Polarization dependence of coherent phonon generation and detection in highly-aligned single-walled carbon nanotubes

TL;DR

This study explores how the polarization of light affects the generation and detection of coherent phonons in highly-aligned single-walled carbon nanotubes, revealing nearly complete quenching when polarization is perpendicular.

Contribution

It provides a microscopic theoretical model that accurately describes polarization dependence of coherent phonon signals in aligned nanotubes, validated by experimental data.

Findings

RBM coherent phonons are quenched when pump polarization is perpendicular to nanotubes.

Theoretical model matches experimental polarization dependence.

Signal decreases from 0 to 90 degrees relative to nanotube axis.

Abstract

We have investigated the polarization dependence of the generation and detection of radial breathing mode (RBM) coherent phonons (CP) in highly-aligned single-walled carbon nanotubes. Using polarization-dependent pump-probe differential-transmission spectroscopy, we measured RBM CPs as a function of angle for two different geometries. In Type I geometry, the pump and probe polarizations were fixed, and the sample orientation was rotated, whereas, in Type II geometry, the probe polarization and sample orientation were fixed, and the pump polarization was rotated. In both geometries, we observed a very nearly complete quenching of the RBM CPs when the pump polarization was perpendicular to the nanotubes. For both Type I and II geometries, we have developed a microscopic theoretical model to simulate CP generation and detection as a function of polarization angle and found that the CP signal decreases as the angle goes from 0 degrees (parallel to the tube) to 90 degrees (perpendicular to the tube). We compare theory with experiment in detail for RBM CPs created by pumping at the E44 optical transition in an ensemble of single-walled carbon nanotubes with a diameter distribution centered around 3 nm, taking into account realistic band structure and imperfect nanotube alignment in the sample.