Vertical-cavity surface-emitting lasers (VCSELs) are small, semiconductor-based lasers that emit optical beams from their top surface, and one of their main applications is in gas sensing. Gases each have a unique set of energies they can absorb, derived from their molecular structure. These sets of absorption lines are akin to fingerprints, which enables unambiguous and sensitive detection with a suitable tunable laser like a tunable VCSEL.