Quantum Control of Coherent EUV Radiation: New Methods for Phase Matching at Short Wavelengths

University Of Colorado At Boulder, Boulder CO

Investigators

Abstract

In this project, new techniqucs to extend nonlinear optics into the soft-x-ray region of the spectrum will be explored. Specifically, precisely controlled optical waveforms, structured wave guides, and quasi phase-matching at short wavelengths will be used to increase the brightness of laser-based coherent x-ray sources. In the past 2 years. dramatic progress has been made in this arca, demonstrating new methods for efficient conversion of laser light into the extreme ultraviolet (EUV) region of the spectrum, at wavelengths around 50eV. It is now possible to generate short- wavelength light pulses 1000 times shorter than can be generated by synchrotrons-short enough (10 femtoseconds) to directly probe atomic motion. It is also possible to dramatically improve the conversion efficiency of these very high-order nonlinear processes by using phase matching techniques. For example, by propagating the laser beam through a hollow fiber, the phase velocity of the optical pulse can be made to match that of the generated x-ray beam, thus improving thc conversion efficiency. Finally, very recently it was shown that feedback-control algorithms can fine- tune the shape in time of the laser pulse driving high-harmonic generation, making it possible to optimize the process and selectively enhance a particular x-ray photon energy. This is a fundamentally new type of phase matching that occurs within a single atom, where the laser pulse shape is optimized so that x-rays generated from one half-cycle of the laser interfere constructively with x-rays generated by adjacent half-cycles. This is in contrast to more-conventional phase matching techniques, where emission from a large number of individual atoms is arranged to interfere constructively by matching the phase velocities of the driving and harmonic waves. In the proposed work, several significant remaining challenges for generating coherent light in the EUV will be addressed. New techniques will he developed that will allow us to apply phase- matching techniques to higher photon energies, from 50 - 500eV. Simply extending previous work will not suffice because higher-energy x-ray photons are emitted at higher levels of ionization, introducing a very large phase velocity mismatch between the laser and x-ray beams. Possible new techniques include the use of structured waveguides for modulating the nonlinear response of the system to obtain quasi phase matching, and the use of two-color excitation. This work, when combined with continuing work on the use of temporally-shaped pulses, will greatly enhance the understanding of laser-atom interactions in this highly nonlinear-regime, and our ability to optimally control the x-ray generation process. This area of research presents a unique and challenging combination of forefront basic research and advanced technology. Ultrafast, broad bandwidth laser pulses and feedback algorithms will be used to coherently control and engineer the electron wave function of a radiating atom, with the very practical objective of developing bright, coherent, soft-x-ray light sources. This control of matter on the sub-nanometer, sub-femtosecond, distance- and time-scales explores the limits of fundamental atomic and molecular processes, as well as of optical technology. This work will provide excellent training for students in optical, computer, electronic, and EUV technologies- technologically- significant fields where the needs of industry far outpaee the availability of graduates. Furthermore, this new light-source has potential future applications in nanotechnology, microscopy, metrology, lithography, the characterization of x-ray optics, and in the study of ultrafast dynamic processes using x-rays. We and other are actively pursuing many of these applications.