Ultraviolet‐C Vertical‐Cavity Surface‐Emitting Lasers with Precise Cavity Length Control

Abstract In vertical‐cavity surface‐emitting lasers (VCSELs), the cavity length defines the resonance wavelength, which is directly related to the laser detuning, that is, the difference between resonance wavelength and gain peak. A low detuning maximizes the modal gain leading to a reduction of the threshold. Therefore, controlling the cavity length of VCSELs is of great importance. Here optically pumped ultraviolet‐C (wavelength 280 nm) VCSELs with precise cavity length control are demonstrated. The VCSEL structure is formed by an AlN cavity with 5 Al 0.40 Ga 0.60 /Al 0.70 Ga 0.30 N quantum wells and a top HfO 2 spacer layer with dielectric SiO 2 /HfO 2 distributed Bragg reflectors on both sides of the cavity. To access the N‐face side of the cavity, a new methodology referred to as photo‐assisted electrochemical etching is employed for substrate removal. Across a 0.9 mm 1.2 mm area, the lasing wavelength varies a maximum of 1.17 nm between different UVC VCSELs, exhibiting threshold pump power densities from 0.7 MW/cm 2 to 3.7 MW/cm 2 and detuning values between 0 to 2 nm. The results show that VCSELs with a cavity length variation lower than 1 can be obtained with this technology.