Title Extreme Polarization Anisotropy in Resonant Third-Harmonic Generation from Aligned Carbon Nanotube Films.

Carbon nanotubes (CNTs) possess extremely anisotropic electronic, thermal, and optical properties owing to their one-dimensional character. While their linear optical properties have been extensively studied, nonlinear optical processes, such as harmonic generation for frequency conversion, remain largely unexplored in CNTs, particularly in macroscopic CNT assemblies. In this work, w e synthesized macroscopic films of aligned and type-separated (semiconducting and metallic) CNTs and studied polarization-dependent third-harmonic generation (THG) from the films with fundamental wavelengths ranging from 1.5 to 2.5 μm. Both films exhibited strongly wavelength-dependent, intense THG signals, enhanced through exciton resonances, and w e found third-order nonlinear optical susceptibilities of 2.50 × 10 m /V (semiconducting CNTs) and 1.23 × 10 m /V (metallic CNTs), respectively, for 1.8 μm excitation. Further, through systematic polarization-dependent THG measurements, w e determined the values of all elements of the susceptibility tensor, verifying the macroscopically one-dimensional nature of the films. Finally, w e performed polarized THG imaging to demonstrate the nonlinear anisotropy in the large-size CNT film with good alignment. These findings promise applications of aligned CNT films in mid-infrared frequency conversion, nonlinear optical switching, polarized pulsed lasers, polarized long-wave detection, and high-performance anisotropic nonlinear photonic devices. This article is protected by copyright. All rights reserved.