Giant resonant light forces in microspherical photonics

Resonant light pressure effects can open new degrees of freedom in optical manipulation with microparticles, but they have been traditionally considered as relatively subtle effects. Using a simplified two-dimensional model of surface electromagnetic waves evanescently coupled to whispering gallery modes (WGMs) in transparent circular cavities, we show that under resonant conditions the peaks of the optical forces can approach theoretical limits imposed by the momentum conservation law on totally absorbing particles. Experimentally, we proved the existence of strong peaks of the optical forces by studying the optical propulsion of dielectric microspheres along tapered microfibers. We observed giant optical propelling velocities ∼0.45 mm s−1 for some of the 15-20 µm polystyrene microspheres in water for guided powers limited at ∼43 mW. Such velocities exceed previous observations by more than an order of magnitude, thereby providing evidence for the strongly enhanced resonant optical forces. We analyzed the statistical properties of the velocity distribution function measured for slightly disordered (∼1% size variations) ensembles of microspheres with mean diameters varying from 3 to 20 µm. These results demonstrate a principal possibility of optical sorting of microspheres with the positions of WGM resonances overlapped at the wavelength of the laser source. They can be used as building blocks of the lossless coupled resonator optical waveguides and various integrated optoelectronics devices

In situ size sorting in CVD synthesis of Si microspheres

[EN] Silicon microspheres produced in gas-phase by hot-wall CVD offer unique quality in terms of sphericity, surface smoothness, and size. However, the spheres produced are polydisperse in size, which typically range from 0.5 mu m to 5 mu m. In this work we show through experiments and calculations that thermophoretic forces arising from strong temperature gradients inside the reactor volume effectively sort the particles in size along the reactor. These temperature gradients are shown to be produced by a convective gas flow. The results prove that it is possible to select the particle size by collecting them in a particular reactor region, opening new possibilities towards the production by CVD of size-controlled high-quality silicon microspheres.The authors acknowledge financial support from the following projects: ENE2013-49984-EXP, MAT2012-35040, MAT2015-69669-P and ESP2014-54256-C4-2-R of the Spanish Ministry of Economy and Competitiveness (MINECO), and PROMETEOII/2014/026 of the Regional Valencian Government.Garín Escrivá, M.; Fenollosa Esteve, R.; Kowalski, L. (2016). In situ size sorting in CVD synthesis of Si microspheres. Scientific Reports. 6:1-10. https://doi.org/10.1038/srep38719S110

Heavy photon dispersions in photonic crystal waveguides

Heavy photon dispersion curves exhibiting group velocities suppressed by two orders of magnitude are measured directly for deeply etched AlGaAs waveguide structures by means of surface coupling techniques. It is shown that due to the wave vector-selective nature of surface coupling, such techniques permit the excitation of modes of specific, known dispersion in photonic crystal waveguides. Coupling to regions of very strong anomalous dispersion is demonstrated, with potential to be developed into a method for excitation of gap solitons. (C) 2000 American Institute of Physics. [S0003-6951(00)04728-8].</p

Polarisation conversion in the reflectivity properties of photonic crystal waveguides

Strong resonant polarization conversion is observed in the reflectivity properties of one-dimensional (1-D) lattices of air trenches deeply etched in AlGaAs surface waveguides. The symmetry properties and the magnitudes of the observed effects are found to be in good agreement with the results of scattering matrix calculations.</p

Determination of the band structure of photonic crystal waveguides

We report an experimental and theoretical investigation of the near infra-red reflectivity properties of two-dimensional (2D) periodically patterned semiconductor waveguides. The coupling of incident electromagnetic radiation to leaky modes of the photonic crystal waveguide is shown to clearly manifest itself by the appearance of sharp features in the reflectivity spectra. By determining the energy dependence of the resonant features on the angle of incidence along different symmetry directions we are able to map out the photonic band structure. Theoretical spectra obtained using an advanced scattering matrix theory are found to agree well with experiment. The vertical confinement provided by the waveguide is found to have a profound effect on the in-plane band structure of the photonic crystal. (C) 2000 Elsevier Science B.V. All rights reserved.</p

Reflectivity studies of photonic band structure effects in two-dimensional air/semiconductor lattices

We report an experimental and theoretical investigation of the near infra-red reflectivity properties of two-dimensional (2D) photonic crystals obtained by deep reactive ion etching of AlGaAs planar waveguides. It is shown that coupling of incident radiation to the folded band structure of photonic crystal waveguides allows sharp resonance features to be observed in the reflectivity spectra. By determining the energy dependence of the resonant features on the angle of incidence along different symmetry directions we are able to map out the photonic band structure. Theoretical spectra obtained from a numerical solution of Maxwell's equations for the periodically patterned waveguide were found to be in very good agreement with experiment.</p

Resonant coupling of near-infrared radiation to photonic band structure waveguides

Sharp resonance features are observed in the polarized reflectivity spectra of semiconductor photonic crystals fabricated by deep periodic patterning of AlGaAs surface waveguides. Both one- (1-D) and two-dimensional (2-D) lattices are studied by angular dependent reflectivity. By comparison with theory me show that the sharp features in reflectivity arise from resonant coupling of the external radiation to the folded band structure of the photonic crystal waveguides. Wavevector selective coupling to &quot;heavy photon&quot; states at the edge of the photonic Brillouin zone is demonstrated for the 1-D lattices. In the case of the 2-D lattices me observe polarization mixing of the photonic bands. Theoretical reflectivity spectra were obtained from a numerical solution of Maxwells equations for the patterned waveguide and were found to be in very good agreement with experiment.</p