The researchers at the University of Pennsylvania have worked to decrease the emission lifetime of semiconductors for developing ultra-fast photonic devices by designing unique nanowires.
An associate professor, Ritesh Agarwal stated that when a semiconductor is excited, it returns to the ground state in a few nanoseconds along with the emission of light. That period is regarded as the emission lifetime, he added. A semiconductor is regarded to be in the excited state when there is energy in it and in the ground state when there is no energy. Generally, a semiconductor in its high-energy excited state first cools down, emits energy in the form of heat and then returns to the ground state. The left out energy is discharged in the form of light.
Semiconductors nanowires developed by Pennsylvania University researchers can make the semiconductors to skip the cooling down process by shifting it directly from excited state to the ground state. This improvement in emission lifetime was because of the innovative design of the nanowires. The core material used was cadmium sulfide. Besides, these nanowires were coated with a silicon dioxide buffer layer and an outer silver layer. The silver layer supports specific waves called surface plasmons that are formed by oscillation of light and metal electrons. The surface plasmons are restricted to the region at the junction of the silicon dioxide and silver layer.
In the case of nanowires having specific sizes, resonance is created by the silver layer resulting in the formation of highly-focused electromagnetic fields inside the nanostructure. The emission lifetime can be manipulated by accurately controlling the highly-focused electromagnetic fields within the light-emitting area, the cadmium sulfide core.
In order to attain an emission lifetime determined in femtoseconds, the high-intensity electromagnetic field must be balanced with a suitable quality factor that measures the performance of the cavity in accumulating energy. By selecting a good quality factor, the research team were able to increase the electric field intensity within the nanowire by utilizing resonant surface plasmons and hence considerably reduce emission lifetime. This advancement can be helpful in numerous applications such as nanophotonic devices detectors, and LEDs.