E. Devaux

Channel plasmon subwavelength waveguide components including interferometers and ring resonators

Photonic components are superior to electronic ones in terms of operational bandwidth, but the diffraction limit of light poses a significant challenge to the miniaturization and high-density integration of optical circuits. The main approach to circumvent this problem is to exploit the hybrid nature of surface plasmon polaritons (SPPs), which are light waves coupled to free electron oscillations in a metal that can be laterally confined below the diffraction limit using subwavelength metal structures. However, the simultaneous realization of strong confinement and a propagation loss sufficiently low for practical applications has long been out of reach. Channel SPP modes—channel plasmon polaritons (CPPs)—are electromagnetic waves that are bound to and propagate along the bottom of V-shaped grooves milled in a metal film. They are expected to exhibit useful subwavelength confinement, relatively low propagation loss, single-mode operation and efficient transmission around sharp bends. Our previous experiments showed that CPPs do exist and that they propagate over tens of micrometres along straight subwavelength grooves. Here we report the design, fabrication and characterization of CPP-based subwavelength waveguide components operating at telecom wavelengths: Y-splitters, Mach–Zehnder interferometers and waveguide–ring resonators. We demonstrate that CPP guides can indeed be used for large-angle bending and splitting of radiation, thereby enabling the realization of ultracompact plasmonic components and paving the way for a new class of integrated optical circuits.

Efficient unidirectional nanoslit couplers for surface plasmons

The emerging field of plasmonics is based on exploiting the coupling between light and collective electronic excitations within conducting materials known as surface plasmons. Because the so-called surface plasmon polariton (SPP) modes that arise from this coupling are not constrained by the optical diffraction limit, it is hoped that they could enable the construction of ultracompact optical components1,2. But in order that such potential can be realized, it is vital that the relatively poor light–SPP coupling be improved. This is made worse by the fact that the incident light that is conventionally used to launch SPPs in a metal film 3,4,5,6 is a significant source of noise, unless directed away from a region of interest, which then decreases the signal and increases the system’s size. Back-side illumination of subwavelength apertures in optically thick metal films7,8,9,10,11,12,13 eliminates this problem but does not ensure a unique propagation direction for the SPP. We propose a novel back-side slit-illumination method that incorporates a periodic array of grooves carved into the front side of a thick metal film. Bragg reflection enhances the propagation of SPPs away from the array, enabling them to be unidirectionally launched from, and focused to, a localized point.

Bright Unidirectional Fluorescence Emission of Molecules in a Nanoaperture with Plasmonic Corrugations

Controlling the fluorescence emission from nanoscale quantum emitters is a key element for a wide range of applications, from efficient analytical sensing to quantum information processing. Enhancing the fluorescence intensity and narrowing the emission directivity are both essential features to achieve a full control of fluorescence, yet this is rarely obtained simultaneously with optical nanoantennas. Here we report that gold nanoapertures surrounded by periodic corrugations transform standard fluorescent molecules into bright unidirectional sources. We obtain enhancement factors of the fluorescence count rate per molecule up to 120 fold simultaneously with a directional emission of the fluorescence into a narrow angular cone in the direction normal to the sample plane. The bright emission and narrow directionality enable the detection of single molecules with a low numerical aperture objective, and improve the effectiveness of fluorescence-based applications. We thoroughly quantify the increased light-matter coupling as well as the radiation pattern intensity. These results are highly relevant for the development of single molecule sensing, single-photon sources, and light emitting devices.

Plasmonic Antennas for Directional Sorting of Fluorescence Emission

Spontaneous emission of fluorescent molecules or quantum dots is radiated along all directions when emitters are diluted in a liquid solution, which severely limits the amount of collected light. Besides, the emission direction does not carry any useful information and cannot be used to sort different molecules. To go beyond these limits, optical antennas have been recently introduced as conceptual tools to control the radiation properties for nanoemitters fixed on a substrate. Despite intense recent research, controlling the luminescence directivity remains a challenge for emitters with random positions and orientations, which is a key for several biomolecular screening applications. Here, we present full directional control of the fluorescence emission from molecules in water solution by an optical antenna made of a nanoaperture surrounded by a periodic set of shallow grooves in a gold film. For each emission wavelength, the fluorescence beam can be directed along a specific direction with a given angular width, hereby realizing a micrometer-size dispersive antenna. We demonstrate the fluorescence beaming results from an interference phenomenon and provide physical optics guidelines to control the fluorescence directivity by tuning the groove-nanoaperture distance. This photon-sorting capability provides a new approach for high-sensitivity screening of molecular species in solution.

Nanofocusing with channel plasmon polaritons.

We investigate radiation nanofocusing with channel plasmon polaritons (CPPs) propagating along subwavelength metal grooves that are tapered synchronously in depth and in width. Efficient CPP nanofocusing at telecom wavelengths with the estimated field intensity enhancement of up to approximately 90 is directly demonstrated using near-field microscopy. Experimental observations are concurred with electromagnetic simulations, predicting the possibility of reaching the intensity enhancements of approximately 1200 and opening thereby exciting perspectives for practical applications of CPP nanofocusing.

Conductivity in organic semiconductors hybridized with the vacuum field.

Much effort over the past decades has been focused on improving carrier mobility in organic thin-film transistors by optimizing the organization of the material or the device architecture. Here we take a different path to solving this problem, by injecting carriers into states that are hybridized to the vacuum electromagnetic field. To test this idea, organic semiconductors were strongly coupled to plasmonic modes to form coherent states that can extend over as many as 10(5) molecules and should thereby favour conductivity. Experiments show that indeed the current does increase by an order of magnitude at resonance in the coupled state, reflecting mostly a change in field-effect mobility. A theoretical quantum model confirms the delocalization of the wavefunctions of the hybridized states and its effect on the conductivity. Our findings illustrate the potential of engineering the vacuum electromagnetic environment to modify and to improve properties of materials.

Resonant plasmon nanofocusing by closed tapered gaps.

We study radiation nanofocusing by closed tapered gaps, i.e. metal V-grooves, under normal illumination, and discover that the local field inside a groove can be resonantly enhanced due to interference of counter-propagating gap plasmons. Considering V-grooves milled in gold, we analyze this phenomenon theoretically, deriving an analytic expression for the resonance condition and predicting more than 550-fold intensity enhancements at resonance, and observe it experimentally with two-photon photoluminescence microscopy, demonstrating more than 100-fold intensity enhancements.

Optimization of surface plasmons launching from subwavelength hole arrays: modelling and experiments

The launching of surface plasmons by micro-gratings of subwavelength apertures milled in a thick metal film is important for the development of surface plasmon based circuits. By comparing the near-field optical images of such surface plasmon sources with the results of a Huygens-Fresnel principle based scattering model, we show that the properties of the locally launched SP beams such as divergence or uniformity can be tuned by adjusting the shape of the micro-gratings. This allows us to propose an optimized source array well adapted for providing a narrow, collimated and uniform beam.

Nanoaperture-Enhanced Signal-to-Noise Ratio in Fluorescence Correlation Spectroscopy

The fluorescence enhancement found in gold nanoapertures is demonstrated to increase the signal-to-noise ratio (SNR) in fluorescence correlation spectroscopy (FCS). Starting from a general discussion on noise in FCS experiments, we show that fluorescence enhancement leads to a dramatic increase in the SNR. This prediction is confirmed by experiments where we report an experimental gain in SNR of about 1 order of magnitude, corresponding to a 100-fold reduction of the experiment duration. This technique is then applied to monitor the kinetics of a fast enzymatic cleavage reaction. This set of experiments evidence the feasibility of FCS analysis with fast integration times of about 1 s, opening the way to the monitoring of a variety of biochemical reactions at reduced time scales.

Sorting nanoparticles with intertwined plasmonic and thermo-hydrodynamical forces.

We exploit plasmonic and thermo-hydrodynamical forces to sort gold nanoparticles in a microfluidic environment. In the appropriate regime, the experimental data extracted from a Brownian statistical analysis of the kinetic motions are in good agreement with Mie-type theoretical evaluations of the optical forces acting on the nanoparticles in the plasmonic near field. This analysis enables us to demonstrate the importance of thermal and hydrodynamical effects in a sorting perspective.