Steady‐State and Time‐Resolved Optical Properties of Multilayer Film of Titanium Dioxide Sandwiched by Gold Nanoparticles and Gold Thin Film

We proposed metal‐insulator (MI) and metal‐insulator‐metal (MIM) structures of titanium dioxide (TiO 2 ) sandwiched by gold nanoparticles (AuNPs) layer and gold sputtered thin film (only for the MIM film) to couple localized plasmon mode of AuNP with multi‐reflection mode and/or cavity resonator mode of TiO 2 . The optical extinctions of MI and MIM with differing TiO 2 thickness were studied theoretically by finite‐element method simulation and experimentally by optical spectrometry. The extinction peaks of MI and MIM shifted by exchanging the surrounding medium from air to TiO 2 . The interference of TiO 2 in MI structure also affected the extinction spectra showing the oscillation along the spectrum of AuNP in TiO 2 . Then, the extinction degree of MIM was higher than that of MI because of the coupling between cavity resonance mode with localized plasmon mode and interband transition in AuNPs. In addition, the cross section of MI and MIM films were observed by scanning electron microscopy. The surface of thinner film was rough because TiO 2 heterogeneously grew from AuNP. The irregular growth of TiO 2 might have induced the wide‐range extinction in 300–2500 nm after Au thin film deposition. The transient absorption spectra using a femtosecond laser were also carried out under the condition of 800 nm for excitation laser and 950 nm for probe laser. The long‐lived electron (∼1 ns) was observed in thick MIM film as a result of hot electron transfer from the gold nanostructure in the film.