Ve 7800

For SEM (VE-7800; Keyence, Tokyo, Japan) observations, fresh leaves of the Empire and Salinas type were collected and mounted on a pedestal with carbon tape. The epidermal cells of leaf surface was visualized using SEM.

Figure 1 shows the experimental setup for the irradiation of an optical vortex beam on a Si target. A Q-switched Nd:YAG laser (Spectra Physics, Quanta-Ray, LAB-150) with a wavelength of 1064 nm and a pulse duration of 17 ns was used as the ablation laser. A linearly polarized Gaussian beam from the laser was converted into a circularly optical vortex beam through the vortex phase plate with ℓ =  − 1 and the quarter wave plate (QWP), as shown in Fig. 1. A single pulse of the doughnut beam was focused by a plano-convex lens (f = 50 mm) on the surface of the Si target. The irradiation spot size and laser fluence are key factors for the droplet ejection. Thus, in this experiment, the spot size was changed by increasing the distance between the Si target and the focus lens from the focal point (z = 0 µm) to z = 600 µm at 100 µm intervals. The pulse energy was fixed at 0.08 mJ. The irradiated surface was observed by a scanning electron microscope (KEYENCE, VE7800). The ablated structures were gradually increased with defocusing, as shown in Fig. 1. Cone structures were formed at the center of the irradiation spots at z = 0–400 µm.