Controlling the Collective Behaviors of Ultrasound-Driven Nanomotors via Frequency Regulation.

Controlling the collective behavior of micro/nanomotors with ultrasound may enable new functionality in robotics, medicine, and other engineering disciplines. Currently, various collective behaviors of nanomotors, such as assembly, reconfiguration, and disassembly, have been explored by using acoustic fields with a fixed frequency, while regulating their collective behaviors by varying the ultrasound frequency still remains challenging. In this work, we designed an ultrasound manipulation methodology that allows nanomotors to exhibit different collective behaviors by regulating the applied ultrasound frequency. The experimental results and FEM simulations demonstrate that the secondary ultrasonic waves produced from the edge of the sample cell lead to the formation of complex acoustic pressure fields and microfluidic patterns, which causes these collective behaviors. This work has important implications for the design of artificial actuated nanomotors and optimize their performances.

Highly Efficient Magnetic Propulsion of NiFe Nanorod-Based Miniature Swimmers in Three Dimensions.

Magnetically actuated miniature robots have attracted the attention of the scientific community over the past two decades, but the confined workspace of their manipulation system (typically a tri-axial coil or eight electromagnetic coils) and the low efficiency of propulsion have limited their utility. Here, we describe a highly efficient NiFe nanorod-based magnetic miniature swimmer that can be manipulated in 3D spaces using two pairs of coils placed in the x-y horizontal plane. In the new swimmer, the shape symmetry is broken along its body, and the asymmetry in magnetizations is introduced perpendicular to the long axis of its body simultaneously. Such a combined asymmetry design offers favorable controllability in planar magnetic fields, which relaxes the multi-axial coil requirement of the commonly used manipulation system and thus reduces the restriction on the shape and size of the workspaces. The new swimmers display efficient 3D propulsion, with a speed of over 5000 µm s-1 (∼3 body length s-1) and powerful locomotion in biological media such as raw human blood. The fuel utilization efficiency of the swimmer, defined as the ratio of the distance to the net input work in one period, was estimated to be approximately from 10-2 to 10-3 m/J, which is significantly higher than that of magnetic motors with a slender body. Moreover, to provide practical support for further potential use, we demonstrated that the swimmer is able to perform incision operations as a minimally invasive microsurgical tool. Such a swimmer actuation strategy provides a simple and efficient way for 3D manipulation of magnetic miniature robots, offering great potential for future biomedical and other applications.

Augmented reality enabled Galilean sighting system with an adjustable reticle.

A sighting system based on the Galileo telescope structure generally suffers from the lack of a reticle, which poses great challenges to target aiming. In this work, a hybrid Galilean sighting system integrated with an augmented reality (AR) component is proposed, which produces an image of the reticle to serve as a reticle. The depth of the reticle can be adjusted flexibly to superpose with the image of the observed object. The designed system features a fourfold magnification, a large modulation transfer function (MTF)>0.3 at 40 lp/mm, and a small distortion less than 0.3% in the full field of view. The superb imaging performance combined with the targeting function of the AR reticle renders the proposed design a powerful tool in the applications of target tracking.

Magnetically Powered Shape-Transformable Liquid Metal Micromotors.

Shape-transformable liquid metal (LM) micromachines have attracted the attention of the scientific community over the past 5 years, but the inconvenience of transfer routes and the use of corrosive fuels have limited their potential applications. In this work, a shape-transformable LM micromotor that is fabricated by a simple, versatile ice-assisted transfer printing method is demonstrated, in which an ice layer is employed as a "sacrificial" substrate that can enable the direct transfer of LM micromotors to arbitrary target substrates conveniently. The resulting LM microswimmers display efficient propulsion of over 60 µm s-1 (≈3 bodylength s-1 ) under elliptically polarized magnetic fields, comparable to that of the common magnetic micro/nanomotors with rigid bodies. Moreover, these LM micromotors can undergo dramatic morphological transformation in an aqueous environment under the irradiation of an alternating magnetic field. The ability to transform the shape and efficiently propel LM microswimmers holds great promise for chemical sensing, controlled cargo transport, materials science, and even artificial intelligence in ways that are not possible with rigid-bodies microrobots.

Magnetically Powered Annelid-Worm-Like Microswimmers.

A bioinspired magnetically powered microswimmer is designed and experimentally demonstrated by mimicking the morphology of annelid worms. The structural parameters of the microswimmer, such as the surface wrinkling, can be controlled by applying prestrain on substrate for the precise fabrication and consistent performance of the microswimmers. The resulting annelid-worm-like microswimmers display efficient propulsion under an oscillating magnetic field, reaching a peak speed of ≈100 µm s-1 . The speed and directionality of the microswimmer can be readily controlled by changing the parameters of the field inputs. Additionally, it is demonstrated that the microswimmers are able to transport microparticles toward a predefined destination, although the translation velocity is inevitably reduced due to the additional hydrodynamic resistance of the microparticles. These annelid-worm-like microswimmers have excellent mobility, good maneuverability, and strong transport capacity, and they hold considerable promise for diverse biomedical, chemical sensing, and environmental applications.

Polarization-insensitive and wide-incident-angle optical absorber with periodically patterned graphene-dielectric arrays.

A polarization-insensitive and angle-independent graphene absorber (GA) with periodically patterned grating is demonstrated. A periodic nanocavity composed of multilayer subwavelength grating and metal substrate supports a strongly localized mode inside the cavity, where the mode helps to absorb more electromagnetic waves. The proposed GA exhibits polarization-insensitive behavior and maintains the high absorption above 90% within a wide range of incident angle (more than 80°). We attribute the high absorption to the excitation of the cavity mode resonance and magnetic resonance for the transverse electric and transverse magnetic polarizations, respectively. The proposed GA has potential applications in the design of various devices, such as optical modulators or tunable absorption filters because of its remarkable angle-insensitive absorption performance.

Angularly dense comb-like enhanced absorption of graphene monolayer with attenuated-total-reflection configuration.

A multiline absorber based on the excitation of guided-mode resonance of one-dimensional photonic crystals (1D-PhCs), including a surface graphene monolayer under the attenuated-total-reflection configuration, is proposed and demonstrated. By carefully designing the structure parameters of the 1D-PhCs, the guided mode can be modulated by the periodic distribution of the refractive index. Our results reveal that the critical coupling of the guided resonance in periodical PhCs to graphene produces the perfect absorption. The number of absorption peaks within the photonic band corresponds to the number of unit cells. An ultrahigh Q-factor value of 4.75×106 is obtained at resonance with unity absorption, which could serve as a promising replacement of metallic thin film as a sensor probe for future biosensing applications.

Broadband visible-light absorber via hybridization of propagating surface plasmon.

We demonstrate a broadband visible-light absorber based on excitation of multiple propagating surface plasmon (PSP) resonances. The simple structure is constructed of continuous gold/silica multi-layers covered by a one-dimensional gold grating. The broadening of bandwidth arises from the inter-layer hybridization and spectral superposition of PSPs, which is predicted with the analytical coupled oscillator model and validated using the RCWA simulation. The average absorption increases with the number of gold/silica pairs and exceeds 95% over the whole visible spectrum when only five pairs are included. Moreover, results show that the absorption can be further enhanced by grading the thickness of silica layers. The presented design might enable promising applications in the fields of photovoltaic cells and thermal emitters, owing to its advantages of wideband, near-unity absorption and simple fabrication simultaneously.

Angle-insensitive and narrow band grating filter with a gradient-index layer.

We demonstrate the design of an efficient angle-insensitive guided mode resonance filter (GMRF), with narrow bandwidth and low sideband reflection, for TE-polarized waves. The reflection properties of the multilayer structure have been studied, and the results verify that the thin film design of the gradient-index layer is important for the realization of an angle-insensitive filter. Various gradient coefficients of the thin film have distinct effects on the reflection spectrum. For an increasing incident angle, although the line-shape symmetry becomes less perfect, the positions of the resonant peak remain the same. The GMRF proposed here has many desirable attributes that lends itself to being an excellent platform, for devices such as lasers, detectors, filters, and sensors.

The ratio of the kinetic inductance to the geometric inductance: a key parameter for the frequency tuning of the THz semiconductor split-ring resonator.

By introducing the frequency tuning sensitivity, an analytical model based on equivalent LC circuit is developed for the relative frequency tuning range of THz semiconductor split-ring resonator (SRR). And the model reveals that the relative tuning range is determined by the ratio of the kinetic inductance to the geometric inductance (RKG). The results show that under the same carrier density variation, a larger RKG results in a larger relative tuning range. Based on this model, a stacked SRR-dimer structure with larger RKG compared to the single SRR due to the inductive coupling is proposed, which improves the relative tuning range effectively. And the results obtained by the simple analytical model agree well with the numerical FDTD results. The presented analytical model is robust and can be used to analyze the relative frequency tuning of other tunable THz devices.

Negative-index metamaterial at visible frequencies based on high order plasmon resonance.

A type of negative-index metamaterial composed of periodic arrays of SRRs is proposed and numerically investigated in the visible frequencies. Employing the high order magnetic resonance to induce negative permeability, negative refractive index is obtained between 395 THz and 430 THz with the maximum FOM=4.59. The effective permeability exhibits a rapid convergence with increasing number of metamaterial layers. Different responses from the electric and magnetic resonances to the changing geometric parameters are compared and analyzed in terms of the field distribution. Simulation results show that the high order magnetic resonance can be greatly enhanced at visible frequencies as well as effectively tuned over a wide spectral range without notably altering the coupling between unit cells.