Aurélien Bruyant

Efficient Directional Coupling between Silicon and Copper Plasmonic Nanoslot Waveguides: toward Metal−Oxide−Silicon Nanophotonics

Coupling plasmonics and silicon photonics is the best way to bridge the size gap between macroscopic optics and nanodevices in general and especially nanoelectronic devices. We report on the realization of key blocks for future plasmonic planar integrated optics, nano-optical couplers, and nanoslot waveguides that are compatible both with the silicon photonics and the CMOS microelectronics. Copper-based devices provide for very efficient optical coupling, unexpectedly low propagation losses and a broadband sub-50 nm optical confinement. The fabrication in a standard frontline microelectronic facilities hints broad possibilities of hybrid opto-electronic very large scale integration.

All-silicon omnidirectional mirrors based on one-dimensional photonic crystals

We report on the fabrication of monolithic omnidirectional mirrors based on one-dimensional photonic crystals. The mirrors are comprised of chirped and unchirped multiple layers of microporous silicon. Porosities have been chosen to achieve an optimal low refractive index nL∼1.5 and a high refractive index nH∼2.55. Unchirped structures, centered in the near-infrared, exhibit an omnidirectional reflection band of 100 nm, in agreement with the calculated photonic band structure. Chirped structures exhibit an enlarged omnidirectional stop band (340 nm). Given the possibility of easily tailoring the optical thickness of porous silicon, this material is shown to be very practical for engineering omnidirectional mirrors.

Implementation of PT symmetric devices using plasmonics: principle and applications

The so-called PT symmetric devices, which feature ε((-x)) = ε((x))* associated with parity-time symmetry, incorporate both gain and loss and can present a singular eigenvalue behaviour around a critical transition point. The scheme, typically based on co-directional coupled waveguides, is here transposed to the case of variable gain on one arm with fixed losses on the other arm. In this configuration, the scheme exploits the full potential of plasmonics by making a beneficial use of their losses to attain a critical regime that makes switching possible with much lowered gain excursions. Practical implementations are discussed based on existing attempts to elaborate coupled waveguide in plasmonics, and based also on the recently proposed hybrid plasmonics waveguide structure with a small low-index gap, the PIROW (Plasmonic Inverse-Rib Optical Waveguide).

Heterodyne detection of guided waves using a scattering-type Scanning Near-Field Optical Microscope.

An inherent problem to the study of waveguides with strong propagation losses by Scattering-type Scanning Near field Optical Microscopy is the coherent optical background field which disrupts strongly the weak detected near-field signal. We present a technique of heterodyne detection allowing us to overcome this difficulty while amplifying the near field signal. As illustrated in the case of a highly confined SOI structure, this technique, besides the amplitude, provides the local phase variation of the guided field. The knowledge of the complex field cartography leads to the modal analysis of the propagating radiation.

Enlarged atomic force microscopy scanning scope: Novel sample-holder device with millimeter range

We propose a homemade sample-holder unit used for nanopositionning in two dimensions with a millimeter traveling range. For each displacement axis, the system includes a long range traveling stage and a piezoelectric actuator for accurate positioning. Specific electronics is integrated according to metrological considerations, enhancing the repeatability performances. The aim of this work is to demonstrate that near-field microscopy at the scale of a chip is possible. For this we chose to characterize highly integrated optical structures. For this purpose, the sample holder was integrated into an atomic force microscope. A millimeter scale topographical image demonstrates the overall performances of the combined system.

Analysis of the interferometric effect of the background light in apertureless scanning near-field optical microscopy

We investigate in detail the interferometric nature of the signal delivered by an apertureless scanning near-field optical microscope (SNOM). This nature is first brought to the fore by near-field images of an integrated waveguide. The detection process of an evanescent wave generated by total internal reflection is then studied by both lateral near-field scans and signal detection as a function of the tip-to-sample distance. This study permits interpretation of fringe patterns appearing in apertureless SNOM images and provides important information about the nature of the signal. In particular, both experimental data and simple calculations show that, because of interference with background light coming from the sample, the detected signal can describe the complex field amplitude, or the field intensity, or a subtle mix of both, depending on the tip environment and the tip position.

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.

Phase sensitive optical near-field mapping using frequency-shifted laser optical feedback interferometry

The use of laser optical feedback Imaging (LOFI) for scattering-type scanning near-field optical microscopy (sSNOM) is proposed and investigated. We implement this sensitive imaging method by combining a sSNOM with optical heterodyne interferometry and the dynamic properties of a B class laser source which is here used both as source and detector. Compared with previous near field optical heterodyne experiments, this detection scheme provides an optical amplification that is several orders of magnitude higher, while keeping a low noise phase-sensitive detection. Successful demonstration of this complex field imaging technique is done on Silicon on Insulator (SOI) optical waveguides revealing phase singularities and directional leakage.

Apertureless scanning near-field optical microscopy for ion exchange channel waveguide characterization.

We report the characterization of an integrated Ag+/Na+ ion exchange waveguide realized in a silicate glass substrate using apertureless scanning near‐field optical microscopy. Our experimental set‐up is based on the combination of a commercial atomic force microscope with an optical confocal detection system. Thanks to this system, the topography and evanescent optical field at the waveguide top surface are mapped simultaneously. Also, the process of apertureless scanning near‐field optical microscopy image formation is analysed. In particular, fringe patterns appearing in the image reveal the intrinsic interferometric nature of the collected signal, due to interference between the field scattered by the tip end and background fields related to guide losses. We give a quantitative interpretation of these fringes. Evanescent intensity mapping on the sample surface allowed us to extract physical waveguide parameters. In particular, it shows an unambiguous multimode beat along the waveguide propagation axis. Furthermore, we show that analysis of this intensity profile reveals back‐reflection effects from the waveguide exit facet. The resulting standing waves pattern allows us to evaluate the eigenmode propagation constants.

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.