Multidimensional Femtosecond Spectroscopy

Multidimensional optical spectroscopy has over the past decade become an important and increasingly utilised technique for revealing dynamics in complex systems. This technique has revealed details of energy transfer in photosynthetic light-harvesting complexes, resolved different many-body effects in semiconductor quantum wells, explored high order correlations between excitons in semiconductor quantum wells, and identified long-lived coherences in conjugated polymers and light harvesting complexes.

In order to gain even greater insights we ahve developed a technique that allows us to obtain phase information without interferometry, which opens up experimental possibilities previously inaccessible. To date we have used these techniques to explore a range of different systems as described under the other projects.

We continue to develop our phase retrieval approach for multidimensional spectroscopy, as well as other new experiments that will, for example, allow us to perform quantitative multi-colour multidimensional spectroscopy and access precise details of the quantum state evolution of complex systems.

• J.A. Davis, C.R. Hall, H.M. Quiney, K.A. Nugent, H.H. Tan, C. Jagadish, Three-Dimensional Electronic Spectroscopy of Excitons in Asymmetric Double Quantum Wells, J. Chem. Phys. 135, 044510 (2011).  
• J.A. Davis, H.M. Quiney, T. Calhoun and K.A. Nugent, Ultrafast Optical multi-dimensional spectroscopy without interferometry, J. Chem. Phys. 134, 024504 (2011).
• J.A. Davis , L.V. Dao, M.T. Do, P. Hannaford, K.A. Nugent, and H.M. Quiney, Noninterferometric Two-Dimensional Fourier-Transform Spectroscopy of Multilevel Systems, Phys. Rev. Lett. 100, 227401 (2008).

Quantum Coherence in Photosynthetic Light Havesting Complexes

Photosynthesis, the process by which energy from the sun is collected and stored as chemical energy, is responsible for all life on this planet. Until recently it was assumed that classical physics was sufficient to be able to understand these inherently biological processes. In 2007, however, two papers showed that quantum coupling between different states in light-harvesting complexes can remain coherent for relatively long times (>200 fs). These initial experiments were performed at 77K, however, subsequent experiments at room temperature were also able to observe similar long-lived coherences.

These results have sparked much speculation and subsequent research by theoretical physicists and chemists regarding the potential role of quantum effects and the role played by the protein matrix in maintaining coherence and facilitating efficient energy transfer. To date there have been no experiments to clearly confirm or refute any of the predictions.

We are utilising our unique two-colour multi-dimensional spectroscopy experiment to probe these long-lived coherences in greater detail than previously possible. In this technique we are able to selectively excite specific pathways and directly probe the quantum mechanical interactions between the electronic states and the vibrational modes of the chromophores nad protein matrix. We aim to gain unprecedented insight into the role of the protein matrix and the significance of intrinsically quantum mechanical process for energy transfer in the early stages of photosynthesis.

• G.H. Richards, K. Wilk, P.M.G Curmi, H.M. Quiney and J.A. Davis, Coherent vibronic coupling in light-harvesting complexes from photosynthetic marine algae, J. Phys. Chem. Lett. 3, 272-277 (2012).
• N.S. Ginsberg J.A. Davis , M. Ballottari, Y-C Chen, R. Bassi and G.R. Fleming, Solving structure in the CP29 light harvesting complex with polarization-phased 2D electronic spectroscopy, Proc. Natl Acad. Sci. USA 108 , 3848 (2011).

Coherently coupled quantum wells and quantum dots

Semiconductor quantum wells and quantum dots have been of significant interest for studying fundamental physics and for device applications since epitaxial growth techniques were able to be controlled to produce such structures. Recent experiments exploring coherent coupling between spatially separated quantum dots have revealed strong coupling over distances up to 1µm, with very little dependence on the separation. Such strong coupling without any discernible distance dependence cannot be explained by any of the standard coupling mechanisms. . Such a long-range coupling mechanism is potentially significant for a range of disparate fields including quantum information and energy transfer in photosynthesis. The best explanation to date attributes the coupling to a biexcitonic renormalization, and the long-range nature of the coupling is attributed to the existence of spatially extended exciton states. There remains significant conjecture over this mechanism and so it is important that this phenomenon is fully understood.

We have recently utilised our unique multidimensional femtosecond spectroscopy techniques to reveal coupling between different exciton states in an asymetric double quantum well sample. The observatioin of oscillating population density and the peak shapes in the 2D spectrum reveals details of the coupling between the spatially separated excitons and the role of many body effects.

We continue to explore the nature of coupling between spatailly separated excitons in coupled quantum wells and quantum dots, and with the new experimental capabilities being developed we will be able to reveal further details of the mechanism for long-range coupling described above.

J.A. Davis, C.R. Hall, H.M. Quiney, K.A. Nugent, H.H. Tan, C. Jagadish, Three-Dimensional Electronic Spectroscopy of Excitons in Asymmetric Double Quantum Wells, J. Chem. Phys. 135, 044510 (2011).

Resolving the Excited State Structure and Dynamics in Carotenoids

The electronic structure of carotenoids allows them to participate in seemingly opposing roles in photosynthesis as both light harvestors, absorbing and donating light energy, and as photoprotectors, safely dissipating excess energy. The bright colors of carotenoids arise from the strong S0 to S2 transition near 500 nm because these linear polyene molecules have C2h-like symmetry, resulting in an optically "dark" (i.e., the transition is not allowed for one-photon absorption from S0) first excited singlet state (S1). The lifetime of the S2 state is very short, <200 fs, while the lower energy S1, populated via relaxation, lasts into the picosecond regime as probed through the S1 to S1n excited state absorption (ESA)

Despite the importance of this class of molecule and their relative simplicity, there is still much conjecture over the nature of the excited states and the relaxation pathways. We seek to use both broadband and narrowband two-colour multidimensional spectroscopy to reveal the physics behind the excitation and subsequent relaxation of carotenoids both isolated in solution and when incorporated within biological light-harvesting complexes.

Recent Papers:

• T.R. Calhoun, J.A. Davis , M.W. Graham, G.R. Fleming, The Separation of Overlapping Transitions in ß-carotene with Broadband 2D Electronic Spectroscopy, Chem. Phys. Lett. (In Press). DOI:10.1016/j.cplett.2011.10.051
• J.A. Davis , E. Cannon, L.V. Dao, P. Hannaford, H.M. Quiney and K.A. Nugent, Long-lived coherence in carotenoids, New J. Phys. 12, 085015 (2010).

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:
C.R. Hall et al., Physical Review B 80, 235316 (2009) 
J.A. Davis and C. Jagadish, Laser and Photonics Review 3, 85-97 (2009) 
J.A. Davis, et al. Nanotechnology 19, 055205 (2008)
J.A. Davis et al., Appl. Phys. Lett. 89, 182109 (2007) 
X. Wen et al., Appl. Phys. Lett. 90, 221914 (2007)