A double feature: Phase behaviour of a bio-lipid monolayer


Terahertz studies of exciton and charge carrier dynamics

Mischa Bonn

Leiden Institute of Chemistry, Einsteinweg 55, P.O. Box 9502, 2300 RA Leiden, The Netherlands.
FOM-Institute AMOLF, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands

Phospholipids are key constituents of cell membranes. Thanks to their amphiphilicity and ability to self-organize they form the skeleton of living cells. Knowledge of the behaviour of the self-organization of these phospholipids thus forms an important aspect in the understanding of cell structure. We have investigated the self-organizing behaviour of a phospholipid monolayer, in contact with water. We can map the order inside the alkyl chains using Vibrational Sum-Frequency Generation (VSFG), and the mesoscopic properties of the monolayer using fluorescence microscopy. This unique combination of optical tools allows us to observe, as the molecular surface area is increased, a novel, sharp phase transition, attributed to the curling up of the apolar alkyl chains as their exposure to the polar water environment increases. We also present recent results on the novel application of VSFG to non-planar surfaces.

Although whole new classes of materials such as (semi-)conducting polymers and nanoparticles are finding applications in novel opto-electronic devices (solar cells, LEDís, lasers), the elementary photo-physics of many of these materials has remained subject of intense debate. These unresolved issues include: What is the primary photo-product following optical excitation: free charges or bound electron-hole pairs (excitons); What is the polarizability of quantum-confined excitons in nanoparticles (determining the color tuning in nanoparticles through the Stark shift); what limits charge transport in these materials? We will demonstrate the merit of time-domain Terahertz (THz) spectroscopy in addressing these issues. THz spectroscopy provides a non-contact (all-optical) probe of both the real and imaginary conductivity, with sub-picosecond time resolution. As such, the dynamics of excitons and charges in these novel materials can be followed with unprecedented resolution.