A method of modifying the spectral width of extreme ultraviolet light (EUV) has been developed by researchers at the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy. The scientists used a novel phase-matching scheme in four-wave mixing, which allowed them to compress the spectral width of the initial broadband light by more than a hundred times. The researchers specifically combined broadband white light in the visible portion of the spectrum with light exhibiting a broad spectrum in the vacuum ultraviolet (VUV) region. The light was then passed through a dense jet of krypton atoms, creating a new laser pulse in the EUV range. The spectral width of the new pulse was more than a hundred times narrower when compared to the initial visible and VUV pulses. The refractive index as a function of the photon energy is shown by the red dashed curve. In the region around 9.2 eV it changes comparably slowly (left side), whereas it changes very fast in the region around 12.365 eV. Therefore, a broadband absorption (blue area) can lead to a narrowband emission (violet area) with the help of two visible photons (shown by the arrows). Courtesy of MBI. In the four-wave mixing technique used by the researchers, one krypton atom absorbs two visible photons and one VUV photon, which leads to the emission of one EUV photon. Because of how energy is conserved, the emitted EUV photon would have a frequency equal to the frequencies of all three absorbed photons. At the same time, due to momentum conservation, the velocity of the incoming lightwave has to match the velocity of the outgoing wave inside the mixing medium. This velocity changes quickly — close to an atomic resonance. To generate the EUV light, the researchers selected a VUV spectral range far from any resonance, and a target EUV range between two resonances. This allowed them to match the velocities of a broad range of incoming wavelengths to a narrow region of outgoing wavelengths. The generation of narrowband EUV could support applications including the investigation of resonant transitions and the coherent diffractive imaging of structures in the nanoscale. In the future, the technique could be used in the other direction — for example, to spectrally broaden EUV pulses, which may result in the generation of very short EUV pulses from sources such as free-electron lasers and soft x-ray lasers. The research was published in Nature Photonics (http://dx.doi.org/10.1038/s41566-020-00758-8).