Photothermal assembly (PTA) is a technique to assemble microscale and nanoscale dispersoids in a suspension. When a solid-liquid interface of a light-absorbing substrate and a suspension liquid is irradiated with a laser, dispersoids are carried toward vicinity of the laser irradiation spot by light-induced convection under the local heating. The transported dispersoids can be accumulated between the micro bubble and substrate. Here, in order to generate light-induced convection and bubble efficiently, we developed a bowl-shaped plasmonic substrate (BPS) which exhibits characteristic optical response in a facile and inexpensive manner based on the self-assembling. Remarkably, we have succeeded in PTA with our developed BPS at a considerably low laser power on the order of several milliwatt (6.1 mW). The obtained characteristic optical response of BPS will open the way to a potential application as a compact and portable sensor for harmful particles and biological targets.