Unveiling Local Optical Properties Using Nanoimaging Phase Mapping in High-Index Topological Insulator Bi₂Se₃...

Topological insulators are materials characterized by an insulating bulk and high mobility topologically protected surface states, making them promising candidates for future optoelectronic and quantum devices. Although their electronic properties have been extensively studied, their mid-infrared (MIR) 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 chemical vapor deposition-grown Bi₂Se₃ nanobeams (NBs). We extract the 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 2π phase-shift across the resonance, in excellent agreement with finite-difference time-domain simulations. This work highlights the potential of Bi₂Se₃ for quantum circuitry, nonlinear generation, high-Q metaphotonics, and photodetection.