Zero-phonon lines in the spectra of dysprosium atoms in superfluid helium

We present an experimental study of a zero-phonon line (ZPL) in the absorption spectrum of Dy atoms solvated in superfluid He. The dopants reside in nanometer-sized spherical cavities known as atomic bubbles. We observe a temperature-dependent broadening of ZPL in the absorption spectrum. The effect is attributed to the scattering of thermal phonons on the atomic bubble that leads to the dephasing of the Dy transition dipole. The extrapolated ZPL intrinsic spectral width at zero temperature is 2300 times larger than the natural linewidth in a free atom. This can be assigned to a fast radiationless quenching of the upper state of the studied transition.

Figure 1

Experimental setup. 1, cryostat; 2, sample cell; 3, ablation target; 4, pulsed DPSS laser $\left(\lambda =355\mathrm{nm}\right)$; 5, motorized $XY$ translation stage; 6, frequency-doubled pulsed Nd:YAG laser $\left(\lambda =532\mathrm{nm}\right)$ or frequency-tripled Nd:YAG laser $\left(\lambda =355\mathrm{nm}\right)$; 7, cw DPSS laser $\left(\lambda =532\mathrm{nm}\right)$; 8, cw tunable Ti:sapphire laser; 9, second harmonic generator (SHG); 10, dichroic mirror; 11, wavelength meter; 12, scanning confocal Fabry-Pérot etalon; 13, oscilloscope; 14, laser-induced fluorescence; 15, grating spectrograph; 16, CCD camera; 17, PMT; 18, video camera; PD1 and PD2, photodiodes.

Figure 2

(a) High-resolution scan of ZPL in the excitation spectrum. $T=1.5\mathrm{K}$. Dots, experimental data; solid red line, fitted Lorentzian. (b) Temperature dependence of the ZPL spectral width (FWHM). Dots, experimental data; solid red line, fit according to Eq. (1); solid green line, fit according to Eq. (2).

Figure 3

Dispersion diagram of superfluid He $\omega \left(k\right)/2\pi$ [27] (black, right axis); calculated scattering cross section $\sigma \left(k\right)$ (red, left axis) and the density of elementary excitations at $T=1.5$ K (blue, arb. units). Two shaded bands correspond to the two types of elementary excitations: phonons and rotons. Two horizontal dashed lines mark the calculated frequencies of the atomic bubble oscillation modes: ${\mathrm{\Omega }}_0/2\pi$ and ${\mathrm{\Omega }}_2/2\pi$.

Figure 4

Temperature dependence of the ZPL spectral width (FWHM). Dots and solid line 1 (green), experimental data; curve 2 (blue), calculation according to the atomic bubble model; Eqs. (3, 4, 5, 6, 7) including only the excitations from the phonon branch; curve 3 (red), calculations including both phonons and rotons.