7 Micro-hole Arrays and Net-like Structure Fabrication …
213
femtosecond lasers can be separated into two categories: (1) metal materials (2) and
non-metallic materials. Metal materials have good thermal and electric conductivity
because of the existence of a large number of free electrons within the material, and
metal materials have absorption characteristics for near-infrared and visible light,
while transparent materials and semiconductors do not absorb these two bands of
light. However, it will be a metal similar absorption material due to multi-photon
ionization when femtosecond laser power density reaches a certain extent.
7.2.1 Interaction Principle Between Femtosecond Laser
and Metallic Materials
Most of the metal materials are opaque to visible light. When the laser initially
interacts with the metal, the energy is deposited at a penetration depth of l s = 1
α
(α is the absorption coefficient) [6]. Then the laser energy gradually spreads to
the depth of l d =
√
Dτ (D is the thermal diffusion coefficient, and τ is the laser
pulse width) during the process. If the material is processed by a long-pulse laser
(l d > l s ), the thermal diffusion effect is very obvious; however, when processed by
an ultrashort pulse laser (l d < l s ), the laser energy is unable to spread quickly which
causes material directly evaporation around the focus. The laser heat-affected zone
is very small during the process, allowing high-precision machining.
When there is a strong laser interaction with metal materials, the free electrons in
the metal are all heated instantaneously. The high-temperature electrons transfer their
energy to low-temperature particles via collision. The energy transfer time between
electrons is in the order of 100 fs, during which time the high-temperature electrons
can transfer energy to the non-laser affected region. Relaxation time (the collision
heating time between electron and lattice) of the metal is in the order of picoseconds.
The relaxation time of a metal material with strong electron–phonon coupling (e.g.,
Fe) is one to two orders shorter than that of metal with a weak electron–phonon
coupling. However, the pulse width of a femtosecond laser is much smaller than the
relaxation time, so there is not enough time to heat up the metal lattice during the
process. Electrons are instantly heated up to high temperature, causing the electron
and lattice to exist at different temperatures. So, a two-temperature model is used to
replace the classical thermodynamic model in solving the changes of temperature in
ion T i and electron T e during the process [7, 8],
C e (T e )
∂ T e
∂t
= K ∇
2 T e − g(T e − T i ) + A(r, t)
(1)
C i
∂ T i
∂t
= g(T e − T i )
(2)
213
femtosecond lasers can be separated into two categories: (1) metal materials (2) and
non-metallic materials. Metal materials have good thermal and electric conductivity
because of the existence of a large number of free electrons within the material, and
metal materials have absorption characteristics for near-infrared and visible light,
while transparent materials and semiconductors do not absorb these two bands of
light. However, it will be a metal similar absorption material due to multi-photon
ionization when femtosecond laser power density reaches a certain extent.
7.2.1 Interaction Principle Between Femtosecond Laser
and Metallic Materials
Most of the metal materials are opaque to visible light. When the laser initially
interacts with the metal, the energy is deposited at a penetration depth of l s = 1
α
(α is the absorption coefficient) [6]. Then the laser energy gradually spreads to
the depth of l d =
√
Dτ (D is the thermal diffusion coefficient, and τ is the laser
pulse width) during the process. If the material is processed by a long-pulse laser
(l d > l s ), the thermal diffusion effect is very obvious; however, when processed by
an ultrashort pulse laser (l d < l s ), the laser energy is unable to spread quickly which
causes material directly evaporation around the focus. The laser heat-affected zone
is very small during the process, allowing high-precision machining.
When there is a strong laser interaction with metal materials, the free electrons in
the metal are all heated instantaneously. The high-temperature electrons transfer their
energy to low-temperature particles via collision. The energy transfer time between
electrons is in the order of 100 fs, during which time the high-temperature electrons
can transfer energy to the non-laser affected region. Relaxation time (the collision
heating time between electron and lattice) of the metal is in the order of picoseconds.
The relaxation time of a metal material with strong electron–phonon coupling (e.g.,
Fe) is one to two orders shorter than that of metal with a weak electron–phonon
coupling. However, the pulse width of a femtosecond laser is much smaller than the
relaxation time, so there is not enough time to heat up the metal lattice during the
process. Electrons are instantly heated up to high temperature, causing the electron
and lattice to exist at different temperatures. So, a two-temperature model is used to
replace the classical thermodynamic model in solving the changes of temperature in
ion T i and electron T e during the process [7, 8],
C e (T e )
∂ T e
∂t
= K ∇
2 T e − g(T e − T i ) + A(r, t)
(1)
C i
∂ T i
∂t
= g(T e − T i )
(2)