It is well known that the period d of nanostructures depends on the peak power density I and scanning speed v (i.e., the number of laser pulses N) (Yasumaru et al., 2003; Miyaji and Miyazaki, 2006). We investigated the formation process of nanostructures on SUS304 surfaces with 7-fs laser pulses, changing both I and v. Figure 2 shows SEM images of SUS304 surfaces irradiated with 7-fs pulses at I = 12.4 TW/cm², 15.5 TW/cm², and 20.6 TW/cm² for v = 5 μm/s and 10 μm/s. The figure shows line-like nanostructures perpendicular to the polarization direction formed over the entire ablation trace. To make it easier to see the change in d for various I and v, d is plotted as a function of I for v = 5 μm/s and 10 μm/s in Figure 3. As I increases and v decreases (i.e., increasing N), d monotonically increases in the range d = 60–110 nm. In previous experiments, periodic nanostructures with d = 280–560 nm were formed with 800-nm, 100-fs laser pulses (Qi et al., 2009; Hou et al., 2011; Yasumaru et al., 2013; Miyazaki et al., 2015). Compared to these results, d for the 7-fs pulses is about 1/5 as large as that for the 100-fs pulses. Furthermore, the trend of increasing d with increasing I is different from that of nanostructure formation on DLC films with 7-fs pulses (Nikaido et al., 2018), while it is the same as that of nanostructure formation on SUS304 surfaces with 100-fs pulses (Qi et al., 2009; Hou et al., 2011; Yasumaru et al., 2013; Miyazaki et al., 2015). The depth of the nanostructures ranged from 10 to 40 nm and increased monotonically with increasing I.

To investigate the bonding structure of SUS304 surfaces by irradiation with 7-fs pulses, we measured the Raman spectra of the irradiated surfaces. Figure 4 shows the Raman spectra of an SUS304 surface irradiated with 7-fs pulses at I = 15.5 TW/cm² and 19.6 TW/cm² at v = 5 μm/s and 10 μm/s. The Raman spectrum of a non-irradiated surface is also shown for comparison. The peaks at ∼690 cm⁻¹ and ∼1,320 cm⁻¹ indicate an A_1g mode of Fe₃O₄ (Verble, 1974) and a second-order-longitudinal-optical (2LO) phonon mode of Fe₂O₃ (Ohtsuka et al., 1986; Marshall et al., 2020), respectively. At v = 5 μm/s, these peaks monotonically increased with increasing I. At v = 10 μm/s, the Fe₃O₄ peak was observed, while the Fe₂O₃ peak was small. On the other hand, the peaks of Cr₂O₃ [305, 348, 552, 612 cm⁻¹ (Dostovalov et al., 2018)] were not observed in our measurement. These results indicate that a layer of iron oxide is formed on the SUS304 surface by 7-fs pulse irradiation and that the layer becomes thicker as I increases and v decreases.

Figure 5 shows SEM images of GaAs surfaces irradiated with 7-fs pulses at I = 17.0 TW/cm², 18.1 TW/cm², and 19.6 TW/cm² at v = 5 μm/s and 10 μm/s. The figure shows line-like nanostructures perpendicular to the polarization direction formed over the entire ablation traces. Plots of d as a function of I at v = 5 μm/s and 10 μm/s are shown in Figure 6. As I increases and v decreases (i.e., increasing N) d decreases monotonically in the range d = 130–165 nm. In previous experiments, nanostructures with d = 600–650 nm were observed to be formed with 800-nm, 100-fs laser pulses (Chakravarty et al., 2011; Ionin et al., 2013). Compared to these results, d for the 7-fs pulses was about 1/5–1/4 as large as that for the 100-fs pulses. Furthermore, the trend of increasing d with increasing I was different from that on SUS304 surfaces for 7-fs pulses. The depth of the nanostructures was in the range 30–50 nm and increased monotonically with increasing I.

Figure 7 shows Raman spectra of GaAs surfaces irradiated with 7-fs pulses at I = 15.5 TW/cm² and 19.6 TW/cm² at v = 5 μm/s and 10 μm/s. The Raman spectrum of a non-irradiated surface is also shown for comparison. The spectrum of the non-irradiated surface contains a peak at ∼265 cm⁻¹, ∼269 cm⁻¹, and ∼290 cm⁻¹, which indicates a longitudinal-optical-phonon-plasmon-coupled (LOPC) mode (Mooradian and Wright, 1966), a transverse-optical (TO) phonon mode (Abstreiter et al., 1978), and a LO phonon mode of crystalline GaAs (c-GaAs) (Abstreiter et al., 1978). The spectral peak at 290 cm⁻¹ was reduced after laser irradiation, indicating that irradiation with multiple consecutive 7-fs pulses can change c-GaAs to amorphous GaAs (a-GaAs) (Jakata et al., 2012).

Based on the results obtained, we first discuss the origin of the periodicity of nanostructure formation on a SUS304 surface. In our previous report for diamond-like carbon (DLC) films, it has been shown that the few-cycle 7-fs laser pulses can form a metallic layer with a much thinner thickness (a few nm) than that with the 100-fs pulses due to the high-peak-power density of the laser pulse with low fluence and the nonlinear absorption of the target (Iida et al., 2021). It is assumed that the 7-fs pulses create a thin layer of iron oxide on the SUS304 surface, a high-density electron layer then forms in the iron oxide layer surface by large optical energy through linear and nonlinear optical absorption process (Iida et al., 2021), and SPPs are excited on the electron layer. The nanostructure can then be formed through ablation induced by the fine distribution of high-density electromagnetic energy. If the thickness of the high-density electron layer is several nanometers to several tens of nanometers, SPPs are excited at both the air/high-density electron layer and high-density electron layer/iron oxide layer interfaces and are coupled coherently. Short-range propagating SPPs (SR-SPPs) with SPP wavelength λ _spp = 2π/Re[k _spp] can then be excited (Fukui et al., 1979; Raether, 1988). In this scenario, the following dispersion relation equation is satisfied: k z 1 ε 1 + k z 2 ε 2 k z 2 ε 2 + k z 3 ε 3 + k z 2 ε 2 − k z 1 ε 1 k z 3 ε 3 − k z 2 ε 2 exp 2 i k z 2 δ = 0 , (1) where ε ₁, ε ₂, and ε ₃ are the dielectric constants of air, iron oxide with high-density electrons, and iron oxide, respectively, k _zj ² = ε _j (ω/c)² – k _spp ² (j = 1, 2, 3) is the wavenumber of SPPs in each medium, and δ is the thickness of the high-density electron layer. c and ω = 2πc/λ are the speed of light in vacuum and the angular frequency, respectively. Using the Drude model (Sokolowski-Tinten and von der Linde, 2000; Danilov et al., 2015), ε ₂ is represented by ε 2 = ε IB 1 − N e N bf − ω p 2 ω 2 + 1 / τ 2 + i ω p 2 ω τ ω 2 + 1 / τ 2 , (2) where ε _IB is the dielectric constant of unexcited iron oxide, N _e is the free electron density, N _bf is the characteristic band capacity of the specific photoexcited regions of the first Brillouin zone in the k-space associated with the band-filling effects (Danilov et al., 2015), τ is the damping time, and ω _p ² = N _e e ²/(m*mε ₀) is the square of the plasma frequency with the dielectric constant of vacuum ε ₀, the electron mass m, and the optical relative effective mass of the electrons m*. Figure 8 shows the calculated period d _cal of a nanostructure plotted as a function of electron density N _e. Since the exited SPPs are propagating in two directions along the laser polarization direction to create a standing wave mode, d _cal can be λ _spp/2 (Novotny et al., 1997; Luo and Ishihara, 2004; Miyaji and Miyazaki, 2008). Taking ε ₁ = 1, we have ε _IB = ε₃ = 6.66 + i0.29 (Karlsson and Ribbing, 1982), N _bf = 10²³ cm⁻³ (Danilov et al., 2015), m* = 0.8 (Perevalov and Gritsenko, 2011), and τ = 1 fs (Sokolowski-Tinten and von der Linde, 2000). For comparison, we also show d _cal for SPPs excited only at the interface between the iron oxide with high-density electrons and the iron oxide. In previous reports, we have shown that N _e reaches 10²² cm⁻³ during nanostructure formation (Miyazaki et al., 2015). In Figure 8, d _cal > 30 nm can be seen for δ = 5–10 nm, while d _cal increases to >60 nm as δ increases from 10 to 25 nm. For δ > 25 nm, d _cal is consistent with that obtained from SPPs, which are excited only at the iron oxide with high-density electrons/iron oxide interface. As shown in our previous study by using DLC (Iida et al., 2021), the thickness of the excited layer should be shallower than the ablation depth per pulse, because SPPs could be excited on the surface having high-density electrons and their near-field could ablate the surface. Because the depth of the observed nanostructures was 10–40 nm, the thickness of the exited layer could be below these values. Based on this, d _cal calculated by this model is in good agreement with the period d of the observed nanostructures. As the number of pulses irradiated per unit area increases with decreasing v, the oxide layer should become thicker and the thickness δ of the excited layer should also increase. Therefore, d _cal increases with an increase in δ, as shown in Figure 8. On the other hand, an increase in I could increase both δ and N _e. The experimentally observed increase in d with an increase in I, as shown in Figure 3, indicates that the increase in d _cal with increasing δ was dominant for SUS304.

Next, we discuss the origin of the periodicity of nanostructure formation on a GaAs surface. Assuming that the formation process of nanostructures on GaAs irradiated with 7-fs laser pulses is the same as that on stainless steel, SPPs are excited at two interfaces, air/excited a-GaAs and excited a-GaAs/c-GaAs, and are coupled coherently. Since a-GaAs has no bandgap, the bandgap renormalization effect can be neglected. Figure 9 shows d _cal plotted as a function of electron density N _e, where ε ₁ = 1, the dielectric constant of a-GaAs ε _IB = 18.7 + i6.8 (Erman et al., 1984), the dielectric constant of c-GaAs ε ₃ = 13.55 + i0.63 (Erman et al., 1984; Palik and Palik, 1985), N _bf = 10²³ cm⁻³ (Danilov et al., 2015), m* = 0.061 (Solymar and Walsh, 1993), and τ = 1 fs (Sokolowski-Tinten and von der Linde, 2000). For comparison, d _cal is also shown for SPPs excited only at the excited a-GaAs/c-GaAs interface. In nanostructure formation on GaAs with 100-fs laser pulses with fluences of 70–200 mJ/cm², N _e = 2.0–2.6 × 10²¹ cm⁻³ has been reported (Margiolakis et al., 2018). In Figure 9, d _cal > 100 nm can be seen for δ = 5–10 nm, while d _cal increases to >160 nm as δ increases from 10 nm to 25 nm. For δ > 50 nm, d _cal = 120–140 nm is consistent with that obtained from SPPs excited only at the excited a-GaAs/c-GaAs interface. Because the depth of the observed nanostructures was 30–50 nm, the thickness of the exited layer could be below these values. Based on this, d _cal calculated by this model is in good agreement with the period d of the observed nanostructures. As the number of pulses irradiated per unit area increases with decreasing v, the modified layer should become thicker and the thickness δ of the excited layer should also increase. Therefore, as shown in Figure 9, d _cal decreases with increasing δ and N _e. This result can be well explained by the decrease in d with increasing I, as shown in Figure 6.