Abstract
Understanding the dynamics of atomic vibrations confined in quasi-zero dimensional systems is crucial from both a fundamental point-of-view and a technological perspective. Using ultrafast electron diffraction, we monitored the lattice dynamics of GaAs quantum dots—grown by Droplet Epitaxy on AlGaAs—with sub-picosecond and sub-picometer resolutions. An ultrafast laser pulse nearly resonantly excites a confined exciton, which efficiently couples to high-energy acoustic phonons through the deformation potential mechanism. The transient behavior of the measured diffraction pattern reveals the nonequilibrium phonon dynamics both within the dots and in the region surrounding them. The experimental results are interpreted within the theoretical framework of a non-Markovian decoherence, according to which the optical excitation creates a localized polaron within the dot and a travelling phonon wavepacket that leaves the dot at the speed of sound. These findings indicate that integration of a phononic emitter in opto-electronic devices based on quantum dots for controlled communication processes can be fundamentally feasible.
Highlights
The understanding and active control of quantum materials are crucial aspects for addressing the technological challenges of the 21st century, mainly associated with the pressing demands for sustainable energy, high-speed communication and high-capacity data storage
GaAs quantum dots without wetting layer were grown by droplet epitaxy (DE)46 on a 90nm thick Al0.3Ga0.7As layer deposed on an n-type doped GaAs(001) substrate
Which turns out to be about 31 ps. This is contrary to our experimental observations, where a time constant of 2.5–4 ps is measured and can rather be interpreted according to the following scenario. It has been theoretically predicted43–45 that the deformation potential mechanism responsible for the excitation of acoustic phonons within the dots is associated to the buildup of a nonequilibrium phonon distribution consisting of two parts: a localized one that remains within the dots and reflects the polaronic nature of the excited state, as discussed above, and another part that, because of the spatial dispersion of the acoustic phonons, leaves the dot and propagates in the surrounding regions as a phonon wavepacket (WP) travelling at the speed of sound
Summary
The understanding and active control of quantum materials are crucial aspects for addressing the technological challenges of the 21st century, mainly associated with the pressing demands for sustainable energy, high-speed communication and high-capacity data storage. Despite their electronic structure is made of discrete levels similar to isolated atoms, QDs are generally grown in a solid-state environment, which is responsible for strong dephasing and decoherence effects detrimental for their optical, electronic, and thermal properties.. In order to fully exploit their potential, it is crucial to understand the interaction between the electronic and atomic degrees of freedom both within the dots and with the surrounding crystal lattice. We employ UED in the reflection geometry to investigate the lattice dynamics in GaAs quantum dots (QDs) grown on AlGaAs. An ultrafast optical excitation nearly resonant with the lowest electronic transition of the dots is adopted to create a confined exciton. In agreement with previous theoretical calculations, we interpret our observations as a result of a nonequilibrium phonon population composed of two parts: one localized within the dots, as detected from the Bragg reflections, and one traveling at the speed of sound in the surrounding region as a phonon wavepacket (WP), as retrieved from the dynamics of surface wave resonance (SWR) features
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