Photonic metamaterials are man-made structures composed of tailored micro- or nanostructured metallodielectric subwavelength building blocks. This deceptively simple yet powerful concept allows the realization of many new and unusual optical properties, such as magnetism at optical frequencies, negative refractive index, large positive refractive index, zero reflection through impedance matching, perfect absorption, giant circular dichroism and enhanced nonlinear optical properties. Possible applications of metamaterials include ultrahigh-resolution imaging systems, compact polarization optics and cloaking devices. This Review describes recent progress in the fabrication of three-dimensional metamaterial structures and discusses some of the remaining challenges.

Past achievements and future challenges in the development of three-dimensional photonic metamaterials

Passive radiative cooling draws heat from surfaces and radiates it into space as infrared radiation to which the atmosphere is transparent. However, the energy density mismatch between solar irradiance and the low infrared radiation flux from a near-ambient-temperature surface requires materials that strongly emit thermal energy and barely absorb sunlight. We embedded resonant polar dielectric microspheres randomly in a polymeric matrix, resulting in a metamaterial that is fully transparent to the solar spectrum while having an infrared emissivity greater than 0.93 across the atmospheric window. When backed with a silver coating, the metamaterial shows a noontime radiative cooling power of 93 watts per square meter under direct sunshine. More critically, we demonstrated high-throughput, economical roll-to-roll manufacturing of the metamaterial, which is vital for promoting radiative cooling as a viable energy technology.

http://science.sciencemag.org/content/sci/early/2017/02/08/science.aai7899.full.pdf

Scalable-manufactured randomized glass-polymer hybrid metamaterial for daytime radiative cooling

Wave propagation and group velocity

Mie resonance-based dielectric metamaterials

Electromagnetic Metamaterials: Transmission Line Theory and Microwave Applications. By Christophe Caloz and Tatsuo Itoh.

A reflective impedance element is connected to a port of a composite right/left-handed transmission line, and operates at a predetermined operating frequency so that an impedance when the reflective impedance element is seen from the port becomes a pure imaginary number jB. A reflective impedance component is connected to a port of the composite right/left-handed transmission line, and operates at the predetermined operating frequency so that an impedance when the reflective impedance element is seen from the port becomes −jB.

Microwave resonator configured by composite right/left-handed meta-material and antenna apparatus provided with the microwave resonator

Propagation and radiation characteristics of 1-D dielectric-resonator-based composite right/left-handed metamaterial structures with open mesh plates are investigated as a function of configuration parameters. The complex effective permittivity and permeability are estimated from the scattering parameters of the unit-cell structure. Based on this configuration, the single-mode transmission line is designed and the backfire-to-endfire frequency-scanned leaky wave antenna is implemented. It is found from numerical simulations that the 30-cell leaky wave antenna has a gain of 10 dBi with a scanning angle of over 90deg covering backfire-to-endfire directions. The measurement results show as wide beam-steering angles as numerically predicted.

Dielectric-Resonator-Based Composite Right/Left-Handed Transmission Lines and Their Application to Leaky Wave Antenna

A new type of nonreciprocal phase-shift transmission line is proposed; it realizes the dominant right-handed mode in one direction of the power flow along the line, and the dominant left-handed mode in the opposite power direction. These right and left handed modes can have the same wave number vector at a specific frequency but there is no significant coupling effect. An equivalent circuit model is provided to explain its nonreciprocal transmission characteristics. A nonreciprocal phase-shift composite right/left handed transmission line is designed to satisfy specific performance parameters and fabricated as a ferrite microstrip line. The leaky wave radiation characteristics of the fabricated nonreciprocal line are investigated. An experiment and analyses confirm obliquely unidirectional leaky wave radiation from the line for the two different directions of the transmitted power.

Nonreciprocal Phase-Shift Composite Right/Left Handed Transmission Lines and Their Application to Leaky Wave Antennas

A ferrite left-handed transmission line has been proposed and investigated for the first time, which is composed of a ferrite microstrip line loaded periodically with capacitances and inductances. The nonreciprocal transmission characteristics with the isolation of greater than 20 dB are numerically obtained by finite element method and demonstrated experimentally in the positive and negative permeability frequency region for edge guided modes when the passband in the left-handed transmission overlaps with the negative permeability frequency region.

Nonreciprocal left-handed microstrip lines using ferrite substrate

A pseudo-traveling-wave resonator with magnetically tunable phase gradient of field distribution is investigated, and a new type of beam-steering antenna based on the resonator is experimentally demonstrated for the first time. It is a short-ended transmission-line resonator and is composed of a nonreciprocal phase-shift composite right/left-handed transmission line using a polycrystalline yttrium-iron-garnet rod. The resonator operates as zeroth-order resonator if there is no dc magnetic field, and the radiation beam directs to broadside. By increasing an externally applied dc magnetic field normal to the substrate, the effective dc magnetization in the ferrite increases under the unsaturated regime. The phase gradient of the field distribution along the resonator is then continuously increased. As a result, the radiation beam direction changes obliquely with respect to broadside. Continuous backfire-to-endfire beam steering with more than 40° was achieved with almost constant gain of 5 dBi. In addition, numerical simulation results show considerably high radiation efficiency of 85%-95%, and the measured beam angle and gain were found almost constant within the relative bandwidth of 2%.

Tetsuya Ueda

Papers

Microwave resonator configured by composite right/left-handed meta-material and antenna apparatus provided with the microwave resonator

Dielectric-Resonator-Based Composite Right/Left-Handed Transmission Lines and Their Application to Leaky Wave Antenna

Nonreciprocal Phase-Shift Composite Right/Left Handed Transmission Lines and Their Application to Leaky Wave Antennas

Nonreciprocal left-handed microstrip lines using ferrite substrate

Pseudo-Traveling-Wave Resonator With Magnetically Tunable Phase Gradient of Fields and Its Applications to Beam-Steering Antennas