Polarization-independent, high-efficiency broadband metal-dielectric double-layer gratings for the 1053 nm band

Abstract

The present invention discloses a polarization-independent, high-efficiency, broadband metal dielectric double-layer grating for the 1053nm band, which comprises, from top to bottom, a grating ridge composed of two high-refractive-index dielectric layers, a grating matching layer, a metal reflector, and a grating substrate. Any layer of the metal dielectric double-layer grating is a rectangular parallelepiped structure. The period of the dielectric double-layer grating is 700-750nm, and the duty cycle is 0.4-0.48. The thickness of the metal reflector is greater than the skin depth of the metal. When TE or TM polarized light is incident at the Littrow angle, its 1st-order average diffraction efficiency exceeds 98% in the 1000-1100nm band, and the average value of the polarization-dependent loss over the entire band is 0.016dB. The present invention can be made by electron beam direct writing device and microelectronic deep etching processing, has a simple structure, large tolerance, is suitable for large-scale manufacturing, and can be applied to spectral beam combining and pulse compression systems.

Description

Technical Field

[0001] The present invention relates to the technical field of metal-dielectric double-layer gratings, and more particularly to a polarization-independent, high-efficiency, broadband metal-dielectric double-layer grating for the 1053nm band, particularly a double-layer reflective grating in which the refractive index of one dielectric layer can be changed. The metal-dielectric reflective double-layer grating is suitable for the 1053nm band and has high diffraction efficiency and low polarization-dependent loss in the -1 diffraction order at the Littrow angle of incidence. Background Art

[0002] Chirped pulse amplification technology and spectral beam combining technology are currently the main means of obtaining high-power laser output. The core components of these systems are gratings with high efficiency, broadband, and high laser damage threshold. Diffraction efficiency is an important parameter to measure the quality of diffraction gratings, because these systems often use multiple diffraction gratings. When the grating diffraction efficiency is low, the efficiency of the pulse will be significantly reduced after diffraction by multiple diffraction gratings. Moreover, the laser pulses that need to be compressed usually have wide spectral characteristics, so the grating needs to have high diffraction efficiency over a wider bandwidth. In addition, in the spectral beam combining system, a grating with polarization-independent characteristics can improve the system's efficiency in using the incident light.

[0003] Cao Hongchao et al. [Prior Art 1Cao Hong Chao et al., Appl. Opt., 57(4), 900(2018)] designed and manufactured a polarization-independent multilayer dielectric grating with a special structure. The diffraction efficiency of the tilted grating was calculated using a rigorous coupled wave algorithm [Prior Art 2Moharam MG et al., JOSA a, 12(5), 1068-1076, (1995)], and the grating structure was optimized by combining the simulated annealing algorithm [Prior Art 3Kirkpatrick S., et al, science, 220(4598), 671-680(1983)]. Finally, the diffraction efficiency measured in the wavelength range of 1040nm-1090nm was greater than 91%. However, the design of the all-dielectric grating requires following a special strategy, which makes the design work complicated. In addition, the all-dielectric grating is made of different materials stacked together, which causes mechanical stress between the thin films, which will cause cracks to appear on the grating surface. The use of metal dielectric gratings can solve this problem. The principle is that adding a metal layer as a reflector can reduce the number of stacked thin films with different refractive indices, thereby reducing the mechanical stress between the thin films. At the same time, the metal reflector can also increase the bandwidth of the grating. Hu An Duo et al. [Prior Art 4Hu An Duo et al., Applied Optics, 51(20), 4902-4906(2012)] reported a polarization-independent, broadband, high-efficiency metal-dielectric hybrid grating. At around 800nm, the -1 order diffraction efficiency under TE and TM polarized light incidence over a bandwidth of more than 120nm is more than 90%. However, the diffraction efficiency is not high enough, resulting in the system's laser output power being difficult to reach its peak value. As far as the preliminary research and development proposed by the present invention is concerned, there is currently no design for a polarization-independent, high-efficiency, broadband, metal-dielectric grating under the Littrow angle of incidence for the 1053nm band. Summary of the Invention

[0004] The present invention aims to address the aforementioned deficiencies in the prior art by providing a high-efficiency, broadband, polarization-independent metal-dielectric double-layer grating with a novel structure suitable for the 1053nm band. The grating ridge of this invention is a double-layer structure, with the first layer being a dielectric material with a refractive index of 1.8 to 2.3. A matching layer and a metal reflector are stacked on the grating substrate. This invention also features a wide manufacturing tolerance for ease of processing, achieving high-efficiency, broadband diffraction efficiency in the -1 diffraction order within the 1053nm band for both TE and TM polarized light at the Littrow angle of incidence.

[0005] The purpose of the present invention can be achieved by taking the following technical solutions:

[0006] A metal-dielectric double-layer grating with broadband, high diffraction efficiency, and polarization-independent characteristics suitable for a central wavelength of 1053 nm. The metal-dielectric double-layer grating comprises, from top to bottom, a grating ridge composed of two high-refractive-index dielectric layers, a grating matching layer, a metal reflector, and a grating substrate. Any layer of the metal-dielectric double-layer grating is a rectangular parallelepiped structure.

[0007] As a further preferred embodiment, the grating ridges are a first grating layer and a second grating layer arranged one above the other, wherein the first grating layer is made of a dielectric material with a refractive index between 1.8 and 2.3, and the above dielectric material is selected from one or more combinations of hafnium dioxide HfO2, zinc sulfide ZnS, tantalum pentoxide Ta2O5, and titanium dioxide TiO2; the material of the second grating layer is Si.

[0008] As a further preferred embodiment, the grating matching layer is made of SiO2 material.

[0009] As a further preferred embodiment, the metal reflector is made of a metal material with high reflectivity in the 1053 nm band, and the metal material includes but is not limited to one or more combinations of gold, silver, and aluminum.

[0010] As a further preferred embodiment, the substrate material of the grating substrate includes but is not limited to fused quartz or silicon.

[0011] As a further preferred embodiment, the grating period of the metal dielectric double-layer grating is 700-750 nm, and the duty cycle is 0.4-0.48.

[0012] As a further preferred embodiment, the thicknesses of the first grating layer, the second grating layer, and the grating matching layer are 204-242 nm, 104-140 nm, and 240-278 nm, respectively.

[0013] As a further preferred embodiment, the thickness of the metal reflector is greater than the skin depth of the metal.

[0014] As a further preferred embodiment, the metal dielectric double-layer grating has the characteristics of obtaining high diffraction efficiency and polarization independence over a wide bandwidth, and is suitable for pulse compression systems in chirped pulse amplification technology. It can also be used as a key beam combining device in a spectral beam combining system. It is a reflective grating with -1 order high diffraction efficiency at the Littrow incident angle.

[0015] The present invention has the following advantages and effects compared to the prior art:

[0016] (1) This invention proposes a broadband, high-efficiency, polarization-independent metal-dielectric grating suitable for the 1053nm band. The grating ridge is composed of two layers of high-refractive-index dielectric films. The first layer of the grating can be made of a dielectric material with a refractive index between 1.8 and 2.3, including hafnium dioxide (HfO2), zinc sulfide (ZnS), tantalum pentoxide (Ta2O5), and titanium dioxide (TiO2). The second layer of the grating is made of Si, significantly increasing the selectivity in actual production.

[0017] (2) When the grating period of the present invention is 727.6 nm, a ZnS layer with a thickness of 225.7 nm is used as the first grating layer, a Si layer with a thickness of 132.3 nm is used as the second grating layer, a SiO2 layer with a thickness of 264.6 nm is used as the grating matching layer, and a silver mirror with a thickness of 200 nm is used as the metal reflector, the -1 order diffraction efficiency of TE polarized light at the Littrow angle of incidence is greater than 90% in the range of 985 nm to 1120 nm and greater than 90% at 100 nm. The diffraction efficiency is greater than 98% from 1 to 1092 nm, greater than 99% from 1028 to 1080 nm, and averages 98.9% over the 1000 to 1100 nm bandwidth. The -1 order diffraction efficiency for TM polarized light is greater than 92% from 980 to 1120 nm, greater than 98% from 1008 to 1113 nm, and greater than 99% from 1027 to 1068 nm, with an average diffraction efficiency of 98.8% over the 1000 to 1100 nm bandwidth. The average -1 order polarization-dependent loss over the 1000 to 1100 nm bandwidth is 0.016 dB. Meanwhile, the diffraction efficiency of the 0th order diffracted light is extremely low, with the energy of the reflected light primarily concentrated in the -1 order.

[0018] (3) The metal dielectric double-layer grating designed in the present invention has a simple structure, a large manufacturing tolerance, is easy to manufacture, and is suitable for large-scale manufacturing. It is applicable to pulse compression systems and spectral beam combining systems in chirped pulse amplification technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The drawings described herein are used to provide a further understanding of the present invention and constitute a part of this application. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0020] Figure 1 This is a geometric structure diagram of the broadband, high-efficiency, polarization-independent metal-dielectric double-layer grating proposed in the present invention for the 1053 nm band;

[0021] Figure 1In the figure, 1 represents the incident light, 2 represents the -1st order diffracted light, and 3 represents the 0th order diffracted light. 4 represents the first layer of the grating with a refractive index of n1, 5 represents the second layer of the grating with a refractive index of n2, 6 represents the grating matching layer with a refractive index of n3, 7 represents the metal reflector of the grating, and 8 represents the grating substrate. In addition, Λ represents the grating period, f represents the duty cycle, h1 represents the thickness of the first layer of the grating, h2 represents the thickness of the second layer of the grating, h3 represents the thickness of the grating matching layer, and α represents the incident angle of the grating.

[0022] Figure 2 Schematic diagram of the -1st and 0th order diffraction efficiencies and polarization-dependent loss (PDL) within the required range of the present invention (980 nm to 1120 nm) for TE and TM polarized light incident at the Littrow angle, when the period Λ of the metal-dielectric double-layer grating is 727.6 nm, the incident angle α of the grating is 46.35°, the first grating layer is a ZnS (n1=2.29) layer with a thickness of h1=225.7 nm, the second grating layer is a Si (n2=3.56) layer with a thickness of h2=132.3 nm, the SiO2 (n3=1.45) matching layer has a thickness of h3=264.6 nm, and the metal reflector is made of silver with a thickness of 200 nm;

[0023] Figure 3 This is a contour plot of the -1st order diffraction efficiency as a function of the grating period Λ and the incident wavelength when TE and TM polarized light are incident at the Littrow angle within the 1000-1100 nm wavelength range of the present invention;

[0024] Figure 4 The present invention is a contour map showing the variation of the -1st order diffraction efficiency with the grating duty cycle f and the incident wavelength when TE and TM polarized light are incident at the Littrow angle within the wavelength range of 1000-1100 nm.

[0025] Figure 5 This is a contour plot of the -1st order diffraction efficiency of TE and TM polarized light incident at the Littrow angle in the 1000-1100 nm band of the present invention as a function of the first layer thickness h1 of the grating and the incident wavelength;

[0026] Figure 6 This is a contour plot of the -1st order diffraction efficiency of TE and TM polarized light incident at the Littrow angle in the 1000-1100 nm band of the present invention as a function of the thickness h2 of the second layer of the grating and the incident wavelength;

[0027] Figure 7 It is a contour diagram of the present invention showing the variation of -1 order diffraction efficiency with grating matching layer thickness h3 and incident wavelength when TE and TM polarized light are incident at the Littrow angle within the 1000-1100 nm band. DETAILED DESCRIPTION

[0028] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0029] Example 1

[0030] See also Figure 1 , Figure 1 This diagram shows the geometric structure of the broadband, high-efficiency, polarization-independent metal-dielectric double layer disclosed in this embodiment, suitable for the 1053 nm wavelength band. TM-polarized incident light corresponds to a magnetic field vector oscillating perpendicular to the plane of incidence, while TE-polarized incident light corresponds to an electric field vector oscillating perpendicular to the plane of incidence. The incident light strikes the grating at the Littrow angle.

[0031] This embodiment discloses a metal-dielectric double-layer grating suitable for broadband, high diffraction efficiency, and polarization-independent characteristics with a center wavelength of 1053 nm. The metal-dielectric double-layer grating comprises, from top to bottom, a grating ridge composed of two high-refractive-index dielectric layers, a grating matching layer, a metal reflector, and a grating substrate. Any layer of the metal-dielectric double-layer grating is a rectangular parallelepiped structure.

[0032] The grating ridges consist of a first and second grating layer arranged one above the other. The first grating layer is made of a dielectric material with a refractive index between 1.8 and 2.3, including hafnium dioxide (HfO2), zinc sulfide (ZnS), tantalum pentoxide (Ta2O5), and titanium dioxide (TiO2). The second grating layer is made of Si. The grating matching layer is made of SiO2. The metal reflector is made of a metal material with high reflectivity in the 1053nm band, including but not limited to one or more combinations of gold, silver, and aluminum. The grating substrate includes but is not limited to fused silica or silicon.

[0033] Depend on Figure 1 As shown, the present invention is suitable for broadband, high-efficiency, polarization-independent metal-dielectric double-layer gratings in the 1053nm band. The period is 700-750nm, the duty cycle is 0.4-0.48, and the thicknesses of the first grating layer, second grating layer, and grating matching layer are 204-242nm, 104-140nm, and 240-278nm, respectively. The thickness of the metal reflector is greater than the metal's skin depth.

[0034] In such Figure 1Under the grating structure shown, the present invention uses rigorous coupled wave theory to simulate the grating structure, calculate the diffraction efficiency, and optimizes the grating structure in combination with a simulated annealing algorithm, so that its -1 order diffraction efficiency in the 1008-1092nm band is greater than 98% when TE and TM polarized light is incident at the Littrow angle.

[0035] Figure 2 This diagram shows the -1st and 0th order diffraction efficiencies and polarization-dependent loss (PDL) for TE and TM polarized light incident at the Littrow angle within the specified range (980 nm to 1120 nm) for a metal-dielectric double-layer grating with a period Λ of 727.6 nm and an incident angle α of 46.35°. The first grating layer is a ZnS (n1 = 2.29) layer with a thickness of h1 = 225.7 nm, the second grating layer is a Si (n2 = 3.56) layer with a thickness of h2 = 132.3 nm, the SiO2 (n3 = 1.45) matching layer has a thickness of h3 = 264.6 nm, and the metal reflector is made of silver with a thickness of 200 nm.

[0036] Figure 2 The polarization-dependent loss is calculated as follows:

[0037] Where DE -1，TE and DE -1，TM represent the diffraction efficiency of -1 order under TE polarization and TM polarization, respectively;

[0038] Figure 2 The Littrow angle of incidence is calculated as follows:

[0039] Where λ is the operating wavelength of the grating.

[0040] Figures 3 to 7 The other parameters of the metal dielectric double-layer grating calculated in Figure 2 The calculated gratings are consistent.

[0041] Figure 3 and Figure 4 They are contour plots of the -1st order diffraction efficiency of TE and TM polarized light incident at the Littrow angle in the 1000-1100 nm band of the present invention as a function of the grating period Λ, the grating duty cycle f and the incident wavelength.

[0042] Figure 3 When Λ varies in the range of 719-749nm, the -1 order diffraction efficiency is greater than 96% under TE and TM polarized light incidence.

[0043] Figure 4 When f varies in the range of 0.4-0.46, the -1 order diffraction efficiency is greater than 90% under TE and TM polarized light incidence.

[0044] Figure 5 and Figure 6 They are contour diagrams of the -1st order diffraction efficiency of TE and TM polarized light incident at the Littrow angle in the 1000-1100nm band of the present invention as a function of the thickness h1 of the first layer, the thickness h2 of the second layer, and the incident wavelength.

[0045] Figure 5 When h1 varies in the range of 220-232 nm, the -1 order diffraction efficiency is greater than 96% for both TE and TM polarized light.

[0046] Figure 6 When h2 varies in the range of 124-134nm, the -1st order diffraction efficiency is greater than 96% under TE and TM polarized light incidence.

[0047] Figure 7 It is a contour diagram of the present invention showing the variation of the -1 order diffraction efficiency with the thickness h3 of the grating matching layer and the incident wavelength when TE and TM polarized light are incident at the Littrow angle within the 1000-1100 nm band. Figure 7 When h3 varies in the range of 244-272nm, the -1st order diffraction efficiency is greater than 96% under TE and TM polarized light incidence.

[0048] like Figure 2 As shown, under the Littrow angle of incidence, the -1 order diffraction efficiency of TE polarized and TM polarized light of the present invention is greater than 97% at 1000nm-1100nm, greater than 98% at 1008-1092nm, and greater than 99% at 1028-1068nm. The average diffraction efficiency is 98.9%, and the average value of polarization-dependent loss is 0.016dB.

[0049] like Figure 3-Figure 7 As shown, when the grating period Λ of the present invention is 719-749nm, the duty cycle f is 0.4-0.46, the thickness h1 of the first layer is 204-242nm, the thickness h2 of the second layer is 104-136nm, and the thickness h3 of the matching layer is 240-278nm, the -1 order diffraction efficiency of both TE and TM polarized light exceeds 90%.

[0050] Example 2

[0051] Table 1 lists the parameters used in Example 2. The incident angle of the metal-dielectric double-layer grating is the Littrow angle α = 46.35°. The refractive index of the first grating layer is n1, the second grating layer is Si (n2 = 3.56), the grating matching layer is SiO2 (n3 = 1.45), and the metal reflector is silver with a thickness of 200 nm. In the table, Λ represents the grating period, f is the grating duty cycle, h1 and h2 are the thicknesses of the first and second grating layers, respectively, and h3 is the thickness of the grating matching layer. It is the average diffraction efficiency in the 1000-1100nm band.

[0052] Average diffraction efficiency Calculated by the following formula:

[0053]

[0054] Where DE -1，TE (λ i ) and DE -1，TM (λ i ) represent the -1 diffraction order at λ under TE polarization and TM polarization, respectively. i Diffraction efficiency at nm.

[0055] When manufacturing the broadband, high-efficiency, polarization-independent metal-dielectric double-layer grating suitable for the 1053 nm band of the present invention, the period, duty cycle, and thickness of each layer of the grating should be appropriately selected according to the actual effect to be achieved.

[0056] Table 1. Diffraction efficiency values ​​of metal-dielectric double-layer gratings with different parameters

[0057]

[0058]

[0059] In summary, the high-efficiency, broadband, polarization-independent metal-dielectric double-layer grating suitable for the 1053nm band disclosed in the above embodiments is made by an electron beam direct writing device and microelectronic deep etching. It has a simple structure, a large manufacturing tolerance, is easy to manufacture, and can be manufactured on a large scale. The etched grating has stable and reliable performance and is suitable for pulse compression systems and spectral beam combining systems in chirped pulse amplification technology, and has important application prospects.

[0060] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A polarization-independent, high-efficiency, broadband metal-dielectric double-layer grating for the 1053 nm band, characterized in that: The metal dielectric double-layer grating comprises, from top to bottom, a grating ridge composed of two high-refractive-index dielectric layers, a grating matching layer, a metal reflector, and a grating substrate. Any layer of the metal dielectric double-layer grating is a rectangular parallelepiped structure. The grating ridge comprises the first and second grating layers arranged from top to bottom. The period of the metal-dielectric double-layer grating is 727.6nm, and the incident angle of the metal-dielectric double-layer grating is the Littrow angle , Cape Littrow The first grating layer is a ZnS layer with a thickness of h1 = 225.7 nm and a refractive index of n1 = 2.

29. The second grating layer is a Si layer with a thickness of h2 = 132.3 nm and a refractive index of n2 = 3.

56. The grating matching layer is SiO2 with a thickness of h3 = 264.6 nm and a refractive index of n3 = 1.

45. The metal reflector is made of silver and has a thickness of 200 nm. At Cape Littrow Under incident light, the -1-order diffraction efficiency of TE polarized light and TM polarized light is greater than 97% in the range of 1000nm-1100nm, the -1-order diffraction efficiency of TE polarized light and TM polarized light is greater than 98% in the range of 1008nm-1092nm, and the -1-order diffraction efficiency of TE polarized light and TM polarized light is greater than 99% in the range of 1028nm-1068nm.

2. The polarization-independent, high-efficiency, broadband metal-dielectric double-layer grating for the 1053 nm band according to claim 1, characterized in that: The material of the grating substrate is fused quartz or silicon.

3. The polarization-independent, high-efficiency, broadband metal-dielectric double-layer grating for the 1053 nm band according to claim 1, characterized in that: The metal dielectric double layer grating has the characteristics of high diffraction efficiency and polarization independence in a broadband, and is suitable for the pulse compression system in the chirped pulse amplification technology, or can be used as a beam combining device in the spectrum combining system. The metal dielectric double layer grating is a grating at the Littrow angle. A reflection grating with high diffraction efficiency of -1st order for TE and TM polarized light.

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