Uniformity tests of individual segments of interband cascade diode laser Nanostacks®.

We investigate optoelectronic properties of monolithically stacked diode lasers, so-called Nanostack® devices that include two nominally identical waveguide segments separated by a specially designed tunnel junction. Near-field optical microscopy provides straightforward and separate access to the properties of both optically active segments. Device emission, namely electroluminescence and lasing, as well as photoluminescence and photocurrent data, are recorded with high spatial resolution and consistently interpreted. We find reduced laser emission from the laser segment that is situated closer to the substrate. We show that this is not caused by thermal effects but most likely due to a larger trap concentration within or in the vicinity of the quantum wells of this laser segment. Furthermore, we show that in the unbiased devices the potential gradient in this segment is significantly larger than in the one close to the heat sink. In addition it is shown that the coupling between both waveguides is marginal. The results underline the potential and unique advantages of near-field optical microscopy for nondestructive analysis of optoelectronic device. [ABSTRACT FROM AUTHOR]