Efficient $2{}^{3}S$ positronium production by stimulated decay from the $3{}^{3}P$ level

We investigate experimentally the possibility of enhancing the production of $2{}^{3}S$ positronium atoms by driving the $1{}^{3}S$–$3{}^{3}P$ and $3{}^{3}P$–$2{}^{3}S$ transitions, overcoming the natural branching ratio limitation of spontaneous decay from $3{}^{3}P$ to $2{}^{3}S$. The decay of $3{}^{3}P$ positronium atoms toward the $2{}^{3}S$ level has been efficiently stimulated by a 1312.2 nm broadband IR laser pulse. The dependence of the stimulating transition efficiency on the intensity of the IR pulse has been measured to find the optimal enhancement conditions. A maximum relative increase of $\times (3.1\pm 1.0)$ in the $2{}^{3}S$ production efficiency, with respect to the case in which only spontaneous decay is present, was obtained.

Figure 1

Laser spectra (arbitrary units) measured in the two OPG temperature set points optimized for stimulating the $1{}^{3}S$–$3{}^{3}P$–$2{}^{3}S$ transition (white squares, ${172.4}^{\circ }$) and for the highest $1{}^{3}S$–$3{}^{3}P$ excitation efficiency (black circles, ${175.6}^{\circ }$). The dotted lines mark the transition resonances. Inset: measured laser detuning from the theoretical resonance frequency as a function of the OPG crystal temperature for the $1{}^{3}S$–$3{}^{3}P$ 205 nm laser (blue band) and the $3{}^{3}P$–$2{}^{3}S$ 1312 nm laser (red band), respectively, compared to the reference Ps Doppler distribution as measured in [19] (gray band).

Figure 2

(a) Distributions of Ps impact positions, shown superimposed upon the 3D drawing of the chamber walls, for the $T_1={175.6}^{\circ }$ (blue circles) and $T_2={172.4}^{\circ }$ (red circles) set points, emphasizing the different Doppler selection in the two detuning conditions. (b) Measurement of the annihilation time distributions of spontaneously decaying $2{}^{3}S$ Ps atoms from the $3{}^{3}P$ level without stimulated transition to $2{}^{3}S$ for the two temperature set points. The graph shows the curve $-S\left(t\right)=[A^{\mathrm{UV}}\left(t\right)-A^{\mathrm{off}}\left(t\right)]/A^{\mathrm{off}}\left(t\right)$ (see the text). Each time distribution has been fitted with the Monte Carlo model discussed in the text (solid lines).

Figure 3

Main panel: the $S_{\text{peak}}$ value, as defined in the text, vs the attenuation in dB of the IR laser pulse starting from the maximum energy $405\mu \mathrm{J}$ (black points). Above: the $-S\left(t\right)$ parameter in the case of the 5 dB attenuation point.

Figure 4

The experimental curve of the $-S$ parameter ($=[A^{\mathrm{UV}+\mathrm{IR}}\left(t\right)-A^{\mathrm{off}}\left(t\right)]/A^{\mathrm{off}}\left(t\right)$) measured with the empirically determined optimal 1312.2 nm laser intensity, compared to the reference $-S$ obtained without the IR laser.