Near-field Nanoscale Investigation of Optical Properties of Bi 2 Se 3 Thin-films
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Near-field Nanoscale Investigation of Optical Properties of Bi2Se3 Thin-films Sarah E. Grefe,1 Malinda Tan,2 Shahab Derakhshan2 and Yohannes Abate1 1 California State University Long Beach, Department of Physics & Astronomy, 1250 Bellflower Blvd., Long Beach, CA 90840-9505, USA. 2 California State University Long Beach, Department of Chemistry and Biochemistry, 1250 Bellflower Blvd., Long Beach, CA 90840-9507, USA. ABSTRACT Bi2Se3 thin films are imaged in the near-field using spectroscopic scattering type near-field optical microscopy (s-SNOM) at mid infrared laser wavelength region (9-11μm). Single phases Bi2Se3 thin film structures were prepared by mechanical exfoliation on silicon wafers. We report size and wavelength dependent near-field interaction contrasts in both optical amplitude and phase. We show that near-field optical imaging allows material specific identification and characterization of Bi2Se3 exfoliated samples including the confirmation of residual tape presence or removal in stacked films. We describe an alternative “shear exfoliation” sample preparation method which reliably deposits Bi2Se3 without the possibility of adhesive contaminants. INTRODUCTION Among some of the most exciting discoveries in the past few years in condensed matter research is the theoretical foundations of topological insulating electronic phases based on strong spin-orbit coupling and observations of the these states in model materials [1,2]. On the surface of topological insulators (TIs) such as Bi2Se3 the electronic spectrum is characterized by a single helical Dirac dispersion such that counter-propagating electrons carry opposite spin allowing propagation of pure spin currents [3]. In addition to their fundamental scientific novelty, these states are predicted to have special properties arising from charge dynamics that could be driven optically, making them potentially useful for applications ranging from opto-spintronic devices, quantum computation, nanoscale electronics and nanophotonics [4-6]. The novel properties of these 2D surface states have yet to be fully explored, their response to direct near-field optical excitations and direct real-space optical mapping of the surface states remains to be reported. Among surface sensitive techniques, scattering type scanning near-field optical microscopy (s-SNOM), is an ideal technique to directly map surface states. This is because the generation and dynamics of surface states manifests itself strongly in the near-field, and requires distance dimensions below the diffraction limit, therefore high spatial resolution experimental techniques become invaluable methods for the investigation of such systems. SSNOM offers direct experimental access to the optical near-field distribution of surface electrons in TIs, enabling them to be visualized and investigated in a manner similar to the studies recently conducted on graphene [7]. Previously, the mechanical exfoliation technique has been successful in fabricating thin samples of Bi2Se3 and Bi2Te3 nanofilm owing to the repeated
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