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Abstract
Sapphire nanostructures with a high aspect-ratio have broad applications in photoelectronic devices, which are difficult to be fabricated due to the properties of high transparency and hardness, remarkable thermal and chemical stability. Although the phenomenon of laser-induced periodic surface structures (LIPSS) provides an extraordinary idea for surface nanotexturing, it suffers from the limitation of the small depth of the nanostructures. Here, a high-efficiency self-modulated femtosecond laser hybrid technology was proposed to fabricate nanostructures with high aspect-ratios on the sapphire surface, which was combined backside laser modification and subsequent wet etching. Due to the refractive index mismatch, the focal length of the laser could be elongated when focused inside sapphire. Thus, periodic nanostructures with high-quality aspect ratios of more than 55 were prepared on the sapphire surface by using this hybrid fabrication method. As a proof-of-concept, wafer-scale (∼2 inches) periodic nanostripes with a high aspect-ratio were realized on a sapphire surface, which possesses unique diffractive properties compared to typical shallow gratings. The results indicate that the self-modulated femtosecond laser hybrid technology is an efficient and versatile technique for producing high aspect-ratio nanostructures on hard and transparent materials, which would propel the potential applications in optics and surface engineering, sensing, etc.
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1. Introduction
High aspect-ratio micro/nanostructures of rigid materials have essential applications in microfluidic, biomedical and optoelectronic devices [1–6]. For example, due to the excellent optical transparency, ultra-hardness, and physicochemical stability, sapphire materials are widely used in high-quality wear-resistant optical windows and GaN led substrates [7–11]. And it is these unique material properties that make the preparation of sapphire microstructures extremely difficult, especially the preparation of sapphire high aspect-ratio microstructures [12]. Light, especially laser, has excellent applications in material processing, device preparation and material property analysis [13–18]. Ultrafast laser processing has emerged as an advanced micro and nano processing method with the advantages of high precision, high designability and applicability to cover almost all materials [19–22]. Hong et al. proposed a novel femtosecond laser direct ablation technique to prepare high aspect ratio (up to 10) crack-free sapphire microstructures [23]. In addition, the femtosecond laser direct ablation technique combined with the wet etching process demonstrated unparalleled advantages for the preparation of high aspect-ratio sapphire surface microstructures and internal microchannels [2,8,24,25]. The accuracy of laser direct writing processing is usually limited to the sub-micron level due to the optical diffraction limit and the effect of debris caused by surface damage, in addition to the efficiency problems caused by the point-to-point processing method, makes this processing method unable to cope with the needs of large-area structure preparation.
Laser-induced periodic surface structuring (LIPSS) can break the diffraction limit and efficiently produce large area regular nanoscale structures [26,27]. Since its first discovery by scientists in 1965 [28], extensive theoretical and experimental work has made it a laser micro and nano processing technology with a controlled preparation structure and a wide range of materials (dielectrics, semiconductors, metals, polymers) [29–32]. For example, Qiu et al. demonstrated an optical localization-induced nonlinear competition mechanism. A large-scale but extremely regular LIPSS structure was prepared using femtosecond laser-induced low-threshold surface oxidation on silicon thin films. Moreover, these structures are stable and durable, showing great potential in structural color and antireflection applications [33]. Wide-field scanning through optical field modulation techniques results in several orders of magnitude improvement in preparation efficiency [27,34]. However, when laser surface texturing of solid surfaces, the induced groove structure and a large number of particles and debris can prevent the laser energy from propagating downward, making the depth of the grooves generally less than 1 µm, which severely limits the application of femtosecond lasers in inducing high aspect ratio fine structures [35,36]. Therefore, it is a great challenge to prepare periodic nanostructures with high aspect ratios (more than 10) by LIPSS technology.
Here, we proposed a self-modulated femtosecond laser hybrid technology for fabricating sapphire micro/nanostructures with a high aspect ratio. Compared to the top surface focusing process, the high repetition frequency femtosecond laser is focused on the backside of the sapphire, resulting in self-modulation of the laser focused optical field due to refractive index mismatch, avoiding the problems of structure and debris that prevent the laser energy from propagating to the interior of the sample and inducing nanostripe structures that are deeper and adjustable. Large area periodic nanostructures with a high aspect ratio of more than 55 were obtained on the sapphire surface in combination femtosecond laser with wet etching. Laser-matter interaction excitation produces a transition from spherical to planar nano-plasma bubbles leading to the induced generation of bulk nanostripes with a wavelength of 1030 nm femtosecond laser-induced nanostripes with a period of about 292 nm. The high aspect ratio stripe structure can be used as a grating and exhibits different diffractive properties than a typical shallow grating.
2. Experimentals
In this experiment, a femtosecond laser (Pharos, Light Conversion Ltd.) is used to prepare fine periodic structures with a high aspect ratio of sapphire. The laser has a pulse duration τp = 230 fs with the wavelength λ = 1030 nm, and the largest repeat frequency is 1 MHz. A focusing lens (SAGA, magnification 4×, NA = 0.1) delivers the laser to a spot, which focal length f = 66 mm and gaussian beam diameter at the entrance of focusing lens D = 12 mm. And the diameter of the light spot can be calculated by this formula: d = 4λf/(πD) = 4*1.03 µm* 66 mm/(3.14*12 mm) = 7.22 µm [37]. The use of the lower numerical aperture objective lens can obtain a larger laser spot to improve processing efficiency. The laser power measured before the entrance to the objective lens was from 0.76 - 1.18 W (or pulse energy Ep = 0.76 - 1.18 µJ). This corresponds to the peak laser fluence per pulse of Fp = Ep / [ π(d/2)2/2] = 3.70–5.82 J/cm2 [38,39]. Unspecified, the laser light passing through the sample is focused on the backside surface of the sapphire. The polarization direction can be adjusted by a λ/2 waveplate (Union Optic, WPZ2420-1030). The sample was placed on the two-dimensional pneumatic platform (Aerotech, ABL 10100-LN-HALAR) with the biggest scan linear speed is 40 mm/s, and the z-axis position of the spot focus was controlled by a single-axis stepper motor (BOCIC, MVS301). In the case of the line laser scanning procedure, the Neff, line is defined by Neff, line = d*f/v, here the f is the repetition frequency of the laser and the v is the scanning speed. An objective lens with correction collars (Plan Fluor Nikon, 40× magnification) is used to adjust the shape of the focused spot.
A sapphire c-cut (He Fei KeJing Material Technology, Ltd.) sample thickness of 380 µm was ultrasonic cleaned by ethanol and DI water and then air-dried. The sample was placed on the platform carefully and the laser parameters and scanning parameters can be adjusted on the homemade fabrication software. After laser processing, the sample was wet etched in 20% HF acid at room temperature to remove the micro debris generated during laser processing. The debris was deposited in the middle of the sapphire fine periodic structures and covered on the structure surface. The high aspect ratio fine periodic sapphire structures were characterized using scanning electron microscopy (SEM, Jeol JSM-7500F). 2D-FFT analysis was performed based on SEM images using ImageJ software. Monochromatic continuous lasers of different wavelengths (450 nm, 515 nm and 650 nm) were used to test the spectroscopy of the nanostripe structures. Diffraction maps of the different structures on the screen were obtained through the camera.
3. Results and discussion
The schematic diagram of the experimental setup is shown in Fig. 1(a). By adjusting the position of the objective lens in the Z-direction, the laser can be focused on the top or backside surface of the sapphire sample to obtain the surface structure. The sample was placed on a high-precision, large-stroke pneumatic displacement stage. The stage movement was controlled by a computer program, which enabled the precise preparation of the sample. Focusing a laser on the surface of a sample to create a specific pattern by the relative motion of the laser spot, and which has become a common solution for surface structure preparation. However, when processing hard and brittle materials, the sample debris generated during the ablation process can fall on the sample surface, especially when processing structures over large areas where large amounts of debris accumulation can block the laser energy causing incomplete structures (Supplement 1, Fig. S1). In contrast, processing by focusing the laser on the backside surface of the transparent material would avoid this problem because the generation of surface debris did not affect the laser propagation. Also, the laser can induce nanostripe structures on the surface regardless of whether it is processed on the top or backside surface (Supplement 1, Fig. S1). However, the height of nanostructures induced by top surface processing was generally small (about several hundred nanometers), so it was challenging to prepare high aspect ratio nanostructures. Unlike top surface processing, which the laser energy is absorbed or scattered by the damaged structure and cannot propagate the laser energy more profound into the sapphire sample, backside processing not only does not block the laser propagation but due to the refractive index mismatch, the focal depth of the femtosecond laser may be elongated when focused inside the sapphire, this self-modulation phenomenon more favorable for the preparation of structures with larger aspect ratios. After backside surface processing, a large amount of debris were present on the surface of the processed sapphire sample. The periodic distribution of laser energy during laser processing, this part will be explained in detail in the next paragraph, caused the sapphire in the laser processing area to be periodically modified into amorphous, and the remaining unmodified part will remain crystalline (Fig. 1(b)). The material modified by the femtosecond laser can be removed by a certain etching solution [40]. Amorphous sapphire and sapphire debris can be etched off by a certain hydrofluoric acid (HF) concentration at room temperature that essentially did not affect the sapphire crystal material [24]. Therefore, after penetration processing, the samples were ultrasonically etched in a concentration of 20% HF for 1 h to prepare high aspect ratio nanostripe structures. The morphology of the front and cross-section of the nanostripe structure after etching with HF was shown in Fig. 1(c).
Fig. 1. The schematic diagram of the femtosecond laser processing system and the mechanism of high aspect ratio stripes by femtosecond laser-induced processing. (a)The schematic diagram of the femtosecond laser processing system. SEM images of sapphire nanostructures after backside laser irradiation (b) and wet etching (c). (d)The schematic diagram of the formation mechanism of laser-induced high-aspect-ratio nanostripes. (e) The cross-section SEM images of the nanostripe structures with different effective pulse numbers (Neff).
Many theories have been proposed to explain LIPSS [29,41]. Here, we argue that backside femtosecond laser-induced high aspect ratio periodic nanostructures can be explained by the theoretical model of volumetric nanogratings to surface ripples on transparent materials [42,43]. Under the action of multiple pulses of the femtosecond laser, the nonlinear ionization and localized defect excitation of the sapphire produced nano-plasma spheres. Furthermore, as the plasma density increased due to the boundary conditions, the electric field around the equator was enhanced, the electric field at the poles of the plasmasphere was suppressed, and the nano-plasma gradually formed nano-planes. As the nano-plane evolved during the pulse, a standing wave resonant cavity with the maximum electric field was formed at the nano-plane with an interval of λ/2n, here λ is the wavelength of the laser and n is the refractive index of the sapphire. This standing wave resonant cavity provided positive feedback for the ionization at the nano-plane of the plasma, which had a great value of electric field and intensity. The nanoplanes were at the subsurface of the sapphire side due to the air-sapphire interface, and once they reached the surface, ablation and breakdown will occur. The size of the nano-plasma plane was in the tens of nanometers, and the nano-plane grew towards the incident light through ionization. In the case of penetration processing, the nanoplanes grew from the surface to the interior of the sapphire, which made it possible to induce striped nanostructures with high aspect ratios (Fig. 1(d)). The cross-section of the nanostripe structure with different effective pulse numbers (Neff) was shown in Fig. 1(e). As the pulse number increased, it can be observed that the depth of the stripe gradually became more prolonged, and the depth of the stripe can reach 4.6 µm when the equivalent pulse number was 2.5 × 105. This depth was tens of times greater than the depth of top surface induced sapphire nanostripe structures (generally around 100-200 nm).
Different processing parameters affected the morphology of the induced nanostripe structure. The effect of laser fluence and scanning speed on the nanostripe structure was investigated in detail. Some structures processed with the representative parameters were shown in Fig. 2, and more structures on detail parameters were presented in Supplement 1, Fig. S2. Figure 2(a) showed the top-view (x-y plane) SEM images of a single line structure processed at different laser fluences and scanning speeds. The corresponding cross-section SEM images were shown in Supplement 1, Fig. S3. Excessive laser fluence (more than 5.76 J/cm2) causes the continuity of the nanostripe to become poor. By analyzing the data on the depth, line width, and period of the structure, the effect of the different processing parameters on the structure can be derived. Different laser fluences (Fig. 2(b–d)) and different scanning speeds (Fig. 2(e–g)) were investigated at a scanning speed of 4 mm/s and a laser fluence of 4.20 J/cm2, respectively. With the increase of the laser fluence, the stripe depth gradually increased, and the line width of the single line also gradually increased. A larger line width improved the processing efficiency of machining 2D structures, and the depth directly affected their aspect ratio. However, too high energy will deteriorate the stripe morphology, so the maximum energy density was 5.76 J/cm2. Increasing the scanning speed increases the processing efficiency but reduces the depth and line width of the structure, so it is necessary to use a more compromised scanning speed for processing, such as 4 mm/s. The period of the stripes was slightly muscled around Λ= λ/2n = 292 nm, and the change was not apparent. The width of the nanostripe was determined by the width of the nanoplanes formed by the nano-plasma [42]. It was known from SEM images of nanostripe structures with different parameters that the stripe width did not exceed 100 nm, so for statistical convenience, the width of the stripe was fixed as 100 nm when calculating the aspect ratio. Therefore, structures with different aspect ratios can be prepared by changing the laser processing parameters. At a scanning speed of 4 mm/s and a laser fluence of 5.76 J/cm2, the aspect ratio of the sapphire nanostripe structure exceeded 55.
Fig. 2. Induced nanostripe structures at different laser fluences and scanning speeds (a) The top-view SEM images of the nanostripe structures at different laser fluences and scanning speeds. At scanning speed of 4 mm/s, (b) the cross-section SEM images of nanostripes for different laser fluences; (c) the effect of laser fluences on the depth and linewidth of nanostripe structures; (d) the effect of laser fluences on the aspect ratio and period of nanostripe structures. At laser fluence of 4.20 J/cm2, (e) the cross-section SEM images of nanostripes for different scanning speeds; (f) the effect of scanning speeds on the depth and linewidth of nanostripe structures; (g) the effect of scanning speeds on the aspect ratio and period of nanostripe structures. Scale bar = 1 µm.
The distribution of the focused energy near the surface of the Gaussian spot at different focusing positions relative to the backside surface will have a significant difference. Theoretically, without considering the influence of the sapphire material on the spot, the diameter of the focused spot is d = 4λf/(πD) = 5.25 µm; the length of the focused spot (Z-direction) is 2*ZR = 2*(π(d/2)2/λ) = 79.5 µm. However, in practice, the spot was elongated after the laser passes through the high refractive index sapphire sample. The relative position of the focused spot and the backside surface of the sample can be achieved by adjusting the displacement of the sample in the Z-axis direction. The processing at different out-of-focus positions was studied at a laser fluence of 4.20 J/cm2 and a scanning speed of 4 mm/s (Fig. 3). At these processing parameters, the length of the damageable spot in the z-direction reached more than 80 µm. We showed the relative positional relationship between the spot center position and the backside surface of the sample (Fig. 3(a)), and the cross-section and front SEM of the nanostripe structure fabricated at different defocus positions were shown in Fig. 3(b) and 3(c). By studying the top-section (XY plane) and the cross-section (XZ plane) images of the single-line processing, the maximum depth and width of the stripes can be obtained near the center of the focused Gaussian spot. The maximum linewidth of laser damage to sapphire was about 4.5 µm, and the depth of the nanostrips was about 4.2 µm. The linewidth and depth of the nanostripes decrease to a certain extent with the gradual increase of the defocusing degree (Fig. 3(d) and 3(e)). For different out-of-focus positions, the period of the stripe varies within a range around Λ= λ/2n = 292 nm (Fig. 3(f)).
Fig. 3. Induced nanostripe structures at different out-of-focus positions. (a) Schematic diagram of the relative position relationship between the focus spot and the backside surface for processing at different defocus positions. SEM images of the cross-section (b) and the top-view (c) of nanostripe structures at different out-of-focus positions (-40, -20, 0, 20, 40 µm from left to right). The effect of out-of-focus positions on the depth (d), line width (e) and period (f) of nanostripe structures. Scale bar = 1 µm.
The direction of laser polarization had a decisive influence on the direction of induced high aspect ratio nanostripe structures. SEM images of the processed nanostripe structures with different polarization directions at the same laser fluence (4.20 J/cm2) and scanning speed (4 mm/s) with the same scanning direction were shown in Fig. 4(a), where the direction of the induced stripe was perpendicular to the laser polarization. By adjusting the scanning direction during processing, curved nanostripe structures can be prepared (Fig. 4(b) and 4(c)). In addition, geometric phase grating structures (Fig. 4(d)) can also be prepared by adjusting the polarization direction, demonstrating the potential of femtosecond laser-induced nanostripe to prepare geometric phase optical elements. In addition, the difference in the angle between the polarization direction and the scanning direction also led to the variation of the processed linewidth, which was maximum (4.7 µm) when the scanning direction was parallel to the polarization direction (0°) and decreased with the increasing angle until it reached a minimum (3.5 µm) when the scanning direction was at an angle of 90 degrees to the polarization direction.
Fig. 4. The effect of laser polarization variation on induced nanostripe structures. (a) SEM images of the induced nanostripes with different directions of laser polarization. The red double arrow is the direction of polarization. (b) SEM images of the curved nanostripe structures prepared by changing the laser polarization at the line spacing of 3 µm. (c)A local enlarged SEM image of (b). (d) Geometric phase grating structure was prepared by changing the laser polarization at a line spacing of 5 µm, and the inset showed a local enlargement. (e) The angles between the scanning direction and the polarization direction on the linewidth and period of nanostripe structures. Scale bar = 5 µm.
The effect of the repetition frequency of the laser on the stripe structure was demonstrated in Fig. 5. With a fixed laser fluence and scanning speed (4.20 J/cm2 and 4 mm/s), the repetition frequency of the laser had a significant influence on the induced structure, as a more regular stripe structure cannot be formed at the scanning speed was 4 mm/s when the repetition frequency was less than 250 kHz. No significant stripe structure was formed when the repetition frequency was further reduced to 25 kHz and the scanning speed was 4 mm/s. The incomplete structure obtained by lower repetition frequency machining is caused by the insufficient number of effective pulses. The nanostripe structure can be processed by a scanning speed of 0.01 mm/s and a laser repetition frequency of 1 kHz (Supplement 1, Fig. S4); however, this is not conducive to the rapid preparation. Therefore, high repetition frequency was vital for efficient regular high aspect ratio nanostripe structure.
Fig. 5. The effect of the repetition frequency on the induced nanostripe structures at a laser fluence of 4.20 J/cm2 and scanning speed of 4 mm/s. Scale bar = 5 µm.
By laser scanning processing (4.20 J/cm2 and 4 mm/s), circular large-area nanostripe structures with a high aspect ratio of 4 cm in diameter were prepared on a 2-inch sapphire sample (Fig. 6(a)). It can be observed that under white light illumination, the nanostripe structures area was still relatively transparent. The line spacing of the scan was set to 3.5 µm due to the small spot focused by the objective so that the large-area preparation would be time-consuming. By increasing the area of the scanning spot through light field modulation techniques, the processing efficiency of high aspect ratio nanostructures can be significantly improved. Figures 6(b) and 6(c) showed the SEM images of the high aspect ratio stripe structure, whereas Fig. 6(c) was a magnified image, which showed that the stripe structure was more regular and continuous. The fast Fourier transform image (Fig. 6(d)) showed the stripe period and the standard deviation corresponding to Λ = 280 ± 20 nm. Nanopillar structures with a high aspect ratio can be obtained by cross-scanning (Fig. 6(e)). The height information of the nanopillar structure can be demonstrated by ICP etching (Supplement 1, Fig. S5). The sapphire nanopillar structure was etched using BCl3 and Cl2, and after 20 mins of etching, the stripe was etched off by 2.83 µm. As the etching time increased, the etching depth became larger, and after 60 mins, the etching depth reached 3.98 µm. The nanopillar structure did not change from the top-view SEM after 20 mins of etching, indicating that the height of the nanopillar structure was at least more than 2.8 µm.
Fig. 6. Large area induced nanostripe structures on sapphire. (a) Photograph of the large area induced nanostripe structure on a 2-inch sapphire sample. (b) SEM image of the large area induced nanostripe structure, scale bar = 10 µm. (c) local magnification SEM image of (b), scale bar = 5 µm. and (d) the fast Fourier transform image. (d) SEM image of the nanostripe structure obtained by cross-scanning, scale bar = 5 µm. The red double arrow is the direction of polarization.
Tuning the laser processing parameters has been shown to be able to adjust the aspect ratio of the nanostripe structure within a certain range. Modulation of the laser light field can more actively regulate the aspect ratio of nanostripes. The Bessel light generated by the spatial light modulator can be used to elongate the focused spot for directly processing nanostructures with high aspect ratios. However, considering the high damage threshold of sapphire and the equipment cost, a simple focusing objective with a correction ring was used to modulate the focused spot's focus actively and verify the effect of active light field modulation on the induced light field streak structure. Since the thickness of the penetrating processed sapphire is 380 µm, which is thicker than a conventional coverslip, and the refractive index of the sapphire is larger, the sapphire cannot be damaged because of the insufficient degree of focusing when the focusing ring is less than 0.3 mm. Increasing the compensation of the correction change, the focusing degree of the spot will be strengthened, and the depth of the induced streak structure will increase (Fig. 7). When the compensation of the correction ring reaches 0.6 mm, the maximum depth of the induced streak can reach 9.5 µm, which indicates the best focusing degree of the focused spot. Continuing to increase the compensation of the correction ring, the laser spot will be scattered, the focused spot size becomes larger, and the focused energy at the focal point becomes smaller, resulting in a shallow streak depth. Active modulation of the optical field will provide greater freedom for preparing high aspect ratio nanostripes.
Fig. 7. The schematic illustration (a) and cross-section SEM images (b) of nanostripe structures fabricated by varying correction ring compensation of focusing objective. The number in (a) is the value of the correction ring, the unit is mm. Scale bar = 2 µm.
Due to the effect of the height of the grating structure on the diffraction efficiency of the grating, the high aspect ratio stripe structure exhibits different diffractive properties than a shallow grating [44,45]. The SEM images of the microstructures after laser processing and etching were shown in Fig. 1(b) and Fig. 1(c). The SEM images of the cross-sections showed that the laser processed structure had a large amount of debris or amorphous sapphire in the high aspect ratio stripe, resulting in a shallow depth of the structure. In contrast, after wet etching, the high aspect ratio stripe structure was exposed to achieve a high aspect ratio grating structure. The diffraction patterns of the two gratings for different wavelengths of light were shown in Fig. 8, Fig. 8(a–b), Fig. 8(c–d) and Fig. 8(e–f) were red, green and blue light, respectively. A comparison of the normalized diffraction intensity distribution of diffracted light at different wavelengths was shown in Figs. 8(g–i) for red, green and blue light, respectively. High aspect ratio stripe structure had more diffraction orders of diffraction spots than shallower structured sapphire stripe structure. The beam splitting energy of the laser-processed grating was mainly concentrated in the 0-order diffracted light, and then the beam splitting energy decreased order by order. The energy of the 0-level diffracted light was more diffracted in the grating with a high aspect ratio so that the energy of the 0-level diffracted light was not the highest compared to other levels. Furthermore, we found that the energy distribution of diffracted light was not the same for different wavelengths. For green light at 515 nm, the highest energy of the diffracted spot was order-2, while for light at 650 nm and 450 nm, the highest energy of the diffracted spot was order-1. By adjusting the height of the nano-grating to achieve custom scaling for specific wavelengths, the designability of the grating was greatly enhanced.
Fig. 8. Diffractive properties of the laser processed shallow grating and HF etched high aspect ratio nanostripe grating. Diffraction patterns of sapphire gratings of different wavelengths for red, green and blue light are tested (a), (c) and (e) and (b), (d) and (f) for red, green and blue light shallow gratings and high aspect ratio gratings, respectively. (g-i) Comparison graphs of red, green and blue normalized diffraction intensity distribution.
4. Conclusions
The proposed self-modulated femtosecond laser hybrid technology has successfully overcome the small depth of structures obtained by conventional LIPSS methods, resulting the fabrication of sapphire periodic nanostructures with high aspect ratios of more than 55, which provides a new insight for femtosecond laser-based nanofabrication. We have comprehensively investigated the effects of parameters including focusing position, laser fluence, and scanning speed on the nanostructure morphology explained the reasons for the generation of high aspect ratio stripes, and successfully prepared large-area (2 inches) regular high aspect ratio nanostripe structures on sapphire. Curved and multi-directional stripe structures can also be realized using the dependence of the induced stripe on polarization. The sapphire high aspect ratio nanostripe as a grating demonstrates a different diffraction effect on light from a shallow grating and provides a reasonable basis for realizing specific optical functions. The self-modulated femtosecond laser hybrid technology is an effective means of laser preparation of high aspect ratio nanostructures that can be applied to other transparent materials, which will have a wide range of applications in optics, functional surfaces, sensing, et al.
Funding
National Natural Science Foundation of China (62105117, 61827826, 62075081); Scientific Research Project of the Education Department of Jilin Province (JJKH20221005KJ).
Disclosures
The authors declare no conflicts of interest
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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