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1. Static elastic cloaking, low-frequency elastic wave transparency and neutral inclusions

   https://doi.org/10.1098/rspa.2019.0725  

   Norris, Andrew N. ; Parnell, William J. ( August 2020 , Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences)

   null (Ed.)

   New connections between static elastic cloaking, low-frequency elastic wave scattering and neutral inclusions (NIs) are established in the context of two-dimensional elasticity. A cylindrical core surrounded by a cylindrical shell is embedded in a uniform elastic matrix. Given the core and matrix properties, we answer the questions of how to select the shell material such that (i) it acts as a static elastic cloak, and (ii) it eliminates low-frequency scattering of incident elastic waves. It is shown that static cloaking (i) requires an anisotropic shell, whereas scattering reduction (ii) can be satisfied more simply with isotropic materials. Implicit solutions for the shell material are obtained by considering the core–shell composite cylinder as a neutral elastic inclusion. Two types of NI are distinguished, weak and strong with the former equivalent to low-frequency transparency and the classical Christensen and Lo generalized self-consistent result for in-plane shear from 1979. Our introduction of the strong NI is an important extension of this result in that we show that standard anisotropic shells can act as perfect static cloaks, contrasting previous work that has employed ‘unphysical’ materials. The relationships between low-frequency transparency, static cloaking and NIs provide the material designer with options for achieving elastic cloaking in the quasi-static limit. 

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2. Inverse-Designed Metastructures Together with Reconfigurable Couplers to Compute Forward Scattering

   https://doi.org/10.1021/acsphotonics.2c00373  

   Nikkhah, Vahid ; Tzarouchis, Dimitrios C. ; Hoorfar, Ahmad ; Engheta, Nader ( September 2022 , ACS Photonics)

   Wave-based analog computing in the forms of inversedesigned metastructures and the meshes of Mach−Zehnder interferometers (MZI) have recently received considerable attention due to their capability in emulating linear operators, performing vector-matrix multiplication, inverting matrices, and solving integral and differential equations via electromagnetic wave interaction and manipulation in such structures. Here, we combine these two platforms to propose a wave-based metadevice that can compute scattered fields in electromagnetic forward scattering problems. The proposed device consists of two subsystems: a set of reconfigurable couplers with a proper feedback system and an inverse-designed inhomogeneous material block. The first subsystem computes the magnitude and phase of the dipole polarization induced in the scatterers when illuminated with a given incident wave (matrix inversion). The second subsystem computes the magnitude and phase of the scattered fields at given detection points (vector-matrix multiplication). We discuss the functionality of this metadevice, and through several examples, we theoretically evaluate its performance by comparing the simulation results of this device with fullwave numerical simulations and numerically evaluated matrix inversion. We also highlight that since the first section is reconfigurable, the proposed device can be used for different permittivity distributions of the scatterer and incident excitations without changing the inverse-designed section. Our proposed device may provide a versatile platform for rapid computation in various scattering scenarios. 

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3. Ultrasonic imaging in highly heterogeneous backgrounds

   https://doi.org/10.1098/rspa.2022.0721  

   Pourahmadian, F. ; Haddar, H. ( March 2023 , Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences)

   This work formally investigates the differential evolution indicators as a tool for ultrasonic tracking of elastic transformation and fracturing in randomly heterogeneous solids. Within the framework of periodic sensing, it is assumed that the background at time t ∘ contains (i) a multiply connected set of viscoelastic, anisotropic and piecewise homogeneous inclusions, and (ii) a union of possibly disjoint fractures and pores. The support, material properties and interfacial condition of scatterers in (i) and (ii) are unknown, while elastic constants of the matrix are provided. The domain undergoes progressive variations of arbitrary chemo-mechanical origins such that its geometric configuration and elastic properties at future times are distinct. At every sensing step t ∘ , t 1 , … , multi-modal incidents are generated by a set of boundary excitations, and the resulting scattered fields are captured over the observation surface. The test data are then used to construct a sequence of wavefront densities by solving the spectral scattering equation. The incident fields affiliated with distinct pairs of obtained wavefronts are analysed over the stationary and evolving scatterers for a suit of geometric and elastic evolution scenarios entailing both interfacial and volumetric transformations. The main theorem establishes the invariance of pertinent incident fields at the loci of static fractures and inclusions between a given pair of time steps, while certifying variation of the same fields over the modified regions. These results furnish a basisfor theoretical justification of differential evolution indicators for imaging in complex composites which, in turn, enable the exclusive tomography of evolution in a background endowed with many unknown features. 

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4. On the effects of suitably designed space microstructures in the propagation of waves in time modulated composites

   https://doi.org/10.1063/5.0132899  

   Mattei, O. ; Gulizzi, V. ( February 2023 , Applied Physics Letters)

   In the one-dimensional case, the amplitude of a pulse that propagates in a homogeneous material whose properties are instantaneously changed in time will undergo an exponential increase due to the interference between the reflected and transmitted pulses generated at each sudden switch. Here, we resolve the issue by designing suitable reciprocal PT-symmetric space-time microstructures so that the interference between the scattered waves is such that the overall amplitude of the wave will be constant in time in each constituent material. Remarkably, for the geometries proposed here, a pulse will propagate with constant amplitude regardless of the impedance between the constituent materials, and for some, regardless of the wave speed mismatch. We extend, then, these results to the two-dimensional case, by proposing suitable geometries that avoid the blow up of the wave amplitude at the source point due to the scattering associated with time modulation. Given that the energy associated with the wave will increase exponentially in time, this creates the possibility to exploit the stable propagation of the pulse to accumulate energy for harvesting. 

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5. Strategies to Enhance the Capability of Carrier Injection to the Effective Channel for Bottom-gated Amorphous Oxide Thin Films Transistors

   Liu, Mingyuan ; Kim, Hyeonghun ; Song, Han Wook ; Lee, Sunghwan ( November 2021 , Materials Research Society)

   Over the two decades, amorphous oxide semiconductors (AOSs) and their thin film transistor (TFT) channel application have been intensely explored to realize high performance, transparent and flexible displays due to their high field effect mobility (μFE=5-20 cm2/Vs), visible range optical transparency, and low temperature processability (25-300 °C).[1-2] The metastable amorphous phase is to be maintained during operation by the addition of Zn and additional third cation species (e.g., Ga, Hf, or Al) as an amorphous phase stabilizer.[3-5] To limit TFT off-state currents, a thin channel layer (10-20 nm) was employed for InZnO (IZO)-based TFTs, or third cations were added to suppress carrier generations in the TFT channel. To resolve bias stress-induced instabilities in TFT performance, approaches to employ defect passivation layers or enhance channel/dielectric interfacial compatibility were demonstrated.[6-7] Metallization contact is also a dominating factor that determines the performance of TFTs. Particularly, it has been reported that high electrical contact resistance significantly sacrifices drain bias applied to the channel, which leads to undesirable power loss during TFT operation and issues for the measurement of TFT field effect mobilities. [2, 8] However, only a few reports that suggest strategies to enhance contact behaviors are available in the literature. Furthermore, the previous approaches (1) require an additional fabrication complexity due to the use of additional treatments at relatively harsh conditions such as UV, plasma, or high temperatures, and (2) may lead to adverse effects on the channel material attributed to the chemical incompatibility between dissimilar materials, and exposures to harsh environments. Therefore, a simple and easy but effective buffer strategy, which does not require any additional process complexities and not sacrifice chemical compatibility, needs to be established to mitigate the contact issues and therefore achieve high performance and low power consumption AOS TFTs. The present study aims to demonstrate an approach utilizing an interfacial buffer layer, which is compositionally homogeneous to the channel to better align work functions between channel and metallization without a significant fabrication complexity and harsh treatment conditions. Photoelectron spectroscopic measurements reveal that the conducting IZO buffer, of which the work function (Φ) is 4.37 eV, relaxes a relatively large Φ difference between channel IZO (Φ=4.81 eV) and Ti (Φ=4.2-4.3 eV) metallization. The buffer is found to lower the energy barrier for charge carriers at the source to reach the effective channel region near the dielectric. In addition, the higher carrier density of the buffer and favorable chemical compatibility with the channel (compositionally the same) further contribute to a significant reduction in specific contact resistance as much as more than 2.5 orders of magnitude. The improved contact and carrier supply performance from the source to the channel lead to an enhanced field effect mobility of up to 56.49 cm2/Vs and a threshold voltage of 1.18 V, compared to 13.41 cm2/Vs and 7.44 V of IZO TFTs without a buffer. The present work is unique in that an approach to lower the potential barrier between the source and the effective channel region (located near the channel/dielectric interface, behaving similar to a buried-channel MOSFET [9]) by introducing a contact buffer layer that enhances the field effect mobility and facilitates carrier supply from the source to the effective channel region. 

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