High-order harmonic generation by an ultrafast infrared pulse

Life science and biology have advanced with the progress of microscope technology. A strong desire of life scientists is to observe living cells with high spatial resolution. The development of soft x-ray microscopy using a special wavelength called the water window started in the 1970s in order to fulfill this desire. However, the photon fluxes realized so far are still insufficient for this application. Here, we review our work on a coherent water window x-ray generation based on high-order harmonics driven by an infrared pulse. Our proposed generation scheme, combining an infrared laser driver and a neutral gas medium, is efficient and scalable in output yields of the water window x-ray.