J. Eur. Opt. Society-Rapid Publ. 21, 26( 2025) 255
Figure 8. Optical pictures of the two polymeric microbottles with a diameter of about 240 lm( a) and 490 lm( b). Courtesy from CNR-IREA research group headed by Dr. Romeo Bernini.
Figure
7.( a – b) Pictures of the fabrication setup implementing the flame fabrication technique( a: lateral view, b: top view),( c – d) microspheres with diameters of about 100 lm,( e) overexposed microsphere,( f) underexposed microsphere.
Figure 7f, instead, shows a microsphere which has been underexposed.
2.2.2 Microbottles
Sumetsky et al. pioneered two very interesting and promising cavity geometries: the microbottle [ 43 ] and the microbubble [ 44 ]. The whispering-gallery bottle( WGB, as introduced by Sumetsky) is obtained by deforming a glass cylinder along its long axis, so to have the cylinder radius increasing smoothly up to a maximum and then decreasing again. WGBs combine standard WGMs with“ bouncing ball” modes, localized near the stable closed rays that experience multiple reflection from the cavity wall. In their simplest form, WGBs, which routinely achieve Q-factors on the order of 10 7, can be created by heating an optical fiber( e. g. with a CO 2 laser) to modify its radius and obtain the characteristic prolate shape of the microbottle. In practice, to produce a microbottle, two adjacent fiber stems are heated to their softening point and then pushed against each other to form the microbottle. By carefully tuning the push procedure and adding also a slight stretching, the geometric parameters of the microcavity can be tuned. This represents an advantage of WGB resonators with respect to the conventional fiber-based microspheres. Even more simply, polymeric microbottles can be fabricated by dispensing a polymer drop onto the surface of a glass optical fiber. In this case, a series of effects linked to the polymer surface energy and the surface tension between the polymer and the glass fiber lead to the formation of the microbottle( see Fig. 8). After this initial formation, the microbottle is then hardened by UV curing [ 45, 46 ]. A similar tecnique was used
Figure
9. Fabrication steps to produce an active glass microbottle. Reproduced from [ 26 ] under Creative Commons License.
by Ward et al. [ 22 ] to fabricate active microbottles: an active glass rod was placed in contact with a silica capillary and then it was heated by a CO 2 laser. Due to the different melting points of the materials, the rod glass melted and flowed on the outer surface of the silica capillary, forming a bottle shaped structure. Figure 9 shows the steps Ward et al. used for fabricating the active glass microbottle.
2.2.3 Microbubbles
Microbubble resonators( MBRs) are hollow spherical WGMR fabricated by means of glass blowing techniques. Generally, a glass capillary is inflated by pressuring it with an inert gas( e. g. nitrogen) and then rapidly heating it( e. g. through a laser pulse [ 44 ] oranelectricdischarge [ 47 ]). This rapid heating produces a softening of the capillary walls, which expand to a spherical bulge due to the internal pressure: this shell, or bubble, is the resonator itself. The arc discharge technique for fabricating MBR was first described in [ 48 ]. Immediately after the authors modified the setup using four electrodes [ 47, 49 ]. Figures 10a and 10b show a sketch of the fabrication technique, whereas panel c shows a detail of the arc discharge unit and panel d shows one of the MBRs mounted on the discharge system after the inflation process. The MBRs were produced from fused silica capillaries and their typical radius fell in the 200 – 250 lm range.
Finally, it is important to highlight that the MBRs optical features follow not only from the very low losses( both intrinsic and coupling ones), but also from their little difference( critical coupling condition). The intrinsic losses( g 2)