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Register a new userSimultaneous Existence of Confined and Delocalized Vibrational Modes in Colloidal Quantum Dots
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Coupling to phonon modes is a primary mechanism of excitonic dephasing and energy loss in semiconductors. However, low-energy phonons in colloidal quantum dots and their coupling to excitons are poorly understood, since their experimental signatures are weak and usually obscured by unavoidable inhomogeneous broadening of colloidal dot ensembles. We use multi-dimensional coherent spectroscopy at cryogenic temperatures to extract the homogeneous nonlinear optical response of excitons in a CdSe/CdZnS core/shell colloidal quantum dot ensemble. Comparison to simulation provides evidence that the observed lineshapes arise from the co-existence of confined and delocalized vibrational modes, both of which couple strongly to excitons in CdSe/CdZnS colloidal quantum dots.
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The dephasing time of the lowest bright exciton in CdSe/ZnS wurtzite quantum dots is measured from 5 K to 170 K and compared with density dynamics within the exciton fine structure using a sensitive three-beam four-wave-mixing technique unaffected by spectral diffusion. Pure dephasing via acoustic phonons dominates the initial dynamics, followed by an exponential zero-phonon line dephasing of 109 ps at 5 K, much faster than the ~10 ns exciton radiative lifetime. The zero-phonon line dephasing is explained by phonon-assisted spin-flip from the lowest bright state to dark exciton states. This is confirmed by the temperature dependence of the exciton lifetime and by direct measurements of the bright-dark exciton relaxation. Our results give an unambiguous evidence of the physical origin of the exciton dephasing in these nanocrystals.
Polariton emission from optical cavities integrated with various luminophores has been extensively studied recently due to the wide variety of possible applications in photonics, particularly promising in terms of fabrication of low-threshold sources of coherent emission. Tuneable microcavities allow extensive investigation of the photophysical properties of matter placed inside the cavity by deterministically changing the coupling strength and controllable switching from weak to strong and ultra-strong coupling regimes. Here we demonstrate room temperature strong coupling of exciton transitions in CdSe/ZnS/CdS/ZnS colloidal quantum dots with the optical modes of a tuneable low-mode-volume microcavity. Strong coupling is evidenced by a large Rabi splitting of the photoluminescence spectra depending on the detuning of the microcavity. A coupling strength of 154 meV has been achieved. High quantum yields, excellent photostability, and scalability of fabrication of QDs paves the way to practical applications of coupled systems based on colloidal QDs in photonics, optoelectronics, and sensing.
Lateral quantum dot molecules consist of at least two closely-spaced InGaAs quantum dots arranged such that the axis connecting the quantum dots is perpendicular to the growth direction. These quantum dot complexes are called molecules because the small spacing between the quantum dots is expected to lead to the formation of molecular-like delocalized states. We present optical spectroscopy of ensembles and individual lateral quantum dot molecules as a function of electric fields applied along the growth direction. The results allow us to characterize the energy level structure of lateral quantum dot molecules and the spectral signatures of both charging and many-body interactions. We present experimental evidence for the existence of molecular-like delocalized states for electrons in the first excited energy shell.
In this work we theoretically study properties of electric current driven by a temperature gradient through a quantum dot/molecule coupled to the source and drain charge reservoirs. We analyze the effect of Coulomb interactions between electrons on the dot/molecule and of thermal environment on the thermocurrent. The environment is simulated by two thermal baths associated with the reservoirs and kept at different temperatures. The scattering matrix formalism is employed to compute electron transmission through the system. This approach is further developed and combined with nonequilibrium Greens functions formalism, so that scattering probabilities are expressed in terms of relevant energies including the thermal energy, strengths of coupling between the dot/molecule and charge reservoirs and characteristic energies of electron-phonon interactions. It is shown that one may bring the considered system into regime favorable for heat-to-electric energy conversion by varying the applied bias and gate voltages.
We investigate phonon-induced spin and charge relaxation mediated by spin-orbit and hyperfine interactions for a single electron confined within a double quantum dot. A simple toy model incorporating both direct decay to the ground state of the double dot and indirect decay via an intermediate excited state yields an electron spin relaxation rate that varies non-monotonically with the detuning between the dots. We confirm this model with experiments performed on a GaAs double dot, demonstrating that the relaxation rate exhibits the expected detuning dependence and can be electrically tuned over several orders of magnitude. Our analysis suggests that spin-orbit mediated relaxation via phonons serves as the dominant mechanism through which the double-dot electron spin-flip rate varies with detuning.
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