arXiv subjects

Cristian Cassella

Publications and source records attributed to Cristian Cassella.

11 recordsLinked to original sources

arXiv2026

Topologically Protected Surface Acoustic Wave Edge States in Phononic Integrated Circuits

Phononic integrated circuits enable the development of compact systems to manipulate radio frequency signals through interconnected networks of acoustic waveguides. Introducing topologically protected robustness in these circuits to counter fabrication imperfections and local perturbations, remains a challenge. This work presents a validated modeling framework for topologically protected edge states using surface acoustic waves in etch-defined waveguides consisting of scandium aluminum nitride on silicon carbide, directly comparing analytical, full-wave numerical simulation, and laser Doppler vibrometer measured experimental approaches and their agreement. This work represents the first experimental demonstration of topological edge states on a ScAlN on SiC platform, validated through direct quantitative comparison with analytical and numerical analyses, with future implications for topological protection in phononic and optomechanical integrated circuit platforms.

physics.app-ph↗

arXiv2025

Toward Enhanced Inertial Sensing via Dynamically Soft Topological States in Piezoelectric Microacoustic Metamaterials

In recent decades, microelectromechanical systems (MEMS)-based gyroscopes have been employed to meet positioning and navigation demands of a plethora of commercially available devices. Most of such gyroscopes rely on electrostatic actuators with nanometer-scale air gaps$\unicode{x2013}$an architecture that enables large particle velocities in a proof mass and, consequently, high Coriolis-force sensitivity to angular velocity$\unicode{x2013}$but is inherently susceptible to damage under shock and vibration. This vulnerability is typically mitigated by purposely reducing gyroscopic sensitivity, thereby compromising readout accuracy. Microacoustic gyroscopes, by contrast, offer greater resilience to shock and vibration but currently exhibit significantly lower sensitivities. This limitation stems from the low dynamic compliance of the modes they employ$\unicode{x2013}$typically Lamb or Rayleigh modes$\unicode{x2013}$which restricts their maximum achievable particle velocity. This work presents a piezoelectric microacoustic device that overcomes this fundamental constraint by harnessing a topological interface state at the boundary between two microscale metamaterial structures. We theoretically and experimentally show that this state exhibits much higher modal compliance than Lamb or Rayleigh modes. This enables record-high particle velocities (>51 m/s) never reached, due to material limits, by any previously demonstrated piezoelectric gyroscope.

physics.app-ph↗

arXiv2025

Analyzing Parametric Oscillator Ising Machines through the Kuramoto Lens

Networks of coupled nonlinear oscillators are emerging as powerful physical platforms for implementing Ising machines. Yet the relationship between parametric-oscillator implementations and traditional oscillator-based Ising machines remains underexplored. In this work, we develop a Kuramoto-style, canonical phase description of parametric oscillator Ising machines by starting from the Stuart-Landau oscillator model -- the canonical normal form near a Hopf bifurcation, and a natural reduced description for many parametric oscillator implementations such as the degenerate optical parametric oscillator (DOPO) among others. The resulting phase dynamics combine the usual phase-difference coupling observed in the standard Kuramoto model along with an intrinsic phase sum term that is generated when conjugate coupling is considered. Moreover, our formulation helps explain why explicit second-harmonic driving is unnecessary in parametric oscillators and also reveals how quasi-steady amplitude heterogeneity scales the original strength of the spin interaction with potentially adverse impacts on the solution quality. Our work helps develop a unifying view of the oscillator-based approach to designing Ising machines.

eess.SY↗

arXiv2024

Localized Topological States beyond Fano Resonances via Counter-Propagating Wave Mode Conversion in Piezoelectric Microelectromechanical Devices

A variety of scientific fields like proteomics and spintronics have created a new demand for on-chip devices capable of sensing parameters localized within a few tens of micrometers. Nano and microelectromechanical systems (NEMS/MEMS) are extensively employed for monitoring parameters that exert uniform forces over hundreds of micrometers or more, such as acceleration, pressure, and magnetic fields. However, they can show significantly degraded sensing performance when targeting more localized parameters, like the mass of a single cell. To address this challenge, we present a new MEMS device that leverages the destructive interference of two topological radiofrequency (RF) counter-propagating wave modes along a piezoelectric Aluminum Scandium Nitride (AlScN) Su-Schrieffer-Heeger (SSH) interface. The reported MEMS device opens up opportunities for further purposes, including achieving more stable frequency sources for communication and timing applications.

physics.app-ph↗

arXiv2024

Parametric Frequency Divider based Ising Machines

We report on a new class of Ising Machines (IMs) that rely on coupled parametric frequency dividers (PFDs) as macroscopic artificial spins. Unlike the IM counterparts based on subharmonic injection locking (SHIL), PFD IMs do not require strong injected continuous wave signals or applied DC voltages. Therefore, they show a significantly lower power consumption per spin compared to SHIL based IMs, making it feasible to accurately solve large scale combinatorial optimization problems (COPs) that are hard or even impossible to solve by using the current von Neumann computing architectures. Furthermore, using high quality (Q) factor resonators in the PFD design makes PFD IMs able to exhibit a nanoWatt level power per spin. Also, it remarkably allows a speed up of the phase synchronization among the PFDs, resulting in shorter time to solution and lower energy to solution despite the resonators' longer relaxation time. As a proof of concept, a 4 node PFD IM has been demonstrated. This IM correctly solves a set of MaxCut problems while consuming just 600 nanoWatts per spin. This power consumption is two orders of magnitude lower than the power per spin of state of the art SHIL based IMs operating at the same frequency.

physics.app-ph↗

arXiv2022

A 5.3 GHz Al0.76Sc0.24N Two-Dimensional Resonant Rods Resonator with a kt2 of 23.9%

This work reports on the measured performance of an Aluminum Scandium Nitride (AlScN) Two-Dimensional Resonant Rods resonator (2DRR), fabricated by using a Sc-doping concentration of 24%, characterized by a low off-resonance impedance (~25 Ohm) and exhibiting a record electromechanical coupling coefficient (kt2) of 23.9% for AlScN resonators. In order to achieve such performance, we identified and relied on optimized deposition and etching processes for highly-doped AlScN films, aiming at achieving high crystalline quality, low density of abnormally oriented grains in the 2DRR's active region and sharp lateral sidewalls. Also, the 2DRR's unit-cell has been acoustically engineered to maximize the piezo-generated mechanical energy within each rod and to ensure a low transduction of spurious modes around resonance. Due to its unprecedented kt2, the reported 2DRR opens exciting scenarios towards the development of next generation monolithic integrated radio-frequency (RF) filtering components. In fact, we show that 5th-order 2DRR-based ladder filters with fractional bandwidths (BW) of ~11%, insertion-loss (I.L) values of ~2.5 dB and with >30 dB out-of-band rejections can now be envisioned, paving an unprecedented path towards the development of ultra-wide band (UWB) filters for next-generation Super-High-Frequency (SHF) radio front-ends.

eess.SP↗

arXiv2020

Reflective Parametric Frequency Selective Limiters with sub-dB Loss and $μ$Watts Power Thresholds

This article describes the design methodology to achieve reflective diode-based parametric frequency selective limiters (pFSLs) with low power thresholds ($P_{th}$) and sub-dB insertion-loss values ($IL^{s.s}$) for driving power levels ($P_{in}$) lower than $P_{th}$. In addition, we present the measured performance of a reflective pFSL designed through the discussed methodology and assembled on a FR-4 printed circuit board (PCB). Thanks to its optimally engineered dynamics, the built pFSL can operate around $\sim$2.1 GHz while exhibiting record-low $P_{th}$ (-3.4 dBm) and $IL^{s.s}$ (0.94 dB) values. Furthermore, while the pFSL can selectively attenuate undesired signals with power ranging from -3.4 dBm to 13 dBm, it provides a strong suppression level (IS > 12.0 dB) even when driven by much higher $P_{in}$ values approaching 28 dBm. Such measured performance metrics demonstrate how the unique nonlinear dynamics of parametric-based FSLs can be leveraged through components and systems compatible with conventional chip-scale manufacturing processes in order to increase the resilience to electromagnetic interference (EMI), even of wireless radios designed for a low-power consumption and consequently characterized by a narrow dynamic range.

eess.SY↗

arXiv2020

Aluminum Nitride Two-Dimensional-Resonant-Rods

In the last decades, Bulk-Acoustic-Wave (BAW) filters have been essential components of 1G-to-4G radios. These devices rely on the high electromechanical coupling coefficient (kt2~7%), attained by Aluminum Nitride (AlN) Film-Bulk-Acoustic-Resonators (FBARs), to achieve a wideband and low-loss frequency response. As the resonance frequency of FBARs is set by their thickness, the integration of multiple FBARs, to form filters, can only be attained through the adoption of frequency tuning fabrication steps, such as mass loading or trimming. However, as the ability to reliably control these steps significantly decays for thinner FBARs, manufacturing FBARs-based filters, addressing the needs of emerging IoT and 5G high-frequency applications, is becoming more and more challenging. Consequently, there is a quest for new acoustic resonant components, simultaneously exhibiting high kt2 and a lithographic frequency tunability. In this work, a novel class of AlN resonators is presented. These radio-frequency devices, labelled as Two-Dimensional-Resonant-Rods (2DRRs), exploit, for the first time, the unconventional acoustic behavior exhibited by a forest of locally resonant rods, built in the body of a profiled AlN layer that is sandwiched between a bottom un-patterned metal plate and a top metallic grating. 2DRRs exhibit unexplored modal features that make them able to achieve high kt2, a significant lithographic frequency tunability and a relaxed lithographic resolution, while relying on an optimal AlN crystalline orientation. The operation of 2DRRs is discussed, in this work, by means of analytical and finite-element (FE) investigations. The measured performance of the first fabricated 2DRR, showing a kt2 in excess of 7.4%, are also reported.

eess.AS↗

arXiv2020

Systematic Synthesis and Design of Ultra-Low Threshold Parametric Frequency Dividers

A new method is discussed for the systematic synthesis, design and performance optimization of varactor-based parametric frequency dividers (PFDs) exhibiting an ultra-low power threshold ($P_{th}$). For the first time, it is analytically shown that the $P_{th}$-value exhibited by any PFD can always be expressed as an explicit closed-form function of the different impedances forming its network. Such a unique and unexplored property permits to rely on linear models, during the PFD design and performance optimization. The validity of our analytical model has been verified, in a commercial circuit simulator, through time-domain and frequency-domain algorithms. To demonstrate the effectiveness of our new synthesis approach, we also report on a lumped prototype of a 200:100MHz PFD, realized on a printed circuit board (PCB). Although inductors with quality factors lower than 50 were used, the PFD prototype exhibits a $P_{th}$-value lower than $-$15dBm. Such a low $P_{th}$-value is the lowest one ever reported for passive varactor-based PFDs, operating in the same frequency range.

eess.SP↗

arXiv2019

Highly-Linear Magnet-Free Microelectromechanical Circulators

This paper reports the first demonstration of a magnet-free, high performance microelectromechanical system (MEMS) based circulator. An innovative circuit based on the commutation of MEMS resonators with high quality (Q) factor using RF switches is designed and implemented. Thanks to the high Q factor, a much smaller modulation frequency can be achieved compared to the previous demonstrations, reducing the power consumption and enabling the use of high power-handling switches. Furthermore, the MEMS resonators greatly reduce the required inductance value, guaranteeing much smaller form factor compared to the previous LC demonstrations. The demonstrated circulator shows broad BW (15 dB-IX BW=34.7 MHz for an operational frequency around 2.5 GHz), low IL (4 dB), high IX (30 dB), high linearity (P1dB=28 dBm; IIP3=40 dBm) and at the same time low power consumption, addressing several of the current limitations hindering the full development of magnet-free circulators.

physics.app-ph↗

arXiv2019

Radio Frequency Magnet-free Circulators Based on Spatiotemporal Modulation of Surface Acoustic Wave Filters

In this paper, a new generation of magnet-free circulators with high performance is proposed. Circulators are crucial devices in modern communication systems due to their ability to enable full-duplexing and double the spectral efficiency directly in the physical layer of the radio-frequency (RF) front-end. Traditionally, Lorentz reciprocity is broken by applying magnetic bias to ferrite materials, therefore conventional circulators are bulky and expensive. In this paper, this problem is addressed by replacing the magnetic bias with periodic spatiotemporal modulation. Compared to previous works, the proposed circulator is constructed using surface acoustic wave (SAW) filters instead of transmission lines (TL), which reduces the modulation frequency by at least a factor of 20 and ensures ultra-low power consumption and high linearity. The miniaturized high quality (Q) factor SAW filters also lead to a low-loss non-reciprocal band with strong isolation (IX) and broad bandwidth (BW) on a chip scale, therefore addressing such limitations in previous magnet-free demonstrations. Furthermore, compared to the conventional differential circuit configuration, a novel quad configuration is developed, which doubles the intermodulation-free bandwidth.

eess.SP↗