Jan 20, 2009
Nanoparticle arrays extract light on-demand
Since the pioneering work of Purcell in 1946, it is well established that fluorescence properties of light emitters such as molecules and quantum dots may be strongly modified by the presence of a photonic structure in their proximity. In this context plasmonic nanoantennae - metallic nanostructures that convert localized energy into free propagating radiation - are receiving much attention for their potential ability to transform bad light emitters into highly efficient fluorescent sources.
Large enhancement of the radiative recombination rates can be achieved by coupling the emitter electromagnetic field to the radiative plasmon oscillations supported by the nanoantenna, thus greatly increasing the overall fluorescence quantum efficiency. Careful tailoring of the geometry and composition of the antenna allows opens the door to remarkable light extraction performances, nevertheless strong ohmic losses and broad plasmon resonances limit the flexibility and the possible application of these devices.
In a very recent study, published in Nanotechnology, the authors show by full-field electromagnetic calculations that such limitations may be overcome if regular arrays of noble metal nanoparticles are employed as plasmonic nanoantennae. Such extended structures support extremely narrow mixed Bragg-plasmonic modes, which may be exploited for light extraction applications.
The above modes depends linearly on lattice parameters of the system, and they may be further tuned simply by changing the dielectric environment surrounding the nanoantenna array. Likewise the radiation patterns are strongly directional and highly sensitive the structure geometric and dielectric parameters.
In their current paper, the authors show that even in the case of bad emitters (1% initial quantum efficiency), efficiencies above 50% are attainable in the visible and near infrared regime, including wavelengths that are relevant in lighting (λ~400 nm) and telecommunication (λ~1.5 μm) applications. Furthermore it is found that even slight variations in the matrix refractive index, down to 0.1%, are able trigger a major redistribution of the radiated power, making nanoantenna arrays extremely promising also for possible optical sensing applications.
A systematic exploration of other free parameters such as antenna materials, interparticle spacing and particle shapes could lead to structures with even better performances, highly desirable for efficient light emitting devices and sensors.
About the author
Dr. Giovanni Pellegrini is a postdoctoral researcher in the Condensed Matter Physics group at the University of Padova, Department of Physics. He is currently investigating the employment of nanocluster-based functional plasmonic materials for photonics applications. The group is led by Prof. Giovanni Mattei and Prof. Paolo Mazzoldi, who are together author or co-author of more than 400 scientific publications.