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arXiv:2601.03455

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

Fabian Becker, Anna Selzer, Lorenz J. J. Sauerzopf, Catherine L. Curtin, Sudip KC, Tim Schneider, Kai Müller

cond-mat.mtrl-sciquant-phSubmitted Jan 6, 2026• Announced Jan 8, 2026• 0 citations
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Abstract

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.

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Comparative Optical Spectroscopy of Er3+^{3+}3+, Tm3+^{3+}3+, and Eu3+^{3+}3+ Molecular Complexes at Cryogenic Temperature
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We present a detailed spectroscopic study of the I 13 / 2 4 and I 15 / 2 4 Er 3 + multiplets of Er: CaWO 4 grown without a co-dopant. Using photoluminescence and photoluminescence excitation measurements, we find multiple environments into which erbium ions are incorporated in the crystal. For the four most prevalent environments, we provide a complete assignment of the I 13 / 2 4 and I 15 / 2 4 sublevel energies. Polarization-dependent absorption and emission spectroscopy allow us to identify the irreducible representations of the states and determine the dipole nature of transitions. In addition, we compare the different relaxation paths within I 13 / 2 4 and I 15 / 2 4 multiplets via line broadening and lifetime measurements. Finally, spectral, lifetime, and polarization measurements indicate similarities between the studied environments.
Rare-earth-ion doped crystals are a promising platform for applications in microwave-to-optical transduction. Erbium is of special interest because of its telecom wavelength optical transitions. This work investigates the optical and electron-spin properties of Er3+{}^{3+}3+:YVO4{}_{4}4​. An initial transduction of classical microwave signal using this material is demonstrated. The measured strong optical dipole moment and narrow ensemble inhomogeneities make Er3+{}^{3+}3+:YVO4{}_{4}4​ a promising material for microwave-to-optical transduction.
Polarisation and Temperature Dependence of Er3+^{3+}3+:CaWO4_44​ -- Towards a Solid-State Rare-Earth Ion-Doped Quantum Memory
In the endeavour of developing quantum memories, Er3+^{3+}3+:CaWO4_44​ has emerged as a promising rare-earth ion-doped (REID) crystal platform due to its long optical coherence times and compatibility with the 1550 nm telecommunications band. This work investigates the effects of polarisation and temperature on the absorption strength, central wavelength, and linewidth of the Z1→Y1Z_1\to Y_1Z1​→Y1​ and Z1→Y2Z_1\to Y_2Z1​→Y2​ optical transitions, with light incident along the crystal aaa and ccc axes. It is found that the Z1→Y1Z_1\to Y_1Z1​→Y1​ transition at 1532.6 nm with the incident laser along the ccc-axis at cryogenic temperatures ($\sim$3 K) is particularly favourable. The transition exhibits a stable central wavelength, narrower linewidth, polarisation independence, larger absorption cross-section, and lies within the C-band -- attributes that make it highly suitable for quantum memory applications.
Harnessing rare-earth ions in oxides for quantum networks requires integration with bright emitters in III-V semiconductors, but local disorder and interfacial noise limit their optical coherence. Here, we investigate the microscopic origins of the ensemble spectrum in Er3+^{3+}3+:TiO2_22​ epitaxial thin films on GaAs and GaSb substrates. Ab initio calculations combined with noise-Hamiltonian modeling and Monte Carlo simulations quantify the effects of interfacial and bulk spin noise and local strain on erbium crystal-field energies and inhomogeneous linewidths. Photoluminescence excitation spectroscopy reveals that Er3+^{3+}3+ ions positioned at increasing distances from the III-V/oxide interface produce a systematic blue shift of the Y1→Z1Y_1\rightarrow Z_1Y1​→Z1​ transition, consistent with strain relaxation predicted by theory. Thermal annealing produces a compensating redshift and linewidth narrowing, isolating the roles of oxygen-vacancy and gallium-diffusion noise. These results provide microscopic insight into disorder-driven decoherence, offering pathways for precise control of hybrid quantum systems for scalable quantum technologies.