Aniello Apuzzo

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.

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.

Observation of Near-Field Dipolar Interactions Involved in a Metal Nanoparticle Chain Waveguide

We present near-field measurements of transverse plasmonic wave propagation in a chain of gold elliptical nanocylinders fed by a silicon refractive waveguide at optical telecommunication wavelengths. Eigenmode amplitude and phase imaging by apertureless scanning near-field optical microscopy allows us to measure the local out-of-plane electric field components and to reveal the exact nature of the excited localized surface plasmon resonances. Furthermore, the coupling mechanism between subsequent metal nanoparticles along the chain is experimentally analyzed by spatial Fourier transformation on the complex near-field cartography, giving a direct experimental proof of plasmonic Bloch mode propagation along array of localized surface plasmons. Our work demonstrates the possibility to characterize multielement plasmonic nanostructures coupled to a photonic waveguide with a spatial resolution of less than 30 nm. This experimental work constitutes a prerequisite for the development of integrated nanophotonic devices.

Quantitative analysis and near-field observation of strong coupling between plasmonic nanogap and silicon waveguides

We present a near field optical study of a plasmonic gap waveguide vertically integrated on silicon. The experimental study is based on a near field scanning optical microscope configured in perturbation mode. This operation mode is described and modeled to give a physical insight into the measured signal. A high spatial resolution allows for the characteristics of the plasmonic gap modes, such as near field distributions, effective indices, direction of propagation, and coupling between perpendicularly polarized modes, to be imaged and analyzed with accuracy. This experimental work is supported by numerical simulations based on finite element optical mode solvers and by the application of the strongly coupled-mode theory to the device.

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.

Theoretical analysis of Bloch mode propagation in an integrated chain of gold nanowires

The eigenmodes analysis of Bloch modes in a chain of metallic nanowires (MNWs) provides a significant physical understanding about the light propagation phenomena involved in such structures. However, most of these analyses have been done above the light line in the dispersion relation, where the Bloch modes can only be excited with radiative modes. By making use of the Fourier modal method, in this paper we rigorously calculate the eigenmode and mode excitation of a chain of MNWs via the fundamental transverse magnetic (TM) mode of a dielectric waveguide. Quadrupolar and dipolar transversal Bloch modes were obtained in an MNW chain embedded in a dielectric material. These modes can be coupled efficiently with the fundamental TM mode of the waveguide. Since the eigenmodes supported by the integrated plasmonic structure exhibit strong localized surface plasmon (LSP) resonances, they could serve as a nanodevice for sensing applications. Also, the analysis opens a direction for novel nanostructures, potentially helpful for the efficient excitation of LSPs and strong field enhancement.

Light confinement and propagation characteristics in plasmonic gap waveguides on silicon

Plasmonic waveguiding structures have the ability to confine and propagate light over short distances, typically less than a hundred micrometers. This short propagation length is the price that is paid for confining light to dimensions on the order of a hundred of nanometers. With these scales in mind, several plasmonic devices can be proposed (e.g. wavelength multiplexors) and some of them have been already demonstrated such as Y junctions and directional couplers. Although the dimensions involved in such structures are below the diffraction limit, large-scale optical characterization techniques, such as transmitted power, are still employed. In this contribution, we present a characterization technique for the study of the guided modes in plasmonic gap waveguiding structures that resolves subwavelength-scale features, as it is based on atomic force microscope and on near field scattering optical microscope in guided detection.

Optical near field in silicon photonics

Optical devices based on SOI have been fabricated and tested for the last decade by using far field optics. Alternatively, near-field scanning optical microscopes (NSOM) have the ability to reach unique optical resolution by converting the evanescent waves into radiation waves that can be detected by conventional far field optics. Thus, the aim of this paper is to show the most recent capabilities of the NSOM in a guided detection to probe SOI-based structures. By using this simple yet powerful configuration, we can observe the propagation of the light in Si-based devices and thus measure the propagation characteristics of the guided modes.

Numerical analysis of tip-localized surface plasmon resonances in periodic arrays of gold nanowires with triangular cross section

In this contribution, we numerically study the tip enhancement of localized surface plasmons in periodic arrays of gold nanowires with triangular cross section under different illumination configurations. We found that the plasmonic resonance in a single nanowire is excited with a transverse magnetic (TM) plane wave impinging from the substrate at the critical angle, whereas grazing angles are required for the excitation of resonant propagating modes in periodic arrays of triangular-shaped nanowires. Moreover, we found that resonant plasmonic quasi-Bloch modes are efficiently excited with the fundamental TM mode of a dielectric waveguide placed underneath the array. The integrated plasmonic structure allows a strong enhancement of the electromagnetic field at the tip of the nanowires, hence its potential application in the development of new nanophotonic devices.

Optical near field imaging of localized surface plasmons modes in metallic nanostructures integrated on dielectric waveguides

The study of surface plasmon-polaritons interactions in metallic nanostructures has been a topic of interest during last years due to their use in various areas such as the photonics, chemistry and biology. Example of use is found in biosensors for the efficient detection of biological analyte and in nanophotonic elements for on-chip photonics. Here, we study the interactions properties of localized surface plasmons in a hybrid waveguiding structure made of bi-dimensional array of gold nanowires vertically integrated on silicon-on-insulator waveguides across the near infrared spectrum. With the use of near-field scanning optical microscopy (NSOM) in perturbation mode, we qualitatively obtained the spectral response of such hybrid structure through intensity near field maps of the light propagation. These experimental results demonstrate that metallic nanostructures integrated on silicon are suitable for the development of localized surface plasmon integrated devices or metallic metamaterials.