Beam steering in high-power CW quantum-cascade lasers

We report the light-current (L-I), spectral, and farfield characteristics of quantum cascade lasers (QCLs) with seven different wavelengths in the λ = 4.3 to 6.3 μm range. In continuous-wave (CW) mode, the narrow-stripe (≈ 13 μm) epitaxial-side-up devices operated at temperatures up to 340 K, while at 295 K the CW output power was as high as 640 mW with a wallplug efficiency of 4.5%. All devices with λ ≥ 4.7 μm achieved room-temperature CW operation, and at T = 200 K several produced powers exceeding 1 W with ≈ 10% wallplug efficiency. The data indicated both spectral and spatial instabilities of the optical modes. For example, minor variations of the current often produced non-monotonic hopping between spectra with envelopes as narrow as 5-10 nm or as broad as 200-250 nm. Bistable beam steering, by far-field angles of up to ±12 ° from the facet normal, also occurred, although even in extreme cases the beam quality never became worse than twice the diffraction limit. The observed steering is consistent with a theory for interference and beating between the two lowest order lateral modes. We also describe simulations of a wide-stripe photonic-crystal distributed-feedback QCL, which based on the current material quality is projected to emit multiple watts of CW power into a single-mode beam at T = 200 K.