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.”

Abstract-Amplitude Modulation of Anomalously Refracted Terahertz Waves with Gated-Graphene Metasurfaces

Teun-Teun Kim, Hyunjun Kim, Mitchell Kenney, Hyun Sung Park, Hyeon-Don Kim, Bumki Min, Shuang Zhang

http://onlinelibrary.wiley.com/doi/10.1002/adom.201700507/full

Although recent progress in metasurfaces has shown great promise for applications, optical properties in metasurfaces are typically fixed by their structural geometry and dimensions. Here, an electrically controllable amplitude of anomalously-refracted waves in a hybrid graphene/metasurface system are experimentally demonstrated, which consists of an artificially constructed two-dimensional metallic apertures array and naturally occurring two-dimensional carbon atoms (graphene) in the subwavelength-scale (< λ/10). Based on Pancharatnam–Berry phase and by careful design of a spatially linear phase profile, it is shown that the amplitude of anomalously refracted circularly cross-polarized terahertz waves can be effectively modulated by an applied gate voltage. The developed electrically tunable graphene metasurfaces may lead to various advanced applications that require dynamical control over electromagnetic waves, such as amplitude tunable active focusing lenses, vortex phase plates and dynamic holography.

Ultrathin and flat graphene metalenses gain morace properties

Lenses made of graphene and precisely pierced gold sheets are able to concentrate terahertz beams to a spot, flip its polarization and modulate its intensity.

https://www.sciencedaily.com/releases/2017/11/171127124731.htm

On the quest for miniaturization, scientists at the Center for Integrated Nanostructure Physics, within the Institute for Basic Science (IBS, South Korea), in collaboration with researchers from the University of Birmingham and the Korea Advanced Institute of Science and Technology (KAIST), develop credit card-thick, flat lenses with tunable features. These optical devices, made of graphene and a punctured gold surface, could become optical components for advanced applications, such as amplitude tunable lenses, lasers (i.e. vortex phase plates), and dynamic holography.

Metasurfaces are new 2D materials that can effectively control the electric and magnetic components of light (and other electromagnetic waves) and bend them to bespoken directions. Controlling the beam's direction can bring out interesting phenomena; the most incredible being the "invisibility cloak effect," where light waves bypass an object recreating the image beyond the object, as flowing water in a river would bypass a stone.

Published in Advanced Optical Materials, the study presents the properties of a metasurface which works as a convex lens. Specifically, it is made of a gold sheet pierced with micrometer-sized U-shaped holes and covered with graphene. As the shape of common convex lenses allows light to be concentrated on a spot (or focus), think about a magnifying glass which can concentrate a light beam and even start a fire, so the particular pattern of the tiny apertures of the metalenses works by focusing the incoming beam.

In addition, these microholes can also change light polarization. While natural light is generally unpolarized before being reflected, the team used circularly polarized waves, that is a light beam where the direction of the electric field is corkscrew spiraling. This metalens can convert the left-circular polarization wave (going counterclockwise if seen straight in front) to right-circular polarization (clockwise). The researchers managed to obtain a conversion rate of 35%. Converting circular polarization could be useful in a number of fields, for example biosensing and telecommunications.

In order to control even more properties, the scientists took advantage of graphene's unique electronic features and used them to tune the output beam's intensity or amplitude. Here graphene plays the role of the exposure of a camera. 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. These metalenses instead, regulate the 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. "Using metalenses, you can make microscopes, cameras, and tools used in very sensitive optical measurements, much more compact," clarifies Teun-Teun Kim, the first author of the study.

The metalenses were designed for a type of electromagnetic wave, which falls in-between infrared radiation and microwave radiation, called terahertz radiation. This type of radiation can pass through some materials (like fabrics and plastics), but at a shorter depth than microwave radiation, for this reason it is employed for surveillance and security screening.

"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," highlights the scientists.Teun-Teun Kim, the first author of the study.

Semi- OT Abstract-Graphene–ferroelectric metadevices for nonvolatile memory and reconfigurable logic-gate operations

• 
  
• 
  • 
    
  Woo Young Kim,
• Hyeon-Don Kim,
• Teun-Teun Kim,
• Hyun-Sung Park,
• Kanghee Lee,
• Hyun Joo Choi,
• Seung Hoon Lee,
• Jaehyeon Son,
• Namkyoo Park
• & Bumki Min
• 
  
• 
  • http://www.nature.com/ncomms/2016/160127/ncomms10429/abs/ncomms10429.html

Memory metamaterials are artificial media that sustain transformed electromagnetic properties without persistent external stimuli. Previous memory metamaterials were realized with phase-change materials, such as vanadium dioxide or chalcogenide glasses, which exhibit memory behaviour with respect to electrically/optically induced thermal stimuli. However, they require a thermally isolated environment for longer retention or strong optical pump for phase-change. Here we demonstrate electrically programmable nonvolatile memory metadevices realised by the hybridization of graphene, a ferroelectric and meta-atoms/meta-molecules, and extend the concept further to establish reconfigurable logic-gate metadevices. For a memory metadevice having a single electrical input, amplitude, phase and even the polarization multi-states were clearly distinguishable with a retention time of over 10 years at room temperature. Furthermore, logic-gate functionalities were demonstrated with reconfigurable logic-gate metadevices having two electrical inputs, with each connected to separate ferroelectric layers that act as the multi-level controller for the doping level of the sandwiched graphene layer.

Switching terahertz waves with gate-controlled active graphene metamaterials

http://graphenetimes.com/2012/09/switching-terahertz-waves-with-gate-controlled-active-graphene-metamaterials/

Authors: Seung Hoon Lee, Muhan Choi, Teun-Teun Kim, Seungwoo Lee, Ming Liu, Xiaobo Yin, Hong Kyw Choi, Seung S. Lee, Choon-Gi Choi, Sung-Yool Choi, Xiang Zhang & Bumki Min

The extraordinary electronic properties of graphene provided the main thrusts for the rapid advance of graphene electronics. In photonics, the gate-controllable electronic properties of graphene provide a route to efficiently manipulate the interaction of photons with graphene, which has recently sparked keen interest in graphene plasmonics. However, the electro-optic tuning capability of unpatterned graphene alone is still not strong enough for practical optoelectronic applications owing to its non-resonant Drude-like behaviour. Here, we demonstrate that substantial gate-induced persistent switching and linear modulation of terahertz waves can be achieved in a two-dimensional metamaterial, into which an atomically thin, gated two-dimensional graphene layer is integrated. The gate-controllable light–matter interaction in the graphene layer can be greatly enhanced by the strong resonances of the metamaterial. Although the thickness of the embedded single-layer graphene is more than six orders of magnitude smaller than the wavelength (<λ/1,000,000), the one-atom-thick layer, in conjunction with the metamaterial, can modulate both the amplitude of the transmitted wave by up to 47% and its phase by 32.2° at room temperature. More interestingly, the gate-controlled active graphene metamaterials show hysteretic behaviour in the transmission of terahertz waves, which is indicative of persistent photonic memory effects.

Nature Materials. doi:10.1038/nmat3433

Abstract-Metamaterials: Reversibly Stretchable and Tunable Terahertz Metamaterials with Wrinkled Layouts

Seungwoo Lee,  Seongnam Kim,  Teun-Teun Kim,  Yushin Kim,  Muhan Choi,  Seung Hoon Lee,  Ju-Young Kim,  Bumki Min 

http://onlinelibrary.wiley.com/doi/10.1002/adma.201290159/abstract

When metamaterials are arranged in a stretchable platform, their practical applications (e.g., reconfigurable optoelectronic devices and biological sensors) can be greatly extended. B. Min and co-workers take inspiration from the wrinkled structures of various bio-organisms to develop a generic strategy for reversibly stretchable metamaterials. The structural integrities of the wrinkled THz metamaterials described, remain intact after repeated stretching/relaxing, and the wrinkled metamaterial makes a mechanically reversible transition between artificial homogeneous and inhomogeneous broadenings

Abstract-Reversibly Stretchable and Tunable Terahertz Metamaterials with Wrinkled Layouts

http://onlinelibrary.wiley.com/doi/10.1002/adma.201200419/abstract
The use of wrinkling provides a generic route to stretchable metamaterials, with unprecedented terahertz tunability. The wrinkled metamaterial can be stretched reversibly up to 52.5%; the structural integrity can be maintained during at least 100 stretching/relaxing cycles. Importantly, the wrinkling of meta-atoms offers a deterministic way to achieve controlled broadening of electrical resonance.