Giant gate-tunability of complex refractive index in semiconducting carbon nanotubes

TL;DR

This study measures and demonstrates giant gate-tunable changes in the complex refractive index of high-purity semiconducting carbon nanotube films, surpassing existing electro-optic materials, with potential applications in infrared photonics and telecommunications.

Contribution

It provides the first measurement of gate-tunable optical constants of high-purity s-SWCNT films and designs a multilayer modulator achieving significant phase shifts in the IR range.

Findings

Giant modulation of refractive index (~11.2%) and extinction coefficient (~11.6%) in near-IR.

Achieved >45° reflection phase modulation at 1600 nm with a <200 nm stack.

Demonstrated s-SWCNT as a promising material for IR electro-optic devices.

Abstract

Electrically-tunable optical properties in materials are desirable for many applications ranging from displays to lasing and optical communication. In most two-dimensional thin-films and other quantum confined materials, these constants have been measured accurately. However, the optical constants of single wall nanotubes (SWCNT) as a function of electrostatic tuning are yet to be measured due to lack of electronic purity and spatial homogeneity over large areas. Here, we measure the basic optical constants of ultrathin high-purity (>99%) semiconducting single wall carbon nanotube (s-SWCNT) films with spectroscopic ellipsometry. We extract the gate-tunable complex refractive index of s-SWCNT films and observe giant modulation of the real refractive index (~11.2% or an absolute value of >0.2) and extinction coefficient (~11.6%) in the near-infrared (IR) region (1.3-1.55 {\mu}m) induced by the applied electric field significantly higher than all existing electro-optic semiconductors in this wavelength range. We further design a multilayer IR reflection phase modulator stack by combining s-SWCNT and monolayer MoS2 heterostructures that can attain >45{\deg} reflection phase modulation at 1600 nm wavelength for < 200 nm total stack thickness. Our results highlight s-SWCNT as a promising material system for infrared photonics and electro-optics in telecommunication applications.