High-power, surface-emitting quantum cascade laser operating in a symmetric grating mode

• Published in: Applied physics letters 2016-03, Vol.108 (12)
• Main Authors: Boyle, C., Sigler, C., Kirch, J. D., Lindberg, D. F., Earles, T., Botez, D., Mawst, L. J.
• Format: Article
• Language: English
• Subjects: Applied physics; CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; Coupled modes; Coupling; DIFFRACTION; Diffraction patterns; Far fields; FEEDBACK; GRATINGS; LANTHANUM SELENIDES; Laser beams; LASERS; LENGTH; METALS; PLASMONS; Quantum cascade lasers; SEMICONDUCTOR MATERIALS; SIMULATION; SURFACES; SYMMETRY
• ISSN: 0003-6951; 1077-3118
• DOI: 10.1063/1.4944846

Grating-coupled surface-emitting (GCSE) lasers generally operate with a double-lobed far-field beam pattern along the cavity-length direction, which is a result of lasing being favored in the antisymmetric grating mode. We experimentally demonstrate a GCSE quantum-cascade laser design allowing high-power, nearly single-lobed surface emission parallel to the longitudinal cavity. A 2nd-order Au-semiconductor distributed-feedback (DFB)/distributed-Bragg-reflector (DBR) grating is used for feedback and out-coupling. The DFB and DBR grating regions are 2.55 mm- and 1.28 mm-long, respectively, for a total grating length of 5.1 mm. The lasers are designed to operate in a symmetric (longitudinal) grating mode by causing resonant coupling of the guided optical mode to the antisymmetric surface-plasmon modes of the 2nd-order metal/semiconductor grating. Then, the antisymmetric modes are strongly absorbed by the metal in the grating, causing the symmetric mode to be favored to lase, which, in turn, produces a single-lobed beam over a range of grating duty-cycle values of 36%–41%. Simulations indicate that the symmetric mode is always favored to lase, independent of the random phase of reflections from the device's cleaved ends. Peak pulsed output powers of ∼0.4 W were measured with nearly single-lobe beam-pattern (in the longitudinal direction), single-spatial-mode operation near 4.75 μm wavelength. Far-field measurements confirm a diffraction-limited beam pattern, in agreement with simulations, for a source-to-detector separation of 2 m.