Jongsuck Bae

Scanning near-field millimeter-wave microscopy using a metal slit as a scanning probe

In this paper, a novel type of scanning near-field millimeter-wave microscopy using a metal slit-type probe is proposed. A tapered reduced-height rectangular waveguide forms the slit aperture, which has a width much smaller than one wavelength /spl lambda/ and length of the order of /spl lambda/. The slit probe can be operated in the TE/sub 10/ mode and, thus, results in high transmission efficiency, even when the width is exceedingly small. An image reconstruction algorithm based on computerized tomographic imaging is used to obtain two-dimensional near-field images. Experiments performed at 60 GHz (/spl lambda/=5 mm) show that image resolution equal to the slit width (/spl sim/80 /spl mu/m) is achieved. As an application of this scanning slit microscopy, visualization of transition phenomena of photoexcited free carriers in silicon have been successfully demonstrated, yielding useful information on the dynamics of free carriers in semiconductor materials.

Active control of the emission current of field emitter arrays

The current control and the stabilization in field emission of gated field emitter arrays (FEAs) are the highest demand for applications to a flat panel display and other beam devices. The concept of the field‐effect‐controlled field emission cathode is very useful for these purposes. We carried out preliminary experiments of the idea and showed that the controllability and the stability of emission current of FEA were significantly improved by an actively controlled FEA. Additionally, we discussed beam focusing of FEA for a flat panel display application.

Experimental demonstration for scanning near-field optical microscopy using a metal micro-slit probe at millimeter wavelengths

Scanning near-field optical microscopy using a slit-type probe is discussed. The slit-type probe has a width of much less than a wavelength, λ, and a length on the order of λ, and thus has high transmission efficiency. Two dimensional near-field images of objects have been constructed using an image reconstruction algorithm based on computerized tomographic imaging. Experiments performed at 60 GHz (λ=5 mm) show that this type of near-field microscopy can achieve a spatial resolution of better than λ/45 for two dimensional imaging. A method for fabricating a submicron width slit probe at the end of an optical fiber is presented for extending this microscopy to optical waves.

Refractive index of nematic liquid crystals in the submillimeter wave region

Large electro-optic effects of liquid-crystal materials are attractive in applications to various optical devices in a wider wavelength region. Fundamental optical properties in the submillimeter wave region, such as refractive indices and transmission losses for some cyanobiphenyl nematic liquid crystals, have been investigated for the first time, to our knowledge, with a submillimeter laser. Refractive indices of the liquid crystal materials for ordinary and extraordinary rays are a little larger than those in the visible region, and a larger birefringence comparable with the visible region can also be obtained. Although the loss level is larger by ~2 orders of magnitude than that of quartz plate, which is an excellent window in the submillimeter wave region, the transmission of the liquid crystal cell is high enough.

Spatial power combining of Gunn diodes using an overmoded waveguide resonator at millimeter wavelengths

An oscillator which incorporates an overmoded-waveguide resonator with an array of TE/sub 10/-mode waveguides containing Gunn diodes, has been developed as a means for achieving highly efficient spatial power combing. This oscillator makes use of mode conversion of radiation power from the Gunn diodes in the waveguide array to the overmoded-waveguide resonator, to produce high power at millimeter wavelengths. An efficiency of about 83% and an output power of 1.5 W (CW) at 61.4 GHz, has been achieved with a 3/spl times/3 waveguide Gunn diode array.

Millimeter and submillimeter wave quasi-optical oscillator with Gunn diodes

The grooved-mirror-type Fabry-Perot (GFP) oscillator was used for coherent power-combining of multiple elements in the millimeter- and submillimeter-wave region. The admittance of the Gunn diode in oscillation was measured experimentally in the millimeter-wave region to design the GFP oscillator. The gain characteristics of the diode were found at the frequencies from 42 to 48 GHz from the measured results. With this Gunn diode in the GFP resonator, oscillation was observed. The experimental results indicated that for impedance matching between the diode and the resonant cavity, the groove height must be adjusted. >

A W-band overmoded-waveguide oscillator with Gunn diodes

Spatial power combining of Gunn diodes having an efficiency of greater than 80% has been demonstrated at W-band using an overmoded-waveguide resonator having an array of fundamental mode (TE/sub 10/) waveguides. Nine Gunn diodes contained in the 3/spl times/3 TE/sub 10/-waveguide array have oscillated in a single TE/sub 30/ mode in the overmoded-waveguide resonator, and have produced 0.45-W output power (continuous wave) with a combining efficiency of 55% at 98.8 GHz. This efficiency has been improved to 84% using a small and compact resonator that reduces the number of undesirable modes in the overmoded waveguide. The output mode of TE/sub 30/ in the oscillator has been converted to TE/sub 10/ using a mitered-waveguide junction mode converter.

Millimeter-wave scanning near-field anisotropy microscopy

A millimeter-wave scanning near-field microscopy using a slit-type probe, which permits the observation of electrical anisotropy in the viewed object, is proposed. The slit probe is sensitive to any electrical anisotropy along the object surface direction that is inherent in the object to be imaged, because the electric field at its aperture is linearly polarized. An electrical anisotropy model is incorporated into the image reconstruction process that enables two-dimensional image reconstruction. The details of the model and the reconstruction method adopted in this work are described and experimental results to demonstrate the feasibility of this microscopy format are presented.

Experimental verification of the theory on the inverse Smith–Purcell effect at a submillimeter wavelength

The inverse Smith–Purcell effect is a candidate for laser‐driven linacs utilizing the interaction between laser light and an electron beam traveling just in front of a metallic grating. We have performed experiments to study electron energy spread as a function of electron beam position above the grating. A submillimeter wave laser (CH3F, 496 μm) is used as a driving source. It is found that the energy spread characteristics show exponential decay of the interaction strength (field intensity) in the direction perpendicular to the grating surface, as a classical theory on the effect predicts.

Visualization of photoexcited free carriers by scanning near-field millimeter-wave microscopy

Visualization of transition phenomena of photoexcited free carriers by scanning near-field millimeter-wave microscopy has been demonstrated. A scanning millimeter-wave microscope using a metal slit-type probe and an image reconstruction algorithm based on computerized tomographic imaging has been used in the experiment to achieve two-dimensional time-resolved imaging. Experiments performed at 60 GHz (λ=5 mm) under room temperature conditions show that generation, extinction, and diffusion processes of photoexcited free carriers generated in the silicon layer of a silicon on quartz substrate can be imaged with a time division of one nanosecond and a spatial resolution of 110 μm (∼λ/45).