  The possibility of steady-state population inversion in a small sample of
strongly driven two-level emitters like quantum dots (QDs) in micro-cavities,
and its utilization towards amplification of light and acoustic waves is
investigated theoretically. We find that inversion and absorption spectrum of
photons, and phonons crucially depend on the interplay between the intrinsic
vacuum and phonon environments. The absorption profiles of photons and phonons
show marked novel features like gain instead of transparency and absorption
reversed to gain, respectively. Furthermore, we report collectivity induced
substantial enhancement of inversion and pronounced gain in the photon, and
phonon absorption spectrum for a wavelength size QD ensemble.
