Intersubband Electroluminescence From Long-Side-Cleaved Quantum Cascade Lasers Above Threshold: Investigation of Phonon Bottleneck Effects

A new technique is reported which allows the observation of intersubband spontaneous emission in unipolar quantum cascade lasers above threshold. The techniqie consists of cleaving the laser stripe in the direction perpendicular to its facets. This does not negatively affect the operation of the lasers thanks to their unipolar nature. To show the potential of the method we apply it to superlattice quantum cascade lasers with various active region designs. we directly observe the saturation of the luminescence intensity at the laser transition, and a bottleneck effect for transitions separated from the lasing one by less than one optical phonon. The new technique should help in the optimization of QC lasers.