Method for bandwidth expansion of diffraction efficiency of pulse compressed gold gratings

Abstract

A method for expanding the diffraction efficiency bandwidth of a pulse-compressed gold grating, characterized by utilizing the self-shielding effect of grating coating and the local melting of ultra-short pulse laser irradiation to regulate the structure of the light-incident surface of the grating ridge. The method comprises the following steps: grating mask preparation, magnetron sputtering gold film plating to generate a raised structure at the bottom of the groove, constructing a data set of grating groove shape and diffraction efficiency and plating parameters, ultra-short pulse laser irradiation, constructing a data set of grating groove shape and diffraction efficiency and irradiation parameters, and two-dimensional plane scanning grating by laser. The raised structure at the bottom of the grating groove can be precisely regulated, and the inclined surface structure can be precisely regulated, so that the high diffraction efficiency region of the original grating is expanded to the short wave. The method is compatible with the whole set of processes of holographic interference lithography gold grating, the product aperture can be expanded to meters, and the method has important economic and practical value in the field of high-energy lasers.

Description

Technical Field

[0001] This invention pertains to reflective gratings, and specifically to a method for extending the diffraction efficiency bandwidth of pulse-compressed gold gratings. Background Technology

[0002] Research institutions around the world are using chirped pulse amplification (CPA) and optical parameter chirped pulse amplification (OPCPA) to push the peak output energy of lasers to the petawatt (PW) level.

[0003] In both CPA and OPCPA laser amplification technologies, the grating compressor is the core module. The key component of the grating compressor is the grating. Gold gratings are widely used due to their advantages of wide spectrum, wide angle spectrum, large aperture, good wavefront and environmental stability.

[0004] Gold gratings have been used for pulse compression for half a century. Currently, lasers need spectral bandwidths exceeding 100 nm to compress ultrashort pulses around a center wavelength of 810 or 910 nanometers (nm), but improving the efficiency of gold gratings at short wavelengths is difficult. Large-area holographic interference lithography fabricates gold gratings with sinusoidal groove profiles, and the fixed surface profile of the fabricated gold grating results in definite and unmodifiable optical properties. With the development of high-power laser devices, compressors require gold gratings to exhibit wider spectral bandwidths. Therefore, methods to further expand the spectral bandwidth of finished gold gratings or to control their high diffraction efficiency spectrum range have significant research prospects and application value. To our knowledge, there is currently no method for modifying the groove profile of large-aperture gold gratings to expand their diffraction efficiency bandwidth. Summary of the Invention

[0005] The technical problem this invention aims to solve is that the groove contour of gold gratings fabricated by large-area holographic interference lithography has a sinusoidal structure, and the surface contour of the fabricated gold grating is fixed, resulting in its fixed optical performance that cannot be modified. To address this problem, a method for expanding the diffraction efficiency bandwidth of pulse-compressed gold gratings is provided. This method combines self-masking in magnetron sputtering coating with ultrashort pulse laser irradiation to modify the original grating contour, resulting in a gold grating with a wider bandwidth. This method is compatible with the complete process of large-aperture holographic interference lithography gold gratings and has significant economic and application value in the field of large-aperture pulse-compressed gratings.

[0006] The technical solution of the present invention is as follows:

[0007] A method for extending the diffraction efficiency bandwidth of pulse-compressed gold gratings is characterized by utilizing the self-shading effect of the grating coating and the local melting of the grating ridge under ultrashort pulse laser irradiation to control the structure of the grating ridge facing surface. The method includes the following steps:

[0008] 1) Based on the compressor's requirements for the spectrum, angular spectrum, and polarization of the gold grating, the linear density, duty cycle, groove depth, and shape are optimized using vector diffraction theory, and grating masks with corresponding parameters are prepared using holographic interference lithography.

[0009] 2) The prepared grating mask is mounted on a sample disk that can move horizontally in a magnetron sputtering coating machine. Using magnetron sputtering coating technology, under the condition of a fixed set of conventional process parameters, the coating time t and the sample disk moving speed v are changed to deposit a gold film on the grating mask, so that the grating strips gradually form a pure gold protrusion structure due to the self-shading effect.

[0010] 3) The groove profile p and diffraction efficiency η of the gold grating prepared in step 2) were detected using an atomic force microscope and a spectrophotometer, and a dataset Q was established to connect t, v, p, η and the profile. + [t,v,p,η];

[0011] 4) Select a set of parameters Q from the dataset created in step 3). i + [t,v,p,η] is used for the preparation of large batches or large-diameter samples;

[0012] 5) Irradiate the sample from step 4) with an ultrashort pulse laser to cause local ablation of the light-facing surface of the grating ridge. The ablated area and the raised structure at the bottom of the groove are combined to partially or completely fill the groove between the original grating ridge and the raised structure to form a slope. The degree of filling is controlled by adjusting the laser single pulse energy density F, repetition frequency f, and pulse number N.

[0013] 6) The groove profile and diffraction efficiency of the gold grating modified in step 5) were detected using an atomic force microscope and a spectrophotometer, and a dataset Q was established for the gold grating before and after modification, based on F, f, N, p, and η. + [l x ,l y ,F,f,N,p,η];

[0014] 7) Using a diffraction efficiency greater than 90% as the standard, parameters with a wider diffraction efficiency spectral bandwidth than the unmodified gold grating are selected from the dataset established in step 6).

[0015] 8) Load the gold grating from step 4) onto a two-dimensional displacement stage, according to the selected set of parameters Q i +[F,N,f,p,η] is used to modify the grating, and the diffraction efficiency of the finished product is tested and verified. If the diffraction efficiency performance does not meet the standard, steps 5-7 are repeated.

[0016] The optimization calculation of line density, duty cycle, groove depth and shape using vector diffraction theory can be found in patent CN111580205A.

[0017] Compared with the prior art, the technical effects of the present invention are as follows:

[0018] 1) The method of the present invention can precisely control the raised structure and inclined structure of the bottom of the grating groove.

[0019] 2) The method of the present invention can extend the high diffraction efficiency region of the original grating to the short wavelength range.

[0020] 3) The method of the present invention is compatible with the complete process of holographic interference photolithography gold grating, and the product aperture can be extended to the meter level, which has important economic and practical value in the field of high-energy lasers. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the method for expanding the diffraction efficiency bandwidth of a pulse-compressed gold grating according to the present invention.

[0022] Figure 2 This is an atomic force map of the grating mask in Example 1.

[0023] Figure 3 This is an atomic force diagram showing the raised structure at the bottom of the groove after the grating is gold-plated in Example 1.

[0024] Figure 4 This is the atomic force map of the gold grating in Example 1 after being irradiated by laser.

[0025] Figure 5 This is a comparison diagram of the original grating in Example 1 and the -1st order diffraction efficiency after being processed by the method of the present invention.

[0026] Figure 1 middle:

[0027] 1-Raster mask, 2-Gold film, 3-Raised structure, 4-Sloping structure. Detailed Implementation

[0028] 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.

[0029] Example 1:

[0030] A method for extending the diffraction efficiency bandwidth of pulse-compressed gold gratings, the general process is as follows: Figure 1 As shown.

[0031] The grating parameters in this embodiment are 1400 lines per second, 0.7 duty cycle, 220 nm groove depth, and 3 shape factor. The grating with these specifications was fabricated using holographic interference exposure technology. The substrate was wiped with alcohol or acetone. A photoresist layer of approximately 220 nm thickness was coated onto the substrate using a spin coater at a rate of 2800 r / min for 30 s, and then baked at 100 °C for 2 min. Next, the substrate with the photoresist layer was exposed to 325 nm light at an exposure power of 50 μW for 200 s using a two-beam interference exposure method. The exposed sample was then immersed in a 4‰ sodium hydroxide solution for 50 s. The results are as follows: Figure 2 As shown.

[0032] A 200nm thick pure gold grating was deposited using magnetron sputtering technology. Figure 3 The raised structure formed at the bottom of the gold grating groove due to the self-occlusion effect is shown.

[0033] The method utilizes a center wavelength of 925 nm, a pulse width of 15 femtoseconds (fs), a repetition frequency of 1 Hz, and an energy density of 0.18 J / cm². 2 The sample is irradiated with a single-pulse laser using an ultrashort pulse. Figure 4 The diagram shows that after irradiation, a sloping structure is formed between the grating protrusions and the grating ridge.

[0034] The -1st order diffraction efficiency of the sample in the 700–900 nm wavelength range was tested at a fixed angle of 62°. For example... Figure 5 As shown, compared to the original gold grating, the processed gold grating has a first-order diffraction efficiency of over 90% in the 770–900 nm range, and its bandwidth extends to shorter wavelengths by nearly 20 nm.

[0035] The above embodiments will help those skilled in the art to further understand the present invention, but do not limit the present invention in any way. Those skilled in the art can modify or make equivalent substitutions to the technical solutions of the present invention without departing from the concept of the present invention, and these modifications and substitutions all fall within the protection scope of the present invention. The protection scope of the present invention should be determined by the claims.

Claims

1. A method for extending the diffraction efficiency bandwidth of pulse-compressed gold gratings, characterized in that... The method includes the following steps: 1) According to the compressor’s requirements for the spectrum, angular spectrum and polarization of the gold grating, the linear density, duty cycle, groove depth and shape are optimized by using vector diffraction theory, and grating masks with corresponding parameters are prepared by using holographic interference lithography (1); 2) The grating mask (1) prepared in step 1) is mounted on a sample disk that can move horizontally in a magnetron sputtering coating machine. Using magnetron sputtering coating technology, under the condition of a fixed set of conventional process parameters, the coating time t and the sample disk moving speed v are changed to deposit a gold film (2) on the grating mask, so that a pure gold protrusion structure (3) is gradually formed between the grating strips due to the self-shading effect. 3) The groove profile p and diffraction efficiency η of the gold grating prepared in step 2) were detected using an atomic force microscope and a spectrophotometer, and a dataset Q was established to connect t, v, p, η and the profile. + [t,v,p,η]; 4) In the dataset Q mentioned above + Choose a set of parameters from [t,v,p,η]. Prepare large batches or large-aperture gold grating samples; 5) Irradiate the sample in step 4) with an ultrashort pulse laser to cause local ablation of the light-facing surface of the grating ridge; combine the ablation area with the groove bottom protrusion structure (3) to partially or completely fill the groove between the original grating ridge and the protrusion to form a slope (4); the filling degree is controlled by controlling the laser single pulse energy density F, repetition frequency f, and pulse number N. 6) The groove profile and diffraction efficiency of the gold grating modified in step 5) were detected using an atomic force microscope and a spectrophotometer, and a dataset Q was established for the gold grating before and after modification, based on F, f, N, p, and η. + [F,N,f,p,η]; 7) Using a diffraction efficiency greater than 90% as the standard, parameters with a wider diffraction efficiency spectral bandwidth than the unmodified gold grating are selected from the dataset established in step 6). 8) Load the gold grating from step 4) onto a two-dimensional displacement stage, according to the selected set of parameters. Perform grating reshaping and verify the diffraction efficiency of the finished product. If the diffraction efficiency performance does not meet the standard, repeat steps 5-7 until the standard is met.

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