Controlling the spectral shape of coherent soft X-rays

We demonstrate the generation of arbitrarily shaped spectra of coherent soft X-ray radiation by adaptive control of the driving laser pulse. The selection of both narrow and wide spectral ranges as well as suppression of high-harmonic emission in a selected wavelength region is achieved. Feedback-controlled spectral engineering of coherent soft X-rays paves the way to shaping of soft X-ray pulses and coherent control of electron dynamics in the sub-femtosecond regime.