Sb-free quantum cascade lasers (QCLs) in the 3-4 µm spectral range (Vol. 43 No. 3)

The mid-IR spectral range is a region of great interest for numerous scientific and industrial applications such as environmental sensing, metrology and clinical diagnosis. In particular the first atmospheric window between 3-5µm is crucial where a large number of gases e.g. methane, nitric oxide, carbon mono-/dioxide or formaldehyde can be detected. The presence of very strong fundamental stretching modes of O-H, C-H and N-H bonds that can be orders of magnitude stronger than the overtones in the near-IR, brings the detection limits down to sub-ppb concentrations.

The unique feature of QCLs to tailor the emission wavelength makes them appealing sources for this kind of applications. Unfortunately the realization of QCLs in the first atmospheric window is especially challenging because a large conduction band discontinuity is needed to obtain high-energy photons. This is solved by using antimony in the lasing material. However, the growth of Sb-containing devices can be difficult and the fabrication techniques used for high performance QCLs lack compatibility.

Therefore the ETH team has focused on developing a Sb-free system by using strain-compensated InGaAs/InAlAs-AlAs on InP. In pulsed-operation watt-level emission at 3.3µm was obtained at room temperature, and lasing above 350 K could be observed. The laser performance is comparable to Sb-containing QCLs. Tunable single-mode emission between 3.15-3.4µm (Figure 1) was observed in a Littrow external-cavity configuration. The team has begun to develop buried heterostructure QCLs to obtain continuous wave operation. By incorporating first-order distributed feedback gratings, for the first time in this spectral range single-mode emitting buried heterostructure QCLs could be realized.