Xiao-Wei Wu

Taper-microsphere coupling with numerical calculation of coupled-mode theory

We have numerically studied a taper-microsphere coupling system with coupled-mode theory by introducing a mode-coupling mechanism. Transmission and reflection coefficients have been obtained directly through numerical calculation for a spherical silica microcavity. In the presence of the modal coupling mechanism, the optimal condition for transferring the light energy into the microsphere is no longer the critical coupling point. Instead, it should be in the under-coupling regime and can be reached by optimizing the size of the fiber taper and the taper-microsphere gap.

High-Q and Unidirectional Emission Whispering Gallery Modes: Principles and Design

The general principles of the unidirectional emission of a high-Q whispering gallery mode (WGM) in deformed circular microcavities are studied and presented in this paper. In the phase space of deformed microcavities, light in the chaotic sea leaks out from the cavity through specific refraction regions, the positions of which are determined by the stable islands and the unstable manifolds. By changing the cavity shape, we can tune the positions of the refraction regions, as well as fixed points, to achieve unidirectional emission with narrow angular divergence. The microcavity with high-Q and unidirectional emission makes high-efficient free-space excitation and collection possible; thus, it can be applied for far-field detection of nanoparticles and low-threshold lasers with collimated emission.

Anti-bunching and luminescence blinking suppression from plasmon-interacted single CdSe/ZnS quantum dot

CdSe/ZnS colloidal quantum dots generally exist as blinking phenomena during the luminescence process that remarkably influences its applications. In this work, we used the surface plasmonic effect to effectively modulate single quantum dots. Obvious contrasts have been observed by comparing single quantum dots on silica and gold films. The surface plasmon is shown to obviously suppress the blinking of single quantum dots. With further demonstrated second- order correlation measurements, an anti-bunching effect was observed. The anti-bunching dip gives the smallest value of g(2)(0) = 0.15, and the lifetime of the exciton has been reduced. This method presents the applications potential towards tunable high-emitting-speed single photon sources at room temperature.

Modal coupling strength in a fibre taper coupled silica microsphere

The character of the modal coupling strength β between clockwise (CW) and counterclockwise (CCW) whispering gallery modes (WGMs) represents an intrinsic property of a microsphere cavity, which is an important factor for the application of both micro-laser and cavity QED. Depending on a fibre taper coupled microsphere with quality factor larger than 107, modal coupling has been observed, while reflection spectra have been captured to derive the practical modal coupling strength. Upon obtaining results the modal coupling strength is confirmed to be a constant determined only by a microcavity and modes under excitation. This measurement system and results enable ways not only to acquire the material character of a microcavity, but also to analyse microsphere-based cavity QED research.

Practical non-Poissonian light source for passive decoy state quantum key distribution

Passive decoy state quantum key distribution (QKD) has enormous potential in high-speed applications. In this Letter, an intrinsic-stable non-Poissonian light source was implemented with Faraday mirrors and could be a crucial element in realizing passive decoy state QKD. The stable g((2))(0) was obtained through a Hanbury Brown-Twiss experiment, and the results fit well with the theoretical value according to Curty et al.s theory [Opt. Lett.34, 3238 (2009)].

Photoluminescence from site-selected coupling between quantum dots and microtoroid cavities

By using CdSe/ZnS quantum dots (QDs), we study the effect of cavity quantum electrodynamics on the coupling of the microtoroid cavity. When with whispering gallery (WG) modes, the microtoroid cavity demonstrates high quality factor and small mode volume at visible wavelengths. Accordingly, fiber tapers allow QDs to adhere into the cavity and further permit the control of site-selected coupling. From the luminescence spectra, QDs are modulated effectively by cavity modes. Variable modulations are observed by changing QD coupling conditions. Therefore, based on experimental and theoretical research, strong and tunable Purcell enhancement can be realized by this system.The authors thank Jinming Cui and Chunhua Dong for their helpful discussion. This work was supported by the National Fundamental Research Program of China (No. 2006CB921900), the National Natural Science Foundation of China (Nos. 60537020 and 60621064), and the Knowledge Innovation Project of the Chinese Academy of Sciences.

Triple differential cross sections of Li+ (1s2) and K+ (3p6) for electron-impact ionization

The triple differential cross sections (TDCSs) of electron-impact ionization for ions (Li+ (1s2) and K+ (3p6)) and atoms (He (1s2) and Ar (3p6)) have been calculated at coplanar asymmetric geometry using the distorted wave Born approximation method. The features of binary peaks in the TDCSs of ions are compared with those of corresponding atoms. Differences of positions of binary peaks between ions and atoms indicate that the usual single-binary encounter mechanism, which is a good description of the electron collision with atoms, cannot describe the electron collision with ions well. A double-binary encounter mechanism is proposed to describe the electron collision with ions.

Mode coupling strength in a microsphere cavity coupled with fiber taper

With the coupled-mode theory, we analyzed the energy transmission between the microsphere cavity and waveguide system, and revealed the mode coupling strength in a bare microsphere coupled with a fiber taper theoretically and experimentally. In a practical taper-microsphere coupling system, the traveling wave clockwise (CW) and counterclockwise (CCW) whispering gallery modes (WGMs) should existed simultaneity. Those two modes are coupled in the microsphere cavity with a coupling strength (beta). In experiment, the reflection spectra of the taper-microsphere system were investigated, By modifying various coupling condition for microspheres, different groups of data were obtained from the spectra, and used as fitting data for coupling strength estimation. The theory will be valuable in evaluating the upper limit of quality factor (Q) of a microcavity basing on the WGMs. Referring to the result and consequence of beta, it enables multiple ways to improve the Q-factor of a real cavity like annealing. Because the experiment device is compact and sensitive, such method is also promising for detecting the propagating loss in fiber instead of traditional way.