In this study, the application of thermal lens spectroscopy to detection of analytes in agarose gels was examined. It was found that a long-term refractive index gradient is produced in the sample, in addition to the thermally induced refractive index gradient, and gives rise to a long-term thermal lens signal. It is argued that the most likely source of the long-term signal is a concentration gradient formed in the sample. The long-term refractive index gradient results in a focusing lens, and is thus opposite to the thermal gradient. The formation of this gradient requires as much as 500 s to reach equilibrium during constant irradiation. The existence of the long-term refractive index gradient is verified by time-resolved Z-scan experiments. The signal resulting from this gradient is termed the long-term thermal lens and is shown to depend on temperature, is observed only for gelled agarose solutions, and is sensitive to anisotropies in the sample.

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