# High-resolution optical spectroscopy of europium nitrate hexahydrate crystals

Federico Chiossi, Diana Serrano, Alexey Tiranov, Philippe Goldner

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Federico Chiossi, Diana Serrano, Alexey Tiranov, Philippe Goldner. High-resolution optical spectroscopy of europium nitrate hexahydrate crystals. Journal of Luminescence, 2025, 288, pp.121500. 10.1016/j.jlumin.2025.121500. hal-05377405

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Full Length Article

# High-resolution optical spectroscopy of europium nitrate hexahydrate crystals

Federico Chiossi<sup>1,*</sup>, Diana Serrano, Alexey Tiranov, Philippe Goldner

Chimie ParisTech, PSL University, CNRS, Institut de Recherche de Chimie, Paris, 75005, France

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## ARTICLE INFO

**Keywords:**

- Stoichiometric crystals
- Spectral hole burning
- Optical coherence time
- Rare Earth spectroscopy

## ABSTRACT

We report the optical study of europium nitrate hexahydrate crystals Eu(NO₃)₃·6H₂O and their deuterated version Eu(NO₃)₃·6D₂O conducted at 50 mK. The crystals show sub-GHz inhomogeneous linewidths for the ⁷F₀ ↔⁵D₀ transition and optical coherence times up to 136 µs for the non-deuterated crystal and 750 µs for the deuterated one. In addition, using the spectral hole burning technique, the hyperfine splittings in the absence of an external field have been estimated for the ground and optical levels. These results suggest that rare-earth nitrate crystals could represent a promising class of materials for quantum information applications as well as for fundamental physics experiments requiring narrow optical transitions and good coherence properties.

## 1. Introduction

Trivalent europium-doped crystals have attracted significant attention for their potential applications in quantum information science [1–3] and, more recently, in fundamental physics [4]. This interest is primarily due to the long coherence times achievable with the optical [5] and nuclear spin [6,7] transitions of trivalent europium ions, combined with narrow optical and spin inhomogeneous linewidths that enable efficient, coherent transfer between optical and spin states [8]. These characteristics make europium crystals particularly suitable for applications requiring high-fidelity, long-lived quantum states, such as quantum information processing [9] and storage [10,11]. However, the low optical density in doped crystals, resulting from both the low dopant concentration and the relatively weak optical transition strength, limits their quantum storage efficiency and multimodal capacity. Stoichiometric europium crystals, with their significantly higher ion density, have emerged as an attractive solution for overcoming these limitations [12].

Crystals with europium in non-centrosymmetrical sites have also been proposed for high-sensitivity experiments aimed at detecting new particles and testing violations of fundamental symmetries, specifically time-reversal (T) and parity (P) symmetries. An electric dipole moment (EDM) can arise from P- and T-odd forces within the nucleus, inducing a small, permanent EDM in the respective atom [13,14]. The measurement of the atomic energy level shifts due to this permanent EDM in the presence of strong electric fields can thus give access to the investigation of these symmetry-breaking nuclear forces, potentially leading

to physics beyond the standard model. Additionally, oscillations of the atomic EDM could reveal interactions between the nucleus and hypothetical axion particles, which are strong candidates for dark matter [15]. Europium-153, which constitutes 52% of naturally occurring europium, is a particularly promising system for these experiments [16,4], as it is among the few atoms predicted to exhibit enhanced EDM [17] and possesses atom-like 4f-4f transitions with good coherent properties. In fact, new limits on the axion-like particle-gluon coupling have been very recently determined using europium-doped crystals [18].

The higher ion density in stoichiometric europium crystals is certainly beneficial for enhancing the sensitivity of these experiments. Yet, what has truly underscored the potential of fully concentrated crystals was the report of exceptionally narrow optical linewidths in an isotopically purified Eu³⁵Cl₃·6H₂O crystal. For the Eu³⁺ 4f⁷F₀ ↔ 4f⁵D₀ transition and a flawless region of the crystal, an inhomogeneously broadened optical linewidths as narrow as 25 MHz allowed for the first time the resolution of hyperfine splitting of the Eu³⁺ optical transitions, thereby enabling the polarization of the europium spins via optical pumping [12]. It has been calculated that the spin polarization may lead to an improved sensitivity to the axion interactions via EDM moment oscillations [19]. Additionally, the possibility of increasing the optical density of a specific optical hyperfine transition is attractive for applications in quantum information science as well [12], taking also into account that by deuterating the europium chloride crystals, the optical coherence time was extended from 77 µs to 740 µs at liquid helium temperatures.

<footnote>* Corresponding author.
E-mail address: federico.chiossi@chimieparistech.eu (F. Chiossi).</footnote>
Besides europium chloride hydrate, which is highly hygroscopic and requires isotopic purification, several other europium-based stoichiometric crystals have been investigated, with a focus on their optical transitions and linewidths [20–24]. Unfortunately, only a few crystals, namely europium ethylsulfate nonahydrate and europium hydroxide, exhibited optical transition linewidths in the order of hundreds of MHz, but still insufficient to resolve the hyperfine splittings [23,25].

In this work, we investigate the inhomogeneous transition linewidth, the coherence time and the hyperfine splittings in another promising stoichiometric europium crystal, namely europium nitrate hexahydrate crystal Eu(NO₃)₃·6H₂O, and its deuterated version Eu(NO₃)₃·6D₂O. In these crystals, the europium ions occupy a non-centrosymmetric site, making them particularly interesting for experiments searching for new physics related to nuclear EDM. Moreover, nitrate crystals are stable in air, and except for europium, their constituent atoms naturally occur with a single isotope in over 99.6% abundance, overcoming the need for isotopic purification in the search for extremely narrow optical linewidths.

## 2. Crystal preparation

Stoichiometric Eu(NO₃)₃·6H₂O and Eu(NO₃)₃·6D₂O crystals were grown from saturated aqueous solutions using the slow-cooling method. Initially, 10 g of Eu(NO₃)₃·6H₂O powder (99.99% purity, ThermoFisher) were dissolved in pure water at room temperature. Several crystal seeds with a few millimeters size were obtained through spontaneous nucleation by gradually cooling the solution from room temperature down to -5 °C. The solution was subsequently evaporated at 35 °C under reduced pressure, and the precipitate was dissolved again in an aqueous solution containing 5% nitric acid. Large transparent Eu(NO₃)₃·6H₂O crystals were then grown by immersing a seed in the acid solution maintained at 5 °C and gradually lowering their temperature by 2 °C per day until reaching -19 °C. The Eu(NO₃)₃·6D₂O crystal was grown similarly, starting from a deuterated europium nitrate solution containing nitric acid. The solution was prepared starting from a Eu(NO₃)₃·6H₂O water solution by repeatedly removing the water molecules via evaporation and adding 99.96% deuterated water multiple times. For the last step, a deuterated water and nitric acid solution at 5% was used (Eurisotop). The solution was acidified and the crystal was grown at relatively low temperatures to minimize the formation of hydroxide complexes in the rare-earth nitrate solutions, which are believed to significantly contribute to the formation of defects in the crystal structure [26].

The europium nitrate hexahydrate crystallizes in the triclinic space group P ¯1 with each europium ion coordinated to ten oxygen atoms, six from the bidentate nitrate group and four from the water molecules. The two free water molecules per unit cell couple to the large [Eu(NO₃)₃(H₂O)₄] complexes in the crystal via hydrogen bonding [27]. In water solutions, only the hexahydrate solid phase forms at temperatures below 30 °C [28]. The hexahydrate phase was also confirmed for the crystal grown in the nitric acid solution by checking the frequency of the europium transitions [23,27].

Disk-shaped crystals with typical sizes of a few centimeters were obtained (Fig. 1b). The crystals were not oriented and samples with thicknesses of a few hundred micrometers were extracted from the central region and cut along one of the edge directions, parallel to the flat surface.

## 3. Experimental results

The optical spectroscopy measurements were performed with the crystals cooled by a dilution refrigerator (Bluefors SD) operating at the base temperature of 50 mK. To prevent dehydration and ensure a good thermalization, the crystal slices were glued using silver paint inside a sealed, oxygen-free copper chamber equipped with two optical windows. Optical excitation was provided by a tunable continuous-wave

<figure><img src="image_2.png" /><figcaption>**Fig. 1.** (a) Absorption spectrum of the $ ^{7}\text{F}_{0} \leftrightarrow ^{5}\text{D}_{0} $ transition for the Eu(NO₃)₃·6D₂O crystal recorded at 50 mK. (b) Photo of the as-grown deuterated crystal. The shape of the crystal partially reflects that of the container, which was a cylinder with a diameter of 1 inch. (c) Absorption spectrum of the Eu(NO₃)₃·6H₂O crystal. The arrows indicate the frequency used to record the hole burning spectra in Fig. 2. (d) Transition satellite lines observed in the Eu(NO₃)₃·6H₂O crystal at -2.9 GHz. The red line is a bi-Lorentzian fit. The spectra in (a) and (c) were recorded with the laser polarization aligned to the direction of minimum absorption, while the spectrum in (d) was recorded with the laser polarization aligned to the direction of maximum absorption.</figcaption></figure>

dye laser (Sirah Matisse DS) with a linewidth of 300 kHz. Additional details on the experimental apparatus can be found in Ref. [29]. The laser beam was sent to the samples inside the dilution refrigerator, and the transmitted and partially scattered light was refocused onto the active area of a low-noise avalanche photodiode (Hamamatsu c5460) for the absorption, fluorescence, and spectral hole burning spectroscopy. Optical coherence times were instead measured by performing photon echo experiments with heterodyne detection, using a fast avalanche photodiode (Thorlabs APD110A, 50 MHz bandwidth).

The position of the transition $ ^{7}\text{F}_{0} \leftrightarrow ^{5}\text{D}_{0} $ is found for both samples around 579.78 nm in wavelength (vac), in coincidence with the spectroscopic data reported previously by P. Hill and S. Hüfner for a Eu(NO₃)₃·6H₂O crystal [23]. The fluorescence lifetimes of 180 µs and 1.87 ms measured for the Eu(NO₃)₃·6H₂O and Eu(NO₃)₃·6D₂O crystals respectively are also in fair agreement with the values in Ref. [30]. The absorption spectra of both samples were recorded by scanning the laser frequency around the central absorption peak for two laser polarizations, corresponding to the minimum and maximum absorption conditions. In both cases, the absorption was very strong. The spectrum was saturated for the maximum absorption condition, whereas the spectra obtained under minimum absorption for both deuterated and non-deuterated crystals are shown in Fig. 1. The absorption profile in both crystals showed negligible variation across different regions, indicating uniform optical properties throughout the samples. For the deuterated sample, the absorption peak exhibits a Full Width at Half Maximum (FWHM) of approximately 500 MHz. Europium has two naturally occurring isotopes with nuclear spin 5/2, which are expected to produce nine distinct optical hyperfine transitions per isotope. However, the measured inhomogeneous linewidths are too broad to resolve the individual optical hyperfine transitions, although the central frequencies of the transitions of the two isotopes can be faintly discerned.

In the non-deuterated sample, the absorption peak is shifted by 5.8 GHz and is broader, with a FWHM of approximately 700 MHz. Unexpect-
<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Hole-burning spectra obtained from the transmitted light at 50 mK for two burning frequencies (black and red lines). The blue (green) lines indicate the positions of the expected holes and antiholes, calculated using the assumed hyperfine splittings for ¹⁵³Eu (¹⁵¹Eu). The dark blue and green lines mark the holes used to estimate the hyperfine splittings of the optical levels. The two spectra have been normalized independently.</figcaption></figure>

edly, the isotope splitting is much clearly observed in weak redshifted satellite lines located at -2.9 GHz relative to the center of the main peak (see Fig. 1d). As noted for the europium chloride hydrated crystals [31], these satellite lines can be attributed to the absorption of europium isotopes with a single spurious deuterium atom replacing one hydrogen atom in a nearest-neighbor site. The spectrum of the deuterated crystals exhibits analogous satellite lines blueshifted by +3.0 GHz because of the ¹H impurities. A bi-Lorentzian fit to the satellite lines (Fig. 1d) yields a peak separation of 200 MHz and FWHMs of 185 MHz for the lower-energy peak and 255 MHz for the higher-energy peak. This separation aligns with the isotopic shift of +200 MHz reported for ¹⁵³Eu relative to ¹⁵¹Eu in europium chloride hydrate crystals [32]. Similarly, for europium nitrate hexahydrate, the transitions of ¹⁵³Eu are found at higher energy, as demonstrated in the following analysis of the spectral hole burning data. Furthermore, the obtained hyperfine splittings explain the broader linewidth of the ¹⁵³Eu peak and allow us to estimate a linewidth for the individual optical hyperfine transitions in the satellite lines on the order of 100 MHz. For the main peaks, however, an approximate estimation is 200 MHz for the deuterated crystal and below 500 MHz for the non-deuterated one.

The hyperfine splittings were estimated using spectral hole-burning (SHB) spectroscopy. The sample was excited with millisecond-long laser burning pulses at a fixed frequency, after which the laser frequency was scanned from the initial excitation frequency up to +200 MHz using an acousto-optic modulator in a double-pass configuration. The absorption spectrum was recorded for this frequency range both with and without the burning pulses, and the SHB spectrum, obtained as the ratio of the two spectra, is shown in Fig. 2 for two burning frequencies separated by 0.9 GHz using a non-deuterated sample. The positions of the spectral features in the SHB spectrum correspond to specific algebraic sums of the hyperfine splittings of the ground and excited levels. Two series of three holes were identified at 30.9, 37.1, and their sum 68.0 MHz, and at 78.4, 94.8, and their sum 173.2 MHz. These features, indicated with dark green and blue lines can be unambiguously assigned to the energy separations between the ± 1/2, ± 3/2 and ± 5/2 nuclear spin states of the optical level ⁵D₀ of the ¹⁵¹Eu and ¹⁵³Eu isotopes. The ⁵D₀ level is sufficiently isolated from other energy levels, and its hyperfine splitting is generally determined predominantly by the quadrupole moment interaction of the europium nucleus [33]. Consistent with this, the ratios of the splittings for the two isotopes, namely 31/78.4 = 0.395 and 37.2/94.8 = 0.392, closely correspond to the ratio of the quadrupole moments of ¹⁵¹Eu and ¹⁵³Eu, which is 0.374 [34]. It is noteworthy that in the SHB spectrum obtained when burning with the higher frequency (indicated by the black arrow in Fig. 1), the holes associated with ¹⁵³Eu exhibit significantly higher contrast compared to those of ¹⁵¹Eu, while they are strongly reduced when burning at a lower frequency (red arrow). This indicates that fewer ¹⁵¹Eu ions contribute to the spectral density in the higher-energy region of the absorption band, confirming

<table><caption>**Table 1**
Estimated hyperfine splittings for ¹⁵¹Eu and ¹⁵³Eu in the ⁵D₀ and ⁷F₀ states.</caption><thead><tr><td>Isotope State</td><td>Hyperfine splitting [MHz]</td></tr></thead><tbody><tr><td>¹⁵¹Eu ⁷F₀</td><td>11.0, 21.2</td></tr><tr><td>¹⁵³Eu ⁷F₀</td><td>28.4, 54.6</td></tr><tr><td>¹⁵¹Eu ⁵D₀</td><td>30.9, 37.1</td></tr><tr><td>¹⁵³Eu ⁵D₀</td><td>78.4, 94.8</td></tr></tbody></table>

that the optical inhomogeneous linewidths are not significantly larger than the isotope shift.

All other features in the spectra include contributions from the splitting of the ⁷F₀ ground state, making their assignment more challenging. The differing contrast of these features also helps in associating them with their respective Eu isotopes. However, a reliable assignment of the ground-state hyperfine splittings was made possible only thanks to the unpublished Raman-Heterodyne measurements performed on europium nitrate hexahydrate crystals.<fnref n="1" /> The values we report in Table 1 for the non-deuterated crystal with an estimated uncertainty of ≤ 1 MHz are in agreement with the Raman-Heterodyne measurements, preserve the expected isotopic ratio of ~0.37, and show excellent correspondence with the observed positions of holes and anti-holes in the spectral hole-burning spectrum. Shifts of a few hundred kilohertz have been observed between the deuterated and non-deuterated samples. We also mention that the holes at 50 mK exhibited lifetimes exceeding one hour.

Finally, the coherence time of the optical transition in the two samples was measured using heterodyne photon echo techniques. A sequence of three laser pulses was employed: the first two pulses, with durations of 2.5 and 4.5 µs respectively, shared the same optical frequency and were separated by a delay time $τ_d$, acting as $π/2$ and $π$ pulses. A third pulse, sent after $τ_d$ following the second pulse, had the same duration as the first but was frequency-shifted by 30 MHz. The rephased atomic coherence was detected as a beating signal between the generated photon echo and the third laser pulse. For this experiment, the avalanche photodetector was used, with its signal filtered using a radiofrequency bandpass filter centered at 30 MHz. The beating signal was analyzed using the fast Fourier transform (FFT) function of an oscilloscope (Lecroy Wavesurfer - bandwidth 200 MHz). The amplitude of the beats for each $τ_d$ was then determined by either averaging the spectral intensity at 30 MHz over 40 FFT spectra or taking the highest value at 30 MHz among 40 FFT spectra. This was done by using either the oscilloscope's average or roof functions over 40 traces after the FFT function. To avoid the spectral hole burning effects that may artificially reduce the echo amplitude, the laser frequency was slowly scanned over tens of MHz during the measurements.

The amplitude of the beating signals $A$ is expected to decay as $A(τ_d) = A(0)\exp(-2τ_d/T_2)$, with $T_2$ the coherence time of the atomic transition. The amplitude of the heterodyne echo signal was thus recorded for various $τ_d$ values and the $T_2$ extracted accordingly via exponential fitting (Fig. 3). Coherence times ranging between 80 and 140 µs were obtained for Eu(NO₃)₃)·6H₂O crystals, with longer values achieved when the laser frequency was tuned to the tail of the absorption peak or to the satellite lines, and shorter times at the peak's maximum. We attribute this effect to instantaneous spectral diffusion [35,36]. At the peak, the resonant ion density is larger and the relative distance of the ions excited by the laser pulses is shorter. Therefore, the excitation of the europium ions during the second pulse has a bigger impact on the frequency of the ions just excited via dipole-dipole interaction, affecting their participation in the rephasing. In case of the satellite lines, the presence of a nearby deuterium atom may have also contributed to increase the coherence time.

<footnote n="1">¹ These data were kindly provided by one of the referees and refer to samples grown independently of those used in this study.</footnote>
<figure><img src="image_4.png" /><figcaption>**Fig. 3.** Decay of the heterodyne beat amplitude recorded at 50 mK as a function of the delay time for the two samples. The FFT and roof function were used to collect these datasets. The laser frequency was 517074.0 GHz for the deuterated crystals and 517081.4 GHz for the non-deuterated one. The exponential fitting gives $T_2 = 652 \pm 30$ µs for the former crystal and $T_2 = 134 \pm 5$ µs for the latter.</figcaption></figure>

The heterodyne echo signal decay was recorded firstly with the average function and then with the roof function, obtaining similar $T_2$ values for the $Eu(NO_3)_3 \cdot 6H_2O$ crystals. In contrast, significant differences were found for the deuterated crystals. Coherence times between 430 and 570 µs were measured on the tail of the absorption using the standard averaging function, whereas values between 650 and 750 µs were obtained with the roof function. This discrepancy, also found in other crystals with long coherence times [37], is attributed to vibrations induced by the cryocooler of the dry dilution refrigerator. The roof function selectively records the best signals, i.e., those least affected by these vibrations.

## 4. Discussion and conclusions

In this work, large transparent stoichiometric europium nitrate hexahydrate crystals were successfully grown and their optical properties were characterized. The absorption linewidths related to the $^7F_0 \leftrightarrow ^5D_0$ optical transition were found to be approximately 700 MHz and 500 MHz for the non-deuterated and deuterated crystals, respectively. These values are well below the several-GHz values reported for similar crystals in Ref [23]. Taking into account the hyperfine splittings of several tens of MHz and the 200 MHz isotope shift, the individual hyperfine optical transitions are definitely narrower but not sufficiently so to resolve them.

The origin of the inhomogeneous broadening may be attributed to several factors, including growth-related defects, impurities introduced through the nitric acid solution or the undesired presence of ¹H and ²H in the respective crystals. Additionally, strain induced by gluing the crystal onto the copper holder may also contribute to the observed broadening [38], even though contact was made only on one side. This strain-induced broadening can be avoided by immersing the crystal in a liquid helium bath, which ensures thermal contact without mechanical stress. Further reductions in the inhomogeneous linewidth could be achieved by using higher-purity starting materials and exploring alternative growth conditions, such as slowing the crystal growth rate and adjusting the acidity of the solution. Excess concentrations of nitric acid in rare-earth nitrate solutions can suppress the formation of impurity phases, such as hydroxides [26] but at the same time, they may promote the formation of lower hydrates, such as europium nitrate tetrahydrate [28]. Therefore, fine-tuning the solution's acidity could also improve the crystal quality.

Coherence times as long as 136 µs for the $Eu(NO_3)_3 \cdot 6H_2O$ and 750 µs for the $Eu(NO_3)_3 \cdot 6D_2O$ crystals were measured, corresponding to homogeneous linewidth (FWHM) of 2.34 kHz and 0.42 kHz. Their difference could be partly attributed to the different excited state lifetime $T_{ex}$, strongly accelerated by multiphonon relaxation in the case of the non-deuterated sample. The relative contribution to the linewidth, given by $(1/2\pi T_{ex})$, account for 0.88 kHz and 0.09 kHz for the two crystals,

respectively. The long lifetimes of the spectral holes observed at 50 mK indicate that the lifetime of the ground state is very long, making its contribution to the linewidth negligible. For the deuterated crystal, the pure dephasing component is therefore estimated to be 0.33 kHz. Spin flips of ¹⁴N (nuclear spin I=1 and moment $\mu$ = 0.40), ¹⁵¹Eu (I=5/2, $\mu$ = 3.47), ¹⁵³Eu (I=5/2, $\mu$ = 1.53) and D (I=1, $\mu$ = 0.85) nuclei are likely the main responsible for the optical dephasing of the europium ions via magnetic interaction. Instantaneous spectral diffusion and crystal vibrations could also play a role. For the non-deuterated sample, the pure dephasing rate is found to be 1.46 kHz, significantly larger than its deuterated counterpart. An enhanced local magnetic noise and decoherence rate can be expected since ¹H possesses a larger magnetic moment (I=1/2, $\mu$ = 2.79) compared to ²H. Nonetheless, the potential impact of a higher defect density, as suggested by the larger optical inhomogeneous linewidth, cannot be ruled out. However, the fact that the longest coherence times measured in these crystals are comparable to or even exceed those reported for europium chloride hexahydrates (740 µs and 77 µs for the deuterated and non-deuterated crystals, respectively [31]), despite their much narrower inhomogeneous linewidth (25 MHz), strongly suggests that dephasing is primarily governed by interactions with magnetic nuclei, rather than by structural defects or chemical impurities [12]. This hypothesis could be further tested by measuring the coherence times of $Eu^{3+}$ ions diluted in a $Y(NO_3)_3 \cdot 6H_2O$ matrix, where the host $Y^{3+}$ ions possess a much weaker nuclear magnetic moment (I=1/2, $\mu$ = -0.14) than $Eu^{3+}$, allowing for an approximate estimation of the magnetic contribution from the europium ions themselves.

The results presented here are encouraging. Although the full potential of europium nitrate crystals, characterized by their isotopically pure environment, has not yet been realized in terms of resolving individual optical hyperfine transitions, significant progress has been made. Hyperfine transitions with linewidths of hundreds of MHz have been observed, and further reductions in linewidth might be still achievable by optimizing the growth processes and by avoiding strain effects associated with the use of silver paint. In fact, having fully resolved optical hyperfine transitions would significantly enhance the suitability of these crystals for both precision measurements, such as searches for the nuclear electric dipole moment, and quantum information applications. In particular, resolved optical lines are a strict requirement for quantum memory protocols like gradient echo memory [39]. In this regard, $EuCl_3 \cdot 6H_2O$ remains the most promising option to date, although there is still room for improvement in europium nitrate hexahydrate crystals. Long nuclear spin coherence times are another key requirement for both EDM searches and several quantum memory protocols. While these parameters were not directly measured in this study, our observations suggest that they could be sufficiently long for these applications.

The observed absorption spectra, along with those obtained with europium chlorates [12] and hydroxides [25], demonstrate that crystals grown through hydrothermal growth or slow-cooling at room temperature can achieve extremely narrow linewidths and encourage the exploration of other similar crystals. For instance, it would be worthwhile to explore other hydrates of europium nitrate, such as the penta- and tetrahydrates, which can be grown at slightly higher temperatures and may exhibit improved spectroscopic properties.

Finally, we note that the high optical absorption, narrow homogeneous linewidths, and long spectral hole lifetimes make the material studied here particularly well suited for implementing slow-light effects based on transient spectral hole burning. By exploiting the isoelectronic transition $^7F_0 \rightarrow ^5D_0$ of $Eu^{3+}$ in a crystal of BaFCl doped with $Sm^{2+}$, light was successfully slowed to a group velocity of 4000 m/s [40]. It would therefore be of interest to explore whether comparable slow-light effects can be observed in europium nitrate crystals under similar conditions.
CRediT authorship contribution statement

**Federico Chiossi:** Writing – original draft, Project administration, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. **Diana Serrano:** Writing – review & editing, Validation, Supervision, Software, Resources, Methodology, Investigation, Funding acquisition, Conceptualization. **Alexey Tiranov:** Writing – review & editing, Validation. **Philippe Goldner:** Writing – review & editing, Validation, Supervision, Software, Resources, Project administration.

Declaration of competing interest

One of the authors, Philippe Goldner, is an Associate Editor of the Journal of Luminescence. He had no involvement in the peer review of this article. There are no other conflicts of interest to declare.

Acknowledgements

This work was supported by the European Marie Curie postdoctoral fellowship HARCOTREC (Grants No. 101066781). We thank the French Agence Nationale de la Recherche (ANR) under grants ANR-23-CE47-0011-01 (MolEquBe) and ANR-20-CE09-0022-01 (UltraNanOSpec). The regional network on quantum technologies of Ile-de-France region (QuanTip, former Sirteq) is also gratefully acknowledged for funding (NoREQ project).

Data availability

Data will be made available on request.

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