  Laser cooling of semiconductors has been an elusive goal for many years, and
while attempts to cool the narrow gap semiconductors such as GaAs are yet to
succeed, recently, net cooling has been attained in a wider gap CdS. This
raises the question of whether wider gap semiconductors with higher phonon
energies and stronger electron-phonon coupling are better suitable for laser
cooling. In this work we develop a straightforward theory of phonon-assisted
absorption and photoluminescence of semiconductors that involves more than one
phonon and use to examine wide gap materials, such as GaN and CdS and compare
them with GaAs. The results indicate that while strong electron-phonon coupling
in both GaN and CdS definitely improves the prospects of laser cooling, large
phonon energy in GaN may be a limitation, which makes CdS a better prospect for
laser cooling.
