CN111992890B - Method for machining phase-tunable optical super-surface based on femtosecond laser - Google Patents
Method for machining phase-tunable optical super-surface based on femtosecond laser Download PDFInfo
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- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
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Abstract
The invention provides a phase-tunable optical super-surface processing method based on femtosecond laser, which directly writes and processes the surface of a processed sample by using femtosecond laser, regulates and controls various different phases of the processed sample by accurately controlling pulse energy under the condition of not damaging materials, so that various grating structures with different crystalline degrees and excellent consistency and uniformity are generated on the surface of the sample, thereby forming an optical super-surface; the method has the advantages of realizing high efficiency of super-surface grating processing, overcoming the defects of high processing cost, low processing efficiency, long processing period and the like of FIB cutting, electron beam mask processing and other processing modes, and overcoming the problems of poor structural uniformity and consistency in the traditional ablation processing.
Description
Technical Field
The invention relates to the technical field of femtosecond laser application, in particular to a phase-tunable optical super-surface method based on femtosecond laser processing.
Background
The super surface can regulate and control the characteristics of polarization, phase, amplitude, frequency and the like of electromagnetic waves through a sub-wavelength microstructure, and is a new technology combining optics and nanotechnology. At present, with the continuous progress of micro-nano processing technology and numerical simulation technology, the research on optical super-surface has been developed to a new height, which departs from the research category of static devices and gradually develops towards the direction of novel dynamic tunable optical devices. GST (germanium antimony tellurium alloy Ge) 2 Sb 2 Te 5 ) The material is a phase change material, and as a special functional material, the amorphous state and the crystalline state have optical constants n (refractive index) and k (extinction coefficient) which are greatly different, and the two states can be reversibly transformed by femtosecond laser induction, and the transformation speed is very high. Such non-volatile, optically switchable all-dielectric materials provide the possibility of implementation of dynamically tunable optical devices.
The periodic micro-nano structure with the period approximate to the incident light wavelength can be generated on the surface of the material irradiated by the laser, the periodic micro-nano structure of the sub-wavelength surface with the period obviously smaller than the incident light wavelength is discovered by people due to the emergence of the femtosecond laser, and the femtosecond laser induced surface periodic structure (LIPSS) provides a non-contact high-efficiency processing method for processing the super surface of the grating. The femtosecond laser induced dielectric surface periodic structure is formed because the femtosecond laser excites the surface of the material to a high excited state and then forms a periodic structure through self-organization. The conventional method for processing ablation stripes is generally adopted by the grating super surface, the ablation stripes are generated on the surface of a material by high laser power, and the material ablation is usually accompanied by the formation of a large amount of fragments and surface defects, so that the structural consistency and uniformity are extremely poor, and the performance of the grating super surface is seriously influenced. In contrast, the femtosecond laser irradiates the surface of the GST material below the ablation threshold to induce the rapid crystallization of amorphous GST, and crystalline periodic fringes with excellent structural consistency and uniformity are generated. At present, methods commonly used for processing a grating structure include a photolithography technique, a Focused Ion beam milling method, an electron beam lithography method, and the like, and these processing methods have the defects of expensive processing equipment, incapability of directly realizing tunability of a device, poor flexibility, and the like, for example, in the document "All-dielectric phase-change configurable metrology", artemis karvoid, and the like, a grating structure with a nano-scale period is processed on the surface of GST by using a Focused Ion Beam (FIB) cutting method, thereby realizing an All-dielectric super-surface grating structure. However, this method is expensive, has a long production cycle and low efficiency, and cannot directly realize the tunability of the super-surface.
Disclosure of Invention
The invention provides a phase-tunable optical super-surface processing method based on femtosecond laser, which mainly solves the technical problems that: the current super-surface grating structure has low processing efficiency, expensive production cost and poor tunability.
In order to solve the technical problem, the invention provides a phase-tunable optical super-surface processing method based on femtosecond laser, which comprises the following steps:
the energy of the femtosecond laser is adjusted by using the attenuation sheet to be lower than the ablation threshold of the processed sample, and the laser energy can be continuously adjusted;
adjusting an included angle alpha between the femtosecond laser pulse polarization direction and the laser scanning direction by using a half-wave plate;
fixing a processed sample on a six-dimensional mobile platform, shaping and focusing the femtosecond laser by using a cylindrical lens in front of an imaging light path and a focusing lens, and adjusting the six-dimensional mobile platform to focus the femtosecond laser on the surface of the processed sample by observing an imaging CCD (Charge Coupled Device);
by moving the six-dimensional moving platform, the processed sample and the femtosecond laser focus do relative motion, and a periodic micro-nano structure is processed on the surface of the processed sample.
Further, the method for processing the periodic micro-nano structure on the surface of the processed sample by using the attenuation sheet to adjust the energy of the femtosecond laser comprises the following steps:
under the conditions that the femtosecond laser pulse repetition frequency is fixed, the scanning speed is fixed, the included angle alpha between the laser pulse polarization direction and the laser scanning direction is kept fixed, and the laser scanning direction is kept consistent, and under the energy just lower than the ablation threshold value, a complete crystalline state area is generated; generating a modified periodic stripe structure from two ends of a direct writing area vertical to a scanning direction along with the reduction of the laser pulse energy to a modification threshold value; the energy is continuously reduced to the condition of E1, and a complete crystalline periodic stripe structure is generated; the energy is continuously reduced to the condition of E2, and complete crystalline state periodic stripes continuously exist, and the optical constants of the processed sample are changed relative to the values under the condition of E1 energy; when the energy is continuously reduced to the condition of E3, the optical constant of the processed sample is continuously changed; when the energy is reduced to En, the processed sample keeps an amorphous state because the energy is too small; the processed sample can obtain crystalline state periodic stripes of various different phases under the action of the direct-writing femtosecond laser pulse; the energy corresponding to the ablation threshold is greater than the modification threshold, the modification threshold is greater than the E1, the E1 is greater than the E2, the E2 is greater than the E3, and the E3 is greater than the En.
Further, the method also comprises the following steps of carrying out secondary direct writing processing on the periodic micro-nano structure:
keeping the repetition frequency and the scanning speed unchanged, adjusting an included angle alpha between the femtosecond laser pulse polarization direction and the laser scanning direction, and performing secondary direct writing processing on the periodic micro-nano structure by controlling the femtosecond laser energy to obtain the grid-shaped super-surface structure.
Further, the processed sample is plated on the monocrystalline silicon by adopting a magnetron sputtering mode for phase change materials.
Further, the phase change material is germanium antimony tellurium alloy Ge2Sb2Te 5.
Furthermore, the femtosecond laser pulse frequency is 1000 Hz.
Further, the scanning speed is 500 μm/s.
The beneficial effects of the invention are:
according to the method for processing the phase-tunable optical super-surface based on the femtosecond laser, the femtosecond laser is used for directly writing and processing the surface of the processed sample, and under the condition of not damaging materials, the precise control of pulse energy is adopted to regulate and control various different phases of the processed sample, so that various grating structures with different crystalline degrees and excellent consistency and uniformity are generated on the surface of the sample, and the optical super-surface is formed; the method realizes the high efficiency of super-surface grating processing, overcomes the defects of high processing cost, low processing efficiency, long processing period and the like of processing modes such as FIB cutting, electron beam mask processing and the like, and solves the problems of poor structural uniformity and consistency in the traditional ablation processing.
Drawings
FIG. 1 is a view showing the structure of a processing optical path according to the present invention;
FIG. 2 is a schematic diagram of a femtosecond laser single-time direct-writing preparation method of a grating super-surface wave absorber with adjustable absorption peak positions;
FIG. 3 is a schematic diagram of the femtosecond laser double direct writing processing of a grid-shaped super surface.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail with reference to the following detailed description and accompanying drawings. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
In order to solve the problems of low processing efficiency, high production cost and poor tunability of a super-surface grating structure, the invention provides a method for processing a phase-tunable optical super-surface based on femtosecond laser. In the processing mode, a processing method of femtosecond laser induced surface periodic structure is adopted, and through the self-assembly process, crystalline state periodic stripes which are free of damage and good in uniformity and consistency are formed on the surface of the phase-change material, so that the optical super-surface processing is realized; in the aspect of device tunability, the phase modulation of a processed sample and the control of the direction of crystalline fringes are accurately realized by controlling the femtosecond laser pulse energy and the polarization direction, and crystalline periodic fringes with different optical properties (different n and k values) and surface micro-nano grid structures with different shapes are directly generated in the processing process.
Referring to fig. 1, fig. 1 is a structural diagram of a processing optical path of the present invention, wherein: 1-a femtosecond laser; 2-a first half wave plate; 3-a polarizing plate; 4-a second half-wave plate; 5-an attenuation sheet; 6-mechanical switch; a 7-cylindrical lens; an 8-dichroic mirror; 9-a beam splitter; 10-an illumination white light source; 11-a focusing lens; 12-imaging CCD; 13-focusing objective (objective or plano-convex lens); 14-the sample being processed; 15-six-dimensional moving platform.
The femtosecond laser device 1 generates femtosecond laser pulses, the femtosecond laser pulses pass through the first half-wave plate 2, the polarizing plate 3, the second half-wave plate 4, the attenuation plate 5, the mechanical switch 6 and the cylindrical lens 7, are reflected by the dichroic mirror 8, are focused by the focusing objective lens 13 to the surface of a processed sample 14, and the processed sample 14 is fixed on the six-dimensional moving platform 15; the illumination white light source 10 passes through the beam splitter 9 and the dichroic mirror 8, is reflected by the beam splitter 9, passes through the focusing lens 11, and then enters the imaging CCD 12.
The parameters of the femtosecond laser used in the experimental process are as follows: the central wavelength is 800nm, the pulse width is 50fs, the repetition frequency is 1000Hz, and linear polarization is realized; in the experiment, a processed sample is a GST material (the thickness is 100nm) and is plated on the monocrystalline silicon (100) in a magnetron sputtering mode.
The energy of the femtosecond laser is adjusted by using the attenuation sheet 5 to be lower than an ablation threshold (FTH) of a processed sample, and the laser energy can be continuously adjusted; adjusting an included angle alpha between the polarization direction of the femtosecond laser pulse and the laser scanning direction (namely, the direct writing direction) by using a half-wave plate; fixing a processed sample 14 on a six-dimensional moving platform 15, shaping and focusing the femtosecond laser by using a cylindrical lens 7 in front of an imaging light path and a focusing objective lens 13, and adjusting the six-dimensional moving platform 15 to focus the femtosecond laser on the surface of the processed sample 14 through observation of an imaging CCD12 (Charge Coupled Device); by moving the six-dimensional moving platform 15, the processed sample 14 and the femtosecond laser focus do relative motion, and a periodic micro-nano structure is processed on the surface of the processed sample 14.
The method for processing the periodic micro-nano structure on the surface of the processed sample 14 by utilizing the attenuation sheet 5 to adjust the energy of the femtosecond laser comprises the following steps:
under the conditions that the repetition frequency of the femtosecond laser pulse is 1000Hz and the scanning speed is 500 muW/s, the shape characteristics of the periodic micro-nano structure of the surface are adjusted by adjusting the energy of the laser pulse: at energies below the ablation threshold (120.1 nJ for a single laser pulse), a complete crystalline region is generated; as the energy of a single laser pulse decreases to the modification threshold (84.1nJ), a modified periodic fringe structure is created starting from both ends of the write-through region perpendicular to the scan direction; the energy is continuously reduced to E1(51.6nJ), a complete crystalline periodic stripe structure with good consistency and uniformity can be generated; when the energy is continuously reduced to the E2(45.6nJ) condition, complete and consistent crystalline period stripes with good uniformity continuously exist, and the optical constant of the crystalline GST is changed relative to the value under the E1 energy condition; the crystalline GST optical constants continue to change as the energy continues to drop to E3(37.2 nJ); when the energy is reduced to En (30.1nJ), the GST material is kept in the original state (amorphous state) because the energy is too small.
Optionally, the method further comprises performing secondary direct writing processing on the periodic micro-nano structure: keeping the repetition frequency and the scanning speed unchanged, adjusting the included angle alpha between the polarization direction of the femtosecond laser pulse and the laser scanning direction, and performing secondary direct writing processing on the periodic micro-nano structure by controlling the energy of the femtosecond laser to obtain the grid-shaped super-surface structure.
The femtosecond laser pulse frequency is 1000 Hz; the scanning speed is 500 μm/s.
Example 1: processing of grating super-surface wave absorber with adjustable absorption rate wave crest position
The super surface absorption rate is greatly related to the properties of the material. The femtosecond laser with different pulse energy acts on the surface of the GST material to generate crystalline stripes with different crystalline degrees, and the crystalline degrees are different, and the properties of the crystalline stripes are also greatly different. Since the n value of the GST material tends to be stable when the laser wavelength is larger than 1200nm, the n value gradually becomes larger as the crystallization degree is improved. This example defines the degree of crystallization of the crystalline stripe processed with energy E1(51.6nJ) as 1 and the degree of crystallization (amorphous state) of the material processed under the energy condition of E10(38.2nJ) as 0, based on the value of n at a wavelength of 1300 nm. The energy in the interval 0 to 1, 10, was equally divided to correspond to 10 different phases of the GST material. Through the quantitative and accurate control of the laser pulse energy, phases of different GST material crystalline state stripes are regulated and controlled, and further the tunability of the grating super-surface processing is realized.
The specific processing steps of this example are as follows:
(1) adjusting a light path to ensure that the incident direction of laser is vertical to the surface of the processed sample;
(2) adjusting the second half-wave plate 4 to enable the included angle alpha between the polarization direction of the femtosecond laser pulse and the direct writing direction to be 90 degrees; adjusting the attenuation sheet 5 to make the pulse energy of the femtosecond laser slightly lower than the modification threshold (approximately equal to 0.7 FTH);
(3) setting the relative speed (namely scanning speed) of the six-dimensional moving platform and the laser focus to be 500 mu m/s;
(4) and moving the translation stage, and adjusting the rotary attenuation sheet 5 to adjust the energy of the laser pulse to E1, so that the crystalline state periodic stripes with the first crystalline state degree are processed.
(5) Repeating the step (4), and adjusting the energy to E1-E10 respectively, so that 10 kinds of periodic stripes with different crystalline degrees can be processed, and ten kinds of wave absorbers with different absorption peak positions can be prepared.
Since the periodic micro-nano structure regulation and control mode related to the embodiment 1 is the regulation and control based on the femtosecond laser pulse energy, the implementation process of the mode is specifically described as follows:
under the specific scanning speed of 500 mu m/s, the energy deposited on the surface of the processed sample in unit area is further adjusted by controlling the size of femtosecond laser pulse energy, so that the regulation and control of the periodic micro-nano structure are realized. Specifically, a cylindrical lens is matched with a 100mm plano-convex lens (objective lens), under the condition that the pulse repetition frequency is 1000Hz, the laser energy is reduced from E1(51.6nJ) to E10(38.2nJ), the linear polarization direction of the laser pulse is kept to be vertical to the laser scanning direction, and the laser scanning direction is kept consistent, so that the processed sample can obtain crystalline state periodic stripes of various different phases under the action of direct-writing femtosecond laser pulses. With the increase of energy, optical constants (n and k values) are changed, and the crystallization degree of crystalline strips is higher and higher, so that the position of the absorption peak of the super surface of the prepared grating is red-shifted, and the possibility of realizing the direct tunability of the super surface wave absorber of the grating is provided.
Example 2: processing of grid-shaped super-surface wave absorber with controllable shape
The super surface absorptivity has a great relationship with the geometric shape, period and size of the surface micro-nano structure. The femtosecond laser generates crystalline stripes vertical to the polarization direction on the surface of the phase-change material during the first processing. The polarizing film 3 is adjusted to change the polarization direction of the laser, and after optimization control, secondary direct writing processing is performed along the original path in a specific energy range, so that crystalline stripes generated by the two times of direct writing processing can be overlapped, thereby forming grid-shaped (such as square, rhombus and the like) structures with different shapes.
The specific processing steps of this example are as follows:
(1) adjusting the light path to ensure that the incident direction of the laser is vertical to the surface of the processed sample 14;
(2) adjusting the second half-wave plate 4 to enable the included angle alpha between the polarization direction of the femtosecond laser pulse and the direct writing direction to be 90 degrees; adjusting the attenuation sheet 5 to enable the pulse energy of the femtosecond laser to be slightly lower than a modification threshold value;
(3) setting the relative speed (scanning speed) of the six-dimensional moving platform and the laser focus to be 500 mu m/s; and moving the six-dimensional moving platform, and adjusting the energy of the laser pulse to 42.8nJ by adjusting the rotary attenuation sheet 5, so that the crystalline state periodic stripes in the first direction (vertical to the direct writing direction of the laser) are processed.
(4) Adjusting a second half-wave plate 4, and changing an included angle alpha (such as 0 degree/45 degree) between the polarization direction of the femtosecond laser pulse and the direct writing direction; setting the relative speed (scanning speed) of the six-dimensional moving platform and the laser focus to be 500 mu m/s; and moving the six-dimensional moving platform, adjusting the energy of the laser pulse to a specific energy range (40.2-42.8 nJ) by adjusting the rotary attenuation sheet 5, wherein the processed crystalline state periodic stripes in the second direction (which are parallel to the laser direct writing direction or form an included angle of 45 degrees) are overlapped with the crystalline state stripes processed by the first direct writing to form a checkerboard-shaped (square and rhombic) grid-shaped super surface.
Since the preparation of the lattice-shaped wave absorber in example 2 involves the second direct-write processing of a femtosecond laser, the implementation of this method is described in detail below:
under the specific scanning speed (500 mu m/s) and the single pulse energy (42.8nJ) of the first direct-write processing, the size of the single pulse energy of the second femtosecond laser direct-write processing is accurately controlled, and then the stripes generated by the two processing are crossed, so that the preparation of the grid-shaped wave absorber is realized. Because the adopted laser is a Gaussian beam, the energy of the laser is gradually reduced from the circle center to the periphery, and the precise control of the energy is particularly critical in the secondary direct-writing processing. When the energy is too large, the energy of the central position of the laser is larger than the threshold value of the laser for generating the modified stripe, so that the processed area covers the crystalline stripe processed by the first direct writing, and becomes a complete crystalline area, and the energy at the peripheral edge of the laser possibly generates a grid-shaped structure with a small area in a proper range; when the energy is too low, the area generated by the grid-shaped structure is too small, or the grid-shaped structure cannot be generated; when the energy is in a proper range (40.2-42.8 nJ), the surface of the material after the secondary direct writing processing can generate a large-area grid-shaped structure. By adjusting the polarization of the laser and accurately controlling the energy during the secondary direct writing processing, grid-shaped super-surface structures (such as squares, diamonds and the like) with different shapes can be processed, and the possibility is provided for realizing the structure controllability of the optical super-surface wave absorber.
Because the large-area periodic micro-nano structure processing related to the two embodiments both relate to shaping and focusing of femtosecond laser by the cylindrical lens 7 and the focusing objective lens 13, the implementation process of the mode is specifically described as follows:
the femtosecond laser is shaped by the cylindrical lens 7 and then changed into a long-strip-shaped beam from a round beam, so that a processing area is changed into a surface from a line in the processing process, and large-area processing is realized. The white illumination source 10 is also shaped after passing through the cylindrical lens 7, so that the beam returning to the CCD imaging 12 is no longer a circular area but a long strip, resulting in too small an imaging area to see the processed area. In contrast, the difficulty is overcome by shaping the cylindrical lens 7 and focusing the cylindrical lens with the focusing objective lens 13. And a cylindrical lens 7 is arranged on the light path before the imaging light path, and the laser light shaped by the cylindrical lens 7 is reflected by a dichroic mirror 8 to enter the imaging light path. Because the focal length of the cylindrical lens 7 is limited, the focal point of the cylindrical lens 7 cannot be well controlled to be on the surface of the processed sample, and therefore, the light beam which is focused by the cylindrical lens 7 and then diverges is focused on the surface of the processed sample by adding the focusing objective lens 13. By utilizing the method of matching the cylindrical lens 7 with the focusing objective lens 13, the efficient processing of the large-area periodic micro-nano structure is effectively realized.
According to the phase-tunable optical super-surface method based on femtosecond laser processing, the periodic micro-nano structure is generated on the surface of the GST material through femtosecond laser induction, and the periodic micro-nano structures on the surfaces of various forms can be efficiently and accurately designed and processed. The invention starts from femtosecond laser direct writing, realizes the high efficiency of super-surface grating processing, overcomes the defects of expensive processing cost, low processing efficiency, long processing period and the like of FIB cutting, electron beam mask processing and other processing modes, and solves the problems of poor structural uniformity and consistency in the traditional ablation processing.
Through accurately controlling the pulse energy of the femtosecond laser, various phases of the GST material are regulated and controlled, so that the GST material directly generates crystalline stripes with various different n and k values in the processing process, and the tunability of the grating super surface is realized; by regulating the polarization direction of femtosecond laser, various surface micro-nano structures are regulated, and the tunability of a grid-shaped super surface is realized; the phase of the GST crystalline state stripe is prevented from being changed by other subsequent processing modes (such as heating or voltage application) so as to simplify the processing flow and improve the processing efficiency.
The foregoing is a more detailed description of the present invention that is presented in conjunction with specific embodiments, and the practice of the invention is not to be considered limited to those descriptions. For those skilled in the art to which the invention pertains, several simple deductions or substitutions can be made without departing from the spirit of the invention, and all shall be considered as belonging to the protection scope of the invention.
Claims (6)
1. A method for processing a phase-tunable optical super-surface based on femtosecond laser is characterized by comprising the following steps:
the energy of the femtosecond laser is adjusted by using the attenuation sheet to be lower than the ablation threshold of the processed sample, and the laser energy can be continuously adjusted;
adjusting an included angle alpha between the femtosecond laser pulse polarization direction and the laser scanning direction by using a half-wave plate;
fixing a processed sample on a six-dimensional moving platform, shaping and focusing the femtosecond laser by using a cylindrical lens in front of an imaging light path and a focusing lens, and adjusting the six-dimensional moving platform to focus the femtosecond laser on the surface of the processed sample through observation of an imaging CCD (charge coupled device);
moving a six-dimensional moving platform to enable a processed sample and a femtosecond laser focus to move relatively, and directly writing a processing periodic micro-nano structure on the surface of the processed sample; the method comprises the following steps of adjusting the energy of femtosecond laser by using an attenuation sheet, and processing a periodic micro-nano structure on the surface of a processed sample, wherein the periodic micro-nano structure comprises the following steps:
under the conditions that the femtosecond laser pulse repetition frequency is fixed, the scanning speed is fixed, the included angle alpha between the laser pulse polarization direction and the laser scanning direction is kept fixed, and the laser scanning direction is kept consistent, and under the energy just lower than the ablation threshold value, a complete crystalline state area is generated; generating a modified periodic stripe structure from two ends of a direct writing area vertical to a scanning direction along with the reduction of the laser pulse energy to a modification threshold value; the energy is continuously reduced to the condition of E1, and a complete crystalline periodic stripe structure is generated; the energy is continuously reduced to the condition of E2, and complete crystalline state periodic stripes continuously exist, and the optical constants of the processed sample are changed relative to the values under the condition of E1 energy; the energy is continuously reduced to the condition of E3, and the optical constant of the processed sample is continuously changed; when the energy is reduced to En, the processed sample keeps an amorphous state because the energy is too small; the processed sample obtains a plurality of crystalline state periodic stripes with different phases under the action of direct-writing femtosecond laser pulses; the energy corresponding to the ablation threshold is greater than the modification threshold, the modification threshold is greater than the E1, the E1 is greater than the E2, the E2 is greater than the E3, and the E3 is greater than the En.
2. The femtosecond laser processing phase-tunable optical super-surface-based method according to claim 1, further comprising performing secondary direct-write processing on the periodic micro-nano structure:
keeping the repetition frequency and the scanning speed unchanged, adjusting an included angle alpha between the femtosecond laser pulse polarization direction and the laser scanning direction, and performing secondary direct writing processing on the periodic micro-nano structure by controlling the femtosecond laser energy to obtain the grid-shaped super-surface structure.
3. The method for phase-tunable optical super-surface based on femtosecond laser according to claim 1 or 2, wherein the processed sample is plated on single crystal silicon by magnetron sputtering of phase-change material.
4. The femtosecond laser processing phase-tunable optical super-surface-based method according to claim 3, wherein the phase-change material is germanium antimony tellurium alloy Ge 2 Sb 2 Te5。
5. The method of claim 1 or 2, wherein the femtosecond laser pulse frequency is 1000 Hz.
6. The method of claim 1, wherein the scanning speed is 500 μm/s.
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