Person:

Yeung, Yan Mui Kitty

Metallic optical systems can confine light to deep subwavelength dimensions, but verifying the level of confinement at these length scales typically requires specialized techniques and equipment for probing the near field of the structure. We experimentally measured the confinement of a metal-based optical cavity by using the cavity modes themselves as a sensitive probe of the cavity characteristics. By perturbing the cavity modes with conformal dielectric layers of subnanometer thickness using atomic layer deposition, we find the exponential decay length of the modes to be less than 5% of the free-space wavelength ((\lambda)) and the mode volume to be of order (\lambda^3/1000). These results provide experimental confirmation of the deep subwavelength confinement capabilities of metal-based optical cavities.

Plasmonic waves are waves of mobile charge carriers caused by their collective oscillations. They can be excited in solid-state conducting materials and behave distinctively in different numbers of dimensions. With fabrication technologies available for solid-state materials, one can functionalize the dimensional properties by engineering the boundaries and interfaces of the plasmonic wave medium. For instance, plasmonic waves in two-dimensional (2D) conductors, such as semiconductor heterojunction and graphene, exhibit strong subwavelength confinement – with a wavelength about a factor of 100 below the electromagnetic wavelength at the same frequency. Hence, 2D plasmonic devices can be constructed below the diffraction limit of light. To utilize this ultra-subwavelength confinement is the main motivation of this thesis. This thesis establishes the machinery behind the unique behaviors of 2D plasmons, and compares them to plasmons in higher dimensions, namely plasma oscillations in bulk materials and surface plasmons on conducting-insulating interfaces. The Coulomb restoring force and mobile charge carrier inertia causing the collective oscillations are formulated into a transmission-line model. This formulation is used to engineer ultra-subwavelength plasmonic circuits in gigahertz integrated electronics and terahertz metamaterials. As one of the demonstration platforms, we use GaAs/AlGaAs 2D electron gas. Amongst a variety of devices, the thesis focuses on an on-chip solid-state 2D plasmonic Mach-Zehnder interferometer operating at microwave frequencies. The gated 2D plasmonic waves achieve a velocity of ~c/300 (c: free-space speed of light). Due to this ultra-subwavelength confinement, the resolution of the 2D plasmonic interferometer is two orders of magnitude higher than that of its electromagnetic counterpart at a given frequency. Another material we use, which hosts mobile charge carriers in 2D, is graphene. We fabricate metamaterials in the form of graphene plasmonic crystals in a continuous graphene sheet with periodic structural perturbations. Plasmonic bands in the far-infrared are formed and excited via symmetry-based selection rules, in a manner akin to photonic crystals. The plasmonic bands can be engineered by manipulating the charge carrier concentration, the dimensions of the periodic lattice, the shape of the perturbation and the lattice symmetry. These demonstrations may generate new avenues for a wealth of subwavelength graphene plasmonic devices, such as band gap filters, modulators and switches.