Stand-alone optical spinning tweezers with tunable rotation frequency.

Advances in optical trapping design principles have led to tremendous progress in manipulating nanoparticles (NPs) with diverse functionalities in different environments using bulky systems. However, efficient control and manipulation of NPs in harsh environments require a careful design of contactless optical tweezers. Here, we propose a simple design of a fibered optical probe allowing the trapping of dielectric NP as well as a transfer of the angular momentum of light to the NP inducing its mechanical rotation. A polarization conversion from linearly-polarized incident guided to circularly transmitted beam is provoked geometrically by breaking the cylindrical symmetry of a coaxial nano-aperture that is engraved at the apex of a tapered metal coated optical fiber. Numerical simulations show that this simple geometry tip allows powerful light transmission together with efficient polarization conversion. This guarantees very stable trapping of quasi spherical NPs in a non-contact regime as well as potentially very tunable and reversible rotation frequencies in both directions (up to 45 Hz in water and 5.3 MHz in air for 10 mW injected power in the fiber). This type of fiber probe opens the way to a new generation of miniaturized tools for total manipulation (trapping, sorting, spinning) of NPs.

Transmission properties of slanted annular aperture arrays. Giant energy deviation over sub-wavelength distance.

This paper is devoted to the study of the transmission properties of Slanted Annular Aperture Arrays made in perfectly conducting metal. More precisely, we consider the transmission based on the excitation of the cutoff-less guided mode, namely the TEM mode. We numerically and analytically demonstrate some intrinsic properties of the structure showing a transmission coefficient of at least 50% of an unpolarized incident beam independently of the illumination configuration (angle and plane of incidence). The central symmetry exhibited by the structure is analytically exploited to demonstrate the existence of a polarization state for which all the incident energy is transmitted through the sub-wavelength apertures when the eigenmode is excited, whatever are the illumination and the geometrical parameters. For this state of polarization, the laminar flow of the energy through the structure can exhibit giant deviation over very small distances. An example of energy flow deviation of 220° per wavelength is presented for illustration. The results presented in this paper could be considered as an important contribution to the understanding of the enhanced transmission phenomenon based on the excitation of guided modes.

Optical horn antennas for efficiently transferring photons from a quantum emitter to a single-mode optical fiber.

We theoretically demonstrate highly efficient optical coupling between a single quantum emitter and a monomode optical fiber over remarkably broad spectral ranges by extending the concept of horn antenna to optics. The optical horn antenna directs the radiation from the emitter toward the optical fiber and efficiently phase-matches the photon emission with the fiber mode. Numerical results show that an optical horn antenna can funnel up to 85% of the radiation from a dipolar source within an emission cone semi-angle as small as 7 degrees (antenna directivity of 300). It is also shown that 50% of the emitted power from the dipolar source can be collected and coupled to an SMF-28 fiber mode over spectral ranges larger than 1000 nm, with a maximum energy transfer reaching 70 %. This approach may open new perspectives in quantum optics and sensing.

6-micron interaction length electro-optic modulation based on lithium niobate photonic crystal cavity.

We report on electro-optic modulation using a Lithium Niobate (LN) Photonic Crystal (PC) cavity structure. The compact device (6 µm in length) consists of a 2D photonic crystal cavity made on an Annealed Proton Exchange (APE) LN waveguide with vertical deposited electrodes. Experimental results show a tunability of 0.6 nm/V. This compact design opens a way towards micro and nano-scale tunable photonic devices with low driving electrical power.

Enhanced electro-optical lithium niobate photonic crystal wire waveguide on a smart-cut thin film.

We report an electro-optically tunable photonic crystal linear cavity etched on a 200 nm lithium niobate waveguide ridge. The photonic crystal cavity and the ridge are both fabricated on a 1 µm thin film of lithium niobate obtained by smart-cut technology. The photonic crystal, of area 4x0.8 µm2, has been engineered to work in a slow light configuration so that the electro-optic effect is 20 times more important than in bulk material.

Diabolo nanoantenna for enhancing and confining the magnetic optical field.

In this Letter, we introduce a new nanoantenna concept aimed at generating a single magnetic hot spot in the optical frequency range, thus confining and enhancing the magnetic optical field on the background of a much lower electric field. This nanoantenna, designed by applying Babinet's principle to the bowtie nanoaperture, takes the shape of a diabolo. It differs from the well-known bowtie nanoantenna in that the opposing pair of metal triangles are electrically connected through their facing tips. Thus instead of a large charge density accumulating at the air gap of the bowtie nanoantenna, leading to a large electric field, a high optical current density develops within the central "metal gap" of the diabolo nanoantenna, leading to a large magnetic field. Numerical simulation results on the first nanodiabolo geometries show a 2900-fold enhancement of the magnetic field at a wavelength of 2540 nm, confined to a 40-by-40 nm region near the center of the nanoantenna.

Enhanced optical transmission by light coaxing: mechanism of the TEM-mode excitation.

This paper presents a theoretical study showing the mechanism of light transmission through opaque metallic films perforated with nanocoaxial apertures thanks to the excitation of their cutoff-free TEM (Transverse ElectroMagnetic) guided mode. Full three-dimensional Finite Difference Time Domain (3D-FDTD) together with a Body-Of-Revolution FDTD simulation results are presented and discussed in order to optimize this extraordinary transmission. Very promising findings are pointed out opening the path to the design of new devices for both nano-optic and photovoltaic applications.

Bowtie-shaped nanoaperture: a modal study.

Using the N-order finite-difference time-domain (FDTD) method, we show that optical resonances of the bowtie nanoaperture (BNA) are due to the combination of a guided mode inside the aperture and Fabry-Perot modes along the metal thickness. The resonance of lower energy, which leads to the well-known light confinement in the gap zone, occurs at the cutoff wavelength of the fundamental guided mode. No plasmon resonance is directly involved in the generation of the light hot spot. We also define a straightforward relationship between the resonance wavelengths of the BNA and its geometrical parameters. This brings a simple tool for the optimization of the BNA design.

Implementation of dispersion models in the split-field-finite-difference-time-domain algorithm for the study of metallic periodic structures at oblique incidence.

We extend here the finite-difference-time-domain (FDTD) algorithm working in oblique incidence to dispersive media. The split-field method (SFM) is used and adapted for taking into account the metal dispersion. The additional equations to the FDTD algorithm are given. Instead of the 24 field components usually used in the SFM, 38 and 112 field components are needed to implement the cases of Drude and Drude-Lorentz dispersion models, respectively. Some tests are presented to validate our code as long as the angle of incidence is lower than 76 degrees in addition to an application dealing with enhanced transmission and showing original results.

Second harmonic generation enhancement by use of annular aperture arrays embedded into silver and filled by lithium niobate.

We propose and theoretically study a metallo-dielectric photonic crystal (MDPhC) based on metallic annular aperture arrays (AAA) associated to a nonlinear material (LiNbO(3)) for the second harmonic generation (SHG). An optimal structure design can be found thanks to the relations that link the geometrical parameters to the operating point namely the wavelength of the fundamental and SHG signals. A slow light phenomenon, which occurs at the cut-off frequency of the guided mode through the annular cavities, is at the origin of the SHG signal enhancement. The benefit of the AAA is demonstrated through a comparison with cylindrical aperture arrays.

Split-field FDTD method for oblique incidence study of periodic dispersive metallic structures.

The study of periodic structures illuminated by a normally incident plane wave is a simple task that can be numerically simulated by the finite-difference time-domain (FDTD) method. On the contrary, for off-normal incidence, a widely modified algorithm must be developed in order to bypass the frequency dependence appearing in the periodic boundary conditions. After recently implementing this FDTD algorithm for pure dielectric materials, we here extend it to the study of metallic structures where dispersion can be described by analytical models. The accuracy of our code is demonstrated through comparisons with already-published results in the case of 1D and 3D structures.

Three-dimensional finite-difference time-domain algorithm for oblique incidence with adaptation of perfectly matched layers and nonuniform meshing: application to the study of a radar dome.

The three-dimensional finite-difference time-domain (3D-FDTD) method is developed and implemented in the case of oblique incidence in order to study biperiodic structures that are finished according to the third direction. The perfectly matched layer (PML) is adapted to the developed algorithm. The electromagnetic fields of Maxwell's equations in the main grid and in the PML media are transferred from the E-H domain to the mapped P-Q domain. The modified Maxwell's equations are implemented by the split-field method (SFM). Several tests are made and presented in order to verify and demonstrate the accuracy of our codes. The obtained results are in good agreement with published ones obtained by other methods. The originality of this paper comes, first from the fact that it brings a complete development of the used algorithm, and second, from the study of the spectral response of a radar dome based on annular aperture arrays perforated into a perfect conductor plate.

Near-field optical images of subwavelength annular aperture arrays exhibiting an extraordinary transmission.

In this paper, we present near-field optical images of nanostructures exhibiting an extraordinary transmission. These structures consist of annular aperture arrays engraved in a metallic film: they are quite promising structures for nanophotonics because of their high transmission directly linked to a guided mode mediated by each annular aperture. We first briefly explain our fabrication process (focused ion beam milling), then we expose the experimental setup of the near-field optical microscope working both in reflection and transmission modes. For the reflection mode, the 'coffee-bean' structure of the electromagnetic field predicted by the theory has been quite well reproduced. For the transmission mode, we present preliminary experimental results concerning the influence of the wavelength and the polarization of the incident beam on the obtained near-field images.

Near- and far-field verification of electro-optic effect enhancement on a tunable lithium niobate photonic crystal.

In this paper, we present the optical response of a tunable lithium niobate photonic crystal (PC) using the electro-optic effect of the material. The band gap tunability is 300 times higher than what one could expect for a bulk lithium niobate device of the same characteristics. Theoretical calculations based on the finite-difference time domain technique have allowed us to determine the physical origin of this enhanced electro-optic coefficient. Indeed, the effective second-order susceptibility in the LN nanostructure increases, giving rise to an ultra-compact low-voltage photonic crystal modulator when it operates at its band edge. In addition, the theoretical and far-field transmission results are confirmed by near-field optical microscopy images of the structure at different excitation voltages.

Annular aperture arrays: study in the visible region of the electromagnetic spectrum.

Baida and Van Labeke recently proposed a structure that exhibits a supertransmission of light through an array of nanometric coaxial apertures in a metallic film that has been named an annular aperture array (AAA) [Opt. Commun. 209, 17 (2002); Phys. Rev. B 67, 155314 (2003); J. Microsc. 213, 140 (2003)]. We present the first experimental study, to our knowledge, of an AAA structure in the visible region. For technological reasons, the structure under study does not produce a supertransmission of 80% as in Baida and Van Labeke [Opt. Commun. 209, 17 (2002)]. We built the nanostructure and experimentally recorded its far-field spectral response. This transmission shows only one broad band with a maximum around lambda = 700 nm, giving a maximum efficiency around 17%. A finite-difference time-domain simulation reproduces quite well the obtained transmission spectrum.

Waveguiding through a two-dimensional metallic photonic crystal.

We present a two-dimensional (2D) finite-difference time domain simulation of the propagation of light through linear and bent channels in metallic photonic crystals. We took as a starting point the Bozhevolnyi experiment, consisting of the scattering of surface plasmons by a 2D structure of finitely sized periodic gold dots arranged in a triangular lattice of 400-nm period. We model injection and propagation of light through linear channels of different widths. We also study the behaviour of light in the presence of a 90 degrees bent line defect made in the structure. We show that the confinement depends on the orientation of the input and output line defects. The two cases of GammaM and GammaK orientations are considered and a spectral study for five different wavelengths is carried out.

A new structure for enhanced transmission through a two-dimensional metallic grating.

The spectral response of metallic two-dimensional periodic structures in which circular apertures are engraved has been extensively studied. We show that in such devices transmission can be highly enhanced by partially shutting the central parts of each aperture in order to make a periodic array of subwavelength coaxial structures.

Propagation and diffraction of locally excited surface plasmons.

With the use of optical near-field techniques, it is now possible to excite or observe surface plasmons with high lateral resolution. A theoretical study is presented of surface plasmon excitation by near-field optical probes and the influence of well-defined structures on surface plasmon propagation and surface plasmon detection in the far field. The generation and the diffraction of the surface plasmon is calculated by using a theoretical scheme founded upon a first-order perturbation expansion of the Rayleigh-Fano method. A very good agreement is obtained between numerical and experimental results. The theoretical tools used should prove a useful guideline for future experiments of nanooptics with surface plasmons.

Theoretical study of transient phenomena in near-field optics.

A time-domain study of the propagation of a light pulse is made by the finite-difference time-domain method. This method is described briefly and then two applications are presented: creation and diffraction of surface plasmons in the time-domain, and propagation of a light pulse through two tip models, a dielectric one and a metal-coated one. Results on propagation speed of surface plasmons, spatio-temporal shape and spectral study of the field emitted by a tip are presented.

Near-field effects of focused illumination on periodic structures in scanning tunneling optical microscopy.

The influence of a focused polarized Gaussian beam on image formation was studied. We show that the position of the tip with reference to the center of the beam involves asymmetry in the intensity map. A comparison between s and p polarization can be made, owing to the definition of both a three-dimensional polarized Gaussian beam and a three-dimensional object. This result implies that the best way to scan a sample consists in moving it and not the tip; moreover, focusing the incident light to get a higher signal-to-noise ratio must be done carefully with respect to the sample period.