Electro-Optical Properties of Chiral-Pure, Excitonic Single-Walled Carbon Nanotubes

Single-walled carbon nanotubes (SWCNTs) are highly applicable to telecommunication systems due to their diameter-dependent band gaps ranging from 0 to 2 eV. Their strong exciton resonances allow for efficient electro-optical control of the amplitude and phase of light, and in ordered arrays of SWCNTs, their optical anisotropy allows for the control of the polarization of light. Here, we demonstrate that quantum-confined excitons in centimeter-scale, near-monolayer films of chiral-pure SWCNTs lead to epsilon-near-zero (ENZ) regions in the near infrared at room temperature. We develop theory to generate the excitonic-ENZ criterion of which shows that ideal materials have a small background permittivity () and exciton linewidth () and a large oscillator strength (f) and energy (E_x). By electrostatically doping the SWCNTs, we find that the sign of the real part of the permittivity (ε₁) can controlled at-will with a maximum modulation of Δε₁ = 2.1. A larger modulation of Δε₁ = 2.5 is also observed below the SWCNT band gap. Over an injected carrier range of ±5 x 10¹² cm^-2, the real and imaginary parts of the refractive index are modulated by 15% and 26%, respectively. Further, we study the effects of nanotube alignment on the micron-scale optical properties of these chiral-pure films. By selection rules, the primary exciton will only be excited for light polarized along the nanotube axis (extraordinary direction); accordingly, we observe in-plane hyperbolicity in the near-infrared owing to sufficient biaxial alignment of neighboring SWCNTs. This is the first demonstration of excitonic in-plane hyperbolicity in the near infrared at room temperature. Additionally, we observe record values in the in-plane birefringence (Δn = 1.07) and dichroism (Δk = 2.17) in the near infrared (λ = 1,000 nm). The work shows the extraordinary potential for semiconducting SWCNTs to efficiently control all the parameters of light (amplitude, phase, and polarization) at vital telecommunication wavelengths.