Fiber Optic Excitation of Silicon Microspheres in Amorphous and Crystalline Fluids

This study investigates the optical resonance spectra of free-standing monolithic single crystal silicon microspheres immersed in various amorphous fluids, such as air, water, ethylene glycol, and 4-Cyano-4’-pentylbiphenyl nematic liquid crystal. For the various amorphous fluids, morphology-dependent resonances with quality factors on the order of 105 are observed at 1428 nm. The mode spacing is always on the order of 0.23 nm. The immersion in various amorphous fluids affects the spectral response of the silicon microsphere and heralds this technique for use in novel optofluidics applications. Even though the nematic liquid crystal is a highly birefringent, scattering, and high-index optical medium, morphology-dependent resonances with quality factors on the order of 105 are observed at 1300 nm in the elastic scattering spectra of the silicon microsphere, realizing a liquid-crystal-on-silicon geometry. The relative refractive index and the size parameter of the silicon microsphere are the parameters that affect the resonance structure. The more 4-Cyano-4’-pentylbiphenyl interacting with the silicon microsphere, the lower the quality factor of the resonances is. The more 4-Cyano-4’-pentylbiphenyl is interacting with the silicon microsphere, the lower the mode spacing Δλ of the resonances is. The silicon microspheres wetted with nematic liquid crystal can be used for optically addressed liquid-crystal-on-silicon displays, light valve applications, or reconfigurable optical networks

Controlled photoluminescence in amorphous-silicon-nitride microcavities

Narrow-band and enhanced photoluminescence have been observed in hydrogenated amorphous-silicon-nitride microcavities. The distributed Bragg reflectors were fabricated using alternating layers of hydrogenated amorphous-silicon nitride and hydrogenated amorphous-silicon oxide. The microcavity resonance wavelength was designed to be at the maximum of the bulk hydrogenated amorphous-silicon-nitride luminescence spectrum. At the microcavity resonance, the phololuminescence amplitude is enhanced, while the photoluminescence linewidth is reduced with respect to the bulk hydrogenated amorphous-silicon nitride. © 2001 American Institute of Physics

Optical microsphere resonators: optimal coupling to high-Q whispering gallery modes

A general model is presented for coupling of high-$Q$ whispering-gallery modes in optical microsphere resonators with coupler devices possessing discrete and continuous spectrum of propagating modes. By contrast to conventional high-Q optical cavities, in microspheres independence of high intrinsic quality-factor and controllable parameters of coupling via evanescent field offer variety of regimes earlier available in RF devices. The theory is applied to the earlier-reported data on different types of couplers to microsphere resonators and complemented by experimental demonstration of enhanced coupling efficiency (about 80%) and variable loading regimes with Q>10^8 fused silica microspheres.Comment: 14 pages, 4 figure

Dye lasing in optically manipulated liquid aerosols

We report lasing in airborne, rhodamine B-doped glycerol-water droplets with diameters ranging between 7.7 and 11.0 mu m, which were localized using optical tweezers. While being trapped near the focal point of an infrared laser, the droplets were pumped with a Q-switched green laser. Our experiments revealed nonlinear dependence of the intensity of the droplet whispering gallery modes (WGMs) on the pump laser fluence, indicating dye lasing. The average wavelength of the lasing WGMs could be tuned between 600 and 630 nm by changing the droplet size. These results may lead to new ways of probing airborne particles, exploiting the high sensitivity of stimulated emission to small perturbations in the droplet laser cavity and the gain medium

Second-Harmonic Generation in Silicon Nitride Ring Resonators

The emerging field of silicon photonics seeks to unify the high bandwidth of optical communications with CMOS microelectronic circuits. Many components have been demonstrated for on-chip optical communications, including those that utilize the nonlinear optical properties of silicon[1, 2], silicon dioxide[3, 4] and silicon nitride[5, 6]. Processes such as second harmonic generation, which are enabled by the second-order susceptibility, have not been developed since the bulk $\chi^{(2)}$ vanishes in these centrosymmetric CMOS materials. Generating the lowest-order nonlinearity would open the window to a new array of CMOS-compatible optical devices capable of nonlinear functionalities not achievable with the?$\chi^{(3)}$ response such as electro-optic modulation, sum frequency up-conversion, and difference frequency generation. Here we demonstrate second harmonic (SH) generation in CMOS compatible integrated silicon nitride (Si3N4) waveguides. The $\chi^{(2)}$ response is induced in the centrosymmetric material by using the nanoscale structure to break the bulk symmetry. We use a high quality factor Q ring resonator cavity to enhance the efficiency of the nonlinear optical process and detect SH output with milliwatt input powers.Comment: 4 pages, 3 figure

A method for volume stabilization of single, dye-doped water microdroplets with femtoliter resolution

A self-control mechanism that stabilizes the size of Rhodamine B-doped water microdroplets standing on a superhydrophobic surface is demonstrated. The mechanism relies on the interplay between the condensation rate that was kept constant and evaporation rate induced by laser excitation which critically depends on the size of the microdroplets. The radii of individual water microdroplets (>5 um) stayed within a few nanometers during long time periods (up to 455 seconds). By blocking the laser excitation for 500 msec, the stable volume of individual microdroplets was shown to change stepwise.Comment: to appear in the J. Op. Soc. Am.

Radiative donor-acceptor pair recombination in TlInS2 single crystals

Photoluminescence (PL) spectra of TlInS2 layered single crystals were investigated in the 500-860 nm wavelength region and in the 11.5-100 K temperature range. We observed two PL bands centred at 515 nm (2.41 eV, A band) and 816 nm (1.52 eV, B band) at T = 11.5 K and an excitation intensity of 7.24 W cm-2. A detailed study of the A band was carried out as a function of temperature and excitation laser intensity. A red shift of the A band position was observed for both increasing temperature and decreasing excitation laser intensity in the range from 0.12 to 7.24 W cm-2. Analysis of the data indicates that the A band is due to radiative transitions from the moderately deep donor level located at 0.25 eV below the bottom of the conduction band to the shallow acceptor level located at 0.02 eV above the top of the valence band. An energy-level diagram for radiative donor-acceptor pair transitions in TlInS2 layered single crystals is proposed

Theory of free space coupling to high-Q whispering gallery modes

A theoretical study of free space coupling to high-Q whispering gallery modes both in circular and deformed microcavities are presented. In the case of a circular cavity, both analytical solutions and asymptotic formulas are derived. The coupling efficiencies at different coupling regimes for cylinder incoming wave are discussed, and the maximum efficiency is estimated for the practical Gaussian beam excitation. In the case of a deformed cavity, the coupling efficiency can be higher if the excitation beam can match the intrinsic emission well and the radiation loss can be tuned by adjusting the degree of deformation. Employing an abstract model of slightly deformed cavity, we found that the asymmetric and peak like line shapes instead of the Lorentz-shape dip are universal in transmission spectra due to multi-mode interference, and the coupling efficiency can not be estimated from the absolute depth of the dip. Our results provide guidelines for free space coupling in experiments, suggesting that the high-Q ARCs can be efficiently excited through free space which will stimulate further experiments and applications of WGMs based on free space coupling.Comment: 8 pages, 4 figure

Super-radiant surface emission from a quasi-cavity hot electron light emitter

The Hot Electron Light Emitting and Lasing in Semiconductor Heterostructure (HELLISH-1) device is a novel surface emitter which utilises hot carrier transport parallel to the layers of a Ga1 - xAlxAs p-n junction incorporating a single GaAs quantum well on the n-side of the junction plane. Non-equilibrium electrons are injected into the quantum well via tunnelling from the n-layer. In order to preserve the charge neutrality in the depletion region, the junction undergoes a self-induced internal biasing. As a result the built-in potential on the p-side is reduced and hence the injection of non-equilibrium holes into the quantum well in the active region is enhanced. The work presented here shows that a distributed Bragg reflector grown below the active region of the HELLISH device increases the emitted light intensity by two orders of magnitude and reduces the emission line-width by about a factor of 3 in comparison with the original HELLISH-1 structure. Therefore, the device can be operated as an ultrabright emitter with higher spectral purity