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Nuclear Lasing via Laser-Assisted Electronic Bridge Excitation and Lasing Without Inversion in ²²⁹Th:LiSAF

Randy Gummelt

Published May 13, 2026• 0 citations
Publisher

Abstract

A scheme is proposed for achieving coherent emission from the 8.355 eV nuclear isomeric transition in ²²⁹Th doped into LiSrAlF₆ (LiSAF), combining laser-assisted electronic bridge excitation for isomer population with lasing without inversion via RF-driven ground-state coherence. Gain calculations show lasing threshold is achievable with cavity finesse as low as 12. Three critical unknowns are identified for experimental verification.

DOI
10.5281/zenodo.20171860

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A scheme is proposed for achieving coherent emission from the 8.355 eV nuclear isomeric transition in ²²⁹Th doped into LiSrAlF₆ (LiSAF), combining laser-assisted electronic bridge excitation for isomer population with lasing without inversion via RF-driven ground-state coherence. Gain calculations show lasing threshold is achievable with cavity finesse as low as 12. Three critical unknowns are identified for experimental verification.
We demonstrate theoretically that nuclear lasing is achievable in ²²⁹Th-doped CaF₂ at 4.2 K using standard laboratory equipment. The 1,740-second excited-state lifetime enables population inversion with milliwatt VUV pumping, while superradiant collective emission in the dense crystal (10¹⁹ atoms/cm³) provides sufficient gain enhancement to reach threshold with < 5% inversion. A parameter sweep identifies 96 viable configurations. We further show that a second-generation XFEL-pumped ⁵⁷Fe amplifier, combined with multilayer Laue lens focusing, can achieve Schwinger-limit field intensities. Seventy-seven computational physics checks confirm no known law is violated. Total prototype cost: $150K–$330K.
We experimentally demonstrate the existence of long-lived coherent polarizations coupling simultaneously ground state X (X2Σg+) to excited states A (A2Πu) and B (B2Σu+) of N2+ inside a plasma created by a short intense laser pulse at 800 nm. This three-level V-scheme arrangement is responsible for a strong optical gain without population inversion at the B–X transition at 391.4 nm. Simulations based on Maxwell-Bloch equations reproduce well the kinetics and the pressure dependence of the gain.
The direct interaction between nuclei and strong electromagnetic fields generated by novel coherent x-ray light sources such as the x-ray free electron laser is studied. We investigate the case of various stable or long-lived isotopes with energetically low-lying excited states, where the laser drives the nuclear transition from the ground to the first excited state. We therefore consider the nucleus as a two-level system including the corresponding hyperfine levels in our theoretical description. The performance of present and upcoming x-ray free electron laser facilities in coherently controlling the nuclear excited state population is analyzed for setups involving both solid state as well as accelerated nuclear targets to bridge the gap between photon and transition frequency. Current and predicted laser parameters are applied to calculate excited state populations and signal photon rates of nuclei after a radiation pulse, finding significant improvements compared to former calculations. Furthermore, we determine the parameters necessary to transfer substantial amounts of the nuclear ground state population to the excited state, and therefore to bring nuclear Rabi-oscillation within reach. Additionally, we take into account for the first time collective effects that take place in solid state targets and show that they may provide a significant effect in broadening natural linewidths of nuclear levels depending on the experimental setup. Our findings support the case of nuclear quantum optics for XFEL applications. Therefore, one hopes that eventually coherent control of nuclear transitions can be successfully applied in a number of areas in nuclear physics including preparation, control and detection methods.
The authors leverage the recently proposed optonuclear quadrupolar effect to develop a two-photon pumping scheme for the isomeric state of 229{}^{229}229Th, an exotic nucleus with an ultralow excitation energy of 8.3 eV. Additionally, the authors demonstrate that the lasing between nuclear states, which has been pursued for decades, may be realized based on the two-photon pumping scheme in ultrawide-band-gap compounds like ThF4{}_{4}4​, Na2{}_{2}2​ThF6{}_{6}6​, or K2{}_{2}2​ThF6{}_{6}6​.
It has been known for two decades that coherence can yield lasing without inversion (LWI), which could be useful for making lasers at shorter wavelength. However, excitation of extreme ultraviolet and x-ray atomic transitions typically requires a plasma medium with rapid collisions, which destroy atomic coherence. Here we demonstrate LWI on a time scale shorter than the decoherence time. We show that in such a regime LWI is possible in a V -scheme with a strong coherent drive on the low-frequency transition and obtain an analytical expression for the gain of the laser pulse at high frequency. We propose an experiment in which such LWI can be realized in He plasma.