# A Review on Ultrafast Laser Enabled Excellent Superhydrophobic Anti-Icing Performances
## 2 Ultrafast Laser-Fabricating Micro-Nanostructured Superhydrophobic Surfaces
### Ultrafast Laser Micro-Nano Fabricating  
As discussed above, ultrafast lasers have many incomparable advantages in micro-nanofabrication. These advantages are not only ascribed to the convenience and programmability of processing methods but also attributed to the complicated light-matter interactions [39; 93]. Generally, ultrafast lasers are defined as lasers with ultrashort pulse duration from several tens of femtoseconds to tens of picoseconds [94]. Figure 5a shows the timescales and intensity of the main phenomena during and after irradiating a solid with a femtosecond laser. The light-matter interacting processes could be divided mainly into three types: electronic excitation, nonequilibrium phase transformation, and morphology formation [95], as illustrated in Figure 5a. Overall, in most cases, electronic excitation occurs during the laser pulse (fs--ps), and nonequilibrium phase transformations take place on the picosecond timescale (about ps--100 ps). The timescale of morphology formation reaches up to nanoseconds (about ns--100 ns). There also exist some differences when processing different materials. For semiconductors or dielectrics, the irradiated laser on the one hand can prompt the generation of high-concentration free electrons across the bandgap, which might further induce non-thermal melting. Meanwhile, on the other hand, the laser also provides the excited electrons with more energy, triggering the switching to the plasma state or producing a coulomb explosion. When reaching the electron lattice equilibration, rapid heating at a rate of \(>\)10\({}^{14}\) K/s occurs to further lead to homogeneous melting or the formation of a supercritical fluid [96]. Many new surface morphologies or metastable phases are produced in extreme environments. After the temperature decreases below material melting points, the melts start to solidify or resolidify to form surface micro-nanostructures. For metals, laser-induced electron heating dominates, resulting in localized high-temperature melting. However, the high thermal conductivities of metals can additionally result in a high supercooling state, where some unexpected structures or defects will be constructed, such as some amorphous structures or nanocrystals [97; 98]. Following the interaction mechanisms between ultrafast lasers and matter, numerous diverse micro-nanostructures can be easily fabricated and can be further tuned by varying  
Figure 4: Three classical wetting states and the superhydrophobic phenomena in nature. (**a**–**c**) Schematics of the three states: (**a**) Young’s equation; (**b**) Wenzel equation; (**c**) Cassie–Baxter equation. (**d**–**i**) Creatures with superhydrophobicity: (**d1,d2**) lots leaves (reproduced with permission from [88]. Copyright (2007) Elsevier); (**e1,e2**) water striders (reproduced with permission from [89]. Copyright (2007) American Chemical Society); (**f1,f2**) rose petals (reproduced with permission from [90]. Copyright (2008) American Chemical Society); (**g1,g2**) rice leaves (reproduced with permission from [88]. Copyright (2007) Elsevier); (**h1,h2**) butterflies (reproduced with permission from [91]. Copyright (2018) American Chemical Society); (**i1,i2**) mosquito eyes (reproduced with permission from [92]. Copyright (2007) John Wiley and Sons). laser processing parameters or adjusting processing environments. Some common laser parameters include laser wavelengths, pulse duration, pulse repetition frequency, pulse input number, laser fluence, and so on [39; 99]. Here, we take two important parameters (laser pulse duration and pulse input number) as examples to introduce their effects on micro-nanostructures. Generally, different laser pulse durations result in different heat affected zones, eventually affecting the processing quality. As shown in Figure 5b, with the shortening of pulse duration from \(-10^{-9}\) to \(10^{-15}\), the processing quality gradually improves as a result of the smaller heat-affected zone [94].