Marta Castro-Lopez

Aluminum for nonlinear plasmonics: resonance-driven polarized luminescence of Al, Ag, and Au nanoantennas.

Resonant optical antennas are ideal for nanoscale nonlinear optical interactions due to their inherent strong local field enhancement. Indeed second- and third-order nonlinear response of gold nanoparticles has been reported. Here we compare the on- and off-resonance properties of aluminum, silver, and gold nanoantennas, by measuring two-photon photoluminescence (TPPL). Remarkably, aluminum shows 2 orders of magnitude higher luminescence efficiency than silver or gold. Moreover, in striking contrast to gold, the aluminum emission largely preserves the linear incident polarization. Finally, we show the systematic resonance control of two-photon excitation and luminescence polarization by tuning the antenna width and length independently. Furthermore, we analyze this tuning of the polarization with the rod dimensions by measuring the angular emission of TPPL via back focal plane imaging. Our findings point to aluminum as a promising metal for nonlinear plasmonics.

Multipolar Interference for Directed Light Emission

By directing light, optical antennas can enhance light-matter interaction and improve the efficiency of nanophotonic devices. Here we exploit the interference among the electric dipole, quadrupole, and magnetic dipole moments of a split-ring resonator to experimentally realize a compact directional optical antenna. This single-element antenna design robustly directs emission even when covered with nanometric emitters at random positions, outperforming previously demonstrated nanoantennas with a bandwidth of 200 nm and a directivity of 10.1 dB from a subwavelength structure. The advantages of this approach bring directional optical antennas closer to practical applications.

Plasmonic antennas as design elements for coherent ultrafast nanophotonics

Significance The focus of ultrafast science is rapidly moving toward increasingly complex systems, both on a fundamental level (such as quantum networks in diamond or excitonic coherence in photosynthesis) and in applied physics (such as multiphoton microscopy in membranes and cells). However, to disentangle the heterogeneous contributions to dynamics in such complex structures, femtosecond time resolution needs to be accompanied by nanometric spatial excitation volumes. Here we present preengineered plasmonic structures to amplitude-phase shape excitation pulses in a designed way, and thus deliver simultaneous deterministic spatial and temporal control. We expect these results to establish the reproducibility that ultrafast plasmonics needs to serve as a reliable and accurate tool for the investigation of femtosecond nanoscopic dynamics in complex systems. Broadband excitation of plasmons allows control of light-matter interaction with nanometric precision at femtosecond timescales. Research in the field has spiked in the past decade in an effort to turn ultrafast plasmonics into a diagnostic, microscopy, computational, and engineering tool for this novel nanometric–femtosecond regime. Despite great developments, this goal has yet to materialize. Previous work failed to provide the ability to engineer and control the ultrafast response of a plasmonic system at will, needed to fully realize the potential of ultrafast nanophotonics in physical, biological, and chemical applications. Here, we perform systematic measurements of the coherent response of plasmonic nanoantennas at femtosecond timescales and use them as building blocks in ultrafast plasmonic structures. We determine the coherent response of individual nanoantennas to femtosecond excitation. By mixing localized resonances of characterized antennas, we design coupled plasmonic structures to achieve well-defined ultrafast and phase-stable field dynamics in a predetermined nanoscale hotspot. We present two examples of the application of such structures: control of the spectral amplitude and phase of a pulse in the near field, and ultrafast switching of mutually coherent hotspots. This simple, reproducible and scalable approach transforms ultrafast plasmonics into a straightforward tool for use in fields as diverse as room temperature quantum optics, nanoscale solid-state physics, and quantum biology.

Modal Coupling of Single Photon Emitters Within Nanofiber Waveguides

Nanoscale generation of individual photons in confined geometries is an exciting research field aiming at exploiting localized electromagnetic fields for light manipulation. One of the outstanding challenges of photonic systems combining emitters with nanostructured media is the selective channelling of photons emitted by embedded sources into specific optical modes and their transport at distant locations in integrated systems. Here, we show that soft-matter nanofibers, electrospun with embedded emitters, combine subwavelength field localization and large broadband near-field coupling with low propagation losses. By momentum spectroscopy, we quantify the modal coupling efficiency identifying the regime of single-mode coupling. These nanofibers do not rely on resonant interactions, making them ideal for room-temperature operation, and offer a scalable platform for future quantum information technology.

Reciprocal space engineering with hyperuniform gold disordered surfaces

Hyperuniform geometries feature correlated disordered topologies which follow from a tailored k-space design. Here, we study gold plasmonic hyperuniform disordered surfaces and, by momentum spectroscopy, we report evidence of k-space engineering on both light scattering and light emission. Even if the structures lack a well-defined periodicity, emission and scattering are directional in ring-shaped patterns. The opening of these rotational-symmetric patterns scales with the hyperuniform correlation length parameter as predicted via the spectral function method.

Magneto-electric antennas for directed light emission

Summary form only given. By directing light, optical antennas can enhance light-matter interaction and improve the efficiency of nanophotonic devices. Though several designs have been presented1, most of them have been scaled-down radio frequency antennas which have narrow bandwidths and are sensitive to the position of the coupled emitters. Here we present a magneto-electric antenna: by exploiting the interference between magnetic and electric modes in a split-ring resonator (SRR) we experimentally realize a compact and robust optical antenna which outperforms larger, multi-element antennas in both bandwidth and directionality.Our magneto-electric antenna is in essence a U-shaped SRR2 with a narrow gap, optimized to enhance the coupling of electric dipole sources to the antenna modes (Fig. 1a). Thus, if a local luminescence source is coupled to this antenna then it will simultaneously drive perpendicular electric and magnetic3,4 dipole moments whose inverted symmetries produce a directed light emission5. We experimentally demonstrate this concept using a microscope objective to collect the emission of quantum dots coupled to the magneto-electric antennas. By imaging at the back focal plane, momentum-space images of single antennas are recorded and the directionality can be characterized. The collected luminescence around λ = 800 nm is observed to be highly directed over the entire quantum dot emission spectrum (Fig. 1b), in contrast to the relatively narrowband YagiUda antenna6 based on an array of dephased electric dipoles.Through FDTD simulations we find that the radiated power enhancement and unidirectional emission are both maintained over a broad bandwidth even when the emitter position is displaced within the gap, resulting in the robust performance observed experimentally when the emitters are attached to the antenna at random positions. Finally we analyze the electromagnetic near field and discuss the resonant modes which cause this directed emission. Our results bring directional optical antennas a step closer to practical applications in light emission, sensing and light harvesting thanks to the more compact (λ2/10), more broadband and more robust single-element design. Furthermore, the directional response of split-ring resonator unit cells is an intriguing property to exploit in metamaterials.

Light coupling in polymer nanofibers: from single-photon emission to random lasing

The understanding of the phenomena underlying the interaction of photons with dielectric, metallic and hybrid microand nano-structures and the development of advanced fabrication tools have paved the way to the realization of complex, nanostructured photonic structures, with tailored and exotic absorption and emission properties. Among such nanostructured materials, polymer nanofibers have intriguing and specific properties: they can embed molecular and quantum dot light sources, they can transport light among distant emitters and they can be arranged in 2-dimensional and 3-dimensional architectures in a controlled fashion, forming complex networks of interacting light emitters. However, coupling of light with polymer nanofibers depends on many variables, being often limited by the arrangement and positioning of the nanoscale light-sources, and by the fiber geometry. Here we report on the fabrication of active polymer nanofibers with improved surface properties and controlled geometry by electrospinning. Polarization and momentum spectroscopy of light emitted by molecular compounds and single quantum dots embedded in electrospun polymer fibers, evidence that efficient, nanostructured photon sources with targeted polarization and coupling efficiency can be realized in nanofiber-based photonic environments.

Hyperuniform plasmonic metasurfaces, controlling light with correlated disorder

Coherent control of optical waves by scattering from 2D plasmonic surfaces is a very active field with the goal of wavefront shaping an incoming light beam via nanostructured surfaces. Here we report gold hyperuniform surfaces fabricated by electron beam lithography and designed to interact with visible radiation. We observe omnidirectional light diffraction in the far-field indicating successful k-space design with rotational symmetry, and spontaneous emission directional control of near-field coupled emitters.

Propagation and localization of quantum dot emission along a gap-plasmonic transmission line

Plasmonic transmission lines have great potential to serve as direct interconnects between nanoscale light spots. The guiding of gap plasmons in the slot between adjacent nanowire pairs provides improved propagation of surface plasmon polaritons while keeping strong light confinement. Yet propagation is fundamentally limited by losses in the metal. Here we show a workaround operation of the gap-plasmon transmission line, exploiting both gap and external modes present in the structure. Interference between these modes allows us to take advantage of the larger propagation distance of the external mode while preserving the high confinement of the gap mode, resulting in nanoscale confinement of the optical field over a longer distance. The performance of the gap-plasmon transmission line is probed experimentally by recording the propagation of quantum dots luminescence over distances of more than 4 μm. We observe a 35% increase in the effective propagation length of this multimode system compared to the theoretical limit for a pure gap mode. The applicability of this simple method to nanofabricated structures is theoretically confirmed and offers a realistic way to combine longer propagation distances with lateral plasmon confinement for far field nanoscale interconnects.