In laser processing, so-called "voxel" corresponds to the volume where the light beam is sufficiently focused to actually reach the desired energy level. It is typically an ellipsoid extending over the depth of focus and located at the beam waist. We can represent it as follows:

Suppose that we need to obtain a machining voxel of  um diameter and  um height. We are working with a Gaussian beam (see Lecture 3, slide 54 in the final version of the slides). Note that the depth of focus is two times the Rayleigh length.

Select the right analytical expression giving the wavelength that one could use to reach these dimensions as a function of and .