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1. Energy trapping in a phononic crystal cavity enhanced by nonreciprocal acoustic wave transmission

   https://doi.org/10.1016/j.apacoust.2022.109192  

   Dhillon, Jyotsna; Walker, Ezekiel; Krokhin, Arkadii; Neogi, Arup (February 2023, Applied Acoustics)

   Defect mode induced energy trapping at the bandgap frequency of a phononic crystal has been widely explored. Unlike this extensively used mechanism, this work reports the use of nonreciprocity in the transmission band to trap energy inside a phononic crystal cavity. Passive nonreciprocity is due to natural viscosity of the background liquid (water) and asymmetry of aluminum scatterers. The level of nonresonant energy trapping was compared for three cavities with different symmetry. Enhancement of energy trapping at a frequency of 624 kHz was observed experimentally for the cavity where nonreciprocity suppresses acoustic radiation into environment. Experimental results were further investigated and confirmed using finite element numerical analysis. 

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2. On the non-dispersive and pseudo-homogeneous behavior of harmonically-graded phononic crystals at high frequency regimes

   https://doi.org/10.1088/1361-6463/ae6d0b  

   Soliman, K; Jafari, A; Yousef, H; Fuad, M; Singh, T; Nouh, M (June 2026, Journal of Physics D: Applied Physics)

   Phononic crystals represent a class of architected materials with spatially periodic unit cells. In their most common form, they involve distinct layers with sharply contrasting properties (e.g., hard-soft materials). These systems exhibit wide frequency bandgaps which are understood to culminate from impedance mismatches between the neighboring layers. This work focuses on a less-commonly studied configuration of phononic crystals in which the unit cell is formed via a continuous harmonic variation of material properties. While multi-layered solid phononic crystals display an infinite number of bandgaps, we show that an elastically-graded phononic crystal exhibits a single, well-defined bandgap that can be tuned by varying the parameters of the harmonically-varying stiffness. Additionally, we reveal that such graded phononic crystals transition to a fully non-dispersive behavior above a frequency threshold, rendering them pseudo-homogeneous structures despite their physical heterogeneity. As the unit cell transforms from distinct layers to a smoothly graded profile, we seek to understand the system’s evolution leading to the closing of higher frequency bandgaps and the loss of dispersive wave response. We employ a segmented discretization of the harmonic profile to approximate its continuous form with the goal of deriving a semi-analytical solution that systematically captures the emergence, width, and eventual closure of the bandgaps. By varying the resolution of discretization, we gain valuable insight into the changes in bandgap widths and the subsequent morphing of the dispersion bands, which enables the design of broadband phononic crystals with tailored and reconfigurable wave manipulation capabilities. 

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3. Experimental demonstration of tunable spectral flow of elastic localized modes

   https://doi.org/10.1063/5.0293535  

   Kuchibhatla, Sai_Aditya Raman; Gupta, Aakash; Miniaci, Marco; Leamy, Michael J (November 2025, Applied Physics Letters)

   We experimentally demonstrate the tunability of a topological defect mode and its spectral flow within the bandgap of a periodic structure made of beams and cylindrical masses and hosting a topological bandgap in its frequency spectrum. A defect is introduced by incorporating a pair of piezoelectric plates into a single unit cell of the lattice, giving rise to a localized mode inside the bandgap. We experimentally observe this mode and its spectral flow in the bandgap by altering the elastic modulus of the piezoelectric plates using negative capacitance shunt circuits. Further, through finite element simulations and experiments, we show that the spectral flow can be tuned by modulating the mass of the defect cell alone. This work introduces additional degrees of freedom in the design of elastic wave-based devices for applications in acoustic logic, waveguiding, and energy harvesting. 

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4. Coupled Resonator Acoustic Waveguides‐Based Acoustic Interferometers Designed within 2D Phononic Crystals: Experiment and Theory

   https://doi.org/10.1002/apxr.202300093  

   Martínez‐Esquivel, David; Méndez‐Sánchez, Rafael Alberto; Heo, Hyeonu; Martínez‐Argüello, Angel Marbel; Mayorga‐Rojas, Miguel; Neogi, Arup; Reyes‐Contreras, Delfino (March 2024, Advanced Physics Research)

   Abstract The acoustic response of defect‐based acoustic interferometer‐like designs, known as Coupled Resonator Acoustic Waveguides (CRAWs), in 2D phononic crystals (PnCs) is reported. The PnC is composed of steel cylinders arranged in a square lattice within a water matrix with defects induced by selectively removing cylinders to create Mach‐Zehnder‐like (MZ) defect‐based interferometers. Two defect‐based acoustic interferometers of MZ‐type are fabricated, one with arms oriented horizontally and another one with arms oriented diagonally, and their transmission features are experimentally characterized using ultrasonic spectroscopy. The experimental data are compared with finite element method (FEM) simulations and with tight‐binding (TB) calculations in which each defect is treated as a resonator coupled to its neighboring ones. Significantly, the results exhibit excellent agreement indicating the reliability of the proposed approach. This comprehensive match is of paramount importance for accurately predicting and optimizing resonant modes supported by defect arrays, thus enabling the tailoring of phononic structures and defect‐based waveguides to meet specific requirements. This successful implementation of FEM and TB calculations in investigating CRAWs systems within PnCs paves the way for designing advanced acoustic devices with desired functionalities for various practical applications, demonstrating the application of solid‐state electronics principles to underwater acoustic devices description. 

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5. The Role of Frequency and Impedance Contrasts in Bandgap Closing and Formation Patterns of Axially-Vibrating Phononic Crystals

   https://doi.org/10.1115/1.4063815  

   Al Ba’ba’a, Hasan B.; Nouh, Mostafa (March 2024, Journal of Applied Mechanics)

   Bandgaps, or frequency ranges of forbidden wave propagation, are a hallmark of phononic crystals (PnCs). Unlike their lattice counterparts, PnCs taking the form of continuous structures exhibit an infinite number of bandgaps of varying location, bandwidth, and distribution along the frequency spectrum. While these bandgaps are commonly predicted from benchmark tools such as the Bloch-wave theory, the conditions that dictate the patterns associated with bandgap symmetry, attenuation, or even closing in multi-bandgap PnCs remain an enigma. In this work, we establish these patterns in one-dimensional rods undergoing longitudinal motion via a canonical transfer-matrix-based approach. In doing so, we connect the conditions governing bandgap formation and closing to their physical origins in the context of the Bragg condition (for infinite media) and natural resonances (for finite counterparts). The developed framework uniquely characterizes individual bandgaps within a larger dispersion spectrum regardless of their parity (i.e., odd versus even bandgaps) or location (low versus high-frequency), by exploiting dimensionless constants of the PnC unit cell which quantify the different contrasts between its constitutive layers. These developments are detailed for a bi-layered PnC and then generalized for a PnC of any number of layers by increasing the model complexity. We envision this mathematical development to be a future standard for the realization of hierarchically structured PnCs with prescribed and finely tailored bandgap profiles. 

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