Fig. 1: Quantel Brilliant YAG lasers with higher-harmonic generator units.
Fig. 2: The YAG laser mounted at the bottom of the chamber. The black box in front of the chamber is the RHEED camera housing
Fig. 3: Target surface after 800 laser pulses. A deep track is clearly visible.
The ablation laser is a high-power YAG unit from Quantel (Fig. 1). The third harmonic output pulse energy is about 100 mJ / pulse. The pulse length is about 5 times shorter (4 ns) than an excimer, so even when the energy density at the target is the same, the YAG gives a much higher power density, up to several GW/cm2. This is enough to cut through a target very quickly if the laser light is focused to a small spot (Fig. 3). The laser spot size should therefore be at least a few mm in diameter to prevent such surface erosion.
The YAG laser is mounted at the bottom of the laser rack (Fig. 2). Light is directed into the chamber with two mirrors, one just in front of the laser and the other mounted on a rod in front of the laser viewport. The focusing lens is mounted on the same rod. The power density at the ablation target surface can be adjusted by changing the lens position and the focal length of the lens (300 or 400 mm). The laser viewport is fused silica, which doesn't absorb the laser light but the viewport is not AR coated, which means that there is some back-reflection. Care must be taken that the reflected light is not refocused at the mirror.