We demonstrate a tuneable nanocube-on-mirror Fabry-Perot cavity that combines the record breaking plasmonic confinement of the nanocube-on-mirror (NCoM) system with the high quality factors and tuneability of microcavity systems. We demonstrate selective addressing of individual molecular vibrational lines with robust SERS enhancements on par with those of the seminal NCoM system, reaching sideband resolved SERS at Q/V values above 1 million inverse cubic wavelengths. We envision this as a platform for sideband-resolved molecular optomechanics, polariton chemistry and vibrational strong coupling.
Controlling near-field in space and time is crucial to applications of high-Q nonlocal metasurfaces, for instance for their use in nonlinear frequency conversion. We discuss near-field interferometric autocorrelation (IAC) measurements that reveal the dynamics of optical fields of quasi-bound states in the continuum (quasi-BICs) in plasmonic-dielectric metasurfaces. Using two-photon excited luminescence (TPEL) from quantum dots as local probes, our IAC measurements probe resonant near-field enhancement. Femtosecond laser pulse excitation of quasi-BICs produces coherent oscillations visible in TPEL, offering insights into resonances and their temporal beating. We discuss application scenarios for frequency-converting nonlinear metasurfaces in XUV generation and wafer metrology, as well as strategies for achieving high metasurface Q factors in metallic and dielectric systems.
We show spontaneous symmetry breaking (SSB) in a nonlocal metasurface laser. The system is a hexagonal plasmonic distributed feedback laser that lases at the six K-points in momentum space, or more exactly at the K and K’ modes. A unique properties is that these modes are exactly degenerate in both spatial distribution and energy. By simultaneous real-space and Fourier-space measurements, we map both the relative amplitude (parity symmetry breaking) and phase (rotational symmetry breaking ) of the two symmetry-broken modes. Our results open new perspectives on studying SSB and emergence of spatial coherence in photonic systems.
Plasmon antenna lattices enjoy a long history in the domain of fluorescence control, SERS, sensing and plasmon lasers, due to the combination of high Q and strong field enhancement, by virtue of diffractive resonances. Recent theory for honeycomb, Kagomé and distributed loss and gain lattices point at the potential for topological and pseudochiral bandstructure physics. For this regime, sub-diffractive periodicities are particularly interesting, pushing the array bandstructure beyond the light line and in a regime where plasmon antennas interact strongly. I will present experiments that access this band structure on basis of far-field reflective, fluorescence, and nonlinear Fourier microscopy.
The high Q of microcavities, and the ultimate confinement of plasmonics may be advantageously combined in hybrid photonic-plasmonic resonators for ultrastrong light-matter interactions for SERS, polaritonic chemistry, and novel light sources. I will present the unique merits of hybrids on basis of plasmonic-nanoparticle-on-mirror systems in cavities as compared to simple metasurface etalons and constructs on basis of dielectric microdisk cavities and simple dipole antennas. I will discuss experimental evidence from linear mode spectroscopy, femtosecond SHG and two photon luminescence studies, and SERS, and will provide a viewpoint on potential uses for sideband-resolved molecular optomechanics.
Obtaining nanoscale spatial information without real-space imaging, but from diffraction patterns, is already a valuable tool in metrology We investigate the potential of metasurfaces for nano-optical sensing and metrology with subwavelength resolution. We aim to exploit complex light scattering from a metasurface, programmable illumination, and retrieval of spatial information of a sample from far-field scattering images. We demonstrate an inversion technique based on singular value decomposition whereby we can retrieve the spatial position of a pointlike light source in a plasmon antenna with lambda/50 resolution just on basis of measured far field radiation patterns. Also, we argue that wavefront-shaping applied to plasmon oligomers enables selective generation of sub-diffractive field patterns that could form an optimal and complete basis for spatially-resolved sensing at the nanoscale. Our endeavours use stochastic optimization to choose wavefronts and plasmonic structures.
We report on nonlinear eigenmode and scattering properties of lattice resonances in 2D plasmonic antenna arrays and metasurfaces coupled to spatially distributed loss and gain. Dispersive lattice resonances have already been used to realize plasmon antenna array lasers. Here we go beyond lasers and homogeneously distributed gain. We report on band structures of plasmon lattices with spatially distributed loss and gain, where we show under which conditions topological features related to PT-symmetry occur despite the fact that plasmon arrays are very far from usual tight-binding descriptions, due to dominant far-field retarded interactions. Our experimental work includes first results in which we spatially program gain and loss by shaping of pump light incident on the arrays, and ultrafast response properties probed by two-photon luminescence and interferometric autocorrelation generated in the metasurfaces.
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.
Perovskite quantum dots have shown to be promising bright, tunable, high oscillator strength quantum emitters with potential in quantum optical applications. However, they suffer from intermittency. While superficially similar to other quantum dots, we show that the mechanisms behind intermittency must be fundamentally different, and the models commonly used to describe quantum dot intermittency are insufficient for these systems.
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.
Introducing thin, light-weight and high efficiency photovoltaics will make solar cells more suitable to be integrated in urban landscapes or even small gadgets and would largely contribute to solving the global warming threat that we are facing today. Stacking of solar cells with different characteristic bandgaps is the most common strategy to surpass the Shockley-Queisser efficiency limit, but such tandem devices are typically heavy weight, rigid and costly. Thinning down of absorber materials is a good strategy to overcome these restrictions. However, nano- and micro-meter thicknesses come down to the expense of light absorption. An effective approach to tackle the absorption problem in thin materials is nanopatterning the absorbing layer.
In this work we introduce hyperuniform designs as an effective way to control scattered light into particular range of angles (revealed as a ring in k-space of the reflected/transmitted light), with the aim to efficiently trap light in μm-thick Silicon (Si) cells. We first consider the –theoretical and experimental- case of a single Si solar cell, and thanks to an optimization algorithm, we show the highest light absorption in 1 μm-thick Si film to date. We also compare different designs for best anti-reflection effect on top of light trapping and characterize the increased absorption in photoelectrochemical devices. Second, we incorporate a similar light trapping strategy in a tandem solar cell, by using a periodic GaAs nanowire array as a top cell. We introduce two waveguiding effects in GaAs NW-Si thin film architectures to explain the 4-fold light absorption in the Si ultrathin bottom cell for tailored geometries of the NW array. These results represent significant light trapping scheme that is obtained “for free” when using a nanostructured top cell.
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 [1] 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.
[1] 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 [1]. 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.
[1] Zhen, B., et al. (2014). Physical review letters, 113(25), 257401.
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.
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.
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