Jeong Min Woo

Graphene based salisbury screen for terahertz absorber

A graphene-based, multiband absorber operating in terahertz (THz) frequency range was demonstrated. Graphene film was transferred onto the top of a flexible polymer substrate backed with a gold reflector. The graphene acts as a resistive film that partially attenuates and reflects THz waves. The destructive interference between THz waves reflected from graphene and backside reflector gives rise to perfect absorbance at multiple frequencies. To enhance the absorbance on/off ratio (AR), the conductivity of graphene was varied using a chemical doping method. The resulting p-doped, graphene-based THz absorber exhibited absorbance at maxima and AR higher than 0.95 and 25 dB, respectively.

Dual-band terahertz metamaterials based on nested split ring resonators

Two dual-band terahertz metamaterials based on nested split ring resonators (SRRs) were designed and fabricated on a flexible plastic substrate. Each nested SRR structure composed of two electric field coupled resonators exhibited two transmission minimums, which inherently come from the LC resonances of the respective SRRs. The primary and secondary resonance frequencies can be individually fine-tuned by adjusting the geometry of the respective resonator. The fabricated devices exhibited very low insertion loss of 3 dB in the transmission band and the high attenuation of 27 dB in the stop band.

Terahertz Modulator based on Metamaterials integrated with Metal-Semiconductor-Metal Varactors

The terahertz (THz) band of the electromagnetic spectrum, with frequencies ranging from 300 GHz to 3 THz, has attracted wide interest in recent years owing to its potential applications in numerous areas. Significant progress has been made toward the development of devices capable of actively controlling terahertz waves; nonetheless, further advances in device functionality are necessary for employment of these devices in practical terahertz systems. Here, we demonstrate a low voltage, sharp switching terahertz modulator device based on metamaterials integrated with metal semiconductor metal (MSM) varactors, fabricated on an AlGaAs/InGaAs based heterostructure. By varying the applied voltage to the MSM-varactor located at the center of split ring resonator (SRR), the resonance frequency of the SRR-based metamaterial is altered. Upon varying the bias voltage from 0 V to 3 V, the resonance frequency exhibits a transition from 0.52 THz to 0.56 THz, resulting in a modulation depth of 45 percent with an insertion loss of 4.3 dB at 0.58 THz. This work demonstrates a new approach for realizing active terahertz devices with improved functionalities.

High Volumetric Energy Density Hybrid Supercapacitors Based on Reduced Graphene Oxide Scrolls

The low volumetric energy density of reduced graphene oxide (rGO)-based electrodes limits its application in commercial electrochemical energy storage devices that require high-performance energy storage capacities in small volumes. The volumetric energy density of rGO-based electrode materials is very low due to their low packing density. A supercapacitor with enhanced packing density and high volumetric energy density is fabricated using doped rGO scrolls (GFNSs) as the electrode material. The restacking of rGO sheets is successfully controlled through synthesizing the doped scroll structures while increasing the packing density. The fabricated cell exhibits an ultrahigh volumetric energy density of 49.66 Wh/L with excellent cycling stability (>10 000 cycles). This unique design strategy for the electrode material has significant potential for the future supercapacitors with high volumetric energy densities.

Graphene Oxide–Phosphor Hybrid Nanoscrolls with High Luminescent Quantum Yield: Synthesis, Structural, and X-ray Absorption Studies

Highly luminescent graphene oxide (GO)-phosphor hybrid thin films with a maximum quantum yield of 9.6% were synthesized via a simple chemical method. An intense luminescence emission peak at 537 nm and a broad emission peak at 400 nm were observed from the GO-phosphor hybrid films. The maximum quantum yield of the emissions from the hybrid films was found to be 9.6%, which is 48 times higher than that of pristine GO films. The GO-phosphor hybrids were prepared via spin-coating and subsequent postannealing of the films, resulting in scrolling of the GO sheets. The resulting GO nanoscrolls exhibited a length of ∼2 μm with nanoscale interior cavities. Transmission electron microscopy and selected-area electron diffraction analyses revealed that the lattice structure of the tubular scrolls is similar to that of carbon nanotubes. While pristine GO films are p-type, in the GO-phosphor hybrids, the Fermi level shifted upward and fell between the HOMO-LUMO gap due to phosphor attachment via C-N bonding. The highly luminescent GO-phosphor hybrids will find important applications in graphene-based optoelectronic devices.

Low-loss flexible bilayer metamaterials in THz regime

Low insertion-loss single-layer and bilayer metamaterial filters in terahertz (THz) frequency regime were demonstrated on top of low cost flexible Scotch tape by utilizing pattern transfer method. The transmittance of the flexible 51-μm-thick Scotch tape was found out to be higher than 0.85 in the range of 0.2 to 3 THz, which is excellent for the substrate materials for THz applications. Free standing filters exhibited record low insertion loss of 0.6 dB and band rejection ratio as high as 30 dB. The resonance reflection characteristics of the bilayer filters were maintained when they were attached on top of curved PET bottle or metallic surfaces, providing promising application in THz identifications.

A terahertz in-line polarization converter based on through-via connected double layer slot structures

A terahertz (THz) in-line polarization converter that yields a polarization conversion ratio as high as 99.9% is demonstrated at 1 THz. It has double-layer slot structures oriented in orthogonal directions that are electrically connected by 1/8-wavelngth-long through-via holes beside the slot structures. The slots on the front metal-plane respond to the incident THz wave with polarization orthogonal to the slots and generates a circulating surface current around the slots. The surface current propagates along a pair of through-via holes that function as a two-wire transmission line. The propagating current generates a surface current around the backside slot structures oriented orthogonal to the slot structures on the front metal layer. The circulating current generates a terahertz wave polarized orthogonal to the backside slot structures and the 90° polarization conversion is completed. The re-radiating THz wave with 90° converted polarization propagates in the same direction as the incident THz wave.

Quad-band THz frequency selective surface based on a split ring resonator loaded with multiple slots

A quad-band terahertz (THz) frequency selective surface with bandstop response is designed, fabricated, and characterized. The unit cell of the filter consists of a split ring resonator (SRR) and surrounding four groups of slots. Each group of slots has three slots with different lengths. The lowest frequency band is determined by the resonance of the SRR and the other three upper bands are selected by the resonance of the slots which exhibit bandstop characteristics rather than bandpass characteristics. The filter transformation characteristic originates from the slits placed between the unit cells which act as series-connected capacitive components. The fabricated quad-band THz filters exhibited bandstop characteristics with attenuation higher than 20 dB in all the operating frequency bands.

Terahertz filter integrated with a subwavelength structured antireflection coating

Micro-pyramid shaped subwavelength structures (SWSs) were integrated on both sides of a terahertz (THz) filter by means of stamping methods. Two silicon-based stamping molds fabricated via crystallographic wet etching were utilized to replicate SWSs onto cyclo-olefin copolymer (COC) films coated onto both sides of a THz filter at the same time. The SWSs act as an broadband antireflection coating to reduce the surface reflection loss in a frequency range of 0.2 THz to 1.4 THz. Compared to a THz filter without SWSs, the filter integrated with double-sided SWSs exhibits a low standing wave ratio inside the substrate and THz signal transmission enhancement of up to 10.8%.