Ultrafast Laser Micromachining Handbook
Machining materials with lasers - a technology first introduced in the early 1970's - is now used routinely in many industries. Laser micromachining is a more recent development. First demonstrated in the 1980's, micromachining with lasers is an evolving technology. Initially laser micromachining was based on continuous wave or long-pulse lasers. With these "conventional" lasers, the heat transferred from the laser beam to the work piece introduced numerous restrictions that limit the precision and the quality of the machining process. In other words, laser micromachining is... well, not so micro, but rather course by some of today's standards. Machinists have learned ways to minimize the negative effects associated with heat transfer through various types of pre- and post-processing. These additional steps considerably increase the complexity and cost of the machining operation.
Note that heat-diffusion is not limited to laser machining. Tool bits deposit mechanical energy into the material that is being machined, a portion of which is converted to heat. This heat energy does not stay localized where it was initially deposited. It moves away in a characteristic time - the so-called "heat-diffusion time." This is a familiar phenomenon. If you turn on the heating element on an electric stove, it will take a few seconds to warm up. The same happens at the microscopic level, but the time scales involved are quite different. The typical "heat-diffusion time" encountered in laser machining is not counted in seconds, but rather in picoseconds (a picosecond is a millionth of a millionth of a second).
In the early nineties, scientists at the University of Michigan discovered that the transfer of heat from the laser beam to the work piece could be defeated using ultrafast laser pulses instead of standard long-pulse lasers. Essentially machining with laser pulses of very short duration eliminates heat flow to surrounding materials. This discovery opened the way for fine laser micromachining.
Before looking in detail at some samples that were machined with ultrafast pulses, let's first take a closer look at the way lasers interact with matter. To make this complex science reasonably understandable, we have simplified or ignored many issues: we arbitrarily divide the physics of how light interacts with materials into two time regimes - one in which the laser pulse is either very, very short (called ultrafast or ultrashort), and another in which the laser pulse is not so short (which we call "long"). Ultrafast, or ultrashort, means that the laser pulse has a duration that is somewhat less that about 10 picoseconds - usually some fraction of a picosecond (femtosecond). "Long" means that the pulse is longer that about 10 picoseconds, that is, longer than the heat-diffusion time. These long pulse lasers may be continuous, quasi-continuous, or Q-switched, but in any case they are generating long pulses compared to the heat-diffusion time.