Plasmon polaritons in topological insulators attract attention from a fundamental perspective and for potential THz photonic applications. Although polaritons have been observed by THz far-field spectroscopy on topological insulator microstructures, real-space imaging of propagating THz polaritons has been elusive so far. Here, we show spectroscopic THz near-field images of thin Bi2Se3 layers (prototypical topological insulators) revealing polaritons with up to 12 times increased momenta as compared to photons of the same energy and decay times of about 0.48 ps, yet short propagation lengths. From the images we determine and analyze the polariton dispersion, showing that the polaritons can be explained by the coupling of THz radiation to various combinations of Dirac and massive carriers at the Bi2Se3 surfaces, massive bulk carriers and optical phonons. Our work provides critical insights into the nature of THz polaritons in topological insulators and establishes instrumentation and methodology for imaging of THz polaritons.

Replacing some Bi with In in Bi 2 Se 3 transforms it from a topological insulator to a band insulator. Here, we have used time-resolved terahertz spectroscopy to investigate photoexcited carrier dynamics in (Bi 1−x In x ) 2 Se 3 films with indium concentration x = 0%, 25%, and 50%. In Bi 2 Se 3 , optically excited carriers scatter from the bulk conduction band states into high mobility topological surface states within picoseconds after excitation. We demonstrate that a second set of Dirac surface states, located ∼1.5−1.8 eV above the conduction band minimum and accessible to carriers excited by 3.1 eV pulses, is characterized by a higher mobility than the surface states within the band gap that dominate equilibrium conductivity. In (Bi 0.75 In 0.25 ) 2 Se 3 and (Bi 0.50 In 0.50 ) 2 Se 3 , which are insulating without photoexcitation, the dynamics of photoexcited free carriers are affected by the twin domain boundaries and are sensitive to the disorder introduced by indium substitution. Transient conductivity rise time, as well as the mobility and lifetime of the photoexcited carriers in (Bi 1−x In x ) 2 Se 3 films, can be tuned by the indium content, enabling tailoring of band insulators that have the desired optoelectronic properties and are fully structurally compatible with the topological insulator Bi 2 Se 3 for applications in high-speed photonic devices based on topological insulator/band insulator heterostructures.

No abstract

Two-dimensional (2D) material research is rapidly evolving to broaden the spectrum of emergent 2D systems. Here, we review recent advances in the theory, synthesis, characterization, device, and quantum physics of 2D materials and their heterostructures. First, we shed insight into modeling of defects and intercalants, focusing on their formation pathways and strategic functionalities. We also review machine learning for synthesis and sensing applications of 2D materials. In addition, we highlight important development in the synthesis, processing, and characterization of various 2D materials (e.g., MXnenes, magnetic compounds, epitaxial layers, low-symmetry crystals, etc.) and discuss oxidation and strain gradient engineering in 2D materials. Next, we discuss the optical and phonon properties of 2D materials controlled by material inhomogeneity and give examples of multidimensional imaging and biosensing equipped with machine learning analysis based on 2D platforms. We then provide updates on mix-dimensional heterostructures using 2D building blocks for next-generation logic/memory devices and the quantum anomalous Hall devices of high-quality magnetic topological insulators, followed by advances in small twist-angle homojunctions and their exciting quantum transport. Finally, we provide the perspectives and future work on several topics mentioned in this review.