its manufacturing method

Abstract

The present invention provides a packaged sub-wavelength grating polarization filter and a manufacturing method thereof. The polarizer comprises a double-layer anti-reflection film, an infrared material substrate, a dielectric grating, and a double-layer metal film. The double-layer metal film covers the bottom of the dielectric grating groove and the top of the grating line respectively. The grating period is much smaller than the wavelength of the incident light. The rectangular grating groove is directly manufactured on the substrate and then the metal film is plated on the grating surface. The grating groove is encapsulated with the same material as the substrate. Because it has the same refractive index, the encapsulation layer can reduce light reflection loss at the grating side wall interface, thereby improving the transmittance of TM polarized light. At the same time, it can also greatly protect the metal film. The double-layer anti-reflection film is plated on both sides of the packaged grating to solve the problem of low transmittance of long-wave infrared materials. In the long-wave infrared band, the polarization filter has the characteristics of wide band, high transmittance, high extinction ratio, etc.

Description

Technical Field

[0001] The present invention belongs to the technical field of micro-nano processing of sub-wavelength gratings, and in particular relates to a packaged sub-wavelength grating polarization filter and a manufacturing method thereof. Background Art

[0002] Traditional infrared subwavelength grating polarizers suffer from low transmittance for TM polarized light, a low extinction ratio, and the inability to simultaneously improve both transmittance and extinction ratio. Currently, infrared polarization filters primarily operate in the 3-5 μm band, but research and design efforts in the long-wave infrared band are scarce. Designing polarization filters for the 7-14 μm band, one of the three critical atmospheric windows, holds significant application value in target identification and military applications. To meet these application needs, there is an urgent need to design polarization filters with both high transmittance and high extinction ratio.

[0003] One of the key challenges in the development of subwavelength metallic polarization gratings is balancing extinction ratio and TM transmittance. Achieving a high extinction ratio and low insertion loss requires each metal grating line to have a high aspect ratio and smooth sidewalls. Current polarizers primarily utilize metal nanowire structures fabricated on the surface of infrared-transmitting materials, such as KRS-5, GaF2, BaF2, and Si. KRS-5, GaF2, and BaF2 are brittle and easily hygroscopic, and these halide materials lack chemical durability, mechanical strength, processability, and adhesion to metal. Si's high refractive index requires an anti-reflection coating. One way to improve polarizer performance is to add an anti-reflection coating between the grating structure and the substrate. Possible materials include SiO, ZnS, and polymer films. Alternatively, surface anti-reflection structures can be applied on both sides of the substrate to improve polarization transmittance. For the long-wave infrared, the main substrate materials available include ZnSe, ZnS, Ge, and Si. ZnSe has high transmittance but is relatively toxic and soft, requiring a hard coating. The manufacturing process of infrared polarization filters mainly includes: holographic exposure, electron beam lithography, nanoimprint lithography, etc.

[0004] In order to improve the transmittance, the methods usually adopted are: adding an anti-reflection layer to utilize FP-like cavity resonance to enhance transmittance, selecting a substrate material with high transmittance, coating a multi-layer anti-reflection film, etc.

[0005] In order to improve the extinction ratio, the commonly used methods include: selecting aluminum film with greater long-wave infrared absorption and reflection for coating, reducing the grating period, improving the steepness of the rectangular side wall, and increasing the aspect ratio of the grating groove.

[0006] The current performance constraints of infrared polarization filters include: deformation of the grating groove, residual aluminum film on the sidewalls, high reflection of the substrate material, the influence of test system noise, and easy oxidation of the aluminum film.

[0007] However, there are few reports on the use of packaged gratings to achieve polarization filtering in the infrared band. Summary of the Invention

[0008] To solve the above technical problems, the present invention provides a packaged sub-wavelength grating polarization filter and a manufacturing method thereof, which operates in the long-wave infrared band and overcomes the problem that existing infrared polarization filters are difficult to achieve both high transmittance and high extinction ratio.

[0009] A packaged sub-wavelength grating polarization filter comprises a base material made of infrared material, a grating made of the same material as the base material, a double-layer metal film, double-layer first and second anti-reflection films, and a packaging material identical to the base material. Starting from the incident direction of the incident light, the first anti-reflection film, the second anti-reflection film, the base material, the second anti-reflection film, and the first anti-reflection film are sequentially arranged. The base material has rectangular sub-wavelength grating grooves within it. A layer of metal film is evaporated on the bottom and top of the sub-wavelength grating grooves. The sub-wavelength grating grooves are packaged with the same packaging material as the base material to form a packaging layer. The double-layer first and second anti-reflection films are symmetrically plated on both sides of the packaged substrate.

[0010] Furthermore, the packaging material has the same refractive index as the grating material and the substrate material.

[0011] Furthermore, the base material has good optical properties in the long-wave infrared band.

[0012] Furthermore, the thickness of the packaging layer is greater than the depth of the grating groove.

[0013] Furthermore, the grating period is 200-1000 nm; the grating groove depth is 200-600 nm; and the thickness of each layer of the double-layer metal film is 60-120 nm.

[0014] Furthermore, the substrate material is one of the following: silicon, germanium; the metal film material is one of the following: aluminum, gold, chromium.

[0015] Furthermore, the two high and low refractive index materials of the first and second double-layer antireflection films are zinc sulfide and ytterbium fluoride respectively; wherein the first antireflection film is zinc sulfide and the second antireflection film is ytterbium fluoride.

[0016] The present invention also provides a method for manufacturing a packaged sub-wavelength grating polarization filter, comprising the following steps:

[0017] (1) Directly fabricating rectangular sub-wavelength grating grooves on the substrate material;

[0018] (2) evaporating a layer of metal film on the bottom of the grating groove and the top of the grating;

[0019] (3) Encapsulating the grating groove with the same encapsulation material as the substrate material;

[0020] (4) A double layer of first and second antireflection films are symmetrically plated on both sides of the packaged substrate.

[0021] Beneficial effects:

[0022] The present invention directly manufactures a subwavelength grating structure on an infrared material substrate, then evaporates a layer of aluminum film, and then encapsulates the grating with the same material as the substrate. On the one hand, this can protect the metal film and improve the extinction ratio. On the other hand, since the encapsulation material and the grating material have the same refractive index, reflection loss is reduced, which can greatly improve the transmittance. A double-layer infrared anti-reflection film is plated on both sides of the encapsulated grating, which can reduce the reflection at the double-layer interface of the infrared material substrate. At the same time, it can improve the mechanical strength of the polarization filter and extend the service life. It has important significance for fields such as polarization imaging in harsh environments. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 4 is a schematic structural diagram of a packaged sub-wavelength grating polarization filter in an embodiment of the present invention.

[0024] Figure 2 This is a schematic diagram of the structure of a packaged sub-wavelength grating polarization filter without an anti-reflection film.

[0025] Figure 3 This is a graph showing how the transmittance of TM polarized light of the packaged sub-wavelength grating polarization filter in the first embodiment of the present invention changes with the wavelength of the incident light when the grating groove depth changes.

[0026] Figure 4 This is a graph showing how the extinction ratio of the packaged sub-wavelength grating polarization filter in the first embodiment of the present invention changes with the wavelength of the incident light when the grating groove depth changes.

[0027] Figure 5 This is a graph showing how the transmittance of TM polarized light of the packaged sub-wavelength grating polarization filter in the second embodiment of the present invention varies with the wavelength of the incident light at different grating periods.

[0028] Figure 6 4 is a graph showing how the extinction ratio of the packaged sub-wavelength grating polarization filter in the second embodiment of the present invention varies with the wavelength of the incident light at different periods.

[0029] Figure 7 This is a graph showing how the transmittance of TM polarized light of the packaged sub-wavelength grating polarization filter in Example 3 of the present invention varies with the wavelength of the incident light when the aluminum film thickness is different.

[0030] Figure 8 This is a graph showing how the extinction ratio of the packaged sub-wavelength grating polarization filter in the third embodiment of the present invention varies with the wavelength of the incident light at different aluminum film thicknesses.

[0031] Figure 93 is a graph showing how the transmittance of TM polarized light of the packaged sub-wavelength grating polarization filter in the fourth embodiment of the present invention varies with the wavelength of the incident light at different duty ratios.

[0032] Figure 10 4 is a graph showing how the extinction ratio of the packaged sub-wavelength grating polarization filter in the fourth embodiment of the present invention varies with the wavelength of the incident light at different duty ratios.

[0033] Figure 11 This is a graph showing how the transmittance of TM polarized light of the packaged sub-wavelength grating polarization filter in Example 5 of the present invention changes with the wavelength of the incident light when the filter is coated with an anti-reflection film and when it is not coated with an anti-reflection film. DETAILED DESCRIPTION

[0034] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely for the purpose of explaining the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0035] like Figure 2 As shown, the packaged subwavelength grating polarization filter of the present invention comprises a substrate material 14 and a metal film 15 with a thickness of h4. Subwavelength grating rectangular grooves are directly fabricated on the substrate material 14 made of infrared material. A layer of metal film 15 is evaporated on the bottom and top of the subwavelength grating grooves. The rectangular grating grooves are then encapsulated with the same material as the substrate material 14. The depth of the rectangular grating grooves is h3.

[0036] like Figure 1 As shown, a double-layer infrared anti-reflection film - a first material 12 and a second material 13 - is symmetrically coated on both sides of the packaged grating, which can reduce the reflection of the double-layer interface of the substrate material 14 and improve the mechanical strength of the polarization filter. Here, p is the grating period, w represents the grating groove width, and θ represents the angle between the incident light and the normal direction of the substrate plane.

[0037] The packaged sub-wavelength grating polarization filter of the present invention includes two layers of first material 12 with a thickness of h1 and two layers of second material 13 with a thickness of h2. Starting from the incidence of incident light 11, the first material 12, the second material 13, the base material 14, the second material 13, and the first material 12 are arranged in sequence.

[0038] The substrate is made of silicon (Si) or germanium (Ge), and the metal film 15 is made of aluminum, gold, or chromium. The double-layer infrared broadband antireflection film is made of zinc sulfide (ZnS) and ytterbium fluoride (YbF3), both common infrared antireflection film materials with excellent optical properties.

[0039] The grating period is 200-1000nm; the grating groove depth is 200-600nm; the thickness of the metal film 15 is 60-120nm; the packaging material is the same as the base material 14, and the thickness is greater than the grating groove depth; the grating width ratio is 0.3-0.7.

[0040] The manufacturing method for the packaged sub-wavelength grating polarization filter includes three steps: holographic exposure, ICP etching, and metal thermal evaporation. The desired grating is etched onto a substrate 14 using ICP, followed by thermal evaporation of a metal film 15. Finally, the rectangular grooves of the grating are filled with the same material as the substrate 14 to encapsulate the grating.

[0041] The infrared broadband antireflection coating requires at least two layers of thin films with high and low refractive indices. To maintain the simplicity of the design, two layers were used: ZnS for the high refractive index and YbF3 for the low refractive index. Using a silicon substrate as an example, film system software optimization yielded a ZnS thickness of 857nm and a YbF3 thickness of 1267nm.

[0042] The subwavelength grating of the present invention can achieve excellent transmittance at 7-14 μm when considering the structure alone without considering the double-interface reflection of the substrate. The substrate material 14 is silicon, the grating period is 600 nm, the grating groove depth is 450 nm, the thickness of the metal film 15 using aluminum film is 120 nm, the aspect ratio is 0.5, the TM transmittance is higher than 90%, and the extinction ratio is higher than 47 dB. Compared with the current design of high line density to achieve high extinction ratio, it has lower processing difficulty. Taking into account the absorption of the anti-reflection film, the single-side transmittance can reach up to 96%. In theory, for the anti-reflection design of the infrared wide band, it should be composed of five or more layers of film, which can greatly reduce reflection loss, thereby further improving the transmittance of TM polarized light.

[0043] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0044] Example 1:

[0045] See also Figure 2 , which is a schematic diagram of a packaged sub-wavelength grating polarization filter (no anti-reflection film on both sides).

[0046] The substrate material 14 is silicon, the grating period p = 600 nm, w = 300 nm, the incident light 11 is TM or TE polarized light incident normally on the substrate with a wavelength of 7-14 μm, the metal film 15 is aluminum film with a thickness h4 = 120 nm, and the grating groove depth h3 varies.

[0047] The extinction ratio ER is defined as:

[0048]

[0049] TTM is the transmittance of TM polarized light, T TE is the transmittance of TE polarized light, where TM polarization represents linear polarization with the electric field vector perpendicular to the grating lines, and TE polarization represents linear polarization with the electric field vector parallel to the grating lines. The unit of extinction ratio is dB.

[0050] Considering the transmittance of the grating structure alone (not considering the interface reflection on both sides), the relationship between the TM polarization transmittance and extinction ratio with wavelength at different grating groove depths h3 is as follows: Figure 3 、 Figure 4 In the 7-14 μm band, when the grating groove depth h3 is 400-500 nm, the TM polarized light transmittance is greater than 85%, and the extinction ratio is greater than 46 dB. High extinction ratio and transmittance can also be achieved at low line density.

[0051] Example 2:

[0052] See also Figure 2 The substrate material 14 is silicon, the grating period p varies, and the duty ratio remains constant at 0.5. The incident light 11 is TM or TE polarized light incident normally on the substrate with a wavelength of 7-14 μm. The metal film 15 is aluminum with a thickness h4 = 120 nm. The grating groove depth h3 is 450 nm. Considering the transmittance of the grating structure alone (ignoring the interface reflection on both sides), the relationship between the TM polarized light transmittance and extinction ratio with wavelength at different periods is as follows: Figure 5 、 Figure 6 As shown in Figure 2, with the decrease of the grating period p, the transmittance and extinction ratio of TM polarized light gradually increase.

[0053] Example 3:

[0054] See also Figure 2 The substrate material 14 is silicon, the grating period p = 600nm, the duty ratio is 0.5, the incident light 11 is TM or TE polarized light incident normally on the substrate with a wavelength of 7-14μm, the metal film 15 is aluminum film, the aluminum film thickness h4 is 60-120nm, and the grating groove depth is 450nm. Considering the transmittance of the grating structure alone (ignoring the interface reflection on both sides), the relationship between the TM polarized light transmittance and extinction ratio with wavelength under different aluminum film thicknesses is as follows: Figure 7 、 Figure 8 As shown in the figure, as the aluminum film thickness increases, the transmittance of TM polarized light gradually decreases, but the extinction ratio gradually increases. Taking all factors into consideration, an aluminum film thickness of 80nm can be selected for manufacturing, with a transmittance greater than 90%, reaching a maximum of over 96%, and an extinction ratio greater than 40dB.

[0055] Example 4:

[0056] See also Figure 2The substrate material 14 is silicon, the grating period p = 600nm, the duty ratio is 0.3-0.7, the incident light 11 is TM or TE polarized light incident normally on the substrate with a wavelength of 7-14μm, the metal film 15 is aluminum film with a thickness h4 of 120nm, and the grating groove depth h3 is 450nm. Considering the transmittance of the grating structure alone (ignoring the interface reflection on both sides), the relationship between the TM polarized light transmittance and extinction ratio as a function of wavelength at different duty ratios is as follows: Figure 9 、 Figure 10 As shown in Figure 2, the transmittance curves and extinction ratios of gratings with complementary duty ratios (adding up to 1) are essentially identical. This is a unique property of this design. This is because the encapsulation medium and substrate material are the same. A duty ratio of 0.5 is generally chosen for ease of processing, while ensuring the highest transmittance and a high extinction ratio.

[0057] Embodiment 5:

[0058] See also Figure 1 , the base material 14 is silicon, the second material 13 is zinc sulfide film, the first material 12 is ytterbium fluoride film, Figure 2 Two layers of symmetrically thick antireflection films are coated on the upper and lower sides of the packaging substrate. The thickness of the first material 12 is h1 = 1267nm, and the thickness of the second material 13 is h2 = 857nm. The grating period p = 250nm, the duty ratio is 0.5, the incident light 11 is TM or TE polarized light incident from the substrate, with a wavelength of 7-14um. The metal film 15 is an aluminum film with a thickness of h4 = 120nm, and the grating groove depth is 450nm. Considering the TM polarized light transmittance of the entire polarization filter (including the interface reflection on both sides), the relationship between the TM polarized light transmittance and wavelength is as follows: Figure 11 As shown in the figure. Without an antireflection structure, the transmittance is low, with transmittance below 50% in most bands. Although the packaged grating structure has high transmittance, the interface reflection of silicon itself is greater, so antireflection coating is still required. After considering a simple double-layer film design, the transmittance can reach up to 88%. At 7-12μm, the transmittance of TM polarized light exceeds 70%. Because the extinction ratio at normal incidence is mainly determined by the packaged grating structure, it is generally believed that the extinction ratio after antireflection coating can refer to Example 2 and should be greater than 60dB.

[0059] It will be easily understood by those skilled in the art that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A packaged sub-wavelength grating polarization filter, characterized by: The invention comprises a base material made of infrared material, a grating made of the same material as the base material, a double-layer metal film, double-layer first and second anti-reflection films, and a packaging material made of the same material as the base material; starting from the incident direction of the incident light, the first anti-reflection film, the second anti-reflection film, the base material, the second anti-reflection film, and the first anti-reflection film are sequentially arranged; the base material has a rectangular sub-wavelength grating groove, the bottom of the sub-wavelength grating groove and the top of the grating are respectively evaporated with a layer of metal film, and the packaging layer is formed by packaging with the same packaging material as the sub-wavelength grating groove and the base material. The double-layer first anti-reflection film and the second anti-reflection film are symmetrically plated on both sides of the packaged substrate; The first antireflection film is zinc sulfide, and the second antireflection film is ytterbium fluoride; The thickness of the first antireflection film is 857nm, and the thickness of the second antireflection film is 1267nm; When considering the subwavelength grating structure alone without considering the substrate double interface reflection, it achieves excellent transmittance at 7-14 μm. A sub-wavelength grating structure is directly manufactured on an infrared material substrate, and then a layer of aluminum film is evaporated, and the grating is then encapsulated with the same material as the substrate; The packaging material is the same as the substrate material, and its thickness is greater than the depth of the grating groove; The substrate material is silicon, the grating period is 600nm, the grating groove depth is 450nm, the thickness of the aluminum film is 120nm, the duty ratio is 0.5, the TM transmittance is higher than 90%, and the extinction ratio is higher than 47dB.

2. A method for manufacturing the packaged sub-wavelength grating polarization filter according to claim 1, characterized in that: The manufacturing method comprises: (1) Directly fabricating subwavelength grating grooves on substrate materials; (2) evaporating a layer of metal film on the bottom and top of the grating groove respectively; (3) Encapsulating the grating groove with the same encapsulation material as the substrate material; (4) A double layer of first and second antireflection films are symmetrically plated on both sides of the packaged substrate.

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