Mode-matching in multiresonant plasmonic nanoantennas for enhanced second harmonic generation (1412.0698v2)

Abstract: Boosting nonlinear frequency conversion in extremely confined volumes remains a key challenge in nano-optics, nanomedicine, photocatalysis, and background-free biosensing. To this aim, field enhancements in plasmonic nanostructures are often exploited to effectively compensate for the lack of phase-matching at the nanoscale. Second harmonic generation (SHG) is, however, strongly quenched by the high degree of symmetry in plasmonic materials at the atomic scale and in nanoantenna designs. Here, we devise a plasmonic nanoantenna lacking axial symmetry, which exhibits spatial and frequency mode overlap at both the excitation and the SHG wavelengths. The effective combination of these features in a single device allows obtaining unprecedented SHG conversion efficiency. Our results shed new light on the optimization of SHG at the nanoscale, paving the way to new classes of nanoscale coherent light sources and molecular sensing devices based on nonlinear plasmonic platforms.

• The paper demonstrates that doubly-resonant, asymmetrically designed plasmonic nanoantennas significantly boost second harmonic generation efficiency.
• The study employs FDTD simulations to optimize V-shaped nanoantenna and nanorod geometries for precise spatial and frequency mode overlap.
• The enhanced SHG performance highlights potential applications in label-free biomedical imaging and coherent control in nonlinear photonics.

Mode-Matching in Multiresonant Plasmonic Nanoantennas for Enhanced Second Harmonic Generation

This paper addresses the challenge of enhancing nonlinear frequency conversion processes such as second harmonic generation (SHG) within highly confined nanoscale environments. The work takes advantage of plasmonic nanoantennas, particularly those lacking axial symmetry, to achieve substantial improvements in SHG efficiency. The achievement of spatial and frequency mode overlap at both excitation and SHG wavelengths constitutes the focal point of the research.

The researchers introduce a nanostructured device leveraging the plasmonic properties of gold nanoantennas that exhibits multiple narrowly spaced resonances. These resonances occur simultaneously at the excitation wavelength and at the wavelength of the second harmonic, thereby enhancing SHG via what is termed a doubly-resonant regime. This mode matching is crucial to effective nonlinear frequency conversion at the nanoscale, emulating phase-matching conditions known from bulk materials.

The nanoantennas are meticulously engineered using Finite Difference Time Domain (FDTD) simulations to ensure optimized geometrical parameters and plasmonic behavior. Detailed attention is given to the spatial overlap of localized fields at the key operative wavelengths and the strategic design to favor dipole-allowed SHG emissions. The paper underscores the significance of multipolar hierarchical light-matter interactions and engineering structures that facilitate electric-dipole-allowed transitions, which are crucial due to their higher transition rates compared to magnetic multipolar transitions.

The designed structure comprises a V-shaped nanoantenna coupled with a nanorod, optimized to exhibit both multifunctional plasmonic resonances and spatial mode overlap. This configuration enhances absorption at the excitation frequency while bolstering emission at the SH frequency, maintaining the SHG process's efficiency. The outcome is an SHG rate superior to those achieved with prior designs, largely due to the disjoint interplay of resonances and symmetry-breaking geometric configurations.

From a practical perspective, the device operates in the near-infrared (NIR) region, minimizing absorption by gold and the biological constituents at these wavelengths, with potential applicability to label-free biomedical imaging. It manifests high coherence emission properties desirable for coherent control applications in light-matter interactions and paves the way for enhanced performance in second-order nonlinear processes like parametric down-conversion and difference frequency generation.

The empirical results, substantiated by rigorous simulations, signify a substantial advancement in SHG efficiency facilitated by the outlined methodology. Specifically, the SHG efficiency presented displays superiority over previous plasmonic structures under comparable conditions, credited to the strategic combination of factors: non-symmetrical arrangement, multiresonance response, and excellent spatial mode overlap.

In conclusion, this research streamlines the path for creating novel classes of nanoscale devices implementing coherent nonlinear light sources. The developed plasmonic nanoantenna configuration presents a benchmark in the pursuit of advanced nanoscale photonic applications and provides foundational insights with theoretical and applied implications for future developments in quantum optics and nonlinear plasmonics. Furthermore, the work provides a framework by which similar methodologies could enhance other nonlinear photonic processes, potentially leading to innovations in nanoscale devices within quantum optics and beyond.