I understand at high pulse energies you can machine visible patterns on quartz. How do we tune the energy/other parameters to form patterns which are only visible after etching with HF or KOH?
You have to control the following:
1. Spot diameter of the beam, which is controlled by the focal length and input light diameter.
2. The energy of the LASER. By controlling the input energy you can actually make a pattern in the quartz that is smaller than the diffraction limit.
3. Beam shape (profile) of your input LASER beam; typically a Gaussian beam is used.
4. Ellipticity of the beam.
5. The LASER beam pulse width; a shorter pulse beam will have higher fluence. In general it is the peak fluence that causes index of refraction difference in the crystal from the high photon flux. At a molecular level the substrate cannot handle the thermal in flux of photons, so energy still has to be conserved, therefore the electric dipoles (quantum level) absorb the energy and cause the dipoles to be displaced. This displacement of the dipoles in the short pulse focus beam area causes an index of refraction change from the rest of the substrate.
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