Femtosecond laser-induced periodic structures: mechanisms, techniques, and applications

Over the past two decades, femtosecond laser-induced periodic structures (femtosecond-LIPSs) have become ubiquitous in a variety of materials, including metals, semiconductors, dielectrics, and polymers. Femtosecond-LIPSs have become a useful laser processing method, with broad prospects in adjusting material properties such as structural color, data storage, light absorption, and luminescence. This review discusses the formation mechanism of LIPSs, specifically the LIPS formation processes based on the pump-probe imaging method. The pulse shaping of a femtosecond laser in terms of the time/frequency, polarization, and spatial distribution is an efficient method for fabricating high-quality LIPSs. Various LIPS applications are also briefly introduced. The last part of this paper discusses the LIPS formation mechanism, as well as the high-efficiency and high-quality processing of LIPSs using shaped ultrafast lasers and their applications. Sci 1 , 220005


Introduction
Femtosecond lasers have ultrashort pulse durations and high peak powers 1 . The interaction between femtosecond laser and matter has the advantages of minimal thermal effect and no material selectivity 2,3 . Compared with nanosecond and other long-pulse lasers, femtosecond lasers could induce more regular and deeper periodic structures on different types of materials, such as metals, semiconductors, dielectrics, and polymers, without being overwhelmed by huge thermal effects 4−14 .
Laser-induced periodic structures (LIPSs) have been intensively studied since 1965 when Birnbaum et al. found LIPSs on the surface of germanium and gallium arsenide 15,16 . Over the past two decades, ultrafast laser technology has developed rapidly. Femtosecond LIPSs (Λ) to the laser wavelength (λ), LIPSs are divided into low-spatial-frequency LIPSs (LSFLs, ) and high-spatial-frequency LIPSs (HSFLs, ) 4 , as shown in Fig. 1. However, the formation mechanism of femtosecond-LIPSs does not yet have a clear explanation.
Femtosecond-LIPSs have become a useful laser processing method, with broad prospects for adjusting material properties 9,32−38 . HSFLs on the surfaces and interiors of dielectrics demonstrate birefringence effects, which have been used in data storage and waveplates 34,36,38 . LIPSs produce a significant dispersion of incident light and have been used to prepare pure and bright structural color surface 32,35 . Composite structures with LIPSs can enhance light absorption and luminescence and adjust the wetting properties of a material surface 33,37,39 . The anisotropic electrical properties of LIPSs are an emerging research topic 40,41 .
The main contents of this review are as follows. Section Several models of LIPS formation discusses the formation mechanism of LIPSs. Section Ultrafast dynamics of LIPS formation based on pump-probe imaging presents the ultrafast dynamics studied by the pump-probe imaging method. The pulse shaping of a femtosecond laser in the temporal -frequency domain, polarization, and spatial distribution is an efficient method for fabricating regular LIPSs with high efficiency, as described in Section Efficient fabrication of high-quality LIPSs with shaped ultrafast laser pulses. Section Femtosecond laser direct writing of <100 nm nanostructures introduces promising methods for laser processing nanostructures with feature sizes smaller than 100 nm. In the Section Applications of LIPSs, we briefly introduce LIPS applications in the fields of data storage, structural color surfaces, light absorption and luminescence enhancement, and the adjustment of conductive properties. The last section presents the outlook on the LIPSs. storage, structural color surfaces, light absorption and luminescence enhancement, and the adjustment of conductive properties. The last section presents the outlook on the LIPSs.

Several models of LIPS formation
The formation of periodic nanostructures after irradiation with a single laser beam is an interesting phenomenon. In particular, the periods of femtosecond-LIPSs change significantly with the laser fluence and the number of overlapping pulses, which renders the formation mechanism of LIPSs more complicated to determine. Numerous experimental, theoretical, and numerical studies have been performed, and several models have been proposed to explain the formation of LIPSs 9,16−18,20,21,25,42 , which are described below.

Scattering light model
After irradiation with continuous and long-pulse lasers, LSFLs perpendicular or parallel to the polarization direction formed on the surfaces of semiconductor materials, where the periods were nearly equal to the laser wavelength, λ. It was proposed that these LSFLs were caused by the interference between the incident light and scattered light from the ablated surface 16,18,43 , as shown in Fig. 2. When the laser was perpendicular to the sample, period Λ was equal to laser wavelength λ. When the laser was obliquely incident at an angle θ, period Λ = λ/ (1 ± sin θ), where + and -represent the forward and backward scattered light, respectively 44 . This model was refined and developed by Sipe et al. They performed detailed theoretical studies and found that the generation of periodic structures depended significantly on the a b LSFL HSFL 1 μm 2 μm  roughness of the material surface 16 . Rough structures with a height significantly smaller than the laser wavelength resulted in a periodic distribution of the laser field itself. The incident angle and polarization direction highly influenced the period and orientation of the LS-FLs. This model has often been used to explain classical LSFLs induced by nanosecond or continuous wave (CW) lasers. Over the last two decades, scattering models have been developed and improved to explain the formation of femtosecond-LIPSs 45,46 . According to the Drude model, the dielectric constants of semiconductors and dielectrics depend on the numerical density of free electrons excited by femtosecond laser. According to the Sipe model, the periodic deposition of laser energy depends on the surface roughness and dielectric constants. The formation of femtosecond-LIPSs on semiconductors and dielectrics was studied by comprehensively considering the Sipe and Drude models 22,23,47,48 , and developed into the Sipe-Drude model 22 . The numerical density of free electrons and dielectric constants varied with the laser fluence, wavelength, and pulse number, which further tuned the periods of LIPSs, as shown in Fig. 2.

Surface plasmon polariton model
Under femtosecond laser irradiation, a large number of free electrons are excited, forming a plasma layer on the surface of a material. The femtosecond laser further causes the collective oscillation of the surface plasma and forms surface plasmon polaritons (SPPs), as shown in Fig. 3(a). The excitation of SPPs causes a periodic distribution of the laser field and energy deposition in the free electrons. The lattice is heated, melted, or even ablated via electron-phonon coupling, which further induces the formation of LIPSs 8,21,25,48,50−62 . However, in the case of vertical incidence (Fig. 3(b)), the wave vector of the SPPs is always larger than that of the incidence light. Therefore, grating structures 42 or a single nanogroove 25,52 are The SPP model was used to explain the formation of LSFLs on semiconductors. The periods are expressed as follows 21 where λ SPP is the SPP wavelength, θ is the angle of incidence, ε is the dielectric constant of the metal, and ε d is the dielectric constant of the dielectric. Under femtosecond laser irradiation, a large number of free electrons are excited, resulting in a significant change in the dielectric constant, according to the Drude model. The SPP  wavelength and LSFL period can be calculated based on   the dielectric constant in the excited state, which agrees  well  with  the  experimental  results  for  semiconductors 42,51,52 . The LSFL periods induced by femtosecond laser pulses on semiconductors and metals are always smaller than the SPP wavelengths. With more laser pulse irradiation, the LSFL-assisted SPP is coupled with the laser field, forming a positive feedback effect, and leading to the formation of shorter-period LSFLs 21 .

Model of nanoplasmonic enhancement of laser field
The groove widths of HSFLs induced by femtosecond laser pulses are usually smaller than 100 nm, whereas the depth can reach hundreds of nanometers to micrometers. The widths of these periodically arranged nanogrooves are significantly smaller than the diffraction limit of the laser 17,20,63−72 . Figure 4(a) shows 20 nmthick nanosheet-like nanostructures with a period of λ/2n in fused silica (refractive index n) induced by linearly polarized 800 nm femtosecond laser pulses. It was proposed that these nanosheets were produced by the asymmetrically localized laser-field enhancement of the nanoplasma 20 , as shown in Fig. 4(b).
The periodic distribution of the nanoplanes evolved from a random distribution of nanoplasma over many shots owing to the memory mechanism and mode selection. The period, λ 0 /2n, is the minimum spacing required in planar metal waveguides to support this mode with the field maxima at the metal-dielectric interface 20 .
Another mechanism that has been proposed for the formation of HSFLs in fused silica is photon -plasmon scattering 17 . By adjusting the pulse energy and cumulative pulse number of the 800 nm femtosecond laser, the HSFLs period varied from 140 to 320 nm. This phenomenon was interpreted in terms of the interference between the incident light field and the electric field of the bulk plasma wave, resulting in the periodic modulation of the electron concentration and structural changes in the fused silica 17 .

Other models
In recent years, several other models have been proposed to explain the formation of LIPSs, such as self-organization 24,26,73 , evanescent waves 74 , and Coulomb explosions 75,76 . During the femtosecond laser irradiation of SiC, ZnSe, and ZnO crystals, the generation of second harmonics induces the formation of HSFLs 9,77,78 . The ripples of LSFLs on metal surfaces gradually split and evolve into HSFLs with an increase in femtosecond laser pulse irradiation 79,80 .
During the femtosecond laser irradiation of semiconductor materials such as Si and ZnS, the surface plasmon layer supports two types of SPPs: those at the plasma -air interface and those at the plasma -substrate interface 81 . The local laser fluence and accumulated pulse number are key factors affecting the formation of LSFLs and HSFLs. Two SPP waves interfere with each other to form standing waves that induce the formation of HS-FLs 82,83 . A femtosecond laser was obliquely incident on a stainless steel surface and induced two periodic structures, LSFLs and HSFLs, which were caused by forwardand backward-propagating SPPs, respectively 84 . Numerical simulation results using the finite-difference timedomain (FDTD) method indicated that the coherent superposition of the scattering far-field (propagation) of the microstructured surface and refracted fields induced LSFLs, while the scattering near-field (evanescent) and refracted fields induced HSFLs 85 .

Ultrafast dynamics of LIPS formation based on pump-probe imaging
The phenomena of femtosecond-LIPSs are very rich, and the formation mechanisms are very complicated. Are LIPSs caused by the periodic deposition of laser energy or by self-organization during laser excitation, melting, and solidification? The propagating SPP model has been widely accepted to explain the formation of LSFLs on semiconductor and metal surfaces, however several key issues remain to be addressed. First, the SPPs on the surfaces of gold and silver are very strong 86 , however it is difficult to form regular LSFLs on these noble metals 10 . Second, the periods of the LIPSs on metal surfaces are significantly smaller than the SPP wavelengths 49,87 . The further development of new methods is required to solve these problems.
Scanning electron microscopy (SEM) and atomic force microscopy (AFM) are typically used to observe LIPSs. The resolutions of these methods are very high, however they cannot be used to study the dynamics of LIPS formation. Pump-probe spectroscopy is an effective method to study the kinetics of femtosecond laser ablation 88 . However, transient images of LIPSs cannot be observed directly 19,79 . Therefore, we developed a collinear pumpprobe imaging method to study the transient processes of LIPS formation 25,49,52,55,56,89 . The spatial resolution is 300 nm and the temporal resolution is 0.6 ps. To show clearly the changes in the surface microstructure, the same spot was observed at three different times: before the arrival of the pump pulse, at the designed delay time, and after the ablation spot solidified. Figure 5 shows the optical microscope (OM) images of the transient LSFLs on the Si surface at different delay times 52 . The LSFL period was 680 ± 15 nm, and the direction was parallel to the nanogroove. The annular structures were caused by the diffraction effect of the concave lens. Surprisingly, periodic ripples quickly emerged. The transient ripples were clear at a delay time of 4.0 ps, which indicated that these ripples were due to surface melting, excluding other thermodynamics such as ablation and hydrodynamics. The AFM measurements further showed that the height fluctuation in the LSFL region was less than 0.6 nm, and had no spatial periodicity. The LSFL period was predicted based on the SPP model, and it coincided with the experimental results. The formation of LSFLs on silicon was due to the periodic energy deposition caused by the SPP excitation, rather than  self-organization processes.
The pump-probe imaging experiments were further carried out to investigate the role of SPPs and thermal effects on the formation of LIPSs on noble-metal surfaces 49 . Under the irradiation of the second femtosecond laser pulse, transient periodic ripples were clearly observed on the gold film, but no obvious ripples remained after the surface solidified. The surface defects produced by the first laser pulse launched SPPs on the Au film, which caused further modulated energy deposition and induced transient LIPSs. However, because of the small electron -phonon coupling coefficient of Au, the slow heat process and strong melting effect caused the ripples to disappear. When the Au film was immersed in water, the LIPSs were retained because of the high cooling rate of the molten layers 49 . These experimental results indicated that the SPPs played an important role in the formation of LIPSs, while the thermal effect greatly influenced whether the transient ripples were retained after the molten surface solidified 49 .
However, the period dependence of the LSFLs on metal surfaces was much different from the SPP model 87,90,91 . The SPP wavelengths were calculated by using the dielectric constants of silver, gold, copper, and aluminum in the ground states, which were significantly larger than the periods of LSFL ripples 10,49,92 . The d-band electrons in gold and silver could be excited to states above the Fermi surface 93,94 . The free-electron density and plasma frequency both increased with laser fluence, resulting in longer SPP wavelengths. The LSFL periods were very different from those of the SPP model, which is a difficult problem to solve.
LSFLs are typically induced by multiple laser pulses. The material at the laser focus is partially ablated after each laser pulse irradiation, which makes the formation mechanisms more difficult to understand because of the effects of the scattered light and grating coupling feedback. Therefore, theoretical and experimental studies on the LSFLs induced by a single femtosecond laser pulse provide fundamental information on the origin of LSFLs on metal surfaces 52,95−97 . Figure 6 shows the evolution of the LSFLs formed on an Au film. The transient LSFLs began to appear at 45 ± 10 ps, and become more numerous and clearer over time 25 . Between 400 and 600 ps, the ripples appear to be the most distinct and regular. The ripples are perpendicular to the laser polarization with a period of 740 ± 10 nm. Figure 7 shows that the LSFL period increased from 685 nm to 770 nm when the fluence increased from 0.73 to 3.42 J/cm 2 25 , which was similar to the results for Si, GaP, and ZnO crystals 51,52,98,99 . The SPP wavelength of gold was 780 nm for 800 nm light, which was significantly larger than the experimental value. With increasing laser fluence, free-electron density became larger, which led to longer SPP wavelength and LSFL period. Obviously, there is a great deviation between the SPP theory and the experimental results.
The dielectric constant is a key factor affecting the SPP wavelength 21,25 . The dielectric constant in the ground state was usually used to study SPP on metal surfaces, which was an important factor that caused the deviation between the SPP theory and the experimental results. Therefore, Cheng et al. proposed and studied in detail the effects of hot electron localization and d-band transitions on the dielectric constant and SPP wavelength for gold at the highly excited states during femtosecond laser irradiation 25 . The LSFL period calculated by the developed SPP model decreased from 780 nm to 685 nm as the fluence increased to 0.75 J/cm 2 . However, as F was  heat, and avoidance of debris deposited on the ablation spots 8 . A temporally shaped ultrafast laser is an efficient tool for fabricating regular LIPSs 102 . By changing the subpulse interval, pulse number, and sub-pulse energy distribution of the burst pulse train, the regularity and uniformity of LIPSs were obviously improved, and the structural color was more vivid 103,104 . Double temporally delayed femtosecond laser beams can efficiently induce uniform LSFLs in semiconductors, metals and dielectrics 105,106 . A femtosecond laser was further shaped into symmetrical three-and four-pulse sequences for the fabrication of double-grating structures and HSFLs on fused silica 107 .
By controlling the voltage of each element of the spatial light modulator (SLM) via computer programming, the phase and/or amplitude of the laser spectrum can be modulated, and the 4f zero-dispersion pulse-shaping system can generate shaped laser pulses with arbitrary temporal distributions 8 . Specifically, the number of subpulses, interval between adjacent subpulses, and pulse energy of each subpulse can be flexibly adjusted.
Regular LSFLs on silicon were processed using a 4f configuration zero-dispersion pulse-shaping system 8 . A Gaussian pulse is shaped into pulse trains with an interval of 16.2 ps by using periodic π-phase step modulation. The transient LIPSs started appearing on the Si surface at a delay time of 4 ps 52 , which was shorter than the interval between adjacent sub-pulses. Thus, the transient LIPSs have started to appear under the illumination of the two main subpulses. When the subsequent subpulse reached the sample surface, the transient LIPSs induced by the previous subpulses enhanced the excitation of the SPPs, as well as the periodic distribution of the laser field.
When the subsequent small sub-pulses reached, the surface layer remained at a very high temperature. It was further excited and partially ablated, taking away some of the remaining heat (ablative cooling effect). Moreover, the ablated plume was further excited by the subsequent subpulses, and the debris was further ionized and vaporized, resulting in fewer deposited particles. Therefore, regular and uniform LSFLs were induced on the Si surface using a shaped pulse of 16.2 ps 8 , as shown in Fig. 8.
The fabrication efficiency, depth, and regularity of the LSFLs when using shaped pulses of 16.2 ps were significantly better than those using Gaussian femtosecond laser pulses 8 . The scan velocity for fabricating regular LSFLs was 2.3 times faster, while the LSFLs depth was 2 times deeper, and the diffraction efficiency was 3 times higher than that of LSFLs using Gaussian femtosecond laser pulses.
Spatially shaped femtosecond laser-induced LIPSs with high efficiency Arbitrary spatial intensity distribution of femtosecond laser pulses can be modulated by loading a computergenerated hologram (CGH) on a SLM, and LIPSs can be processed in parallel with high efficiency 108 . For example, Hayasaki et al. successfully realized the parallel processing of microstructures by loading a mixed-phase Fresnel lens on the SLM 109,110 .
Maskless flexible spatial shaping based on spatiotemporal interference can modulate the interference intensity distribution into arbitrary patterns 111 . It was verified using a Michelson interferometer, where the phase of one laser beam was controlled via SLM.
Lin et al. developed a correction method for the spatial distortion and light intensity, which could greatly improve the accuracy of the spatial pattern and uniformity of the light intensity simply by changing the phase map loaded on the SLM 112 . This method was used to efficiently process colorful two-dimensional codes on silicon and shark-skin-like structures with superhydrophobic properties on stainless steel 112, 113 .
The laser focal point of a cylindrical lens is only a few to tens of micrometers wide, while its length is on the order of 1 -10 mm 114,115 . Compared with ordinary circular lenses, the efficiency of femtosecond-LIPSs can be improved by a factor of 10 3 -10 4 by using cylindrical lenses, which have been demonstrated in materials such as silicon and graphene oxide 11,57,116−118 . This method is very simple, and has important implications for the industrial application of femtosecond-LIPSs in the future.
Polarization shaped femtosecond-LIPSs By using a liquid crystal SLM and/or Q-plate, a femtosecond Gaussian laser beam could be modulated into vortex light, radially polarized or angularly polarized beam 119−122 , and used to directly fabricate complex LIPS patterns 77,121−129 . Two laser beams with s and p polarizations were collinearly focused on the SiC surface, and nanostructures with different morphologies were fabricated by adjusting the energy ratio of these two laser beams 77 . Radially and angularly polarized femtosecond lasers were focused on transparent materials and HSFL patterns consistent with the polarization distribution were fabricated 125 .

Efficient fabrication of complex femtosecond-LIPS patterns by multi-beam interference
Multibeam interference is an efficient etching technology for processing regular periodic patterns, where the light intensity distribution in the interference field is usually imprinted on the materials. By combining laser interference technology with femtosecond-LIPSs, many complex LIPS patterns were fabricated with high efficiency and quality by simultaneously controlling the light intensity distribution and polarization distribution 37,130 . Experimental and theoretical results showed that the longer-period structure was determined by the intensity distribution, whereas the LIPS pattern was determined by the polarization distribution 131 .
Two-beam femtosecond laser interference was used to fabricate grating structures. By changing the laser polarization direction, interesting complex periodic structures covered with LIPSs have been efficiently fabricated 130,132 . Recently, large-area regular LSFLs were efficiently fabricated on a silicon wafer using doublebeam femtosecond laser interference focused by cylindrical lens. The space of the double-beam interference fringes was an integer multiple of the SPP wavelength, thereby effectively exciting the resonance-enhanced SPPs. The fabricated nanogratings were very regular, uniform, and smooth, and the resolution of the diffraction light was almost the same as that with commercial gold gratings 57 . Using a similar method, a large area of regular and uniform LIPSs was processed on the surface of quartz glass 118 .
A wide variety of composite LIPSs such as hexagonally distributed nanogratings, hexagonal nanoflowers, and nanorings were efficiently fabricated on widebandgap SiC and ZnO semiconductors by changing the polarization combination of the three beams 37,131 , as shown in Fig. 9. The four-beam interference method was further developed, and composite LIPSs such as square nanostructures, symmetric petal structures, and asymmetric helix-like structures were prepared on the surface of a ZnO crystal 133 .
Femtosecond laser direct writing of <100 nm nanostructures  laser processing with high numerical aperture objectives is also on the order of hundreds of nanometers. The groove widths of HSFLs on semiconductor, dielectric, and metal surfaces induced by a femtosecond laser are typically less than 50 nm and less than 1/10 of the laser wavelength, opening up new avenues for laser processing with feature sizes smaller than 100 nm 134−138 . Liao et al. reported a single nanohollow channel with a width of 40 nm in porous glass using femtosecond laser-induced HSFL technology. It was used in nanofluidics in the field of single-molecule detection and biochip preparation. By precisely controlling the laser focus, fluences, and scanning speed, straight nanogrooves with widths smaller than 100 nm were prepared on the surfaces of fused silica and sapphire crystals 135 .
A single nanogroove with a width of 30 nm and a double-groove structure with an interval of 150 nm were fabricated on a ZnO crystal surface immersed in water by precisely controlling the 800-nm femtosecond laser fluence and scanning speed 136 , as shown in Fig. 10. Doublegroove grating structures and single-groove grating structures with different spacings were fabricated, with groove depths ranging from 200 to 300 nm and groove widths smaller than 40 nm. Nanosquare structures with dimensions of 150 nm × 150 nm and 150 nm × 250 nm were fabricated by direct writing twice along the vertical direction.
By controlling the laser polarization always perpendicular to the scanning direction, curved nanogrooves with widths smaller than 20 nm and curvature radii smaller than 100 nm were fabricated on a silicon surface 139 . Lin et al. processed curved nanogrooves with a feature size of 12 nm on silicon surfaces using 800 nm femtosecond laser direct writing with orthogonally polarized dual beams 27 .

Applications of LIPSs
Femtosecond-LIPSs provide a mask-free, high-efficiency processing method for tuning material properties. This method has been used in many fields, with applications that include structural colors 8,32,35 , birefringent optical components 34,36,38 , optical absorption and luminescence 33,37,39 , and electrical properties 40 .

Structural colors
Structural colors originate from the interference, diffraction, or scattering produced by micro/nanostructures on the surface of a material 32 . LIPSs have been widely used to produce structural colors on metals, semiconductors and transparent materials 8,35,118,140−151 .
The properties of such structural colors are determined by the period, depth, and orientation of the LIPSs. LIPSs with periods in the range of 400 -1500 nm were prepared on the surface of stainless steel using femtosecond lasers with different wavelengths 140 . These LIPSs demonstrated completely different colors under each diffraction order, such that the structural color of the stainless steel surface could cover most of the color gamut. The same LIPSs displayed different colors at different angles 118,142,148 . Therefore, surfaces with different colors were prepared by controlling the orientation of the LIPSs. Liu et al. proposed to control the orientation of LIPSs in real time using a femtosecond laser doublepulse sequence with orthogonal polarization and equal pulse energy and fabricated LIPSs with arbitrary orientations on silicon surfaces that were not affected by the scanning path 147 . This is of great significance for multilayer encryption, anti-counterfeiting, and so on.
The brightness and purity of the structural colors were affected by the regularity and depth of the LIPSs. The high-efficiency preparation of high-quality LIPSs has attracted increasing attention. Zhang et al. 8 fabricated very regular and deep LSFLs on a silicon surface using temporally shaped pulses via a 4f zero-dispersion system and demonstrated very bright and pure structural colors, as shown in Fig. 11. Vivid structural colors on fused silica surfaces were prepared with high efficiency using the double-beam interference of femtosecond laser focused    The writing speed has been increased to 225 kB/s 153−155 , which is mainly a result of the higher laser power and repetition frequency with pulse energy modulation, as shown in Fig. 12.
Mechanisms and applications based on femtosecond laser-induced birefringence have been extensively studied, including various polarization-sensitive elements 70,156−158 such as polarization beam splitters 159 , polarization diffraction gratings 160 , and polarization optical vortex rotation converters 123 . HSFLs fabricated in amorphous silicon 161,162 , silicon carbide 163 and indium tin oxide (ITO) thin films 164,165 exhibit birefringence effects two orders of magnitude higher than those of quartz glass, demonstrating that high-refractive-index materials can be used to fabricate birefringent elements. Largearea HSFLs have been efficiently fabricated on a glass surface coated with an ITO thin film using femtosecond laser direct writing focused via a cylindrical lens 166 . The optical retardation reached 44 nm, which was eight times that of the HSFLs processed directly on a bare glass surface.

Enhancement of optical absorption and photoluminescence
The optical properties were significantly tuned after forming complex micro-nanostructures covered with LIPSs on the surfaces of metals, semiconductors, and dielectrics 33,35,37,39,118,167−174 . "Black " silicon has been extensively studied and was shown to exhibit ultra-low reflectivity and very high absorption in the UV -NIR region as a result of nanostructures fabricated by femtosecond lasers 33,39,171,172 .
Various composite LIPSs have been efficiently fabricated on wide-bandgap SiC and ZnO semiconductors by the multi-beam interference of femtosecond lasers 174

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These composite nanostructures exhibited greatly enhanced light absorption in the UV-NIR region, as well as an intense blue emission when excited by infrared femtosecond lasers. Compared with the smooth surface of ZnSe crystals, the blue light intensity of the composite nanostructures excited by a 1200 nm femtosecond laser was enhanced by 30-100 times 175 , as shown in Fig. 13.  178,179 analyzed the effect of laser irradiation on the critical current value of superconductors by measuring hysteresis loops, and proved that LIPSs could change the superconducting properties of niobium. The secondary electron emission from a copper surface with LIPSs was effectively reduced 181 , which was very effective in suppressing secondary electron emission in accelerators.

Other applications
After femtosecond laser processing, the entire ablation area is covered with nanostructures, which can effect-ively increase the surface roughness and reduce the contact area with the droplet. This is an efficient method to alter the hydrophilicity and hydrophobicity of sample surfaces, and has been successfully demonstrated on various materials including metals, semiconductors, dielectrics, and polymers 182−192 .
Femtosecond-LIPSs were optimized on steel, titanium alloy, and atomic layer deposition cemented carbide surfaces. The friction coefficient and the wear were reduced significantly, which revealed the great potential for tribological applications 193−199 . Outlook In this paper, the formation mechanisms, high-quality and high-efficiency processing methods, and applications of LIPSs were reviewed. We believe that the following problems that may be worth further investigation.

Formation mechanisms of LIPSs
The formation mechanism of LIPSs has always been a hot topic in this field. The scattered light model and propagating SPP model are continuously being developed and refined and have been used to explain the formation of LSFLs on dielectrics, semiconductors, and metals. However, the formation mechanism of LSFLs requires further experimental and theoretical studies, especially ultrafast dynamics.
HSFLs are typically formed after irradiation with multiple pulses. Their formation mechanism is more complex and difficult to determine than that of LSFLs, requiring extensive experimental and theoretical studies, including numerical simulations of the dynamics using the COMSOL and FDTD methods. In recent years, various far-field super-resolution imaging methods have been reported 200−202 . Combining the pump-probe method with far-field super-resolution imaging is very interesting and may be helpful in revealing the formation mechanism of HSFLs irradiated by a femtosecond laser.

Patterns with features of <30 nm obtained by direct writing
The groove width of HSFLs is only tens of nanometers, which provides a new method of nanofabrication using ultrafast laser direct writing. However, before this can be widely applied, there are still many problems to be solved such as the slow scanning speed and poor uniformity of the nanogrooves. In the last decade, multi-dimensional laser shaping technologies in time/frequency and polarization have been rapidly developed 108,203 . Synchronizing laser direct writing and multi-dimensional laser shaping in real time will ensure that the nanogrooves grow along the direction of the direct laser writing, improve the local optical field enhancement, and reduce the residual heat of ablation. This is expected to efficiently fabricate high-quality nanopatterns with features less than 30 nm.

LIPSs based on multi-dimensional laser shaping and its applications
Large-area LIPSs fabricated with high quality, efficiency, and stability can greatly promote their applications in many fields. By developing pulse-shaping methods based on the 4f configuration zero-dispersion system or Fabry-Perot interferometer 204 , the regularity and depth of LIPSs can be effectively improved, which will improve the phase retardation and writing speed and reduce the bit  error rate when reading out. Combining time/frequency shaping and spatial shaping can improve the regularity and depth of LIPSs fabricated with high efficiency, thereby improving the purity and brightness of the structural colors. By simultaneously controlling the polarization and spatial distribution of the laser field, nanostructures with designed patterns and specific nanogroove distributions can be processed with high efficiency. Combining time/frequency domain shaping and polarization shaping can improve the quality and efficiency of far-field nanofabrication and allow LIPS applications in new fields such as metasurfaces and microelectronics.
Self-organization of surfaces on the nanoscale by topographymediated selection of quasi-cylindrical and plasmonic waves.