Inserting self-assembled InAs quantum dots into quantum cascade lasers to achieve a broadband free-running frequency comb and effective radio-frequency injection

Abstract

We present what we belive to be a new band design in which self-assembled InAs quantum dots (QD) are embedded in InGaAs quantum wells (QW) to fabricate broadband single-core quantum dot cascade lasers (QDCLs) operating as frequency combs. The hybrid active region scheme was exploited to form upper hybrid QW/QD energy states and lower pure QD energy states, which expanded the total laser bandwidth by up to 55 cm-1 due to a broad gain medium provided by the inherent spectral inhomogeneity of self-assembled QDs. The continuous-wave (CW) output power of these devices was as high as 470 mW with optical spectra centered at ∼7 µm, which allowed CW operation at temperatures up to 45 °C . Remarkably, measurement of the intermode beatnote map revealed a clear frequency comb regime extending over a continuous 200 mA current range. Moreover, the modes were self-stabilized with intermode beatnote linewidths of approximately 1.6 kHz. Furthermore, what we believe to be a novel π-shaped electrode design and coplanar waveguide transition way were used for RF signal injection. We found that RF injection modified the laser spectral bandwidth by up to 62 cm-1. The developing characteristics indicate the potential for comb operation based on QDCLs as well as the realization of ultrafast mid-infrared pulse.