Abstract-Graphene-based crack lithography for high-throughput fabrication of terahertz metamaterials

Sejeong Won, Hyun-June Jung, Dasom Kim, Sang-Hun Lee, DoVan Lam, Hyeon-Don Kim, Kwang-Seop Kim, Seung-Mo Lee, Minah Seo, Dai-Sik Kim, Hak-Joo Lee, Jae-Hyun Kim,



https://www.sciencedirect.com/science/article/abs/pii/S000862231931142X

Terahertz (THz) nanoantennas have significant potential for versatile applications in THz spectroscopy because of their capability for strong electromagnetic field localization. Electron-beam lithography or focused ion beam machining is typically employed to fabricate nanoantenna structures. These nanolithography methods present limitations in the widespread utilization of THz nanoantennas because of their high cost and low productivity. In this work, we proposed graphene-based crack lithography as a high throughput fabrication method for nanoantenna structures. A double-layer graphene interface was introduced to enable independent control of the nanoantenna dimensions and provide graphene-based nanoantenna structures. We analyzed the underlying mechanism of graphene-based cracking and developed an analytical model governing the geometric parameters of the fabricated nanostructures. As a vital application of the fabricated nanoantenna structures, we demonstrated the highly sensitive detection of d-Glucose molecules. Graphene-based crack lithography can provide a cost-effective method for generating nanoantenna structures with the desired characteristics and can accelerate the development of practical applications of electromagnetic metamaterials.

Abstract-Enhanced terahertz conductivity in ultra-thin gold film deposited onto (3-mercaptopropyl) trimethoxysilane (MPTMS)-coated Si substrates

Youjin Lee, Dasom Kim, Jeeyoon Jeong, Jugyoung Kim, Volodymyr Shmid, Oleg Korotchenkov, Parinda Vasa, Young-Mi Bahk,  Dai-Sik Kim

https://www.nature.com/articles/s41598-019-51085-0

Various material properties change considerably when material is thinned down to nanometer thicknesses. Accordingly, researchers have been trying to obtain homogeneous thin films with nanometer thickness but depositing homogeneous few nanometers thick gold film is challenging as it tends to form islands rather than homogenous film. Recently, studies have revealed that treating the substrate with an organic buffer, (3-mercaptopropyl) trimethoxysilane (MPTMS) enables deposition of ultra-thin gold film having thickness as low as 5 nm. Different aspects of MPTMS treatment for ultra-thin gold films like its effect on the structure and optical properties at visible wavelengths have been investigated. However, the effect of the MPTMS treatment on electrical conductivity of ultra-thin gold film at terahertz frequency remains unexplored. Here, we measure the complex conductivity of nanometer-thick gold films deposited onto an MPTMS-coated silicon substrate using terahertz time-domain spectroscopy. Following the MPTMS treatment of the substrate, the conductivity of the films was found to increase compared to those deposited onto uncoated substrate for gold films having the thickness less than 11 nm. We observed 5-fold enhancement in the conductivity for a 7 nm-thick gold film. We also demonstrate the fabrication of nanoslot-antenna arrays in 8.2-nm-thick gold films. The nanoslot-antenna with MPTMS coating has resonance at around 0.5 THz with an electric field enhancement of 44, whereas the nanoslot-antenna without MPTMS coating does not show resonant properties. Our results demonstrate that gold films deposited onto MPTMS-coated silicon substrates are promising advanced materials for fabricating ultra-thin terahertz plasmonic devices.

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-Terahertz wave interaction with metallic nanostructures

Ji-Hun Kang, Dai-Sik Kim,  Minah Seo,

https://www.degruyter.com/downloadpdf/j/nanoph.2018.7.issue-5/nanoph-2017-0093/nanoph-2017-0093.pdf

Understanding light interaction with metallic structures provides opportunities of manipulation of light, and is at the core of various research areas including terahertz (THz) optics from which diverse applications are now emerging. For instance, THz waves take full advantage of the interaction to have strong field enhancement that compensates their relatively low photon energy. As the THz field enhancement have boosted THz nonlinear studies and relevant applications, further understanding of light interaction with metallic structures is essential for advanced manipulation of light that will bring about subsequent development of THz optics. In this review, we discuss THz wave interaction with deep sub-wavelength nano structures. With focusing on the THz field enhancement by nano structures, we review fundamentals of giant field enhancement that emerges from non-resonant and resonant interactions of THz waves with nano structures in both sub- and super- skin-depth thicknesses. From that, we introduce surprisingly simple description of the field enhancement valid over many orders of magnitudes of conductivity of metal as well as many orders of magnitudes of the metal thickness. We also discuss THz interaction with structures in angstrom scale, by reviewing plasmonic quantum effect and electron tunneling with consequent nonlinear behaviors. Finally, as applications of THz interaction with nano structures, we introduce new types of THz molecule sensors, exhibiting ultrasensitive and highly selective functionalities

Abstract-Cross sectional enhancements in terahertz nano antennas

Dai-Sik Kim

https://www.osapublishing.org/abstract.cfm?URI=sensors-2018-SeTh4E.3

We take advantage of recent advances in nano technology to study quantum scale light matter interaction. Terahertz waves are squeezed into extreme aspect ratio slot antennas. In the middle of a nano slot antennas, the electric field intensity can be a million times stronger than the incident one. Thereby, cross sections of molecules can be hugely enhanced [1] while the probing depth decreases [2]. Quantum tunneling of electrons through nanometer and Angstrom sized gaps are inevitable, rendering different dielectric constants compared with bulk counterparts [3-5]. These efforts originated from nearly perfect transmission through terahertz slot antennas with tens of microns of feature sizes, together with its nanometer sized counterparts [6, 7]. We will discuss our recent results on tunneling through closed, macroscopic rings of quantum barriers. Surface current driven tunneling provides a new paradigm compared with the conventional voltage driven tunneling, being ultrasensitive to the global geometry and symmetry of the planar geometry. Macroscopic geometries can be used to govern total tunneling currents, to be used to sensing as well.

© 2018 The Author(s)

Abstract-Terahertz-driven polymerization of resists in nanoantennas

Woongkyu Park, Youjin Lee, Taehee Kang, Jeeyoon Jeong,  Dai-Sik Kim

https://www.nature.com/articles/s41598-018-26214-w


Plasmon-mediated polymerization has been intensively studied for various applications including nanolithography, near-field mapping, and selective functionalization. However, these studies have been limited from the near-infrared to the ultraviolet regime. Here, we report a resist polymerization using intense terahertz pulses and various nanoantennas. The resist is polymerized near the nanoantennas, where giant field enhancement occurs. We experimentally show that the physical origin of the cross-linking is a terahertz electron emission from the nanoantenna, rather than multiphoton absorption. Our work extends nano-photochemistry into the terahertz frequencies.

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-Electromagnon with Sensitive Terahertz Magnetochromism in a Room-Temperature Magnetoelectric Hexaferrite

Sae Hwan Chun, Kwang Woo Shin, Hyung Joon Kim, Seonghoon Jung, Jaehun Park, Young-Mi Bahk, Hyeong-Ryeol Park, Jisoo Kyoung, Da-Hye Choi, Dai-Sik Kim, Gun-Sik Park, J. F. Mitchell, and Kee Hoon Kim

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.120.027202

An electromagnon in the magnetoelectric (ME) hexaferrite ${\mathrm{Ba}}_{0.5}{\mathrm{Sr}}_{2.5}{\mathrm{Co}}_2{\mathrm{Fe}}_{24}{O}_{41}$ (${\mathrm{Co}}_{2}Z$-type) single crystal is identified by time-domain terahertz (THz) spectroscopy. The associated THz resonance is active on the electric field ($E^{\omega }$) of the THz light parallel to the $c$ axis ($\parallel \left[001\right]$), whose spectral weight develops at a markedly high temperature, coinciding with a transverse conical magnetic order below 410 K. The resonance frequency of 1.03 THz at 20 K changes $-8.7\%$ and $+5.8\%$ under external magnetic field ($H$) of 2 kOe along [001] and [120], respectively. A model Hamiltonian describing the conical magnetic order elucidates that the dynamical ME effect arises from antiphase motion of spins which are coupled with modulating electric dipoles through the exchange striction mechanism. Moreover, the calculated frequency shift points to the key role of the Dzyaloshinskii-Moriya interaction that is altered by static electric polarization change under different $H$.

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Abstract-Colossal Terahertz Field Enhancement using Split-Ring Resonators with a Sub-10 nm Gap

Nayeon Kim, Sungjun In, Dukhyung Lee, Jiyeah Rhie, Jeeyoon Jeong, Dai-Sik Kim, Namkyoo Park


http://pubs.acs.org/doi/abs/10.1021/acsphotonics.7b00627?mi=aayia761&af=R&AllField=nano&target=default&targetTab=std

Terahertz (THz) nanogap structures have emerged as versatile platforms for THz science and applications by virtue of their strong in-gap field enhancements and accompanying high levels of sensitivity to gap environments. However, despite their potential, reliable fabrication methods by which to create THz structures with sub-10 nm gaps remain limited. In this work, we fabricated THz split-ring resonator (SRR) arrays featuring a sub-10 nm split gap. Our fabrication method, involving photolithography, argon ion milling, and atomic layer deposition, is a high-throughput technique which is also applicable to the fabrication of other THz structures with sub-10 nm gaps. Through THz-time domain spectroscopy and a numerical simulation, we identified the fundamental magnetic resonances of the nanogap SRRs, at which the electric field enhancement factor is experimentally estimated to be around 7000. This substantial field enhancement makes SRRs with a sub-10 nm gap suitable for the study of high-field phenomena and related applications.

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-Ultimate terahertz field enhancement of single nanoslits

Young-Mi Bahk, Sanghoon Han, Jiyeah Rhie, Joohyun Park, Hyeongtag Jeon, Namkyoo Park, and Dai-Sik Kim

Phys. Rev. B 95, 075424

http://journals.aps.org/prb/abstract/10.1103/PhysRevB.95.075424

A single metallic slit is the simplest plasmonic structure for basic physical understanding of electromagnetic field confinement. By reducing the gap size, the field enhancement is expected to first go up and then go down when the gap width becomes subnanometer because of the quantum tunneling effects. A fundamental question is whether we reach the classical limit of field enhancement before entering the quantum regime, i.e., whether the quantum effects undercut the highest field enhancement classically possible. Here, by performing terahertz time domain spectroscopy on single slits of widths varying from 1.5 nm to 50 µm, we show that ultimate field enhancement determined by the wavelength of light and film thickness can be reached before we hit the quantum regime. Our paper paves way toward designing a quantum plasmonic system with maximum control yet without sacrificing the classical field enhancements.

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Abstract-Terahertz field enhancement in asymmetric and tapered nano-gaps

Sanghoon Han, Young-Mi Bahk, Namkyoo Park, and Dai-Sik Kim
https://www.osapublishing.org/oe/abstract.cfm?uri=oe-24-3-2065

We investigate field enhancement inside metal-insulator-metal gaps with asymmetric thicknesses and tapered shapes in the terahertz regime. Finite-difference time-domain simulations were conducted for calculation of field enhancement factor. The calculation indicates that for asymmetric sample, field enhancement increases proportionally with the decrease of the thinner of the two metal film thicknesses surrounding the gap. Concomitantly, angle variation has little effect on the field enhancement if the thickness of the narrowest gap region is fixed. A model based on the capacitor concept is proposed for intuitive understanding of the phenomena.

© 2016 Optical Society of America

Full Article  |  PDF Article

Abstract-Electromagnetic Saturation of Angstrom-Sized Quantum Barriers at Terahertz Frequencies

Young-Mi Bahk, Bong Joo Kang, Yong Seung Kim, Joon-Yeon Kim, Won Tae Kim, Tae Yun Kim, Taehee Kang, Jiyeah Rhie, Sanghoon Han, Cheol-Hwan Park, Fabian Rotermund, and Dai-Sik Kim

Phys. Rev. Lett. 115, 125501 – Published 16 September 2015

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.115.125501

Metal-graphene-metal hybrid structures allow angstrom-scale van der Waals gaps, across which electron tunneling occurs. We squeeze terahertz electromagnetic waves through these λ/10 000 000 gaps, accompanied by giant field enhancements. Unprecedented transmission reduction of 97% is achieved with the transient voltage across the gap saturating at 5 V. Electron tunneling facilitated by the transient electric field strongly modifies the gap index, starting a self-limiting process related to the barrier height. Our work enables greater interplay between classical optics and quantum tunneling, and provides optical indices to the van der Waals gaps.

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Abstract-Terahertz-triggered phase transition and hysteresis narrowing in a nanoantenna patterned vanadium dioxide film

ZACHARY J. THOMPSON, Andrew J. Stickel, YOUNG-GYUN JEONG, SANGHOON HAN, BYUNG HEE SON, MICHAEL J. PAUL, BYOUNGHWAK LEE, ALI MOUSAVIAN, GIWAN SEO, Hyun-Tak Kim, Yun-Shik Lee, and Dai-Sik Kim

Nano Lett., Just Accepted Manuscript

DOI: 10.1021/acs.nanolett.5b01970

Publication Date (Web): August 24, 2015

Copyright © 2015 American Chemical Society

http://pubs.acs.org/doi/abs/10.1021/acs.nanolett.5b01970

We demonstrate that high-field THz pulses trigger transient insulator-to-metal transition in a nanoantenna patterned vanadium dioxide thin film. THz transmission of vanadium dioxide instantaneously decreases in the presence of strong THz fields. The transient THz absorption indicates that strong THz fields induce electronic insulator-to-metal transition without causing a structural transformation. The transient phase transition is activated on the sub-cycle time scale during which the THz pulse drives the electron distribution of vanadium dioxide far from equilibrium and disturb the electron correlation. The strong THz fields lower the activation energy in the insulating phase. The THz-triggered insulator-to-metal transition gives rise to hysteresis loop narrowing, while lowering the transition temperature both for heating and cooling sequences. THz nanoantennas enhance the field-induced phase transition by intensifying the field strength and improve the detection sensitivity via antenna resonance. The experimental results demonstrate a potential that plasmonic nanostructures incorporating vanadium dioxide can be the basis for ultrafast, energy-efficient electronic and photonic devices.

Abstract-High-throughput fabrication of infinitely long 10 nm slit arrays for terahertz applications

Jeeyoon Jeong, 

Jiyeah Rhie, 

Woojin Jeon, 

Cheol Seong Hwang, 

Dai-Sik Kim

http://link.springer.com/article/10.1007/s10762-014-0135-3

In pursuit of higher field enhancement and applications in terahertz frequency regime, many techniques have been developed and reported for fabrication of high-aspect-ratio metallic nanostructures. While techniques utilizing spacer deposition has successfully overcome the size limit of conventional fabrication tools, they suffer from low throughput or vulnerability to mechanical and chemical treatment, limiting their further application to various fields. In this Letter we report a high-throughput scheme for fabricating metallic gap structures, free from all the aforementioned shortcomings. Vertically aligned gaps are first defined with photolithography and atomic layer deposition, and then made suitable for transmission measurements by etching out predefined sacrificial layers. Existence of the sacrificial layers alleviates many requirements associated with fabrication steps, thereby increasing the overall reliability of the whole process. Using this method we fabricate arrays of 10 nm wide metallic slits whose length is only limited by the substrate size, here 1 cm, and then characterize the sample with terahertz time domain spectroscopy. The sample show steady performance of up to 2500-fold field enhancement even after sonication under various solvents.

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.