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Journal of Experimental and Theoretical Physics Letters

Two-Photon Mechanism of the Excitation of Exciton States in Noble Gas Cryocrystals

D. A. Myzin, P. V. Borisyuk, K. L. Gubskii, N. V. Didenko, F. A. Korneev, A. P. Kuznetsov, Yu. Yu. Lebedinskii, Zh. M. Nauryzbaev, S. V. Popruzhenko, E. V. Tkalya, E. V. Chubunova, V. A. Shilov

Published Oct 1, 2025• 1 citation
Publisher

Abstract

The results of an experiment on the two-photon excitation of exciton states in krypton cryocrystals by the fifth harmonic of 209-nm laser radiation have been presented. It has been shown that the two-photon excitation mechanism allows reaching a density of 1016 cm−3 of free excitons with an excitation energy of ~8.4 eV at a focal radiation intensity of $$ \approx {{10}^{{11}}}{\kern 1pt} $$ W/cm2. The observed broad photoluminescence line arising from the relaxation of excited exciton states covers the $$3{\text{/}}{{2}^{ + }}(8.4{\kern 1pt} \;{\text{eV}}) \to 5{\text{/}}{{2}^{ + }}(0.0)$$ low-energy nuclear isomeric transition in 229Th. Analytical estimates showing that two-photon laser generation of exciton states in noble gas crystals can be used as a new method for the excitation of the nuclear isomeric state of thorium at a pump intensity of ~1011 W/cm2 have been provided.

DOI
10.1134/s002136402560853x

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The results of an experiment on the two-photon excitation of exciton states in krypton cryocrystals by the fifth harmonic of 209-nm laser radiation have been presented. It has been shown that the two-photon excitation mechanism allows reaching a density of 1016 cm−3 of free excitons with an excitation energy of ~8.4 eV at a focal radiation intensity of ≈1011\approx {{10}^{{11}}}{\kern 1pt}≈1011 W/cm2. The observed broad photoluminescence line arising from the relaxation of excited exciton states covers the 3/2+(8.4  eV)→5/2+(0.0)3{\text{/}}{{2}^{ + }}(8.4{\kern 1pt} \;{\text{eV}}) \to 5{\text{/}}{{2}^{ + }}(0.0)3/2+(8.4eV)→5/2+(0.0) low-energy nuclear isomeric transition in 229Th. Analytical estimates showing that two-photon laser generation of exciton states in noble gas crystals can be used as a new method for the excitation of the nuclear isomeric state of thorium at a pump intensity of ~1011 W/cm2 have been provided.
A possibility of the two-photon excitation of an isomeric state in a nucleus of thorium-229 has been discussed. The fluorescence intensity of the excitation is demonstrated to be identical for the irradiation of nuclei with either monochromatic light or polychromatic radiation consisting of a sequence of short lightpulses of the same intensity. The two-photon excitation of Th3+ ion in an electromagnetic trap with a focused laser beam with a wavelength of about 320 nm and power of 100 mW can lead to the absorption saturation, at which the fluorescence emission with the frequency of the transition in a nucleus is maximal. In crystals doped with Th4+ to a concentration of about 1018 cm-3 and irradiated with a laser radiation 10 W in power, the emission of several photons persecond with a wavelength of about 160 nm becomes possible.
Through the studies of excitation spectra of emissions in CuCl crystals by a frequency tunable dye laser, excitonic molecules have been found to be generated directly by the giant two-photon absorption at the photon energy of 3.187 eV, as suggested by Hanamura. Two-photon excitation of crystals at 1.6 K enhanced very narrow emission bands at 3.1647 eV and at 3.1709 eV. These photon energies coincide with those of the high energy edges of the M L and M T bands due to the radiative annihilation of excitonic molecules having K =0. From these facts the Bose condensation of excitonic molecules at K ∼0 has been suggested.
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​.
Experiments on one- and two-photon laser excitation of 232{}^{232}232Th+{}^{+}+ ions in a radio-frequency ion trap are reported. As the first excitation step, the strongest resonance line at 402 nm from the (6d27s)J=3/2(6{d}^{2}7s)J=3/2(6d27s)J=3/2 ground state to the (6d7s7p)J=5/2(6d7s7p)J=5/2(6d7s7p)J=5/2 state at 24874 cm${}^{\ensuremath{-}1}$ is driven by radiation from an extended-cavity diode laser. Spontaneous decay of the intermediate state populates a number of low-lying metastable states, thus limiting the excited state population and fluorescence signal obtainable with continuous laser excitation. We study the collisional quenching efficiency of helium, argon, and nitrogen buffer gases, and the effect of repumping laser excitation from the three lowest-lying metastable levels. The experimental results are compared with a four-level rate equation model, which allows us to deduce quenching rates for these buffer gases. Using laser radiation at 399 nm for the second step, we demonstrate two-photon excitation to the state at 49 960 cm${}^{\ensuremath{-}1}$, among the highest-lying classified levels of Th+{}^{+}+. This is of interest as a test case for the search for higher-lying levels in the range above 55 000 cm${}^{\ensuremath{-}1}$ which can resonantly enhance the excitation of the 229{}^{229}229Th+{}^{+}+ nuclear resonance through an inverse two-photon electronic bridge process.
In the light of the creation of a diamond laser [1], an urgent task is to determine the characteristics of diamonds in order to determine the range in which such lasers can be created not in isolated cases. This work is aimed at studying the mechanisms of creating superluminescence in diamond under the action of optical pumping by the second harmonic of an ND:YAG laser (λ = 532 nm). It was found that, upon pulsed excitation of a diamond by an ND:YAG laser with a pump intensity above ~2.0 MW/cm2 in the spectral range 700–750 nm, a nonlinear increase in intensity appears against the background of the spontaneous photoluminescence spectrum, which, with a further increase in the pump intensity, turns into a pronounced superluminescence peak with a maximum at about 718 nm. An increase in the pump intensity from 2.7 to 46 MW/cm2leads to the broadening of this peak at half maximum from 13 to 19 nm. At high levels of pump intensity, nonlinear absorption of pump radiation and accumulation of NV centers in the excited state were found. The position of the photoluminescence band was calculated as a function of various values of the inversion density of the populations of color centers, taking into account the intrinsic absorption spectrum of diamond. The calculation results are close to the experimental data.
Creation of the inverse population in the 229^{229}229Th ground-state doublet
A new method for obtaining of inverse population between quadrupole sublevels of the ground state 5/2+(0.0)5/2^+(0.0)5/2+(0.0) and the isomeric state 3/2+(7.63/2^+(7.63/2+(7.6 eV) of the 229^{229}229Th nucleus in a dielectric crystal with a large band gap by means of external source of the VUV radiation is proposed. The method is based on an efficient depopulation of the upper sublevels of the ground state of 229^{229}229Th by resonant photons from narrowband laser or broader tunable free electron laser. Sublevels of the isomeric state play the role of intermediate states. In addition, we have considered a case of excitation of the isomeric state (without creation of inverse population) by broadband source of light with the anti-Stokes scattering. The proposed scheme of optical pumping results in (1) inverse population of nuclear sublevels without using extremely low temperatures, and (2) shows a new way for the creation of the gamma-ray laser of optical range at the nuclear transition in 229^{229}229Th (a) in crystals with isovalent substitution of host ions (eg Si4+^{4+}4+ ions in the SiO2_22​ crystal replaced by the 229^{229}229Th4+^{4+}4+ ions) and (b) in crystals such as Na2_22​ThF6_66​, where the substitution is not necessary.