🔬 applied physics

Push-Pull acousto-optic modulator based on non-suspended thin-film lithium niobate on silicon substrate

This paper demonstrates a high-efficiency, low-loss, non-suspended push-pull acousto-optic modulator on a thin-film lithium niobate-on-silicon substrate that achieves a half-wave voltage-length product of 1.004 V cm and a 132.5 MHz bandwidth, offering a stable and practical solution for chip-scale microwave-to-optical conversion and quantum transduction.

Original authors: Haorui Ni, Sunil Bhave, Mengyue Xu

Published 2026-06-05

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Haorui Ni, Sunil Bhave, Mengyue Xu

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you have a beam of light traveling through a tiny glass highway (a waveguide) on a computer chip. Usually, to change the properties of this light—like its brightness or timing—you might use electricity directly. But this paper introduces a clever trick: using sound to control light.

Think of it like this: If light is a runner on a track, sound is a series of invisible, rhythmic bumps on the track. When the runner hits these bumps, they speed up, slow down, or change their stride. In the world of physics, this is called an Acousto-Optic Modulator (AOM).

Here is what the researchers built, explained simply:

1. The Problem: Floating vs. Sturdy

Previous versions of these "sound-controlled light" devices were like trapeze artists. To make the sound waves strong enough to move the light, scientists had to build the devices on "suspended" structures (hanging in the air) or on very expensive, hard-to-work-with materials.

The downside: These floating devices are fragile, hard to make, and often only work for a very narrow range of sounds (like a radio station that only plays one specific song).

2. The Solution: The "Push-Pull" Sandwich

The team at Purdue and Michigan built a new version that is sturdy and flat. They didn't suspend the chip; they built it right on top of a standard silicon substrate (like a normal computer chip), using a special material called Thin-Film Lithium Niobate.

To make it super efficient, they used a "Push-Pull" strategy:

The Setup: Imagine a race with two lanes (two light paths) running side-by-side.
The Sound: They created a sound wave that acts like a giant, rhythmic seesaw. On one side of the seesaw, the track is pushed up; on the other side, it's pulled down.
The Magic: They placed one light lane on the "push" side and the other on the "pull" side.
The Result: Instead of just nudging the light, the sound pushes one lane forward while pulling the other back. This doubles the effect, making the device much more powerful and efficient than if they had just used one lane.

3. Finding the Perfect Angle

The researchers realized that the direction the sound travels matters a lot. It's like trying to push a swing; if you push at the wrong angle, nothing happens. If you push at the perfect angle, the swing goes high.

They tested three different angles for the sound waves:

Angle A: Weak push.
Angle B: Medium push.
Angle C (The Winner): A perfect 0-degree angle relative to the crystal's structure. This angle acted like the perfect push on the swing, creating the strongest connection between the electricity, the sound, and the light.

4. The Results: Fast, Strong, and Stable

Because they got the angle right and used the "Push-Pull" design, their device achieved some impressive numbers:

High Efficiency: It takes very little electrical power to make the sound waves strong enough to control the light. They measured this as a "half-wave voltage-length product" of 1.004 V·cm. In plain English, this means the device is very good at its job without needing a lot of energy.
Broad Bandwidth: Unlike the old "trapeze" devices that only worked on one specific sound frequency, this one works across a wide range of frequencies (132.5 MHz). Think of it as a radio that can tune into a whole station's worth of channels, not just one song.
Stability: Because it's built flat on a solid silicon base (not floating), it's much more stable and easier to mass-produce using standard factory methods.

Summary

The researchers built a sound-controlled light switch that is:

Sturdy: It sits flat on a standard chip, not hanging in the air.
Smart: It uses a "push-pull" technique to double its effectiveness.
Tuned: They found the exact angle to make the sound waves work best.
Versatile: It can handle a wide variety of frequencies, making it ready for real-world use in communication and advanced computing systems.

They didn't just make a theoretical idea; they built it, tested it, and proved it works better than previous attempts at making these devices stable and efficient.

Technical Summary: Push-Pull Acousto-Optic Modulator Based on Non-Suspended Thin-Film Lithium Niobate on Silicon

Problem Statement
Acousto-optic modulators (AOMs) are critical for microwave-to-optical conversion, quantum transduction, and optical frequency manipulation. While integrated piezoelectric platforms offer compact geometries and strong light-sound interactions, previous implementations of thin-film lithium niobate (TFLN) AOMs have faced significant trade-offs. Devices utilizing suspended structures or non-standard substrates (e.g., sapphire) to confine acoustic energy often suffer from stability issues, limited bandwidth, and high fabrication complexity, making them difficult to integrate with standard foundry processes. Conversely, achieving efficient acousto-optic interaction on a standard, non-suspended TFLN-on-insulator platform compatible with electro-optic device fabrication has remained a challenge.

Methodology
The authors propose and demonstrate a non-suspended, built-in push-pull AOM on an X-cut TFLN platform. The device architecture integrates a Mach-Zehnder interferometer (MZI) with an interdigital transducer (IDT) to generate surface acoustic waves (SAWs). Key design features include:

Push-Pull Configuration: Both optical waveguides of the MZI are embedded directly within the IDT aperture. By positioning the waveguides between signal and ground electrodes, the acoustic strain induces phase shifts of opposite signs in the two arms, effectively doubling modulation efficiency.
Acoustic Confinement: Metal reflectors are placed outside the IDT region to reinforce mechanical resonance and maximize the acoustic field interacting with the optical modes.
Optimization of Orientation: To address electromechanical coupling efficiency, the authors fabricated devices with three different acoustic propagation angles ( $\alpha = 135^\circ, 15^\circ, 0^\circ$ ) relative to the crystal Y-axis. They utilized the quasi-static approximation to calculate the electromechanical coupling coefficient ( $k^2_{16}$ ) for shear-horizontal (SH) acoustic waves to identify the optimal orientation.
Fabrication: The device was fabricated on a 360 nm X-cut TFLN wafer with a 4.7 µm buried oxide. The process involved electron-beam lithography (EBL), Ar+ ion milling for rib waveguides, and the strategic omission of IDT fingers closest to the waveguides to minimize metal-induced optical absorption while maintaining acoustic confinement.

Key Contributions

Non-Suspended Architecture: The work demonstrates a high-efficiency AOM on a standard TFLN-on-silicon stack without suspended structures, offering a fabrication-friendly and scalable platform.
Orientation Dependence Analysis: The study systematically investigates the orientation dependence of electromechanical coupling in X-cut TFLN, identifying the optimal acoustic propagation direction for enhanced transduction.
Integrated Push-Pull Design: The implementation of a built-in push-pull configuration where both MZI arms interact with the acoustic field simultaneously, overcoming the limitations of single-sided acoustic excitation found in prior designs.

Results

Optimal Orientation: Device characterization confirmed that the acoustic propagation angle $\alpha = 0^\circ$ yields the strongest electromechanical coupling and acousto-optic transduction, aligning with theoretical predictions of $k^2_{16}$ .
Modulation Efficiency: The optimized device (Device 3) achieved a half-wave voltage-length product ( $V_\pi L$ ) of 1.004 V·cm at a center frequency of 0.842 GHz with an interaction length of 400 µm.
Bandwidth: The device demonstrated a relatively wide acousto-optic modulation bandwidth of 132.5 MHz (3-dB). This broad bandwidth is attributed to the excitation of multiple closely spaced acoustic modes within the acoustic cavity formed by the IDT and reflectors.
Loss and Performance: The on-chip propagation loss was measured at 0.76 dB at 1550 nm. The microwave-to-acoustic conversion efficiency was estimated at approximately 99.96% of input RF power, with an acousto-optic transmission peak of -40.4 dB.

Significance
The paper claims that these results pave the way for efficient, practical integrated photonic-phononic links. By combining high efficiency, broad bandwidth, and low optical loss on a non-suspended, standard substrate, the demonstrated device addresses the stability and integration challenges of previous suspended or resonant AOMs. The work establishes a foundation for future integrated acousto-optic systems suitable for advanced photonic applications in communication, signal processing, and quantum technologies.

1. The Problem: Floating vs. Sturdy

2. The Solution: The "Push-Pull" Sandwich

3. Finding the Perfect Angle

4. The Results: Fast, Strong, and Stable

Summary

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