Intense femtosecond laser pulses are a common tool in precision processing of a variety of materials, which are needed in the fabrication of, for example, electro-optical devices and micro-electromechanical systems. The advantage of using femtosecond laser pulses for material processing is the ultrafast deposition of high-density energy onto the target and the resulting suppression of undesirable thermal and mechanical effects in the light-matter interaction. In addition, observations of self-organized, surface nanostructure formations in femtosecond-laser ablation experiments have found the size of these structures is typically 1/10–1/5 of the laser wavelength λ,1–4 which suggests that femtosecond lasers have the potential to sculpt, or ‘nano-process,’ structures smaller than the diffraction limit.
To find a new route to femtosecond-laser nano-processing, we focused our attention on the nanoscale, ultrafast light-matter interaction responsible for nanostructuring. We explored in particular the use of superimposed multiple shots of low-fluence femtosecond laser pulses. Our preliminary studies with dielectric and semiconductor materials have shown that the laser-induced near-field plays a fundamental role in the nanoscale ablation of a corrugated surface,5, 6 and the origin of periodicity can be attributed to the excitation of surface plasmon polaritons (SPPs) in the surface layer.7, 8
Based on the model of nanostructuring, we successfully fabricated a nanograting with a uniform period on a crystalline gallium nitride (GaN) surface. We performed our experiment in air (as opposed to vacuum) using linearly polarized 800-nm, 100-fs laser pulses from a Ti:sapphire laser system operated at a repetition rate of 10Hz. In the first-step of a two-step process, we split the femtosecond laser output into two beams. As schematically illustrated in Figure 1(a), Beam 1 is normally incident on the target while Beam 2 is incident at an angle θ with respect to the normal. The two beams overlap on the target surface to create interference fringes in the direction perpendicular to the laser polarization. The deposited laser energy ablates the GaN surface at the peaks in the interference pattern, as shown in the scanning probe microscope (SPM) image of Figure 1(b), where the fluence of each beam was F = 400mJ/cm2 and the angle separation was θ=59°. In this example, the resulting groove period and groove depth were, respectively, Λ ∼ 940nm and h ∼ 75nm.
Figure 1. (a) This schematic of the optical configuration shows how Beam 1 (normal incidence) overlaps with Beam 2 (incidence angle θ) on the target surface. (b) The interference from the two beams (coming from a single femtosecond pulse) forms a periodic structure on a gallium nitride (GaN) surface, as seen in this scanning probe microscope (SPM) image. The fluence in each beam is F = 400mJ/cm2.
In the second step in our experiment, we reduce the period in the surface structure by cutting new grooves with multiple shots from a single laser beam (Beam 1). We decrease the fluence F in the laser pulses to well-below the single-pulse ablation threshold of GaN. Figure 2 shows the scanning electron microscope (SEM) images of the surface as the number of shots N is increased. The bright and dark stripes in the image correspond to the ridges and grooves in the surface structure, respectively. The starting point in Figure 2(a) with N = 0 is the same as in Figure 1(b). After 10 shots from the laser, the surface shown in Figure 2(b) exhibits parallel lines of ablation at both edges of the grooves in the initial interference pattern, while no strong ablation takes place on the ridges. We explain this preferential ablation as being due to a lower ablation threshold on groove surfaces, as a result of defect sites created in the first step of our experiment.4 Furthermore, the high curvature along the groove edge induces a strong near-field that can initiate the ablation.
Figure 2. These scanning electron microscope (SEM) images show a GaN surface after a number Nof superimposed shots of femtosecond laser pulses: (a) N = 0, (b) N = 10, (c) N = 20, and (d) N = 40. The fluence is F = 400mJ/cm2, and the laser polarization is horizontal.
With increasing N, as seen in Figure 2(c), the femtosecond laser pulses start to ablate each ridge with a pair of parallel narrow valleys, while a third ablation trace is added to each groove. When N reaches 40 in Figure 2(d), the structure is a uniform nanograting with a period of d ∼ 190nm, which is roughly one-fifth of the original groove period (∼ Λ/5). An SPM image of the final nanograting in Figure 2(d) shows the deep grooves, which were measured to be ∼ 550nm in depth on average (see Figure 3).
Figure 3. This SPM image shows the final nanograting formed on GaN by the two-step process with femtosecond laser pulses.
We observed that the laser fluence affects the fine period d of the nanogratings. For the pre-structured surface with Λ ∼ 940nm, for example, multiple femtosecond laser pulses at F = 480mJ/cm2 produced a nanograting with d ∼ 230nm, or ∼Λ/4.
Our earlier work suggests that the standing SPP wave in the surface layer plays a crucial role in downsizing the groove period from Λ to Λ/m for m = 4 and 5. The preliminary results from a model calculation of the standing SPP wave predicted a periodic structure with d = 150–300nm,7, 8 which is consistent with the experiments reported here. We notice that different phenomena contributing to the interaction process should carefully be examined to account for the detail of the nanostructuring mechanism, as well as for the origin of the observed deep valley.
In summary, we demonstrated a simple two-step process to fabricate a nanograting on a GaN surface irradiated with femtosecond laser pulses in air. This technique is applicable to various kinds of solid targets. In the next step we plan to demonstrate its adaptability to different materials, aiming at the development of a new class of versatile nano-processing techniques using femtosecond laser pulses.
Godai Miyaji, Kenzo Miyazaki
Institute of Advanced Energy (IAE)
Godai Miyaji received his doctorate degree in engineering from Osaka University in 2004. Since then he has been working as an assistant professor.
Kenzo Miyazaki is a professor. He received his doctorate degree in engineering from Kyoto University in 1976. Current research in his group is concerned with ultrafast, strong-field interactions with matter. He is a fellow of the Optical Society of America (OSA) and the Japan Society of Applied Physics (JSAP).
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