Thermo-acoustic Phase Modulator based on Y36-cut LiNbO3 Thin Film

Abstract

Microwave acoustic components have higher quality factors and less crosstalk than electromagnetic components. Efficient modulation of acoustic devices is essential for building large-scale multifunctional acoustic circuits. Here, we demonstrate a thermo-acoustic phase modulator based on a Y36-cut LiNbO₃ (LN) thin-film platform. The proposed structure integrates a 460 MHz SH₀ mode acoustic delay line and an on-chip microheater for locally changing the temperature and thus controlling the phase of the ADL. Using this approach, we achieve a phase change of more than 281° at a heating power of 20 mW, and a modulation ability of 17 °/mW in the linear modulation range, which is a 6.5 times improvement over previously reported bulk-LN platforms. Our thermo-acoustic modulators enable reconfigurable acoustic signal processing for next-generation wireless communication and microwave systems.

Keywords: Thermo-acoustic phase modulator, SH₀ mode acoustic delay line, LiNbO₃ thin film

A. Design of thermo-acoustic phase modulator

Fig. 1. Schematic of the thermo-acoustic phase modulator. The RF signal convert into acoustic wave via a single-phase unidirectional transducer (SPUDT), and modulated via a heater, when the heater is turned on, the elastic constant decreases, causing the acoustic velocity to decrease, resulting in a shorter wavelength in the modulation region, and eventually a phase lag occurs.

The fabrication process started with a 750 nm Y36-cut LN on a silicon wafer. Firstly, the release boundaries of the device are defined using photoresist AZ5214, then the exposed areas were etched by ion beam etching (IBE) with a biased voltage of 300 V. Electrodes are defined in a second lithography step, and a 100 nm layer of Al is sputtered, followed by a lift-off process. Finally, the silicon underneath the device was released via XeF₂, creating a suspended LN platform. The optical image of the fabricated thermo-acoustic phase modulator and the experimental setup are illustrated in Fig. 4. The S-parameters of the devices were measured using a vector network analyzer (Keysight N5234B) equipped with two ports connected to ground-signal-ground (GSG) RF probes. To assess the DC bias phase control, a DC source meter (Keysight B2901A) was utilized to supply the DC voltage. In this setup, an RF signal is transformed into a shear horizontal acoustic wave by a SPUDT at Port1. This acoustic wave then propagates through the modulation area and is received by another SPUDT at Port2, where it is converted back into an RF signal for evaluating the phase change. The design parameters of the fabricated device are detailed in Table II. The propagation length (LPropagation) and modulation length (LModulation) are designed to be 400 µm and 200 µm, respectively. The wavelength (λ) is designed to be 8 µm, targeting operation at a frequency of approximately 460 MHz. The number of cells (Ncell) is set to 10 to ensure effective transceiving of the acoustic wave.

TABLE II. DESIGN PARAMETERS

Fig. 5. (a) The measured S21 parameters under different input power of the heater. (b) The unwrapped phase angles. (c) The phase angle lag compared with the unmodulated phase angle.

B. DC phase modulation

The measured S21 parameters under different heating powers are shown in Fig. 5(a). The test results demonstrate a stable frequency response despite changes in modulation power, indicating that the thermal energy is effectively localized around the modulation area and has minimal effect on the SPUDT area. Fig. 5(b) presents the unwrapped phase angles and the phase angle lag compared with the unmodulated phase angle, illustrating the acoustic phase shift as the modulation power increases. This shift indicates that the phase velocity of the acoustic wave decreases as the modulation temperature rises. Fig. 5(c) shows the modulated phases relative to the unmodulated phase across the working frequencies. The stability of the modulated phase across these frequencies is noteworthy, as it suggests that the modulator can operate consistently over a range of frequencies. Notably, at a modulation power of 20 mW, which corresponds to a heating voltage of 1 V, the modulation phase angle reaches 281°. The thin-film LN thermo-acoustic modulator demonstrates a significant modulation capability of 17 °/mW, which represents a 6.5 times improvement compared to previously reported bulk-LN platforms. The enhancement in modulation capability is attributed to the unique properties of the suspended thin-film structure. The thin-film design offers excellent thermal isolation, which minimizes the dissipation of thermal energy to surrounding areas, and a low thermal capacity, which allows for rapid and efficient temperature changes with minimal power input. These characteristics result in significantly reduced power consumption and increased modulation capability, making the thin-film LN thermo-acoustic modulator an attractive option for high-efficiency, low-power modulation applications in advanced RF and acoustic wave devices.

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