  Topological insulators (TIs) are a class of materials characterized by an
insulting bulk and high mobility topologically protected surface states, making
them promising candidates for future optoelectronic and quantum devices.
Although their electronic and transport properties have been extensively
studied, their optical properties and prospective photonic capabilities have
not been fully uncovered. Here, we use a combination of far-field and
near-field nanoscale imaging and spectroscopy, to study CVD grown Bi2Se3
nanobeams (NBs). We first extract the mid-infrared (MIR) optical constants of
Bi2Se3, revealing refractive index values as high as n ~6.4, and demonstrate
that the NBs support Mie-resonances across the MIR. Local near-field reflection
phase mapping reveals domains of various phase shifts, providing information on
the local optical properties of the NBs. We experimentally measure up to 2{\pi}
phase-shift across the resonance, in excellent agreement with FDTD simulations.
This work highlights the potential of TI Bi2Se3 for quantum circuitry,
non-linear generation, high-Q metaphotonics, and IR photodetection.
