Room temperature lasing of multi-stage quantum-cascade lasers at 8 $\mu$m wavelength

Room-temperature lasing at a wavelength of 8 $\mu$m in multistage quantum-cascade lasers pumped by current pulses is demonstrated. A quantum-cascade laser heterostructure based on the In$_{0.53}$Ga$_{0.47}$As/Al$_{0.48}$In$_{0.52}$As alloy heteropair, matched to an InP substrate, is grown by molecular-beam epitaxy and consists of 50 identical cascades placed in a waveguide with air as the top cladding. A threshold current density of $\sim$5.1 kA/cm$^2$ at a temperature of 300 K is obtained in ridge lasers with a cavity length of 1.4 mm and a ridge width of 24 $\mu$m.