Quantum cascade lasers based on quantum dot superlattice

The possibility of an optical amplification by a quantum well (QW) superlattice subject to a strong DC electric field was predicted more than 30 years ago [1]. This idea stimulated a 15-year research effort at Bell Labs which culminated in 1994 with the invention of a quantum cascade (QC) laser [2]. Now, the QC lasers greatly overperform mid-IR diode lasers and are used in a growing number of industrial and scientific applications [3]. The performance of a QW QC laser is, however, fundamentally limited mainly due to a wide QW subband spectrum, corresponding to a free electronic in-plane motion (motion in the perpendicular direction is quantized by QW potential and DC field). The wide spectrum gives rise to a fast non-radiative decay due to phonon emission whose rate is three orders of magnitude larger than the radiative one. Thus, the population inversion needed for a laser operation can be achieved at a high threshold current of a few kA/cm(2) only [2]. The continuous in-plane spectrum of QWs also results in a strong free-carrier absorption and corresponding losses due to current excited by the in-plane electric field component of a laser mode. Here we exploit two alternative schemes of a cascade laser based on a quantum dot (QD) superlattice (first proposed in Ref. [4]). The purely discrete spectrum of QDs allows for a significant reduction of the non-radiative decay rate, threshold current, and optical losses.