This work presents a major advancement in mmWave Bulk Acoustic Wave (BAW) resonator technology by achieving a record-high Quality factor $(Q)$ of 184 at 51 GHz using a 35% Scandium alloyed Aluminum Nitride Overmoded Bulk Acoustic Resonator (ScAlN OBAR). Achieving high $Q$ at such elevated frequencies is extremely challenging due to intrinsic material losses and energy leakage. To address this major bottleneck for radio frequency (RF) front-end filter applications, we isolate and mitigate key loss contributors through a systematic design and fabrication of four OBAR variants. The final design not only demonstrates superior $Q$ performance but also maintains fabrication simplicity, standing out against competing technologies which require significantly more complex architectures to achieve comparable $Q$.

Integration of nano-electromechanical systems (NEMS) actuators into multi-physics platforms, such as with photonic integrated circuits (PICs), can provide unique advantages in power consumption, form factor, and performance. However, characterization of these systems is challenging due to their coupled transduction mechanisms. This paper discusses the interactions between electro-mechanical and opto-mechanical transduction in nano-scale photonic modulators. Herein, pull-in instability and spring softening is leveraged as a self-characterization mechanism within a novel high-sensitivity NEMS actuator that is monolithically integrated with a photonic resonator. Through this method, measurements using existing input/output (I/O) connections to the device can provide new insights into internal device interactions between the electrical, optical, and mechanical domains. This study shows a novel demonstration of the measurement of optical force effects generated between the photonic resonator and the NEMS actuator. Extraction of the electro-mechanical actuation shows that the device achieves an order of magnitude enhancement in mechanical sensitivity (>550 nm/V) over the state-of-the-art by using a non-linear electrostatic design while maintaining the opto-mechanical performance compared to previous high quality-factor (Q >10 k) monolithic devices. This study provides new experimental approaches for extracting photonic NEMS parameters and serves as a model for characterizing other multi-physics systems.

This work presents the first demonstration of impedance tuning in Frequency Range 3 (FR3) Bulk Acoustic Wave (BAW) resonators through ferroelectric state control. Previous efforts to adjust BAW resonator impedance characteristics through ferroelectric states have been focused on binary switching or activating specific layers in multilayer stacks rather than achieving precise modulation of the resonator's impedance characteristics. By leveraging an ultra-thin 50 nm Y36 cut Lithium Niobate (LN) film with low coercive field in a Tunable Bulk Acoustic Wave resonator (TBAW), we achieve fine-tuning of impedance at both series and parallel resonance frequencies. The impedance variation spans an impressive 18 dB range with switching voltages below 2.2 V. The combination of low-voltage operation and wide tunability positions the LN TBAW as a promising pathway for dynamic signal modulation and adaptive Radio Frequency $(\text{RF})$ front-end architectures.

The era of intelligent machines demands advanced hardware capable of high-density data collection and processing within compact platforms. Thermal-infrared emission, with its dual functionalities of heat and light, enables promising applications in optical data acquisition and information processing. However, the inherent stochastic nature and slow thermal response speed of thermal emission limit practical applications. In this work, we demonstrate electrically programmable, pixelated metasurfaces based on GeTe phase-change materials that enable dynamic and localized control of thermal-infrared emission. By integrating GeTe into hybrid plasmonic meta-atoms with strong field confinement, we achieve fast, nonvolatile switching with large optical contrast using minimal active material. This approach allows multidimensional tunability, establishing a versatile platform for reconfigurable photonic systems with high integration density, adaptive functionality, and embedded intelligence.

We introduce a novel electro-optomechanic neural sensor for realizing ultra-compact neural recording probes that can detect and relay electrophysiology signals from within neural tissue. This technology addresses outstanding challenges faced by existing neural recording technologies, including the resolution trade-off with signal-to-noise-ratio (SNR) due to the high impedances of small electrodes, and lingering stimulation artifacts. The sensor employs a highly miniaturized NEMS (nano-electromechanical systems) electrostatic transducer that modulates a silicon photonic microdisk resonator to convert electrical signals to an optical signal modulation. We have been able to achieve a limit of detection down to 110 microvolts, making the sensor sensitive enough to detect neural signals. This sensitive electro-optomechanic sensor directly detects electrophysiology signals and converts them to optomechanic modulation for effective transmission to outside the brain, which provides the unique potential for massive multiplexing of neural recordings. This design eliminates the need for bulky backend headstages that limit neural recording on awake free-roaming subjects. The ability of the device to record electrophysiological signals has been demonstrated using benchtop characterization and ex-vivo recordings from live neural tissue.

In this work, we report an efficient high-gain ovenization system for a thin-film ST-cut quartz MEMS resonator. The high-performance ovenization is enabled by the use of thin films (1 mu m) of ST-cut quartz on silicon, optimized suspension design, and strategic placement of the heater in close proximity to the resonator. The suspended body, supported by meandering anchors, effectively rejects environmental temperature fluctuations, achieving a measured oven gain of 195 while maintaining an oven efficiency of 89 K/mW. The demonstrated devices are capable of raising the resonator temperature to 80 degrees C with only 0.6 mW of power.

Scandium containing aluminum nitride (ScAlN) combines strong piezoelectricity and second-order nonlinearities with compatibility to CMOS backend processes, making it a promising material for integrated photonics. In this work, we demonstrate electro-optic modulation in high-concentration Sc\textsubscript{0.30}Al\textsubscript{0.70}N using Mach–Zehnder interferometer and ring resonator modulators fabricated on a CMOS-compatible platform. Devices were characterized under AC and DC excitation with lock-in detection, and simulation-calibrated measurements yielded an $r_{13}$ value of approximately 0.19 pm/V. While this coefficient is lower than values reported for ScAlN with similar Sc content, it was consistent across device geometries and reflects films grown under conditions optimized for piezoelectric rather than electro-optic performance. These results establish a clear baseline for the Pockels effect in highly Sc concentrated ScAlN and highlight the need for growth strategies specifically tailored to electro-optic applications in scalable photonic–electronic integration.

Lithium Niobate (LN) has been a piezoelectric material of interest for various types of acoustic resonators operating at GHz frequencies as it offers very high electromechanical coupling ($k_t^2$). However, very little is known about the incorporation of thin-film LN into Bulk Acoustic Wave (BAW) resonators above 7 GHz, and how its ferroelectric properties affect the resonance characteristics. In this work, we present the first Lithium Niobate Tunable Bulk Acoustic Wave (LN TBAW) resonator, which is a BAW type resonator that incorporates a 50 nm thick Y36 cut LN film as the piezoelectric film and has its resonance frequency response tuned through ferroelectric switching. The fabricated device achieves a $k_t^2$ up to 16.5% and a maximum Quality factor (Q) of 105.8 at 17.6 GHz in its "on" state, while its series and parallel resonances are suppressed when tuned to an "off" state. Ferroelectric switching to the "off" state is attained with a voltage less than 1.5 V. This novel demonstration opens a new path towards the implementation of TBAW into tunable Radio Frequency (RF) front-end filters.

This work demonstrates an Overmoded Bulk Acoustic Resonator (OBAR) design that incorporates 35% Scandium doped Aluminum Nitride (Sc0.35Al0.65N) as the piezoelectric layer. The ScAlN OBAR presented here is a Bulk Acoustic Wave (BAW) resonator that excites a second overtone within a stack formed by a ScAlN layer and a set of alternating metallic layers. The metal electrodes act simultaneously as the acoustic cavity and as acoustic Bragg mirrors. Individual resonators are connected to each other by thick floating electrodes and top interconnects to form the devices demonstrated herein. The fabricated ScAlN OBAR with best performance exhibits a series resonant frequency of 51.3 GHz, electromechanical coupling ( $k_{t}^{2}$ ) of 6.1% and a Quality factor (Q) at series resonance of 108. The measurements of various ScAlN OBAR devices with different geometries show that Q is increasing as the perimeter and area of the individual resonator increases and $k_{t}^{2}$ is increasing as the number of resonators in series increases. Material losses and surface roughness with associated acoustic energy leakage are discussed as possible sources of damping in these mmWave resonators. The investigations trace a path for further technological improvement and show that the ScAlN OBAR is a promising device for mmWave acoustics and filtering applications. [2025-0071]

In-memory computing (IMC) has emerged as an alternative to the Von Neumann architecture, enabling computation directly in memory. Emerging non-volatile memories (eNVMs) are being investigated because of their non-volatility and multi-state capabilities that are necessary for IMC. Lithium niobate $(\text{LiNbO}_{3})$, with its large polarization and low switching voltages, is a promising candidate for eNVMs. To our knowledge, there is no demonstration of metal-ferroelectric-metal (MFM) capacitors with sub-100 nm thick $\text{LiNbO}_{3}$ films utilizing its ferroelectric properties for IMC. Herein, we demonstrate the multi-state conductance of ultra-thin $43 \text{nm} \mathrm{Y}-36$ cut $\text{LiNbO}_{3}$ (Y-36 LN) films in a MFM capacitor structure. The Y-36 LN MFM device is characterized to have an ON/OFF ratio of $\sim 6$ with 40 states while maintaining write pulses below 4 V. Our results showcase the potential of $\text{LiNbO}_{3}$ as a highly scaled and tunable ferroelectric material for IMC application.

This work presents an analysis of the first mmWaves-operating Cross-sectional Lame Mode Resonators (CLMRs), investigating the intrinsic quality factor limit of the technology. By leveraging the Finite Element Modeling-simulated energy distributions in the piezoelectric and metal layer, an accurate matching of theoretical and experimental quality factor is achieved, thus identifying the main source of CLMR performance degradation in the 20 to 35 GHz frequency range. Furthermore, excellent quality factors are recorded, achieving the largest frequency and quality factor product (f(s) center dot Q = 14.4 THz) ever demonstrated on sputtered thin-films and among the largest ever reported for the microacoustic technology. In conclusion, the present results explore the feasibility of enabling mmWaves-operating CLMRs to extend the use cases of the microacoustic technology to a virtually new spectrum territory, while outlining design trade-offs aiming to maximize their quality factors.

In this work, we demonstrate a simple microfabri-cation process for 1 mu m thick ST-cut MEMS quartz resonators. A variety of resonators with different geometrical parameters designed to operate at around 180 MHz were fabricated and measured. The devices exhibit a quality factor as high as 6,600 in vacuum. An on-chip ovenization system is integrated with the resonator. The engineering of high thermal isolation suspensions facilitated the demonstration of an ovenization method capable of raising the resonator temperature by 63.9 K with 1 mW.

Scandium doped Aluminum Nitride (ScAlN) has become a piezoelectric material of interest for Bulk Acoustic Wave (BAW) resonators as it offers an intrinsically high electromechanical coupling (k(t)(2)). However, little is known about ScAlN at mm-wave frequencies. In this work, we demonstrate an innovative Overmoded Bulk Acoustic Resonator (OBAR) design that incorporates 35% Scandium doped Aluminum Nitride (Sc0.35Al0.65N) piezoelectric layer. The ScAlN OBAR presented herein is a BAW device that is excited in the 2(nd) overtone by means of the ScAlN layer and a stack of metal electrodes that act simultaneously as acoustic cavity and as acoustic Bragg mirrors. The fabricated device demonstrates k(t)(2) of 6.1% and Quality factor (Q) of 114 at 52 GHz - a substantial improvement in the k(t)(2). Q figure of merit of ScAlN acoustic resonators at mm-wave frequencies.

A novel hybrid plasmonic‐germanium telluride (GeTe) metasurface design, which demonstrates single‐peak on/off tunability across a spectral range of 2.5–7.5 μm is developed. Unlike previous metasurface designs utilizing GeTe layers as spacers, this study integrates GeTe as part of the meta‐atoms with gold (Au), significantly reducing the amount of GeTe required while achieving substantial on/off contrast. By varying the sizes of the meta‐atoms, different resonant wavelengths are achieved. This work represents a significant advancement in chalcogenide‐based active metasurfaces, providing precise and dynamic control over thermal emission properties.

Although Sc doped AlN (ScAlN) has been used extensively in micro-electro-mechanical systems (MEMS) devices and more recently in optical devices, there have not been thorough studies of its intrinsic optical losses. Here we explore the optical losses of the Sc0.30Al0.70N waveguide system by observing racetrack resonator waveguide quality factors. Using a partial physical etch, we fabricate waveguides and extract propagation losses as low as 1.6 ± 0.3 dB/cm at wavelengths around 1550 nm, mostly dominated by intrinsic material absorption from the Sc0.30Al0.70N thin film layer. The highest quality factor of the resonators was greater than 87,000. The propagation loss value is lower than any value previously published and shows that this material can be broadly used in optical modulators without significant loss.

We present a novel NEMS Optomechanic modulator on a silicon photonics platform capable of resolving sub-millivolt analog signals, making it suitable for the recording and multiplexing of electrophysiological neural signals.

This paper reviews the latest developments in microwave acoustic wave devices.After an introduction and brief history of bulk acoustic wave (BAW) and surface acoustic wave (SAW) devices, a review is given for guided SAWs and XBARs -two new technologies, which are promising for future 5G applications.Following this, we discuss recent simulation techniques, such as 3D finite element method (3D FEM) and simulation of nonlinearities, as well as filter synthesis.Next, a review on tunable and reconfigurable acoustics is given.Finally, we present the latest developments in microwave acoustics for millimeter-wave (mm-wave) operation as well as BAW oscillators.

In this work we measure, for the first time, mechanical loss in an Al thin film from 3 GHz to 25 GHz through the use of high overtone bulk acoustic resonator (HBAR) spectroscopy. This is made possible by scaling down a HBAR to use a thin (200 nm) aluminum nitride (AlN) piezoelectric transducer and replacement of the thick substrate with a sputtered, suspended metal film (~1 µm). Resonant overtones are transduced piezoelectrically while >90% of the mechanical energy is confined to the Al, allowing Al mechanical loss to be accurately determined from quality factor at series resonance (Qs). Measured Qs for 7 overtones ranging from 3-25 GHz with values from 140 to 50 was found to be ~30% less than predicted by analytical models considering thermoelastic and phonon phonon damping. Additionally, measured frequency dependence of Q (f -0.46 ) aligned well with the dependence predicted by thermoelastic and Landau Rumer regime phonon phonon damping (f -0.56 ) and was well above the f -1 dependence predicted by thermoelastic and Akhiezer regime phonon phonon damping. This technique can be applied to measure mechanical loss in metal thin films up to K band, allowing loss characterization and enabling informed decision making for design of many high frequency acoustic devices.

In this work we present a solidly mounted overmoded bulk acoustic resonator (OBAR) designed to operate at millimeter wave (mmWave) frequencies. This device uses a combination of overmoded operation, a layer transfer based fabrication process, all metal Bragg mirrors, and series arrays to allow acoustic resonator frequency scaling through V band. The solidly mounted OBAR can have up to 2/3 the $k_t^2$ of a fundamental mode (~4% for AlN, >10% for ScAlN or LN), reasonable piezoelectric film thicknesses (>100 nm at 50 GHz for AlN), arbitrarily thick electrodes to minimize ohmic loss, and be made in series arrays to allow 50 Ω matched devices with reasonable area to perimeter ratios. We demonstrate a proof of concept device using a 110 nm AlN piezoelectric layer operating at 55 GHz with an electromechanical-coupling coefficient $\left( {k_t^2} \right)$ of 2.2%, and a series resonance quality factor (Qs) of 90.