A uniform current injection is important in long-wavelength vertical-cavity surface-emitting lasers (VCSELs) for lowthreshold, high-efficiency, and stable-mode operation. The utilization of a tunnel junction can solve these problems by employing an n-type highly conductive material for an anode. In this study, the electrical property of a long-wavelength VCSEL with a tunnel junction is analyzed using an equivalent circuit model. The optical absorption loss of a tunnel-junction-embedded VCSEL is also investigated. This results in the quantitative estimation of lasers with a tunnel junction in terms of device resistance, carrier distribution in the active region and optical absorption loss. The introduction of a tunnel junction may improve the output power, power-conversion efficiency, and mode stability of long-wavelength VCSELs.
We had proposed and developed a THz-wave apparatus for the nondestructive detection of illicit drugs and hazardous substances hidden in sealed envelopes. In actual inspection, we have proposed the following procedure: First, Xray pre-inspection is carried out; second, the measurement of scattered THz waves is applied to pick up suspicious mails; and, third, the identification of the drugs is done by spectral fingerprinting. In this paper, our recent developments of this application and the prototype apparatus for the inspection will be presented.
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