Compound Structures of Periodic Holes and Curved Ripples Fabricated by the Interference between the Converging Surface Plasmon Polaritons and Femtosecond Laser

Abstract

Non-cylindrical vectorial femtosecond lasers are employed to irradiate tungsten surfaces. Compound nanopatterns composed of periodic nanoholes and semi-circular curved ripples are produced by scanning the target relative to the laser beam. The tangential direction of the curved ripples is perpendicular to the local polarization direction of the vectorial femtosecond laser beam. Therefore, the formation mechanism of the curved ripples can be attributed to the interference between the incident femtosecond laser and the laser-induced surface plasmon polaritons (SPPs). We found that, in addition to the curved ripples, periodic nanoholes with an average diameter of 406 nm also appeared on the target surface, and they all tended to appear at the vertexes of the semi-circular curved ripples, i.e., the converging point of SPPs. Further experiments demonstrated that the location of the periodic nanoholes was totally determined by the polarization state of the incident femtosecond laser. Therefore, we deduced that the convergent SPPs induced by the non-cylindrical vectorial femtosecond laser interfered with the incident laser at the convergent point, leading to the generation of periodic nanoholes. The investigations in this work exhibited the important role of manipulating the propagation of SPPs in femtosecond laser surface structuring, which not only diversifies the surface patterns that can be produced by laser-induced periodic surface structuring (LIPSS) but also provides deep insights in the excitation and propagation dynamics of SPPs.

1. Introduction

The generation of femtosecond laser-induced periodic surface structures (fs-LIPSS) is a high-throughput, parallel surface-structuring method with subwavelength or deep-subwavelength resolution [1,2,3,4,5,6,7]. The modified surfaces with LIPSS patterns have important applications in the fields of structural color [8,9,10,11], super-hydrophobicity [12,13], self-cleaning [12,14], and anti-bacterial [15,16], among others. One commonly accepted formation mechanism of fs-LIPSSs is the interference between the incident femtosecond laser and the laser-induced surface plasmon polaritons (SPPs) [17,18,19]. Therefore, compared with the scalar femtosecond laser, the vectorial femtosecond laser can locally manipulate the propagation direction of SPPs and produce more versatile surface patterns [20,21,22,23,24,25,26,27,28]. In addition, the employment of the vectorial femtosecond laser to produce LIPSSs on a target surface is also an effective method used to investigate the unique excitation/propagation modes of SPPs by freezing the SPPs into the surface patterns.

In this paper, a non-cylindrical femtosecond vector beam was generated by two phase gratings used correspondingly as the beam splitter and combiner. The polarization of the femtosecond vector beam at the beam cross-section (xoy plane) varied only along the vertical (y) direction and was kept constant along the horizontal (x) direction. Periodic curved ripples were produced on the tungsten surface after it was irradiated by the vectorial femtosecond laser. By increasing the polarization variation speed along the vertical direction, we found that uniformly distributed periodic nanoholes with an average diameter of 406 nm were formed at the vertexes of the curved ripples. The vertexes of the curved ripples were actually the converging points of the SPPs induced by the vectorial femtosecond laser. Therefore, we deduced that the converging SPPs induced by the non-cylindrical vectorial femtosecond laser interfered with the incident laser, leading to the generation of the semi-circular curved ripples and the nanoholes at the vertexes of the ripples. The results in this paper demonstrated a high-throughput nanohole fabrication method and exhibited the SPPs’ propagation dynamics by freezing the SPPs into the final LIPSS patterns.

2. Methods

The experimental setup is presented in Figure 1. A Ti:sapphire femtosecond laser amplifier system (Legend Elite HE+, Coherent Inc., Santa Clara, CA, USA) was employed to generate 800 nm, 35 fs, 500 Hz laser pulses with a horizontal linear polarization and a single-pulse energy up to 4 mJ. The combination of the half-wave plate (HWP1) and the linear polarizer (LP1) was used to continuously adjust the laser power. A Ronchi phase grating (RG1) with a period of 64 μm was loaded on the spatial light modulator (SLM, PLUTO-NIR-015, Holoeye Inc., Berlin, Germany) to split the incident laser into ±1 diffraction orders. The total power of the ±1 diffraction orders contained more than 80% of the incident laser power. Two plano-convex lenses (L1 and L2) with focal lengths of 30 cm formed a 4f system. A pinhole filter (F) was located at the Fourier plane of the 4f system to block other diffraction orders (except ±1 orders) of RG1. The fast axes of the two quarter-wave plates (QWP1 and QWP2) behind the pinhole filter were perpendicular to each other, making the linearly polarized ±1 diffraction orders convert to the left- and right-handed circularly polarized beams. Another Ronchi grating (RG2) made of fused silica and with the same period as that loaded on the SLM was conjugated with RG1 relative to the 4f system, acting as a beam combiner.

When the grating vectors of RG1 and RG2 were conjugated relative to the 4f system, we found that the combined femtosecond laser beam after RG2 was a scalar beam with horizontally linear polarization. When rotating RG2 around its normal direction, we generated a non-cylindrical vector beam with spatially changing linear polarization. The polarization distribution of the non-cylindrical vector beam was investigated by the setup shown in the dashed frame of Figure 1. When LP2 with horizontal transmission axis was employed, the beam intensity distribution recorded by the CCD camera (LU135M, Lumenera Inc., Ottawa, ON, Canada) was shown, as presented in Figure 2a. During the experiments, as the angle θ between the transmission axis of LP2 and the x-axis increased (see Figure 2a), the periodic intensity distribution in Figure 2a gradually moved towards the negative direction of the y-axis. The dependence of the dark stripe’s position in Figure 2a on θ is presented in Figure 2b, which quantitatively shows the polarization state of the femtosecond vector beam. The goodness of fit (R²) was 0.99, showing that a perfect linear relationship existed between the dark stripe position and θ. Thus, this concludes that the local polarization direction of the femtosecond vector beam can be indicated by the tangential direction of the blue solid curve shown in Figure 2a. The polarization variation speed along the vertical direction increased with the increase in the rotation angle of RG2. A further explanation of the formation of the vector beam by the setup in Figure 1 can be found in [29,30].

When we removed the setup shown in the dashed frame in Figure 1, the apparatus in Figure 1 was able to be used to fabricate LIPSSs patterns on the tungsten surface. A 10× objective (NA = 0.25) was employed to focus the femtosecond vector beam. After taking the dispersion of the optical elements in the experimental setup and the dispersion in air into account, we calculated the pulse duration at the target surface to be 55 fs. The tungsten sample had a purity of 99.95% and was polished by the abrasive paper of #1200. During the experiments, the tungsten target was mounted on an electrical controlled 3D translation stage (WNSC400, Winner Optical Co., Ltd., Beijing, China), and the target surface was 100 μm away from the geometrical focus of the objective. A scanning electron microscope (VE9800, Keyence Inc., Osaka, Japan) was employed to investigate the sample surface morphology after it was irradiated by a femtosecond vector beam.

3. Results and Discussion

Figure 3 shows the nanostructures on the tungsten surface that were fabricated by femtosecond laser beams with different polarization states. As shown in Figure 1, when the grating vectors of RG1 and RG2 were conjugated relative to the 4f system, the combined beam after RG2 was a scalar beam with a vertical polarization. Using the scalar femtosecond laser beam (Figure 3a), we produced periodic ripples on the tungsten surface, which is presented in Figure 3f. By rotating the RG2 around its normal direction, we were able to generate a femtosecond vector beam with different polarization distributions. Using these vector beams, we fabricated curved periodic ripples with different curvature radii on the tungsten surface, which is shown in Figure 3g–j. Since the tangential direction of the curved ripples was perpendicular to the local polarization of the femtosecond laser beam, the formation mechanism of the curved ripples was able to be attributed to the interference between the incident femtosecond laser and laser-induced SPPs.

It was also noted that besides curved ripples, as shown in Figure 3g–j, nanoholes were also fabricated on the target surface. It is interesting to find that as the variation period of the polarization of the femtosecond laser decreased, the nanoholes fabricated on the tungsten surface become increasingly more regular in the spatial distribution. In Figure 3j, the nanoholes all appear at the vertexes of the curved ripples, regularly arranged in the horizontal direction. However, in Figure 3f, the nanoholes have a more random spatial distribution. In order to investigate the formation mechanism of these nanoholes, we fabricated the compound surface patterns composed of nanoholes and curved ripples when the half-wave plate (HWP2) shown in Figure 1 was rotated at different angles. The experimental results are shown in Figure 4.

When we gradually rotated the HWP2 shown in Figure 1, the local linear polarization of the femtosecond vector beam rotated as a whole, inducing the shift of the nanostructures on the tungsten surface along the direction perpendicular to the target scanning direction, i.e., along the vertical direction shown in Figure 4a. Taking the case that the rotation angle of HWP2 was 25° as an example (see Figure 4b), using ImageJ software [31], we were able to extract the nanoholes from the SEM picture in Figure 4b. The processed picture is presented in Figure 4c. By counting the holes in Figure 4c, we were able to provide the dependence of the hole number on the location along the vertical direction, which is given in Figure 4d. It was seen that the nanoholes tended to appear at the vertexes of the curved ripples. The spatial interval between adjacent spikes shown in Figure 4d was simply the spatial period of the polarization variation of the focused femtosecond laser beam on the tungsten surface. Therefore, the location of the spike with the maximal number of holes was chosen to indicate the location of the nanoholes. The dependence of the nanoholes’ location on the angle of HWP2 is shown as the square dots in Figure 4e. The curved ripples’ location is indicated by the bordering between adjacent curved ripples, which is presented by the red solid line in Figure 4a. The dependence of the curved ripples’ location on the angle of HWP2 is presented as circular dots in Figure 4d. It can be seen that the locations of the nanoholes and the curved ripples changed synchronously with the increase in the angle of HWP2. Therefore, it was concluded that the nanoholes were generated due to the polarization distribution of the femtosecond vector beam.

It is commonly accepted that the periodic ripples are generated due to the interference between the femtosecond laser beam and its induced SPPs [18]. Therefore, the ripples’ radius of curvature can be controlled by the polarization distribution of the incident femtosecond laser beam, as shown in Figure 3. When the orientation of the linear polarization rotated quickly at the beam cross-section, all of the nanoholes appeared at the vertexes of the curved ripples. The vertexes of the curved ripples were the converging point of SPPs, which is schematically presented in Figure 5. Therefore, on the tungsten surface, after irradiation by the femtosecond laser, the vertexes of the curved ripples had the most intensive SPPs. The nanoholes are considered to be generated due to the interference between the femtosecond laser and the focused SPPs [32].

Finally, non-cylindrical femtosecond vector beams with different average powers were employed to irradiate the tungsten target. The morphologies of the irradiated tungsten surfaces are presented in Figure 6. The target scanning speed was kept at 0.06 mm/s, and the scanning direction is indicated by the arrow in Figure 6d. It was seen that as the single pulse energy decreased from 20 μJ to 8 μJ, the region covered by the curved ripples and the nanoholes shrunk to the bottom part of the irradiated region, as shown in Figure 6d. This indicates that the intensity distribution on the cross section of the laser beam was non-uniform and relatively large at the bottom part of the beam cross-section. Therefore, this explains why the nanoholes were more evident in the bottom part of the irradiated surface, as shown in Figure 4b and Figure 6, inferring that the formation of nanoholes not only depends on the polarization distribution of the incident femtosecond laser but also requires proper optical intensity.

4. Conclusions

In this paper, a non-cylindrical femtosecond vector beam was employed to irradiate the tungsten target. Compound surface patterns composed of periodic curved ripples and uniformly distributed nanoholes were produced on the tungsten surface. Experimental results show that the uniformity of the spatial distribution of the nanoholes increased as the polarization of the femtosecond vector beam varied increasingly more quickly at the beam cross-section. It was also found that the nanoholes always appeared at the vertexes of the curved ripples. The formation mechanism of the periodic nanoholes could be attributed to the interference between the femtosecond laser and the converging SPPs induced by the non-cylindrical femtosecond vector beam. The experimental results not only demonstrate the feasibility of fabricating large-area surface patterns via manipulating SPPs by the non-cylindrical vectorial femtosecond laser but also intuitively exhibit the SPPs’ propagation dynamics by freezing the SPPs into the final surface patterns.