Abstract-Giant terahertz polarization rotation in ultrathin films of aligned carbon nanotubes

 

Andrey Baydin, Natsumi Komatsu, Fuyang Tay, Saunab Ghosh, Takuma Makihara, G. Timothy Noe, and Junichiro Kono

Experimental setup for showing giant THz polarization rotation in an aligned CNT film. (a) Schematic of THz transmission and reflection through the CNT film and substrate; (b) THz waveform in the time domain indicating the existence of a second pulse due to reflections in the substrate as shown in (a); (c) experimental configuration showing wire-grid polarizer, the sample, and the schematic of the polarization rotation of the propagating THz pulse; (d) polarization angle 𝜃$\theta$ defined as the angle between the CNT alignment direction and the polarization of the incident THz electric field.

https://www.osapublishing.org/optica/fulltext.cfm?uri=optica-8-5-760&id=451230

For easy manipulation of polarization states of light for applications in communications, imaging, and information processing, an efficient mechanism is desired for rotating light polarization with a minimum interaction length. Here, we report giant polarization rotations for terahertz (THz) electromagnetic waves in ultrathin (∼45nm$\sim 45\mathrm{n}\mathrm{m}$), high-density films of aligned carbon nanotubes. We observed polarization rotations of up to ∼20∘$\sim {20}^{\circ }$ and ∼110∘$\sim {110}^{\circ }$ for transmitted and reflected THz pulses, respectively. The amount of polarization rotation was a sensitive function of the angle between the incident THz polarization and the nanotube alignment direction, exhibiting a “magic” angle at which the total rotation through transmission and reflection becomes exactly 90°. Our model quantitatively explains these giant rotations as a result of extremely anisotropic optical constants, demonstrating that aligned carbon nanotubes promise ultrathin, broadband, and tunable THz polarization devices.

© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Terahertz Excitonics in Carbon Nanotubes: Exciton Autoionization and Multiplication

Filchito Bagsican, Michael Wais, Natsumi Komatsu, Weilu Gao, Weilu Gao, Lincoln W. Weber, Kazunori Serita,  Hironaru Murakami, Karsten. Held, Frank A. Hegmann, Masayoshi Tonouchi, Junichiro Kono, Iwao Kawaya, Marco Battiato

https://pubs.acs.org/doi/10.1021/acs.nanolett.9b05082

Excitons play major roles in optical processes in modern semiconductors, such as single-wall carbon nanotubes (CNTs), transition metal dichalcogenides, and 2D perovskite quantum wells. They possess extremely large binding energies (>100 meV), dominating absorption and emission spectra even at high temperatures. The large binding energies imply that they are stable, that is, hard to ionize, rendering them seemingly unsuited for optoelectronic devices that require mobile charge carriers, especially terahertz emitters and solar cells. Here, we have conducted terahertz emission and photocurrent studies on films of aligned single-chirality semiconducting CNTs and find that excitons autoionize, i.e., spontaneously dissociate into electrons and holes. This process naturally occurs ultrafast (<1 ps) while conserving energy and momentum. The created carriers can then be accelerated to emit a burst of terahertz radiation when a dc bias is applied, with promising efficiency in comparison to standard GaAs-based emitters. Furthermore, at high bias, the accelerated carriers acquire high enough kinetic energy to create secondary excitons through impact exciton generation, again in a fully energy and momentum conserving fashion. This exciton multiplication process leads to a nonlinear photocurrent increase as a function of bias. Our theoretical simulations based on nonequilibrium Boltzmann transport equations, taking into account all possible scattering pathways and a realistic band structure, reproduce all of our experimental data semiquantitatively. These results not only elucidate the momentum-dependent ultrafast dynamics of excitons and carriers in CNTs but also suggest promising routes toward terahertz excitonics despite the orders-of-magnitude mismatch between the exciton binding energies and the terahertz photon energies.