A technique is proposed for determining the spatio-temporal dynamics of current in semiconductor heterostructures. This technique is based on the modulation of external radiation during passage through the crystal under study. Approbation of the technique was carried out on semiconductor laser-thyristor based on AlGaAs/ InGaAs/GaAs heterostructures. The experimental results are in a good qualitative agreement with previous measurements of the spatio-temporal dynamics in the laser-thyristor.