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Open Access Publications from the University of California

Hyperbolic Metamaterials for Super-Resolution Imaging and Deep Sub-Wavelength Cavities

  • Author(s): Rho, Junsuk
  • Advisor(s): Zhang, Xiang
  • et al.

Metamaterials, artificially structured nanomaterials, have enabled unprecedented and extraordinary phenomena such as invisibility cloaking due to their unique optical properties which do not exist in naturally available materials or traditional composites materials.

Especially, hyperbolic metamaterials, also known as indefinite metamaterials, have a unique dispersion relation where the principal components of the permittivity tensors do not all have the same signs and magnitudes. Such an extraordinary dispersion relation results in hyperbolic dispersion relations which leads to a number of interesting phenomena, such as the super-resolution effect, in which evanescent waves are transferred to propagating waves at the interface with normal materials. This results in the propagation of electromagnetic waves with very large wavevectors which would otherwise be evanescent waves and thus decay quickly in natural materials.

Thanks to the development of nanofabrication techniques, I have successfully realized such hyperbolic metamaterials into experimental devices. The first hyperbolic metamaterals device is a super-resolution imaging device called the "hyperlens", which is the first experimental demonstration of near- to far-field imaging using visible light with resolution beyond the diffraction limit in two lateral dimensions. This allows real-time sub-diffractional imaging without any support of image reconstruction and localization. Thus, it would offer a platform to perform very easy and strong biomolecular imaging.

The other unique application of hyperbolic metamaterials is metamaterial optical nanocavities, a key component to scale lasers down to the nanoscale. Unlike traditional dielectric cavities, which resonate at higher frequencies when the cavity size reduces, indefinite cavities with sizes different by several orders may resonate at the same frequency and same mode order. Furthermore, the size dependence of quality factor due to the radiation loss also shows a reversed behavior compared to traditional dielectric cavities. Large wavevectors supported by hyperbolic metamaterials successfully captured light in 20nm dimension and show a very high figure of merit due to their extremely small mode volume. Such a theoretical and experimental demonstration could have strong potential to achieve a truly nanometer scale low threshold laser whose size is compatible to biomolecules. In addition, the tunable refractive index characteristic of nanocavities is promising for high refractive index surface coatings and cloaking metasurfaces.

I have also studied several interesting metamaterials applications based on various schemes such as negative index, metasurface and plasmonic lens. First of all, I demonstrate a photo-induced chiral switching of reconfigurable negative index metamaterial device as a new class of custom-designed composite with deep sub-wavelength building blocks in response to external optical stimuli. This metamaterial device allows electromagnetic control of the polarization of light and will find important applications in manipulation of terahertz waves, such as dynamically tunable terahertz circular polarizers and polarization modulators for terahertz radiations. Also, with a large scale negative index metasurface, I am able to observe a very strong photonic spin-hall effect, which could provide a route for exploiting the spin and orbit angular momentum of light for information processing and communication. Last, plasmonic nanolithography is studied as a new low-cost high-throughput approach to maskless nanolithography, providing both 22-nm high-resolution direct patterning and 10m/s high-throughput writing speed at the same time.

All topics covered needed very serious effort in nanofabrication technique development. For this reason, I include state-of-the-art nanofabrication techniques and tips which are used to demonstrate such metamaterials and metadevices shown in this dissertation. Utilizing very special top-down and bottom-up processes such as electron beam lithography overlay, super high resolution lift-off process, quantum dot lithography, metamaterial and plasmonic devices that need very high resolution and high-accuracy are demonstrated successfully.

I believe my efforts in developing different types of sub-wavelength metamaterials including hyperbolic metamaterials for extraordinary optical properties and demonstrating the devices experimentally will advance future nanoscale optics and photonics, materials science and broad nanoscience and nanotechnology.

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