Aluminum plasmonic waveguides co-integrated with Si3N4 photonics using CMOS processes

Co-integrating CMOS plasmonics and photonics became the “sweet spot” to hit in order to combine their benefits and allow for volume manufacturing of plasmo-photonic integrated circuits. Plasmonics can naturally interface photonics with electronics while offering strong mode confinement, enabling in this way on-chip data interconnects when tailored to single-mode waveguides, as well as high-sensitivity biosensors when exposing Surface-Plasmon-Polariton (SPP) modes in aqueous environment. Their synergy with low-loss photonics can tolerate the high plasmonic propagation losses in interconnect applications, offering at the same time a powerful portfolio of passive photonic functions towards avoiding the use of bulk optics for SPP excitation and facilitating compact biosensor setups. The co-integration roadmap has to proceed, however, over the utilization of fully CMOS compatible material platforms and manufacturing processes in order to allow for a practical deployment route. Herein, we demonstrate for the first time Aluminum plasmonic waveguides co-integrated with Si₃N₄ photonics using CMOS manufacturing processes. We validate the data carrying credentials of CMOS plasmonics with 25 Gb/s data traffic and we confirm successful plasmonic propagation in both air and water-cladded waveguide configurations. This platform can potentially fuel the deployment of co-integrated plasmonic and photonic structures using CMOS processes for biosensing and on-chip interconnect applications.

Electromigrated electrical optical antennas for transducing electrons and photons at the nanoscale

Background: Electrically controlled optical metal antennas are an emerging class of nanodevices enabling a bilateral transduction between electrons and photons. At the heart of the device is a tunnel junction that may either emit light upon injection of electrons or generate an electrical current when excited by a light wave. The current study explores a technological route for producing these functional units based upon the electromigration of metal constrictions. Results: We combine multiple nanofabrication steps to realize in-plane tunneling junctions made of two gold electrodes, separated by a sub-nanometer gap acting as the feedgap of an optical antenna. We electrically characterize the transport properties of the junctions in the light of the Fowler-Nordheim representation and the Simmons model for electron tunneling. We demonstrate light emission from the feedgap upon electron injection and show examples of how this nanoscale light source can be coupled to waveguiding structures. Conclusion: Electromigrated in-plane tunneling optical antennas feature interesting properties with their unique functionality enabling interfacing electrons and photons at the atomic scale and with the same device. This technology may open new routes for device-to-device communication and for interconnecting an electronic control layer to a photonic architecture.

Broadband etching-free metal grating couplers embedded in titanium dioxide waveguides

Metal grating couplers embedded into a titanium dioxide layer are proposed. A coupling efficiency better than 20% is experimentally demonstrated with a 3 dB bandwidth of 86 nm which is in agreement with simulation results. This allowed us to perform error-free transmissions of 10 Gbit/s wavelength multiplexed signals in the C-band.

Low-Power consumption Franz-Keldysh effect plasmonic modulator

In this paper we report on a low energy consumption CMOS-compatible plasmonic modulator based on Franz-Keldysh effect in germanium on silicon. We performed integrated electro-optical simulations in order to optimize the main characteristics of the modulator. A 3.3 dB extinction ratio for a 30 µm long modulator is demonstrated under 3 V bias voltage at an operation wavelength of 1647 nm. The estimated energy consumption is as low as 20 fJ/bit.

Silicon-loaded surface plasmon polariton waveguides for nanosecond thermo-optical switching

A MHz-bandwidth thermo-optical (TO) plasmonic switch operating at telecommunication wavelengths and based on a hybrid solid-state silicon-loaded surface plasmon polariton waveguide design is demonstrated numerically. The nanosecond (ns) TO response of the switch is due to the high thermal conductivities of the employed materials and we demonstrate specifically a 10 dB extinction ratio in the time-dependent switch transmission which features a pulsed 1 ns rise time followed by a 25 ns fall time when the switch is photo-thermally activated by a ns pulse at 532 nm wavelength.

Active plasmonics in WDM traffic switching applications

With metal stripes being intrinsic components of plasmonic waveguides, plasmonics provides a “naturally” energy-efficient platform for merging broadband optical links with intelligent electronic processing, instigating a great promise for low-power and small-footprint active functional circuitry. The first active Dielectric-Loaded Surface Plasmon Polariton (DLSPP) thermo-optic (TO) switches with successful performance in single-channel 10 Gb/s data traffic environments have led the inroad towards bringing low-power active plasmonics in practical traffic applications. In this article, we introduce active plasmonics into Wavelength Division Multiplexed (WDM) switching applications, using the smallest TO DLSPP-based Mach-Zehnder interferometric switch reported so far and showing its successful performance in 4×10 Gb/s low-power and fast switching operation. The demonstration of the WDM-enabling characteristics of active plasmonic circuits with an ultra-low power × response time product represents a crucial milestone in the development of active plasmonics towards real telecom and datacom applications, where low-energy and fast TO operation with small-size circuitry is targeted.

Excitation of a one-dimensional evanescent wave by conical edge diffraction of surface plasmon

The experimental observation of a one-dimensional evanescent wave supported by a 90◦ metal edge is reported. Through a measurement of in-plane momenta, we clearly demonstrate the dimensional character of this surface wave and show that it is non-radiative in the superstrate. Excitation conditions, lateral extension and polarization properties of this wave are discussed. Finally, we explore the effect of the surrounding dielectric medium and demonstrate that a single edge can sustain distinct excitations.

Refractive micro-optical elements for surface plasmons: from classical to gradient index optics

Controlling the propagation of surface plasmons along a metal-dielectric interface is a key feature for the development of surface plasmon based circuits. We have designed various two-dimensional refractive dielectric optical elements for surface plasmons (SP) and characterized their capacity to route SP, using near- or far-field techniques. We first present basic devices analogous to usual optical components and the associated challenges for SP optics. We then use a metamaterial approach to locally vary the refractive index and fabricate gradient index structures for SP circuitry.

Tuning of an optical dimer nanoantenna by electrically controlling its load impedance

Optical antennas are elementary units used to direct optical radiation to the nanoscale. Here we demonstrate an active control over individual antenna performances by an external electrical trigger. We find that by an in-plane command of an anisotropic load medium, the electromagnetic interaction between individual elements constituting an optical antenna can be controlled, resulting in a strong polarization and tuning response. An active command of the antenna is a prerequisite for directing light wave through the utilization of such a device.

Gain-assisted propagation in a plasmonic waveguide at telecom wavelength

The spatial confinement of surface plasmon polaritons is a promising route for realizing optical on-board interconnects. However, mode losses increase with the confinement factor. To overcome this road block, we investigate propagation assisted by stimulated emission in a polymer strip-loaded plasmonic waveguide doped with nanocrystals. We achieve 27% increase of the propagation length at telecom wavelength corresponding to a 160 cm(-1) optical gain coefficient. Such a configuration is a step toward integrated plasmonic amplifiers.

Fluorescence relaxation in the near-field of a mesoscopic metallic particle: distance dependence and role of plasmon modes

We analytically and numerically analyze the fluorescence decay rate of a quantum emitter placed in the vicinity of a spherical metallic particle of mesoscopic size (i.e with dimensions comparable to the emission wavelength). We discuss the efficiency of the radiative decay rate and non-radiative coupling to the particle as well as their distance dependence. The electromagnetic coupling mechanisms between the emitter and the particle are investigated by analyzing the role of the plasmon modes and their nature (dipole, multipole or interface mode). We demonstrate that near-field coupling can be expressed in a simple form verifying the optical theorem for each particle modes.

Analysis of the angular acceptance of surface plasmon Bragg mirrors

We analyze an important aspect of the behavior of surface plasmon polariton (SPP) Bragg mirrors: the dependence of the angular acceptance for reflection on the incidence angle. By means of leakage radiation microscopy, both in direct and Fourier space, we observe that the angular acceptance diminishes for increasing incidence angles. This effect, which can considerably affect the design of devices based on these elements, is shown to be the consequence of the decrease of the bandgap width with increasing incidence angle.

Sub-wavelength patterning of the optical near-field

We report the sub-wavelength patterning of the optical near-field by total internal reflection illumination of a regular array of resonant gold nano-particles. Under appropriate conditions, the in-plane coupling between Localized Surface Plasmon (LSP) fields gives rise to sub-wavelength light spots between the structures. Measurements performed with an Apertureless Scanning Near-Field Optical Microscope (ASNOM) show a good agreement with theoretical predictions based on the Green dyadic method. This concept might offer a convenient way to elaborate extended optical trap landscapes for manipulation of sub-micrometer systems.

Photonic nanopatterns of gold nanostructures indicate the excitation of surface plasmon modes of a wavelength of 50-100 nm by scanning near-field optical microscopy

Scanning near-field optical microscopy images of metal nanostructures taken with the tetrahedral tip (T-tip) show a distribution of dark and bright spots at distances in the order of 25-50 nm. The images are interpreted as photonic nanopatterns defined as calculated scanning near-field optical microscopy images using a dipole serving as a light-emitting scanning near-field optical microscopy probe. Changing from a positive to a negative value of the dielectric function of a sample leads to the partition of one spot into several spots in the photonic nanopatterns, indicating the excitation of surface plasmons of a wavelength in the order of 50-100 nm in metal nanostructures.

Imaging the local density of states of optical corrals

This paper reports the experimental observation, at optical frequencies, of the electromagnetic local density of states established by nanostructures corresponding to the recently introduced concept of optical corral [G. Colas des Francs et al., Phys. Rev. Lett. 86, 4950 (2001)]. The images obtained by a scanning near-field optical microscope under specific operational conditions are found in agreement with the theoretical maps of the optical local density of states. A clear functionality of detection by the scanning near-field optical microscope is thereby identified since the theoretical maps are computed without including any specific tip model.

Addressing and imaging high optical index dielectric ridges in the optical near field

Experimental observation of light coupling between TiO(2) integrated waveguides of subwavelength cross section and pure three-dimensional evanescent light fields is reported. This near-field optical phenomenon is produced by controlling the location of the focusing of a laser beam totally reflected at the surface of the sample. The phenomenon is observed in direct space with a photon scanning tunneling microscope. Dielectric ridges several tens of micrometers long have been efficiently excited with this technique. Upon excitation, the extremities of the linear dielectric wires display intense light spots localized both inside and around the ridge. For ridge lengths up to 30 microm, the observed phenomenon has been reproduced numerically with a parallel implementation based on the three-dimensional Green dyadic method.

Optical analogy to electronic quantum corrals

We describe full multiple-scattering calculations of localized surface photonic states set up by lithographically designed nanostructures made of a finite number of dielectric pads deposited on a planar surface. The method is based on a numerical solution of the dyadic Dyson's equation. When the pads are arranged to form a closed circle, we find field patterns that look like the electronic charge density recently observed above quantum corrals. We propose two experimental techniques that could be used to observe these electromagnetic modes in direct space.

Direct interpretation of near-field optical images

The interpretation of the detection process in near-field optical microscopy is reviewed on the basis of a discussion about the possibility of establishing direct comparisons between experimental images and the solutions of Maxwell equations or the electromagnetic local density of states. On the basis of simple physical arguments, it is expected that the solutions of Maxwell equations should agree with images obtained by collecting mode near-field microscopes, while the electromagnetic local density of states should be considered to provide a practical interpretation of illumination mode near-field microscopes. We review collecting mode near-field microscope images where the conditions to obtain good agreement with the solutions of Maxwell equations have indeed been identified. In this context of collecting mode near-field microscopes, a fundamentally different functionality between dielectric and gold-coated tips has been clearly identified experimentally by checking against the solutions of Maxwell equations. It turns out that dielectric tips detect a signal proportional to the optical electric field intensity, whereas gold-coated tips detect a signal proportional to the optical magnetic field intensity. The possible implications of this surprising phenomenon are discussed.

Near-field optical addressing of single molecules in coplanar geometry: a theoretical study

Photonic transfer through elongated optical structures of submicrometre section microfabricated at the surface of dielectric or semiconductor samples can be enhanced by an appropriate structuring of the local refraction index. We show from computerized simulations that both the light localization and the spectroscopic properties of such structures can be used to selectively excite, in coplanar geometry, individuals molecules located in the near-field.

Light field propagation by metal micro- and nanostructures

The ability to sustain plasmon oscillations gives rise to unique properties of metal nanostructures, which can be exploited for the controlled manipulation of light fields on the nanoscale. In this context we investigate electromagnetic coupling effects within lithographically produced ensembles of gold nanoparticles with a photon scanning tunnelling microscope. To provide an interface between these nano-optical devices and classical far-field optics, we investigate surface plasmon propagation on microstructured metal thin films.

Optical addressing at the subwavelength scale

The Green dyadic formalism is applied to the study of the optical properties of dielectric subwavelength structures integrated in coplanar geometry. We first consider homogeneous wires with high refractive index featuring subwavelength cross sections. We show that such wires may have guiding properties and that they may be coupled with a local illumination produced by a focused Gaussian beam totally reflected at the substrate interface. When excited by the focused beam, these subwavelength optical waveguides (SOW's) provide a confined source of light that could be used to excite a single nanoscopic object. Well designed heteregeneous wires resulting from the alignment of dielectric particles separated from each other by a subwavelength distance are also found to propagate a Gaussian beam excitation over several micrometers. This propagation occurs with reasonable damping for incident beams in the visible frequency range. The computed transmission spectra of these heterowires may exhibit narrow gaps. Finally, we discuss the relation between the optical properties of the SOW and the calculated electromagnetic local density of states.