The near and far fields of quantum cascade lasers emitting in the spectral range near 8 µm have been studied. Lasing in the fundamental waveguide mode along the "fast" axis with a divergence of ~ 50º was observed in all samples of lasers with stripe widths of 22 and 50 µm in the entire range of pump currents. On the "slow" axis, multimode lasing and competition between the fundamental and higher waveguide modes were observed, which manifests itself for lasers with a 50 µm stripe in an increase of the radiation divergence from ~ 10º near the threshold to > 50º with a two-fold excess of the lasing threshold. The divergence along the slow axis for lasers with a stripe width of 22 µm remained unchanged over the entire range of pump currents and amounted to ~ 50º. The significant contribution of the fundamental mode to the total radiation intensity of quantum cascade lasers, being obvious from the far-field intensity distribution, cannot be established solely from the results of measurements of the near-field.
Near-Field microscopy with phase sensitive coherent detection employing quantum cascade lasers
