Directional escape from a high-Q deformed microsphere induced by short CO2 laser pulses

Nonaxisymmetric fused-silica microspheres were fabricated by using only one microsphere by virtue of short pulses of a CO2 laser. Highly directional emission from whispering-gallery modes (WGMs) was studied through direct free-space evanescent excitation. The observed quality factors of WGMs are high, up to 2x10(7), despite this deformation. We show that WGMs in slightly deformed microspheres possibly play an important role in cavity quantum electrodynamics research.     

Excitation and interrogation of whispering-gallery modes in optical microresonators using a single fused-tapered fiber tip

We show that whispering-gallery modes (WGMs) in optical microresonators can be excited and detected using a fused-tapered fiber tip (FTFT). The fabrication of FTFTs is simple and inexpensive; they are robust and allow the excitation and interrogation of the resonances with a single fiber. Excitation of high-Q WGMs in silica microcylinders and microspheres is demonstrated.     

Mode coupling between first- and second-order whispering-gallery modes in coupled microdisks

The mode characteristics for two coupled microdisks are investigated by the finite-difference time-domain technique. In the two coupled micodisks, mode coupling between the same order whispering-gallery modes (WGMs) results in coupled WGMs with split mode wavelengths. The numerical results show that the split mode wavelengths of the coupled first- and second-order WGMs can have a crossing point in some cases, which can induce anticrossing mode coupling between them and greatly reduce the mode Q factor of the coupled first-order WGMs. The time variation of mode field pattern shows the transformation between the coupled first- and second-order WGMs.     

Whispering-gallery modes in glass microspheres: optimization of pumping in a modified confocal microscope

Whispering-gallery modes (WGMs) on Nd3+-doped glass microspheres with a radius of ～15?μm were measured in a modified confocal microscope, where a dual spatial resolution in both excitation and detection zones was possible. As an alternative to the standard excitation mechanism by an evanescent wave, we used an efficient pumping/detecting scheme, focusing a laser in the microsphere and exciting the Nd3+ ions, whose fluorescent emission produces the WGMs. We have also measured the generated WGMs by changing the detection zone, where higher amplitude resonances were found when exciting in the center and detecting at the edge of the microsphere.     

Whispering gallery modes and effect of coating on Raman spectra of single microspheres

Raman spectra and fluorescence of BaTiO₃ and silica microspheres of different sizes have been studied. The observed whispering gallery modes (WGMs) have been assigned using theoretical simulations based on the Lorenz–Mie theory. The WGMs are found to have selective enhancements in the Raman spectra. The variations in the Raman spectra with the radial position of the excitation spot and excitation wavelength have been correlated with the morphology‐dependent internal field distributions of the microspheres. The effect of a thin dye coating on the fluorescence and Raman spectra was studied, and a coating thickness of ∼200 nm was estimated from the theoretical simulation of experimentally observed data based on the Aden and Kerker theory. Copyright © 2011 John Wiley & Sons, Ltd.     

Mode characteristics of silver-coated inverted-wedge silica microdisks

The characteristics of whispering gallery modes(WGM) in silver-coated inverted-wedge silica microdisks are theoretically investigated by using finite element method. Dielectric TE mode always exists in silver-coated inverted-wedge resonators; dielectric TM mode tends to couple with SPP modes; only pure interior surface plasmonic polariton(SPP) mode but not pure exterior SPP mode is observed in contrast to the metal-coated cylindrical and toroidal resonators. The dependence of quality factor of different kinds of WGMs on the radius of the resonator and the thickness of the coated silver layer are systematically analyzed. We find that the quality factors of the hybrid WGMs associated with SPP mode can reach 104. The maximum light intensity enhancement in ambient for a hybrid mode consisting of a dielectric TM mode and an exterior SPP mode can be obtained when a silver film of thickness ~40 nm is deposited. The silver-coated inverted-wedge silica resonators may be widely applied in sensing and surface enhanced Raman scattering.     

Quality-factor enhancement of supermodes in coupled microdisks

We investigate the optical modes in a coupled pair of semiconductor microdisks in symmetric and asymmetric configurations both experimentally and theoretically. While the quality factors of coupled first- and second-order whispering gallery modes (WGMs) show a conventional crossing, the quality factors of the same-order WGMs reveal an interesting splitting behavior, leading to the formation of high- and low-quality supermodes. Our results are reproduced by numerical simulations, and an explanation based on optical interference is suggested. Quality-factor splitting is a subtle phenomenon that might help to design microarchitectures for efficient optical coupling in cavity quantum electrodynamic experiments.     

消逝场耦合圆柱形微腔中回音壁模式结构的实验研究

采用石英圆柱形微腔与锥形光纤通过消逝场耦合的方式,在实验上观察到了不同半径的柱形微腔中清晰的回音壁模式,耦合效率接近10%. 利用柱形微腔回音壁模式位置和间隔的解析近似公式,对实验所得的透射光谱做了模式的精确标定,共振波长的实验值和标定值间的误差小于0.05nm. 引入回音壁模式的“模场半径”概念,由“模场半径”计算出的模式间隔和实验测量值之间符合很好. 利用透射光谱中的共振波长和理论标定波长间的差值,得到了实验光谱范围内(1295—1320nm)石英光纤的色散曲线,折射率精度达10^-5. 关键词： 圆柱形微腔 回音壁模式 模式标定 模场半径     

Observation of microlaser with Er-doped phosphate glass coated microsphere pumped by 780 nm

The silica microspheres coated with a thin layer of Er-doped phosphate glass are fabricated and whispering-gallery modes (WGMs) with quality factor of 8 × 10⁶ in the 780 nm band are obtained. We demonstrate that the microsphere can form single-mode and multi-mode microlaser in communication-band with a pumping laser of wavelength 780 nm band. Moreover, the representative lateral emission distributed modes in the microsphere can be directly observed via the upconverted photoluminescence.     

Asymmetric plasmonic-dielectric coupler with short coupling length, high extinction ratio, and low insertion loss

Asymmetric directional coupling between a hybrid plasmonic waveguide with subwavelength field confinement and a conventional dielectric waveguide is investigated. The proposed hybrid coupler features short coupling length, high coupling efficiency, high extinction ratio, and low insertion loss; it can also be integrated into a silicon-based platform. This coupler can be potentially adopted for signal routing between plasmonic waveguides and dielectric waveguides in photonic integrated circuits. Furthermore, it can be exploited to efficiently excite hybrid plasmonic modes with conventional dielectric modes.     

Fiber taper coupled high-quality-factor planar microdisk

A fiber taper can evanescently couple to whispering gallery modes (WGMs) in a planar silica microdisk for observing the optical properties of the microdisk cavity. It is revealed that WGMs have very high quality (Q) factors by controlling the air gap between the taper and the microdisk. The best coupling efficiency from the taper to the microdisk is as high as 98%, and can be continuously adjusted from the under-coupling, critical-coupling to over-coupling regimes. The influence of the laboratory circumstance such as surface contamination on the microdisk is also discussed. It is experimentally shown that the high-Q-factor (105) modes can be kept for a long period in a general laboratory circumstance.     

Deciphering output coupling mechanisms in spiral microcavities with femtosecond light bullets

A pump-probe two-photon-excited fluorescence technique deciphers in space and time the propagation of ballistic wave packets sustained by whispering-gallery modes (WGMs) in a spiral-shaped microcavity. Diffraction on the spiral discontinuity does not prevent the WGMs from closing. The resultant average Q of the resonator is 3 x 10(4) +/- 50%. Experimental results are compared with numerical simulations, providing evidence of a new contribution to output coupling: Part of the WGM evanescent wave is reflected at the spiral notch and leads to a propagating wave at an angle that matches the previously observed laser emission direction in 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran-doped poly(methyl methacrylate) and InGaN spiral lasers.     

光学微盘腔与三能级量子点系统中的模耦合研究

用一种全量子理论方法研究了波导、光学微盘腔与三能级量子点耦合系统的动力学过程,求出其耦合后的透射模和反射模的解析解. 由于微腔表面粗糙引起反向散射,在微腔内形成两简并回音壁耦合共振模,其耦合率为β;量子点的两激发态分别以耦合率g₁,g₂与回音壁耦合共振模产生耦合. 在实数空间里,得出透射光谱和反射光谱的数值解,这些三能级模型结果比二能级模型结果更接近真实光学微盘腔系统,能更好地显示耦合系统的动力学特性. 关键词： 模耦合 光学微盘腔 三能级量子点 全量子理论     

Dual-band plasmon induced transparency controlled by the polarization of the incident wave

In this paper, a hybridized plasmonic waveguide system is investigated numerically. By coupling plasmonic modes with different waveguide modes, two plasmon induced transparency (PIT) windows with high transmittance and deep modulation can be realized. Through the polarization of the incident wave, the transparency windows can be dynamically controlled.     

Modification of photon states in photonic molecules with semiconductor nanocrystals

We report on the coherent coupling of whispering gallery modes (WGMs) in a photonic molecule formed from two melamine formaldehyde spherical microcavities coated with a thin shell of CdTe nanocrystals. Utilizing different excitation conditions, the splitting of the WGM resonances originating from bonding and antibonding branches of the photonic states is observed and a fine structure consisting of very sharp peaks resulting from lifting of the WGM degeneracy has been detected. Observation of optical wavelength switching with controllable spectral position and separation is reported.     

Controlled observation of nondegenerate cavity modes in a microdroplet on a superhydrophobic surface

We demonstrate controlled lifting of the azimuthal degeneracy of the whispering gallery modes (WGMs) of single glycerol-water microdroplets standing on a superhydrophobic surface by using a uniform electric field. A good agreement is observed between the measured spectral positions of the nondegenerate WGMs and predictions made for a prolate spheroid. Our results reveal fewer azimuthal modes than expected from an ideal spherical microdroplet due to the truncation by the surface. We use this difference to estimate the contact angles of the microdroplets.     

Optical manipulation in optofluidic microbubble resonators

An optical manipulation system based on optofluidic microbubble resonators(MBR) is proposed. As the high-Q whispering gallery modes(WGMs) are excited in an MBR, the buildup of the field intensity inside the resonator is large enough to trap nanoscale particles. The optical gradient forces generated by the WGMs with different radial orders are investigated numerically. The negative effect of the resonance detuning induced by the particles is taken into account to investigate the optical gradient forces exerting on the particles. By the stability analysis, the WGMs with high radial orders show a better trapping stability under Brownian motion since most of the optical fields reside within the water core.     

Optical imaging with subnanometric vertical resolution using nanoparticle-based plasmonic probes

We present images with subnanometric optical resolution using novel metal nanoparticle-based probes in a Field Enhancement Scanning Optical Microscope. The probe is built of silica microspheres decorated with silver nanoparticles, which act as enhancers of the optical field. The optical resolution is dependent on the wavelength and polarization of the exciting field, and the approach curves reveal the enhancement due to cavity modes between the plasmonic probe and the sample.     

Manipulating electromagnetic resonances in meta-molecules with double split ring resonators

We proposed meta-molecules structure composed of stacked double split ring resonators (DSRRs) and studied its electromagnetic resonances in the optical frequency range. We demonstrated that, for the first order plasmonic modes, the coupling between the outer and the inner SRRs in plane strongly influences on the resonant frequency splitting of the stacked DSRRs. And their resonant dips change with the arrangement of SRRs. However, the resonant frequencies for the high order plasmonic modes always remain immobile as the configuration varies. Our investigation offers an effective way to manipulate the resonant behavior in metamaterials.     

Whispering-gallery mode terahertz pulses

We report what is to our knowledge the first observation of a subpicosecond terahertz (THz) pulse propagating as a superposition of the whispering-gallery modes (WGMs) of a dielectric cylinder. The WGM THz pulses are coupled into and out of a 5-mm-diameter silicon cylinder via a 100-mum -thick, single-mode silicon slab waveguide. We observed two round-trip cylindrical cavity pulses from this coupling structure, which cover a continuous frequency range from 0.4 to 1.8 THz. The coupled-mode equations are used to analyze this system and give reasonably good agreement with the experiment in both the frequency and the time domains.