arXiv:2610.27223

Comparative Optical Spectroscopy of Er$^{3+}$, Tm$^{3+}$, and Eu$^{3+}$ Molecular Complexes at Cryogenic Temperature

Farhad Rasekh, Sahil, Arsalan Mansourzadeh, Diogo Alves Gálico, David Hunger, Muralee Murugesu, Daniel Oblak

quant-phSubmitted Oct 19, 2026• Announced Oct 21, 2026• 0 citations

Abstract

Rare-earth molecular complexes are attracting increasing interest for quantum information applications because of their structural flexibility and chemical tunability. Here, we compare the cryogenic optical properties of Er$^{3+}$, Tm$^{3+}$, and Eu$^{3+}$ complexes in the same ligand environment. Optical spectroscopy shows differing behavior across the three systems: spectral hole burning is observed in the Eu$^{3+}$ complex, whereas no measurable hole is detected in the Er$^{3+}$ and Tm$^{3+}$ complexes. We measure laser-induced local heating and rethermalization in the Er$^{3+}$ and Tm$^{3+}$ powder-like samples, observing strong local heating and slow thermal relaxation in the cold-plate cryostat. The recovery occurs over several seconds and, for some Er$^{3+}$ excitation conditions, extends over tens of seconds. The comparison across the three ions nevertheless suggests that heating alone is unlikely to account for the full contrast. Rather, the different behaviors of Eu$^{3+}$, Tm$^{3+}$, and Er$^{3+}$ point to ion-dependent relaxation and decoherence processes associated with their electronic level structure and coupling to the molecular environment. Temperature-dependent linewidth and line-shift measurements are further consistent with phonon-mediated broadening through coupling to crystal-field levels.

Comments: 14 pages and 5 figures

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Judd-Ofelt analysis and coherent spectroscopy of europium-based molecular complexes for quantum information applications

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quant-pharXiv:2606.11947Jun 11, 20260 citations

Towards Rare-Earth Molecular Crystals as a New Platform in Quantum Networks

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IEEE International Conference on Quantum Computing and Engineering (QCE)Sep 15, 20240 citations

Optical Spectroscopy of Waveguide coupled Er$^{3+}$ ensembles in CaWO$_4$ and YVO$_4$

Fabian Becker, Anna Selzer, Lorenz J. J. Sauerzopf +4 more

cond-mat.mtrl-sciCross-listedarXiv:2601.03455Jan 8, 20260 citations

Optical Spectroscopy of Waveguide Coupled Er 3+ Ensembles in CaWO 4 and YVO 4

Fabian Becker, Anna Selzer, Lorenz J. J. Sauerzopf +4 more

Advanced Quantum TechnologiesApr 1, 20260 citations

Spectroscopy and Coherence of an Excited-State Transition in Tm$^{3+}$:YAlO$_3$ at Telecommunication Wavelength

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quant-pharXiv:2511.05370Physical Review ResearchNov 10, 20250 citations

arXiv:2610.27223

Comparative Optical Spectroscopy of Er $^{3+}$ , Tm $^{3+}$ , and Eu $^{3+}$ Molecular Complexes at Cryogenic TemperatureComparative Optical Spectroscopy of Er$^{3+}$, Tm$^{3+}$, and Eu$^{3+}$ Molecular Complexes at Cryogenic Temperature

Farhad Rasekh, Sahil, Arsalan Mansourzadeh, Diogo Alves Gálico, David Hunger, Muralee Murugesu, Daniel Oblak

quant-phSubmitted Oct 19, 2026• Announced Oct 21, 2026• 0 citations

arXiv HTML PDF

Abstract

^{3+}

, Tm

^{3+}

, and Eu

^{3+}

complexes in the same ligand environment. Optical spectroscopy shows differing behavior across the three systems: spectral hole burning is observed in the Eu

^{3+}

complex, whereas no measurable hole is detected in the Er

^{3+}

and Tm

^{3+}

complexes. We measure laser-induced local heating and rethermalization in the Er

^{3+}

and Tm

^{3+}

powder-like samples, observing strong local heating and slow thermal relaxation in the cold-plate cryostat. The recovery occurs over several seconds and, for some Er

^{3+}

excitation conditions, extends over tens of seconds. The comparison across the three ions nevertheless suggests that heating alone is unlikely to account for the full contrast. Rather, the different behaviors of Eu

^{3+}

, Tm

^{3+}

, and Er

^{3+}

point to ion-dependent relaxation and decoherence processes associated with their electronic level structure and coupling to the molecular environment. Temperature-dependent linewidth and line-shift measurements are further consistent with phonon-mediated broadening through coupling to crystal-field levels.

Comments: 14 pages and 5 figures

Related papers

Judd-Ofelt analysis and coherent spectroscopy of europium-based molecular complexes for quantum information applications

Nicholas Lester Jobbitt, Sascha Weis, Timo Schweizer +9 more

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 Eu $^{3+}$ -based molecular complexes, with a focus on the coherent $^{5}\text{D}_{0} \longrightarrow \ ^{7}\text{F}_{0}$ transition at $\sim $580 nm. Judd-Ofelt analyses are performed across all complexes, providing insights into ligand-field environments, local point-group symmetries, and the resulting branching ratios of the coherent transition. We find that lower point-group symmetry (C$ _1 $, C$ _2$) yields higher branching ratios of the $^{5}\text{D}_{0} \longrightarrow \ ^{7}\text{F}_{0}$ 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 $T_{\text{2}}^{\text{opt}} = 2.3~μ\mathrm{s}$ 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.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 Eu$^{3+}$-based molecular complexes, with a focus on the coherent $^{5}\text{D}_{0} \longrightarrow \ ^{7}\text{F}_{0}$ transition at $\sim$580 nm. Judd-Ofelt analyses are performed across all complexes, providing insights into ligand-field environments, local point-group symmetries, and the resulting branching ratios of the coherent transition. We find that lower point-group symmetry (C$_1$, C$_2$) yields higher branching ratios of the $^{5}\text{D}_{0} \longrightarrow \ ^{7}\text{F}_{0}$ 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 $T_{\text{2}}^{\text{opt}} = 2.3~μ\mathrm{s}$ 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.

quant-pharXiv:2610.13664Oct 12, 20260 citations

Controlled ion-ion interactions and cavity-enhanced emission of a coherent dinuclear Eu $^{3+}$ complexControlled ion-ion interactions and cavity-enhanced emission of a coherent dinuclear Eu$^{3+}$ complex

Evgenij Vasilenko, Vishnu Unni Chorakkunnath, Barbora Brachnakova +4 more

Molecular rare-earth-ion complexes offer unique opportunities for quantum technologies by combining the intrinsic coherence properties of rare-earth ions with chemically tunable molecular environments. A crucial capability is the realization of multi-qubit architectures with defined qubit couplings to enable two-qubit quantum gates. Here, we investigate the optical coherence properties and excitation-induced interactions of two Eu $^{3+}$ -based molecular complexes, comparing a mononuclear reference system with a dinuclear analogue in which two Eu $^{3+}$ ions are positioned at a well-defined intramolecular distance of about 7 Angstrom. Using cryogenic ensemble spectroscopy, including spectral hole burning, free-induction decay, and photon echo measurements at temperatures down to 100 mK, we demonstrate long optical coherence times $T_{2,\text{o}}$ of up to 9 $μ$ s. As a key step toward scalable multi-qubit architectures, a control-target sequence was implemented to probe conditional ion-ion interactions, revealing a stronger interaction-induced dephasing in the dinuclear complex. Finally, we show the integration of the dinuclear complex into a fiber-based optical microcavity, and observe an 380-fold emission enhancement of the $\mathrm{}^5\mathrm{D}_0\rightarrow\mathrm{}^7\mathrm{F}_0$ transition. Together, these results position molecular rare-earth complexes as versatile and chemically tunable building blocks for scalable quantum technologies.Molecular rare-earth-ion complexes offer unique opportunities for quantum technologies by combining the intrinsic coherence properties of rare-earth ions with chemically tunable molecular environments. A crucial capability is the realization of multi-qubit architectures with defined qubit couplings to enable two-qubit quantum gates. Here, we investigate the optical coherence properties and excitation-induced interactions of two Eu$^{3+}$-based molecular complexes, comparing a mononuclear reference system with a dinuclear analogue in which two Eu$^{3+}$ ions are positioned at a well-defined intramolecular distance of about 7 Angstrom. Using cryogenic ensemble spectroscopy, including spectral hole burning, free-induction decay, and photon echo measurements at temperatures down to 100 mK, we demonstrate long optical coherence times $T_{2,\text{o}}$ of up to 9 $μ$s. As a key step toward scalable multi-qubit architectures, a control-target sequence was implemented to probe conditional ion-ion interactions, revealing a stronger interaction-induced dephasing in the dinuclear complex. Finally, we show the integration of the dinuclear complex into a fiber-based optical microcavity, and observe an 380-fold emission enhancement of the $\mathrm{}^5\mathrm{D}_0\rightarrow\mathrm{}^7\mathrm{F}_0$ transition. Together, these results position molecular rare-earth complexes as versatile and chemically tunable building blocks for scalable quantum technologies.

quant-pharXiv:2606.11947Jun 11, 20260 citations

Towards Rare-Earth Molecular Crystals as a New Platform in Quantum Networks

Farhad Rasekh, Diogo A. Gálico, Nasser Gohari Kamel +6 more

Rare-Earth (RE) atoms possess a unique atomic energy level structure, leading to excellent electronic and spin coherence properties [1]. Consequently, RE-ion-doped materials are promising candidates for implementing light-matter interfaces for photonic quantum technologies, such as quantum memories. To exploit these properties in quantum technologies, the REs must be hosted in a solid-state material [1]. RE organic molecular crystals offer advantages over the well-known RE-doped inorganic crystals. In organic molecular crystals, RE ions are integral parts of the molecular structure, unlike in inorganic crystals, in which the RE ions are distributed randomly into the crystal lattice structure. This can potentially lead to narrower inhomogeneous broadening. These crystals provide unmatched flexibility in combining with photonic structures and in material composition, allowing the choice of different ligands and point group symmetries around the RE ion [2], [10], [4]. Our research focuses on crystals formed from a molecular complex hosting erbium ions $\lceil{E}r(C_{5}H_{7}O_{2})_{3}(C_{12}H_{8}N_{2})\rceil$ . As these crystals have not yet been studied for quantum information purposes, we conducted a series of optical spectroscopy measurements to characterize the platform. We performed absorption measurements and obtained the absorption profiles at room and cryogenic temperatures. The results show a narrow inhomogeneous broadening in samples with different relative concentrations, even at 100%, for which all molecules contain an erbium ion. We measured the lifetime of the second lowest crystal field level of the ground state manifold $(\boldsymbol{30}\ cm^{-1}$ above ground state) to be 1.759 seconds. Additionally, we measured the inhomogeneous broadening of the lowest crystal field level versus temperature to investigate the effects of phonon processes. Finally, under an external magnetic field, we observed Zeeman splitting and determined the g-factor to be 3.23 for erbium ions in this molecular complex.Rare-Earth (RE) atoms possess a unique atomic energy level structure, leading to excellent electronic and spin coherence properties [1]. Consequently, RE-ion-doped materials are promising candidates for implementing light-matter interfaces for photonic quantum technologies, such as quantum memories. To exploit these properties in quantum technologies, the REs must be hosted in a solid-state material [1]. RE organic molecular crystals offer advantages over the well-known RE-doped inorganic crystals. In organic molecular crystals, RE ions are integral parts of the molecular structure, unlike in inorganic crystals, in which the RE ions are distributed randomly into the crystal lattice structure. This can potentially lead to narrower inhomogeneous broadening. These crystals provide unmatched flexibility in combining with photonic structures and in material composition, allowing the choice of different ligands and point group symmetries around the RE ion [2], [10], [4]. Our research focuses on crystals formed from a molecular complex hosting erbium ions$\lceil{E}r(C_{5}H_{7}O_{2})_{3}(C_{12}H_{8}N_{2})\rceil$. As these crystals have not yet been studied for quantum information purposes, we conducted a series of optical spectroscopy measurements to characterize the platform. We performed absorption measurements and obtained the absorption profiles at room and cryogenic temperatures. The results show a narrow inhomogeneous broadening in samples with different relative concentrations, even at 100%, for which all molecules contain an erbium ion. We measured the lifetime of the second lowest crystal field level of the ground state manifold$(\boldsymbol{30}\ cm^{-1}$above ground state) to be 1.759 seconds. Additionally, we measured the inhomogeneous broadening of the lowest crystal field level versus temperature to investigate the effects of phonon processes. Finally, under an external magnetic field, we observed Zeeman splitting and determined the g-factor to be 3.23 for erbium ions in this molecular complex.

IEEE International Conference on Quantum Computing and Engineering (QCE)Sep 15, 20240 citations

Optical Spectroscopy of Waveguide coupled Er $^{3+}$ ensembles in CaWO $_4$ and YVO $_4$ Optical Spectroscopy of Waveguide coupled Er$^{3+}$ ensembles in CaWO$_4$ and YVO$_4$

Fabian Becker, Anna Selzer, Lorenz J. J. Sauerzopf +4 more

We present an optical study of near-surface Er $^{3+}$ ensembles in waveguide-integrated CaWO $_4$ and YVO $_4$ , investigating how nanophotonic coupling modifies rare-earth spectroscopy. In particular, we compare bulk excitation with evanescently coupled TE and TM waveguide modes. In Er $^{3+}$ :CaWO $_4$ , we observe a pronounced polarization-dependent surface effect. TE-coupled spectra closely reproduce bulk behavior. In contrast, TM coupling induces strong inhomogeneous broadening and an asymmetric low-energy shoulder of the site S1 Y1Z1 transition, with linewidths exceeding those of the bulk by more than a factor of four. Temperature-dependent measurements and surface termination studies indicate that surface charges are the dominant mechanism. Er $^{3+}$ :YVO $_4$ remains largely unaffected by mode polarization, and surface termination leads only to minor spectral shifts. These observations suggest that non-charge-neutral rare-earth systems are more susceptible to surface-induced decoherence sources than charge-neutral hosts.We present an optical study of near-surface Er$^{3+}$ ensembles in waveguide-integrated CaWO$_4$ and YVO$_4$, investigating how nanophotonic coupling modifies rare-earth spectroscopy. In particular, we compare bulk excitation with evanescently coupled TE and TM waveguide modes. In Er$^{3+}$:CaWO$_4$, we observe a pronounced polarization-dependent surface effect. TE-coupled spectra closely reproduce bulk behavior. In contrast, TM coupling induces strong inhomogeneous broadening and an asymmetric low-energy shoulder of the site S1 Y1Z1 transition, with linewidths exceeding those of the bulk by more than a factor of four. Temperature-dependent measurements and surface termination studies indicate that surface charges are the dominant mechanism. Er$^{3+}$:YVO$_4$ remains largely unaffected by mode polarization, and surface termination leads only to minor spectral shifts. These observations suggest that non-charge-neutral rare-earth systems are more susceptible to surface-induced decoherence sources than charge-neutral hosts.

cond-mat.mtrl-sciCross-listedarXiv:2601.03455Jan 8, 20260 citations

Optical Spectroscopy of Waveguide Coupled Er 3+ Ensembles in CaWO 4 and YVO 4

Fabian Becker, Anna Selzer, Lorenz J. J. Sauerzopf +4 more

ABSTRACT We present an optical study of near‐surface ensembles in waveguide‐integrated and , investigating how nanophotonic coupling modifies rare‐earth spectroscopy. In particular, we compare bulk excitation with evanescently coupled TE and TM waveguide modes. In , we observe a pronounced polarization‐dependent surface effect. TE‐coupled spectra closely reproduce bulk behavior. In contrast, TM coupling induces strong inhomogeneous broadening and an asymmetric low‐energy shoulder of the site S1 Y1Z1 transition, with linewidths exceeding those of the bulk by more than a factor of four. Temperature‐dependent measurements and surface termination studies indicate that surface charges are the dominant mechanism. remains largely unaffected by mode polarization, and surface termination leads only to minor spectral shifts. These observations suggest that non‐charge‐neutral rare‐earth systems are more susceptible to surface‐induced decoherence sources than charge‐neutral hosts.ABSTRACT We present an optical study of near‐surface ensembles in waveguide‐integrated and , investigating how nanophotonic coupling modifies rare‐earth spectroscopy. In particular, we compare bulk excitation with evanescently coupled TE and TM waveguide modes. In , we observe a pronounced polarization‐dependent surface effect. TE‐coupled spectra closely reproduce bulk behavior. In contrast, TM coupling induces strong inhomogeneous broadening and an asymmetric low‐energy shoulder of the site S1 Y1Z1 transition, with linewidths exceeding those of the bulk by more than a factor of four. Temperature‐dependent measurements and surface termination studies indicate that surface charges are the dominant mechanism. remains largely unaffected by mode polarization, and surface termination leads only to minor spectral shifts. These observations suggest that non‐charge‐neutral rare‐earth systems are more susceptible to surface‐induced decoherence sources than charge‐neutral hosts.

Advanced Quantum TechnologiesApr 1, 20260 citations

Spectroscopy and Coherence of an Excited-State Transition in Tm $^{3+}$ :YAlO $_3$ at Telecommunication WavelengthSpectroscopy and Coherence of an Excited-State Transition in Tm$^{3+}$:YAlO$_3$ at Telecommunication Wavelength

Luozhen Li, Akshay Babu Karyath, Julien Bertrand +12 more

We characterize spectroscopic and coherence properties of the 1451.37 nm excited-state zero-phonon line (ZPL) between the $^{3}F_{4}$ and the $^{3}H_{4}$ manifolds of a thulium-doped yttrium aluminum perovskite (Tm $^{3+}$ :YAlO $_3$ ) crystal at temperatures around 1.5 K. We measure the absorption spectrum between the $^{3}H_{6}$ - $^{3}F_{4}$ and $^{3}F_{4}$ - $^{3}H_{4}$ manifolds, the inhomogeneous broadening of the $^{3}F_{4}$ - $^{3}H_{4}$ (excited-state) ZPL, and the lifetimes of the higher-lying and lower-lying excited states. We also investigate level shifts caused by the quadratic Zeeman interaction as well as spectral hole-burning spectra with varying magnetic fields, providing insights into hyperfine interactions. Using again spectral holes but also optical free induction decays (FIDs), we assess optical coherence times, finding a maximum of $4.75 \pm 0.07~μs$ at B=2T and low ion concentration in the $^{3}F_{4}$ level. Our results -- the first to demonstrate coherence of an excited-state transition in a rare-earth crystal -- suggest the possibility of exploiting such transitions for quantum technology.We characterize spectroscopic and coherence properties of the 1451.37 nm excited-state zero-phonon line (ZPL) between the $^{3}F_{4}$ and the $^{3}H_{4}$ manifolds of a thulium-doped yttrium aluminum perovskite (Tm$^{3+}$:YAlO$_3$) crystal at temperatures around 1.5 K. We measure the absorption spectrum between the $^{3}H_{6}$ - $^{3}F_{4}$ and $^{3}F_{4}$ - $^{3}H_{4}$ manifolds, the inhomogeneous broadening of the $^{3}F_{4}$ - $^{3}H_{4}$ (excited-state) ZPL, and the lifetimes of the higher-lying and lower-lying excited states. We also investigate level shifts caused by the quadratic Zeeman interaction as well as spectral hole-burning spectra with varying magnetic fields, providing insights into hyperfine interactions. Using again spectral holes but also optical free induction decays (FIDs), we assess optical coherence times, finding a maximum of $4.75 \pm 0.07~μs$ at B=2T and low ion concentration in the $^{3}F_{4}$ level. Our results -- the first to demonstrate coherence of an excited-state transition in a rare-earth crystal -- suggest the possibility of exploiting such transitions for quantum technology.

quant-pharXiv:2511.05370Physical Review ResearchNov 10, 20250 citations

Comparative Optical Spectroscopy of Er

^{3+}

, Tm

^{3+}

, and Eu

^{3+}

Molecular Complexes at Cryogenic Temperature

^{3+}

, Tm

^{3+}

, and Eu

^{3+}

complexes in the same ligand environment. Optical spectroscopy shows differing behavior across the three systems: spectral hole burning is observed in the Eu

^{3+}

complex, whereas no measurable hole is detected in the Er

^{3+}

and Tm

^{3+}

complexes. We measure laser-induced local heating and rethermalization in the Er

^{3+}

and Tm

^{3+}

powder-like samples, observing strong local heating and slow thermal relaxation in the cold-plate cryostat. The recovery occurs over several seconds and, for some Er

^{3+}

^{3+}

, Tm

^{3+}

, and Er

^{3+}

^{3+}

-based molecular complexes, with a focus on the coherent

^{5}\text{D}_{0} \longrightarrow \ ^{7}\text{F}_{0}

transition at $\sim

580 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 (C

, C

_2$) yields higher branching ratios of the

^{5}\text{D}_{0} \longrightarrow \ ^{7}\text{F}_{0}

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

T_{\text{2}}^{\text{opt}} = 2.3~μ\mathrm{s}

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.

Controlled ion-ion interactions and cavity-enhanced emission of a coherent dinuclear Eu

^{3+}

complex

^{3+}

-based molecular complexes, comparing a mononuclear reference system with a dinuclear analogue in which two Eu

^{3+}

ions are positioned at a well-defined intramolecular distance of about 7 Angstrom. Using cryogenic ensemble spectroscopy, including spectral hole burning, free-induction decay, and photon echo measurements at temperatures down to 100 mK, we demonstrate long optical coherence times

T_{2,\text{o}}

of up to 9

μ

s. As a key step toward scalable multi-qubit architectures, a control-target sequence was implemented to probe conditional ion-ion interactions, revealing a stronger interaction-induced dephasing in the dinuclear complex. Finally, we show the integration of the dinuclear complex into a fiber-based optical microcavity, and observe an 380-fold emission enhancement of the

\mathrm{}^5\mathrm{D}_0\rightarrow\mathrm{}^7\mathrm{F}_0

transition. Together, these results position molecular rare-earth complexes as versatile and chemically tunable building blocks for scalable quantum technologies.

\lceil{E}r(C_{5}H_{7}O_{2})_{3}(C_{12}H_{8}N_{2})\rceil

. As these crystals have not yet been studied for quantum information purposes, we conducted a series of optical spectroscopy measurements to characterize the platform. We performed absorption measurements and obtained the absorption profiles at room and cryogenic temperatures. The results show a narrow inhomogeneous broadening in samples with different relative concentrations, even at 100%, for which all molecules contain an erbium ion. We measured the lifetime of the second lowest crystal field level of the ground state manifold

(\boldsymbol{30}\ cm^{-1}

above ground state) to be 1.759 seconds. Additionally, we measured the inhomogeneous broadening of the lowest crystal field level versus temperature to investigate the effects of phonon processes. Finally, under an external magnetic field, we observed Zeeman splitting and determined the g-factor to be 3.23 for erbium ions in this molecular complex.

Optical Spectroscopy of Waveguide coupled Er

^{3+}

ensembles in CaWO

_4

and YVO

_4

We present an optical study of near-surface Er

^{3+}

ensembles in waveguide-integrated CaWO

_4

and YVO

_4

, investigating how nanophotonic coupling modifies rare-earth spectroscopy. In particular, we compare bulk excitation with evanescently coupled TE and TM waveguide modes. In Er

^{3+}

:CaWO

_4

, we observe a pronounced polarization-dependent surface effect. TE-coupled spectra closely reproduce bulk behavior. In contrast, TM coupling induces strong inhomogeneous broadening and an asymmetric low-energy shoulder of the site S1 Y1Z1 transition, with linewidths exceeding those of the bulk by more than a factor of four. Temperature-dependent measurements and surface termination studies indicate that surface charges are the dominant mechanism. Er

^{3+}

:YVO

_4

remains largely unaffected by mode polarization, and surface termination leads only to minor spectral shifts. These observations suggest that non-charge-neutral rare-earth systems are more susceptible to surface-induced decoherence sources than charge-neutral hosts.

Spectroscopy and Coherence of an Excited-State Transition in Tm

^{3+}

:YAlO

_3

at Telecommunication Wavelength

We characterize spectroscopic and coherence properties of the 1451.37 nm excited-state zero-phonon line (ZPL) between the

^{3}F_{4}

and the

^{3}H_{4}

manifolds of a thulium-doped yttrium aluminum perovskite (Tm

^{3+}

:YAlO

_3

) crystal at temperatures around 1.5 K. We measure the absorption spectrum between the

^{3}H_{6}

^{3}F_{4}

and

^{3}F_{4}

^{3}H_{4}

manifolds, the inhomogeneous broadening of the

^{3}F_{4}

^{3}H_{4}

(excited-state) ZPL, and the lifetimes of the higher-lying and lower-lying excited states. We also investigate level shifts caused by the quadratic Zeeman interaction as well as spectral hole-burning spectra with varying magnetic fields, providing insights into hyperfine interactions. Using again spectral holes but also optical free induction decays (FIDs), we assess optical coherence times, finding a maximum of

4.75 \pm 0.07~μs

at B=2T and low ion concentration in the

^{3}F_{4}

level. Our results -- the first to demonstrate coherence of an excited-state transition in a rare-earth crystal -- suggest the possibility of exploiting such transitions for quantum technology.