  Semiconductor lasers are strongly altered by adding spin-polarized carriers.
Such spin lasers could overcome many limitations of their conventional
(spin-unpolarized) counterparts. While the vast majority of experiments in spin
lasers employed zinc-blende semiconductors, the room temperature electrical
manipulation was first demonstrated in wurtzite GaN-based lasers. However, the
underlying theoretical description of wurtzite spin lasers is still missing. To
address this situation, focusing on (In,Ga)N-based wurtzite quantum wells, we
develop a theoretical framework in which the calculated microscopic
spin-dependent gain is combined with a simple rate equation model. A small
spin-orbit coupling in these wurtzites supports simultaneous spin polarizations
of electrons and holes, providing unexplored opportunities to control spin
lasers. For example, the gain asymmetry, as one of the key figures of merit
related to spin amplification, can change the sign by simply increasing the
carrier density. The lasing threshold reduction has a nonmonotonic depenedence
on electron spin polarization, even for a nonvanishing hole spin polarization.
