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The formation and control of surface creases in soft materials under compression have intrigued the mechanics community for decades and recently found many applications in tissue biomechanics, soft robotics and tunable devices. In spite of a rapidly growing literature in this field, existing methods of analysis often rely on a presumed crease configuration and consequently there is still a lack of profound theoretical understanding on crease nucleation. In this study, we propose a force based perturbation approach to predicting the occurrence of crease nucleation without assuming a post-instability configuration. In a set of carefully controlled FEM simulations, by considering the relative magnitudes among the element size, perturbation displacement and sample size, we find that beyond a critical strain around , a flat surface under uniform deformation becomes metastable, while the creased configuration becomes stable, with energy barrier for creasing proportional to the square of the FEM element size and therefore vanishing in the continuum limit. Beyond the Biot critical strain of , the uniformly deformed configuration of a flat surface becomes unstable. Our force-based instability criterion also enabled us to determine the critical conditions of crease formation for different materials under general loading conditions, leading to a set of crease diagrams. Interestingly, it is shown theoretically and validated experimentally that some highly compressible soft materials do not undergo creasing under loading conditions close to equibiaxial compression.

MATERIALS FOR HUMANITY (MH 21)

A perturbation force based approach to creasing instability in soft materials under general loading conditions

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MH 21 is arranged around 6 themes that focus attention on pressing needs that should be addressed to enhance the well-being of communities and provide a forum for presenting breakthrough discoveries.

technical paper

Light is a ubiquitous renewable energy resource that nature has implemented as the foundational power supply for biomass generation on Earth. Human society is also built upon an intense reliance on chemical demands for our food, materials, and pharmaceuticals; yet, the chemical industry has evolved to power those industrial processes with energy derived from the combustion of fossil fuels. While the chemical industry is a cornerstone of the modern world, it also exists as one of the most polluting sectors on the planet. Is it possible to develop novel nanophotonic materials that effectively harness optical energy to power chemical processes and transition to a sustainable energy supply of the global chemical industry? In this talk, I will detail recent efforts in the development of plasmonic photocatalysts. These nanophotonic materials have demonstrated unique pathways to activate and engage chemical bond formation when illuminated with visible light, primarily due to non-thermal contributions to chemical processes at interfaces. The talk will introduce fundamental concepts in the rational design of plasmonic catalysts, highlight several important demonstrations of plasmonic photocatalysts in reactions key for global sustainability (e.g., H2 generation and greenhouse gas mitigation), and describe recent developments in optically coupled transmission electron microscopy to reveal the influence of light on atomic-scale phase transformations. Important attention will be paid to describe work where the influence of light results in unique mechanistic pathways for chemical reactions, particularly hydrogenation chemistry and nanoscale phase transformations. Finally, this talk will conclude with an outlook on the future of nanophotonic chemical processes in the next decade.

Making and Breaking Chemical Bonds with Light

Meta-lenses consist of a large number of optical nano-antennas which are capable of focusing the incoming wavefront of light [1-6]. We use a 60 × 60 dielectric achromatic meta-lens array to capture multidimensional optical information. The highest efficiency can be up to 74% at a wavelength of 420 nm, while the average efficiency is approximately 39% over the whole working bandwidth. The light field images and the depth information of objects can be determined by reorganizing the patches of sub-images and calculating the disparity of neighbor sub-images, respectively. The depth information can be used to optimize the patch sizes to render the all-in-focus images without artifacts. The smallest feature of objects that could be resolved in our system is 1.95 μm under the incoherent white light. Our work provides several advantages associated with light field imaging: elimination of chromatic aberration, polarization selectivity and compatibility of the semiconductor process. Considering the flexibility, the achromatic multiplexed meta-lens array with integrated functionalities may be promising for multi-focusing microscopy, high-dimension quantum technology, hyperspectral microscopy, micro robotic vision, autonomous sensing, virtual and augmented reality (VR and AR), drones, and miniature personal security systems [7-10].
References: 
[1] S. M. Wang, et al., Nature Comm. 8, 187 (2017). 
[2] B. H. Chen, et al., Nature Nanotechnology 13, 227 (2018). 
[3] V.-C. Su, et al., Adv. Optical Mater. 6, 1800554 (2018). 
[4] H.‐H. Hsiao, et al., Adv. Optical Mater. 6, 1800031 (2018).
[5] R. J. Lin, et al., Nature Nanotechnology, 14, 227 (2019). 
[6] L. Li, et al., Science 368, 1487–1490 (2020).
[7] G. Qu, et al., Nature Communications 11, 5484 (2020).
[8] M. K. Chen, et al., Advanced Optical Materials 4, 202001414 (2021). 
Acknowledgment: This work was supported by RGC Areas of Excellence Scheme 2020/21 (AoE/P-502/20).


Meta-optics: Meta-lens for imaging, sensing, and quantum chip

Coherent optical excitations in van der Waals materials (2D polaritons) span a large range of optical phenomena which arise from their extraordinary dispersion relations [1]. Among the different polaritons, it is particularly interesting to examine polaritons in hyperbolic materials (hyperbolic phonon-polaritons, hPhP) which show rich physical behavior, ranging from negative refraction and subdiffraction imaging. The phononic resonance creates a dispersion relation that contains multiple branches, which were shown to be tunable by the 2D material geometry, thickness, and surrounding environment [2], reaching relatively low losses in room temperature [3].
Among the various experimental techniques that are used in the field of 2D polaritons, scanning nearfield optical microscopy (SNOM) and its variants dominates the direct nearfield imaging of 2D polaritons. Specifically, recent advances in time-resolved SNOM allowed adressing the polariton’s dynamical properties as group velocity from the changes of interference of scattered polaritons with different time delays [4]. However, this interferometric technique cannot image the wavepacket dynamics, as it excludes the phase difference between different wavelengths. Different imaging techniques such as electron energy loss spectroscopy (EELS), where the 2D polariton dispersion was recently extracted [5], obtains only time-independent information on the polaritonic modes. 
Here we present how an ultrafast transmission electron microscope (UTEM), driven by femtosecond mid-infrared pulses, can probe the spatiotemporal dynamics of 2D wavepackets.
We record how the mid-infrared pulses excites hPhP wavepacket and measure the wavepacket's trajectory inside the sample, revealing unforseen physical behaviors: multi-branch wavepacket splitting and wavepacket acceleration and deceleration. These phenomena are especially surprising since conventional wavepackets propagate in a fixed group velocity. We show that it is the unique dispersion relations of the hyperbolic PhPs that facilitates these phenomena, while recording group velocities between c/45 to c/850. These results examplify the wide physical spatiotemporal dynamical phenomena that can be probed through a non-destructive measurement using the unique combination of femtosecond temporal and nanometer spatial resolution of the UTEM.

Refs:
[1] D. N. Basov, A. Asenjo-Garcia, P. J. Schuck, X. Zhu, A. Rubio, Polariton panorama. Nanophotonics 10, 549–577 (2020).
[2] S. Dai et al. Tunable Phonon Polaritons in Atomically Thin van der Waals Crystals of Boron Nitride. Science. 343, 1125–1129 (2014).
[3] A. J. Giles et al. Ultralow-loss polaritons in isotopically pure boron nitride. Nat. Mater. 17, 134–139 (2018).
[4] E. Yoxall et al. Direct observation of ultraslow hyperbolic polariton propagation with negative phase velocity. Nat. Photonics. 9, 674–678 (2015).
[5] N. Li et al. Direct observation of highly confined phonon polaritons in suspended monolayer hexagonal boron nitride. Nat. Mater. 20, 43–48 (2021).

Imaging the dynamics of 2D polariton wavepackets

We will discuss research on generating a wide range of colors by nanostructuring materials that are colorless in bulk. Unlike pigments and dyes that have specific colors due to their chemical composition, we generate colors by precise nanopatterning structures using two-photon polymerization (TPL) and electron beam lithography (EBL), shaping them into a variety on geometry out of metals, semiconductors, stacks of metal/dielectrics, and most recently low refractive-index polymers. For instance, we introduce a heat-shrinking method to produce 3D-printed photonic crystals with a 5x reduction in lattice constants, achieving sub-100-nm features exhibiting a full range of colors. We recently demonstrate four-dimensional (4D) printing of shape memory polymer (SMP) that imparts time responsive properties to 3D structures. A new SMP photoresist based on Vero Clear was used achieving print features at a resolution of~ 300 nm half pitch using TPL. Prints consisting of grids with size-tunable multi-colours enabled the study of shape memory effects to achieve large visual shifts through nanoscale structure deformation. These structural colors could have an impact on humanity by providing potentially alternative solutions to spectral control in sensors to decorative colors in consumer products.

Colorless Materials For Humanity’s Color Needs

Unidirectional magnetoresistance (UMR) is a new type of MR effect, which is strongly related to the spin-orbit coupling (SOC) in various types of magnetic heterostructures, consisting of either conventional transition heavy metals or emergent materials systems such as topological insulators (TIs). UMR differs from other well-reported spin-dependent MR effects, such as the spin Hall magnetoresistance (SMR), that the resistance is current direction dependent. Although some seminal works have proposed its origin, especially for the UMR in normal metal/ferromagnetic metal (NM/FM) and TI/FM bilayer systems, the physical mechanisms that can lead to UMR still remain elusive. In this presentation, I will first introduce some of the common features and measurement protocols for UMR. Then I will discuss the possible scenarios that can lead to a sizable spin-dependent UMR in magnetic heterostructures: the spin Hall effect (SHE), Rashba-Edelstein effect (REE), and topological surface states (TSSs). Employing UMR for fast-track spin-orbit torque (SOT) characterization in a pump-probe fashion will also be demonstrated.

Unidirectional Spin-Dependent Magnetoresistance: Physics and Applications

Spin-orbit interaction (SOI) has been playing one of the most pivotal roles in condensed matter physics. Especially, the role of the SOI in spintronics and spin-orbitronics is quite significant and much effort has been paid to explore novel physics related with SOI in these research fields. 
Although a magnitude of the SOI of solids has been thought to be inherently determined and unchanged, our finding of gate-tunable SOI, resulting in control of the inverse spin Hall effect, opened a new field of gate-tuneable SOI [1,2]. Furthermore, we have recently discovered that a strong electric field application to a solid with inherently small SOI enables creation of synthetic Rashba fields and gate-tunability of them [3]. 
In this presentation, the recently-found gate modulation of SOI in solids will be discussed. 

References:
[1] S. Dushenko, M. Shiraishi et al., Nature Communications 9, 3118 (2018). 
[2] S. Yoshitake, M. Shiraishi et al., Appl. Phys. Lett. 117, 092406 (2020). 
[3] S. Lee, M. Shiraishi et al., to appear. 


Gate modulation of spin-orbit interaction in solids

Two-dimensional magnets based on van der Waals materials are currently fostering great expectations for the advancement of spin-orbitronics, which aims to exploit the spin-orbit coupling in non-centrosymmetric magnetic heterostructures to enable current-driven magnetic torques and stabilize homochiral magnetic textures. Van der Waals magnets are particularly appealing for this purpose as their properties can be tuned by surface engineering at the atomic level. In this talk, I will present our investigation of the spin-orbitronics properties of selected highly promising van der Waals candidates, both in pristine and Janus configurations, from first principles and effective tight-binding models. I will first discuss the spin-orbit torque that emerges from inversion symmetry breaking in magnetic transition metal dichalcogenide monolayers [1] and demonstrate that sizable torques can be obtained, whose magnitude is controlled by the electric dipole due to the chalcogen elements. Most importantly, I will show the existence of a unique spin-orbit torque component that allows for field-free current-driven switching, of highest interest for applications. Then, I will discuss the nature of Dzyaloshinskii-Moriya interaction in these systems, and how they can stabilize magnetic skyrmions, but also magnetic bimerons depending on the magnetic anisotropy properties [2]. Finally, I will discuss the spin-orbitronics properties of Fe3Ge2Te2 and show that in spite of its high symmetry, its inherent mirror symmetry breaking enables the onset of an in-plane Dzyaloshinskii-Moriya interaction that can stabilize planar chiral textures [3].

[1] Smaili et al., arXiv:2007.07579
[2] Laref et al., arXiv:2011.07813 
[3] Laref et al., Physical Review B 102, 060402(R) (2020)


Theory of spin-orbit torque and Dzyaloshinskii-Moriya interaction in van der Walls magnets

The discovery of the spin Hall effect (SHE) enabled the efficient generation and manipulation of the spin current. The magnetic spin Hall effect provides a unique opportunity to control the spin current and relevant device performance with controllable magnetization. In this talk, we report the magnetic spin Hall effect both in non-collinear antiferromagnet Mn3Pt and a collinear antiferromagnet Mn2Au. We generate tiny out-of-plane polarized spin current when the charge current is applied along the axis perpendicular to the magnetic mirror plane of Mn3Pt/permalloy bilayers, but robust when the current is parallel to the magnetic mirror plane in all of the Mn3Pt films with different orientations. In Mn2Au, the spin currents are generated at two spin sublattices with broken spatial symmetry, and the antiparallel antiferromagnetic moments play an important role. Therefore, we term the Néel vector-dependent spin Hal effect the ‘antiferromagnetic spin Hall effect’. The out-of-plane spins from the antiferromagnetic spin Hall effect are favourable for the efficient switching of perpendicular magnetized devices, which is required for high-density applications. The antiferromagnetic spin Hall effect adds another twist to the atomic-level control of spin currents via the antiferromagnetic spin structure.

Antiferromagnetic spin Hall effect

In this presentation, microscopic structures and magnetic properties of the Fe5−xGeTe2 single crystal, recently discovered as a promising van der Waals (vdW) ferromagnet, are introduced. Our study demonstrate that the √3 × √3 superstructures have two different phases due to the symmetry of Fe(1) ordering. Intriguingly, the observed √3 × √3 ordering breaks the inversion symmetry of crystal, resulting in substantial antisymmetric exchange interaction. The temperature dependence of magnetization reveals a sharp magnetic anomaly suggesting helical magnetism of the Fe5−xGeTe2 due to its non‐centrosymmetricity. Analytical study also supports that the observed ordering can give rise to the helimagnetism. The work will provide essential information to understand the complex magnetic properties of the new vdW ferromagnet, Fe5−xGeTe2 for future topology‐based spin devices.

Helimagnetism of the Fe5GeTe2 crystal

The rising demand for computing power driven by real-world applications is driving significant research and development effort into discovery of new device technologies (e.g., mem-transistor, spintronics, etc.) that better meet the requirements of novel computing paradigms (e.g., bio-inspired neural networks, Ising machines, etc.). In this talk, we will present an overview of our research in exploring emerging non-volatile computing device technologies for novel computing paradigms from a devices-to-algorithms perspective. We will be highlighting some of the research gaps that need to be closed to enable architecture-level simulations that better capture the effects of stochasticity in emerging device technologies, and discussing the exploitation of physical phenomena in novel device technologies for enabling alternative computing paradigms.

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