Kouki Totsuka

Tunable Fano interference effect in coupled-microsphere resonator-induced transparency

We investigated interference effect between two whispering gallery modes in a system of ultrahigh-Q silica microspheres in which the resonance frequencies of spheres were precisely controlled through thermal tuning. A symmetric transmission peak of coupled—resonator—induced transparency reshaped into a sharp asymmetric spectrum similar to Fano effect in atomic system as the resonance frequency of the second sphere was detuned. The resonance modes showed frequency shifts as a function of the coupling strength between the two spheres, indicating that two whispering gallery modes were configurationally mixed. The observations were compared with calculations and discussed using double-spiral structures in the phase space in the transmitted field.

Slow and fast light in a microsphere-optical fiber system

The dispersion relation in a system of a fiber taper coupled with a microsphere is investigated. On the undercoupling condition, in which the coupling strength between the sphere and the fiber is weak compared with the round-trip loss in the sphere, the system shows anomalous dispersion. On the other hand, on the overcoupling condition, in which the coupling strength is strong compared with the loss, the system shows normal dispersion. We performed pulse propagation experiments and observed both the slow light and the fast light relevant to the normal and anomalous dispersions in the single microsphere-fiber taper system, controlling the coupling strength between the sphere and the fiber. The observed pulse propagation times show good agreement with theoretical calculations based on a directional coupling theory.

Optical microsphere amplification system

We demonstrated a compact optical amplification system using a fiber taper and rare-earth-doped microsphere. Both the signal and pump beams were injected into the microsphere via the fiber taper to increase the spatial matching between two whispering gallery modes. As the pump power was increased, the resonance dip in the transmission spectra due to intrinsic attenuation in the microsphere became filled and then transited to the resonance gain peak. An amplification factor of up to 2.5 was realized with a microsphere of 63 microm in diameter.

Amplification and diffusion of spontaneous emission in strongly scattering medium

We experimentally examined the threshold of the spectral collapse in dye embedded in a strongly scattering medium as a function of the excitation beam diameter and the transport mean free path in order to find a condition to minimize the threshold. We found a critical transport mean free path, below which the threshold pulse energy is almost independent of the mean free path. Experimental observations are well explained by a theoretical model based on the rate and diffusion equations, which shows that the luminescence spectra are also dependent on the position in the medium.

High Spatial Resolution Atom Detector with Two-Color Optical Near Fields

We describe a slit-type detector for ground-state atoms with nanometer optical near fields. The detector with a slit width of 70 nm and a slit length of 100 µm is fabricated by processing a silicon-on-insulator wafer with photolithography and anisotropic etching. Measurement with a fiber probe of the intensity distribution on the slit reveals that the spatial resolution is 96 nm. Rubidium atoms can be selectively detected by means of two-step photoionization. The detection efficiency is estimated at 25% for Rb atoms with a mean velocity of 10 m/s. The atom detector can be used for atom deflection experiments with a fiber probe.

A slit-type atom deflector with near-field light

We developed a near-field optical deflector for precise direction control of atomic motion using a dipole force. The blue-detuned, near-field light used to deflect atoms was generated near the edge of a 100-nm-wide slit and had a spatial distribution of 126 nm at a distance of 10 nm from the top edge. The deflection angle for a Rb atom was a function of light intensity, frequency detuning, and atomic velocity.

Observation of whispering gallery modes in cathode luminescence in TiO2:Eu3+ microspheres

We report cavity enhanced periodic structures in cathode luminescence spectra in TiO2:Eu3+ microspheres ranging from 6.2to12.2μm in diameter. The spectral structures observed at the accelerating voltage below 10kV are attributed to the lower order whispering galley modes, while the additional components appearing at 20kV are to the higher order modes within the microsphere. These observations illustrate that the basic properties of phosphors can be advanced from the point of view of cavity quantum electrodynamics by fabricating a microcavity structure onto a single phosphor crystal.

Propagation of arbitrarily shaped femtosecond laser pulses through a photonic crystal fiber

We produce femtosecond optical pulses of different shapes to investigate the linear pulse propagation through a photonic crystal fiber in a regime where the conventional group velocity has no meaning and show that the temporal positions of centroid of energy are the same for both arbitrary pulses and transform-limited coherent pulses. By the concept of net group delay we explain the propagation delay for both the cases despite the fact that transmitted pulses suffer a severe distortion in the fiber.

Observation of Normal and Anomalous Dispersions in a Microsphere Taper Fiber System

A dielectric microsphere may be considered as an artificial photonic atom. The electromagnetic modes in the microsphere are completely assigned with indexes similar to those used to characterize simple atomic systems. Microsphere modes, as well as microring or microdisk, trap light in circular orbits near the sphere surface and strongly modify the phase of the interacting light, resulting in very steep dispersions. Pulse propagation experiments were performed in a system consisting of a fiber taper coupled with a silica glass microsphere, and it was shown that the light velocity in the system can be largely changed through the control of the coupling strength. Slow light was also demonstrated in a system in which two ultrahigh-Q silica microspheres of different diameters were coupled in tandem to a fiber taper demonstrating classical analog of electromagnetically induced transparency in atomic system. Here, we employed interferometric techniques based on the Mach-Zehnder interferometer for examining extremely steep dispersions induced by whispering gallery modes with a quality factor on the order of Q 1⁄4 10. Such a narrow resonance in the dielectric microsphere is comparable to that in cooled atomic systems. We successfully observed both normal and anomalous dispersions at the resonant frequency, depending on the coupling strength between the sphere and the fiber. Figure 1 shows our experimental system based on an interferometric setup. We used the second harmonics of a single mode ring cavity Nd3þ yttrium aluminum garnet (Nd3þ:YAG) laser with a line width of 1 kHz at 532 nm. The laser frequency was tuned using thermal control of the ring cavity length, over a mode-hop-free range of greater than 10GHz. The reference arm was empty, while the sample arm contained the microsphere-fiber system. Silica microspheres were fabricated from standard telecommunication optical fiber. The end of the etched fiber was fused using a CO2 laser and the microsphere formed due to the surface tension. The experiments were performed with a microsphere 60 mm in diameter. The fiber taper was also fabricated from the same telecommunication fiber. The typical fiber waist diameter at the coupling region was 0.5 mm. A large number of spectrally overlapping resonance modes exist within a microsphere. The fiber taper method is essential for coupling the incident light to a specific whispering gallery mode selectively and efficiently. The microsphere was attached to a translation stage controlled by a piezo actuator. The relative position between the sphere and the fiber were carefully adjusted so that we could observe spectrally isolated whispering gallery modes with proper coupling strength. The optical path length and dispersion were equalized in each arm as completely as possible. The fringe pattern was captured using a charge-coupled device (CCD) camera for each frequency increment. The observed interference patterns appeared as concentric circles due to the interference between the plain reference wave and the spherical wave from the sample arm. A least square fit for each fringe pattern was performed to extract the phase information in the transmitted light. Figures 2(a) and 2(b) show examples of transmission spectra through the microsphere fiber taper coupled system. These spectra were taken in two different whispering gallery modes at different laser frequencies in the mode-hop-free laser tuning range. The transmission dips appear when the incident laser frequencies are resonant at specific whispering gallery modes in the microsphere. The two whispering gallery modes shown in Figs. 2(a) and 2(b) are denoted as WG1 and WG2, respectively. The transmission minimum was Tmin 1⁄4 0:12, and the resonance width was 1⁄4 BS

Deflecting, detecting, and funneling atoms using near-field light

A slit-type atom deflector is developed, that sends atoms at the aimed point on a substrate. In addition, a multi-slit atom detector with high spatial resolution is made. For the atom deflection by near-field light, a cold atomic beam is required. Cold atomic beam is generated with an atom funnel composed of evanescent-light mirrors.