Tunable-laser-induced grating dip for measuring sub-picosecond relaxation

This paper describes the experimental verification of a laser-induced double-dip resonance predicted theoretically for two-wave saturation spectroscopy. Fine structure in the form of a central dip with a width (FWHM) near 20 cm⁻¹ is found in the broad hole-burning feature induced by 942 cm⁻¹ wavelength radiation inp-type germanium. The central dip is ascribed to Bragg scattering caused by, and thus measuring the decay of, the excited-state population grating set up by interference of the saturating and the probing waves. From the composite spectrum, therefore, apopulation difference lifetimeT ₁=0.6 ps can be directly obtained as the inverse width of the central dip. Likewise the broad background feature yields the homogeneous ordipole lifetimeT ₂=0.1 ps.