{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,7,16]],"date-time":"2026-07-16T02:47:15Z","timestamp":1784170035227,"version":"3.55.0"},"reference-count":63,"publisher":"MDPI AG","issue":"3","license":[{"start":{"date-parts":[[2026,2,3]],"date-time":"2026-02-03T00:00:00Z","timestamp":1770076800000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"DOI":"10.13039\/501100002383","name":"King Saud University","doi-asserted-by":"crossref","award":["ORF-2025-482"],"award-info":[{"award-number":["ORF-2025-482"]}],"id":[{"id":"10.13039\/501100002383","id-type":"DOI","asserted-by":"crossref"}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>Near-field scanning microwave microscopy (NSMM) offers the ability to probe local electromagnetic properties beyond the classical Abbe diffraction limit, but achieving high resolution over practical scan areas remains challenging. In this work, we introduce a novel three-dimensional (3D) NSMM probe consisting of a split-ring resonator (SRR) coupled to a microstrip line and loaded with vertically extended metallic bars. The 3D loading enhances electric-field localization in the sensing region by introducing field singularities. Full-wave numerical simulations are used to extract the field-spread function (FSF) of the probe and to quantify how probe geometry, stand-off distance, and bar dimensions control the FSF and its spatial-frequency (k-space) content. An imaging model is then developed in which the NSMM image is represented as a convolution between the object and FSF in one and two dimensions. This framework demonstrates that progressively localized FSFs, obtained through 3D loading and resonator miniaturization, systematically improve image fidelity and preserve higher spatial frequencies. The probe is fabricated using printed circuit board technology (PCB) with vertically attached metallic bars, and its performance is validated by imaging a dielectric slab containing a cylindrical air-filled void. The measured line profiles and two-dimensional images are in good agreement in general characteristics with the convolution-based model, confirming that the proposed 3D SRR-based probe operates as a spatial filter whose engineered near-field distribution governs the achievable resolution in NSMM imaging.<\/jats:p>","DOI":"10.3390\/s26030995","type":"journal-article","created":{"date-parts":[[2026,2,3]],"date-time":"2026-02-03T13:58:49Z","timestamp":1770127129000},"page":"995","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":1,"title":["A Novel 3D Probe for Near-Field Scanning Microwave Microscopy"],"prefix":"10.3390","volume":"26","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-9184-9936","authenticated-orcid":false,"given":"Ali M.","family":"Almuhlafi","sequence":"first","affiliation":[{"name":"Department of Electrical Engineering, College of Engineering, King Saud University, Riyadh 12372, Saudi Arabia"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-9403-0029","authenticated-orcid":false,"given":"Omar M.","family":"Ramahi","sequence":"additional","affiliation":[{"name":"Department of Electrical and Computer Engineering, University of Waterloo, Waterloo, ON N2L 3G1, Canada"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2026,2,3]]},"reference":[{"key":"ref_1","unstructured":"Irving, P., and Soutis, C. (2019). Polymer Composites in the Aerospace Industry, Woodhead Publishing."},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"6709","DOI":"10.1039\/C5RA18712E","article-title":"Composite materials for supersonic aircraft radomes with ameliorated radio frequency transmission-a review","volume":"6","author":"Khatavkar","year":"2016","journal-title":"RSC Adv."},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"1700","DOI":"10.1364\/JOSAA.32.001700","article-title":"Electrically thin flat lenses and reflectors","volume":"32","author":"Ruphuy","year":"2015","journal-title":"J. Opt. Soc. Am. A"},{"key":"ref_4","doi-asserted-by":"crossref","first-page":"014901","DOI":"10.1063\/1.5095410","article-title":"Practical approaches to designing and fabricating flat lenses","volume":"126","author":"Sedaghat","year":"2019","journal-title":"J. Appl. Phys."},{"key":"ref_5","doi-asserted-by":"crossref","first-page":"22714","DOI":"10.1038\/srep22714","article-title":"Microwave dielectric characterisation of 3D-printed BaTiO3\/ABS polymer composites","volume":"6","author":"Castles","year":"2016","journal-title":"Sci. Rep."},{"key":"ref_6","doi-asserted-by":"crossref","first-page":"506","DOI":"10.1109\/OJAP.2023.3274782","article-title":"3D Printable Synthetic Metasurface to Realize 2D Beam-Steering Antenna","volume":"4","author":"Ahmed","year":"2023","journal-title":"IEEE Open J. Antennas Propag."},{"key":"ref_7","doi-asserted-by":"crossref","first-page":"413","DOI":"10.1007\/BF02956173","article-title":"Beitr\u00e4ge zur Theorie des Mikroskops und der mikroskopischen Wahrnehmung","volume":"9","author":"Abbe","year":"1873","journal-title":"Arch. Mikrosk. Anat."},{"key":"ref_8","first-page":"25","article-title":"Superresolution optical microscopy","volume":"227","author":"Masters","year":"2020","journal-title":"Springer Ser. Opt. Sci."},{"key":"ref_9","doi-asserted-by":"crossref","first-page":"356","DOI":"10.1080\/14786440808564615","article-title":"XXXVIII. A suggested method for extending microscopic resolution into the ultra-microscopic region","volume":"6","author":"Synge","year":"1928","journal-title":"Lond. Edinb. Dublin Philos. Mag. J. Sci."},{"key":"ref_10","doi-asserted-by":"crossref","first-page":"510","DOI":"10.1038\/237510a0","article-title":"Super-resolution aperture scanning microscope","volume":"237","author":"Ash","year":"1972","journal-title":"Nature"},{"key":"ref_11","doi-asserted-by":"crossref","unstructured":"Zhou, W., and Wang, Z.L. (2007). Scanning Microscopy for Nanotechnology: Techniques and Applications, Springer Science & Business Media.","DOI":"10.1007\/978-0-387-39620-0"},{"key":"ref_12","doi-asserted-by":"crossref","first-page":"930","DOI":"10.1103\/PhysRevLett.56.930","article-title":"Atomic force microscope","volume":"56","author":"Binnig","year":"1986","journal-title":"Phys. Rev. Lett."},{"key":"ref_13","doi-asserted-by":"crossref","first-page":"321","DOI":"10.1016\/S0924-4247(01)00602-1","article-title":"Atomic force microscope for planetary applications","volume":"91","author":"Akiyama","year":"2001","journal-title":"Sens. Actuators A Phys."},{"key":"ref_14","doi-asserted-by":"crossref","first-page":"2599","DOI":"10.1063\/1.339435","article-title":"Atomic resolution imaging of a nonconductor by atomic force microscopy","volume":"62","author":"Albrecht","year":"1987","journal-title":"J. Appl. Phys."},{"key":"ref_15","doi-asserted-by":"crossref","first-page":"113402","DOI":"10.1103\/PhysRevB.64.113402","article-title":"Curvature, hybridization, and STM images of carbon nanotubes","volume":"64","author":"Kleiner","year":"2001","journal-title":"Phys. Rev. B"},{"key":"ref_16","doi-asserted-by":"crossref","first-page":"115421","DOI":"10.1103\/PhysRevB.65.115421","article-title":"Tip influence on plasmon excitations in single gold particles in an STM","volume":"65","author":"Nilius","year":"2002","journal-title":"Phys. Rev. B"},{"key":"ref_17","doi-asserted-by":"crossref","first-page":"165421","DOI":"10.1103\/PhysRevB.66.165421","article-title":"Contrast changes in STM images and relations between different tunneling models","volume":"66","author":"Nieminen","year":"2002","journal-title":"Phys. Rev. B"},{"key":"ref_18","first-page":"3103","article-title":"Near-field scanning optical microscopy II","volume":"9","author":"Isaacson","year":"1991","journal-title":"J. Vac. Sci. Technol. B Microelectron. Nanometer Struct. Process. Meas. Phenom."},{"key":"ref_19","doi-asserted-by":"crossref","first-page":"674","DOI":"10.1063\/1.97565","article-title":"Super-resolution fluorescence near-field scanning optical microscopy","volume":"49","author":"Harootunian","year":"1986","journal-title":"Appl. Phys. Lett."},{"key":"ref_20","doi-asserted-by":"crossref","first-page":"28","DOI":"10.1002\/sca.4950170105","article-title":"The fourier optics of near-field microscopy","volume":"17","author":"Sheppard","year":"1995","journal-title":"Scanning"},{"key":"ref_21","doi-asserted-by":"crossref","first-page":"1437","DOI":"10.1063\/1.334506","article-title":"Scanning capacitance microscopy","volume":"57","author":"Matey","year":"1985","journal-title":"J. Appl. Phys."},{"key":"ref_22","doi-asserted-by":"crossref","first-page":"203","DOI":"10.1063\/1.102096","article-title":"Scanning capacitance microscopy on a 25 nm scale","volume":"55","author":"Williams","year":"1989","journal-title":"Appl. Phys. Lett."},{"key":"ref_23","doi-asserted-by":"crossref","first-page":"2980","DOI":"10.1038\/s41467-021-23253-2","article-title":"The limits of near field immersion microwave microscopy evaluated by imaging bilayer graphene moir\u00e9 patterns","volume":"12","author":"Ohlberg","year":"2021","journal-title":"Nat. Commun."},{"key":"ref_24","doi-asserted-by":"crossref","first-page":"971","DOI":"10.1109\/TMTT.2004.823596","article-title":"Design and fabrication of scanning near-field microwave probes compatible with atomic force microscopy to image embedded nanostructures","volume":"52","author":"Wang","year":"2004","journal-title":"IEEE Trans. Microw. Theory Tech."},{"key":"ref_25","doi-asserted-by":"crossref","first-page":"52","DOI":"10.1109\/MMM.2013.2288711","article-title":"Near-Field Scanning Microwave Microscopy: An Emerging Research Tool for Nanoscale Metrology","volume":"15","author":"Imtiaz","year":"2014","journal-title":"IEEE Microw. Mag."},{"key":"ref_26","doi-asserted-by":"crossref","unstructured":"Anlage, S.M., Talanov, V.V., and Schwartz, A.R. (2007). Principles of Near-Field Microwave Microscopy. Scanning Probe Microscopy, Springer.","DOI":"10.1007\/978-0-387-28668-6_8"},{"key":"ref_27","doi-asserted-by":"crossref","unstructured":"Weinstock, H., and Nisenoff, M. (2001). Near-Field Microwave Microscopy of Materials Properties. Microwave Superconductivity, Springer.","DOI":"10.1007\/978-94-010-0450-3"},{"key":"ref_28","doi-asserted-by":"crossref","first-page":"1778","DOI":"10.1063\/1.121182","article-title":"Quantitative topographic imaging using a near-field scanning microwave microscope","volume":"72","author":"Vlahacos","year":"1998","journal-title":"Appl. Phys. Lett."},{"key":"ref_29","doi-asserted-by":"crossref","first-page":"3846","DOI":"10.1063\/1.1149189","article-title":"Quantitative microwave near-field microscopy of dielectric properties","volume":"69","author":"Gao","year":"1998","journal-title":"Rev. Sci. Instrum."},{"key":"ref_30","doi-asserted-by":"crossref","first-page":"121409","DOI":"10.1103\/PhysRevB.83.121409","article-title":"Detection of defects buried in metallic samples by scanning microwave microscopy","volume":"83","author":"Plassard","year":"2011","journal-title":"Phys. Rev. B"},{"key":"ref_31","doi-asserted-by":"crossref","first-page":"134106","DOI":"10.1063\/1.2189147","article-title":"A near-field scanned microwave probe for spatially localized electrical metrology","volume":"88","author":"Talanov","year":"2006","journal-title":"Appl. Phys. Lett."},{"key":"ref_32","doi-asserted-by":"crossref","first-page":"1032","DOI":"10.1063\/1.1597984","article-title":"Near-field scanning microwave microscope using a dielectric resonator","volume":"83","author":"Kim","year":"2003","journal-title":"Appl. Phys. Lett."},{"key":"ref_33","doi-asserted-by":"crossref","first-page":"1725","DOI":"10.1063\/1.1149658","article-title":"0.4 \u03bcm spatial resolution with 1 GHz (\u03bb = 30 cm) evanescent microwave probe","volume":"70","author":"Su","year":"1999","journal-title":"Rev. Sci. Instrum."},{"key":"ref_34","doi-asserted-by":"crossref","first-page":"114901","DOI":"10.1063\/1.3259366","article-title":"Direct imaging of photoconductivity of solar cells by using a near-field scanning microwave microprobe","volume":"106","author":"Hovsepyan","year":"2009","journal-title":"J. Appl. Phys."},{"key":"ref_35","doi-asserted-by":"crossref","unstructured":"Imtiaz, A. (2005). Quantitative Materials Contrast at High Spatial Resolution with a Novel Near-Field Scanning Microwave Microscope, University of Maryland.","DOI":"10.1063\/1.2234801"},{"key":"ref_36","doi-asserted-by":"crossref","first-page":"1736","DOI":"10.1063\/1.120020","article-title":"Surface resistance imaging with a scanning near-field microwave microscope","volume":"71","author":"Steinhauer","year":"1997","journal-title":"Appl. Phys. Lett."},{"key":"ref_37","doi-asserted-by":"crossref","first-page":"1877","DOI":"10.1109\/TIM.2014.2317295","article-title":"On the crack characteristic signal from an open-ended coaxial probe","volume":"63","author":"Donnell","year":"2014","journal-title":"IEEE Trans. Instrum. Meas."},{"key":"ref_38","doi-asserted-by":"crossref","first-page":"183111","DOI":"10.1063\/1.3514243","article-title":"Atomic resolution imaging at 2.5 GHz using near-field microwave microscopy","volume":"97","author":"Lee","year":"2010","journal-title":"Appl. Phys. Lett."},{"key":"ref_39","doi-asserted-by":"crossref","first-page":"2038","DOI":"10.1038\/s41598-017-02176-3","article-title":"A split ring resonator dielectric probe for near-field dielectric imaging","volume":"7","author":"Isakov","year":"2017","journal-title":"Sci. Rep."},{"key":"ref_40","doi-asserted-by":"crossref","first-page":"2783","DOI":"10.1063\/1.1149795","article-title":"Nondestructive superresolution imaging of defects and nonuniformities in metals, semiconductors, dielectrics, composites, and plants using evanescent microwaves","volume":"70","author":"Pathak","year":"1999","journal-title":"Rev. Sci. Instrum."},{"key":"ref_41","doi-asserted-by":"crossref","first-page":"35","DOI":"10.1080\/09349849409409679","article-title":"Microwave detection of finite surface cracks in metals using rectangular waveguides","volume":"6","author":"Yeh","year":"1994","journal-title":"Res. Nondestruct. Eval."},{"key":"ref_42","doi-asserted-by":"crossref","first-page":"323","DOI":"10.1080\/10589759808953057","article-title":"Microwave measurement-parameter optimization for detection of durface breaking hairline cracks In metals","volume":"14","author":"Zoughi","year":"1998","journal-title":"Nondestruct. Test. Eval."},{"key":"ref_43","doi-asserted-by":"crossref","first-page":"1579","DOI":"10.1063\/1.116685","article-title":"Novel millimeter-wave near-field resistivity microscope","volume":"68","author":"Golosovsky","year":"1996","journal-title":"Appl. Phys. Lett."},{"key":"ref_44","doi-asserted-by":"crossref","first-page":"2073","DOI":"10.1063\/1.1351837","article-title":"Near-field scanning microwave probe based on a dielectric resonator","volume":"72","author":"Golosovsky","year":"2001","journal-title":"Rev. Sci. Instrum."},{"key":"ref_45","doi-asserted-by":"crossref","first-page":"695","DOI":"10.1111\/j.1460-2695.2008.01255.x","article-title":"Microwave and millimetre wave sensors for crack detection","volume":"31","author":"Zoughi","year":"2008","journal-title":"Fatigue Fract. Eng. Mater. Struct."},{"key":"ref_46","doi-asserted-by":"crossref","unstructured":"Wiwatcharagoses, N., Park, K.Y., and Chahal, P. (June, January 29). Metamaterial-inspired miniaturized microwave sensing probes. Proceedings of the 2012 IEEE 62nd Electronic Components and Technology Conference, San Diego, CA, USA.","DOI":"10.1109\/ECTC.2012.6249132"},{"key":"ref_47","doi-asserted-by":"crossref","first-page":"7066","DOI":"10.1109\/JSEN.2018.2852657","article-title":"Design of a Split-Ring Resonator Sensor for Near-Field Microwave Imaging","volume":"18","author":"Mukherjee","year":"2018","journal-title":"IEEE Sens. J."},{"key":"ref_48","doi-asserted-by":"crossref","unstructured":"Abdolrazzaghi, M., Zarifi, M.H., and Daneshmand, M. (November, January 30). Sensitivity enhancement of split ring resonator based liquid sensors. Proceedings of the 2016 IEEE SENSORS, Orlando, FL, USA.","DOI":"10.1109\/ICSENS.2016.7808957"},{"key":"ref_49","doi-asserted-by":"crossref","first-page":"287","DOI":"10.1109\/LMWC.2018.2805866","article-title":"Highly Sensitive Microwaves Sensors for Fluid Concentration Measurements","volume":"28","author":"Albishi","year":"2018","journal-title":"IEEE Microw. Wirel. Compon. Lett."},{"key":"ref_50","doi-asserted-by":"crossref","first-page":"9138","DOI":"10.1109\/JSEN.2021.3053938","article-title":"Three-Dimensional Split-Ring Resonators-Based Sensors for Fluid Detection","volume":"21","author":"Albishi","year":"2021","journal-title":"IEEE Sens. J."},{"key":"ref_51","doi-asserted-by":"crossref","unstructured":"Puentes, M., Sch\u00fc\u00dfler, M., and Jakoby, R. (2011, January 28\u201331). 2D sensor array based on Split Ring Resonators for monitoring of organic tissue. Proceedings of the Sensors, 2011 IEEE, Limerick, Ireland.","DOI":"10.1109\/ICSENS.2011.6126955"},{"key":"ref_52","unstructured":"HFSS-ANSYS, Version 15.0. Available online: https:\/\/www.ansys.com\/products\/electronics\/ansys-hfss."},{"key":"ref_53","doi-asserted-by":"crossref","first-page":"2075","DOI":"10.1109\/22.798002","article-title":"Magnetism from conductors and enhanced nonlinear phenomena","volume":"47","author":"Pendry","year":"1999","journal-title":"IEEE Trans. Microw. Theory Tech."},{"key":"ref_54","doi-asserted-by":"crossref","first-page":"130899","DOI":"10.1016\/j.snb.2021.130899","article-title":"Planar single and dual-resonant microwave biosensors for label-free bacteria detection","volume":"351","author":"Piekarz","year":"2022","journal-title":"Sens. Actuators B Chem."},{"key":"ref_55","doi-asserted-by":"crossref","first-page":"18742","DOI":"10.1109\/JSEN.2021.3090050","article-title":"Noninvasive Glucose Sensing in Aqueous Solutions Using an Active Split-Ring Resonator","volume":"21","author":"Abdolrazzaghi","year":"2021","journal-title":"IEEE Sens. J."},{"key":"ref_56","doi-asserted-by":"crossref","first-page":"174667","DOI":"10.1109\/ACCESS.2020.3025672","article-title":"Metamaterial-Inspired Antenna Array for Application in Microwave Breast Imaging Systems for Tumor Detection","volume":"8","author":"Alibakhshikenari","year":"2020","journal-title":"IEEE Access"},{"key":"ref_57","doi-asserted-by":"crossref","unstructured":"Haq, T.u., Ruan, C., Zhang, X., and Ullah, S. (2019). Complementary Metamaterial Sensor for Nondestructive Evaluation of Dielectric Substrates. Sensors, 19.","DOI":"10.3390\/s19092100"},{"key":"ref_58","doi-asserted-by":"crossref","first-page":"48324","DOI":"10.1109\/ACCESS.2018.2867077","article-title":"Highly-Sensitive Microwave Sensors Based on Open Complementary Split Ring Resonators OCSRRs for Dielectric Characterization and Solute Concentration Measurement in Liquids","volume":"6","author":"Velez","year":"2018","journal-title":"IEEE Access"},{"key":"ref_59","doi-asserted-by":"crossref","first-page":"13310","DOI":"10.1038\/s41598-019-49767-w","article-title":"A pixelated microwave near-field sensor for precise characterization of dielectric materials","volume":"9","author":"Nayyeri","year":"2019","journal-title":"Sci. Rep."},{"key":"ref_60","doi-asserted-by":"crossref","first-page":"2562","DOI":"10.1109\/JSEN.2018.2890089","article-title":"Split Ring Resonator-Based Microwave Fluidic Sensors for Electrolyte Concentration Measurements","volume":"19","author":"Velez","year":"2019","journal-title":"IEEE Sens. J."},{"key":"ref_61","doi-asserted-by":"crossref","first-page":"112112","DOI":"10.1016\/j.sna.2020.112112","article-title":"PCB-integrated non-destructive microwave sensor for liquid dielectric spectroscopy based on planar metamaterial resonator","volume":"312","author":"Chuma","year":"2020","journal-title":"Sens. Actuators A Phys."},{"key":"ref_62","doi-asserted-by":"crossref","first-page":"1451","DOI":"10.1109\/TMTT.2005.845211","article-title":"Equivalent-circuit models for split-ring resonators and complementary split-ring resonators coupled to planar transmission lines","volume":"53","author":"Baena","year":"2005","journal-title":"IEEE Trans. Microw. Theory Tech."},{"key":"ref_63","doi-asserted-by":"crossref","first-page":"442","DOI":"10.1109\/TAP.1972.1140243","article-title":"The behavior of electromagnetic fields at edges","volume":"20","author":"Meixner","year":"1972","journal-title":"IEEE Trans. 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