ZnO based quantum wells

ZnO is a material of significant recent interest due to its large band bap (3.63 eV) and large exciton binding energy (~60 meV). We are studying the dynamics of electrons, holes, and excitons in ZnO/ZnMgO quantum wells, which are important for understanding the processes that occur in these materials that are of interest for potential light emitting devices. In ZnO quantum structures there exists a large internal electric field. We have recently shown that inducing intermixing of the Zn and Mg atoms the effects of this electric field can be reduced, thereby increasing the magnitude of the optical gain achievable. We are currently investigating this further, and the possibility of designing the well profile during the QW growth to minimise the effects of the electric field.

Papers: J.A. Davis et al, Appl. Phys. Lett. 89, 182109 (2007) 
X. Wen, et al., Appl. Phys. Lett. 90, 221914 (2007)
X.M. Wen, et al., Nanotechnology 18, 315403 (2007)

Silicon quantum dots

Quantum dots are of significant interest because they are able to confine carriers in three dimensions, thereby quantising the energy levels, and allowing access to the fundamental quantum behaviours. They are also of significant interest for potential applicatins in light emitting devices and photovoltaic devices. In a collaboration with UNSW we are studying the carrier transport dynamics and optical properties in Si QDs within different matrixes. This is done with a view to designing 3rd generation photovoltaic devices.

Paper: L.V. Dao, et al., J. Mat. Sci. Mater. Electron. (in press)
L.V. Dao, et al., Appl. Phys. Lett. 90, 081105 (2007)

Carotenoid molecules

Carotenoids are a class of molecule that play an important role as light harvesting for photosynthesis. The relaxation pathways following photoexcitation and the role and location of dark-states in these processes is not well known we are using the powerful spectrally resolved 2-colour 3-pulse photon echo experiment to study the relaxation pathways and dynamics in these biologically important molecules.

Light emitting polymers

Conjugated photoluminescent polymers based on poly(p-phenylene-vinylene) PPV have become an important class of compound in recent times due to their semiconductor-like properties and have been used as components of organic light emitting devices (OLEDs) and for chemical sensing. The photophysics and excited state dynamics are strongly dependent on the structural conformation of the polymer chains. We are studying two water-soluble polyanionic PPV derivatives in the solid state and in solution to investigate the ultrafast photoinduced dynamics and the influence of the polymer conformation and interchain and intrachain interaction by modification of solvent composition. Measurement techniques include pump-probe anisotropy and temperature dependent laser induced photoluminescence.

Structural changes in DCM

Using pump-probe anisotropy measurements we are studying the transient structure of DCM (a luminescent dye molecule) in a range of solvents. The anisotropy measurements are able to identify changes in the structure, and the directional dipole moment of the ensemble of molecules. This will give us information that will help determine the presence of a charge transfer state, and the structure of the molecule in this state.

Phase Retreival from spectrally resolved 2-colour 3-pulse photon echo data

The 2-colour 3-pulse photon echo experiment is a very powerful tool to determine the dynamics of charge transfer, energy transfer, coherence transfer, carrier transport dynamics and lifetimes. However, it does not resolve the phase of the polarization generated in the sample. However, by following well known phase retrieval algorithms, used in coherent diffractive imaging (and several other engineering applications), we have developed a means to obtain the phase from our 2-dimensional measurements. This gives us the complete evolution of the macroscopic polarization in the sample, and allows us to acces all the information on any coupling and carrier transfer between states. We are continuing to further develop this method adn increasing the compexity of the algorithms to make the process more robust.