Low power optical saturation of bound excitons with giant oscillator strength

The optical saturation of the I2 bound exciton in cadmium sulfide, an exciton bound to a donor, is reported. This system, which decays radiatively in 500 ps, can be saturated with a cw power of only 3.6 μW, which is equivalent to a saturation intensity of 58 W/cm2. Good agreement is found with the predictions of an inhomogeneously broadened two-level system. Thermal effects appear to play a minimal role.