for an unfocused CW laser beam (e.g. 20 μm diameter in air). The temperature measured from the optical spectroscopy is a time average of the temperature from the rising of the temperature to partially cooled down. Therefore, the temperature measured from the optical measurements would not catch the maximum temperature that the Au and TiO2 reached. To check this hypothesis, spectra were taken every 25 ms from when the laser was on. The Eg mode shifts (Figure ) from 387 cm -1 at 75 ms to 410 cm -1 at 4150 ms, representing the decay of the temperature from about 1150 ºC to 715 ºC (Figure ). Note that 25 ms is the smallest time interval for our spectrometer. A pump and probe experiment should be able to catch the transient temperature. Our results show that most optical methods measure the time-averaged temperature and miss the transient temperature (Tglobal) due to the collective photothermal heating even under CW laser irradiation. Similar temperature inconsistency has been encountered in laser-based additive manufacturing. Recently, a high-speed (100 kHz) coaxial camera imaging setup has been developed to resolve the time and spatial evolution of the laser melting and melt pool cooling. The time scale of the metal powder reaching maximum temperature was about 50 μs, much less than the integration time in the Raman spectrometry.