Structural Correlations of the Nonlinear Optical Response in Polydiacetylene Nanotubes Hybridized with Gold Nanoparticles

The nonlinear optical (NLO) properties of π-conjugated polydiacetylene (PDA) nanotubes (NTs) were enhanced by the localized surface plasmon resonance (LSPR) effect from gold (Au) nanoparticles (NPs). Three types of PDA NTs hybridized with Au NPs have been fabricated successfully, i.e., Au NPs deposited on the outer surface (outer type), on both outer and inner surfaces (outer/inner type), and on the inner surface (inner type). Third-order NLO susceptibility, χ⁽³⁾(ω), has been evaluated by transient transmission spectroscopy and spectroscopic ellipsometry, and then structural correlations of these optical data were discussed in detail. χ⁽³⁾(ω) was compared by newly proposing a figure of merit (FOM), which may reflect the generation efficiency for χ⁽³⁾(ω) based on the intensity of the excitonic absorption peak in PDA NTs because the NLO properties of PDA were originated from excitons. As a result, the outer/inner type has numerically provided the maximum of FOM by a factor of about 5 compared with original PDA NTs. The gap mode effect in a metal-insulator-metal-like nanostructure as well as the LSPR effect would be responsible for the increase in the FOM of the outer/inner type. Consequently, hybridized PDA NTs would open a new field toward all-optical and photonic device applications.