Highly sensitive lock-in thermoreflectance temperature measurement using thermochromic liquid crystal

We show that the temperature measurement sensitivity in the lock-in thermoreflectance (LITR) can be improved by incorporating a thermochromic liquid crystal (TLC) into a transducer. The quantitatively estimated thermoreflectance coefficient of a TLC/Pt hybrid film depends on the excitation frequency and reaches >2 x 10(-2) K-1 at low excitation frequencies, which is two orders of magnitude greater than typical values of 10(-4) for metallic films. Using the TLC/Pt film, we detected the temperature changes due to Joule heating and the spin Peltier effect with the temperature resolution of similar to 10 mu K by the LITR method. We also performed the same measurements for an Au film and found that the temperature resolution for the TLC/Pt film is increased by a factor of >10 compared with that for the Au film despite the low reflected light intensity of the TLC/Pt film.