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Abstract
Subwavelength nanogratings for optical modulation have attracted significant attention for efficient large-area fabrication. We developed a far-field laser nanofabrication method, called line-shaped laser lithography, to ablate thin films with separated line-shaped ultrafast laser pulses on substrates into multiple nanogratings. The millimeter pulse length and high-speed scanning for a single-pulse ablation ensure exceptionally high large-area fabrication efficiency. Moreover, nanogratings with a sub-100 nm linewidth (e.g., a 57 nm linewidth (${\sim}\lambda /{9}$) nanograting on 10 nm Au film) were generated based on the Marangoni effect. An ultra-uniform large-area (${6} \times {15}\;{{\rm mm}^2}$) nanograting with a width of 132 nm and a length-to-width ratio of ${\sim}{45},\!{500}$ was achieved in only 1.5 s. In particular, diversified nanogratings can be obtained by designing scanning strategies, including moiré, blaze-like, Dammann, and 2D nanogratings, revealing light modulations in diffraction and beam splitting.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. INTRODUCTION
Thanks to their outstanding optical performance and exceptional compatibility in microsystems, ultrathin optical devices such as metagratings are in high demand as perfect absorbers, polarization filters, and phase modulators [1,2]. The subwavelength scale is significant for effective optical modulations, especially for ultraviolet to near-IR wavelengths [3,4]. In particular, mask patterning plus ion-etching techniques (e.g., electron beam lithography), and template duplication (e.g., nanoimprint lithography), are core approaches to fabricate optical nanodevices [5]. Achieving a subwavelength scale depends on the resolution of the mask or template and is contradictory to expeditious large-area fabrication at low cost. Laser processing can balance the contradiction under specific demands by laser interference lithography to produce nanogratings with low requirements for period modulation and microsphere laser nanoprocessing to form nanocone arrays [6]. An excellent approach to creating subwavelength nanogratings is the fabrication of laser-induced periodic surface structures (LIPSS) from the excitation of surface plasmon polaritons by an ultrafast laser. This approach possesses pattern diversity, feature size controllability, and uniformity to a certain extent [7–9]. Additionally, dual-beam super-resolution direct laser writing [10] and near-field lithography [11–13] are used to create 2D or even 3D structures on photoresists or certain materials with sub-100 nm linewidths, limiting the fabrication efficiency for large-area and appliable materials.
Multifocal or line-shaped laser scanning improves processing efficiency [14–16]. Previous work considered line-shaped laser scanning for fabricating LIPSS with a single laser spot [14,17,18] or interference spots of two line-shaped laser beams from two cylindrical lenses [19]. A spatial light modulator (SLM) was used to imitate a cylindrical lens to form a line-shaped laser and optimize the uniformity by an additional window function on the phase images [20]. Moreover, laser processing by separated line-shaped laser pulse ablation is seldom reported because it is difficult to obtain a line-shaped laser pulse with a long efficient length, high uniformity, and enough pulse energy [20,21].
We developed a far-field line-shaped laser lithography (LLL) approach under ambient conditions to achieve diverse large-area subwavelength 1D/2D nanogratings. Separated ultrafast line-shaped laser pulses focused by a cylindrical lens were applied to ablate thin films on substrates to periodically form nanogratings. The high repetition rate guarantees high efficiency in the nano-structure fabrication [22]. Based on the Marangoni effect, we present a material residue effect to explain the generation of nanogratings with a sub-100 nm (${\lt}\lambda /{5}$) linewidth from the recast layer. The achievable minimum linewidth can be improved by decreasing the width of the recast layer, which is directly proportional to the laser fluence and the film thickness. In addition, customized scanning speed and path manipulation broaden the achievable structures of nanogratings, revealing their applications in the modulation of light in diffraction.
2. RESULTS AND DISCUSSION
A. Fabrication Method
During the LLL process, shown in Fig. 1(d), we applied a femtosecond line-shaped beam to produce nanogratings using an ultrafast laser (Spirit One, Newport) with a pulse width of 300 fs, a wavelength of 520 nm, a maximum pulse energy of 19 µJ, and a repetition rate of 10 kHz. A Gaussian beam was expanded to obtain a relatively energy-flattened region in the middle, tailored into a rectangular shape, and finally focused by a cylindrical lens (focal length of 50 mm, LJ1695RM, Thorlabs). A relatively uniform line-shaped laser with a length of approximately 6 mm along the long axis was obtained after focusing with a ${10} \times$ objective (NA of 0.25).
Fig. 1. Schematic and demonstration for LLL processing. (a) Surface morphologies of large-area 1D nanogratings on GST film (10 nm on quartz substrates) with an area of ${6} \times {15}\;{{\rm mm}^2}$, a linewidth of 132 nm, and a period of 1.4 µm measured by SEM in the top, middle, and bottom place of the gratings with frequency spectrum in the corner by a fast Fourier transform algorithm (FFT). The scanning speed was 14 mm/s, and the pulse energy was 1.5 µJ. (b) Enlarged view of the large-area 1D nanogratings. (c) Photograph of the nanograting demonstration. (d) Schematic of LLL processing.
The modulated laser ablates metal and alloy thin films on substrates using Au (with a Cr adhesive layer, deposited by e-beam evaporation) and phase change materials ${{\rm Ge}_2}{{\rm Sb}_2}{{\rm Te}_5}$ (GST, deposited by magnetron sputtering) with a thickness of tens of nanometers on quartz substrates (JGS1) as examples. Further discussion on material selection is provided in Supplement 1, Figs. S1 and S2. The quartz substrate offers a low pulse energy absorption, which does not affect the performance of the films during ablation. The adhesion layer is optional for different films and substrates, mainly applied to improve the adhesion between the film and the substrate during film deposition and fabrication.
Laser pulses were spatially separated during high-speed scanning and periodically ablated films on substrates with tunable scanning speeds and paths. The high-speed motion was provided by a motion stage (XMS100-S, Newport) with a velocity range from approximately zero to 300 mm/s. Moreover, the spatial pulse interval, namely the period, as defined in Fig. 2, was equal to the scanning speed divided by the repetition frequency of the ultrafast laser. According to Supplement 1 and Fig. S3, the films are almost entirely removed, with a negligible trace of the material remaining in the ablation areas. Thus, producing large-area nanogratings in one step with high controllability of the shape, period, and the duty cycle is feasible. In particular, we obtained a uniform nanograting with a large area of ${6} \times {15}\;{{\rm mm}^2}$ and a linewidth of 132 nm in just 1.5 s, as shown in Figs. 1(a)–1(c), revealing excellent uniformity in the long axis and a super-large length-to-width ratio of approximately 45,500.
Fig. 2. Schematics and SEM images of the formative and evolutionary stages of structures during the LLL process. (a1)–(a2) Formation process under a single pulse ablation. (b1)–(b4) Formative process of the ridge after the second pulse ablation with a period close to ablating width. Ablation results in Au and GST films with pulse energy slightly below (c1)–(c3) and beyond (c4) the ablation threshold. (d) Schematic of nanogratings with $a$ as the short axis ablating width, $p$ as the period or spatial pulse interval, ${h_1}$, ${h_2}$ as the heights of the successively generated recast layer, $D$ as the width of the recast layer, and $d$ as the linewidth of nanogratings. (e) Schematic of the four major stages along the changing of the spatial pulse interval; namely, period. (f1)–(f5) Nanogratings on 10 nm Au film with a pulse energy of 6.12 µJ and speed of 20 to 12.5 mm/s. (g1)–(g3) LIPSS-like structures on 10 nm Au films with pulse energy of 3.2 µJ and speed of 1 to 0.5 mm/s. (h1–h5) Nanogratings on 30 nm GST film with pulse energy of 6.12 µJ and speed of 23 to 10 mm/s (i1)–(i4) LIPSS-like structures on 30 nm GST films with pulse energy of 2.23 µJ and speed of 6.5 to 0.5 mm/s. All scale bars are 1 µm.
B. Generation of Sub-100 nm Nanogratings
Figure 2(a) presents the formation process of the nanogratings. The gradient color represents the temperature distribution with a dark red for the highest temperature. The abstract diagram that illustrates the sectional profile of the film depicts the movement of materials under the irradiation of a Gaussian pulse. After a single ultrafast laser pulse is imposed, the materials undergo melting, ablation, evaporation, and resolidification sequentially in several nanoseconds to microseconds [23]. According to the thermal Marangoni effect (or thermal-capillary effect), molten metal forms a raised recast layer, as shown in the light gray section of Fig. 2(a2), by flowing along the temperature gradient, shown by the gray arrow in Fig. 2(a1) [24–26]. The recast layer is in the resolidification state for some noble metal films (e.g., Au and Ag). However, phase-change materials such as germanium–antimony–tellurium (GST) become crystalline in the recast layer from amorphous under single ultrafast pulse irradiation [27]. Films with poor adhesion to substrates cannot support the generation of recast layers because the film peels when heated, as shown in Supplement 1, Fig. S4. Materials in the middle of the pulse would accumulate under the instantaneous high temperature owing to dewetting, as shown by the red arrow in Fig. 2(a1). Then the materials evaporate, resulting in total material removal [26]. Ablations with different pulse energies near the threshold demonstrate the material removal process, as shown in Figs. 2(c1)–2(c4).
According to Fig. 2(b2), when the second pulse is imposed with a period $p$, as shown in Fig. 2(d), close to or even smaller than the ablating width $a$ in Fig. (2(d)), the materials between the two ablation areas are not entirely removed. A ridge, which is shown in Figs. 2(b3) and 2(b4) remains because of the thermal Marangoni effect, where the molten material flows along the temperature gradient to the low-temperature region. This unique material residue phenomenon supports the generation of sub-100 nm nanogratings and dot arrays. The motion of the liquid film forming the recast layer can be determined as [28]
Fig. 3. Detailed data on linewidth, ablating width, and cross-sectional profile. (a) and (b) Linewidth under different grating periods, pulse energies, and films with pulse energy of 3.2 µJ (Au low), 6.12 µJ (Au high), 2.25 µJ (GST low), and 6.12 µJ (GST high). (c) Schematic of the formation of recast layer under different pulse energy. (d) Various recast layer widths under different film materials, film thicknesses, and pulse energies. (e) Minimum linewidth related to the recast layer width on different samples. (f) and (g) Cross-sectional profile of the raster, ridge, and dot chain on Au and GST films, respectively, with high pulse energy under different periods.
Fig. 4. Multiple nanogratings from different scanning strategies on 10 nm Au film with silica substrate. (a1)–(a4) 1D moiré nanogratings by overlapping scanning with large and small nanograting period differences. (b1) and (b2) 2D nanogratings by cross scanning under various angles. (c1)–(c3) 1D Dammann-like nanogratings and gratings with smaller duty cycles fabricated by multiple times of same-speed overlapping scanning with different path displacements. (d1) and (d2) Complex scanning path combination for blaze-like nanograting.
We used two different film materials as examples to analyze the factors contributing to the different nanogratings: Au and GST. These materials exhibit different thermal and mechanical characteristics, as shown in Supplement 1, Tables S1 and S2. Figures 2(f)–2(i) show that the nanogratings and other nanostructures created by the LLL method reveal a continuous process on these two films, along with the changing of the period control by scanning speed variation [Figs. 3(a) and 3(b)]:
As for the raster, the period and linewidth follow a proportional relationship, and the X intercept presents the ablating width $a$, as shown by the star mark in Figs. 3(a) and 3(b). The cross-sectional profile of the raster is saddle-shaped. Because the spatial pulse interval is sufficiently small between the first and second pulses, the recast layers generating between adjacent pulses approach each other. The preformed recast layer is ablated once again, revealing a lower height of the newly generated recast layer, which is more evident for GST films with a lower ablation threshold, as shown in the purple and green areas in Figs. 3(f) and 3(g). When the period is equal to $a + {2}D$, a ridge with two hills is just formed. With a slightly further approach, the two recast layers merge rapidly and become a ridge with only a single hill, as shown in the blue areas in Figs. 3(f) and 3(g), whose linewidth should be $\sqrt 2 D$ without considering the material loss. The ridge shows a perfect single-hill structure on the Au film but with a two-stage structure on the GST film, owing to the higher surface tension of Au film, as shown in the blue areas in Figs. 3(f) and 3(g). Subsequently, the ridge shrinks under heavier ablation and becomes a dot chain, as shown in Figs. 2(f3), 2(h4), and 2(h5), and then a dot array in Figs. 2(f4) and 2(i1) because of the Plateau–Rayleigh instability [29].
Fig. 5. Light diffraction performances of different nanogratings with surface morphology, green light (520 nm) or white light diffraction spectrum, and optical simulations results in (a2), (d2), and (g2). (a1) and (a2) 1D nanogratings as the first-order diffraction gratings. (b) Blaze-like nanogratings with mixed diffraction orders. (c) Dammann nanogratings as the second-order diffraction gratings. (d1) and (d2) Dammann nanogratings for seven-point beam splitting. (e) Moiré nanogratings with substantially large nanograting period differences (1 µm and 1.7 µm) as diffraction gratings with high-order strengthening. (f) Moiré nanogratings with slight nanograting period differences showing composite diffraction orders. (g1), (g2), and (h) Cross scanning with four and three scanning paths, respectively. (i) Photograph of the nanograting device with an efficient large area. All scale bars are 5 µm.
Considering the proportional relationship during the raster, we can ideally predict that materials should be removed entirely when $p\; \le \;a$. However, the film materials remain following smaller spatial pulse intervals, even at the overlap ($p\; \lt \;a$). This aspect verifies the material residue effect, which supports the formation of sub-100 nm nanogratings. In detail, the GST film exhibits a broader ridge/dot chain range than Au film, revealing a more challenging generation of dot arrays. It is harder to generate separate dots on GST film with smaller surface tension [larger $|{C_1}|$ in Eq. (2)] than Au film due to Plateau–Rayleigh instability. In addition, the Au film exhibits a wider dot array range between the dot chain and the clean situation [larger $|{C_2}|$ in Eq. (2)] due to the higher ablation threshold.
A 57 nm nanograting was obtained on the Au film, as shown in Fig. 2(f3). We define the minimum linewidth of the nanograting as the linewidth of the dot chain grating without any break. Therefore, we can forecast that the width of the continuous recast layer is nearly the achievable minimum linewidth of the dot chain nanogratings owing to Plateau–Rayleigh instability. Thus, the predicted linewidth of the dot chain can be written as
According to Eq. (3), creating a smaller linewidth for the recast layer ($D$) is required to reach the minimum linewidth of the nanogratings. As shown in Fig. 3(d), the smaller $D$ is due to the smaller pulse energy, film thickness, and film materials with higher dynamic viscosity. First, as shown in Fig. 3(c), the recast layer under high pulse energy becomes flatter and wider, and an additional heat-affected zone (HAZ, the region with grating stripes) emerges during the motion of the liquid metal. Similarly, the film thickness is proportional to the moving velocity, so a thinner film results in a lower moving velocity and a narrower recast layer. In addition to compare the thermal characteristics, the dynamic viscosity (${\eta \rm _{{m,\:Au}}} = {11.3}\;{\rm mPa}\cdot{\rm s}$, ${\eta \rm_{{m,\:GST}}} = {1 - 2}\;{\rm mPa}\cdot{\rm s}$) exhibits greater dominance than the surface tension temperature coefficient (${({\rm d}\sigma /{\rm d}T)\rm _{{m,\:Au}}} = - {0.18}\;{\rm mN}\;{{\rm m}^{- 1}}\;{{\rm K}^{- 1}}$, ${({\rm d}\sigma /{\rm d}T)\rm _{{m,\:GST}}}\; \approx \; - {0.1}\;{\rm mN}\;{{\rm m}^{- 1}}\;{{\rm K}^{- 1}}$) in Marangoni motion. In particular, a larger dynamic velocity reduces the Marangoni velocity, as shown in Eq. (1). Thus, the Au film exhibits a smaller recast layer.
Furthermore, when the spatial pulse interval is small enough to cause heavy pulse overlap, LIPSS and disordered LIPSS [Figs. 2(g2), 2(i2)–2(i4), and Supplement 1, Fig. S6] are generated due to the combined action of the Plateau–Rayleigh instability and the Marangoni effect [18]. Note that the uniform LIPSS originates from ablation with pulse energy near the ablation threshold.
C. Various Types of Nanogratings
Line-shaped laser lithography with designable scanning paths and material residue effects provides significant richness and resolution in achievable patterns. The material residue effect helps to achieve nanogratings involving ridges, dot chains, and dot arrays with sub-100 nm (${\lt}\lambda /{5}$) linewidths. We further produced the moiré grating, blaze-like grating, Dammann-like grating, and 2D dot arrays in Fig. 4 with designed scanning paths. The higher duty cycle of the nanogratings provides excellent efficiency in light modulation, including diffraction, dispersion, and beam splitting, as shown in Fig. 5.
The moiré grating has multiple periods showing superposition in diffraction orders, which can be achieved by the same-directional repetitive scanning with different scanning speeds (21 mm/s and 20 mm/s, 23 mm/s and 20 mm/s) or cross scanning with small included angles (2.5°), as shown in Fig. 4(a) and Supplement 1, Figs. S7 and S10(a)–S10(c). The smaller speed differences for overlapping scanning or smaller included angles for cross scanning increase the period. The material residue effect guarantees the generation of sub-100 nm ridges, dot chains, and dot arrays filling the area between significant periods, as shown by the green circles in Figs. 4(a2) and 4(a4).
Concerning the cross scanning with large included angles [Fig. 4(b) with a scanning speed of 20 mm/s, and included angles of 90° and 60°], 2D rectangular and rhombus arrays were produced with smoother boundaries than previous work by direct laser writing [30]. Note that a slightly engrailed boundary appears after the second scanning due to the Plateau–Rayleigh instability [Fig. 4(b1)], which debilitates when the included angle is smaller [Fig. 4(b2), with an in-depth explanation in Supplement 1, Fig. S8].
The same-speed repetitive scanning in the same direction with a slight path offset introduces a displacement between the nanogratings [Fig. 4(c) with scanning speeds of 40, 55, and 80 mm/s, and path offsets of 1.5, 2, and 0.4 µm, respectively]. Thus, the 1D Dammann-like grating with complex units and a controllable duty cycle [Figs. 4(c) and Supplement 1, Fig. S9] were created to enrich the types of nanogratings for different path displacements. As shown in Fig. 4(c2), the secondary ridges and dot arrays form part of the Dammann patterns.
Furthermore, a customized curving scanning path with a constant scanning speed can produce blaze-like nanogratings [Fig. 4(d) with a scanning speed of 16 mm/s and quadrant radius of 30 µm, and Supplement 1, Fig. S10(d)] due to the horizontal gradient components of the scanning speed. These components further enrich the pattern types. Moreover, nanogratings with a gradient linewidth in a single period, as shown in Fig. 4(d2), reveal the complete evolutionary process of raster, ridge, dot chain, and dot array.
D. Metagratings for Light Modulations
We proceeded with several reflective metagrating devices to reveal the excellent light diffraction performance of the nanogratings by LLL. We aim to show the potential applications in light modulations, including dispersion in spectroscopy in the visible wavelength range. Each had an efficiently large area, as shown in Fig. 5(i). The substrate comprises a sandwich structure on a quartz substrate (5 nm Cr, 70 nm Au, 35 nm ${{\rm Al}_2}{{\rm O}_3}$, and 30 nm GST from bottom to top). The 30 nm GST film is structured into the grating. The sample forms a resonant cavity to enhance the reflection efficiency of this reflective metagrating device with thin surface structures. The resonant cavity design and further diffraction simulations were processed and verified using the finite-difference time-domain method (Lumerical, Ansys). Detailed simulation data on the electric-field distribution for the substrate design are provided in Supplement 1, Fig. S11.
The normal 1D nanogratings shown in Figs. 5(a1) and 5(a2) reveal good uniformity in the first degree of diffraction. Moreover, Dammann nanogratings can enhance second-order diffraction and achieve seven-point beam splitting [Figs. 5(c), 5(d1), and 5(d2)].
Moiré nanogratings have riveting light-modulation performance and exhibit potential applications in metasurfaces [31]. Moiré nanogratings can strengthen higher-order diffraction with a significant difference in the period of the two scanning paths [1 µm and 1.7 µm in Fig. 5(e)]. In particular, the 1D moiré nanograting with a small period difference shows an interesting composite diffraction pattern [Fig. 5(f)].
A blaze-like nanograting presents a more complex diffraction composition to mix the spectrum, resulting in a more continuous natural spectrum, namely continuous color distribution [Fig. 5(b)].
Further 2D three- and four-path cross-scanning analyses reveal significant 2D diffraction patterns [Figs. 5(g1), 5(g2), and 5(h)]. Moreover, when two layers of nanogratings are superimposed, bulk-tunable nanogratings with motor driving are created to offer tunable light modulations (Supplement 1, Fig. S12) [32,33].
3. CONCLUSION
In summary, far-field line-shaped laser lithography (LLL) is feasible to efficiently fabricate large-area nanogratings on films with substrates on a subwavelength scale. Nanograting with an extreme linewidth of 57 nm was demonstrated. We further provided a large-area (${6} \times {15}\;{{\rm mm}^2}$) nanograting with an ultra-large length-to-width ratio of approximately 45,500, revealing great accessibility in the nanoscale linewidth and a large macrosize. We proposed a Marangoni-based material residue effect and predicted the achievable minimum linewidth range related to the recast layer. In particular, narrowing the recast layer reduced the achievable linewidth of the gratings, which was mainly due to the low Marangoni velocity, determined by the low pulse energy, film thickness, and materials with considerable surface tension and dynamic viscosity at the melting point. Designable scanning paths supported the manufacture of multiple nanogratings, including blaze-like, multiperiod, Dammann nanogratings, and rectangular arrays, resulting in functional nanogratings, such as nanograting rulers, optical absorbers, and diffraction nanogratings, which substantially simplified the processing of diverse nanogratings into one step. We demonstrated several nanogratings by LLL that exhibit engaging light modulation performance and reveal great potential applications in metagratings and diffraction gratings, such as beam splitting, different diffraction order enhancement, and 2D diffraction patterns.
Funding
Guangdong Provincial University Science and Technology Program (2020KTSCX119); Shenzhen Science and Technology Program (20200925155508001, GJHZ20190820151801786, JCYJ20210324115608024, KQTD20170810110250357).
Acknowledgment
The authors thank the Materials Characterization and Preparation Center of the Southern University of Science and Technology for SEM image measurements.
Author Contributions. Author Kang Xu performed the experiments, analyzed the data, and wrote the manuscript. Author Lingyu Huang helped perform the experiments, and author Shaolin Xu guided the project and improved the manuscript. All authors participated in the experimental analyses and discussed the results.
Disclosures
The authors declare no conflicts of interest.
Data availability
The 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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