  Coherent sources of attosecond extreme ultraviolet (XUV) radiation present
many challenges if their full potential is to be realized. While many
applications benefit from the broadband nature of these sources, it is also
desirable to produce narrow band XUV pulses, or to study autoionizing
resonances in a manner that is free of the broad ionization background that
accompanies above-threshold XUV excitation. Here we demonstrate a method for
controlling the coherent XUV free induction decay that results from using
attosecond pulses to excite a gas, yielding a fully functional modulator for
XUV wavelengths. We use an infrared (IR) control pulse to manipulate both the
spatial and spectral phase of the XUV emission, sending the light in a
direction of our choosing at a time of our choosing. This allows us to tailor
the light using opto-optical modulation, similar to devices available in the IR
and visible wavelength regions.
