Abstract: Femtosecond spectroscopy such as two color pump-probe and four-wave mixing (FWM) on GaAs quantum wells has been performed using two partially synchronized, independently tunable lasers with external jitter compensation. In the two beam and three beam (approximately ega$-1$/, approximately ega$-1$/; approximately ega$-2$/) four wave mixing experiments, heavy-and-light-hold beatings are observed with these two mutually incoherent lasers. FWM signals are observed when approximately ega$-2$/ is completely below the exciton energies, with no spectral overlap with the absorption profile. These off-resonant signals are stronger than the interband continuum signals for equivalent detunings. Femtosecond frequency mixing (2 approximately ega$-1$/- approximately ega$-2$/) is also demonstrated when the grating is formed by two different frequencies.!19
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