%0 journal article
%@ 0146-9592
%A Fohrmann, L.S., Lotfi, N., Alzein, B., Gaafar, M.A., Petrov, A.Y.U., Eich, M.

%D 2019
%J Optics Letters
%N 1
%P 175-178 
%R doi:10.1364/OL.44.000175
%T Free-carrier detection in a silicon slab via absorption measurement in 2D integrating cells
%U https://doi.org/10.1364/OL.44.000175
1 
%X 2D integrating cells provide long optical path lengths on a chip by multiple reflections at highly reflective mirrors similar to integrating spheres in free space. Therefore, they build a promising platform for integrated optical absorption sensing. Here, we present first absorption measurements of free carriers generated by a modulated pump laser inside a 2D integrating cell in a silicon slab. The results can be used to evaluate the lifetimes of free carriers in silicon slabs for integrated optics. Employing a silicon-on-insulator platform with a silicon thickness of 220 nm, we demonstrate measurements of the access free-carrier concentration on the order of 1−8·1015 cm−3 with lifetimes on the order of 0.1–1 μs governed by surface recombination at the silicon interfaces. The measured lifetimes are dependent on free-carrier concentration, which confirms previous observations. The presented free-carrier absorption experiment verifies the sensitivity of 2D integrating cells to changes in the absorption coefficient and thus demonstrates the potential of 2D integrating cells for absorption sensing.