Phononic and photonic properties of shape-engineered silicon nanoscale pillar arrays.

We report the results of Brillouin-Mandelstam spectroscopy and Mueller matrix spectroscopic ellipsometry of the nanoscale 'pillar with the hat' periodic silicon structures, revealing intriguing phononic and photonic-phoxonic-properties. It has been theoretically shown that periodic structures with properly tuned dimensions can act simultaneously as phononic and photonic crystals, strongly affecting the light-matter interactions. Acoustic phonon states can be tuned by external boundaries, either as a result of phonon confinement effects in individual nanostructures, or as a result of artificially induced external periodicity, as in the phononic crystals. The shape of the nanoscale pillar array was engineered to ensure the interplay of both effects. The Brillouin-Mandelstam spectroscopy data indicated strong flattening of the acoustic phonon dispersion in the frequency range from 2 GHz to 20 GHz and the phonon wave vector extending to the higher-order Brillouin zones. The specifics of the phonon dispersion dependence on the pillar arrays' orientation suggest the presence of both periodic modulation and spatial localization effects for the acoustic phonons. The ellipsometry data reveal a distinct scatter pattern of four-fold symmetry due to nanoscale periodicity of the pillar arrays. Our results confirm the dual functionality of the nanostructured shape-engineered structure and indicate a possible new direction for fine-tuning the light-matter interaction in the next generation of photonic, optoelectronic, and phononic devices.

Tunable Hyperbolic Metamaterials Based on Self-Assembled Carbon Nanotubes.

We show that packed, horizontally aligned films of single-walled carbon nanotubes are hyperbolic metamaterials with ultrasubwavelength unit cells and dynamic tunability. Using Mueller matrix ellipsometry, we characterize the films' optical properties, which are doping level dependent, and find a broadband hyperbolic region tunable in the mid-infrared. To characterize the dispersion of in-plane hyperbolic plasmon modes, we etch the nanotube films into nanoribbons with differing widths and orientations relative to the nanotube axis, and we observe that the hyperbolic modes support strong light localization. An agreement between the experiments and theoretical models using the ellipsometry data indicates that the packed carbon nanotubes support bulk anisotropic responses at the nanoscale. Self-assembled films of carbon nanotubes are well-suited for applications in thermal emission and photodetection, and they serve as model systems for studying light-matter interactions in the deep subwavelength regime.

Robust Extraction of Hyperbolic Metamaterial Permittivity using Total Internal Reflection Ellipsometry.

Hyperbolic metamaterials are optical materials characterized by highly anisotropic effective permittivity tensor components having opposite signs along orthogonal directions. The techniques currently employed for characterizing the optical properties of hyperbolic metamaterials are limited in their capability for robust extraction of the complex permittivity tensor. Here we demonstrate how an ellipsometry technique based on total internal reflection can be leveraged to extract the permittivity of hyperbolic metamaterials with improved robustness and accuracy. By enhancing the interaction of light with the metamaterial stacks, improved ellipsometric sensitivity for subsequent permittivity extraction is obtained. The technique does not require any modification of the hyperbolic metamaterial sample or sophisticated ellipsometry set-up, and could therefore serve as a reliable and easy-to-adopt technique for characterization of a broad class of anisotropic metamaterials.