Saejune Park

Queen Mary, University of London | QMUL

Vibrational modes in the terahertz (THz) frequency range are good indicators of lead halide perovskite's crystallization phase. We performed real-time THz spectroscopy to monitor the crystallization kinetics in lead halide perovskite films. First, THz absorptance was measured while the perovskite film was annealed at different temperatures. During...

We report on both experiments and theory of low-terahertz frequency range (up to 400 GHz) magnetoplasmons in a gated two-dimensional electron gas at low (<4K) temperatures. The evolution of magnetoplasmon resonances was observed as a function of magnetic field at frequencies up to ∼400 GHz. Full-wave 3D simulations of the system predicted the spati...

We demonstrate highly sensitive detection of viruses using terahertz split-ring resonators with various capacitive gap widths. Two types of viruses, with sizes ranging from 60 nm (PRD1) to 30 nm (MS2), were detected at low densities on the metamaterial surface. The dielectric constants of the virus layers in the THz frequency range were first measu...

In this paper, we demonstrate that terahertz (THz) metamaterials are powerful tools for determination of dielectric constants of polymer films and polar liquids. As we deposit a dielectric film on a metamaterial, the resonant frequency shifts, but saturates at a specific thickness due to the limited sensing volume of the metamaterial. From the satu...

Surface plasmon polaritons (SPPs) are electromagnetic waves that have attracted significant interest owing to their subwavelength confinement and the strong field enhancement that they provide. Yet in the terahertz (THz) frequency region of the spectrum, which is well below the plasma frequency of metals, these surface waves are characterized by ex...

Terahertz (THz) electromagnetically induced transparency-like (EIT-like) metasurfaces have been extensively explored and frequently used for sensing, switching, slow light, and enhanced nonlinear effects. Reducing radiation and non-radiation losses in EIT-like systems contributes to increased electromagnetic (EM) field confinement, higher transmiss...

The potential of terahertz (THz) spectroscopy and imaging in skin cancer detection and mainly basal cell carcinoma (BCC), the most common type of skin cancer, was previously studied. Thanks to the significant absorption of THz waves by hydrogen bonds of water and the higher hydration of tumors than normal skin, it is considered an ideal non-invasiv...

We propose a novel method for detecting microplastic particles in water using terahertz metamaterials. Fluidic channels are employed to flow the water, containing polystyrene spheres, on the surface of the metamaterials. Polystyrene spheres are captured only near the gap structure of the metamaterials as the gap areas are functionalized. The resona...

Finite element method (FEM) simulations (employing ANSYS High Frequency Structure Simulator, HFSS) are used to investigate the response of terahertz (THz) frequency range split‐ring resonators (SRRs) integrated with on‐chip THz waveguides to cancerous tissues. Two‐port S‐parameter simulations are performed to obtain the transmission spectra (S21) o...

We investigate using finite element methods how sub‐micrometer to micrometer‐scale coplanar waveguide (CPW) can be used for the detection of fingerprint spectra of very small (of order 10⁻¹⁴ mL) volumes of analytes in the terahertz (THz) frequency range. The electric field distribution is investigated near the waveguide for various gap widths betwe...

We demonstrate engineering of the low-terahertz range plasmonic spectra of two-dimensional electron systems by modifying their geometry. Specifically, we have modelled, fabricated, and measured two devices for comparison. The first device has a rectangular channel, while the second is trapezoidal, designed to support a richer plasmonic spectrum by c...

Near-field microscopy techniques operating in the terahertz (THz) frequency band offer the tantalizing possibility of visualizing with nanometric resolution the localized THz fields supported by individual resonators, micro-structured surfaces, and metamaterials. Such capabilities promise to underpin the future development and characterization of a...

Optically controlled spectral-band selective terahertz (THz) modulators based on metal-organic hybrid metamaterials were investigated. An artificially structured material, which consists of two single split-ring resonators put together on the split gap side, was patterned on a silicon substrate to generate frequency-selective properties. An active...

We have used finite-element methods to design and simulate a tunable terahertz frequency range band-stop filter based on coupled split-ring resonators integrated into planar Goubau-line waveguide. Two split-ring resonators with different geometrical lengths and gap widths were designed to resonate at the same frequency. When the two resonators were...

We investigate the effect of the effective substrate refractive index on the sensitivity of terahertz (THz) metamaterials for dielectric sensing by substrate etching. We found the effective refractive index near the metamaterials can be actively controlled by etching, allowing optimization of the sensitivity. Our experimental findings are in good a...

We report observations of magnetoplasmons at frequencies up to 400 GHz in gated two-dimensional electron gases (2DEG) at low temperature (∼ 2K). We observed bulk magnetoplasmons for frequencies greater than the cyclotron resonance frequency, and attributed resonances occurring at frequencies below the cyclotron resonance to formation of edge magnet...

We investigate the use of finite-element simulations as a novel method for determining the dielectric property of target materials in the terahertz (THz) frequency range using split-ring resonator (SRR) sensing elements integrated into a planar Goubau line (PGL) waveguide. Five such SRRs were designed to support resonances at specific target freque...

We investigate the effect of substrate etching on terahertz frequency range metamaterials using finite-element method simulations. A blue shift was found in the metamaterial resonance with increasing substrate etch depth, caused by a decrease in the effective refractive index. The relative contribution of the substrate’s refractive index to the eff...

Most microbial detection techniques require pretreatment, such as fluorescent labeling and cultivation processes. Here, we propose novel tools for classifying and identifying microorganisms such as molds, yeasts, and bacteria based on their intrinsic dielectric constants in the THz frequency range. We first measured the dielectric constant of films...

We demonstrate a significant enhancement in the sensitivity of split ring resonator terahertz metamaterial dielectric sensors by the introduction of etched trenches into their inductive-capacitive gap area, both through finite element simulations and in experiments performed using terahertz time-domain spectroscopy. The enhanced sensitivity is demo...

A facile synthesis method for the heterostructures of single‐walled carbon nanotubes (SWCNTs) and few‐layer MoS2 is reported. The heterostructures are realized by in situ chemical vapor deposition of MoS2 on individual SWCNTs. Field effect transistors based on the heterostructures display different transfer characteristics depending on the formatio...

We present dielectric constant measurements of thin films using THz metamaterials fabricated on a quartz substrate. The resonance shifts of the metamaterials exhibit saturation behavior with increasing film thickness. The saturation frequency shift varies with the real part of the dielectric constant, from which the numerical expression for the par...

We investigate the interaction of Carbon Nanotubes (CNTs) embedded in a polymer matrix [Poly(methyl methacrylate) (PMMA)] with Ar plasma, which results in the formation of PMMA nanostructures as CNTs act as an etching mask. Due to the large differences in the Ar ion sputtering yields between CNTs and PMMA, PMMA lines with the width comparable to th...

We demonstrate that the local resonance of metamaterials can be tuned by the effects of organic thin films under photoexcitation. Tris (8-hydroxyquinolinato) aluminum (Alq3) layers are deposited on metamaterial/silicon hybrid structures. By varying the thickness of the Alq3 layer on the subwavelength scale, the resonant peak of the metamaterial bec...

We fabricated plasmonic devices operating in the terahertz (THz) frequency range using silver nanowire (AgNW) network films. AgNW films exhibit high conductivity and good transparency in the visible range, with a figure of merit comparable to that of conventional transparent conducting oxide films. The THz conductivity of AgNW films can be improved...

We demonstrated the fabrication of terahertz metamaterial sensor for the accurate and on-site detection of yeast using electrohydrodynamic jet printing, which is inexpensive, simple, and environmentally friendly. The very small sized pattern up to 5 µm-width of electrical split ring resonator unit structures could be printed on a large area on both...

We explored single-layer graphene and graphene field-effect transistors immersed in nitric acid using in-situ Raman spectroscopy. Two distinct stages were observed in the chemical doping process. The first stage involved blue shifts of the G and 2D peaks, whose saturation occurred rapidly with a time constant in the range of 10–25 s depending on...

We demonstrate dielectric substrate effects on the resonance shift of terahertz metamaterials with various metal thicknesses by using finite-difference time-domain simulations. We found a small red shift in the metamaterial resonance with increasing metal thickness for the free-standing case. Conversely, when the metamaterial pattern was supported...

The shape dependence of target materials on the sensitivity of terahertz metamaterial sensors was investigated. Polystyrene microbeads with a known dielectric constant and spherical, ovular, lens-shaped, and star-shaped structures were studied. The resonant frequency showed a clear red-shift after the deposition of low-density microbeads owing to t...

GaN-based ultraviolet (UV) LEDs are widely used in numerous applications, including white light pump sources and high-density optical data storage. However, one notorious issue is low hole injection rate in p-type transport layer due to poorly activated holes and spontaneous polarization, giving rise to insufficient light emission efficiency. There...

We demonstrated sensitive detection of individual yeast cells and yeast films by using slot antenna arrays operating in the terahertz frequency range. Microorganisms located at the slot area cause a shift in the resonant frequency of the THz transmission. The shift was investigated as a function of the surface number density for a set of devices fa...

We report on the effect of dielectric substrate on the resonance shift of metallic metamaterials operating in the terahertz frequency region. The resonance frequencies for various metamaterials obtained by time-domain spectroscopy agree well with calculations based on the finite-difference time-domain method. The dependencies of the resonance frequ...

We performed time-domain terahertz (THz) spectroscopy on reduced graphene oxide (rGO) network films coated on quartz substrates from dispersion solutions by spraying method. The rGO network films demonstrate high conductivity of about 900 S/cm in the THz frequency range after a high temperature reduction process. The frequency-dependent conductivit...