The “water window” of biological tissue can be exploited to achieve high-contrast microscope images, as well as precision spectroscopy, according to a new theoretical study. Researchers from the Center for Pulsed Lasers said they have identified the physical mechanism to efficiently generate high-order harmonic radiation at high laser intensities beyond the saturation threshold of atoms and molecules. It should be possible to develop coherent radiation in the 3.3- to 4.4-nm range that is not absorbed by the water in biological tissues, the researchers said. The lack of absorption in that range led to the term “water window.” The researchers predict an increased harmonic yield when laser intensity is increased. This “contradicts the general belief of a progressive degradation of the harmonic emission at ultrahigh intensities,” the researchers wrote in the study. Similar work focusing on hydrogen has been conducted in the past. The current study extends beyond that to argon atoms, which typically provide a high-enough frequency conversion to effect high-order harmonic generation. The researchers combined classical analysis and quantum mechanical calculation resulting from “the numerical integration of the 3-D time-dependent Schröodinger equation complemented with the time-frequency analysis.” The research was published in European Physical Journal D (doi: 10.1140/epjd/e2014-50086-6). For more information, visit www.clpu.es.