Asymmetry of localised states in a single quantum ring: polarization dependence of excitons and biexcitons

We performed spectroscopic studies of a single GaAs quantum ring with an anisotropy in the rim height. The presence of an asymmetric localised state was suggested by the adiabatic potential. The asymmetry was investigated in terms of the polarization dependence of excitons and biexcitons, where a large energy di erence (0.8 meV) in the exciton emission energy for perpendicular polarizations was observed and the oscillator strengths were also compared using the photoluminescence decay rate. For perpendicular polarizations the biexciton exhibits twice the energy di erence seen for the exciton, a fact that may be attributed to a possible change in the selection rules for the lowered symmetry.Comment: accepted in Applied physics Letter

Excited exciton and biexciton localised states in a single quantum ring

We observe excited exciton and biexciton states of localised excitons in an anisotropic quantum ring, where large polarisation asymmetry supports the presence of a crescent-like localised structure. We also find that saturation of the localised ground state exciton with increasing excitation can be attributed to relatively fast dissociation of biexcitons (? 430 ps) compared to slow relaxation from the excited state to the ground state (? 1000 ps). As no significant excitonic Aharonov-Bohm oscillations occur up to 14 T, we conclude that phase coherence around the rim is inhibited as a consequence of height anisotropy in the quantum ring.Comment: 4 pages, 4 figure

Time-resolved spectroscopy of excitons and carriers in GaN and InGaN

delocalised electron-hole pairs from the lowest confined level are responsible for the gain in our sample. The polarization dependence of biexcitonic signals and quantum beats between A-excitons (X sub A) and A-biexcitons (X sub A X sub A) in a high-quality GaN epilayer are measured by spectrally-resolved and time-integrated four-wave mixing measurements. We also measured the polarization dependent B-biexciton (X sub B X sub B) signal. The emission mechanisms in GaN and ln sub x Ga sub 1 sub - sub x N is systematically studied to investigate carrier and exciton dynamics. Reflectance, time-integrated luminescence, and time-resolved reflectance spectroscopy are used to study exciton transitions in GaN, and the saturation of the exciton resonances with increasing carrier density has been measured in the case of resonant and non-resonant excitations. The coincidence between the density for the onset of the stimulated emission and the Mott density leads us to the conclusion that the stimulated emission mechanism in GaN is due to the electron-hole plasma. A detailed study of hot carrier relaxation at low temperature is presented, using pump-probe transmission spectroscopy. At early times, phonon satellites are seen, and a strong non-thermal electron distribution in the region of the LO-phonon energy arising from the remarkably strong electron-LO phonon interaction are observed. A measure of the mean energy of the carriers in non-thermal states agree well with our Monte-Carlo model, and confirms the dominance of hot phonon effect at early times. Time-resolved photoluminescence measurements are performed in a In sub 0 sub . sub 0 sub 2 Ga sub 0 sub . sub 9 sub 8 N/In sub 0 sub . sub 1 sub 6 Ga sub 0 sub . sub 8 sub 4 N multiple quantum well structure to investigate the spontaneous recombination mechanism. The gain spectrum for the different stripe lengths using the VSLM technique suggests that nearl

Hot carrier dynamics and carrier-phonon interaction in GaN

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 remark-ably strong electron-LO phonon interaction. By employing a new technique which involves the integration of the normalized NDS multiplied by the corresponding energy, a measure of the mean energy of the carriers in non-thermal states is obtained. By comparing the time-dependent energy loss with the theoretical energy loss rate, we estimate the effective temperature of the phonon modes as well as the population of phonons

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

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

Time-resolved gain dynamics in InGaN MQW structures

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 Kerr-gate (OKG) technique. Gain dynamics were measured for a range of excitation lengths from short (50 mu m) to long (350 mu m) stripes with the sample under femtosecond photoexcitation. Analysis of the 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

Excitation and temperature dependence of the broad gain spectrum in GaAs/AlGaAs quantum rings

We have employed a variable stripe length method in order to measure the optical gain of GaAs/AlGaAs quantum rings. Although the large lateral diameter of quantum rings (∼ 50 nm) with a few nm size distribution is expected to cause a small spectral inhomogeneity (∼ 1 %), a broad gain width (∼ 300 meV) was observed. This result was attributed to a variation of the vertical heights and variations in localized states that exhibit crescent shaped wavefunctions, whereby the energy levels are distributed over a broad spectral range. When the excitation intensity is decreased, irregular peaks appear in the gain spectrum gradually. Similar phenomena were also observed with increased temperature. We conclude that excited carriers in quantum rings are distributed stochastically at various localized states, and the population inversion is sensitive to excitation intensity and temperature