Hexagonal plasmonic necklaces of silver nanoparticles organized in 2D superlattices on functional ferroelectric templates are fabricated in large-scale spatial regions by using a surfactant-free photo-deposition process. The plasmonic necklaces support broad radiative plasmonic resonances allowing the enhancement of second harmonic generation (SHG) at the ferroelectric domain boundaries. A 400-fold SHG enhancement is achieved at the near-UV spectral region with subsequent interest for technological applications.

develop novel nanophotonic devices with innovative functions for light generation and control. Indeed, the development of coherent light sources with sub-wavelength confined modes is a flourishing area that can yield a revolution in several fields, including physics, chemistry, materials science, biology, and/or imaging and information technologies. [13][14][15][16] In this review, we discuss the progress on the fabrication and performance features of light sources operating at the nanoscale, which can be attained by the interaction of localized surface plasmons (LSP) with optical gain dielectric media. The optical gain is provided by functional ferroelectric platforms (either by stimulated emission or by nonlinear (NL) frequency conversion processes), which in turn are used as templates for the deposition of metallic arrangements. Two types of sources with sub-diffraction spatial confinement are presented: plasmon-assisted solid-state nanolasers, based on the interaction between metallic nanostructures and optically active rare earth (RE 3+ ) doped ferroelectric crystals, and NL radiation sources based on quadratic frequency mixing mechanisms, that are enhanced by means of LSP resonances.Ferroelectrics are nowadays strong actors in the area of light generation and control. In the context of this report, the relevance of employing ferroelectric crystals as optical functional platforms is multiple. On one hand, the presence of a reversible spontaneous polarization in the ferroelectric phase allows to engineer ferroelectric domain patterns with different photonic functionalities. [17][18][19][20] On the other hand, the noncentrosymmetric crystal structure of ferroelectrics enables the generation of quadratic frequency conversion processes, which makes these systems useful as frequency doublers and optical parametric oscillators. [21,22] Further, the possibility of some ferroelectric crystals to host luminescent trivalent RE 3+ ions, from which laser action can be achieved, is an additional advantage exploited in the context of this work. [23] In fact, some of the materials that will be the subject of discussion in this report constitute true solid-state lasers due to the incorporation of optically active ions such as Nd 3+ or Yb 3+ during the crystal growth. Finally, the presence of surface charges on ferroelectrics is a key factor that allows ferroelectric surfaces to act as templates to assemble different type of nanostructures Coherent light sources providing sub-wavelength confined modes are in ever more demand to face new challenges in a variety of disciplines. Scalability and cost-effective production of these systems are also highly desired. The use of ferroelectrics in functional optical platforms, on which plasmonic arrangements can be formed, is revealed as a simple and powerful method to develop coherent light sources with improved and novel functionalities at the nanoscale. Two types of sources with subdiffraction spatial confinement and improved performances are presented: i) plasmon-assisted solid-sta...

The nonlinear properties of hybrid metallic-dielectric systems are attracting great interest due to their potential for the enhancement of frequency conversion processes at nanoscale dimensions. In this work, we theoretically and experimentally address the correlation between the near field distribution of hexagonal plasmonic necklaces of silver nanoparticles formed on the surface of a LiNbO crystal and the second harmonic generation (SHG) produced by this nonlinear crystal in the vicinities of the necklaces. The spectral response of the hexagonal necklaces does not depend on the polarization direction and is characterized by two main modes, the absorptive high-energy mode located in the UV spectral region and the lower energy mode, which is strongly radiant and extends from the visible to the near infrared region. We show that the spatial distribution of the enhanced SHG is consistent with the local field related to the low energy plasmon mode, which spectrally overlaps the fundamental beam. The results are in agreement with the low absorption losses of this mode and the two-photon character of the nonlinear process and provide deeper insight in the connection between the linear and nonlinear optical properties of the hybrid plasmonic-ferroelectric system. The study also highlights the potential of hexagonal necklaces as useful plasmonic platforms for enhanced optical processes at the nanoscale.

Plasmonic structures have been revealed as efficient units to enhance localized nonlinear phenomena generated at dielectric–metal interfaces. However, their effect on the nonlinear interactions provided by quasi-phase matching processes in χ(2) modulated dielectric crystals have been scarcely addressed, mainly due to the complexity in manufacturing appropriate periodic plasmonic structures overlying the χ(2) dielectric structure. Here, by a simple method we have fabricated a periodic structure based on the combination of two commensurate lattices: a periodic lattice of chains of Ag nanoparticles and a periodic lattice of χ(2)-modulation based on a ferroelectric domains structure. The hybrid system supports multiple surface plasmon lattice resonances (SLRs) at the technologically relevant NIR spectral region, which yield the enhancement of the nonlinear diffraction pattern generated by the χ(2) structure. The superposition of the plasmonic and the χ(2)-modulation lattice results in a 20-fold enhancement of the directional SHG due to the excitation of SLRs by the interacting waves involved in the nonlinear process. The results are obtained in lithium niobate, a widely used crystal in optoelectronics, and demonstrate the potential of the approach to design integrated solid-state platforms for on-chip optical steering, multiplexing or quantum technologies.

The association of plasmonic nanostructures with nonlinear dielectric systems has been shown to provide useful platforms for boosting frequency conversion processes at metal-dielectric interfaces. Here, we report on an efficient route for engineering light–matter interaction processes in hybrid plasmonic-χ(2) dielectric systems to enhance second harmonic generation (SHG) processes confined in small spatial regions. By means of ferroelectric lithography, we have fabricated scalable micrometric arrangements of interacting silver nanoparticles compactly distributed on hexagonal regions. The fabricated polygonal microstructures support both localized and extended plasmonic modes, providing large spatial regions of field enhancement at the optical frequencies involved in the SHG process. We experimentally demonstrate that the resonant excitation of the plasmonic modes supported by the Ag nanoparticle-filled hexagons in the near infrared region produces an extraordinary 104-fold enhancement of the blue second harmonic intensity generated in the surface of a LiNbO3 crystal. The results open new perspectives for the design of efficient hybrid plasmonic frequency converters in miniaturized devices.