High-longitudinal-width-ratio focusing light field modulation method based on double diaphragms

Abstract

The invention discloses a double-diaphragm high-longitudinal-width-ratio focusing light field modulation method, and belongs to the technical field of femtosecond laser micro-nano machining. According to the method, based on the combination of two diaphragms with continuously adjustable apertures, light field modulation of Gaussian laser beams is realized, a high-longitudinal-width-ratio focusing light spot in the optical axis direction is formed through a microscopic focusing objective lens, the axial size of a two-photon polymer voxel is greatly improved, and the precision of the two-photon polymer voxel is improved. Therefore, the problems of low axial processing efficiency and complex light field modulation algorithm of the current two-photon polymerization are effectively solved. Hardware implementation of the method needs a femtosecond laser system, a double-diaphragm assembly and a microscopic focusing system thereof, wherein the double-diaphragm assembly plays a decisive role in light field modulation. By adjusting the size and spacing of the diaphragm, the modulation of focusing light fields with different longitudinal-width ratios can be realized. The invention provides an effective technical scheme for two-photon polymerization axial high-efficiency processing, and meanwhile, the light field modulation method is also suitable for the fields of optical imaging, photoacoustic imaging and the like with similar requirements on axial light field distribution.

Description

Technical Field

[0001] The present invention relates to the technical field of ultrafast laser micro-nano processing, and particularly relates to a method for modulating a high aspect ratio focused light field based on a double aperture. It has broad potential applications in the fields of optoelectronics and biomedicine, such as flexible optoelectronic devices, micro-nano robots, and the regulation of three-dimensional growth characteristics of cells, etc. Background Art

[0002] As one of the important methods of micro-nano additive manufacturing, femtosecond laser two-photon polymerization has unique advantages in the fabrication of high-resolution three-dimensional complex micro-nano structures. The pulse width of femtosecond laser can reach dozens to hundreds of femtoseconds. The ultrashort pulse time will essentially change the interaction mechanism between the laser and the sample, so that the energy can act quickly and precisely inside the material. Utilizing the ultra-high peak power (10 12 ~10 15 W / cm 2 ) of femtosecond laser, extremely small heat-affected area and energy threshold effect, two-photon polymerization based on femtosecond laser can fabricate three-dimensional micro / nano structures with a resolution less than the diffraction limit inside transparent materials. In recent years, femtosecond laser two-photon polymerization has been widely applied to the preparation of functional devices with complex three-dimensional micro-nano structures, such as soft robots, flexible sensors, micro-nano optical devices, etc.

[0003] At present, the realization of high-throughput micro-structure fabrication based on femtosecond laser two-photon polymerization is mainly achieved through two methods: sequential scanning processing mode and light field projection plane processing mode. The sequential scanning processing mode uses one-dimensional and two-dimensional scanning galvanometers to achieve point-by-point scanning processing of complex structures, which can greatly improve the two-photon polymerization processing efficiency while ensuring extremely high resolution. However, in the application of fabricating millimeter- or even centimeter-scale micro-structures, the sequential scanning mode often requires dozens of hours or even days, severely restricting the application fields of femtosecond laser processing technology. The femtosecond laser light field projection plane processing mode usually uses a digital micromirror device (DMD) and a liquid crystal spatial light modulator (LC-SLM) to modulate the spatial light field, and realizes high-efficiency patterning processing of micro-structures through patterned focused spots, greatly improving the femtosecond laser processing speed. However, in the application of fabricating micro-structures with a large axial height, the surface projection processing method based on DMD and LC-SLM faces two challenges. On the one hand, the surface projection processing mode focuses more on improving the lateral processing efficiency and lacks an effective method for improving the axial processing efficiency. The surface projection mode needs to perform layer cutting on the three-dimensional structure and realizes the fabrication of micro-structures with a large height through a three-dimensional layer-by-layer scanning method, resulting in a low processing efficiency for high aspect ratio micro-structures. On the other hand, the algorithm for realizing axial light field modulation in the surface projection processing mode is complex, and the quality of the algorithm has a great impact on the processing results. The above factors severely limit the application fields and commercialization process of femtosecond laser two-photon polymerization. Therefore, there is an urgent need for a simple and efficient axial light field modulation method to achieve high-efficiency axial processing of two-photon polymerization. Summary of the Invention

[0004] In view of the deficiencies of the prior art, the present invention provides a high aspect ratio focused light field modulation method with double apertures. An axial light modulation component is composed of two apertures with adjustable diameters (utilizing the combined action of the Fresnel diffraction effects of two apertures with different diameters) to modulate the original laser light field with a Gaussian distribution, forming a high aspect ratio focused light field distributed along the axis, greatly increasing the axial two-photon polymerization voxel size, and further realizing high-efficiency axial processing of two-photon polymerization.

[0005] The technical solution adopted by the present invention to achieve the above object is: A high aspect ratio focused light field modulation method based on double apertures, comprising the following steps:

[0006] S1. Build a high aspect ratio focused light field modulation device based on double apertures; the device includes a femtosecond laser system, a double aperture modulation component, and a microscopic focusing system; the femtosecond laser system outputs a femtosecond laser beam, which is modulated by the double aperture modulation component to adjust the light field distribution, and then the modulated light field is focused by the microscopic focusing system to adjust the two-photon polymerization focal plane;

[0007] S2. Adjust the parameters, calculate the light field distribution after diffraction by the double-aperture modulation component, and deduce the light field distribution irradiated at the sample focus according to the light propagation path;

[0008] S3. Change the laser power to adjust the two-photon polymerization voxels and output a customized spot voxel shape.

[0009] The femtosecond laser system is a femtosecond laser (1) for outputting a femtosecond laser beam with a Gaussian distribution; the double-aperture modulation component includes an adjustable aperture I (2) and an adjustable aperture II (3) sequentially placed on the optical path for modulating the light field distribution; the microscopic focusing system includes a microscopic objective lens (5) and a sample platform (6) sequentially placed in the direction of the modulated light field for focusing the modulated light field and adjusting the two-photon polymerization focal plane.

[0010] A dichroic mirror (4) is also provided between the adjustable aperture II (3) and the microscopic objective lens (5) for changing the optical path to make the device structure compact.

[0011] The adjusted parameters include: the apertures of the adjustable aperture I (2) and the adjustable aperture II (3), the distance between the aperture I (2) and the aperture II (3), and the distance between the aperture II (3) and the microscopic objective lens (5); different light field energy distributions are realized by adjusting the through-hole apertures of the aperture I (2) and the aperture II (3), and different light field energy distributions are realized by adjusting the distance between the aperture II (3) and the distance between the aperture II (3) and the microscopic objective lens (5); by adjusting the diameters and distances of the two apertures, a focused spot with different aspect ratios is formed.

[0012] The Gaussian beam emitted from the femtosecond laser system passes through the adjustable aperture I (2) and the adjustable aperture II (3) in sequence, undergoes two Fresnel diffractions respectively, and is focused by the microscopic objective lens to form different high-aspect-ratio focused spots along the optical axis direction.

[0013] The light field distribution irradiated at the sample focus is:

[0014]

[0015]

[0016]

[0017] where C is a constant, is the light field polar coordinate at the entrance pupil 5-1 of the microscopic objective lens 5, obtained from the light field distribution after diffraction by the double-aperture modulation component, is the light field polar coordinate of the focused wavefront 5-2 formed by focusing of the microscopic objective lens 5, is the angle between the projection of the optical vector in the xy plane and the x-axis, θ is the projection of the optical vector on the z-axis, is the numerical aperture of the microscope objective 5, and n is the refractive index of the medium between the microscope objective 5 and the sample 6.

[0018] The method of changing the laser power to adjust the two-photon polymerization voxel is as follows: when the high aspect ratio focused spot along the optical axis direction is completely immersed in the photoresist sample, two-photon polymerization is carried out by a single femtosecond laser pulse to obtain a high aspect ratio two-photon polymerization voxel, and the height and diameter of the voxel will increase with the increase of the laser power.

[0019] The height of the spot voxel microcolumn customized according to the method is 0-143 μm, the width range of the microcolumn is 1.9-6 μm, and the highest aspect ratio is 23.8.

[0020] The present invention has the following beneficial effects and advantages:

[0021] 1. The present invention proposes a new method for modulating a high aspect ratio focused light field with a double aperture. By using two consecutive adjustable apertures, the incident femtosecond laser beam with a Gaussian distribution is modulated by Fresnel diffraction light field twice, so that the spot formed after focusing by the microscope objective has a high aspect ratio along the axis. This light field modulation method has low cost and is easy to operate, greatly simplifying the light field modulation process of two-photon polymerization.

[0022] 2. The high aspect ratio focused spot formed by the double aperture modulation of the present invention can realize single-pulse polymerization of axially ultra-high aspect ratio voxels. For example, for a micro-structure with a height of more than one hundred micrometers, the polymerization time only needs 217 fs, greatly improving the axial processing efficiency of the micro-structure.

[0023] 3. The present invention provides an effective technical solution for high-efficiency axial processing of two-photon polymerization, but the light field modulation method is not limited to two-photon polymerization. This method is also applicable to optical imaging, photoacoustic imaging and other fields with similar requirements for axial light field distribution. Description of the drawings:

[0024] Figure 1 is a schematic structural diagram of the optical system of the present invention;

[0025] Figure 2 is a schematic diagram of the double aperture light field modulation principle;

[0026] Figure 3 is the modulation simulation result of the aperture I on the Gaussian light field;

[0027] Figure 4 is the modulation simulation result of the aperture II on the light beam emitted by the aperture I;

[0028] Figure 5 is the experimental effect diagram of the present invention.

[0029] In the figure: 1 is a femtosecond laser, 2 is a variable aperture I, 3 is a variable aperture II, 4 is a dichroic mirror, 5 is a microscope objective, 5-1 is the incident surface of the microscope objective, 5-2 is the apodized light field distribution formed after the microscope objective focuses, 6 is a sample stage, 7 is a three-dimensional piezoelectric ceramic displacement platform, and 8 is a three-dimensional macro motion platform. Specific implementation manner:

[0030] To make the above objects, features, and advantages of the present invention more obvious and understandable, the following will describe the specific implementation method of the present invention in detail with reference to the accompanying drawings. Many specific details are set forth in the following description in order to fully understand the present invention. However, the present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without departing from the connotation of the invention. Therefore, the present invention is not limited by the specific implementations disclosed below.

[0031] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which this invention belongs. The terms used in the description of the invention herein are for the purpose of describing specific embodiments only and are not intended to limit the present invention.

[0032] The present invention provides a method for modulating a high aspect ratio focused light field based on a double aperture. Figure 1 The following is a schematic diagram of the optical system of the present invention. The double aperture light field modulation system includes a femtosecond laser system, a double aperture light field modulator, and a microscopic focusing system. The specific structures and connection relationships of each part are as follows:

[0033] The femtosecond laser system 1 is used to output an original femtosecond laser beam with a Gaussian distribution; the double aperture optical modulation component is composed of two apertures with continuously adjustable light transmission apertures (adjustable aperture I 2 and adjustable aperture II 3), and the Gaussian laser beam is subjected to two Fresnel diffractions through the adjustable aperture I 2 and the adjustable aperture II 3 respectively, that is, two modulations of the spatial distribution are performed; the microscopic focusing system includes a dichroic mirror 4, a microscope objective 5, a sample stage 6, a piezoelectric ceramic displacement platform 7, and a macro motion platform 8, and is used to focus the modulated light field and adjust the sample position.

[0034] The sample stage 6 is arranged on the three-dimensional piezoelectric ceramic displacement platform 7, and the three-dimensional piezoelectric ceramic displacement platform 7 is fixed on the three-dimensional macro motion platform 8, and large-range and high-precision motion are realized through the combined motion of the two platforms.

[0035] The effective aperture of the adjustable aperture I 2 is 1 - 4 mm, and the aperture of the adjustable aperture II 3 is 1 - 4 mm. By adjusting the light-passing apertures of the aperture I 2 and the aperture II 3, different light field energy distributions can be achieved; by adjusting the distance between the aperture I 2 and the aperture II 3 and the distance between the aperture II 3 and the microscope objective 5, different light field energy distributions can be achieved. By adjusting the two aperture parameters, focused spots with different aspect ratios can be formed. Typically, the light-passing diameter of the adjustable aperture I is set to 3 mm, and the light-passing diameter of the adjustable aperture II is set to 2 mm.

[0036] This method utilizes the femtosecond laser energy threshold effect. By precisely adjusting the sizes and spatial relative positions of the aperture I and the aperture II, single-pulse two-photon polymerization of voxels with different heights and diameters can be achieved. After two-photon polymerization is completed, the excess photoresist needs to be removed through development to obtain the expected high-aspect-ratio voxel structure.

[0037] Based on the single-pulse exposure processing of this method, the height of the micro-columns can reach 143 μm, the width range of the micro-columns is 2.5 - 7 μm, and the highest aspect ratio is up to 23.8.

[0038] Next, a brief analysis and derivation of the modulation mechanism of the double aperture on the light field will be carried out.

[0039] The beam emitted by the femtosecond laser is Gaussian-distributed. When the Gaussian light passes through the aperture I 2, the light field E at any point (x 0 , y 0 ) on the aperture I 2 can be expressed as: 0 It can be expressed as:

[0040]

[0041] Among them, R 1 is the radius of the adjustable aperture I 2, A 0 represents the light field amplitude, w is the waist radius of the Gaussian beam, and r represents the distance of the observation point from the optical axis At the aperture, z 0 = 0. The z direction represents the light field transmission direction.

[0042] The light field E at the position (x 1 , y 1 , z 1 ) after Fresnel diffraction by the adjustable aperture I 2 is: 1 It is:

[0043]

[0044] Among them, the wave number r 1 represents the distance of the observation point from the aperture I 2 The spatial frequencies in the x and y directions are respectively is the Fourier transform function.

[0045] The distance between the adjustable aperture I2 and the adjustable aperture II3 is L1. At the position of the adjustable aperture II3, z 1 = L 1 , and the radius of the adjustable aperture II3 is R 2 . The light field distribution at the aperture II3 can be expressed as:

[0046]

[0047] The distance between the adjustable aperture II3 and the objective lens is L 2 . After Fresnel diffraction by the adjustable aperture II3, the light field distribution at a position L 2 behind the adjustable aperture II3 at the position (x 2 , y 2 , L 2 ) of the light field distribution E 2 (x 2 , y 2 , L 2 ) can be expressed as:

[0048]

[0049] According to the schematic diagram of light propagation Figure 2 , the light field distribution at the focus on the sample stage 6 is derived as (Debye vector integral):

[0050]

[0051]

[0052]

[0053]

[0054] where C is a constant, is the polar coordinates of the light field at the entrance pupil 5-1 of the microscope objective 5 (which can be obtained by converting from the three-dimensional coordinates (x 2 , y 2 , z 2 ))), is the polar coordinates of the light field of the focusing wavefront 5-2 formed by the focusing of the microscope objective 5 (which can be obtained by converting from the three-dimensional coordinates (x 4 , y 4 , z 4 ))), is the angle between the projection of the optical vector in the xy plane and the x-axis, θ is the projection of the optical vector on the z-axis, is the numerical aperture of the microscope objective 5, and n is the refractive index of the medium between the microscope objective 5 and the sample 6.

[0055] The wavelength of the femtosecond laser is 1030 nm. When the size of the adjustable aperture I2 is set to 3 mm and the distance between the adjustable aperture I2 and the adjustable aperture II3 is 100 mm - 800 mm, the light field energy distribution at different distances is calculated by Matlab simulation as follows Figure 3 shown. When the distance between the adjustable aperture I2 and the adjustable aperture II3 is set to 500 mm, the light field energy distribution at 100 mm - 800 mm behind the adjustable aperture II3 is as follows Figure 4 shown. It can be seen from the simulation results that the aperture sizes of both the adjustable aperture I2 and the adjustable aperture II3 have an impact on the light field distribution. When the positions of the apertures are fixed, the final light field distribution can be modulated by adjusting the apertures of the double apertures.

[0056] After the light field modulated by the double apertures is focused by the microscope objective 5, the energy distribution of the focused spot is ellipsoidal, and the major axis of the ellipsoid is along the optical axis direction. To verify this result, by finely adjusting the three-dimensional motion platform 8 and the piezoelectric ceramic nano-displacement platform 7, the spot focused by the microscope objective 5 is completely within the sample 6. Using the femtosecond laser single-pulse two-photon polymerization method, the photoresist within the focused spot range is polymerized. After the photoresist is developed, a spindle-shaped microstructure with a high aspect ratio can be obtained, as follows Figure 5 shown. The length of the microcolumn is 143 μm, the diameter is 6 μm, and the aspect ratio can reach 23.8. This experimental result can indirectly prove the effectiveness of the double-aperture high-aspect-ratio light field modulation method. The height and diameter of the microstructure are affected not only by the aperture modulation but also by the laser power. The higher the laser power, the larger the height and diameter of the microstructure.

[0057] This method modulates the incident laser light field through two consecutive Fresnel diffractions of the double apertures, and then forms a high-aspect-ratio focused light field through the focusing of the microscope objective, effectively solving the problem of the low axial processing efficiency of the existing femtosecond laser two-photon polymerization, greatly improving the microstructure processing efficiency of the femtosecond laser two-photon polymerization, providing an effective solution for the commercial application of the femtosecond laser processing technology, and also helping to broaden the application field of the femtosecond laser.

[0058] The above is only a preferred embodiment of the present invention, and does not impose any limitation on the present invention. Any simple modification, change, and equivalent structural change made to the above embodiments according to the technical essence of the present invention still fall within the protection scope of the technical solution of the present invention.

Claims

1. A high aspect ratio focusing light field modulation method based on double apertures, characterized in that: The steps include: S1. Build a high aspect ratio focusing light field modulation device based on double apertures; the device comprises a femtosecond laser system, a double aperture modulation component and a microscopic focusing system; the femtosecond laser system outputs a femtosecond laser beam, which is modulated by the double aperture modulation component to modulate the light field distribution, and then the modulated light field is focused by the microscopic focusing system to adjust the two-photon polymerization focal plane; S2, adjusting parameters, calculating the light field distribution after diffraction by the double aperture modulation component, and deriving the light field distribution irradiated at the focus of the sample according to the light propagation path; S3. Change the laser power to adjust the two-photon polymerization voxel and output a customized spot voxel shape.

2. The high aspect ratio focusing light field modulation method based on double apertures according to claim 1, characterized in that: The femtosecond laser system is a femtosecond laser (1) for outputting a femtosecond laser beam with a Gaussian distribution; the double aperture modulation component comprises an adjustable aperture I (2) and an adjustable aperture II (3) sequentially placed on an optical path, for modulating the light field distribution; the microscopic focusing system comprises a microscope objective lens (5) and a sample platform (6) sequentially placed in the direction of the modulated light field, for focusing the modulated light field and adjusting the two-photon polymerization focal plane.

3. The high aspect ratio focusing light field modulation method based on double apertures according to claim 2, characterized in that: A dichroic mirror (4) is also arranged between the adjustable diaphragm II (3) and the microscope objective lens (5) to change the optical path so that the device structure is compact.

4. The dual aperture modulation assembly according to claim 2, characterized in that: The adjustment parameters include: the aperture of the adjustable diaphragm I (2), the aperture of the adjustable diaphragm II (3), the spacing between the diaphragm I (2) and the diaphragm II (3), and the spacing between the diaphragm II (3) and the microscope objective (5); different light field energy distributions are achieved by adjusting the light apertures of the diaphragm I (2) and the diaphragm II (3); different light field energy distributions are achieved by adjusting the spacing between the diaphragm II (3) and the spacing between the diaphragm II (3) and the microscope objective (5); and focused light spots with different aspect ratios are formed by adjusting the diameters and spacings of the two diaphragms.

5. The high aspect ratio focusing light field modulation method based on double apertures according to claim 1, characterized in that: The Gaussian beam emitted from the femtosecond laser system passes through the adjustable aperture Ⅰ(2) and the adjustable aperture Ⅱ(3) in sequence, undergoes two Fresnel diffractions respectively, and is then focused by the microscope objective to form different high aspect ratio focused spots along the optical axis.

6. The high aspect ratio focusing light field modulation method based on double apertures according to claim 1, characterized in that: The light field distribution at the sample focus is: Among them, C is a constant, is the polar coordinate of the light field at the entrance pupil 5-1 of the microscope objective 5, The light field distribution after diffraction by the double aperture modulation component is obtained. is the light field polar coordinate of the focusing wavefront 5-2 formed by focusing of the microscope objective lens 5, is the angle between the projection of the light vector in the xy plane and the x-axis, θ is the projection of the light vector and the z-axis, is the numerical aperture of the microscope objective 5, and n is the refractive index of the medium between the microscope objective 5 and the sample 6.

7. The high aspect ratio focusing light field modulation method based on double apertures according to claim 1, characterized in that: The method of changing the laser power to adjust the two-photon polymerized voxel is as follows: when a focused light spot with a high aspect ratio along the optical axis is completely immersed in the photoresist sample, two-photon polymerization is performed by a single femtosecond laser pulse to obtain a two-photon polymerized voxel with a high aspect ratio, and the height and diameter of the voxel increase with the increase of the laser power.

8. The high aspect ratio focusing light field modulation method based on double apertures according to claim 1, characterized in that: The height of the light spot voxel microcolumn customized according to the method is 0-143 μm, the microcolumn width ranges from 1.9 to 6 μm, and the highest aspect ratio is 23.8.

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