Amplified all-optical polarization phase modulator assisted by a local surface plasmon in Au-hybrid CdSe quantum dots.

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>

Sub-picosecond spin relaxation of bright excitons and imbalance suppression in asymmetric Cdse/Zns nanocrystal quantum dots under an applied magnetic field.

Kyhm, Kwangseuk,Kim, Jihoon,Yang, Ho-Soon,Je, Koo-Chul,Murayama, Akihiro American Scientific Publishers 2012 Journal of Nanoscience and Nanotechnology Vol.12 No.3

<P>The ultrafast spin dynamics of the bright exciton in CdSe/ZnS nanocrystal quantum dots has been investigated using a circularly polarized pump-probe experiment. A remarkably fast spin flip (-500 fs) of the bright exciton was observed at 4 K, which is attributed to the anisotropic electron-hole exchange interaction and the random positioning of nanocrystal quantum dots. In the presence of an applied magnetic field (5 T), the exciton spin parallel to the external magnetic field was favored due to Zeeman splitting. We found that this imbalance can possibly be suppressed by the state-blocking and the mixing of the 1(L) and 1(U) states in asymmetric quantum dots.</P>

Time-resolved spectroscopy of non-thermal carrier dynamics in GaN

K. Kyhm,R. Lota,R.A. Taylor,J.F. Ryan,N.J. Cain 한국물리학회 2006 Current Applied Physics Vol.6 No.5

The dynamics of carriers in GaN epilayers is investigated using femtosecond pump-probe spectroscopy. After the residual chirpon the continuum probe is removed, the normalized dierence spectra (NDS) for dierent probe energies are synchronized, recov-ering the full time resolution of our laser pulse. Our Monte-Carlo simulation agrees well with the unchirped NDS spectrum, whichshows the development of the carrier distribution at early times, where phonon satellites are seen, together with a strong non-ther-mal electron distribution in the region of the LO-phonon energy arising from the remarkably strong electronLO phonon interac-tion. Employing a new technique which involves the integration of the normalized NDS multiplied by the corresponding energy, ameasure of the mean energy of the carriers in non-thermal states is obtained. By comparing the time-dependent energy loss with thetheoretical energy loss rate, we estimate the eective temperature of the phonon modes as well as the population of phonons.

Electron-Hole Plasma Mott Transition and Stimulated Emission in GaN

K. KYHM,B. BEAUMONT,J. F. RYAN,P. GIBART,R. A. TAYLOR 한국물리학회 2004 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.45 No.3

We present femtosecond pump-probe re ectance measurements of excitons in GaN for various pump intensities. Saturation of the excitonic absorption with increasing carrier density has been measured in the case of resonant and non-resonant excitations during the rising times, and the exciton bleaching densities for various excitation energies have also been measured. We found that the resonant excitons are bleached at higher densities (10 times) than the non-resonant excitons due to the absence of free carriers. The stimulated emission mechanism is investigated by measuring simultaneously the photoluminescence and the time-resolved re ectance near the band edge, over a range of excitation densities. The onset of the stimulated emission coincides with the bleaching density of the non-resonant excitons as well as a theoretical gain threshold density. These results suggest that the stimulated emission in GaN is due to the electron-hole plasma.

Time-resolved gain dynamics in InGaN MQW structures

K. Kyhm,J.D. Smith,R.A. Taylor 한국물리학회 2005 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.47 No.2

Transient gain spectra were measured for an In0.02Ga0.98N / In0.16Ga0.84N multiple quantum well by using the variable-stripe-length method (VSLM) in combination with the ultrafast optical Kerrgate (OKG) technique. Gain dynamics were measured for a range of excitation lengths from short (50 μm) to long (350 μm) stripes with the sample under femtosecond photoexcitation. Analysis ofthe temporal behaviour of gain and chemical potential suggests that stimulated emission originates from a photoexcited electron-hole plasma at early times; at later times, localized states dominate as the electron-hole plasma becomes exhausted. Gain reduction at early times is attributable to coupling of the electron-hole plasma with photons along the stripe, whilst localized states are less susceptible to gain saturation.

Hot carrier dynamics and carrier-phonon interaction in GaN

K. Kyhm,N.J. Cain,R.A. Taylor 한국물리학회 2005 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.47 No.2

The dynamics of carriers in GaN epilayers is investigated by using femtosecond pump-probe spectroscopy. After the residual chirp on the continuum probe is removed, the normalized difference spectra (NDS) for different probe energies are synchronized, recovering the full time resolution of our laser pulse. Our Monte-Carlo simulation agrees well with the unchirped NDS spectrum, which shows the development of the carrier distribution at early times, where phonon satellites are seen, together with a strong non-thermal electron distribution in the region of the LO-phonon energy arising from the remarkably strong electron-LO phonon interaction. By employing a new technique which involves the integration of the normalized NDS multiplied by the corresponding energy, ameasure of the mean energy of the carriers in non-thermal states is obtained. By comparing the timedependent energy loss with the theoretical energy loss rate, we estimate the effective temperature of the phonon modes as well as the population of phonons.

Dynamics of Localized Carriers in InGaN Multi-Quantum Wells

Kwangseuk Kyhm,Robert Taylor 한국물리학회 2006 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.49 No.2I

Time-resolved photoluminescence measurements are performed in a In0.02Ga0.98N/In0.16Ga0.84N multiple-quantum well structure in order to investigate the spontaneous emission mechanism. The radiative and the non-radiative recombination times are deduced from the photoluminescence decay times using the temperature-dependent internal quantum efficiency measured from the temperaturedependent normalized photoluminescence intensity. We found that the radiative recombination time has a linear temperature-dependence for T 100 K, consistent with the two-dimensional nature of excitons. This suggests that at low temperatures 2D excitons are weakly localized at the potential minima in the quantum well.0

Comparison of Exciton Optical Nonlinearities for Resonant and Non-Resonant Excitation

Kwangseuk Kyhm,R. A. Taylor 한국물리학회 2007 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.51 No.I

The saturation of exciton resonances with increasing carrier density has been measured in the case of resonant and non-resonant excitations by using a pump-probe technique. We observed that the bleaching density for resonantly excited excitons is considerably higher than that for non-resonantly excited excitons and that with increasing excitation energy, the exciton bleaching density converges to the Mott density of free carrier model. This results suggest that resonant excitons show weaker screening than an unbound electron-hole plasma mixed with excitons and that the transient optical nonlinearities associated with screening and bandgap renormalization are governed for the ratio of excitons to free carriers with different interactions. Also, we observed that the non-resonantly excited excitons had longer lifetimes than the resonantly excited excitons, which we attributed to the relaxation of free carriers and to the polariton bottle-neck effect.

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>

Observation of a Biexciton Wigner Molecule by Fractional Optical Aharonov-Bohm Oscillations in a Single Quantum Ring

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>