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Author: search.filters.author.Cheng, Shun-JenHas files: No

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    Distinctive Signatures of the Spin- and Momentum-Forbidden Dark Exciton States in the Photoluminescence of Strained WSe2 Monolayers under Thermalization
    (2019-04-01)
    Peng, Guan-Hao
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    Lo, Ping-Yuan
    ;
    Li, Wei-Hua
    ;
    Huang, Yan-Chen
    ;
    Chen, Yan-Hong
    With both spin and valley degrees of freedom, the low-lying excitonic spectra of photoexcited transition-metal dichalcogenide monolayers (TMDC-MLs) are featured by rich fine structures, comprising the intravalley bright exciton states as well as various intra- and intervalley dark ones. The latter states can be classified as those of the spin- and momentum-forbidden dark excitons according to the violated optical selection rules. Because of their optical invisibility, these two types of the dark states are in principle hardly observed and even distinguished in conventional spectroscopies although their impacts on the optical and dynamical properties of TMDC-MLs have been well noticed. In this Letter, we present a theoretical and computational investigation of the exciton fine structures and the temperature-dependent photoluminescence spectra of strained tungsten diselenide monolayers (WSe2-MLs) where the intravalley spin-forbidden dark exciton lies in the lowest exciton states and other momentum-forbidden states are in the higher energies that are tunable by external stress. The numerical computations are carried out by solving the Bethe-Salpeter equation for an exciton in a WSe2-ML under the stress-control in the tight-binding scheme established from the first principle computation in the density functional theory. According to the numerical computation and supportive model analysis, we reveal the distinctive signatures of the spin- and momentum-forbidden exciton states of strained WSe2-MLs in the temperature-dependent photoluminescences and present the guiding principle to infer the relative energetic locations of the two types of dark excitons.
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    Effects of electrostatic environment on the electrically triggered production of entangled photon pairs from droplet epitaxial quantum dots
    (2019-02-07)
    Ramirez, Hanz Y.
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    Chou, Ying-Lin
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    Cheng, Shun-Jen
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    電子物理學系
    ;
    Department of Electrophysics
    Entangled photon pair generation is a crucial task for development of quantum information based technologies, and production of entangled pairs by biexciton cascade decays in semiconductor quantum dots is so far one of the most advanced techniques to achieve it. However, its scalability toward massive implementation requires further understanding and better tuning mechanisms to suppress the fine structure splitting between polarized exciton states, which persists as a major obstacle for entanglement generation from most quantum dot samples. In this work, the influence of electrostatic environment arising from electrically biased electrodes and/or charged impurities on the fine structure splitting of GaAs/AlGaAs droplet epitaxial quantum dots is studied, by means of numerical simulations considering a realistic quantum dot confining potential and electron-hole exchange interaction within a multiband k.p framework. We find that reduction of the fine structure splitting can be substantially optimized by tilting the field and seeding impurities along the droplet elongation axis. Furthermore, our results provide evidence of how the presence of charged impurities and in-plane bias components, may account for different degrees of splitting manipulation in dots with similar shape, size and growth conditions.
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    Optimal electromechanical control of the excitonic fine structures of droplet epitaxial quantum dots
    (2018-10-19)
    Cheng, Shun-Jen
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    Yang, Yi
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    Wu, Yu-Nien
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    Liao, Yu-Huai
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    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.
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    Tailoring of the Wave Function Overlaps InAs/GaAs1-xSbx Type-II Quantum Dots
    (2010-09-01)
    Hsu, Wei-Ting
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    Liao, Yu-An
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    Lu, Shu-Kai
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    Cheng, Shun-Jen
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    Chiu, Pei-Chin
    Effects of thermal annealing on the emission properties of type-II InAs quantum dots (QDs) covered by a thin GaAs1-xSbx layer are investigated by photoluminescence (PL) and time-resolved PL measurements. Apart from large blueshifts and a pronounced narrowing of the QD emission peak, the annealing induced alloy intermixing also leads to enhanced radiative recombination rates and reduced localized states in the GaAs1-xSbx layer. We find that the type-II QD structure can sustain thermal annealing up, to 850 degrees C. In particular, we find that it is possible to manipulate between type-I and type-II recombinations in annealed QDs by using different excitation powers. We demonstrate that postgrowth thermal annealing can be used to tailor the band alignment, the wave function overlaps, and hence the recombination dynamics in the InAs/GaAs1-xSbx type-II QDs. (C) 2009 Elsevier B.V. All rights reserved.
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    Electron-hole symmetry breakings in optical fine structures of single self-assembled quantum dots (vol 42, pg 1155, 2010)
    (2010-11-01)
    Ramirez, Hanz Y.
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    Lin, Chia-Hsien
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    You, Wen Ting
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    Huang, Shan-Yu
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    Chang, Wen-Hao
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    The Maintaining of the Exciton State in the Two-QD System with Dipole Dipole Interaction and the Intrinsic Electron-Hole Exchange Interaction
    (2017-01-01)
    Huang, Shiang-Yu
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    Chen, Guang-Yin
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    Chen, Yueg-Nan
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    Cheng, Shun-Jen
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    電機學院
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    Wavelength extension beyond 1.5 mu m in symmetric InAs quantum dots grown on InP(111)A using droplet epitaxy
    (2016-10)
    Ha, Neul
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    Mano, Takaaki
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    Wu, Yu-Nien
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    Ou, Ya-Wen
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    Cheng, Shun-Jen
    By using a C-3v symmetric (111) surface as a growth substrate, we can achieve high structural symmetry in self-assembled quantum dots, which are suitable for use as quantum-entangled-photon emitters. Here, we report on the wavelength controllability of InAs dots on InP(111)A, which we realized by tuning the ternary alloy composition of In(Al, Ga) As barriers that were lattice-matched to InP. We changed the peak emission wavelength systematically from 1.3 to 1.7 mu m by barrier band gap tuning. The observed spectral shift agreed with the result of numerical simulations that assumed a measured shape distribution independent of the barrier choice. (C) 2016 The Japan Society of Applied Physics
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    Finger-gate manipulated quantum transport in Dirac materials
    (2015-05-27)
    Kleftogiannis, Ioannis
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    Tang, Chi-Shung
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    Cheng, Shun-Jen
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    電子物理學系
    ;
    Department of Electrophysics
    We investigate the quantum transport properties of multichannel nanoribbons made of materials described by the Dirac equation, under an in-plane magnetic field. In the low energy regime, positive and negative finger-gate potentials allow the electrons to make intra-subband transitions via hole-like or electron-like quasibound states (QBS), respectively, resulting in dips in the conductance. In the high energy regime, double dip structures in the conductance are found, attributed to spin-flip or spin-nonflip inter-subband transitions through the QBSs. Inverting the finger-gate polarity offers the possibility to manipulate the spin polarized electronic transport to achieve a controlled spin-switch.
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    Double-finger-gate controlled spin-resolved resonant quantum transport in the presence of a Rashba-Zeeman gap
    (2015-03-04)
    Tang, Chi-Shung
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    Tseng, Shu-Ting
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    Gudmundsson, Vidar
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    Cheng, Shun-Jen
    ;
    電子物理學系
    We investigate double finger gate (DFG) controlled spin-resolved resonant transport properties in an n-type quantum channel with a Rashba-Zeeman (RZ) subband energy gap. By appropriately tuning the DFG in the strong Rashba coupling regime, resonant state structures in conductance can be found that are sensitive to the length of the DFG system. Furthermore, a hole-like bound state feature below the RZ gap and an electron-like quasi-bound state feature at the threshold of the upper spin branch can be found that is insensitive to the length of the DFG system.