• Photonics Research
  • Vol. 9, Issue 5, 839 (2021)
Yuchan Zhang1, Qilin Jiang1, Kaiqiang Cao1, Tianqi Chen1, Ke Cheng1, Shian Zhang1, Donghai Feng1, Tianqing Jia1、2、*, Zhenrong Sun1, and Jianrong Qiu3
Author Affiliations
  • 1State Key Laboratory of Precision Spectroscopy, School of Physics and Materials Science, East China Normal University, Shanghai 200062, China
  • 2Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006, China
  • 3State Key Laboratory of Optical Instrumentation, Zhejiang University, Hangzhou 310027, China
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    DOI: 10.1364/PRJ.418937 Cite this Article Set citation alerts
    Yuchan Zhang, Qilin Jiang, Kaiqiang Cao, Tianqi Chen, Ke Cheng, Shian Zhang, Donghai Feng, Tianqing Jia, Zhenrong Sun, Jianrong Qiu. Extremely regular periodic surface structures in a large area efficiently induced on silicon by temporally shaped femtosecond laser[J]. Photonics Research, 2021, 9(5): 839 Copy Citation Text show less

    Abstract

    Femtosecond laser-induced periodic surface structures (LIPSS) have several applications in surface structuring and functionalization. Three major challenges exist in the fabrication of regular and uniform LIPSS: enhancing the periodic energy deposition, reducing the residual heat, and avoiding the deposited debris. Herein, we fabricate an extremely regular low-spatial-frequency LIPSS (LSFL) on a silicon surface by a temporally shaped femtosecond laser. Based on a 4f configuration zero-dispersion pulse shaping system, a Fourier transform limit (FTL) pulse is shaped into a pulse train with varying intervals in the range of 0.25–16.2 ps using periodic π-phase step modulation. Under the irradiation of the shaped pulse with an interval of 16.2 ps, extremely regular LSFLs are efficiently fabricated on silicon. The scan velocity for fabricating regular LSFL is 2.3 times faster, while the LSFL depth is 2 times deeper, and the diffraction efficiency is 3 times higher than those of LSFL using the FTL pulse. The formation mechanisms of regular LSFL have been studied experimentally and theoretically. The results show that the temporally shaped pulse enhances the excitation of surface plasmon polaritons and the periodic energy deposition while reducing the residual thermal effects and avoiding the deposition of the ejected debris, eventually resulting in regular and deeper LSFL on the silicon surface.

    1. INTRODUCTION

    Femtosecond laser-induced periodic surface structures (LIPSS) have been widely studied over the last 20 years [14]. These periodic nanostructures efficiently modify the properties of materials and have many applications in surface coloring [5,6], large-area grating [7,8], birefringence optical elements [9], data storage [10], and surface wettability [11,12]. In general, three main characteristics are considered for evaluating the quality of LIPSS: uniformity, depth, and orientation, which have a significant influence on their functions.

    Numerous experimental and theoretical studies investigated the formation mechanism of low-spatial-frequency LIPSS (LSFL) with a spatial period Λ>λ/2, where λ is the laser wavelength [1,2,4,13,14]. For semiconductors and metals, the orientation of LSFL was usually perpendicular to the incident laser polarization, which was widely accepted as a result of the laser energy periodic distribution induced by surface plasmon polaritons (SPPs) [1,14]. Therefore, the control and enhancement of SPPs is a fundamental challenge for obtaining regular LSFL. Jiang et al. [15] attempted to control the localized transient electron dynamics by temporally or spatially shaping femtosecond pulses and further attempted to modify the properties of transient materials. Jalil et al. [16] fabricated uniform LSFL on nickel using two collinear femtosecond lasers at various temporal delays and attributed the regular LSFL to the reduction in the propagation length of SPP. The research to effectively regulate and enhance SPPs is still in progress.

    During LSFL formation on the semiconductor and metal surfaces induced by Fourier transform limit (FTL) laser pulses, a part of the ejected ablation plume deposited on the surface and formed a massive amount of debris [17]. Yang et al. [18] experimentally demonstrated the generation and erasure of LIPSS on a Si surface under irradiation with a single femtosecond laser pulse, which was due to the opposition between periodic surface structuring and surface smoothing associated with surface melting. Cheng et al. [19,20] clearly demonstrated the transient LSFL at a delay time of 150–400 ps on gold, silver, and nickel. These ripples were totally/partly submerged after solidification owing to the residual thermal effect. The deposited debris and the residual heat significantly affected the SPP excitation, propagation, and light field distribution during the subsequent laser irradiation, resulting in significant distortions and bifurcations on LIPSS.

    The enhancement of the strength of SPPs along with reducing the ablation debris and residual heat so as to fabricate LSFL with a high homogeneity and depth is a significant and fundamental research topic. In this paper, based on a 4f configuration zero-dispersion pulse shaping system, an FTL pulse is shaped into a pulse train using periodic π-phase step modulation. The LSFL fabricated by a shaped pulse of 16.2 ps is very straight and regular, with much less ablation debris. The LSFL depth fabricated by a shaped pulse of 16.2 ps is much larger than that of the FTL pulse, and the scanning velocity is also much faster. Large-area extremely regular LSFLs are fabricated by direct writing with a shaped pulse of 16.2 ps. The diffraction spectra are 36–20 nm wide in the range of 400–700 nm, and the diffraction efficiency is 3 times higher than that of the FTL pulse. The fabricated pattern of “Chinese knot” on the Si surface shows a vivid structural color. The formation mechanisms of regular LSFL were studied in detail through investigating the formation processes by changing the laser fluences and scanning velocity, and by theoretical calculation with the two-temperature-Drude model (TTM–Drude model).

    2. EXPERIMENTAL SETUP AND THE PROPOSED MODEL

    A. Experimental Setup

    (a) Experimental setup used for processing LSFL on silicon with a temporally shaped femtosecond laser. (b) Periodic π-phase step modulation applied to the laser spectrum. (c) Temporal intensity profile of the shaped pulse of 16.2 ps with a modulation period of 4.5 cm−1.

    Figure 1.(a) Experimental setup used for processing LSFL on silicon with a temporally shaped femtosecond laser. (b) Periodic π-phase step modulation applied to the laser spectrum. (c) Temporal intensity profile of the shaped pulse of 16.2 ps with a modulation period of 4.5  cm1.

    The 4f configuration zero-dispersion pulse shaping system consists of a pair of diffraction gratings with 1200 lines/mm (G1 and G2) and a pair of convex cylindrical lenses with a 400 mm focal length (CL1 and CL2) [22,23]. A one-dimensional programmable liquid crystal spatial light modulator (LC-SLM, SLM-S320d, Jenoptik) is placed at the Fourier plane to modulate the spectral phase. As shown in Fig. 1(b), a periodic π-phase step modulation is applied to the laser spectrum to generate femtosecond laser pulse trains [24]. Figure 1(c) shows the temporal intensity profile of a shaped pulse with a modulation period of 4.5  cm1, while the interval between two adjacent sub-pulses is 16.2 ps, which can be varied by changing the modulation period. The two primary sub-pulses exhibit a maximum normalized peak intensity of 0.63, and the intensities of the three outward sub-pulses are 0.21, 0.13, and 0.09, respectively. In this paper, the initial femtosecond laser pulses are denoted by the FTL pulse, and the pulse trains are denoted by the corresponding interval, such as a shaped pulse of 16.2 ps. The interval of the sub-pulses can be adjusted in a range of 0.25–16.2 ps.

    The sample was a commercial 0.5 mm thick undoped Si wafer (100) (MTI-group, China). The surface was optically polished with a roughness of <1  nm. The morphology and depth of the LSFL were measured using a scanning electron microscope (SEM) (S-4800, Hitachi, Japan) and a confocal microscope (Smartproof 5 Widefield Confocal Microscope, Zeiss, Germany), respectively. A fiber optical spectrometer (Ocean-2000, Ocean Optics, USA) was utilized to measure the diffraction spectrum of the larger-area LSFL. Structural colors ranging from red to purple were captured by a camera at different angles.

    B. Proposed Model of Shaped Pulse Laser-Induced Regular and Deep LSFL

    Schematic diagram of shaped pulse laser-induced regular and deep LSFL. The red area in the Si substrate presents the high temperature region when the sub-pulse reaches the surface.

    Figure 2.Schematic diagram of shaped pulse laser-induced regular and deep LSFL. The red area in the Si substrate presents the high temperature region when the sub-pulse reaches the surface.

    3. EXPERIMENTAL RESULTS

    A. Large-Area Extremely Regular LSFL Fabricated by Laser Direct Writing with a Shaped Pulse of 16.2 ps

    (a)–(d) SEM images of LSFL fabricated by shaped pulse of 16.2 ps through laser direct writing in parallel lines. (e) SEM image of the cross section of the LSFL. (f) 2D-FFT image of (b). (g) Spectrum of the FFT along the x axis. The scale bar in (e) is 500 nm long. The dash arrow in (a) presents the laser polarization, and the solid arrow presents the scan direction.

    Figure 3.(a)–(d) SEM images of LSFL fabricated by shaped pulse of 16.2 ps through laser direct writing in parallel lines. (e) SEM image of the cross section of the LSFL. (f) 2D-FFT image of (b). (g) Spectrum of the FFT along the x axis. The scale bar in (e) is 500 nm long. The dash arrow in (a) presents the laser polarization, and the solid arrow presents the scan direction.

    Figure 3(e) shows an SEM image of the cross section of the LSFL cut with a focused ion beam (FIB). The edges of the LSFL are significantly straight. The grooves are very regular and consistent with a depth of 97.6±2  nm. The white stripe is a Pt film coated before FIB cutting.

    (a) Optical characterization measurement for testing the diffractive properties of large-area LSFL. The diffraction spectra from the LSFL fabricated (b) by shaped pulse of 16.2 ps and (c) by FTL pulse. The orderly multicolor diffraction pattern from the LSFL fabricated (d) by shaped pulse of 16.2 ps and (e) by FTL pulse.

    Figure 4.(a) Optical characterization measurement for testing the diffractive properties of large-area LSFL. The diffraction spectra from the LSFL fabricated (b) by shaped pulse of 16.2 ps and (c) by FTL pulse. The orderly multicolor diffraction pattern from the LSFL fabricated (d) by shaped pulse of 16.2 ps and (e) by FTL pulse.

    The large-area LSFLs fabricated by the shaped pulse of 16.2 ps (F=0.70  J/cm2, vscan=8  mm/s) and FTL pulse (F=0.48  J/cm2, vscan=8  mm/s) with a line space of 21 μm were mounted on the same stage and measured in sequence. The diffraction spectra at different angles are depicted in Figs. 4(b) and 4(c), respectively. The diffraction spectra can be characterized by the diffraction grating equation as dsinα=mλ, where m is the diffraction order, d is the grating constant (LSFL period), and α is the diffraction angle. For the LSFL fabricated by a shaped pulse of 16.2 ps, the FWHM values of the diffraction peaks at 450, 500, 550, 600, and 650 nm are only 36, 28, 25, 22 and 20 nm, respectively. Moreover, the diffractive efficiency is 3 times larger than that using the FTL pulse. Figure 4(d) presents the orderly multicolor diffraction patterns from the LSFL fabricated by the shaped pulse of 16.2 ps. The colors range from red to purple at different diffraction angles, which are obviously brighter, clearly distinguished, and much narrower than those from the FTL pulse [Fig. 4(e)]. These results further demonstrate that the quality of the LSFL fabricated by the shaped pulse of 16.2 ps is significantly better than that by the FTL pulse.

    Structural colors of “Chinese knot” pattern made up of LSFL fabricated by shaped pulse of 16.2 ps on Si surface. The white scale bars are 5 mm long.

    Figure 5.Structural colors of “Chinese knot” pattern made up of LSFL fabricated by shaped pulse of 16.2 ps on Si surface. The white scale bars are 5 mm long.

    B. LSFL Fabricated by a Shaped Pulse of 16.2 ps and FTL Pulse

    1. Window of the Laser Fluence for Fabricating Regular LSFL at a Different Scan Velocity

    Laser fluence and scan velocity are two key parameters in femtosecond laser processing of uniform and regular LSFL. The homogeneous alternating amorphous-crystalline LSFL was induced by an FTL pulse under a low laser fluence, and the scan velocity was usually less than 2.0 mm/s under 1 kHz repetition [17], which indicates a low fabricating efficiency. In order to increase the scan velocity and LSFL depth, it is necessary to increase the laser fluence. However, this will partially enhance laser ablation and cause massive debris and surface defects. Debris and surface defects substantially affect the generation of SPPs and the distribution of the light field of subsequent laser pulses, resulting in curved and cracked LSFL, even partially smoothed by the molten layer [18].

    (a) At different scan velocity, the laser fluence windows for fabricating spaced (green), regular (orange), and partly damaged (blue) LSFLs by shaped pulse of 16.2 ps (the upper area) and by FTL pulse (the lower area). The SEM images (s1)–(s6) and (f1)–(f6) are the corresponding LSFLs of the marked points in (a). The dash arrow in (s1) presents the laser polarization, and the solid arrow presents the scan direction. The scale bars have a length of 3 μm.

    Figure 6.(a) At different scan velocity, the laser fluence windows for fabricating spaced (green), regular (orange), and partly damaged (blue) LSFLs by shaped pulse of 16.2 ps (the upper area) and by FTL pulse (the lower area). The SEM images (s1)–(s6) and (f1)–(f6) are the corresponding LSFLs of the marked points in (a). The dash arrow in (s1) presents the laser polarization, and the solid arrow presents the scan direction. The scale bars have a length of 3 μm.

    When the scan velocity increases to 8.0  mm/s, the overlapped pulse number is too low to support the formation of a regular LSFL fabricated by an FTL pulse. When the laser fluence is less than 0.46  J/cm2, only spaced and curved ripples are formed as shown in Fig. 6(f4). When the laser fluence is 0.48  J/cm2, the spaced ripples start to connect with each other but are much curved with many bifurcations because of debris and residual thermal effects as shown in Figs. 6(f5) and 6(f6).

    Because each shaped pulse contains many sub-pulses, when the scan velocity gradually increases, the laser fluence window of the regular LSFL becomes larger, which is very different from the FTL pulse. When the scan velocity increases to 8.0 mm/s, regular LSFLs are fabricated in the range of 0.721.00  J/cm2. The maximum window was 0.28  J/cm2, which is 7 times larger than the maximal window of the FTL pulse (0.04  J/cm2 at a scan velocity of 5 mm/s). Figure 6(s5) depicts a very regular LSFL with a period of 770 nm for a laser fluence F=0.84  J/cm2. When the laser fluence is 0.68  J/cm2, the spaced LSFLs are still very straight without any debris as shown in Fig. 6(s4). When the laser fluence is larger than 1.00  J/cm2, the LSFL with a part of the damaged area is still in a good arrangement as shown in Fig. 6(s6). When the scan velocity increased to 12 mm/s, the laser fluence window of the regular LSFL is still as large as 0.14  J/cm2. Under the irradiation of the temporally shaped pulse of 16.2 ps, the laser fluence window of regular LSFL is larger and the scan velocity is faster, which indicates that the temporally shaped pulse is a good method to achieve high efficiency and stability in fabricating regular LSFLs.

    2. Groove Depth of LSFL

    Confocal microscopy images of the regular LSFL fabricated by (a) FTL pulse (F=0.43 J/cm2) and (b) shaped pulse of 16.2 ps (F=0.69 J/cm2). The scan velocity is 5.0 mm/s. (c) and (d) 2D plots of the cross sections along the yellow arrows in (a) and (b), respectively. (e) Average depth of the LSFL along the cross section perpendicular to the ablation trace.

    Figure 7.Confocal microscopy images of the regular LSFL fabricated by (a) FTL pulse (F=0.43  J/cm2) and (b) shaped pulse of 16.2 ps (F=0.69  J/cm2). The scan velocity is 5.0 mm/s. (c) and (d) 2D plots of the cross sections along the yellow arrows in (a) and (b), respectively. (e) Average depth of the LSFL along the cross section perpendicular to the ablation trace.

    3. Orientation of LSFL

    To characterize the uniformity of LSFL, the local orientations of the structures in Figs. 6(s2),6(s3),6(f2),6(f3) and 6(s5), 6(s6), 6(f5), 6(f6) are analyzed through an open software “ImageJ” with the “OrientationJ” plugin [28] as shown in Table 1. The divergence of the structure orientation angle (DSOA) is the half-width at half-maximum value of the distribution. The DSOA of the regular LSFL in Fig. 6(s2) is 3.7°, which is slightly smaller than that of Fig. 6(f2). However, for the partly damaged LSFL, the DSOA of Fig. 6(s3) is only 6.7°, which is significantly less than that of Fig. 6(f3). These results indicate that the partly damaged ripples fabricated by the shaped pulse of 16.2 ps are considerably more homogeneous and regular than those fabricated by the FTL pulse at lower scan velocities. When the scanning velocity is increased to 8.0 mm/s, the DSOA of the regular ripples fabricated by the shaped pulse of 16.2 ps surprisingly drops to 2.5°, which is even smaller than that of the alternating amorphous-crystalline LSFL [7]. These results further demonstrate that the LSFLs induced by the shaped pulse of 16.2 ps are much more homogeneous and straighter than that of the FTL pulse, especially for high-speed fabrication.

    DSOA of the Regular Ripples and Partly Damaged Ripples in Fig. 6

    LSFLs2s3s5s6
    DSOA3.7°6.7°2.5°5.8°
    LSFLf2f3f5f6
    DSOA4.0°11.5°17.5°22.5°

    C. Mechanisms of the Regular LSFL Fabricated by a Temporally Shaped Laser Pulse

    To investigate the formation mechanism of the regular LSFL induced by the shaped pulse of 16.2 ps, the ripples fabricated at different scanning velocities under the irradiation of an FTL pulse and the shaped pulse of 16.2 ps are studied in detail. The two-temperature model and Drude model (TTM–Drude model) were used to study the evolution of electron density, electron temperature, lattice temperature, and dielectric constant on the Si surface after irradiation with the shaped pulse of 16.2 ps or FTL pulse. The experimental and theoretical results elucidate the formation mechanisms of regular LSFL.

    1. Formation of LSFL by FTL Pulse

    SEM images of the surface nanostructures fabricated by FTL pulse at different scanning velocities of (a) 16 mm/s, (b) 14 mm/s, (c) 10 mm/s, and (d) 8 mm/s. The laser fluence was fixed at 0.54 J/cm2. The dashed arrow presents the laser polarization, and the solid arrow presents the scanning direction. The scale bars are all 5 μm in length.

    Figure 8.SEM images of the surface nanostructures fabricated by FTL pulse at different scanning velocities of (a) 16 mm/s, (b) 14 mm/s, (c) 10 mm/s, and (d) 8 mm/s. The laser fluence was fixed at 0.54  J/cm2. The dashed arrow presents the laser polarization, and the solid arrow presents the scanning direction. The scale bars are all 5 μm in length.

    When the scan velocity further reduced to 8 mm/s, the curved LSFL filled in the ablation trace. A considerable amount of ablative materials are deposited on the surface. The ripples are partly melted and submerged owing to the thermal effect as shown in Fig. 8(d). When the scan velocity decreases to 6  mm/s, most of the ripples at the center of the ablation trace are submerged, owing to the thermal effect. The results show that, under the irradiation of the FTL pulse, the edge of the ablation spot is the origin of the SPPs, which induces curved and broken ripples. In addition, the deposited debris disturbs the excitation and propagation of SPPs [8], and the residual heat partly submerges the ripples. Therefore, the conditions to fabricate regular LSFLs under the irradiation of an FTL pulse are very strict.

    2. Formation of LSFL by a Shaped Pulse of 16.2 ps

    SEM images of the surface nanostructures fabricated by shape pulse of 16.2 ps at different scanning velocities of (a) 17 mm/s, (b) 15 mm/s, (c) 13 mm/s, and (d) 11 mm/s. The laser fluence was fixed at 1.13 J/cm2. The dashed arrow presents the laser polarization, and the solid arrow presents the scanning direction. The scale bars are all 5 μm long.

    Figure 9.SEM images of the surface nanostructures fabricated by shape pulse of 16.2 ps at different scanning velocities of (a) 17 mm/s, (b) 15 mm/s, (c) 13 mm/s, and (d) 11 mm/s. The laser fluence was fixed at 1.13  J/cm2. The dashed arrow presents the laser polarization, and the solid arrow presents the scanning direction. The scale bars are all 5 μm long.

    It is worth noting that the LSFL appears in the entire ablation spot irradiated by the shaped pulse of 16.2 ps, rather than only in the vicinity outside the edge of the ablation spot by the FTL pulse. Moreover, these ripples are very straight and regular, considerably better than those obtained by the FTL pulse. The main influencing factors are as follows. Under the irradiation of an FTL pulse, the energy is mainly used to excite a plasma layer; only a small portion of the energy in the tail of the laser pulse is used to excite SPPs [1,25]. SPPs propagate simultaneously from the edge to the outside and inside of the ablation spots. Owing to the thermal effect caused by the higher local laser fluence, the LSFLs inside the ablation spots are submerged [25]. The LSFLs outside the ablation spots are reserved owing to the appropriate local laser fluence. However, irradiated by the shaped pulse of 16.2 ps, the first strongest sub-pulse excites the Si surface from the semiconductor to the metal-like state, which supports the excitation of SPPs [18,25]. The transient LSFLs induced by SPPs are the origin of the SPPs for the subsequent sub-pulses [1,25,29], which further enhance the excitation of the SPPs propagating perpendicularly to the laser polarization, resulting in the formation of straight and regular LSFL.The experimental results of ultrafast pump-probe imaging show that the Si surface begins to melt and ablate from a delay time of 1–30 ps after femtosecond laser irradiation [25,29]. The electron and lattice on the surface remain at a high temperature when the subsequent sub-pulses arrive at the surface irradiated by the shaped pulse of 16.2 ps. The subsequent sub-pulses further excite the high-temperature surface and effectively ablate materials [26]. Under the irradiation of the shaped pulse of 16.2 ps, the residual thermal effect on the ablation spot is significantly reduced because of the “ablation cooling” effect [26], and the SPPs are considerably enhanced because of the “grating-light coupling” effect [1]; thus, the regular LSFLs are well preserved. This further explains the much deeper LSFL induced by the shaped pulse of 16.2 ps as shown in Fig. 6.As shown in Fig. 8, under the irradiation of the FTL pulse, a significant amount of ablative debris and particles were deposited on the surface, which substantially disturbed the formation of LSFL. In contrast, Fig. 9 shows that the Si surface is very clear, without any debris or particles after irradiation by the shaped pulse of 16.2 ps. The energy of each sub-pulse in the shaped pulse of 16.2 ps is small, the surface is gently ablated, and the amount of ejected materials is also very small for each sub-pulse. The ablation dynamics shows that, at a delay time of 3–4 ps after irradiation by a femtosecond laser pulse, the LSFLs appear on a Si surface [25]. Some ejected materials are observed on the surface at a delay time of dozens of picoseconds and will deposit back to the surface [29]. However, under the irradiation of the shaped pulse of 16.2 ps, the ejected materials will be excited by the subsequent sub-pulses, and the plume is further ionized and vaporized, resulting in less deposited debris [29,30].

    When the scan velocity decreased to 13 mm/s, continuous, regular, and straight LSFLs are formed on the ablation trace as shown in Fig. 9(c). When the scan velocity decreased to 11 mm/s, the LSFL melted slightly as shown in Fig. 9(d). It is worth noting that, under the irradiation of the shaped pulse of 16.2 ps with such a high laser fluence, although the ripples are partly melted and ablated, no ablation debris is observed on the surface, and the ripples are still very straight.

    3. LSFL Fabricated by Temporally Shaped Pulses with Different Intervals

    Confocal optical images of LSFL fabricated by shaped pulse of (a) 16.2 ps, 0.65 J/cm2; (b) 4.1 ps, 0.44 J/cm2; (c) 1.0 ps, 0.43 J/cm2; and (d) 0.25 ps, 0.42 J/cm2. The scan velocity was fixed as 5 mm/s. (e) The period and depth of the LSFLs for different intervals of sub-pulse.

    Figure 10.Confocal optical images of LSFL fabricated by shaped pulse of (a) 16.2 ps, 0.65  J/cm2; (b) 4.1 ps, 0.44  J/cm2; (c) 1.0 ps, 0.43  J/cm2; and (d) 0.25 ps, 0.42  J/cm2. The scan velocity was fixed as 5 mm/s. (e) The period and depth of the LSFLs for different intervals of sub-pulse.

    When the interval of the sub-pulses is less than 1.0 ps, that is, less than the electron-phonon coupling time [25], the temporally shaped pulse will induce an intense thermal accumulation effect rather than an ablation-cooling effect. Thus, the strong thermal hydrodynamics will cause the LSFL to bend and partially submerge. When the interval of sub-pulse increases from 0.25 to 16.2 ps, Fig. 10(e) shows that the period of the LSFL increases from 772 to 793 nm, and the depth increases from 34 to 103 nm. These results show that the period and depth of the LSFL can be well adjusted by a temporally shaped pulse.

    4. Theoretical Study of the Ultrafast Dynamics of a Si Surface Irradiated by a Temporally Shaped Pulse

    When silicon is irradiated by a femtosecond laser pulse with a fluence F, electrons are excited to the conduction band via single-photon absorption, two-photon absorption, free-carrier absorption, and impact ionization. A high-density free carrier Ne is generated on the surface, and the optical properties of Si change from the semiconductor to the metallic state [30,31]. The carriers are rapidly heated by strong electron-electron scattering within tens of femtoseconds [32], and the energy is transferred to the lattice via a carrier-phonon scattering process within several to tens of picoseconds to achieve a thermal equilibrium between electron and lattice systems [32]. To elucidate the mechanism of the formation of LSFL irradiated by a temporally shaped pulse, the evolution of the carrier excitation, the carrier and lattice temperatures, and the transient dielectric constant on the Si surface were studied using the TTM–Drude model [18,25,33]. While numerically solving the TTM–Drude model, the time range was set as 500 to +500  ps with a step size of 1 fs.

    Evolution of (a) the carrier density, (b) the carrier temperature, (c) the lattice temperature, and (d) the real part of the dielectric constant on the Si surface irradiated with shaped pulse of 16.2 ps.

    Figure 11.Evolution of (a) the carrier density, (b) the carrier temperature, (c) the lattice temperature, and (d) the real part of the dielectric constant on the Si surface irradiated with shaped pulse of 16.2 ps.

    Figure 11(b) shows the evolution of the carrier temperature on the Si surface. The carrier temperature exceeds 10,200 K after the two strongest sub-pulses have arrived at the Si surface. Owing to electron-phonon coupling, the lattice temperature rapidly increases to the melting temperature (1687 K) at t=8.3  ps as shown in Fig. 11(c). The lattice temperature decreases by 40–70 K between two adjacent sub-pulses because of the lattice thermal conductivity [33]. When t>150.0  ps, the lattice temperature reaches a plateau at approximately 2300 K, indicating that the Si surface maintains a mild melting state.

    Figure 11(d) shows the evolution of the real part of the dielectric constant ε of the Si surface. When t<56.0  ps, the real part of the dielectric constant decreases slowly because the electrons are excited by the front weak sub-pulses. When t=8.1  ps, the first strongest sub-pulse reaches the surface, and ε decreases to 7.88 owing to a high carrier density of 8.05×1021  cm3. The surface changes from a semiconductor to a metal-like state and effectively supports the excitation of SPPs. The subsequent nine sub-pulses can also excite the carrier density to a high level, causing ε to be less than 1. The width of the temporal envelope for ε<1 is 145.8 ps.

    The ultrafast dynamics of periodic ripple formation on a Si surface was studied using imaging systems with high spatial-temporal resolution [25,29]. The LSFLs emerge very rapidly, only 3 ps after a single femtosecond laser pulse [25]. This indicates that after irradiation by the strongest sub-pulse at t=8.1  ps, the Si changes from a semiconductor to a metal-like state. When the second strongest sub-pulse (t=+8.1  ps) arrives at the surface, the LSFLs appear. These transient ripples enhance the SPPs and lead to a periodic distribution of the laser energy [1]. During the excitation of the SPPs [145.8 ps, as shown in Fig. 11(d)], both melting and ablation can occur [34,35]. Thus, a part of the plasma excited by the strongest pulse is ejected from the surface, removes the deposited heat, and enables ablation cooling [26]. The subsequent sub-pulses further excite the SPPs, enhancing the periodic laser energy deposition. In this case, the SPPs and the periodic laser energy deposition are enhanced, and the residual heat is reduced; thus, clear LSFLs are formed at the center of the ablation spot. Hence, the temporally shaped pulse of 16.2 ps can significantly increase the scan velocity, the window of laser fluence of regular LSFL, and the regularity and LSFL depth.

    The theoretically calculated results show that when the Si surface is irradiated by a shaped pulse with an interval less than 1 ps, the width of the temporal envelope of ε<1 is less than 10 ps, which means that the excitation of the SPPs lasts for a short time. The interval between two adjacent sub-pulses is less than the time of electron-phonon coupling, and the energy carried away by the lattice heat conduction is very small. Moreover, the lattice temperature only decreases by 0–3 K between two adjacent sub-pulses, and the material ablation is negligible. These results show that when the interval is less than 1 ps, the shaped pulse can enhance the SPPs but to a lesser extent than the shaped pulse of 16.2 ps. Meanwhile, the thermal effect is strong, and the ablation-cooling effect is weak; thus, the regularity of LSFLs is substantially reduced.

    4. CONCLUSIONS

    In this study, based on a 4f configuration zero-dispersion pulse shaping system, an FTL pulse is shaped into a pulse train with varying temporal intervals in the range of 0.25–16.2 ps by periodic π-phase step modulation. The large-area LSFL is efficiently fabricated by the shaped pulse of 16.2 ps by laser direct writing. The grooves of LSFL are very straight with DSOA as low as 2.5°, very uniform with a period of 774±5  nm, and considerably deep with a depth of 97.6±2  nm. The LSFL gratings demonstrate a very bright and pure structural color ranging from blue to red, observed at different angles. The diffraction peaks from 450 to 700 nm are all significantly narrow, only 26 nm on an average, and the diffractive efficiency is 3 times larger than that fabricated by the FTL pulse.

    The fabrication efficiency, LSFL depth, and DSOA obtained by using the shaped pulse of 16.2 ps are all considerably better than those by using the FTL pulse. The formation mechanisms of regular LSFLs by using the shaped pulse are experimentally and theoretically studied in detail. The results show that the temporally shaped pulse can enhance the excitation of the SPPs and periodic energy deposition, while reducing the residual thermal effects and the deposition of the ejected debris on the Si surface, eventually resulting in regular and deep LSFL.

    Acknowledgment

    Acknowledgment. The English text of this manuscript was edited using the OSA Language Editing Services [36].

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    Yuchan Zhang, Qilin Jiang, Kaiqiang Cao, Tianqi Chen, Ke Cheng, Shian Zhang, Donghai Feng, Tianqing Jia, Zhenrong Sun, Jianrong Qiu. Extremely regular periodic surface structures in a large area efficiently induced on silicon by temporally shaped femtosecond laser[J]. Photonics Research, 2021, 9(5): 839
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