Homodyne detection technique using spontaneously generated reference signal in picosecond thermoreflectance measurements

Abstract

A new detection technique for picosecond thermoreflectance measurements has been developed. Conventional picosecond thermoreflectance measurements detect the signal amplitude of a reflected probe laser beam synchronized with the modulation frequency of the pump laser beam using a lock-in amplifier. Our new detection technique uses the signal phase of the lock-in output instead of the signal amplitude. Thermoreflectance signals for molybdenum thin films deposited on glass substrates were observed at room temperature. The signal-to-noise ratio of the phase detection was about four times better than that of the amplitude detection. The signal phase detected by this model is insensitive to fluctuation of both pump and probe beams. According to this technique, picosecond thermoreflectance measurements can apply not only to single layer metal thin films of around 100 nm thickness, but also to thicker metal films, metal–nonmetal multilayer thin films, and many other thin films.