Polarization-controlled tunable directional coupling of surface plasmon polaritons

Broadband and efficient plasmonic control in the near-infrared and visible via strong interference of surface plasmon polaritons

Broadband and tunable control of surface plasmon polaritons in the near-infrared and visible spectrum is demonstrated theoretically and numerically with a pair of phased nanoslits. We establish, with simulations supported by a coupled wave model, that by dividing the incident power equally between two input channels, the maximum plasmon intensity deliverable to either side of the nanoslit pair is twice that for an isolated slit. For a broadband source, a compact device with nanoslit separation of the order of a tenth of the wavelength is shown to steer nearly all the generated plasmons to one side for the same phase delay, thereby achieving a broadband unidirectional plasmon launcher. The reported effect can be applied to the design of ultra-broadband and efficient tunable plasmonic devices.

Compact aperiodic metallic groove arrays for unidirectional launching of surface plasmons

The ever-increasing power of computers and the development of new optimization methodologies have enabled the ability to design complex aperiodic devices, which can outperform periodic ones and offer new functionalities. Here, we describe the realization of an ultracompact aperiodic grating coupler capable of selectively launching surface plasmon polaritons (SPPs) in a desired direction. We use a transfer matrix model to facilitate the rapid optimization of such structures. We demonstrate both numerically and experimentally that a structure consisting of five subwavelength grooves patterned into silver can unidirectionally launch SPPs in the visible spectral range with a record right-to-left contrast ratio of 55. The general design principles behind this study can readily be extended to a great diversity of sophisticated aperiodic nanophotonic structures.

Plasmonic metal-to-semiconductor switching in Au nanorod-ZnO nanocomposite films

We demonstrate conductivity switching from a metal to semiconductor using plasmonic excitation and charge injection in Au-nanorod (AuNRs)-ZnO nanocomposite films. ZnO films 12.6, 20.3, and 35.6 nm were deposited over AuNRs using atomic layer deposition. In dark conditions, the films transitioned from metallic to semiconducting behavior between 150 and 200 K. However, under sub-bandgap, white light illumination, all films behaved as semiconductors from 80 to 320 K. Photoresponse (light/dark conductivity) was strongly dependent on the thickness of ZnO, which was 94.4 for AuNR-12.6 nm ZnO and negligible for AuNR-35.6 nm ZnO. Conductivity switching and thickness dependence of photoresponse were attributed to plasmonically excited electrons injected from AuNRs into ZnO. Activation energies for conduction were extracted for these processes.

Plasmonic nanogap tilings: light-concentrating surfaces for low-loss photonic integration

Owing to their ability to concentrate light on nanometer scales, plasmonic surface structures are ideally suited for on-chip functionalization with nonlinear or gain materials. However, achieving a high effective quantum yield across a surface requires not only strong light localization but also control over losses. Here, we report on a particular class of tunable low-loss metasurfaces featuring dense arrangements of nanometer-sized focal points on a photonic chip with an underlying waveguide channel. Guided within the plane, the photonic wave evanescently couples to the nanogaps, concentrating light in a lattice of hot-spots. In studying the energy transfer between photonic and plasmonic channels of single trimer molecules and triangular nanogap tilings in dependence on element size, we identify different regimes of operation. We show that the product of field enhancement, propagation length, and element size is close to constant in both the radiative and subwavelength regimes, opening pathways for device designs that combine high-field enhancements with large propagation lengths.

Active unidirectional propagation of surface plasmons at subwavelength slits

Highly efficient, active and compact, unidirectional surface plasmon (SP) propagator composed of double subwavelength slits; filled with organic electro-optic (EO) material is proposed and investigated. By selecting appropriate structure parameters, obtained by solving phase relations between slits, the relative phase of SP generated at the slit exit aperture can be tailored. Simulation results show under normal illumination and external voltage of 8.7 V, SP launching efficiency of 55% and unidirectional SP extinction ratio about 47dB at wavelength of 632.8 nm is achieved. The power consumption of the structure is on the order of 9 fJ/bit which meet the power consumption limitation for optical devices. Moreover, the structure is very compact with effective total length of 1.2 µm and thickness of 0.6 µm.