UC Berkeley

Light Propagation and Imaging in Indefinite Metamaterials

by

Jie Yao

Doctor of Philosophy in Applied Science & Technology

University of California, Berkeley

Professor Xiang Zhang, Chair

Unlike any traditional materials, indefinite materials possess very unique optical properties. The principal components of its permittivity and/or permeability tensors are not all with the same signs, resulting in a hyperbolic dispersion relation. Such extraordinary dispersion leads to a number of interesting phenomena, such as negative refraction of light at its interface with normal materials, the propagation of electromagnetic waves with very large wave vectors which are evanescent in natural dielectric materials.

Due to the development of metamaterial research and nanofabrication techniques, we have successfully created a type of metamaterial made of parallel silver nanowires embedded in porous alumina matrix. Both effective media theory and finite element simulations showed that such metamaterial possesses indefinite permittivities. We performed optical measurements of visible and infrared light transmitted through such metamaterials. By using an NSOM system capable of mapping the light field distribution with sub-wavelength resolution at both the surface of the nanowire metamaterial and the 3D space near the surface, we have demonstrated all-angle negative refraction and slab lens imaging of visible light in nanowire metamaterials.

The nanowire metamaterials hold the indefinite permittivity over a broad frequency range from visible to infrared, therefore negative refraction phenomena are not restricted in a narrow band any more. By avoiding magnetic resonance, the nanowire metamaterials show substantially reduced energy loss and make negative refraction possible in bulk materials.

Since the indefinite materials support the propagation of very large k waves, deep sub-wavelength 3D nanocavities can be made from them. Theoretically there is no lower limit to the size of the cavity with idea indefinite materials. Unlike traditional dielectric cavities which resonate at higher frequencies when the cavity size reduces, indefinite cavities with sizes several orders different may resonate at the same frequency and same mode order. Furthermore, the size dependence of quality factor due to the radiation loss also shows an inversed behavior compared to traditional dielectric cavities.

Another interesting phenomenon we have discovered is the abnormal diffraction inside the nanowire metamaterials. The hyperbolic iso-frequency contours of such materials are "flatter" for light with longer wavelengths, because of the strong dispersive behavior of the metal in the metamterials. Since the Poynting vector is normal to the iso-frequency contour, the diverging angle of diffracted light inside the nanowire metamterial decreases as the wavelength gets longer, which is the opposite to the diffraction in dielectrics. The flat iso-frequency contour together with the large k wave transmission may lead to the possibility of deep sub-wavelength imaging or lithography with the nanowire metamaterials.

Besides the research on indefinite metamaterials, I also studied the "2D optics" of surface plasmons. Based on the interference of surface plasmons, sub-wavelength patterns can be lithographically created on a metal surface. We also predicted the existence of surface plasmon beats formed by the interference of two different modes excited on the same thin metal film.