Observation of two different types of optical supercontinua: Structured and structureless

The dynamics of nonlinear optical conversion in the high-intensity femtosecond pump regime has been studied in gaseous media over a wide pressure range. The existence of two different types of optical supercontinua, structured and structureless, has been found in a number of atomic (argon, krypton) and molecular (methane) gases. The structured optical supercontinuum exhibits a typical oscillationlike spectral structure, resulting from self-phase modulation in neutrals and plasma. The structureless optical supercontinuum is completely dense, and its main properties cannot be explained within the self-phase modulation mechanism. The structureless optical supercontinuum is attributed to Bremsstrahlung radiation from electrons that tunnel through the bound potential inside the atoms or molecules under a strong optical field. The basic properties of the structureless optical supercontinuum have now been revealed experimentally.