Ultrafast Spectroscopy of Fano-Like Resonance between Optical Phonon and Excitons in CdSe Quantum Dots: Dependence of Coherent Vibrational Wave-Packet Dynamics on Pump Fluence.

and Dmitry Cherepanov,Fedor Gostev,Stanislav Umanskiy,Andrey Titov,Nikolay Denisov,Victor Nadtochenko,Arseniy Aybush,Ivan Shelaev

doi:10.3390/nano7110371

and Dmitry Cherepanov, Fedor Gostev + Show 6 more

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https://doi.org/10.3390/nano7110371

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Abstract

The main goal of the present work is to study the coherent phonon in strongly confined CdSe quantum dots (QDs) under varied pump fluences. The main characteristics of coherent phonons (amplitude, frequency, phase, spectrogram) of CdSe QDs under the red-edge pump of the excitonic band [1S(e)-1S3/2(h)] are reported. We demonstrate for the first time that the amplitude of the coherent optical longitudinal-optical (LO) phonon at 6.16 THz excited in CdSe nanoparticles by a femtosecond unchirped pulse shows a non-monotone dependence on the pump fluence. This dependence exhibits the maximum at pump fluence ~0.8 mJ/cm2. At the same time, the amplitudes of the longitudinal acoustic (LA) phonon mode at 0.55 THz and of the coherent wave packet of toluene at 15.6, 23.6 THz show a monotonic rise with the increase of pump fluence. The time frequency representation of an oscillating signal corresponding to LO phonons revealed by continuous wavelet transform (CWT) shows a profound destructive quantum interference close to the origin of distinct (optical phonon) and continuum-like (exciton) quasiparticles. The CWT spectrogram demonstrates a nonlinear chirp at short time delays, where the chirp sign depends on the pump pulse fluence. The CWT spectrogram reveals an anharmonic coupling between optical and acoustic phonons.

Highlights

Quantum dots (QDs), and in particular CdSe QDs, have broad applications in novel electronic and photonic devices due to their unique optical and electronic properties
We report the main characteristic of coherent phonons in strongly confined CdSe QDs under the red-edge pump of the excitonic band [1S(e)-1S3/2(h)]
We demonstrate for the first time that the amplitude of the coherent optical phonon in CdSe nanoparticles excited by a femtosecond unchirped pulse shows a non-monotone dependence on the pump fluence

Summary

Introduction

Quantum dots (QDs), and in particular CdSe QDs, have broad applications in novel electronic and photonic devices due to their unique optical and electronic properties. The studies on CdSe QDs longitudinal-optical (LO) [5,6,7,8,9,10,11,12,13], surface optical [12], transverse [24], and longitudinal acoustic (LA) phonon modes have been reported [4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23] In these works, electron-phonon coupling for optical phonons has been attributed to the Fröhlich interaction(s) [13]. The CWT spectrogram demonstrates a nonlinear chirp at short time delays, where the chirp sign depends on the pump pulse fluence

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