{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,3,19]],"date-time":"2026-03-19T19:25:55Z","timestamp":1773948355362,"version":"3.50.1"},"reference-count":21,"publisher":"MDPI AG","issue":"9","license":[{"start":{"date-parts":[[2018,9,7]],"date-time":"2018-09-07T00:00:00Z","timestamp":1536278400000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"DOI":"10.13039\/501100000923","name":"Australian Research Council","doi-asserted-by":"publisher","award":["FL130100044"],"award-info":[{"award-number":["FL130100044"]}],"id":[{"id":"10.13039\/501100000923","id-type":"DOI","asserted-by":"publisher"}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>Whispering gallery mode (WGM) resonators have become increasingly diverse in terms of both architecture and applications, especially as refractometric sensors, allowing for unprecedented levels of sensitivity. However, like every refractometric sensor, a single WGM resonator cannot distinguish temperature variations from changes in the refractive index of the surrounding environment. Here, we investigate how breaking the symmetry of an otherwise perfect fluorescent microsphere, by covering half of the resonator with a high-refractive-index (RI) glue, might enable discrimination of changes in temperature from variations in the surrounding refractive index. This novel approach takes advantage of the difference of optical pathway experienced by WGMs circulating in different equatorial planes of a single microsphere resonator, which induces mode-splitting. We investigated the influence of the surrounding RI of the microsphere on mode-splitting through an evaluation of the sphere\u2019s WGM spectrum and quality factor (Q-factor). Our results reveal that the magnitude of the mode-splitting increases as the refractive index contrast between the high-refractive-index (RI) glue and the surrounding environment increases, and that when they are equal no mode-splitting can be seen. Investigating the refractive index sensitivity of the individual sub modes resulting from the mode-splitting unveils a new methodology for RI sensing, and enables discrimination between surrounding refractive index changes and temperature changes, although it comes at the cost of an overall reduced refractive index sensitivity.<\/jats:p>","DOI":"10.3390\/s18092987","type":"journal-article","created":{"date-parts":[[2018,9,7]],"date-time":"2018-09-07T11:47:41Z","timestamp":1536320861000},"page":"2987","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":18,"title":["Mode-Splitting for Refractive Index Sensing in Fluorescent Whispering Gallery Mode Microspheres with Broken Symmetry"],"prefix":"10.3390","volume":"18","author":[{"ORCID":"https:\/\/orcid.org\/0000-0003-2533-5213","authenticated-orcid":false,"given":"Yvonne Q.","family":"Kang","sequence":"first","affiliation":[{"name":"School of Engineering, University of South Australia, Mawson Lakes, SA 5095, Australia"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Alexandre","family":"Fran\u00e7ois","sequence":"additional","affiliation":[{"name":"School of Engineering, University of South Australia, Mawson Lakes, SA 5095, Australia"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-6803-0666","authenticated-orcid":false,"given":"Nicolas","family":"Riesen","sequence":"additional","affiliation":[{"name":"School of Engineering, University of South Australia, Mawson Lakes, SA 5095, Australia"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Tanya M.","family":"Monro","sequence":"additional","affiliation":[{"name":"School of Engineering, University of South Australia, Mawson Lakes, SA 5095, Australia"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"1968","published-online":{"date-parts":[[2018,9,7]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","unstructured":"Fran\u00e7ois, A., Zhi, Y., and Meldrum, A. (2016). Whispering gallery mode devices for sensing and biosensing. Photonic Materials for Sensing, Biosensing and Display Devices, Springer.","DOI":"10.1007\/978-3-319-24990-2_9"},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"17067","DOI":"10.1364\/OE.23.017067","article-title":"Optimization of whispering gallery resonator design for biosensing applications","volume":"23","author":"Reynolds","year":"2015","journal-title":"Opt. Express"},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"4057","DOI":"10.1063\/1.1482797","article-title":"Protein detection by optical shift of a resonant microcavity","volume":"80","author":"Vollmer","year":"2002","journal-title":"Appl. Phys. Lett."},{"key":"ref_4","doi-asserted-by":"crossref","first-page":"933","DOI":"10.1038\/nnano.2014.180","article-title":"Single-molecule nucleic acid interactions monitored on a label-free microcavity biosensor platform","volume":"9","author":"Baaske","year":"2014","journal-title":"Nat. Nanotechnol."},{"key":"ref_5","doi-asserted-by":"crossref","first-page":"168","DOI":"10.1364\/AOP.7.000168","article-title":"Whispering gallery mode sensors","volume":"7","author":"Foreman","year":"2015","journal-title":"Adv. Opt. Photonics"},{"key":"ref_6","doi-asserted-by":"crossref","first-page":"4036","DOI":"10.1021\/acs.analchem.6b00365","article-title":"Dynamic self-referencing approach to whispering gallery mode biosensing and its application to measurement within undiluted serum","volume":"88","author":"Reynolds","year":"2016","journal-title":"Anal. Chem."},{"key":"ref_7","doi-asserted-by":"crossref","first-page":"2577","DOI":"10.1364\/OE.23.002577","article-title":"Protein biosensing with fluorescent microcapillaries","volume":"23","author":"Lane","year":"2015","journal-title":"Opt. Express"},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"1104","DOI":"10.1364\/OL.18.001104","article-title":"Strain-sensitive elastic scattering from cylinders","volume":"18","author":"Huston","year":"1993","journal-title":"Opt. Lett."},{"key":"ref_9","doi-asserted-by":"crossref","first-page":"418","DOI":"10.1016\/j.bios.2009.07.021","article-title":"In-vitro sensing of biomechanical forces in live cells by a whispering gallery mode biosensor","volume":"25","author":"Himmelhaus","year":"2009","journal-title":"Biosens. Bioelectron."},{"key":"ref_10","doi-asserted-by":"crossref","first-page":"024011","DOI":"10.1103\/PhysRevApplied.5.024011","article-title":"Detection of single nanoparticles using the dissipative interaction in a high-Q microcavity","volume":"5","author":"Shen","year":"2016","journal-title":"Phys. Rev. Appl."},{"key":"ref_11","doi-asserted-by":"crossref","first-page":"043847","DOI":"10.1103\/PhysRevA.90.043847","article-title":"Mode broadening induced by nanoparticles in an optical whispering-gallery microcavity","volume":"90","author":"Hu","year":"2014","journal-title":"Phys. Rev. A"},{"key":"ref_12","doi-asserted-by":"crossref","first-page":"1051","DOI":"10.1364\/JOSAB.17.001051","article-title":"Rayleigh scattering in high-Q microspheres","volume":"17","author":"Gorodetsky","year":"2000","journal-title":"JOSA B"},{"key":"ref_13","doi-asserted-by":"crossref","first-page":"46","DOI":"10.1038\/nphoton.2009.237","article-title":"On-chip single nanoparticle detection and sizing by mode splitting in an ultrahigh-Q microresonator","volume":"4","author":"Zhu","year":"2010","journal-title":"Nat. Photonics"},{"key":"ref_14","doi-asserted-by":"crossref","first-page":"428","DOI":"10.1038\/nnano.2011.99","article-title":"Detecting single viruses and nanoparticles using whispering gallery microlasers","volume":"6","author":"He","year":"2011","journal-title":"Nat. Nanotechnol."},{"key":"ref_15","doi-asserted-by":"crossref","first-page":"1600265","DOI":"10.1002\/lpor.201600265","article-title":"Fluorescent and lasing whispering gallery mode microresonators for sensing applications","volume":"11","author":"Reynolds","year":"2017","journal-title":"Laser Photonics Rev."},{"key":"ref_16","doi-asserted-by":"crossref","first-page":"28896","DOI":"10.1364\/OE.23.028896","article-title":"Q-factor limits for far-field detection of whispering gallery modes in active microspheres","volume":"23","author":"Riesen","year":"2015","journal-title":"Opt. Express"},{"key":"ref_17","doi-asserted-by":"crossref","first-page":"031104","DOI":"10.1063\/1.4905931","article-title":"Polymer based whispering gallery mode laser for biosensing applications","volume":"106","author":"Riesen","year":"2015","journal-title":"Appl. Phys. Lett."},{"key":"ref_18","doi-asserted-by":"crossref","first-page":"1168","DOI":"10.3390\/s150101168","article-title":"A fiber-tip label-free biological sensing platform: A practical approach toward in-vivo sensing","volume":"15","author":"Reynolds","year":"2015","journal-title":"Sensors"},{"key":"ref_19","doi-asserted-by":"crossref","first-page":"1381","DOI":"10.1364\/JOSAB.23.001381","article-title":"Theory of resonance shifts in TE and TM whispering gallery modes by nonradial perturbations for sensing applications","volume":"23","author":"Teraoka","year":"2006","journal-title":"JOSA B"},{"key":"ref_20","doi-asserted-by":"crossref","first-page":"6836","DOI":"10.3390\/s90906836","article-title":"Optical sensors based on whispering gallery modes in fluorescent microbeads: Size dependence and influence of substrate","volume":"9","author":"Francois","year":"2009","journal-title":"Sensors"},{"key":"ref_21","doi-asserted-by":"crossref","first-page":"212","DOI":"10.1364\/JOSAB.23.000212","article-title":"Effect of changes in size and index of refraction on the resonance wavelength of microspheres","volume":"23","author":"Schweiger","year":"2006","journal-title":"JOSA B"}],"container-title":["Sensors"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/1424-8220\/18\/9\/2987\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2025,10,11]],"date-time":"2025-10-11T15:19:17Z","timestamp":1760195957000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/1424-8220\/18\/9\/2987"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2018,9,7]]},"references-count":21,"journal-issue":{"issue":"9","published-online":{"date-parts":[[2018,9]]}},"alternative-id":["s18092987"],"URL":"https:\/\/doi.org\/10.3390\/s18092987","relation":{},"ISSN":["1424-8220"],"issn-type":[{"value":"1424-8220","type":"electronic"}],"subject":[],"published":{"date-parts":[[2018,9,7]]}}}