Experimental confirmation at visible light wavelengths of the backscattering enhancement phenomenon of the photonic nanojet.

We report what we believe is the first experimental confirmation at visible light wavelengths of the backscattering enhancement phenomenon of the photonic nanojet. A specially designed sample stage consisting of a multilayered sandwich of glass, solid polydimethylsiloxane (PDMS), and liquid PDMS, permitted the precise positioning of a gold nanoparticle of diameter between 50 and 100 nm within the nanojet emitted by a 4.4 µm diameter BaTiO(3) microsphere embedded within the PDMS. We determined that, when the gold nanoparticle is optimally positioned within the nanojet, the backscattering of the microsphere can greatly increase: for example, by 3:1 (200%) for the 50 nm gold nanoparticle. The increased backscattering is strongly dependent upon the illumination wavelength and the numerical aperture of the imaging system, and occurs for nonresonant illuminations of the isolated microsphere. Low objective numerical apertures of approximately 0.075 yield the maximum observed increases in backscattering. The measured data agree well with numerical calculations incorporating Mie-based theory and Fourier optics.

Spectroscopy of coherently coupled whispering-gallery modes in size-matched bispheres assembled on a substrate.

Coupling between whispering-gallery modes is studied using imaging spectroscopy in pairs of size-selected (size deviation of <0.03%) spheres in a touching position. In a special geometrical configuration, peculiar shapes resembling kites are observed in the spectral images of bispheres by coupling of multiple pairs of azimuthal modes. The maximal WGM coupling is observed in the equatorial plane of spheres determined by the substrate. The WGM coupling constant is quantified for spheres from 2.9 to 6 microm for such modes.