Optical properties of chalcogenide topological insulators (TIs), namely, Bi2Se3 (BS) and Bi2Te3 (BT) were studied across the NIR to MIR spectral ranges. In this spectral range, the experimentally measured optical constants revealed an extremely high permittivity values amounting to refractive indices as high as n≈11 and n≈6.4, for BT and BS respectively. These ultra-high index values were then utilized for demonstrating ultracompact, deep-subwavelength nanostructures (NSs), with unit cell sizes down to ~λ/10. Finally, using scattering-type Scanning Near-field Optical Microscopy (s-SNOM), local variations in the optical constants of these nanostructured TIs were studied. Nanoscale phase mapping on a BS NS revealed the role of the imaginary component of the refractive index in the observed phase shifts, varying from as low as ~0.37π to a maximum of ~2π radians across a resonance. This work thus highlights the potential of TIs as a low-loss, high index material for ultracompact nanophotonics.
We present a study of various compositions of the chalcogenide family used for static and active metasurfaces. We start with large area CVD grown amorphous spherical Selenium nanoparticles on various substrates and show that their Mie-resonant response spans the entire mid-infrared (MIR) range. By coupling Se Mie-resonators to ENZ substrates we demonstrate an order of magnitude increase in quality factor. Next, we investigate topological insulators Bi2Se3 and Bi2Te3 metasurfaces. We study the optical constants of single crystal Bi2Te3 in the NIR to the MIR range, followed by fabrication and characterization of metasurface disk arrays. We show that these high permittivity metasurfaces can yield very large absorption resonances using deep subwavelength structures. Finally, we demonstrate ultra-wide dynamic tuning of PbTe meta-atoms and metasurfaces, utilizing the anomalously large thermo-optic coefficient and high refractive index of this material.
Efficient light manipulation at subwavelength scales in the mid-infrared (MIR) region is essential for various applications and can be harnessed from intrinsic low-loss dielectric resonators. Here, we demonstrate the fabrication of truncated spherical selenium (Se) resonators with tunable high-quality (Q) factor Mie resonances. Large area amorphous Se subwavelength resonators of varying sizes were grown on different substrates, using a novel CVD process. We demonstrate size-tunable Mie resonances spanning the 2-16 µm range, for single isolated resonators and large area ensembles, respectively. We show strong tunable absorption resonances (90%) in ensembles of resonators in a significantly broad MIR range. Moreover, by coupling resonators to epsilon-near-zero (ENZ) substrates, we engineer high-Q resonances as high as Q=40. We also show the resonance pinning effect near the substrate ENZ value, which is manifested in size-independent resonances.
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