Real-space nanoimaging of THz polaritons in the topological insulator Bi2Se3

Abstract

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.

Self‐Induced Phase Locking of Terahertz Frequency Combs in a Phase‐Sensitive Hyperspectral Near‐Field Nanoscope

“…The bulk model (black curve, Figure 4h), which predicts a minimum at 3.7 THz, is not able to capture the 𝜂 3 measured with our FC s-SNOM, which instead, in the investigated 2.29-3.60 THz range, is very well replicated by including [59] deeply subdiffractional, highly directional hyperbolic phonon polaritons interacting with the electrons of the topological surface-states of Bi 2 Se 3 flakes, which can be activated by the s-SNOM tip (magenta line in Figure 4h). [61,62] We also measured the intermode FC beatnote map as a function of L on both Au (Figure 4h) and the Bi 2 Se 3 flake (Figure 4i), and observed an unexpected behavior as the feedback phase was varied. In both cases, the beatnote splits into two separated lines, which periodically merge into a more intense single beatnote.…”

“…The bulk model (black curve, Figure 4h ), which predicts a minimum at 3.7 THz, is not able to capture the η 3 measured with our FC s‐SNOM, which instead, in the investigated 2.29‐3.60 THz range, is very well replicated by including [ 59 ] deeply subdiffractional, highly directional hyperbolic phonon polaritons interacting with the electrons of the topological surface‐states of Bi 2 Se 3 flakes, which can be activated by the s‐SNOM tip (magenta line in Figure 4h ). [ 61 , 62 ]…”

THz Aperture Near-Field Spectroscopy of Dirac Plasmons in Topological Insulator Bi2Se3

“…Whilst Dirac plasmons could offer a whole host of useful functionalities, previous spectroscopic investigations of Bi2Se3 have revealed very complex optoelectronic behaviour, with contributions to the spectroscopic signature not only from Dirac plasmons, but also plasmons of massive 2DEG and bulk states as well as coupled phonon-plasmon polaritons [1][2][3][4]. Disentangling these separate contributions is of great importance for using Bi2Se3 in practical devices.…”

“…In contrast to far field methods, near-field techniques have the potential to increase the sensitivity and contrast of Bi2Se3 plasmon measurements [1,2]. In particular, aperture near-field microscopy enables the probing of evanescent fields on the sample surface at a spatial resolution determined by aperture size [5].…”

“…The measured spectra can be used to experimentally map the dispersion relationship, which is compared to theoretical predictions in order to determine the nature of the supported plasmons. The aperture method is a complimentary technique to previously used far and near-field measurements of topological insulators [1][2][3][4] and provides interesting insight into the near-field behaviour of Bi2Se3 plasmons.…”