Abstract-Terahertz rectification in ring-shaped quantum barriers

Taehee Kang, R. H. Joon-Yeon Kim, Geunchang Choi, Jaiu Lee, Hyunwoo Park, Hyeongtag Jeon, Cheol-Hwan Park, Dai-Sik Kim, 

https://www.nature.com/articles/s41467-018-07365-w

Tunneling is the most fundamental quantum mechanical phenomenon with wide-ranging applications. Matter waves such as electrons in solids can tunnel through a one-dimensional potential barrier, e.g. an insulating layer sandwiched between conductors. A general approach to control tunneling currents is to apply voltage across the barrier. Here, we form closed loops of tunneling barriers exposed to external optical control to manipulate ultrafast tunneling electrons. Eddy currents induced by incoming electromagnetic pulses project upon the ring, spatiotemporally changing the local potential. The total tunneling current which is determined by the sum of contributions from all the parts along the perimeter is critically dependent upon the symmetry of the loop and the polarization of the incident fields, enabling full-wave rectification of terahertz pulses. By introducing global geometry and local operation to current-driven circuitry, our work provides a novel platform for ultrafast optoelectronics, macroscopic quantum phenomena, energy harvesting, and multi-functional quantum devices.

Abstract-Giant field enhancements in ultrathin nanoslots above 1 terahertz

Dasom Kim, Jeeyoon Jeong, Geunchang Choi, Young-Mi Bahk, Taehee Kang, Dukhyung Lee, Bidhek Thusa, Dai-Sik Kim,

https://pubs.acs.org/doi/abs/10.1021/acsphotonics.8b00151?journalCode=apchd5

Strong demand for plasmonic devices with an enormously enhanced electric field and desired resonance frequencies has led to extensive investigations of metallic slot structures. While strong field enhancement can be achieved by reducing the width of the slot, the effect of the gap surface plasmon limits the maximum achievable field enhancement at higher frequencies. Specifically, the effect of the gap surface plasmon becomes stronger as the gap width decreases and strongly suppresses the transmission while causing a red-shift of the resonance. Here, we overcome these issues and realize strong field enhancements at higher frequencies, by managing the metal thickness of the nanoslots. We show that as the nanoslots become as thin as 10 nm, they show a giant electric field enhancement of up to 7600. Moreover, the resonances are strongly blue-shifted to above 1 THz from 0.33 THz. Our work provides a novel route to achieving high field enhancements at desired frequencies, as well as a means by which to characterize the slot as the gap-sensitive or substrate-sensitive type

Abstract-Enhanced Terahertz Shielding of MXenes with Nano-Metamaterials

Geunchang Choi, Faisal Shahzad, Young-Mi Bahk, Young Min Jhon, Hyunchul Park, Mohamed Alhabeb, Babak Anasori, Dai-Sik Kim, Chong Min Koo, Yury Gogotsi, Minah Seo,

http://onlinelibrary.wiley.com/doi/10.1002/adom.201701076/full

Terahertz (THz) shielding becomes increasingly important with the growing development of THz electronics and devices. Primarily materials based on carbon nanostructures or polymer–carbon nanocomposites have been explored for this application. Herein, significantly enhanced THz shielding efficiencies for 2D titanium carbide (Ti3C2 MXene) thin films with nanoscale THz metamaterials are presented. Nanoscale slot antenna arrays with strong resonances at certain frequencies enhance THz electromagnetic waves up to three orders of magnitude in transmission, which in turn enormously increases the shielding performance in combination with MXene films. Drop-casting of a colloidal solution of MXene (a few micrograms of dry material) can produce an ultrathin film (several tens of nanometers in thickness) on a slot antenna array. Consequently, THz waves strongly localized in the near-field regime by the slot antenna undergo enhanced absorption through the film with a magnified effective refractive index. Finally, the combination of an ultrathin MXene film and a nano-metamaterial shows excellent shielding performance in the THz range.

Abstract-Enhanced Terahertz Shielding of MXenes with Nano-Metamaterials

Geunchang Choi, Faisal Shahzad, Young-Mi Bahk, Young Min Jhon, Hyunchul Park, Mohamed Alhabeb, Babak Anasori, Dai-Sik Kim, Chong Min Koo, Yury Gogotsi,

http://onlinelibrary.wiley.com/doi/10.1002/adom.201701076/abstract

Terahertz (THz) shielding becomes increasingly important with the growing development of THz electronics and devices. Primarily materials based on carbon nanostructures or polymer–carbon nanocomposites have been explored for this application. Herein, significantly enhanced THz shielding efficiencies for 2D titanium carbide (Ti3C2 MXene) thin films with nanoscale THz metamaterials are presented. Nanoscale slot antenna arrays with strong resonances at certain frequencies enhance THz electromagnetic waves up to three orders of magnitude in transmission, which in turn enormously increases the shielding performance in combination with MXene films. Drop-casting of a colloidal solution of MXene (a few micrograms of dry material) can produce an ultrathin film (several tens of nanometers in thickness) on a slot antenna array. Consequently, THz waves strongly localized in the near-field regime by the slot antenna undergo enhanced absorption through the film with a magnified effective refractive index. Finally, the combination of an ultrathin MXene film and a nano-metamaterial shows excellent shielding performance in the THz range.

Abstract-Terahertz Nanoprobing of Semiconductor Surface Dynamics

Geunchang Choi, Young-Mi Bahk, Taehee Kang, Yoojin Lee, Byung Hee Son, Yeong Hwan Ahnn,  Minah Seo, Dai-Sik Kim,

http://pubs.acs.org/doi/10.1021/acs.nanolett.7b03289

Most semiconductors have surface dynamics radically different from its bulk counterpart due to surface defect, doping level, and symmetry breaking. Because of the technical challenge of direct observation of the surface carrier dynamics, however, experimental studies have been allowed in severely shrunk structures including nanowires, thin films, or quantum wells where the surface-to-volume ratio is very high. Here, we develop a new type of terahertz (THz) nanoprobing system to investigate the surface dynamics of bulk semiconductors, using metallic nanogap accompanying strong THz field confinement. We observed that carrier lifetimes of InP and GaAs dramatically decrease close to the limit of THz time resolution (∼1 ps) as the gap size decreases down to nanoscale and that they return to their original values once the nanogap patterns are removed. Our THz nanoprobing system will open up pathways toward direct and nondestructive measurements of surface dynamics of bulk semiconductors

Abstract-Terahertz nano probing of semiconductor surface dynamics

Geunchang Choi, Young-Mi Bahk, Taehee Kang, Yoojin Lee, Byung Hee Son, Yeong Hwan Ahn, Minah Seo, and Dai-Sik Kim

http://pubs.acs.org/doi/abs/10.1021/acs.nanolett.7b03289

Most semiconductors have surface dynamics radically different from its bulk counterpart due to surface defect, doping level, and symmetry breaking. Due to the technical challenge of direct observation of the surface carrier dynamics, however, experimental studies have been allowed in severely shrunk structures including nanowires, thin films, or quantum wells where the surface-to-volume ratio is very high. Here, we develop a new type of terahertz (THz) nano probing system to investigate the surface dynamics of bulk semiconductors, using metallic nano gap accompanying strong THz field confinement. We observed that carrier lifetimes of InP and GaAs dramatically decrease close to the limit of THz time resolution (~1 ps) as the gap size decreases down to nano scale, and that they return to their original values once the nano gap patterns are removed. Our THz nano probing system will open up pathways towards direct, and nondestructive measurements of surface dynamics of bulk semiconductors

Abstract-Plasmon Enhanced Terahertz Emission from Single Layer Graphene

Young-Mi Bahk , Gopakumar Ramakrishnan ,Jongho Choi , Hyelynn Song , Geunchang Choi ,Yong Hyup Kim , Kwang Jun Ahn , Dai-Sik Kim , and Paul C. M. Planken

ACS Nano, Just Accepted Manuscript

DOI: 10.1021/nn5025237

Publication Date (Web): August 19, 2014

Copyright © 2014 American Chemical Society

http://pubs.acs.org/doi/abs/10.1021/nn5025237

We show that surface plasmons, excited with femtosecond laser pulses on continuous or discontinuous gold substrates, strongly enhance the generation and emission of ultrashort, broadband terahertz pulses from single layer graphene. Without surface plasmon excitation, for graphene on glass, ‘non-resonant laser-pulse-induced photon drag currents’ appear to be responsible for the relatively weak emission of both s- and p-polarized terahertz pulses. For graphene on a discontinuous layer of gold, only the emission of the p-polarized terahertz electric field is enhanced, whereas the s-polarized component remains largely unaffected, suggesting the presence of an additional terahertz generation mechanism. We argue that in the latter case, ‘surface-plasmon-enhanced optical rectification’, made possible by the lack of inversion symmetry at the graphene on gold surface, is responsible for the strongly enhanced emission. The enhancement occurs because the electric field of surface plasmons is localized and enhanced where the graphene is located: at the surface of the metal. We believe that our results point the way to small, thin and more efficient terahertz photonic devices.