Honeycomb pattern formation by laser-beam filamentation in atomic sodium vapor

Summary form only given. There is currently great interest in phenomena involving the spontaneous emergence of a regular spatial pattern when a laser beam propagates through a nonlinear medium. Such phenomena have been studied previously in a variety of media, including atomic vapors, liquid crystals, photorefractive crystals, organic liquids, glasses, semiconductors, and bacteriorodopsin. Most observations of regular patterns were obtained in systems with optical feedback (including optical resonators) and/or with counterpropagating beams. Here we report on a striking from of pattern formation in sodium vapor that occurs with a single laser beam (i.e. with no feedback mechanism) and which appears to have been previously unreported, even though there have been studies of sodium vapor under similar experimental conditions, as well as studies of small-scale self-focusing with a single beam in Kerr (non-saturable) media.