Temperature Dependence of Optical Gain in CdSe/ZnS Quantum Rods

We studied the optical gain characteristics of CdSe/ZnS core/shell colloidal quantum rods, investigated their temperature dependence, and compared the gain properties with quantum dots (QD). The gain was measured systematically for close-packed films of rods and dots under quasi-CW nanosecond optical pumping, using the variable stripe length method measuring the amplified spontaneous emission (ASE). Tunable ASE can be achieved by changing the rod diameter. Optical gain factors of up to 350 cm<sup>-1</sup> at a temperature range of 10−120 K were measured for quantum rods. Above 120 K, the gain decreased sharply, but by increasing the pump power, ASE was easily achieved also at room temperature. The temperature dependence was assigned to the Auger heating process and phonon assisted thermal relaxation. QD of similar diameters as the rods showed much smaller gain values (∼50 cm<sup>-1</sup>) and a sharp decrease in gain at lowered temperatures (∼50 K), and ASE could not be detected at room temperature even at high pump powers. The significantly improved gain values in quantum rods as compared with dots were attributed to the slower Auger relaxation rates, the higher absorption cross-section, and the reduced self-absorption due to the larger Stokes shift. The temperature dependence of the threshold power for the quantum rods, used to characterize the thermal insensitivity of the system, showed two distinct temperature regions. In the low-temperature region, a very high <i>T</i><sub>0</sub> value of 3500 K was measured, as predicted for a low-dimensional quantum confined system.