Nonlinear absorption spectroscopy of InGaAs single quantum dots (QDs) was realized by means of a low-temperature near-field optical microscope. The spatial distribution of the nonlinear absorption change in single QDs was determined. The dependence of the nonlinear absorption change on the pump power density demonstrates that the nonlinearity originates from the state filling of the ground state. The nonlinear absorption spectrum showed a homogeneous broadening of the ground state of about 5 meV. Furthermore, the change in the absorption cross section of the single QD when the ground state is saturated with carriers is estimated to be 2.8±0.6nm2, which agrees with the result predicted on the basis of theoretical consideration.
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