Dephasing ofG-band phonons in single-wall carbon nanotubes probed via impulsive stimulated Raman scattering

Abstract

We study the coherent dynamics of G-band phonons in single-wall carbon nanotubes through impulsive stimulated Stokes and anti-Stokes Raman scattering. The probe energy dependence of the phonon amplitude as well as the preferential occurrence between Stokes and anti-Stokes components in response to chirpedpulse excitation are well explained within our model. The temperature dependence of the observed dephasing rate clearly exhibits a thermally activated component, with an activation energy that coincides with the frequency of the radial breathing mode (RBM). This fact provides a clear picture for the dephasing of G-band phonons by random frequency modulation via interaction with the RBM through anharmonicity. Optical phonons often play major roles in dynamical phenomena in solids. Due to their strong coupling with electrons, they are behind virtually all energy and phase relaxation processes that occur in the presence of high electric fields and/or nonequilibrium carrier distributions. Recently, much attention has been paid to nonperturbative electronoptical-phonon coupling in single-wall carbon nanotubes (SWCNTs).1-3 Such strong coupling is believed to be responsible for the current saturation behaviors observed in high-field electronic transport 4,5 as well as for the appearance of a broad and red-shifted Raman feature due to Kohn anomalies. 6 In both cases, dynamical quantities of optical phonons such as lifetimes and dephasing times are the key parameters that characterize the processes.Raman scattering spectroscopy has been an indispensable tool for characterizing the electronic structure of carbonbased nanomaterials such as SWCNTs.7 While an extensive literature exists on CW Raman studies, time-domain vibrational measurements directly probing lattice dynamics in SWCNTs have only recently begun. [8][9][10][11][12][13][14] Real-time observations of coherent oscillations have been made of both the low-frequency (100-300 cm −1 ) radial-breathing mode (RBM) and the high-frequency (1550-1600 cm −1 ) phonon mode, the so-called G band, providing important insight into chirality dependence, 8 generation mechanisms, 12,15 and electron-phonon coupling strength.13 A population lifetime of 1.1 ± 0.2 ps was also measured for optical phonons in SWCNTs at room temperature using time-resolved anti-Stokes Raman spectroscopy.11 However, exactly how optical phonons decay in energy and phase in SWCNTs is still an open question.Here, we report results of coherent phonon spectroscopy of SWCNTs using spectrally resolved and temperaturedependent ultrafast pump-probe spectroscopy. A pump pulse initiates coherent lattice vibrations, and then a delayed, spectrally broad probe pulse is incident on the sample, where it induces additional lattice vibrations through impulsive stimulated Raman scattering. The probe photon-energy dependence of phonon amplitude for both transformed-limited and chirped pulses can be well explained within our model based on impulsive stimulated Stokes Raman scattering (SSRS) and anti-Stokes Raman scattering (S...

Strong Acoustic Phonon Localization in Copolymer-Wrapped Carbon Nanotubes

“…Previously, one could distinguish Raman and PL signals from individual SWCNTs simply based on the spectral line width. At typical G-band line widths of about 1 meV, corresponding to rather fast phonon decay times of about 2 ps, − the Raman signal was found to be much narrower than typical exciton emission spectra. However, the recent observation of extremely narrow spectral exciton line widths in ultraclean SWCNTs , makes this assumption no longer reliable.…”

Selective coherent phonon-mode generation in single-wall carbon nanotubes

“…We expect that if the phonon damping is included, an additional damping term modifying the coherent phonon amplitude will appear due to the dephasing time of the coherent phonon, which could then modulate the coherent phonon amplitude. However, looking at some experimental measurements on the phonon dephasing in SWNTs by singlepulse pump-probe spectroscopy, we estimate that even the G-band, which has a high frequency, can survive up to 5 ps [24]. Therefore, we would not expect the phonon damping to be a key factor contributing to the selective phonon excitation.…”

Third Harmonic Generation in Thin NbOI2 and TaOI2

“…Moreover, ultrafast third-order nonlinear process in the femtosecond scale holds potential for telecommunication, quantum photonics and optical sensing applications. This encompasses processes like THG [ 19 ], four-wave mixing (FWM) [ 23 ], self-phase modulation (SPM) [ 24 ] and stimulated Raman scattering [ 25 ].…”