|Physics Department | Center For Optical Technologies | Lehigh University|
We use the light intensity pattern produced by two intersecting picosecond pulses inside an insulating crystal to excite a spatially modulated distribution of charge carriers in the valence and/or in the conduction band. The aim of this research is to investigate charge transport parameters by observing the evolution of the charge-carrier distribution in time. Most often, the excitation happens from impurity states in the bandgap and the laser wavelength is tuned so that the photoexcitation process is optimized. The final density of charge carriers obtained by this process is realtively small. So small in fact that their presence cannot change in any detectable way the refractive index or the absorption in the crystal
So, how do we observe the moving charge carriers if their presence does not directly affect any detectable material properties? The solution is found in the ability of certain materials to change their refractive index in the presence of an electric field. We exploit the fact that any movement of the photoexcited charge carriers away from their photoexcitation place is a displacement of charge that leads to an electric field. It is the refractive index modulation induced by this electric field that allows us to detect (by diffraction of an optical wave) how fast the charge carriers move and consequently determine their mobility, or the free carrier lifetime. These are the essential principles of an all-optical, contact-less technique to determine carrier mobilities inside any volumne of an electro-optic insulator, the Holographic Time of Flight method. Look here for an example of its use.
We are currently applying this method to the fundamental investigation of charge transport in polar inorganic and organic crystals where the mobility is limited by formation of polarons and by a small overlap between electronic states.
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