214
G. Feng et al.
where C e and C i are the specific heat capacity (per volume) of lattices and electrons,
respectively, K is thermal conductivity, A(r, t) is the corresponding heat source of
laser pulse, and g is the electron and phonon coupling constant.
The main differences between long-pulse-laser and ultrashort-pulse-laser
processing of metal materials are as follows: The thermal effect time of long-pulselaser interaction with matter is determined by the laser pulse width, but the thermal
effect time of ultrashort-pulse-laser interaction with matter depends on the relaxation
time of the material. The electron temperature reaches extremely high temperatures
during ultrashort-pulse-laser interaction, while the ion temperature is delayed. So,
the interaction between femtosecond lasers and the material is “cold.” The lattice
temperature will still rise after the laser pulse has stopped. Laser thermal effects
can be appropriately reduced but unable to be avoided by using ultrashort laser
pulse interaction for micromachining. The main reasons that affect the precision
of femtosecond laser micromachining include the melting of the material caused
by lattice heating, the thermal infiltration caused by electron interaction, and the
accumulation of molten material.
It should be noted that the dual-temperature model does not take into account the
material nonlinear absorption effect; therefore, the dual-temperature model is only
applicable to metal materials.
7.2.2 The Interaction of an Intense Femtosecond Laser Pulse
with Dielectric Materials
7.2.2.1 Ionization Mechanism
The interaction between the laser and the solid matter is the stimulated process of
valence electrons reaching their excited state by absorbing photons. The generation of stimulated processes can be single-photon resonance transitions, two-photon
and high-order multi-photon transitions, tunnel ionization, and so on. The singlephoton resonant transitions obey Beer’s law, a linear process, while the others are
nonlinear processes which are closely related to the intensity of the laser beam.
Avalanche ionization and photoionization are the main reasons for the generation of
free electrons, but their dominance in dielectric processing remains unclear.
1. Avalanche ionization process
Avalanche ionization is a phenomenon in which free-charge carriers absorb laser
energy after impact ionization occurs in the material. A small number of free
electrons in the medium are used as seed electrons to absorb the laser photon
energy and enhance their kinetic energy through inverse bremsstrahlung absorption. When the kinetic energy is larger than that of bound electrons, the electrons collide with the valence band electrons, causing the valence electrons to
be excited to the conduction band, becoming free electrons and leading to the
formation of an additional free electron. Thus, a seeded electron with higher
G. Feng et al.
where C e and C i are the specific heat capacity (per volume) of lattices and electrons,
respectively, K is thermal conductivity, A(r, t) is the corresponding heat source of
laser pulse, and g is the electron and phonon coupling constant.
The main differences between long-pulse-laser and ultrashort-pulse-laser
processing of metal materials are as follows: The thermal effect time of long-pulselaser interaction with matter is determined by the laser pulse width, but the thermal
effect time of ultrashort-pulse-laser interaction with matter depends on the relaxation
time of the material. The electron temperature reaches extremely high temperatures
during ultrashort-pulse-laser interaction, while the ion temperature is delayed. So,
the interaction between femtosecond lasers and the material is “cold.” The lattice
temperature will still rise after the laser pulse has stopped. Laser thermal effects
can be appropriately reduced but unable to be avoided by using ultrashort laser
pulse interaction for micromachining. The main reasons that affect the precision
of femtosecond laser micromachining include the melting of the material caused
by lattice heating, the thermal infiltration caused by electron interaction, and the
accumulation of molten material.
It should be noted that the dual-temperature model does not take into account the
material nonlinear absorption effect; therefore, the dual-temperature model is only
applicable to metal materials.
7.2.2 The Interaction of an Intense Femtosecond Laser Pulse
with Dielectric Materials
7.2.2.1 Ionization Mechanism
The interaction between the laser and the solid matter is the stimulated process of
valence electrons reaching their excited state by absorbing photons. The generation of stimulated processes can be single-photon resonance transitions, two-photon
and high-order multi-photon transitions, tunnel ionization, and so on. The singlephoton resonant transitions obey Beer’s law, a linear process, while the others are
nonlinear processes which are closely related to the intensity of the laser beam.
Avalanche ionization and photoionization are the main reasons for the generation of
free electrons, but their dominance in dielectric processing remains unclear.
1. Avalanche ionization process
Avalanche ionization is a phenomenon in which free-charge carriers absorb laser
energy after impact ionization occurs in the material. A small number of free
electrons in the medium are used as seed electrons to absorb the laser photon
energy and enhance their kinetic energy through inverse bremsstrahlung absorption. When the kinetic energy is larger than that of bound electrons, the electrons collide with the valence band electrons, causing the valence electrons to
be excited to the conduction band, becoming free electrons and leading to the
formation of an additional free electron. Thus, a seeded electron with higher