Ratio of photo-acoustic responses P
(black squares) and P
(green circles) as a function of laser modulation frequency. Pc, Psio2, and Ppsi represent the photo-acoustic response of the local plasmon resonator with and without a porous column layer and that of nanoPS + Cu, respectively. The red and blue dashed lines show the calculation result for Pc/Psio2 and Ppsi/Psio2, respectively.