Popmintchev, T., Chen, M.-C., Popmintchev, D., Arpin, P., Brown, S., Alisauskas, S., Andriukaitis, G., Balciunas, T., Mücke, O. D., Pugzlys, A., Baltuska, A., Shim, B., Schrauth, S. E., Gaeta, A. L., Hernández-García, C., Plaja, L., Becker, A., Jaron-Becker, A., Murnane, M. M., & Kapteyn, H. C. (2012). Bright Coherent Ultrahigh Harmonics in the keV X-ray Regime from Mid-Infrared Femtosecond Lasers. Science, 336(6086), 1287–1291. https://doi.org/10.1126/science.1218497