Machining with Ultrafast Laser Pulses
This regime has numerous advantages as illustrated in the Figure 5.1 below.
Figure 5.1: Ultrafast-Pulse Laser-Matter Interaction. Click here to view an animation of the process.
You will notice that this figure is much simpler than Figure 3.1, and there is a good reason for this. Physical processes like heat conduction, etc. don't have time to leach energy away from the process of plasma formation and subsequent material ejection.
Because the energy does not have the time to diffuse away, the efficiency of the machining process is high. If you remember the analogy we made in Chapter 3, this is similar to filling a bucket with no holes! The laser energy has nowhere to go (or more precisely does not have the time to move away). It just piles up at the level of the working spot, whose temperature rises instantly past the melting point of the material and goes, very quickly, well beyond even the evaporation point. In fact, the temperature keeps on climbing into what is called the plasma regime. This may seem strange. It is certainly not a common experience. How can this happen?
Femtosecond lasers, like our model CPA-2101, deliver an incredible amount of peak power. These systems routinely deliver 5 to 10 Gigawatts of peak power (this is more than the average power delivered by a large nuclear plant).The laser intensity easily reaches the hundreds of Terawatts per square centimeter range at the work spot. Absolutely, positively nothing else that is man-made gets anywhere close to this power density.
No materials can withstand the forces at work at these power densities. This means that with ultrafast laser pulses we can machine very hard materials, as well as materials with extremely high melting points such as Molybdenum, Rhenium, etc.
What else does this lack of thermal diffusion do to the machining process?
After the ultrafast laser pulse creates the plasma in the surface of the material, the pressures created by the forces within it cause the material to expand outward from the surface in a highly energetic plume or gas. The internal forces that previously held the material together are vastly insufficient to contain this expansion of highly ionized atoms (physicists call these charged atoms "ions") and electrons from the surface. Because the electrons are lighter and more energetic than the ions, they come off the material first, followed later by the ions. And because the ions all have some positive charge, they repel each other as they expand away from the material. Consequently, there are no droplets that condense onto the surrounding material. Additionally, since there is no melt phase, there is no splattering of material onto the surrounding surface.
Micromachining with femtosecond pulses offer some additional advantages as shown in subsequent chapters.