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Kim, Hee Dae,Okuyama, Rin,Kyhm, Kwangseuk,Eto, Mikio,Taylor, Robert A.,Nicolet, Aurelien L.,Potemski, Marek,Nogues, Gilles,Dang, Le Si,Je, Ku-Chul,Kim, Jongsu,Kyhm, Ji-Hoon,Yoen, Kyu Hyoek,Lee, Eun Hy American Chemical Society 2016 Nano letters Vol.16 No.1
<P>The Aharonov-Bohm effect in ring structures in the presence of electronic correlation and disorder is an open issue. We report novel oscillations of a strongly correlated exciton pair, similar to a Wigner molecule, in a single nanoquantum ring, where the emission energy changes abruptly at the transition magnetic field with a fractional oscillation period compared to that of the exciton, a so-called fractional optical Aharonov-Bohm oscillation. We have also observed modulated optical Aharonov-Bohm oscillations of an electron-hole pair and an anticrossing of the photoluminescence spectrum at the transition magnetic field, which are associated with disorder effects such as localization, built-in electric field, and impurities.</P>
Light Controlled Optical Aharonov-Bohm Oscillations in a Single Quantum Ring
Kim, Heedae,Park, Seongho,Okuyama, Rin,Kyhm, Kwangseuk,Eto, Mikio,Taylor, Robert A.,Nogues, Gilles,Dang, Le Si,Potemski, Marek,Je, Koochul,Kim, Jongsu,Kyhm, Jihoon,Song, Jindong American Chemical Society 2018 NANO LETTERS Vol.18 No.10
<P>We found that optical Aharonov-Bohm oscillations in a single GaAs/GaAlAs quantum ring can be controlled by excitation intensity. With a weak excitation intensity of 1.2 kW cm<SUP>-2</SUP>, the optical Aharonov-Bohm oscillation period of biexcitons was observed to be half that of excitons in accordance with the period expected for a two-exciton Wigner molecule. When the excitation intensity is increased by an order of magnitude (12 kW cm<SUP>-2</SUP>), a gradual deviation of the Wigner molecule condition occurs with decreased oscillation periods and diamagnetic coefficients for both excitons and biexcitons along with a spectral shift. These results suggest that the effective orbit radii and rim widths of electrons and holes in a single quantum ring can be modified by light intensity via photoexcited carriers, which are possibly trapped at interface defects resulting in a local electric field.</P> [FIG OMISSION]</BR>
Exciton Dipole–Dipole Interaction in a Single Coupled-Quantum-Dot Structure via Polarized Excitation
Kim, Heedae,Kim, Inhong,Kyhm, Kwangseuk,Taylor, Robert A.,Kim, Jong Su,Song, Jin Dong,Je, Koo Chul,Dang, Le Si American Chemical Society 2016 NANO LETTERS Vol.16 No.12
<P>We find that the exciton dipole dipole interaction in a single laterally coupled GaAs/AlGaAs quantum dot structure can be controlled by the linear polarization of a nonresonant optical excitation. When the excitation intensity is increased with the linearly polarized light parallel to the lateral coupling direction [1 (1) over bar0], excitons (X-1 and X-2) and local biexcitons (X1X1 and X2X2) of the two separate quantum dots (QD(1) and QD(2)) show a redshift along with coupled biexcitons (X1X2), while neither coupled biexcitons nor a redshift are observed when the polarization of the exciting beam is perpendicular to the coupling direction. The polarization dependence and the redshift are attributed to an optical nonlinearity in the exciton Forster resonant energy transfer interaction, whereby exciton population transfer between the two quantum dots also becomes significant with increasing excitation intensity. We have further distinguished coupled biexcitons from local biexcitons by their large diamagnetic coefficient.</P>
Kyhm, Kwangseuk,Je, Koo-Chul,Taylor, Robert A Optical Society of America 2012 Optics express Vol.20 No.18
<P>We propose an amplified all-optical polarization phase modulator assisted by a local surface plasmon in Au-hybrid CdSe quantum dots. When the local surface plasmon of a spherical Au quantum dot is in resonance with the exciton energy level of a CdSe quantum dot, a significant enhancement of the linear and nonlinear refractive index is found in both the real and imaginary terms via the interaction with the dipole field of the local surface plasmon. Given a gating pulse intensity, an elliptical polarization induced by the phase retardation is described in terms of elliptical and rotational angles. In the case that a larger excitation than the bleaching intensity is applied, the signal light can be amplified due to the presence of gain in the CdSe quantum dot. This enables a longer propagation of the signal light relative to the metal loss, resulting in more feasible polarization modulation.</P>
Quantum confined Stark effect of InGaN/GaN multi-quantum disks grown on top of GaN nanorods
Park, Young S,Holmes, Mark J,Kang, Tae W,Taylor, Robert A IOP Pub 2010 Nanotechnology Vol.21 No.11
<P>We have investigated, using micro-photoluminescence, the quantum confined Stark effect in an In<SUB><I>x</I></SUB>Ga<SUB>1 − <I>x</I></SUB>N/GaN multi-quantum disk structure at the tip of a single GaN nanorod. A strong and sharp emission line from the In<SUB><I>x</I></SUB>Ga<SUB>1 − <I>x</I></SUB>N/GaN quantum disks near 3.26 eV was observed. The peak energy of the emission line was observed to blue-shift with increasing excitation power, indicating a quantum confined Stark effect. Furthermore, both the blue-shift and the intensity of the emission saturate with increasing excitation power. The temperature-dependence of the 3.26 eV emission line has also been investigated. </P>
Surface-Effect-Induced Optical Bandgap Shrinkage in GaN Nanotubes
Park, Young S.,Lee, Geunsik,Holmes, Mark J.,Chan, Christopher C. S.,Reid, Benjamin P. L.,Alexander-Webber, Jack A.,Nicholas, Robin J.,Taylor, Robert A.,Kim, Kwang S.,Han, Sang W.,Yang, Woochul,Jo, Y. American Chemical Society 2015 NANO LETTERS Vol.15 No.7
<P>We investigate nontrivial surface effects on the optical properties of self-assembled crystalline GaN nanotubes grown on Si substrates. The excitonic emission is observed to redshift by ∼100 meV with respect to that of bulk GaN. We find that the conduction band edge is mainly dominated by surface atoms, and that a larger number of surface atoms for the tube is likely to increase the bandwidth, thus reducing the optical bandgap. The experimental findings can have important impacts in the understanding of the role of surfaces in nanostructured semiconductors with an enhanced surface/volume ratio.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/nalefd/2015/nalefd.2015.15.issue-7/acs.nanolett.5b00924/production/images/medium/nl-2015-00924p_0006.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nl5b00924'>ACS Electronic Supporting Info</A></P>
Micro- and Time-resolved Photoluminescence in GaN Nanorods with Different Diameters
박영신,윤임택,이선균,조용훈,Robert A. Taylor,임현식 한국물리학회 2010 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.57 No.4
We have investigated the optical properties of GaN nanorods with different diameters by using micro- and time-resolved photoluminescence measurements. Donor-bound and free exciton peaks are observed in GaN nanorods with diameters larger than 100 nm. While the relative magnitude of the free exciton emission gradually increases with decreasing nanorod diameter, there is a relative decrease in the emission from the donor bound exciton. Using time-resolved photoluminescence, the diameter dependence of the decay times for these exciton peaks is measured to be a few tens of ps. With decreasing diameter, the decay time decreases due to surface recombination.
Park, Youngsin,Chan, Christopher C.S.,Taylor, Robert A.,Kim, Nammee,Jo, Yongcheol,Lee, Seung W.,Yang, Woochul,Im, Hyunsik Elsevier 2018 Optical materials Vol.78 No.-
<P><B>Abstract</B></P> <P>Structural and optical properties of In<SUB> <I>x</I> </SUB>Ga<SUB>1-<I>x</I> </SUB>N/GaN multi quantum disks (QDisks) grown on GaN nanorods by molecular beam epitaxy have been investigated by transmission electron microscopy and micro-photoluminescence (PL) spectroscopy. Two types of InGaN QDisks were grown: a pseudo-3D confined InGaN pillar-type QDisks embedded in GaN nanorods; and QDisks in flanged cone type GaN nanorods. The PL emission peak and excitation dependent PL behavior of the pillar-type Qdisks differ greatly from those of the flanged cone type QDisks. Time resolved PL was carried out to probe the differences in charge carrier dynamics. The results suggest that by constraining the formation of InGaN QDisks within the centre of the nanorod, carriers are restricted from migrating to the surface, decreasing the surface recombination at high carrier densities.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Two types of InGaN/GaN quantum disks (QDisks) are grown on GaN nanorods by MBE. </LI> <LI> Time resolved PL is performed to probe the charge carrier dynamics in QDisks. </LI> <LI> GaN barriers around the sidewalls of QDisks effectively reduce surfaces effects. </LI> </UL> </P>
Selective self-assembly and characterization of GaN nanopyramids on m-plane InGaN/GaN quantum disks
Park, Young S,Holmes, Mark J,Taylor, Robert A,Kim, Kwang S,Lee, Seung-Woong,Ju, HaeRi,Im, Hyunsik IOP Pub 2012 Nanotechnology Vol.23 No.40
<P>Semiconductor nanopyramids (NPs) provide advantages in the development of novel functional optoelectronic devices due to their unique size-dependent properties. Here we demonstrate a new method for the fabrication of selectively self-assembled single-crystalline GaN NPs on the m-plane of periodically strained GaN/InGaN multiquantum disks embedded in the middle of GaN nanorods. The GaN NPs, which have ∼100 nm diameters and heights, are observed by scanning electron microscopy and their crystalline structure is confirmed by high-resolution transmission electron microscopy. Experimental analysis directly reveals the strain distribution along the growth direction of the NPs. Cathodoluminescence measurements on a single NP show that its emission energy redshifts compared with that of bulk GaN, corroborating the results showing the formation of tensile strain in the NP. Observations of the uniform distribution and localization of these NPs show the possibility of further tuning their size and density by controlling periodically strained nanorod surfaces. </P>
Optical polarization in mono and bilayer MoS2
박영신,Nannan Li,Christopher C.S. Chan,Benjamin P.L. Reid,Robert A. Taylor,임현식 한국물리학회 2017 Current Applied Physics Vol.17 No.9
Optical anisotropy in monolayer- and bilayer-MoS2 was investigated by polarization resolved photoluminescence measurements. The photoluminescence of monolayer-MoS2 is found to be partially polarized at 4.2 K and maintains this polarization characteristic up to room temperature, while the photoluminescence of bilayer-MoS2 shows no obvious polarization. This polarization anisotropy is due to strain effects at the interface between the MoS2 layer and the SiO2 substrate, causing symmetry breaking of the MoS2 charge distribution. Calculations using density functional theory of the electron density distribution of the monolayer- and bilayer-MoS2 in the in-plane direction are also presented, giving support to our qualitative analysis.