Nanoscale nonlinear optics using silica nanowires

Abstract:

We have fabricated silica fibers with diameters less than one micrometer and molecularly smooth side walls. We modeled the waveguide properties and demonstrated low-loss light propagation over tens of millimeters, sufficient for microphotonic device applications. We manipulated silica nanowires into geometries to demonstrate waveguiding around tight bends as small as 5 micrometers, evanescent coupling, and wavelength filtering as a ring resonator. The linear waveguiding properties produced by the large index contrast between silica and air yield a tight confinement of the mode, which, when combined with the high electric field intensity in an ultrashort laser pulse, can produce significant nonlinear effects over lengths of about one millimeter. We studied nonlinear optical properties by observing the spectral broadening as a probe for the diameter-dependent nonlinearity of the silica nanowire. Our measurements confirmed the dependence of the generated spectra on the theoretically calculated effective nonlinearity and diameter-dependent dispersion. Our results reveal a diameter range for silica nanowires with an enhanced nonlinearity which may be employed in nonlinear devices. We fabricate a nonlinear Sagnac interferometer using silica nanowires for optical switching and discuss possibilities for optical logic. Our results confirm light-by-light modulation, with pulse energies less than a couple of nJ. Combining top-down and bottom-up fabrication techniques, we use tapered silica fibers to couple light directly into the waveguiding modes of ZnO nanowires. We experimentally confirm simulations for the coupling efficiencies and propagation of higher order modes. We also excite ZnO nanowires with ultrashort laser pulses and compare the spectrum transmitted along the nanowire waveguide with the spectrum at the excitation spot. Our results show a shifting of the band edge that indicates a local heating of the nanowire by a few hundred degrees, which is confirmed by finite-element simulation. Finally, we discuss the outlook for nanoscale nonlinear optical devices.