{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,1,25]],"date-time":"2026-01-25T22:43:59Z","timestamp":1769381039602,"version":"3.49.0"},"reference-count":45,"publisher":"MDPI AG","issue":"4","license":[{"start":{"date-parts":[[2015,4,16]],"date-time":"2015-04-16T00:00:00Z","timestamp":1429142400000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"DOI":"10.13039\/501100004189","name":"Max Planck Society","doi-asserted-by":"publisher","id":[{"id":"10.13039\/501100004189","id-type":"DOI","asserted-by":"publisher"}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>Asymmetric microsphere resonant cavities (ARCs) allow for free-space coupling to high quality (Q) whispering gallery modes (WGMs) while exhibiting highly directional light emission, enabling WGM resonance measurements in the far-field. These remarkable characteristics make \u201cstand-off\u201d biodetection in which no coupling device is required in near-field contact with the resonator possible. Here we show asymmetric microsphere resonators fabricated from optical fibers which support dynamical tunneling to excite  high-Q WGMs, and demonstrate free-space coupling to modes in an aqueous environment. We characterize the directional emission by fluorescence imaging, demonstrate coupled mode effects due to free space coupling by dynamical tunneling, and detect adsorption kinetics of a protein in aqueous solution. Based on our approach, new, more robust WGM biodetection schemes involving microfluidics and in-vivo measurements can be designed.<\/jats:p>","DOI":"10.3390\/s150408968","type":"journal-article","created":{"date-parts":[[2015,4,16]],"date-time":"2015-04-16T10:37:57Z","timestamp":1429180677000},"page":"8968-8980","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":31,"title":["Stand-Off Biodetection with Free-Space Coupled Asymmetric Microsphere Cavities"],"prefix":"10.3390","volume":"15","author":[{"given":"Zachary","family":"Ballard","sequence":"first","affiliation":[{"name":"Max Planck Institute for the Science of Light, Laboratory of Nanophotonics & Biosensing,  Guenther-Scharowsky-Str. 1\/Bldg. 24, Erlangen D-91058, Germany"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Martin","family":"Baaske","sequence":"additional","affiliation":[{"name":"Max Planck Institute for the Science of Light, Laboratory of Nanophotonics & Biosensing,  Guenther-Scharowsky-Str. 1\/Bldg. 24, Erlangen D-91058, Germany"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Frank","family":"Vollmer","sequence":"additional","affiliation":[{"name":"Max Planck Institute for the Science of Light, Laboratory of Nanophotonics & Biosensing,  Guenther-Scharowsky-Str. 1\/Bldg. 24, Erlangen D-91058, Germany"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"1968","published-online":{"date-parts":[[2015,4,16]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"14657","DOI":"10.1073\/pnas.1408453111","article-title":"Single nanoparticle detection using split-mode microcavity Raman lasers","volume":"111","author":"Li","year":"2014","journal-title":"Proc. Natl. Acad. Sci. USA"},{"key":"ref_2","first-page":"E3836","article-title":"Highly sensitive detection of nanoparticles with a self-referenced and self-heterodyned whispering-gallery Raman microlaser","volume":"111","author":"Zhu","year":"2014","journal-title":"Proc. Natl. Acad. Sci. USA"},{"key":"ref_3","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_4","doi-asserted-by":"crossref","first-page":"20701","DOI":"10.1073\/pnas.0808988106","article-title":"Single virus detection from the reactive shift of a whispering-gallery mode","volume":"105","author":"Vollmer","year":"2008","journal-title":"Proc. Natl. Acad. Sci. USA"},{"key":"ref_5","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_6","doi-asserted-by":"crossref","first-page":"5616","DOI":"10.1002\/adma201302572","article-title":"Detection of Single Nanoparticles and Lentiviruses Using Microcavity Resonance Broadening","volume":"25","author":"Shao","year":"2013","journal-title":"Adv. Mater."},{"key":"ref_7","doi-asserted-by":"crossref","first-page":"453","DOI":"10.1364\/OL.21.000453","article-title":"Ultimate Q of optical microsphere resonators","volume":"21","author":"Gorodetsky","year":"1996","journal-title":"Opt. Lett."},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"1129","DOI":"10.1364\/OL.22.001129","article-title":"Phase-matched excitation of whispering-gallery-mode resonances by a fiber taper","volume":"22","author":"Knight","year":"1997","journal-title":"Opt. Lett."},{"key":"ref_9","doi-asserted-by":"crossref","first-page":"327","DOI":"10.1209\/0295-5075\/23\/5\/005","article-title":"Very High-Q Whispering-Gallery Mode Resonances Observed on Fused Silica Microspheres","volume":"23","author":"Collot","year":"2007","journal-title":"Europhys. Lett."},{"key":"ref_10","doi-asserted-by":"crossref","first-page":"043902","DOI":"10.1103\/PhysRevLett.91.043902","article-title":"Ideality in a Fiber-Taper-Coupled Microresonator System for Application to Cavity Quantum Electrodynamics","volume":"91","author":"Spillane","year":"2003","journal-title":"Phys. Rev. Lett."},{"key":"ref_11","doi-asserted-by":"crossref","first-page":"147","DOI":"10.1364\/JOSAB.16.000147","article-title":"Optical microsphere resonators: Optimal coupling to high-Q whispering-gallery modes","volume":"16","author":"Gorodetsky","year":"1999","journal-title":"J. Opt. Soc. Am. B"},{"key":"ref_12","doi-asserted-by":"crossref","first-page":"723","DOI":"10.1364\/OL.24.000723","article-title":"Pigtailing the high-Q microsphere cavity: A simple fiber coupler for optical whispering-gallery modes","volume":"24","author":"Ilchenko","year":"1999","journal-title":"Opt. Lett."},{"key":"ref_13","doi-asserted-by":"crossref","first-page":"427","DOI":"10.1002\/cphc.201100757","article-title":"Optical Resonator Biosensors: Molecular Diagnostic and Nanoparticle Detection on an Integrated Platform","volume":"13","author":"Baaske","year":"2012","journal-title":"Chem. Phys. Chem."},{"key":"ref_14","doi-asserted-by":"crossref","first-page":"225","DOI":"10.1016\/j.biomaterials.2011.09.036","article-title":"Whispering gallery mode biosensor quantification of fibronectin adsorption kinetics onto alkylsilane monolayers and interpretation of resultant cellular response","volume":"33","author":"Wilson","year":"2012","journal-title":"Biomaterials"},{"key":"ref_15","doi-asserted-by":"crossref","first-page":"183106","DOI":"10.1063\/1.4804243","article-title":"Detection of nanoparticles with a frequency locked whispering gallery mode microresonator","volume":"102","author":"Swaim","year":"2013","journal-title":"Appl. Phys. Lett."},{"key":"ref_16","doi-asserted-by":"crossref","first-page":"1346","DOI":"10.1109\/LPT.2011.2160847","article-title":"Optical Detection of Single Nanoparticles with a Subwavelength Fiber-Taper","volume":"23","author":"Zhu","year":"2011","journal-title":"IEEE Photonics Technol. Lett."},{"key":"ref_17","doi-asserted-by":"crossref","first-page":"2075","DOI":"10.1021\/nl061342r","article-title":"Cavity QED with Diamond Nanocrystals and Silica Microspheres","volume":"6","author":"Park","year":"2006","journal-title":"Nano Lett."},{"key":"ref_18","unstructured":"Conti, G.N., Berneschi, S., Brenci, M., Cosi, F., Pelli, S., Soria, S., and Righini, G.C. (2011, January 26\u201330). WGM microresonators: Light coupling issues and integration perspectives. Proceedings of 2011 13th International Conference on Transparent Optical Networks (ICTON), Stockholm, Sweden."},{"key":"ref_19","doi-asserted-by":"crossref","first-page":"9982","DOI":"10.1364\/OE.21.009982","article-title":"Theory of free space coupling to high-Q whispering gallery modes","volume":"21","author":"Zou","year":"2013","journal-title":"Opt. Express"},{"key":"ref_20","doi-asserted-by":"crossref","first-page":"474","DOI":"10.1364\/OL.30.000474","article-title":"Chaos-assisted tunneling in dielectric microcavities","volume":"30","author":"Podolskiy","year":"2005","journal-title":"Opt. Lett."},{"key":"ref_21","doi-asserted-by":"crossref","first-page":"023810","DOI":"10.1103\/PhysRevA.88.023810","article-title":"Dynamical tunneling-assisted coupling of high-Q deformed microcavities using a free-space beam","volume":"88","author":"Yang","year":"2013","journal-title":"Phys. Rev. A"},{"key":"ref_22","doi-asserted-by":"crossref","first-page":"61","DOI":"10.1103\/RevModPhys.87.61","article-title":"Dielectric microcavities: Model systems for wave chaos and non-Hermitian physics","volume":"87","author":"Cao","year":"2015","journal-title":"Rev. Mod. Phys."},{"key":"ref_23","doi-asserted-by":"crossref","first-page":"644","DOI":"10.1364\/OL.32.000644","article-title":"Directional escape from a high-Q deformed microsphere induced by short CO2 laser pulses","volume":"32","author":"Xiao","year":"2007","journal-title":"Opt. Lett."},{"key":"ref_24","doi-asserted-by":"crossref","first-page":"1609","DOI":"10.1364\/OL.21.001609","article-title":"Directional emission from asymmetric resonant cavities","volume":"21","author":"Stone","year":"1996","journal-title":"Opt. Lett."},{"key":"ref_25","doi-asserted-by":"crossref","first-page":"251101","DOI":"10.1063\/1.3153276","article-title":"Directional emission and universal far-field behavior from semiconductor lasers with lima\u00e7on-shaped microcavity","volume":"94","author":"Yan","year":"2009","journal-title":"Appl. Phys. Lett."},{"key":"ref_26","doi-asserted-by":"crossref","first-page":"22407","DOI":"10.1073\/pnas.1015386107","article-title":"Whispering-gallery mode resonators for highly unidirectional laser action","volume":"107","author":"Wang","year":"2010","journal-title":"Proc. Natl. Acad. Sci. USA"},{"key":"ref_27","doi-asserted-by":"crossref","first-page":"284","DOI":"10.1364\/OL.38.000284","article-title":"Whispering gallery mode resonators augmented with engraved diffraction gratings","volume":"38","author":"Aveline","year":"2013","journal-title":"Opt. Lett."},{"key":"ref_28","doi-asserted-by":"crossref","first-page":"6396","DOI":"10.1038\/srep06396","article-title":"Interfacing whispering-gallery microresonators and free space light with cavity enhanced Rayleigh scattering","volume":"4","author":"Zhu","year":"2014","journal-title":"Sci. Rep."},{"key":"ref_29","doi-asserted-by":"crossref","first-page":"300","DOI":"10.3788\/COL20080604.0300","article-title":"Directly mapping whispering gallery modes in a microsphere through modal coupling and directional emission","volume":"6","author":"Dong","year":"2008","journal-title":"Chin. Opt. Lett."},{"key":"ref_30","doi-asserted-by":"crossref","first-page":"16471","DOI":"10.1364\/OE.15.016471","article-title":"Radiation pressure driven mechanical oscillation in deformed silica microspheres via free-space evanescent excitation","volume":"15","author":"Park","year":"2007","journal-title":"Opt. Express"},{"key":"ref_31","first-page":"OP260","article-title":"Highly Unidirectional Emission and Ultralow-Threshold Lasing from On-Chip Ultrahigh-Q Microcavities","volume":"24","author":"Jiang","year":"2012","journal-title":"Adv. Mater."},{"key":"ref_32","doi-asserted-by":"crossref","first-page":"45","DOI":"10.1038\/385045a0","article-title":"Ray and wave chaos in asymmetric resonant optical cavities","volume":"385","author":"Stone","year":"1997","journal-title":"Nature"},{"key":"ref_33","doi-asserted-by":"crossref","unstructured":"Young, K. (2010). Advances in Optical and Photonic Devices, INTECH Open Access Publisher.","DOI":"10.5772\/127"},{"key":"ref_34","first-page":"1","article-title":"High-Q and Unidirectional Emission Whispering Gallery Modes: Principles and Design","volume":"19","author":"Zou","year":"2013","journal-title":"IEEE J. Sel. Top. Quantum Electron."},{"key":"ref_35","doi-asserted-by":"crossref","first-page":"1943","DOI":"10.1364\/OL.26.001943","article-title":"Directional emission from whispering-gallery modes in deformed fused-silica microspheres","volume":"26","author":"Lacey","year":"2001","journal-title":"Opt. Lett."},{"key":"ref_36","doi-asserted-by":"crossref","first-page":"923","DOI":"10.1364\/JOSAB.21.000923","article-title":"Dramatic shape sensitivity of directional emission patterns from similarly deformed cylindrical polymer lasers","volume":"21","author":"Schwefel","year":"2004","journal-title":"J. Opt. Soc. Am. B"},{"key":"ref_37","doi-asserted-by":"crossref","first-page":"163902","DOI":"10.1103\/PhysRevLett.104.163902","article-title":"Chaos-Assisted Directional Light Emission from Microcavity Lasers","volume":"104","author":"Shinohara","year":"2010","journal-title":"Phys. Rev. Lett."},{"key":"ref_38","doi-asserted-by":"crossref","first-page":"509","DOI":"10.1364\/OL.34.000509","article-title":"Low-threshold microlaser in a high-Q asymmetrical microcavity","volume":"34","author":"Xiao","year":"2009","journal-title":"Opt. Lett."},{"key":"ref_39","doi-asserted-by":"crossref","first-page":"298","DOI":"10.1364\/AO.52.000298","article-title":"Controlling deformation in a high quality factor silica microsphere toward single directional emission","volume":"52","author":"Cui","year":"2013","journal-title":"Appl. Opt."},{"key":"ref_40","doi-asserted-by":"crossref","first-page":"033902","DOI":"10.1103\/PhysRevLett.91.033902","article-title":"Directional Tunneling Escape from Nearly Spherical Optical Resonators","volume":"91","author":"Lacey","year":"2003","journal-title":"Phys. Rev. Lett."},{"key":"ref_41","doi-asserted-by":"crossref","first-page":"L51","DOI":"10.1002\/lpor.201300042","article-title":"Chaos-induced transparency in an ultrahigh-Q optical microcavity","volume":"7","author":"Xiao","year":"2013","journal-title":"Laser Photonics Rev."},{"key":"ref_42","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_43","doi-asserted-by":"crossref","first-page":"5491","DOI":"10.1364\/OE.22.005491","article-title":"Optimizing detection limits in whispering gallery mode biosensing","volume":"22","author":"Foreman","year":"2014","journal-title":"Opt. Express"},{"key":"ref_44","doi-asserted-by":"crossref","first-page":"10907","DOI":"10.3390\/s111110907","article-title":"Plasmonic Nanostructures for Nano-Scale Bio-Sensing","volume":"11","author":"Chung","year":"2011","journal-title":"Sensors"},{"key":"ref_45","doi-asserted-by":"crossref","first-page":"5980","DOI":"10.1021\/ac900819j","article-title":"Multispectral Thin Film Biosensing and Quantitative Imaging Using 3D Plasmonic Crystals","volume":"81","author":"Stewart","year":"2009","journal-title":"Anal. Chem."}],"container-title":["Sensors"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/1424-8220\/15\/4\/8968\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2025,10,11]],"date-time":"2025-10-11T20:44:51Z","timestamp":1760215491000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/1424-8220\/15\/4\/8968"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2015,4,16]]},"references-count":45,"journal-issue":{"issue":"4","published-online":{"date-parts":[[2015,4]]}},"alternative-id":["s150408968"],"URL":"https:\/\/doi.org\/10.3390\/s150408968","relation":{},"ISSN":["1424-8220"],"issn-type":[{"value":"1424-8220","type":"electronic"}],"subject":[],"published":{"date-parts":[[2015,4,16]]}}}