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Subwavelength Optical Devices

Recent progress in near--field optics instrumentation led to a new class of subwavelength optical experiments in which it is intended to use either the optical tunnel effect (OTE) or the lower mode based transmission (LMBT) in order to control the optical transfer between several delocalized detection or injection centers.

In our group, we are testing different concepts from extensive numerical simulations performed on massively parallel computers. Our simulations are based on the Green functions formalism. All this computational work is developed in collaboration with several experimental groups.

In the context of nanosciences, a searched functionality concerns the optical addressing of individual nanostructures such as molecules quantum dots or any others microscopic systems. When working in coplanar geometry, no simple technique is available to achieve this goal on a routine basis. Indeed, standard optical focussing techniques or optical waveguides do not lead to illumination area which are commensurable with the above mentionned nanoscale objects (see for example ``Coplanar devices for the optical addressing of single molecules'', Nanotechnology 12 (2001) 75-79).


In optical technologies, transferring optical energy from a dielectric medium to another one is achieved by an optical waveguide connexion. Such devices guide optical waves with low losses over very large distances. Most waveguides developed up to now have the following characteristics:
(i) Similar topology: a core is surrounded by one or several layers. The guiding properties mostly rely on the core.

The surrounding layers ensure several functionalities from the optimization of the guiding properties to the mechanical and thermal protection of the device. The typical diameter of the core is of several micrometers. The smallest core diameter expected on the basis of the Rayleigh criterion is of the order of the incident wavelength. For transverse sizes smaller than the wavelength, the incoming electromagnetic wave decays exponentially inside the guide along the direction of propagation (longitudinal axis of the guide).
(ii) An efficient coupling of light into such an optical waveguide requires a perfect alignment of the wavevector of the incoming wave with the longitudinal axis of the guide.
(iii) Homogeneity of the index of refraction along the longitudinal direction. This homogeneity must be conserved over large distances to ensure an optimal guiding efficiency.

With the advent of Near-Field Optics, new solutions can be investigated by, for example, tailoring surface evanescent waves.

The need for reliable computational data to support the development of subwavelength optics is generating new numerical tools. Our real-space methodology well--suited to describe the non--trivial near--field optical phenomena close to resonant metallic particles supported by dielectric surfaces as well as the coupling between evanescent light source and surface dielectric patterns of arbitrary shape and optical index is also well-suited to describe subwavelength optical devices. In this page, we present some recent applications realized in coplanar geometry.

(i) One-dimensional photonic crystals addressed in coplanar geometry by surface evanescent waves.
In this case, matrices or invidual lines of dielectric pads are addressed by the evanescent waves that tails off an integrated optical waveguide. (Figure 1)

(ii) Subwavelength Optical Waveguides (SOW).
In this field area, solutions are proposed to achieve efficient optical addressings of integrated waveguides featuring: (a) transverse sizes ranging in the subwavelength domain; modes confined laterally within a width of about the half of the incident wavelength; (b) demonstration of detection in a subwavelength volume at the exit of the guide. Recently, the pertinence of this coupling mode was checked on various configurations by extensive numerical simulations based on the Green dyadic technique. When shining a Gaussian
beam in the total internal reflection setup one may expect to exploit the Goos--Hanschen shift which results in the fact that the incident and reflected beams are not symmetrical with respect to the focal point. Figure 2 represents the device described in ``Optical addressing at the subwevalength scale'', Phys. Rev. E62, 7381 (2000).

By scanning a pointed tip, it is possible to image the near-field distribution generated by the whole device. This experimental work has been developed with a Photon Scanning Tunneling Microscope (also called STOM for Scanning Tunneling Optical Microscope) in the team of Dijon (cf. figure 3 ). A part of this work is reported in the reference ``Addressing and imaging high optical index dielectric ridges in the optical near-field published in Phys. Rev E64 (2001) 066607-1.

This intriguing optical addressing mode can be numerically reproduced by using a peculiar numerical implementation of the Green Dyadic formalism (see for example ``Optical Addressing at the subwavelength scale'', Phys. Rev E62, 7381 (2000)). An example of simulation above a 30000 nm long SOW is given in figure 4.

Figure 1 : Transport and localization of light in two adjacent subwavelength optical waveguides.
(a) the two lines of pads have the same period.
(b) same calculation with two different periods.

Figure 2 : Schematic illustration of a SOW addressed by a focused evanescent light spot.




Figure 3 : PSTM image recorded above the SOW exit (courtesy R. Quidant and J.C. Weeber LPUB/CNRS Dijon).




Figure 4 : Simulation of the guiding process. These maps represent two PSTM images calculated at, respectively, 50nm and 350nm above the guide.