Semiconductor-based layered hyperbolic metamaterials (HMMs) house high-wavevector volume plasmon polariton (VPP) modes in the infrared spectral range. VPP modes have successfully been exploited in the weak-coupling regime through the enhanced Purcell
effect. In this paper, we experimentally demonstrate strong coupling between the VPP modes in a semiconductor HMM and the intersubband transition of epitaxially-embedded quantum wells. We observe clear anticrossings in the dispersion curves for the zeroth-, first-, second-, and third-order VPP modes, resulting in upper and lower polariton branches for each mode. This demonstration sets the stage for the creation of novel infrared optoelectronic structures combining HMMs with embedded epitaxial emitter or detector structures.
Dirac plasmon polaritons in topological insulators (TIs),light coupled to massless Dirac electrons, have been attracting a large amount of attention, both from a fundamental perspective and for potential terahertz (THz) photonic applications. Althoug
h THz polaritons have been observed by far-field THz spectroscopy on TI microstructures, real-space imaging of propagating THz polaritons in unstructured TIs has been elusive so far. Here, we show the very first spectroscopic THz near-field images of thin Bi2Se3 layers (prototypical TIs) revealing polaritons with up to 12 times increased momenta as compared to photons of the same energy and decay times of about 0.24 ps, yet short propagation lengths. From the near-field images we determine the polariton dispersions in layers from 120 to 25 nm thickness and perform a systematic theoretical dispersion analysis, showing that the observed polaritons can be explained only by the simultaneous coupling of THz radiation to Dirac carriers at the TI surfaces, massive bulk carriers and optical phonons. Our work does not only provide critical insights into the nature of THz polaritons in TIs, but also establishes instrumentation of unprecedented sensitivity for imaging of THz polaritons.
The naturally existing chalcogenide Bi2Se3 is topologically nontrivial due to the band inversion caused by strong spin-orbit coupling inside the bulk of the material. The surface states are spin polarized, protected by the time-inversion symmetry, an
d thus robust to the scattering caused by non-magnetic defects. A high purity topological insulator thin film can be easily grown via molecular beam epitaxy (MBE) on various substrates to enable novel electronics, optics, and spintronics applications. However, the unique surface state properties have historically been limited by the film quality, which is evaluated by crystallinity, surface morphology, and transport data. Here we propose and investigate different MBE growth strategies to improve the quality of Bi2Se3 thin films grown by MBE. In addition, growths of topological trivial insulator (Bi0.5In0.5)2Se3 (BIS) are also investigated. BIS is often used as a buffer layer or separation layer for topological insulator heterostructures. Based on the surface passivation status, we have classified the substrates into two categories, self-passivated or unpassivated, and determine the optimal growth mechanisms on the representative sapphire and GaAs, respectively. Growth temperature is a crucial control parameter for the van der Waals epitaxy for both types of substrates. For Bi2Se3 on GaAs, the surface passivation status determines the dominant growth mechanism.
