Abstract-Electrically Controllable Terahertz Second‐Harmonic Generation in GaAs

Joo Kang, Won Tae Kim, Hyeon‐Don Kim, Soojeong Baek, Kwang Jun Ahn, Bumki Min,Fabian Rotermund

https://onlinelibrary.wiley.com/doi/abs/10.1002/adom.202000359?af=R

Terahertz radiation and its nonlinear optical manipulation may possess potential for a variety of applications in next‐generation electronics and optics. Pioneering studies have shown that the nonlinearity of carrier drift in semiconductors and graphene can be utilized for nonlinear optical processes at terahertz frequencies. However, because of the symmetric response of carriers to the terahertz field direction, most experiments have confirmed only the presence of odd‐order nonlinear processes. In this study, electric‐field‐induced terahertz second‐harmonic generation (SHG) in photoexcited gallium arsenide is demonstrated, where an applied bias field breaks the directional symmetry of the drift transport of electrons. The amplitudes of odd‐ and even‐harmonic waves are found to be highly controllable using the bias field. The measured conversion efficiency of SHG reaches beyond 10−5, substantially higher than the value previously reported. This terahertz harmonic generation platform with electrical controllability may be useful for future nonlinear applications at terahertz frequencies.

Abstract-Electrical control of terahertz frequency conversion from time-varying surfaces

Kanghee Lee, Jagang Park, Jaehyeon Son, Bong Joo Kang, Won Tae Kim, Seong Cheol Lee, Bumki Min,  Fabian Rotermund,

Fig. 1 Experimental information. (a) Schematic of experimental setup. (b) Spectrum of single-cycle THz waves generated from LiNbO3 crystal with waveform in time domain (inset). (c) Spectrum of multi-cycle THz waves modulated through stacked band-pass filters with waveform in time domain (inset).

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-9-12762

We investigate the electrical control of frequency conversion from a time-varying interdigitated photo-conductive antenna (IPCA) and time-varying metasurface in the terahertz (THz) frequency range. Ultrafast near-infrared (NIR) optical pulses rapidly modify the conductivities of the IPCA and metasurface; however, external voltages can retard this conductivity transition. Thus, external voltages can be used to control the frequency conversion process based on the interaction between the THz waves and the time-varying surfaces. In the IPCA, both frequency up- and down-conversion processes are suppressed by external voltages. However, in the metasurface, the down-conversion is dramatically suppressed by external voltages, whereas the suppression on the up-conversion is less effective.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

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.

Abstract-Linear frequency conversion via sudden merging of resonances in time-variant terahertz metasurfaces

Kanghee Lee,   Jaehyeon Son,   Byungsoo Kang,   Jagang Park,  Fabian Rotermund,  Bumki Min

http://ieeexplore.ieee.org/document/8067111/


We experimentally demonstrate linear frequency conversion on time-variant terahertz (THz) metasurface. The proposed metasurface is originally designed to have two resonances, and with optical pumping, the resonances are merged into one. When a THz pulse passes through the metasurface during this merging process, a frequency component corresponding to the merged mode is generated.

Abstract-Electrically Controlled Second-Order Nonlinear Generation of Terahertz Waves

Kanghee Lee, Jagang Park, Bong Joo Kang, Won Tae Kim, Bumki Min, and Fabian Rotermund
https://www.osapublishing.org/abstract.cfm?uri=NLO-2017-NW3A.2

We report electric-field-induced second-harmonic generation of terahertz waves in photodoped gallium arsenide with direct-current bias fields. A fundamental wave of 0.6 THz was selectively incident and its second-harmonic wave of 1.2 THz was clearly observed.

© 2017 OSA

Abstract-Control of terahertz nonlinear transmission with electrically gated graphene metadevices

• Hyun Joo Choi
• , In Hyung Baek
• , Bong Joo Kang
• , Hyeon-Don Kim
• , Sang Soon Oh
• , Joachim M. Hamm
• , Andreas Pusch
• , Jagang Park
• , Kanghee Lee
• , Jaehyeon Son
• , Young U. k. Jeong
• , Ortwin Hess
• , Fabian Rotermund
•  & Bumki Min

http://www.nature.com/articles/srep42833?WT.feed_name=subjects_materials-science

Graphene, which is a two-dimensional crystal of carbon atoms arranged in a hexagonal lattice, has attracted a great amount of attention due to its outstanding mechanical, thermal and electronic properties. Moreover, graphene shows an exceptionally strong tunable light-matter interaction that depends on the Fermi level - a function of chemical doping and external gate voltage - and the electromagnetic resonance provided by intentionally engineered structures. In the optical regime, the nonlinearities of graphene originated from the Pauli blocking have already been exploited for mode-locking device applications in ultrafast laser technology, whereas nonlinearities in the terahertz regime, which arise from a reduction in conductivity due to carrier heating, have only recently been confirmed experimentally. Here, we investigated two key factors for controlling nonlinear interactions of graphene with an intense terahertz field. The induced transparencies of graphene can be controlled effectively by engineering meta-atoms and/or changing the number of charge carriers through electrical gating. Additionally, nonlinear phase changes of the transmitted terahertz field can be observed by introducing the resonances of the meta-atoms.

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.

Abstract Photoinduced nonlinear mixing of terahertz dipole resonances in graphene metadevice

Chihu

https://www.osapublishing.org/abstract.cfm?uri=CLEOPR-2015-28E1_3

Bumki Min, Chihun In, Hyeon-Don Kim, Hyunyong Choi

The plethora of nonlinear optical phenomena can offer an innovative route for developing subwavelength-scale optical components. Here, using graphene-integrated metadevices, nonlinear interaction between two electric dipole resonances is demonstrated by ultrafast terahertz spectroscopy.

© 2015 IEEE

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Abstract-Photoinduced Nonlinear Mixing of Terahertz Dipole Resonances in Graphene Metadevices

http://onlinelibrary.wiley.com/doi/10.1002/adma.201504444/abstract

The first experimental demonstration of nonlinear terahertz difference-frequency generation in a hybrid graphene metadevice is reported. Decades of research have revealed that terahertz-wave generation is impossible in single-layer graphene. This limitation is overcome and nonlinear terahertz generation by ultra-short optical pulse injection is demonstrated. This device is an essential step toward atomically-thin, nonlinear terahertz optoelectronic components.

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.

Terahertz Metamaterial for cancer diagnosis, security scanning and even invisibility cloaks

Metamaterial breaks refraction record
http://physicsworld.com/cws/article/news/45115
Feb 16, 2011







Record-breaking metamaterial
Researchers in Korea have created a new metamaterial with the most extreme positive index of refraction yet – a whopping 38.6. The metamaterial operates at terahertz frequencies and the team believes that it could find use in a number of applications including high-resolution imaging.
The refractive index of a material defines the angle through which light is bent when it travels between a material and the vacuum. Ordinary materials such as glass have refractive indices between one and three at optical frequencies, with a few materials like silicon approaching four. Over the past decade or so, physicists have been developing artificial materials with negative indices of refraction. These metamaterials bend light in the opposite direction to normal materials and can be used to make invisibility cloaks and superlenses.
While this new material has a positive index of refraction, its value is so large that it could lead to new terahertz technologies for security scanning and cancer diagnosis. The researchers also believe that the metamaterial could find use in invisibility cloaks.
Lattice of I-shapes
Created by Bumki Min of the Korean Advanced Institute for Science and Technology, the metamaterial is a polymer film inset with a pattern of thin gold or aluminium shapes. The team set the I-shapes into the polymer using lithography techniques. The I-shapes were slightly less than 60 µm tall and wide and repeated every 60 µm in a square lattice so that the individual shapes don't touch each other (see figure).
The team found that the material achieves its peak refractive index of 38.6 at frequencies near 0.3 THz. The value drops away at other frequencies, but the refractive index remains above 20 for frequencies near 0.35 THz.
A material's index of refraction is a function of two electromagnetic properties. These are its permittivity, or the ease with which it is electrically polarized, and its permeability, which is the ease at which it can be magnetized. In Min's metamaterial, the permeability remained ordinary, while the metal patterning boosted the permittivity by a considerable amount.
Resonating with the gap
When linearly polarized light is shone through the material, the metal pieces become electrically polarized. The tops and bottoms of the I-shapes act as capacitor plates with their oppositely-charged neighbours, setting up an electric field in the gap between the shapes. The strength of this field indicates the material's permittivity. At certain frequencies, the wavelength of the light resonates with the size of the gap, resulting in a stronger electric field. This increases the permittivity further and results in a high maximum refractive index.
By changing the size of the I-shaped pieces, and so changing their distance from one another, the researchers tested gaps between the metal edges of 30 µm down to 80 nm. This smallest spacing was achieved with aluminium, which was set into the polyimide with the help of more precise electron-beam lithography. The refractive index rose rapidly for gaps below about 5 µm, and the aluminium material, with its tiny gaps, had the highest maximum index.
The researchers also made thicker versions of their metamaterial, generating up to five layers of the gold pattern. In this material, each layer's response is superimposed on the others, and as a result Min says it "exhibits a completely different bulk refractive index profile from that of a single-layer profile".
For the five-layer material, the I-shapes were only 40 µm square, and the maximum refractive index of 33 was well above other measurements for gold. The high index was also maintained for a broader band of frequencies, remaining above 15 between 0.7 and 1.8 THz.
Skin cancer treatment
Jung-Tsung Shen of Washington University in Missouri calls the work "very significant", noting that the Korean team's high-index material is also flexible rather than rigid. "I believe their results could find potential applications in many situations where terahertz frequencies are used," he says, citing security checkpoints and skin cancer diagnosis.
The metamaterial's refractive index scales with that of the metal's host. The researchers believe that higher refractive indices can be constructed by replacing the polymer with a material whose own refractive index is high, such as lead sulphide. Moreover, as demonstrated by the aluminium prototype, the thinner gaps between the metal pieces strengthen their capacitive behaviour – and hence the permittivity of the material.
And, Min says that the development of high-index materials may still lead to invisibility. "The broadened index spectrum will provide more design freedom in the path control of electromagnetic waves," he says, potentially shrinking cloaking devices. "Positive high refractive index will also be useful for various applications such as high-resolution imaging."
The work is reported in Nature 470 369.
About the author
Kate McAlpine is a science writer based in the UK