Mid-infrared Tunable Surface-Emitting Lasers for Gas Spectroscopy

Abstract

We describe the development of a tunable semiconductor disk laser, also called a vertical external-cavity surface-emitting laser (VECSEL) for the mid-infrared range (wavelength ~2.5 μm to 10 μm). The intended applications are mainly gas spectroscopy for trace gas analysis, air pollution monitoring, or medical use. The mid-infrared range is especially suited for gas spectroscopy since the fundamental modes of many common gases lie in this range. This includes gases such as CO2, CO, nitric oxides (NOx), many organic compounds (including C-H vibration lines), SO2, or H2O. Gas spectroscopy in this range is therefore especially interesting since the fundamental modes yield to much higher sensitivity than when using shorter wavelengths in the near-infrared or even visible range, where only higher-order lines can be analyzed. For the VECSEL, lead chalcogenide (IV-VI) narrow-gap semiconductor layers are employed, as they are perfectly suited for the mid-infrared wavelength range, while VECSELs fabricated with the well established III-V technologies are restricted to wavelengths below 2.8 μm. Generally, VECSELs offer several advantages compared to conventional edge-emitting laser diodes: VECSELs emit perpendicular to the surface, so batch processing is possible leading to extremely low costs. The emitted beam is circular with a cone angle of only a few degrees. Large continuous tuning without any mode hops is easily attained by simply changing the length of the cavity. Note that quantum cascade lasers (QCLs) as well as interband cascade lasers (ICLs) fabricated with III-V technology emit in the mid-infrared, but both are edge-emitting lasers. They therefore emit a strongly astigmatic beam with an aperture angle in the fast axis of up to 60 deg. This requires elaborate optics for beam conditioning. Edge-emitting laser diodes additionally need individual delineation of the edge mirrors. Tuning can be done with an external cavity or by using QCL arrays; however, the design of the cavity is quite elaborate in order to obtain a mode-hop-free tuning. An additional advantage of the lead chalcogenide VECSEL is that the layers are epitaxially grown by molecular beam epitaxy (MBE) on easily available and robust Si substrates. The chapter is organized as follows: Since lead chalcogenides (IV-VI semiconductors) are presently not widely employed, we first summarize some of their properties. The reasons the IV-VIs can be grown by MBE on Si substrate material, despite the huge lattice-thermal-expansion mismatch, are outlined in Section 11.2. Some infrared applications are reviewed in Section 11.3. In Section 11.4, the design and realization of IV-VI VECSELs on Si is described, followed by their experimental and theoretically feasible properties. True monomode emission is achieved with short cavities and end-pumping (Section 4.3). Finally, a spectroscopic application is presented as an example.