CN114879293A - Large-bottom wide-small-sharp-angle pulse compression metal grating and preparation method and application thereof - Google Patents

Abstract

Compatible with metal pulse compression gratings with large azimuth angle and super 400nm bandwidth, large bottom width and small sharp angle, including optimized design and fabrication process of grating parameters. The grating should have a bottom-width ratio greater than 0.6, a shape factor of 0.5-2.5, a groove depth of 160-250 nm, and a linear density of 1300-1650 g/mm. The preparation process includes substrate cleaning, gluing, baking, exposure, development and metal film plating. The broadband high-efficiency grating of the present invention has a structure with a large base and wide small sharp angles. In a large azimuth angle of plus or minus 15 or 20°, when the TM polarized light is incident at a Littrow angle of ‑1 order, the efficiency of the grating in the ultra-400nm bandwidth exceeds 90%. %. The grating and related process parameters of the present invention can support hundreds of petawatts of single-cycle pulse compression grating reserves.

Description

Pulse-compressed metal grating with large base width and small sharp angle and its preparation method and application

technical field

The invention belongs to a reflective grating, in particular to a metal pulse compression grating compatible with a large azimuth angle, a bandwidth of over 400 nm, a large bottom width and a small sharp angle, and a preparation method and application thereof.

Background technique

The field of ultra-intense and ultra-short lasers is at a critical stage of making major breakthroughs and developing applications. Internationally, ultra-intense ultra-short laser light sources and cutting-edge technology innovation platforms relying on them are being vigorously developed. Research institutes and scientific research institutions around the world effectively use chirped pulse amplification (CPA) and optical parameter chirped pulse amplification (OPCPA) to push the peak power of lasers to dozens of Petawatt (PW) scale. In the next ten years, 100PW ultra-intense and ultra-short laser devices will be put into use in succession around the world. The impact of higher peak power has become a racing field for major countries.

In CPA and OPCPA two laser amplification technologies, grating compressor is the core module. The key element of a raster compressor is the grating. Gratings widely used in high peak power laser systems include metal gratings, all-dielectric gratings and metal-dielectric hybrid gratings. Among them, all-dielectric gratings and metal-dielectric hybrid gratings are limited by spectral bandwidth and size, and have not yet been widely used in femtosecond pulse compression systems. Metal gratings are favored in small to large laser devices due to their advantages of large bandwidth, high efficiency, excellent surface shape, and wide angular spectrum.

At present, the pulse width can be compressed to the order of hundreds of femtoseconds (fs) or even 18.6fs under the order of 100joules or even kilojoules of pulse energy, and the peak power of the output pulse can reach the order of PW or even 10PW. In the planning of ultra-intense and ultra-short laser systems with higher pulse peak power, such as 100 PW or even EW, the compressed pulse width is further required to reach 10 fs, few cycles or even a single cycle. At this time, the spectral bandwidth of the pulse is required to reach 400 nanometers (nm).

At present, commercially available standard gold grating line densities (g/mm) are mainly 1400g/mm, 1480g/mm and 1740g/mm, the center wavelength is 800nm or 910nm, and the bandwidth is generally 100nm and 200nm [J.A.Britten et al. , Optics Letters 21, 540 (1996); US20200142107A1; CN111580205A; D.A. Alessi et al., Optics & Laser Technology 117, 239 (2019)]. Furthermore, the design and application of gold gratings is limited to no azimuth, whether for grating manufacturers or laser builders. In addition, although the gold grating process is mature, no detailed preparation process parameters have been disclosed. To the best of our knowledge, no one has designed, fabricated, and tested metal gratings with large azimuth angles (greater than plus or minus 15°) azimuth beyond 400 nm bandwidth.

SUMMARY OF THE INVENTION

The technical problem to be solved by the present invention is to realize a metal grating compatible with a large azimuth angle and a spectral width covering 400 nm. The diffraction efficiency of the grating is greater than 90% in the wavelength range from 750nm to greater than 1150nm under the condition that TM polarized light is incident at a large azimuth angle of ±15-20° and a -1 order Littrow angle. The detailed process parameters of the grating are suitable for metal gratings with a wide range of linear density (1300-1600 g/mm), and have practical value. The grating can be used for single-period (3fs) pulse spectrum broadening and compression compatible with larger pulse width, and has important economic and application value.

The technical solution of the present invention is as follows:

In one aspect, the present invention provides a pulse compression metal grating with large bottom width and small sharp angle, which is characterized in that the bottom width ratio of the metal grating is greater than 0.6, the shape factor is 0.5-2.5, the groove depth is 160-250 nm, and the line density is 160-250 nm. It is 1300～1650g/mm.

On the other hand, the present invention also provides a method for preparing the above-mentioned metal grating, which is characterized in that the method comprises the following steps:

1) The optimized design of the grating parameters: according to the requirements, use a strict coupled wave calculation program to determine the optimal grating parameters to achieve high diffraction efficiency within a specific spectral bandwidth, select the optimized single-layer grating structure or two-layer grating structure, optimize The initial wavelength is set in the range of 700nm～more than 1150nm. According to the calculation amount and the effect of the actual optimization efficiency curve, the number of layers of the selected grating structure is greater than 1, and the number of wavelength sampling points of the selected evaluation function is greater than 1. The metal grating mask structure is prone to high diffraction efficiency with a bandwidth of over 400 nm in the range of the bottom width ratio of more than 0.6, the shape factor of 0.5 to 2.5, the groove depth of 160 to 250 nm, and the linear density of 1300 to 1650 g/mm;

2) Substrate cleaning; that is, wipe the substrate with alcohol or acetone to remove impurities and contamination on the surface of the substrate to obtain a clean substrate;

3) Coating: Coat the photoresist on the cleaning substrate, the coating speed is 1800-3800r/r/min, the duration is 10-30s, the thickness of the coating is 150-300nm, and the coating is good. The base of the resist;

4) Baking: Bake the photoresist-coated substrate at 100° C. for 2-4 minutes:

5) Exposure: the described exposure is to interferometrically expose the substrate coated with the photoresist layer with a 325nm double beam with an exposure power of 50-60μW for 200 seconds or with a 413nm double beam with an exposure power of 13-16gW for 300 seconds. Obtain post-exposure samples;

6) Development: use a sodium hydroxide solution with a mass fraction of 4‰ to soak the exposed sample for 50-90s or a sodium hydroxide solution with a mass fraction of 6‰ for 5-14s to prepare a latent image grating;

7) Metal film plating: a metal film with a thickness of 100-300 nm is plated on the latent image grating by magnetron sputtering or electron beam evaporation technology, and the material of the metal film is gold, silver or gold-silver alloy .

The present invention also provides an application of the above-mentioned parallel-placed pair of pulse-compressed metal gratings with large bottom width and small sharp angle in a laser.

The technical effect of the present invention is as follows:

1) Under the condition that TM polarized light is incident at the -1 order Littrow angle, the grating of the present invention has a diffraction efficiency higher than 90% in a spectral bandwidth exceeding 400 nm.

2) The grating of the present invention has a -1st order diffraction efficiency higher than 90% in a wide-angle spectrum of about 10°.

3) The grating of the present invention has a -1st order diffraction efficiency higher than 90% under the condition of a large azimuth angle of plus or minus 15-20°.

4) The grating process parameters of the present invention are compatible with a wide range of grating line densities.

5) The grating of the present invention has a clear structure and stable process parameters, and the grating and related process parameters of the present invention can support hundreds of petawatts of single-cycle pulse compression grating reserves. It has important economic and practical value in spectrometer, high-power laser and other fields.

Description of drawings

FIG. 1 is a schematic diagram of the grating optimization structure of Embodiment 1. FIG.

FIG. 2 is a graph showing the grating azimuth angle and -1st order diffraction efficiency of Example 1. FIG. The incident angle is 50.3°, the grating line density is 1443g/mm, the duty cycle is 0.634, the groove depth is 225nm, and the shape factor is 1.91. The azimuth angle is ±20°.

FIG. 3 is a graph of the grating incident angle and -1st order diffraction efficiency of Example 1. FIG. No azimuth, grating linear density 1443g/mm, duty cycle 0.634, groove depth 225nm, shape factor 1.91. The incident angle is taken from 0 to 90°.

FIG. 4 is a graph showing the grating groove depth and -1st order diffraction efficiency of Example 1. FIG. Without azimuth, the incident angle is 50.3°, the grating linear density is 1443g/mm, the duty cycle is 0.634, and the shape factor is 1.91. The groove depth is 150 to 300 nm.

FIG. 5 is a graph of the grating duty ratio and -1st order diffraction efficiency of Example 1. FIG. There is no azimuth angle, the incident angle is 50.3°, the grating linear density is 1443g/mm, the groove depth is 225nm, and the shape factor is 1.91. The duty cycle is 0.5～1.

FIG. 6 is a graph of grating line density and -1st order diffraction efficiency of Example 1. FIG. Without azimuth, the incident angle is 50.3°, the grating duty cycle is 0.634, the groove depth is 225nm, and the shape factor is 1.91. Linear density 1300 ~ 1800g/mm.

FIG. 7 is a graph of the grating shape factor and -1st order diffraction efficiency of Example 1. FIG. There is no azimuth angle, the incident angle is 50.3°, the grating linear density is 1443g/mm, the duty cycle is 0.634, and the groove depth is 225nm. Azimuth shape factor 1-3.

8 is a graph of the measured-first-order diffraction efficiency of the grating of Example 1. FIG.

9 is a graph of the measured-first-order diffraction efficiency of the grating of Example 2. FIG.

FIG. 10 is a schematic diagram of a grating optimized structure in Embodiment 3. FIG.

11 is a graph of grating azimuth angle and -1st order diffraction efficiency of Example 3. FIG. The incident angle is 63.4°, the grating line density is 1527g/mm, the duty cycle is 0.765, the groove depth is 216nm, the shape factor is 1.36, and the gold film thickness is 236nm. The azimuth angle is ±20°.

FIG. 12 is a graph of the grating incident angle and -1st order diffraction efficiency of Example 3. FIG. No azimuth angle, grating linear density 1527g/mm, duty cycle 0.765, groove depth 216nm, shape factor 1.36, gold film thickness 236nm. The incident angle is taken from 0 to 90°.

FIG. 13 is a graph showing the grating groove depth and -1st order diffraction efficiency of Example 3. FIG. There is no azimuth angle, the incident angle is 63.4°, the grating linear density is 1527g/mm, the duty cycle is 0.765, the shape factor is 1.36, and the gold film thickness is 236nm. The groove depth is 150 to 300 nm.

FIG. 14 is a graph of the grating duty ratio and -1st order diffraction efficiency of Example 3. FIG. There is no azimuth angle, the incident angle is 63.4°, the grating linear density is 1527g/mm, the groove depth is 216nm, the shape factor is 1.36, and the gold film thickness is 236nm. The duty cycle is 0.5～1.

FIG. 15 is a graph of grating line density and -1st order diffraction efficiency of Example 3. FIG. Without azimuth, the incident angle is 63.4°, the duty cycle is 0.765, the groove depth is 216nm, the shape factor is 1.36, and the gold film thickness is 236nm. Linear density 1300 ~ 1800g/mm.

FIG. 16 is a graph of the grating shape factor and -1st order diffraction efficiency of Example 3. FIG. There is no azimuth angle, the incident angle is 63.4°, the grating linear density is 1527g/mm, the duty cycle is 0.765, the groove depth is 216nm, and the gold film thickness is 236nm. The shape factor is 0.5 to 3.

FIG. 17 is a graph of the -1st-order diffraction efficiency measured for the grating of Example 3. FIG.

Fig. 18 is a schematic diagram of a pulse compressor composed of a metal grating with a large base and wide small sharp angles.

In the figure: 1-metal layer, 2-mask layer, 3-metal grating with large bottom width and small sharp angle, 4-input pulse, 5-output pulse.

Detailed ways

The present invention will be further described below with reference to the embodiments and accompanying drawings, but the protection scope of the present invention should not be limited by this.

Example 1:

The single-layer grating structure is selected, the number of layers of the grating structure is 15, the optimized starting wavelengths are 750 nm and 1150 nm, and the number of wavelength sampling points of the evaluation function is 50. As shown in Figure 1, through optimization, the obtained grating parameters are a linear density of 1443 g/mm, a duty cycle of 0.634, a groove depth of 225 nm, and a shape factor of 1.91. As shown in Figure 2, in the presence of an azimuth angle of -15° to 15°, the -1st order diffraction efficiency bandwidth of the grating can still reach 400 nm. As shown in Figure 3, when the TM polarized light is incident in the range of 45-61°, the -1st-order diffraction efficiency bandwidth of the grating can still reach 400 nm without the azimuth angle. As shown in Figure 4, the tolerance range of the groove depth is 183-235 nm. As shown in Fig. 5, the large bottom width (duty ratio greater than 0.6) contributes to the -1st order diffraction efficiency bandwidth reaching 400 nm. As shown in Figure 6, the linear density tolerance is 1400-1500 g/mm. As shown in Fig. 7, the small sharp corners (shape factors between 1.3 and 2) contribute to the -1st order diffraction efficiency bandwidth reaching 400 nm. Prepare gratings of the above specifications. Wipe the substrate with alcohol or acetone. The substrate was spin-coated at a rate of 2500 r/min for 30 s with a spin coater to coat a photoresist layer with a thickness of about 230 nm on the substrate, and then the substrate was baked at 100 °C for 2 min. Next, the substrate coated with the photoresist layer was exposed to light of 325 nm for 200 s at an exposure power of 50 μW by means of double beam interference exposure. The exposed samples were immersed for 50s in a sodium hydroxide solution with a concentration of 4‰. Finally, a gold film with a thickness of about 150 nm is plated by magnetron sputtering technology. As shown in FIG. 8 , the -1st order diffraction efficiency of the prepared grating is higher than 90% in the range of 750 to more than 1150 nm.

Example 2:

The single-layer grating structure is selected, the number of layers of the grating structure is 15, the optimized starting wavelengths are 750m and 1150m, and the number of wavelength sampling points of the evaluation function is 100. As shown in Figure 1, through optimization, the obtained grating parameters are a linear density of 1443 g/mm, a duty cycle of 0.634, a groove depth of 225 nm, and a shape factor of 1.91. As shown in Figure 2, in the presence of an azimuth angle of -15° to 15°, the -1st order diffraction efficiency bandwidth of the grating can still reach 400 nm. As shown in Figure 3, when the TM polarized light is incident in the range of 45-61°, the -1st-order diffraction efficiency bandwidth of the grating can still reach 400 nm without the azimuth angle. As shown in Figure 4, the tolerance range of the groove depth is 183-235 nm. As shown in Fig. 5, the large bottom width (duty ratio greater than 0.6) contributes to the -1st order diffraction efficiency bandwidth reaching 400 nm. As shown in Figure 6, the linear density tolerance is 1400-1500 g/mm. As shown in Fig. 7, the small sharp corners (shape factors between 1.3 and 2) contribute to the -1st order diffraction efficiency bandwidth reaching 400 nm. Prepare gratings of the above specifications. Wipe the substrate with alcohol or acetone. The substrate was spin-coated at a rate of 3200 r/min for 30 s with a spin coater, and a photoresist layer with a thickness of about 200 nm was coated on the substrate, and then the substrate was baked at 100 °C for 2 min. Next, the substrate coated with the photoresist layer was exposed to light of 325 nm for 300 s at an exposure power of 35 μW using the method of double-beam interference exposure. The exposed samples were soaked for 12 s in a sodium hydroxide solution with a concentration of 6‰. Finally, a gold film with a thickness of about 210 nm is plated by magnetron sputtering technology. As shown in Fig. 9, the -1st order diffraction efficiency of the prepared grating is higher than 90% in the range of 750 to more than 1150 nm.

Example 3:

The double-layer grating structure is selected, the number of layers of the grating structure is 50, the optimized starting wavelengths are 750 nm and 1150 nm, and the number of wavelength sampling points of the evaluation function is 50. As shown in Figure 10, through optimization, the obtained grating parameters are the linear density of 1527g/mm, the duty cycle of 0.765, the groove depth of 216nm, the shape factor of 1.36, and the thickness of the gold film of 236nm. As shown in Fig. 11, in the presence of an azimuth angle of -20° to 20°, the -1st order diffraction efficiency bandwidth of the grating can still reach 400 nm. As shown in Figure 12, when the TM polarized light is incident in the range of 61.4-71°, the -1st order diffraction efficiency bandwidth of the grating can still reach 400 nm without azimuth angle. As shown in Fig. 13, the tolerance range of the groove depth is 166-238 nm. As shown in Figure 14, the large bottom width (duty ratio greater than 0.5) contributes to the -1st order diffraction efficiency bandwidth reaching 400 nm. As shown in Figure 15, the linear density tolerance is between 1384 and 1611 g/mm. As shown in Fig. 16, the small sharp corners (shape factors ranging from 0.83 to 2.21) contribute to the -1st order diffraction efficiency bandwidth reaching 400 nm. Prepare gratings of the above specifications. Wipe the substrate with alcohol or acetone. The substrate was spin-coated at a rate of 2100 r/min for 30 s with a spin coater to coat a photoresist layer with a thickness of about 250 nm on the substrate, and then the substrate was baked at 100 °C for 2 min. Next, the substrate coated with the photoresist layer was exposed to light of 325 nm for 200 s at an exposure power of 55 μW by using the method of double-beam interference exposure. The exposed samples were soaked for 12 s in a sodium hydroxide solution with a mass fraction of 6‰. Finally, a gold film with a thickness of 236 nm is plated by magnetron sputtering technology. As shown in FIG. 17 , the -1st order diffraction efficiency of the prepared grating is higher than 90% in the range of 742 to more than 1150 nm.

As shown in FIG. 18 , an embodiment of the present invention further provides a pulse compressor, which includes at least two metal gratings placed in parallel, and the wide-spectrum short pulse input from the front end is compressed into a single pair of ultra-wideband metal gratings after passing through two pairs of ultra-wideband metal gratings. Periodic or near-single-period laser pulses. In the compressor provided by the embodiment of the present invention, a metal pulse compression grating with a large bottom width and a small sharp angle is used as a pulse compression element, which has high -1 order diffraction efficiency, flexible use and wide bandwidth, and is an ideal diffraction grating. Optical devices have good practical prospects in the field of high-power lasers.

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 form. Those of ordinary skill in the art can modify or equivalently replace the technical solutions of the present invention without departing from the concept of the present invention, which all belong to the protection scope of the present invention. The protection scope of the present invention should be based on the claims.

Claims (7)

1. A pulse-compressed metal grating with large bottom width and small sharp angle, characterized in that the bottom width ratio of the metal grating is greater than 0.6, the shape factor is 0.5 to 2.5, the groove depth is 160 to 250 nm, and the linear density is 1300 to 1650 g /mm. 2. The pulse-compressed metal grating with large bottom width and small sharp angle according to claim 1, characterized in that, under the condition that TM polarized light is incident at -1-order Littrow angle, the diffraction efficiency is higher than 90 in the spectral bandwidth exceeding 400 nm. %. 3. The pulse-compressed metal grating according to claim 1, characterized in that, under the condition of large azimuth angle within plus or minus 15° to 20°, it has a -1 order diffraction efficiency higher than 90% . 4. A preparation method of the pulse-compressed metal grating with large bottom width and small sharp angle according to any one of claims 1-3, wherein the preparation method comprises the following steps: Step 1) Optimal design of grating parameters: according to the requirements, determine the optimized single-layer grating structure or two-layer grating structure. The number of sampling points is greater than 1; The designed metal grating has a bottom width ratio of more than 0.6, a shape factor of 0.5 to 2.5, a groove depth of 160 to 250 nm, and a linear density of 1300 to 1650 g/mm; Step 2) substrate cleaning and gluing: photoresist coating is performed on the cleaned substrate at a gluing rate of 1800-3800 r/min, the coating time is 10-30 s, and the thickness is 150-300 nm; Step 4) Baking: Bake the photoresist-coated substrate at 100° C. for 2-4 minutes: Step 5) Exposure: interference exposure with 325nm double beams at 50-60μW exposure power for 200 seconds or 413nm double beams at 13-16μW exposure power for 300 seconds to obtain exposed samples; Step 6) development: soak the exposed sample in a sodium hydroxide solution with a mass fraction of 4‰ for 50-90s or a sodium hydroxide solution with a mass fraction of 6‰ for 5-14s to obtain a latent image grating; Step 7) Metal film plating: plating a metal film with a thickness of 100-300 nm on the latent image grating by magnetron sputtering or electron beam evaporation technology. 5 . The method for preparing a pulse-compressed metal grating with a large bottom and a wide small sharp angle according to claim 4 , wherein the material of the metal film is gold, silver or gold-silver alloy. 6 . 6. the preparation method of the pulse compression metal grating of large bottom width small sharp angle according to claim 4, is characterized in that, described substrate cleaning refers to wiping substrate with alcohol or acetone, removes the impurity and pollution of substrate surface, obtains Clean substrate. 7 . A pulse compressor, characterized in that it comprises at least two pulse-compressed metal gratings with large base width and small sharp angle according to any one of claims 1 to 6 placed in parallel. 8 .

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