Shallow slot depth broadband all-dielectric reflective grating for medium-long wave infrared and preparation method thereof

Abstract

The invention discloses a medium-long wave infrared shallow slot depth broadband all-dielectric reflective grating and a preparation method thereof, and belongs to the technical field of infrared optical devices. The grating is composed of a bottom grating film and a top shallow slot deep diffraction grating, the bottom grating film and the top shallow slot deep diffraction grating cooperatively modulate the Bloch mode reflection phase, and the-1-level diffraction efficiency is higher than 99% within the waveband range of 3-14 microns. The preparation method comprises the following steps: sequentially depositing the functional film layer on the substrate, and patterning and etching to form the shallow groove deep grating structure. According to the invention, the design of the all-dielectric shallow-slot deep grating is adopted, the preparation difficulty of a medium-long wave infrared diffraction grating is avoided, and the optical grating has the performance expressions of wide angular spectrum, ultra wide band, high diffraction efficiency and high damage threshold. The invention provides a solution that the preparation conditions of the medium-long wave diffraction grating are limited, the application of the diffraction grating in medium-long wave infrared is accelerated, and the development of a medium-long wave infrared high-energy laser spectrum beam combining system, ultra-strong and ultra-short laser pulse compression and a spectrometer system is promoted.

Description

Technical Field

[0001] The present invention relates to mid-wave and long-wave infrared reflective diffraction gratings in the fields of high-energy lasers and spectrometer applications, and particularly to a mid-wave and long-wave infrared shallow-groove deep-wideband all-dielectric reflective grating. Technical Background

[0002] Mid-wave and long-wave infrared, as an "atmospheric window", covers the absorption spectral lines of a variety of molecules. Lasers in this band have important application values in the fields of infrared spectroscopy, infrared medicine, infrared countermeasure, atmospheric monitoring, infrared remote sensing, industrial gas detection, etc. Diffraction gratings, as the core components in optical systems such as high-energy laser systems and spectrometers, directly determine the performance of the entire optical system. However, currently, most of the research on diffraction gratings focuses on the short-wave infrared band, and the research on mid-wave and long-wave infrared diffraction grating technology is relatively lacking. So far, some diffraction gratings applied to mid-wave and long-wave infrared are based on metal coating structures. However, due to the inherent absorption characteristics of metal gratings, not only the highest diffraction efficiency is limited to less than 95%. In 2014, the diffraction efficiency of the grating prepared by the Pennsylvania State University in the United States with gold and alumina coatings was at most 92%, and the theoretical maximum was only 94.2% [Prior Art 1: Chen T B, Liu B, McCoy J A, et al. Optical Materials Express, 2024, 14(5): 1336-1348]. In addition, due to the heat absorbed by the metal, the laser damage threshold is greatly reduced, resulting in that it is very difficult for the final laser output power to reach a relatively high level, and the service life of the diffraction grating is also severely limited.

[0003] In the application of diffraction gratings in the short-wave infrared range, non-destructive all-dielectric materials are usually used to design gratings to improve the diffraction efficiency (>99%) and the high laser damage threshold. There has also been some research on mid- and long-wave infrared dielectric gratings. However, in the mid- and long-wave infrared bands, due to the wavelength effect, both the thickness of the dielectric film and the groove depth of the dielectric grating will increase. Since the designed groove depth of the grating is relatively deep and limited by the current process conditions, it cannot be actually fabricated, and much work only stays at the theoretical and design stages. In 2022, the Changchun Institute of Optics, Fine Mechanics and Physics first designed a high-diffraction-efficiency reflective all-dielectric grating for 4.7 μm using Si and SiO2 as the top-layer grating materials [Prior Art 2: Wang Y, Fu X, Chen Y, et al. Micromachines, 2022, 13(4):632.]. The ratio of the groove depth of the grating to the wavelength in this work is 0.123, which is equivalent to that of the grating in the short wave and belongs to the design category of conventional gratings. When extended to the long-wave band, this design will fail due to difficult fabrication. In 2024, Tongji University designed an all-dielectric reflective diffraction grating for the 9.3-9.6 μm band using germanium as the top-layer grating material [Prior Art 3: Zhou S, Dong S, He T, et al. Micromachines, 2024, 15(4):538.]. However, since the groove depth of the grating is 2.5 μm, the existing fabrication processes cannot meet the fabrication requirements of this grating. Although there are also studies on the realization of sample fabrication [Prior Art 4: Zhu J, Zhou S, He T, et al. Nanophotonics, 2025(0).], this work is based on an aperiodic grating with a very small processing tolerance, resulting in a low diffraction efficiency and a narrow bandwidth in actual tests, which cannot meet the application requirements in fields such as high-energy lasers.

[0004] Based on this, since the mid- and long-wave infrared all-dielectric deep-groove diffraction gratings have demanding fabrication conditions, the currently mainstream diffraction gratings used in the mid- and long-wave infrared bands are still metal-coated gratings. There is currently no report on the successful fabrication of mid- and long-wave high-efficiency infrared all-dielectric diffraction gratings. The design of shallow-groove diffraction gratings can circumvent the deficiencies of the process and accelerate the fabrication and application of mid- and long-wave infrared diffraction gratings. Summary of the Invention

[0005] The object of the present invention is to circumvent the design and fabrication difficulties of mid-wave infrared all-dielectric diffraction gratings, and to provide a mid-long wave infrared broadband all-dielectric shallow-groove deep-reflection diffraction grating and a fabrication method thereof, filling the research gap of mid-long wave all-dielectric diffraction gratings. This diffraction grating has an easy-to-fabricate shallow-groove deep-grating structure, uses all-dielectric materials, has the characteristics of low absorption and high laser damage threshold, has a -1st order diffraction efficiency higher than 99% in the operating wavelength band, and has a large operating angular spectrum range. Therefore, this diffraction grating has important practical value in mid-long wave infrared high-power spectral beam combining technology and chirped pulse amplification and compression technology. In addition, this type of diffraction grating can also be applied to spectrometers.

[0006] The technical solution of the present invention is to provide a mid-long wave infrared shallow-groove deep broadband all-dielectric reflection grating, which is composed of a bottom grating film and a top-layer shallow-groove deep diffraction grating. The bottom grating film and the top-layer shallow-groove deep diffraction grating jointly modulate the reflection phase of the Bloch mode propagating in the grating, and achieve a -1st order diffraction efficiency higher than 99% in the effective operating wavelength band range of 3 - 14 μm.

[0007] For the top-layer shallow-groove deep diffraction grating, the grating ridge structure profile is described by a single amplitude grating profile function y(x;d,Λ,α,β)=d(1 - {1 - [sin 2 (πx / Λ)] α} β ), where α and β are real numbers and α≥1, β≥1, Λ and d are the grating period and groove depth respectively, and 14μm>Λ>1.5μm, 1μm>d>50nm, the grating duty cycle f is 0.2 - 0.8, and the ratio of the groove depth d to the operating wavelength λ is less than 0.05;

[0008] The top-layer shallow-groove deep diffraction grating uses a low-absorption, high-refractive-index dielectric material, preferably a single layer of germanium (Ge) or a multi-layer structure of alternating deposition of germanium and ytterbium fluoride (Ge / YbF3).

[0009] The bottom grating film is composed of a multi-layer dielectric structure with low expansion, high thermal conductivity, and low absorption, and has a reflectivity higher than 99% in the incident angle range of 0 - 89° and the effective operating wavelength band range of 3 - 14 μm. Its layered structure sequentially includes a substrate, a high-reflection film stack, a phase-matching layer, and an etching residue layer from bottom to top.

[0010] The substrate uses a low-expansion, high-thermal-conductivity material, preferably fused quartz, diamond, or silicon carbide.

[0011] The high-reflection film stack is a multi-layer film structure, contains two or more materials, the optical thickness of a single layer does not exceed one-fourth of the operating wavelength, and provides a reflectivity higher than 99%. The film system structure is preferably but not limited to the following structures:

[0012] Regular periodic film system: composed of high refractive index material layers and low refractive index material layers alternating with a fixed period and thickness;

[0013] Irregular film system: composed of high refractive index material layers and low refractive index material layers alternating, where the thickness of at least some of the material layers varies non-periodically;

[0014] Composite film system: includes a regular periodic film system and at least one medium refractive index material layer, and the medium refractive index material layer is combined with the regular periodic film system in one of the following ways:

[0015] Insertion type: embedding the medium refractive index material layer in the regular periodic sequence;

[0016] Terminal type: adding a medium refractive index material layer at the end of the regular periodic stack;

[0017] Hybrid type: inserting multiple medium refractive index material layers in the periodic sequence according to a predetermined pattern;

[0018] The high refractive index material is preferably germanium (Ge), silicon (Si), the low refractive index material is preferably silicon dioxide (SiO2), ytterbium fluoride (YbF3), and the medium refractive index material is preferably zinc selenide (ZnSe), zinc sulfide (ZnS).

[0019] The phase matching layer is a one-layer or multi-layer structure that modulates the reflection phase of the top shallow groove deep diffraction grating. The material is preferably at least one of germanium (Ge), silicon dioxide (SiO2), ytterbium fluoride (YbF3), barium fluoride (BaF2), or zinc selenide (ZnSe), and the thickness of each layer is less than 1.5 μm.

[0020] The etched residue layer uses the same material as the top shallow groove deep diffraction grating, which is used to increase the preparation tolerance of the grating, and the thickness is less than 1.5 μm.

[0021] The present invention also provides a preparation method for a mid-long wave infrared shallow groove deep broadband all-dielectric reflective grating, which mainly includes the following steps:

[0022] (1) Sequentially deposit a high reflection film stack, a phase matching layer, an etched residue layer, and a grating layer to be etched on the grating substrate;

[0023] (2) Form a photoresist patterned mask structure on the surface of the grating layer to be etched;

[0024] (3) Use the mask structure as a barrier layer for etching until the target groove depth is etched to form the top shallow groove deep diffraction grating;

[0025] (4) Remove the residual photoresist mask to obtain a reflective diffraction grating composed of a bottom grating film containing a substrate, a high-reflection film stack, a phase-matching layer, and an etching residual layer, and a top shallow groove deep diffraction grating. The diffraction efficiency of the grating in the -1st order in the 3-14 μm band exceeds 99%.

[0026] The present invention has the following beneficial technical effects compared with the prior art:

[0027] The groove depth of the top diffraction grating structure of the present invention is preferably less than 1 μm, and the ratio of the groove depth to the operating wavelength is controlled to be less than 0.05, avoiding the difficulties in the preparation of mid- and long-wave infrared, and accelerating the application of the diffraction grating in mid- and long-wave infrared.

[0028] The diffraction grating of the present invention uses a low-loss all-dielectric material, overcomes the absorption characteristics of the material, and has a high damage threshold and high diffraction efficiency.

[0029] The -1st order diffraction efficiency of the present invention within the operating band is higher than 99%, and it has a large operating angular spectrum range.

[0030] The present invention has the characteristics of broadband, high diffraction efficiency, and easy preparation. It has important practical prospects in the fields of mid- and long-wave infrared high-energy laser beam combining, pulse compression, and spectrometers, etc. Description of the Drawings

[0031] Figure 1 is a structural sectional view of the mid- and long-wave infrared shallow groove depth broadband all-dielectric reflective grating of the present invention.

[0032] Figure 2 is a design structural sectional view of Example 1 and Example 2.

[0033] Figure 3 is a graph showing the relationship between the reflectivity and the wavelength and incident angle under the condition of TE polarization incidence in the wavelength range of 4300 nm to 4600 nm in the operating band of the bottom grating film of Example 1.

[0034] Figure 4 is a graph showing the relationship between the phase difference of two Bloch modes propagating in the grating and the grating groove depth at the center wavelength used in Example 1.

[0035] Figure 5 is a graph showing the relationship between the diffraction efficiency of the diffraction grating and the wavelength under the condition of TE polarization incidence in the wavelength range of 4300 nm to 4600 nm in the operating band of Example 1.

[0036] Figure 6 is a graph showing the relationship between the reflectivity and the wavelength and incident angle under the condition of TE polarization incidence in the wavelength range of 8300 nm to 8700 nm in the operating band of the bottom grating film of Example 2.

[0037] Figure 7It is a diagram showing the variation of the phase difference between two Bloch modes propagating in the grating with respect to the grating groove depth at the central wavelength in Example 2.

[0038] Figure 8 It is a diagram showing the variation of the diffraction efficiency of a diffraction grating with respect to wavelength under the condition of TE polarization incidence in the wavelength range of 8300 nm to 8700 nm in Example 2.

[0039] Figure 9 It is a diagram showing the variation of the reflectivity of the bottom grating film with respect to wavelength and incident angle under the condition of TE polarization incidence in the wavelength range of 12000 nm to 12500 nm in Example 3.

[0040] Figure 10 It is a diagram showing the variation of the phase difference between two Bloch modes propagating in the grating with respect to the grating groove depth at the central wavelength in Example 3.

[0041] Figure 11 It is a diagram showing the variation of the diffraction efficiency of a diffraction grating with respect to wavelength under the condition of TE polarization incidence in the wavelength range of 12000 nm to 12500 nm in Example 3.

[0042] Figure 12 It is a sectional view of the design structure in Example 4.

[0043] Figure 13 It is a diagram showing the variation of the reflectivity of the bottom grating film with respect to wavelength and incident angle under the condition of TM polarization incidence in the wavelength range of 8300 nm to 8700 nm in Example 4.

[0044] Figure 14 It is a diagram showing the variation of the phase difference between two Bloch modes propagating in the grating with respect to the grating groove depth at the central wavelength in Example 4.

[0045] Figure 15 It is a diagram showing the variation of the diffraction efficiency of a diffraction grating with respect to wavelength under the condition of TM polarization incidence in the wavelength range of 8300 nm to 8700 nm in Example 4. Specific implementation manners

[0046] The embodiments of the present invention will be further described below in conjunction with the embodiments and the drawings, but the protection scope of the present invention should not be limited thereby.

[0047] Please refer to Figure 1 , Figure 1This is the structural sectional view of the medium- and long-wave infrared shallow-groove deep broadband all-dielectric reflective diffraction grating of the present invention. As can be seen from the figure, the medium- and long-wave infrared broadband all-dielectric reflective diffraction grating of the present invention is composed of a bottom grating film 1 and a top shallow-groove deep diffraction grating 2. The bottom grating film and the top shallow-groove deep diffraction grating jointly modulate the reflection phase of the Bloch mode propagating in the grating, and achieve a diffraction efficiency of higher than 99% in the effective working wavelength range of 3-14μm.

[0048] For the top shallow-groove deep diffraction grating, the grating ridge structure profile is described by a single amplitude grating profile function y(x;d,Λ,α,β)=d(1-{1-[sin 2 (πx / Λ)] α} β ) where α and β are real numbers and α≥1,β≥1, Λ and d are the grating period and groove depth respectively, and 14μm>Λ>1.5μm, 1μm>d>50nm. The grating duty cycle f is 0.2-0.8, and the ratio of the groove depth d to the working wavelength λ is less than 0.05.

[0049] The top shallow-groove deep diffraction grating uses a dielectric material with low absorption and high refractive index, including but not limited to a single layer of Ge or a multi-layer combination structure such as Ge / SiO2.

[0050] The bottom grating film 1 is composed of a multi-layer dielectric structure with low expansion, high thermal conductivity, and low absorption, and has a reflectivity higher than 99% within the incident angle range of 0-89° and an ultra-wide bandwidth. Its structure includes a substrate 3, a high-reflection film stack 4, a phase-matching layer 5, and an etching residual layer 6 in sequence from bottom to top.

[0051] The substrate 3 uses a material with low expansion and high thermal conductivity, preferably fused quartz, diamond, or silicon carbide.

[0052] The high-reflection film stack 4 is a multi-layer film structure, containing two or more materials. The optical thickness of a single layer of film does not exceed one-fourth of the working wavelength, providing a reflectivity higher than 99%. The film system structure is preferably but not limited to the following structures:

[0053] Regular periodic film system: The high-refractive-index material layer and the low-refractive-index material layer are alternately composed with a fixed period and thickness.

[0054] Irregular film system: The high-refractive-index material layer and the low-refractive-index material layer are alternately composed, and the thickness of at least some of the material layers changes non-periodically.

[0055] Composite film system: It contains a regular periodic film system and at least one medium-refractive-index material layer. The medium-refractive-index material layer is combined with the regular periodic film system in one of the following ways:

[0056] Insertion type: Insert the medium-refractive-index material layer into the regular periodic sequence.

[0057] Terminal type: Add a medium refractive index material layer at the end of the regular period stack;

[0058] Hybrid type: Insert multiple medium refractive index material layers into the periodic sequence according to a predetermined pattern;

[0059] The high refractive index material is preferably Ge or Si, the low refractive index material is preferably SiO2 or YbF3, and the medium refractive index material is preferably ZnSe or ZnS.

[0060] The phase matching layer is a one - layer or multi - layer structure that modulates the reflection phase of the top - layer shallow - groove deep diffraction grating. The material is preferably at least one of Ge, SiO2, YbF3, BaF2 or ZnSe, and the thickness of each layer is less than 1.5μm.

[0061] The etched residue layer is made of the same material as the top - layer shallow - groove deep diffraction grating, which is used to increase the preparation tolerance of the grating, and its thickness is less than 1.5μm.

[0062] The above - mentioned method for preparing a mid - to - long - wave infrared shallow - groove deep broadband all - dielectric reflective grating includes first depositing a high - reflection film stack 4, a phase matching layer 5, an etched residue layer 6 and an etched grating layer on the grating substrate 3 in sequence, then forming a photoresist patterning mask structure on the surface of the etched grating layer, and then etching with the mask structure as a barrier layer until the target groove depth is reached to form a top - layer shallow - groove deep diffraction grating 2; finally, removing the residual photoresist mask to obtain a reflective diffraction grating composed of a bottom grating film 1 containing the substrate 3, the high - reflection film stack 4, the phase matching layer 5, and the etched residue layer 6 and the top - layer shallow - groove deep diffraction grating 2. The diffraction efficiency of the grating in the 3 - 14μm band in the - 1st order exceeds 99%.

[0063] Figure 1 It is a structural cross - sectional view of the mid - to - long - wave infrared shallow - groove deep broadband all - dielectric reflective diffraction grating of the present invention. The TE - polarized incident light corresponds to the vibration direction of the electric field vector perpendicular to the incident plane, and the TM - polarized incident light corresponds to the vibration direction of the electric field vector in the incident plane. As can be seen from the figure, the incident medium above the grating and the medium inside the grating grooves are both air. When light is incident from the air to the grating surface, through the modulation of the Bloch mode phase by the top - layer shallow - groove deep diffraction grating layer and the bottom grating film, high - efficiency emission of diffracted light can be achieved.

[0064] The present invention calculates the effective refractive index of the Bloch modes propagating in the grating using the simplified mode theory, and calculates the propagation phase of the Bloch modes and the propagation phase difference between the two Bloch modes supported for propagation in the grating through the transfer matrix. When the propagation phase difference between the two Bloch modes supported for propagation in the grating is close to π (the difference from π is less than 0.5π), the designed grating meets the condition of high diffraction efficiency. In addition, the present invention calculates the -1st order diffraction efficiency of the mid- and long-wave infrared broadband all-dielectric reflective diffraction grating using the rigorous coupled-wave theory, and optimizes the structural parameters of the bottom grating film and the top shallow groove deep diffraction grating of the mid- and long-wave infrared shallow groove deep broadband all-dielectric reflective diffraction grating through the genetic algorithm, and obtains the conclusion that for the shallow groove deep grating, high -1st order diffraction efficiency of the incident light can be achieved within a large processing tolerance, a large incident angle range, and a wide bandwidth range.

[0065] Example 1:

[0066] Under the grating structure as shown in Figure 2 , 2 is the top shallow groove deep diffraction grating, the grating period Λ is 3333.3 nm, the grating groove depth d is 115 nm, the grating duty cycle f is 0.382, the thickness d1 of the etched residual layer 6 is 30 nm, the material is germanium, the thickness d2 of the phase matching layer 5 is 346 nm, the material is silicon dioxide, and the high - reflection structure composed of the grating substrate 3 and the periodic film system 4 is S|(HL) m H, where H and L are the high - refractive index material layer and the low - refractive index material layer respectively, S is the grating base layer, the material is fused quartz, the high - refractive index material of the periodic film system is Ge, the thickness is 278.1 nm, the low - refractive index material of the periodic film system is SiO2, the thickness is 794.6 nm, m is the number of film layers and m = 3; the coupled output angle θ = 41.87° (the Littrow angle corresponding to the central wavelength of 4450 nanometers). As shown in Figure 3 , within the angular spectrum range of 0 - 89°, the reflectivity of the grating film before etching is higher than 99% within the operating bandwidth. As shown in Figure 4 , the phase difference between the two Bloch modes propagating in the grating has a value close to π under the shallow groove depth, and the corresponding groove depth is basically the same as the designed grating groove depth. As shown in Figure 5 , within the wavelength range of 4300 nm to 4600 nm, the minimum value of the -1st order diffraction efficiency of the grating TE polarization exceeds 99.5%, and the peak value of the diffraction efficiency exceeds 99.9%.

[0067] Fabricate the above mid- and long-wave infrared shallow-groove deep broadband all-dielectric reflective diffraction grating. First, deposit the high-reflection film stack 4, phase-matching layer 5, etching residual layer 6, and the grating layer to be etched on the grating substrate 1 with a size of 50mm * 50mm * 1.5mm in sequence by electron beam evaporation and ion-assisted deposition techniques. Use a spin-coating device to spin-coat a layer of photoresist with a thickness of 600 nanometers on the surface of the coated film layer at a rotation speed of 3800 revolutions per minute, then bake it at 100 °C for 2 minutes, and then use the holographic interference exposure method to expose the coated photoresist. The exposure power is 110 μW, and the exposure time is 300 s. Then develop the exposed sample with a sodium hydroxide solution with a concentration of 1.6% to obtain a photoresist mask structure. Next, use a reactive ion beam etching machine with the photoresist mask structure as a barrier layer for etching until the target groove depth is reached to form the top-layer shallow-groove deep diffraction grating 2; finally, remove the residual photoresist mask to obtain a reflective diffraction grating composed of the bottom grating film 1 containing the substrate 3, high-reflection film stack 4, phase-matching layer 5, and etching residual layer 6 and the top-layer shallow-groove deep diffraction grating 2.

[0068] Example 2:

[0069] Under the grating structure as shown in Figure 2 In this Example 2, it is the grating layer. The grating period Λ is 6250 nm, the grating groove depth d is 226.8 nm, the grating duty cycle f is 0.35, the thickness d1 of the etching residual layer 6 is 25.9 nm, and the material is germanium. The thickness d2 of the phase-matching layer 5 is 891.4 nm, and the material is ytterbium fluoride. The high-reflection structure composed of the grating substrate 3 and the periodic film system 4 is S|(HL) m H, where H and L are the high-refractive-index material layer and the low-refractive-index material layer respectively, S is the grating substrate layer, and the material is fused quartz. The high-refractive-index material of the periodic film system is germanium, with a thickness of 673 nm, and the low-refractive-index material of the periodic film system is ytterbium fluoride, with a thickness of 900 nm. m is the number of film layer periods and m = 3; the coupled output angle θ = 37.25° (the Littrow angle corresponding to the central wavelength of 8500 nanometers). As shown in Figure 6 Before etching, the reflectivity of the grating film within the angular spectrum range of 0 - 89° is higher than 99% within the operating bandwidth. As shown in Figure 7 The phase difference between the two Bloch modes propagating in the grating has a value close to π under the shallow groove depth, and the corresponding groove depth is basically consistent with the designed grating groove depth. As shown in Figure 8 Within the wavelength range of 8300 nm - 8700 nm, the minimum value of the -1st order diffraction efficiency of the grating for TE polarization exceeds 99.4%, and the peak value of the diffraction efficiency exceeds 99.7%.

[0070] Fabricate the above mid-long wave infrared shallow groove deep broadband all-dielectric reflective diffraction grating. First, deposit the high-reflection film stack 4, phase matching layer 5, etch residue layer 6, and the grating layer to be etched on the grating substrate 1 with a size of 50mm * 50mm * 1.5mm in sequence by electron beam evaporation and ion-assisted deposition techniques. Use a spin-coating device to spin-coat a layer of photoresist with a thickness of 600 nanometers on the surface of the coated film layer at a speed of 3800 revolutions per minute, then bake it at 100 °C for 2 minutes. Then, use the holographic interference exposure method to expose the coated photoresist. The exposure power is 110 μW, and the exposure time is 300 s. Then, develop the exposed sample with a sodium hydroxide solution with a concentration of 1.6% to obtain a photoresist mask structure. Next, use a reactive ion beam etching machine with the photoresist mask structure as the blocking layer for etching until the target groove depth is reached to form the top-layer shallow groove deep diffraction grating 2; finally, remove the residual photoresist mask to obtain a reflective diffraction grating composed of the bottom grating film 1 containing the substrate 3, high-reflection film stack 4, phase matching layer 5, and etch residue layer 6 and the top-layer shallow groove deep diffraction grating 2.

[0071] Example 3:

[0072] Under the grating structure as shown in Figure 2 , in this Example 2, it is the grating layer. The grating period Λ is 8696 nm, the grating groove depth d is 320.3 nm, the grating duty cycle f is 0.3045, the thickness d1 of the etch residue layer 6 is 50 nm, and the material is germanium. The thickness d2 of the phase matching layer 5 is 1200 nm, and the material is barium fluoride. The high-reflection structure composed of the grating substrate 3 and the periodic film system 4 is S|(HL) m H, where H and L are the high refractive index material layer and the low refractive index material layer respectively, S is the grating substrate layer, and the material is fused quartz. The high refractive index material of the periodic film system is germanium, with a thickness of 1103.8 nm, and the low refractive index material of the periodic film system is barium fluoride, with a thickness of 1400 nm. m is the number of film layer periods and m = 3; the coupled output angle θ = 44.77° (the Littrow angle corresponding to the central wavelength of 12250 nanometers). As shown in Figure 9 , within the angular spectrum range of 0 - 89°, the reflectivity of the grating film before etching is higher than 99% within the operating bandwidth. As shown in Figure 10 , the phase difference between the two Bloch modes propagating in the grating has a value close to π under the shallow groove depth, and the corresponding groove depth is basically consistent with the designed grating groove depth. As shown in Figure 11 , within the wavelength range of 12000 nm - 12500 nm, the minimum value of the -1st order diffraction efficiency of the grating TE polarization exceeds 99.67%, and the peak value of the diffraction efficiency exceeds 99.79%.

[0073] Fabricate the above mid- and long-wave infrared shallow-groove deep broadband all-dielectric reflective diffraction grating. First, deposit a high-reflection film stack 4, a phase-matching layer 5, an etching residual layer 6, and an etched grating layer on a grating substrate 1 with dimensions of 50 mm * 50 mm * 1.5 mm in sequence using electron beam evaporation and ion-assisted deposition techniques. Use a spin-coating device to spin-coat a layer of photoresist with a thickness of 600 nm on the surface of the coated film layer at a rotation speed of 3800 r / min, then bake it at 100 °C for 2 min. Then, use the holographic interference exposure method to expose the coated photoresist, with an exposure power of 110 μW and an exposure time of 300 s. Then, develop the exposed sample with a sodium hydroxide solution with a concentration of 1.6% to obtain a photoresist mask structure. Next, use a reactive ion beam etching machine with the photoresist mask structure as a barrier layer for etching until the target groove depth is reached to form a top-layer shallow-groove deep diffraction grating 2. Finally, remove the residual photoresist mask to obtain a reflective diffraction grating composed of a bottom grating film 1 containing a substrate 3, a high-reflection film stack 4, a phase-matching layer 5, and an etching residual layer 6 and a top-layer shallow-groove deep diffraction grating 2.

[0074] Example 4:

[0075] Under the grating structure as shown in Figure 12 , in this Example 2, the grating layer has a grating period Λ of 5586.7 nm, a grating groove depth d of 394 nm, a grating duty cycle f of 0.837, an etching residual layer 6 thickness d1 of 882.1 nm, the material is germanium, the first phase-matching layer 51 material is ytterbium fluoride, the thickness d2 is 750.18 nm, the second phase-matching layer 52 material is zinc selenide, the thickness d3 is 100 nm, and the high-reflection structure composed of the grating substrate 3 and the periodic film system 4 is S|BHB(LBHB) m , where H and L are high-refractive-index material layers and low-refractive-index material layers respectively, B is a medium-refractive-index material layer with a refractive index between the high-refractive-index material and the low-refractive-index material, the material is zinc selenide, the thickness is 100 nm, S is the grating substrate layer, the material is fused quartz, the high-refractive-index material of the periodic film system is germanium, the thickness is 617.8 nm, the low-refractive-index material of the periodic film system is ytterbium fluoride, the thickness is 1000 nm, m is the number of film layer periods and m = 3; the coupled output angle θ = 50°. As shown in Figure 13 , within the angular spectrum range of 0 - 89°, the reflectivity of the grating film before etching is higher than 99% within the operating bandwidth. As shown in Figure 14 , the phase difference between the two Bloch modes propagating in the grating has a value close to π under the shallow groove depth, and the corresponding groove depth is basically consistent with the designed grating groove depth. As shown in Figure 15 , within the wavelength range of 8300 nm to 8700 nm, when TM polarization is incident, the highest diffraction efficiency within the operating bandwidth is 99.47%, and the average diffraction efficiency of the bandwidth is 99.52%.

[0076] Fabricate the above mid-wave and long-wave infrared shallow-groove deep broadband all-dielectric reflective diffraction grating. First, deposit the high-reflection film stack 4, phase-matching layer 5, etching residual layer 6, and the grating layer to be etched on the grating substrate 1 with a size of 50mm * 50mm * 1.5mm in sequence by using electron beam evaporation and ion-assisted deposition techniques. Use a spin-coating device to spin-coat a layer of photoresist with a thickness of 600 nanometers on the surface of the coated film layer at a rotation speed of 3800 r / min, then bake it at 100 °C for 2 min, and then use the holographic interference exposure method to expose the coated photoresist. The exposure power is 110 μW, and the exposure time is 300 s. Then develop the exposed sample with a sodium hydroxide solution with a concentration of 1.6% to obtain a photoresist mask structure. Next, use a reactive ion beam etching machine with the photoresist mask structure as the barrier layer for etching until the target groove depth is etched to form the top-layer shallow-groove deep diffraction grating 2; finally, remove the residual photoresist mask to obtain a reflective diffraction grating composed of the bottom grating film 1 containing the substrate 3, high-reflection film stack 4, phase-matching layer 5, and etching residual layer 6 and the top-layer shallow-groove deep diffraction grating 2.

[0077] In summary, the present invention is used for mid-wave and long-wave infrared shallow-groove deep broadband all-dielectric reflective diffraction gratings. By adopting shallow-groove deep gratings and all-dielectric structures, it has the characteristics of ultra-wideband, high diffraction efficiency, and high damage threshold, circumvents the preparation difficulties of mid-infrared diffraction gratings, accelerates the application of diffraction gratings in the mid-infrared band, and especially can meet the requirements of high laser damage threshold and broadband high diffraction efficiency diffraction gratings for high-energy laser systems and spectrometer systems.

Claims

1. A shallow-groove deep-wideband all-dielectric reflective grating for mid-wave and long-wave infrared, characterized in that, It consists of a bottom grating film (1) and a top shallow groove deep diffraction grating (2). The bottom grating film (1) and the top shallow groove deep diffraction grating (2) jointly modulate the reflection phase of the Bloch mode propagating in the grating, and achieve a -1st order diffraction efficiency higher than 99% within the effective working wavelength range of 3 - 14 μm. The top shallow groove deep diffraction grating (2) is made of a dielectric material with low absorption and high refractive index. The grating ridge structure profile is described by a single amplitude grating profile function y(x; d, Λ, α, β) = d(1 - {1 - [sin 2 [(πx / Λ)] α} β ), where α and β are real numbers and α≥1, β≥1, Λ and d are the grating period and groove depth respectively, 14μm > Λ > 1.5μm, 1μm > d > 50nm, the grating duty cycle f is 0.2 - 0.8, and the ratio of the groove depth d to the working wavelength λ is less than 0.05; The bottom grating film (1) is composed of a multi-layer dielectric structure with low expansion, high thermal conductivity, and low absorption, and has a reflectivity higher than 99% within the incident angle range of 0 - 89° and the effective working wavelength range of 3 - 14 μm. Its layered structure includes a substrate (3), a high reflection film stack (4), a phase matching layer (5), and an etching residual layer (6) from bottom to top in sequence.

2. The shallow-groove deep-wideband all-dielectric reflective grating for mid-wave and long-wave infrared according to claim 1, wherein The structure of the top shallow groove deep diffraction grating (2) is preferably a single Ge layer structure or a multi-layer structure with alternating deposition of Ge / YbF3.

3. The shallow-groove deep-wideband all-dielectric reflective grating for mid-wave and long-wave infrared according to claim 1, wherein The substrate (3) is made of a material with low expansion and high thermal conductivity, preferably fused quartz, diamond, or silicon carbide.

4. A shallow-groove deep-wideband all-dielectric reflective grating for mid- and long-wave infrared, characterized in that, The high reflection film stack (4) is a multi-layer film structure, containing two or more materials. The optical thickness of a single layer does not exceed one quarter of the working wavelength, providing a reflectivity higher than 99%. The film system structure is preferably but not limited to the following structures: Regular periodic film system: The high refractive index material layer and the low refractive index material layer alternate with a fixed period and thickness. Irregular film system: The high refractive index material layer and the low refractive index material layer alternate, and the thickness of at least some of the material layers changes non-periodically. Composite film system: It includes a regular periodic film system and at least one medium refractive index material layer. The medium refractive index material layer is combined with the regular periodic film system in one of the following ways: Insertion type: Insert the medium refractive index material layer into the regular periodic sequence. Terminal type: Add a medium refractive index material layer at the end of the regular periodic stack. Hybrid type: Insert multiple medium refractive index material layers into the periodic sequence according to a predetermined pattern.

5. A shallow groove deep broadband all-dielectric reflective grating for mid- and long-wave infrared, characterized in that, The high refractive index material is preferably Ge or Si, the low refractive index material is preferably SiO2 or YbF3, and the medium refractive index material is preferably ZnSe or ZnS.

6. The shallow-groove deep-wideband all-dielectric reflective grating for mid- and long-wave infrared according to claim 1, wherein The phase matching layer (5) is a single-layer or multi-layer structure, which modulates the reflection phase of the top shallow groove deep diffraction grating. The material is preferably at least one of Ge, SiO2, YbF3, BaF2, or ZnSe, and the single-layer thickness is less than 1.5 μm.

7. The shallow-groove deep-wideband all-dielectric reflective grating for mid-wave and long-wave infrared according to claim 1, wherein The etching residual layer (6) uses the same material as the top shallow groove deep diffraction grating (2), which is used to increase the preparation tolerance of the grating, and the thickness is less than 1.5 μm.

8. A preparation method for a mid- and long-wave infrared shallow groove deep broadband all-dielectric reflective grating according to any one of claims 1 to 7, characterized in that, It mainly includes the following steps: S1: Deposit a high reflection film stack (4), a phase matching layer (5), an etching residual layer (6), and an etched grating layer on the grating substrate (3) in sequence. S2: Form a photoresist patterned mask structure on the surface of the etched grating layer. S3: Use the mask structure as a barrier layer for etching until the target groove depth is reached to form the top shallow groove deep diffraction grating (2). S4: Remove the residual photoresist mask to obtain a reflective diffraction grating composed of the bottom grating film (1) containing the substrate (3), the high reflection film stack (4), the phase matching layer (5), and the etching residual layer (6) and the top shallow groove deep diffraction grating (2). The diffraction efficiency of the -1st order of the grating exceeds 99% in the 3 - 14 μm band.

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