Controlling x-rays with light

Light can been used to control how radiation itself interacts with matter: optical lasers coherently couple energy states within a material and produce a light/matter system with novel and controllable properties. A probing radiation pulse interacting with this system experiences material properties that are quite different from those experienced in the absence of the ‘controlling’ light pulse. This has led to new research directions in areas such as quantum computing and non-linear optics, while also spawning entirely new research areas such as electromagnetically induced transparency and slow light.^1,2 To date, this approach to controlling how radiation interacts with matter has been confined to long wavelength (e.g. visible) radiation. For researchers who probe matter at the microscopic level, a relevant question arises: can light also be used to control how x-rays interact with matter? If so, this control might create new opportunities in x-ray science, including new approaches for x-ray optics and nonlinear x-ray spectroscopy.

While controlling light with light is now well established, the extension to short wavelength probe radiation is non-trivial: ‘optical control’ relies on coherence, yet dephasing becomes more important as the probe radiation wavelength is shortened. Specifically, 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. This core-hole filling disrupts the optically imposed material coherence and destroys the ‘control.’ Core-hole filling occurs on a timescale comparable to the optical period (~1fs). This makes it impossible to maintain coherence over multiple optical cycles and creates fundamental uncertainties about whether light can be used to manipulate x-ray interactions. While the problems associated with rapid dephasing can be mitigated by using intense optical control pulses, the high intensity creates additional problems that add to the overall uncertainty about whether optical control is feasible for x-ray transitions.

My colleagues and I have recently shown that ultrashort light pulses can be used to induce x-ray transparency in a nominally opaque medium.³ This finding lays the foundation for optical control of x-ray/matter interactions. Such studies could spawn new ideas for how to use x-rays at a time when next-generation sources featuring intense, ultrashort x-ray pulses are coming online.

Figure 1. The Advanced Light Source at Berkeley, California. Photo credit: Roy Kaltschmidt, Berkeley Lab Public Affairs.

We performed a series of experiments at the Advanced Light Source (ALS) Femtosecond Spectroscopy beamline 6.0.2, which is one of only three places worldwide where wavelength-tuneable, femtosecond-duration x-ray pulses are available (see Figure 1). In the experiment, a femtosecond x-ray (~900eV) pulse propagated through a neon-filled gas cell and was heavily attenuated. When the pulse was contained within an intense femtosecond co-propagating optical pulse, the x-ray transmission increased dramatically (~ factor of three; see Figure 2).

Figure 2. Fractional change in x-ray transmission (ΔT/T) through a ~110torr-cm thick neon gas cell. X-ray transmission is induced by an optical laser at peak intensities of 2.5 × 10¹³W/cm² (left), 1.8 × 10¹³W/cm² (middle) and 1.1 × 10¹³W/cm² (right). At the highest intensity, x-ray transmission is increased threefold. The solid lines show theoretical simulations of the expected change in transmission.³

The light pulse essentially created a window through which x-rays could pass with significantly lower material attenuation (i.e., transparency). Since this window lasts for a short period of time (down to ~70fs for the ALS experiments), it can be used to characterize x-rays on ultrashort timescales and was used to measure the duration of the femtosecond x-ray pulse. Measuring the duration of such short pulses has previously proven difficult, so this technique should help to further develop ultrafast x-ray spectroscopy.

Additionally, since techniques for shaping optical pulses on a femtosecond timescale are well developed, optically driven x-ray transparency can be used to sculpt the temporal profile of ultrashort x-ray pulses. The ability to generate shaped, femtosecond duration pulses may create new x-ray research opportunities such as ‘quantum control.’ (Quantum control is a well-developed discipline in the visible regime that uses shaped pulses to control the evolution of chemical reactions.)

The recent ALS experiments establish that light can be used to efficiently manipulate the amplitude of an x-ray pulse. More broadly, the findings lay a foundation for investigating a wider spectrum of applications that rely on using light to control how x-rays interact with matter. One open question is whether light can be used to control the phase of an x-ray pulse. Such phase control could inspire new approaches to, for instance, making x-ray lenses or viewing matter at the microscopic level via x-ray scattering. In a similar vein, whether ‘control’ techniques can be used to significantly enhance cross-sections for nonlinear x-ray scattering has yet to be determined. Such enhancement would prove important to developing nonlinear x-ray spectroscopy, a goal currently hampered by the fact that cross sections for non-linear scattering are intrinsically weak at these wavelengths. Our next steps will likely be analysis and modeling to assess whether it is possible to significantly modify the phase of an x-ray pulse with light. If such phase control seems possible, we hope to demonstrate light-driven focusing of an x-ray beam.

These experiments were the result of a collaborative effort led by L. Young of Argonne National Laboratory, and A. Belkacem and T.E. Glover of Lawrence Berkeley National Laboratory.