Optical nonlinearity originating in photothermal effects in guest-host liquid crystals (GHLC's), which caused self-phase modulation in the GHLC's, was characterized by use of heat-conduction analysis. The refractive-index distributions that are due to the photothermal optical nonlinearity in the GHLC's were determined by two different methods. One method (approach 1) is analytical solution of Kirchhoff's diffraction integral including the higher-order nonlinear phase distribution. Another method (approach 2) is to start by analytical solution of a heat-conducting equation. The refractive-index distributions determined from approach 1 were in good agreement with those from approach 2, and the physical origin of the higher-order optical nonlinearity caused by the photothermal effects in the GHLC's is discussed.
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