Accelerating the Spontaneous Emission of X Rays from Atoms in a Cavity

We have investigated the spontaneous radiative decay of resonant nuclei in a planar x-ray waveguide after excitation by synchrotron radiation pulses. The waveguide acts as a cavity and modifies the mode structure of the electromagnetic field. As a result, the rate of spontaneous emission is enhanced by a factor proportional to the density of photon states in the cavity. In this experiment, we have observed a sixfold acceleration of the coherent radiative decay of ${}^{57}\mathrm{Fe}$ nuclei located in the center of the first-order guided mode. This is in very good agreement with theoretical predictions.

Figure 1

Grazing incidence x-ray reflection from a waveguide consisting of a Fe guiding layer sandwiched between a Ta and a FePt layer. The right graph shows the spatial distribution of the field intensity $I(z)$ in the first-order guided mode as given by Eq. (1).

Figure 2

(a) Intensity of the radiation electronically reflected from the layer system shown in Fig. 1. The radiation couples into the guided mode which appears as a minimum at $q=0.63{\mathrm{nm}}^{-1}$. (b) Intensity of the radiation resonantly reflected from the ${}^{57}\mathrm{Fe}$ probe layer, obtained by recording the time-integrated delayed photons in a time window from 15–160 ns after excitation. (c) Normalized intensity of the electric field at the position of the probe layer as calculated via Eq. (1) (labels on the right scale). The solid line in (b) was obtained by squaring the curve in (c) and scaling it to the measured data [11].

Figure 3

Temporal evolution of the intensity scattered resonantly from the ${}^{57}\mathrm{Fe}$ probe layer that is embedded in the waveguide structure. Data were recorded at selected angular positions around the first-order guided mode. From the slope of the envelopes (dashed lines) the reduced decay width $\gamma$ is derived. Solid lines are fits to the data according to Eq. (2) with $p=0.7$, 0.8, 0.95, 0.91, and 0.88 (top to bottom) [12].

Figure 4

Angular dependence of the reduced decay width $\gamma =\Gamma /{\Gamma }_0$ as determined from the measured time spectra. The solid line is a fit to the data according to Eq. (9).