Cascaded-transition Quantum Cascade laser

The Quantum Cascade (QC) laser is a semiconductor laser based on intersubband transitions in the conduction band of a multiple quantum-well heterostructure, and is a highperformance light source in the mid-infrared and terahertz region. With the aim of improving the quantum efficiency and power efficiency of long-wavelength QC lasers, based on the principle of conventional single-transition QC lasers, we demonstrate a cascaded-transition, long-wavelength (λ ∼ 14.6 μm) QC laser where an electron makes two consecutive, cascading optical transitions and emits two energy-matched photons in each active region. The laser structure grown consists of a cascaded-transition active core with 25 stages sandwiched between two conventional single-transition active cores with 30 stages each. From the grown samples, laser emission from ∼ 680 cm^-1 to ∼ 700 cm ^-1 (λ ∼ 14.7-14.3 μm) with threshold current density of 4.8 kA/cm² at 80 K is observed. The laser functions in pulsed mode up to ∼ 240 K, with a characteristic temperature of 190-230 K for ridge widths around 20 μm. From the current-voltage (I-V) measurement, two turn-on's, defined as the electric field at which the subbands line up to create a low-resistive state, are observed, which is consistent with the I-V of a cascaded-transition only mesa, which, in turn, is suggestive of the functionality of the cascaded-transition active core; each turn-on relates to electron transport through the upper laser subbands and the middle subband respectively.