## 4 Discussion  
When the CYTOP-glass distance was short, fibrous network structures composed of NP aggregates were deposited onto the glass slide. At longer CYTOP-glass distances, a monolayer structure of NPs was deposited. In previous studies of fs laser ablation of matter [24; 25], when fs laser ablation of CYTOP occurs, critical local expansion dynamics at the focal area lead to the direct fragmentation and ejection of the molten CYTOP. The ejected liquid species from the ablation plume solidifies rapidly and the subsequent growth of the solid fragments will form CYTOP NPs. In fact, agglomerates  
Figure 8: (a) Schematic of the CYTOP NP deposition and removal procedures using holographic fs laser processing on a glass slide. A line-shaped fs laser beam was scanned on the NP-deposited areas to divide the glass and NP-network structures (note that the scales do not relatively agree with each other). SEM images showing (b) an overall view of the fabricated glass slide and (c) an enlarged view of the boundaries between the glass and the deposited network structures. Figure 9: Microscope images showing (a) HeLa cells selectively cultured on the fabricated glass and (b) an enlarged view of the deposited NP-glass boundaries. are also formed as a result of collisions of individual NPs during the ablation. Since the network structures composed of NP aggregates were heavier than the individual NPs, the structures could only deposit onto the glass slide when the CYTOP-glass slide distance was short. Only the individual light NPs could be transported longer CYTOP-glass distances, resulting in the deposition of a monolayer structure (Figure 10). Furthermore, according to [PERSON] and [PERSON] [26], the pulse frequency plays a critical role in the formation of NP aggregates. They found that network structures of Si NPs were likely to be formed when MHz fs laser processing was used rather than kHz processing, even though we observed the same fibrous dense structures of CYTOP NPs in the present work by using 1 kHz fs laser processing. The reason why the kHz fs laser enables the generation of dense network structures is still unclear; however, we speculate that melted liquid species that eventually form NPs are more likely to be ejected from the ablation plume of CYTOP than from that of Si because the CYTOP is thermoplastic and exhibits a lower melting point than Si. In addition, the holographic fs laser may play an important role in forming network structures. Figure 11 shows SEM images of CYTOP NPs deposited using a single-dot fs laser beam for comparison with the results obtained using five-dot parallel-beam irradiation (Figure 4a). The pulse energy, number of pulses, and the CYTOP-glass distance were 1 \(\upmu\)J/pulse, 10,000 pulses, and 100 \(\upmu\)m, respectively, and the irradiation was repeated five times by shifting the focal position 8 \(\upmu\)m along the \(x\)-direction to be consistent with the irradiation conditions represented in Figure 4a. Figure 11 shows that the short chain-like structures were deposited onto the glass, unlike the dense fibrous network structures observed in Figure 4a. This result suggests that the holographic  
Figure 10: Schematics of the NP deposition via CYTOP ablation, where the deposited structure is dependent on the CYTOP–glass distance. fs laser processing--specifically, the multibeam fs laser irradiation--contributed to the generation of dense fibrous network structures because the collisions and the agglomerates of ejected species from the ablation, which led to the formation of the network structure, were more likely to occur as a consequence of the multiple simultaneous ablations. Given the current experimental results, we cannot conclusively determine the influence of the holographic fs laser processing on NP generation. However, we speculate that these simultaneous particle interactions by holographic laser processing may enable the fabrication of functional nanostructures.