Single-fundamental-mode vertical-cavity surface-emitting lasers (VCSELs) could benefit data transmission applications, but designing such a device to function over an entire pump range is difficult because of eventual material deformations such as thermal lensing. A team from the Korea Advanced Institute of Science and Technology in Taejon has integrated a single-defect photonic crystal -- with a 5-µm-hole pitch and a 3.5-µm-hole diameter -- into the top mirror of an 850-nm VCSEL to produce a side-mode suppression ratio of 35 to 40 dB. This method has allowed continuous-wave, single-mode operation in the entire current range. The researchers say it can be adapted for any VCSEL. The technique is reported in the May 27 issue of Applied Physics Letters.