Observation of Free-Electron-Laser-Induced Collective Spontaneous Emission (Superfluorescence)

We have observed and characterized 501.6 nm collective spontaneous emission (superfluorescence) following $1s^2\to 1s3p$ excitation of helium atoms by 53.7 nm free-electron laser radiation. Emitted pulse energies of up to 100 nJ are observed, corresponding to a photon number conversion efficiency of up to $10\%$. We observe the peak intensity to scale as ${\rho }^2$ and the emitted pulse width and delay to scale as ${\rho }^{-1}$, where $\rho$ is the atom number density. Emitted pulses as short as 1 ps are observed, which corresponds to a rate around 75 000 times faster than the spontaneous $1s3p\to 1s2s$ decay rate. To our knowledge, this is the first observation of superfluorescence following pumping in the extreme ultraviolet wavelength region, and extension of the technique to the generation of extreme ultraviolet and x-ray superfluorescence pulses should be straightforward by using suitable atomic systems and pump wavelengths.

Figure 1

The excitation scheme. The $1s3p$ state is excited by 53.7 nm wavelength EUV-FEL radiation (red arrow), and superfluorescence is observed at 501.6 nm on the $1s3p\to 1s2s$ transition (green arrow).

Figure 2

Representative streak camera data. Each trace is the average of several traces recorded at each gas pressure. The time corresponding to zero delay has been found by fitting—see text and Fig. 3. The inset shows the pressure dependence of the peak and integrated intensity for pressures up to 2 atm (see text), with error bars corresponding to the statistical variance of the data.

Figure 3

Pressure dependence of the observed pulse delay and width. The solid lines represent fits to the data of the form $\tau =a{P}^b+c$ for the delay and ${\tau }^2=a{P}^{2b}+c$ for the width. The offsets $c$ have been subtracted from the data for clarity, and the error bars represent the statistical variance of the extracted values.