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Robust Optical Coherence in a Chemically Tunable Eu(III) Molecular Crystal

Suma Al-Hunaishi, Senthil Kumar Kuppusamy, Yisheng Lei, Idris Tlemsani, Burak Gurlek, Jordan L. Appleton, Sergei Lebedkin, Alexandre Tallaire, Mario Ruben, Philippe Goldner, Diana Serrano

Published Aug 27, 2026• 0 citations
Publisher

Abstract

Chemical substitution o!ers a powerful route to tune molecular quantum materials, but its e!ects on optical decoherence extend beyond the simple dilution of interacting emitters; here, we investigate these e!ects in the molecular crystal [Eu(III)(TMHD)3(phen)], its dilution series in the isostructural host [Y(TMHD)3(phen)], and a partially deuterated analogue. The optical inhomogeneous linewidth of the 7F0 ↔ 5D0 transition near 580.4 nm exhibits a pronounced concentration dependence, ranging from 6 to 20 GHz, while the transition frequency shifts by up to ↗25 GHz across the dilution series. In contrast, the homogeneous linewidth measured by two-pulse photon echoes remains in the 50–130 kHz regime at 1.4 K, and shows a non-monotonic dependence on Eu(III) content. Partial phenanthroline deuteration also leaves the homogeneous linewidth essentially unchanged. Spectral-di!usion and temperature-dependent photon-echo measurements identify non-magnetic fluctuations of the molecular environment as the dominant residual dephasing channel. In contrast to rigid inorganic rare-earth hosts, the molecular crystal lattice supports low-frequency lattice modes that can dynamically modulate the anisotropic Eu(III) crystal field, while the extrapolated zero-temperature linewidth is most likely limited by electric-field perturbations. These results establish Eu(III) molecular crystals as chemically tunable materials with robust optical coherence and show that chemical dilution simultaneously reshapes static disorder, lattice dynamics, and decoherence, identifying molecular-lattice rigidity and electrostatic stability as central design parame-

DOI
10.26434/chemrxiv.15007936/v1

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Chemical substitution o!ers a powerful route to tune molecular quantum materials, but its e!ects on optical decoherence extend beyond the simple dilution of interacting emitters; here, we investigate these e!ects in the molecular crystal [Eu(III)(TMHD)3(phen)], its dilution series in the isostructural host [Y(TMHD)3(phen)], and a partially deuterated analogue. The optical inhomogeneous linewidth of the 7F0 ↔ 5D0 transition near 580.4 nm exhibits a pronounced concentration dependence, ranging from 6 to 20 GHz, while the transition frequency shifts by up to ↗25 GHz across the dilution series. In contrast, the homogeneous linewidth measured by two-pulse photon echoes remains in the 50–130 kHz regime at 1.4 K, and shows a non-monotonic dependence on Eu(III) content. Partial phenanthroline deuteration also leaves the homogeneous linewidth essentially unchanged. Spectral-di!usion and temperature-dependent photon-echo measurements identify non-magnetic fluctuations of the molecular environment as the dominant residual dephasing channel. In contrast to rigid inorganic rare-earth hosts, the molecular crystal lattice supports low-frequency lattice modes that can dynamically modulate the anisotropic Eu(III) crystal field, while the extrapolated zero-temperature linewidth is most likely limited by electric-field perturbations. These results establish Eu(III) molecular crystals as chemically tunable materials with robust optical coherence and show that chemical dilution simultaneously reshapes static disorder, lattice dynamics, and decoherence, identifying molecular-lattice rigidity and electrostatic stability as central design parame-
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Rare-earth-ion based molecular crystals are emerging as a promising platform for quantum information applications, combining long optical coherence times with the chemical tunability of coordination chemistry. Here we present a comprehensive optical characterisation of twelve Eu3+^{3+}3+-based molecular complexes, with a focus on the coherent 5D0⟶ 7F0^{5}\text{D}_{0} \longrightarrow \ ^{7}\text{F}_{0}5D0​⟶ 7F0​ transition at $\sim580nm.Judd−Ofeltanalysesareperformedacrossallcomplexes,providinginsightsintoligand−fieldenvironments,localpoint−groupsymmetries,andtheresultingbranchingratiosofthecoherenttransition.Wefindthatlowerpoint−groupsymmetry(C580 nm. Judd-Ofelt analyses are performed across all complexes, providing insights into ligand-field environments, local point-group symmetries, and the resulting branching ratios of the coherent transition. We find that lower point-group symmetry (C580nm.Judd−Ofeltanalysesareperformedacrossallcomplexes,providinginsightsintoligand−fieldenvironments,localpoint−groupsymmetries,andtheresultingbranchingratiosofthecoherenttransition.Wefindthatlowerpoint−groupsymmetry(C_1,C, C,C_2$) yields higher branching ratios of the 5D0⟶ 7F0^{5}\text{D}_{0} \longrightarrow \ ^{7}\text{F}_{0}5D0​⟶ 7F0​ transition, exceeding 1\% in several complexes. Optical inhomogeneous linewidths measured at 4.2 K reveal narrow linewidths for some complexes of below ten GHz, indicative of a highly ordered crystalline environment. Optical coherence measurements of one of the complexes, 5-Eu, yields an optical coherence time of T2opt=2.3 μsT_{\text{2}}^{\text{opt}} = 2.3~μ\mathrm{s}T2opt​=2.3 μs at 4.2 K, comparable to the best-performing molecular systems reported to date. Systematic characterisation and quantitative comparison will be key to find structure-property relations guiding rational design of optimised complexes for quantum information applications.
The optical spectroscopy and dynamics of three materials; Eu^3+:Y2SiO5, Pr^3+:Y2SiO5, and Tm^3+:Y2SiO5 that are of interest for application in persistent spectral holeburning optical memory and signal processing devices were investigated using linear and nonlinear spectroscopic techniques.Two-pulse photon echoes have been used to measure the homogeneous linewidths.The effects of instantaneous spectral diffusion were systematically studied to allow accurate determination of the optical resonance widths, and external magnetic fields were applied to characterize the effects of fluctuating nuclear spins of the host lattice on dephasing of the dopant ion.The 7F0 → 5D0 transition of 0.1% Eu^3+ in the two crystallographic sites of Y2SiO5 was studied using photon echoes.Using this material, we succeeded in measuring the narrowest optical resonance in a solid by shielding the crystal from stray electromagnetic fields and using sufficiently low laser power to minimize contributions from instantaneous diffusion.The homogeneous linewidths measured in a magnetic field of 100 G are 122 Hz (site 1) and 167 Hz (site 2).These widths are dominated by population decay, but up to 20 Hz is attributed to 89Y spin fluctuations which have been reduced by the applied magnetic field.Similar studies were performed to determine the contributions to the homogeneous linewidth of the 3H4(1) → 1D2(1) transition for both sites of 0.02% Pr^3+ in Y2SiO5.Linewidths of 2.1 kHz (site 1) and 0.85 kHz (site 2) were obtained with an applied magnetic field of 77 G.A significant contribution of 89Y nuclear spin fluctuations to the zero-field homogeneous linewidth was found.Optically detected nuclear magnetic resonance measurements determined the hyperfine structure of the 3H4 ground state for each site, and photon echo nuclear double resonance was used to determine the hyperfine levels of the lowest component of the 1D2 manifold. 3) 2.1? Ec,2S
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Eu-doped yttrium orthosilicate (Eu(3+) : Y2SiO5) had been a subject being investigated for coherent time-domain optical memory and information processing applications since its ultraslow optical dephasing was discovered several years ago. In this crystal the weakly allowed (7)F0 - (5)D0 transition of europium ions exhibits a sufficient long dephasing time and no spectral difli.usion on a time scale of several hours at low temperature, thus an information pattern or data can be stored as a population grating in the ground state hyperfine levels. On the other hand, the study on photon-echo relaxation shows that the dephasing time T2 of Eu (3+) and other rare-earth ions doped YAG, YAlO3 strongly depends on the intensity of the excitation pulses. In Eu (3+) :YAlO3, an exponential decay of photon-echo with T2 = 53 microseconds if the excitation pulses are weak (5 vJ/pulse) was observed. However, when the excitation pulses are strong (80 pJ/pulse) they observed a much shortened T2 with a highly nonexponential decay pattern. The conclusion they derived is that the intensity-dependent dephasing rate effects are quite general, and it depends on how much the excitation intensity varies. In this paper we use transient grating formation technique showing that a temporal lattice distortion may only occur along crystal c axis, caused by EU (3+) excitation. At high excitation level the produced exciton in conduction band may also couple to the dynamical lattice relaxation process, giving rise to an apparently much shortened dephasing time.