Abstract

Large amplification of 1-μm ps pulses is achieved by two-beam mixing in GaAs. Here two 28-ps (FWHM) pulses are spatially and temporally coincident in an undoped semi-insulating sample of GaAs with an angle 2θ between the two beams. The energy transferred between the two beams is measured as a function of fluence, crystal orientation, pump–probe ratio, and time delay between pump and probe. The two-photon absorption of the two interferring pulses produces a spatially periodic distribution of electron-hole pairs. The sign of the Drude-Lorentz index change associated with these free carriers is negative, and the magnitude is proportional to the carrier density. Thus this tree-carrier index grating is spatially shifted by one-half a period (or π) with respect to the incident irradiance pattern. It is well known that such an index grating cannot transfer energy in two-beam coupling experiments in steady state or for equal beams. During picosecond two-beam mixing experiments, however, significant energy transfer is expected because of the highly transient nature of the grating formation. This process is called transient energy transfer, and for fluences larger than ~5 mJ/cm², it is observed to dominate the twobeam mixing process. At fluences of 25 mJ/cm², weak beam gains >10 are measured at room temperature. Similar gains were recently reported in Si1 at energy densities of ~100 mJ/cm².

© 1987 Optical Society of America