Chae-Hwa Shon

Fast scanning of a pulsed terahertz signal using an oscillating optical delay line

We describe a fast measurement of a pulsed terahertz signal generated by a femtosecond laser and a photoconductive antenna using an oscillating optical delay line. The method to measure the amplitude of the retroreflector in the oscillating optical delay line is proposed and the displacement of the retroreflector is exactly calculated to acquire the optical delay time in the fast scan mode. With the different oscillation frequency and amplitude of the retroreflector, the pulsed terahertz signals are measured and analyzed. The comparison of the temporal waveform and frequency spectrum between the fast scan mode and the slow scan mode shows a good agreement with the decrease in the scanning time from 60 to 1 s at a signal to noise ratio of 430.

Three-Dimensional Particle-in-Cell Simulation of Fast Oscillation Startup in Strapped Magnetron Using Electrically Primed Electrons

Fast oscillation startup in a strapped magnetron using electrically primed electrons is demonstrated by a three-dimensional particle-in-cell simulation. The electrons are electrically primed by five-fold perturbation of the radial electric field along the azimuthal direction in a 10-vane strapped magnetron, and this leads to fast oscillation startup in the desired π-mode. The start of oscillation is advanced from 62 to 52 ns, and the time at which the steady state is reached from 130 to 85 ns when the radial variation of the protrusion and recession of the central region of the anode is 0.3 mm with an angular width of 0.5 mm.

Three-dimensional particle-in-cell simulations of 300GHz reflex klystrons

Three-dimensional (3D) particle-in-cell simulations of 300GHz reflex klystrons are presented. 300GHz electromagnetic wave generation in a resonant cavity is analyzed by using a 3D simulation model in which all the geometric parameters (such as the grid thickness, repeller shape, beam radius, etc.) are described. When an electron beam of an energy of 1.0keV and a net current of 8.9mA is used, the maximum electronic efficiency of energy transfer is observed when the gap transit angle is 0.7πrad, and the efficiency saturates when the beam current is over 10mA. Space charge forces produce a shift in the optimum repeller voltage. It is also shown that the effect of the beam temperature is not critical, even though the bunching wavelength of the electron beam is several times smaller than that in conventional vacuum electron devices. Our simulation results show that a microfabricated 300GHz reflex klystron can directly generate electromagnetic waves with output power levels of several tens of milliwatts.

Localized Degradation of an MgO Layer in an Over-Frequency Accelerated Discharge of a Plasma Display Panel

The experimental and numerical studies of the degradation process of an MgO layer were performed in order to obtain a fundamental guideline for the lifetime evaluation of a plasma display panel (PDP). From the experimental results of an over-frequency accelerated discharge test and surface analyses of the MgO layer, it was found that the degradation process was divided into the sputter-erosion by the bombarding ions and the deposited impurities progressed on the MgO surface during the accelerated discharge. The numerical analysis of the plasma behavior and structure during PDP discharge was performed to explain these experimental results. The localized degradation of the MgO surface was due to the unevenness of the ion bombardment, which was further expedited by the wall charge behavior. The combination of these factors was found to explain the localized degradation process in the MgO layer during the accelerated PDP discharge.

Compression of Pulsed Terahertz Image Using Discrete Wavelet Transform

We present the result of data compression using discrete wavelet transform (DWT) in terahertz pulsed imaging (TPI). A test object composed of dielectric materials is imaged by a fast oscillating delay line in reflection geometry. In general, because the size of the original image is from several megabytes (MB) to tens of MB, an adequate compression algorithm that prevents the loss of relevant information is indispensable. We analyze the image by multilevel DWT and obtain the wavelet coefficients of the levels. The images reconstructed from some of the coefficients are compared with the original image. The reconstructed pulse signals and their frequency-domain spectrums for each pixel are compared with the original signals to obtain the optimum decomposition level. The image could be compressed below 15% of its original size without loss of image quality and significant errors in the time and frequency domains.

Rapid Cellulose-Mediated Microwave Sintering for High-Conductivity Ag Patterns on Paper

Cellulose-based paper is essential in everyday life, but it also has further potentials for use in low-cost, printable, disposable, and eco-friendly electronics. Here, a method is developed for the cellulose-mediated microwave sintering of Ag patterns on conventional paper, in which the paper plays a significant role both as a flexible insulating substrate for the conductive Ag pattern and as a lossy dielectric media for rapid microwave heating. The anisotropic dielectric properties of the cellulose fibers mean that a microwave electric field applied parallel to the paper substrate provides sufficient heating to produce Ag patterns with a conductivity 29-38% that of bulk Ag in a short period of time (∼1 s) at 250-300 °C. Significantly, there is little thermal degradation of the substrate during this process. The microwave-sintered Ag patterns exhibit good mechanical stability against 10 000 bending cycles and can be easily soldered with lead-free solder. Therefore, cellulose-mediated microwave sintering presents a promising means of achieving short processing times and high electrical performance in flexible paper electronics.

Improvement of Two-Dimensional Terahertz Image by Digital Image Processing

Two-dimensional (2D) images that are produced by terahertz (THz) irradiation we presented. It is possible to obtain 2D image of various materials by observing the amplitude and the phase of the THz signals which go through them. Better images are produced by combining the amplitude and phase of the signal rather than using only one of these. Homomorphic filtering that is one elf the well-known technique of digital image signal processing is effective to reduce the noise signal and can provide better quality images. The results can be applied to real-time imaging afterwards.

Back-irradiation photonic sintering for defect-free high-conductivity metal patterns on transparent plastic

Photonic sintering has attracted considerable attention for printed electronics. It irradiates high-intensity light onto the front surface of metal nanoparticle patterns, which often causes defects such as delamination, cavities, and cracks in the patterns. Here, a back-irradiation photonic sintering method is developed for obtaining defect-free high-conductivity metal patterns on a transparent plastic substrate, through which high-intensity light is irradiated onto the back surface of the patterns for a few milliseconds. Ag patterns back-irradiated with ∼10.0 J cm−2 are defect-free in contrast to front-irradiated patterns and exhibited an electrical conductivity of ∼2.3 × 107 S m−1. Furthermore, real-time high-speed observation reveals that the mechanisms that generate defects in the front-irradiated patterns and prevent defects in the back-irradiated patterns are closely related to vapor trapping. In contrast to the latter, in the former, vapor is trapped and delaminates the patterns from the substrate ...

π-mode formation of hybrid metallic photonic-band-gap coaxial resonator

pi-mode formation of hybrid metallic photonic-band-gap (PBG) coaxial resonator for single mode selection is demonstrated by using a finite- difference-time-domain simulation. The TE911-mode as a pi-mode in 18-vane hybrid metallic PBG coaxial resonator is filtered by the TE011-mode in the defect area at the resonant frequency of 30.9 GHz.

Fast scanning and real time monitoring of a pulsed terahertz signal

We present a fast measurement and real time monitoring of a pulsed terahertz signal generated by a femtosecond laser and a photoconductive antenna. Two types of high speed optical delay lines (ODLs) are considered: oscillating type and rotary type. The method to measure the amplitude of the retro-reflector in the oscillating optical delay line is proposed and the displacement of the retro-reflector is exactly calculated to acquire the optical delay time in the fast scan mode. A rotary optical delay line which gives a linear displacement with rotating angle will be described. The rotary ODL was successfully applied to scan a terahertz pulse. The linearity of rotary ODL was confirmed more than 200 ps by comparison with a linear stage ODL. The requirements and possibility of the rotary ODL for application in fast scanning and monitoring of terahertz pulse will be given.