Transient-grating frequency-resolved optical gating.

We introduce a transient-grating beam geometry for frequency-resolved optical-gating measurements of ultrashort laser pulses and show that it offers significant advantages over currently used geometries. Background free and phase matched over a long interaction length, it is the most sensitive third-order pulse-measurement geometry. In addition, for pulses greater than ~300 fs in length and ~1 microJ in energy, the nonlinear medium can be removed and the nonlinearity of air can be used to measure the pulse.