Brian O. Raeker

The ability to transmit wireless signals into the body is important for medical communication, sensing, and powering technologies. However, the efficiency of signal transmission is limited by the large reflection encountered at the interface between air and biological tissue. Here, we demonstrate an approach to overcome this reflection using cascaded metasurfaces (MSs) with deeply subwavelength thickness to transform the wave impedance at the air–tissue interface. We develop a procedure to systematically synthesize these MSs using a multilayer tissue model that can account for different body compositions. Using this procedure, we design a three-layer MS operating at 2.45 GHz with $\lambda /24.5$ thickness. Full-wave simulations in a computational human body model show that the MS provides transmission equivalent to replacing the background with a tissue-matched medium, while experimental measurements demonstrate 12.1 dB enhancement of transmission into a multilayer tissue phantom.

Optical metasurfaces, composed of subwavelength scattering elements, demonstrate remarkable control over the transmitted amplitude, phase, and polarization of light. However, manipulating the amplitude upon transmission, without the use of surface waves, requires loss if a single metasurface is used. Herein, high‐efficiency independent manipulation of the amplitude and phase of a beam is described using two lossless phase‐only metasurfaces separated by a distance. With this configuration, optical components such as combined beam‐forming and splitting devices are experimentally demonstrated, as well as those for forming complex‐valued, 3D holograms. The compound meta‐optic platform provides a promising approach for achieving high‐performance holographic displays and compact optical components, while exhibiting a high overall efficiency.

Lossless, reciprocal bianisotropic metasurfaces have the ability to control the amplitude, phase, and polarization of electromagnetic wavefronts. However, producing the responses that are necessary for achieving this control with physically realizable surfaces is a challenging task. Here, several design approaches for bianisotropic metasurfaces are reviewed that produce physically realizable metasurfaces using cascaded impedance sheets. In practice, three or four impedance sheets are often used to realize bianisotropic responses, which can result in narrowband designs that require the unit cells to be optimized in order to improve the performance of the metasurface. The notion of a metasurface quality factor is introduced for three-sheet metasurfaces to address these issues in a systematic manner. It is shown that the quality factor can be used to predict the bandwidth of a homogeneous metasurface, and it can also be used to locate problematic unit cells when designing inhomogeneous metasurfaces. Several design examples are provided to demonstrate the utility of the quality factor, including an impedance matching layer with maximal bandwidth and a gradient metasurface for plane wave refraction. In addition to these examples, several metasurfaces for polarization control are also reported, including an isotropic polarization rotator and an asymmetric circular polarizer.

Forming a desired optical-field distribution from a given source requires precise spatial control of a field’s amplitude and phase. Low-loss metasurfaces that allow extreme phase and polarization control of optical fields have been demonstrated over the past few years. However, metasurfaces that provide amplitude control have remained lossy, utilizing mechanisms such as reflection, absorption, or polarization loss to control amplitude. Here, we describe the amplitude and phase manipulation of optical fields without loss, by using two lossless phase-only metasurfaces separated by a distance. We first demonstrate a combined beam former and splitter optical component using this approach. Next, we show a high-quality computer-generated three-dimensional hologram. The proposed metaoptic platform combines the advantage of lossless complex-valued field control with a physically small thickness. This approach could lead to low-profile, three-dimensional holographic displays, compact optical components, and high precision optical tweezers for micro-particle manipulation.

Metasurfaces, composed of subwavelength scattering elements, have demonstrated remarkable control over the transmitted amplitude, phase, and polarization of light. However, manipulating the amplitude upon transmission has required loss if a single metasurface is used. Here, we describe high-efficiency independent manipulation of the amplitude and phase of a beam using two lossless phase-only metasurfaces separated by a distance. With this configuration, we experimentally demonstrate optical components such as combined beam-forming and splitting devices, as well as those for forming complex-valued, three-dimensional holograms. The compound meta-optic platform provides a promising approach for achieving high performance optical holographic displays and compact optical components, while exhibiting a high overall efficiency.

Spatial amplitude and phase control across an optical wavefront with high efficiency is demonstrated using all-dielectric paired-metasurface devices. Experimental results of meta-optics forming three-dimensional holograms are reported.

We demonstrate multi-metasurface devices with high efficiency that provide independent amplitude and phase control across an optical wavefront. Experimental results of metaoptics implementing a three-dimensional point source hologram are reported.

Layered structures are widely used in optics to control wave reflection and propagation at interfaces but their thickness relative to the wavelength hinders their application in the radio-frequency regime. Here, we design and experimentally demonstrate cascaded metasurfaces with deeply subwavelength thickness that provide both antireflection and wavefront manipulation. We describe the systematic synthesis of the metasurface layers from a prescribed scattering response and demonstrate designs capable of providing near-perfect microwave transmission through glass as well as antireflection focusing of wireless signals from air into water.

Lossless, reflectionless metasurfaces are generally limited to impressing phase shifts onto incident wavefronts, and can therefore only manipulate their phase profiles. However, two such metasurfaces placed along a common axis form a compound metaoptic, and can convert an incident complex-valued field to a desired complex-valued field. Here, we show that compound metaoptics at optical frequencies can form desired field patterns when the metasurfaces are separated by a few wavelengths, without relying on reflection, absorption, or polarization losses. With these properties, compound metaoptics are likely to find applications in complex-valued holography, camouflage, and forming custom near- and far-field radiation patterns.

We present a method to transform the radiation pattern of an antenna into an arbitrarily defined pattern through the use of an inhomogeneous spherical impedance metasurface that completely encloses the radiating source.We derive a general procedure to determine the sheet impedance distribution required to form the desired electric field image pattern. As an example, we demonstrate the ability to transform the radiation pattern of a dipole into the image of a nautical anchor in both the near field and far field. Full-wave simulation results provide good agreement with calculated expectations, confirming the ability to accurately form arbitrary radiation patterns using closed metasurfaces.