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Understanding metal-semiconductor interfaces is critical to the advancement of photocatalysis and sub-bandgap solar energy harvesting where sub-bandgap photons can be excited and extracted into the semiconductor. In this work, we compare the electron extraction efficiency across Au/TiO2 and titanium oxynitride/TiO2-x interfaces, where in the latter case the spontaneously forming oxide layer (TiO2-x) creates a metal-semiconductor contact. Time-resolved pump-probe spectroscopy is used to study the electron recombination rates in both cases. Unlike the nanosecond recombination lifetimes in Au/TiO2, we find a bottleneck in the electron relaxation in the TiON system, which we explain using a trap-mediated recombination model. Using this model, we investigate the tunability of the relaxation dynamics with oxygen content in the parent film. The optimized film (TiO0.5N0.5) exhibits the highest carrier extraction efficiency, slowest trapping and an appreciable hot electron population reaching the surface oxide. Our results demonstrate the productive role oxygen can play in enhancing electron harvesting and elongating electron lifetimes providing an optimized metal-semiconductor interface using only the native oxide of titanium oxynitride., Comment: 21 words, 5 figures + 4 supplementary figures

We experimentally investigate second harmonic generation from strongly coupled localized and propagative phonon polariton modes in arrays of silicon carbide nanopillars. Our results clearly demonstrate the hybrid nature of the system's eigenmodes and distinct manifestation of strong coupling in the linear and nonlinear response. While in linear reflectivity the intensity of the two strongly-coupled branches is essentially symmetric and well explained by their respective localized or propagative components, the second harmonic signal presents a strong asymmetry. Analyzing it in detail, we reveal the importance of interference effects between the nonlinear polarization terms originating in the bulk and in the phonon polariton modes, respectively., Comment: 7 pages, 4 figures

Quantum emitters radiate light omni-directionally, making it hard to collect and use the generated photons. Here we propose a 3D metal-dielectric parabolic antenna surrounding an individual quantum dot as a source of collimated single photons which can then be easily extracted and manipulated. Our fabrication method relies on a single optically-induced polymerization step, once the selected emitter has been localized by confocal microscopy. Compared to conventional nano-antennas, our geometry does not require near-field coupling and it is therefore very robust against misalignment issues, and minimally affected by absorption in the metal. The parabolic antenna provides one of the largest reported experimental directivities (D=106) and the lowest beam divergences ({\Theta}=13.5 deg), a broadband operation over all the visible and near-IR range, together with more than 96% extraction efficiency, offering a practical advantage for quantum technological applications., Comment: accepted for publication in Nano Letters

In the search for alternative plasmonic materials SrMoO3 has recently been identified as possessing a number of desirable optical properties. Owing to the requirement for many plasmonic devices to operate at elevated temperatures however, it is essential to characterize the degradation of these properties upon heating. Here, SrMoO3 thin films are annealed in air at temperatures ranging from 75 - 500{\deg} C. Characterizations by AFM, XRD, and spectroscopic ellipsometry after each anneal identify a loss of metallic behaviour after annealing at 500{\deg} C, together with the underlying mechanism. Moreover, it is shown that by annealing the films in nitrogen following deposition, an additional crystalline phase of SrMoO4 is induced at the film surface, which suppresses oxidation at elevated temperatures.

Materials such as W, TiN, and SrRuO3 (SRO) have been suggested as promising alternatives to Au and Ag in plasmonic applications owing to their refractory properties. However, investigation of the reproducibility of the optical properties after thermal cycling at high operational temperatures is so far lacking. Here, thin films of W, Mo, Ti, TiN, TiON, Ag, Au, and SrRuO3 are investigated to assess their viability for robust refractory plasmonic applications. Films ranging in thickness from 50 - 180 nm are deposited on MgO and Si substrates by RF magnetron sputtering and, in the case of SrRuO3, pulsed laser deposition, prior to characterisation by means of AFM, XRD, spectroscopic ellipsometry, and DC resistivity. Measurements are conducted before and after annealing in air at temperatures ranging from 300 - 1000{\deg} C for one hour, to establish the maximum cycling temperature and potential longevity at temperature for each material. It is found that SrRuO3 retains metallic behaviour after annealing at 800{\deg} C, however, importantly, the optical properties of TiN and TiON are degraded as a result of oxidation. Nevertheless, both TiN and TiON may be better suited than Au or SRO for high temperature applications operating under vacuum conditions., Comment: 15 pages, 4 Figures, 3 Tables

The inherent crystal anisotropy of hexagonal boron nitride (hBN) sustains naturally hyperbolic phonon polaritons, i.e. polaritons that can propagate with very large wavevectors within the material volume, thereby enabling optical confinement to exceedingly small dimensions. Indeed, previous research has shown that nanometer-scale truncated nanocone hBN cavities, with deep subwavelength dimensions, support three-dimensionally confined optical modes in the mid-infrared. Due to optical selection rules, only a few of many such modes predicted theoretically have been observed experimentally via far-field reflection and scattering-type scanning near-field optical microscopy. The Photothermal induced resonance (PTIR) technique probes optical and vibrational resonances overcoming weak far-field emission by leveraging an atomic force microscope (AFM) probe to transduce local sample expansion due to light absorption. Here we show that PTIR enables the direct observation of previously unobserved, dark hyperbolic modes of hBN nanostructures. Leveraging these optical modes could yield a new degree of control over the electromagnetic near-field concentration, polarization and angular momentum in nanophotonic applications., Comment: 14 pages with references, 4 figures

Self-assembly processes allow us to design and create complex nanostructures using molecules as building blocks and surfaces as scaffolds. This autonomous driven construction is possible due to a complex thermodynamic balance of molecule-surface interactions. As such, nanoscale guidance and control over this process is hard to achieve. Here we use the highly localized light-to-chemical-energy conversion of plasmonic materials to spatially cleave Au-S bonds on pre-determined locations within a single nanoparticle, enabling unprecedented control over this archetypal system for molecular self-assembly. Our method offers nanoscale precision and high-throughput light-induced tailoring of the surface chemistry of individual and packed nano-sized metallic structures by simply varying wavelength and polarization of the incident light. Assisted by single-molecule super-resolution fluorescence microscopy, we image, quantify and shed light onto the plasmon-induced desorption mechanism. We show that plasmon-induced photo-desorption enables sub-diffraction multiplexing, selective positioning of nanomaterials and even sorting and purification of colloidal nanoparticles.

Nonlinear optics, especially frequency mixing, underpins modern optical technology and scientific exploration in quantum optics, materials and life sciences, and optical communications. Since nonlinear effects are weak, efficient frequency mixing must accumulate over large interaction lengths restricting the integration of nonlinear photonics with electronics and establishing limitations on mixing processes due to the requirement of phase matching. In this work we report efficient four-wave mixing over micron-scale interaction lengths at telecoms wavelengths. We use an integrated plasmonic gap waveguide on silicon that strongly confines light within a nonlinear organic polymer in the gap. Our approach is so effective because the gap waveguide intensifies light by efficiently nanofocusing it to a mode cross-section of a few tens of nanometres, generating a nonlinear response so strong that efficient four-wave mixing accumulates in just a micron. This is significant as our technique opens up nonlinear optics to a regime where phase matching and dispersion considerations are relaxed, giving rise to the possibility of compact, broadband, and efficient frequency mixing on a platform that can be integrated with silicon photonics., Comment: 9 pages, 4 figures

Titanium Oxynitride (TiOxNy) thin films are fabricated using reactive magnetron sputtering. The mechanism of their growth formation is explained and their optical properties are presented. The films grown when the level of residual Oxygen in the background vacuum was between 5E-9Torr to 20E-9Torr exhibit double Epsilon-Near-Zero (2-ENZ) behaviour with ENZ1 and ENZ2 wavelengths tunable in the 700-850 nm and in the 1100-1350 nm spectral ranges, respectively. Samples fabricated when the level of residual Oxygen in the background vacuum was above 2E-8Torr exhibit non-metallic behaviour, while the layers deposited when the level of residual Oxygen in the background vacuum was below 5E-9Torr, show metallic behaviour with a single ENZ value. The double ENZ phenomenon is related to the level of residual Oxygen in the background vacuum and is attributed to the mixture of TiN and TiOxNy/TiOx phases in the films. Varying the partial pressure of nitrogen during the deposition can further control the amount of TiN, TiOx and TiOxNy compounds in the films and, therefore, tune the screened plasma wavelength. A good approximation of the ellipsometric behaviour is achieved with Maxwell-Garnett theory for a composite film formed by a mixture of TiO2 and TiN phases suggesting that double ENZ TiOxNy films are formed by inclusions of TiN within a TiO2 matrix. These oxynitride compounds could be considered as new materials exhibiting double ENZ in the visible and near-IR spectral ranges.

The absorption process of an emitter close to a plasmonic antenna is enhanced due to strong local electromagnetic (EM) fields. The emission process, if resonant with the plasmonic system, re-radiates to the far-field by coupling with the antenna due to the availability of plasmonic states. This increases the local density of states (LDOS), effectively providing more, or alternate, pathways for emission. Through the mapping of localized emission events from single molecules close to plasmonic antennas, performed using far-field data, one gains combined information on both the local EM field strength and the LDOS available. The localization from these emission-coupled events generally do not, therefore, report the real position of the molecules, nor the EM enhancement distribution at the illuminating wavelength. Here we propose the use of a large Stokes shift fluorescent molecule in order to spectrally decouple the emission process of the dye from the plasmonic system, leaving only the absorption strongly in resonance with the enhanced EM field in the antennas vicinity. We show that the real position of the emitters in this complex, but interesting, scenario can be found directly. Moreover, we demonstrate that this technique provides an effective way of exploring either the EM field or the LDOS with nanometre spatial resolution., Comment: 17 pages, 4 figures

Strontium molybdate (SrMoO3) thin films are shown to exhibit plasmonic behaviour with a zero crossover wavelength of the real part of the dielectric permittivity tunable between 600 and 950 nm (2.05 eV and 1.31 eV). The films are grown epitaxially on strontium titanate (SrTiO3), magnesium oxide (MgO), and lanthanum aluminate (LaAlO3) substrates by pulsed laser deposition. SrMoO3 is shown to have optical losses lower than those of gold at the point at which the real part of the dielectric permittivity is equal to -2, while possessing low electrical resistivity of 100E-6 Ohm cm at room temperature. Spectroscopic ellipsometry measurements reveal that SrMoO3 shows plasmonic behaviour, moreover we demonstrate that the epsilon near zero (ENZ) wavelength is tunable by engineering the residual strain in the films. The relatively broadband ENZ behaviour observed in SrMoO3 demonstrates its potential suitability for transformation optics along with plasmonic applications in the visible to near infrared spectral range., Comment: 13 pages, 5 figures

Plasmon hybridization between closely spaced nanoparticles yields new hybrid modes not found in individual constituents, allowing for the engineering of resonance properties and field enhancement capabilities of metallic nanostructure. Experimental verifications of plasmon hybridization have been thus far mostly limited to optical frequencies, as metals cannot support surface plasmons at longer wavelengths. Here, we introduce the concept of 'spoof plasmon hybridization' in highly conductive metal structures and investigate experimentally the interaction of localized surface plasmon resonances (LSPR) in adjacent metal disks corrugated with subwavelength spiral patterns. We show that the hybridization results in the splitting of spoof plasmon modes into bonding and antibonding resonances analogous to molecular orbital rule and plasmonic hybridization in optical spectrum. These hybrid modes can be manipulated to produce enormous field enhancements (larger than 5000) by tuning the separation between disks or alternatively, the disk size, which effectively changes the relative gap size. The impact of the radiation loss is considered to find out the optimum disk size that maximizes field enhancement capabilities. Our investigation not only extends the range of applicability of the hybridization model, but also provides insightful guidance to exporting the exciting applications associated with plasmon hybridization to lower spectral range., Comment: 23 pages, 6 figures

Plasmonic nanoparticles influence the absorption and emission processes of nearby emitters due to local enhancements of the illuminating radiation and the photonic density of states. Here, we use the plasmon resonance of metal nanoparticles in order to enhance the stimulated depletion of excited molecules for super-resolved microscopy. We demonstrate stimulated emission depletion (STED) microscopy with gold nanorods with a long axis of only 26 nm and a width of 8 nm that provide an enhancement of the resolution compared to fluorescent-only probes without plasmonic components irradiated with the same depletion power. These novel nanoparticle-assisted STED probes represent a ~2x10^3 reduction in probe volume compared to previously used nanoparticles and we demonstrate their application to the first plasmon-assisted STED cellular imaging. We also discuss their current limitations., Comment: 30 pages, 15 figures, main text and supplementary information

Superfluidity---the suppression of scattering in a quantum fluid at velocities below a critical value---is one of the most striking manifestations of the collective behaviour typical of Bose-Einstein condensates. This phenomenon, akin to superconductivity in metals, has until now only been observed at prohibitively low cryogenic temperatures. For atoms, this limit is imposed by the small thermal de Broglie wavelength, which is inversely related to the particle mass. Even in the case of ultralight quasiparticles such as exciton-polaritons, superfluidity has only been demonstrated at liquid helium temperatures. In this case, the limit is not imposed by the mass, but instead by the small exciton binding energy of Wannier-Mott excitons, which places the upper temperature limit. Here we demonstrate a transition from normal to superfluid flow in an organic microcavity supporting stable Frenkel exciton-polaritons at room temperature. This result paves the way not only to table-top studies of quantum hydrodynamics, but also to room-temperature polariton devices that can be robustly protected from scattering., Comment: 14 pages, 5 figures, supporting information 7 pages, 9 figures

Of late there has been a surge of interest in localised phonon polariton resonators which allow for sub-diffraction confinement of light in the mid-infrared spectral region by coupling to optical phonons at the surface of polar dielectrics. Resonators are generally etched on deep substrates which support propagative surface phonon polariton resonances. Recent experimental work has shown that understanding the coupling between localised and propagative surface phonon polaritons in these systems is vital to correctly describe the system resonances. In this paper we comprehensively investigate resonators composed of arrays of cylindrical SiC resonators on SiC substrates. Our bottom-up approach, starting from the resonances of single, free standing cylinders and isolated substrates, and exploiting both numerical and analytical techniques, allows us to develop a consistent understanding of the parameter space of those resonators, putting on firmer ground this blossoming technology., Comment: 10 Pages, 8 Figures

Nanoscale localization of electromagnetic fields near metallic nanostructures underpins the fundamentals and applications of plasmonics. The unavoidable energy loss from plasmon decay, initially seen as a detriment, has now expanded the scope of plasmonic applications to exploit the generated hot carriers. However, quantitative understanding of the spatial localization of these hot carriers, akin to electromagnetic near-field maps, has been elusive. Here we spatially map hot-electron-driven reduction chemistry with 15 nanometre resolution as a function of time and electromagnetic field polarization for different plasmonic nanostructures. We combine experiments employing a six-electron photo-recycling process that modify the terminal group of a self-assembled monolayer on plasmonic silver nanoantennas, with theoretical predictions from first-principles calculations of non-equilibrium hot-carrier transport in these systems. The resulting localization of reactive regions, determined by hot carrier transport from high-field regions, paves the way for hot-carrier extraction science and nanoscale regio-selective surface chemistry., Comment: 18 pages, 5 figures

We report on strong enhancement of mid-infrared second harmonic generation (SHG) from SiC nanopillars due to the resonant excitation of localized surface phonon-polaritons within the Reststrahlen band. The magnitude of the SHG peak at the monopole mode experiences a strong dependence on the resonant frequency beyond that described by the field localization degree and the dispersion of linear and nonlinear-optical SiC properties. Comparing the results for the identical nanostructures made of 4H and 6H SiC polytypes, we demonstrate the interplay of localized surface phonon polaritons with zone-folded weak phonon modes of the anisotropic crystal. Tuning the monopole mode in and out of the region where the zone-folded phonon is excited in 6H-SiC, we observe a prominent increase of the already monopole-enhanced SHG output when the two modes are coupled. Envisioning this interplay as one of the showcase features of mid-infrared nonlinear nanophononics, we discuss its prospects for the effective engineering of nonlinear-optical materials with desired properties in the infrared spectral range., Comment: 16 pages, 3 figures

We introduce a real-space technique able to extend the standard Hopfield approach commonly used in quantum polaritonics to the case of inhomogeneous lossless materials interacting with the electromagnetic field. We derive the creation and annihilation polaritonic operators for the system normal modes as linear, space-dependent superpositions of the microscopic light and matter fields, and we invert the Hopfield transformation expressing the microscopic fields as functions of the polaritonic operators. As an example, we apply our approach to the case of a planar interface between vacuum and a polar dielectric, showing how we can consistently treat both propagative and surface modes, and express their nonlinear interactions, arising from phonon anharmonicity, as polaritonic scattering terms. We also show that our theory can be naturally extended to the case of dissipative materials.

According to the hydrodynamic Drude model, surface-plasmon resonances of metallic nanostructures blueshift owing to the nonlocal response of the metal's electron gas. The screening length characterising the nonlocal effect is often small relative to the overall dimensions of the metallic structure, which enables us to derive a coarse-grained nonlocal description using matched asymptotic expansions; a perturbation theory for the blueshifts of arbitrary shaped nanometallic structures is then developed. The effect of nonlocality is not always a perturbation and we present a detailed analysis of the "bonding" modes of a dimer of nearly touching nanowires where the leading-order eigenfrequencies and eigenmode distributions are shown to be a renormalisation of those predicted assuming a local metal permittivity.

By imaging single-shot realizations of an organic polariton quantum fluid, we observe the long-sought dynamical instability of non-equilibrium condensates. Without any free parameters, we find an excellent agreement between the experimental data and a numerical simulation of the open-dissipative Gross-Pitaevskii equation, which allows us to draw several important conclusions about the physics of the system. We find that the reservoir dynamics are in the strongly nonadiabatic regime, which renders the complex Ginzburg-Landau description invalid. The observed transition from stable to unstable fluid can only be explained by taking into account the specific form of reservoir-mediated instability as well as particle currents induced by the finite extent of the pump spot., Comment: 9 pages, 8 figures

Here we study subwavelength gratings for coupling into graphene plasmons by means of an an- alytical model based on transformation optics that is not limited to very shallow gratings. We consider gratings that consist of a periodic modulation of the charge density in the graphene sheet, and gratings formed by this conductivity modulation together with a dielectric grating placed in close vicinity of the graphene. Explicit expressions for the dispersion relation of the plasmon po- laritons supported by the system, and reflectance and transmittance under plane wave illumination are given. We discuss the conditions for maximising the coupling between incident radiation and plasmons in the graphene, finding the optimal modulation strength for a conductivity grating., Comment: Accepted for Journal of Optics, special issue on Transformation Optics, in November 2015

Photonic sensors have many applications in a range of physical settings, from measuring mechanical pressure in manufacturing to detecting protein concentration in biomedical samples. A variety of sensing approaches exist, and plasmonic systems in particular have received much attention due to their ability to confine light below the diffraction limit, greatly enhancing sensitivity. Recently, quantum techniques have been identified that can outperform classical sensing methods and achieve sensitivity below the so-called shot-noise limit. Despite this significant potential, the use of definite photon number states in lossy plasmonic systems for further improving sensing capabilities is not well studied. Here, we investigate the sensing performance of a plasmonic interferometer that simultaneously exploits the quantum nature of light and its electromagnetic field confinement. We show that, despite the presence of loss, specialised quantum resources can provide improved sensitivity and resolution beyond the shot-noise limit within a compact plasmonic device operating below the diffraction limit., Comment: 9 pages, 5 figures, ACS Photonics, 2016

Many promising nanophotonics endeavours hinge upon the unique plasmonic properties of nanometallic structures with narrow non-metallic gaps, which support super-concentrated bonding modes that singularly redshift with decreasing separations. In this letter, we present a descriptive physical picture, complemented by elementary asymptotic formulae, of a nonlocal mechanism for plasmon-redshift saturation at subnanometric gap widths. Thus, by considering the electron-charge and field distributions in the close vicinity of the metal-vacuum interface, we show that nonlocality is asymptotically manifested as an effective potential discontinuity. For bonding modes in the near-contact limit, the latter discontinuity is shown to be effectively equivalent to a widening of the gap. As a consequence, the resonance-frequency near-contact asymptotics are a renormalisation of the corresponding local ones. Specifically, the renormalisation furnishes an asymptotic plasmon-frequency lower bound that scales with the $1/4$-power of the Fermi wavelength. We demonstrate these remarkable features in the prototypical cases of nanowire and nanosphere dimers, showing agreement between our elementary expressions and previously reported numerical computations.

We propose a theory of metals at the spin-density wave quantum critical point in spatial dimension $d=2$. We provide a first estimate of the full set of critical exponents (dynamical exponent $z=2.13$, correlation length $\nu =1.02$, spin susceptibility $\gamma = 0.96$, electronic non-Fermi liquid $\eta^f_\tau = 0.53$, spin-wave Landau damping $\eta^b_\tau = 1.06$), which determine the universal power-laws in thermodynamics and response functions in the quantum-critical regime relevant for experiments in heavy-fermion systems and iron pnictides. We present approximate numerical and analytical solutions of Polchinski-Wetterich type flow equations with soft frequency regulators for an effective action of electrons coupled to spin-wave bosons. Performing the renormalization group in frequency -instead of momentum- space allows to track changes of the Fermi surface shape and to capture Landau damping during the flow. The technique is easily generalizable from models retaining only patches of the Fermi surface to full, compact Fermi surfaces., Comment: 46 pages, 13 figures, typos fixed; as accepted to Physical Review B