An Ultra-Thin Terahertz Metalens

Illustration of a terahertz metasurface ultra-thin collimator for power enhancement. (T. Suzuki, TUAT)

https://www.photonicsviews.com/an-ultra-thin-terahertz-metalens/

Terahertz radiation has barely been exploited compared to most of the rest of the electro­magnetic spectrum. Yet T-rays potentially have appli­cations in next-generation wireless communi­cations (6G/7G), security systems, biome­dicine, and even art history. A new device for controlling T-rays using a specially designed meta­surface with properties not found in nature could begin to realize this potential.

Terahertz technology that allows gene­ration, detection, and appli­cation of terahertz waves has taken off in the last decade or so, closing the terahertz gap somewhat. But the performance and dimensions of conventional optical components able to mani­pulate terahertz waves have not kept up with this rapid development. One reason is the lack of naturally occurring materials suitable for the terahertz waveband. However, researchers at Tokyo Uni­versity of Agriculture and Techno­logy (TUAT) led by terahertz wave engineer Takehito Suzuki have recently developed an optical component that can more easily mani­pulate T-rays and in a practical fashion by using a material that doesn’t occur in nature.

Conventionally, a colli­mator, typically consisting of a curved lens or mirror, can mani­pulate T-rays is a bulky three-dimensional structure made of naturally occurring materials. But the researchers Takehito Suzuki, Kota Endo, and Satoshi Kondoh, have devised a collimator as an ultra-thin (2.22 micro­meters) plane made from a meta­surface. These properties come not from whatever metal or plastic base substance they are composed of, but instead from the geometry and arrangement of the material in tiny repeating patterns that can bend electro­magnetic waves in a way that natural substances cannot.

In this case, the material has an extremely high refrac­tive index and low reflec­tance. The collimator consists of 339 pairs of meta-atoms arranged so that the refractive index concen­trically increases from the outside to the center of the device. “The meta­surface design is unpre­cedented,” said Suzuki, “delivering a much higher performance that should acce­lerate the development of a wide range of appli­cations, including next-generation wireless communi­cations (6G/7G) and even thermal radiation control devices.” (Source: U. Tokyo)

Reference: T. Suzuki et al.: Terahertz metasurface ultra-thin collimator for power enhancement, Opt. Exp. 28, 22165 (2020); DOI: 10.1364/OE.392814

Abstract-Terahertz metasurface ultra-thin collimator for power enhancement

Takehito Suzuki, Kota Endo, and Satoshi Kondoh

(a) Terahertz metasurface ultra-thin collimator for power enhancement and (b) the design model of a single meta-atom extracted from the full structure assuming periodicity.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-28-15-22165

Manipulation of electromagnetic waves from radio to visible wavelengths could lead to technology to investigate unexplored wavebands. However, flexible control of terahertz waves is difficult, because few naturally occurring, appropriate materials and sophisticated optical components exist. We propose a 2.28-µm (0.02λ) ultra-thin terahertz metasurface collimator with a high directivity of 4.6 times (6.6 dB) consisting of 339 pairs of meta-atoms compared with a single terahertz continuous-wave source. The metasurface exhibits an extremely high refractive index of 15.0 and a low reflectance of 15.5% at 3.0 THz, and with Fresnel reflections for naturally occurring dielectric materials with high refractive indices avoided. This metasurface collimator should facilitate ground-breaking applications such as arbitrary phase converters, solid immersion lenses, and cloaking.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Underused part of the electromagnetic spectrum gets optics boost from metamaterial

Terahertz metasurface ultra-thin collimator for power enhancement Credit: Takehito Suzuki, TUAT

https://phys.org/news/2020-07-underused-electromagnetic-spectrum-optics-boost.html

Terahertz radiation, or T-rays, has barely been exploited compared to most of the rest of the electromagnetic spectrum. Yet T-rays potentially have applications in next-generation wireless communications (6G/7G), security systems, biomedicine, and even art history. A new device for controlling T-rays using a specially designed 'metasurface' with properties not found in nature could begin to realize this potential. 

The findings are published in the peer-reviewed journal Optics Express on July 13th, 2020.

The 'terahertz gap' is a term used by engineers to describe how very little technology exists that makes use of the frequency band in the electromagnetic spectrum that lies between microwaves and infrared radiation: terahertz radiation (also called T-rays).

While it is straightforward to generate and manipulate microwaves and infrared radiation, practical technologies that operate at room temperature and that are able to do the same with T-rays are inefficient and impractical. This is a great shame, as the properties of T-rays would make them extremely useful if we could indeed harness them.

T-rays can penetrate opaque objects like X-rays, but they are non-ionizing, so much safer. They can also go through clothing, wood, plastics, and ceramics, so are of interest for the security and surveillance sector for real-time imaging to identify concealed guns or explosives. For this same reason, terahertz radiation applications are also promising for cultural heritage science, offering art historians and museums a no-radiation risk option for investigation of artifacts ranging from paintings to mummies.

Terahertz technology that allows generation, detection, and application of terahertz waves has taken off in the last decade or so, closing the terahertz gap somewhat. But the performance and dimensions of conventional optical components able to manipulate terahertz waves have not kept up with this rapid development. One reason is the lack of naturally occurring materials suitable for the terahertz waveband.

However, researchers at Tokyo University of Agriculture and Technology (TUAT) led by Associate Professor and terahertz wave engineer Takehito Suzuki have recently developed an optical component that can more easily manipulate T-rays and in a practical fashion—by using a material that doesn't occur in nature.

Conventionally, a collimator—a device that narrows beams or waves, typically consisting of a curved lens or mirror—that can manipulate T-rays is a bulky three-dimensional structure made of naturally occurring materials.

But TUAT researchers Takehito Suzuki, Kota Endo, and Satoshi Kondoh have devised a collimator as an ultra-thin (2.22 micrometers) plane made from a 'metasurface'—a material that is engineered to have properties that are impossible or difficult to find in nature. These properties come not from whatever metal or plastic base substance they are composed of, but instead from the geometry and arrangement of the material in tiny repeating patterns that can bend electromagnetic waves in a way that natural substances cannot.

In this case, the material has an extremely high refractive index (how slow light travels through it) and low reflectance (proportion of light reflected after striking a surface). The collimator consists of 339 pairs of meta-atoms arranged so that the refractive index concentrically increases from the outside to the center of the device.

"The metasurface design is unprecedented," said Suzuki, "delivering a much higher performance that should accelerate the development of a wide range of applications, including next-generation wireless communications (6G/7G) and even thermal radiation control devices."

Abstract-Terahertz beam focusing through designed oblique metal-slit array

Takehito Suzuki, Masashi Sekiya,  Hideaki Kitahara,

https://www.osapublishing.org/ao/abstract.cfm?uri=ao-58-15-4007

Manipulation of propagating beams is essential in applications, and the potentially arising phenomena offer attractive optical components. However, the design of optical components using only naturally occurring materials has approached physical limits, and artificial materials such as metamaterials and metasurfaces are a way forward to open the door to sophisticated optical components. This paper shows manipulation of terahertz beams through designed oblique metal-slit arrays where a common metal-slit array does not perform as a lens. The oblique metal-slit array has a refractive index determined as a function of a steep angle. The lens consists of multiple metal plates with a designed oblique angle, and a convex output structure produces a focusing effect. We also suggest that the Brewster phenomenon in the lens can simply enhance the electric field intensity at the focal point. The Brewster condition of the lens is correlated with a jagged edged face on the input side with an appropriate metal-slit spacing and thickness. The phenomenon would be applicable to numerous promising components and applications such as gain-enhancement optical components and perfect impedance-matching polarizers.

© 2019 Optical Society of America

Abstract-Negative refractive index metamaterial with high transmission, low reflection, and low loss in the terahertz waveband

Takehito Suzuki, Masashi Sekiya, Tatsuya Sato,Yuki Takebayashi,

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-26-7-8314

The refractive index is a basic parameter of materials which it is essential to know for the manipulation of electromagnetic waves. However, there are no naturally occurring materials with negative refractive indices, and high-performance materials with negative refractive indices and low losses are demanded in the terahertz waveband. In this paper, measurements by terahertz time-domain spectroscopy (THz-TDS) demonstrate a metamaterial with a negative refractive index n of −4.2 + j0.17, high transmitted power of 81.5%, low reflected power of 4.3%, and a high figure of merit (FOM = |Re(n)/Im(n)|) of 24.2 at 0.42 THz. The terahertz metamaterial with these unprecedented properties can provide various attractive terahertz applications such as superlenses with resolutions beyond the diffraction limit in terahertz continuous wave imaging.

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

Abstract-Quasi-three-dimensional post-array for propagation and focusing of a terahertz spoof surface plasmon polariton

Nozomu Koja, 

John C. Young,  

Takehito Suzuki

http://link.springer.com/article/10.1007%2Fs00339-015-9259-0

This paper presents a quasi-three-dimensional post-array designed to propagate a terahertz spoof surface plasmon polariton (terahertz spoof SPP) with confinement. A transmission line making use of a terahertz spoof SPP is a promising device in the terahertz wave band, and there are many previous reports of two-dimensional structures. Three-dimensional structures provide sophisticated designs for transmission lines propagating a terahertz spoof SPP. Eigenmode analysis is used to derive a dispersion diagram for one post with boundary conditions extracted from the full model. The propagation frequency of the terahertz spoof SPP increases with lower heights or smaller diameters, and that remains virtually unchanged for post-spacing and thickness of a substrate. The analysis of the full model confirms the confinement of a terahertz spoof SPP vertically on the post-array. The magnitude of the electric field is strong around the top and bottom and weak at approximately one-third height. The terahertz spoof SPP is confined in the space around the post-array as well as a substrate, while it is confined only on substrates in conventional two-dimensional structures. The designed post-array can control the three-dimensional focusing of a terahertz spoof SPP in an arbitrary volume of space.

Abstract-Achromatic wave plate in THz frequency region based on parallel metal plate waveguides with a pillar array

Masaya Nagai, Noriyuki Mukai, Yosuke Minowa, Masaaki Ashida, Takehito Suzuki, Jun Takayanagi, and Hideyuki Ohtake  »View Author Affiliations

http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-23-4-4641

Optics Express, Vol. 23, Issue 4, pp. 4641-4649 (2015)
http://dx.doi.org/10.1364/OE.23.004641

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We demonstrated an achromatic wave plate based on parallel metal plate waveguides in the high THz frequency region. The metal plates have periodic rough structures on the surface, which allow slow transverse magnetic wave propagation and fast transverse electric wave propagation. A numerical simulation showed that the height of the periodic roughness is important for optimizing the birefringence. We fabricated stacked metal plates containing two types of structures by chemical etching. An array of small pillars on the metal plates allows higher frequency optimization. We experimentally demonstrated an achromatic quarter-wave plate in the frequency region from 2.0 to 3.1 THz.

© 2015 Optical Society of America