Topological-insulator-based terahertz modulator

Abstract

Three dimensional topological insulators, as a new phase of quantum matters, are characterized by an insulating gap in the bulk and a metallic state on the surface. Particularly, most of the topological insulators have narrow band gaps, and hence have promising applications in the area of terahertz optoelectronics. In this work, we experimentally demonstrate an electronically-tunable terahertz intensity modulator based on Bi_1:5Sb_0:5Te_1:8Se_1:2 single crystal, one of the most insulating topological insulators. A relative frequency-independent modulation depth of ~62% over a wide frequency range from 0.3 to 1.4 THz has been achieved at room temperature, by applying a bias current of 100 mA. The modulation in the low current regime can be further enhanced at low temperature. We propose that the extraordinarily large modulation is a consequence of thermally-activated carrier absorption in the semiconducting bulk states. Our work provides a new application of topological insulators for terahertz technology.

Figure 1

(a) Schematic illustration of Kapton/BSTS/Kapton sandwich-structure THz modulator. (b) Transmittance of 30-$\mu$m-thick BSTS crystal and single layer Kapton tape at room temperature.

Figure 2

(a) Measured THz waveforms transmitted through the device under different bias current from 0 to 100 mA in a step of 20 mA at room temperature. (b) The corresponding THz transmittance spectra normalized to the spectrum at zero bias.

Figure 3

(a) Modulation depth at 0.5 THz and temperature change of the device and (b) Normalized modulation depth and heating power as a function of applied in plane current at room temperature. Inset: Current–Voltage (I-V) characteristic of the modulator at room temperature.

Figure 4

(a) Normalized THz transmittance spectra under various bias current at 5 K. (b) Modulation depth at 0.5 THz for various temperatures (filled circle) and normalized heating power at 5 K (open circle) versus the applied in-plane current.