1. Introduction

Recent laser ablation experiments for solid materials have demonstrated that superimposed multiple shots of low-fluence femtosecond (fs) laser pulses can produce a self-organized, periodic nanostructure (PNS) on the surface [1–7]. The structure size observed is typically 100 – 300 nm for the most commonly used fs laser pulses at the wavelength λ ~800 nm, indicating possible formation of nanoscale structures beyond the diffraction limit of light. For fabricating a well-defined PNS or nanograting, one of the most important subjects has been to understand and control the interaction processes for nanostructuring at the surface [1–17].

The experimental studies of present authors and their collaborators for dielectrics [8–12,16], semiconductors [13–15] and metals [17] have shown that the PNS formation develops through the bonding structure change [8–10], the near-field ablation [10–12], and the excitation of surface plasmon polaritons (SPP) in the thin layer on the target surface [12–17]. It has also been demonstrated that use of multiple shots of low-fluence fs pulses is important for suppressing undesirable thermal processes in the PNS formation through the nanoscale ablation [13–15]. Based on the mechanism we have recently developed a two-step ablation technique [16] and an ablation technique using interfering fs laser beams [17], where the interaction processes were controlled to fabricate a homogeneous nanograting on gallium nitride (GaN) and metals (stainless steel and titanium).

The mechanism for PNS formation suggests that nanogratings with much smaller periods can be fabricated in air by taking advantage of the wavelength-dependent periodicity of SPP fields [18]. In this paper, we report the successful formation of a homogenous nanograting with the period of d ~50 nm on GaN in air, using ultraviolet (UV) fs laser pulses at λ ~266 nm for the two-step ablation [16]. The results have shown that the excitation of SPPs is certainly the dominant process to form the nanograting, as well as in the previous experiment with 800-nm fs laser pulses [16]. The grating period at different fluences in the second step was 1/6 or 1/5 of the initial fringe period created in the first step. We have calculated the grating period and its wavelength dependence for a model target. The results are in good agreement with the experimental, confirming the mechanism for nanostructuring.

2. Experimental

The ablation experiment was made for polished crystalline GaN (0001) with linearly-polarized fs laser pulses in UV at λ ~266 nm. The UV pulses were generated by frequency tripling of the fundamental 800-nm, 100-fs pulses from a Ti:sapphire laser system operated at 10 Hz, using two thin β-BaB₂O₄ crystals of 0.5 mm and 0.3 mm in thickness for the type-I second and third harmonic generation. The UV pulse energy was 0.27-mJ in the 300-fs width (FWHM) for the fundamental pulse energy of 2.9 mJ.

The experimental setup for the two-step ablation was almost the same as in our previous study [16]. Briefly, the horizontally-polarized UV fs laser beam was split into two beams (beam 1 and 2). Beam 1 and 2 focused with a 300-mm focal-length lens were normally and obliquely incident at the relative angle of θ = 62° on the target, respectively. The focal spot size of both beams was 14 μm in 1/e² radius on the surface. The pulse energy of each beam was independently controlled so as to achieve an identical fluence F⁽¹⁾ = F⁽²⁾ of beam 1 and 2. The temporal and spatial overlap between beam1 and 2 was confirmed by observing interference fringes on the target with a CCD camera. In the first step, a single intense fs laser pulse produced an interference pattern with the fringe period Λ = λ/sinθ = 295 nm on the surface. In the second step, the interference pattern was irradiated with the superimposed multiple number of shots N at F⁽¹⁾ < F₁ from the beam 1. Meanwhile, the interference fringe periods were downsized to form a number of parallel line-like ablation traces.

Morphological change of the target surface was observed with a scanning electron microscope (SEM) and a scanning probe microscope (SPM). With two-dimensional Fourier transform, we analyzed the SEM image to see the spatial frequency distribution, or the spatial period d, in the surface structure.

3. Results and discussion

In the preliminary experiment using a single fs-laser beam (beam 1), the single-shot ablation threshold F₁ was measured to be F₁ = 790 ( ± 10) mJ/cm² for the GaN target. We confirmed that the self-organized PNS can be formed when the fresh surface is irradiated at lower fluence than F₁ with superimposed multiple pulses from the single beam. Figure 1 shows an example of SEM image of the PNS and its frequency distribution observed with the superimposed 50 shots of UV fs pulses at the fluence F = 660 mJ/cm². The PNS image represents line-like patterns extending to the direction perpendicular to the laser polarization, and the periodicity is rather non-uniform with the broad maximum including the random peaks at d = 46 – 63 nm. We observed that the period increased to d = 50 – 69 nm at F = 770 mJ/cm², and the higher fluence started to form a ripple structure with d ~λ [19].

 

Fig. 1 SEM image of the self-organized PNS on GaN irradiated with N = 50 at F = 660 mJ/cm² (left) and its spatial frequency spectrum (right). The fs laser polarization direction is horizontal.

Download Full Size | PDF

We found that the two-step ablation techinique produces more uniform nanostrutures with the well-defined period on the GaN surface. Figure 2 shows a pair of the SEM image and its Fourier spectrum of the surface observed with the technique for (a) N = 0, i.e., the interference pattern produced at F⁽¹⁾ = F⁽²⁾ = 550 mJ/cm² in the first step, and (b) N = 10 and (c) N = 30 at F⁽¹⁾ = 660 mJ/cm² in the second step, where the bright and dark stripes in the SEM image correspond to the ridge and the groove in the surface structures, respectively. The initial fringe pattern [Fig. 2(a)] represents the frequency peak at Λ = 295 nm, together with the peaks of its harmonic components. The ablated fringe depth was measured to be ~30 nm with the SPM. For N = 10, the interference pattern is periodically grooved along the direction perpendicular to the field polarization [Fig. 2(b)] . After N = 30, the surface structure is a homogeneous nanograting with the period of d = 49 nm ~Λ/6, which is also seen with the isolated peak in the spectrum shown in Fig. 2(c). It is noted that the nanograting is formed at the harmonic frequency q/Λ with the integer q = 6. This indicates that the periodic near-field ablation is induced to downsize Λ through the excitation of a single standing SPP wave mode [12,16] between the interference fringes.

 

Fig. 2 SEM image and frequency spectrum of the periodic structure on GaN surface irradiated with (a) N = 0, (b) N = 10, and (c) N = 30 at F⁽¹⁾ = 660 mJ/cm² from beam 1 in the second step. The fs laser polarization direction is horizontal. The number at each peak in the spectrum denotes the spatial period d in nm.

Download Full Size | PDF

At the higher fluence F⁽¹⁾ = 770 mJ/cm², we observed that the second-step ablation produced a nanograting with q = 5. The results are shown in Fig. 3. With N = 5, the narrow line-like ablation traces start to be created with the multiple frequency peaks on the initial fringe pattern [Fig. 3(a)]. After N = 30, a homogeneous nanograting is formed with d = 59 nm ~Λ/5, while the other frequency peaks are greatly suppressed [Fig. 3(b)]. The discrete change in q (from 6 to 5) with increasing F⁽¹⁾ is also the nature of a single standing SPP wave mode excited in the fringe period Λ [16,17].

 

Fig. 3 SEM image and frequency spectrum of the periodic structure on GaN surface irradiated with (a) N = 5 and (b) N = 30 at F⁽¹⁾ = 770 mJ/cm² from beam 1 in the second step. The fs laser polarization direction is horizontal. The number at each peak in the spectrum denotes the spatial period d in nm.

Download Full Size | PDF

The mechanism based on SPPs has been confirmed by calculating the SPP wavelength λ_spp for the model surface given in [16]. The method of calculation is similar to that in our previous studies [12,13,16,17], where the fs laser pulse incident on the GaN target in air is considered to produce a high density N_e of free electrons at the surface to form a thin metal-like excited layer on the substrate, and then SPPs are excited at the interface between the excited layer and the substrate. Using the relation between the incident light and SPPs [18], the SPP wave number is represented by k_spp = k₀[ε_sε*/(ε_s + ε*)]^1/2, where k₀ is the wave number of the incident light in vacuum, ε* and ε_s are the dielectric constants of the excited layer and the substrate of GaN, respectively. This relation leads to the grating period D = λ_spp /2 = π/Re[k_spp] that is produced by the near-field ablation at every half period of the standing SPP waves [12,13,16]. For the calculation, we used ε_s = 6.7 + i 2.2 for GaN [21] and ε* = ε_s – [ω_p²/(ω² + iω/τ)] during the laser-matter interaction, with the laser frequency ω in vacuum, the Drude damping time of free electrons τ = 1 fs [23], and the plasma frequency ω_p = [e²N_e/(ε₀ m*m)]^1/2 with the dielectric constant of vacuum ε₀, the electron charge e and mass m, and the optical effective mass of carriers m* = 0.2 [24].

Figure 4(a) shows the period D calculated as a function of N_e, which ranges from 45 nm to 100 nm, as the SPPs can be excited in the region of Re[ε*] < 0 with N_e > 1.7 × 10²² cm⁻³. This suggests that the nanograting period d to be created should be 45 nm < Λ/q < 100 nm, or the possible values of q for Λ = 295 nm are 3, 4, 5, and 6. This is consistent with the observation of q = 5 or 6. In addition, the calculated result of D reconciles with d = 46 – 69 nm observed in the self-organized PNS formation [Fig. 1], whereas the uncontrolled excitation process leads to the non-uniform distribution of d.

 

Fig. 4 (a) Period D of the surface structure (thick curve) and skin depth δ (thin curve), calculated as a function of N_e in an excited GaN layer. The excitation of SPPs is allowed in the region of Re[ε*] < 0 with N_e > 1.7 × 10²² cm⁻³, where D is drawn with a solid curve. (b) The maximum (open circles) and minimum values of D (open triangles) as a function of λ, where the hatched area marks the possible D between them, and the solid circles represent d observed at λ ~800 nm and 266 nm.

Download Full Size | PDF

Compared with D shown in Fig. 4(a), the period d = 50 – 70 nm observed in the experiment would be created at N_e = (3 – 4) × 10²² cm⁻³. This value of N_e for the nanograting formation is larger by an order of magnitude than N_e = (2 – 4) × 10²¹ cm⁻³ at λ ~800 nm [16], due to the possible excitation of SPPs with the UV pulses. In the excited GaN layer with the large N_e, the skin depth δ decreases down to δ = 7 – 9 nm, as shown in Fig. 4(a), which would be comparable to or slightly larger than the ablation rate in the second step.

To confirm the λ-dependent period of nanogratings, we calculated D as a function of N_e for λ = 200 – 900 nm, using ε_s for GaN [21,22]. In Fig. 4(b), the maximum and minimum values of D at λ are plotted with the open circles and triangles, respectively, together with d observed at λ ~266 nm and 800 nm. The shorter wavelength is shown to decrease the grating period.

4. Conclusion

We have shown that the excitation of SPPs is the dominant process for the nanograting formation with fs laser pulses, and the UV pulses at λ ~266 nm can certainly reduce the grating period according to the λ-dependent field size of SPPs. The two-step ablation technique using the UV fs laser pulses successfully controlled the interaction process to create a homogeneous nanograting with d ~50 nm or 60 nm, corresponding to 1/6 or 1/5 of Λ in the first step. Calculation for the model target has reproduced well the observed period and confirmed the characteristic properties of nanograting formation through the excitation of SPPs.