Wet-assisted femtosecond laser processing of nanoholes in optically isotropic crystals

Abstract

The present invention discloses a method for wet-assisted femtosecond laser processing of nanopores in optically isotropic crystals. A thicker optically isotropic crystal is placed on the optically isotropic crystal of the target object through optical contact as a spherical aberration enhancement auxiliary. The femtosecond laser is transmitted through the spherical aberration enhancement auxiliary and then incident and focused into the target object to produce a self-focusing wiredrawing effect, thereby inducing the local modification of the material into a nanowiredrawing induction region; then, wet etching is performed with a specific acidic solution to etch the nanowiredrawing induction region to form a nanopore with controllable aperture and a high aspect ratio. The present invention can induce processing of the three-dimensional space inside any optically isotropic crystal, and can also adjust and prepare nanopore structures with a high aspect ratio, overcoming the disadvantage that traditional methods can only process nanopore structures with a low aspect ratio on the surface of the material, and preparing smaller, uniform, and controllable nanopore structures, with the characteristics of efficient, stable, and low-cost three-dimensional space controllable nanoprocessing.

Description

Technical Field

[0001] The present invention relates to a femtosecond laser processing method in the field of laser processing, and in particular to a method for processing "nanopores with controllable aperture and high aspect ratio" in optically isotropic crystals using a wet-assisted femtosecond laser. Background Art

[0002] As we all know, the development of silicon-based semiconductor electronic chips is constrained by Moore's Law, and chip manufacturing processes below 5nm are fraught with challenges. To prepare for the advent of the post-Moore era, research on photonic chips has attracted significant attention. The heart of photonic chips consists of a variety of novel nanophotonic devices, including nanolasers, photon sensors, optical amplifiers, optical modulators, and optical switches. Therefore, to ultimately achieve the successful development of high-performance photonic chips, intensified research into novel nanophotonic devices is essential. The fabrication of novel nanophotonic devices is fundamental to the fabrication of finely controlled nanostructures. Traditionally, these nanostructures have relied on methods such as UV lithography, EBL, FIB, and RIE. There are many problems: 1) Nanostructures can only be processed on the surface of the material, and the required nanostructures cannot be processed in the three-dimensional space inside the material; 2) Only nanostructures with a relatively low aspect ratio can be processed, which limits the development of nanophotonic devices. For example, nanostructures with a high aspect ratio have always been pursued in the preparation of large-area, achromatic superlenses; 3) The processing technology is complex, expensive, time-consuming, and the processing stability deteriorates over time; 4) It is difficult to process nanostructures smaller than 100nm.

[0003] The existing femtosecond laser direct writing technology has achieved the processing of different nanophotonic structures in many hard and transparent materials, including self-assembled nanogratings, distributed Bragg gratings, nanostructures prepared by beam shaping, nanoholes, etc.

[0004] However, there are still many fundamental problems with femtosecond laser direct writing:

[0005] 1) It is difficult to directly write arbitrary 3D shape-controllable, sub-wavelength-resolved (λ / 10), low-loss nanophotonic structures;

[0006] 2) It is difficult to induce a large refractive index difference (Δn>0.5), which hinders its development in nanophotonic devices;

[0007] 3) Femtosecond laser direct writing easily causes longitudinal depth elongation, which is not conducive to the processing of nanostructures with uniform and controllable dimensions in any direction;

[0008] 4) Femtosecond laser uses beam shaping to process nanostructures, which can easily lead to serious sidelobe effects, resulting in unevenness around the nanostructure and even serious defects such as microcracks. Summary of the Invention

[0009] In response to the problems in the above-mentioned background technology, the present invention utilizes femtosecond laser direct writing technology combined with the high efficiency, high precision and strong flexibility of femtosecond laser processing, and proposes a wet-assisted femtosecond laser nano-processing method for preparing "nanopores with controllable aperture and high aspect ratio" in optically isotropic crystals. It is an efficient, stable and low-cost three-dimensional space controllable processing method.

[0010] The method of the present invention utilizes a femtosecond laser to generate a self-focusing nano-wire drawing effect through spherical aberration enhancement in a thicker optically isotropic crystal. The generated femtosecond laser nano-filaments propagate and focus into the interior of a thinner optically isotropic crystal. The femtosecond laser parameters are regulated to locally induce, draw, and modify the optically isotropic crystal material. Finally, with the aid of an acidic solution, the modified area of ​​the optically isotropic crystal material induced by the femtosecond laser can be selectively wet-etched to prepare a "nanopore with controllable aperture and high aspect ratio" structure.

[0011] The technical solution adopted in the present invention is:

[0012] The method of the present invention is directed to a piece of optically isotropic crystal to be processed as a target object, and specifically includes the following steps:

[0013] First, another optically isotropic crystal that is thicker than the target workpiece and has an adjustable thickness is placed in optical contact on one side of the target workpiece as a spherical aberration enhancement auxiliary to form an optical contact stack. A femtosecond laser is applied from the spherical aberration enhancement auxiliary to the target workpiece, passes through the spherical aberration enhancement auxiliary, and is incident and focused into the interior of the target workpiece. Based on the spherical aberration enhancement effect of the femtosecond laser, the self-focusing wiredrawing effect of the femtosecond laser is enhanced, and the self-focusing wiredrawing inducing material is locally modified inside the optically isotropic crystal of the target workpiece to form a nanowiredrawing inducing region; the diameter of the nanowiredrawing inducing region is on the order of nanometers, and the direction of the filaments is consistent with the direction in which the femtosecond laser is incident on the isotropic crystal, and is perpendicular to the surface of the optically isotropic crystal.

[0014] The target object is then wet-etched with a specific acidic solution, so that the nanowire drawing induction area is etched to form nanopores with controllable pore size and high aspect ratio.

[0015] The high aspect ratio mentioned in the present invention refers to a aspect ratio greater than 3000:1 or even higher, which can be achieved by properly adjusting parameters such as the thickness of the spherical aberration enhancement auxiliary, the working distance of the focusing objective lens, and the power of the femtosecond laser.

[0016] Specifically, a thin optically isotropic crystal is used as the target workpiece, while a relatively thicker optically isotropic crystal is stacked in optical contact on the upper surface of the target workpiece as an auxiliary material for spherical aberration enhancement. The two optically isotropic crystals maintain optical contact. When the femtosecond laser beam passes through the thicker optically isotropic crystal, it produces significant spherical aberration enhancement, prompting the femtosecond laser to form a uniform localized nanofilament region as an induction region during the self-focusing process. The crystal material in the localized nanofilament region is modified to form a nanowire induction region.

[0017] The optically isotropic crystal materials of the target workpiece and the spherical aberration enhancement auxiliary are selected from one of the crystal materials such as YAG, LuAG, diamond crystal, etc., and their crystal axes are optically isotropic and have no birefringence effect. The propagation of laser light in this type of crystal will not be affected by the crystal axis, and the light beam always maintains uniform and directional propagation.

[0018] The materials of the optically isotropic crystal of the target object and the spherical aberration enhancement auxiliary may be the same or different.

[0019] The relative thickness of the spherical aberration enhancement auxiliary compared to the target workpiece is greater than the range of 1-10 mm.

[0020] In specific implementation, femtosecond lasers use different output wavelengths such as 355, 532, 800, and 1030 nm, different pulse widths (50 fs–10 ps), different repetition frequencies (1 Hz–100 kHz), and pulse train output modes with high repetition rates, different powers, different polarizations (including linear polarization and circular polarization), and different numbers of exposure pulses.

[0021] Focusing is achieved through aspherical focusing objectives with different magnifications and numerical apertures NA.

[0022] The femtosecond laser is focused on any relative position inside the optically isotropic crystal as the target processing object through the focusing objective of the aspheric mirror for exposure, so that the microscopic atomic arrangement and looseness of the local space material of the crystal after exposure are changed, thereby forming a slender nano-wire drawing induction area.

[0023] The aperture of the nanopore is regulated by parameters such as the output wavelength, pulse width, repetition rate, power, and exposure pulse number of the femtosecond laser, and the aperture range is 50-400nm.

[0024] The depth-to-diameter ratio of the nanopore is controlled by parameters such as the refractive index of the optically isotropic crystal used as the target object, the thickness of the optically isotropic crystal used as the auxiliary object, the working distance of the focusing objective lens, and the power of the femtosecond laser. The depth-to-diameter ratio is as high as 3000:1.

[0025] The "nanopore with controllable aperture and high aspect ratio" can achieve adjustable aperture of the nanopore in the range of 50-400nm by regulating parameters such as the output wavelength, pulse width, repetition rate, power, and number of exposure pulses of the femtosecond laser. In addition, by selecting optically isotropic crystal target processing materials with different refractive indices and regulating parameters such as the thickness of the optically contact stacked optically isotropic crystal material used as a spherical aberration enhancement auxiliary, the working distance of the focusing mirror, and the power of the femtosecond laser, nanopore processing with different aspect ratios can be achieved, for example: nanopore processing with a aspect ratio of up to 3000:1 can be achieved.

[0026] The wet etching using an acidic solution is specifically as follows:

[0027] First, before the target workpiece is placed in a container of an acidic aqueous solution, the upper and lower surfaces of the target workpiece are polished until the nanowire drawing induction area is exposed on the surface;

[0028] Secondly, the target workpiece was ultrasonically cleaned in deionized water, acetone, and alcohol for 5 min respectively;

[0029] Then, the target object is placed in a container of acidic solution, and the container is etched under magnetic stirring. After etching, the target object is taken out and ultrasonically cleaned with deionized water, acetone, and alcohol for 5 minutes respectively to obtain a nanopore structure.

[0030] In wet etching with an acidic solution, the choice and concentration of the acidic solution, the presence or absence of heating, the presence and selection of oscillation conditions (magnetic stirring, ultrasound), and the etching time are all crucial factors in the successful formation of uniform, diameter-controlled nanopores with a high aspect ratio. By adjusting the optically isotropic crystal, such as the acidic solution concentration, heating temperature, the magnetic speed that accelerates the solution perturbation, and etching time, the femtosecond laser-induced nanofilamentous regions can be selectively controlled to form "nanopores with controllable pore size and a high aspect ratio."

[0031] The principle of the present invention is that the crystal axes of an optically isotropic crystal are optically isotropic, and when a femtosecond laser beam is transmitted in an optically isotropic crystal, no deflection or bifurcation occurs. Therefore, when the femtosecond laser passes through a thick optically isotropic crystal as an auxiliary material, the spherical aberration is significantly enhanced, thereby generating a plasma self-focusing effect to form a wiredrawing effect. The formed femtosecond laser nanofilaments are then focused into the optically isotropic crystal of the target workpiece to form a localized nanofilament region, i.e., the induced region. By adjusting the focus depth of the femtosecond laser, the nanofilaments caused by the heat effect of the localized nanofilament region are ensured to act within the optically isotropic crystal of the target workpiece. The femtosecond laser parameters are further adjusted to induce localized wiredrawing of the optically isotropic crystal of the target workpiece. After the femtosecond laser induced wiredrawing, the atomic arrangement, chemical bond structure, and atomic looseness of the localized spatial material have changed, making the crystal material in the induced region completely different from that in the non-induced region.

[0032] The method further requires the use of acidic solution wet etching to quickly and selectively etch the femtosecond laser-induced nanofilamentous modified area without etching the crystalline material in the non-induced area, thereby forming a uniform nanopore structure with controllable pore size (50-400nm) and high aspect ratio (>3000:1) in the optically isotropic crystal.

[0033] After the nanopores with controllable aperture and high aspect ratio are prepared by the present invention, they can be further used in various new nanophotonic devices, including: photonic crystal microcavity lasers, topological nanophotonic devices, continuous bound state nanophotonic devices, supersurface nanophotonic devices, etc. The present invention not only has high processing efficiency, good stability, relatively low cost, and can easily achieve the preparation of uniform 50nm deep holes, but also overcomes the shortcomings of traditional technology that can only process nanostructures on the surface of the material and the depth-to-diameter ratio of the processed nanostructures is very low. In addition, the nanopores with controllable aperture and high aspect ratio prepared by the present invention play a more unique and superior function in some new nanophotonic devices. For example: in the field of supersurface nanophotonic devices, traditional methods can only process low-depth-to-diameter-ratio convex supersurface nanophotonic structures, which hinders the development of large-scale, achromatic superlenses, broadband adjustable, and high laser damage threshold supersurface nanophotonic devices. However, the nanopores with controllable aperture and high aspect ratio prepared by the present invention not only provide a new solution for the design and preparation of hole-based metasurface nanophotonic devices, but also solve many problems faced by traditional methods in processing metasurface nanophotonic devices.

[0034] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0035] (1) Compared with traditional processing methods such as UV lithography, EBL, FIB, and RIE, the present invention is based on wet-assisted femtosecond laser processing of nanopore structures. The processing technology is simple, efficient, and low-cost. It can also induce processing of the three-dimensional space inside any optically isotropic crystal and can adjust the preparation of nanopore structures with high aspect ratio.

[0036] (2) Compared with the general femtosecond laser direct writing technology, the present invention can directly process a "nanopore structure with controllable aperture and high aspect ratio" inside an optically isotropic crystal, overcoming the disadvantage that traditional technology can usually only process nanopore structures with low aspect ratio on the surface of the material.

[0037] Furthermore, the gentle wet etching process can be fully utilized to not only produce smaller (<λ / 20), uniform, and controllable nanopore structures, but also effectively avoid the depth-directed elongation effect caused by traditional femtosecond laser direct writing techniques. Furthermore, the large refractive index contrast (>0.8) between the formed nanopores and the non-induced crystal facilitates the design and fabrication of high-performance nanophotonic devices.

[0038] (3) Compared with the femtosecond laser direct writing technology after beam shaping (such as Bessel beam), the shaped femtosecond laser beam is usually accompanied by severe sidelobe effects, resulting in uneven edges around the nanopore structure and even defects such as microcracks, which is not conducive to the design, preparation and performance improvement of nanophotonic devices. The present invention is simple to operate and can effectively overcome this shortcoming. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 Schematic diagram of the wet-assisted femtosecond laser nanomachining method of the "nanopores with controllable aperture and high aspect ratio" in the optically isotropic crystal of the present invention.

[0040] Figure 2 This is a scanning electron microscope (SEM) image of the approximately 50 nm nanopores manipulated in an optically isotropic YAG crystal using the wet-assisted femtosecond laser nanomachining method.

[0041] Figure 3 This is a scanning electron microscope (SEM) image of a nanopore of approximately 400 nm fabricated in an optically isotropic YAG crystal using wet-assisted femtosecond laser nanomachining.

[0042] Figure 4 Optical microscope photos of the cross-sections at different stages of processing nanoholes in optically isotropic YAG crystals using the wet-assisted femtosecond laser nanomachining method. DETAILED DESCRIPTION

[0043] The present invention will be further described in detail below with reference to the examples, but the embodiments of the present invention are not limited thereto.

[0044] The embodiments of the present invention are as follows:

[0045] Example 1

[0046] like Figure 1 Figure 2 shows a schematic diagram of wet-assisted femtosecond laser processing of a "nanopore with controllable aperture and high aspect ratio" in an optically isotropic crystal. In this embodiment, an optically isotropic YAG crystal is used as the material for wet-assisted femtosecond laser processing of a "nanopore with controllable aperture and high aspect ratio."

[0047] The wet-assisted femtosecond laser processing method of this embodiment for "nanopores with controllable aperture and high aspect ratio" in YAG crystal mainly includes the following two steps:

[0048] 1) Based on the self-focusing enhancement effect of femtosecond laser, localized wire drawing is induced in bulk YAG crystals;

[0049] The femtosecond laser parameters used in step 1) are: output laser wavelength 1030 nm, pulse width 226 fs, repetition frequency 10 kHz, laser power 120 mW, linear polarization, and laser exposure pulse number 3 pulses.

[0050] The focusing objective lens parameters are: magnification 50x, numerical aperture NA=0.55, working distance WD=8.9 mm.

[0051] The bulk YAG crystal used is a thick piece (thickness of about 3 mm) and a thin piece (thickness of about 230 μm) stacked in optical contact.

[0052] The femtosecond laser is focused on the upper surface of the bottom thin YAG crystal by focusing the objective lens, and local exposure drawing is performed to obtain a nanowire induction area with a length of about 150 μm. Figure 4 As shown in a.

[0053] 2) Wet etching the wire drawing induction area using a phosphoric acid solution to form a nanopore structure.

[0054] Step 2) ultrasonically clean the surface of the substrate with deionized water, acetone, and alcohol for 5 minutes each. Then, a 44% phosphoric acid aqueous solution was used, the solution temperature was controlled at 80°C, the speed of the magnetic rotor was adjusted to 1000 r / min, and the etching time was 3 hours. After the etching was completed, ultrasonically clean the surface of the substrate with deionized water, acetone, and alcohol for 5 minutes each, and a nanopore structure with a pore size of about 50 nm was obtained. Figure 2 As shown in the SEM photos.

[0055] In this embodiment, a nanopore structure with a depth-to-diameter ratio of up to 3000:1 is obtained by simply extending the wet etching time to ensure that the 150μm YAG crystal material modification area induced by femtosecond laser drawing is formed. At the same time, wet etching is started from both end faces, and both sides of the YAG crystal are polished until the material modification area induced by drawing is exposed ( Figure 4 b), and then through the wet etching process, it can be ensured that the nano-through holes are uniformly etched ( Figure 4 c).

[0056] Example 2

[0057] like Figure 1 Figure 2 shows a schematic diagram of wet-assisted femtosecond laser processing of a "nanopore with controllable aperture and high aspect ratio" in an optically isotropic crystal. In this embodiment, an optically isotropic YAG crystal is used as the material for wet-assisted femtosecond laser processing of a "nanopore with controllable aperture and high aspect ratio."

[0058] The wet-assisted femtosecond laser processing method of this embodiment for "nanopores with controllable aperture and high aspect ratio" in YAG crystal mainly includes the following two steps:

[0059] 1) Based on the self-focusing enhancement effect of femtosecond laser, localized wire drawing is induced in bulk YAG crystals;

[0060] The parameters of the femtosecond laser used in step 1) are: output laser wavelength 1030nm, pulse width 226fs, repetition rate 10kHz, laser power 180mW, linear polarization, and laser exposure pulse number 5pulse. The focusing objective parameters are: magnification 50x, numerical aperture NA=0.55, working distance WD=8.9mm. The bulk YAG crystal used is a thick piece (thickness of about 3mm) and a thin piece (thickness of about 230μm) stacked in optical contact. The femtosecond laser is focused on a distance of about 80μm from the upper surface of the bottom thin-sheet YAG crystal through the focusing objective lens, and local exposure drawing is performed to obtain a nanowire induction area with a length of about 150μm, such as Figure 4 As shown in a.

[0061] 2) Wet etching the wire drawing induction area using a phosphoric acid solution to form a nanopore structure.

[0062] Step 2) ultrasonically clean the sample with deionized water, acetone, and alcohol for 5 minutes each. Then, a 44% phosphoric acid aqueous solution was used, the solution temperature was controlled at 80°C, the speed of the magnetic rotor was adjusted to 1000 r / min, and the etching time was 3 hours. After the etching was completed, ultrasonically clean the sample with deionized water, acetone, and alcohol for 5 minutes each, and a nanopore structure with a pore size of about 400 nm was obtained. Figure 3As shown in the SEM photos.

[0063] In this embodiment, a nanopore structure with a depth-to-diameter ratio of up to 375:1 is obtained by simply extending the wet etching time to ensure that the 150μm YAG crystal material modification area induced by femtosecond laser drawing is formed. At the same time, wet etching is started from both end faces, and both sides of the YAG crystal are polished until the material modification area induced by drawing is exposed ( Figure 4 b), and then through the wet etching process, it can be ensured that the nano-through holes are uniformly etched ( Figure 4 c).

[0064] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A method for wet-assisted femtosecond laser processing of nanopores in optically isotropic crystals, characterized in that: The method takes an optically isotropic crystal to be processed as the target object, and specifically The following processes are included: First, another optically isotropic crystal thicker than the target object is placed in optical contact on one side of the target object as a spherical aberration enhancement auxiliary. A femtosecond laser is applied from the spherical aberration enhancement auxiliary toward the target object, passes through the spherical aberration enhancement auxiliary, and then enters and focuses into the interior of the target object. Then, the self-focusing wiredrawing-inducing material in the optically isotropic crystal of the target object is locally modified to form a nanowiredrawing-inducing region. The femtosecond laser uses different numbers of exposure pulses. The target object is then wet-etched with an acidic solution, so that the nanowire drawing induction area is etched to form nanopores with controllable pore size and high aspect ratio.

2. The wet-assisted femtosecond laser processing method for nanopores in optically isotropic crystals according to claim 1, characterized in that: The optically isotropic crystal materials of the target workpiece and the spherical aberration enhancement auxiliary are both selected from one of the crystal materials of YAG, LuAG, and diamond crystal.

3. The wet-assisted femtosecond laser processing method for nanopores in optically isotropic crystals according to claim 1, characterized in that: The femtosecond laser is focused on any relative position inside the optically isotropic crystal as the target processing object through the focusing objective of the aspheric mirror for exposure, so that the microscopic atomic arrangement and looseness of the local space material of the crystal after exposure are changed, thereby forming a nano-wire drawing induction area.

4. The wet-assisted femtosecond laser processing method for nanopores in optically isotropic crystals according to claim 1, characterized in that: The aperture of the nanopore is regulated by the output wavelength, pulse width, repetition rate, power and exposure pulse number parameters of the femtosecond laser.

5. The wet-assisted femtosecond laser processing method for nanopores in optically isotropic crystals according to claim 3, characterized in that: The depth-to-diameter ratio of the nanopore is regulated by the refractive index of the optically isotropic crystal as the target object, the thickness of the optically isotropic crystal as the auxiliary object, the working distance of the focusing objective lens, and the power parameters of the femtosecond laser.

6. The wet-assisted femtosecond laser processing method for nanopores in optically isotropic crystals according to claim 1, characterized in that: The wet etching using an acidic solution is specifically as follows: First, before the target workpiece is placed in a container of an acidic aqueous solution, the upper and lower surfaces of the target workpiece are polished until the nanowire drawing induction area is exposed on the surface; Secondly, the target workpiece was ultrasonically cleaned in deionized water, acetone, and alcohol respectively; Then, the target object is placed in a container of acidic solution, and the container is etched under magnetic stirring; after the etching is completed, the target object is taken out and ultrasonically cleaned with deionized water, acetone, and alcohol respectively to obtain a nanopore structure.

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