Mid-infrared ballistic metamaterials and subdiffraction limited photonic funnels




Li, Kun, Ph. D.

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This thesis focuses on engineering light matter interaction in the mid-IR wavelength range by demonstrating “ballistic metamaterials” and “subdiffraction limited photonic funnels”. The first part of this thesis provides the theoretical foundation for, and experimental realization of, the optical phenomenon we term “ballistic resonance”, which results from the interplay between free charge carrier motion in confining geometries and the periodic driving electromagnetic fields. The ballistic resonance can be used to achieve negative permittivity at frequencies well above the plasma frequency of the bulk material. As a proof of principle, we have demonstrated all-semiconductor hyperbolic metamaterials leveraging the “ballistic resonance” and operating at frequencies 60 percent above the plasma frequency of the constituent doped semiconductor “metallic” layer. The proposed and demonstrated ballistic resonance can be used to enable the realization and deployment of various applications that rely on local field enhancement and emission modulation, typically associated with plasmonic materials, in new material platforms. The second part of this thesis proposes a possible solution to overcome the diffraction limit in the mid-IR wavelength range by experimentally fabricating, characterizing and theoretically analyzing nano-scale 3D waveguides we term “Photonic Funnels”. The photonic funnel can achieve the efficient optical coupling between nano- and macroscale areas which would otherwise be suppressed by the diffraction limit. The nanostructured funnels represent a novel composite material platform that combines hyperbolic optical response with geometry-assisted optical confinement. We have experimentally demonstrated the funneling of mid-infrared light through openings with diameters as small as 1/25th of the free space wavelength (λ0). By analyzing the optical response of the fabricated funnels, both confinement of mid-infrared radiation to the λ/25 areas and efficient outcoupling of light from deep subwavelength areas are confirmed. The future work will be to experimentally use a photonic funnel as the nano-tip to do sub-diffraction-limited mapping in the mid-IR wavelength range. According to our simulations, the energy confinement at the funnel tip is much stronger than the isotropic core with the same geometry. The application of this subwavelength nano tip can be used to achieve the mid-IR subdiffraction limited signal distinguishing. The third part of this thesis is focused on the interrogation of the hyperbolic metamaterials with integrated intersubband transitions using thermal emission spectroscopy. We have provided a possible characterization method to probe the intersubband transitions of active quantum well devices such as quantum cascade lasers. This method is potentially useful when it is difficult to extract information on the intersubband transitions of the active devices using traditional reflection and transmission methods, for instance in the case of devices grown on doped substrates. We have experimentally used the thermal emission setup to get the absorption spectra from the 4 HMM samples which are presented in the first part of this thesis. The absorption spectra from the thermal emission method match well with the spectra extracted from the transmission and reflection measurement setup.


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