Optical Microspherical Resonators for Biomedical Sensing

Abstract

Optical resonators play an ubiquitous role in modern optics. A particular class of optical resonators is constituted by spherical dielectric structures, where optical rays are total internal reflected. Due to minimal reflection losses and to potentially very low material absorption, these guided modes, known as whispering gallery modes, can confer the resonator an exceptionally high quality factor Q, leading to high energy density, narrow resonant-wavelength lines and a lengthy cavity ringdown. These attractive characteristics make these miniaturized optical resonators especially suited as laser cavities and resonant filters, but also as very sensitive sensors. First, a brief analysis is presented of the characteristics of microspherical resonators, of their fabrication methods, and of the light coupling techniques. Then, we attempt to overview some of the recent advances in the development of microspherical biosensors, underlining a number of important applications in the biomedical field.

Photonic studies on polymer-coated sapphire-spheres: A model system for biological ligands

“…This is possibly due to the fact that higher polymer concentrations lead to the formation of an inhomogeneous coating, with the presence of clusters [13]. This scatters the photons out of the WGM and decreases the Q as shown in Table 1 [47].…”

“…Label-free detection of protein-protein interaction was shown recently using nanowires [9], nanoparticle probes [10], biochips [11], and mechanical cantilevers [12]. Optical measurement techniques can provide high sensitivity, compactness, fast response and real-time measurements [13]. They are non-destructive to the sample, and the transduction processes generally take place on a surface and can be tailored to sense almost any kind of (bio)chemical molecules [14].…”

Two photon versus one photon fluorescence excitation in whispering gallery mode microresonators

“…Whispering gallery mode resonators (WGMR) are evanescent wave sensors that can be used both as refractometres or label-free sensors and fluorescence based sensors [1,2]. The disadvantages of the labeled system-namely, cost and possibly reduced reactivity-are normally compensated by lower limit of detection (LOD), while in the case of direct monitoring, the limitations lie in the ineffectiveness of detecting small 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 molecular weight analytes and in the sensitivity to non-specific binding.…”

“…In order to overcome such a difficulty, i.e. to achieve the needed energies and yet avoid tight focusing, we resort to low-loss, high-quality factor whispering gallery mode resonators (WGMR) [1,14]. Another reason to use TPF is the available wide range of visible fluorescent dyes with very high quantum yields and molar extinction coefficients.…”

Special Section Guest Editorial: Integrated Optics

“…6; and (iii) the combination of ultrahigh-Q whispering gallery mode microresonators with optical waveguides for the implementation of very sensitive biosensors; see, for instance, Ref. 7. Several books [8][9][10][11][12][13] and a huge number of papers on this subject have been published in the 40 years since the seminal paper by P. K. Tien in 1971, which described not only the physics of guided waves in thin films and the prismcoupling method, but also some very early nonlinear optical phenomena in integrated optics.…”