Ali Serpengüzel

Excitation of resonances of microspheres on an optical fiber

Morphology-dependent resonances (MDRs) of solid microspheres are excited by using an optical fiber coupler. The narrowest measured MDR linewidths are limited by the excitation laser linewidth (<0.025 nm). Only MDRs, with an on-resonance to off-resonance intensity ratio of 10(4), contribute to scattering. The intensity of various resonance orders is understood by the localization principle and the recently developed generalized Lorentz-Mie theory. The microsphere fiber system has potential for becoming a building block in dispersive microphotonics. The basic physics underlying our approach may be considered a harbinger for the coupling of active photonic microstructures such as microdisk lasers.

Enhanced Coupling to Microsphere Resonances with Optical Fibers

Morphology-dependent resonances (MDRs) of polystyrene microspheres were excited by an optical fiber coupler. For optical elimination of the air–cladding interface at the optical fiber coupler surface, the microsphere was immersed in an index-matching oil. MDRs were observed, even though the relative refractive index between the microsphere and the oil was only 1.09. The observed MDR spectra are in good agreement with the generalized Lorenz–Mie theory and the localization principle. The scattering efficiency into each MDR is estimated as a function of the impact parameter by means of generalized Lorenz–Mie theory.

Morphology-dependent resonances of a microsphere–optical fiber system

Morphology-dependent resonances of microspheres sitting upon an index-matched single-mode fiber half-coupler are excited by a tunable 753-nm distributed-feedback laser. Resonance peaks in the scattering spectra and associated dips in the transmission spectra for the TE and TM modes are observed. We present a new model that describes this interaction in terms of the fiber-sphere coupling coefficient and the microspheres intrinsic quality factor Q(0). This model enables us to obtain expressions for the finesse and the Q factor of the composite particle-fiber system, the resonance width, and the depth of the dips measured in the transmission spectra. Our model shows that index matching improves the coupling efficiency by more than a factor of 2 compared with that of a non-index-matched system.

Wavelength variation of laser emission along the entire rim of slightly deformed microdroplets

The frequencies of the normal azimuthal modes of a slightly deformed droplet that is axisymmetric about its flow direction are no longer degenerate but vary with position along the droplet rim. We measured the wavelength variation along the entire rim of a dye-lasing droplet with a spectrograph and a CCD array. We determined the amplitude and the shape of the droplet deformation from the observed and predicted parabolic dependence of wavelength variation with distance along the spectrograph slit.

Two-photon-pumped lasing in microdroplets

Lasing is observed in laser-dye-doped ethanol droplets after two-photon absorption by the dye molecules. The two-photon-pumped lasing emission by the droplets is at a higher frequency than the input laser. Competitive nonlinear-optical effects that occur at high input-laser intensity are discussed.

Two-dimensional imaging of sprays with fluorescence, lasing, and stimulated Raman scattering

Two-dimensional fluorescence, lasing, and stimulated Raman scattering images of a hollow-cone nozzle spray are observed. The various constituents of the spray, such as vapor, liquid ligaments, small droplets, and large droplets, are distinguished by selectively imaging different colors associated with the inelastic light-scattering processes.

Optical channel dropping with a silicon microsphere

We report the observation of morphology-dependent resonances from silicon microspheres. The dropped channels are observed both in the transmission and elastic scattering spectra in the O-band. The filter drops approximately 23% of the power at the resonance wavelength. The highest observed quality factor for the morphology-dependent resonances was on the order of 10/sup 5/. These resonances have a linewidth of 0.007 nm and a mode spacing of 0.19 nm.

Luminescence of black silicon

Room temperature visible and near-infrared photoluminescence from black silicon has been observed. The black silicon is manufactured by shining femtosecond laser pulses on silicon wafers in air, which were later annealed in vacuum. The photoluminescence is quenched above 120 K due to thermalization and competing nonradiative recombination of the carriers. The photoluminescence intensity at 10K depends sublinearly on the excitation laser intensity confirming band tail recombination at the defect sites.

Microsphere-based channel dropping filter with an integrated photodetector

Morphology-dependent resonances of dielectric microspheres are used for polarization-insensitive optical channel dropping from an optical fiber half coupler to a silicon photodetector in the M-band. The dropped channels are observed both in the elastic scattering and the transmission spectra. The highest quality factor morphology-dependent resonances have repetitive channel separations of 0.14 nm and linewidths of 0.06 nm. The filter drops approximately 10% (0.5 dB) of the power at the resonance wavelength. The power detected by the photodiode is estimated to be 3.5% of the power in the fiber.

Donor-acceptor pair recombination in AgIn5S8 single crystals

Photoluminescence (PL) spectra of AgIn5S8 single crystals were investigated in the 1.44–1.91 eV energy region and in the 10–170 K temperature range. The PL band was observed to be centered at 1.65 eV at 10 K and an excitation intensity of 0.97 W cm−2. The redshift of this band with increasing temperature and with decreasing excitation intensity was observed. To explain the observed PL behavior, we propose that the emission is due to radiative recombination of a donor-acceptor pair, with an electron occupying a donor level located at 0.06 eV below the conduction band, and a hole occupying an acceptor level located at 0.32 eV above the valence band.