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Extending the Limits of Optical Metamaterials


For centuries, the ability to control light is bounded by the limited range of materials’ refractive index available in nature. This limit is finally broken with the advent of artificially-engineered optical metamaterials. Not only have metamaterials significantly extended the boundaries of refractive index, they have led to many novel properties and applications never thought to be possible. In optics, some of the most fascinating promises of metamaterials include the negative refractive index materials, superlenses and invisibility cloaks. However, optical metamaterials typically suffer enormous loss, which hinders their practical applications. This dissertation not only serves to tackle the optical loss problem, but also offers unique strategies to build practical optical devices with unprecedented functionalities.

The first part of the dissertation shows how the real part of the refractive index can be engineered to cover the entire range from positive to negative. Low-loss bulk three-dimensional optical metamaterials are experimentally demonstrated in both the near-infrared and visible wavelengths, with a maximum transmission exceeding 40%. As metamaterials are typically anisotropic, we can further engineer the components of the permittivity and permeability tensors to attain unique isofrequency contours in the Fourier space. Through rigorous measurement of the transmission and reflection, both amplitude and phase for a range of wavevectors, a novel magnetic hyperbolic dispersion metamaterial is demonstrated, enabling a new path to enhance photon density of states and light-matter interaction. In addition, I will discuss the realization of a zero index metamaterial (ZIM), and its huge potential for nonlinear light generation owing to the uniform phase distribution within the bulk medium. In particular, a phase-mismatch free nonlinear propagation is achieved for the very first time using the ZIM, opening new opportunities for efficient multi-directional nonlinear light generation. The second part of the dissertation details the engineering of the imaginary part of refractive index, taking advantage of the usually undesirable loss, to attain novel device functionalities. The interplay between loss and gain based on the parity-time (PT) symmetry concept can lead to complex conjugate eigenmodes, where the electric fields can be confined either in the gain or loss region at the same (degenerate) frequency. Here I present two novel on-chip PT metamaterial devices: A single-mode microring laser with unique mode manipulation capability is achieved by careful positioning of the loss and gain nanostructures, where all the unwanted resonant modes are spatially overlapping with the loss regions and hence suppressed. Similarly, by introducing the loss and gain modulation in a straight waveguide configuration, a unique coherent optical amplifier-absorber (COAA) device is demonstrated. Importantly, COAA synergizes both the lasing and anti-lasing (i.e. time-reversed lasing) modes, meaning the device can either amplify or absorb coherent signals at the same frequency depending on the relative input phase, thus potentially approaching the ultimate limit of signal modulation depth in optical communications. The third section of the dissertation presents a subwavelength local phase engineering approach to realize novel optical properties. Using spatially varying plasmonic nanoantennas with strong light confinement to locally modify the phase of the electromagnetic waves, I show how a metasurface optical cloak working in the visible wavelength is carefully designed and built. Unlike previous cloaks, our cloak is ultra-thin and conformal, with the unique ability to restore both the wavefront and phase of the reflected light, rendering a 3D object of arbitrary shape invisible.

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