Dr Jenny Clark
VC Advanced Fellow
- Room: D16
- Phone: +44 (0)114 22 23526
- email: firstname.lastname@example.org
My research interests involve understanding the photophysics of carbon-based materials such as biological materials, organic semiconductors and graphene. I do this using a variety of spectroscopic techniques. For more information see my personal webpage or contact me.
Advanced Vice-Chancellor's Fellow, University of Sheffield, UK
2009-2013 Royal Society Dorothy Hodgkin Fellow, Cambridge University, UK.
2012 Maternity leave (9 months)
2003-2007 PhD in Physics, Cambridge University, UK. Supervisor: Prof. C. Silva.
Fellowships and Awards
2014 University of Sheffield Vice-Chancellor's Fellowship (5 years).
New laser facility at Sheffield university
The University of Sheffield has a new laser facility that I help run. The facility is currently being built and will have several spectroscopy stations including:
Ultrafast spectroscopy of carbon-based materials
To study organic semiconductors and biological samples, we mainly use transient absorption spectroscopy. This technique uses laser pulses as short as 7 femtoseconds (7 millionths of a billionth of a second) to take snapshots of the electronic and vibrational state of the molecules after they have absorbed light.
In organic solar cells, absorption of light typically creates a tightly bound electron-hole pair. To collect the charges and produce current, the e-h pair must separate. This happens at a junction between an electron accepting and an electron donating material. By directly tracking the separation of the charges, we showed that they separate to 4nm within 40fs thereby overcoming the electronic Coulomb potential between the electron and hole. This fast separation occurs due to the wavelike nature of electrons which are governed by fundamental laws of quantum mechanics.
We were able to track the electron-hole separation using the electric field generated between them as they separate. This electric field perturbs the spectra of the surrounding material and leads to an electro-absorption feature. Measuring the evolution of the electroabsorption using transient absorption spectroscopy allowed us to track the charge separation in a polymer:PCBM blend and a small molecule:PCBM blend.
A single photon can alter the shape of a molecule. In the eye for example, a photon drives the cis-trans isomerisation which allows us to see. Using a pump-push-probe technique, we showed that quantum effects can play an important role in this change leading to conformation relaxation rates hundreds of times faster than previously expected.
In general, conformational change occurs on a timescale defined by the energy of the main vibrational mode and the rate of energy dissipation.
Typically, for a conformational change such as a twist around the backbone of a conjugated molecule, this occurs on the tens of picoseconds timescale. However, we demonstrated experimentally that in certain circumstances the molecule, in this case an oligofluorene, can change conformation over two orders of magnitude faster (that is sub-100 fs) in a manner analogous to inertial solvent reorganization demonstrated in the 1990s.
Theoretical simulations demonstrate that non-adiabatic transitions during internal conversion can efficiently convert electronic potential energy into torsional kinetic energy, providing the ‘kick’ that prompts sub-100 fs torsional reorganization.
See Ref.  and the News and Views by Prof. Mukamel here.
Ultrafast triplet formation: two for the price of one [3,4]
In general, photoexcitation leads to an excited state with spin-0 character known as a singlet exciton. This can convert to a spin-1 excited state (a triplet exciton) through intersystem crossing. In systems made up entirely of Carbon, Nitrogen, Oxygen, Hydrogen or even Sulphur, intersystem crossing generally takes 10s of nanoseconds or more. However, in certain systems, when the triplet energy is roughly half the singlet energy or less and there is enough space, the singlet can split into two triplet excitons.
The splitting process is known as singlet exciton fission. The ability to generate two excited states from one photon could be used in solar cells to dramatically improve their efficiency. The fission process is still not fully understood. We have studied a range of materials with the aim of understanding - and one day hopefully controlling - singlet exciton fission. In polycrystalline pentacene films, singlet fission occurs with a 80fs time-constant . In a polymer poly(3-thienylene-vinylene), it has a time-constant of roughly 45fs  and in carotenoid aggregates it ranges from 50-100fs depending on aggregate type .
Broadband optical limiting in graphene 
Strong solvent/matrix effect on the nonlinear optical properties of dispersed sub-GOx. a, Plot of output versus input fluence for a neat film of
The large non-linear effect is due to the formation of localised exciton-like states which we attribute to triplet excitons due to the heavy-atom effect. We speculate that the initial electron–hole gas condenses to triplet-like excitons when promoted by spin–orbit coupling
Ultrafast all-optical switching in plastic optical fibers 
The ability to switch light with sub-picosecond time-scales in an optical fiber network is important for improved signal data communication speed and potentially for optical computing. We made a logical NOT gate using gain switching in isolated conjugated fluorene oligomers. We use the fact that charge absorption overlaps with stimulated emission and generate charges on the oligomers which recombine within a few tens of femtoseconds. The change from amplification through stimulated emission to loss through charge absorption produces a large switching signal and the recombination allows for ultrafast sub 100fs switching.
Change in transmission when excited with a gating pulse. Moving from ON (gain) to OFF (loss) and back again within 150fs.
 Ultrafast Long-Range Charge Separation in Organic Semiconductor Photovoltaic Diodes S. Gélinas, A. Rao, A. Kumar, S.L. Smith, A.W. Chin, J. Clark, T.S. van der Poll, G.C. Bazan, R.H. Friend Science, 343, 512 (2014)
Recent key publications
The Nature of Singlet Exciton Fission in Carotenoid Aggregates A. J. Musser, M. Mauri, D. Brida, G. Cerullo, R. H. Friend and J. Clark Journal of the American Chemical Society, 137, 5130 (2015)