Heterogeneous Optically-Detected Spin-Acoustic Resonance in Solid-State Molecular Thin-film

We report an implementation of spin-acoustic resonance in pentacene thin films integrated on a high-quality-factor (high-Q) surface acoustic wave (SAW) resonator on a lithium niobate substrate. Heterogeneous optically detected spin-acoustic resonance (HODSAR) is an optically detected spin-resonance measurement in which the resonant drive is delivered mechanically by a surface acoustic wave (SAW). By leveraging the photo-excited triplet state of pentacene at room temperature, we demonstrate coherent spin manipulation via acoustic driving under zero externally applied magnetic field. The heterogeneously integrated device, referred to as HODSAR, utilizes spin-phonon coupling to achieve mechanically driven, zero-field spin resonance, opening avenues for room-temperature mechanically addressable spin control and device integration. We show that the high-Q multimode response of the SAW resonator enables spectrally selective acoustic addressing of triplet transitions near 105 MHz. Coherent control is evidenced by Rabi oscillations, with a Rabi frequency that increases linearly with the square root of the applied RF input power over the measured drive range, consistent with driven two-level dynamics under acoustic excitation. These results establish spin-acoustic resonance in a heterogeneously integrated molecular thin-film platform and provide a quantitative basis for benchmarking mechanically mediated spin control.

💡 Research Summary

The authors present a novel platform, termed Heterogeneous Optically‑Detected Spin‑Acoustic Resonance (HODSAR), that achieves coherent, room‑temperature manipulation of organic spin systems without any external magnetic field. The core of the device is a high‑quality‑factor (Q≈10⁴) multimode surface acoustic wave (SAW) resonator fabricated on a lithium‑niobate (LiNbO₃) substrate. Interdigital transducers (IDTs) generate Rayleigh SAWs in the 100–110 MHz range, with several narrow resonances (linewidths of a few tens of kHz) that can be precisely aligned with the zero‑field splitting (ZFS) of the photo‑excited triplet manifold of pentacene (Pc:PTP) thin films deposited directly on the SAW surface.

Optical pumping at 532 nm excites pentacene from its singlet ground state (S₀) to the first excited singlet (S₁). Intersystem crossing then populates the triplet manifold (T₂ → T₁), producing a non‑thermal, spin‑polarized distribution among the three sub‑levels (Tₓ, Tᵧ, T_z). Because the population is inverted between certain sub‑levels, the system is amenable to optically detected magnetic resonance (ODMR)‑type readout: any redistribution of sub‑level populations changes the spin‑dependent non‑radiative decay pathways, which is observed as a change in photoluminescence (ΔPL) collected by a single‑photon detector.

The mechanical drive is provided by the SAW‑induced strain field εᵢⱼ(t). The authors model the interaction with an effective spin‑strain Hamiltonian ˆH_strain(t)=∑₍ᵢⱼ₎ hᵢⱼ εᵢⱼ(t) ˆSᵢ ˆSⱼ, where hᵢⱼ are phenomenological coupling constants constrained by the D₂h symmetry of the pentacene crystal. Finite‑element COMSOL simulations confirm that the magnetic field component associated with the SAW is negligible (coupling efficiency η≈5×10⁻¹²), ruling out conventional microwave B₁ driving. Consequently, the observed resonant response is attributed to strain‑mediated modulation of the ZFS parameters (D, E) and direct mixing of the triplet sub‑levels.

Two measurement modalities are demonstrated. In continuous‑wave (CW) HODSAR, a steady RF tone excites the SAW while the laser continuously pumps the triplet population. Sweeping the RF frequency reveals distinct ΔPL peaks that coincide with the high‑Q SAW modes and match the zero‑field Tₓ–Tᵧ transition (~105 MHz). The spectral shape mirrors that obtained from conventional CW‑EPR, confirming that the acoustic drive accesses the same spin transition but with a mechanically mediated drive field.

In pulsed experiments, short RF bursts generate acoustic pulses of controlled duration. By varying the pulse length, the authors observe clear Rabi oscillations in the ΔPL signal, demonstrating coherent control of the spin‑triplet system. The extracted Rabi frequency Ω scales linearly with the square root of the applied RF power, consistent with the expected Ω∝√P dependence for a driven two‑level system. The decay of the Rabi envelope (~5 µs) reflects the intrinsic spin‑lattice relaxation rates of the pentacene triplet and the finite strain amplitude delivered by the SAW.

The work establishes several key advances. First, it shows that organic spin systems can be integrated with MEMS‑scale acoustic resonators, eliminating the need for bulky microwave cavities and enabling chip‑scale quantum devices. Second, the strain‑driven mechanism operates at room temperature and zero external magnetic field, opening pathways for practical quantum sensors that can be addressed mechanically. Third, the multimode high‑Q SAW platform provides spectral selectivity, allowing individual spin transitions to be addressed without cross‑talk.

Limitations are also discussed. The spin‑strain coupling constants hᵢⱼ are not directly measured, and the contribution of the piezoelectric electric field (possible Stark shifts) remains a secondary, unquantified effect. Coherence times are limited to a few microseconds, which, while sufficient for proof‑of‑concept Rabi experiments, would need improvement for quantum memory or long‑distance entanglement protocols. Finally, the optical excitation volume defines the active spin ensemble, leading to spatial inhomogeneity in both strain amplitude and optical pumping, which may broaden the observed resonances.

In conclusion, HODSAR demonstrates a viable route to mechanically mediated, optically readable spin control in organic thin films. By leveraging high‑Q SAW resonators, the authors provide a compact, scalable platform that bridges molecular quantum materials with established MEMS technologies, promising new hybrid quantum devices for sensing, information processing, and interfacing with other quantum systems such as superconducting qubits or color‑center defects.