Quantum-Dot Cascade Laser: Proposal for an Ultra-Low-Threshold   Semiconductor Laser
  We propose a quantum-dot version of the quantum-well cascade laser of Faist et al. [Science {\bf 264}, 553 (1994)]. The elimination of single phonon decays by the three-dimensional confinement implies a several order-of-magnitude reduction in the threshold current. The requirements on dot size (10-20nm) and on dot density and uniformity [one coupled pair of dots per (180nm)^3 with 5% nonuniformity] are close to current technology. 
Ned S. Wingreen and Charles A. Stafford