{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,3,20]],"date-time":"2026-03-20T23:42:42Z","timestamp":1774050162529,"version":"3.50.1"},"reference-count":36,"publisher":"MDPI AG","issue":"21","license":[{"start":{"date-parts":[[2021,10,21]],"date-time":"2021-10-21T00:00:00Z","timestamp":1634774400000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"name":"The Ministry of Trade, Industry, and Energy (MOTIE) and the Ministry of SMEs and Startups (MSS) of the Korean Government","award":["S2524372"],"award-info":[{"award-number":["S2524372"]}]},{"DOI":"10.13039\/501100003696","name":"Electronics and Telecommunications Research Institute","doi-asserted-by":"publisher","award":["21ZB1100, 21ZH1100"],"award-info":[{"award-number":["21ZB1100, 21ZH1100"]}],"id":[{"id":"10.13039\/501100003696","id-type":"DOI","asserted-by":"publisher"}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>Terahertz (THz) imaging techniques are attractive for a wide range of applications, such as non-destructive testing, biological sensing, and security imaging. We investigate practical issues in THz imaging systems based on a solid immersion lens (SIL). The system stability in terms of longitudinal misalignment of the SIL is experimentally verified by showing that the diffraction-limited sub-wavelength beam size (0.7 \u03bb) is maintained as long as the SIL is axially located within the depth-of-focus (~13 \u03bb) of the objective lens. The origin of the fringe patterns, which are undesirable but inevitable in THz imaging systems that use continuous waves, is analytically studied, and a method for minimizing the interference patterns is proposed. By combining two THz images obtained at different axial positions of the object and separated by \u03bb\/4, the interference patterns are significantly reduced, and the information hidden under the interference patterns is unveiled. The broad applicability of the proposed method is demonstrated by imaging objects with different surface profiles. Our work proves that the resolution of conventional THz imaging systems can easily be enhanced by simply inserting a SIL in front of the object with high tolerance in the longitudinal misalignment and provides a method enabling THz imaging for objects with different surface profiles.<\/jats:p>","DOI":"10.3390\/s21216990","type":"journal-article","created":{"date-parts":[[2021,10,21]],"date-time":"2021-10-21T23:27:39Z","timestamp":1634858859000},"page":"6990","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":10,"title":["Investigations on Practical Issues in Solid Immersion Lens Based Sub-Wavelength Terahertz Imaging Technique: System Stability Verification and Interference Pattern Removal"],"prefix":"10.3390","volume":"21","author":[{"given":"Da-Hye","family":"Choi","sequence":"first","affiliation":[{"name":"Terahertz Research Section, Electronics and Telecommunications Research Institute, Daejeon 34129, Korea"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Jun-Hwan","family":"Shin","sequence":"additional","affiliation":[{"name":"Terahertz Research Section, Electronics and Telecommunications Research Institute, Daejeon 34129, Korea"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Il-Min","family":"Lee","sequence":"additional","affiliation":[{"name":"Terahertz Research Section, Electronics and Telecommunications Research Institute, Daejeon 34129, Korea"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Kyung Hyun","family":"Park","sequence":"additional","affiliation":[{"name":"Terahertz Research Section, Electronics and Telecommunications Research Institute, Daejeon 34129, Korea"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"1968","published-online":{"date-parts":[[2021,10,21]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"043001","DOI":"10.1088\/1361-6463\/50\/4\/043001","article-title":"The 2017 terahertz science and technology roadmap","volume":"50","author":"Dhillon","year":"2017","journal-title":"J. Phys. D Appl. Phys."},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"106","DOI":"10.1016\/j.ndteint.2009.09.005","article-title":"Nondestructive evaluation of aircraft composites using reflective terahertz time domain spectroscopy","volume":"43","author":"Stoik","year":"2010","journal-title":"NDT Int."},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"021105","DOI":"10.1063\/1.4955407","article-title":"Highly accurate thickness measurement of multi-layered automotive paints using terahertz technology","volume":"109","author":"Krimi","year":"2016","journal-title":"Appl. Phys. Lett."},{"key":"ref_4","doi-asserted-by":"crossref","first-page":"6849","DOI":"10.1364\/AO.44.006849","article-title":"Terahertz paintmeter for noncontact monitoring of thickness and drying progress in paint film","volume":"44","author":"Yasui","year":"2005","journal-title":"Appl. Opt."},{"key":"ref_5","doi-asserted-by":"crossref","first-page":"585","DOI":"10.1007\/s00339-010-5642-z","article-title":"Non-invasive investigation of art paintings by terahertz imaging","volume":"100","author":"Abraham","year":"2010","journal-title":"Appl. Phys. A"},{"key":"ref_6","doi-asserted-by":"crossref","unstructured":"Cosentino, A. (2016). Terahertz and cultural heritage science: Examination of art and archaeology. Technologies, 4.","DOI":"10.3390\/technologies4010006"},{"key":"ref_7","doi-asserted-by":"crossref","first-page":"3584","DOI":"10.1364\/BOE.10.003584","article-title":"In vivo terahertz imaging to evaluate scar treatment strategies: Silicone gel sheeting","volume":"10","author":"Wang","year":"2019","journal-title":"Biomed. Opt. Express"},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"201","DOI":"10.1109\/TTHZ.2011.2159551","article-title":"THz medical imaging: In vivo hydration sensing","volume":"1","author":"Taylor","year":"2011","journal-title":"IEEE Trans. THz Sci. Technol."},{"key":"ref_9","doi-asserted-by":"crossref","first-page":"6467","DOI":"10.1364\/BOE.9.006467","article-title":"THz in vivo measurements: The effects of pressure on skin reflectivity","volume":"9","author":"Wang","year":"2018","journal-title":"Biomed. Opt. Express"},{"key":"ref_10","doi-asserted-by":"crossref","first-page":"e201700111","DOI":"10.1002\/jbio.201700111","article-title":"In vivo THz imaging of human skin: Accounting for occlusion effects","volume":"11","author":"Sun","year":"2018","journal-title":"J. Biophotonics"},{"key":"ref_11","doi-asserted-by":"crossref","first-page":"3766","DOI":"10.1021\/nl802086x","article-title":"Terahertz Near-Field Nanoscopy of Mobile Carriers in Single Semiconductor Nanodevices","volume":"8","author":"Huber","year":"2008","journal-title":"Nano Lett."},{"key":"ref_12","doi-asserted-by":"crossref","first-page":"3009","DOI":"10.1063\/1.1616668","article-title":"Terahertz imaging with nanometer resolution","volume":"83","author":"Chen","year":"2003","journal-title":"Appl. Phys. Lett."},{"key":"ref_13","doi-asserted-by":"crossref","first-page":"18423","DOI":"10.1364\/OE.26.018423","article-title":"Phase-resolved terahertz self-detection near-field microscopy","volume":"26","author":"Giordano","year":"2018","journal-title":"Opt. Express"},{"key":"ref_14","doi-asserted-by":"crossref","first-page":"056106","DOI":"10.1063\/1.4983114","article-title":"Enhancement of spatial resolution of terahertz imaging systems based on terajet generation by dielectric cube","volume":"2","author":"Pham","year":"2017","journal-title":"APL Photonics"},{"key":"ref_15","doi-asserted-by":"crossref","first-page":"031105","DOI":"10.1063\/1.5026758","article-title":"Dielectric sphere-coupled THz super-resolution imaging","volume":"113","author":"Yang","year":"2018","journal-title":"Appl. Phys. Lett."},{"key":"ref_16","doi-asserted-by":"crossref","first-page":"2615","DOI":"10.1063\/1.103828","article-title":"Solid immersion microscope","volume":"57","author":"Mansfield","year":"1990","journal-title":"Appl. Phys. Lett."},{"key":"ref_17","doi-asserted-by":"crossref","first-page":"388","DOI":"10.1063\/1.112341","article-title":"Near-field optical data storage using a solid immersion lens","volume":"65","author":"Terris","year":"1994","journal-title":"Appl. Phys. Lett."},{"key":"ref_18","doi-asserted-by":"crossref","first-page":"1","DOI":"10.1038\/s42003-019-0317-6","article-title":"Solid immersion microscopy images cells under cryogenic conditions with 12 nm resolution","volume":"2","author":"Wang","year":"2019","journal-title":"Commun. Biol."},{"key":"ref_19","doi-asserted-by":"crossref","first-page":"4122","DOI":"10.1063\/1.1627474","article-title":"Focusing of millimeter-wave radiation beyond the Abbe barrier","volume":"83","author":"Pimenov","year":"2003","journal-title":"Appl. Phys. Lett."},{"key":"ref_20","doi-asserted-by":"crossref","first-page":"128","DOI":"10.1016\/j.infrared.2006.01.021","article-title":"THz-micro-spectroscopy with backward-wave oscillators","volume":"49","author":"Gompf","year":"2006","journal-title":"Infrared Phys. Technol."},{"key":"ref_21","doi-asserted-by":"crossref","first-page":"111102","DOI":"10.1063\/1.5045480","article-title":"Reflection-mode continuous-wave 0.15 \u03bb-resolution terahertz solid immersion microscopy of soft biological tissues","volume":"113","author":"Chernomyrdin","year":"2018","journal-title":"Appl. Phys. Lett."},{"key":"ref_22","doi-asserted-by":"crossref","first-page":"221109","DOI":"10.1063\/1.4984952","article-title":"Solid immersion terahertz imaging with sub-wavelength resolution","volume":"110","author":"Chernomyrdin","year":"2017","journal-title":"Appl. Phys. Lett."},{"key":"ref_23","doi-asserted-by":"crossref","first-page":"1379","DOI":"10.1364\/JOSAB.21.001379","article-title":"Terahertz time-domain spectroscopy characterization of the far-infrared absorption and index of refraction of high-resistivity, float-zone silicon","volume":"21","author":"Dai","year":"2004","journal-title":"J. Opt. Soc. Am. B"},{"key":"ref_24","doi-asserted-by":"crossref","first-page":"061605","DOI":"10.1117\/1.OE.59.6.061605","article-title":"Numerical analysis and experimental study of terahertz solid immersion microscopy","volume":"59","author":"Chernomyrdin","year":"2019","journal-title":"Opt. Eng."},{"key":"ref_25","doi-asserted-by":"crossref","first-page":"864","DOI":"10.1109\/34.709612","article-title":"Selecting the optimal focus measure for autofocusing and depth-from-focus","volume":"20","author":"Subbarao","year":"1998","journal-title":"IEEE Trans. Pattern Anal. Mach. Intell."},{"key":"ref_26","doi-asserted-by":"crossref","first-page":"1","DOI":"10.1002\/(SICI)1097-0320(20000101)39:1<1::AID-CYTO2>3.0.CO;2-J","article-title":"Robust autofocusing in microscopy","volume":"39","author":"Geusebroek","year":"2000","journal-title":"Cytom. A"},{"key":"ref_27","first-page":"023106","article-title":"Focus measure method based on the modulus of the gradient of the color planes for digital microscopy","volume":"57","year":"2018","journal-title":"Opt. Eng."},{"key":"ref_28","doi-asserted-by":"crossref","first-page":"612","DOI":"10.1016\/j.optcom.2018.06.011","article-title":"Continuous-wave off-axis and in-line terahertz digital holography with phase unwrapping and phase autofocusing","volume":"426","author":"Huang","year":"2018","journal-title":"Opt. Comm."},{"key":"ref_29","doi-asserted-by":"crossref","first-page":"81","DOI":"10.1002\/cyto.990060202","article-title":"A Comparison of Different Focus Functions for Use in Autofocus Algorithms","volume":"6","author":"Groen","year":"1985","journal-title":"Cytometry"},{"key":"ref_30","unstructured":"Young, I.T., Zagers, R., van Vliet, L.J., Mullikin, J., Boddeke, F., and Netten, H. (1993, January 25\u201328). Depth-of-Focus in microscopy. Proceedings of the 8th Scandinavian Conference on Image Analysis, Tromso, Norway."},{"key":"ref_31","doi-asserted-by":"crossref","first-page":"483","DOI":"10.1364\/OL.44.000483","article-title":"Shape-from-focus for real-time terahertz 3D imaging","volume":"44","author":"Perraud","year":"2019","journal-title":"Opt. Lett."},{"key":"ref_32","doi-asserted-by":"crossref","first-page":"3865","DOI":"10.1088\/0031-9155\/47\/21\/326","article-title":"Evaluation of image quality in terahertz pulsed imaging using test objects","volume":"47","author":"Fitzgerald","year":"2002","journal-title":"Phys. Med. Biol."},{"key":"ref_33","doi-asserted-by":"crossref","first-page":"325101","DOI":"10.1088\/1361-6463\/aad0ca","article-title":"Interference elimination in terahertz imaging based on inverse image processing","volume":"51","author":"Wang","year":"2018","journal-title":"J. Phys. D Appl. Phys."},{"key":"ref_34","doi-asserted-by":"crossref","first-page":"2492","DOI":"10.1002\/mop.31375","article-title":"Origin of interference fringes and its countermeasures in coherent terahertz imaging","volume":"60","author":"Ning","year":"2018","journal-title":"Microw. Opt. Technol. Lett."},{"key":"ref_35","doi-asserted-by":"crossref","first-page":"877","DOI":"10.1016\/S0031-3203(98)00107-1","article-title":"Fingerprint minutiae extraction from skeletonized binary images","volume":"32","author":"Farina","year":"1999","journal-title":"Pattern Recognit."},{"key":"ref_36","doi-asserted-by":"crossref","first-page":"14695","DOI":"10.1364\/OE.27.014695","article-title":"Terahertz continuous wave system using phase shift interferometry for measuring the thickness of sub-100-\u03bcm-thick samples without frequency sweep","volume":"27","author":"Choi","year":"2019","journal-title":"Opt. Express"}],"container-title":["Sensors"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/1424-8220\/21\/21\/6990\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2025,10,11]],"date-time":"2025-10-11T07:20:15Z","timestamp":1760167215000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/1424-8220\/21\/21\/6990"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2021,10,21]]},"references-count":36,"journal-issue":{"issue":"21","published-online":{"date-parts":[[2021,11]]}},"alternative-id":["s21216990"],"URL":"https:\/\/doi.org\/10.3390\/s21216990","relation":{},"ISSN":["1424-8220"],"issn-type":[{"value":"1424-8220","type":"electronic"}],"subject":[],"published":{"date-parts":[[2021,10,21]]}}}