Evaluation of the thermal conductivity of porous silicon layers by an optical pump-probe method

Measurements of the thermal conductivity for free-standing porous silicon l ayers were performed by means of an optical pump-probe experimental set-up. By transient heating due to laser pumping, a refractive index modulation w as induced in the sample and, solving the heat propagation equation for ins ide the solid film, it is shown that the time decay of the nonlinear transm ittance can be related to the thermal conductivity. The optical technique d emonstrated here is contactless, quite simple and does not require much eff ort in data analysis, and is therefore very useful for thin-film characteri zation. The thermal conductivities of the porous silicon samples, whose por osities lay in the range 60-70%, were taken into account, and we found good agreement with results obtained with different techniques.