Abstract-Electrically Controllable Molecularization of Terahertz Meta‐Atoms

Hyunseung Jung,   Jaemok Koo,   Eunah Heo,   Boeun Cho,   Chihun In,   Wonwoo Lee,   Hyunwoo Jo,  Jeong Ho Cho,   Hyunyong Choi,   Moon Sung Kang,  Hojin Lee,

https://onlinelibrary.wiley.com/doi/abs/10.1002/adma.201802760

Active control of metamaterial properties is critical for advanced terahertz (THz) applications. However, the tunability of THz properties, such as the resonance frequency and phase of the wave, remains challenging. Here, a new device design is provided for extensively tuning the resonance properties of THz metamaterials. Unlike previous approaches, the design is intended to control the electrical interconnections between the metallic unit structures of metamaterials. This strategy is referred to as the molecularization of the meta‐atoms and is accomplished by placing graphene bridges between the metallic unit structures whose conductivity is modulated by an electrolyte gating. Because of the scalable nature of the molecularization, the resonance frequency of the terahertz metamaterials can be tuned as a function of the number of meta‐atoms constituting a unit metamolecule. At the same time, the voltage‐controlled molecularization allows delicate control over the phase shift of the transmitted THz, without changing the high transmission of the materials significantly.

Abstract-Composition control of plasmon-phonon interaction using topological quantum-phase transition in photoexcited (Bi1-xInx)2Se3

Sangwan Sim, Jun Park, nikesh koirala, Seungmin Lee, Matthew J. Brahlek, Jisoo Moon, Maryam Salehi,Jaeseok Kim, Soonyoung Cha, Ji Ho Sung, Moon-Ho Jo, Seongshik Oh, and Hyunyong Choi

ACS Photonics, Just Accepted Manuscript

DOI: 10.1021/acsphotonics.6b00021

Publication Date (Web): May 24, 2016

Copyright © 2016 American Chemical Society

http://pubs.acs.org/doi/abs/10.1021/acsphotonics.6b00021?journalCode=apchd5

Plasmonics is a technology aiming at light modulation via collective charge oscillations. Topological insulators, where Dirac-like metallic surfaces coexist with normal insulating bulk, have recently attracted great attentions in plasmonics due to their topology-originated outstanding properties. Here, we introduce a new methodology for controlling the interaction of plasmon with phonon in topological insulators, which is a key for utilizing the unique spectral profiles for photonic applications. By using both static and ultrafast terahertz spectroscopy, we show that the interaction can be tuned by controlling the chemical composition of (Bi1-xInx)2Se3micro-ribbon arrays. The topological quantum-phase transition induced by varying the composition drives a dramatic change in the strength of the plasmon-phonon interaction. This was possible due to the availability of manipulating the spatial overlap between topological surface plasmonic states and underlying bulk phonon. Especially, we control the laser-induced ultrafast evolution of the transient spectral peaks arising from the plasmon-phonon interaction by varying the spatial overlap across the topological phase transition. This study may provide a new platform for realizing topological insulator-based ultrafast plasmonic devices.

Abstract-Electromagnetically Induced Transparency Analogue by Self-Complementary Terahertz Meta-Atom

1. Hyunseung Jung¹, 
2. Chihun In², 
3. Hyunyong Choi² and
4. Hojin Lee^1,*

Article first published online: 19 JAN 2016

DOI: 10.1002/adom.201500620

© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

http://onlinelibrary.wiley.com/doi/10.1002/adom.201500620/abstract

In the past years, many researchers have tried to realize the electromagnetically induced transparency (EIT) phenomena from the metamaterial arrays for the attractable applications such as slow light devices, novel optical communication systems, and nonlinear optical devices. Most of metamaterial enabled EIT analogues reported so far are based on the destructive interference between the bright and dark meta-atoms. Whether they are located on the same plane or off the plane, EIT-like properties can be achieved only by breaking the symmetry between the bright and dark meta-atoms. In this study, a novel self-complimentary terahertz meta-atom is developed by combining cut-wire resonators and its pseudo-complimentary pattern within a single meta-atom. From the measurement results, it is verified that the proposed meta-atom can exhibit EIT-like phenomenon within a single unit cell by controlling the focus of the electric field induced by the incident terahertz waves within the self-complimentary meta-atom cell as predicted by the numerical simulation as well as by the theoretical forced oscillation model.

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.

Abstract-Anisotropy Modeling of Terahertz Metamaterials: Polarization Dependent Resonance Manipulation by Meta-Atom Cluster

Hyunseung Jung,

Chihun In,

Hyunyong Choi

& Hojin Lee

• http://www.nature.com/srep/2014/140609/srep05217/full/srep05217.html

Recently metamaterials have inspired worldwide researches due to their exotic properties in transmitting, reflecting, absorbing or refracting specific electromagnetic waves. Most metamaterials are known to have anisotropic properties, but existing anisotropy models are applicable only to a single meta-atom and its properties. Here we propose an anisotropy model for asymmetrical meta-atom clusters and their polarization dependency. The proposed anisotropic meta-atom clusters show a unique resonance property in which their frequencies can be altered for parallel polarization, but fixed to a single resonance frequency for perpendicular polarization. The proposed anisotropic metamaterials are expected to pave the way for novel optical systems.

Abstract-Unconventional Terahertz Carrier Relaxation in Graphene Oxide: Observation of Enhanced Auger Recombination due to Defect Saturation

Jaeseok Kim , Juyeong Oh , Chihun In , Yun-Shik Lee , Theodore B. Norris , Seong Chan Jun , and Hyunyong Choi

http://pubs.acs.org/doi/abs/10.1021/nn406066f

ACS Nano, Just Accepted Manuscript

DOI: 10.1021/nn406066f

Publication Date (Web): February 4, 2014

Copyright © 2014 American Chemical Society

Photoexcited carrier relaxation is a recurring topic in understanding the transient conductivity dynamics of graphene-based devices. For atomically-thin graphene oxide (GO), a simple free-carrier Drude response is expected to govern the terahertz (THz) conductivity dynamics ―same dynamics observed in conventional CVD-grown graphene. However, to date no experimental testimony has been provided on the origin of photo-induced conductivity increase in GO. Here, using ultrafast terahertz (THz) spectroscopy, we show that the photoexcited carrier relaxation in GO exhibits a peculiar non-Drude behavior. Unlike graphene, the THz dynamics of GO show percolation behaviors: as the annealing temperature increases, transient THz conductivity rapidly increases and the associated carrier relaxation changes from mono- to bi-exponential decay. After saturating the recombination decay through defect trapping, a new ultrafast decay channel characterized by multi-particle Auger scattering is observed whose threshold pump fluence is found to be 50 µJ/cm2. The increased conductivity is rapidly suppressed within 1 ps due to the Auger recombination, and non-Drude THz absorptions are subsequently emerged as a result of the defect-trapped high-frequency oscillators.