| An antenna is a device enhancing the coupling between a local electric oscillator and propagating electromagnetic waves. While antennas are widely used for the emission and reception of radio frequencies, their counterparts in the visible range have only recently emerged as their dimensions must be controlled at length scales much smaller than the wavelength. This question has raised great interest in the perspective of enhancing the absorption (solar cells, detectors…) and the emission of light (bio-imaging, single-photon emission…). Positioning the light emitter near the antenna is an especially delicate aspect of coupling. The group “Nanostructures and optics” at INSP has studied [1] the fluorescence of semiconductor nanocrystals positioned deterministically at the center of a « patch » nano-antenna consisting in a metallic disk, and showed an acceleration and redirection of light emission. |
« Patch » antennas consist in a plane surface (square, disk…) as the radiating element, separated from a conductor reflecting plane by a dielectric layer. They are used in mobile phones for the versatility of their industrial production and integration in networks. Greffet et al. [2] have shown in 2010 that, for an emitter coupled to a metallic disk, it is possible to obtain both a strong localization of the electromagnetic field at the position of the emitter, thus a good coupling between the emitter and the antenna, and a good directionnality of the emission. The quality factor of the antenna resonant modes is of the order of 10, so that a very broadband coupling can be obtained.
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a) schematic of the antenna geometry
b) schematic of the in-situ lithography principle, adapted at the LPN for the fabrication of the patch antennas.
The group has chosen an antenna geometry where the emitter is located between a gold plane and a gold disk (fig 1a), separated by a 30-nm dielectric spacing layer. The “in situ lithography”[3] developed by P. Senellart (Laboratoire de Photonique et de Nanostructures), consisting in locating the emitters before fabricating the gold disks (fig 1b), has provided a positioning of the emitters (very bright aggregates of CdSe/CdS nanocrystals) at the center of the antenna with a lateral precision of 25 nm.
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c | Figure 2 a) Microphotoluminescence image of a patch antenna coupled to nanocrystals. b) Decay curves (response to a pulsed excitation) of nanocrystals coupled to antennas of respective diameters 1.6 (1), 1.9 (2) and 2.1 (3) µm, and of nanocrystals in a homogeneous reference medium (silica). c) Angular distribution of the emission by a 1.6-µm-diameter antenna. |
The photoluminescence from the antenna (fig. 2a) is about 10 times brighter than the one from emitters without antenna (smaller dots on the image). The measured radiation diagrams show that the emission (fig. 2c) is concentrated in a 30° cone, which facilitates coupling to a microscope objective or an optical fiber. This diagram is in agreement with the model developed in J.-J. Greffet’s group. The decay curves of the emitters coupled to an antenna show a decay 5 to 14 times faster than for an emitter in a homogeneous medium (fig 2b). These measurements were performed for an ensemble of nanoemitters of random orientations and are in agreement with the decay enhancement of a factor 70 calculated for a dipole oriented along the antenna symmetry axis.
A stronger light emission enhancement can be obtained by replacing gold by silver or by working in the near infrared in order to limit the ohmic losses inside the antenna. The group is presently setting up the in situ lithography protocol at the INSP and adapting it to the coupling of a single nanocrystal. The goal is then to study to which extent the quantum optical emission properties of the nanocrystal are modified by the antenna.
Reference
“Controlling Spontaneous Emission with Plasmonic Optical Patch Antennas”
C. Belacel, B. Habert, F. Bigourdan, F. Marquier, J.-P. Hugonin, S.-M. de Vasconcellos, X. Lafosse, L. Coolen, C. Schwob, C. Javaux, B. Dubertret, J.-J. Greffet, P. Senellart, A. Maître
Nano Letters 13, 1516 (2013)
Contact
Agnès Maître
1 In collaboration with P. Senellart (Laboratoire Photonique et de Nanostructures), J.-J.Greffet, F. Marquier (Laboratoire Charles Fabry) et B. Dubertret (Laboratoire de Physique et d’Étude des Matériaux) dans le cadre du projet ANR Delight.
2 R. Esteban, T. V. Teperik et J.-J. Greffet, « Optical patch antennas for single photon emission using surface plasmon resonances », Phys. Rev. Lett. 104, 026802 (2010).
3 A. Dousse, L. Lanco, J. Suffczynski, E. Semenova, A. Miard, A. Lemaitre, I. Sagnes, C. Roblin, J. Bloch et P. Senellart, « Controlled Light-Matter Coupling for a Single Quantum Dot Embedded in a Pillar Micro cavity Using Far-Field Optical Lithography », Physical Review Letters 101, 267404 (2008).