A beyond-collimated low internal electric field enhanced multilayer film grating

Abstract

The application discloses a low internal electric field enhanced multilayer film grating beyond self-collimation, which comprises, from top to bottom, an inverted trapezoidal microstructure, an electric field regulation layer, a double-angle multilayer film and a substrate. The inverted trapezoidal microstructure period Lambda and the wavelength lambda satisfy 0.62<=Lambda / lambda<=0.88, and the angles between the left and right side walls of the inverted trapezoidal microstructure and the grating plane are alpha and beta. The electric field regulation layer cooperates with the inverted trapezoidal microstructure to regulate the internal electric field distribution of the grating. The double-angle multilayer film is stacked by multilayer films with different positions of angle spectrum reflection peaks, and the film system formula is (aHaL) x (bHbL) y wherein H and L are high and low refractive index layers respectively, and a and b are thickness scaling factors for adjusting the position of the angle spectrum reflection peak. Through the synergistic effect of the inverted trapezoidal microstructure, the electric field regulation layer and the double-angle multilayer film, the application can realize low internal electric field enhancement under the condition of beyond self-collimation, effectively suppress local electric field hot spots, improve the damage threshold, and is suitable for super strong and super short laser systems.

Description

Technical Field

[0001] This invention relates to the field of ultra-intense and ultra-short lasers, specifically to a multilayer grating for pulse compression or broadening, and more particularly to a low internal electric field enhanced multilayer grating that transcends self-collimation. Background Technology

[0002] Over the past few decades, the invention and development of pulse compression technology have propelled the field of ultraintense and ultrashort lasers into the petawatt era. Currently, pulse compression and broadening processes in mainstream ultraintense and ultrashort laser systems rely heavily on gratings with dispersive properties. Multilayer gratings, by avoiding intrinsic absorption of materials, possess high diffraction efficiency and damage thresholds, making them crucial for applications in ultraintense and ultrashort lasers. However, with the continuous development of ultraintense and ultrashort laser technology and the increasing peak power, multilayer gratings now require further improvement in their damage threshold. [Prior Technology 1: US Patent US8349226B2] improves the damage threshold through laser pretreatment. [Prior Technology 2: US Patent US11815705B2] improves the damage threshold by using materials with higher band gaps. [Prior Technology 3: Chinese Patent CN112596137B] improves the damage threshold by introducing nanoimprint technology to reduce defects on the grating surface and by introducing a rectangular structure. Current technologies are primarily based on self-collimation and rarely explore new structures that can further suppress electric field enhancement. Under self-collimation conditions, the incident light strongly interferes with the -1st order diffracted light, resulting in localized electric field intensity hotspots on the grating ridge sidewalls, which limits further improvement of the damage threshold. Therefore, there is an urgent need for a novel multilayer grating structure that can further suppress the enhancement of the electric field inside the grating and improve the damage threshold. Summary of the Invention

[0003] The technical problem this invention aims to solve is overcoming the technical bottleneck of the difficulty in further improving the damage threshold of existing multilayer film gratings. To this end, this invention firstly ensures high reflectivity of both the 0th and -1st order diffracted beams under conditions exceeding self-collimation by setting a dual-angle multilayer film. Secondly, the strategy of setting the incident angle in the angular spectrum region exceeding self-collimation significantly reduces the electric field intensity of the -1st order diffracted beam, thereby weakening the interference electric field formed between it and the incident beam, giving the grating the potential to achieve low electric field intensity. Finally, an inverted trapezoidal microstructure and an electric field modulation layer are used to modulate the electric field inside the grating, achieving a -1st order diffraction efficiency of over 99% while suppressing electric field enhancement within the microstructure.

[0004] The technical solution of the present invention is as follows: A low internal electric field enhanced multilayer film grating with super-collimation is provided, wherein the grating comprises, from top to bottom, an inverted trapezoidal microstructure, an electric field modulation layer, a dual-angle multilayer film and a substrate; The inverted trapezoidal microstructure has an inverted trapezoidal shape. The angle between its left outer wall and the grating plane is α, and the angle between its right outer wall and the grating plane is β. The ratio of its bottom width to its period Λ is the duty cycle f. Its structural height is d. Its period Λ and the wavelength λ satisfy 0.62 ≤ Λ / λ ≤ 0.88. The operating angle θ satisfies the following relationship with λ and Λ. ,and The aforementioned self-collimation means that the angle θ used satisfies the above formula; The electric field modulation layer is composed of a dielectric film layer and is a modulation layer used to coordinate with the inverted trapezoidal microstructure to modulate the enhancement of the electric field inside the grating structure. The dual-angle multilayer film is composed of stacked multilayer films with two angular spectral reflection peaks at different positions, and its film system formula is (aHaL). x (bHbL) y Where H is the refractive index n H A high refractive index layer with a thickness of λ / (4n) H L is the refractive index n L A low-refractive-index layer with a thickness of λ / (4n) L ); a and b are film thickness scaling factors that can adjust the position of the angular spectrum reflection peak, and a>0, b>0, x and y are the number of stacking cycles, and x≥1, y≥1.

[0005] The electric field modulation layer preferably consists of 1 to 5 film layers with a thickness ranging from 30 to 1000 nanometers. The selected thickness considers both practical fabrication feasibility and ensures that the transmission phase difference of the 0th and -1st order diffracted waves beyond the self-collimation condition can basically cover 0 to π / 2 in each film, maximizing the modulation capability. The dielectric film layer of the electric field modulation layer preferably includes alternating Ta2O5 layers and SiO2 layers.

[0006] The grating is preferably used under TM polarization.

[0007] The angle α between the left outer wall of the inverted trapezoidal microstructure and the grating plane is preferably in the range of 91° to 107°, the angle β between the right outer wall and the grating plane is preferably in the range of 68° to 81°, the ratio f of its bottom width to its period Λ is preferably in the range of 0.19 to 0.36, and its structural height d is preferably 1.18 to 1.45 times λ.

[0008] The thickness scaling factor 'a' is preferably in the range of 1.15-1.3, and the thickness scaling factor 'b' is preferably in the range of 1.01-1.15. The number of stacking periods is preferably in the range of x≥7, y≥7. The high refractive index layer H is preferably made of Ta2O5 or HfO2, and the low refractive index layer L is preferably made of SiO2.

[0009] The preferred materials for the inverted trapezoidal microstructure are HfO2, Ta2O5, SiO2, Ge, and ZnS.

[0010] The substrate (4) is preferably made of fused silica or glass.

[0011] The preferred wavelength range for the wavelength λ is 1050–1056 nanometers.

[0012] This invention achieves low internal electric field enhancement through the following mechanism: I. Regulation Mechanism of Electric Field Regulation Layer The inverted trapezoidal microstructure and the electric field modulation layer synergistically suppress electric field enhancement through the following three effects: First, the inverted trapezoidal microstructure avoids local hotspots in electric field intensity: The interference fringes formed by the incident light and diffracted light in the incident medium are perpendicular to the axis of symmetry of the incident light and diffracted light. That is, the interference fringes will tilt in the opposite direction to the direction of the incident light. This often causes local hotspots in electric field intensity to appear in the lower part of the backlight side of the grating ridge. The inverted trapezoidal microstructure can reduce the proportion of the bottom structure, effectively avoiding local hotspots in this position. Furthermore, the asymmetrical tilt angles on both sides can further avoid local hotspots on the backlight side.

[0013] Second, the standing wave node modulation effect: by precisely designing the optical thickness of each film layer of the electric field modulation layer, the electric field standing wave nodes near the bottom of the microstructure can be modulated into the air, thereby reducing the peak intensity of local hot spots within the structure.

[0014] Third, the diffraction intensity suppression effect: The selected inverted trapezoidal microstructure and the electric field modulation layer synergistically modulate the diffraction characteristics of the incident wave, enabling the grating to be used under conditions exceeding self-collimation. Under this condition, the angle of the -1st order diffracted light is much smaller than the incident angle. According to the law of conservation of energy and the projection ratio relationship, the intensity of the -1st order diffracted light will be suppressed, thereby reducing the electric field intensity of its interference fringes with the incident light.

[0015] II. Design Principles of Dual-Angle Multilayer Films The dual-angle multilayer film is composed of two films stacked at different angular reflectance peak positions, from bottom to top as (aHaL). x Membrane stack and (bHbL) y Membrane stack. Among them, the lower layer (aHaL) x The angular reflection peak of the film stack is designed near the incident angle θ, mainly to enhance the -1st order diffraction efficiency; while the upper layer (bHbL) y The angular reflection peak of the film stack is designed near the angle corresponding to the 0th order diffraction light, mainly to suppress the transmission loss of the 0th order diffraction light.

[0016] The adjustment principles of thickness scaling factors a and b are as follows: The reflection peak of the (HL) film stack is at a 0-degree incident angle. Increasing the film thickness leads to an increase in optical path length, while increasing the incident angle reduces the vertical wave vector component, offsetting the phase shift caused by the increased optical path length. Therefore, increasing the film thickness causes the reflection peak to shift towards a larger angle. Thus, by independently adjusting a and b, the reflection peaks of the two film stacks (aHaL) and (bHbL) can be aligned with their respective operating angles.

[0017] Compared with the prior art, the beneficial technical effects of the present invention are as follows: Enhanced Low Internal Electric Field: Through the synergistic effect of the inverted trapezoidal microstructure, the electric field modulation layer, and the dual-angle multilayer film, this invention can effectively control the electric field distribution inside the grating, reducing the maximum normalized electric field strength inside the microstructure to below 1.0. Under TM polarized laser irradiation at a wavelength of 1053 nm and an incident angle of 70°, the maximum normalized electric field strength is only 0.83, far lower than that of traditional self-collimating multilayer gratings (typically greater than 1.5).

[0018] High laser damage threshold: Due to the significant suppression of the internal electric field intensity, the present invention is expected to have a higher laser damage threshold, which is suitable for petawatt-level ultra-intense and ultra-short laser systems.

[0019] High diffraction efficiency: Under conditions of super-collimation, the dual-angle multilayer film of the present invention can significantly reduce the 0th and -1st order transmission losses during diffraction, so that the final -1st order diffraction efficiency can still reach more than 99%.

[0020] Large process tolerance: This invention has a wide tolerance range for key geometric parameters. It can achieve a diffraction efficiency of over 90% in the range of α=91°~107°, β=68°~81°, f=0.19~0.36, and d=1.18λ~1.45λ, which reduces the difficulty of nanofabrication.

[0021] Avoid beam obstruction: When used under conditions of super-collimation, the -1st order diffracted light does not return along the incident light path, thus avoiding spatial overlap between the incident beam and the diffracted beam, which facilitates optical path integration and system debugging. Attached Figure Description

[0022] Figure 1 This is a schematic cross-sectional view of the grating structure of the present invention.

[0023] Figure 2 This is a schematic cross-sectional view of the grating described in Example 1.

[0024] Figure 3 This is a graph showing the relationship between the -1st order diffraction efficiency of the grating described in Example 1 and the wavelength in the range of 1048-1058 nanometers with TM polarization.

[0025] Figure 4 This is a simulation diagram of the normalized electric field intensity inside the structure of the grating described in Example 1 under irradiation with a TM polarized laser at an incident angle of 70° and a wavelength of 1053 nm.

[0026] Figure 5 This is a parameter scan diagram of the diffraction efficiency at a wavelength of 1053 nm, showing the angle α between the left outer wall and the grating plane and the angle β between the right outer wall and the grating plane.

[0027] Figure 6 This is a parameter scan diagram of the ratio of bottom width to period f and the structure height d for the diffraction efficiency at a wavelength of 1053 nm.

[0028] Figure 7 This is a parameter scan diagram of the diffraction efficiency at a wavelength of 1053 nm with respect to thickness scaling factors a and b. Detailed Implementation

[0029] The present invention will be further described below with reference to embodiments and accompanying drawings, but this should not be construed as limiting the scope of protection of the present invention. Figure 1 The basic stacked structure of the grating of the present invention is shown, and the specific materials and parameters of each layer are described in detail in the following embodiments.

[0030] Example 1 A low internal electric field enhanced multilayer grating with super-autocollimation is proposed for use at 1053±3 nm, an incident angle θ of 70°, and a period Λ of 765 nm. The Littoral angle of the grating under this period is 43°, and 70° falls within the super-autocollimation angle range. Its structure is as follows... Figure 2 As shown. In this embodiment, the inverted trapezoidal microstructure 1 is made of Ta2O5, with a structural height d of 1420 nm, a bottom width to period Λ ratio f of 0.26, an angle α between the left outer wall and the grating plane of 102°, and an angle β between the right outer wall and the grating plane of 70°. The electric field modulation layer 2 consists of four dielectric films: film 2-1 is made of Ta2O5 with a thickness of 404 nm, film 2-2 is made of SiO2 with a thickness of 206 nm, film 2-3 is made of Ta2O5 with a thickness of 128 nm, and film 2-4 is made of SiO2 with a thickness of 823 nm. The thickness was determined by the following principles: (1) Standing wave node control: moving the electric field standing wave nodes near the bottom of the inverted trapezoidal microstructure into the air to reduce the peak electric field intensity; (2) Co-optimization with the inverted trapezoidal microstructure: achieving the dual objectives of maximizing the -1st order diffraction efficiency (≥99%) and minimizing the normalized electric field intensity (≤1.0), which was obtained through electromagnetic field simulation scanning optimization; (3) Adaptation beyond self-collimation conditions: ensuring that the grating is in Further suppression of the interference electric field intensity is achieved through the following process. The dual-angle multilayer film system is (aHaL). x (bHbL)y , where H represents a high refractive index layer made of Ta₂O₅ with a thickness of 150 nanometers, L represents a low refractive index layer made of SiO₂ with a thickness of 212 nanometers, a=1.24, b=1.15, x=13, y=13. The substrate 4 is a fused silica substrate plate. In this embodiment, the thicknesses of each film layer of the electric field regulation layer (404 nm, 206 nm, 128 nm, 823 nm) and the scaling factors of the dual-angle multilayer film (a=1.24, b=1.15) are all obtained by electromagnetic field simulation optimization, following the above design principles.

[0031] This embodiment is applied under TM polarization, as shown in Figure 3 , its diffraction efficiency at 1053±3 nanometers is greater than 99%. In addition, the simulation diagram of the normalized electric field intensity inside the structure under 1053 nanometer laser irradiation is as shown in Figure 4 , its maximum normalized electric field intensity is 0.83. Figure 5 is a parameter scanning diagram of the included angle α between the left outer wall and the grating plane and the included angle β between the right outer wall and the grating plane on the diffraction efficiency at the wavelength of 1053 nanometers. There are parameter combinations with diffraction efficiency >90% in the range of 91°<α<107° and 68°<β<81°. Figure 6 is a parameter scanning diagram of the bottom duty ratio f and the structure height d on the diffraction efficiency at the wavelength of 1053 nanometers. There are parameter combinations with diffraction efficiency >90% in the range of 0.19<f<0.36 and 1250 nm<d<1530 nm. Figure 7 is a parameter scanning diagram of the scaling factors a and b on the diffraction efficiency at the wavelength of 1053 nanometers. There are parameter combinations with diffraction efficiency >90% in the range of 1.15<a<1.3 and 1.01<b<1.15.

[0032] All electromagnetic field simulations in this embodiment adopt the following tools and conditions: Simulation software: RETICOLO (version V9a), using the Rigorous Coupled Wave Analysis (RCWA) method; Incidence conditions: TM polarized plane wave, incidence angle θ=70°, wavelength range 1048~1058 nm; Material refractive index data (λ=1053 nm): refractive index of Ta₂O₅ is 2.05, refractive index of SiO₂ is 1.45, refractive index of air is 1.00, refractive index of fused silica substrate is 1.52, and the extinction coefficient of each material is 0; Under the above simulation conditions, it is reproducible Figure 3 the diffraction efficiency curve (≥99% in the range of 1053±3 nm) and the Figure 4 normalized electric field intensity distribution (maximum value 0.83) as shown.

[0033] The grating described in this embodiment can be prepared by the following method: (1) Substrate cleaning: The fused silica substrate was ultrasonically cleaned in acetone, ethanol and deionized water in sequence, and then dried with nitrogen. (2) Dual-angle multilayer film deposition: using an ion beam sputtering deposition system, (aHaL) is sequentially deposited on a fused silica substrate. x Membrane stack and (bHbL) y Membrane stack. Taking this embodiment as an example, firstly, 13 cycles are deposited, in each cycle a Ta2O5 layer with a thickness of 186 nm and a SiO2 layer with a thickness of 263 nm are deposited sequentially; then 13 cycles are deposited, in each cycle a Ta2O5 layer with a thickness of 172.5 nm and a SiO2 layer with a thickness of 244 nm are deposited sequentially. (3) Deposition of electric field control layer and etching layer: 823nm SiO2, 128nm Ta2O5, 206nm SiO2, and 1824nm Ta2O5 are deposited sequentially on the dual-angle multilayer film. Among them, the upper 1420nm of the 1824nm Ta2O5 layer will be used as the grating etching layer, which will be etched to form an inverted trapezoidal microstructure. The remaining 404nm will be used as part of the electric field control layer. (4) Fabrication of inverted trapezoidal microstructure: Based on the etching rate ratio of photoresist to Ta2O5, a photoresist of appropriate thickness is spin-coated. After baking, the photoresist is exposed using a two-beam holographic interference exposure method. The grating period Λ=765nm is determined by adjusting the beam angle. Subsequently, the exposed sample is developed using a 0.5% sodium hydroxide solution to obtain the mask structure. Finally, reactive ion beam etching is used to obtain an inverted trapezoidal cross-sectional structure with α=102°, β=70°, and d=1420nm by dynamically controlling the etching parameters and the substrate tilt angle. (5) Cleaning and inspection: Remove residual resist, clean and dry, and then inspect the morphology and diffraction efficiency.

Claims

1. A low internal electric field enhanced multilayer grating that surpasses self-collimation, characterized in that: The grating consists of an inverted trapezoidal microstructure (1), an electric field modulation layer (2), a dual-angle multilayer film (3), and a substrate (4) from top to bottom. The inverted trapezoidal microstructure (1) has an inverted trapezoidal shape. The angle between its left outer wall and the grating plane is α, and the angle between its right outer wall and the grating plane is β. The ratio of its bottom width to its period Λ is the duty cycle f. Its structural height is d. Its period Λ and the wavelength λ satisfy 0.62 ≤ Λ / λ ≤ 0.

88. The angle θ satisfies the relationship between λ and Λ. ,and ; The electric field modulation layer (2) is composed of a dielectric film layer and is used to coordinate with the inverted trapezoidal microstructure (1) to modulate the electric field enhancement inside the grating; The dual-angle multilayer film (3) is composed of stacked multilayer films with two angular spectral reflection peaks at different positions, and its film system formula is (aHaL). x (bHbL) y Where H is the refractive index n H A high refractive index layer with a thickness of λ / (4n) H L is the refractive index n L A low-refractive-index layer with a thickness of λ / (4n) L ), where a and b are film thickness scaling factors that can adjust the position of the angular spectrum reflection peak, and a>0, b>0, and x and y are the number of stacking cycles, and x≥1, y≥1.

2. The low internal electric field enhancement multilayer grating with super-self-collimation according to claim 1, characterized in that: The electric field control layer (2) is composed of 1 to 5 dielectric film layers, and the thickness of each dielectric film layer is in the range of 30 to 1000 nanometers; when the number of layers is more than 1, the dielectric film layer is composed of two materials with different refractive indices alternately.

3. The grating according to claim 2, characterized in that: The two materials with different refractive indices are Ta2O5 and SiO2.

4. The low internal electric field enhancement multilayer grating with super-self-collimation according to claim 1, characterized in that: The grating is used under TM polarization.

5. A super-self-collimated low internal electric field enhanced multilayer grating according to claim 1, characterized in that: The included angle α ranges from 91° to 107°, the included angle β ranges from 68° to 81°, the duty cycle f ranges from 0.19 to 0.36, and the structural height d is 1.18 to 1.45 times λ.

6. The super-self-collimated low internal electric field enhanced multilayer grating according to claim 1, characterized in that: The thickness scaling factor a ranges from 1.15 to 1.3, and the value of b ranges from 1.01 to 1.15; the number of stacking cycles x ≥ 7, y ≥ 7.

7. The low internal electric field enhancement multilayer grating with super-self-collimation according to claim 1, characterized in that: The inverted trapezoidal microstructure (1) is made of HfO2, Ta2O5, SiO2, Ge or ZnS.

8. A super-self-collimated low internal electric field enhanced multilayer grating according to claim 1, characterized in that: In the dual-angle multilayer film (3), the material of the high refractive index layer H is Ta2O5 or HfO2, and the material of the low refractive index layer L is SiO2.

9. A super-self-collimated low internal electric field enhanced multilayer grating according to claim 1, characterized in that: The substrate (4) is made of fused silica or glass.

10. A low internal electric field enhanced multilayer grating with super-autocollimation according to claim 1, characterized in that: The wavelength λ used is in the range of 1050–1056 nanometers.

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