Nhng tin b trong Quang hc, Quang ph và ng dng VI ISSN 1859 - 4271

Advances in Optics, Photonics, Spectroscopy & Applications VI ISSN 1859 - 4271Here, Γ(g) and Γ(r) are emission rate of fluorescence <strong>ph</strong>otons into the guided modes andradiation modes (free space), respectively. G is the enhancement factor due to the cavitystructure. Note that the enhancement factor is almost equal to the finesse of the cavity. Weassume that the coupling efficiency ηc ~ 10 % without cavity; namely, for G = 1. Then, if onesubstitutes G = 30 to the above equation, one can obtain ηc ~ 80 %. It means that once a cavitystructure with a moderate finesse of 30 is fabricated on the nanofiber, most of the fluorescence<strong>ph</strong>otons are emitted into the guided modes. It means that one atom in the nanofiber cavity shouldwork as a very efficient single <strong>ph</strong>oton generation system. One may wonder that single-<strong>ph</strong>otongeneration with cavity in free space is quite a standard method of cavity-QED. But, in such astandard method, the cavity must have a very high finesse cavity of F ~ 50,000 or even higher[1], which is more than 1,000 times bigger than that for nanofiber cavity. This is simply due to afact that for the free space cavity the s<strong>trong</strong> field-confinement must be established by using onlya pair of mirrors. On the other hand, for the nanofiber cavity, the field confinement is controlledby both nanofiber itself and cavity structure. Since the nanofiber confines the field in thetransverse plane to a <s<strong>trong</strong>>tin</s<strong>trong</strong>>y area smaller than (λ)2, additional requirement to the cavity can becomerather moderate with low finesse of F ~ 30.V. REALIZATION OF THE NANOFIBER CAVITYAs discussed above, optical nanofibers may give a very promising method for genera<s<strong>trong</strong>>tin</s<strong>trong</strong>>gsingle <strong>ph</strong>otons. In order to really demonstrate the method, the key issue is to experimentallyrealize the highly efficient single <strong>ph</strong>oton generation by incorpora<s<strong>trong</strong>>tin</s<strong>trong</strong>>g a cavity structure on thenanofiber. The cavity can be created by fabrica<s<strong>trong</strong>>tin</s<strong>trong</strong>>g a pair of fiber Bragg gra<s<strong>trong</strong>>tin</s<strong>trong</strong>>gs (FBGs) onnanofibers. Although FBG technologies are quite well-established for the conventional opticalfibers, any method has not been reported so far regarding the FBG-fabrications on nanofibers.There may be two approaches to create FBGs on nanofiber. One is to directly mill periodicgrooves on nanofibers, and the other is to induce periodic refractive index change on nanofibers.Presently we are working on the direct-milling method. We use focused ion-beam (FIB)technology to mill grooves on nanofibers. One example of milled nanofiber is exhibited in Fig.5. The diameter of nanofiber is about 500 nm. One can see that grooves are periodically milledon both sides of nanofiber. The pitch is 360 nm with groove depth of 100 nm.Fig. 5: Fiber Bragg gra<s<strong>trong</strong>>tin</s<strong>trong</strong>>g fabricated on optical nanofiber. Grooves aremilled by focused ion-beam method.16