Liquid-crystal phase transitions of thin layers: A photothermal analysis

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Abstract
A numerical analysis of the photothermal probe-beam signal generated at first- and second-order phase transitions in 4-n-octyl-4’-cyanobiphenyl (8CB) is proposed. A criterion for photothermal signal sensitivity in detection of phase transitions is given. The influences of probe-beam transverse offset and modulation frequency, as well as thermal parameters of the probed medium, on this sensitivity are investigated experimentally and theoretically. It is shown that the photothermal signal may be insensitive to the phase transitions for certain values of the modulation frequency and the probe-beam transverse offset, while a maximum of sensitivity is obtained for a range of values of these parameters. It is also shown that this sensitivity is improved if we use a probed medium with an effusivity smaller than the sample effusivity. A comparison of some of the results of the theory with experimental results is given.