Laser and Optical Properties of Green-Emitting ZnCdSe Quantum Dot Based Heterostructures
Green-emitting laser diodes are in great demand for mobile projection media (pico-projector), navigation, underwater communication but they are still absent on the market. InGaN/GaN-based quantum well structures are approaching green spectral region by use of polar, semipolar as well as free-standing GaN substrates. However such heterostructures suffer from high laser thresholds with increase of indium content. A promising alternative way is the use of highly efficient green-emitting undoped ZnCdSe based quantum dot (QD) laser heterostructures optically pumped by blue InGaN laser diodes. Operation of blue-green laser converter based on MBE grown heterostructure with two ZnCdSe QD layers was shown for the first time in .
In this work, the results of improvement of the converter performance are presented. Active region of a new heterostructure is comprised of five ZnCdSe QD sheets each placed in ZnSe quantum well and separated from each other by 5 nm ZnSSe layers that is necessary for effective compensation of elastic strain. Adjacent QD sheets are tunnel-coupled due to small thickness of ZnSSe layer that results in homogeneity of optical pumping. The active region is placed non-symmetrically in optical waveguide to reach high optical confinement factor. Optical waveguide contains a set of ZnMgSSe/ZnSe superlattices specially designed for decrease of effective band gap toward QD sheets position in order to stimulate charge carrier transport into the active region. The new heterostructure showed considerable increase of photoluminescence efficiency and effective laser action at 540–550 nm wavelengths. The use of this heterostructure as an active medium of blue-green laser converter results in substantial improvement of its performance. Maximal quantum efficiency of 25 % and relatively high output pulse power (τ = 50 ns, f = 1 kHz) of 150 mW were achieved.