Schemes that use one light pulse to manipulate interactions of another with matter are well developed in the visible-light regime where an optical control pulse influences how an optical probe pulse interacts with a medium. This approach has opened new research directions in fields like quantum computing and nonlinear optics, while also spawning entirely new research areas, such as electromagnetically induced transparency and slow light. However, it has been unclear whether similar optical control schemes could be used to modify how x rays interact with matter. In a dramatic breakthrough demonstration at the ALS, a Berkeley Lab–Argonne National Laboratory group has now used powerful visible-light lasers to render a nominally opaque material transparent to x rays. While x-ray transparency will have immediate applications at x-ray light sources, the important result is that the findings lay a foundation for a broader spectrum of applications.

The underlying control mechanism is the light-induced creation of coherence between eigenstates of electrons in a material. This means that, for a two-state system, light forces electrons to exist in both states simultaneously rather than in one state or the other. This light–matter system can have fundamentally different properties from the matter system in isolation, properties that can be controlled by varying the properties of the optical control beam. Problems arise, however, when the probe pulse lies in the x-ray regime because x rays interact most strongly with the tightly bound (core) electrons in a material, leaving behind a "core hole" that is rapidly filled by more weakly bound electrons, thereby disrupting the optically imposed coherence and destroying the control.

Similar disruptive events occur for visible probe pulses, but the key difference for x-ray interactions is that the disruptive events occur about a thousand times more rapidly than for visible-light interactions. While the problems associated with a short coherence time can be mitigated by using very intense optical control pulses, the high intensity creates a different set of scientific problems. New technical problems also arise. For example, ultrashort optical pulses are needed to achieve high intensity and the x-ray pulse duration should be matched to the optical pulse duration, thereby requiring femtosecond x-ray pulses.

To address this issue, the Berkeley Lab–Argonne team collaborated on a series of experiments at ALS Beamline 6.0.2, which offers femtosecond-duration x-ray pulses and is one of only a handful of places worldwide where the experiment could be done. In the experiment, an intense femtosecond optical pulse co-propagated with a femtosecond x-ray (~900-eV) pulse through a neon-filled gas cell. Without the optical pulse, x rays were heavily attenuated by the neon gas, but with it, the x-ray transmission increased dramatically (by about a factor of three). The transparent window created by the light pulse lasted just 70 fs, short enough to be used to characterize x rays on ultrashort time scales. For example, the research team used this window to measure the duration of a femtosecond x-ray pulse, a heretofore difficult challenge. This capability should help to further develop ultrafast x-ray spectroscopy.

ALS femtosecond spectroscopy beamline layout. Femtosecond x-ray and laser pulses derive from a single 800-nm laser oscillator. Femtosecond x rays result from the interaction of a laser pulse with an electron bunch as it passes through the first of two insertion devices (the e-beam modulator) followed by the passage of the modified bunch through the second insertion device (the x-ray radiator), a technique known as laser slicing. Control-laser pulses (70–300 fs) are focused to attain the required intensity (~10¹³ W/cm²). A waveplate and delay stage regulate the polarization and delay of the laser pulses relative to the x-ray pulses. The x-ray and laser pulses co-propagate through the neon sample gas cell.

Optically induced x-ray transparency data for three peak laser intensities and a neon gas target with a thickness of 108 torr-cm. The laser (~1.55 eV) coherently couples core-excited 3s and 3p states, thereby inducing transparency on the 1s → 3p x-ray absorption resonance. At the highest intensity (top), the transmission increased by a factor of three relative to the transmission at the lowest intensity. The solid lines show theoretical simulations of the expected change in transmission. The degree of transparency varies linearly with intensity within error.

Future research will focus on using similarly transparent windows to shape x-ray pulses on a femtosecond time scale, a capability that opens the door to new areas of x-ray research such as quantum control of chemical reactions, a discipline now well developed in the visible regime, where shaped pulses are used to control the evolution of chemical reactions. It also lays a foundation for investigating a broader spectrum of ways to use light to control how x rays interact with matter. One interesting possibility is using light to enhance cross sections for nonlinear x-ray scattering. Developing nonlinear spectroscopy in the x-ray regime is currently hampered by weak cross sections for nonlinear x-ray scattering.

Research conducted by T.E. Glover, M.P. Hertlein, and B. Rude (ALS); J. van Tilborg and A. Belkacem (Berkeley Lab); S. Southworth, E.P. Kanter, B. Krässig, H. Varma, and L. Young (Argonne National Laboratory); T.K. Allison (Berkeley Lab and University of California, Berkeley); and R. Santra (Argonne National Laboratory and University of Chicago).

Research funding: U.S. Department of Energy, Office of Basic Energy Sciences (BES). Operation of the ALS is supported by BES.

Publication about this research: T.E. Glover, M.P. Hertlein, S.H. Southworth, T.K. Allison, J. van Tilborg, E.P. Kanter, B. Krässig, H.R. Varma, B. Rude, R. Santra, A. Belkacem, and L. Young, "Controlling x-rays with light," Nature Physics 6, 69 (2009).

ALSNews Vol. 305