Phased arrays and plasmon lasers

Most of the sources of light that nature provides are electronic transitions in atoms and molecules. This means that elementary emitters of photons are by themselves deeply subwavelength in size, almost pointlike. As consequences quantum emitters that emit single photons are not very bright, and their emission is not very directional. There are several techniques that allow for quantum emitters emit more light per unit of time, and to make them do so in just one preferred spatial mode. These generally rely on narrowband dielectric microcavities, or on plasmon nano-antennas that provide strong field enhancement. In a recent review paper, we outlined the different merits and drawbacks of plasmon antennas for single photon sourcesĀ  as summarized by the figure below.

In our work, we mainly focus on phased array antennas, as opposed to high-local-field antennas. Phased arrays are arrays of radiating elements which are phase shifted with respect to each other. For optical phase array antennas the building blocks are large metal particles. These provide strong scattering as metal particles provide very large polarizability in a small volume, and thereby very large resonant scattering cross sections. When cleverly arranged, oligomers of such strongly scattering particles will make emitters radiate in a strongly directive fashion. A single driving emitter will excite plasmon resonances in all the antenna particles around it, which also can strongly drive each other. When done correctly, the induced currents in all the antenna particles will constitute a strongly directional source by interference that is controlled through geometry. The most famous example is the plasmon Yagi-Uda antenna for beaming along the antenna axis. More practical examples are hole array antennas, and periodic lattice antenna structures for directing light out of plane. In our group we have performed intense studies on all these types of antennas, asking ourselves how to integrate them with a controlled number of emitters (missing reference), how to measure radiation patterns how to trade off geometry, bandwidth, directionality and loss, and how to integrate phased arrays in cavities and waveguides.

Phased array antenas have application potential in several areas. In fluorescence microscopy, a substrate that makes single fluorophores emit more light in a low NA is highly desirable. We have worked on phased-array antenna substrates for bright and directional emission that enhance collection efficiencies by up to two orders of magnitude [63]. In solid-state lighting, phased-array antennas embedded in phosphor layers can improve the conversion of blue LED light to a bright and directed white. For this, the inherent losses in metal must be minimized, to maintain high internal quantum efficiency, while beaming light. Together with the group of Rivas at DIFFER, we study plasmon lattice structures for fluorescence (missing reference), and for lasing . We study 2D organic waveguides with gain, in which periodic plasmon lattices are embedded. These structures are on par with organic DFB lasers in terms of threshold, but remarkably robust against disorder, and remarkably flexible in terms of re-arranging the phased array into quasiperiodic, aperiodic and even randomized structures.