Systematic gain measurements of CdSe quantum dot stacks were carried out using the variablestripe-length method at various pump densities and temperatures. The fivefold quantum dot stacks show an effective optical gain at double threshold of the order of 125 cm À1 (150 cm À1 ) for the doped (undoped) sample. A maximum optical gain of 400 cm À1 was achieved at a pump density of 980 kW/cm À2 and a temperature of 10 K. High-resolution transmission electron microscope and micro-photoluminescence measurements were carried out. Temperature depending measurements yield a strong increase of the threshold densities for temperatures above 100 K.Introduction Semiconductor lasers with quantum dots (QDs) in their active region are expected to exhibit ultra-low threshold currents and temperature insensitive operation due to the d-like density of states [1]. For very small QDs, the energy separation of the electronic states is much larger than the available thermal energy, inhibiting thermal depopulation of the lowest electronic states. This should result in a lasing threshold that is temperature-insensitive at an excitation level of only one electron-hole pair per dot [1], and in a modal gain which is inverse proportional to the energy broadening of the luminescence due to volume fluctuation of the QD ensemble [2]. CdSe/ZnSe has attracted much interest because of emission in the blue/green spectral range. The selfassembled growth of quantum dots as a consequence of the large lattice mismatch and their optical properties have been widely studied (e.g. [3][4][5][6][7]). Here, we present systematic gain measurements which give clear evidence for dealing with QD structures in the active region.

Nanowires can successfully be used as building blocks for nanoscaled laser devices. Calculations predict an extremely large modal gain for nanowires made up of semiconductors such as GaN or ZnO. We determine experimentally the modal gain of single-ZnO nano-and microwires to approach 5000 cm −1 under particular size conditions. We demonstrate the distinct and sensitive dependence of the modal gain on the wire diameter and discuss optimizations for lasing of these wires.

No abstract

No abstract

No abstract