J. T. Hong

Detection of microorganisms using terahertz metamaterials

Microorganisms such as fungi and bacteria cause many human diseases and therefore rapid and accurate identification of these substances is essential for effective treatment and prevention of further infections. In particular, contemporary microbial detection technique is limited by the low detection speed which usually extends over a couple of days. Here we demonstrate that metamaterials operating in the terahertz frequency range shows promising potential for use in fabricating the highly sensitive and selective microbial sensors that are capable of high-speed on-site detection of microorganisms in both ambient and aqueous environments. We were able to detect extremely small amounts of the microorganisms, because their sizes are on the same scale as the micro-gaps of the terahertz metamaterials. The resonant frequency shift of the metamaterials was investigated in terms of the number density and the dielectric constants of the microorganisms, which was successfully interpreted by the change in the effective dielectric constant of a gap area.

Terahertz conductivity of reduced graphene oxide films

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 conductivities and the refractive indexes of the rGO films have been obtained and analyzed with respect to the Drude free-electron model, which is characterized by large scattering rate. Finally, we demonstrate that the THz conductivities can be manipulated by controlling the reduction process, which correlates well with the DC conductivity above the percolation limit.

Terahertz Wave Applications of Single-Walled Carbon Nanotube Films with High Shielding Effectiveness

We demonstrate that a filtration method is efficient for the fabrication of thick single-walled nanotube films and is capable of shielding terahertz waves. Shielding effectiveness can be engineered by controlling the film thickness and we achieved 38 dB for a 950-nm-thick film. In addition, we found that the films exhibit a dispersion of dielectric constant obeying the Drude free-electron model, whereas the plasma frequency decreases with increasing film thickness. Based on the nanotube films with a thickness greater than the skin depth, we fabricated grid polarizers by laser-machining process, which enable us to achieve a large polarization extinction ratio.

Imaging Ultrafast Carrier Transport in Nanoscale Field-Effect Transistors

One-dimensional nanoscale devices, such as semiconductor nanowires (NWs) and single- walled carbon nanotubes (SWNTs), have been intensively investigated because of their potential application of future high-speed electronic, optoelectronic, and sensing devices. To overcome current limitations on the speed of contemporary devices, investigation of charge carrier dynamics with an ultrashort time scale is one of the primary steps necessary for developing high-speed devices. In the present study, we visualize ultrafast carrier dynamics in nanoscale devices using a combination of scanning photocurrent microscopy and time- resolved pump-probe techniques. We investigate transit times of carriers that are generated near one metallic electrode and subsequently transported toward the opposite electrode based on drift and diffusion motions. Carrier dynamics have been measured for various working conditions. In particular, the carrier velocities extracted from transit times increase for a larger negative gate bias, because of the increased field strength at the Schottky barrier.In the present study, we visualize ultrafast carrier dynamics in one-dimensional nanoscale devices, such as Si nanowire and carbon nanotube transistors using femtosecond photocurrent microscopy. We investigate transit times of ultrashort carriers that are generated near one metallic electrode and subsequently transported toward the opposite electrode based on drift and diffusion motions. Conversely, pure diffusion motion is observed when the pump pulse is located in the middle of the nanowires. Carrier dynamics have been addressed for various working conditions, in which we found that the carrier velocity and pulse width can be manipulated by the external electrodes. In particular, the carrier velocities extracted from transit times increase for a larger negative gate bias because of the increased field strength at the Schottky barrier.

Crystallization Kinetics of Lead Halide Perovskite Film Monitored by In Situ Terahertz Spectroscopy

Vibrational modes in the terahertz (THz) frequency range are good indicators of lead halide perovskites crystallization phase. We performed real-time THz spectroscopy to monitor the crystallization kinetics in the perovskite films. First, THz absorptance was measured while the perovskite film was annealed at different temperatures. By analyzing the Avrami exponent, we observed an abrupt dimensionality switch (from 1D to 2D) with increasing temperature starting at approximately 90 °C. We also monitored the laser-induced crystallinity enhancement of the preannealed perovskite film. The THz absorptance increased initially, then subsequently decayed over a couple of hours, although the enhancement factor varies depending on the film crystallinity. In particular, the Avrami analysis implied that the light-induced crystallization was assisted by the 1D diffusion processes. The activation photon energy was measured at 2.3 eV, which indicated that enhanced crystallization originated from the photoinduced structural change of residual lead iodide at the grain boundary.

Terahertz slot antenna devices fabricated on silver nanowire network films

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 by post-treatment procedures such as welding using graphene oxide flakes. Using photolithography, we fabricated the slot antenna arrays whose resonance behaviors are determined by geometric parameters such as the length of individual elements. The plasmonic resonance varied with the sheet resistances of the film, enabling us to manipulate the quality factors and the peak position of the resonance, in particular, by controlling the films thickness and by the post-procedures such as the chemical vapor treatment.

Electronic Band Alignment at Complex Oxide Interfaces Measured by Scanning Photocurrent Microscopy

The band alignment at an Al2O3/SrTiO3 heterointerface forming a two-dimensional electron gas (2DEG) was investigated using scanning photocurrent microscopy (SPCM) in an electrolyte-gated environment. We used a focused UV laser source for above-the-bandgap illumination on the SrTiO3 layer, creating electron-hole pairs that contributed to the photocurrent through migration towards the metal electrodes. The polarity of the SPCM signals of a bare SrTiO3 device shows typical p-type behavior at zero gate bias, in which the photogenerated electrons are collected by the electrodes. In contrast, the SPCM polarity of 2DEG device indicates that the hole carriers were collected by the metal electrodes. Careful transport measurements revealed that the gate-dependent conductance of the 2DEG devices exhibits n-type switching behavior. More importantly, the SPCM signals in 2DEG devices demonstrated very unique gate-responses that cannot be found in conventional semiconducting devices, based on which we were able to perform detailed investigation into the electronic band alignment of the 2DEG devices and obtain the valence band offset at the heterointerface.

UV-induced terahertz wave modulation in free-standing ZnO nanowire films

We present terahertz (THz) wave modulation by using free-standing ZnO nanowire (NW) network films. The ZnO NW films were virtually transparent against THz waves without UV illumination. Conversely, the THz waves were attenuated under very low-intensity UV illumination, making the ZnO NW film a promising platform for low-loss, low-power and all-optical THz modulators. The complex dielectric constants measurements reveal that the UV laser induces an enhancement in the ac conductivity while leaving the real part of the dielectric constant unchanged. The relatively slow time response implies that the UV-induced modulation is closely linked to the surface trap states. The THz attenuations showed clear saturation behavior with respect to the UV intensity, from which we extracted the ZnO NW surface trap density.

Enhanced sensitivity in THz plasmonic sensors with silver nanowires

We developed hybrid slot antenna structures for microbial sensing in the THz frequency range, where silver nanowires (AgNWs) were employed to increase the sensitivity. In order to fabricate the hybrid devices, we partially etched the AgNW in the slot antenna region, where we can expect the field enhancement effect at the AgNW tip. We measured the resonant-frequency shift observed upon the deposition of a polymer layer, and observed that the sensitivity increased upon the introduction of AgNWs, with an enhancement factor of more than four times (approximately six times in terms of figure-of-merit). The sensitivity increased with the AgNW density until saturation. In addition, we tested devices with PRD1 viruses, and obtained an enhancement factor of 3.4 for a slot antenna width of 3 μm. Furthermore, we performed finite-difference time-domain simulations, which confirmed the experimental results. The sensitivity enhancement factor decreased with the decrease of the slot width, consistent with the experimental findings. Two-dimensional mapping of the electric field confirmed the strong field localization and enhancement at the AgNW tips.

Enhanced transmission of terahertz waves through subwavelength apertures in carbon nanotube network films

We demonstrate enhanced terahertz wave transmission through carbon nanotube films with subwavelength apertures. The peak frequency of the transmitted waves matches well with the shape resonance, mainly determined by the length of the apertures and the refractive index of surrounding media.