Lubos Hvozdara

We demonstrate what we believe is the first application of the recently developed electrically pumped GaAs/AlGaAs quantum-cascade lasers in a spectroscopic gas-sensing system by use of hollow waveguides. Laser light with an emission maximum at 10.009 µm is used to investigate the mid-infrared absorption of ethene at atmospheric pressure. We used a 434-mm-long silver-coated silica hollow waveguide as a sensing element, which served as a gas absorption cell. Different mixtures of helium and ethene with known concentrations are flushed through the waveguide while the laser radiation that passes through the waveguide is analyzed with a Fourier-transform infrared spectrometer. The experimentally obtained discrete ethene spectrum agrees well with the calculated spectrum. A detection threshold of 250 parts per million is achieved with the current setup.

Highly improved quantum cascade lasers based on intraband transitions in a chirped AlGaAs-GaAs superlattice have been realized. Lasing in a liquid nitrogen environment with a laser wavelength centered at 13 /spl mu/m and with a peak optical power per facet exceeding 80 mW is achieved. A threshold current density of 8.6 kA/cm/sup 2/ at 80 K has been measured. Multimode lasing spectra ranging from 12.4 to 13.3 /spl mu/m and single mode spectra in a short cavity at 13 /spl mu/m are obtained. To improve the gain in the structure, the design of the injector is optimized. The minibands in the active region are more strongly confined beneath the conduction band edge of the barriers to decrease electron losses into the continuum. The maximum operating temperature exceeds 230 K with T/sub 0/ values of 170 K.

In this paper, we present experimental results of our investigations on deformed GaAs-AlGaAs microlasers emitting around /spl lambda/=10 /spl mu/m. These quantum cascade lasers exhibit interesting features regarding the threshold current densities, optical output, and far-field pattern. A slight aberration from a circular cross section decreases the threshold current density for microlasers (which have a radius of 50 /spl mu/m). For larger deformations /spl epsiv/, the threshold starts to increase because of the increasing mirror losses. For smaller microlasers (radii between 22 and 34 /spl mu/m), the threshold current density increases already for slight deformations due to the increase of the mirror losses. The experimental results can be fitted very well with the mirror losses as a fitting parameter using a well-known and simple model. Threshold currents as low as 170 mA are measured for a cylindrical microlaser with a radius of 22 /spl mu/m. The peak optical output is increasing quasi-exponentially with rising deformation. Lasing emission from slightly concave resonator shapes is detected. The bow-tie mode and other modes-different from Whispering-gallery modes-are responsible for highly directed emission along the diagonal axis and along the short axis, respectively, of the microlasers. Single-mode emission with a side-mode suppression ratio larger than 25 dB is shown over the entire drive current range for a highly deformed microlaser. The laser line can be temperature tuned with /spl Delta//spl nu//T=-0.027 cm/sup -1//K. A dual mode switching depending on the drive current with a mode spacing of /spl Delta//spl nu/=8.1 cm/sup -1/ between 999 and 1007.1 cm/sup -1/ is observed for a less deformed microlaser.