Y. De Wilde

Fluctuations of the local density of states probe localized surface plasmons on disordered metal films.

We measure the statistical distribution of the local density of optical states (LDOS) on disordered semicontinuous metal films. We show that LDOS fluctuations exhibit a maximum in a regime where fractal clusters dominate the film surface. These large fluctuations are a signature of surface-plasmon localization on the nanometer scale.

Second-harmonic generation from coupled plasmon modes in a single dimer of gold nanospheres

We show that a dimer made of two gold nanospheres exhibits a remarkable efficiency for second-harmonic generation under femtosecond optical excitation. The detectable nonlinear emission for the given particle size and excitation wavelength arises when the two nanoparticles are as close as possible to contact, as in situ controlled and measured using the tip of an atomic force microscope. The excitation wavelength dependence of the second-harmonic signal supports a coupled plasmon resonance origin with radiation from the dimer gap. This nanometer-size light source might be used for high-resolution near-field optical microscopy.

Local optical imaging of nanoholes using a single fluorescent rare-earth-doped glass particle as a probe

We have developed a local optical imaging technique that uses a fluorescent rare-earth-doped fluoride glass particle as a probe. This particle is glued at the end of an atomic force microscope tip and scanned over the surface of a nanostructured sample illuminated by a laser beam. The intensity of the laser-induced fluorescence of the particle is then recorded as a function of the position on the sample surface. This method has enabled us to image the light scattered by 250-nm large nanoholes made in a thin chromium film. The advantages of this material over other fluorescent probes is that it has a strong fluorescence when excited at 980 nm, it operates at room temperature, and does not present any evidence of photobleaching.

Thermo-optical dynamics in an optically pumped Photonic Crystal nano-cavity

Linear and non-linear thermo-optical dynamical regimes were investigated in a photonic crystal cavity. First, we have measured the thermal relaxation time in an InP-based nano-cavity with quantum dots in the presence of optical pumping. The experimental method presented here allows one to obtain the dynamics of temperature in a nanocavity based on reflectivity measurements of a cw probe beam coupled through an adiabatically tapered fiber. Characteristic times of 1.0+/-0.2 micros and 0.9+/-0.2 micros for the heating and the cooling processes were obtained. Finally, thermal dynamics were also investigated in a thermo-optical bistable regime. Switch-on/off times of 2 micros and 4 micros respectively were measured, which could be explained in terms of a simple non-linear dynamical representation.

A semiconductor laser device for the generation of surface-plasmons upon electrical injection

Surface plasmons are electromagnetic waves originating from electrons and light oscillations at metallic surfaces. Since freely propagating light cannot be coupled directly into surface-plasmon modes, a compact, semiconductor electrical device capable of generating SPs on the device top metallic surface would represent an advantage: not only SP manipulation would become easier, but Au-metalized surfaces can be easily functionalized for applications. Here, we report a demonstration of such a device. The direct proof of surface-plasmon generation is obtained with apertureless near-field scanning optical microscopy, which detects the presence of an intense, evanescent electric field above the device metallic surface upon electrical injection.

In Situ Generation of Surface Plasmon Polaritons Using a Near-Infrared Laser Diode

We demonstrate a semiconductor laser-based approach which enables plasmonic active devices in the telecom wavelength range. We show that optimized laser structures based on tensile-strained InGaAlAs quantum wells-coupled to integrated metallic patternings-enable surface plasmon generation in an electrically driven compact device. Experimental evidence of surface plasmon generation is obtained with the slit-doublet experiment in the near-field, using near-field scanning optical microscopy measurements.

Imaging subwavelength holes using an apertureless near-field scanning optical microscope

We present investigations of the light scattered by subwavelength holes in a chromium film using an apertureless near-field scanning optical microscope, which operates either in the visible (λ=655 nm) or in the infrared (λ=10.6 μm). The near-field optical images exhibit patterns around the holes that seem to coincide with the component of the stray electrical field parallel to the tip axis. A tip–sample dipole coupling model provides a satisfactory description of the experimental data recorded in the infrared with light polarized normally to the sample surface.

Injection of midinfrared surface plasmon polaritons with an integrated device

We demonstrate a compact, integrated device in which surface plasmon polaritons (SPPs) are injected into a passive metal waveguide. We directly excite a SPP mode at a metal-air interface using a room-temperature midinfrared quantum cascade laser which is integrated onto the microchip. The SPP generation relies on end-fire coupling and is demonstrated via both far-field and near-field imaging techniques in the midinfrared. On one hand, a metallic diffraction grating is used to scatter in the far-field a portion of the propagating SPPs, thus allowing their detection with a microbolometer camera. On the other hand, direct images of the generated SPPs in the near-field were collected with a scanning optical microscope.We demonstrate a compact, integrated device in which surface plasmon polaritons (SPPs) are injected into a passive metal waveguide. We directly excite a SPP mode at a metal-air interface using a room-temperature midinfrared quantum cascade laser which is integrated onto the microchip. The SPP generation relies on end-fire coupling and is demonstrated via both far-field and near-field imaging techniques in the midinfrared. On one hand, a metallic diffraction grating is used to scatter in the far-field a portion of the propagating SPPs, thus allowing their detection with a microbolometer camera. On the other hand, direct images of the generated SPPs in the near-field were collected with a scanning optical microscope.

Towards a full characterization of a plasmonic nanostructure with a fluorescent near-field probe

We report on the experimental and theoretical study of the spatial fluctuations of the local density of states (EM-LDOS) and of the fluorescence intensity in the near-field of a gold nanoantenna. EM-LDOS, fluorescence intensity and topography maps are acquired simultaneously by scanning a fluorescent nanosource grafted on the tip of an atomic force microscope at the surface of the sample. The results are in good quantitative agreement with numerical simulations. This work paves the way for a full near-field characterization of an optical nanoantenna.

A hybrid plasmonic semiconductor laser

We present a semiconductor-based approach to compensate plasmonic losses. The core idea is to employ an electrically pumped laser diode and to overlap its active region with the evanescent field of a surface plasmon wave. In order to keep the losses at a manageable level, we rely on hybrid waveguide modes stemming from the coupling of a dielectric and a plasmonic mode. The laser device we demonstrate operates—at telecom wavelengths—on such a hybrid plasmonic mode. The device operates by electrical injection at room temperature. The near-field imaging of the laser facet provides evidence of the stimulated emission into the hybrid mode and confirms the prediction of the numerical simulations.