Fano resonance in a tapered optical fiber in contact with a high-Q microsphere is demonstrated. Multimode waves propagating in a 2.3 m diameter taper were coupled with a single whispering gallery mode of a 220 m sphere, and their coherent interaction resulted in Fano resonance. The asymmetric line shapes of the transmission spectra changed periodically with scanning of the coupling position along the taper. The observed 24 m period was due to modal dispersion in the tapered fiber.

We propose a direct electron-beam excitation assisted optical microscope with a resolution of a few tens of nanometers and it can be applied for observation of dynamic movements of nanoparticles in liquid. The technique is also useful for live cell imaging under physiological conditions as well as observation of colloidal solution, microcrystal growth in solutions, etc. In the microscope, fluorescent materials are directly excited with a focused electron beam. The direct excitation with an electron beam yields high spatial resolution since the electron beam can be focused to a few tens of nanometers in the specimens. In order to demonstrate the potential of our proposed microscope, we observed the movements of fluorescent nanoparticles, which can be used for labelling specimens, in a water-based solution. We also demonstrated an observation result of living CHO cells.

We demonstrate frequency tuning of whispering-gallery mode resonances in a fused-silica microsphere by means of temperature control. Scattering spectra of the microsphere are measured using a tunable narrow-linewidth diode laser, and the frequency scale is precisely calibrated from a simultaneously observed spectrum of rubidium vapor. The resonant frequencies of whispering-gallery modes can be varied linearly by temperature elevation and lowering, at a rate of -2.6 GHz/K for a 190 µm sphere.

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Asymmetric lineshapes of resonant dips were observed in transmission spectra of a tapered optical fiber contacted with a high-Q microsphere, and the spectral shapes were changed with scanning the coupling position along the taper.A transparent microsphere induces high-Q optical resonances, so-called whispering-gallery (WG) mode resonances. The WG modes originate from the total internal reflection inside the microsphere, and light interfering at the surface of the microsphere can be confined strongly. The microspherical cavity possesses extremely high-Q values and small mode volumes, in addition, highly efficient coupling can be achieved with a tapered optical fiber via an evanescent field[1]. Recently, a microring resonator coupled with a waveguide was demonstrated[2,3] to exhibit asymmetric lineshape called as Fano resonance [4], which is applicable to high sensitive sensing device. In this system, additional elements for partial reflection were employed to induce phase shift of the light propagating in the waveguide, which is indispensable for providing the asymmetric lineshape.In this paper, we report Fano resonances induced in a multimode tapered fiber waveguide coupled with a high-Q microspherical cavity without use of any additional elements such as reflectors and delay optics.We fabricated a tapered fiber with a waist diameter of 2.3 µm, which was formed by heating and stretching a length of single-mode fiber. And we produce a microsphere at a tip of a single mode fiber by irradiating with a CO 2 laser. A diameter of the microsphere with a stem was 220 µm, whose lowest radial-order mode can be coupled in phase matching with HE 11 HE 21 , TM 01 , and TE 01 modes of the 2.3-µm tapered fiber [5,6]. A micrograph of the spherical microcavity attached to the multimode tapered fiber is shown in Figure 1.A tunable external-cavity diode laser with the linewidth of 300 kHz was used for transmission spectrum measurement. While the laser frequency was scanned within a range of 18 GHz around 780-nm wavelength, intensity of the light passing through the tapered fiber was measured with a photodiode. The output intensity from the fiber without coupling to the microsphere was 50 µW. We also measured transmission spectra of a rubidium-vapor cell and of a Fabry-Perot etalon (free spectral range = 2.5 GHz) for precise frequency calibration. The 5 2 S 1/2 (F=3)→5 2 P 3/2 transition peak of 85 Rb was set to the origin of frequency detuning. The microsphere was positioned by a piezo-electric stage three-dimensionally and kept touching on the taper waist. The microsphere/tapered-fiber-waveguide system was placed in a chamber, which was filled with dry air (humidity<1.0%) to reduce water adsorption causing adhesion forces at the surfaces. In addition, the chamber reduces temperature change inside itself, which was indispensable for avoiding the resonant frequency shift of the microspherical cavity [7]. Figure 2 shows four typical transmission spectra of the tapered fiber coupled with a microsphere. These spectra were measured at d...

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