Optical Resonators

Label–free optical biosensors based on Whispering Gallery Mode (WGM) microresonators have been proposed as special optical platforms capable to reach single molecule detection, with the first example given by biosensors implemented on glass microspheres carrying on their surface the selective sensing layer. Particular attention is devoted to the design, development and charaterisation of microbubble resonators constituted by spherical bulges realized in hollow silica microcapillaries.

They present the advantage of incorporating in the same device the exclusive properties of WGM resonators (i.e.: high Q factors – typically > 107 in air – and small mode volumes), with embedded microfluidics ensured by the microcapillary itself.






Main Publications on the subject

Grimaldi I. A., Berneschi S., Testa G., Baldini F., Nunzi Conti G., Bernini R., Whispering gallery mode in self-assembled bottle microresonators coupled to planar waveguide, Proc.SPIE 9750, Integrated Optics: Devices, Materials, and Technologies XX (13-18 Febbraio 2016, San Francisco, USA), pp. 97501H-1 -97501H-1, 2016

Abstract:Polymer micro-bottle resonators are fabricated by means of a simple self-assembling process. High optical quality SU-8 negative resist is chosen as bottle material. SU-8 is dispensed onto a fiber stem and is kept in rotation during the UV photo-polymerization. Micro-bottles with different geometrical sizes have been obtained by changing the SU-8 dispensed volume, with a repeatability of about 2%. Planar waveguides is chosen as interrogation system for our SU-8 self-assembled micro-bottle resonators. The highest quality factor Q associated to the resonance peak in the transmission spectrum is about 3.8 × 104.

Berneschi S., Baldini F., Barucci A., Cosci A., Cosi F., Farnesi D., Nunzi Conti G., Righini G. C., Soria S., Tombelli S., Trono C., Pelli S., Giannetti A. , Localized biomolecules immobilization in optical microbubble resonators, Proc.SPIE 9727, Laser Resonators, Microresonators, and Beam Control XVIII (13-18 Febbraio 2016, San Francisco, USA), pp. 972719-1 -972719-8, 2016

Abstract:In order to optimize the performance of an optical microbubble resonator (OMBR) as biosensor, the chemical functionalization of its inner surface plays a key role. Here we report on a spatially selective photo – chemical procedure able to bind fluorescent biomolecules only in correspondence of the OMBR inner surface. This abruptly reduces the occurrence of an undesired specific biochemical bond event all along the microfluidic section of the device. The evidence of this method, which maintains high Q factor (> 105) for the OMBR in buffer solution, is proved by fluorescence microscopy and real time measurement of the resonance broadening. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

A.Giannetti, A. Barucci,S. Berneschi, A. Cosci, F. Cosi, D. Farnesi, G. Nunzi Conti, S. Pelli, S. Soria, S. Tombelli, C. Trono, G.C. Righini, F. Baldini, Optical micro-bubble resonators as promising biosensors, Proc.SPIE 9506, Optical Sensors (13-16 Aprile 2015, Prague, Czech Republic), pp. 950617-1 - 950617-7, 2015

Abstract:Recently, optical micro-bubble resonators (OMBRs) have gained an increasing interest in many fields of photonics thanks to their particular properties. These hollow microstructures can be suitable for the realization of label – free optical biosensors by combining the whispering gallery mode (WGM) resonator properties with the intrinsic capability of integrated microfluidics. In fact, the WGMs are morphology-dependent modes: any change on the OMBR inner surface (due to chemical and/or biochemical binding) causes a shift of the resonance position and reduces the Q factor value of the cavity. By measuring this shift, it is possible to obtain information on the concentration of the analyte to be detected. A crucial step for the development of an OMBR-based biosensor is constituted by the functionalization of its inner surface. In this work we report on the development of a physical and chemical process able to guarantee a good homogeneity of the deposed bio-layer and, contemporary, to preserve a high quality factor Q of the cavity. The OMBR capability of working as bioassay was proved by different optical techniques, such as the real time measurement of the resonance broadening after each functionalization step and fluorescence microscopy.

I. A. Grimaldi, S. Berneschi, G. Testa, F. Baldini, G. Nunzi Conti, and R. Bernini: Polymer based planar coupling of self-assembled bottle microresonators, Appl. Phys. Lett., 105, 231114 1-4, 2014

Abstract:The investigation of a simple and self-assembling method for realizing polymeric micro-bottle resonators is reported. By dispensing precise amounts of SU-8 onto a cleaved optical fiber, employed as mechanical support, bottle microcavities with different shapes and diameters are fabricated. The balancing of surface energy between glass fiber and polymeric microresonator with surface tension of SU-8 confers different shape to these microstructures. Planar single-mode SU-8 based waveguide, realized on polymethylmethacrylate, is chosen for exciting the micro-bottle resonators by evanescent wave. The reliability of the fabrication process and the shape of the bottle microcavities are investigated through optical analysis. We observe whispering gallery modes in these resonant microstructures by a robust coupling with single mode planar waveguides around 1.5 μm wavelength. The resonance spectra of micro-bottle resonators and the spectral characteristics, such as Quality-factor (Q factor) and free spectral range, are evaluated for all the realized microstructures. SU-8 micro-bottle resonators show high Q-factors up to 3.8 × 104 and present a good mechanical stability. These features make these microcavities attractive for sensing and/or lasing applications in a planar platform.

A. Giannetti, S. Berneschi, F. Baldini, F. Cosi, G. Nunzi Conti, S. Soria: Performance of Eudragit Coated Whispering Gallery Mode Resonator-Based Immunosensors, Sensors, 12, pp. 14604-14611, 2012

Abstract:Whispering gallery mode resonators (WGMR) are an efficient tool for the realization of optical biosensors. A high Q factor preservation is a crucial requirement for good biosensor performances. In this work we present an Eudragit®L100 coated microspherical WGMR as an efficient immunosensor. The developed resonator was morphologically characterized using fluorescence microscopy. The functionalization process was tuned to preserve the high Q factor of the resonator. The protein binding assay was optically characterized in terms of specificity in buffer solution.

S. Soria, F. Baldini, S. Berneschi, F. Cosi, A. Giannetti, G. Nunzi Conti, S. Pelli, G.C. Righini, B. Tiribilli: High-Q polymer-coated microspheres for immunosensing applications, Optics Express, 17, pp. 14694–14699, 2009

Abstract: Homogeneous polymeric thin layers have been used as functionalizing agents on silica microspherical resonators in view of the implementation of an immunosensor. We have characterized the microspheres functionalised with poly-L-lactic acid and Eudragit® L100, as an alternative to the commonly used 3-Aminopropyltrimethoxysilane. It is shown that polymeric functionalization does not affect the high quality factor (Q greater than 107) of the silica microspheres, and that the Q factor is about 3x105 after chemical activation and covalent binding of immunogammaglobulin (IgG). This functionalizing process of the microresonator constitutes a promising step towards the achievement of an ultra sensitive immunosensor.