Accurate measurements of the cross-plane thermal conductivity {\Lambda}_crossof a high-thermal-conductivity thin film on a low-thermal-conductivity({\Lambda}_s) substrate (e.g., {\Lambda}_cross/{\Lambda}_s>20) are challenging,due to the low thermal resistance of the thin film compared to that of thesubstrate. In principle, {\Lambda}_cross could be measured by time-domainthermoreflectance (TDTR), using a high modulation frequency f_h and a largelaser spot size. However, with one TDTR measurement at f_h, the uncertainty ofthe TDTR measurement is usually high due to low sensitivity of TDTR signals to{\Lambda}_cross and high sensitivity to the thickness h_Al of Al transducerdeposited on the sample for TDTR measurements. We observe that in most TDTRmeasurements, the sensitivity to h_Al only depends weakly on the modulationfrequency f. Thus, we performed an additional TDTR measurement at a lowmodulation frequency f_0, such that the sensitivity to h_Al is comparable butthe sensitivity to {\Lambda}cross is near zero. We then analyze the ratio ofthe TDTR signals at f_h to that at f_0, and thus significantly improve theaccuracy of our {\Lambda}cross measurements. As a demonstration of thedual-frequency approach, we measured the cross-plane thermal conductivity of a400-nm-thick nickel-iron alloy film and a 3-{\mu}m-thick Cu film, both with anaccuracy of ~10%. The dual-frequency TDTR approach is useful for future studiesof thin films.
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