Prior to the TDTR measurement, a thin layer of Al optical transducer (≈90 nm) was deposited (via magnetron sputtering) onto the sample. The water contents in the sample after Al coating were expected to be negligible, due to the long-time pumping in vacuum before deposition (90 °C baking for 2 h at 1× 10^-4 Torr, followed by pumping at