Recent progress for blue VCSELs and challenges to move to UV

We will summarize state-of-the-art results in III-nitride-based vertical-cavity surface-emitting lasers (VCSELs) and the main challenges in going from blue to ultraviolet-emitting devices. In particular we will highlight how to reduce threshold current densities by using optically guided cavities and our method to simultaneously achieve high-reflectivity mirrors and good control over the cavity length for AlGaN-based ultraviolet VCSELs. The latter has resulted in the shortest emission wavelength from a VCSEL at around 310 nm. This claim of lasing is supported by a nonlinear increase in output power versus pump power and a simultaneous beam width and spectral narrowing around threshold.