Shaped beams in vertically emitting quantum cascade ring lasers

Full text: Since the mid-infrared (MIR) and terahertz (THz) regime of the electromagnetic spectrum is rich in absorption resonances, applications like chemical sensing and spectral imaging call for reliable coherent emitters with well defined beam profiles. We demonstrate the simulation, design, fabrication and operation of surface emitting MIR and THz quantum cascade ring lasers (QCL) that hold second-order gratings to allow for radiation out-coupling. The emitting area naturally forms a circularly shaped far field and the overall large emission area narrows the beam, making bulky and expensive optics obsolete. The capability of beam shaping is achieved by tuning the grating period, resulting in spot- and ring-shaped symmetric far-field patterns. The emitters exhibit robust single mode operation with a side mode suppression ratio higher than 25 dB, for all bias currents and temperatures. A strong reduction of the beam divergence was observed for MIR and THz QCLs, with a full-width-at-half-maximum of 3o and 15o, respectively. (author)