We cooled ultrathin tapered fibers to cryogenic temperatures and controllably coupled them with high-Q microsphere resonators at a wavelength close to the optical transition of diamond nitrogen vacancy centers. The 310-nm-diameter tapered fibers were
stably nanopositioned close to the microspheres with a positioning stability of approximately 10 nm over a temperature range of 7-28 K. A cavity-induced phase shift was observed in this temperature range, demonstrating a discrete transition from undercoupling to overcoupling.
Highly efficient coupling of photons from nanoemitters into single-mode optical fibers is demonstrated using tapered fibers. 7.4 +/- 1.2 % of the total emitted photons from single CdSe/ZnS nanocrystals were coupled into a 300-nm-diameter tapered fibe
r. The dependence of the coupling efficiency on the taper diameter was investigated and the coupling efficiency was found to increase exponentially with decreasing diameter. This method is very promising for nanoparticle sensing and single-photon sources.
We investigated the cause of optical transmittance degradation in tapered fibers. Degradation commences immediately after fabrication and it eventually reduces the transmittance to almost zero. It is a major problem that limits applications of tapere
d fibers. We systematically investigated the effect of the dust-particle density and the humidity on the degradation dynamics. The results clearly show that the degradation is mostly due to dust particles and that it is not related to the humidity. In a dust free environment it is possible to preserve the transmittance with a degradation of less than the noise (+/- ?0.02) over 1 week.
