Lead-salt mid-infrared micro cavity lasers

Within this project semiconductor lasers will be developed giving coherent light in the spectral range of the mid- infrared. The spectral range of the mid-infrared covers the finger-print region of many molecules and is therefore of special interest for several applications like pollution monitoring and trace gas analysis. Semiconductor lasers working in this special wavelength range are usually based on lead-salt materials. Lead-salt based diode lasers are commercially available, but these lasers have some severe disadvantages for applications like an strongly divergent output beam with an unsymmetric beam profile. These unfortunate properties of the beam are solely caused by the fact that the laser light is emitting across the cleavage edge of the samples. Much better beam profiles are achieved by vertical emitting lasers such as the lead salt microcavity lasers developed within this project. Our mid-infrared lasers will work at room temperature at wavelength between 3 000 and 5 000 nm and will be used for the detection of C-H stretching modes. These C-H stretching modes are present not only in many organic compounds but also in several thin films of technical relevance. Therefore, our devices could be applied for the in-situ control of the growth of technical coatings as well as the growth of semiconductor layers. We will apply different concepts for developing lead salt microcavity lasers. On one hand we will apply commercially available diode lasers from III-V materials for pumping our devices optically. In this case, our microcavity structures are used to convert the emission generated in the pump laser to longer wavelength in the mid-infrared. On the other hand we will also produce electrically pumped laser structures driven by current. As laser active material not only epitaxially grown lead salt layers will be used. As an alternative we will also test films containing lead salt nanocrystals fabricated by wet chemical methods or lead salt layers buried under III-V materials. Finally, different geometries of the laser cavity will be tested and optimized for laser emission at these long wavelength. Among these we will test cavities based on micro-stadions and micro-pillars which actually result in a very high finesses and therefore such devices are very promising for obtaining laser operation at small excitation powers.

In this project, vertical emitting semiconductor lasers were developed with emission wavelength in the mid-infrared spectral region. Because this spectral region covers the finger-print absorption and emission lines of many gaseous molecules, these lasers are important for many applications such as pollution monitoring, trace gas analysis and medical diagnostics. The lasers developed in this project consist of a vertical emitter structure that is obtained by monolithic integration of highly efficient Bragg interference mirrors with the active laser region. The mirrors as well as the active region are made from narrow gap IV-VI semiconductors and related compounds, which exhibit very favorable electronic and optical properties for efficient infrared emitters. The laser structures were grown by molecular beam epitaxy and consists of several hundred individual layers. The special feature of the lasers is that the light is coupled out in the vertical direction through the top Bragg mirror, which yields circular beams with narrow beam divergence. In this project, vertical emitting lasers with the longest emission wavelengths of up to 8 micrometer, the highest operation temperatures as well as the narrowest laser line widths were achieved. In addition, the lasers exhibit ultra narrow line widths as well as a large wavelength tuning range that is of particular importance for spectroscopic applications. In addition, it was shown that a tuning of the laser wavelength can be also achieved by using magnetic fields. Apart from the lasers and their detailed characterization, also passive optical components such as ultra-high reflectivity infrared Bragg mirrors were developed in this project, as well as wavelength selective infrared detectors based on microcavity structures. For further improvements, several alternative new materials were investigated as active materials in infrared devices. This included as self-assembled quantum dots, nanocrystals as well as heterostructures combining different groups of semiconductor compounds. The results indicate that the latter have a particularly high potential for realization of high performance infrared laser devices for a wide range of applications.