Holograms, the optical devices to reconstruct predesigned images, show many applications in our daily life. However, applications of hologram are still limited by the constituent materials and therefore their working range is trapped at a particular electromagnetic region. In recent years, the metasurfaces, an array of subwavelength antenna with varying sizes, show the abilities to manipulate the phase of incident electromagnetic wave from visible to microwave frequencies. Here, we present a reflective-type and high-efficiency meta-hologram fabricated by metasurface for visible wavelength. Using gold cross nanoantennas as building blocks to construct our meta-hologram devices with thickness ∼ λ/4, the reconstructed images of meta-hologram show polarization-controlled dual images with high contrast, functioning for both coherent and incoherent light sources within a broad spectral range and under a wide range of incidence angles. The flexibility demonstrated here for our meta-hologram paves the road to a wide range of applications related to holographic images at arbitrary electromagnetic wave region.
There is a recognized and growing need for rapid and efficient cell assays, where the size of microfluidic devices lend themselves to the manipulation of cellular populations down to the single cell level. An exceptional way to analyze cells independently is to encapsulate them within aqueous droplets surrounded by an immiscible fluid, so that reagents and reaction products are contained within a controlled microenvironment. Most cell encapsulation work has focused on the development and use of passive methods, where droplets are produced continuously at high rates by pumping fluids from external pressure-driven reservoirs through defined microfluidic geometries. With limited exceptions, the number of cells encapsulated per droplet in these systems is dictated by Poisson statistics, reducing the proportion of droplets that contain the desired number of cells and thus the effective rate at which single cells can be encapsulated. Nevertheless, a number of recently developed actively-controlled droplet production methods present an alternative route to the production of droplets at similar rates and with the potential to improve the efficiency of single-cell encapsulation. In this critical review, we examine both passive and active methods for droplet production and explore how these can be used to deterministically and non-deterministically encapsulate cells.
Metamaterials have attracted intensive research interest in recent years because their optical properties have a strong dependence on the geometry of metamaterial molecules rather than the material composition. [1][2][3] This feature has inspired the creation and tailoring of exotic properties, such as a negative refractive index, [ 4 , 5 ] perfect absorption, [ 6 ] and super lensing, [ 7 , 8 ] which are not readily available in nature. For many practical applications such as data storage [ 9 ] and optical switching, [ 10 ] switchable metamaterials that possess very different states are almost a necessity. [ 11 ] Most of the tunable metamaterials that have been demonstrated rely on tuning constituent materials or changing surrounding media by introducing natural materials with higher tunability, such as liquid crystals and phase changing materials. [12][13][14][15][16][17][18][19] However, this limits the choices of materials and becomes increasingly diffi cult to implement at higher frequencies. Moreover, the tuning range is usually too limited to achieve a switching effect between strikingly different states.A complementary approach is to mechanically reconfi gure the metamaterial molecules. [ 20 , 21 ] Micromachining technology has been developed for fabrication and actuation of micromechanical devices [22][23][24][25][26] with switching frequencies up to the GHz level. [ 27 ] An attempt was made to adjust the distance between several planar metamaterial layers in which effi cient transmission change was achieved but the tuning originated from a change in the layer structure rather than a change in metamaterial molecule. [ 22 ] Recently, another interesting work demonstrated the modifi cation of the optical properties of a metamaterial by reorienting the metamaterial molecules. [ 23 ] Inspired by these prior studies, we report the concept and design of switchable magnetic metamaterials by directly reshaping the metamaterial molecules using the micromachining technology and present working devices with switchable magnetic responses.The schematic diagram of the switchable magnetic metamaterial is shown in Figure 1 a. Each metamaterial molecule consists of two semi-square split rings. One is anchored on the substrate while the other can be moved by micromachined actuators. As a result, the gap between the split rings can be altered and thus the geometric shape of the metamaterial molecule can be changed. Figure 1 b-d illustrates the two semi-square spit rings in different states. In Figure 1 b, the two split rings are separated by a small gap, resulting in a geometric shape "[]". This is a typical split ring resonator. [ 28 ] For simple notation, this state is called the open-ring state. Figure 1 c,d show two extreme cases. In the former, the gap between the two split rings is closed and the actual metamaterial molecule becomes a closed ring in the "ٗ" shape. This is called the closed-ring state. In the latter, the movable ring is moved away until it touches the back side of the fi xed ring in the next metama...
The toroidal dipole is a localized electromagnetic excitation independent from the familiar magnetic and electric dipoles. It corresponds to currents flowing along minor loops of a torus. Interference of radiating induced toroidal and electric dipoles leads to anapole, a nonradiating charge-current configuration. Interactions of induced toroidal dipoles with electromagnetic waves have recently been observed in artificial media at microwave, terahertz, and optical frequencies. Here, we demonstrate a quasi-planar plasmonic metamaterial, a combination of dumbbell aperture and vertical split-ring resonator, that exhibits transverse toroidal moment and resonant anapole behavior in the optical part of the spectrum upon excitation with a normally incident electromagnetic wave. Our results prove experimentally that toroidal modes and anapole modes can provide distinct and physically significant contributions to the absorption and dispersion of slabs of matter in the optical part of the spectrum in conventional transmission and reflection experiments.
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