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Dhritiman Bhattacharyya

Electronic Sum Frequency Generation
Dhritiman Bhattacharyya

A photovoltaic cell is a specialized semiconductor device that converts energy of light into DC electricity. These cells are made by sandwiching layers of organic electronic materials (typically one is donor and the other one is acceptor) between two metallic conductors. The whole operation requires three basic attributes:

     1.  Light is absorbed to generate electron hole pair or excitons.
     2.  These opposite charge carriers are separated.
     3.  After separation they are directed to flow through an external circuit.

The second step, exciton dissociation, or the separation of an electron-hole pair typically occurs at the donor-acceptor interface. So, understanding the electronic properties of molecules at the interface is important. A molecule may not behave the same way at an interface as it does in the bulk. To obtain specific information at the surface or interface, we have adopted a useful surface specific technique — SFG (sum frequency generation) spectroscopy. It is a powerful tool for not only identifying molecular species, but also analyzing orientation configurations on the surface or interface. This technique requires two incident beams of frequency w1 and w2 overlap spatially and temporarily at the interface and the sum frequency light is monitored to get useful information. My particular work is based on electronic SFG which deals with electronic properties at interface. Here two visible/near infrared beams are guided to the sample where the sum frequency light is resonant with a molecular electronic state. By changing the polarization combination of the input and output lights, we can investigate relative molecular orientation at the interface. We also compare SFG spectra with one photon and two photon absorption spectra, to show how much the surface properties of molecules deviate from that of bulk.