Beyond 5G: Wireless communications may get a boost from ultra-short collimating metalens

 

https://phys.org/news/2021-07-5g-wireless-boost-ultra-short-collimating.html

Screens may be larger on smartphones now, but nearly every other component is designed to be thinner, flatter and tinier than ever before. The engineering requires a shift from shapely, and bulky lenses to the development of miniaturized, two-dimensional metalenses. They might look better, but do they work better?

A team of Japan-based researchers says yes, thanks to a solution they published on July 7th in Applied Physics Express.

The researchers previously developed a low-reflection metasurface—an ultra-thin interface that can manipulate electromagnetic waves—specifically to control terahertz waves. These waves overlap millimeter waves and infrared waves, and, while they can transmit a significant amount of data, they easily attenuate in the atmosphere.

The technology may not be suitable for long-range wireless communications, but could improve short-range data exchanges, such as residential internet speeds, said paper author Takehito Suzuki, associate professor in the Institute of Engineering at Tokyo University of Agriculture and Technology. According to Suzuki, the researchers have taken a step toward such application developments by using their metasurface to craft the world's best ultra-short metalens that collimates to align an optical system with a distance of only one millimeter. The metalens is capable of increasing transmitted power by three at the far field, where the signal strength typically weakens.

"Terahertz flat optics based on our originally developed low-reflection metasurface with a high-refractive index can offer attractive two-dimensional optical components for the manipulation of terahertz waves," Suzuki said.

The challenge was whether the collimating lens, which converts approximately spherical-shaped terahertz waves to aligned terahertz waves, made with the metasurface, could be mounted closely to the electronics—called a resonant tunneling diode—that transmits terahertz waves at the right frequency and in the right direction. The minimal distance between the diode and the metalens is the necessary ingredient in current and future electronic devices, Suzuki said.

"We resolved this problem," Suzuki said. "We integrated a fabricated collimating metalens made with our original metasurface with a resonant tunneling diode at a distance of one millimeter." Measurements verify that the collimating metalens integrated with the resonant tunneling diode enhances the directivity to three times that of a single resonant tunneling diode.

The researchers tuned their device to 0.3 terahertz, a band at a higher frequency than the one used for 5G wireless communications. The manipulation of higher-frequency electromagnetic waves allows the upload and download of huge amounts of data in 6G wireless communications, according to Suzuki.

"The 0.3 terahertz band is a promising candidate for 6G offering advanced cyber-physical systems," Suzuki said. "And our presented collimating metalens can be simply integrated with various terahertz continuous-wave sources to accelerate the growth of emerging terahertz industry such as 6G wireless communications."

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Abstract-Graphene aperture-based metalens for dynamic focusing of terahertz waves

Pei Ding, Yan Li, Li Shao, Ximin Tian, Junqiao Wang, and Chunzhen Fan

Fig. 3 (a) Phase distributions 𝑝(𝑥)$p(x)$of the metalens with a focal length of F = 150 um (p1) and 190 um (p2), respectively, at the incident frequency of f0 = 5 THz. The phase difference between the two lenses (𝑝2−𝑝1$p_2-p_1$) along the x axis is also illustrated. (b) E-field intensity distribution of the reflection field on the x-z plane for the metalens with a designed focal length F = 150 um at f0 = 5 THz for Ef = 1.0 eV. The simulated focal length is 140 um. (c) E-field intensity distribution of the reflected wave along the z-axis for different Ef. The simulated focal lengths remain unchanged. (d) The corresponding E-field intensity distributions of the reflected wave on the focal plane (z = 140 um) for different Ef.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-26-21-28038

We theoretically study a tunable reflective focusing lens, based on graphene metasurface, which consists of rectangle aperture array. Dynamic control of either the focal intensity or focal length for terahertz circular polarized waves can be achieved by uniformly tuning the graphene Fermi energy. We demonstrate the graphene apertures with the same geometry; however, spatially varying orientations can only control the focal intensity. To change the focal length, the spatially varying aperture lengths are also required. A comparative study between the metalenses, which generate only geometric or both gradient and geometric phase changes, has shown that the apertures’ spatially varying length distribution is the key factor for determining the modulation level, rather than the focal length’s modulation range. This kind of metalens provides tunable, high-efficiency, broadband, and wide-angle off-axis focusing, thereby offering great application potential in lightweight and integrated terahertz devices.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Metalenses developed for MEMS chips

                                             Metasurface-based flat lens integrated onto a MEMS scanner.

Harvard-Argonne partnership develops technology, with flexible features such as fast scanning and beam steering.

http://optics.org/news/9/2/32

In recent years, lens technologies have advanced across all scales, from digital cameras and high bandwidth in fiber optics to the LIGO lab instruments. Now, a new lens technology, which could be produced using standard computer-chip technology, is emerging and could replace the bulky layers and complex geometries of traditional curved lenses.

Flat lenses, unlike their traditional counterparts, are relatively lightweight, based on optical nanomaterials known as metasurfaces. When the subwavelength nanostructures of a metasurface form certain repeated patterns, they mimic the complex curvatures that refract light, but with less bulk and an improved ability to focus light with reduced distortion. However, most of these nanostructured devices are static, which limits their functionality.

Federico Capasso, an applied physicist at Harvard University who pioneered metalens technology, and Daniel Lopez, group leader of nanofabrication and devices at Argonne National Laboratory and an early developer of microelectromechanical systems (MEMS), brainstormed about adding motion capabilities like fast scanning and beam steering to metalenses for new applications.

Capasso and Lopez developed a device that integrates mid-infrared spectrum metalenses onto MEMS. The researchers have reported their findings in APL Photonics.

Development and applications

"Dense integration of thousands of individually controlled lens-on-MEMS devices onto a single silicon chip would allow an unprecedented degree of control and manipulation of the optical field," Lopez said.

The researchers formed the metasurface lens using standard photolithography techniques on a silicon-on-insulator wafer with a 2 µ-thick top device layer, a 200 nm buried oxide layer, and a 600 µm-thick handle layer. Then, they placed the flat lens onto a MEMS scanner, essentially a micromirror that deflects light for high-speed optical path length modulation. They aligned the lens with the MEMS' central platform and fixed them together by depositing small platinum patches.

"Our MEMS-integrated metasurface lens prototype can be electrically controlled to vary the angular rotation of a flat lens and can scan the focal spot by several degrees," said Lopez. "Furthermore, this proof-of-concept integration of metasurface-based flat lenses with MEMS scanners can be extended to the visible and other parts of the electromagnetic spectrum, implying the potential for application across wider fields, such as MEMS-based microscope systems, holographic and projection imaging, LIDAR scanners and laser printing."

When electrostatically-actuated, the MEMS platform controls the angle of the lens along two orthogonal axes, allowing the scanning of the flat lens focal spot by about 9 degrees in each direction. The researchers estimate that the focusing efficiency is about 85 percent.

"Such metalenses can be mass produced with the same computer-chip fabrication technology and in the future, will replace conventional lenses in a wide range of applications," Capasso said.

Abstract-Experimental Realization of an Epsilon-Near-Zero Graded-Index Metalens at Terahertz Frequencies

Victor Pacheco-Peña, Nader Engheta, Sergei Kuznetsov, Alexandr Gentselev, and Miguel Beruete

https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.8.034036

The terahertz band has been historically hindered by the lack of efficient generators and detectors, but a series of recent breakthroughs have helped to effectively close the “terahertz gap.” A rapid development of terahertz technology has been possible thanks to the translation of revolutionary concepts from other regions of the electromagnetic spectrum. Among them, metamaterials stand out for their unprecedented ability to control wave propagation and manipulate electromagnetic response of matter. They have become a workhorse in the development of terahertz devices such as lenses, polarizers, etc., with fascinating features. In particular, epsilon-near-zero (ENZ) metamaterials have attracted much attention in the past several years due to their unusual properties such as squeezing, tunneling, and supercoupling where a wave traveling inside an electrically small channel filled with an ENZ medium can be tunneled through it, reducing reflections and coupling most of its energy. Here, we design and experimentally demonstrate an ENZ graded-index (GRIN) metamaterial lens operating at terahertz with a power enhancement of 16.2 dB, using an array of narrow hollow rectangular waveguides working near their cutoff frequencies. This is a demonstration of an ENZ GRIN device at terahertz and can open the path towards other realizations of similar devices enabling full quasioptical processing of terahertz signals.

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Thin, flat meta-lenses with tunable features developed

Korean-UK group makes “credit card-thick” metasurface lenses from graphene and gold, to focus terahertz beams.  

http://optics.org/news/8/11/44?webSyncID=776b28d9-cc7c-7cae-388c-544e71035da4&sessionGUID=1ccd516a-90a0-0b40-8604-5dc1a6e754b4&_ga=2.190507624.2107229762.1512083484-178576186.1505831485

                                                                                      

Thinner, flatter: Conventional lens and metalens.

Credit card-thick, flat lenses with tunable features based on graphene and gold have been developed by a partnership of Korean- and UK-based researchers. They say that such optical devices “could become optical components for advanced applications, such as amplitude-tunable lenses, lasers (so-called vortex phase plates), and dynamic holography.”

The scientists work at the Center for Integrated Nanostructure Physics, in the Institute for Basic Science, the Korea Advanced Institute of Science and  Technology and the University of Birmingham. The work has been published in Advanced Optical Materials.

The paper describes the properties of a newly-developed metasurface (a 2D material that can control the electric and magnetic components of light and direct them as wanted) which works as a convex lens. It is made of a gold sheet pierced with micrometer-sized U-shaped holes and covered with graphene.

Conventional solid convex lenses concentrate light on a spot. Similarly with this metasurface, the pattern of apertures of the metalenses focusing the incoming beam. In addition, the microholes can also change light polarization. For example, the metalens can convert the left-circular polarization wave to right-circular polarization (clockwise).

Graphene advantages

The researchers have achieved a conversion rate of 35%. They comment that converting circular polarization could be useful in a number of fields, for example biosensing and telecommunications. To be able to control a range of optical properties, the scientists took advantage of graphene’s unique electronic features and used them to tune the output beam’s intensity or amplitude. The scientists liken graphene’s function to the exposure operation of a camera.                                                                                                                   

        Metalenses’ features.

In the case of the camera, a mechanical control allows a certain shutter’s opening time and size to determine the amount of light entering the instrument. The metalens instead regulates exposure via an electric tension applied to the graphene sheet, without the need for bulky components. When voltage is applied to the graphene layer, the output beam becomes weaker.

'Very sensitive'

“Using metalenses, you can make microscopes, cameras, and tools used in very sensitive optical measurements, much more compact,” clarifies Teun-Teun Kim, lead author of the study.

The metalenses were designed specifically for terahertz radiation. This radiation can pass through some materials such as fabrics and plastics, but at a shorter depth than microwave radiation. For this reason it is employed for surveillance and security screening.

Kim added, “While conventional optical lenses have a thickness of several centimeters to several millimeters, this metalens is just a few tens of micrometers thick. The intensity of the focused light can be effectively controlled and it could find useful applications in ultra-small optical instruments.”