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Figure 5.( a) Optical picture of an angle-polished fiber( polishing angle highlighted in blue),( b) picture of a microsphere coupled to an angle polished fiber.
diameter, a microdisk-waveguide system made in LN and the same disk when using a prism for light coupling.
The angle-polished fiber coupler is very similar to a prism and it consists of a fiber whose end facet has been polished to a specific angle to allow phase matching [ 36 ]. The fiber couples light into the WGMR via total internal reflection at the angle-polished facet. The advantages of this system are the waveguide light insertion and the robustness of prism coupling, without the bulk of the prism itself( see Fig. 5). The main disadvantage of the prism and of the angle-polished strategies is their need of a second coupler to collect the transmitted light, since, after the interaction with the WGMR, it propagates in free space. Anglepolished fiber couplers are conveniently used in the adddrop configuration, in which a second coupler is used to extract the light from the resonator.
Once the coupling is achieved, laser light can be injected into the resonator to excite its WGM resonances. By scanning the laser wavelength of a tunable laser and recording the transmission with a detector, it is then possible to observe the lineshape of the WGM resonances and define a series of parameters to characterize the resonator( see Fig. 6). This list of figures-of-merit is inherited from interferometry and comprises, for example, the quality factor( Q-factor), the finesse, the contrast( or visibility), the fullwidth half-maximum( FWHM) and the free spectral range( FSR). Among these parameters, the quality factor, the finesse and the contrast are the most important in applications, since they quantify the sharpness of the WGM resonances, the average number of photon round-trips and the loaded energy in the WGM, respectively.
2.2
Fabrication techniques
We will discuss here the fabrication techniques of four different types of WGMR: microspheres, microbottles, microbubbles and microtoroids. The shapes and the material used for their fabrication are crucial for the applications. The optical characteristics of the chosen material will determine its optical properties and will enable the functionalization of the surface( see Sect. 5.1), which is the most important step for specific biochemical sensing aimed at accurately detecting the desired target.
Figure 6. Sketch of a WGMR characterization setup showing a typical resonance with a Q-factor of about 10 7 and contrast( or visibility) close to 1.
2.2.1 Microspheres
Silica microspheres are usually fabricated by melting the end of a stripped standard telecommunication fiber. After melting, the tip solidifies through surface tension and a sphere with a radius of hundred of micrometers can be obtained. The melting can be achieved either with a hydrogen torch, a CO 2 laser or a fiber splicer( see [ 26 ] andreferences therein). In all cases, the glass is heated and softened so that surface tension forms a spherical object with very low surface roughness at the end of the fiber stem. If one starts from a conventional telecom fiber, with clad diameter of 125 lm, spheres with diameter in the range of 125 – 350 lm can be easily produced. To obtain smaller spheres the fiber needs to be tapered first: in this case the taper is obtained by heating and stretching the fiber until it breaks. By melting the tip of the broken tapered fiber, spheres with diameter down to about 25 lm can be obtained [ 37 ]. A detailed protocol of fabrication using a fiber splicer can be found at JOVE [ 38 ]. Another class of( almost) spherical WGMRs is that of droplet resonators: after the seminal papers by Chang and Campillo [ 39, 40 ], these liquid resonators are again gaining attention [ 41, 42 ].
Figure 7 shows the fabrication set-up of microspheres using an oxygen-butane flame and a series of pictures of the microspheres obtained with this method. This particular set-up was built in our labs for the fabrication of active microspheres. We first prepared an active glass filament and then obtained microspheres with diameter in the range of 60 – 120 lm, depending of the initial diameter of the filament and the exposure time( below 2 s). Figures 7d and 7e show microspheres with their centers aligned with the supporting glass filament, while the panel e shows a microsphere that has been overexposed. In this case, the ratio between the microsphere diameter and the supporting stem diameter is too high and the stem is no longer able to support the weight of the microsphere. This, in turn, causes the bending of the stem, which assumes a“ pipe” shape.