New plasmonic materials are needed to overcome limitations of noble metals, particularly their high Ohmic losses, to develop metamaterial-based devices for nanophotonics, data processing circuits, sensors, etc. Recently, topological insulators were identified as a potential platform for broadband plamonics [1]. Here we study a topological insulator compound, BSTS, which supports broadband plasmonic response and has appealing photonic properties in the mid-IR to UV spectral range.
BSTS single crystals were grown by modified Bridgeman method. BSTS dielectric constants were derived by ellipsometric measurements and are in excellent agreement with first principle DFT calculations. Unlike common direct or indirect bandgap semiconductors, the anomalous dispersion region falls in the visible part of the spectrum, leading to negative values of the permittivity. This behavior of the optical response is attributed to a combination of bulk interband transitions and surface contribution of the topologically protected states.
To prove the plasmonic behavior of BSTS, we fabricated metamaterials and gratings on crystal flakes and registered strong plasmonic response from UV to NIR (Fig. 1a) both via optical and cathodoluminescence spectroscopy. The coexistence of plasmonic response of the topological surface with dielectric properties of the semiconducting bulk enables broadband (up to to mid-IR) and ultrafast (t>100 fs) photo-modulation of the optical response, which shows a two-fold enhancement by metamaterial structuring of the BSTS surface (Fig. 1b).
These findings confirm the potential of topological insulators as a platform for high-frequency switchable plasmonic metamaterials.
Figure 1. Plasmonic response of nano-slit array metamaterials fabricated on topological insulator semiconductor. (a) Absorption spectra, 1-R, of nano-slit arrays with lengths D ranging from 100 to 225 nm, for light polarized perpendicular to the slits nano-slits and (b) Ultrafast optical modulation of the reflectivity of D=250 nm slit array with pulsed (t=100 fs) off-resonance circularly polarized pump (400 nm), and resonant linearly polarized probe (600 nm).
[1] J.-Y. Ou, J.-K. So, G. Adamo, A. Sulaev, L. Wang, and N. Zheludev, Ultraviolet and visible range plasmonics in the topological insulator Bi1.5Sb0.5Te1.8Se1.2, Nat. Comm., online, (2014)
csoci@ntu.edu.sg