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Register a new userThe dynamics of amplified spontaneous emission in CdSe/ZnSe quantum dots
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We have used the variable stripe technique and pump-probe spectroscopy to investigate both gain and the dynamics of amplified spontaneous emission from CdSe quantum dot structures. We have found modal gain coefficients of 75 and 32 1/cm for asymmetric and symmetric waveguide structures, respectively. Amplified spontaneous emission decay times of 150 and 300 ps and carrier capture times of 15 and 40 ps were measured for the structures with high and low material gains respectively. The difference in the capture times are related to the fact that for the symmetric waveguide, carriers diffuse into the active region from the uppermost ZnMgSSe cladding layer, yielding a longer rise time for the pump-probe signals for this sample.
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We show that two major carrier excitation mechanisms are present in II-VI self-assembled quantum dots. The first one is related to direct excited state - ground state transition. It manifests itself by the presence of sharp and intense lines in the excitation spectrum measured from single quantum dots. Apart from these lines, we also observe up to four much broader excitation lines. The energy spacing between these lines indicates that they are associated with absorption related to longitudinal optical phonons. By analyzing resonantly excited photoluminescence spectra, we are able to separate the contributions from these two mechanisms. In the case of CdTe dots, the excited state - ground state relaxation is important for all dots in ensemble, while phonon - assisted processes are dominant for the dots with smaller lateral size.
Using micro- and nano-scale resonantly excited PL and PLE, we study the excitonic structure of CdSe/ZnSe and CdTe/ZnTe self assembled quantum dots (SAQD). Strong resonantly enhanced PL is seen at one to four optic phonon energies below the laser excitation energy. The maximum enhancement is not just one phonon energy above the peak energy distribution of QDs, but rather is 50 meV (for CdSe dots) or 100 meV (for CdTe) above the peak distribution. We interpret this unusual result as from double resonances associated with excited state to ground state energies being commensurate with LO phonons. Such a situation appears to occur only for the high-energy quantum dots.
We study spin dynamics of excitons confined in self-assembled CdSe quantum dots by means of optical orientation in magnetic field. At zero field the exciton emission from QDs populated via LO phonon-assisted absorption shows a circular polarization of 14%. The polarization degree of the excitonic emission increases dramatically when a magnetic field is applied. Using a simple model, we extract the exciton spin relaxation times of 100 ps and 2.2 ns in the absence and presence of magnetic field, respectively. With increasing temperature the polarization of the QD emission gradually decreases. Remarkably, the activation energy which describes this decay is independent of the external magnetic field, and, therefore, of the degeneracy of the exciton levels in QDs. This observation implies that the temperature-induced enhancement of the exciton spin relaxation is insensitive to the energy level degeneracy and can be attributed to the same excited state distribution.
We report semi-empirical pseudopotential calculations of emission spectra of charged excitons and biexcitons in CdSe nanocrystals. We find that the main emission peak of charged multiexcitons - originating from the recombination of an electron in an s-like state with a hole in an s-like state - is blue shifted with respect to the neutral mono exciton. In the case of the negatively charged biexciton, we observe additional emission peaks of lower intensity at higher energy, which we attribute to the recombination of an electron in a p state with a hole in a p state.
We have studied the electronic structure of CdS/ZnSe coupled quantum dot, a novel heterostructure at the nano-scale. Our calculations reveal CdS/ZnSe coupled quantum dots to be of type-II in nature where the anion-p states play an important role in deciding the band offset for the highest occupied molecular orbitals (HOMO). We show that the offsets of HOMO as well as the lowest unoccupied molecular orbitals (LUMO) can be tuned by changing the sizes of the components of the coupled quantum dot, thereby providing an additional control parameter to tune the band gap and the optical properties. Our investigations also suggest that formation of alloy near the interface has very little influence on the band offsets, although it affects the spatial localization of the quantum states from the individual components. Comparing the influence of strain on coupled quantum dots and core/shell nanowires, we find strain practically has no role in the electronic structure of coupled quantum dots as the small effective area of the interface in a coupled quantum dot helps a large part of the structure remain free from any substantial strain. We argue that in contrast to core-shell nanowires, quantum confinement is the key parameter that controls the electronic properties of coupled quantum dot and should therefore be an ideal candidate for the design of a quantum device.
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