Two-Photon Spontaneous Emission in Atomically Thin Plasmonic Nanostructures.

The ability to harness light-matter interactions at the few-photon level plays a pivotal role in quantum technologies. Single photons-the most elementary states of light-can be generated on demand in atomic and solid state emitters. Two-photon states are also key quantum assets, but achieving them in individual emitters is challenging because their generation rate is much slower than competing one-photon processes. We demonstrate that atomically thin plasmonic nanostructures can harness two-photon spontaneous emission, resulting in giant far field two-photon production, a wealth of resonant modes enabling tailored photonic and plasmonic entangled states, and plasmon-assisted single-photon creation orders of magnitude more efficient than standard one-photon emission. We unravel the two-photon spontaneous emission channels and show that their spectral line shapes emerge from an intricate interplay between Fano and Lorentzian resonances. Enhanced two-photon spontaneous emission in two-dimensional nanostructures paves the way to an alternative efficient source of light-matter entanglement for on-chip quantum information processing and free-space quantum communications.

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.

Tunable plasmons in atomically thin gold nanodisks.

The ability to modulate light at high speeds is of paramount importance for telecommunications, information processing and medical imaging technologies. This has stimulated intense efforts to master optoelectronic switching at visible and near-infrared frequencies, although coping with current computer speeds in integrated architectures still remains a major challenge. As a partial success, mid-infrared light modulation has been recently achieved through gating patterned graphene. Here we show that atomically thin noble metal nanoislands can extend optical modulation to the visible and near-infrared spectral range. We find plasmons in thin metal nanodisks to produce similar absorption cross-sections as spherical particles of the same diameter. Using realistic levels of electrical doping, plasmons are shifted by about half their width, thus leading to a factor-of-two change in light absorption. These results, which we substantiate on microscopic quantum theory of the optical response, hold great potential for the development of electrical visible and near-infrared light modulation in integrable, nanoscale devices.

Magnetic polarization in the optical absorption of metallic nanoparticles.

We find remarkably strong absorption due to magnetic polarization in common colloidal and lithographic metallic nanoparticles. Our analysis is based upon a thorough examination of the dipolar electric and magnetic polarizabilities for representative combinations of nanoparticle composition, size, and morphology. We illustrate this concept by first discussing absorption in metallic spheres and then exploring ellipsoids, disks, and rings. Magnetic polarization reaches ~ 90% of the total absorption in 100 nm disks and rings for wavelengths above 1 µm under co-linear electric and magnetic irradiation. Our results demonstrate that the magnetic contribution to absorption cannot be naively overlooked, as it can largely exceed the contribution of electric polarization.

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Quantum plexcitonics: strongly interacting plasmons and excitons.

We present a fully quantum mechanical approach to describe the coupling between plasmons and excitonic systems such as molecules or quantum dots. The formalism relies on Zubarev's Green functions, which allow us to go beyond the perturbative regime within the internal evolution of a plasmonic nanostructure and to fully account for quantum aspects of the optical response and Fano resonances in plasmon-excition (plexcitonic) systems. We illustrate this method with two examples consisting of an exciton-supporting quantum emitter placed either in the vicinity of a single metal nanoparticle or in the gap of a nanoparticle dimer. The optical absorption of the combined emitter-dimer structure is shown to undergo dramatic changes when the emitter excitation level is tuned across the gap-plasmon resonance. Our work opens a new avenue to deal with strongly interacting plasmon-excition hybrid systems.

Beam width of highly-focused radially-polarized fields.

On the basis of a formal analogy with the irradiance moments, analytical definitions are proposed for the width of both the transverse and the longitudinal component of rotationally-symmetric radially-polarized fields at the focal plane of a high-focusing optical system. The beam width of the whole field is also introduced. The transverse beam size is thus associated with the overall spatial structure of the field. The beam-width definitions are applied to an illustrative example, which enables us to show that, at the focal plane, the power contained within a circle whose radius is given by the proposed beam widths represents the main part of the total power.

Vacuum friction in rotating particles.

We study the frictional torque acting on particles rotating in empty space. At zero temperature, vacuum friction transforms mechanical energy into light emission and produces particle heating. However, particle cooling relative to the environment occurs at finite temperatures and low rotation velocities. Radiation emission is boosted and its spectrum significantly departed from a hot-body emission profile as the velocity increases. Stopping times ranging from hours to billions of years are predicted for materials, particle sizes, and temperatures accessible to experiment. Implications for the behavior of cosmic dust are discussed.

Coupling of gap plasmons in multi-wire waveguides.

We investigate the coupling of gap plasmons in various configurations of neighboring metallic nanowires. Starting with the basic element defining a gap plasmon, consisting of two neighboring silver wires, we study the energy splitting and symmetry properties of hybridized plasmons resulting from the interaction of two wire pairs. The system is shown to display non-avoided crossings of hybridized modes, and it evolves at short distances towards a degenerate system consisting of four wires arranged in a square, where two new gap plasmons emerge from redshifted higher-energy modes. The gap modes of three neighboring wires are also described in a continuous transition from a coplanar configuration to an equilateral triangle arrangement. The interaction between wire pairs is shown to be weak enough to prevent efficient transfer of plasmon signal from a pair to the other one, which is beneficial to avoid crosstalking, but not to produce waveguide couplers. The coupling is significantly increased by placing a wire of rectangular cross section in between the wire pairs, thus allowing us to achieve large plasmon-signal transfers within propagation distances below the attenuation length. Our results can find application in the design of signal-processing devices based upon gap plasmons.

Cross-correlation between spiral modesand its influence on the overall spatial characteristics of partially coherent beams.

The overall spatial structure of a general partially coherent field is shown to be connected with the cross-correlation between the so-called spiral modes, understood as the terms of the spiral-harmonics series expansion of the field. The formalism based on the beam irradiance-moments is used, and the light field is globally described by the beam width, the far-field divergence, the beam quality factor, the orientation of the beam profile and the angular orbital momentum, given as the sum of its asymmetrical and vortex parts. This overall spatial description is expressed in terms of the intermodal coherence features (cross-correlation between spiral modes). The above analytical results are also illustrated by means of an example.

Near-field focusing with optical phase antennas.

We investigate the near-field focusing properties of three-dimensional phase antennas consisting of concentric rings designed to have source and image spots separated by several microns from the lens. Tight focal spots are obtained for silicon or gold rings patterned in a silica matrix. We analyze in detail the dependence of the performance of these lenses on geometrical parameters such as the number of rings, the ring thickness, and the focal distance. Subwavelength focal spots are found to form at distances of tens of wavelengths from the lens, thus suggesting applications to remote sensing and penlight microscopy and lithography.

Robust plasmon waveguides in strongly interacting nanowire arrays.

Arrays of parallel metallic nanowires are shown to provide a tunable, robust, and versatile platform for plasmon interconnects, including high-curvature turns with minimum signal loss. The proposed guiding mechanism relies on gap plasmons existing in the region between adjacent nanowire pairs and multiwire arrays. We focus on square and circular silver nanowires in silica, which we demonstrate to perform much better than previous schemes in terms of a relevant figure of merit measuring the degree of allowed integration. Our work provides the tools for designing plasmon-based interconnects and achieving a high degree of integration with minimum cross talk between adjacent plasmon guides, which should be relevant ingredients for future multiplexed biosensors.