ABSORPTION-SPECTRUM AND GAIN WITHOUT INVERSION OF A DRIVEN 2-LEVEL ATOM WITH ARBITRARY PROBE INTENSITY IN A SQUEEZED VACUUM

We examine the response of a coherently driven two-level atom interact ing with a broadband squeezed vacuum to a probe beam of arbitrary inte nsity. The equations of motion for the reduced density matrix elements are numerically solved by nonperturbative matrix inversion methods. F or large driving intensities, the absorption spectrum exhibits a numbe r of multiphoton absorption peaks with increasing probe strength in a standard vacuum and in a squeezed vacuum with small photon numbers, wh ile the multiphoton absorption peaks are wiped out for large squeezed photon numbers. Strong gain without inversion at line center can be ob tained in the presence of the squeezed vacuum when the probe strength does not exceed the driving one. The absorption spectrum is phase sens itive. We also find the presence of remarkable, very narrow structures at line center. in general, the spectrum is symmetric when Phi=0 and pi; otherwise, it is asymmetric and displays a dispersivelike profile at line center.