Theoretical calculations showed that collective heating could increase the temperature by orders of magnitude higher for the plasmonic nanoparticle assemblies than for the single particles. However, the associated temperature increase was often difficult to predict. Whether the temperature was confined to the vicinity of the NPs or delocalized was related to the spatial distribution of the NPs. The time scale of the temperature increases and heat dissipation in thermodynamics was also challenging to estimate because of the nonuniformity of the surroundings. Additionally, the temperature of the NPs was difficult to measure. Most of the temperature microscopy methods utilize optical spectroscopy. Although the melting of the plasmonic materials and the support and the surface evaporation of plasmonic materials have been observed, the experimentally obtained melting temperature was much lower than the high temperature (thousands of Kevin) predicted. Since 2010, plasmonic chemistry has become an attractive approach to enhance the photoreactivity of the catalysts. For example, plasmonic gold NPs supported on titanium dioxide (TiO2) are widely reported to have better photocatalytic performance than pristine TiO2 systems. The primary mechanism of plasmon-assisted photoreactivity has long been connected to hot carriers. Recently, alternative mechanisms, such as photothermal effects and enhanced near field, have been proposed to make a significant contribution. Theoretical studies suggest that the photothermal effect can explain the observed enhanced photocatalytic reactivity. However, little experimental evidence showed the contribution of plasmonic heating because of the difficulty of measuring the temperature and separating plasmonic heating from other mechanisms. Degussa P25, composed of anatase and rutile nanocrystallites, is the golden standard for the TiO2 photocatalyst. Among TiO2 polymorphs, anatase is the metastable phase, whereas rutile is the stable polymorph under ambient conditions. Anatase-rutile phase transition (ART) occurs at a wide temperature range between 400 °C and 1200 °C. Laser-induced phase transition has been reported using femtosecond laser, and nanosecond UV laser . Anatase NPs transformed into an amorphous phase, or rutile phase, depending on the laser and the surrounding matrix. Plasmonic heating enabled the local phase transition of amorphous TiO2 thin film into the anatase phase, observed on polymetric beads coated with Au thin film by a low-power CW laser irradiation. Here, the plasmonic heating alone did not introduce phase transition. The heat was attributed to the plasmonic heating, which was enhanced by the exothermic decomposition of the polymetric beads. In general, the melting of the surrounding media with a melting point as high as ~1800 °C has not been reported from plasmonic heating by a CW laser.