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

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Advances in Optics, Photonics, Spectroscopy & Applications VI ISSN 1859 - 4271efficiency of spontaneous emission into each direction of the nanofiber guided-modes. Diameterof nanofiber is chosen to satisfy a condition of k0a=1.45, where k0 is the free-space propagationconstant and a is the radius of the nanofiber. Condition of k0a=1.45 is the optimum condition forcoupling the fluorescence into the guided mode [4]. One can see that the nanofiber can collectfluorescence <strong>ph</strong>otons very efficiently. For atoms on the nanofiber surface, 11 % of thefluorescence <strong>ph</strong>otons are channeled into each side of the guided mode, and for atoms at oneradius away from the surface it is still 3 %.(a)(b)Fig. 1: a) Conceptual diagram of the scheme. The nanofiber locates at the mid oftapered optical fiber. b) Coupling efficiency of spontaneous emission into eachdirection of nanofiber propagation mode, η g , versus atom position r/a, where r and aare distance from nanofiber axis and radius of nanofiber, respectively.III. ATOMS ON A NANOFIBERAs quantum emitters, we use laser-cooled Cs-atoms and CdSeTe nanocrystal quantum-dots.Here, we describe the results on Cs-atoms. Experimental setup is sketched in Fig. 2.Fig. 2: Schematics for <strong>ph</strong>oton –correlation measurements.Cold atom cloud is overlapped to the nanofiber, and the atoms are excited by a probe laserbeam which is focused to the nanofiber. Fluorescence <strong>ph</strong>otons which are emitted into the fibermode are detected by two avalanche <strong>ph</strong>otodiodes APD1 and APD2, and <strong>ph</strong>oton coincidences attwo APDs are measured. Results are shown in the left column of Fig. 3. Right column exhibitsthe coincidences measured for <strong>ph</strong>otons emitted to both sides of the fiber. Parameter Ib denotesthe heater current to generate Cs-atoms.14
Những tiến bộ <strong>trong</strong> <strong>Quang</strong> học, <strong>Quang</strong> <strong>ph</strong>ổ và Ứng dụng VI ISSN 1859 - 4271Fig. 3: Left Column; One-end correlations; Right Column; Both-end correlationsOne can readily see a dip at the center for the coincidences for atom number less than one forone-end coincidences and for both-side coincidences. The dip corresponds to so-called antibunchingdip which is a signature of single <strong>ph</strong>oton emitter. It means that one can readily generatesingle <strong>ph</strong>otons into a fiber mode by using nanofiber/one-atom system. Details of the observationsand discussions, especially on the different behaviors for one-end and for both-end correlations,are found in Ref. [5].IV. ENHANCEMENT OF THE COUPLING EFFICIENCYThe nanofiber method would become useful for genera<s<strong>trong</strong>>tin</s<strong>trong</strong>>g single <strong>ph</strong>otons, especially itsfeature of emit<s<strong>trong</strong>>tin</s<strong>trong</strong>>g single <strong>ph</strong>otons into the fiber guided mode should be beneficial for realapplications. However, in order to extend the nanofiber technology to the widely usable method,one difficulty is that the coupling efficiency to the fiber mode is limited to 28 % at maximum [4].In order to extend the nanofiber technology to overcome this difficulty, we discuss here amethod to enhance the coupling efficiency greater than 80% so that the nanofiber method maybecome much more versatile technology.The key idea is to incorporate a cavity structure on the nanofiber as briefly sketched in Fig. 4.Due to the cavity structure, the fluorescence emission rate into the guided modes should beenhanced although emission rate into free space should not have any change.Fig. 4: Conceptual sketch for “single atom in the optical nanofiber cavity”.The coupling efficiency of fluorescence <strong>ph</strong>otons from atoms on the nanofiber into the guidedmode in the above structure can be expressed as below [6].15
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Nhng tin b trong Quang hc, Quang ph vÃ ng dng VI ISSN 1859 - 4271

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