Recent advances in materials, manufacturing, biotechnology, and microelectromechanical systems (MEMS) have fostered many exciting biosensors and bioactuators that are based on biocompatible piezoelectric materials. These biodevices can be safely integrated with biological systems for applications such as sensing biological forces, stimulating tissue growth and healing, as well as diagnosing medical problems. Herein, the principles, applications, future opportunities, and challenges of piezoelectric biomaterials for medical uses are reviewed thoroughly. Modern piezoelectric biosensors/bioactuators are developed with new materials and advanced methods in microfabrication/encapsulation to avoid the toxicity of conventional lead‐based piezoelectric materials. Intriguingly, some piezoelectric materials are biodegradable in nature, which eliminates the need for invasive implant extraction. Together, these advancements in the field of piezoelectric materials and microsystems can spark a new age in the field of medicine.

Dielectric-resonator-based nanophotonic devices show promise owing to their low intrinsic losses, support of multipolar resonances, and efficient operation in both reflection and transmission configurations. A key challenge is to make such devices dynamically switchable, such that optical behavior can be instantaneously reconfigured. In this work we experimentally demonstrate large, broadband, and continuous electrical tuning of reflection resonances in hybrid dielectric–VO2 devices. Our calculations, in strong agreement with experimental reflectance measurements, also indicate the presence of large transmission and absorption modulation. We additionally demonstrate independent modulation of both reflection amplitude and phase at Fabry–Pérot anti-nodes and nodes, respectively, a key requirement for metasurface design. We conclude with a temporal characterization, in which we achieve rapid electronic modulation rates of approximately 3 kHz, substantially faster than other recent approaches. These findings greatly expand the potential of designing nanophotonic devices that exploit the tunable behavior of hybrid dielectric–VO2 resonators.

materials which show near-zero thermal expansion, 3D materials are expected to have a larger thermal expansion coefficient (TEC). [9,10] The large thermoelectric power factor, [11] high carrier mobility, [12] and resistance against oxidation, [13] similarly make Cd 3 As 2 a superb candidate for optoelectronic applications. [7,8,[14][15][16] From a basic scientific perspective, Cd 3 As 2 is an excellent playground for studying exotic and nontrivial topological phases of matter and intriguing condensed matter phenomena, such as linear quantum magnetoresistance, [17] and chiral anomaly. [18,19] Thermoelectric signatures of the chiral anomaly in Cd 3 As 2 has recently been reported. [20] Additionally, Cd 3 As 2 has recently been exploited to realize a Weyl semimetal state. [16][17][18] To get a Weyl semimetal phase, either time reversal or inversion symmetry needs to be broken. Unlike graphene, the Dirac points in the Brillouin zone are protected by the point group (C 4 symmetry for Cd 3 As 2 ) and cannot be gapped via spin-orbit coupling (SOC).Interesting properties of this Dirac material system have been revealed via a variety of experimental techniques, such as surface tunneling microscopy (STM) to determine the Lifshitz gap energy, [15] angle-resolved photoemission spectroscopy (ARPES), and transport measurements to study the linear dispersion, [21] and time-resolved optical pump and terahertz (THz) probe spectroscopy to examine the relaxation dynamics of photoexcited particles. [22] To date, many tunable optical and thermal properties of Cd 3 As 2 remain unexplored. Moreover, characterizing the dynamics of charge carriers subjected to electromagnetic perturbation is essential for the fundamental physics of optical excitations. [23,24] For future photonic, optoelectronic, and thermoelectric Cd 3 As 2 devices, knowledge of the thermo-optic coefficient (TOC), TEC, and the carrier transport under different operating temperatures is crucial.In this paper, we use infrared (IR) spectroscopy to investigate the thermo-optic properties of Cd 3 As 2 and demonstrate large optical tunability in the mid-and far-IR regions. IR and THz spectroscopy are robust techniques for characterizing optical and electronic properties of Dirac and Weyl semimetals. [25][26][27][28][29] For example, optical spectroscopies of Dirac and Weyl semimetals has been used to study the excited transient excitonic instability [23] and chiral anomaly. [19] Through IR spectroscopy, we demonstrate large thermo-optic tuning of the Cd 3 As 2 permittivity. Our modeling of these results supports the In this paper, a detailed analysis of the temperature-dependent optical properties of epitaxially grown cadmium arsenide (Cd 3 As 2 ), a newly discovered 3D Dirac semimetal is reported. Fermi level tuning-instigated from Pauli-blocking in the linear Dirac cone-and varying Drude response, generate large variations in the mid-and far-infrared optical properties. Thermo-optic shifts larger than those of traditional III-V semiconductors are demonstrated. Electron scat...

Metasurfaces are two-dimensional nanostructures that allow unprecedented control of light through engineering the amplitude, phase, and polarization across meta-atom resonators. Adding tunability to metasurface components would boost their potential and unlock a vast array of new application possibilities such as dynamic beam steering, tunable metalenses, and reconfigurable meta-holograms, to name a few. We present here high-index meta-atoms, resonators, and metasurfaces reconfigured by thermal effects, across the near to mid-infrared spectral ranges. We study thermal tunability in group IV and group IV-VI semiconductors, as well as in phase-transition materials, and demonstrate large dynamic resonance frequency shifts accompanied by significant amplitude and phase modulation in metasurfaces and resonators. We highlight the importance of high-Q resonances along with peak performance of thermal and thermo-optic effects, for efficient and practical reconfigurable devices. This paper paves the way to efficient high-Q reconfigurable and active infrared metadevices.

Precisely tailored plasmonic substrates can provide a platform for a variety of enhanced plasmonic applications in sensing and imaging. Despite the significant advances made in plasmonics, most plasmonic devices suffer critically from intrinsic absorption losses at optical frequencies, fatally restricting their efficiency. Here, we describe and engineer plasmonic substrates based on metal-insulator-metal (MIM) plasmon resonances with ultra-sharp optical transmission responses. Due to their sharp transmission spectrum, the proposed substrates can be utilized for high quality (Q)-factor multi-functional plasmonic applications. Analytical and numerical methods are exploited to investigate the optical properties of the substrates. The optical response of the substrate can be tuned by adjusting the periodicity of the nanograting patterned on the substrate. Fabricated substrates present Q-factors as high as ∼40 and refractive index sensing of the surrounding medium as high as 1245 nm/RIU. Our results indicate that by engineering the substrate geometry, the dielectric thickness and incident angle, the radiation losses can be greatly diminished, thus enabling the design of plasmonic substrates with large Q factor and strong sensitivity to the environment.

A facile and straightforward strategy was developed to synthesize hierarchical lotus leaf-like mesoporous silica material (LLMSM) with unique bilayer and hollow sandwich-like folds using hexadecylamine as a structure-directing agent, high molecular weight polyvinylpyrrolidone as a structure-stabilizing agent, and tetraethyl orthosilicate as a silica precursor in the mixed solvents of water and ethanol. The formation mechanism of LLMSM was proposed based on a series of experimental investigations. This new particulate platform exhibited superior adsorption capacity toward organic dye, effective catalytic reduction of 4-nitrophenol as a Ag catalyst supporter, and a multistage sustained drug release profile. Many other applications can be also envisaged for such a hierarchical structure.

Recently, Cd3As2 has attracted intensive research interest as an archetypical Dirac semimetal, hosting three dimensional linear-dispersive electronic bands near the Fermi level. Previous studies have shown that single-crystalline Cd3As2 has an anomalously low lattice thermal conductivity, ranging from 0.3 W/mK to 0.7 W/mK at 300 K, which has been attributed to point defects. In this work, we combine first-principles lattice dynamics calculations and temperature-dependent highresolution Raman spectroscopy of high-quality single-crystal thin films grown by molecular beam epitaxy to reveal the existence of a group of soft optical phonon modes at the Brillouin zone center of Cd3As2. These soft phonon modes significantly increase the scattering phase space of heatcarrying acoustic phonons and are the origin of the low lattice thermal conductivity of Cd3As2. Furthermore, we show that the interplay between the phonon-phonon Umklapp scattering rates and the soft optical phonon frequency explains the unusual non-monotonic temperature dependence of the lattice thermal conductivity of Cd3As2. Our results further suggest that the soft phonon modes are potentially induced by a Kohn anomaly associated with the Dirac nodes, in analogy to similar, nonetheless weaker, effects in graphene and Weyl semimetals.

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