Spectral Control of an X-Ray $L$-Edge Transition via a Thin-Film Cavity

By embedding a thin layer of tantalum in an x-ray cavity, we observe a change in the spectral characteristics of an inner-shell transition of the metal. The interaction between the cavity mode vacuum and the $L_{\mathrm{III}}$-edge transition is enhanced, permitting the observation of the collective Lamb shift, superradiance, and a Fano-like cavity-resonance interference effect. This experiment demonstrates the feasibility of cavity quantum electrodynamics with electronic resonances in the x-ray range with applications to manipulating and probing the electronic structure of condensed matter with high-resolution x-ray spectroscopy in an x-ray cavity setting.

Figure 1

(a) Experimental setup. The multilayer cavity containing a thin tantalum layer is placed on a goniometer and illuminated at grazing incidence angles by x rays from a synchrotron tuned to the Ta $L_{\mathrm{III}}$ edge energy of 9881 eV. An avalanche photodiode (APD) is positioned to collect the reflected radiation. Another detector is placed to the side of the cavity to measure a part of the light emitted into $4\pi$, which is largely fluorescence radiation. (b) The reflectivity curve of the sample, detuned by 70 eV from the edge in order to omit resonant effects. The dips indicate standing wave modes, with the first order mode being the first sharp dip on the left.

Figure 2

Spectroscopic signals of the cavity. The intensity is encoded in the color map. The $x$ axis shows the cavity detuning expressed in terms of both angle (bottom axis) and energy (top axis), and the $y$ axis shows the detuning of the monochromator from the transition energy in eV. (a) Reflectivity of the sample. There is an anticrossinglike behavior which is not entirely resolved; rather, the two branches of the dispersion relation interfere, giving Fano-like effects. (b) The fluorescence signal. As the cavity is tuned through the transition line, the latter undergoes some changes; it is slightly shifted by up to 2.5 eV and its spectral width is enhanced to up to 10 eV. These effects are the collective Lamb shift and the superradiant decay enhancement, respectively. The large signal at positive monochromator detunings is the fluorescence of the steplike electronic continuum. Panels (c) and (d) show simulations of (a) and (b), respectively, without taking the effects of the electronic continuum into account.

Figure 3

Panels (a) and (b) show line cuts at 0.235° and 0.25° through Fig. 2 and 2, respectively. Panels (c) and (d) show details of (a) and (b), confirming the magnitude and angular detuning dependence of the collective Lamb shift. The steplike signature of the electronic continuum was not taken into account in the analysis.

Figure 4

The functional dependence of the superradiant decay enhancement and the collective Lamb shift of the $L_{\mathrm{III}}$-transition level. The latter changes its sign depending on the detuning, and is zero at the point of zero detuning of the cavity; the former has its maximum at that point. The influence of the cavity allows the effective “tuning” of the transition properties. The horizontal dashed line represents the position at which the effect is zero, the vertical dashed line the angular position of the cavity resonance.