Metamaterials are engineered/manmade materials with properties not found in nature. The emergence of metamaterials is expected to have a significant impact on a variety of electronics applications. These unnatural materials could have an impact similar to “MEMS” (micro-electromechanical system) technology, with the potential to bring transformational changes.
This initial FAQ will briefly look into possible near-term applications, including thermal management, sensors, advanced optics, and acoustic applications, before concluding with a look into the future and the possibility of programmable metamaterials. The second FAQ will consider “Metamaterials, mmWave antennas, 3D radar, and holographic beamforming” and the third installment will delve into “Metamaterials for power and energy.” These FAQs include current commercial examples as well as research and development activities related to metamaterials for various applications in electronic systems.
Metamaterials are fashioned from assemblies of multiple elements consisting of composite materials such as metals, plastics, ceramics, etc. The properties of metamaterials are not based on the properties of the base materials. They are based on engineering the physical structure of the material. Metamaterials rely on precise geometry/shape, size, orientation, and arrangement to derive their properties that extend beyond what is possible with naturally occurring materials. The name metamaterial is derived from the Greek word meta meaning “beyond” and the Latin word materia, meaning “matter” or “material”.
Electrical engineering, semiconductors, electromagnetics, microwave and antenna engineering, optoelectronics, classical optics, solid state physics, material sciences, and nanoengineering are among the disciplines contributing to advancements in metamaterials.
Thermal management and metamaterials
Thermal metamaterials could help dissipate heat in a deterministic manner and avoid thermal crosstalk and local hot spots in advanced semiconductor packaging such as 2.5D and 3D devices. Three examples of the use of metamaterials in thermal management include:
- Thermal shields protect an area from transient diffusive heat flow.
- Thermal concentrators focus thermal flux on a small area.
- Thermal inverters (also called thermal rotators) change the direction of the thermal gradient in an area.
The emergence of nanoelectronics, 3D-integrated circuits (ICs), and flexible electronics makes thermal management increasingly difficult. For example, in 2.5D packages, the logic power and the number of high bandwidth memory (HBM) layers, continue to grow. One critical challenge in 2.5D packages is thermal crosstalk, as the logic chip and HBM are placed close to each other while requiring different operating temperatures. Thermal metamaterials can be used to enhance thermal dissipation where need and provide thermal shielding, reducing thermal cross talk.
Other metamaterial films are expected to be highly scalable to the size of buildings or industrial installations with the ability to “self cool” under daylight conditions without the need for electricity or water consumption. This passive cooling architecture is designed to reflect solar light while simultaneously radiating heat to the cold sky through an atmospheric infrared transparency window. Relative to conventional air cooling systems based on conduction and convection, a passive cooling module based on metamaterial films is expected to drop condenser temperatures by at least 13°C, with zero net water dissipation to the atmosphere, which correlates to at least a 3% increase in efficiency.
Acoustic applications
An acoustic metamaterial, sometimes called a sonic crystal or phononic crystal, is a material engineered to control, direct and/or manipulate sound waves, or phonons. Phonons are responsible for thermal conduction in solids. As a result, phononic crystals can be engineered to control heat transfer.
Typical applications for acoustic engineering include noise control, medical ultrasound, and sound reproduction or shielding. The direction of sound through a given medium can be controlled using acoustic metamaterials to manipulate the acoustic refractive index. Also, sound wave control can be achieved by controlling specific material parameters such as the bulk modulus β, density ρ, and chirality. Acoustic metamaterials can be designed to transmit or to trap and amplify specific frequencies of sound waves. It is called an acoustic resonator when the sound waves are trapped and amplified.
The first successful industrial applications of acoustic metamaterials were tested for sound insulation in aircraft. Areas of acoustic metamaterial research range from seismic wave reflection and vibration control technologies related to earthquakes, ultrasonic/acoustic imaging, and precision sensing.
Photonic metamaterials, super lenses, and lidar
A photonic metamaterial (PM), also known as an optical metamaterial, interacts with light, covering terahertz (THz), infrared (IR), or visible wavelengths. As with other metamaterials, a PM employs a periodic, cellular structure, which differentiates it from photonic bandgap or photonic crystal structures. The cells are on a scale that is magnitudes larger than an atom, yet much smaller than the radiated wavelength, are on the order of nanometers. Envisioned PMs applications include cloaking and transformation optics, which produce spatial variations derived from coordinate transformations and can direct chosen bandwidths of electromagnetic radiation. Nearer-term applications include the development of super lenses, advanced holographic devices, and lidar.
A planar super lens (also called a metalens) has been developed that works with high efficiency within the visible spectrum of light covering the range from red to blue, and goes beyond the diffraction limit. The lens can resolve nanoscale features separated by distances smaller than the wavelength of light. It uses an ultrathin array of tiny waveguides, known as a metasurface, which bends light as it passes through. It is fabricated with titanium dioxide, a common material found in paint and sunscreen, to create the nanoscale array of smooth and high-aspect-ratio (planar) nanostructures. Unlike conventional lenses, which require precise polishing, this super lens is produced in a single lithographic step. Metalenses are currently finding application in advanced holographic systems.
Beam steering is an important part of emerging applications ranging from 5G telephony to lidar systems. Currently, most beam steering systems are based on mechanical scanning, resulting in reliability, cost, and form-factor concerns, and can also limit system performance capabilities. A new beam steering technology for lidar systems has been developed using a liquid crystal metasurface, enabling a totally solid-state system with higher resolution, range, and frame rates compared with mechanical scanning counterparts. The internal laser is directed onto the reflective semiconductor chip. The direction of reflection from the chip is programmable. Depending on the metamaterials’ configuration on the chip’s surface (which is under software control), the reflected beam direction is programmable. It can be pointed in any direction and in any sequence.
Programmable metamaterials
Metamaterials and their two-dimensional counterparts, metasurfaces, can provide powerful control over electromagnetic (EM) waves from microwave to visible. A metamaterial can become programmable by developing the capability to introduce explicit control of its sub-wavelength unit cells. It may be possible to manipulate multiple EM functions in a single metamaterial or metasurface through software control. Research is currently being pursued to provide a means for various metamaterials to autonomously adapt to their environment and/or communicate with other metamaterial elements, enabling new classes of devices for sensing, imaging, and communications. One approach is developing a metamaterial equivalent to today’s field programmable gate arrays (FPGAs) that drives a digitized version of a reconfigurable metamaterial.
The goal is the development of intelligent and self-adaptive metamaterials. One of the more difficult challenges in developing programmable metamaterials is to achieve high enough refresh rates to enable meaningful changes in material properties in real time when dealing with high-frequency environments, including mmWaves and THz waves. Depending on the application, refresh rates in the kHz or even MHz range will be required. Initial applications for programmable metamaterials are expected to include wireless communications, medical imaging, and holography.
As shown, metamaterials have the potential to bring transformational impact to a variety of electronic systems and applications. The second FAQ will consider “Metamaterials, mmWave antennas, 3D radar, and holographic beamforming” and the third installment will delve into “Metamaterials for power and energy.”
References
Acoustic metamaterial, Wikipedia
Metalens works in the visible spectrum, sees smaller than a wavelength of light, Harvard
Metamaterial, Wikipedia
Metamaterials-Enabled Passive Radiative Cooling Films, PARC
Programmable Metamaterials for Software-Defined Electromagnetic Control: Circuits, Systems, and Architectures, IEEE Journal on Emerging and Selected Topics in Circuits and Systems
Recent Advances in Thermal Metamaterials and Their Future Applications for Electronics Packaging, Journal of Electronic Packaging
