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

TL;DR

The study demonstrates heterogeneous optically detected spin‑acoustic resonance (HODSAR) by integrating a pentacene:molecular thin film with a high‑Q surface‑acoustic‑wave resonator on LiNbO$_3$. Spin dynamics are driven mechanically via SAW‑induced strain at zero external magnetic field, enabling optical initialization and readout of room‑temperature triplet states with coherent control evidenced by Rabi oscillations. The work establishes spin–phonon coupling as a viable, MEMS‑compatible pathway for mechanically addressable spin control in organic molecular systems, providing a quantitative framework for benchmarking acoustically mediated spin manipulation. It also outlines substrates and device considerations that influence film texture, spin properties, and ensemble versus single‑spin sensitivity, setting the stage for future developments in molecular spin‑photon–phonon hybrids and quantum sensing applications.

Abstract

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.

Figures (9)

• Figure 1: (a) Jablonski diagram illustrating the electronic state cycle in pentacene, responsible for spin-polarized triplet-state sublevels in pentacene:p-terphenyl. Optical excitation by a 532 nm laser promotes pentacene molecules from the ground state $S_0$ to the first singlet state $S_1$. Spin-orbit coupling mediates spin-selective intersystem crossing from the excited singlet state into the $T_2$ triplet manifold. The $T_2$ triplet states then decay non-radiatively to the $T_1$ triplet state, preserving the spin-polarized populations. Spin-lattice relaxation can occur between the triplet sublevels with rates $\gamma_{XZ}$, $\gamma_{XY}$, and $\gamma_{YZ}$, representing the $T_{X \rightarrow Z}$, $T_{X \rightarrow Y}$, and $T_{Y \rightarrow Z}$ transitions, respectively. The triplet sublevels ultimately decay back to the $S_0$ ground state with rates $k_X$, $k_Y$, $k_Z$, completing the cycle. Resonant acoustic driving redistributes the triplet sublevel populations, which changes the spin-dependent decay pathways and is detected as a change in the measured photon counts. Our ESR readout targets the $T_x$-$T_y$ transition near 105 MHz under zero externally applied magnetic field, leveraging the distinct resonance frequencies of the triplet sublevels for frequency-selective detection at zero externally applied field. (b) Structural depiction of a pentacene thin film deposited on a LiNbO$_3$ substrate, indicating the molecular arrangement and orientation. (c) Schematic of the HODSAR device, featuring a high-Q multimode resonator that enables laser pumping and photon readout for triplet state detection. (d) Alternative schematic of the device architecture, illustrating the integration of an acoustic transducer and RF input for resonance excitation.
• Figure 2: (a) Continuous-wave (CW) HODSAR measurements of the pentacene-deposited MMSAR device with 60 repetitions. The fluorescence data are presented with error bars indicating the standard deviation, with a resolution of 100 kHz. The dashed black box highlights the high-$Q$ resonance band around $\sim 105$ MHz. Blue triangles are guide-to-the-eye markers indicating representative modes within this band. (b) $S_{21}$ transmission measurements of the MMSAR device, focusing on the region within the black dashed box in (a). Four distinct high-Q resonant modes are observed in the band relevant to the $T_x-T_y$ transition of the pentacene triplet state. (c) HODSAR readout of the spin-acoustic coupling, shown in comparison with continuous-wave electron paramagnetic resonance (CW-EPR) spectroscopy measurements of the hyperfine-coupled ESR. Notably, both readouts exhibit a similar bandwidth, though the line shape in (c) may be influenced by the limited high-Q multimode response of the MMSAR device across the 100-110 MHz range. The electro-mechanical properties of the MMSAR device are detailed in the Supplementary Materials.
• Figure 3: Characterization of Rabi oscillations in the pentacene thin-film-based HODSAR device. (a) Time-domain measurement of the HODSAR signal, showing normalized signal intensity as a function of time, with the fitted Rabi oscillation overlaid in red. (b) Rabi frequency plotted as a function of the square root of applied power, with experimental data points and a linear fit illustrating power dependence. (c) Schematic of the experimental setup for Rabi oscillation measurements, indicating the timing of laser polarization, acoustic pulse length, and photon counting readout. This characterization validates the coherence and control of Rabi oscillations in the device.
• Figure 4: (a) Schematic illustration of the simulation geometry with the used mesh showing the pentacene sample on the lithium niobate substrate. The corresponding simulation results are depicted in the subsequent three figures: (a) the displacement field, (b) the electric field, and (c) the magnetic field around the pentacene sample. Deformation is added to the displacement field for visualization purposes.
• Figure 5: Fabrication process for pentacene thin-film-based high-Q organic dielectric surface acoustic resonator devices (as mentioned as HODSAR devices) with gold electrodes. The procedure starts with wafer cleaning, followed by photoresist spin-coating. Photoresist exposure and development define the electrode pattern, followed by gold deposition and lift-off to establish the electrode structure. A shadow mask is then applied to enable selective deposition of the pentacene thin film on targeted areas.