WHAT DO YOU MEAN BY METASTABLE ENERGY?

A metastable state may thus be considered a kind of temporary energy trap or a somewhat stable intermediate stage of a system the energy of which may be lost in discrete amounts. In quantum mechanical terms, transitions from metastable states are “forbidden” and are much less probable than the “allowed” transitions from other excited states.

Why is optical pumping not used in gas?

Optical pumping methods are very ineffective for gas lasers because gas atoms show sharp energy levels as compared to solids, which are characterized by energy bands rather than energy levels. Thus for gas laser the electrical discharge is used for pumping.

Why is hologram called wave front reconstruction?

Because the very complex waves that are reflected from an object when it was sitting there are reproduced(reconstructed) the very same way in the hologram. The hologram appears to be a window behind which the object is sitting, but the object is not there.

All of the light that was reflected off the object was Reconstructed, rebuilt, duplicated by the hologram.