In order to design high Purcell-factor systems, hybrid cavity-antenna systems have shown to be highly promising. We show in what way a system of a high-Q cavity combined with a low-V antenna can outperform its constituents, and give rules of thumb for their design for several applications. In particular, we will present experimental results of different hybrid systems including microdisk resonators and integrated photonic crystal nanobeams coupled to rod and dimer antennas respectively. By placing single quantum dots, we experimentally measure the high Purcell factors these systems promise.
We report on experiments where high-Q silicon nitride microdisks couple to arrays of plasmon antennas. Such hybrids promise `best of both world’ performances, i.e., subwavelength mode volume yet high Q, with exciting repercussions for spontaneous emission control, sensing, and plasmon-enhanced Raman scattering. In this framework it is particularly interesting to understand antenna-antenna interactions mediated through a resonant cavity. In a first experiment we examine cooperative dipole-dipole coupling of antenna dimers coupled through a whispering gallery mode, and demonstrate implications for high-Purcell factors with `chiral’, properties, i.e., unidirectional circulation. In a second experiment we studied far-field OAM generation by rings of antennas on cavities, demonstrating simultaneous pure OAM and pure polarization control through unit-cell design. Finally we report on the potential of such structures for molecular optomechanics.
Historically, strong light-matter interaction is achieved by using either high quality factor (Q) micro-resonators such as photonic crystal cavities which enable long photon lifetimes, or metallic nanoresonators which allow for strong field enhancements provided by localized plasmon resonances. However, it has been recently demonstrated that a hybrid system, which combines both a dielectric cavity and a dipolar plasmonic antenna, can achieve stronger emission enhancements than the cavity or antenna alone [ACS Photonics, 3 (10) (2016)].
We propose to use arrays of N plasmonic antennas to further engineer the directionality of this enhanced emission. We analyze the resonant mode structure and local density of states in high-Q hybrid plasmonic-photonic resonators composed of a dielectric disk, perturbed by dimers of plasmon antennas, systematically swept in position through the cavity mode. A simple cavity-perturbation-theory model shows how the degenerate clockwise and anticlockwise whispering gallery modes (WGMs) of the unperturbed cavity split into two new hybrid modes with different complex eigenfrequencies, showing an interesting evolution of the resonance frequencies and Q's as the antenna spacing is varied. We find that one may construct large LDOS enhancements exceeding those given by a single antenna, which are `chiral' in the sense of correlating with unidirectional injection into the cavity. We report an experiment probing the resonances of silicon nitride (Si3N4) microdisks decorated with Aluminium antenna dimers that confirms the predicted mode properties as function of antenna spacing.
Strong interaction between light and a single quantum emitter is pivotal to many applications, including single photon sources and quantum information processing. Typically, plasmonic antennas or optical cavities are used to boost this interaction. The former can focus light in a deeply subwavelength region, whereas the latter can store light for up to billions of oscillations.
In our work, we combine these two opposite elements into a single coupled system. First, we show theoretically  that hybrid cavity-antenna systems can achieve Purcell enhancements far exceeding those of the bare cavity and antenna, and can do so at any desired bandwidth. This requires a delicate balance between spoiling the cavity with the antenna on the one hand, and cooperative and interference effects on the other.
We then present our experimental results on hybrid systems using a whispering-gallery mode cavity and an aluminum plasmonic antenna. Using taper-coupled excitation of the hybrid mode, we study quality factors and radiation patterns, demonstrating that we can control the antenna-cavity coupling strength by varying their respective frequency detuning. We show that we can achieve modes that retain quality factors around 10^4, while creating a strongly localized field around the antenna. As such, we can exploit the benefits of plasmonic confinement without suffering from the usual losses. Finally, we present first studies of fluorescent emitters coupled to the hybrid modes.
 Doeleman, H. M., Verhagen, E., & Koenderink, A. F., "Antenna–Cavity Hybrids: Matching Polar Opposites for Purcell Enhancements at Any Linewidth." ACS Photonics 3.10 (2016): 1943-1951.
Bound states in the continuum (BICs) are modes that, although energy and momentum conservation allow coupling to far-field radiation, do not show any radiation loss. As such, energy can theoretically be stored in the mode for infinite time. Such states have been shown to exist for e.g. photonic and acoustic waves, and show great promise for applications including lasing, (bio)sensing and filtering. Despite intense research, the mechanism behind these states and their robustness is still poorly understood.
Recently it was proposed theoretically that BICs occur at points where the far-field polarization of the radiated waves shows a vortex, i.e. points where the polarization is undefined . Due to the integer winding number associated to such vortices, the modes should be topologically protected against disorder. In this work, we verify this claim experimentally. We fabricate a SiN grating and use reflection measurements to show that it supports an optical BIC around 700 nm wavelength. We then perform polarimetry measurements in a Fourier reflection microscopy scheme to map the far-field polarization at every angle and wavelength, demonstrating the existence of a vortex at the BIC. We use a simple dipole model to characterize the BIC as a Friedrich-Wintgen type, arising from the interference between two electromagnetic dipoles induced in the grating. Our method can be used to characterize the polarization structure of any leaky photonic mode, including those supporting polarization vortices of arbitrary winding numbers.
 Zhen, B., et al. (2014). Physical review letters, 113(25), 257401.
Hybrid nanophotonic structures are structures that integrate different nanoscale platforms to harness light-matter interaction. We propose that combinations of plasmonic antennas inside modest-Q dielectric cavities can lead to very high Purcell factors, yielding plasmonic mode volumes at essentially cavity quality factors. The underlying physics is subtle: for instance, how plasmon antennas with large cross sections spoil or improve cavities and vice
versa, contains physics beyond perturbation theory, depending on interplays of back-action, and interferences. This is evident from the fact that the local density of states of hybrid systems shows the rich physics of Fano interferences. I will discuss recent scattering experiments performed on toroidal microcavities coupled to plasmon particle arrays that probe both cavity resonance shifts and particle polarizability changes illustrating these insights. Furthermore I will present our efforts to probe single plasmon antennas coupled to emitters and complex environments using scatterometry. An integral part of this approach is the recently developed measurement method of `k-space polarimetry’, a microscopy technique to completely classify the intensity and polarization state of light radiated by a single nano-object into any emission direction that is based on back focal plane imaging and Stokes polarimetry. I show benchmarks of this technique for the cases of scattering, fluorescence, and cathodoluminescence applied to directional surface plasmon polariton antennas.