Casimir force searches provide among the most sensitive laboratory probes of new short range interactions. Existing constraints rely almost exclusively on a single geometry. We show that Casimir geometry constitutes an independent observable, as Yukawa-type interactions and Casimir background exhibit different geometric scaling for bulk forces and surface quantum effects. We derive the first constraints from sphere-sphere and plate-plate geometries, thereby completing the canonical set of Casimir geometries, obtaining the most stringent Casimir-based bounds for λ≲ 10^-8 m. Our results establish geometry as a new handle for systematic searches for short range forces.

Topological mechanics can exhibit localized softness through geometrically programmed unit cells, and a promising application of this property is impact mitigation. We experimentally demonstrate efficient impact mitigation achieved through localized softness in a topological lattice, where the lattices are fabricated via 3D printing of hinged structures, and their force responses under dynamic loading are measured. The introduced soft surface produces a plateau in the force-displacement response, thereby suppressing peak forces relative to the hard surface and enhancing energy absorption efficiency. These soft responses are consistent with the predicted zero-mode counts. Furthermore, the design relies solely on periodic structures rather than material and geometrical gradation, preserving modularity and simplifying fabrication. These results indicate that localized topological mechanics can effectively tune contact compliance and mitigate impact loads, offering an efficient strategy for impact attenuation.

Multi-band topological states enable robust and versatile wave manipulation across a variety of physical platforms. However, the emergence of multi-band topological states has relied on higher-frequency modes with complex spatial profiles, which constrains the realization of robust topological states due to fragile symmetry and pseudospin hybridization in these modes. Here, we show a general design principle for scalable multi-band topological states by replicating a robust fundamental topological mode in the frequency domain. By introducing hierarchical resonators as an internal degree of freedom into a quantum spin Hall-based lattice, multiple topological states emerge discretely in correspondence with the hierarchical levels while preserving the spatial profile of the fundamental mode at the host lattice. Implementing this design principle in a versatile microelectromechanical platform, we experimentally demonstrate that the fundamental and replicated topological modes propagate simultaneously in a single waveguide while suppressing mutual cross-talk. Our results establish topology replication as a universal strategy for designing multi-band topological systems and open routes toward multi-channel topological wave devices.

A broadband electromagnetic source is important for scientific and technological applications. Quantum vacuum fluctuations, which manifest most prominently in the Casimir effect, provide a fundamentally broadband electromagnetic source. Here we explore a potential consequence of the broadband nature of quantum vacuum fluctuations, by showing that such fluctuations can enable measurement of material permittivity over a broad frequency range. Specifically, we consider the Casimir force in a parallel-plate geometry, with one plate covered by a nanoscopic thin film. Using a machine learning approach, we show that one can infer both the thickness of the film and its permittivity over a broad frequency range, starting from the dependency of the Casimir forces on the spacing between the two plates. Our work highlights the application potential of using vacuum fluctuations as a naturally-existing broadband electromagnetic source for material characterization, and shows that the inverse problem in Casimir force calculation can be solved with machine learning.

We give a robust formulation to calculate the Casimir energy and Casimir force for plane-parallel multilayer setups with general dielectric constants. We derive recursion relations for multilayer reflection and transmission coefficients in the most general setups, which are essential ingredients for evaluating the Casimir energy. With the use of complex analysis techniques involving the argument principle, we carefully treat and subtract UV divergences and make clear the relation between the subtraction procedure and an actual physical setup. We also clarify the physical operational meaning of pole subtraction, which is required to utilize the argument principle in the calculation of the Casimir energy. Our formula is applicable to more general situations including chiral medium or Weyl semimetals.

Topological insulators have taken classical wave systems to the next level by their unique characteristics such as highly efficient, robust, and unidirectional wave propagation. As a step further, tunability in fundamental building blocks such as couplers is desired toward topological integrated circuits and communication systems. However, most of existing topologically protected couplers exhibit a constant transmission coefficient within the bandgap, and achieving continuously tunable transmission coefficients remains challenging. Here it is experimentally showed that a four-port coupler consisting of quantum valley Hall effect-based waveguides can exhibit continuously tunable transmission coefficients from one input port to each of the two output ports over the bandgap, while the rest port is consistently isolated by the topological protection. Further topologically protected digital signal transfer is demonstrated with frequency shift keying modulation in the coupler, revealing the data-transfer capability close to the theoretical limit established in the digital communication scheme. These results pave the way for topological data transfer and signal processing.

Predicting the position of a magnetic noise source in the kilohertz/megahertz range is of interest in academia and importance in industries. We consider a system consisting of one transmitting and four receiving coils around a frequency of 10 MHz. The transmitting coil is assumed to be a magnetic noise source that needs to be found in our study. We theoretically and experimentally show that the position of the transmitting coil can be predicted using Fano resonance, i.e., the four receiving coils are strongly coupled with each other, and weakly coupled to the transmitting coil via magnetic fields, by employing supervised machine learning. A coupled mode theory is built to elucidate such magnetic resonance coupling among coils. The characteristics of the system are investigated by using a method-of-moment based electromagnetic simulator, with sufficiently large volume of scattering parameters for different positions of the transmitting coil. Measured spectra of scattering parameters reasonably agree with analytical and numerical results. Experimental results reveal that the position of the transmitting coil is predicted in a range of 0.4 m to 2 m for the distance and ± 60º for the angle, with resolutions of 0.048 m and 8.8º, respectively.

We demonstrate the direction sensing of surface acoustic waves (SAWs) using a single subwavelength micropillar resonator by monitoring its displacements at multiple points on its top surface. Our findings reveal that the displacements of the micropillar depend on the direction of the SAWs. This directional dependence originates from interference of bending and longitudinal resonant modes. These modes are excited by SAWs through a rotational displacement field, where the vertical component induces longitudinal resonance and the horizontal component induces bending resonance. By selecting the appropriate micropillar geometry, the frequencies of these two modes can be aligned, enabling effective mode interference. This study enhances the fundamental understanding of the interaction between SAWs and resonators, offering a practical approach for monitoring SAWs in compact electronic systems.

The spin qubits of color centers are extensively investigated for quantum sensing, communication, and information processing, with their states generally controlled using lasers and microwaves. However, it is challenging to effectively irradiate both lasers and microwaves onto color centers using small-footprint microwave waveguides or antennas that are compatible with semiconductor devices. We show experimentally that, by introducing a compact coplanar waveguide with a thin slit in its signal line, effective irradiation of both lasers and microwaves is enabled, allowing spin-state control of color centers created around the slit. Microwave magnetic fields parallel to the surface, intrinsically generated by a standard coplanar waveguide, persist even after loading the slit, which is necessary to control the color centers whose spin quantization axes are oriented perpendicular to the surface, while laser light for the initialization and readout of spin states can access the color centers through the slit. Continuous and pulsed optically detected magnetic resonance measurements are performed for the silicon vacancies (VSi) in silicon carbide 4H-SiC(0001). Experimental results indicate that the spin states of VSi are effectively controlled by the microwave magnetic fields parallel to the surface, which agrees with numerical results from electromagnetic field simulations. Our small-footprint waveguide is suitable for integrating color-center-based quantum sensors into semiconductor electronic devices and other small-scale systems.

Reversing near-field thermal radiation between a rotating pair of hot and cold dipolar objects has recently been theoretically reported at low temperature. We demonstrate that such a reversal between two indium antimonide (InSb) nanoparticles occurs at lower rotation frequency at room temperature under an external magnetic field. Additionally, a nearby InSb substrate significantly relaxes the requirement of high rotation by acting as a heat sink and exciting surface modes that couple with particle resonances, both of which are tuned by the magnetic field. Our results provide a critical understanding about reversing near-field heat transfer between nanostructures with reduced rotation frequency, pointing to the possibility of experimental observation of heat reversal around room temperature.

Topological artificial crystals can exhibit one-way wave-propagation along the boundary with the wave being localized perpendicular to the boundary. The control of localization of such topological wave propagation is of great importance for enhancing coupling or avoiding unwanted coupling among neighboring boundaries toward topological integrated circuits. However, the effect of the geometry of topological boundaries on localization properties is not yet fully clear. Here, we experimentally and numerically demonstrate valley-topological transport on representative valley-topological boundaries with micro-electro-mechanical systems. We show that the zigzag and bridge boundaries, which have highly efficient wave transport, exhibit frequency independent and dependent wave localization, respectively. A simple analytic model is presented to capture the different behaviors of the two boundaries observed in the experiments. Our results provide opportunities to engineer frequency responses in topological circuits including frequency selective couplers through proper selection of boundary geometries.

Electromagnetic fields confined in thin photocatalyst layers in stacked slabs having periodic holes enable the enhancement of the photocatalytic activities.

The computation of Casimir forces, in principle, requires knowledge about the permittivity of the materials over a very wide range of frequencies. In particular, the permittivity at the high frequency limit should approach unity for the numerical integration to converge. In practice, however, the permittivities at high frequencies beyond a certain frequency cutoff, especially for many emerging materials, are not yet known experimentally or accounted for in existing models, which presents a challenge for the computation of Casimir forces for these material systems. Here we consider a scheme to overcome such a challenge by artificially adding extra Lorentz-Drude poles at frequencies beyond the frequency cutoff from measurements or existing models. We provide a systematic evaluation of the range of the validity of such a scheme by calculations of Casimir forces between parallel plates. We show that there is a distance scale inversely proportional to the frequency cutoff. The scheme of adding extra poles gives reliable results if the distances between the plates are larger than the distance scale. We validate our results by considering the cases of plasmonic materials and magnetic Weyl semimetals.

We show that square-root topological insulators can be utilized as a platform for dual-band topological waveguides in 2-dimensional photonic crystals. Valley states for each of two frequency band gaps in differently perturbed decorated honeycomb lattices have opposite directions of the phase rotations, revealing opportunities for dual-band unidirectional topological valley transport. The topological valley edge states exist at the boundary between differently perturbed photonic crystals in the two frequency bands. In addition, unidirectional, and robust propagation of electromagnetic waves along the boundary in the two frequency bands are numerically performed. Our work provides extensions of square-root topological insulators to robust, unidirectional, and dual-band photonic waveguides.

We demonstrate tunable thermal rectification enabled by near-field radiative heat transfer between two Weyl semimetal nanoparticles above graphene nanoribbons by rotating the particles and varying their loss factors. We show that, through the rotation, the thermal rectification, quantified by the ratio between the heat powers in the forward and backward directions, is significantly enhanced thanks to the nonre-ciprocal particles and the substrate that are more strongly coupled than the counterpart systems having reciprocal particles. We reveal that nonreciprocal particles induce spinning Poynting vectors, whose di-rections can be controlled by rotating the particles and can selectively couple to surface modes excited in the graphene nanoribbons. In addition, by optimizing the loss factor of the particles, the ratio is fur-ther improved. Interestingly, we find that when the rotation angles and the loss factor are appropriately chosen, thermal rectification disappears despite nonreciprocal materials in the system. Similarly, the rec-tification disappears when certain symmetry is satisfied in the system. Our results provide important understanding of nonreciprocal near-field radiative heat transfer in particle-substrate systems involving Weyl semimetals that offer multiple control knobs.& COPY; 2023 Elsevier Ltd. All rights reserved.

Motors arise as a heart of the mobility society, and wirelessly operated motors may improve our standard of living. Wireless power transfer in the kilohertz and megahertz range has been extensively explored, finding various potential applications in consumer electronics, electric vehicles, and medical implants. However, stable operation of wirelessly powered motors remains challenging due to voltage fluctuations for motors occurring in dynamic scenarios, e.g., the rotating speed of the motors is varied. Here, we theoretically and experimentally demonstrate the operation of a motor, where the power is wirelessly transferred via coils, is robust against the rotating speed by employing the analogy with non-Hermitian parity-time (PT) symmetry. In addition, our system is robust for misalignment of the coils. Our results open up opportunities for the robust operation of motors via wireless power transfer in dynamic scenarios towards autonomous vehicles.

We investigate the Casimir forces in a planar two-body system with each backed by a plate of perfect electric conductor. We show that for the equilibrium Casimir forces, the spectra of the Casmir pressure are symmetric in the wave-vector space for reciprocal systems, and become asymmetric when reciprocity is broken. Therefore, there is a distinct signature of reciprocity in the spectra of equilibrium Casimir forces. The same signature also holds for the lateral force, which is nonzero only in the nonequilibrium case. On the other hand, for the pressure in the nonequilibrium scenario, such an asymmetry can arise for both reciprocal and nonreciprocal cases. We also elucidate a relation on the Casmir force that is connected to Newton's third law. We show that Newton's third law holds for every frequency and wave vector, as long as no exchange of photons occurs between the two-body system and the environment. Newton's third law is distinct from the reciprocity constraints. We illustrate the theoretical results with numerical calculations described by anisotropic permittivity tensors that are either symmetric and hence reciprocal, or asymmetric and hence nonreciprocal.

Higher-order topological insulators are attracting attention from fundamental interest to fascinating applications, owing to the topological properties with higher-order topological corner states. Breathing kagome lattice is a prospective platform which can support higher-order topological corner states. Here, we experimentally demonstrate that higher-order topological corner states are supported in a breathing kagome lattice consisting of magnetically coupled resonant coils. The winding direction of each coil is determined to hold C3 symmetry for each triangle unit cell, enabling to emerge higher-order topological corner states. In addition, topological and trivial phases can be switched by changing the distances between the coils. The emergence of corner states in the topological phase is experimentally observed through admittance measurements. As an illustration, wireless power transfer is performed between the corner states, and between the bulk and corner states. The proposed configuration is a promising platform for not only investigating topological properties of the breathing kagome lattice but also an alternative mechanism of selective wireless power transfer.

The levitation of a metallic nanoflake has been experimentally demonstrated in liquid by manipulating the Casimir force with a proper choice of materials, and as a step further, dynamic control of the Casimir force acting on a nanoflake is of great importance. We show that the Casimir force acting on a nanoflake in ethanol can be dynamically controlled by adjusting the chemical potential of a graphene layer sandwiched by the Teflon film and the SiO2 substrate at a distance away from the nanoflake, where the nanoflake can consist of either monolayer graphene or solid material. We develop a harmonic oscillator model that captures the dynamics of the nanoflake observed in numerical calculations. The time constant in the dynamics for a graphene nanoflake is 3 to 6 times longer than that for a gold nanoflake. The upper limit of the zero-force position of the nanoflake is discussed. Our scheme, through the use of the tunable graphene layer, significantly extends the degrees of freedom for dynamic control of nanoflakes in liquid.

Non-Hermitian physical systems have enriched wave control abilities and dynamics of wave matter, especially around exceptional points (EPs). In this work, we theoretically, numerically, and experimentally studied an LR-shunted mechanical resonator for the observation of the exceptional point. The shunted resonator was composed of a mechanical resonator with a bonded piezoelectric patch and an LR-shunted circuit. A theoretical model for the LR-shunted resonator was first developed, and the exceptional point was identified from this theoretical model via a parameter space study. It was found that by varying the shunting circuit parameters, i.e., inductance and resistance, such a simple design supports the non-Hermitian degeneracy, namely the exceptional point. At the vicinity of the EP, the LR-shunted mechanical resonator had a square-root dependence on the external perturbation related to the nominal resistance. Next, we designed and fabricated a shunted resonator to verify our design through numerical analysis and experimental measurements. Moreover, our proposed LR-shunted resonator provides the possibility to dynamically encircle an exceptional point due to external stimuli. Our approach sheds light on the designs of chiral effect systems and dynamically tailoring losses in elasticity, and it enables alternative solutions for nondestructive structural health monitoring with enhanced sensitivity.