Nonlinear optical methods to determine the transverse relaxation constants of excitonic molecules making use of two-photon echo and a new type of excited state photon echo are proposed. The two-photon echo is a seventh order nonlinear effect at the lowest and one needs three pulses to observe it, while the excited state photon echo is a fifth order nonlinear effect and requires four pulses. In these nonlinear optical methods we can satisfy the phase matching condition rather easily due to the large dispersion of excitonic polaritons. We can determine directly the transverse relaxation constants associated with the excitonic molecule from the correlation trace of the two-photon echo and the excited state photon echo.
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