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We pursue a nanophotonic platform for strong light-matter interaction that combines plasmonic mode volumes, i.e., deep subwavelength confinement, with cavity quality factors (Q = 1000 to 100000). To this end we study the physics of resonator structures in which plasmon antennas are placed inside microcavities, like microdisks and photonic crystal cavities. Coupled oscillator theory for the local density of optical states in such systems shows a rich family of Fano-type line shapes, meaning that interferences lead to both transparency windows (very low LDOS, even when both antenna and cavity are separately on resonance) and to Purcell factors that far exceed those of antenna and cavity alone. These results are further confirmed by full-wave modelling.
We will report experiments that probe the system from several viewpoints. First, we show that it is not true, even for high-Q cavities, that plasmon scatterers necessarily reduce Q, as evident from probing the cavity response in an experiment where we approach a cluster of plasmon nanorods to a microtoroid with a Q of 10^6. Second, we show that the polarizability of an antenna is strongly dependent on whether it is coupled to a microcavity, as evident from antenna extinction in the same experimental system. Thirdly, we show that in Si3N4 microdisk-antenna structures made by lithography that we decorate with single nanoantennas as well as phased arrays, dominantly plasmonic modes can be obtained even at Q’s well above 10.000. The richness of the physics that is evident from the experiments clearly goes well beyond simple perturbation models. The underlying mechanism is that both the cavity and the antenna are essentially open systems that have radiation as their main loss mechanism. Interaction and interference through these radiative channels leads to unexpected performance characteristics for light-matter interaction that in terms of coupled mode theory map on non-hermitian coupled oscillator properties. We believe that this can be captured by casting the problem in language of Quasi-Normal Modes. Our current efforts are devoted to matching these systems to near-infrared quantum emitters such as dibenzo-terrylene in anthracene for low-temperature quantum optics studies.
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