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-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.

Abstract-Free-Standing Guided-Mode Resonance Humidity Sensor in Terahertz

Hee JunShin, Sae-HyungKim, KisangPark, Min-CheolLim, Sung-WookChoi, Gyeongsik Ok, 

http://www.sciencedirect.com/science/article/pii/S0924424717308646

We theoretically and experimentally investigate a polymer-based guided-mode resonance (GMR) structure and its application for relative humidity (RH) sensing in the terahertz region. We obtained RH-dependent resonance frequency shifts and transmittance changes of the free-standing polymer GMR sensors with PVA material coating at 20 °C. We numerically simulated the GMR structure in the same frequency region by using the FDTD method. It shows that the water contents in the polyvinyl alcohol (PVA) film are increased up to 6% as RH is increased up to 70%. As a result, we propose a highly sensitive RH sensor, which can be fabricated by a simple method at low cost for widespread usage.

Abstract-High-speed terahertz imaging toward food quality inspection

Gyeongsik Ok, Kisang Park, Hyun Jung Kim, Hyang Sook Chun, and Sung-Wook Choi  
»View Author Affiliations

Applied Optics, Vol. 53, Issue 7, pp. 1406-1412 (2014)
http://dx.doi.org/10.1364/AO.53.001406

http://www.opticsinfobase.org/ao/abstract.cfm?uri=ao-53-7-1406

In contrast to conventional x-ray food inspection systems that have difficulty in detecting low-density materials, a terahertz imaging system can even identify insects and plastics embedded in a food matrix. A reflection-mode continuous-wave terahertz imaging system was therefore developed for application to food quality inspection, which requires fast, compact, and low-cost detection. High-speed operation of the terahertz imaging system was achieved through the use of a beam-steering tool. A reasonable compromise between the spatial resolution and the scan length of an aspheric f-theta scanning lens could be achieved by optimizing the lens parameters.

© 2014 Optical Society of America

Abstract-High-speed terahertz imaging toward food quality inspection

                         Gyeongsik Ok, Kisang Park, Hyun Jung Kim, Hyang Sook , and Sung-Wook Choi

received 12/06/2013; accepted 01/27/2014; posted 01/28/2014; Doc. ID 202672

http://www.opticsinfobase.org/ao/upcoming_pdf.cfm?id=202672

In contrast to conventional X-ray food inspection systems that have difficulty detecting low-density materials, a terahertz (THz) imaging system can even identify insects and plastics embedded in a food matrix. A reflection mode continuous-wave THz imaging system was therefore developed for application in food quality inspection, which requires fast, compact, and low-cost detection. High-speed operation of the THz imaging system was achieved through the use of a beam-steering tool. A reasonable compromise between the spatial resolution and the scan length of an aspheric f-theta scanning lens could be achieved by optimizing the lens parameters.

Abstract-Foreign Object Detection by Sub-Terahertz Quasi-Bessel Beam Imaging

Gyeongsik Ok ¹ , Sung-Wook Choi ¹ , Kyung Hyun Park ²  and Hyang Sook Chun ^1,* 

Abstract: Food quality monitoring, particularly foreign object detection, has recently become a critical issue for the food industry. In contrast to X-ray imaging, terahertz imaging can provide a safe and ionizing-radiation-free nondestructive inspection method for foreign object sensing. In this work, a quasi-Bessel beam (QBB) known to be nondiffracting was generated by a conical dielectric lens to detect foreign objects in food samples. Using numerical evaluation via the finite-difference time-domain (FDTD) method, the beam profiles of a QBB were evaluated and compared with the results obtained via analytical calculation and experimental characterization (knife edge method, point scanning method). The FDTD method enables a more precise estimation of the beam profile. Foreign objects in food samples, namely crickets, were then detected with the QBB, which had a deep focus and a high spatial resolution at 210 GHz. Transmitted images using a Gaussian beam obtained with a conventional lens were compared in the sub-terahertz frequency experimentally with those using a QBB generated using an axicon.