Optical study of single InAs on In0.12Ga0.88As self-assembled quantum dots: biexciton binding energy dependence on the dots size
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Biexciton
Electro-absorption modulator
We report unusual excitonic absorption spectra with dip-shaped structures caused by exciton–biexciton coherent interactions in quantum dots. The exciton absorption spectrum is measured with the micro-photoluminescence excitation technique in a single InGaAs quantum dot. The spectrum changes from Lorenztian-shaped to dip-shaped with increasing excitation intensity. A theoretical analysis with the density matrix method reveals that exciton–biexciton coherent interactions create dressed states in the exciton–biexciton system and lead to the unusual absorption spectra.
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Photoluminescence excitation
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Abstract The investigation results of exciton, exciton-magnon and biexciton absorption by α-oxygen at T = 1.5 K are reported. It is shown that the doublet structure of exciton-magnon bands is due to anisotropy of the exciton and rnagnon bands; the doublet structure of the bands in the two-particle absorption results from the bound two-exciton states, the biexcitons, present in α-O2.
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Magnon
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The influence of excitonic localization on the biexcitonic binding energy in quasi-3D and quasi-2D structures is investigated. We consider here localization energies smaller than the exciton binding energy, not significantly changing the exciton internal motion. We show, that the biexciton binding energy increases for localization energies comparable to or larger than the biexciton binding energy. In the limit of large localization energies, the biexciton binding energy reaches about 40% of the exciton binding energy, independent of the initial dimensionality of the system. This is discussed considering the zero-point kinetic energy of the exciton–exciton motion in the biexciton. The biexciton binding energy accordingly shows a dependence on the localization depth and on the shape of the exciton localization potential.
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We have investigated photoluminescence (PL) properties of GaAs (15 nm)/AlAs (15 nm) and GaAs (20 nm)/AlAs (20 nm) multiple quantum wells at 10 K under high density excitation conditions at excitation energies in the region of the fundamental excitons. It has been found that the PL due to exciton-exciton scattering, the so-called P emission, is observed with a threshold nature in addition to the appearance of the biexciton PL. The energy spacing between the P-PL band and the heavy-hole exciton depends on the layer thickness, which reflects the change of the exciton binding energy by the quantum size effect. The intensity of the biexciton-PL band is saturated by the appearance of the P-PL band. Both the exciton-exciton scattering process and the biexciton formation process require the collision of two excitons. Thus, the exciton-exciton scattering process prevents the formation of biexcitons, which leads to the saturation behavior of the biexciton-PL intensity. Furthermore, we have confirmed the existence of optical gain leading to stimulated emission due to the exciton-exciton scattering process with use of a variable-stripe-length method.
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Abstract Optical properties in (C 4 H 9 NH 3 ) 2 PbBr 4 single crystals under high density excitation have been investigated by photoluminescence (PL) spectroscopy. The PL bands associated with the Γ 1 , Γ 2 and Γ 5 excitons and the biexcitons have been observed. While the Γ 2 exciton and biexciton PL intensities are proportional to the 0.9 and 1.7 power of the excitation density, respectively, the Γ 5 exciton PL intensity shows the nonlinear dependence on the excitation density. The excitation‐density dependence of the PL bands is calculated by taking account of the dynamical change of the exciton and biexciton populations in an exciton‐biexciton system consisting of two exciton states and a biexciton state. The calculated results well explain the nonlinear dependence of the Γ 5 exciton PL intensity on the excitation density. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
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A comprehensive experimental investigation of exciton and biexciton recombination in GaAs/AlGaAs quantum wells is presented. Exciton and biexciton recombination times are found to be 16 and 55 ps, respectively. A method of determining the dynamics of the exciton temperature is developed. It is shown that both exciton and biexciton recombination processes increase the exciton temperature by an amount as high as $\ensuremath{\sim}10$ K. These processes impose a new restriction on the possibility of exciton Bose-Einstein condensation and make impossible its achievement in a system of direct excitons in GaAs quantum wells even for resonantly excited exciton gas.
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Transient photoinduced absorption spectra of one-dimensional excitons and biexcitons coupled with lattice vibrations are calculated with a two-band model by molecular dynamics. It is shown that the absorption peaks corresponding to the transition from photoexcited Bu excitons to Ag excitons and the lowest biexciton shift as self-trapping proceeds, where the former shifts to higher energy and the latter to lower energy. This difference is understood by means of the relaxation energy of Bu, Ag excitons and the biexciton. The transient absorbance change near the Bu to Ag peak is also discussed.
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Absorption band
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The emission lines of biexciton and donor-bound biexciton are observed in a high-quality ZnO thin-film sample with time-resolved photoluminescence (TRPL) measurement. The TRPL intensity profiles reveal the formation sequence of various types of exciton. After free excitons are first generated, part of them is trapped by neutral donors to form donor-bound excitons. The other part contributes to the generation of biexcitons through free exciton scattering. Next, a donor-bound biexciton is generated through the trapping of a biexciton or two free excitons by a neutral donor or the trapping of a free exciton by a donor-bound exciton. Except donor-bound exciton, the relaxations of all other exciton states show two decay stages. Either the increasing or decreasing trends of the calibrated decay times in increasing the excitation power are well interpreted with a four-level model.
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