Controlling single rare earth ion emission in an electro-optical nanocavity

The study addresses the challenge of detecting and controlling single rare-earth ions (REIs), whose intra-4f optical transitions have inherently low emission rates. REIs, and erbium in particular, are attractive for quantum networking due to narrow, coherent optical and microwave transitions and telecom-band emission. Purcell enhancement in optical cavities has recently enabled single-ion readout in bulk host crystals, but scalable, dynamically tunable, integrated platforms remain limited. Real-time modulation of ion–cavity coupling could enable tailored single-photon sources and high-efficiency spin–photon interfaces by switching transitions on and off resonance and controlling branching between Zeeman transitions. Lithium niobate (LN) offers strong electro-optic properties and mature integrated photonics, and smart-cut Er:LN films preserve bulk coherence while providing full modal overlap. The work’s purpose is to realize and control single erbium ion emission using an electro-optically tunable photonic crystal cavity in thin-film Er:LN, demonstrating Purcell-enhanced detection, temporal shaping, and storage/retrieval of single-ion excitation.

Prior works have achieved single-REI optical probing via cavity-enhanced emission in systems coupling microcavities to bulk crystals, including single Er in YSO with silicon nanocavities and Fabry–Perot resonators, and single Nd in YVO nanophotonic cavities. Dynamic ion–cavity coupling has been explored using piezo-tunable Fabry–Perot microcavities with Er-doped nanoparticles and Yb-doped LN microdisks. LN integrated photonics provides high-Q, small mode volume resonators with electro-optic tunability (rings, microdisks, photonic crystals), and electro-optic modulators have been demonstrated with integrated electrodes. Incorporation of REIs into LNOI has been realized via ion implantation, flip-chip bonding, and smart-cut, with the latter maintaining coherence and full mode overlap. Nonetheless, most efforts have focused on ensembles; single Er emitters in LN hosts had not been addressed prior to this work. Real-time tunability is advantageous for single-photon source design, efficient spin-photon interfaces, transition-selective enhancement to control spin pumping/initialization, and scalable multi-qubit integration with frequency multiplexing.

Devices are fabricated from 300 nm z-cut smart-cut Er:LiNbO3 thin films (100 ppm Er doping). A half-etched ridge waveguide incorporates periodic holes forming photonic crystal mirrors; the underlying SiO2 is removed via buffered oxide etch through the holes to create a suspended structure for improved optical confinement and high-Q performance. A tapered lattice constant defines a defect cavity mode, and one mirror is tapered for reflectivity access. Gold electrodes are integrated along the waveguide for electro-optic (EO) frequency tuning. The cavity supports a fundamental TE mode with dominant electric field along the crystal y-direction; an in-plane bias uses the LN electro-optic coefficient r22 ≈ 7 pm/V. The half-etched slab increases overlap between optical and electric fields and provides mechanical robustness. Cavity resonance is swept during fabrication by varying the defect lattice constant; ~1 nm lattice change yields ~3 nm resonance shift, and devices span ~7 nm to compensate fabrication variations. Packaged chips are wire-bonded (electrical) and fiber-coupled (optical) and mounted on the 1 K plate of a dilution refrigerator. Device characterization at cryogenic temperature includes reflectivity measurements showing Q ≈ 1.58×10^5 and ~10 dB extinction, with total device optical throughput ~1%. EO tuning is calibrated up to 200 V, yielding a tuning rate of ~1.6 pm/V; with breakdown typically >500 V for this geometry, a bipolar tuning range of ~1.5 nm is feasible. Fluorescence is detected with a superconducting nanowire single-photon detector (SNSPD) in a pulsed measurement setup, with synchronized optical pump, SNSPD gating, and EO voltage pulses. Resonant fluorescence decay measurements extract population lifetimes: τ_wg ≈ 2.5 ms in the waveguide and τ_cav ≈ 14 μs in the cavity. The Purcell factor P ≈ τ_wg/τ_cav ≈ 177, in agreement with simulations (P ~150) given mode volume V_mode ~0.55 μm^2 and high Q. The inhomogeneous linewidth of the Er3+ Y1–Z1 transition (~1532 nm) is measured via waveguide fluorescence versus wavelength, yielding a Gaussian FWHM of ~160 GHz. The cavity resonance is EO-tuned across the inhomogeneous distribution: when tuned near the center, continuous spectra indicate ensembles; when tuned to the tail (e.g., 1534.064 nm), discrete peaks indicate single-ion lines. A second-order autocorrelation g^(2)(0) is measured at a selected peak, using pulsed excitation and single-detector self-correlation, yielding g^(2)(0)=0.38±0.08. EO control sequences manipulate emission dynamics: (1) emission shaping by modulating cavity detuning during fluorescence collection to suppress or enhance emission, showing that on-resonance decay matches the unmodulated case; (2) storage and retrieval by detuning the cavity immediately after excitation to store the excitation (reducing the Purcell rate) for a programmable delay, then tuning back on resonance to retrieve emission. Storage lifetimes increase with detuning voltage, e.g., to ~172 μs at 10 V and ~2.82 ms at 80 V, approaching the waveguide lifetime. Emission spectra measured immediately after excitation and after storage show matching single-ion peaks, indicating minimal perturbation from the EO process.

• High-Q EO-tunable Er:LN photonic crystal nanocavities enable strong Purcell enhancement (Q ≈ 1.58×10^5, V_mode ~0.55 μm^2, τ_wg ≈ 2.5 ms to τ_cav ≈ 14 μs), yielding P ≈ 177. - EO tuning rate ≈ 1.6 pm/V; practical bipolar tuning range ~1.5 nm (limited by LN breakdown >~500 V). - Device optical throughput ~1%; cavity reflectivity extinction ~10 dB. - Inhomogeneous broadening of Er3+ Y1–Z1 transition in LN: FWHM ≈ 160 GHz centered near 1532 nm. - Single-ion emission is spectrally isolated by tuning the cavity to the inhomogeneous tail (e.g., 1534.064 nm), exhibiting discrete lines. - Single-photon nature confirmed via autocorrelation: g^(2)(0)=0.38±0.08, indicating that most collected photons originate from a single ion. - Dynamic emission control demonstrated: pulsed EO detuning shapes emission without altering on-resonance decay dynamics. - Storage and retrieval of single-ion excitation achieved by detuning and realigning the cavity: extended effective lifetimes of ~172 μs (10 V detuning) and ~2.82 ms (80 V detuning), approaching the waveguide lifetime, with emission spectra preserved during storage. - Fabrication sweep of defect lattice constants covers ~7 nm resonance variation; simulated sensitivity ~3 nm per 1 nm lattice change.

The results directly address the challenge of detecting and dynamically controlling single rare-earth ion emission by combining strong Purcell enhancement with fast, deterministic electro-optic tuning in an integrated photonic platform. The high-Q, small-mode-volume Er:LN cavity boosts emission sufficiently to resolve single ions. EO tuning to the tail of the inhomogeneous distribution isolates single-ion transitions and enables temporal control of the emission rate. Autocorrelation verifies predominantly single-ion photon emission, while pulse sequences show emission shaping and storage/retrieval without spectral perturbation, indicating that EO detuning controls the radiative rate rather than altering the emitter’s intrinsic properties. The storage lifetimes increasing toward the waveguide value confirm that cavity detuning effectively suppresses Purcell enhancement, allowing retention of excitation for programmable durations. The demonstrated tunability (1.6 pm/V) suggests further enhancements using geometries or film orientations accessing r33 to increase tuning efficiency while managing Stark effects. The platform’s compatibility with magnetic-field-enhanced coherence, lower Er doping to suppress Er–Er interactions, and integration with microwave resonators points to efficient spin control and readout. EO tunability also supports frequency multiplexing and selective enhancement of different Zeeman transitions, enabling high-fidelity spin initialization and scalable multi-qubit architectures.

This work demonstrates Purcell-enabled detection and real-time electro-optic control of single erbium ion emission in a thin-film lithium niobate photonic crystal nanocavity. Key achievements include P ≈ 177 enhancement, spectral isolation of single ions via EO tuning, verification of single-photon emission with g^(2)(0)=0.38±0.08, emission waveform shaping, and storage/retrieval of single-ion excitations with lifetimes extended to millisecond scales when detuned. The approach establishes a controllable, scalable platform for telecom-wavelength single-photon sources and spin–photon interfaces. Future directions include increasing EO tuning efficiency (e.g., leveraging r33), improving coherence with magnetic fields and reduced doping, integrating on-chip microwave resonators for spin manipulation, multiplexing multiple REIs per cavity, and exploring strain-mediated coupling to mechanical modes.

• Er:LN exhibits worse decoherence and spectral diffusion than optimal hosts like YSO; improvements require applying external magnetic fields and potentially lower Er doping. - The measured g^(2)(0)=0.38±0.08 indicates residual multi-emitter/background contributions rather than ideal single-photon purity. - Device optical throughput is low (~1%), and the LN breakdown field limits the maximum EO tuning range (~1.5 nm in the present geometry). - Potential system drifts (e.g., laser instability, spectral diffusion) cause small relative spectral shifts observed between immediate and delayed measurements. - Using r22 orientation minimizes DC Stark shifts, but non-zero z-field components at the waveguide edge could induce small spectral shifts for weakly Purcell-enhanced ions (estimated well below EO tuning).