Abstract-Electromagnon with Sensitive Terahertz Magnetochromism in a Room-Temperature Magnetoelectric Hexaferrite

Sae Hwan Chun, Kwang Woo Shin, Hyung Joon Kim, Seonghoon Jung, Jaehun Park, Young-Mi Bahk, Hyeong-Ryeol Park, Jisoo Kyoung, Da-Hye Choi, Dai-Sik Kim, Gun-Sik Park, J. F. Mitchell, and Kee Hoon Kim

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.120.027202

An electromagnon in the magnetoelectric (ME) hexaferrite ${\mathrm{Ba}}_{0.5}{\mathrm{Sr}}_{2.5}{\mathrm{Co}}_2{\mathrm{Fe}}_{24}{O}_{41}$ (${\mathrm{Co}}_{2}Z$-type) single crystal is identified by time-domain terahertz (THz) spectroscopy. The associated THz resonance is active on the electric field ($E^{\omega }$) of the THz light parallel to the $c$ axis ($\parallel \left[001\right]$), whose spectral weight develops at a markedly high temperature, coinciding with a transverse conical magnetic order below 410 K. The resonance frequency of 1.03 THz at 20 K changes $-8.7\%$ and $+5.8\%$ under external magnetic field ($H$) of 2 kOe along [001] and [120], respectively. A model Hamiltonian describing the conical magnetic order elucidates that the dynamical ME effect arises from antiphase motion of spins which are coupled with modulating electric dipoles through the exchange striction mechanism. Moreover, the calculated frequency shift points to the key role of the Dzyaloshinskii-Moriya interaction that is altered by static electric polarization change under different $H$.

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Abstract-Electromagnon with sensitive terahertz magnetochromism in a room-temperature magnetoelectric hexaferrite

Sae Hwan Chun, Kwang Woo Shin, Hyung Joon Kim, Seonghoon Jung, Jaehun Park, Young Mi Bahk, Hyeong Ryeol Park, Ji Soo Kyoung, Da-Hye Choi, Dae-Sik Kim, Gun-Sik Park, John F. Mitchell, and Kee Hoon Kim

https://journals.aps.org/prl/accepted/d0079Y40Oe81095739b74618cf6352f7426e0ff08

An electromagnon in the magnetoelectric (ME) hexaferrite Ba0.5Sr2.5Co2Fe24 O41 (Co2Z-type) single crystal is identified by time-domain terahertz (THz) spectroscopy. The associated THz resonance is active on electric field (E\omega ) of the THz light parallel to the c axis (\textbar \textbar [001]), whose spectral weight develops at a markedly high temperature, coinciding with a transverse conical magnetic order below 410 K. The resonance frequency of 1.03 THz at 20 K changes -8.7 {\%} and +5.8 {\%} under external magnetic field (H) of 2 kOe along [001] and [120], respectively. A model Hamiltonian describing the conical magnetic order elucidates that the dynamical ME effect arises from anti-phase motion of spins which are coupled with modulating electric dipoles through exchange striction mechanism. Moreover, the calculated frequency shift points to the key role of Dzyaloshinskii-Moriya interaction that is altered by static electric polarization change under different H.

Abstract-Improved thickness estimation of liquid water using Kramers–Kronig relations for determination of precise optical parameters in terahertz transmission spectroscopy

Heyjin Son, Da-Hye Choi, and Gun-Sik Park

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-4-4509&origin=search

In terahertz transmission spectroscopy, there is a typical problem of thickness uncertainty, which hampers to determine precise optical parameters of samples. In order to resolve this experimental problem, a method optimizing sample thickness using singly subtractive Kramers–Kronig relations is proposed. For tens of micrometers thick water samples, we improved the accuracy of sample thickness by an order of magnitude (up to sub-micrometer) using the algorithm leading to obtain precise optical parameters of water. The broad applicability of the method is demonstrated for measuring various materials in addition to highly absorbing liquid water in the spectral range from 0.3 to 1.6 THz.

© 2017 Optical Society of America

Abstract-Stiffness measurement using terahertz and acoustic waves for biological samples

Jong-Hyun Yoon, Young-Joong Yang, Jinho Park, Heyjin Son, Hochong Park, Gun-Sik Park, and Chang-Beom Ahn

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-23-25-32671

A method is proposed to measure sample stiffness using terahertz wave and acoustic stimulation. The stiffness-dependent vibration is measured using terahertz wave (T-ray) during an acoustic stimulation. To quantify the vibration, time of the peak amplitude of the reflected T-ray is measured. In our experiment, the T-ray is asynchronously applied during the period of the acoustic stimulation, and multiple measurements are taken to use the standard deviation and the maximum difference in the peak times to estimate the amplitude of the vibration. Some preliminary results are shown using biological samples.

© 2015 Optical Society of America

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Vacuum Electronics Serve As Terahertz Power Source

http://www.mwrf.com/Article/ArticleID/23810/23810.html
Capabilities have greatly advanced for vacuum-electronic-device (VED) sources of terahertz and near-terahertz coherent radiation—both continuous wave (CW) and pulsed sources. Quantum-theory models of some terahertz VEDs have been developed and used with some success. Yet all terahertz VEDs can be explained with purely classical models, which is the approach taken by the following group of researchers: John H. Booske from the University of Wisconsin; Richard J. Dobbs from CPI Canada; Colin D. Joye from the US Naval Research Laboratory; Carol L. Kory from Teraphysics, Inc.; George R. Neil with the Thomas Jefferson National Accelerator Facility; Gun-Sik Park with Seoul National University; Jaehun Park from Korea’s Pohang University of Science and Technology; and Richard J. Temkin from the Massachusetts Institute of Technology.
For high-power devices needing to generate high-power electron currents in particular, the vacuum is an ideal propagation medium. VEDs do have tradeoffs, though. These include the need for a three-dimensional (3D), vacuum-tight enclosure.
The researchers note that terahertz devices based on VED technology cover a total bandwidth exceeding 10.0 THz. Terahertz-device choices can be roughly broken down into three classes. Compact sources with high mobility include backwards-wave oscillators (BWOs). They range from 0.1 to 1.0 THz with 10T-3 through 103 W (CW and pulsed) output power. Another option is compact gyrotons with moderate mobility, which cover 0.1 to 1.0 THz with 10-3 through 106 W (CW and pulsed) output power. Stationary accelerator-based sources, including free electron lasers (FELs), range from 0.2 to 10.0 THz and beyond with 10 through 109 W (average and pulsed) output power. See “Vacuum Electronic High Power Terahertz Sources,” IEEE Transactions On Terahertz Science And Technology, Sept. 2011, p. 54.