Solid-state ${}^{229}\mathrm{Th}$ nuclear laser with two-photon pumping

The radiative excitation of the 8.3 eV isomeric state of ${}^{229}\mathrm{Th}$ is an long-standing challenge due to the lack of tunable far-ultraviolet (FUV) sources. In this work, we propose an efficient two-photon pumping scheme for ${}^{229}\mathrm{Th}$ using the optonuclear quadrupolar effect, which only requires a 300 nm $\mathrm{UV}\text{−}\mathrm{B}$ pumping laser. We further demonstrate that population inversion between the nuclear isomeric and ground states can be achieved at room temperature using a two-step pumping process. The nuclear laser, which has been pursued unsuccessfully for decades, may be realized using a watt-level UV-B pumping laser and ultrawide band gap thorium compounds (e.g., $\mathrm{Th}{\mathrm{F}}_4$, ${\mathrm{Na}}_2\mathrm{Th}{\mathrm{F}}_6$, or ${\mathrm{K}}_2\mathrm{Th}{\mathrm{F}}_6$) as the gain medium.

Figure 1

(a) Electronic density of states of ${\mathrm{Na}}_2\mathrm{Th}{\mathrm{F}}_6$ from $G_0{W}_0$ calculations. (b) Nuclear energy level diagram of ${}^{229}\mathrm{Th}$ in ${\mathrm{Na}}_2\mathrm{Th}{\mathrm{F}}_6$.

Figure 2

(a) Two-photon pumping scheme based on the ONQ effect. Two photons pump a (virtual) electronic excitation, which is then swapped to a real nuclear excitation through the nuclear quadrupolar interaction. (b) The two-step pumping scheme to achieve population inversion. Numbers in the boxes indicate normalized populations. States with gray boxes do not participate in the nuclear pumping/lasing process.

Figure 3

(a) A simplified setup of the nuclear laser. (b) Time evolution of the output power of the nuclear laser. The parameters of the nuclear laser are described in the main text. The shaded area indicates a nuclear laser pulse.