Coupling of Excitons and Discrete Acoustic Phonons in Vibrationally Isolated Quantum Emitters

Abstract

The photoluminescence emission by mesoscopic condensed matter is ultimately dictated by the fine-structure splitting of the fundamental exciton into optically allowed and dipole-forbidden states. In epitaxially grown semiconductor quantum dots, nonradiative equilibration between the fine-structure levels is mediated by bulk acoustic phonons, resulting in asymmetric spectral broadening of the excitonic luminescence. In isolated colloidal quantum dots, spatial confinement of the vibrational motion is expected to give rise to an interplay between the quantized electronic and phononic degrees of freedom. In most cases, however, zero-dimensional colloidal nanocrystals are strongly coupled to the substrate such that the charge relaxation processes are still effectively governed by the bulk properties. Here we show that encapsulation of single colloidal CdSe/CdS nanocrystals into individual organic polymer shells allows for systematic vibrational decoupling of the semiconductor nanospheres from the surroundings. In contrast to epitaxially grown quantum dots, simultaneous quantization of both electronic and vibrational degrees of freedom results in a series of strong and narrow acoustic phonon sidebands observed in the photoluminescence. Furthermore, an individual analysis of more than 200 compound particles reveals that enhancement or suppression of the radiative properties of the fundamental exciton is controlled by the interaction between fine-structure states via the discrete vibrational modes. For the first time, pronounced resonances in the scattering rate between the fine-structure states are directly observed, in good agreement with a quantum mechanical model. The unambiguous assignment of mediating acoustic modes to the observed scattering resonances complements the experimental findings. Thus, our results form an attractive basis for future studies on subterahertz quantum opto-mechanics and efficient laser cooling at the nanoscale.

Charge and spin control of ultrafast electron and hole dynamics in single CdSe/ZnSe quantum dots

“…By exploiting colloidal QDs [77][78][79], a new level of control over the position of the quantum emitters may be achieved via nanomanipulation procedures using, e.g., an AFM tip. This step could enable maximally efficient coupling of the quantum emitters to more sophisticated plasmonic nanostructures or nanoresonators [80].…”

Phonon Decoherence of Quantum Dots in Photonic Structures: Broadening of the Zero-Phonon Line and the Role of Dimensionality

“…We start with a 0D nano-sphere cavity, which resembles the geometry of colloidal QDs embedded in spherical shells [52]. The long-time coherence is plotted in Fig.…”

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

“…The PSD spectrum of the LA coherent phonon of 0.55 THz manifests a maximum at 594 nm (see the 3D-presentation of PSD ( ν , λ ) in SI3, Figure S5 ). Electron-phonon coupling of LA phonons in bulk and nanocrystal QDs was studied previously [ 5 , 6 , 7 , 8 , 9 , 10 , 15 , 18 , 23 , 41 ]. In the present work, LA phonons are not analyzed in detail.…”

“…One of the unresolved problems in quantum dot physics is the effect of quantum confinement upon exciton-phonon coupling, especially under high intensity of femtosecond pump pulse. The exciton-phonon coupling is very important to quantum dot photonics because it is a crucial parameter for optical, thermodynamic, and electric properties [ 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 ]. QDs lattice vibrations reveal two types of phonon modes, that are optical and acoustic phonons [ 13 ].…”

“…QDs lattice vibrations reveal two types of phonon modes, that are optical and acoustic phonons [ 13 ]. 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 ].…”