All-optical measurement of the giant ambipolar diffusion constant in a hetero-nipi reflection modulator

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Abstract
We have measured the in-plane ambipolar diffusion constant in a hetero-nipi reflection modulator using a novel all-optical technique. This technique allows us to monitor the time evolution of an excess carrier population at any point on the sample without the need for electrical contacts. The diffusion is found to be enhanced by almost an order of magnitude over that in bulk material (to values of 45 cm2 s-1) due to the 'giant ambipolar diffusion' mechanism arising from perturbations to the periodic nipi potential. This has important implications in the use of nipi structures as fast optical switching elements as it ensures uniform switching across an individual pixel in a device where the in-plane diffusion dynamics are typically the limiting factor in the switching speed.