L. Lalouat

On chip shapeable optical tweezers

Particles manipulation with optical forces is known as optical tweezing. While tweezing in free space with laser beams was established in the 1980s, integrating the optical tweezers on a chip is a challenging task. Recent experiments with plasmonic nanoantennas, microring resonators, and photonic crystal nanocavities have demonstrated optical trapping. However, the optical field of a tweezer made of a single microscopic resonator cannot be shaped. So far, this prevents from optically driven micromanipulations. Here we propose an alternative approach where the shape of the optical trap can be tuned by the wavelength in coupled nanobeam cavities. Using these shapeable tweezers, we present micromanipulation of polystyrene microspheres trapped on a silicon chip. These results show that coupled nanobeam cavities are versatile building blocks for optical near-field engineering. They open the way to much complex integrated tweezers using networks of coupled nanobeam cavities for particles or bio-objects manipulation at a larger scale.

Subwavelength imaging of light confinement in high-Q/small-V photonic crystal nanocavity

The optical near field of a high-Q and ultrasmall volume photonic crystal nanocavity is visualized with a subwavelength resolution by using a scanning near-field optical microscope (SNOM) operating at the same time in collection-scanning mode and in interaction-scanning mode. It is shown that the nanocavity resonant mode is selectively visualized by using the SNOM interaction-scanning mode while the whole electromagnetic field surrounding the nanocavity is probed using the SNOM collection-scanning mode. The different optical near-field images are compared in light of a three-dimensional numerical analysis and we demonstrate an unexpected mode coupling at the cavity resonance.

Lower bound for the spatial extent of localized modes in photonic-crystal waveguides with small random imperfections

Light localization due to random imperfections in periodic media is paramount in photonics research. The group index is known to be a key parameter for localization near photonic band edges, since small group velocities reinforce light interaction with imperfections. Here, we show that the size of the smallest localized mode that is formed at the band edge of a one-dimensional periodic medium is driven instead by the effective photon mass, i.e. the flatness of the dispersion curve. Our theoretical prediction is supported by numerical simulations, which reveal that photonic-crystal waveguides can exhibit surprisingly small localized modes, much smaller than those observed in Bragg stacks thanks to their larger effective photon mass. This possibility is demonstrated experimentally with a photonic-crystal waveguide fabricated without any intentional disorder, for which near-field measurements allow us to distinctly observe a wavelength-scale localized mode despite the smallness (~1/1000 of a wavelength) of the fabrication imperfections.

Measurement of a flat lens focusing in a 2D photonic crystal at optical wavelength

Using scanning near field optical microscopy (SNOM), we observed the focusing effect provided by negative refraction in a 2D photonic crystal. Experimental observations are analyzed in light of FDTD 3D simulations.

Localization of light at vanishingly small disorder-levels with heavy photons

We show that the key parameter driving the spatial extent of localized modes formed in randomly-perturbed periodic media near the band edge is the effective photon mass rather than the group index.

Optical near field interactions

The fundamentals of the near-field interaction between a subwavelength tip and a photonic-crystal nanocavity are investigated experimentally and theoretically. It is shown experimentally that the cavity resonance is tuned without any degradation by the presence of the tip and that the reported near-field interaction is strongly related to the field distribution within the nanostructure. From the interaction between the probe and the cavity, we will show a new kind of microscopy.

Photonic Crystal Based Subwavelength Imaging and Cloaking Optical Devices

Potentialities of 2.5D dielectric photonic crystals for subwavelength imaging or in- visibility cloaking operation at optical wavelengths are studied numerically and experimentally. First, a negative refraction based ∞at lens is designed, fabricated and characterized. Using scan- ning near-fleld optical microscopy, subwavelength imaging (0.8‚) for a deduced refractive index of i1 is obtained at 1.55m. Second, an original design aimed to operate as a cloaking device is proposed by combining photonic lattices operating under difierent regimes, exploiting both pass- and stop-band properties.

Imaging and manipulating confined electromagnetic fields in photonic crystal nanocavities with SNOM probes

By using the optical near-field microscopy technique coupled to microphotoluminescence or transmittance experiments, we investigate the optical near-field properties of photonic crystal nanocavities and evidence the near-field probe ability to manipulate their resonances.