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G. Feng et al.
matter in processed areas is still in a cold state during the femtosecond laser micromachining. Compared with long-pulse laser micromachining, the femtosecond
laser has a small energy diffusion depth and reduced (but unavoidable) thermal
effect during micromachining, which makes the machining accuracy higher and
the processing edge relatively finer, so that it has realized “the cold” processing.
2. It can process a wide range of machinable materials, from metal or nonmetal
to a biological cell. The high intensity femtosecond laser not only contributes
linear absorption to the material matrix but also a strong nonlinear absorption.
Thus, femtosecond lasers can be applied to both the interior and surface of many
metals, dielectrics, semiconductors, and biological tissues in an effort to achieve
material removal or processing.
3. Applications in real three-dimensional processing are achievable because the
laser power density in the laser focus region is extremely high, while the laser
power density in the unfocused region is not significant enough to cause material
damage. Therefore, precise spatial localization can be achieved by adjusting the
spatial orientation of the femtosecond laser. At the same time, the laser intensity
at the focal point can reach a higher level relative to the light intensity at the
non-focal point which is insufficient to cause ionization and multiphoton absorption. Therefore, femtosecond laser can achieve high-precision three-dimensional
machining by focusing laser pulse inside of the transparent material.
4. The processing scale is smaller than the focal spot, achieving
micro/nanofabrication. Femtosecond laser light intensity distribution is generally Gaussian type. The area processed by focused fs laser pulses is far smaller
than the size of the spot. The processing size breaks through optical diffraction
limit, which can achieve real sub-micron or even nano-scale processing, such as
nano-cattle, gears, and micro-chain rotation [5].
The technology used in femtosecond laser micro-processing is mainly
femtosecond laser direct writing (FLDW), projection preparation, and interference.
Among them, FLDW technology is flexible and has a high degree of freedom which
is widely used in a variety of point scanning, line scanning. The projection technique
can be used to process any two-dimensional shapes on the surface of the material.
In addition, interference can be harnessed in creating periodic three-dimensional
structures, specifically, by multi-beam interference.
7.2 Theoretical Analysis of Femtosecond Pulse-Laser
Micromachining
Femtosecond lasers have a high peak power and ultrashort pulse width which exhibits
strong nonlinear effects during matter interaction. Thus, a large variety of materials
can be processed by femtosecond lasers. However, the mechanisms of femtosecond
laser interaction with various semiconductors, metals, ceramics, resins, as well as
others do vary from material to material. The materials that can be processed by
G. Feng et al.
matter in processed areas is still in a cold state during the femtosecond laser micromachining. Compared with long-pulse laser micromachining, the femtosecond
laser has a small energy diffusion depth and reduced (but unavoidable) thermal
effect during micromachining, which makes the machining accuracy higher and
the processing edge relatively finer, so that it has realized “the cold” processing.
2. It can process a wide range of machinable materials, from metal or nonmetal
to a biological cell. The high intensity femtosecond laser not only contributes
linear absorption to the material matrix but also a strong nonlinear absorption.
Thus, femtosecond lasers can be applied to both the interior and surface of many
metals, dielectrics, semiconductors, and biological tissues in an effort to achieve
material removal or processing.
3. Applications in real three-dimensional processing are achievable because the
laser power density in the laser focus region is extremely high, while the laser
power density in the unfocused region is not significant enough to cause material
damage. Therefore, precise spatial localization can be achieved by adjusting the
spatial orientation of the femtosecond laser. At the same time, the laser intensity
at the focal point can reach a higher level relative to the light intensity at the
non-focal point which is insufficient to cause ionization and multiphoton absorption. Therefore, femtosecond laser can achieve high-precision three-dimensional
machining by focusing laser pulse inside of the transparent material.
4. The processing scale is smaller than the focal spot, achieving
micro/nanofabrication. Femtosecond laser light intensity distribution is generally Gaussian type. The area processed by focused fs laser pulses is far smaller
than the size of the spot. The processing size breaks through optical diffraction
limit, which can achieve real sub-micron or even nano-scale processing, such as
nano-cattle, gears, and micro-chain rotation [5].
The technology used in femtosecond laser micro-processing is mainly
femtosecond laser direct writing (FLDW), projection preparation, and interference.
Among them, FLDW technology is flexible and has a high degree of freedom which
is widely used in a variety of point scanning, line scanning. The projection technique
can be used to process any two-dimensional shapes on the surface of the material.
In addition, interference can be harnessed in creating periodic three-dimensional
structures, specifically, by multi-beam interference.
7.2 Theoretical Analysis of Femtosecond Pulse-Laser
Micromachining
Femtosecond lasers have a high peak power and ultrashort pulse width which exhibits
strong nonlinear effects during matter interaction. Thus, a large variety of materials
can be processed by femtosecond lasers. However, the mechanisms of femtosecond
laser interaction with various semiconductors, metals, ceramics, resins, as well as
others do vary from material to material. The materials that can be processed by