Capillary forces arising during the evaporation of liquids from dense carbon nanotube arrays are used to reassemble the nanotubes into two-dimensional contiguous cellular foams. The stable nanotube foams can be elastically deformed, transferred to other substrates, or floated out to produce free-standing macroscopic fabrics. The lightweight cellular foams made of condensed nanotubes could have applications as shock-absorbent structural reinforcements and elastic membranes. The ability to control the length scale, orientation, and shape of the cellular structures and the simplicity of the assembly process make this a particularly attractive system for studying pattern formation in ordered media.C ellular patterns arise frequently in nature on length scales ranging from microscopic to macroscopic as a result of spatially periodic and random perturbations (1-5); examples range from the morphogenesis of embryos to patterns in coffee stains. A film of aligned carbon nanotubes represents a unique, yet unstudied type of system in which pattern formation could arise from the collapse and reassembly of highly ordered, anisotropic, elastic, nanoscale rods with remarkable properties. We report the creation of intriguing two-dimensional cellular foams by the evaporation of liquids from such nanotube films (6, 7). Shrinkage and crack formation in the films caused by strong capillary forces during evaporation and strong van der Waals interactions between condensed nanotubes (8) result in the formation of visually striking, stable cellular patterns and contiguous foams. Patterns formed by nanotube aggregates differ significantly from other polygonal crack patterns (9-13) because of the inherent dimensions, strength, and flexibility of the nanotubes (14, 15). The length scale, orientation, and shape of the cellular structures can be controlled by varying the nanotube height and the rate of evaporation of liquid and by patterning the nanotube array. The nanotube foams also can be floated out to produce free-standing macroscopic films. The outstanding properties of the constituent nanotubes may lead to applications for these structures as shock-absorbent reinforcements and in nanofiltration devices. Materials and MethodsFabrication of Multiwalled Nanotube Arrays. Vertically aligned multiwalled nanotube (MWNT) arrays (Fig. 1a) were grown on rigid silica substrates by using a chemical vapor deposition process (7) based on the decomposition of ferrocene and xylene. Patterned MWNT arrays were fabricated by patterning silica (SiO 2 ) on Si(100) (6) and exposing these patterned substrates to a mixture of ferrocene and xylene at 800°C. Nanotubes grow selectively on the patterned silica regions (6).Formation of Cellular Carbon Nanotube Foams. The aligned nanotube arrays were oxidized in an oxygen plasma created in a glow discharge chamber (Harrick Scientific, Ossining, NY) at room temperature and 0.6 torr (1 torr ϭ 133 Pa) pressure for Ϸ10 min. Characterization of the oxidized MWNTs by Raman spectroscopy confirmed the preservation of the...

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Planar optical elements that can manipulate the multidimensional physical parameters of light efficiently and compactly are highly sought after in modern optics and nanophotonics. In recent years, the geometric phase, induced by the photonic spin–orbit interaction, has attracted extensive attention for planar optics due to its powerful beam shaping capability. The geometric phase can usually be generated via inhomogeneous anisotropic materials, among which liquid crystals (LCs) have been a focus. Their pronounced optical properties and controllable and stimuli‐responsive self‐assembly behavior introduce new possibilities for LCs beyond traditional panel displays. Recent advances in LC‐mediated geometric phase planar optics are briefly reviewed. First, several recently developed photopatterning techniques are presented, enabling the accurate fabrication of complicated LC microstructures. Subsequently, nematic LC‐based transmissive planar optical elements and chiral LC‐based broadband reflective elements are reviewed systematically. Versatile functionalities are revealed, from conventional beam steering and focusing, to advanced structuring. Combining the geometric phase with structured LC materials offers a satisfactory platform for planar optics with desired functionalities and drastically extends exceptional applications of ordered soft matter. Some prospects on this rapidly advancing field are also provided.

Liquid-crystal fork gratings are demonstrated through photopatterning realized on a DMD-based microlithography system. This supplies a new strategy for generating fast switchable, reconfigurable, wavelength-tolerant and polarization-insensitive optical vortices. The technique has great potential in broad fields such as OAM-based quantum computations, optical communications, and micromanipulation.

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Metasurfaces provide a compact and powerful platform for manipulating the fundamental properties of light, and have shown unprecedented capabilities in both optical holographic display and information encryption. For increasing information display/storage capacity, metasurfaces with more polarization manipulation channel and full‐color holographic functionality are now an urgent requirement. Here, a minimalist dielectric metasurface with the capability of full‐color holography encoded with arbitrary polarization is proposed and experimentally demonstrated. Without the daunting exploratory and computational problem in nanostructure searching, full‐color holographic images can be multiplexed into arbitrary polarization channels through vectorial ptychography and k‐space ptychography based on tetratomic macropixel geometric phase metasurfaces. Thanks to the full degree of freedom tuning in polarization and color spaces, the application scenarios such as holographic 3D imaging and information encryption are realized. The strategy exhibits promising potential in applications of 3Dl display, augmented/virtual reality, high‐density data storage, and encryption.

Geometric phases have attracted considerable attention in recent years, due to their capability of arbitrary beam shaping in a most efficient and compact way, while traditional geometric phases are usually limited to handling single-structured beams and lack the capability of parallel manipulation. Here, we propose a digitalized geometric phase enabling parallel optical spin and orbital angular momentum encoding. The concept is demonstrated in inhomogeneous anisotropic media by imprinting a particularly designed binary phase into a space-variant geometric phase. We theoretically analyze its spin−orbit interaction of light and experimentally created higher-order Poincareś phere beam lattices, the order number and symmetry of which can be flexibly manipulated. Special lattices of cylindrical vector beams and orbital angular momentum modes with square and hexagonal symmetry are presented. This work discloses a new insight in programming geometric phases for tailoring the optical field and inspires various photonics applications.