On-chip hybrid photonic-plasmonic light concentrator for nanofocusing in an integrated silicon photonics platform.

The enhancement and confinement of electromagnetic radiation to nanometer scale have improved the performances and decreased the dimensions of optical sources and detectors for several applications including spectroscopy, medical applications, and quantum information. Realization of on-chip nanofocusing devices compatible with silicon photonics platform adds a key functionality and provides opportunities for sensing, trapping, on-chip signal processing, and communications. Here, we discuss the design, fabrication, and experimental demonstration of light nanofocusing in a hybrid plasmonic-photonic nanotaper structure. We discuss the physical mechanisms behind the operation of this device, the coupling mechanisms, and how to engineer the energy transfer from a propagating guided mode to a trapped plasmonic mode at the apex of the plasmonic nanotaper with minimal radiation loss. Optical near-field measurements and Fourier modal analysis carried out using a near-field scanning optical microscope (NSOM) show a tight nanofocusing of light in this structure to an extremely small spot of 0.00563(λ/(2n(rmax)))(3) confined in 3D and an exquisite power input conversion of 92%. Our experiments also verify the mode selectivity of the device (low transmission of a TM-like input mode and high transmission of a TE-like input mode). A large field concentration factor (FCF) of about 4.9 is estimated from our NSOM measurement with a radius of curvature of about 20 nm at the apex of the nanotaper. The agreement between our theory and experimental results reveals helpful insights about the operation mechanism of the device, the interplay of the modes, and the gradual power transfer to the nanotaper apex.

Waveguide-coupled nanowire as an optical antenna.

We study the optical coupling between a gold nanowire and a silver ion-exchanged waveguide, with special emphasis on the nanowire antenna radiation pattern. We measure the radiation patterns of waveguide-coupled gold nanowires with a height of 70 nm and width of 50 or 150 nm in the 450-700 nm spectral range for TE and TM polarizations. We perform a systematic theoretical study on the wavelength, polarization, nanowire size, and material dependences on the properties of the radiation pattern. We also give some elements concerning absorption and near-field. Experiments and calculations show localized plasmon resonance for the polarization orthogonal to the wire (far-field resonance at 580 nm for the smallest wire and 670 nm for the widest). It is shown that a great variety of radiation patterns can be obtained, together with a high sensitivity to a change of one parameter, particularly near-resonance.

Giant coupling effect between metal nanoparticle chain and optical waveguide.

We demonstrate that the optical energy carried by a TE dielectric waveguide mode can be totally transferred into a transverse plasmon mode of a coupled metal nanoparticle chain. Experiments are performed at 1.5 µm. Mode coupling occurs through the evanescent field of the dielectric waveguide mode. Giant coupling effects are evidenced from record coupling lengths as short as ~560 nm. This result opens the way to nanometer scale devices based on localized plasmons in photonic integrated circuits.