AlN structures demonstrate quasi-phase matching within UV range

Laser diodes within the visible and infrared regions of the spectrum are widely available today, but efficient laser diodes within the ultraviolet (UV) range are still hampered by technological difficulties. Now, another alternative to laser diodes directly emitting within the UV is emerging—by using wurtzite-structure quasi-phase matched epitaxial waveguides for frequency doubling.

Alden et al. report that aluminum nitride (AlN) may be a viable solution. Because AlN has large nonlinear susceptibility and is transparent into the UV-C range, it can behave like ferroelectrically poled waveguides used in frequency conversion. The researchers set out to prove that the material, if structured correctly, can generate UV light and replace traditional nonlinear crystals.

To do this, the group grew UV-transparent AlN epitaxially on sapphire and periodically changed its lattice orientation to form a lateral waveguide with an alternating susceptibility coefficient. This allows them to achieve frequency doubling via quasi-phase matching (QPM) in a structure-based waveguide—marking the first time AlN structures have been used to demonstrate QPM within the UV range.

The group demonstrated second-order harmonic generation of UV light at 344 nm, which falls within the UV-A part of the spectrum and may have the potential for applications down to 206-nm (UV-C).

If future work can demonstrate the frequency doubling into the UV-C range and a boost in efficiency, the researchers say this technology will enable coherent light emission/lasers for highly sensitive biochemical spectroscopy applications, DNA manipulation, and materials processing.

Source: “Quasi-phase-matched second harmonic generation of UV light using AlN waveguides,” by Dorian Alden, Tinkara Troha, Ronny Kirste, Seiji Mita, Qiang Guo, Axel Hoffmann, Marko Zgonik, Ramón Collazo, and Zlatko Sitar, Applied Physics Letters (2019). The article can be accessed at https://doi.org/10.1063/1.5087058 .