Controlling the energy flow processes and the associated energy relaxation rates of a light emitter is of high fundamental interest, and has many applications in the fields of quantum optics, photovoltaics, photodetection, biosensing and light emission. While advanced dielectric and metallic systems have been developed to tailor the interaction between an emitter and its environment, active control of the energy flow has remained challenging. Here, we demonstrate in-situ electrical control of the relaxation pathways of excited erbium ions, which emit light at the technologically relevant telecommunication wavelength of 1.5 µm. By placing the erbium at a few nanometres distance from graphene, we modify the relaxation rate by more than a factor of three, and control whether the emitter decays into either electron-hole pairs, emitted photons or graphene near-infrared plasmons, confined to <15 nm to the sheet. These capabilities to dictate optical energy transfer processes through electrical control of the local density of optical states constitute a new paradigm for active (quantum) photonics.Spontaneous emission constitutes a canonical example of energy flow from an excited light emitter into its environment, where energy relaxation takes place via photon emission. Alternatively, for an emitter in the vicinity of a solid, energy relaxation can occur through channels involving electronic excitations, such as electron-hole pairs and collective charge oscillations (plasmons). Tailoring spontaneous emission by modifying the local density of optical states (LDOS), which governs the emitter-environment interactions [1,2], has been achieved using, amongst others, optical cavities [3][4][5][6], photonic crystals [7,8], and metallic nanostructures [9]. In these systems the LDOS available for the light emitters is typically a fixed property that depends only on the type and geometry of the material system. Here, we control electrically and in-situ the local density of optical states and therefore the energy relaxation rate of a nearby emitter, by employing graphene. Specifically, we demonstrate in-situ tuning of the magnitude and character of the energy transfer pathways from optically excited erbium ions -emitters for near-infrared light that are used as a gain medium in telecommunication applications [10,13]. This control enables new avenues in a range of fields, covering photovoltaics [11,12] The ability to control in-situ the LDOS requires a material for which the optical excitations that occur for a specific emission energy can be modified. Because graphene is gapless and it has a Fermi energy that is electrostatically tunable up to optical energies of ∼1 eV, it can effectively behave as a semiconductor, a dielectric, or a metal. Here, we propose to use these material characteristics to electrically control the relaxation rate and energy transfer processes of a dipolar emitter at subwavelength distance from the graphene. The concept of our experiment is shown in Fig. 1a, schematically representing the gate-tunable ener...

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Ultra-small molecule-like AuN nanoclusters made by a number of atoms N less than 30 were produced by ion implantation in silica substrates. Their room temperature photoluminescence properties in the visible and near-infrared range have been investigated and correlated with the Er sensitization effects observed in Er-Au co-implanted samples. The intense photoluminescence emission under 488 nm laser excitation occurs in three different spectral regions around 750 nm (band A), 980 nm (band B) and 1150 nm (band C) as a consequence of the formation of discrete energy levels in the electronic structure of the molecule-like AuN nanoclusters. Indeed, energy maxima of bands A and C scale with N(-1/3) as expected for quantum confined systems. Conversely, the energy maximum of band B appears to be almost independent of size, suggesting a contribution of electronic surface states. A clear correlation between the formation of band B in the samples and Er-related photoemission is demonstrated: the band at 980 nm related to AuN nanoclusters resonant with the corresponding Er(3+) absorption level, is suggested as an effective de-excitation channel through which the Au-related photon energy may be transferred from Au nanoclusters to Er ions (either directly or mediated by photon absorption), eventually producing the Er-related infrared emission at 1540 nm.

Nowadays nanophotonics aims towards low-cost, chip-scale devices that can tailor electromagnetic properties, one of which is the control of the circular polarization at the nanoscale, important for novel optical devices. Here we show that nanosphere lithography, combined with tilted metal deposition, can provide novel metasurfaces with chiral properties.We apply the photo-acoustic technique to characterize the circular dichroism at 633 nm of polystyrene nanospheres covered by three different metals: Au-and Cr-covered samples show extrinsic chiral behavior, while the Ag-covered sample shows circular dichroism at normal incidence, characteristic for intrinsic chirality. As the experimental data are in good agreement with numerical predictions, we believe that such design can be optimized to get efficient circularly polarized detection at the nanoscale. THE MANUSCRIPTChirality, a lack of the mirror symmetry of an object 1 , is an important property of some of the building blocks of our world: many molecules, amino-acids, DNA, sugars, drugs are chiral. Two mirror images of the same object differently interact with circularly polarized light of the opposite handedness, while having other measurable properties equal. In particular, chirality can affect the absorption and/or phase velocity of circularly polarized light, therefore it is possible to measure a difference in absorption directly related to the molecules' chirality. This measurement is known as Circular Dichroism (CD). At the nanoscale, when the nanostructures are comparable or smaller than the light wavelength, and organized periodically, they form a metasurface; generally, if the symmetry of the metasurface is broken, a chiral behavior is expected 2 . Chiral metasurfaces can manipulate electromagnetic fields and enhance the interaction with chiral molecules, important for chiral sensing 3 . On the other hand, they can control the polarization state of the optical field, or emit circularly polarized light, thus leading to applications in optical and quantum communications 4 . Geometric features of intrinsically chiral metasurfaces (the nanostructure in the unit cell is usually helix or gammadion-like) can be complicated to fabricate and implement at the nanoscale. This problem can be solved by a proper experimental set-up following the rule that the impinging light wavevector, the average surface normal, and the sample direction must be nonplanar. Such chiral behavior is called extrinsic chirality as it is governed by both experimental set-up and the a) Electronic mail:

Silver nanostructures are widely employed for Surface Enhanced Raman Scattering (SERS) characterizations owing to their excellent properties of field confinement in plasmonic resonances. However, the strong tendency to oxidation at room temperature of these substrates may represent a major limitation to their performances. In the present work, we investigated in detail the effects of oxidation on the SERS response of a peculiar kind of Ag nanostructured substrates, i.e., bi-dimensional ordered arrangements of Ag nanoprisms synthesized by nanosphere lithography. Particularly, wavelength-scanned SERS measurements were performed on Ag nanoprism arrays with a different level of oxidation to determine the SERS enhancement curves as a function of the excitation wavelength around the dipolar plasmonic resonance of the arrays. The experimental results were compared with those obtained by finite elements method simulations. With this approach, we were able to decouple the effects of spectral shift and decrease of the maximum value of the SERS enhancement observed for the different oxidation conditions. The results could be interpreted taking into account the inhomogeneities of the electromagnetic field distribution around the Ag nanostructures, as demonstrated by the simulations

The control of the spontaneous emission properties of quantum emitters with limited losses by near-field coupling with plasmons-supporting nanostructures is one of the keys for next-generation high-efficiency and high-coherence plasmonic devices. In the present work, gold nanohole arrays are demonstrated to be an effective plasmonic system for controlling radiative rate and quantum efficiency of the 1540 nm emission of Er3+ ions embedded in silica. Finite element method electrodynamic simulations were used to describe the interaction between dipolar Er3+ emitters and the nanohole arrays. The results are in agreement with those of photoluminescence measurements performed in different coupling configurations. Particularly, we demonstrated that owing to the combination of strong emission enhancement and low level of ohmic losses in the metal, nanohole arrays are able to enhance the far-field photon yield up to 74%. This in turn is related to an extremely high far-field quantum efficiency: more than 90% of the emitted photons reach the far-field for the most efficient configurations investigated in which the extraordinary optical transmission peak of the nanohole array is matched with the Er3+ emission.

To unveil the mechanisms of energy transfer between Au-N nanostructures and Er3+ ions in silica is of paramount importance for the possible use of Au molecular clusters as sensitizing agents of the rare-earth luminescence in photonic devices. In the present work a phenomenological model was developed that allowed us to estimate the most important photophysical parameters as the sensitization cross-section, the fraction of sensitized Er ions, and the coupling distance of the energy transfer. The results demonstrate that in spite of very large sensitization cross-sections (more than 3 orders of magnitude higher than the intrinsic Er excitation cross-section in silica) only a limited fraction of Er ions (<1%) are indirectly excited by the Au-N nanoclusters, and the energy transfer occurs via short-range coupling at interatomic distances, in the range 0.4-0.8 n

The nonlinear absorption properties of bidimensional arrays of Au-Ag bilayered nanoprisms have been investigated by z-scan measurements as a function of the bimetallic nanoprism composition. A tunable ps laser system was used to excite the ultrafast, electronic nonlinear response matching the laser wavelength with the quadrupolar surface plasmon resonances, in the visible range, of each nanoprism array. Due to the strong electromagnetic field confinement effects at the nanoprism tips, demonstrated by finite element method simulations, these nanosystems proved to have enhanced nonlinear optical properties. Moreover, a tunable changeover from reverse saturable absorption (RSA) to saturable absorption (SA) can be obtained by properly controlling the bimetallic composition of the nanoprisms, without modifying the overall morphology of the nanosystems. This capability makes these nanosystems extremely interesting for the realization of solid-state nanophotonic devices with enhanced ultrafast nonlinear optical properties.