36
A. Hu et al.
CW laser cannot break a CuNW at all but create a very clear surface deformation on
the nanowire. As shown in Fig. 1.24f, at the laser-focused position, CuNW is partially
melted at high temperatures induced by laser irradiation, and then the melted wire
solidifies and crystalizes again after the laser is off. Furthermore, this thermal effect
is propagated along the CuNW, and cause the increase of surface roughness of the
entire nanowire (see Fig. 1.24f). In a sharp contrast, the surface of the un-irradiated
area is kept as clean and smooth before and after FS laser irradiation. However,
after CW laser irradiation, a clear surface change can be observed even in the unirradiation area. Because the CuNW was exposed to air, serious oxidation could
occur on the surface due to the heat-induced by CW laser, which will be further
confirmed by the chemical analysis later. This oxidization has been confirmed by
checking the oxygen distribution before and after the CW laser irradiation [6]. FS
laser is an ultrafast high repetition pulse laser, with an average laser power of 35
mW, it can generate a peak power density of 1.24 × 10
14 W/cm
2 . On the other hand,
with 35mW power, CW laser only generates a power density of 4.46 × 10
6 W/cm
2 .
Meanwhile, from the aforementioned theoretical analysis, for an ultrafast interaction, the absorption of photons stimulates electrons within a hundred femtoseconds
(fs), which is too short to disturb lattice. The absorbed energy was thus transferred
from electrons to the lattice by electron-lattice scattering after the laser pulse [13,
14]. The thermal coupling between free electrons and lattices typically occurs within
100 ps, depending on the electron-phonon coupling strength of different materials.
The typical electron-phonon coupling time of hundred femtoseconds is much shorter
than the heat transfer period by thermal conduction. Therefore, the thermal diffusion
to the laser-irradiated surrounding area for a FS laser is very limited [15].
According to our computational simulations [6] with the CW laser (35 mW)
irradiation (at x = 0), heat is accumulated, increasing temperature of CuNW with
heating time (t), and the heat is dissipated along the length of CuNW (i.e., in the xdirection) in a nanosecond scale, as shown in Fig. 1.25a. With the CW laser heating
longer than 0.5 μs, the temperature rises beyond the Cu melting point (T m, Cu =
1368 K [126]), and a large portion of CuNW has a temperature above the oxidation
temperature (1073 K) [127, 128]. Figure 1.25b displays that the thermally affected
zone (i.e., the heated portion of the CuNW where the temperature is above the
oxidation temperature) extends to 10 μm after 1 μs of heating time. It can be inferred
that with the increase of irradiation time, the whole CuNW will be heated above the
oxidation temperature. This explains that the oxidization of the entire CuNW with a
CW laser observed from the experiment conducted in an ambient atmosphere in our
study.
With the FS pulse laser irradiation at x = 0, the electron temperature (T e ) reaches
a very high peak value (~27.5 × 10
4 K) within the pulse duration (~300 fs), and then
it decays rapidly, releasing their energy to phonons (or lattice). The electron-lattice
interaction causes the rise of local T l [129], and T e reaches almost equilibrium with
T l shortly (<20 ps) after the pulse at x = 0 (Fig. 1.25c). The lattice temperature
(T l ) rises and decreases, and for most of the wire (>27 μm), it stays below the Cu
melting point (Fig. 1.25d). A non-thermal ablation is expected at the beginning of
the irradiated zone (<1 μm, close to the focus spot size) as the lattice temperature
A. Hu et al.
CW laser cannot break a CuNW at all but create a very clear surface deformation on
the nanowire. As shown in Fig. 1.24f, at the laser-focused position, CuNW is partially
melted at high temperatures induced by laser irradiation, and then the melted wire
solidifies and crystalizes again after the laser is off. Furthermore, this thermal effect
is propagated along the CuNW, and cause the increase of surface roughness of the
entire nanowire (see Fig. 1.24f). In a sharp contrast, the surface of the un-irradiated
area is kept as clean and smooth before and after FS laser irradiation. However,
after CW laser irradiation, a clear surface change can be observed even in the unirradiation area. Because the CuNW was exposed to air, serious oxidation could
occur on the surface due to the heat-induced by CW laser, which will be further
confirmed by the chemical analysis later. This oxidization has been confirmed by
checking the oxygen distribution before and after the CW laser irradiation [6]. FS
laser is an ultrafast high repetition pulse laser, with an average laser power of 35
mW, it can generate a peak power density of 1.24 × 10
14 W/cm
2 . On the other hand,
with 35mW power, CW laser only generates a power density of 4.46 × 10
6 W/cm
2 .
Meanwhile, from the aforementioned theoretical analysis, for an ultrafast interaction, the absorption of photons stimulates electrons within a hundred femtoseconds
(fs), which is too short to disturb lattice. The absorbed energy was thus transferred
from electrons to the lattice by electron-lattice scattering after the laser pulse [13,
14]. The thermal coupling between free electrons and lattices typically occurs within
100 ps, depending on the electron-phonon coupling strength of different materials.
The typical electron-phonon coupling time of hundred femtoseconds is much shorter
than the heat transfer period by thermal conduction. Therefore, the thermal diffusion
to the laser-irradiated surrounding area for a FS laser is very limited [15].
According to our computational simulations [6] with the CW laser (35 mW)
irradiation (at x = 0), heat is accumulated, increasing temperature of CuNW with
heating time (t), and the heat is dissipated along the length of CuNW (i.e., in the xdirection) in a nanosecond scale, as shown in Fig. 1.25a. With the CW laser heating
longer than 0.5 μs, the temperature rises beyond the Cu melting point (T m, Cu =
1368 K [126]), and a large portion of CuNW has a temperature above the oxidation
temperature (1073 K) [127, 128]. Figure 1.25b displays that the thermally affected
zone (i.e., the heated portion of the CuNW where the temperature is above the
oxidation temperature) extends to 10 μm after 1 μs of heating time. It can be inferred
that with the increase of irradiation time, the whole CuNW will be heated above the
oxidation temperature. This explains that the oxidization of the entire CuNW with a
CW laser observed from the experiment conducted in an ambient atmosphere in our
study.
With the FS pulse laser irradiation at x = 0, the electron temperature (T e ) reaches
a very high peak value (~27.5 × 10
4 K) within the pulse duration (~300 fs), and then
it decays rapidly, releasing their energy to phonons (or lattice). The electron-lattice
interaction causes the rise of local T l [129], and T e reaches almost equilibrium with
T l shortly (<20 ps) after the pulse at x = 0 (Fig. 1.25c). The lattice temperature
(T l ) rises and decreases, and for most of the wire (>27 μm), it stays below the Cu
melting point (Fig. 1.25d). A non-thermal ablation is expected at the beginning of
the irradiated zone (<1 μm, close to the focus spot size) as the lattice temperature