Surface-Emitting Lasers in The Ultraviolet Spectrum

The recent demonstrations of edge-emitting lasers in the UVB (280 – 320 nm) operating under pulsed conditions, and in the UVC (&lt; 280 nm) operating continuous-wave bode very well for the development of more complex laser structures in the ultraviolet (UV), such as vertical-cavity surface-emitting lasers (VCSELs) and photonic crystal surface emitting lasers (PCSELs). Recently, optically pumped versions of both of these surface-emitting lasers, emitting in the UVB and UVC, have been realized; there are, however, significant challenges precluding electrical injection. Here, we will summarize the state of the art for these UV surface-emitting lasers and outline the remaining challenges for realizing electrically injected devices.<br/><br/>For VCSELs, we will go in depth into our concept for achieving high reflectivity mirrors with accurate cavity lengths based on substrate removal by selective electrochemical etching. In addition to paving the way to demonstrations of both UVB and UVC VCSELs, this technology has yielded a large reduction in the lasing threshold by allowing us access to both sides of the cavity for post-growth detuning setting. It has also facilitated lasers with an inherently temperature-stable lasing wavelength by enabling the integration of materials with negative thermo-optic coefficients. Moving towards electrically driven UV VCSELs, one major challenge is applying the electrochemical etch technique to heavily doped device structures, since the doping-selective etch process can unintentionally damage doped device layers. Here, we will discuss several important steps towards overcoming this challenge, specifically, the successful substrate removal for thin-film UVB light-emitting diodes (LEDs) and resonant cavity LEDs employing tunnel junctions, without any parasitic etching of device layers. With these results in hand, the only remaining building block to electrically driven UV VCSELs is the development of an aperture scheme for horizontal current confinement.<br/><br/>For PCSELs, we will show single-mode devices that lase in the UVB and in the UVC under optical pumping, despite the challenging surface morphology and the inherently low refractive index of AlGaN. A narrow beam divergence below 1° has been achieved by tailoring PCSEL parameters, such as the hole filling factor. The development of electrically injected PCSELs will benefit from the great progress that has been made for electrically injected edge-emitting lasers, since their epitaxial structures are similar. However, since PCSELs have an electrical injection path that is in the same direction as the outcoupled light they face similar challenges as VCSELs, such as achieving good lateral current spreading without the use of metals that can block the vertical emission.<br/><br/>The question now is, will we first see electrically driven UV PCSELs or VCSELs?