For some applications, nonlinear crystals with a very small thickness of well below 1 mm are used. This may be necessary for minimizing the group velocity mismatch, e.g. in optical autocorrelators for extremely short pulses.
A common method for obtaining ultrathin crystals is to first optically contact a thicker nonlinear crystal with some substrate (e.g. of fused silica) and then to polish the crystal down to the required thickness of e.g. 20 μm. The group velocity mismatch in the thicker substrate material may not matter, as the nonlinear interaction takes place only in the thin crystal. The substrate only serves to mechanically stabilize the thin nonlinear crystal.
It is also possible to fabricate free-standing crystals with a thickness of only 100 μm, sometimes even below 30 μm.
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Laser Components >> Nonlinear Crystals