28 research outputs found
Correlated Roles of Temperature and Dimensionality for Multiple Exciton Generation and Electronic Structures in Quantum Dot Superlattices
Quantum dot superlattices (QDSLs), which are one-, two-, and three-dimensional periodic superlattices composed of QDs, induce dimensionality dependent quantum resonance among component QDs and thus represent a new type of condensed matter exhibiting novel energy, exciton, and carrier dynamics. We focused on the two important parameters, dimensionality and temperature, and identified their correlated roles to determine the electronic and photoexcited properties intrinsic to each QDSL at each dimensionality and temperature. We computationally demonstrated that the multiple exciton generation is significantly accelerated at higher temperature especially in the higher-dimensional QDSLs, indicating their great advantage especially at ambient temperature compared to an isolated zero-dimensional QD. Both dimensionality and temperature can be crucial and correlated parameters for independent tailoring of the properties of the QDSLs without changing the size, shape, and compositions of component QDs. The physical insights and advantage of the QDSLs we found here will lead to designing efficient and space-saving optoelectronic and photovoltaic devices that work at ambient temperature
Control of Rabi-splitting energies of exciton polaritons in CuI microcavities
We have investigated the active-layer-thickness dependence of exciton-photon interactions in CuI microcavities. The active layer thickness was changed from λ/2 to 2λ, where λ corresponds to an effective resonant wavelength of the lowest-lying exciton. In the CuI active layer, thermal strain removes the degeneracy of the heavy-hole (HH) and light-hole (LH) excitons at the Γ point. Angle-resolved reflectance spectra measured at 10 K demonstrate the strong coupling between the HH and LH excitons and cavity photon, resulting in the formation of three cavity-polariton branches: the lower, middle, and upper polariton branches. The energies of the three cavity-polariton modes as a function of incidence angle are reasonably explained using a phenomenological Hamiltonian to describe the exciton-photon strong coupling. It is found that the interaction energies of the cavity-polariton modes, the so-called vacuum Rabi-splitting energies, are systematically controlled from 29 (50) to 48 (84) meV for the LH (HH) exciton by changing the active layer thickness from λ/2 to 2λ. The active-layer-thickness dependence of the Rabi-splitting energies is semi-quantitatively explained by a simple model
Photon-field-shape effects on Rabi splitting energies in CuCl microcavities
We have investigated the photon-field-shape effects on Rabi splitting energies in CuCl microcavities with HfO2/SiO2 distributed Bragg reflectors (DBRs). The CuCl active layer was prepared by vacuum deposition, while HfO2 and SiO2 layers were prepared by rf magnetron sputtering. The photon-field shape was tuned to a node-type or an antinode-type by changing the order of the refractive indices in the DBR. In order to control of the Rabi splitting energies, the active-layer thickness was changed from λ/12 to 9λ/20. In angle-resolved reflectance spectra at 10 K, three cavity polaritons resulting from the strong coupling between the Z3 and Z1,2 excitons and cavity photon were clearly detected. We estimated the energies of the exciton-photon interaction, the so-called vacuum Rabi splitting energies, from the analysis of the cavity polariton dispersions using a phenomenological Hamiltonian for the strong exciton-photon coupling. The active-layer-thickness dependence of the Rabi splitting energies are explained by a semi-quantitative analysis taking account of the overlap between the exciton and photon-field wave functions. We have demonstrated that the photon-field shape drastically affects the active-layer-thickness dependence of the Rabi splitting energies
Control of Multiple Exciton Generation and Electron–Phonon Coupling by Interior Nanospace in Hyperstructured Quantum Dot Superlattice
The
possibility of precisely manipulating interior nanospace, which can be adjusted by ligand-attaching
down to the subnanometer regime, in a hyperstructured quantum dot
(QD) superlattice (QDSL) induces a new kind of collective resonant
coupling among QDs and opens up new opportunities for developing advanced
optoelectric and photovoltaic devices. Here, we report the first real-time
dynamics simulations of the multiple exciton generation (MEG) in one-,
two-, and three-dimensional (1D, 2D, and 3D) hyperstructured H-passivated
Si QDSLs, accounting for thermally fluctuating band energies and phonon
dynamics obtained by finite-temperature ab initio molecular dynamics
simulations. We computationally demonstrated that the MEG was significantly
accelerated, especially in the 3D QDSL compared to the 1D and 2D QDSLs.
The MEG acceleration in the 3D QDSL was almost 1.9 times the isolated
QD case. The dimension-dependent MEG acceleration was attributed not
only to the static density of states but also to the dynamical electron–phonon
couplings depending on the dimensionality of the hyperstructured QDSL,
which is effectively controlled by the interior nanospace. Such dimension-dependent
modifications originated from the short-range quantum resonance among
component QDs and were intrinsic to the hyperstructured QDSL. We propose
that photoexcited dynamics including the MEG process can be effectively
controlled by only manipulating the interior nanospace of the hyperstructured
QDSL without changing component QD size, shape, compositions, ligand,
etc