Abstract-Imaging Polarization in GaN Surfaces by Laser Terahertz Emission Microscopy

Yuji Sakai, Iwao Kawayama, Hidetoshi Nakanishi, and Masayoshi Tonouchi

https://www.osapublishing.org/abstract.cfm?uri=cleo_si-2017-SM4J.6&origin=search

Polarizations in GaN surfaces are visualized using terahertz emission microscopy. A non-radiative-inversion domain that is hardly distinguishable with photoluminescence imaging was clearly observed with this method.

© 2017 OSA

Abstract-Giant Terahertz-Wave Absorption by Monolayer Graphene in a Total Internal Reflection Geometry

Yoichi Harada, Muhammad Shoufie Ukhtary, Minjie Wang, Sanjay K. Srinivasan, Eddwi H. Hasdeo, Ahmad R.T. Nugraha, G. Timothy Noe, Yuji Sakai, Robert Vajtai, Pulickel M Ajayan, Riichiro Saito, and Junichiro Kono

ACS Photonics, Just Accepted Manuscript

DOI: 10.1021/acsphotonics.6b00663

Publication Date (Web): December 12, 2016

Copyright © 2016 American Chemical Society

http://pubsdc3.acs.org/doi/abs/10.1021/acsphotonics.6b00663

We experimentally demonstrated significant enhancement of terahertz-wave absorption in monolayer graphene by simply sandwiching monolayer graphene between two dielectric media in a total internal reflection geometry. In going through this structure, the evanescent wave of the incident terahertz beam interacts with the sandwiched graphene layer multiple (up to four) times at varying incidence angles. We observed extremely large attenuation (up to ~70% per reflection), especially for s-polarized radiation. The experimental results are quantitatively consistent with our calculations, where we modeled the experiment as an electromagnetic wave reflection process in monolayer graphene. We also derived analytical expressions for the absorptance, showing that the absorptance is proportional to the amount of Joule heating on the graphene surface induced by the terahertz radiation.

Abstract-Visualization of GaN surface potential using terahertz emission enhanced by local defects

Yuji Sakai

, Iwao Kawayama

, Hidetoshi Nakanishi

 & Masayoshi Tonouchi

• Scientific Reports 5, Article number: 13860 (2015)
• doi:10.1038/srep13860

http://www.nature.com/articles/srep13860

Wide-gap semiconductors have received significant attention for their advantages over existing semiconductors in energy-efficient power devices. To realize stable and reliable wide-gap semiconductor devices, the basic physical properties, such as the electric properties on the surface and at the interface, should be revealed. Here, we report visualization of terahertz (THz) emission from the surface of GaN, which is excited by ultraviolet femtosecond laser pulses. We found that the THz emission is enhanced by defects related to yellow luminescence, and this phenomenon is explained through the modification of band structures in the surface depletion layer owing to trapped electrons at defect sites. Our results demonstrate that the surface potential in a GaN surface could be detected by laser-induced THz emission. Moreover, this method enables feasible evaluation of the distribution of non-radiative defects, which are undetectable with photoluminescence, and it contributes to the realization normally-off GaN devices.