Self-assembled quantum dot (QD) structures have attracted considerable interest for the realization of broadband superluminescent light emitting diodes (SLEDs) utilizing their naturally broad emission spectrum. Bandwidths of more than 100 nm can be achieved by simultaneously introducing the ground state (GS) and excited state (ES) transitions of QDs in a QD-SLED. However, the resultant emission spectrum is multi-Gaussian with large spectra dips, which is expected to impact on the dynamic range of an OCT imaging system due to the presence of sidelobes in the interferogram. For OCT system applications, a single Gaussian or flat-topped emission spectrum is required. The main factor responsible for the spectral dips in the QD-SLEDs emission is the large energy separation between the GS and ES of the quantum dot.

Researchers at the University of Sheffield, UK, have reported the realization of a broadband QD-SLED with very flat emission spectra obtained by a post-growth annealing process (Nanotechnology 20 055204). In their work, the post-growth annealing of modulation p-doped InAs/InGaAs/GaAs QD structures is employed to fabricate the broadband SLED. The intermixed materials under a GaAs proximity cap show little degradation in optical quality as well as a blue-shift of the GS emission peak and a large reduction of the energy separation between the GS and ES.

The team attributes its findings to the suppression of Ga vacancy propagation in the modulation p-doped QD structures compared with undoped structures. Their intermixed SLEDs exhibit a 132 nm broadband and flat emission with a small spectra dip of ~1.2 dB, and integrated output power of 2 mW as (see diagram).

Back in the lab, the group plans to further improve its QD-SLEDs by broadening the emission spectrum and obtaining high powers in fiber-coupled devices.