Double resonant plasmonic antenna

The study of the nonlinear optical response of matter is important for various disciplines such as biology, information sciences and physics, especially when dealing with nanometer length scales. It is therefore important to have efficient sources of nonlinear optical signals at such length scales. Second harmonic generation (SHG) is one such important nonlinear optical effect, which has the added advantage of being sensitive to symmetry. Under the dipole approximation, SHG is forbidden for centrosymmetric media; however since inversion symmetry is broken at an interface, one can use nonlinear SHG microscopy to study even centrosymmetric structures on surfaces. It is well-known that plasmonic dipole antennae (DA) exhibit so-called hot spots in their gap, where the incident intensity can be enhanced by several orders of magnitude. In recent years, experimental work and simulations dealing with the SHG in dipole antennae have been reported. As the intensity of the second-harmonic signal is proportional to the square of the fundamental field intensity, this second-order nonlinear process can strongly benefit from the high enhancement of the incident field in these structures. Yet, all the previous studies in plasmonics rely on the enhancement of the fundamental field only and do not consider any resonance at the doubled frequency. This approach based on two resonances has been successfully demonstrated in traditional optics, for example in resonant Fabry-Perot microcavities.

In this work, we propose and experimentally demonstrate a novel plasmonic antenna geometry with two gaps that are coupled to each other, both of which exhibit a resonance at the fundamental and doubled frequency, and thus enhance simultaneously both the generation of the second-harmonic and its re-emission into the far-field. The present novel plasmonic antenna geometry – the double resonant antenna (DRA) – is optimized for second-harmonic generation (SHG). This antenna is based on two gaps coupled to each other so that a resonance at the fundamental and at the doubled frequency is obtained, as illustrated in this figure. Furthermore, the proximity of the localized hot spots allows for a coupling and spatial overlap between the two field enhancements at both frequencies. Using such a structure, both the generation of the second-harmonic and its re-emission into the far-field are significantly increased when compared with a standard plasmonic dipole antenna. Such DRA are fabricated in aluminium using electron beam lithography and their linear and nonlinear responses are studied experimentally and theoretically.

Check the corresponding publication: PDF, External link: doi: 10.1364/OE.20.012860