Heterodyne detection of the complete electric field of femtosecond four-wave mixing signals

A novel optimized heterodyne method that recovers the complete electric field of any four-wave mixing signal at its point of origin is demonstrated. A tracer pulse is sent along the signal path and characterized at the sample by frequency-resolved optical gating. Spectral interferometry is used to determine the phase difference between the tracer and a reference pulse, the absorptive change in tracer phase in the sample, and the reference-signal phase difference. Together, these measurements allow calculation of the signal phase at the sample. The phase of three pulse-scattering signals from solutions of the infrared dye IR 144 in methanol determines the absolute signal-emission time within 0.5 fs.