Browsing by Author "Liao, Yu-Huai"

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Dominant channels of exciton spin relaxation in photoexcited self-assembled (In,Ga)As quantum dots
(2011-04-22)
Liao, Yu-Huai
;
Climente, Juan I.
;
Cheng, Shun-Jen
;
電子物理學系
;
Department of Electrophysics
We present a comprehensive theoretical investigation of spin relaxation processes of excitons in photoexcited self-assembled quantum dots. The exciton spin relaxations are considered between dark- and bright-exciton states via the channels created by various spin-admixture mechanisms, including electron Rashba and Dresselhaus spin-orbital couplings (SOCs), hole linear and hole cubic SOCs, and electron hyperfine interactions, incorporated with single-and double-phonon processes. The hole-Dresselhaus SOC is identified as the dominant spin-admixture mechanism, leading to relaxation rates as fast as similar to 10(-2) ns(-1), consistent with recent observations. Moreover, due to significant electron-hole exchange interactions, single-phonon processes are unusually dominant over two-phonon ones in a photoexcited dot even at temperatures as high as 15K.
Geometrical impact on the optical polarization of droplet epitaxial quantum dots
(2012-09-17)
Liao, Yu-Huai
;
Liao, Chien-Chih
;
Ku, Chih-Hao
;
Chang, Yu-Chen
;
Cheng, Shun-Jen
We report on experimental and theoretical investigations of the optical anisotropy of GaAs/AlGaAs quantum dots grown by droplet epitaxy. With in situ annealing in the growth, the shape of quantum dots is systematically controlled from a tall and laterally symmetric shape to a flat and laterally elongated one. Photoluminescence spectroscopy demonstrates an uncommon observation: the more elongated the quantum dots, the lower the degree of linear polarization. Theoretical analysis based on a four-band k center dot p theory reveals a substantial impact of vertical confinement on the valence heavy-hole and light-hole mixing, which leads to the enhancement of polarization anisotropy for taller quantum dots. The influence of Coulomb interactions on polarization anisotropy is studied by using the partial configuration interaction method, and is shown to reduce the polarization anisotropy through the mixing of single-particle configurations with different symmetries.
Mechanically encoded single-photon sources: Stress-controlled excitonic fine structures of droplet epitaxial quantum dots
(2015-03-20)
Cheng, Shun-Jen
;
Liao, Yu-Huai
;
Lin, Pei-Yi
;
電子物理學系
;
Department of Electrophysics
We present numerical investigations based on the Luttinger-Kohn four-band k . p theory and, accordingly, establish a quantitatively valid model of the excitonic fine structures of droplet epitaxial GaAs/AlGaAs quantum dots under uniaxial stress control. In the formalisms, stressing a photoexcited quantum dot is equivalent creating a pseudomagnetic field that is directly coupled to the pseudospin of the exciton doublet and tunable to tailor the polarized fine structure of exciton. The latter feature is associated with the valence-band mixing of exciton, which is especially sensitive to external stress in inherently unstrained droplet epitaxial GaAs/AlGaAs quantum dots and allows us to mechanically design and prepare any desired exciton states of QD photon sources prior to the photon generation.
Optimal electromechanical control of the excitonic fine structures of droplet epitaxial quantum dots
(2018-10-19)
Cheng, Shun-Jen
;
Yang, Yi
;
Wu, Yu-Nien
;
Liao, Yu-Huai
;
Peng, Guan-Hao
The intrinsic fine structure splittings (FSSs) of the exciton states of semiconductor quantum dots (QDs) are known to be the major obstacle for realizing the QD-based entangled photon pair emitters. In this paper, we present a theoretical and computational investigation of the excitonic fine structures of droplet-epitaxial (DE) GaAs/AlGaAs QDs under the electromechanical control of micromachined piezoelectric actuators. From the group theory analysis with numerical confirmation based on the developed exciton theory, we reveal the general principle for the optimal design of micromachined actuators whose application onto an elongated QD can certainly suppress its FSS. We show that the use of two independent tuning stresses is sufficient to achieve the FSS elimination but is not always necessary as widely deemed. The use of a single tuning stress to eliminate the FSS of an elongated QD is possible as long as the crystal structure of the actuator material is in coincidence with that of the QD. As a feasible example, we show that a single symmetric biaxial stress naturally generated from the (001) [Pb(Mg1/3Nb2/3)O-3](0.)(72)-[PbTiO3](0.)(28) (PMN-PT) actuator can be used as a single tuning knob to make the full FSSelimination for elongated DE GaAs QDs.