<8> Angstrom-Scale Active Width Control of Nano Slits for Variable Plasmonic Cavity
Dukhyung Lee, Dohee Lee, Hyeong Seok Yun and Dai-Sik Kim
Nanomaterials 11, 2463 (2021).


Abstract: Nanogap slits can operate as a plasmonic Fabry–Perot cavity in the visible and infrared ranges due to the gap plasmon with an increased wave number. Although the properties of gap plasmon are highly dependent on the gap width, active width tuning of the plasmonic cavity over the wafer length scale was barely realized. Recently, the fabrication of nanogap slits on a flexible substrate was demonstrated to show that the width can be adjusted by bending the flexible substrate. In this work, by conducting finite element method (FEM) simulation, we investigated the structural deformation of nanogap slit arrays on an outer bent polydimethylsiloxane (PDMS) substrate and the change of the optical properties. We found that the tensile deformation is concentrated in the vicinity of the gap bottom to widen the gap width proportionally to the substrate curvature. The width widening leads to resonance blue shift and field enhancement decrease. Displacement ratio ((width change)/(supporting stage translation)), which was identified to be proportional to the substrate thickness and slit period, is on the order of 10−5 enabling angstrom-scale width control. This low displacement ratio comparable to a mechanically controllable break junction highlights the great potential of nanogap slit structures on a flexible substrate, particularly in quantum plasmonics.

<7> Twofold Plasmonic Resonator Based on Polyethylene Terephthalate Thin Films for Terahertz Sensing Applications

Jiyeah Rhie, Sung Ju Hong, Dukhyung Lee, Dohee Lee, Hyeong Seok Yun, Young-Mi Bahk, and Dai-Sik Kim

ACS Appl. Nano Mater. 4, 8753 (2021).


Abstract: We demonstrate a chip-scaled reversible terahertz (THz) resonator in which bending the flexible substrate outward breaks a diabolo array into a bowtie array. The resonance frequency shifts from 0.5 THz to nearly twofold (∼1.1 THz) as the resonator bends outward. Tunable THz spectroscopy achieved by mechanical bending is explained by theoretical simulation. For the cases of flattening/bending/reflattening, we analyze electrical current–voltage characteristic and optical properties in THz transmission spectra. While the current–voltage curves subsequently exhibit metal-, capacitor-, and tunneling-like results, THz transmittance shows twofold resonant behavior. In this behavior, we further demonstrated molecular sensing two materials, α-lactose and caffeine, on a single resonator. Considering the volume of the gap region, the detection limit of the molecules within the gap region for the bowtie antenna array is 80.5 and 64.4 pg for lactose and caffeine, respectively. The detection limit is determined by terahertz transmission change (ΔT/T0) and resonance frequency shift (Δf/fres) caused by the molecules within the gap region due to terahertz field confinement. We expect that this approach provides a more stable and functional platform for THz sensing applications.

<6> High sensitivity bolometers based on metal nanoantenna dimers with a nanogap filled with vanadium dioxide
Dukhyung Lee, Dasom Kim, Dai‑Sik Kim, Hyeong‑Ryeol Park, Changhee Sohn, Seon Namgung, Kunook Chung, Young Chul Jun, Dong Kyun Kim, Hyuck Choo and Young‑Geun Roh
Scientific Reports 11, 15863 (2021).


Abstract: One critical factor for bolometer sensitivity is efficient electromagnetic heating of thermistor materials, which plasmonic nanogap structures can provide through the electric field enhancement. In this report, using finite element method simulation, electromagnetic heating of nanorod dimer antennas with a nanogap filled with vanadium dioxide (VO2) was studied for long-wavelength infrared detection. Because VO2 is a thermistor material, the electrical resistance between the two dimer ends depends on the dimer’s temperature. The simulation results show that, due to the high heating ability of the nanogap, the temperature rise is several times higher than expected from the areal coverage. This excellent performance is observed over various nanorod lengths and gap widths, ensuring wavelength tunability and ultrafast operating speed, thereby making the dimer structures a promising candidate for high sensitivity bolometers.

<5> Effective-zero-thickness terahertz slot antennas using stepped structures

Hyeong Seok Yun, Dukhyung Lee, and Dai-Sik Kim

Opt. Express 29, 21262 (2021).


Abstract: Metallic nanostructures play an essential role in electromagnetic manipulations due to the localization and enhancement of electromagnetic waves in nanogaps. Scaling down the dimensions of the gap, such as the gap width and the thickness, is an effective way to enhance light-matter interaction with colossal field enhancement. However, reducing the thickness below 10 nanometers still suffers from fabrication difficulty and unintended direct transmission through metals. Here, we fabricate effective-zero-thickness slot antennas by stepping metals in the vicinity of the gaps to confine electromagnetic waves in tiny volumes. We analyze and simulate terahertz transmission, and demonstrate the absorption enhancement of molecules in the slot antennas. Our fabrication technique provides a simple but versatile tool for maximum field enhancement and molecular sensing.

<4> Augmented All-Optical Active Terahertz Device Using Graphene-Based Metasurface
Geunchang Choi, Tuan Khanh Chau, Sung Ju Hong, Dasom Kim, Sung Hyuk Kim, Dai-Sik Kim, Dongseok Suh, Young-Mi Bahk, Mun Seok Jeong
Adv. Opt. Mater. 9, 2100462 (2021).


Abstract: Photo-excited graphene has a positive (semiconductor-like) or negative (metal-like) response depending on the Fermi level, which is tuned by gate control, doping, and growth. Both negative and positive photoconductive responses have a potential application as an ultrafast optical modulator in the control of light transmission. However, it is challenging to achieve a high on/off ratio in the photo-excited graphene because of a small absorption of electromagnetic waves and a limitation of photo-induced conductivity change. Here, the negative-type high on/off ratio and ultrafast terahertz modulation are experimentally demonstrated using graphene/metal nanoslot antennas. When the graphene covers the nanoslot antennas, the terahertz waves are completely blocked (off-state). This perfect extinction results from the enhanced intraband absorption in graphene by strong localized fields near the nanogap. However, once the optical pump is applied to the graphene/nanoslot antennas, terahertz transmission becomes recovered resonantly (on-state) due to the photo-induced transparency of graphene that leads to a distinctive modulation from off- to on-resonance. Furthermore, the fast carrier relaxation induced by strong terahertz field-driven carrier redistribution is responsible for the faster modulation of transient terahertz transmission. The results will open up pathways toward negative-response terahertz modulation applications with high on/off ratio and ultrafast time scale.

<3> A Transformative Metasurface Based on Zerogap Embedded Template
Bamadev Das, Hyeong Seok Yun, Namkyoo Park, Jeeyoon Jeong and Dai‐Sik Kim
Adv. Opt. Mater. 9, 2002164 (2021).


Abstract: The ideals of reconfigurable metasurfaces would be operation in a broad frequency range with a high extinction ratio and fatigue resistivity. In this paper, all the above is achieved in the microwave regime by transforming a bare metallic film into well‐controlled nanometer sized gaps in a fully reversible manner. It is shown that adjacent metallic patterns deposited at different times can form “zero‐nanometer gaps,” or “zerogaps,” while maintaining the optical and electrical connectivity. The zerogaps readily open and recover with gentle bending and relaxing of the flexible substrate, precisely along the rims of the pre‐patterns of centimeter lengths. In a prototypical pattern of densely packed slit arrays, these gaps when opened serve as antennas achieving transparency for polarizations perpendicular to the length of the gap and shut off all the incident lights when closed. In such transformation between a polarizer and a mirror, 75% of transmission is observed with polarization extinction ratio of 7500 coming back down to 5 orders of magnitude extinction repeatable over 10 000 times. This work has long‐standing implications to metamaterials and metasurfaces as well as the fundamental aspect of extending a picometer scale distance controllability toward the wafer scale.

<2> Ultra-Narrow Metallic Nano-Trenches Realized by Wet Etching and Critical Point Drying
Jeeyoon Jeong, Hyosim Yang, Seondo Park, Yun Daniel Park and Dai-Sik Kim
Nanomater.11, 783 (2021).


Abstract: A metallic nano-trench is a unique optical structure capable of ultrasensitive detection of molecules, active modulation as well as potential electrochemical applications. Recently, wet-etching the dielectrics of metal–insulator–metal structures has emerged as a reliable method of creating optically active metallic nano-trenches with a gap width of 10 nm or less, opening a new venue for studying the dynamics of nanoconfined molecules. Yet, the high surface tension of water in the process of drying leaves the nano-trenches vulnerable to collapsing, limiting the achievable width to no less than 5 nm. In this work, we overcome the technical limit and realize metallic nano-trenches with widths as small as 1.5 nm. The critical point drying technique significantly alleviates the stress applied to the gap in the drying process, keeping the ultra-narrow gap from collapsing. Terahertz spectroscopy of the trenches clearly reveals the signature of successful wet etching of the dielectrics without apparent damage to the gap. We expect that our work will enable various optical and electrochemical studies at a few-molecules-thick level.

<1> Topology-Changing Broadband Metamaterials Enabled by Closable Nanotrenches
Dasom Kim, Hyeong Seok Yun, Bamadev Das, Jiyeah Rhie, Parinda Vasa, Young-Il Kim, Sung-Hoon Choa, Namkyoo Park, Dukhyung Lee, Young-Mi Bahk, and Dai-Sik Kim.
Nano Lett. 20, 4202 (2021).


Abstract: One of the most straightforward methods to actively control optical functionalities of metamaterials is to apply mechanical strain deforming the geometries. These deformations, however, leave symmetries and topologies largely intact, limiting the multifunctional horizon. Here, we present topology manipulation of metamaterials fabricated on flexible substrates by mechanically closing/opening embedded nanotrenches of various geometries. When an inner bending is applied on the substrate, the nanotrench closes and the accompanying topological change results in abrupt switching of metamaterial functionalities such as resonance,chirality, and polarization selectivity. Closable nanotrenches can be embedded in metamaterials of broadband spectrum, ranging from visible to microwave. The 99.9% extinction performance is robust, enduring more than a thousand bending cycles. Our work provides a wafer-scale platform for active quantum plasmonics and photonic application of subnanometer phenomena.