The University of Texas at Austin UT Acoustic MEMS GroupHall Lab

Research

We design, fabricate, package and test MEMS and photonic sensors. Most projects run the full cycle in-house, from a model on paper to a device in the cleanroom to a measurement on the bench.

A lithium niobate transducer die on a heated stage with probe needles

Sensors and electronics for extreme temperatures

Hypersonic flight, ground-test facilities, engines and deep wells all need sensors that keep working where silicon electronics fail. We build piezoelectric transducers from single-crystal lithium niobate and aluminum nitride, characterize them on hot stages up to 1000 °C, and develop fast measurement methods for the electronics that must sit next to them, including GaN high-electron-mobility transistors tested in open air at 800 °C. This continues the group's longer line of work on pressure and shear-stress sensors for Mach 6 hypersonic flows.

Key papers

Scanning electron micrograph of a triangular AlN wedge resonator with two electrode sets

Lithium niobate and aluminum nitride piezoelectric transducers

Piezoelectric micromachined ultrasonic transducers (PMUTs) and resonators are at the heart of ultrasound imaging, ranging and acoustic communication. With Prof. Ruochen Lu's group we design bimorph lithium niobate PMUTs on transferred periodically poled films. We also drive an AlN bimorph wedge resonator to about 50 m/s, ten times faster than conventional MEMS, and study large-signal whip-mode resonators in LTOI. Measuring motion that fast needed its own technique, so we developed high-velocity laser Doppler vibrometry methods to do it.

Key papers

A photonic accelerometer chip on a circuit board with an optical fiber aligned to it

Photonic integrated circuit (PIC) accelerometers and inertial sensors

Optical readout gives inertial sensors low noise and immunity to electrical interference. We integrate mechanical proof masses directly with silicon nitride photonic waveguides, reading motion with on-chip Mach–Zehnder interferometers. Our monolithic photonic accelerometer with on-chip fringe counting reaches 2.0 µg/√Hz with 39 dB more dynamic range, and was featured on the August 2026 cover of Microsystems & Nanoengineering. The same platform is now being used for optomechanical microphones.

Key papers

  • An integrated monolithic photonic accelerometer with on-chip fringe-counting · Microsystems & Nanoengineering, 2026 · Cover
  • A novel cantilever-based silicon photonic accelerometer using Mach–Zehnder interferometer targeting 10-ng/√Hz noise floor · IEEE Sensors Journal, 2025
  • A novel opto-mechanical accelerometer in low-loss silicon nitride photonics platform with high dynamic range achieved via heterodyne detection · IEEE Sensors Journal, 2025
A packaged MEMS die with a suspended proof mass used for mechanical frequency comb experiments

Mechanical frequency combs for inertial sensing

Optical frequency combs transformed precision timing and spectroscopy. We are building their mechanical counterpart: a MEMS resonator driven by three-wave-mixing parametric excitation produces a comb of evenly spaced tones that can be read out for inertial sensing. This work, with collaborators at UT Austin and the City University of New York, points to a scalable new sensing modality.

Key papers

A square MEMS microphone die with a circular diaphragm and four curved slots

MEMS microphones, including fly-inspired directional designs

The parasitoid fly Ormia ochracea locates sound with eardrums only about half a millimeter apart, by coupling them mechanically. Our microphones borrow that trick: a single diaphragm whose rocking and bending modes act as three microphones, one omnidirectional and two with orthogonal in-plane directivity. We also study lithium niobate microphones with high SNR potential, optimize piezoelectric electrode layouts automatically, and use commercial-style MEMS microphone dies as ultrasonic transmitters. This line of work was featured on NPR in 2014.

Key papers

A cylindrical magnetostrictive hydrophone on an optical table

Underwater acoustic sensors and ultrasound

Listening to the ocean at low frequencies needs sensors with extremely low self-noise. We built the first hydrophone based on a giant magnetostrictive material, with an estimated noise floor below Sea State 0 (featured on the April 2026 JASA cover), along with magnetostrictive accelerometers and jerk sensors. In our scanning ultrasound tanks we characterize immersion PMUTs and develop ultrasound probes, including a transducer probe for direct ultrasonic neuromodulation.

Key papers

The lab you would work in

Laser Doppler vibrometer measuring packaged devices
Four laser Doppler vibrometers (LDVs)
Light guided through silicon nitride waveguides on a chip
PIC characterization: fiber-coupled tunable lasers, photodetectors and a 6-axis precision fiber aligner
A hydrophone positioned over an immersion transducer array
3-axis scanning ultrasound tanks
Rendering of a laser vibrometer measuring a device on a shaker
A suite of electromechanical shakers for IMU testing

Devices are fabricated in the Microelectronics Research Center cleanroom (over 12,000 sq ft of Class 100 and 1000 space, part of the NSF NNCI) and tested in our Acoustical MEMS Test Laboratory in EER. More on facilities.

Earlier work

Projects that shaped the group and several of its alumni.

Fly-inspired hearing

Microphones modeled on the ears of Ormia ochracea. NPR, 2014

Optically read seismometers

Grating-based optical sensors commercialized by Silicon Audio, tested on glaciers in Alaska and Greenland and competing for a mission to Europa. Anchorage Daily News, 2017

Infrasound

Microphones for 0.01 to 20 Hz, the range of earthquakes, volcanoes and explosions, including nuclear test monitoring for treaty verification.

Hypersonic flow sensing

Piezoelectric pressure and shear-stress sensors tested in a Mach 6 facility.

Elastic-wave telemetry

Sending data through vibrations in steel well casing from deep underground storage sites, where radio does not reach.

Capacitive ultrasound

Rotational CMUTs and broadband capacitive microphones with more than 200 kHz bandwidth.

Website designed and built by Zihuan Liu, 2026