External chirality enhancing downconversion in a waveguide-coupled nonlinear plasmonic metasurface.

Metasurfaces, typically constructed from spatial arrangements of localized building blocks, can enhance light-matter interactions through local field enhancement or by coherent coupling to extended photonic modes. Recent works have explored how guided mode resonances influence the performance of nonlinear metasurfaces. Here we investigate the modal impact on difference-frequency generation in a waveguide-coupled metasurface platform. The system is constructed from gold split-ring resonators on a high-index TiO2 waveguide. We find that a symmetric configuration of the metasurface's localized modes and the extended waveguide modes lead to a modest enhancement of the downconversion process. However, when the mirror symmetry of the localized modes with respect to the guided mode propagation breaks, it introduces external chirality. This enables coupling to a higher quality mode, resulting in a 70-fold enhancement of the difference-frequency generation. The capacity to manipulate the nonlocal modes through the design offers broader control over the interaction and new avenues to tailor the nonlinear processes.

Second-Harmonic Enhancement from a Nonlinear Plasmonic Metasurface Coupled to an Optical Waveguide.

Metasurfaces are commonly constructed from two-dimensional arrangements of nanoresonators. Coherent coupling of the nanoresonators through extended photonic modes of the metasurface results in a modified collective optical response, and enhances light-matter interactions. Here we experimentally demonstrate that strong collective resonances can arise also from coupling the metasurface to an optical waveguide. We explore the effect this waveguide-assisted collective interaction has on second-harmonic generation from the hybrid system. Our measurements indicate an enhancement factor of 8 for the transmitted second harmonic in comparison to incoherent collective scattering. In addition, complementary simulations predict about a 100-fold enhancement for the second harmonic that remains confined inside the waveguide. The ability to control the hybrid modes by the waveguide's design provides broader control over the formation of the collective interaction and new tools to tailor the nonlinear interactions. Our findings pave a promising direction to realize nonlinear photonic circuits with metasurfaces.

Strong light-matter interaction in tungsten disulfide nanotubes.

Transition metal dichalcogenide materials have recently been shown to exhibit a variety of intriguing optical and electronic phenomena. Focusing on the optical properties of semiconducting WS2 nanotubes, we show here that these nanostructures exhibit strong light-matter interaction and form exciton-polaritons. Namely, these nanotubes act as quasi 1-D polaritonic nano-systems and sustain both excitonic features and cavity modes in the visible-near infrared range. This ability to confine light to subwavelength dimensions under ambient conditions is induced by the high refractive index of tungsten disulfide. Using "finite-difference time-domain" (FDTD) simulations we investigate the interactions between the excitons and the cavity mode and their effect on the extinction spectrum of these nanostructures. The results of FDTD simulations agree well with the experimental findings as well as with a phenomenological coupled oscillator model which suggests a high Rabi splitting of ∼280 meV. These findings open up possibilities for developing new concepts in nanotube-based photonic devices.

Size-dependent ligand layer dynamics in semiconductor nanocrystals probed by anisotropy measurements.

Colloidal semiconductor nanocrystals (NC) have reached a high level of synthetic control allowing the tuning of their properties, and their use in various applications. However, the surface of NCs and in particular their size-dependent capping organic ligand behavior, which play an important role in the NC synthesis, dispersibility, and optoelectronic properties, is still not well understood. We study the size-dependent properties of the ligand shell on the surface of NCs, by embedding surface bound dyes as a probe within the ligand shell. The reorientation times for these dyes show a linear dependence on the NC surface curvature indicating size-dependent change in viscosity, which is related to a change in the density of the ligand layer because of the geometry of the surface, a unique feature of NCs. Understanding the properties of the ligand shell will allow rational design of the surface to achieve the desired properties, providing an additional important knob for tuning their functionality.