Optical Gain of Vertically Coupled Cd 0.6 Zn 0.4 Te/ZnTe Quantum Dots.

The optical modal gain of Cd 0.6 Zn 0.4 Te/ZnTe double quantum dots was measured using a variable stripe length method, where large and small quantum dots are separated with a ZnTe layer. With a large (~18 nm) separation layer thickness of ZnTe, two gain spectra were observed, which correspond to the confined exciton levels of the large and small quantum dots, respectively. With a small (~6 nm) separation layer thickness of ZnTe, a merged single gain spectrum was observed. This can be attributed to a coupled state between large and small quantum dots. Because the density of large quantum dots (4 × 10 10 cm -2 ) is twice the density of small quantum dots (2 × 10 10 cm -2 ), the density of the coupled quantum dots is determined by that of small quantum dots. As a result, we found that the peak gain (123.9 ± 9.2 cm -1 ) with the 6 nm separation layer is comparable to that (125.2 ± 29.2 cm -1 ) of the small quantum dots with the 18 nm separation layer.