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A quantum cascade laser gets a geometric makeover

A change in the areal cross section and orientation of the laser’s optical cavity collimates an otherwise divergent light beam.

Achieving a high-quality beam—one whose profile is Gaussian and as narrow as diffraction allows—can be a challenge in semiconductor laser design. That’s especially true in the few-terahertz frequency regime, an elusive part of the electromagnetic spectrum just below the reach of optical technologies and just above the reach of electronics. No natural material has an open bandgap small enough to emit coherent photons below a few tens of terahertz. The quantum cascade laser (QCL), one of the few devices able to do so, relies on quantum wells that act as gain material (see the article by Federico Capasso, Claire Gmachl, Deborah Sivco, and Alfred Cho, Physics Today, May 2002, page 34). Like other semiconductor lasers, a QCL emits in-plane, from the end of a relatively long, but exceedingly thin (typically 10 µm) waveguide filled with gain material. Because that width is so much smaller than the wavelength of a few-terahertz photon, the light emission is highly divergent. To collimate the beam, a group led by UCLA’s Benjamin Williams altered the usual geometry of a QCL. In their device, shown here, the single waveguide is replaced by a two-dimensional array of them, each with tapered ends to quench in-plane lasing, and each designed to act as a surface-emitting patch antenna. Collectively, they form a so-called metasurface, which strongly radiates reflected terahertz light outward from the 2-mm2 array without diffraction effects. The team then placed a parallel mirror 6 mm away to create a new, external cavity laser (not shown). That addition amplified any resonant emission in the cavity into coherent 2.9-THz photons. But thanks to the array’s large area, the beam’s divergence was lower than the emission from any previous terahertz QCL. (L. Xu et al., Appl. Phys. Lett. 107, 221105, 2015.)

A quantum cascade laser gets a geometric makeover

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