Interaction of Laser Energy with Metals

For metals, the valence electrons are not constrained by the atomic lattice of the material. Instead, they form a free electron cloud. When a laser beam strikes a metal, it causes all of the electrons in the cloud to oscillate with a period dictated by the wavelength of the laser beam. This oscillation generates an electric field 180 degrees out of phase with the impinging laser beam. This makes it very difficult for a laser beam to penetrate more than a few atomic layers into a metal.

Figure 4. Reflectivity of stainless steel as a function of wavelength¹.
To overcome this issue, a laser beam consisting of high-energy photons is required. The energy of a laser beam (or any electromagnetic wave) is inversely proportional to its wavelength. Referring back to Figure 1, the lasers at the left side of the figure have shorter wavelengths and therefore higher photon energy than the lasers on the right side of the figure. The higher energy photons are able to excite more core electrons, where the excess energy is more readily absorbed by the material.

This is illustrated in Figure 4, which shows the reflectivity of stainless steel as a function of wavelength. At the 10.6μm wavelength, 90% of the energy is reflected. However only 60% of the 1.06μm laser energy is reflected.

Combining Multiple Laser Wavelengths

One existing industrial application that utilizes two distinct laser wavelengths is steel processing. Steel reflects longer wavelength laser beams as shown in Figure 4. Therefore, steel is generally processed with shorter wavelength laser beams (i.e. 1.06μm). An economic advantage is gained by pre-heating the steel with a shorter wavelength laser beam, which is readily absorbed by the steel, then switching to a longer wavelength laser beam, which is more economical to operate.

Figure 5. Absorptivity of several pure metals as a function of temperature².
Heating the steel with a shorter wavelength laser beam causes an increase in lattice vibrations (phonons). This, in turn, increases the probability that the excited electrons in the cloud will contribute their excess energy to the lattice (i.e., absorption), as opposed to re-emitting their excess energy (i.e., reflection). This phenomenon can be extended to many other metals as demonstrated by Prokhorov, et al. in Figure 5[2]. This figure shows a linear increase in the absorptivity of several pure metals as temperature is increased. The stepwise increase in absorptivity that is shown for several of the metals occurs at their melting points.

This ability to use one laser wavelength to modify the properties of a material, thereby enabling another laser wavelength to process the material more efficiently is one of the key advantages that MultiWave Hybrid Technology offers. The primary advantage provided by this new technology is its ability to combine individual laser wavelengths into a single coaxial laser beam. This introduces new capabilities that cannot be achieved with any other laser technology.

Figure 6. Delrin marked with a 10.6μm laser (top), a 1.06μm laser (middle) and with MultiWave Hybrid Technology (bottom).
For example, Ruettimann, et al. showed that combining a 532nm wavelength laser beam with a 1064nm wavelength laser beam produced a copper weld that was superior to any weld that could be obtained with either wavelength independently[3].

In another example, Klotzbach, et al. demonstrated carbon fiber reinforced polymers cannot be cut cleanly with either 10.6μm or 1.07μm laser beams; however, a superior laser cutting quality is achieved if these two wavelengths are combined into a single laser beam[4].

Figure 6 shows a sheet of Delrin™ marked using three different laser technologies. The laser mark at the top of the image was created using a 10.6μm wavelength laser. This mark has a depth of 0.003” (76μm), but no contrast. The laser mark in the center was created using a 1.06μm laser. This mark has contrast, but no depth. MultiWave Hybrid Technology created the laser mark at the bottom, using a laser beam comprised of 10.6μm and 1.06μm wavelengths. This mark has both contrast and a depth of 0.003” (76μm). This represents a new type of laser marking that was not possible in the past. This new type of mark has depth that provides for a permanent mark and contrast that aids in visibility.