Abstract-Direct observation of terahertz emission from ultrafast spin dynamics in thick ferromagnetic films

Applied Physics Letters

 Lin Huang, Ji-Wan Kim,  Sang-Hyuk Lee, Seon-Dae Kim,  Van Manh Tien,  Kiran Prakash Shinde, Je-Ho Shim, Yooleemi Shin, Hee Jun Shin, Seongheun Kim, Jaehun Park, Seung-Young Park, Yeon Suk Choi, Hyun-Joong Kim,  Jung-Il Hong,  Dong Eon Kim, Dong-Hyun Kim

Schematic diagram of THz emission and detection by the electro-optic (EO) sampling method. The dotted box area is zoomed to show the sample region with the axis configuration.

https://aip.scitation.org/doi/abs/10.1063/1.5087236

We have experimentally investigated the field- and fluence-dependent terahertz (THz) emission behaviors by a femtosecond optical pump in Co ferromagnetic films, together with the time-resolved magneto-optical Kerr effect measurement. The thickness of Co and Ta capping layers has been systematically varied. For thick films, THz emission behavior is found to be directly matched with the photoinduced ultrafast demagnetization and remagnetization dynamics. The angle-dependent THz emission measurement also confirms a direct relation between the THz emission and the photoinduced demagnetization/remagnetization dynamics in the case of thick films, while THz emission from the inverse spin Hall effect also plays a role in the case of thin Co films.

This work was supported by the Korea Research Foundation (NRF) (Grant Nos. 2018R1A2B3009569 and 2017R1A6A3A04011173) and by KBSI Grant No. D39614. This work was also supported in part by the Global Research Laboratory Program (Grant No. 2009-00439) and by the Max Planck POSTECH/KOREA Research Initiative Program (Grant No. 2016K1A4A4A01922028) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning.

Abstract-Highly Sensitive Detection of 4-Methylimidazole Using a Terahertz Metamaterial

Hee Jun Shin , Hae Won Jang , Gyeongsik Ok,

https://www.preprints.org/manuscript/201810.0421/v1

We demonstrate, for the first time, a highly sensitive detection of 4-methylimidazole (4-MeI), a carcinogenic material, by applying an electric field enhancement technique using a terahertz (THz) metamaterial at the THz region. The THz metamaterials were fabricated with a metal array using an electric-field-coupled inductor-capacitor (ELC) resonator structure, and FDTD simulation showed good agreement with experimental results. We measured the THz spectra of the metamaterials to detect 4-MeI concentrations of 0, 1, 2, 5, 10, 15, and 20 mg/L. The resonance frequency of the metamaterial was shifted by approximately 8 GHz and transmittance at the resonance frequency increased to 2 x 10-3 as the concentration was increased, up to 20 mg/L. Our study provides new insight into application of metamaterials to detect carcinogen using a THz technique.

Abstract-Ultrafast nonlinear travel of hot carriers driven by high-field terahertz pulse

Hee Jun Shin, Van Luan Nguyen, Seong Chu Lim, and Joo-Hiuk Son

http://iopscience.iop.org/article/10.1088/1361-6455/aac59a

We aim to generate high-intensity terahertz (THz) electric fields and study nonlinear phenomena in GaAs and graphene to investigate their applications. To obtain a high-efficiency intense THz field, we employ the tilted pump-pulse front technique using a LiNbO3 crystal. With this technique, we obtain a THz field strength of over 300 kV cm−1. We investigate the hot-carrier dynamics in n- and p-type GaAs driven by high-field THz pulses. Although both samples show similar carrier concentrations, the nonlinear THz responses show different trends. Owing to hot-carrier generation, intervalley scattering is dominant in n-type GaAs, and intervalence band scattering is the main cause in p-type GaAs. In addition, we study the hot-carrier dynamics in graphene with the grain-size dependency. Although graphene has the same Fermi level regardless of the grain size, the THz responses are different for large- and small-grained graphene: charged impurity scattering in large-grained graphene and defect scattering in small-grained graphene. From these results, our study provides insights into high-speed electronics applications.

Abstract-Dielectric traces of food materials in the terahertz region

Hee Jun Shin, Seung Jae Oh, Min-Cheol Lim, Sung-Wook Choi, Gyeongsik Ok

https://www.sciencedirect.com/science/article/pii/S1350449518301440

We investigated the feasibility of identifying food products using the complex dielectric constant of food materials, and insects as a foreign substance, in the terahertz (THz) frequency range from 0.3–1.2 THz. Although the food and insect materials have unique dielectric properties, several materials could not be distinguished by the dielectric constant. To discriminate all food materials completely, we obtained the dielectric traces, in which the real and imaginary parts of the values are plotted. Consequently, food materials and insects can be separated effectively using this analysis. Our results indicate that food materials and foreign substances can be unambiguously distinguished and detected according to the dielectric traces and independent parameters extracted from the complex dielectric constants, using THz time-domain spectroscopy.

Abstract-Invisible Security Printing on Photoresist Polymer Readable by Terahertz Spectroscopy

Hee Jun Shin, Min-Cheol Lim, Kisang Park, Sae-Hyung Kim , Sung-Wook Choi,  Gyeongsik Ok

http://www.mdpi.com/1424-8220/17/12/2825

We experimentally modulate the refractive index and the absorption coefficient of an SU-8 dry film in the terahertz region by UV light (362 nm) exposure with time dependency. Consequently, the refractive index of SU-8 film is increased by approximately 6% after UV light exposure. Moreover, the absorption coefficient also changes significantly. Using the reflective terahertz imaging technique, in addition, we can read security information printed by UV treatment on an SU-8 film that is transparent in the visible spectrum. From these results, we successfully demonstrate security printing and reading by using photoresist materials and the terahertz technique. This investigation would provide a new insight into anti-counterfeiting applications in fields that need security.

Abstract-Thermally controllable filter at terahertz region

 Hee Jun Shin, Min-Cheol Lim, Sae-Hyung Kim, Kisang Park, Sung-Wook Choi, Gyeongsik Ok

https://www.sciencedirect.com/science/article/pii/S1350449517306722

We experimentally investigated terahertz (THz) filters fabricated using dodecanoic acid (DDA) particles and polyethylene (PE) by performing THz time-domain spectroscopy. Since the refractive indices of DDA and PE are the same in the THz range, no optical scattering occurred when they were mixed. However, heating caused air voids to replace the DDA particles, and the DDA melted into the spaces between the PE particles. Furthermore, as the DDA particle size, heating time, and DDA content increased, the THz band width became narrower. Based on the results, we propose a new, low-cost type of THz filter with a simple manufacturing method.