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

TL;DR

The paper investigates how near-surface Er3+-ion ensembles in waveguide-integrated CaWO4 and YVO4 respond spectroscopically when coupled to TM and TE waveguide modes. Using transfer-printed silicon photonics, the authors measure power- and temperature-dependent optical spectra and examine how surface termination affects the spectra. The key finding is that CaWO4 exhibits strong TM-induced broadening and a high-energy shoulder on the S1 Y1Z1 transition, while TE coupling largely mirrors bulk behavior; YVO4 shows minimal polarization sensitivity. Temperature and surface-termination studies point to surface charges as a dominant decoherence source in the non-charge-neutral CaWO4 host, highlighting the importance of engineering surface environments for reliable on-chip quantum devices.

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.

Figures (9)

• Figure 1: Device transfer printing, design, and performance. a Schematic of transfer printing from patterned Si device layer (i), across wet chemical HF under-etching and critical point drying (ii), towards the actual PIC harvest (iii), release (iv) and PC removal (v) b SEM image of a looped waveguide hung inside a suspended support membrane with inverse-designed grating couplers optimized for normal free-space to fundamental TM mode coupling. c Fraction of the y-polarized light intensity inside CaWO$_4$/total intensity as a function of the waveguide design. Insets show the total TM mode intensity for the applied design (width $=650nm$, height $550nm$). d Example transmission efficiencies of full devices.
• Figure 2: Comparison of pulsed excitation spectra of Er$^{3+}$:YVO$_4$ (a, b, c) and Er$^{3+}$:CaWO$_4$ (d, e, f). a and d Large excitation wavelength scans with highlighted peaks. b and eFWHM of Lorentzian fit Y1 peaks recorded at different excitation powers. Single and double assign a single peak or double peak fit. PLE data taken under cw excitation and recorded with a spectrometer. Transmission data recorded in Becker.10242025. c and f High resolution pulsed excitation comparison of TM and TE modes at different powers with respect to the bulk peak center (ex. wav. for excitation wavelength).
• Figure 3: Crystal temperature variations probed by bulk and guided mode measurements. a YVO$_4$ bulk measurements at $100(2)µ W$. b CaWO$_4$ bulk measurements at $54.1(11)µ W$. c CaWO$_4$ measurements of the S1 Y1Z1 emission peak for different modes with respect to the bulk peak center.
• Figure 4: Comparison of TM mode coupled spectra with different surface terminations. a YVO$_4$ and b CaWO$_4$ large scans. c YVO$_4$ and CaWO$_4$ (S1) high resolution scans of the Y1Z1 peaks with respect to the bulk peak center.
• Figure 5: Polarization dependence of excitation and detection. a Polarization orientations. b Polarization Excitation sweep (raw data for subfigure a) c Supported main transmission polarization (TM mode) with indicated polarizations d Supported minor transmission polarization (TE mode) with indicated polarizations.