2 Ultrafast Laser Enabling Versatile Fabrication of Surface …
85
2.3.2 Fabrication of Metal Surface Micro-nano Dual-scale
Structures
When more laser energies, i.e., higher laser fluence and more laser pulses, are input
into metal surfaces, different kinds of microscale structures can be fabricated by the
high power high repetition rate ultrafast lasers. On the surfaces of the microscale
structures, there covers abundant nanoscale features, naturally forming the unique
micro-nano dual-scale hierarchical structures.
One representative type of structural features for demonstrating this is surface
particle structures. As laser energy input increases, particle structures with sequential
size changes from nano scale to micro scale have been successfully fabricated. Here,
we classify them into five categories: the nano particles with sizes varying from tens
(~10
1 ) to hundreds (~10
2 ) nm (as discussed in Sect. 2.3.1), the sub-micro particles
with sizes in the range of 0.5–1 µm, the fine-micro particles with sizes in the range
of 1–10 µm, the microparticles of 10–50 µm in sizes, and the coarse-micro particles
larger than 50 µm, as shown in Fig. 2.9. Despite that all these sample surfaces are
dominated by particle features, differences in their structural constitutions can be
observed. The nano and sub-micro particles are single-scale structures, while the
surfaces of microscale particles are covered by sub-microscale features, constituting
dual-scale hierarchical structures. Moreover, only the particle feature exists on the
nano and sub-micro particle dominating surfaces, while micro pores and voids occupy
the space among particles in the three microscale particle dominating surfaces. As the
particle sizes change from fine-micro to coarse-micro, the dimensions of the pores
also change.
The microparticle size and surface porosity of the three dual-scale particle structures have also been statistically analyzed. As illustrated in Fig. 2.10, the average
particle size increases from ~7 µm for the fine-micro particle structures to ~56 µm
for the coarse-micro particle structures; meanwhile, the surface porosity shows an
obvious decrease from ~54 to ~21%. Therefore, it demonstrates that the coarsening
of particle structures will compress the space of pores and voids on metal surfaces,
which will alternately influence their optical responses as indicated by the starred
curve in Fig. 2.10.
Figure 2.11 shows the typical SEM images of another representative type of
ultrafast laser fabricated surface micro-nano dual-scale structures, i.e., the arrayed
structures, demonstrating their evolution with laser scanning intervals. As a reference,
the diameter of the ultrafast laser focal spot for this particular investigation is ~30 µm.
With the scanning interval increasing from smaller to greater than of the focal spot,
three kinds of structures were formed on copper surfaces. When scanning intervals
were much smaller than the focal spot, the entire metal surfaces were scanned and
modified by laser more than once. As a result, irregular coral-like surface structures
composed of micro-cavities with random orientations were fabricated (Fig. 2.11a).
Such micro-cavities consist of large hollows with dimensions of 30–100 µm where
small holes with dimensions of 1–30 µm were embedded, with micro protrusions
and particles surrounding them, forming a unique kind of porous structure. When
85
2.3.2 Fabrication of Metal Surface Micro-nano Dual-scale
Structures
When more laser energies, i.e., higher laser fluence and more laser pulses, are input
into metal surfaces, different kinds of microscale structures can be fabricated by the
high power high repetition rate ultrafast lasers. On the surfaces of the microscale
structures, there covers abundant nanoscale features, naturally forming the unique
micro-nano dual-scale hierarchical structures.
One representative type of structural features for demonstrating this is surface
particle structures. As laser energy input increases, particle structures with sequential
size changes from nano scale to micro scale have been successfully fabricated. Here,
we classify them into five categories: the nano particles with sizes varying from tens
(~10
1 ) to hundreds (~10
2 ) nm (as discussed in Sect. 2.3.1), the sub-micro particles
with sizes in the range of 0.5–1 µm, the fine-micro particles with sizes in the range
of 1–10 µm, the microparticles of 10–50 µm in sizes, and the coarse-micro particles
larger than 50 µm, as shown in Fig. 2.9. Despite that all these sample surfaces are
dominated by particle features, differences in their structural constitutions can be
observed. The nano and sub-micro particles are single-scale structures, while the
surfaces of microscale particles are covered by sub-microscale features, constituting
dual-scale hierarchical structures. Moreover, only the particle feature exists on the
nano and sub-micro particle dominating surfaces, while micro pores and voids occupy
the space among particles in the three microscale particle dominating surfaces. As the
particle sizes change from fine-micro to coarse-micro, the dimensions of the pores
also change.
The microparticle size and surface porosity of the three dual-scale particle structures have also been statistically analyzed. As illustrated in Fig. 2.10, the average
particle size increases from ~7 µm for the fine-micro particle structures to ~56 µm
for the coarse-micro particle structures; meanwhile, the surface porosity shows an
obvious decrease from ~54 to ~21%. Therefore, it demonstrates that the coarsening
of particle structures will compress the space of pores and voids on metal surfaces,
which will alternately influence their optical responses as indicated by the starred
curve in Fig. 2.10.
Figure 2.11 shows the typical SEM images of another representative type of
ultrafast laser fabricated surface micro-nano dual-scale structures, i.e., the arrayed
structures, demonstrating their evolution with laser scanning intervals. As a reference,
the diameter of the ultrafast laser focal spot for this particular investigation is ~30 µm.
With the scanning interval increasing from smaller to greater than of the focal spot,
three kinds of structures were formed on copper surfaces. When scanning intervals
were much smaller than the focal spot, the entire metal surfaces were scanned and
modified by laser more than once. As a result, irregular coral-like surface structures
composed of micro-cavities with random orientations were fabricated (Fig. 2.11a).
Such micro-cavities consist of large hollows with dimensions of 30–100 µm where
small holes with dimensions of 1–30 µm were embedded, with micro protrusions
and particles surrounding them, forming a unique kind of porous structure. When