Advanced hybrid plasmonic nano-emitters using smart photopolymer

With the rapid development of integrated optics, nanophotonics, quantum optics, etc., the development of nano-light sources with small size, low cost, and stable and controllable performance has become an urgent problem. Hybrid plasmonic nano-emitters as optical nanosources into photonic nanodevices are of interest for research and technological innovation due to their miniaturization and multi-applications. However, how to integrate the emitters near metallic nanostructures with spatial control and nanometer precision in the three space dimensions remains a challenge.

 

In the early stage, a large number of emitters such as quantum dots, fluorescent dyes, etc. are dispersed randomly in the vicinity of plasmonic nanostructures, without precise control of their relative positions. In recent years, with the development of DNA technology, the use of DNA origami to link nano-emitters and plasmonic nanostructures and control their relative distances has become more and more popular. However, DNA-based hybrid nanosystem are pretty fragile in the sense that, for the survival of DNA origami, one needs to be in a salty liquid environment, which limits the types of available metallic nanoparticles, and requires complicated steps.

 

Near-field plasmonic photopolymerization has proven to be an effective technique to trap light-emitting quantum dots and molecules inside polymer volumes that are integrated at electromagnetic 'hot-spots'. The anisotropic distribution of emitters can be controlled by choosing the plasmonic mode used for nano photopolymerization. However, since the emitters are initially randomly distributed inside the photopolymerizable formulation, the spatial distribution of the emitters is still not precise enough.

 

To address this problem, the group led by Professor Renaud Bachelot from the L2N Laboratory of Universite de Technologie de Troyes developed a method based on "smart" nanopolymers on the basis of their previous near-field plasmonic photopolymerization technology. The new method can successfully solve the technical problems such as the imprecise spatial distribution of the emitter. The relevant research results were published in Photonics Research, Volume. 10, Issue. 7, 2022 (Dandan Ge, Ali Issa, Safi Jradi, Christophe Couteau, Sylvie Marguet, Renaud Bachelot. Advanced hybrid plasmonic nano-emitters using smart photopolymer[J]. Photonics Research, 2022, 10(7): 1522).

 

The smart nature of the polymer is twofold. First, it is a photopolymer that reticulates at the plasmonic hot spot of the metal nanoparticle, allowing one to keep the memory of the selected electromagnetic sites. This "memory" is spatially anisotropic and also decides the distance between the plasmonic nanostructure and the future nano-emitter to be attached. Secondly, it is chemically pre-functionalized to electrostatically "recognize" the nano-emitter that can get selectively attached to the pre-designed sites. The average distance between the metal nanostructure and the emitter to be attached is then determined by the thickness of the smart polymer.

 

In this letter, based on this smart polymer, the hybrid plasmonic nanoemitters composed of quantum dots or polystyrene fluorescent dye spheres and gold nanocubes were studied respectively, as shown in Fig.1. By controlling the excitation dose for the two-photon nanopolymerization, the thickness of the polymer is controlled and thus the corresponding average distance between the emitter and the gold nanocube is controlled, and ultimately the average fluorescence lifetime of the fluorescent spheres/quantum dots attached to the polymer is controlled accordingly.

 

Figure 1 Schematic of the hybrid plasmonic nanoemitters based on gold nanocube using smart polymer

 

"This method retains the advantages of near-field plasmonic photopolymerization," said by Dr. Ge, "the quantum dots/fluorescent spheres attached to smart polymers have anisotropic spatial distribution. The fluorescence emission intensity of the hybrid structure can be regulated by rotating the polarization direction of the excitation light.

 

In particular, it is possible to control the average distance between the metal nanostructure and the emitter to be attached by controlling the thickness of polymer. Finally, the surface attachment method is likely to avoid bad influence from the laser during polymerization, which may damage the emitters or introduce other effects such as light force, two-photon absorption, etc, and also makes the choice of nano-emitters more diverse."

 

This approach will be used for fabricating single-photon hybrid nanosources and multi-color single-photon nanosources by precisely integrating different kinds of QDs through a multistep process. And it will also open new avenues for advanced integrated nanosources based on weak and strong coupling, such as nanolasers, nonlinear plasmonic nanodevices, etc.