Ultrafast gain and refractive index dynamics of semiconductor amplifiers measured by four-wave mixing

Measurements of four-wave mixing in traveling wave semiconductor optical amplifiers are extended up to frequency detunings between pump and probe as high as 4.3 THz. The 35 fs equivalent time resolution is, to our knowledge, the highest achieved to date. Experimental evidence of nonlinear processes faster than spectral hole burning is obtained for the first time by using a frequency domain technique.