ERC Researchers Advance Nonlinear Optics to an Extreme Not Considered Possible Until Now

Achievement date: 

Researchers affiliated with the Center for Extreme Ultraviolet Science and Technology (ERC EUV), an NSF-funded Engineering Research Center headquartered at Colorado State University, have enabled  a coherent X-ray “white light” super-continuum that spans the electromagnetic spectrum from ultraviolet (UV) to the keV region {wavelengths < 7.7Å; [Å equals     10^(-10)m]}. This demonstration represents a modern laser-like, coherent, tabletop version of the vintage Roentgen X-ray tube, but in the soft X-ray region.


These coherent X-ray beams promise revolutionary new capabilities for understanding and controlling how the nanoworld works on its fundamental time and length scales. This knowledge is highly relevant to next-generation electronics, data and energy storage devices, and medical diagnostics. The unique ability of broad bandwidth, ultra-fast X-rays to probe functioning at multiple atomic sites simultaneously is already uncovering new understanding of how electrons, spins, phonons, and photons behave at spatial-temporal limits.


A half century ago the first laser was demonstrated. That device has since been of huge benefit to society. The same revolution that happened decades earlier for visible light sources is now happening for X-rays, which can penetrate thick samples, image small objects, and have the added advantage of elemental and chemical specificity. These unique advantages spurred the development of large-scale X-ray free-electron lasers based on accelerator physics as well as high harmonic generation (HHG) driven by tabletop lasers. The HHG process represents nonlinear optics at an extreme.

When driven by an intense femtosecond [10^(-15) sec]laser field, atoms radiate like microscopic antennae, emitting high-order harmonics of the fundamental laser. Until recently, bright HHG beams were limited to the extreme ultraviolet (EUV) region of the spectrum (< 150eV). However, many inner-shell absorption edges in certain materials (Fe, Co, Ni, Cu) lie at photon energies nearing 1 keV, providing a strong motivation to extend HHG to higher photon energies and correspondingly shorter wavelengths. Fortunately, the remarkable recent breakthrough by the far-flung CBiRC research team takes nonlinear optics to an extreme not before considered possible. This advance was the result of international collaboration by researchers from the University of Colorado Boulder, Cornell University, Technical University Vienna, and University of Salamanca.

Looking to the future, these coherent HHG X-ray beams (see figure) promise revolutionary new insights to the innermost workings of the nanoworld by offering x-rays that are the ultimate "strobe light." The future includes possibilities such as movies that capture the dance of electrons and atoms within a molecule or material and ultrahigh-resolution images of single living cells or nanostructures. Because the lasers are tabletop size, scientists can now use x-ray microscopy at their own facilities rather than facilities at national laboratories. Some of the many questions that can now be investigated include: How fast and dense can we store data; can we speed up catalysis; can designs for solar cells and next-generation circuits be more efficient; can we reduce energy consumption by electronics; can we manipulate electrons to steer chemical reactions; and can we make a 3-D image of the inner workings of a cell in a few minutes for disease diagnostics? Moreover, the limits of HHG are not yet known (e.g., it may be possible to extend HHG to hard X-ray wavelengths).