Waveguiding effects in the measurement of optical gain in a layer of Si nanocrystals

We discuss applicability of the variable stripe length method to experimental investigation of optical gain in a luminescent layer that behaves like a planar waveguide. We show that an interplay between the output direction of guided light modes and the numerical aperture of the collection optics may lead to an artifact manifesting itself as an apparent but false gain. We propose a way to circumvent this inconvenience by using a shifting excitation spot complementary measurement. The method is demonstrated on a layer of Si nanocrystals embedded into a synthetic silica plate. (C) 2002 American Institute of Physics.