Direct observation of the transient thermal-lensing effect using the phase-object Z-scan technique

A phase-object (PO) Z-scan technique, which can conveniently distinguish the transient thermal-lensing effect from the third-order nonlinear refraction, is described. The PO Z-scan trace of the transient thermal-lensing effect shows a valley-peak shape that is very different from the single-valley (peak) shape of the pure negative (positive) third-order nonlinearity. The nonlinear refraction of the C_70/toluene solution is investigated by using the PO Z scan with 8 ns pulses at 532 nm wavelength as a test.