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Now showing 1 - 10 of 49

Original method of GaN and InGaN quantum dots formation on (0001)AlN surface by ammonia molecular beam epitaxy
(2017-01-01)
Zhuravlev, K. S.
;
Gulyaev, D. V.
;
Aleksandrov, I. A.
;
Malin, T. V.
;
Mansurov, V. G.
We report original method of formation Ga(In)N/AlN quantum dots with low density by ammonia MBE on the (0001)AlN surface by using a decomposition process of Ga(In)N thin layer. Low density of quantum dots have been obtained in the range 10(7)-10(9) cm(-2). Single quantum dots photoluminescence lines corresponding to exciton and biexciton transitions were observed in micro-photoluminescence spectra.
Optically initialized robust valley-polarized holes in monolayer WSe2
(2015-11-01)
Hsu, Wei-Ting
;
Chen, Yen-Lun
;
Chen, Chiang-Hsiao
;
Liu, Pang-Shiuan
;
Hou, Tuo-Hung
A robust valley polarization is a key prerequisite for exploiting valley pseudospin to carry information in next-generation electronics and optoelectronics. Although monolayer transition metal dichalcogenides with inherent spin-valley coupling offer a unique platform to develop such valleytronic devices, the anticipated long-lived valley pseudospin has not been observed yet. Here we demonstrate that robust valley-polarized holes in monolayer WSe2 can be initialized by optical pumping. Using time-resolved Kerr rotation spectroscopy, we observe a long-lived valley polarization for positive trion with a lifetime approaching 1 ns at low temperatures, which is much longer than the trion recombination lifetime (similar to 10-20 ps). The long-lived valley polarization arises from the transfer of valley pseudospin from photocarriers to resident holes in a specific valley. The optically initialized valley pseudospin of holes remains robust even at room temperature, which opens up the possibility to realize room-temperature valleytronics based on transition metal dichalcogenides.
Suppressed piezoelectric polarization in single InGaN/GaN heterostructure nanowires
(2013-10-28)
Tsai, Wen-Che
;
Chen, Yen-Ting
;
Lin, Chia-Hsien
;
Hsu, Wei-Ting
;
Hsu, Yung-Sheng
The optical properties of single InGaN/GaN heterostructure nanowires (NWs) with a mean diameter down to 18 nm are investigated. Sharp emission lines originating from the recombination of localized excitons in the InGaN disk layer can be resolved. Excitation-dependent energy shifts, together with spectral diffusions of these emission lines, indicate the presence of a weak quantum confined Stark effect (QCSE) caused by nearby charge fluctuations, rather than the screening of piezoelectric polarizations at the InGaN/GaN interface. The absence of a piezoelectric polarization field is further confirmed by time-resolved photoluminescence measurements. Numerical simulations reveal that the elastic strain relaxation via the NW geometry is marginal and occurs only near the NW sidewalls. Carrier localization preferentially near the periphery of the InGaN disk and alloy intermixing are suggested as possible reasons for the absence of a polarization-induced QCSE in thin InGaN/GaN heterostructure NWs.
Diamagnetic Response of Exciton Complexes in Semiconductor Quantum Dots
(2008-12-31)
Tsai, Ming-Fu
;
Lin, Hsuan
;
Lin, Chia-Hsien
;
Lin, Sheng-Di
;
Wang, Sheng-Yun
We report measurements of diamagnetic shifts for different exciton complexes confined in small InAs quantum dots. The measured diamagnetic responses are sensitive to the number of carriers in the exciton complexes, with systematic differences between neutral excitons, biexcitons, and trions. Theoretical calculations suggest that such systematic differences arise from very different extents of electron and hole wave functions confined in small quantum dots. The measured magnetic response of Coulomb energies is found to vary with the cube of the wave function extent, and can be a sensitive probe to the electron-hole wave function asymmetry.
Impacts of structural asymmetry on the magnetic response of excitons and biexcitons in single self-assembled In(Ga)As quantum rings
(2009-08-01)
Lin, Ta-Chun
;
Lin, Chia-Hsien
;
Ling, Hong-Shi
;
Fu, Ying-Jhe
;
Chang, Wen-Hao
The diamagnetic shifts of neutral excitons and biexcitons confined in single self-assembled In(Ga)As/GaAs quantum rings are investigated. Unlike quantum dots, quantum rings reveal a considerably large biexciton diamagnetic shift, about two times larger than that of single excitons. Based on model calculations, we found that the inherent structural asymmetry and imperfection, combined with the interparticle Coulomb interactions, is the fundamental cause of the more extended biexciton wave function in the quantum rings. The exciton wave function tends to be localized in one of the potential valleys induced by structural imperfections of the quantum ring due to the strong localization of hole and the electron-hole Coulomb attraction, resembling the behavior in single dots. Our results suggest that the phase coherence of neutral excitons in quantum rings will be smeared out by such wave function localizations.
Evidence of indirect gap in monolayer WSe2
(2017-10-13)
Hsu, Wei-Ting
;
Lu, Li-Syuan
;
Wang, Dean
;
Huang, Jing-Kai
;
Li, Ming-Yang
Monolayer transition metal dichalcogenides, such as MoS2 and WSe2, have been known as direct gap semiconductors and emerged as new optically active materials for novel device applications. Here we reexamine their direct gap properties by investigating the strain effects on the photoluminescence of monolayer MoS2 and WSe2. Instead of applying stress, we investigate the strain effects by imaging the direct exciton populations in monolayer WSe2-MoS2 and MoSe2-WSe2 lateral heterojunctions with inherent strain inhomogeneity. We find that unstrained monolayer WSe2 is actually an indirect gap material, as manifested in the observed photoluminescence intensity-energy correlation, from which the difference between the direct and indirect optical gaps can be extracted by analyzing the exciton thermal populations. Our findings combined with the estimated exciton binding energy further indicate that monolayer WSe2 exhibits an indirect quasiparticle gap, which has to be reconsidered in further studies for its fundamental properties and device applications.
Determination of s-d exchange coupling in GaMnN by time-resolved Kerr rotation spectroscopy
(2014-09-29)
Hsu, Wei-Ting
;
Hsieh, Ting-Yen
;
Chen, Hsin-Feng
;
Huang, Feng-Wen
;
Chen, Po-Cheng
Coherent electron-spin dynamics in Ga1-xMnxN has been investigated by time-resolved Kerr rotation spectroscopy. The effective electron g factor shows a linear increase with the Mn concentration due to the s-d exchange coupling between the conduction electrons and the d-shell electrons of Mn3+ impurities. The magnitude and sign of the s-d exchange constant are determined precisely to be N-0 alpha = +0.23 +/- 0.02 eV, indicative of a ferromagnetic s-d exchange coupling in GaMnN. The determined N-0 alpha is consistent with the typical value found in most diluted magnetic semiconductors and reveals that GaMnN is indeed not an exception.
Nonresonant carrier transfer in single InGaAs/GaAs quantum dot molecules
(2008-06-01)
Chang, Wen-Hao
;
Lin, Hsuan
;
Wang, Sheng-Yun
;
Lin, Chia-Hsien
;
Cheng, Shun-Jen
We present a spectroscopic study of single quantum dot molecules (QDMs) formed by two closely stacked In(0.5)Ga(0.5)As layers. The exciton fine structures as well as direct and indirect excitonic species associated with QDMs were identified by power dependent and polarization resolved microphotoluminescence measurements. As the temperature was increased, a directional energy transfer between the direct and indirect excitons in single QDMs was observed. A rate-equation model was developed to explain our data. We show that a phonon-assisted nonresonant tunneling of the hole between the two adjacent dots is responsible for such directional energy transfers in QDMs.
Anticorrelation between the splitting and polarization of the exciton fine structure in single self-assembled InAs/GaAs quantum dots
(2011-02-28)
Lin, Chia-Hsien
;
You, Wen-Ting
;
Chou, Hsiang-Yu
;
Cheng, Shun-Jen
;
Lin, Sheng-Di
We report on a systematic correlation between the fine-structure splitting and polarization anisotropy of excitons in InAs/GaAs quantum dots, but with an unexpected reversal in order of the polarization eigenaxes. Such an anticorrelation is explained by a large valence-band-mixing induced splitting due to the shape and strain anisotropies. The strength and phase of valence band mixing are also found to play an important role in the tuning of fine structure splitting using an in-plane magnetic field.
Strong coupling of different cavity modes in photonic molecules formed by two adjacent microdisk microcavities
(2010-11-08)
Lin, Hsuan
;
Chen, Jhih-Hao
;
Chao, Shih-Shing
;
Lo, Ming-Cheng
;
Lin, Sheng-Di
Strong couplings between cavity modes in photonic molecules formed by two preselected nearly identical microdisk microcavities with embedded quantum dots are investigated. By continuously tuning the refractive index of one microdisk, clear anticrossings in the resonant peak energies associated with crossings in the peak linewidths can be observed. The coupling strengths are extracted by the coupled mode theory and analyzed by the model considering the effective potential confining the electromagnetic waves in the microcavities. (C) 2010 Optical Society of America

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