Stark Effect Control of the Scattering Properties of Plasmonic Nanogaps Containing an Organic Semiconductor

The development of actively tunable plas-monic nanostructures enables real-time reconfigurable and on demand enhancement of optical signals. This is an essential requirement for a wide range of applications such as sensing and nanophotonic devices, for which electrically driven tunability is required. By modifying the transition energies of a material via the application of an electric field, the Stark effect offers a reliable and practical approach to achieve such tunability. In this work, we report on the use of the Stark effect to control the scattering response of a plasmonic nanogap formed between a silver nanoparticle and an extended silver film separated by a thin layer of the organic semiconductor PQT-12. The plasmonic response of such nano-scattering sources follows the quadratic stark shift. Additionally, our approach allows to experimentally determine the polarizability of the semiconductor material embedded in the nanogap region, offering a new approach to probe the excitonic properties of extremely thin semi-conducting materials such as 2D materials under applied external electric field with nanoscale resolution

Thermoplasmonic Effects in Gain-Assisted Nanoparticle Solutions

We report a detailed characterization of the photoinduced heating observed in gain-assisted solutions of gold nanoparticles (AuNPs). AuNPs, with sizes ranging from 14 to 48 nm and concentration of 2.5 × 10^–10 M, are exposed to different intensity values of a resonant continuous laser (532 nm), used to excite their localized surface plasmon resonance (LSPR), responsible for the photogeneration process. In this way the optimal conditions to achieve the maximum temperature variation with the least laser dosage are obtained. By addition of an organic dye to the solutions, whose emission band overlaps to the LSPR, we found that the contribution to the photothermal efficiency is enhanced if the solutions are excited at 405 nm. This happens in the case of smaller NPs, due to a strong coupling effect between the two subunits, which causes an increase of the extinction cross section of the whole gain-assisted system. On the other hand, for the larger AuNPs, an opposite behavior is found: a loss compensation mechanism, based on a resonant energy transfer process from gain units to plasmonic nanoparticles, limits the increase of the absorption cross section with a consequent lowering of the photothermal efficiency. The presented quantitative analysis of a dispersion of AuNPs results as fundamental for biomedical applications as well as for integrated plasmonic devices based on loss compensation effects, where the impact of undesirable thermal effects cannot be ignored