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.
In nanophotonics, temperature variations shift the optical response through the thermo-optic (TO) effect (dn/dT) and may be detrimental in certain applications. We present a comprehensive solution to overcome the problem of optical constants thermal dispersion in nanophotonic and meta-optic devices. By using hybrid meta-atoms composed from two materials with positive and negative TO coefficients, we engineer metasurfaces with zero effective thermal dispersion (dneff/dT≈0). We demonstrate temperature independent resonant frequency, amplitude and phase response in metasurfaces, operating across a broad temperature range (ΔT=500K). Controlling the sign and magnitude of TO dispersion extends the capabilities of light manipulation in nanophotonic systems.
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.
Topological insulators (TIs) are a new class of condensed matter system that host topologically protected surface states, leading to dissipationless electron transport. This intrinsic characteristic makes them potential candidate for quantum computing owing to their ability to preserve quantum coherence. Recently, these systems and the concept of topology have been embraced by the photonics community as well. In this work, we study the mid-infrared optical properties of high index (n~5.2) TI bismuth selenide (Bi2Se3) nanobeams (NBs), grown by chemical vapor deposition. Using Finite-difference time-domain (FDTD) simulations and FTIR nanospectroscopy, we find that these NBs support size-tunable Mie-resonant modes across the infrared (~1-16 µm). Furthermore, polarized measurements reveal that the total optical response of these deep subwavelength NBs is composed of TE and TM resonant mode. Finally, near-filed studies are also carried out to understand the effect of topological phase.
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